Surface Design Of Palladium-based Catalysts And Their Performance In Catalytic Reactions

Noble metal nanomaterials have broad application prospects due to their excellent catalytic performance including stability and selectivity.Since the catalytic reaction occurrs on the surface of catalysts,their catalytic performance can be optimized by regulating the surface atomic distribution of alloy nanocrystals.The catalytic performance of metal alloys can be manipulated by the charge transfer between the two types of metals,so as to alter the electronic structure and activity of the constituent atoms.Since most of the catalytic reactions occur on the surface of the catalysts,the regulation of the surface active sites is beneficial to achieve the atomic utilization of the catalysts.In this dissertation,based on the precisely controlled synthetic method,we sythesized inorganic alloy nanocrystals with different surface structures.The surface state of the catalysts was controlled by changing composition and distribution of the atoms on the catalyst surface,As such,we can investigate the influence of surface and interface regulation on various catalytic reaction processes.The mechanism of different catalytic processes was further explored by controlling the surface state of noble metal catalysts precisely.Specifically,the application of catalysts in electrochemical hydrogen evolution was explored by regulating the distribution of palladium atoms on the surface.,the distribution of different atoms in surface alloy was regulated to examine the effect of surface electron state,and its effect on organic thermocatalytic reaction;by regulating the distribution of surface palladium atoms and introducing the plasmon effect of noble metal Ag,we can study the mechanism of catalysts action in various organic catalytic reactions;by combining the alloys with semiconductors,we can study the mechanism of photo-induced electrons and plasmon effects in photocatalytic reactions.The main research results obtained are summarized as follows:1.We have designed a Pd-implanted Ag nanostructures through a selective etching-deposition approach.The dilution of trace Pd atoms in the near-surface region of Ag nanocrystals has been resolved by synchrotron-radiation characterizations unambiguously.This dilution can alter the adsorption of H atoms from hollow-to top-sites.As a result,the Pd-H bond cleavage was facilitated to dramatically enhance HER activity.Notably the electrocatalytic HER performance has been improved about 14 times as compared with conventional Pd catalysts,approaching the high level of Pt catalysts.This giant HER enhancement overcomes the long-standing bottleneck of Pd catalysts in HER,and provides an alternative strategy for engineering catalytic materials toward hydrogen-related applications given the low cost of Ag matrix material and the ultralow usage of Pd atoms(0.8 mol.%Pd in catalysts).The concept demonstrated here calls for future efforts on near-surface lattice engineering at atomic precision for high-performance and low-cost electrocatalyst design.2.Core-shell Au@AuPd nanocubes have been developed through a coreduction process on the surface of Au nanocubes.This method can control the surface Au/Pd atomic ratios simply by altering the amounts of metal precursors in the synthetic system,which offers the capability of regulating the surface state of Pd sites in Au lattice.Enabled by the control over Pd sites,the Had surface coverage,metal-Had binding,and reactive molecule desorption can be balanced to dramatically enhance the activity and selectivity of semihydrogenation reactions.As a result,an optimal nanocatalyst with both high activity and selectivity for semihydrogenation has been identified by manipulating metal-Had and metal-alkene interactions.The concept demonstrated here calls for future efforts on rationally designing bimetallic catalysts for semihydrogenation reactions.3.Based on the unique LSPR properties of Ag in the visible region,Ag@AgPd core-shell nanocatalysts with AgPd alloy layer on the surface have been synthesized.Contributing to the excellent catalytic activity of Pd in organic reactions and the local surface plasmon resonance(LSPR)effect of Ag,we can investigate the performance of the catalyst in various organic reactions under light irradiation.The comparison experiments was carried out at different temperatures and dark conditions.During the experiment,we chose 40-50 nm silver nanocubes as the model system with a good absorption at around 425 nm.The surface design of the AgPd alloy was beneficial to maximize the use of active Pd sites in catalytic reactions.By changing the Pd2+/Ag+ratio of the added precursors,the Pd atoms in the surface AgPd alloy are dispersed to different extents,and their activities were tuned in the light-driven organic catalytic reactions.Meanwhile,we also investigated the synergistic effect of photo-induced electrons in semiconductor and Ag LSPR in various organic catalytic reactions.4.Based on the previous experimental research,we designed a TiO2-Au@AunPdm semiconductor-metal composite nanostructure.Pd atoms on the catalysis surface served as active sites in catalytic reactions.The local electromagnetic field enhancement and photothermal effect of Au enhanced the catalyst property in the photocatalytic CO2 reduction reaction.Meanwhile,the photoexcited electrons flowed from the semiconductor conduction band to the metal surface and were consumed by the reaction continuously.The photocatalytic CO2 reduction activity of AuPd alloys with different proportions on the surface were also investigated.It was found that the catalyst exhibited the optimal photocatalytic activity when the surface Pd atoms were dispersed to appropriate extent(TiO2-Au@Au2Pd3).The presence of Au nanoparticles on TiO2 broadened the absorption range of sunlight.The Pd on the surface of the nanoparticles acted as a cocatalyst which further improve the catalytic activity.This unique integration design enhanced solar energy utilization efficiency in photocatalytic CO2 reduction reactions.This work also provides a step toward the rational design of semiconductor-metal composite structures for broad-spectrum photocatalysis.