Preparation Of Cu-based Catalyst And Its Catalytic Performance Toward Low-temperature Water Gas Shift Reaction

Regulations on strong metal-support interactions for heterogeneous catalysis is very crucial for the enhancement of catalytic performance of interface-sensitive reactions,e.g.,water gas shift reaction(WGSR).The WGSR(CO+H2O(?)CO2+ H2;△H0=-41.2 kJ mol-1)plays an important role in industrial production of clean hydrogen as well as in eliminating residual CO in feed streams for fuel cells.Recently,oxide supported Cu based catalysts(e.g.,Cu/CeO2,Cu/SiO2,Cu/ZnO,and Cu/TiO2)exhibit tremendous potential for low-temperature water gas shift reaction(LT-WGSR).Great efforts have been devoted to demonstrating the key role of metal-support interfacial sites;however,a precise modulation over active site structure of Cu-based catalysts toward WGSR,and how to figure out the interfacial synergistic mechanism,still remain a challenging goal.In view of the just announced scientific issues,in this paper,a new type of supported Cu-based catalyst(Cu/ZnTi-MMO)was prepared with SMSI-tunable through the topotactic transformation of CuZnTi-LDHs,which brought about a significant improvement of catalytic performance toward WGSR.Firstly,the supported catalyst prepared by LDHs precursor shows the advantages of large specific surface area,good structural stability and high dispersion of metal particles.Secondly,the geometry and electronic structure of the catalyst were studied based on in situ characterizations,and the correlation between catalyst structure and catalytic performance of LT-WGSR was established.Finally,a series of in situ characterization methods were used to monitor the dynamic changes of the active site at the interface in various reaction atmospheres,and an interface synergistic catalysis mechanism was revealed.This work provides a new approach for the structural design and preparation of Cu-based catalysts,which can be potentially applied in LT-WGSR.The main research contents and results are as follows:1.Preparation of Cu/ZnTi-MMO catalyst with tunable SMSI and its catalytic performance towards LT-WGSRThe Cu/ZnTi-MMO catalyst was prepared via a topological transformation of CuZnTi-LDHs.The strong metal-support interaction(SMSI)between Cu nanoparticles and ZnTi-MMO support was confirmed by XRD,TEM,ac-HAADF-STEM.The optimal Cu/ZnTi-MMO(350)catalyst(Cu loading:15%;average size:7.2 nm;dispersion of Cu nanoparticles:15.4%)shows excellent catalytic activity(TOF value:5.7×10-2s-1,reaction rate:19.7 μmolco gcat-1s-1;250℃)toward WGSR,which exceeds the reported other Cu-based catalysts.By means of quasi in situ XPS and in situ XAFS,a strong electron transfer effect was confirmed at the interface site of Cu/ZnTi-MMO,and electron transfer from Cu to Zn induces a large number of Cuδ+species.In addition,quasi in situ XPS reveals a high proportion of Ov-Ti3+at the interface of Cu/ZnTi-MMO.The study on the relationship between active structure and catalytic performance shows a positive correlation between the TOF value and concentration of Cuδ+(or Ov-Ti3+),indicating the Cuδ+-Ov-Ti3+interface site serves as the intrinsic active center.2.Studies on the synergistic catalysis mechanism of Cu/ZnTi-MMO catalyst towards LT-WGSRBased on the regulation of SMSI and the catalytic performance of LT-WGSR,the optimal catalyst Cu/ZnTi-MMO(H350)was chosen as the target sample.In situ DRIFTS,in situ EXAFS and TPSR-MASS were carried out to study the evolution process of the active sites structure of Cu/ZnTi-MMO catalyst towards LT-WGSR.In situ DRIFTS and in situ EXAFS in H2O vapour atmosphere reveal that H2O molecule directly decomposes at the Cuδ1+Ov-Ti3+ interface site into active hydroxyl species;and the interface site structure is oxidized to Cuδ2+-O-Ti4+.Subsequently,a switch to CO/He atmosphere confirm that CO molecule adsorbs at the Cuδ+ active site accompanied with the production of hydroxyl species via H2O dissociation.Finally,the intermediate product(e.g.,formate or carboxylate)decomposes into CO2,resulting in the recovery of the active site structure(Cuδ1+-Ov-Ti3+).Moerover,a comprehensive study including in situ DRIFTS,in situ EXAFS and TPSR-MASS proves an associative mechanism in the WGSR catalyzed by Cu/ZnTi-MMO.This work demonstrates a facile modulation on metal-support interfacial structure via LDHs approach,which paves a way for rational design and preparation of new-type heterogeneous catalysts.