Adsorbate-Induced Structural Evolution of Pd Catalyst for Selective Hydrogenation of Acetylene

Bridging the size gap between experiment and theory: large-scale DFT calculations on realistic sized Pd particles for acetylene hydrogenation

Metal nanoparticles, often supported on metal oxide promoters, are a cornerstone of heterogeneous catalysis. Experimentally, size effects are well-established and are manifested through changes to catalyst selectivity, activity and durability. Density Functional Theory (DFT) calculations have provided an attractive way to study these effects and rationalise the change in nanoparticle properties. However such computational studies are typically limited to smaller nanoparticles (approximately up to 50 atoms) due to the large computational cost of DFT. How well can such simulations describe the electronic properties of the much larger nanoparticles that are often used in practice? In this study, we use the ONETEP code, which is able to achieve more favourable computational scaling for metallic nanoparticles, to bridge this size gap. We present DFT calculations on entire Pd and Pd carbide nanoparticles of more than 300 atoms (approximately 2.5 nm diameter), and find major differences in the electronic structure of such large nanoparticles, in comparison to the commonly investigated smaller clusters. These differences are also manifested in the calculated chemical properties such as adsorption energies for C2H2, C2H4 and C2H6 on the pristine Pd and PdC x nanoparticles which are significantly larger (up to twice in value) for the ∼300 atoms structures. Furthermore, the adsorption of C2H2 and C2H4 on PdC x nanoparticles becomes weaker as more C is introduced in the Pd lattice whilst the impact of C concentration is also observed in the calculated reaction energies towards the hydrogenation of C2H2, where the formation of C2H6 is hindered. Our simulations show that PdC x nanoparticles of about 5% C per atom fraction and diameter of 2.5 nm could be potential candidate catalysts of high activity in hydrogenation reactions. The paradigm presented in this study will enable DFT to be applied on similar sized metal catalyst nanoparticles as in experimental investigations, strengthening the synergy between simulation and experiment in catalysis.

Bi-modified Cu-Based Catalysts for Acetylene Hydrogenation: Leveraging Dispersion and Hydrogen Spillover

Removing trace acetylene from the ethylene stream through selective hydrogenation is a crucial process in the production of polymer-grade ethylene. However, achieving high selectivity while maintaining high activity remains a significant challenge, especially for nonprecious metal catalysts. Herein, the trade-off between activity and selectivity is solved by synergizing enhanced dispersion and hydrogen spillover. Specifically, a bubbling method is proposed for preparing SiO2-supported copper and/or bismuth carbonate with high dispersion, which is then employed to synthesize highly dispersed Bi-modified CuxC-Cu catalyst. The catalyst displays outstanding catalytic performance for acetylene selective hydrogenation, achieving acetylene conversion of 100% and ethylene selectivity of 91.1% at 100 °C. The high activity originates from the enhanced dispersion, and the exceptional selectivity is due to the enhanced spillover capacity of active hydrogen from CuxC to Cu, which is promoted by the Bi addition. The results offer an avenue to design efficient catalysts for selective hydrogenation from nonprecious metals.

A carbon-promoted galvanic replacement method to synthesize efficient PdNi nanoalloy catalyst

PdNi nanoalloy catalysts were prepared by a carbon-promoted galvanic replacement method. Characterizations and control experiments show the increased replacement rate of metal Ni with Pd2+ ion can be attributed to the higher electrode potential and smaller crystalline sizes caused by carbon doping. Introduction of carbon (C) into Ni particles not only accelerates the formation process of PdNi nanoalloys, but also enables C atoms to successfully enter the lattice interstices of PdNi nanoalloys. C regulates the surface electronic properties of PdNi nanoalloys by the electron transfer between different elements and improves their activity. The PdNi@C-650 exhibits extraordinary activity and long-term stability for hydrogenation reduction of hexavalent chromium (Cr (VI)) and hydrodechlorination of chlorophenols in comparison with PdNi/CNTs (carbon nanotubes) and commercial Pd/C. Density functional theory calculations together with investigations of mechanism reveal that the high electron-deficient PdNi nanoalloys from the redistribution of electron between Ni, Pd and C of the PdNi@C-650 promote the surface adsorption of substrate molecules and H2, which accordingly enhances the hydrogenation activity. This study brings a new method for the design and preparation of high active noble metal nanoalloy.

Decoding Active Sites for Highly Efficient Semihydrogenation of Acetylene in Palladium-Copper Nanoalloys

Accurately decoding the three-dimensional atomic structure of surface active sites is essential yet challenging for a rational catalyst design. Here, we used comprehensive techniques combining the pair distribution function and reverse Monte Carlo simulation to reveal the surficial distribution of Pd active sites and adjacent coordination environment in palladium-copper nanoalloys. After the fine-tuning of the atomic arrangement, excellent catalytic performance with 98% ethylene selectivity at complete acetylene conversion was obtained in the Pd34Cu66 nanocatalysts, outperforming most of the reported advanced catalysts. The quantitative deciphering shows a large number of active sites with a Pd-Pd coordination number of 3 distributed on the surface of Pd34Cu66 nanoalloys, which play a decisive role in highly efficient semihydrogenation. This finding not only opens the way for guiding the precise design of bimetal nanocatalysts from atomic-level insight but also provides a method to resolve the spatial structure of active sites.

Atomic Three-Dimensional Investigations of Pd Nanocatalysts for Acetylene Semi-hydrogenation

Deciphering the three-dimensional (3D) insight into nanocatalyst surfaces at the atomic level is crucial to understanding catalytic reaction mechanisms and developing high-performance catalysts. Nevertheless, better understanding the inherent insufficiency of a long-range ordered lattice in nanocatalysts is a big challenge. In this work, we report the local structure of Pd nanocatalysts, which is beneficial for demonstrating the shape-structure-adsorption relationship in acetylene hydrogenation. The 5.27 nm spherical Pd catalyst (Pdsph) shows an ethylene selectivity of 88% at complete acetylene conversion, which is much higher than those of the Pd octahedron and Pd cube and superior to other reported monometallic Pd nanocatalysts so far. By virtue of the local structure revelation combined with the atomic pair distribution function (PDF) and reverse Monte Carlo (RMC) simulation, the atomic surface distribution of the unique compressed strain of Pd-Pd pairs in Pdsph was revealed. Density functional theory calculations verified the obvious weakening of the ethylene adsorption energy on account of the surface strain of Pdsph. It is the main factor to avoid the over-hydrogenation of acetylene. The present work, entailing shape-induced surface strain manipulation and atomic 3D insight, opens a new path to understand and optimize chemical activity and selectivity in the heterogeneous catalysis process.

Encapsulation of Palladium Carbide Subnanometric Species in Zeolite Boosts Highly Selective Semihydrogenation of Alkynes

The selective hydrogenation of alkynes to alkenes is a crucial step in the synthesis of fine chemicals. However, the widely utilized palladium (Pd)-based catalysts often suffer from poor selectivity. In this work, we demonstrate a carbonization-reduction method to create palladium carbide subnanometric species within pure silicate MFI zeolite. The carbon species can modify the electronic and steric characteristics of Pd species by forming the predominant Pd-C4 structure and, meanwhile, facilitate the desorption of alkenes by forming the Si-O-C structure with zeolite framework, as validated by the state-of-the-art characterizations and theoretical calculations. The developed catalyst shows superior performance in the selective hydrogenation of alkynes over mild conditions (298 K, 2 bar H2 ), with 99 % selectivity to styrene at a complete conversion of phenylacetylene. In contrast, the zeolite-encapsulated carbon-free Pd catalyst and the commercial Lindlar catalyst show only 15 % and 14 % selectivity to styrene, respectively, under identical reaction conditions. The zeolite-confined Pd-carbide subnanoclusters promise their superior properties in semihydrogenation of alkynes.

Mechanism Study of Organic Sulfur Hydrogenation over Pt- and Pd-Loaded Alumina-Based Catalysts

The hydrogenation of organic sulfur (CS2) present in industrial off-gases to produce sulfur-free hydrocarbons and H2S can be achieved by using noble-metal catalysts. However, there has been a lack of comprehensive investigation into the underlying reaction mechanisms associated with this process. In this study, we have conducted an in-depth examination of the activity and selectivity of Pt- and Pd-loaded alumina-based catalysts, revealing significant disparities between them. Notably, Pd/Al2O3 catalysts exhibit an enhanced performance at low temperatures. Furthermore, we have observed that CS2 displays a higher propensity for conversion to methane when employing Pt/Al2O3 catalysts, while Pd/Al2O3 catalysts demonstrate a greater tendency for coke deposition. By combining experimental observations with theoretical calculations, we revealed that the capability of H2 spillover along with the adsorption capacity of CS2, play pivotal roles in determining the observed differences. Moreover, the key intermediate species involved in the methanation and coke pathways were identified. The intermediate CH2S* is found to be crucial in the methanation pathway, while the intermediate CSH* is identified as significant in the coke pathway.

Metal-Organic Framework-Derived Ni-S/C Catalysts for Selective Alkyne Hydrogenation

A carbon matrix-supported Ni catalyst with surface/subsurface S species is prepared using a sacrificial metal-organic framework synthesis strategy. The resulting highly dispersed Ni-S/C catalyst contains surface discontinuous and electron-deficient Niδ+ sites modified by p-block S elements. This catalyst proved to be extremely active and selective for alkyne hydrogenation. Specifically, high intrinsic activity (TOF = 0.0351 s-1) and superior selectivity (>90%) at complete conversion were achieved, whereas an analogous S-free sample prepared by the same synthetic route performed poorly. That is, the incorporation of S in Ni particles and the carbon matrix exerts a remarkable positive effect on catalytic behavior for alkyne hydrogenation, breaking the activity-selectivity trade-off. Through comprehensive experimental studies, enhanced performance of Ni-S/C was ascribed to the presence of discontinuous Ni ensembles, which promote desorption of weakly π-bonded ethylene and an optimized electronic structure modified via obvious p-d orbital hybridization.

Dynamics of Pd Subsurface Hydride Formation and Their Impact on the Selectivity Control for Selective Butadiene Hydrogenation Reaction

Structure-sensitive catalyzed reactions can be influenced by a number of parameters. So far, it has been established that the formation of Pd-C species is responsible for the behavior of Pd nanoparticles employed as catalysts in a butadiene partial hydrogenation reaction. In this study, we introduce some experimental evidence indicating that subsurface Pd hydride species are governing the reactivity of this reaction. In particular, we detect that the extent of formation/decomposition of PdHx species is very sensitive to the Pd nanoparticle aggregate dimensions, and this finally controls the selectivity in this process. The main and direct methodology applied to determine this reaction mechanism step is time-resolved high-energy X-ray diffraction (HEXRD).

Atomistic simulations on the carbidisation processes in Pd nanoparticles

The formation of interstitial PdC _x nanoparticles (NPs) is investigated through DFT calculations. Insights on the mechanisms of carbidisation are obtained whilst the material's behaviour under conditions of increasing C-concentration is examined. Incorporation of C atoms in the Pd octahedral interstitial sites is occurring through the [111] facet with an activation energy barrier of 19.3-35.7 kJ mol^-1 whilst migration through the [100] facet corresponds to higher activation energy barriers of 124.5-127.4 kJ mol^-1. Furthermore, interstitial-type diffusion shows that C will preferentially migrate and reside at the octahedral interstitial sites in the subsurface region with limited mobility towards the core of the NP. For low C-concentrations, migration from the surface into the interstitial sites of the NPs is thermodynamically favored, resulting in the formation of interstitial carbide. Carbidisation reaction energies are exothermic up to 11-14% of C-concentration and slightly vary depending on the shape of the structure. The reaction mechanisms turn to endothermic for higher concentration levels showing that C will preferentially reside on the surface making the interstitial carbide formation unfavorable. As experimentally observed, our simulations confirm that there is a maximum concentration of C in Pd carbide NPs opening the way for further computational investigations on the activity of Pd carbides in directed catalysis.

Isoprene selective hydrogenation using AgCu-promoted Pd nanoalloys

Alloying is an effective approach to improve the catalysis performance of Pd-based catalysts for the selective hydrogenation of diolefins towards monoolefines. Herein, PdAgCu ternary nanoalloy catalysts were synthesised by a stepwise impregnation method for isoprene selective hydrogenation. The addition of a moderate amount of Ag and Cu to Pd significantly enhances the isoamylene selectivity in the isoprene hydrogenation, and decreases the non-desired over-hydrogenation. In addition, the loading molar ratio of PdAgCu with 3 : 2 : 3 as the optimal ternary nanoalloy composition maximizes the isoprene conversion (98%) and the monoolefins yield (92%). The surface structure of the catalyst was probed using H₂-TPR, TEM, XRD, and XPS characterization methods, and it was confirmed that the surface Pd composition ratio between the metallic and oxidized states shows significant effects on the monoolefines yield. This work demonstrates the advantages of PdAgCu ternary nanoalloy catalysts for isoprene selective hydrogenation, which also provides guidelines for the development of other Pd-based ternary nanoalloys for diolefins selective hydrogenation.

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.

Single-factor analysis of Ni-B-AC-catalyzed β-pinene hydrogenation based on hierarchical analysis

In the choice of catalysts for the hydrogenation of pinene, nickel-based catalysts show intriguing activity. Here, a Ni-B catalyst supported on activated carbon with Ni as an active component was synthesized by the titration reduction co-impregnation method. The mechanism of such heterogeneous systems has not yet been articulated, and the industrial applications of the potassium borohydride reduction of nickel-based catalysts are limited by their easy agglomeration and poor stability. The materials were analyzed by hierarchical and DFT studies, in situ XPS, BET, XRD, and SEM, which provided insights into the kind of signals in Ni²⁺ reduction to Ni⁰. The hierarchical analysis indicated that Ni/AC (0.4876) and reaction pressure (0.6066) influenced the catalyst preparation and process efficiency changes, respectively. Activated carbon was shown to provide a favorable basis for the stability of the Ni-B activity. In addition, the hierarchical analysis method provides new insights into the data analysis for chemical experiments.

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.

A theoretical study of asymmetric ketone hydrogenation catalyzed by Mn complexes: from the catalytic mechanism to the catalyst design

Herein, a density functional theory (DFT) study was performed to investigate asymmetric ketone hydrogenation (AKH) catalyzed by Mn complexes, from the catalytic mechanism to the catalyst design. The calculated results indicated that the Mn(CO)₂-PSiNSiP (A1, PSiNSiP = P(Ph)₂Si(CH₃)₂NSi(CH₃)₂P(Ph)₂) pincer complex has potential high catalytic activity for ketone hydrogenation. The Mn(CO)-LYB (B, LYB = P(Ph)₂Si(CH₃)₂NSi(CH₃)₂P(Me)₂) pincer complex was then designed to catalyze AKH with good stereoselectivity. The hydrogen transfer (HT) step is the chirality-determining step. To avoid the enantiomer of Mn(CO)₂-LYB, which could eliminate the high stereoselectivity during AKH, novel Mn complexes with quadridentate ligands, such as Mn(CO)-LYC (C, LYC = P(CH₃)₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Ph)₂) and Mn(CO)-LYD (D, LYD = P(CH₃)₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Cy)₂), were designed to drive AKH with medium stereoselectivity. In order to increase the stereoselectivity of AKH, Mn(CO)-LYE (E, LYE = PH₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Ph)₂) and Mn(CO)-LYF (F, LYF = PH₂CH₂Si(CH₃)NSi(CH₃)(Si(CH₃)CH₂P(CH₃)₂)CH₂P(Cy)₂) were further designed and showed very good stereoselectivity, which is due to the lower deformation energy and stronger interactions between the ketone substrates and catalysts. This work may shed light on the design of cheap metal catalysts with a new ligand framework for the asymmetric hydrogenation (AH) of CX bonds (X = O, N).

Rapid detoxification of Microcystin-LR by selective catalytic hydrogenation of the Adda moiety using TiO2-supported Pd catalysts

The hepatotoxicity of Microcystin-LR (MC-LR) is mainly caused by its Adda moiety. In this study, we used TiO₂-supported Pd catalysts to selectively hydrogenate the CC bonds in the Adda moiety, achieving rapid detoxification of MC-LR in water under ambient conditions. MC-LR was removed within 5 min by catalytic hydrogenation on Pd(1.0)/TiO₂ with a catalyst dosage normalized rate constant of 1.3 × 10^-2 L mg_cat^-1 min^-1, significantly more efficient than other catalytic treatment methods. The reactions proceeded in a highly selective manner towards catalytic hydrogenation at the CC bond of the Mdha moiety and subsequently the conjugated double bond of the Adda moiety, yielding two intermediates and one final product. Upon catalytic hydrogenation for 30 min on Pd(0.07)/TiO₂, the toxicity of MC-LR (assessed by protein phosphatase 2A activity assay) drastically decreased by 90.8%, demonstrating effective detoxification. The influence of catalyst support, Pd content, initial MC-LR concentration, reaction pH, and catalytic stability were examined. Surface adsorption and the cationic Pd played a crucial role in the reaction kinetics. Our results suggest that catalytic hydrogenation is a highly effective and safe strategy for detoxifying MC-LR by selective reactions.

Pd/Fe2O3 with Electronic Coupling Single-Site Pd-Fe Pair Sites for Low-Temperature Semihydrogenation of Alkynes

Dispersing single palladium atoms on a support is promising to minimize the usage of palladium and improve the selectivity for alkyne semihydrogenation, but its activity is often very low as a result of unfavorable H₂ activation. Here, we load palladium onto α-Fe₂O₃(012) to construct highly active and stable single-site Pd-Fe pairs with luxuriant d-electron domination near the Fermi level driven by strong electronic coupling and prove that Pd-Fe pairs cooperatively adsorb H₂ and dissociate an H─H bond, whereas solo Pd sites enable preferential desorption of C═C intermediate, thus achieving both high activity and high selectivity for alkyne hydrogenation. This catalyst exhibits state-of-the-art performance in purifying acetylene of ethylene stream, with 99.6% and 100% conversion and 96.7% and 94.7% selectivity at 353 and 393 K, respectively, and excellent stability with negligible activity decay after a 200 h test. This single-site pair inherits the advantage but overcomes the weakness of both Pd ensemble and single Pd atoms, enabling ultralow-Pd-loading catalysts for selective hydrogenation.