Competition of C‐C bond formation and C‐H bond formation For acetylene hydrogenation on transition metals: A density functional theory study

Ethane Dehydrogenation over the Core-Shell Pt-Based Alloy Catalysts: Driven by Engineering the Shell Composition and Thickness

Pt-based catalysts as the commercial catalysts in ethane dehydrogenation (EDH) face one of the main challenges of realizing the balance between coke formation and catalytic activity. In this work, a strategy to drive the catalytic performance of EDH on Pt-Sn alloy catalysts is proposed by rationally engineering the shell surface structure and thickness of core-shell Pt@Pt₃Sn and Pt₃Sn@Pt catalysts from a theoretical perspective. Eight types of Pt@Pt₃Sn and Pt₃Sn@Pt catalysts with different Pt and Pt₃Sn shell thicknesses are considered and compared with the industrially used Pt and Pt₃Sn catalysts. Density functional theory (DFT) calculations completely describe the reaction network of EDH, including the side reactions of deep dehydrogenation and C-C bond cracking. Kinetic Monte Carlo (kMC) simulations reveal the influences of the catalyst surface structure, experimentally related temperatures, and reactant partial pressures. The results show that CHCH* is the main precursor for coke formation, and Pt@Pt₃Sn catalysts generally have higher C₂H₄(g) activity and lower selectivity compared to those of Pt₃Sn@Pt catalysts, which is attributed to the unique surface geometrical and electronic properties. 1Pt₃Sn@4Pt and 1Pt@4Pt₃Sn are screened out as catalysts exhibiting excellent performance; especially, the 1Pt₃Sn@4Pt catalyst has much higher C₂H₄(g) activity and 100% C₂H₄(g) selectivity compared to those of 1Pt@4Pt₃Sn and the widely used Pt and Pt₃Sn catalysts. The two descriptors C₂H₅* adsorption energy and reaction energy of its dehydrogenation to C₂H₄* are proposed to qualitatively evaluate the C₂H₄(g) selectivity and activity, respectively. This work facilitates a valuable exploration for optimizing the catalytic performance of core-shell Pt-based catalysts in EDH and reveals the great importance of the fine control of the catalyst shell surface structure and thickness.

Enhancing Catalytic Performance through Subsurface Chemistry: The Case of C2H2 Semihydrogenation over Pd Catalysts

Subsurface chemistry in heterogeneous catalysis plays an important role in tuning catalytic performance. Aiming to unravel the role of subsurface heteroatoms, C₂H₂ semihydrogenation on a series of Pd catalysts doped with subsurface heteroatom H, B, C, N, P, or S was fully investigated by density functional theory (DFT) calculations together with microkinetic modeling. The obtained results showed that catalytic performance toward C₂H₂ semihydrogenation was affected significantly by the type and coverage of subsurface heteroatoms. The Pd-B_0.5 and Pd-C_0.5 catalysts with 1/2 monolayer (ML) heteroatom coverage, as well as Pd-N, Pd-P, and Pd-S catalysts with 1/16 ML heteroatom coverage, were screened to not only obviously improve C₂H₄ selectivity and activity but also effectively suppress green oil. The essential reason for subsurface heteroatoms in tuning catalytic performance is attributed to the distinctive surface Pd electronic and geometric structures caused by subsurface heteroatoms. In the Pd-B_0.5 and Pd-C_0.5 catalysts, the Pd surface electronic and geometric effects play the dominant role, while the geometric effect plays a key role in the Pd-N, Pd-P, and Pd-S catalysts. The findings provide theoretically valuable information for designing high-performance metal catalysts in alkyne semihydrogenation through subsurface chemistry.

Increasing the Distance of Adjacent Palladium Atoms for Configuration Matching in Selective Hydrogenation

Precisely tailoring the distance between adjacent metal sites to match adsorption configurations of key species for the targeted reaction pathway is a great challenge in heterogeneous catalysis. Here, we report a proof-of-concept study on the atomically sites-tailored pathway in Pd-catalyzed acetylene hydrogenation, i.e., increasing the distance of adjacent Pd atoms (d_Pd-a-Pd ) for configuration matching in acetylene semi-hydrogenation against coupling. d_Pd-a-Pd is identified as a structural descriptor for describing the competitiveness for reaction pathways, and the increased d_Pd-a-Pd prefers the semi-hydrogenation pathway due to simultaneously promoted C₂ H₄ desorption and the destabilized transition state of the C₂ H₃ * coupling. Spectroscopic, kinetics and electronic structure studies reveal that increasing d_Pd-a-Pd to 3.31 Å delivers superior selectivity and stability due to energy matching and appropriate hybridization of Pd 4d with In 2s and, especially, 2p orbitals.

Cu Catalysts Doped with a Heteroatom into the Subsurface: Unraveling the Role of Subsurface Chemistry in Tuning the Catalytic Performance of C2H2 Selective Hydrogenation

Heteroatoms doped into the subsurface of transition metals play a vital role in heterogeneous catalysis via either expressing surface structures or even directly participating in the reaction. Herein, DFT calculations and microkinetic modeling are implemented to examine C₂H₂ selective hydrogenation over heteroatom (H, B, C, N, or P)-doped Cu(111) and Cu(211) subsurfaces, which are compared with pure Cu(111) and Cu(211) to unravel the role of subsurface chemistry in tuning the surface structure and further regulating catalytic performance. Our results indicate that the catalytic performance toward C₂H₂ selective hydrogenation is closely related to the type of doped subsurface heteroatom and the Cu surface coordination environment, which can be attributed to the simultaneous change of Cu surface geometric and electronic structures. Catalytic performance improvement over the heteroatom-doped Cu(111) is generally better than that over the doped Cu(211); especially, B- or N-doped Cu(111) has excellent C₂H₄ activity and selectivity and greatly inhibits green oil. For the heteroatom-doped Cu(211), better performance is only obtained on P-Cu(211), which is still lower than the B- and N-doped Cu(111). The subsurface heteroatom doping should focus on high-coordination Cu(111) instead of low-coordination Cu(211). AIMD simulations verified the thermal stability of B-Cu(111) and N-Cu(111); both were screened out to be the most suitable catalysts toward C₂H₂ hydrogenation. This work clearly unravels the role of subsurface chemistry in heterogeneous catalysis and contributes to the rational design of high-performance metal catalysts by tuning surface structures with the heteroatom into the subsurface.

Mechanism driven design of trimer Ni1Sb2 site delivering superior hydrogenation selectivity to ethylene

Mechanism driven catalyst design with atomically uniform ensemble sites is an important yet challenging issue in heterogeneous catalysis associated with breaking the activity-selectivity trade-off. Herein, a trimer Ni₁Sb₂ site in NiSb intermetallic featuring superior selectivity is elaborated for acetylene semi-hydrogenation via a theoretical guidance with a precise synthesis strategy. The trimer Ni₁Sb₂ site in NiSb intermetallic is predicted to endow acetylene reactant with an adequately but not excessively strong σ-adsorption mode while ethylene product with a weak π-adsorption one, where such compromise delivers higher ethylene formation rate. An in-situ trapping of molten Sb by Ni strategy is developed to realize the construction of Ni₁Sb₂ site in the intermetallic P6₃/mmc NiSb catalysts. Such catalyst exhibits ethylene selectivity up to 93.2% at 100% of acetylene conversion, significantly prevailing over the referred Ni catalyst. These insights shed new lights on rational catalyst design by taming active sites to energetically match targeted reaction pathway.

Designing atomically uniform ensemble sites for matching targeted reaction pathway is important yet challenging in heterogeneous catalysis. Here, the authors fabricate a trimer Ni₁Sb₂ site featuring superior selectivity for acetylene semi-hydrogenation via a mechanism-driven design strategy.

DFT and microkinetic study of acetylene transformation on Pd(111), M(111) and PdM(111) surfaces (M = Cu, Ag, Au)

Density functional calculations and microkinetic simulations were performed on the transformation network of acetylene on Pd(111), M(111) and PdM(111) (M = Cu, Ag, Au) surfaces. It is demonstrated that the adsorption energies on alloy surfaces linearly correlate with the values on the pure metal surfaces. A good linear relationship between the co-adsorption energies of initial states and transition states is revealed with which the barriers of most elementary steps in the reaction network were estimated. To shed light on the transformation of acetylene, microkinetic simulations were conducted on the network. The results show that CHCH and H are dominant species on the surfaces and CCH, CCH₂ and CCH₃ are the main intermediates. Analysis indicates that introduction of coinage metals into Pd reduces the activity, but promotes the selectivity by lowering the barrier of CHCH₂ → CH₂CH₂. The present work provides a comprehensive overview of acetylene transformation on palladium, coinage metals and their alloy surfaces. The linear relationship of adsorption energies between the component metal and alloy surfaces and usage of the TSS relationship to evaluate barriers for microkinetic simulations are worthy of being further studied and extended to other systems.

The influence of palladium on the hydrogenation of acetylene on Ag(111)

We have used reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction (TPR) to study the selective hydrogenation of acetylene on both a clean Ag(111) surface and on a Pd/Ag(111) single-atom-alloy surface. The partial hydrogenation of acetylene to ethylene is an important catalytic process that is often carried out using PdAg alloys. It is challenging to study the reaction with ultrahigh vacuum techniques because H₂ does not dissociate on Ag(111), and while H₂ will dissociate at Pd sites, H-atom spillover from Pd to Ag sites does not generally occur. We bypassed the H₂ dissociation step by exposing the surfaces to atomic hydrogen generated by the hot filament of an ion gauge. We find that hydrogen atoms react with acetylene to produce adsorbed ethylene at 85 K, the lowest temperature studied. This is revealed by the appearance of a RAIRS peak at 950 cm^-1 due to the out-of-plane wagging mode of adsorbed ethylene when acetylene is exposed to a surface on which H atoms are pre-adsorbed. The formation of both ethylene and ethane are detected with TPR, but no acetylene coupling products, such as benzene, were found. From quantitative analysis of the TPR results, the percent conversion and selectivities to ethylene and ethane were determined. Low coverages of Pd enhance the conversion but do so mainly by increasing ethane formation.

Copper-Based Catalysts for Selective Hydrogenation of Acetylene Derived from Cu(OH)2

Replacing precious metals with cheap metals in catalysts is a topic of interest in both industry and academia but challenging. Here, a selective hydrogenation catalyst was prepared by thermal treatment of Cu(OH)₂ nanowires with acetylene-containing gas at 120 °C followed by hydrogen reduction at 150 °C. The characterization by means of transmission electron microscopy observation, X-ray diffraction, and X-ray photoelectron spectroscopy revealed that two crystallites were present in the resultant catalyst. One of the crystal phases was metal Cu, whereas the other crystal phase was ascribed to an interstitial copper carbide (Cu _x C) phase. The reduction of freshly prepared copper (II) acetylide (CuC₂) at 150 °C also afforded the formation of Cu and Cu _x C crystallites, indicating that CuC₂ was the precursor or an intermediate in the formation of Cu _x C. The prepared catalysts consisting of Cu and Cu _x C exhibited a considerably high hydrogenation activity at low temperatures in the selective hydrogenation of acetylene in the ethylene stream. In the presence of a large excess of ethylene, acetylene was completely converted at 110 °C and atmospheric pressure with an ethane selectivity of <15%, and the conversion and selectivity were constant in a 260 h run.

Taming the butterfly effect: modulating catalyst nanostructures for better selectivity control of the catalytic hydrogenation of biomass-derived furan platform chemicals

This minireview highlights versatile routes for catalyst nanostructure modulation for better hydrogenation selectivity control of typical biomass-derived furan platform chemicals to tame the butterfly effect on the catalytic selectivity.

Preparation of an Unsupported Copper-Based Catalyst for Selective Hydrogenation of Acetylene from Cu2O Nanocubes

Designing cheap, earth-abundant, and nontoxic metal catalysts for acetylene hydrogenation is of pivotal importance, but challenging. Here, a nonprecious metal catalyst for selective hydrogenation of acetylene in excess ethylene was prepared from Cu₂O nanocubes. The preparation includes two steps: (1) thermal treatment in acetylene-containing gas at 160 °C and (2) hydrogen reduction at 180 °C. The resultant catalyst showed outstanding performance at low temperature (80-100 °C) and 0.1-0.5 MPa pressure, completely converting acetylene with a low selectivity to undesired ethane (<20%). The characterization results of high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy corroborated that the formation of an interstitial copper carbide (Cu_xC) might give rise to significantly enhanced hydrogenation activity. Preliminary density functional theory calculation demonstrated that the lattice spacing of Cu₃C was nearly identical to that of the new Cu_xC crystallite measured in HRTEM and determined by XRD. The calculated dissociation energy of hydrogen on Cu₃C(0001) was considerably lower than that on Cu(111), suggesting superior hydrogenation activity of Cu₃C(0001). It is experimentally verified that copper(I) acetylide (Cu₂C₂) might be the precursor of Cu_xC. Cu₂C₂ underwent partial hydrogenation to fabricate Cu_xC crystallites and the thermal decomposition to Cu and carbon materials in parallel.

Unravelling the role of active-site isolation in reactivity and reaction pathway control for acetylene hydrogenation

Supported Pd_xCu_y bimetallic catalysts were prepared and characterized to illustrate the active-site isolation effect on controlling the reactivity and reaction pathway of acetylene hydrogenation.