Tuning Pd–In2O3 Interaction and CO2 Hydrogenation Activity for Methanol Synthesis via In2O3 Crystal Phase Engineering

Abstract

Engineering Pd–In₂O₃ interaction is key to developing catalysts with the desired CO₂ hydrogenation activity toward methanol synthesis. Here, the crystalline phase of In₂O₃ nanospheres was tuned by changing the calcination temperature, which was found to affect the Pd–In₂O₃ interaction and thus the supported Pd states and CO₂ hydrogenation performance of the prepared Pd/In₂O₃-a catalysts (where a refers to the calcination temperature for preparing In₂O₃). The fresh Pd/In₂O₃-a catalysts show varied initial activities, and after the induction period, their performance stabilized though being different. During the 100 h catalysis, catalyst microstructures changed, showing Pd aggregation and Pd–In alloying, which was related to the nature of the crystalline phase of In₂O₃. The hexagonal (h-In₂O₃) phase in Pd/In₂O₃-400 possesses concentrated surface OH groups and limited mobility. The relatively poor mobility limits Pd–In alloying, which possibly suppresses the hydrogen spillover effect, causing low CO₂ conversion (8%) and methanol selectivity (45%) under steady-state conditions at 5 MPa and 300 °C. Conversely, the cubic In₂O₃ (c-In₂O₃) phase promotes Pd–In alloying and modifies Pd–In₂O₃ interaction during the reaction. The activity data show that Pd/In₂O₃-600 with the mixed phases of In₂O₃ (h/c-In₂O₃) demonstrated appropriate Pd–In₂O₃ interaction, leading to the Pd core InO_x shell structure with the comparatively best methanol selectivity of about 65% at steady state. Conversely, Pd/In₂O₃-800 with the pure cubic In₂O₃ (c-In₂O₃) phase and a relatively low specific surface area of 16 m² g^–1 encourages the sintering of Pd and thereby the formation of homogeneous Pd–In alloys, having a moderate methanol selectivity of about 50%. These findings highlight the importance of the In₂O₃ crystal phase engineering in the catalytic CO₂ hydrogenation over Pd/In₂O₃ catalysts and the dynamics of Pd–In interactions, which affect the methanol yield.