hydrogenate to CH3OH. Meanwhile, the absence of CO signal may be ascribed to its very low concentration, indicating the generation
of CO from formate can be well restrained on In2O3. The oxygen vacancy on the In2O3 were beneficial for CO2 activation and stabilization
of the key intermediates. CH3OH generation replenished the oxygen vacancy and H2 regenerated the vacancy to achieve the catalytic
cycle. Thus, the generation of CO and CH3OH on the surface of In2O3 can separate to two pathways that redox-path reaction from CO2
to CO and formate-path reaction from CO2 to CH3OH. CO2 and H2 favored to form formate and then hydrogenation to CH3OH on the
surface of In2O3, suggesting the high CH3OH selectivity.
However, due to the thermodynamic restrictions, CO still was the main product at high temperature. With the higher reducibility and
basicity, c-In2O3 showed higher CO2 conversion and CH3OH productivity than rh-In2O3. And the adsorbed CH3OH molecules desorbed
at lower temperature than that of the adsorbed CO molecules on rh-In2O3. In contrast, CO was more easily desorbed from the surface
than CH3OH on c-In2O3. So the rh-In2O3 showed higher CH3OH selectivity during CO2 hydrogenation compared with the c-In2O3.
Above all, the comparative study of c-In2O3 and rh-In2O3 for CO2 hydrogenation was carried out. C-In2O3 showed higher CO2
conversion activity than rh-In2O3, which was ascribed to the higher reducibility and basicity within c-In2O3 and associated with the
generation of the oxygen vacancy. rh-In2O3 showed higher methanol selectivity than c-In2O3. Although different structures showed
different CO2 hydrogenation performance, the hydrogenation mechanism was identical for the two catalysts. CO2 can be directly reduced
to CO through redox mechanism and CO2 hydrogenation to CH3OH was through formate-path. This work demonstrated the fundamental
understanding of the structure-activity relationship for rh-In2O3 and c-In2O3 catalysts and offered some hint for superior catalytic system
for CO2 hydrogenation with high CH3OH selectivity over 340 oC.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgements
The work was financially supported by the National Natural Science Foundation of China (No. 21878116) and Natural Science
Foundation of Hubei Province (No. 2019CFA070). The authors thank the Analysis and Testing Center of Huazhong University of Science
and Technology for analytical support.
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