Study On Catalytic Performance And Reaction Mechanism Of CO₂ Hydrogenation To CH₃OH Over CuO/Ce_1-xZr_xO₂ Catalyst

Hydrogenation of CO2 into fuel CH3OH helps to control excess CO2 emission and reduce the consumption of fossil fuel,thus promoting global sustainable development.The traditional catalyst CuO-ZnO-Al2O3 can catalyze the hydrogenation of CO2 under high temperature((?)300 ?)and pressure((?)5MPa).However,it presents weak CO2 activation ability,low CH3OH selectivity and poor stability.Hence,it is still a big challenge to design highly efficient and stable Cu-based catalysts for CO2 hydrogenation to CH3OH.The ceria-zirconia material has abundant acid-base sites,high stability,better reducibility and tendency to form solid solution with active sites.Therefore,ceria-zirconia solid solution is used as a support to construct Cu-based catalyst and applied in hydrogenation of CO2 to CH3OH.The catalytic performance may be improved by regulating support nature,surface structure and coordination environment of copper species.In the thesis,the influence of preparation methods of copper-cerium-zirconium catalyst on catalyst structure and performance has been systematically investigated.The research focuses on the effect of copper species properties and support nature on catalytic performance.Last but not least,high pressure in situ DRIFTS is used to explore the reaction pathway and deepen the understanding of the reaction mechanism of copper-cerium-zirconium catalyst.The detailed results were listed as below:(1)The effect of preparation methods of copper-cerium-zirconium catalysts(labeled as CCZ)on CO2 hydrogenation to CH3OH was investigated.The copper-cerium-zirconium catalysts were prepared by Urea hydrothermal method,Na2CO3 co-precipitation method and Oxalate gel method,respectively.The results showed that the higher dispersion and better reducibility of copper species as well as a higher concentration of hydroxyl groups over CCZ-Oxalate were beneficial to improve H2 and CO2 adsorption ability.Hence,CCZ-Oxalate had the best catalytic performance of CO2 hydrogenation to CH3OH.At reaction temperature of 200-300?the CO2 conversion and CH3OH selevtivity of CCZ-Oxalate were 5.4%-15.6%and 97.8%-40.6%,respectively,and CO2 conversion of CCZ-Oxalate was stable for 90 h.(2)The effect of copper contents and calcination temperature on types,properties and catalytic performance of copper species was investigated.The results showed that copper-cerium-zirconium catalyst contained surface CuO species and Cu2+ in Cu-Ce-Zr solid solution,which were dependent on CuO contents and calcination temperature.Compared with tradition CuO-ZnO-Al2O3 catalyst,the sample calcined at 450? with 35 wt%CuO had better CO2 conversion(9.8%and 13.2%)and CH3OH selectivity(40.2%and 71.8%)under the same conditions(P=3 MPa,GHSV=10000 h-1,T=280?).Ea and TOFCO2 of the catalyst were 28.5 kJ mol-1 and 13.4×10-2 s-1,respectively.(3)The influence of support properties(CeO2,ZrO2,Ce1-xZrxO2)on CO2/H2 adsorption and activiation ability as well as catalytic performance was investigated.The results showed that the stronger interaction of CuO/CeO2 promoted the formation of oxygen vacancies,which led to form a larger amount of HCOO*intermediate species.Then CuO/CeO2 had a higher CH3OH selectivity.The CuO/ZrO2 with higher concentration of hydroxyl groups was facilitated to produce*COOH intermediate species during CO2 hydrogenation,which resulted in higher selectivity of CO.At reaction temperature of 280?,CH3OH selectivity of CuO/CeO2 and CuO/ZrO2 was 73.5%and 31.1%,respectively while CO selectivity of CuO/CeO2 and CuO/ZrO2 was 17.4%vs 65.4%,respectively.The Zr/Ce ratio played a significant role in CO2/H2 adsorption and intermediate species conversion capabilities over CuO/Ce1-xZrxO2.There were more intermediate species formed on CuO/Ce1-xZrxO2,which resulted in higer catalytic performance.The CH3OH yield(T=280?)with x=0.6 was 1.3 times and 2.5 times higher than that of CuO/CeO2 and CuO/ZrO2.We found that the higher dispersion of copper species and a larger number of oxygen vacancies were observed on the zirconia-rich sampes(x=0.6 and 0.8)compared with ceria-rich samples(x=0.2 and 0.4),leading to superior H2 and CO2 adsorption ability.Additionally,zirconia-rich samples were effective to transform intermediate species into target product CH3OH whereas ceria-rich samples were only favorable for the accumulation of intermediate species.Hence,x=0.6 and 0.8 samples(6.41%-9.47%)showed higher CH3OH yield than that of x=0.2 and 0.4 samples(2.26%-4.14%)at 280?.(4)The reactant adsorption sites and reaction pathway were investigated by high pressure in situ DRIFTS.The dual site reaction mechanism for CO2 hydrogenation to CH3OH over copper-cerium-zirconium catalyst was discussed and the HCOO*reaction route was put forward.The results showed that the surface Cu species was active centre of H2 adsorption and dissociation.The ceria-zirconia support was active for CO2 adsorption and activation.The generation of oxygen vacancies was observed due to the formation of Cu-Ce-Zr on the catalyst,which was conducive to adsorb CO2.The intermediate species of m-HCOO*and bi-HCOO*were active species for CO2 hydrogenation to CH3OH.Specially,b-*OCH3 came from m-HCOO*while t-*OCH3 stemmed from bi-HCOO*.The accumulation and consumption process of m-HCOO*species were observed.The accumulation process of m-HCOO*was slow.