CO2 interaction with violarite (FeNi2S4) surfaces: a dispersion-corrected DFT study

Abstract

The unbridled emissions of gases derived from the use of fossil fuels have led to an excessive concentration of carbon dioxide (CO₂) in the atmosphere with concomitant problems to the environment. It is therefore imperative that new cost-effective catalysts are developed to mitigate the resulting harmful effects through the activation and conversion of CO₂ molecules. In this paper, we have used calculations based on the density functional theory (DFT), including two semi-empirical approaches for the long-range dispersion interactions (-D2 and -D3), to explore the interaction of CO₂ with the surfaces of spinel-structured violarite (FeNi₂S₄). This ternary sulfide contains iron ions in the highest possible oxidation state, while the nickel atoms are in the mixed 2+/3+ valence state. We found that CO₂ interaction with violarite is only moderate due to the repulsion between the oxygen lone pairs and the electronic clouds of the sulfur surface atoms. This suggests that the CO₂ activation is not dictated by the presence of nickel, as compared to the pure iron-isomorph greigite (Fe₃S₄). These results differ from findings in (Ni,Fe) ferredoxin enzymes, where the Ni/Fe ratio influences the redox potential, which suggests that the periodic crystal structure of violarite may hinder its redox capability.