Favorable Role of the Metal-Support Perimeter Region in Electrochemical NH₃ Synthesis: A Density Functional Theory...

The catalytic electrochemical synthesis of NH₃ on Ru/BaCeO₃ was investigated using density functional theory. The competition between NH₃ formation and the hydrogen evolution reaction (HER) is a key for a high NH₃ formation rate. Our calculations show that H adsorbs more strongly than N₂ at the Ru particle moiety, while the adsorption of N₂ is stronger than the H adsorption at the Ru/BaCeO₃ perimeter, a model for the triple-phase boundary that is proposed to be an active site by experimental studies. This indicates that, while the HER is more favorable at the Ru particle moiety, it should be suppressed at the Ru/BaCeO₃ perimeter. We also calculated the Gibbs free energy changes along the NH₃ formation and found that the N₂H formation, the NHNH₂ formation, and the NH₃ formation steps have a relatively large Gibbs energy change. Therefore, these are possible candidates for the potential-determining step. The calculated equilibrium potential (U = -0.70 V, vs RHE) is in reasonable agreement with experiments. We also evaluated the reaction energy (ΔE) and the activation barrier (E _a) of the N₂H formation at several sites. ΔE and E _a were high at the Ru particle moiety (ΔE = 1.18 eV and E _a = 1.38 eV) but became low (ΔE = 0.32 eV and E _a = 1.31 eV) at the Ru/BaCeO₃ perimeter. These provide the atomic-scale mechanism how the proton conduction in BaCeO₃ assists the electrochemical NH₃ synthesis.