Homolytic H2 dissociation for enhanced hydrogenation catalysis on oxides

Abstract

The limited surface coverage and activity of active hydrides on oxide surfaces pose challenges for efficient hydrogenation reactions. Herein, we quantitatively distinguish the long-puzzling homolytic dissociation of hydrogen from the heterolytic pathway on Ga₂O₃, that is useful for enhancing hydrogenation ability of oxides. By combining transient kinetic analysis with infrared and mass spectroscopies, we identify the catalytic role of coordinatively unsaturated Ga³⁺ in homolytic H₂ dissociation, which is formed in-situ during the initial heterolytic dissociation. This site facilitates easy hydrogen dissociation at low temperatures, resulting in a high hydride coverage on Ga₂O₃ (H/surface Ga³⁺ ratio of 1.6 and H/OH ratio of 5.6). The effectiveness of homolytic dissociation is governed by the Ga-Ga distance, which is strongly influenced by the initial coordination of Ga³⁺. Consequently, by tuning the coordination of active Ga³⁺ species as well as the coverage and activity of hydrides, we achieve enhanced hydrogenation of CO₂ to CO, methanol or light olefins by 4-6 times.

Zhu et al. report a quantitative and time-resolved analysis of hydrogen activation on Ga₂O₃, specifically shedding light on the long-standing puzzle of homolytic dissociation as opposed to the heterolytic pathway on oxides.