Favorable Role of the Metal-Support Perimeter Region in Electrochemical NH3Synthesis: A Density Functional Theory Study on Ru/BaCeO3

Abstract

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 (Ea) of the N2H formation at several sites. ΔE and Ea were high at the Ru particle moiety (ΔE = 1.18 eV and Ea = 1.38 eV) but became low (ΔE = 0.32 eV and Ea = 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.