Carbon dioxide hydrogenation over the carbon-terminated niobium carbide (111) surface: a density functional theory study

Abstract

Carbon dioxide (CO₂) hydrogenation is an energetic process which could be made more efficient through the use of effective catalysts, for example transition metal carbides. Here, we have employed calculations based on the density functional theory (DFT) to evaluate the reaction processes of CO₂ hydrogenation to methane (CH₄), carbon monoxide (CO), methanol (CH₃OH), formaldehyde (CH₂O), and formic acid (HCOOH) over the carbon-terminated niobium carbide (111) surface. First, we have studied the adsorption geometries and energies of 25 different surface-adsorbed species, followed by calculations of all of the elementary steps in the CO₂ hydrogenation process. The theoretical findings indicate that the NbC (111) surface has higher catalytic activity towards CO₂ methanation, releasing 4.902 eV in energy. CO represents the second-most preferred product, followed by CH₃OH, CH₂O, and HCOOH, all of which have exothermic reaction energies of 4.107, 2.435, 1.090, and 0.163 eV, respectively. Except for the mechanism that goes through HCOOH to produce CH₂O, all favourable hydrogenation reactions lead to desired compounds through the creation of the dihydroxycarbene (HOCOH) intermediate. Along these routes, CH₃* hydrogenation to CH₄* has the highest endothermic reaction energy of 3.105 eV, while CO production from HCO dehydrogenation causes the highest exothermic reaction energy of −3.049 eV. The surface-adsorbed CO₂ hydrogenation intermediates have minimal effect on the electronic structure and interact only weakly with the surface. Our results are consistent with experimental observations.