Theoretical And Experimental Studies Of CO₂Hydrogenation On The In₂O₃Based Catalyst

Catalytic conversion of CO2has attracted worldwide attentions recently.Hydrogenation of CO2into the liquid fuels and other value-added chemicals isconsidered as the most promising method for controlling CO2emission. Developingcatalysts with a high activity and selectivity for CO2hydrogenation becomes afocused research area in catalysis. In this dissertation,,we studied hydrogenation ofCO2on a novel In2O3-based catalysts through both the theoretical and experimentalapproaches.Theoretically, we studied the adsorption, activation, hydrogenation andprotonation of CO2on In2O3surface using density functional theory study (DFT). Wefound that CO2can be easily adsorbed and activated on the In2O3surface, and thesubsequent studies on CO2hydrogenation showed that In2O3has a high CO2selectivity and low CO productivity, which indicated In2O3will be a promisingcatalyst and/or promoter for methanol synthesis from CO2hydrogenation. Next, westudied the creation of oxygen vacancy on the In2O3surface. We found that differentoxygen vacancies exhibit different activities. The difference in the electron donatingability resulted in the difference in the enhancement for CO2adsorption and activation.Among all the oxygen vacancies, Ov4is the most beneficial site for methanolsynthesis. As a result, we mapped out the complete reaction mechanism of methanolsynthesis from CO2hydrogenation on Ov4. The result showed the facile oxidation-reduction of In2O3enables the catalytic cycles. Finally, we studied the adsorption ofCO2on Pd4/In2O3model catalyst and showd the interface of Pd cluster and In2O3isthe most active site for CO2adsorption and hydrogenation. Based on the synergisticeffect, we studied the complete reaction route for methanol production: HCOO route,RWGS route, HCOOH route. The result showed that HCOO route and RWGS routeare competitive from the perspective of thermodynamics. And HCOO route is thedominant reaction mechanism confirmed by the result of Microkinetics modeling. Inaddition, we studied the influence of reaction condition on the reaction rate ofmethanol synthesis from CO2hydrogenation, such as: temperature, pressure, the ratio of feed gases. Finally, the dehydrogenation of methanol on PdIn alloy surface wasstudied to understand the role of possible alloy formation in methanol synthesis on aPd/In2O3catalyst. At the same time, we studied the influence of using ver Waalsfunctionals on the adsorption and reaction energy of small molecules and, moreimportantly, on the activation barriers.Experimentally, FTIR had been applied to study the In2O3-based catalyst. Theresults confirmed many predictions of the theoretical studies. In2O3indeed enhancedthe adsorption and activation of CO2by formation of carbonate or bicarbonate species,but the adsorption of CO is suppressed by reacting with the surface O to form CO2and desorbing. The results are consistent with our predicitions based on DFT resultsin chapter2and3. The catalytic activity tests for CO2hydrogenation were performedunder the atomspheric pressure. There are interesting observations:(1) the activity ofthe three catalysts toward RWGS follows the order: Pd/SiO2> Pd-In2O3/SiO2>In2O3/SiO2;(2) Pd-In2O3/SiO2and In2O3/SiO2catalysts show100%selectivity towardCO without CH4, indicating that these two catalysts are active for RWGS but inert forCO2methanation. Pd/SiO2catalyst is active for production of both CO and CH4. Theresults of the activity test confirmed the DFT results in chapter2that In2O3stronglysuppressed the RWGS reaction. The results of selectivity test show that the Pd-In2O3/SiO2catalyst is highly selective toward the hydrogenation of CO2and CO: COcan be formed by CO2hydrogenation over Pd-In2O3/SiO2catalyst whereas thehydrogenation of CO to CH4formation is inhibited. The reason for this uniqueselectivity of the Pd-In2O3/SiO2catalyst was unveiled by FTIR analyses and DFTstudies: adsorption of CO is significantly weakened on the PdIn surface, and thebarriers of dissociation of H2and hydrogenation of CO become too high. The catalystcharacterization confirmed the DFT calculation results that the HCOO route, insteadof the RWGS route, is the dominant reaction mechanism not RWGS route formethanol synthesis from CO2hydrogenation.