Synthesis of a hybrid Pd0/Pd-carbide/carbon catalyst material with high selectivity for hydrogenation reactions

Pd catalysts doped with B/C atoms in the subsurface: the positive effect of interstitial atoms in regulating the selectivity of acetylene catalytic hydrogenation

At present, the modification of palladium (Pd) catalysts is an important topic due to its potential to enhance catalytic performance and reduce catalyst costs. In this work, boron (B) and carbon (C) are interstitially doped into the subsurface of Pd to construct Pd4LB2L and Pd4LC2L catalysts. The adsorption properties of acetylene and ethylene, the mechanism of acetylene hydrogenation, and ethylene selectivity are studied based on density functional theory (DFT) calculations. The results show that the ethylene selectivity of the Pd4LB2L catalyst is significantly enhanced compared with that of Pd(111). The adsorption of CH2CH2 is weakened substantially after B element doping, and the ethylene selectivity descriptor (Esel) value of the Pd4LB2L catalyst reaches 19.7 kJ mol-1. It is revealed that non-metallic atoms doped into the subsurface layer of metal catalysts change the adsorption of reactant/intermediate molecules and the selectivity of ethylene by affecting the electronic structure and properties of Pd atoms in the surface layer. This work provides insights into the selectivity of modified Pd-based catalysts for selective hydrogenation of acetylene and realization of cost-effective catalysts.

Implications for the Hydrogenation of Propyne and Propene with Parahydrogen due to the in situ Transformation of Rh2C to Rh0/C

NMR spectroscopy studies using parahydrogen-induced polarization have previously established the existence of the pairwise hydrogen addition route in the hydrogenation of unsaturated hydrocarbons over heterogeneous catalysts, including those based on rhodium (Rh0). This pathway requires the incorporation of both hydrogen atoms from one hydrogen molecule to the same product molecule. However, the underlying mechanism for such pairwise hydrogen addition must be better understood. The involvement of carbon, either in the form of carbonaceous deposits on the surface of a catalyst or as a metal carbide phase, is known to modify catalytic properties significantly and thus could also affect the pairwise hydrogen addition route. Here, we explored carbon's role by studying the hydrogenation of propene and propyne with parahydrogen on a Rh2C catalyst and comparing the results with those for a Rh0/C catalyst obtained from Rh2C via H2 pretreatment. While the catalysts Rh2C and Rh0/C differ notably in the rate of conversion of parahydrogen to normal hydrogen as well as in terms of hydrogenation activity, our findings suggest that the carbide phase does not play a significant role in the pairwise H2 addition route on rhodium catalysts.

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.

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.

Pd on cyclotriphosphazen-hexa imine decorated boehmite as an efficient catalyst for hydrogenation of nitro arenes under mild reaction condition

Cyclotriphosphazen-hexa imine ligand was prepared through successive reactions of phosphonitrilchloride trimer with 4-hydroxybenzaldehyde and (3-aminopropyl) triethoxy silane. The as-prepared ligand was then covalently grafted on boehmite to furnish an efficient support for the immobilization of Pd nanoparticles and synthesis of a novel heterogeneous catalyst for hydrogenation of nitro arenes. The catalytic tests revealed that the catalyst had excellent catalytic activity for hydrogenation of various nitro arenes with different steric and electronic features under mild reaction condition in aqueous media. Noteworthy, the catalyst was highly selective and in the substrates with ketone or aldehyde functionalities, reduction of nitro group was only observed. The catalyst was also recyclable and only slight loss of activity was detected after each recycling run. Hot filtration test also approved true heterogeneous nature of catalysis.