(36e) Challenges in Implementing in2O3 Catalyst for CO2 Hydrogenation to Methanol: H2o Effect and Stability

Catalytic CO₂ conversion to chemical feedstocks and transportation fuels attracts great attention. In₂O₃ is a promising candidate for CO₂ hydrogenation to methanol, and is characteristic of high methanol selectivity. However, an atomic level understanding of surface nature and structural evolution under reaction conditions and changes with time-on-stream (TOS) is lacking. This work studied the activity of In₂O₃ catalysts in the presence of additional moisture in feed gas and long-term stability.

The effect of additional moisture in the feed gas was studied on In₂O₃ and In₂O₃/ZrO₂. Adding 0.1 mol% H₂O significantly enhances CH₃OH formation rate from 2.75 to 3.42 mol-kg^-1-h^-1 over In₂O₃/ZrO₂. Similar enhancement is evident on In₂O₃. Characterization (STEM/EDS and CO₂-TPD) confirms the preservation of In-Zr strong interaction in the presence of additional H₂O and H₂O-induced oxygen vacancies, which improves CO₂ adsorption capacity. XPS reveals the formation of InOOH species due to H₂O addition, which appears to correlate to H₂O-dependant CH₃OH enhancement. Density functional theory calculations demonstrate that adding H₂O is found to facilitate surface InOOH formation, suppress COOH* pathway and thus CO formation, and promote CH₃OH formation via HCOO* intermediate. However, excess H₂O in the feed gas leads to aggregation of In species and reduction of surface In⁰ sites for H₂ dissociation.

Long-term stability tests were conducted as well. After 100-h TOS, CH₃OH formation rate on In₂O₃ drops significantly by 36%, while In₂O₃/ZrO₂ remains stable. In situ X-ray absorption spectroscopy (XAS) results demonstrate that the stability originates from the interaction between In₂O₃/In₂O_3-x and ZrO₂, which prevents deactivation through the over-reduction of In₂O₃ to In⁰. Structurally, ZrO₂ plays a role in kinetically trapping In₂O₃/In₂O_3-x in an oxygen deficient state under reaction conditions.

These findings would be of importance for developing fundamental understanding of reaction chemistry involving in CO₂-to-CH₃OH and for shedding light on implementing In₂O₃ catalysts in practical process technology.