Modeling electrochemical CO2 and N2 reduction reactions on transition metals and metal oxides

Abstract

The main target of this thesis is to use density functional theory-based simulations to study electrochemical CO2 and N2 reductions by employing a recent theoretical model of an electrochemical solid-liquid interface. This model is used to investigate the kinetics of such reactions. However, initially, a simple thermochemical model is used to study electrochemical CO2 reduction reaction (CO2RR) on 12 transition metal oxides (TMOs). We utilize models of rutile oxide (110) surfaces to investigate trends and limitations of CO2RR on those TMOs. We construct scaling law based thermodynamic volcano relation for CO2RR. Accordingly, we propose guidelines for hydrogen and OH binding free energy range where low overpotentials and high selectivity are predicted for CO2RR using certain oxides. Therefore, this provides guidance to future development of oxide catalysts for CO2RR. To get more insight into CO2RR on TMOs more detailed calculations are required which take into account the kinetics involved of various possible branching paths and towards different products. Since such calculations are computationally demanding we focus on the RuO2(110) surface where most experiments have been reported. Ab initio molecular dynamic simulations at room temperature and total energy calculations are used to improve the model system and methodology for CO2RR on RuO2(110) by including both explicit solvation effects and calculate proton-electron transfer energy barriers to elucidate the reaction mechanism towards various products; methanol, methane, CO(g), formic acid, methanediol and hydrogen. A significant difference in energy barriers towards methane and methanol is observed. The formation and role of CO as a spectator species is justified. We conclude that hydrogen is the main product at the potential range of -0.2 V to -0.9 V which is in agreement with recent experimental results. The calculated overpotential for methanol formation is found to be around -1 V. Furthermore, the calculations show why RuO2 also catalyzes CO2RR towards formic acid and CO(g) in a trace amount, in agreement with experimental observations. Finally, the possibility of synthesizing ammonia electrochemically is explored. Density functional theory calculations are used to elucidate the mechanism of the nitrogen reduction reaction (NRR) in an electrochemical double layer on the Ru(0001) electrode. The first protonation step of N2 to NNH is found to be the potential limiting step in agreement with thermodynamic calculations and the additional proton-electron transfer barrier is neglectable. The optimal mechanism of NRR towards ammonia on Ru(0001) follows an associative mechanism where after the third proton−electron transfer, the N−N bond is broken in N−NH3, releasing the first NH3 molecule and leaving N adsorbed on the surface. We find that this detailed kinetic study using a realistic model of the electrochemical solid-liquid interface predicts quiet similar reaction pathway as previously reported using the simple thermochemical model.