Acetylene semi-hydrogenation: recent advances

Catalytic processes have been the subject of research throughout their creation, both from the academic and industrial perspective. The first industrial catalytic processes of nitrogen fixation to ammonia and the development of catalysts for ethylene polymerization marked a new era in the development of chemistry. Many important chemical transformations are catalytic today, e.g., cross-coupling,[1-4] hydrogenation,[5-9] alkylation, and many others.[10-16] The use of transition metal compounds were for these purposes a common case. The variable oxidation state and d-orbital in the atomic structure of these metals provide many opportunities for tuning the electronic structure of the metal centre and increase the number of possible compounds that it can form.

Pd, Pt, Ru, Ni, and Cu-based catalysts are extremely active even in small amounts. Typically, catalytic systems including noble metals are characterized by low selectivity and promote the reaction in all possible directions. At the same time, they are prone to poisoning by impurities that can block their active centres. Yet another issue is sintering (coking) due to high temperatures. All these factors adversely affect the selectivity and performance of catalysts, reducing their durability. The said drawbacks stimulate research to improve the properties of available catalysts and develop new more efficient catalytic systems.

The study of the properties of catalysts is usually carried out on model reactions. Ideally, the most appropriate reactions should proceed completely and selectively, and the product should be clearly identified in the mixture. Hydrogenation is one of the key catalytic reactions in chemical synthesis,[17-27] that meets the listed requirements. In particular, acetylene hydrogenation can be considered as a convenient model for studying the properties of metal catalysts.

Why acetylene hydrogenation? Acetylene is the simplest alkyne consisting of triple-bonded two carbon atoms. Unsaturated character of acetylene is widely used in the synthesis of vinyl monomers, heterocycles, and a wide range of valuable chemicals.[28-33] Hydrogenation of acetylene can proceed with the formation of ethylene or ethane. Semi-hydrogenation of acetylene is a critical process providing selective conversion of acetylene to ethylene, while preventing further hydrogenation to ethane. Ethylene is one of the key raw materials of the chemical industry, representing the main supplier of the C₂ monomer unit. In 2023, global polyethylene production amounted to 141 million tons.[34] Ethylene obtained by high-temperature cracking contains up to 2% of acetylene as an impurity.[35-39] Since acetylene is a catalytic poison in further ethylene polymerization, ethylene must be carefully purified from acetylene, and the residual amount of acetylene should not exceed 5 ppm.[40] Separating ethylene and acetylene is a complicated, labour-intensive task, that requires special approaches.[41-44] Currently, the removal of acetylene from ethylene prior to polymerization is usually realized via selective hydrogenation of acetylene to ethylene.

Acetylene hydrogenation is a tandem reaction including two competing parallel reaction pathways:^* sequential hydrogenation of acetylene to produce ethylene (reaction (1)) and its further hydrogenation to ethane (reactions (2) and (3)).[45]

Prolonged hydrogenation of acetylene in the reducing medium in presence of a catalyst results in the formation of oligomerization products of a linear structure with an even number of carbon atoms (C₄₊, reaction (4)). The liquid fraction of the hydrocarbons is called ‘green oil’.[46] The oil is formed on the surface of catalysts only in the combined presence of acetylene and hydrogen,[47][48] and blocks active centres, deactivating a catalyst.[49][50] Most traditionally employed noble metal-based hydrogenation catalysts [51-60] promote all possible reactions, including the formation of the ‘green oil’. Therefore, the best or ideal catalyst, in addition to high selectivity to acetylene semi-hydrogenation and anti-coking properties, should inhibit the formation of oligomerization products and be resistant to their effects (if green oil is formed). The research of many scientific groups is focused on solving precisely these problems.

According to the SciFinder database, the next trend in publication activity in the field of selective catalysts development and their application in hydrogenation of acetylene occurred over the period 2021 – 2024 (Fig. 1). This increase in the number of works was largely determined by the wide access to the synthesis and study of single-atom catalysts, which significantly changed the ideas about the nature of catalysis per se in general.

Fig. 1

Analysis of publication activity on selective hydrogenation of acetylene in the period 1990 – 2025, carried out using the CAS SciFinder search system

The previous reviews have contributed greatly to the summarization of methods for the selective hydrogenation of acetylene,[61] the systematization of approaches to bimetallic[62][63] and single-atom catalysts,[64] and also revealed the role of solid solutions of insertion in the palladium-catalyzed hydrogenation of acetylene to ethylene.[65][66]

In this review, we summarized information on the achievements in the field of metal-catalyzed selective hydrogenation of acetylene over the last five years. The available data on the nature of the element used as a catalytically active component of the reaction have been systematized. The work is not limited to palladium and its compounds, but includes studies on the activity of nickel, copper, and bimetallic systems. The authors do not limit themselves to considering only heterophase processes, but also jointly consider the hydrogenation of an ethylene mixture with an admixture of acetylene and pure acetylene into ethylene.

* After the reaction equation, the values of their standard enthalpies (DH°) are given.

An effective catalytic system involves obtaining the desired compounds by the chemical reaction in high yields in the absence of by-products. The challenge of catalytic transformations in chemistry is significantly complicated by additional needs, viz., the possibility of catalyst regeneration,[67] the use of minimal catalyst loadings, the absence of stabilizers and expensive ligands,[68] the use of cheap metals.[69] Homogeneous and heterogeneous catalysts are successfully used to solve many problems in organic synthesis.[70-75] Homogeneous catalysts are effective and selective, mainly due to the fact that they generate certain rather stable molecular species in the reaction medium. However, it is usually not possible to reuse such catalysts. Heterogeneous catalysts may be used several times. However, with an increase in the number of cycles, their activity and selectivity may significantly decrease, and the detection of metal-containing species in the solution indicates the transition of part of the catalyst into the solution. This calls into question the heterogeneous nature of the processes and also leads to a decrease in the amount of catalyst and the impossibility of its reuse.

In recent years, there has been a significant accumulation of new knowledge and an evolution of our understanding of catalysis and catalysts. An important conceptual achievement is the development of ideas that expand our understanding of homogeneous and heterogeneous systems and the discovery of dynamic catalyst cocktail systems.[76][77] Catalyst cocktail systems were first described by Ananikov and co-workers.[78] In this Section, mechanistic studies of the dynamic nature of catalysis using various reactions as examples are described, which is necessary for forming ideas about the methods for studying dynamic phenomena in catalysis. At the end of this Section, the effects of ‘cocktail’ systems on the mechanisms of hydro­genation reactions are considered. In the following Sections, specific examples of hydrogenation reactions are given, many of which highlight various aspects of the dynamic behaviour of catalytic systems.

The type (or nature) of catalysis is largely determined by the particle size.[79][80] Nanoparticles and their assemblies [81][82] have fundamentally different properties compared to microparticles. Further reduction in particle size leads to the formation of clusters consisting of several atoms, as well as individual atoms.[83-86] An even greater reduction in the concentration of active sites on the surface of a heterogeneous catalyst is achieved by isolating the active metal with another metal, which can lead to the formation of alloys or intermetallides. All of these particles exhibit different behaviour patterns, and the variability of the processes is astounding. Catalytically active species can stick together, pass into solution as complexes and precipitate back as particles, migrate along the surface of the substrate, be immersed in the substrate, and become coated with a layer of other compounds. All this significantly changes the catalytic activity and selectivity [87][88] and makes the nature of catalysis dynamic.[89][90]

Thus, it was shown that in a model cross-coupling reaction [91][92] the used Pd/NHC (NHC is N-heterocyclic carbene) complex catalysts are transformed in the course of the reaction into metal nanoparticles and clusters due to the rupture of Pd-NHC coordination bonds and the transformation of the ligand, as evidenced by electrospray ionization high resolution mass-spectrometry (ESI-HRMS).[93] It is important to note that the catalyst transformations did not reduce the catalytic activity of the system. This was probably due to the formation of new catalytically active species via the leaching of Pd particles from the surface of the metal catalyst. The reversibility of the transition of species into solution (leaching) and back was calculated using density functional theory (DFT) modelling methods on model reactions of C – H-arylation, Heck, and amination.[94] As was found out, Pd leached into solution from the catalyst was the progenitor of a ‘cocktail’ of compounds that were classified into three main groups (Fig. 2):

1) catalytically active Pd nanoparticles with a high concentration of surface defects,

2) monomeric and oligomeric L[ArPdX]_nL (L is a ligand) particles,

3) irreversibly deactivated Pd species.

Fig. 2

Possible processes of leaching, trapping and oligomerization in Pd catalytic systems. Pd nanoparticles are shown in blue, Pd species that have passed into solution are shown in green. Reproduced from Ref. 94 under a CC-BY-NC-ND license.

The study of complex systems requires a special approach. The traditionally used mercury test, despite its rapidity and versatility for identifying homogeneous and heterogeneous catalysis, demonstrated low analytical accuracy and low reproducibility for Pd and Pt complexes.[95] A low-temperature solid-state NMR spectroscopy technique was proposed to monitor hybrid catalytically active species and study the mechanisms of their reactions.[96] It was shown that for certain nanoparticle sizes, only six NHC ligands are sufficient to cover 80% of the surface of Pd particles measuring ~ 1 nm. Later, the concept of 4D catalysis was proposed, combining the achievements of instrumental analysis methods and the potential of machine learning for studying the mechanisms of dynamic catalysis.[97]

Leaching of a metal catalyst is uneven and depends on the type of facet of the metal catalyst surface. In bimetallic nanocatalysts, predominantly one metal is etched. Surface particles can migrate to the surface, stick together or separate from the surface and settle on it again. The presence of oxygen in the system can contribute to the formation of defects in the crystal lattice of metal particles and clusters, which sometimes plays a decisive role in some catalytic processes. All possible processes are characteristic of the formed catalyst particles: growth and dissolution of particles, movement along the surface and changes in the internal structure at the level of individual atoms, clusters and nanoparticles (NPs). Atoms are also detached from the edges and corners of NPs. Atoms can precipitate from the solution on the catalyst surface both on free surface areas and on NPs. In the first case, atoms can re-enter the catalytic cycle, in the second, they are poisoned by nanoparticles, changing the morphology of the particles and causing their growth.

Not only palladium but also nickel form catalytic systems of the ‘cocktail’ type. In a study,[98] a comparative characteristics and properties of two widely used nickel and palladium catalysts are given. Due to several oxidation states, nickel provides higher activity and larger variety of chemical transformations than palladium. This becomes possible despite the higher activation barrier of nickel catalysts than palladium ones. The authors note that Pd catalytic systems can easily form a ‘cocktail of species’ from various palladium compounds, but with similar oxidation states (Pd₁, Pd_n, Pd NPs), while nickel can behave as a ‘cocktail of species’ in different oxidation states with a smaller number of compounds.

Dynamic transformations of the catalyst in chemical processes significantly complicate the study of reaction mechanisms and catalytic cycles. Nevertheless, these data are necessary for understanding not only the course of reactions, but also the essence and nature of catalysis per se. Hydrogenation of alkynes can be considered as one of the representative examples for studying the nature of catalysis.[99][100]

Metal-catalyzed hydrogenation with hydrogen is carried out in a homogeneous or heterogeneous mode using catalysts based on palladium, nickel and some other transition metals. According to the Horiuti – Polanyi principle (dissociative mechanism), the heterogeneous hydrogenation pathway involves the sorption of the substrate and hydrogen on the catalyst surface. In this case, two variants of the reaction are possible: hydrogenation on the surface of metal particles occurs selectively to an alkene. Sorption of reagents in the entire volume of the catalyst promotes the hydrogenation of an alkyne to an alkane. Adsorption and activation of hydrogen are important steps in the hydrogenation process.

On metal catalysts, the bond in the hydrogen molecule can undergo both homolytic and heterolytic cleavage. This is because transition metals have unoccupied d-orbitals to accommodate hydrogen electrons. At the same time, the heterolytic cleavage of the H – H bond creates a vacant s-orbital for the transfer of electrons from the metal.

In the Langmuir – Hinshelwood mechanism, which remains the most common for heterogeneous catalytic systems, two pathways can be distinguished (Fig. 3). The Horiuti – Polanyi mechanism, or the so-called dissociative mechanism, proposed in 1934, involves the homolytic dissociation of hydrogen on the metal surface, followed by the sequential addition of H atoms to the adsorbed alkynes.[100] Hydrogenation of an alkyne with molecular hydrogen begins with the sorption of the alkyne and hydrogen on the catalyst by bonding the Pd atom with the electrons of the π-bond and forming the [alkyne – Pd] complex (Fig. 3, route A – B).[101] The heat of adsorption of ethyne on palladium through the coordination of both π-bonds of the alkyne molecule with the metal is 112 kJ mol^–1. The next step (route B – C) comprises the migration and addition of the adsorbed H₂ molecule to the [alkyne – Pd] complex. In this case, a σ-bond is formed between the adjacent carbon and Pd, yielding an intermediate (Z)-vinylalkyl compound C or σ-vinyl[Pd]. Upon addition of an additional H atom, σ-vinyl[Pd] is converted to π-alkene[Pd]. This process is thermodynamically favourable (ΔH = −136 kJ mol^–1).

Fig. 3

Mechanisms of hydrogenation of alkynes and alkenes. Red balls are H atoms in molecular hydrogen, green balls are H or D atoms attached to the unsaturated substrate. Reproduced from Ref. 101 with permission from the American Chemical Society.

Further, the hydrogenation process can proceed in two directions. The first consists of the decomposition of the π-alkene[Pd] complex to give free alkene D* (semi-hydrogenation). The second direction is further hydrogenation of the resulting alkene. π-Alkene[Pd] can add adsorbed hydrogen to form a σ-bonded alkyl complex E. This process is energetically less favourable than the formation of a free alkene (route D – E, ΔH = −35 kJ mol^–1). At the final stage E – F, the σ-bond Pd – C is broken with the complete hydrogenation product F.

The mechanism of alkyne hydrogenation over alloy catalysts is somewhat more complex, since it is necessary to study and take into account the effect of each metal in the alloy separately. The mechanism of acetylene hydrogenation on the Pd – Mn/Al₂O₃ alloy catalyst is similar to that on the monometallic Pd catalyst.[102] The authors noted that the initial stage of formation of the π-complex of acetylene with the active centre of the catalyst is fast. The subsequent stages of hydrogen addition there to give ethylene and the formation of the [alkene-metal] complex occur much more slowly. In a study,[103] the mechanism of acetylene hydrogenation over the Pd – Ag/α-Al₂O₃ alloy catalyst was studied using the theory of quantum mechanics and the Monte Carlo method. The doping of the Pd catalyst with Ag reduced the adsorption energies of C₂H₂ and C₂H₄ on the catalyst surface (Fig. 4) and facilitated the desorption of ethylene from the catalyst surface, thereby improving the selectivity of hydrogenation. It is noted that the adsorption energy of acetylene and ethylene decreased with a decrease in the number of Pd atoms or an increase in the number of Ag atoms. Therefore, the activity of the catalysts was proportional to the number of Pd atoms and inversely proportional to the number of Ag atoms, which was consistent with the experimental results.

Fig. 4

Effect of Ag doping on the activation energy of acetylene hydrogenation. Reproduced from Ref. 103 with permission from Taylor & Francis.

During the hydrogenation, the dispersed Pd – Ag/α-Al₂O₃ catalytic particles undergo agglomeration, and their average size increases almost 2-fold.[104] Analysis of images obtained by scanning electron microscopy (SEM) (Fig. 5) showed that in the course of hydrogenation, polymeric compounds and coke accumulate on the catalyst surface, which in turn leads to its deactivation.

Fig. 5

SEM images of fresh (a) and spent (b) catalysts. Darker areas correspond to polymer formations on the catalyst surface. Reproduced from Ref. 104 under a Creative Commons Attribution (CC BY) license.

Homogeneous hydrogenation can involve activation of the alkyne within the metal complex. Pd/NHC complexes are widely used as catalysts in homogeneous hydrogenation reactions. Denisova et al.[105] described the experience of successful use of Pd/NHC complexes in homogeneous-heterogeneous (transfer) catalytic hydrogenation. It was found that using a ¹³C-labeled NHC ligand, the Pd/NHC catalysts are converted to Pd NPs (1 – 9 nm) in the first minutes of the diphenylacetylene transfer hydrogenation, indicating a mixed type of catalysis. The authors proposed a mechanism of mixed hydrogenation, in which oxidative addition occurs on the surface of Pd particles, and the stages of syn-addition of alkynes and reductive elimination occur in the zone of Pd nanoparticles modified with NHC ligands. Using DFT calculations on the Pd₃ catalyst model, a mechanism of three-stage hydrogenation was proposed that includes the reaction of oxidative addition of molecular hydrogen to the Pd cluster, syn-addition of H₂ to the triple CºC bond of acetylene and reductive elimination to give an ethylene molecule. The presence of a ligand on the metal surface complicates the addition of hydrogen.

Similar studies were carried out for the Pd/NHC-catalyzed semi-hydrogenation of 1,2-diphenylacetylene.[106] Using modern NMR, high resolution mass spectrometry with electrospray ionization (ESI-HRMS) and SEM/TEM methods, it was shown that during the hydrogenation process, the metal complexes are transformed into Pd NPs with the formation of a heterogeneous phase. Modification of the surface of these NPs with organic ligands under the reaction conditions significantly affected their catalytic activity.

It is worth noting that studies in the field of homogeneous catalysis confidently demonstrate the presence of dynamic processes. The composition of particles is constantly changing, which means that their properties and the very catalytic activity change. Under conditions of heterogeneous catalysis, the number of possible processes seems more limited. However, even in the solvent-free heterogeneous systems, it has been shown that metal atoms can migrate along the substrate surface and undergo various changes, e.g., combine into larger clusters. This suggests that the heterogeneous process is also an evolutionary system. This raises a question that bridge across the present Section with the following ones: is it possible to discuss the dynamic nature of catalysis in relation to heterogeneous systems? In the authors opinion, the reaction of acetylene hydrogenation is a very convenient tool for proving this fact.

Palladium is a heterogeneous effective hydrogenation catalyst for a wide range of compounds.[107-115] The high activity of Pd catalysts is both an advantage and disadvantage. High-pressure hydrogenation of acetylene in the presence of Pd compounds produces mainly ethane rather than ethylene. Suppression of excess Pd activity to increase selectivity of acetylene semi-hydrogenation reactions is one of the main areas of catalysis. Common tools for varying the properties of a catalyst include reducing the size of catalytic species, the use of bimetallic catalysts, and varying the support (carrier) for the metal particles. Below are the main advances in the field of increasing the selectivity of Pd hydrogenation catalysts.

Single-atom (SA) catalysts have attracted particular attention due to their high reactivity and excellent catalytic efficiency,[116-120] which the small particle size and the resulting weak adsorption of ethylene on its surface. selectivity of acetylene semi-hydrogenation was using the DFT method. The dependence of the increase in selectivity of acetylene semi-hydrogenation on the decrease in the catalyst particle size was calculated using DFT method.[121] Pd/C catalyst obtained by impregnation with subsequent hydrogenation in a hydrogen stream at 500°C was used as a model catalyst. S-containing silane agents served as nanoparticle size regulators. Metal NPs were firmly fixed on the support due to the formation of Pd – S bonds with thiol groups, and were uniformly distributed over the surface. Interestingly, the Pd particle size of 2 nm was crucial, since particles of this size contributed to the formation of C₄ hydrocarbons and ‘green oil’.

For example, 26 nm Pd NPs with a close-packed (Pd(111)) surface demonstrated 16 times greater ethylene selectivity than commercially available Pd/C.[122] At 100% acetylene conversion, the selectivity to acetylene was 76.5% and 4.5% for Pd(111) and Pd/C, respectively (Fig. 6, TEM is transmission electron microscopy). The authors suggested that the increase in the selectivity of nanostructured Pd stemmed from lower hydrogen adsorption on the metal catalyst because of the formation of palladium hydride sites Pd₄H₃ in the hydrogen stream.

Fig. 6

TEM images of a commercially available Pd/C catalyst (a) and octahedral Pd/C (b). Acetylene conversion and selectivity to ethylene in acetylene hydrogenation over commercial Pd/C (c) and octahedral Pd/C (d) catalysts at 293 K, p(C₂H₂) = 0.02 atm, gas hourly space velocity (GHSV) is 120 000 h⁻¹. The insets in the upper corners show enlarged high resolution TEM images. Reproduced Ref. 122 with permission from the American Chemical Society.

In fact, the selective hydrogenation of acetylene requires the reaction to occur on the surface of the catalyst, not in its bulk. The same idea on the need to separate the surface and bulk reactions of acetylene hydrogenation to increase selectivity to ethylene was highlighted in the work[123] (Fig. 7). According to in situ X-ray photoelectron spectroscopic analysis of solid Pd catalysts, non-selective hydrogenation occurred at a low carbon content in the catalyst, and the process itself occurred on hydrogen-saturated β-hydride in the catalyst bulk.

Fig. 7

Schematic view of the processes of non-selective and selective hydrogenation of acetylene. Grey balls represent the metal catalyst, red balls represent hydrogen atoms in molecular H₂ as, blue balls represent hydrogen atoms in acetylene, and black balls represent carbon atoms.

The high selectivity to ethylene in the semi-hydrogenation of acetylene at high temperatures on the Pd(111) catalyst was in good agreement with the calculated value (82% at 300 K), obtained by ab initio molecular dynamics (AIMD) methods. The calculated values of the free energy barriers for ethylene desorption on Pd(111) were 0.51 eV at 300 K and 0.39 eV at 500 K.[124] The surface of the close-packed monoatomic Pd(211) catalyst was more active than that of Pd(111). At the same time, differential chemisorption energy of Pd(211) catalyst was higher with a lower barrier to further hydrogenation of ethylene (Table 1).[125] DFT calculations and coverage-dependent microkinetic simulations showed that ethylene was produced on the close-packed regions of the catalyst, while the defective surface regions were significantly more active in selective ethane formation.

Table 1

\[ \]

Adsorption energies (E_ads, eV) and free adsorption energies (G_ads, eV) of acetylene and ethylene on palladium surfaces.¹²⁵

(1)

According to DFT calculations, hydrogenation of ethylene to ethane on the SA – Pd catalyst required overcoming two barriers of 0.93 and 1.22 eV.[126] The chemical adsorption energy of ethylene is 0.76 eV, while the hydrogenation barriers are only 0.62 and 0.35 eV, located below the ethylene level.

Template method was used for the Pd catalyst design. Isolated Pd atoms (Pd-ISA) were anchored on the inner walls of mesoporous nitrogen-doped carbon foam nanospheres (MPNC).[127] Such catalyst provided complete conversion of acetylene hydrogenation at only 60°C! However, the selectivity of the catalyst was lower than 20%. Increasing the temperature improved the hydrogenation selectivity. The ISA-Pd/MPNC and ISA-Pd/non-MPNC (non-MPNC is non-mesoporous analogue of MPNC) catalysts were used at 120 and 140°C, respectively, and were more selective at lower loadings (200 vs. 300 mg): acetylene conversion was 83% and 32%, respectively, and ethylene selectivity was 82% and 17%, respectively.

Atomically dispersed Pd₁/C₃N₄ catalyst and Pd NP catalyst supported on graphite carbon nitride (gC₃N₄), Al₂O₃ and SiO₂ were synthesized by wet-impregnation of the support with a solution of Pd(acac)₂ (acac is acetyl acetonate) followed by calcination in a flow of 10% O₂/He and H₂.[128] Surprisingly, when hydrogenating acetylene in excess ethylene, the Pd NPs/gC₃N₄ catalysts demonstrated higher selectivity to ethylene than the common Pd/Al₂O₃ and Pd/SiO₂ catalysts. The high catalytic performance was, probably, due to the significant charge transfer from Pd NPs to the gC₃N₄ support. A single-atom Pd₁/C₃N₄ catalyst demonstrated superior catalytic performance compared to Pd NPs/gC₃N₄ catalysts with higher selectivity to ethylene and higher coking stability in both reducing and oxidizing media at high temperatures. Atomically dispersed palladium species Pd-ISAS/CN immobilized on N-doped carbon nanospheres obtained from covalent organic frameworks (COFs) (demonstrated 80% selectivity to ethylene and 92% acetylene conversion at 100°C, with the catalyst activity of 712 mol_C2H2 (mol_Pd h)^–1.[129]

Ultrasmall Pd NPs prepared using H₂ instead of NaBH₄ to reduce NaBH₄ and immobilized in the micropores of layered covalent triazine frameworks (CTFs) demonstrated 100% acetylene conversion with 99.9% ethylene selectivity, as well as excellent stability at 250°C and a space velocity of 110 000 h^–1.[130] The nanocatalyst demonstrated high recycling potential with full conversion and selectivity maintained for 5 cycles. The authors explained the improved selectivity rates by the layering CTFs into thinner and smaller sheets during the reaction, which provides greater access of reagents to the catalytically active Pd sites and increased mass transfer.

Varying the support for immobilization of metals is a convenient tool for controlling the catalyst properties. MgO-supported metal Pd particles (Pd/MgO) showed high selectivity (82%) in the hydrogenation of acetylene to ethylene at 200°C with ultra-low catalyst loading (0.05 wt.%).[131] Such catalyst with a nominal Pd loading of 0.001% demonstrated excellent performance, providing high acetylene conversion (97%) and good selectivity to ethylene (89%).[132] Amazingly, but under the same conditions, the ethylene selectivity of 1% Pd/MgO catalyst was lower than 4%. Kinetic isotope effect and IR spectroscopy data suggested heterolytic H₂ dissociation on isolated Pd atoms to form the O – H bond. This probably determines the high selectivity in the selective hydrogenation of acetylene. Moreover, dispersed Pd atoms were inactive for ethylene hydrogenation, which prevented the formation of ethane.

Single-atom Pd₁/MgO acetylene semi-hydrogenation catalyst was obtained mechanochemically in a ball mill by grinding palladium phthalocyanine and magnesium phthalocyanine, followed by calcination at 600°C.[133] The nature of single-atom Pd species after hydrogenation at 100°C was confirmed by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) (Fig. 8). The strong interaction between Pd atoms and MgO support not only inhibited the aggregation of Pd atoms, but also resulted in high ethylene desorption ability of Pd, thereby providing high selectivity of acetylene semi-hydrogenation: 100% C₂H₂ conversion with 60% selectivity to acetylene at 140°C and TOF = 0.074 s^–1.

Fig. 8

AC-HAADF-STEM images of the surface of Pd₁/MgO (a) and (b) Pd₁/MgO-H100, which is parent Pd₁/MgO particles reduced at 100°C with 10 vol.% H₂ (b). A part of isolated Pd atoms are designated as yellow circles. Reproduced from Ref. 133 with permission from Wiley.

Pd₁/CeO₂ and Pd₁/α-Fe₂O₃ catalysts were prepared by reducing conventional supported Pd/CeO₂ and Pd/α-Fe₂O₃ obtained by impregnating the supports with Pd(NO₃)₂ solution, followed by reduction with 10 vol.% H₂/He at 200, 500, and 600°C and oxidation with 20 vol.% O₂/He at 500°C to selectively encapsulate Pd nanoparticle clusters.[134] Single atoms exhibited strong metal – support interactions only at higher reduction temperatures than for nanoparticles/clusters, thus contributed to increased selectivity and stability in acetylene semi-hydrogenation. Pd/CeO₂ catalyst prepared by impregnation with the initial humidity and calcined at 600°C and 800°C demonstrated excellent ethylene selectivity (~ 100%) in acetylene hydrogenation in large excess of ethylene at 50 – 200°C.[135] This was probably due to the formation of Pd_xCe_{1 – x}O_{2 – y} or Pd – O – Ce phases on the catalyst surface, as confirmed by the results of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy and indicated a strong interaction of Pd and Ce.

SA catalyst coordinated by 1,10-phenanthroline-5,6-dione (PDO) was successfully used for the semi-hydrogenation of acetylene under mild conditions.[136] The hydrogenation selectivity was highly dependent on the metal : ligand molar ratio. Surprisingly, the increase in ligand loading promoted the hydrogenation of acetylene to ethylene. In the absence of an organic ligand, 100% conversion of acetylene to ethane was observed. At a PDO : Pd ratio of 1 : 1, the ethylene selectivity was 30%. The best selectivity (80%) was obtained at a ligand : Pd ratio of 6 : 1, which remained constant after a further increase in the ligand amount (Fig. 9). The authors suggested, that the metal atoms are maximally coordinated and are unable to form bonds with other ligand molecules at a ratio of 6 : 1. According to calculations, the binding of hydrogen to the ligand switches the reaction towards ethylene formation. The measured activation energies for the PDO – Pd(1 : 1)/CeO₂ and PDO – Pd(6 : 1)/CeO₂ catalysts were 36 kJ mol^–1 in both cases. It was later shown that the organic ligand stabilized the single metal atoms through the strong Pd – O or Pd – N interaction.[137] Ethylene molecules were stronger adsorbed on the Pd₁ – PDO/CeO₂(111) catalyst than C₂H₂ molecules, which resulted in higher selectivity in the semi-hydrogenation.

Fig. 9

Conversion (a) and ethylene selectivity (b) of Pd – PDO catalysts as a function of the PDO : Pd ratio in acetylene hydrogenation, T = 120°C. Reproduced from Ref. 136 with permission from Elsevier.

Boron nitride (BN) used as a Pd support acted as a regulator of the electron density of Pd and promoted the desorption of ethylene in acetylene semi-hydrogenation.[138] The Pd/BN catalyst provided 100% acetylene conversion and 93% ethylene selectivity with good stability.

Single-atom Pd species coordinated by dicyandiamide (N_dicy) and supported on hierarchical carbon nanocells (hNCNC) demonstrated an 18-fold increase in activity compared to similar catalysts in the absence of organic ligand (Pd₁/hNCNC) and provided 99% C₂H₂ conversion and 87% selectivity at 160°C due to C₂H₄ desorption on isolated single Pd sites in Pd₁ – N_dicy/hNCNC.[139] N-doped well-organized carbonaceous cells were prepared in situ by the template method at 800°C from MgO and pyridine or benzene as a carbon source. The Pd₁ – N_dicy/hNCNC catalyst was prepared by pyrolysis of a mixture of the support, PdCl₂ and dicyanamide at 700°C.

Single-atom Pd₁/TiO₂ catalyst was prepared by mechanochemical mixing of palladium and titanium phthalocyanines and TiOPc in a ball mill followed by calcination and reduction with hydrogen. For comparison, the same catalyst was obtained by the classical impregnation method. Such catalysts showed low yields in the hydrogenation of acetylene. It should be noted that light irradiation increased the conversion of acetylene hydrogenation from 20 to 80% with a simultaneous decrease in ethylene selectivity. DFT calculations demonstrated, that photoinduced electrons transferred from TiO₂ to adjacent Pd atoms facilitated the activation of acetylene. The selectivity of Pd₁/TiO₂ particles in the form of a small number of encapsulated clusters was significantly higher than that of individual catalytic particles in photothermocatalytic hydro­genation of acetylene at 70°C.[140]

The supported Pd/TiO₂ catalysts containing 0.10 wt.% Pd prepared by precipitation — reduction with NaBH₄ at 300 – 700°C were used in the hydrogenation of acetylene to ethylene.[141] The Pd/TiO₂-HT300^* sample calcined at 300°C performed best, providing 100% conversion of acetylene and 80% selectivity for ethylene at 55°C (TOF = 0.12 s^–1). The authors attributed the catalytic properties to the high content of oxygen vacancies in the sample (71.8%), as confirmed by in situ X-ray photoelectron spectroscopy, in situ Raman spectroscopy and H₂ desorption.

Pd NPs encapsulated in a thin SrTiO₃ shell were synthesized via a hydrothermal method from Sr(OH)₂ · 8 H₂O and Na₂PdCl₄ followed by calcination in air and H₂/Ar at 200°C for 1 h in a tubular furnace. These catalysts featured lower ethylene adsorption and provided 98% conversion and 92% selectivity to ethylene with a specific activity of 5552 mol_C2H2 (mol_Pd h)^–1 at 100°C, significantly exceeding the most catalysts comprising non-encapsulated Pd NPs.[142]

The Pd/Al₂O₃ nanocatalyst with a palladium content of 5% was synthesized by flame spray pyrolysis method and provided 97% conversion of acetylene and 62% selectivity to ethylene, TOF = 5 s^–1.[143] In this case, the catalytic properties depended on the type of Al₂O₃ used. The use of Pd supported on pentacoordinated Al₂O₃ (Pd/meso-Al₂O₃) in the hydrogenation of a mixture of 0.33% C₂H₂ , 0.66% H₂, 33% C₂H₄ demonstrated higher selectivity than Pd/com-Al₂O₃ (commercially available Al₂O₃) at various reaction temperatures (Fig. 10).[144] At acetylene conversion > 99%, the selectivity of Pd/meso-Al₂O₃ catalyst was 83%, whereas with Pd/com-Al₂O₃ it was 25%. According to energy dispersive X-ray spectroscopy (EDS) data, the dispersion of active Pd sites in the Pd/meso-Al₂O₃ catalyst was high and uniform, which contributed to the high catalyst productivity. The dispersion of Pd in the catalyst on the commercial support was low, and the metal formed conglomerates with an average diameter of 5.55 nm according to HR-TEM (HR is high resolution) data.

Fig. 10

The effect of temperature on acetylene conversion and ethylene selectivity to 0.02% Pd/meso-Al₂O₃ and 0.02% Pd/com-Al₂O₃ catalysts (a); and their time dependence under the following reaction conditions: 130°C, gas mixture of 0.33% C₂H₂, 0.66% H₂, 33% C₂H₄ , GHSV = 6000 h^–1). Reproduced from Ref. 144 with permission from Elsevier.

An important role in the manifestation of the properties of a metal catalyst is played by its precursor. The electronic structure of the metal in the starting compound determines the state of the metal in the catalyst. For example, an increase in the electron density on the metal was reported [145] to reduce the adsorption of ethylene on the negatively charged sites of Pd thereby improving the selectivity of the catalyst. This effect was clearly demonstrated on the example of the catalysts obtained from Na₂PdCl₄, PdSO₄ and Pd(acac)₂ by the wetness impregnation method followed by calcination in 10% H₂/N₂ at 400°C for 4 h. The Pd/α-Al₂O₃ catalyst synthesized from Pd(acac)₂ had a higher electron density of the active Pd sites and demonstrated higher selectivity to ethylene than catalysts obtained from Na₂PdCl₄ and PdSO₄ salts (Fig. 11). In addition, Pd/α-Al₂O₃ prepared from Pd(acac)₂ was more stable than similar catalysts obtained from Na₂PdCl₄ and PdSO₄ due to the formation of PdC phase during the prolonged reaction.

Fig. 11

Plots of Acetylene conversion vs. temperature (a), ethylene selectivity vs. acetylene conversion (b), ln(TOF) vs. corrected temperature (c); Arrhenius plots for acetylene conversion and selectivity to ethylene vs. time (d) for Pd/α-Al₂O₃ catalyst obtained from Na₂PdCl₄ (1), PdSO4 (2), and Pd(acac)₂ (3). Fig. 11,c shows apparent activation energy values (E_a), kJ mol^–1. Reproduced from Ref. 145 with permission from Springer Nature.

Highly dispersed stable PdC phase supported on α-Al₂O₃ was prepared using N-containing silane coupling agent as a modifier.[146] The resulting catalyst demonstrated ethylene selectivity 15% higher than the conventional PdAg alloy with total acetylene conversion at 240°C and greater time-on-stream stability for 200 h. The excellent ethylene selectivity was mainly due to the special structure of the PdC phase NPs with a distorted lattice, which contributed to weaker ethylene adsorption and led to strong interaction of Pd NPs with the support surface.

The Pd/MgAl₂O₄ catalyst, intended for the selective hydrogenation of acetylene, was prepared from Na₂PdCl₄ by impregnation and then supported on mixed oxides followed by calcination at 600°C and reduction with H₂.[147] Partially positively charged Pd atoms effectively reduced the adsorption of intermediates during hydrogenation, providing 96% acetylene conversion with 87% ethylene selectivity and excellent stability for a 50-hours test at 120°C.

Using carbon nanotubes (NTs) as a support for Pd instead of α-Al₂O₃ suppressed the formation of palladium hydrides, thereby increasing the yield of ethylene.[148] X-ray absorption spectroscopy, SEM and temperature programmed desorption (TPD) study of C₂H₄ in combination with DFT calculations demonstrated the presence of a unique local environment of Pd containing subsurface carbon atoms, which increased the acetylene conversion. According to kinetic study of the liquid-phase hydrogenation of acetylene over a commercial 0.5 wt% Pd/Al₂O₃ spherical catalyst in N-methylpyrrolidone (NMP) in a reactor at 60 – 100°C and 15 – 250 psi (1 – 17 bar), the reaction occurs on the surface and is limited by minor diffusion resistances between and within the catalyst species.[149]

Pd_0.87/MoS₂ catalyst was synthesized by deposition of Pd particles on MoS₂ nanosheets obtained by ultrasonic exfoliation in formamide.[150] The catalyst provided 75% selectivity to ethylene and 100% conversion at a space velocity of acetylene of 40 000 mL h^–1 g^–1. According to SEM, Pd particles were dispersed on the glossy MoS₂ surface with a low nominal Pd loading. Strong Pd–MoS₂ interactions arising from the charge transfer from Pd to MoS₂ were observed by XPS.

The effective interaction between Pd and the support resulted in high selectivity acetylene hydrogenation to ethylene (100% acetylene conversion at 240°C) of the Fe₅C₂-supported Pd/α-Al₂O₃ catalyst.[151] The catalyst was prepared by reduction of FeCl₃ with NaBH₄ in isopropanol. The resulting Fe NPs reduced Pd²⁺ to Pd⁰ when adding Pd(OAc)₂ to the solution. The addition of oleylamine and heating under reflux at 350°C afforded the desired catalyst. DFT calculations confirmed that the energy barrier for the conversion of ethylene to ethane on this catalyst was very high, making the complete hydrogenation unlikely.

Natural minerals such as complex silicates and alumosilicates have also found application as supports for hydrogenation catalysts. For example, pre-calcined halloysite nanotubes (HNT-c) were used to support Pd NPs from PdCl₂ and Pd(OAc)₂ obtained in solutions of ammonia (NH₃), sodium chloride (NaCl), hydrochloride (HCl) and in acetone (Ac).[152] The highest (~ 65%) ethylene selectivity at acetylene conversion < 70% was achieved using Pd-HNT-c-Ac. The other samples demonstrated close selectivity (55 – 60%), probably due to palladium poisoning with chlorine and amines. Catalysts based on silanized HNTs (HNT-as) provided the selectivity in the range of 45 – 59% with conversion below 80%. The highest selectivity was demonstrated by Pd-Hal-as-HCl. It should be noted that the performance of Pd-HNT catalysts was comparable to that of palladium catalysts supported on Al₂O₃, SiO₂ and TiO₂ . However, their ethylene selectivity is still insufficient for commercial use.

Palygorskite (Pal) is a naturally abundant aluminosilicate mineral that was used as a support for ultralow content Pd (0.8%) hydrogenation catalysts.[153] This mineral was treated ammonia (Pd/N-Pal) or air (A-Pal). The treatment with ammonia to produce Pd/n-Pal ensured easy and strong fixation of metal particles on the support and increased their dispersion due to a decrease in the crystal size and adsorption of H₂ at a temperature of 90 – 110°C, which ultimately contributed to an increase in the selectivity to C₂H₄. On the contrary, air-treated Pd/A-Pal catalysts demonstrated a weak interaction of Pd particles with the support and low dispersion, therefore, the selectivity to ethylene for these catalysts was lower. The TOF values for Pd/A-Pal and Pd/N-Pal in acetylene hydrogenation at 90°C and the reaction time of 360 min were 0.345 and 0.05 s^–1, respectively. It is worth noting that the increase in temperature promoted PdHx hydride phase formation on the Pd/A-Pal catalyst, providing the complete hydrogenation of acetylene to ethane.

N-Doped carbon-supported Pd/NC catalysts were prepared from H₂PdCl₄, activated carbon and 1,3-diethylimidazole acetate/N-methylpyridinone dihydrogen phosphate ionic liquids by wetness impregnation technique followed by calcination at 650°C and hydrogenation at 400°C for 4 h in a tube furnace.[154] Nitrogen doping altered the electronic structure of Pd NPs, allowing rapid desorption of ethylene and preventing ethane formation during overhydrogenation. The ethylene selectivity to this catalyst was 91.3% over 40 h with 94.6% acetylene conversion.

The single-atom Pd₁-N₈/CNT catalyst on multi-walled carbon nanotubes (CNT) was studied by cyclic voltammetry (CVA).[155] Using voltammograms, the azide ion oxidation into azide radicals with subsequent formation of well-ordered polynitrogen species, as well as the palladium reduction from its oxide were identified. PdO/CNT was used as the working electrode. The N₈ fragment was formed from NaN₃ in a buffer solution under electrolysis and attached to the single-atom Pd, stabilizing it. Calculations using acetylene temperature programmed desorption (C₂H₂ – TPD) and DFT confirmed that C₂H₂ favours π-bonding on Pd-SA, and H₂ dissociated on the N atom attached to Pd. This Pd – N synergistic effect favoured the formation of C₂H₄ during acetylene hydrogenation and prevented its complete hydrogenation to C₂H₆ .

Modification of the Pd/SiO₂ catalyst with histidine followed by calcination at 240°C provided 1.0 nm Pd NPs compared to the untreated catalyst.[156] According to the XPS data, Pd NPs of the Pd – His/SiO₂ catalyst were highly dispersed and positively charged, which provided a high semi-hydrogenation selectivity of 81.5% (100% acetylene conversion at 160°C) and high stability at 160°C for 50 hours. The desorption temperature of C₂H₂ decreased from 123°C for Pd/SiO₂ to 112°C for Pd – His/SiO₂, while ethylene was easily desorbed from the latter catalyst during the reaction, which prevented the formation of ethane.

Encapsulated palladium nanoparticles or nanoclusters exhibit high selectivity in selective hydrogenation due to the fact that the thin shell of the support or capsule protects the surface of the active metal. Pd nanoclusters coated with a thin SrTiO₃ shell demonstrated selectivity to ethylene of 92% with acetylene conversion of 98%.[142] The specific activity of such a catalyst was 5552 mol_C2H2 (mol_Pd h)^–1 at 100°C, which significantly exceeds the similar parameter of most palladium-based catalysts. Selectivity was achieved due to the sorption of C₂H₂ on Ti³⁺ capsule sites instead of Pd, and H₂ was sorbed on palladium and transferred to the catalyst shell, where reduction occurred.

An unusual type IL@Pd@NMC palladium catalysts of the core-shell type on N-doped mesoporous carbon (NMC) with a metal content of 20 – 50% was synthesized by wet impregnation and subsequent treatment with MeCN, ionic liquids (ILs) and drying. These catalysts provided ethylene selectivity > 93% with complete acetylene conversion in the temperature range of 100 – 200°C.[157] Importantly, they demonstrated excellent stability, providing 100 h on stream for the hydrogenation of acetylene to ethylene without any obvious decrease in productivity. The efficiency of IL@Pd@NMC was comparable to Pd@NMC, making it a potential candidate for industrial use.

Similar Pd catalysts Pd@S-1@IL obtained by impregnation of zeolite (S-1) with solutions of tetrapropylammonium hydroxide (TPAOH), tetraethyl orthosilicate (TEOS), hexadecyltrimethoxysilane (HTS) and followed by incapsulation with [Prmim][Cl] ionic liquid (Prmim is 1-methyl-3-n-propylimidazolium) exhibited excellent ethylene selectivity (> 90%) regardless of the level of substrate conversion and excellent stability over 800 h on stream even under relatively harsh conditions.[158] The high selectivity of the catalyst was due to the structure of the catalyst, which resembled a kind of ‘filter’ (Fig. 12). Encapsulation of Pd NPs inside the support prevented metal migration, and the IL layer regulated gas sorption-desorption on the catalyst surface, suppressing the process of overhydrogenation to ethane. In addition, the IL layer provided heat removal from the metal catalyst and prevented the formation of hot spots, thereby hampering the formation of polymerization products. The high stability of the atomically dispersed Pd hydrogenation catalysts supported on TiO₂ and coated with 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]) was due to the electrostatic interaction of the IL and Pd/TiO₂.[159] Moreover, the synergism of IL and Pd/TiO₂ suppressed coke deposition on the catalyst, ensuring its higher performance and stability. And the ionic liquid shell prevented the formation of ‘hot spots’, preventing the formation of ‘green oil’, oligomerization products.

Fig. 12

Schematic representation of Pd@S-1@ILs catalysts. Reproduced from Ref. 158 with permission from Elsevier.

An unusual single-atom catalyst was synthesized by in situ confinement of Pd(acac)₂ between two-dimensional layers formed by Cu, 4,4'-bipyridine (bipy) ligand and SiW₁₂O₄₀^4– (SiW) anions in the outer sphere.[160] The small distance (6.25 Å) between two SiW anions restricted the movement of Pd(acac)₂, and the SA-Pd can be directly fixed by oxygen atoms on the SiW surface during the reduction without agglomeration. The resulting Pd₁@Cu – SiW catalyst demonstrated 92.6% ethylene selectivity and complete acetylene conversion at 120°C. Surprisingly, the ethylene selectivity remained > 90% with increasing reaction temperature, whereas it usually drops sharply in the presence of common catalysts.

A useful approach to catalytic hydrogenation using parahydrogen (p-H₂) was shown in a series of works by I.V.Koptyug. Parahydrogen is a hydrogen molecule in a singlet state, in which two nuclear spins are antiparallel. The p-H₂ spin order was transferred to product molecules in hydrogenation, provided that the process involved the simultaneous transfer of two hydrogen atoms of the parent p-H₂ molecule to the substrate. This phenomenon of parahydrogen-induced polarization is used in NMR spectroscopy for mechanistic studies of reactions involving molecular hydrogen, e.g., hydrogenation. Strongly enhanced absorption/emission signals were recorded in the NMR spectra of the hydrogenated products. The observed signal enhancement can theoretically reach 105 times and even more than when using ordinary hydrogen.[161] Thus, catalytic hydrogenation with parahydrogen of propyne,[162-166] propene[167][168] and some other compounds[169] was studied.

Selective hydrogenation of acetylene is also of fundamental importance, since it is practically the only way to enrich the nuclear spin isomers of ethylene using p-H₂ . For example, in the hydrogenation of acetylene with p-H₂ (C₂H₂ : p-H₂ = 1 : 4) over Pd_o37/SiO₂ with octahedral Pd nanoparticles at 150°C (Scheme 1), a strong enhancement of the signals of the oligomerization products was observed in the NMR spectra against the background of the hydrogenation products of the C≡C triple bond (Fig. 13a).[170] Although ethylene was the predominant reaction product, no enhancement of the signals of the protons at the C=C bond was observed. The reason was in blocking of the nuclear spin polarization by the magnetic equivalence of all protons in the ethylene molecule. The NMR signals of the oligomerization products, on the contrary, were significantly enhanced (nuclear spin polarization > 1.7%), despite the low concentration of these compounds in the mixture. For comparison, the spectrum obtained after the nuclear spins returned to thermal equilibrium is shown in Fig. 13b. The authors[170] found, that the signal amplification was higher the larger the catalyst NPs used in hydrogenation. The fact that oligomers accumulated higher polarization than alkenes was additionally confirmed by experiments on propyne hydrogenation.

Scheme 1

Fig. 13

¹H NMR spectra obtained by hyperpolarization of the reaction mixture (a) and after thermal equilibrium has been reached in the stopped-flow experiment (b). Reproduced from Ref. 170 with permission from the American Chemical Society.

Hyperpolarized ethylene was obtained by continuously passing a mixture of acetylene and p-H₂ over a Pd/TiO₂ catalyst.[171] A similar experiment was carried out using [D₂]acetylene, which formed DHC=CHD when hydrogenated with p-H₂. This proved that the addition of parahydrogen occurred in pairs and preserved the correlation of individual nuclear spins of the substrate molecule. It was shown that ethylene obtained with p-H₂ exhibited hyperpolarized signals in an anisotropic medium.

Selective hydrogenation of acetylene with parahydrogen over Pd catalysts and iridium complexes was studied.[172] NMR spectroscopy studies showed that isomers of individual nuclear spin complexes of ethylene formed by the addition of p-H₂ to acetylene equilibrated within 3 – 6 s for spin systems with the same symmetry (syn-addition of hydrogen) and within 1700 – 2200 s for spin systems with different symmetry (anti-addition).

Acetylene molecules contain the isotope ¹³C, the number of which is small and amounts to 2.2% of the total number, according to the natural abundance of the isotope ¹³C. To achieve specific objectives, the content of the ¹³C isotope can be artificially increased by ¹³C-labelling the desired molecule.[173][174] Such molecules, when hydrogenated with parahydrogen, catalyzed by Rh and Ir complexes, would give increased intensity signals of ¹H and ¹³C in the corresponding NMR spectra.[175] The enhancement of the ¹H NMR signal of ¹³C-ethylene protons was 140! And the enhancement of the signals of carbon nuclei was enough to observe ¹³C-ethylene even at extremely low concentrations due to the polarization caused by parahydrogen. The shape of the line of polarized ¹³C-ethylene (Fig. 14) unambiguously indicated that the addition of p-H₂ to the acetylene triple bond on immobilized iridium complexes proceeds stereoselectively as syn-addition. In fact, this method is an efficient approach for enrichment of nuclear spin isomers of ethylene.[175]

Fig. 14

¹H NMR spectra of ethylene formed by hydrogenation of acetylene with the SiO₂-supported catalyst [Ir₂(COD)₂(μ-Cl)₂] (flow rate of the gas mixture 4.6 mL s^–1) before (a) and after relaxation of hyperpolarization to thermal equilibrium (d); (COD is cycloocts-1,5-diene); simulated spectra of ¹³C-ethylene formed by syn- (b) or anti- (c) addition of p-H₂ (c). Reproduced from Ref. 175 with permission from Springer Nature.

* The figures in the designation of the support indicate the reduction temperature - 300°C.

The introduction of another metal into the catalyst to control the geometry and electronic structure of Pd NPs is widely used strategy to effectively improve its overall catalytic activity.[176][177] The comparison, analysis and summary of the synthetic methods of Pd-based bimetallic compounds can provide useful guidelines for the further development of such catalysts.

A mixed PdAg catalyst supported on the mesoporous carbonaceous material Sibunit was obtained by impregnation followed by reduction. Interestingly, the catalysts synthesized by alternating impregnation of the support with solutions of Pd(NO₃)₂ and AgNO₃ in a H₂ medium demonstrated lower ethylene selectivity (68 – 73%) due to the formation of species that were non-uniform in both composition and size (about 4 – 60 nm). This was probably due to the removal of O-containing functional groups from the carbon surface during the hydrogenation process, which contributed to the formation of larger catalyst particles. Impregnation with a mixture of salts followed by reduction provided the formation of more uniform particles.[178]

A series of bimetallic Pd and Ag catalysts were tested in the selective semihydrogenation of acetylene to identify the leading catalysts.[179] Based on the calculations and a series of experiments, the authors were able to find out that the nature of the catalyst surface directly determined its ability to adsorb acetylene and ethylene. It was also obvious that the nature of the surface can be changed depending on the ratio of metals in an alloy. The screening strategy described in this research, allowed for rational engineering, i.e., theoretical determination of those forms of the catalyst that demonstrated maximum activity and selectivity. Based on this strategy, the catalysts Ag₇@Pd, Pd₁@Ag, and Pd₁Ag₃ were proposed. Further, by means of microkinetic analysis, it was found that the Pd₁Ag₃ alloy demonstrated the superior cheracteristics of the tested samples. The calculation methods also confirmed that the presence of silver in the alloy catalyst facilitated the desorption of ethylene, thereby increasing the hydrogenation selectivity.[103] Using the calculations and experimental research, interesting patterns were found for alloy binary PdAg catalysts.[180] The formation energy of palladium alloys was 0.06 eV atom^–1. The role of silver is to limit the dissolution of hydrogen in palladium. The more silver atoms per palladium atom, the more isolated the palladium active sites are on the surface. Silver atoms are combined on the surface, but this segregation is reversible under the action of hydrogen, so the catalyst surface is constantly changing over time. The nature of the palladium species is also changed: they also tend to aggregate, which led to a deterioration of the catalytic properties of the catalyst as a whole. The highest catalytic activity was found in samples of the Pd₁Ag₃ composition, which is also confirmed by other studies.[179]

An alternative approach to the bimetallic PdAg catalyst was based on mechanochemical action on solid starting compounds.[181] Compared to a similar catalyst obtained by the standard wet impregnation procedure, this catalyst was more resistant to deactivation and withstood 15 h on stream rather that 5 – 10 h. Therefore, the mechanochemical approach can be considered as an alternative to the synthesis of complex catalytic systems with increased time-on-stream. A number of bimetallic catalysts with different Pd : Ag ratios was obtained by the original procedure from the corresponding nitrates, a stabilizer and sodium cyanide as an auxiliary reagent.[182] The catalysts were then placed on an alumina support and tested in acetylene hydrogenation. Compared to standard hydrogenation catalysts (Pd Nanoselect LF200 and Pd/Al₂O₃), the bimetallic catalyst demonstrated 80% selectivity to ethylene even after 48 h.

As noted above, non-selective hydrogenation of acetylene can produce ethane and C₄₊ oligomeric compounds, the so-called ‘green oil’, which is a catalyst poison. A study [183] has been conducted to assess the contribution of buta-1,3-diene formed during hydrogenation to the course of the subsequent process. For this purpose, 1,3-butadiene was introduced into the experimental setup for selective hydrogenation of acetylene using PdAg/Al₂O₃ as the model catalyst. It was found that C₄H₅ species were intermediates in the formation of C₆ species. It was also proved that butadiene does not react with acetylene to give longer-chain hydrocarbons, but is actively actively hydrogenated to 1-butene, cis- and trans-2-butenes and butane. The amount of resulting C₄ hydrocarbons directly depends on the reaction time and the catalyst type. Although direct reaction of acetylene and 1,3-butadiene was not observed, two acetylene molecules can react with each other to form C₄H₄ species, which can then react with acetylene to give benzene. In the case of the mixed PdAg catalyst, the treatment of the catalyst with carbon monoxide led to segregation of Pd atoms at room temperature and results in the additional enrichment of the surface with palladium (up to 31%).[184] The IR spectrum of CO on the monometallic Pd catalyst (Fig. 15, the black curve) showed a maximum at 2088 cm^–1, which corresponds to linearly adsorbed CO. The maximum at 1990 cm^–1 and a broad shoulder at 1944 cm^–1 corresponds to multicoordinated CO. Based on the signal intensities, multicoordination clearly dominated, i.e. adsorption on two and three palladium atoms. For the mixed PdAg₂/Al₂O₃ catalyst, linearly adsorbed CO dominated (Fig. 15, red curve), which indicated the disappearance of polyatomic Pd centres and the formation of the single-atom Pd catalytic sites, which are separated by Ag atoms. Thus, pre-treatment of the catalyst with CO increases its in activity at 100% selectivity in the acetylene hydrogenation.

Fig. 15

FT-IR diffuse reflectance spectrum of CO adsorbed by Pd/Al₂O₃ (top); and by PdAg₂/Al₂O₃ (bottom). Reproduced from Ref. 184 with permission from Elsevier.

A series of core-shell catalysts was prepared using a standard impregnation technique followed by deposition on alumina.[185][186] The resulting catalysts consisted of monoatomic alloys of Pd₁Ag₆ and Pd₁Ag and showed selectivity to ethylene 90% at conversion 80%. However, with an increase in conversion to 100%, the selectivity was less than 60%.

A comparative study of the properties of bimetallic Pd catalysts and their analogues modified with S or Si atoms in acetylene semi-hydrogenation using experimental and computational methods has been reported.[187] According to the calculations, the catalytic efficiency of C₂H₂ semi-hydrogenation largely depends on the spatial extension of the active site and the electronic effect, which consists of a change in the density of the d-zone near the Fermi level and has a significant influence on the sorption characteristics of the catalyst. The electronic effect of the catalyst caused by S atoms was more significant than the effect of Si atoms. At the same time, the geometry of the active centre for Si-analogues changed more than in S-analogues. Thus, for bimetallic PdAg catalysts, the synergistic geometric and electronic effects caused by S atoms on S/Pd₆Ag, enhanced the activity and selectivity to C₂H₄ formation and significantly prevented the formation of a ‘green oil’. Modification of the catalysts with silicon was slightly less effective than with sulfur, and they both increased the selectivity to C₂H₄ and prevented the formation of ‘green oil’ in the hydrogenation of acetylene on Si/Pd₁Au₁ and Si/Pd₁Ag₁ catalysts.

Bimetallic supported Ag@Pd/SiO₂ catalysts coated with Ga₂O₃ demonstrated high selectivity to ethylene (up to 95%) with complete conversion of acetylene in ethylene excess.[188] The Ga₂O₃-coated monometallic Pd/SiO₂ catalyst showed moderate activity. At the same time, the activity and high stability of the Ga₂O₃-coated Ag@Pd/SiO₂ catalyst were nearly 50 times higher than those of the reference Pd₁Ag/SiO₂.

Alumina is often used as a support for acetylene hydrogenation catalysts. However, its small surface area limits the dispersion of active metals. To address this issue, alumina was prepared by a special in situ growth method (Fig. 16).[189] Then, the alloy bimetallic catalyst PdAg was applied to the obtained support, resulting in 90% acetylene conversion and 89% selectivity. It was due to even greater isolation of the active Pd sites, a more degradation-resistant substrate structure and a decrease in the rate of heat release. Decreased heat release led to a decrease in the number of hot spots on the catalyst surface, and, consequently, to a decrease in the rate of the endothermic coking reaction. As a result, the catalyst pores remained open for a longer time, thereby increasing the catalyst stability.

Fig. 16

SEM images of the surface of standard spherical aluminum oxide (a) and aluminum oxide with a modified flower-shaped array (b). Reproduced from Ref. 189 with permission from the American Chemical Society.

Liquid-phase hydrogenation of a N₂-stabilized gas mixture consisting of C₂H₂ (8%) CO (20%) and H₂ (50%) was carried out in NMP in the presence of PdAg/SiO₂ showed 92% selectivity to ethylene.[190] The similar characteristics for gas-phase hydrogenation of the same mixture was only 71%.

A mixed SiO₂-supported palladium – nickel catalyst was prepared in several steps.[191] First, the Ni/SiO₂ nanocatalyst was synthesized by impregnation with subsequent reduction in a hydrogen stream. Then, single palladium atoms were deposited on the surface. Acetylene hydrogenation over this catalyst at a temperature of 80°C proceeded with a conversion of 92% and a selectivity for ethylene of 88%. The authors were able to prove the presence of SA-Pd on the catalyst surface by scanning transmission electron microscopy – energy dispersive X-ray spectroscopy (STEM-EDS) (Fig. 17).

Fig. 17

STEM-EDS image of the mixed PdNi/SiO₂ catalyst. Yellow dots indicate Ni atoms; green dots indicate Pd atoms. Reproduced from Ref. 191 with permission from Elsevier.

Metal single-atom alloy (SAA) catalysts based on Pd and Ni were prepared by galvanic replacement (GR) method.[192] For this purpose, nickel nanocrystals were first obtained by the colloidal method. Then, palladium atoms were deposited on nickel via galvanic replacement. And the finished nanocrystalline Pd₁Ni alloy was placed on SiO₂ support. The resulting alloy was tested in the selective hydrogenation of acetylene in comparison with a similar catalyst obtained by common impregnation. It was found that the catalyst obtained by the galvanic replacement showed higher selectivity in the range of 30 – 180°C. According to DFT calculations, electrons were transferred from nickel to palladium atoms in the alloy catalyst.

The efficiency of mixed PdNi catalysts was improved by replacing Pd with monoatomic Pd. Metallic single-atom alloys Pd₁Ni/SiO₂-GR obtained by the galvanic replacement method for selective deposition of colloidal Ni nanocrystals on the SiO₂ support with subsequent deposition of Pd atoms on Ni atoms.[192] The Pd atoms remained atomically dispersed and are located in proximity to Ni atoms (Fig. 18), which improved the ethylene selectivity in the acetylene semi-hydrogenation reaction compared to the reference PdNi/SiO₂ catalyst prepared using the impregnation method. The Pd₁Ni/SiO₂-GR catalyst showed higher C₂H₄ selectivity over a wide temperature range (30 – 180°C), being superior to the Pd₁Ni/SiO₂, Pd/NiO, and Pd/SiO₂ SAA catalysts prepared by the impregnation method. According to DFT calculations, the enhanced selectivity stems from the electron transfer from a Ni nanocrystal to single Pd atoms due to the close proximity of these species.

Fig. 18

STEM-EDS mapping showing the close contact of Pd and Ni in the Pd₁Ni nanocrystal (a); line scan profile of Ni and Pd in Pd_0.1Ni₂/SiO₂-GR-500R (b); AC-HAADF-STEM images of Pd_0.1Ni₂/SiO₂-GR-500R showing the presence of single Pd and Ni atoms in different scale (c, d). Reproduced from Ref. 192 with permission from Elsevier.

The performance of palladium catalyst was significantly improved by inserting tin atoms onto the surface.[193] The resulting Pd₂Sn/C catalyst demonstrated ~ 95% selectivity in hydrogenation of acetylene with almost complete conversion of the substrate. According to experimental studies and DFT calculations, the addition of tin led to a redistribution of Pd atoms over the catalyst surface, which in turn affected the ethylene sorption. Ethylene molecules adsorbed at 300 – 600°C were weakly bound to the surface and easily left it being non-dissociated. At the same time, ethylene molecules that were adsorbed at 439 and 500°C were bound to the surface more strongly and could further react with dissociated hydrogen to form ethane.

Mixed Al₂O₃-supported PdMn catalyst was prepared in hydrogen reducing medium.[194] According to analytical studies, the surface of thus modified catalyst lacks the weakly bound hydrogen, which was explained by the presence of a uniform distribution of Mn atoms over the surface. In tests for selective hydrogenation of acetylene, the catalyst demonstrated increased activity and a 20% increase in selectivity compared to the unmodified palladium catalyst. The complex Pd/In₂O₃@SiO₂ catalyst was prepared using a standard deposition–precipitation procedure.[195] Doping the catalytic system with indium suppressed palladium activity and also facilitated the adsorption of di-σ-bound ethylene, which significantly improved its desorption. As a result, the selectivity to ethylene was 80% with 100% conversion of acetylene at 140°C for 24 h.

The bimetallic PdZn catalyst demonstrated 85% ethylene selectivity in acetylene hydrogenation even after 70 hours on stream.[196] Comparing to other alloy catalysts, that was not the best result; however, it was very promising because of the room temperature reaction conditions. Based on the calculated data, the authors explained the role of zinc by donating electrons from the occupied zinc d-orbitals to the Pd d-orbitals, which facilitated the interaction of acetylene with the surface. Moderate results were obtained for the bimetallic Sibunit-supported PdZn catalyst.[197] The addition of zinc to Pd : Zn in a ratio of 1 : 1 increased the catalyst selectivity to 67% compared to 42% for monometallic palladium. Further increase in zinc content decreased the selectivity to 62%. According to analytical studies, zinc was incorporated into the palladium crystal lattice during the reduction of metals. With an excess of palladium, the interaction occurred more completely with the formation of an intermetallic phase. With an excess of zinc, lesser incorporation was observed, while zinc was retained in the form of large oxide particles. Further studies demonstrated that an increase in the reduction time of Pd – Zn/C samples in H₂ at 500°C from 1 to 20 h was accompanied by an increase in the number of bimetallic sites and an increase in the coordination number of PdZn from 2.4 to 4.3, as well as a decrease in the Pd₀/PdZn ratio from 0.63 to 0.18.[198]An increase in the number of modified active sites led to higher selectivity to ethylene in the process of liquid-phase hydrogenation of acetylene.

Spinel monolithic Al–LDH catalysts with porous woodpile architecture were fabricated by extrusion three-dimensional (3D) printing of layered aluminate-intercalated double hydroxide (Al-LDH) followed by low-temperature calcination.[199] Porous monolithic catalysts with a high specific surface area facilitated mass and heat transfer and exposed active catalyst sites (Fig. 19). Thus, the 3D-printed spinel-based catalyst loaded with Pd (3D-AI-Pd/MMO (MMO is MgAl-mixed metal oxide) provided 100% conversion in acetylene hydrogenation with selectivity to ethylene more than 84% at 60°C.

Fig. 19

Schematic representation of the selective hydrogenation of acetylene to ethylene over a 3D-printed 3D-AI-Pd/MMO catalyst with a hierarchical porous structure (a) and a comparison diagram of different catalysts for this process (b). CNF is carbon nanofibers. Reproduced from Ref. 193 with permission from Springer Nature.

Pd₁ – Cu_n/Al₂O₃ catalysts with single palladium atoms (Pd₁) and copper clusters (Cu_n) were deposited on different supports. The synergistic effect of one palladium atom and a copper cluster provided the isolation effect of catalytic sites and effectively prevented their agglomeration. The activity of the Pd₁ – Cu_n/Al₂O₃ catalyst increased by 60% compared to the Pd₁/Al₂O₃ catalyst, and its stability increased to more than 100 h with the same conversion values for both catalysts with ethylene selectivity of 83%.[200] The PdCu/siloxene catalyst was prepared by direct reduction in the liquid phase and was characterized by a low loading of Pd distributed in ultrafine Cu species.[201] This catalyst provided 91% acetylene conversion and 93% ethylene selectivity at 200°C in the acetylene semi-hydrogenation reaction. Moreover, PdCu/siloxene also demonstrated high activity and stability under ethylene-rich industrial gas conditions. In addition, the catalyst was resistant to coking at high operating temperatures for 39 hours.

The Pd₁Au₁ dimeric catalyst supported on a metal-organic framework demonstrated an outstanding (99.99%) acetylene conversion with a selectivity of over 90% under industrial-scale conditions (1% acetylene, 89% ethylene, 10% H₂ mixture).[202] The catalyst worked efficiently over a wide temperature range (35 – 150°C), with only ethane as a by-product. The reaction proceeded with an apparent activation energy of ∼ 1 kcal mol^–1 even at 35°C and with operating windows (> 100°C) and mass hourly space velocities (66 000 mL g^–1 cat h^–1) within industrial specifications. Experimental and theoretical studies demonstrated that palladium atoms performed the main catalytic function, while gold atoms and thioether moieties of the metal-organic framework participated in hydrogen dissociation.

An interesting technique for regulating the ratio of metals in a bimetallic catalyst was implemented on the example of a PdCo supported on the carbon material Sibunit.[203] The catalysts were prepared by wet impregnation followed by hydrogenation in the stream of hydrogen at various temperatures. At temperatures below 500°C, palladium was reduced to a metallic state, and cobalt was only partially reduced. As the temperature raised to 700°C, larger amount of cobalt was reduced to metal. In this way, catalysts of compositions from Pd_0.6Co_0.4 to Pd_0.45Co_0.55 were obtained. With an increase in the catalyst treatment temperature, the yield of ethylene in the acetylene hydrogenation also increased from 56% to 68%. For comparison, the yield of ethylene in hydrogenation in the presence of monometallic palladium on carbon was only 52%. According to the data obtained, the improvement in the catalyst characteristics was associated with a change in the electronic state of palladium due to its surrounding by cobalt atoms.

The bimetallic Pd-Ga/γ-Al₂O₃ catalyst obtained by impregnation Al₂O₃ with solutions of the appropriate nitrates demonstrated selectivity to ethylene about 66 – 70%.[204] Outstanding results were obtained for the calcite-supported PdBi catalyst (PdBi/Calcite) prepared by multistep synthesis that included of metal salt reduction with NaBH₄ and deposition on the support.[205] This catalyst showed 99% selectivity to ethylene at 100% conversion over the entire temperature range (50 – 300°C)! Thus, the working range with acceptable characteristics was 150°C. The authors carried out a series of experiments to clarify the nature of such high activity. An assumption was made about the influence of bismuth oxide on the activity of Pd metal. However, it was found that palladium and bismuth formed intermetallic compounds that significantly contribute to the reaction mechanism, and the key process parameters depend on their formation. At the same time, the contribution of oxides was insignificant. The intermetallic catalysts contained isolated and electron-rich Pd atoms as active catalytic sites and had an extremely weak ability to absorb ethylene and prevent the formation of palladium hydrides (β-PdH_x) on the catalyst surface.

A bimetallic CuPd catalyst was synthesized especially for the selective hydrogenation of acetylene to ethylene.[201]The undoubted novelty of the research was the use of siloxene as a support. Catalytically active sites consisted of isolated palladium species surrounded by ultra-small copper species, and all the species were uniformly distributed in siloxene, which has high porosity and surface area. The catalyst demonstrated high resistance to sintering for 39 h and provided 91% conversion of acetylene with 93% selectivity to ethylene at 200°C.

The synthesis of mixed hydrogenation catalysts obtained by deposition of bismuth on cubic palladium nanoparticles (Pd – NC) was described.[206] Dotted applicated bismuth poisoned palladium atoms, adjusting the catalyst activity. The PdNCs@Bi_0.06/Al₂O₃ catalyst obtained by applying bismuth to the corners of Pd cubes, provided a selectivity to C₂H₄ of 10.1% with 100% conversion of C₂H₂ , which exceeded the catalytic characteristics of the PdNCs/Al₂O₃ catalyst. With an increase in the degree of bismuth inclusion (i.e., with an increase of poisoning), the C₂H₂ conversion gradually decreased, and the C₂H₄ selectivity increased (Fig. 20), which made it possible to vary the conditions depending on the tasks at hand. For example, the Pd-NC@Bi catalyst with bismuth deposited on the corners and faces of Pd nanocubes had C₂H₂ conversion of 99.7% and selectivity to C₂H₄ 94.3% at 170°C in acetylene semi-hydrogenation in excess of ethylene.

Fig. 20

Temperature dependence of conversion (a) and selectivity (b) of acetylene hydrogenation on Pd-NCs@Bi_0.5/Al₂O₃ catalyst. Reproduced from Ref. 206 under a Creative Commons Attribution (CC BY) license

A mixed Pd – La nanocatalyst was obtained by ultrasonic impregnation using nitrogen-doped biochar derived from Pennisetum giganteum as a support (N-pgBC). PdLa/N-pgBC provided 94.2% selectivity to ethylene with full acetylene conversion in the acetylene hydrogenation.[207] The synergistic effect of the inherent high specific surface area of the carbon support together with the separation of the lanthanum-doped catalytic sites promoted adsorption of acetylene and the rapid desorption of ethylene, blocking overhydrogenation of the latter. In addition, the PdLa/N-pgBC catalyst showed a strong dependence on the component composition, which might be due to electronic or geometric modification of metal particles.

Nickel hydrogenation catalysts as completely exposed small metal nanoclusters (Fig. 21), were obtained on a nanodiamond (ND or graphene (G) support.[208] Calculation methods and H₂/D₂ exchange experiments demonstrated that acetylene and hydrogen are completely activated on the catalytic sites. Moreover, the desorption of ethylene on exposed nickel clusters was more energetically favourable than further dissociation of an H₂ molecule. Therefore, exposed nickel clusters were able to completely convert acetylene to ethylene with 85% selectivity at 190°C. When replacing the carbon substrate with a mesoporous material MCM-41 based on silicates and aluminosilicates, the nickel nanocatalyst provided 96% conversion of acetylene and selectivity to ethylene of 87%.[209]

Fig. 21

High-resolution transmission electron microscope images of nanodiamond and graphene (a), and nickel atoms (Ni₁), nickel NPs (Ni_n) and nickel nanoclusters (Ni_p) supported thereon. Nickel atoms and nickel clusters are circled in white. Reproduced from Ref 208 with permission from the American Chemical Society.

A very interesting idea was realized in the hydrogenation of acetylene to ethylene in the presence of nickel catalysts with a nitrogen-doped carbon support (gC₃N₄).[210] The authors used acetylene obtained by hydrolysis of calcium carbide as a source of ethylene, i.e., ethylene was produced not from the standard cracking of petroleum products. It should be noted, that the use of acetylene from calcium carbide in synthetic transformation is a common practice.[211-218] Nevertheless, the authors managed to obtain impressive results: the catalyst provided 96% conversion of acetylene for 10 hours at 200°C, while the selectivity to ethylene reached 46%.

A similar catalyst was described in a study [219] but it was additionally doped with sulfur. Sulfur-doped nickel supported on gC₃N₄ – T (the T and M locants denote the sulfur-doped support and the sulfur-free support, respectively) showed ethylene selectivity > 63% with complete acetylene conversion at 175 – 300°C. XRD, HRTEM, EDS and XPS revealed that the catalyst represents Ni cations enclosed in gC₃N₄-T cavities or a Ni – S solid solution with surface segregation of S different from the Ni species for Ni/gC₃N₄-M. The Ni²⁺ sites above the gC₃N₄-T support promote the acetylene activation via electrostatic interaction.

Single-atom nickel-dispersed catalysts were obtained by introducing zinc additive.[220] A nitrogen-doped carbon matrix was used as a support. As a result, the presence of the Ni – N₄ structure in the carbon matrix was detected and confirmed. The conversion of acetylene in a real mixture with ethylene was > 97%, which was explained by weak π-adsorption of ethylene on single Ni atoms.

Ni catalysts on various supports (SiO₂, γ-Al₂O₃ and zeolite ZSM-12) were investigated for the hydrogenation of acetylene.[221] The Ni/ZSM-12 catalyst showed low selectivity to ethylene and low stability. However, this catalyst performed better than the Ni/SiO₂ and Ni/γ-Al₂O₃ catalysts. due to the synergistic action of Lewis and Brønsted acid sites. Doping with Sn afforded a more effective Ni₇Sn/ZSM-12 catalyst, which showed 92.51% selectivity to ethylene with complete acetylene conversion at 250°C. These values were 19.4% higher than those in the absence of Sn. According to DFT calculations, the bimetallic catalyst had a rather high energy barrier to the formation of the intermediate compound C₂H₅* in acetylene hydrogenation, which determined its high selectivity.

Outstanding results necessary to understand the effect of additives on nickel catalysts were demonstrated using Au, Ag and Cu additives as an example.[222] It was found, that the energy of initial adsorption of acetylene changed in the series AuNi_0.5/SiO₂ > AgNi_0.5/SiO₂ > CuNi_0.125/SiO₂. Moreover, deactivation of the catalysts was mainly due to coking rather than to a particle size changes. The properties of the NiAg catalyst depended on the depth of penetration of metal species into the matrix, which was definitely a very interesting and unusual marker.[223] Thus, an Al₂O₃-supported complex Ni – Ag@C/Al₂O₃ catalyst on, consisting of carbon-encapsulated Ag NPs and non-encapsulated Ni NPs, was synthesized. Both types of metal NPs were located on the Al₂O₃ support. This was possible due to the catalyst synthesis strategy, which involved the co-precipitation of AgNO₃ and Ni(NO₃)₂ · 6 H₂O onto Al₂O₃, followed by the transformation of salts into carbonates and calcination in an acetylene atmosphere at 120°C to obtain encapsulated silver NPs (Fig. 22). The last step of reduction of NiO – Ag@C/Al₂O₃ with hydrogen at 500°C provided the formation of Ni NPs on the support. After testing, it was found that the Ni – Ag@C/Al₂O₃ catalyst provided ethylene selectivity of up to 81% with complete acetylene conversion within 30 h (Fig. 23). Interestingly, the use of the monometallic catalyst (Ni/Al₂O₃), gave virtually no target product (yield 6%), and the non-encapsulated Ni – Ag/Al₂O₃ catalyst showed only 54% conversion of acetylene. The improved selectivity of the catalyst was achieved due to the ability of carbon particles to provide hydrogen migration across the surface of the Al support of the catalyst. At the same time, the carbon shell blocked the formation of oligomers (i.e. the chain growth) on Ag NPs.

Fig. 22

Schematic illustration of the synthesis of Ni – Ag@C/Al₂O₃. Reproduced from Ref. 223 with permission from Elsevier.

Fig. 23

Temperature dependence of acetylene conversion (a) and ethylene selectivity (b) in acetylene hydrogenation over Ag/Al₂O₃, Ni/Al₂O₃, Ni-Ag/Al₂O₃ and Ni-Ag@C/Al₂O₃ catalysts. Reproduced from Ref. 223 with permission from Elsevier.

Intermetallic NiSb catalyst was used for hydrogenation of acetylene in the presence of ethylene.[224] This catalyst was prepared using the strategy of entrapping molten antimony with nickel (Fig. 24). The authors attributed the impressive results (selectivity up to 93.2% with complete acetylene conversion) not only to the weak adsorption of ethylene and strong adsorption of acetylene on the catalyst surface. The electronegative properties of the ‘guest’ metal (Sb) were used to control the properties of the ‘host’ metal (Ni), so that Ni₁Sb₂ trimers enclosed in NiSb intermetallic particles were formed on the surface. According to theoretical calculations, these regions provided the selective adsorption of acetylene.

Fig. 24

Schematic illustration of the synthesis of a complicated trimeric catalyst Ni₁Sb₂ in the intermetallic compound NiSb. Reproduced from Ref. 224 under a Creative Commons CC BY license.

The carbide-type Ni₃ZnC_0.7/C catalyst was obtained from the NiZn-ZIF-8/LDH precursor (ZIF is zeolite imidazolate framework) and showed 85% selectivity to ethylene at 100% conversion.[225] The Bader atomic charge distribution analysis demonstrated that the subsurface carbon divided the nickel atoms on the surface into two types: electron-depleted with the subsurface carbon atom and electron-rich with the subsurface zinc atom. According to calculations, the nickel atoms arranged into Ni₃ assemblies promoted ethylene desorption and prevented hydrocarbon chain growth, which ultimately led to an increase in the process selectivity.

The bimetallic NiZn/ZnO catalyst performed better in acetylene semi-hydrogenation in comparison with the intermetallic catalyst Ni₃Zn/ZnO of the ligand type. According to research and DFT calculations, the high selectivity of NiZn/ZnO was due to the non-competitive adsorption of acetylene by the adjacent isolated metal atoms.[226]

Features of the influence of the substrate shape on the properties of metal catalyst particles in acetylene hydrogenation were discovered in the Al₂O₃-supported NiCu NPs.[227] It was found that the support atoms govern the arrangement of metal atoms. The spindle-shaped support promoted the formation of NiCu alloys with a predominance of copper on the surface. And the flaky shape led to a switch to monometallic copper and nickel-rich bimetallic systems. The rod-shaped support promoted the appearance of Ni₁Cu₁ alloys, and the coordinatively unsaturated Al³⁺ sites played a key role in the formation of metal NPs, in which nickel atoms were orderly distributed among copper atoms. Uniformly distributed Ni – Cu alloy species with individual nickel sites provided 86% selectivity to ethylene with full conversion at 130°C.

According to calculations, in selective acetylene hydrogenation on intermetallic NiIn catalysts of various compositions, oligomerization products originate exactly from acetylene, not from ethylene.[228] This important conclusion provides deeper insight into the nature of the process and once again emphasizes the need to convert acetylene into ethylene. The calculations also showed that the indium additive to the catalyst improves the selectivity of the process.

NiGa intermetallics were prepared with the aim to elucidate the role of completely isolated nickel sites in the semi-hydrogenation of acetylene.[229] [230] It was found out, that isolated nickel sites preferentially π-adsorbed acetylene and promoted ethylene desorption. Three prepared catalysts based on Ni, Ni₅Ga₃ and NiGa had similar particle sizes, but different atomic arrangements, as well as different environment of nickel sites. As can be seen from Fig. 25a, Ni and Ni₅Ga₃ demonstrated higher activity and lower selectivity to ethylene especially at temperatures above 100°C. This suggests that the complete isolation of nickel particles on the surface led to an increase in selectivity. Temperature desorption profiles (Fig. 25b) clearly indicated three peaks for Ni and Ni₅Ga₃ catalysts. The first two peaks might be attributed to weakly adsorbed ethylene, and the third peak at a higher temperature was a signal of strongly adsorbed acetylene. The absence of a high-temperature peak was observed for the NiGa catalyst with completely isolated nickel sites. Therefore, it can be concluded that isolated nickel sites promoted acetylene desorption. Analysis of acetylene conversion shows that in excess ethylene (Fig. 25c), the catalyst with isolated nickel sites provided higher selectivity to ethylene. It is also evident from Fig. 25d, that the addition of gallium not only increased stability of the catalyst, but also inhibited the formation of ‘green oil’, i.e. hydrocarbons with longer chains (Fig. 25e). In another study,[231] on the properties of multi-intermetallic NiGa with isolated nickel particles, a whole series of additives (Fe, Cu, Ge) were proposed to stabilize the nickel particles on the surface.

Fig. 25

Temperature dependence of acetylene conversion and ethylene selectivity in the absence of ethylene (a); TPD-profiles for acetylene (top) and ethylene (bottom) (b); temperature dependence of acetylene conversion and ethylene selectivity in the presence of ethylene (c); time dependence of acetylene conversion on stream at 190°C (d); thermogravimetric curves of Ni catalysts used: Ni, Ni₅Ga₃ and NiGa (e). Reproduced from Ref. 229 with permission from Wiley.

A series of nickel-based catalysts was synthesized to elucidate the effect of various additives on the selective hydrogenation of acetylene.[232] It was found that the introduction of Mg results in a complete hydrogenation of acetylene to ethane, while the addition of tin improves selectivity, making it possible to stop hydrogenation at the step of ethylene formation. The authors associated these patterns with the facilitation of ethylene desorption from the surface of Ni₇Sn/SiO₂, which performed best: 65% yield of ethylene at 250°C. Mg – Al-supported Ni/MgAl₂O₄ catalyst provided 99% acetylene conversion with an selectivity to ethylene of more than 80%.[233] The authors attributed such outstanding results to the large surface area (309 m² g^–1) at the «tuned’ acidity.

Thus, the conversion and selectivity to ethylene in the acetylene hydrogenation are directly dependent on the size of the nickel species on the substrate surface. Single-atom catalysts demonstrate higher selectivity compared to nano- or microparticles. The catalytic properties of nickel catalysts are significantly affected by the nature of the support, which can either donate electrons to the metal atom or deplete it of electrons. If the support contains more electronegative atoms relative to nickel, the selectivity of such catalysts is superior to those with a support with less electronegative atoms. Lower electron density on the metal atom leads to easier desorption of ethylene and higher hydrogenation selectivity. The presence of a ‘neighbour’ element also has a significant effect on the reaction parameters. Firstly, the second component of the nickel alloy allows for a better distribution of particles over the surface due to greater dilution. Secondly, the nickel atom can transfer part of its electron density to ‘neighbour’ atoms (the number of which may be greater), which prevents the appearance of ‘hot spots’ and ultimately increases the selectivity of the process as a whole.

Similar to Pd- and Ni-based catalysts, the catalytic activity of Cu catalysts depends on the particle size. It was found that the copper particle size played a crucial role in many catalytic transformations.[234] In order to find out how the properties of a copper catalyst are changed in the selective hydrogenation of acetylene, the single-atom catalyst and copper NPs of different sizes (3.4, 7.3, 9.3 nm) were obtained with their subsequent deposition onto aluminium oxide surface. It was found, that with a decrease in the particle size, the catalyst activity decreased. At the same time, selectivity and stability are significantly increased. In particular, single-atom copper catalysts showed 91% selectivity to ethylene with complete conversion and stability for at least 40 hours on stream. The difference in the properties of catalysts with varying particle sizes can be explained by different types of sorption — desorption of ethylene/acetylene (Fig. 26). According to the temperature profiles, acetylene desorption is observed at a lower temperature for the catalyst with a smaller particle size (89°C vs. 105°C). Therefore, the weak binding of acetylene on catalysts with a smaller particle size leads to lower sorption and lower concentration and hence to lower activity. A similar trend is observed for ethylene. In the case of ethylene, this leads to the less sorption of ethylene by the catalyst and, as a result, to less hydrogenation. Therefore, the selectivity of the process as a whole increases.

Fig. 26

TPD profiles of C₂H₂ (top) and C₂H₄ (bottom) on 0.5 Cu/Al₂O₃ and 2.0 Cu/Al₂O₃ (The figures indicate the mass weight of metal in the catalysts). Reproduced from Ref. 234 with permission from the American Chemical Society.

The bimetallic catalyst, Zn – Cu_xC was obtained by co-precipitation of the appropriate starting reagents. The addition of Zn contributed to a decrease in the size and an increase in the number of Cu_xC crystals.[235] In turn, it caused an increase in the activity of the catalyst in transformation of acetylene to ethylene, and the catalyst per se had excellent stability (> 140 h on stream) at 100°C even in the presence of a large excess of ethylene.

A composite catalyst based on copper and iron was prepared to study the mutual influence of the Fe support atoms on copper atoms.[236] Varying the ratio of elements in the catalyst, the authors managed to achieve 95% selectivity to ethylene with full conversion using the CuFe_0.16MgO_x catalyst. Based on the analytical data and calculations, the formation of the Cu^δ–Fe^δ+_0.16MgO_x complex was observed, which played a crucial role in the activation of acetylene and hydrogen. This complex of a certain composition was bonded to both other copper and other iron atoms (with those that were not part of the complex). According to the data obtained, the desorption of ethylene on this complex was more preferable than its further hydrogenation. As the amount of iron in the catalyst increased, its catalytic activity expectedly increased, since the number of complexes and, accordingly, the routes for hydrogen dissociation increased. However, at the atomic ratio Cu/Fe > 0.16, i.e. increasing the proportion of iron, the activity of the catalyst decreased, since the iron particles covered the copper particles. Thus, the activation of acetylene and hydrogen occurred on the copper sites of the catalyst, while the formation of the C – C bond occurred on the Cu/Fe complex, with one carbon atom being bound to a copper atom and the other to an iron atom (Fig. 27).

Fig. 27

The proposed mechanism of acetylene semi-hydrogenation on the Cu/Fe_0.16MgO_x catalyst. Reproduced from Ref. 236 with permission from the American Chemical Society.

Copper atoms were immobilized on a defective nanodiamond–graphene and N-doped nanodiamond graphene to reveal the influence of the coordinated environment (Cu – C or Cu – N bonds) on the catalytic activity in acetylene hydrogenation.[237] It was found, that more oxidized copper atoms with Cu – C bonds showed high selectivity (up to 93%) with quantitative conversion and high stability (> 100 h on stream). According to the DFT calculations and isotope exchange experiments, the presence of a copper atom bound to carbon promoted the binding of hydrogen for subsequent dissociation, which was the rate-limiting step of the reaction. The coordination of three carbon atoms near the copper atom led to rapid charge consumption on the copper atoms, which in turn boosted the catalyst activity and, at the same time, increased its resistance to metal leaching from the surface.

Significant progress has been achieved in reduction of long-chain hydrocarbons during the selective hydrogenation of acetylene.[238] For this purpose, a copper catalyst was modified with a hydrophobic C₁₆ chain, which was grafted to the metal surface using hexadecyltrimethoxysilane. The similar polarity of the alkyl chain of the catalyst and acetylene facilitated the sorption of the substrate on the catalyst surface, thus ultimately increasing the activity of the catalyst. In addition, the presence of a long hydrocarbon chain on the surface led to steric hindrance on the surface and challenges in the formation of hydrocarbons with a longer chain (C₄₊ hydrocarbons), which, in turn, significantly increased the catalyst lifetime. This modification provided an access to a catalyst capable of semi-hydrogenating acetylene with a selectivity of up to 84% at 140°C for 120 h.

The selective hydrogenation of acetylene on copper catalysts produces copper carbide CuC₂, as well as a number of the other carbides (including mixed ones).[239] The catalyst was prepared from copper hydroxide with subsequent exposing to acetylene and reduction with hydrogen (Fig. 28). The resulting catalyst demonstrated moderate 15% selectivity and lower; however, the very fact of the formation of copper carbides is very interesting. Since the presence of copper acetylide in the solid phase was unambiguously proved, this compound turned out to be a precursor of the true catalyst or an intermediate in the hydrogenation reaction. On the one hand, the formation of copper carbide is a reversible process, and, probably, it is through the carbide that acetylene is hydrogenated. On the other hand, copper carbide transforms easily into more complicated carbides when heated, which was an irreversible process and could actually be considered as coking of the metal. Copper carbide immersed in a carbon matrix was also found in the catalysts comprising basic copper carbonate.[240] It was found that copper carbides acted as catalytic sites for the dissociation of hydrogen molecules, and the carbon matrix created steric hindrances for further growth of the hydrocarbon chain, which, accordingly, prevented undesirable oligomerization of acetylene. Copper carbides can also be formed by adding the other metals to the catalyst.[235] For example, the addition of zinc promoted the formation of copper carbides, and at the same time the size of the carbide particles was significantly smaller than in the synthesis of copper carbide without the addition of salts of other metals. In the same work, it was established that the addition of zinc does not affect the electronic structure of the carbides.

Fig. 28

The synthesis of a carbide CuC_x catalyst. Reproduced from Ref. 239 under a CC-BY-NC-ND license.

According to experimental data and DFT calculations, the activation barrier for complete hydrogenation of acetylene on a single-atom copper catalyst was higher than on a copper nanocatalyst.[241] In addition, it was found that ethylene sorption was significantly more complicated on a single-atom catalyst, which led to inhibition of over-hydrogenation.

To prevent the ‘hot spot’ formation and over-hydrogenation, an electrochemical approach carried out at room temperature was proposed.[242] Hydrogen generated in situ during water electrolysis was used as the hydrogenating agent. Copper dendrites, produced electrochemically (electrodeposition from a Cu(NO₃)₂ solution provided an extremely high selectivity of 97% with an acetylene content of only 4 ppm in the final mixture, which corresponds to industrial standards for ethylene purity for further polymerization. These results were achieved using a flow electrochemical cell with a large electrode surface area (Fig. 29).

Fig. 29

Scheme of electrocatalytic purification of ethylene from acetylene. Reproduced from Ref. 242 with permission from Springer Nature.

Another approach for the electrochemical hydrogenation of acetylene to ethylene assumed the diffusion of acetylene through a carbon-based gas diffusion layer (GDL) and then through a microporous layer (MPL) directly to the surface of the catalyst.[243] Thus, the reaction interface was a gas/electrolyte/catalyst system (Fig. 30). The process occurred at room temperature and was completed with 99.9% conversion of acetylene with 90.1% ethylene selectivity. These data allow considering electro­chemical approach involving the use of hydrogen generated in situ by water electrolysis as an alternative to the standard thermal method. Yet another example of electrochemical hydrogenation of acetylene was also demonstrated using a Cu catalyst, which was copper nanodots.[244] In this case, the acetylene content in the final mixture was less than 1 ppm.

Fig. 30

Schematic representation of cathode for acetylene hydrogenation. Reproduced from Ref. 243 with permission from Springer Nature.

A mixed catalyst based on copper and nickel with an aluminosilicate zeolite ZSM-12 support was prepared.[245] During catalytic tests, it was shown that the catalyst can completely convert acetylene with 82.5% selectivity to ethylene. Among the CuNi_x/ZSM-12 catalysts (x = 5, 7, 9, 11), CuNi₇/ZSM-12 performed best. According to the XPS data, the Ni2p peaks of the Ni/ZSM-12 and CuNi₇/ZSM-12 catalysts correspond to three components: the satellite peak of Ni, Ni²⁺, and Ni⁰ (Fig. 31a). The signal intensity in the spectra of Ni/ZSM-12 was much higher than that of CuNi₇/ZSM-12. All the three peaks have a slight shift towards an increase in the binding energy after the formation of CuNi₇ particles, which indicates a strong interaction of copper and nickel particles, expressed in the transfer of electrons. a red shift after the formation of CuNi₇ particles, which indicated a strong interaction of copper and nickel particles, expressed in electron transfer. The Cu2p signals in Cu/ZSM-12 and CuNi₇/ZSM-12 demonstrated binding energies of 935 and 955 eV, which corresponded to the Cu2p_3/2 and Cu2p_1/2 states, respectively (see Fig. 31b). When the support was replaced by ZrO₂, the hydrogenation selectivity increased to 93% also at full conversion of the substrate.[246] The authors have proved the formation of CuNi alloy particles and, using computational methods and analytical studies, explained the improved results by easier adsorption of acetylene and key intermediates.

Fig. 31

XPS spectra of Ni2p (a) and Cu2p (b) in Cu/ZSM-12 and CuNi₇/ZSM-12 catalysts. Reproduced from Ref. 245 under a Creative Commons Attribution (CC BY) license.

A series of copper and iron-based kaolin-supported catalysts were synthesized for testing in the semi-hydrogenation of acetylene.[247] The Cu₃Fe_0.5/kaolin mixture performed best, providing 95% selectivity at 93% conversion. The authors explained this result by the shift of the characteristic Cu peak in the XPS spectra to a lower energy level caused by changes in the electronic structure of the Cu₃Fe_0.5/kaolin catalyst, as well as the interaction of copper with Fe and support atoms (Al – Si). In addition, electrons from the partially occupied 4s orbital of copper were able to transfer to the partially occupied 3d orbital of iron atoms, which led to hybridization of orbitals in the alloy and a change in the electronic structure of active sites. Under hybridization, the Cu2p_3/2 signals in the alloy spectra were slightly shifted to lower energies.

Thus, single-atom copper catalysts show increased selectivity in comparison with copper NPs, which was shown by the authors of the original works using particles of different sizes as an example. The interaction of copper with support atoms and adjacent atoms in the case of alloy catalysts can also be noted. Separately, it is worth mentioning the identified phases of copper carbides formed during the immersion of metal atoms into a carbonaceous support Apparently, the formation of carbides is characteristic of most metals. The role of carbides in the hydrogenation of acetylene has not yet been reliably identified, and it remains to be established how the transition of active forms to carbide (or vice versa) occurs. Using a copper catalyst as an example, it was shown that grafting a long-chain aliphatic substituent to the metal decreases the amount of acetylene oligomerization products, which can most likely be used in the case of other metals.

The common route for removing acetylene from ethylene involves preliminary hydrogen production. An original approach based on the simultaneous production of hydrogen and semi-hydrogenation of acetylene on a Au/α-MoC catalyst was proposed.[55] Hydrogen was obtained by the reaction of carbon monoxide with water. The excess oxygen was removed with CO, which was further oxidized to CO₂, and the released hydrogen was immediately consumed in the hydrogenation reaction. To clarify the mechanism of the process, reagent mixtures with labelled atoms were used: CO + D₂O and ¹³CO + H₂O. Analysis of the kinetic isotope effect of the H/D exchange processes and ¹²C/¹³C in the formation of ethylene revealed the ratios of the formation rates (r):

r(H)/r(D) = 2.59,

r(CO)/r(¹³CO) = 1.14.

In fact, this meant that both the transfer of surface hydrogen to acetylene and the reaction of adsorbed CO with surface oxygen can contribute to the reaction rate. Based on these data, a reaction model was constructed, involving dissociation of water to give hydroxyls, the transfer of hydrogen to acetylene, and the reaction of CO with oxygen with subsequent desorption of CO₂. According to this model, the equilibrium constants of adsorption were determined, which demonstrated a strong competition of acetylene adsorption with the other reagents on the catalyst surface. The authors concluded, that the reaction of the three compounds (CO + H₂O + C₂H₂) occurred by removing of boundary oxygen thereby exposing molybdenum sites for acetylene adsorption and water dissociation to hydroxy anions. Then, hydrogen was transferred from hydroxyls to acetylene (formation of ethylene) with the removal of residual oxygen by carbon monoxide. The best results that were achieved in this process were high acetylene conversion (99%) and very promising ethylene selectivity (83%) at low temperature (80°C). The authors indicated, that the proposed path might be an alternative to traditional energy-intensive hydrogenation processes involving molecular hydrogen.

Alumina-supported gold NPs were used as a catalyst in the selective semi-hydrogenation of acetylene to ethylene.[248] The main idea of the work was to decorate the support with carbon atoms (CA) or carbon and nitrogen atoms (CNA), which were obtained by carbonization of the appropriate precursors on the support surface. The catalysts with doped support demonstrated 5 times higher catalytic activity in the catalytic tests (FIg. 32). According to XPS data, the electron transfer between the Au atom and the C and N atoms can play a crucial role in the activity of gold catalytic sites.

FIg. 32

Acetylene conversion and ethylene selectivity as a function of hydrogenation temperature (a); TOF values for the Au catalysts on various supports at 250°C (b). Reproduced from Ref. 248 with permission from Springer Nature.

Etching the bimetallic AuCu catalyst with acetic acid generates Cu vacancies on the abrupt catalyst surface, which are suitable for alkyne hydrogenation and, in combination with electron-deficient Au atoms, contribute to the increased catalytic properties.[249] The activity of etched Au₃Cu₁/TiO₂ catalysts was 4.6 times higher than that of untreated catalysts, and the ethylene selectivity was 94.0%.

ZnO with the disc-like (ZnO – D) and flower-like (ZnO – F) morphology were prepared hydrothermally from Zn(NO₃)₂ · 6 H₂O and used as a support for Au/ZnO nanocatalysts.[250] Subsequent treatment of the catalysts with a gas mixture of 10 vol.% O₂/Ar furnished Au/ZnO – F – O, Au/ZnO – D – O and their reduced derivatives, Au/ZnO – F – H and Au/ZnO – D – H, when treating the starting catalysts with a mixture of 10 vol.% H₂/Ar (Fig. 33). The catalytic activity in the semi-hydrogenation of acetylene was directly dependent on the Au³⁺ content in the catalyst and decreased in the series Au/ZnO – F > Au/ZnO – F – H > Au/ZnO – D > Au/ZnO – D – H. The authors explained this effect by the ability of Au³⁺ to take electrons from acetylene into vacant 5d orbitals, providing stronger acetylene adsorption than Au⁺ and Au⁰. The best acetylene conversion (~ 58%) and ethylene selectivity (76%) were obtained in the presence of Au/ZnO – F (Fig. 34).

Fig. 33

SEM images of Au/ZnO – F – O (a1, a2), Au/ZnO – F – H (b1, b2), Au/ZnO – D – O (c1, c2), Au/ZnO – D – H (d1, d2). Reproduced from Ref. 250 with permission from the American Chemical Society.

Fig. 34

Dependence of conversion/selectivity of acetylene semi-hydrogenation on the chemical state of Au species. Reproduced from Ref. 250 with permission from the American Chemical Society

The core-shell type composite Au@Pt NPs were synthesized by impregnation and then investigated as catalysts for selective semi-hydrogenation of acetylene.[251] An imidazole framework was used as a catalyst support. The synthetic procedure involved growing a platinum shell on gold nanorods, followed by oxidative etching of the resulting hollow nanocomposite. At the second step, the resulting composite particles were encapsulated into the substrate. The Au@Pt-NTs@ZIF-67 catalyst was the most active in the process and provided the 49.3% yield of ethylene at the acetylene conversion of 69.1%. Probably, high activity of such catalysts was due to their tubular structure with a high content of active sites on the catalyst surface. The existence of highly active platinum particles around gold particles caused a synergistic effect, which provided conversion of acetylene into ethylene. After the catalytic process, the composite nanotubes retained their hollow tubular structure, i.e. the morphology of the particles remained unchanged, opening up abilities for the multiple use of the catalysts.

The bimetallic Pt₃Fe nanocatalyst, individual particles of which were encapsulated in a silicon shell, were also studied in the hydrogenation of acetylene to ethylene. High-temperature (773 – 973 K) hydrogen treatment of the Pt₃Fe surface was carried out, which changed the structure of the catalytic species: disordered Pt species (A1 plane) was formed by ordered Pt₃ assemblies surrounded by Fe atoms (L12 plane) (Fig. 35).[252] The structure of this ensemble structure provided charge transfer from Fe atoms to neighbouring Pt atoms in the catalytic species, facilitating ethylene desorption from it and, accordingly, increasing and maintaining the selectivity of acetylene semi-hydrogenation at 83%.

Fig. 35

Geometry of the arrangement of Pt and Fe atoms in Pt₃Fe nanoparticles. Configurations of atoms in the dominant planes A1 (a) and L12 (b). Reproduced from Ref. 252 with permission from Elsevier.

A systematic and detailed study was carried out for a pair of platinum catalysts PtCu and PtCo.[253] The first important observation concerned the formation of alloy particles. The authors showed that the alloys are formed not only in the course of hydrogenation of metals from their oxides during synthesis of the catalysts, but also in the course of hydrogenation of acetylene. It was also found that larger platinum particles in Co catalysts promoted a higher hydrogenation reaction with increasing temperature thereby improving the yield of ethane. In contrast, in copper catalysts, a decrease in the platinum particle size led to not only to an improved yield of ethylene, but also to losses in the reaction rate. According to the X-ray absorption spectra of the near-threshold fine structure, the threshold energies for PtCu catalysts were 11 566.8 eV, and 11 563.8 eV for PtCo. Accordingly, higher energy implied higher density of unoccupied d-orbitals of Pt, which was a key indicator of improved activity in the semi-hydrogenation of acetylene (87.6% on Cu catalysts vs. 62.9% on Co catalysts). DFT calculations also showed that the redistribution of electron density towards platinum in the PtCu catalyst was more pronounced than in the PtCo catalyst. platinum atoms paired with copper atoms contained more electrons than those paired with cobalt atoms. This redistribution results in weaker π-bonding of ethylene on the surface and, accordingly, inhibition of complete hydrogenation. The authors also pointed out, that in addition to obvious Pt – O and Pt – Cu/Co bonds, Pt – O – Cu/Co bonds were formed.[254] In fact, it’s about unreduced metal oxides that were used as precursors of metal particles on the catalyst surface. On the one hand, the presence of such particles is obvious. On the other hand, their role and their contribution to the reaction are rarely assessed. It was found that oxygen atoms repel platinum atoms from each other, which furnish Pt – O – Cu and Pt – O – Co atomic structures. Under hydrogenation conditions, the coordination number of the Pt – Co alloy was 4.35, and that of the Pt – Cu alloy was 2.21. At the same time, the coordination numbers of Pt – O – CuO_{a(a < 1)} bonds was 7.45, which is 6.48 times greater than that of the Pt – CoO bond (1.15). Another issue was in the absence of the Cu – Cu bond in the Cu catalyst. All these issues provided weak binding of ethylene to the catalyst surface, which in turn decreases the rate of complete hydrogenation.

A series of complicated catalysts containing copper, platinum and cerium simultaneously were prepared for the selective hydrogenation of acetylene.[59] The initially prepared, PtCe alloy was deposited on a support consisting of various zeolites (Y, ZY), as well as on a copper-modified zeolite support to obtain PtCe/Cu–ZY. In addition, the PtCe alloy was deposited on a support of copper-modified oxalate and borohydride ligands (PtCe/CuX – ZY and PtCe/CuB–ZY, respectively). According to experimental data, the ethylene yield was 55.3% using PtCe/ZY catalysts at 120°C; and 71.4% in the presence of PtCe/Cu – ZY at 180°C. At the same time, in the presence of PtCe/CuX – ZY and PtCe/CuB – ZY catalysts, the yields were increased to 81.9 and 92.1% at 180 and 160°C, respectively. Both results demonstrated the positive effect of copper on the catalyst properties.

A silver carbide-like (Ag_xC) catalyst was synthesized by reduction of silver acetylide to Ag_xC under H₂ atmosphere.[255] This catalyst demonstrated promising catalytic activity in the conversion of acetylene to ethylene. The acetylene conversion was 100%, the ethylene selectivity was 88.7%, and the catalyst demonstrated good stability under continuous operation for at least 30 h. High performance was due to the ability of the Ag_xC phase to significantly reduce the activation barrier of H₂ dissociation and easy desorption of ethylene, which prevented overhydrogenation.

It is known, that supported rhodium catalysts are inefficient in semi-hydrogenation reactions. The addition of electron-donating acetylene ligands to atomically dispersed rhodium supported on zeolite provided high selectivity to ethylene (> 90%) in the acetylene semi-hydrogenation at 373 K in the presence of a large excess of ethylene.[256]

Selective hydrogenation of acetylene continues to be a relevant scientific issue. Obviously, the industrial process of ethylene purification from acetylene impurities stimulates research in this area, but it can be noted that the fundamental interest in this reaction is also extremely high. This is largely due to the need to gain deeper insight into the nature of catalysis, and the reaction of selective hydrogenation of acetylene is a kind of ‘textbook’ illustrating the selectivity patterns. Another boost for further research in this area was the development of complex catalysts and the discovery of new possibilities for their synthesis. The state-of-the-art experimental methods allow not only to reduce the size of metal particles to the nanoscale, but also provide uniform distribution of particles over the surface with atomic precision. Tuning the structure of complex species opens an access to single-atom catalysts, as well as ensembles of two or more elements of a certain composition and structure. For example, single atoms of one element can be placed in the corners of nanocubic particles of a second. Much of this precision is due not to the procedures per se, but rather to improvements in analytical techniques and the associated ability to detect the smallest details of the atomic world.

The highest efficiency in semi-hydrogenation of acetylene is demonstrated by catalysts based on palladium, copper, nickel, silver and their alloys. The general patterns of their action consist of an increase in the selectivity index for ethylene and a simultaneous decrease in the degree of acetylene conversion as the size of the metal particles decreases. Due to the small size of the metal particles and their distance from each other, the addition of hydrogen to acetylene occurs on the surface of the catalyst, and not in its bulk. ‘Dilution’ of the active metal with a ‘guest’ element also improves selectivity by transferring excess heat from the catalytic centre to the ‘satellite’ element. Another method for increasing selectivity is to reduce the electron density on the active metal. This is achieved by introducing one (or more) components to form alloy particles, or by appropriately designing the support, the atoms of which can deplete the active metal with electron density. A decrease in the electron density at the catalytic centre leads to easier desorption of ethylene formed during hydrogenation, a shorter time of its coordination with the catalyst surface and, accordingly, complicates further conversion into ethane.

As the types and numbers of active species on the catalyst surface continue to increase, and due to the difficulties in interpreting the catalytic pathway, it is likely that new barriers in the analysis of big catalytic data sets will emerge in the future. From this point of view, research in the field of machine learning and the use of artificial intelligence, which allows for the automatic analysis of hundreds and thousands of spectra with automatic processing of the results obtained, seem promising. Today, machine learning is already actively used in the development of approaches to the selective hydrogenation of acetylene [257] and in the design of catalysts for this process,[179][180] in the development of nanocluster [258] and single-atom [259] catalysts, in the search for active catalytic centres,[260] as well as for the identifying of intermediate structures on the surface of catalysts.[261]

In conclusion of the review, the comparison of the catalytic characteristics of the above-discussed single-atom catalysts with other types of catalytic systems used in the selective hydrogenation reaction of acetylene is given in Table 2.[262-267]

Table 2

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Comparison of single-atom catalysts with other types of catalytic systems used in semi-hydrogenation of acetylene.

(2)

Table 3

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Table 2 (continued)

(3)

E_ads — adsorption energy,

G_ads — adsorption free energy,

ΔH° — standard enthalpy (of reaction),

r — rate of formation,

Ac — acetone,

acac — acetylacetonate,

AC-HAADF-STEM — aberration-corrected high high-angle annular dark-field scanning transmission electron microscopy,

AIMD — initio ab initio molecular dynamics (method),

CN — nitrogen-doped carbon nanospheres obtained from covalent organic frameworks,

CNT — multi-walled carbon nanotubes,

COD — cycloocta-1,5-diene,

COF — covalent organic frameworks,

CTF — layered covalent triazine frameworks,

ESI-HRMS — high resolution mass spectrometry with electrospray ionization,

EDS — energy dispersive X-ray spectroscopy,

G — graphene,

GHSV — gas hourly space velocity,

GR — galvanic replacement (method),

gC₃N₄ — graphene-like carbon nitride,

GDL — carbon-based gas diffusion layer,

hNCNC — hierarchical carbon nanocells,

HR — high resolution,

HTS — hexadecyltrimethoxysilane,

HNT — halloysite nanotubes,

IL — ionic liquid,

ISA — isolated single-atom centres,

L — ligand,

LDH — layered double hydroxides,

MPL — microporous layer,

MPNC — mesoroporous N-doped carbon foam nanospheres,

NC — nanocubes,

ND — nanodiamonds,

NHC — N-heterocyclic carbene,

NMC — N-doped mesoroporous carbon,

NMP — N-methyl-2-pyrrolidon,

NP — nanoparticles,

NT — nanotubes,

Ndicy — dicyandiamide,

Pal — palygorskite,

PDO — 1,10-phenanthroline-5,6-dione,

Prmim — 1-methyl-3-n-propylimidazolium,

SA — single-atom (catalyst),

SAA — single-atom alloy,

SEM — scanning electron microscopy,

TEM — transmission electron microscopy,

TEOS — tetraethyl orthosilicate,

TOF — catalyst turnover frequency, which is a measure of catalytic activity and shows the number of substrate molecules transformed into product on one active centre of the catalyst per unit of time,

TPAOH — tetrapropylammonium hydrozide,

TPD — temperature programmed desorption,

XPS — X-ray photoelectron spectroscopy,

XRD — X-ray (powder) diffraction,

ZIF — zeolite imidazolate framework.