Ammonia (NH3) synthesis plays a significant role as both a synthetic fertilizer and in the symbiosis between plants and rhizobia, and is thus important for human food supply. Currently, NH3 synthesis is mainly achieved through the energy intensive Haber-Bosch process. This process uses Fe and Ru-based heterogeneous thermocatalysts and requires high temperature, 573~773 K, and pressure, 200~300 bar, to ensure both high reaction rate and high reaction equilibrium constant. Therefore, about 1%~1.5% of total energy produced worldwide is consumed to synthesize ammonia. To reduce the energetic cost, electrochemical ammonia synthesis at mild operating condition is promising, inspired by the fact that nitrogenases in microorganisms electrocatalyze N2 to NH3 efficiently. We are using density functional theory methods to examine the elementary electrocatalytic processes involved N2 reduction at the electrode-electrolyte interface. Using the proton-shuttling model developed by our group, we find that certain linear amines and carboxylic acids can reduce the kinetic barrier for N2 associative reduction by coupled proton-electron transfer. The result indicates that nearby ligands/solvent molecules can participate as co-catalysts to accelerate electroreduction through the associative mechanism. Ab initio thermodynamics approach applied to alpha-Fe2O3 and RuO2 surfaces, on the other hand, implies that under N2 electroreduction conditions, the surfaces are not stoichiometric and are terminated by coordinatively unsaturated metal sites or hydroxyl groups, which is crucial for the subsequent proton shuttling. We will further present the effect of peptides and surface terminated species bound on metals/oxides on the kinetics of proton-electron transfer to *N2 and *H, in order to predict the activity and selectivity of these catalysts under certain surface termination and local environment. Full electroreduction mechanisms, including elementary activation barriers, will be presented over metal and metal-oxide surfaces, and used to predict activity and selectivity trends across electrode and near-environmental compositions.

Towards efficient N2 electroreduction to ammonia through DFT investigation on surface environment and surface termination

255th National Meeting (2018)

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