Preparation Of Gold-based Catalysts From Hydrotalcite Precursors Towards Low Temperature Water Gas Shift Reaction

For supported catalysts,the strong electronic metal-support interaction(EMSI)can effectively tune the geometry and electronic structure of active metal and carrier,which is widely used in the field of heterogeneous catalysis(for example,low temperature water gas shift reaction(LT-WGSR)).In 1978,Tauster et al.first proposed the concept of strong metal-support interaction(SMSI),which is usually geometrically represented by the coating of metal oxide support on the surface of metal nanoparticles.Recently,Rodriguez et al.proposed the concept of EMSI for the first time.Studies have shown that EMSI can not only stabilize the metal nanoparticles through the encapsulated structure of the reducible oxide support,but also improve the electron distribution of active metal and the support by modifying the electronic structure of the interface.Although a lot of studies have been carried out on EMSI in recent years,some key problems still remain unresolved.For example,(1)For traditional catalytic systems(eg,Au/TiO2),the production of EMSI usually requires a high pretreatment temperature(>500?),which inevitably leads to metal aggregation,over-coating and the resuling reduced catalytic performance;(2)Effective techniques are rather limited for the regulation of EMSI and for the characterization of physicochemical nature of EMSI;(3)The intrinsic promotion mechanism of EMSI for catalytic reaction is still inconclusive.These unresolved issues severely restrict the structure design and preparation of highly efficient metal catalysts.In view of the above scientific issues,in this thesis,an EMSI-regulated gold-based catalyst Au@TiO2-x/ZnO was prepared,which achieved a significant improvement of the catalytic performance toward LT-WGSR.The active site structure was confirmed,and then a novel synergistic catalytic mechanism was revealed by means of a combination of in situ characterizations.The research of this work provides a new approach for the preparation of high-efficiency gold-based catalysts,and an exploration study on catalytic mechanism toward WGSR.The main research contents and conclusions of this work are as follows:1.Modulation of EMSI in Au@TiO2-x/ZnO Catalytic System and its Catalytic Performance for LT-WGSRZnTi-LDHs was used as the catalyst precursor.By changing the preparation conditions(metal cation molar ratio,hydrotalcite crystallization temperature,hydrotalcite crystallization method,etc.),a gold-based catalyst Au@TiO2-x/ZnO with highly uniform dispersion of Au nanoparticles was prepared.In this part of the work,based on the Au@TiO2-x/ZnO catalytic system,the following studies were performed:(1)By means of experimental techniques(e.g.,TEM,ac-HAADF-STEM,CO low temperature pulse adsorption),a classical SMSI phenomenon was confirmed between the Au nanoparticles and TiO2-x suopport,which leads to a new core-shell gold-based catalytic system Au@TiO2-x/ZnO.(2)A series of characterizations,such as quasi in situ XPS,in situ EXAFS and EPR,demonstrate the formation of interface dual-active-site(Au?--Ov-Ti3+;Ov:oxygen vacancy)based on electron transfer from TiO2-x overlayer to Au atoms.(3)By constructing a kinetic model of EMSI intensity and WGSR catalytic activity,it was confirmed that the Au?--Ov-Ti3+interface structure serves as the intrinsic catalytic active center toward WGSR.Herein,by regulating the strength of EMSI,a gold-based catalyst with high LT-WGSR catalytic activity,stability and recyclability was obtained,which provides a new route for the preparation of high-efficiency gold-based catalysts.2.Study on the synergistic catalytic mechanism in Au@TiO2-x/ZnO catalytic systemBased on the regulation of EMSI in the Au@TiO2-x/ZnO catalytic system and its catalytic performance on LT-WGSR,the optimal catalyst sample Au@TiO2-x/ZnO(H300)was studied in details.By using a comprehensive study including in situ DRIFTS,in situ EXAFS and H2O pulse-MASS in different reaction atmospheres,the changes in interface active site structure during WGSR are revealed:(1)Firstly,H2O molecules are directly adsorb and dissociate at the active site of Au?--Ov-Ti3+,which generates interfacial reactive oxygen species and hydrogen as well as the oxidation of interface structure to Au0-O-Ti4+;(2)Secondly,CO directly captures the interfacial reactive oxygen species to produce CO2,which induces the recovery of interfacial structure(Au?--Ov-Ti3+).The catalytic reaction mechanism of WGSR on Au@TiO2-x/ZnO(H300)was revealed.