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