Reactivity Switch of Platinum with Gallium: From Reverse Water Gas Shift to Methanol Synthesis

Abstract

The development of efficient catalysts for the hydrogenation of CO₂ to methanol using "green" H₂ is foreseen to be a key step to close the carbon cycle. In this study, we show that small and narrowly distributed alloyed PtGa nanoparticles supported on silica, prepared via a surface organometallic chemistry (SOMC) approach, display notable activity for the hydrogenation of CO₂ to methanol, reaching a 7.2 mol_CH3OH h^-1 mol_Pt^-1 methanol formation rate with a 54% intrinsic CH₃OH selectivity. This reactivity sharply contrasts with what is expected for Pt, which favors the reverse water gas shift reaction, albeit with poor activity (2.6 mol_CO2 h^-1 mol_Pt^-1). In situ XAS studies indicate that ca. 50% of Ga is reduced to Ga⁰ yielding alloyed PtGa nanoparticles, while the remaining 50% persist as isolated Ga^III sites. The PtGa catalyst slightly dealloys under CO₂ hydrogenation conditions and displays redox dynamics with PtGa-GaO_x interfaces responsible for promoting both the CO₂ hydrogenation activity and methanol selectivity. Further tailoring the catalyst interface by using a carbon support in place of silica enables to improve the methanol formation rate by a factor of ∼5.