Favorable Role of the Metal–Support Perimeter Region in Electrochemical NH₃ Synthesis: A Density Functional Theory Study on Ru/BaCeO₃

Abstract

The catalytic electrochemical synthesis of NH 3 on Ru/BaCeO 3 was investigated using density functional theory. The competition between NH 3 formation and the hydrogen evolution reaction (HER) is a key for a high NH 3 formation rate. Our calculations show that H adsorbs more strongly than N 2 at the Ru particle moiety, while the adsorption of N 2 is stronger than the H adsorption at the Ru/BaCeO 3 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/BaCeO 3 perimeter. We also calculated the Gibbs free energy changes along the NH 3 formation and found that the N 2 H formation, the NHNH 2 formation, and the NH 3 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 (Δ E ) and the activation barrier ( E a ) of the N 2 H formation at several sites. Δ E and E a were high at the Ru particle moiety (Δ E = 1.18 eV and E a = 1.38 eV) but became low (Δ E = 0.32 eV and E a = 1.31 eV) at the Ru/BaCeO 3 perimeter. These provide the atomic-scale mechanism how the proton conduction in BaCeO 3 assists the electrochemical NH 3 synthesis.

Bird’s-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm

“…The synthesis of ammonia is greatly significant in industry and has continued to attract a great deal of interest. − Because its mechanism on Ru catalysts has been thoroughly studied by experimental and theoretical studies, the reaction could provide a well-defined system for illustrating new theories and concepts. − The surface science technique has been implemented to intuitively study the surface structure of catalysts and revealed that N 2 dissociation activity at the step site was 9 orders of magnitude higher than that on the terrace site . Further theoretical calculations combined with experiments illustrated that the step site with the B5 structure has the highest activity and is the real active center .…”