Hydrogenation of CO₂ on NiGa thin films studied by ambient pressure x-ray photoelectron spectroscopy

Successful catalytic conversion of carbon dioxide into high-value chemicals, as a means of abating global CO₂ emissions is a pressing challenge. Herein, we report on an ambient pressure x-ray photoelectron spectroscopy study of CO₂ hydrogenation reaction using a mixed gas of CO₂ and H₂ at a pressure up to 0.1 mbar on polycrystalline nickel-gallium thin films prepared by vacuum co-deposition that are compositionally equivalent to Ni, Ni₃Ga, Ni₅Ga₃ and NiGa₂, which are some noted low pressure methanol synthesis catalysts. Ni film is very reactive at 300 K, with the reaction sequence initiated by facile CO₂ dissociation, yielding C₂O₄⁻², CO₃⁻², CO₂⁻ and CO, but little HCOO⁻ and HCO₃⁻. The product species observed on NiGa alloy films are not generally different from those observed from using Ni film but are now of different proportions. The NiGa₂ film, with the highest Ga content, is the least reactive, as judged by a dominant molecular adsorption feature of CO₂. Interestingly, the NiGa alloys are found to undergo composition change and surface restructuring induced by the reactant gas and heating during the reaction. A surface depletion of Ni and a surface enrichment of oxidic Ga are found to occur and continue unabated toward high temperature. For example, for the alloy film with a composition similar to that of Ni₅Ga₃ when exposed to the mixed gas at 500 K for the duration of measurements (a few h), the near surface region delimited by an XPS probing depth (3λ) of 36 Å is composed of 29% Ni, 24% Ga, and 47% oxidic Ga vs. 62% Ni, 34% Ga, and 4% oxidic Ga for the freshly prepared film at 300 K. Our study demonstrates that a complex interplay exists between the surface reactivity and the evolving chemical makeup as the alloy films are exposed to the reactive gas at elevated pressure and temperature.