Nature Of Active Sites And Structure-Performance Relationship Of Cu/CeO₂ Catalysts For The Hydrogenation Of Methyl Acetate

Ethanol as a clean fuel or gasoline additive can reduce the emissions of CO,CH_x,and solid particulates from engines.Moreover,a two-step new process consisted of carbonylation of dimethyl ether（DME）to methyl acetate（MA）and MA hydrogenation has been proposed for the synthesis of ethanol from syngas,which characterizes wide raw material sources,mild reaction conditions and high atomic economy.Because of the capability of selective hydrogenation of carbon-oxygen bonds and being inactive towards carbon-carbon bond cleavage,copper catalysts are widely used in MA hydrogenation.In this dissertation,Cu/CeO₂ catalysts have been development for MA hydrogenation reaction.To identify the active sites and improve the catalytic activity and stability of Cu/CeO₂ catalysts,we measured the structure of active species,investigated the structure-performance relationship and discussed the reaction mechanism for MA hydrogenation.In order to identify the active sites of Cu/CeO₂ catalysts in the hydrogenation of carbon-oxygen bonds in MA molecules,a series of rod-shaped ceria supported copper catalysts with different copper sizes ranging from single-atom,1.4 nm nanoclusters to 3.0 nm and 6.8 nm nanoparticles（NPs）were prepared by deposition-precipitation method.The catalyst with 3 nm Cu NPs exhibited the best catalytic performance for MA hydrogenation.The structure and chemical environment of copper species were detected and the surface Cu⁰,Cu^σ+species and defects were quantitatively measured with various characterizations.We also prepared contrast samples by doping Zr and La in ceria or acid treatment.It is demonstrated that the Cu⁰-Cu^σ+species rather than oxygen vacancies or M-[O_x]-Ce solid solution are the primary active sites for MA hydrogenation.It is further evidenced by the linearly increased TOF_Cu value with the increasing Cu⁰-Cu^σ+interfacial perimeter.Combined the in-situ experiment and DFT simulations,we proposed that the carbon-oxygen bonds hydrogenation reaction occurs at the Cu⁰-Cu^σ+interface via a site synergistic mechanism,where the Cu⁰-Cu^σ+interfacial site chemically adsorbs methyl acetate whereas the neighboring Cu⁰ dissociatively activates hydrogen.Based on the above results,increasing the active Cu⁰-Cu^σ+interface is a promising way to enhance the catalytic performance.A N₂O-involved oxidation treatment was used to break the grain boundaries of Cu NPs and facilitate the formation of the active Cu⁰-Cu^σ+sites.However,the catalytic activity rapidly decreased after a temporary enhancement at the beginning of the MA hydrogenation.The switch between oxidation and reduction atmosphere induced the copper aggregation.We further found that 1 wt.%cerium addition into the Cu/CeO₂ catalyst followed by the N₂O treatment could significantly improve the catalytic performance and keep it stable.The introduced cerium species exist in the form of CeO₂nanoparticles over the Cu NPs and could hinder the sintering of Cu species during the oxidation-reduction treatment.Meanwhile,the strong interaction between the reconstructed Cu species and CeO₂ increased the fraction of surface active Cu⁰-Cu^σ+sites,which significantly enhanced the catalytic activity for MA hydrogenation.An atomically dispersed Cu-Pt bimetallic catalyst for MA hydrogenation was prepared by introducing a trace amount of Pt into single-atom Cu/CeO₂ catalyst.With 0.025 wt.%Pt addition,the catalyst exhibited an excellent catalytic performance of 98.4%MA conversion and 97.3%ethanol selectivity at a H₂/MA ratio of 10.Various characterizations demonstrated that the introduced Pt species preferred to deposit onto the supported Cu atoms,resulting in the formation of Cu-Pt atomic pairs.In situ chemisorption experiments were conducted to explore the catalytic mechanism of Cu-Pt atomic pair in the MA hydrogenation.It is shown that the formation of Cu-Pt atomic pairs could promote the adsorption and activation of MA and H₂ molecule,which could be the main reason for enhancing catalytic performance.This understanding may provide guidance for using atomic dispersed catalysts in hydrogenation reactions.