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

A successful catalytic conversion of carbon dioxide into high-value chemicals as a means of abating the global CO2 emission has been a pressing challenge. Herein, we report on an ambient pressure X-ray photoelectron spectroscopy (APXPS) 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, 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 (3 of 36 A 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.