Nitrogen Activation to Ammonia via a Mars-van Krevelen Mechanism on Nitride Electro-Catalysts

Viable design of a green route for production of ammonia represents a cost-effective and environmental solution. One of promising alternatives to the centralized Haber-Bosch process is electroreduction of nitrogen to ammonia renewably where proton is provided by water splitting in anode and both nitrogen and electron are provided in the cathode. For the sake of decentralization, small-scale N2 reduction devices are necessary that are equipped with selective and active catalyst running well at ambient conditions without the sophistications of the Haber-Bosch process. Here, we present the results of density functional theory (DFT) calculations conducted on different facets of transition metal mononitrides of Zr and V for possibility of catalytic conversion of nitrogen to ammonia under ambient conditions. The Mars-van Krevelen mechanism was studied on (100/111) facets of rocksalt and (100/110) facets of zincblende structures. We studied the catalytic activity, calculated free energy of all intermediates along the reaction path for nitrogen reduction to ammonia and constructed free energy diagrams to estimate onset potential necessary for the reaction on each different facet. Ammonia formation was found more energetically favorable on the (100) facets of rocksalt with less numbers of proton/electron pairs and with smaller potential-determining step (PDS) compared to other facets. Therefore, less negative potentials should be required to apply for electrochemical ammonia formation if single crystal (the (100) facet) of the rocksalt structure is utilised.