Synthesis And Catalytic Transfer Hydrogenation Mechanism Of Supported Palladium And Copper Nanocatalysts

Functionalized anilines are important intermediates in the manufacture of various important chemicals including dyes,pharmaceuticals and rubber additives.Selective hydrogenation is one of the most critical transformations in the synthesis of fine chemicals or pharmaceuticals from organic compounds,both in the laboratory and in industry.Generally,two strategies are mainly used in hydrogenation reactions,one is direct hydrogenation with pressurized H₂gas,and the other is catalytic transfer hydrogenation（CTH）with organic hydrogen donors.Handling high-pressure hydrogen results in huge infrastructure costs on an industrial scale,with poor economic efficiency,posing an economic barrier to the development of a sustainable hydrogenation industry.Therefore,there is a great need for environmentally friendly,cost-effective and recyclable hydrogenation processes utilizing H atoms via heterogeneous catalysts.However,the efficient and selective conversion of nitro groups to amine groups in the presence of other sensitive groups such as halogens,alkenes,alkynes,or ketones remains a great challenge.Commercial metal catalysts such as Raney Ni are difficult to achieve high chemoselectivity and even suffer from poisoning or leaching during the process.Therefore,it is imperative to develop catalysts with high selectivity and stable activity in the catalytic hydrogenation of nitroaromatics.In this paper,the catalytic hydrogenation reduction performance of nitrobenzene and p-nitrophenol（4-NP）was investigated by supporting metals on 2D sheet MXene and SnO₂nanospheres,respectively,and calculated by density functional theory（DFT）.The mechanism of catalytic activity improvement was analyzed.The specific research is as follows:1.Pd nanoparticles（NPs）were anchored on layered Ti₃C₂MXenes by a liquid-phase reduction method for efficient and selective catalytic transfer hydrogenation（CTH）of nitrobenzene in the presence of formic acid.Some electrons in Pd NPs transferred to Ti₃C₂MXene upon formation of Ti₃C₂/Pd nanohybrids,as confirmed by XPS and DFT simulations.The electron transfer changed Pd valance electron configure from 4d¹⁰to 4d^10-x.Such electron-deficient Pd NPs tuned reaction pathway and promoted formic acid dissociation,both of which favored the production of active H*atoms,i.e.,the exact reductant for CTH.Compared with Pd NPs,Ti₃C₂/Pd showed stronger adsorption of H*and therefore inhibited the occurrence of HER（2H*→H₂）.Owing to favorable H*production and HER inhibition,Ti₃C₂/Pd（15 wt%Pd）showed enhanced nitrobenzene CTH performance with turnover frequency（TOF）of 351.7 h^-1and 99%aniline selectivity,outperforming most of current catalysts.Our work might inspire designing more advanced CTH catalysts by tuning their valance electron configures with 2D MXene materials.2.SnO₂-supported Cu NPs（SnO₂/Cu）were prepared through a hydrothermal reduction method.The catalytic performance of SnO₂/Cu was investigated with a model reaction,the reduction of p-nitrophenol with sodium borohydride（Na BH₄）as the reductant.XPS combined with DFT analyses confirmed that some electrons in Cu NPs were transferred to SnO₂after the formation of SnO₂/Cu nanocomposites.Such electron transfer changed the Cu electronic structure with its d-band center upshifting from-2.45 e V of to-2.41 e V.The hydrogen adsorption energy(E_ad)of Cu NPs was enhanced after they were supported on SnO₂.The result implied that the SnO₂-supported Cu NPs preferred adsorption of active hydrogen atoms（H*）produced from Na BH₄dissociation,which inhibited their mutual combination to release H₂gas.The inhibited H₂evolution enhanced the reaction rate and the utilization efficiency of sodium borohydride.SC10（SnO₂/Cu with 10 wt%Cu）showed the best catalytic performance for the reduction of p-nitrophenol with Na BH₄with reaction rate constant of 1.843 min^-1and the p-aminophenol selectivity of 99%.Our work might shed new light on designing more advanced hydrogenation catalysts with optimal electronic structure of Cu tuned by transition metal oxides.