Difference in reaction mechanism between ZnZrOx and InZrOx for CO2 hydrogenation

Abstract

Oxide solid-solution catalysts, such as Zn-doped ZrO₂ (ZnZrO_x) and In-doped ZrO₂ (InZrO_x), 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_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.