Difference in reaction mechanism between ZnZrO x and InZrO x for CO 2 hydrogenation.

Oxide solid-solution catalysts, such as Zn-doped ZrO 2 (ZnZrO x ) and In-doped ZrO 2 (InZrO x ), exhibit distinctive catalytic capabilities for CH 3 OH synthesis via CO 2 hydrogenation. We investigated the active site structures of these catalysts and their associated reaction mechanisms using both experimental and computational approaches. Electron microscopy and X-ray absorption spectroscopy reveal that the primary active sites are isolated cations, such as Zn 2+ and In 3+ , dissolved in tetragonal ZrO 2 . Notably, for Zn 2+ , decomposition of the methoxy group, which is an essential intermediate in CH 4 synthesis, is partially suppressed because of the relatively high stability of the methoxy group. Conversely, the methyl group strongly adsorbs on In 3+ , facilitating the conversion of the methoxy species into methyl groups. The decomposition of CH 3 OH is also suggested to contribute to CH 4 synthesis. These results highlight the generation of CH 4 as a byproduct of the InZrO x catalyst. Understanding the active site structure and elucidating the reaction mechanism at the atomic level are anticipated to contribute significantly to the future development of oxide solid-solution catalysts.