DFT insight into the effect of potassium on the adsorption, activation and dissociation of CO2 over Fe-based catalysts

Abstract

Catalytic conversion of CO₂ including hydrogenation has attracted great attention as a method for chemical fixation of CO₂ in combination with other techniques such as CO₂ capture and storage. Potassium is a well-known promotor for many industrial catalytic processes such as in Fischer–Tropsch synthesis. In this work, we performed density functional theory (DFT) calculations to investigate the effect of potassium on the adsorption, activation, and dissociation of CO₂ over Fe(100), Fe₅C₂(510) and Fe₃O₄(111) surfaces. The function of K was analyzed in terms of electronic interactions between co-adsorbed CO₂ and K-surfaces which showed conspicuous promotion in the presence of K of the adsorption and activation of CO₂. The adsorption strength of CO₂ on these surfaces ranks as oct2-Fe₃O₄(111) > Fe(100) > Fe₅C₂(510). Generally, we observed a direct proportional correlation between the adsorption strength and the charges on the adsorbates. Adding K on the catalyst surface also reduces the kinetic barrier for CO₂ dissociation. CO₂ dissociation is more facile to occur on Fe(100) and Fe₅C₂(510) in the presence of K whereas the Fe₃O₄(111) surfaces impede CO₂ dissociation regardless of the existence of K. Instead, a stable CO₃⁻ species is formed upon CO₂ adsorption on Fe₃O₄(111) which will be directly hydrogenated when sufficient H* are available on the surface. Our results highlight the origin of the promotion effect of potassium and provide insight for the future design of K-promoted Fe-based catalysts for CO₂ hydrogenation.