Difference in reaction mechanism between ZnZrO and InZrO for CO₂ hydrogenation

Oxide solid-solution catalysts, such as Zn-doped ZrO₂ (ZnZrO) and In-doped ZrO₂ (InZrO), exhibit distinctive catalytic capabilities for CH₃OH synthesis via CO₂ 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 Zn²⁺ and In³⁺, dissolved in tetragonal ZrO₂. Notably, for Zn²⁺, decomposition of the methoxy group, which is an essential intermediate in CH₄ synthesis, is partially suppressed because of the relatively high stability of the methoxy group. Conversely, the methyl group strongly adsorbs on In³⁺, facilitating the conversion of the methoxy species into methyl groups. The decomposition of CH₃OH is also suggested to contribute to CH₄ synthesis. These results highlight the generation of CH₄ as a byproduct of the InZrO 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.