The role of surface oxidation and Fe-Ni synergy in Fe-Ni-S catalysts for CO hydrogenation.

Increasing carbon dioxide (CO) emissions, resulting in climate change, have driven the motivation to achieve the effective and sustainable conversion of CO into useful chemicals and fuels. Taking inspiration from biological processes, synthetic iron-nickel-sulfides have been proposed as suitable catalysts for the hydrogenation of CO. In order to experimentally validate this hypothesis, here we report violarite (Fe,Ni)S as a cheap and economically viable catalyst for the hydrogenation of CO into formate under mild, alkaline conditions at 125 °C and 20 bar (CO : H = 1 : 1). Calcination of violarite at 200 °C resulted in excellent catalytic activity, far superior to that of Fe-only and Ni-only sulfides. We further report first principles simulations of the CO conversion on the partially oxidised (001) and (111) surfaces of stoichiometric violarite (FeNiS) and polydymite (NiS) to rationalise the experimentally observed trends. We have obtained the thermodynamic and kinetic profiles for the reaction of carbon dioxide (CO) and water (HO) on the catalyst surfaces via substitution and dissociation mechanisms. We report that the partially oxidised (111) surface of FeNiS is the best catalyst in the series and that the dissociation mechanism is the most favourable. Our study reveals that the partial oxidation of the FeNiS surface, as well as the synergy of the Fe and Ni ions, are important in the catalytic activity of the material for the effective hydrogenation of CO to formate.