The Simulation Studies On Catalytic Mechanism Of Methanol Synthesis From CO₂ Hydrogenation And The Surface Structure Over In₂O₃ Catalysts

Catalytic conversion of CO₂ to methanol plays an important role on the relieving of the global CO₂ emission and the greenhouse effect.The development of highly effective CO₂ hydrogenation catalyst plays key role in methanol synthesis from CO₂hydrogenation.The In₂O₃ catalyst has been reported to exhibit better catalytic stability and the selectivity toward methanol than the commercial Cu/Zn O/Al₂O₃ catalyst,thus is ranked as a potential efficient catalyst for methanol synthesis by CO₂ hydrogenation.However,the research on the promotion mechanism of the catalytic activity of In₂O₃catalyst is deficient.In this dissertation,density functional theory(DFT)and kinetic Monte Carlo simulation(k MC)methods were applied to systematically research into the surface structure state of In₂O₃ catalyst under the reaction conditions of CO₂hydrogenation,the reaction routes of CO₂ hydrogenation to methanol,and the promotion mechanism of metal Pd cluster and ZrO₂ as promoter materials.The reaction mechanism of methanol synthesis by CO₂ hydrogenation and the promotion mechanism of methanol production with In₂O₃ catalyst were revealed.By combining the DFT calculation and experimental characterization,the surface structure of In₂O₃ catalyst under the reaction conditions of CO₂ hydrogenation to methanol was investigated systematically.The DFT calculation and the XPS characterization results showed that the surface of In₂O₃ catalyst could be reduced completely in a reducing atmosphere.Through comparing the data on surface free energies of In₂O₃(110)surfaces in different surface states,it was found that the In₂O₃(110)surface with In as the termination surface showed the most stable surface structure.Accordingly,the slab model of In₂O₃ catalyst(In₂O₃(110)-In)under the reaction conditions was built,and was used in the following study on the reaction mechanism of methanol synthesis by CO₂ hydrogenation.By using DFT and k MC simulation methods,the reaction routes of methanol synthesis by CO₂ hydrogenation and the method for promoting methanol production rate were studied.The results indicated that the methanol was formed mainly via the HCOO route from the adsorbed CO₂ molecule,the dissociation of H₂ molecule on the In₂O₃(110)-In surface was the rate determing step of CO₂ hydrogenation,and the hydrogenation of CO₂ to COOH species and the dissociation of CO₂ to CO species were prohibited.The methanol production rate could be promoted by reducing the activation barrier of H₂ dissociation,increasing the barrier of the combination of H to H₂ molecule,and increasing the adsorption energy of CO₂ molecule on the surface of In₂O₃ catalyst.By applying the DFT calculation and k MC simulation methods,the promotion mechanism of H formation on the surface of In₂O₃ catalyst was explored.The results demonstrated that the doping of Zn O on the surface of In₂O₃ catalyst could strengthen the adsorption of H species,and increase the activation barrier of the combination of H to form a H₂ molecule.The research of H₂ dissociation and H transfer on the modeled surface of Pd/In₂O₃ catalyst indicated that the doping of metal Pd clusters on the surface could promote the dissociation of H₂ molecules,and the H transfer from the surface of Pd clusters to the surface of In₂O₃ catalyst was the rate determining step.The promotion effect of metal Pd clusters on H production on the surface of In₂O₃ catalyst was localized.The H production rate reduced rapidly with the increasing of the distance between the Pd cluster and the In sites on In₂O₃ surface.The formation of Pd In alloy phase in a Pd/In₂O₃ catalyst system would inhibit the activation of H₂ and CO₂molecules.Prohibiting the formation of Pd In alloy and maintaining the Pd⁰ phase and In₂O₃ phase could improve the catalytic activity of In₂O₃ catalyst.The DFT calculation method was used to investigate the effects of the doping of ZrO₂ material on the adsorption energy of CO₂ molecule and the surface structure of In₂O₃ surface,and to explore the reaction mechanism of CO₂ hydrogenation to methanol on Pd/In₂O₃-ZrO₂ catalyst.The results revealed that the doping of ZrO₂ on the surface of In₂O₃ catalyst could improve the concentration of surface O atoms on In₂O₃ surface under the reaction conditions of CO₂ hydrogenation,optimize the surface structure of In₂O₃ surface,and increase the CO₂ adsorption energy on the In₂O₃ catalyst.In addition,ZrO₂ doping could reduce the interactions between the Pd cluster and the In₂O₃ surface,and thus inhibit the formation of Pd In alloy.The HCOO route and COOH route were both possible reaction pathways for methanol synthesis by CO₂hydrogenation on Pd/In₂O₃-ZrO₂ catalyst.In the Pd/In₂O₃-ZrO₂ catalyst system,metal Pd served as the H₂ activation center,In₂O₃ served as the CO₂ adsorption and hydrogenation center,and ZrO₂ modified the surface structure of In₂O₃ and enhanced the adsorption of CO₂ molecules.The synergistic effect among Pd,In₂O₃ and ZrO₂helped to achieve the catalytic hydrogenation of CO₂ to form methanol with high efficiency.