Electrochemical Nitrogen Reduction Reaction over Subnano-Clustered Ruthenium Catalysts: A DFT Study

Abstract

The electrochemical nitrogen reduction reaction (NRR) for ammonia production is one of the most spotlighted chemical process for hydrogen storage and transportation. We investigated NRR on the ruthenium-based subnano-clustered catalysts embedded in modified defective graphene (Rux/YC, Ru : ruthenium, x : 1 to 3, Y : boron, carbon, and nitrogen, C : graphitic carbon) using density functional theory (DFT) calculation.We identified the energetically optimized structures of Rux/YCs among all possible geometries. Also we calculated the dissociative adsorption free energy of nitrogen (ΔGDiss) to determine which mechanism to follow for NRR on Rux/YCs. We also calculated the adsorption energy (ΔEAds) of all possible reaction intermediates to identify the detailed mechanism and the limiting potential for NRR on each catalyst. Additionally, we compared the reaction potential for NRR and hydrogen evolution reaction (HER) by calculating the adsorption free energy of H*.In the Rux/C catalysts, ΔEAds of N* tend to become stronger and the ΔGDiss decreased, as the number of Ru atoms increased. In contrast, the reverse tendency was shown in Rux/BC catalysts between ΔGDiss and the number of Ru atoms. In the Rux/NCs, ΔGDiss for the two Ru atoms was lowest. Overall, NRR follow the dissociative mechanism on the Ru3/C, Ru1/BC, and Ru2/BC catalysts, and association mechanism on the others.As shown in figure below, single-atomic Ru1/YCs showed obviously lower NRR potential than double- and triple-atomic Rux/YCs. In the case of Rux/C and Rux/NC, it was due to the relatively weak adsorption of N2H*. However, in Ru1/BC, the ΔEAds of N2H* was so strong that the reaction potential was relatively higher than Ru2/BC and Ru3/BC due to the strong B-N bonding. It was found that the ΔEAds of N2H* can be used as an indicator to predict NRR potential on Rux/YCs.We compared the reaction potential for NRR and HER on each catalysts. In Ru2/NC catalyst, HER potential was lower than NRR potential, and it was found that Ru2/NC with good activity and selectivity is the most promising candidate for NRR catalyst. Figure 1