Brønsted acid sites modulation of InPd/In₂O₃ interfaces for CO₂ hydrogenation to methanol

In₂O₃ is well known for its ability to activate CO₂, forming methanol. Indium oxide (In₂O₃) catalysts are often complemented by Pd to enhance hydrogen activation and spillover during CO₂ activation. In reducing conditions, there is also a tendency for In₂O₃ to be reduced and form InPd alloys. Since the InPd/In₂O₃ interface plays a critical role in CO₂ hydrogenation, the formation of InPd alloys in In-Pd bimetallic catalysts should be carefully controlled to optimize catalytic performance towards forming methanol. This work reports a method exploiting the Brønsted acidity of ZSM-5 zeolites, which acts as a support to modulate the extent of formation of InPd alloys, thereby fine-tuning the formation of InPd/In₂O₃ interfaces and the resulting CO₂ hydrogenation performance. Characterization of Pd-In/ZSM-5 catalysts revealed that a low Si/Al ratio promotes the reduction of In₂O₃ and the formation of InPd alloy. At a Si/Al ratio of 35 (i.e., Pd-In/ZSM-5-35), the molar ratio of metallic In to In₂O₃ was approximately 9:1, while in the absence of any acidic site, the ratio approached 1:1. At 4.0 MPa and 300 °C, Pd-In/Silicate-1 affords 8.2% CO₂ conversion and over 80% methanol selectivity, with a methanol yield tripling that of Pd-In/ZSM-5-35. The results highlight that the synergistic interface of In-Pd alloys and In₂O₃ is crucial for methanol synthesis from CO₂ hydrogenation.