Title: Theoretical Investigation of the Catalytic Mechanism of N2 to NH3 Reduction by an Aluminum Complex
Authors: Ruimeng Wang and Lee-Ping Wang
Abstract: The reduction of nitrogen gas (N2) to NH3 is an essential chemical reaction for modern industry, but energetically highly demanding. In electrocatalytic N2 reduction, a Lewis basic metal catalyst is typically needed to donate electron density to the antibonding orbital of N2, and previously reported metal-catalyzed reactions have very large overpotentials. Recently, an aluminum-centered coordination complex was discovered by the Berben group to catalyze N2 reduction to NH3 at –1.16 V vs. SCE (η=0.28 V). This complex has less negative reduction potential than existing catalysts that typically operate around -1.4 V vs. SCE that a considerable amount of energy could be saved if this electrocatalyst could be utilized in industry. We are conducting a computational investigation into the catalytic mechanism of this N2 reducing aluminum complex. Density functional theory calculations were used to calculate the reduction potentials of the catalyst and compared with experimentally measured values. We found that although the initial reduction potential can be accurately reproduced to within 0.2V, subsequent calculated reduction potentials had larger deviations of 0.85V vs. experiment. The modeling of explicit counterions in the calculation, placed near the complex guided by electrostatic potential (ESP) plots, was found to significantly improve the agreement vs. experiments. Future steps will investigate the thermodynamics of N2 binding, electron transfer, and protonation steps to produce NH3 as well as side reactions that limit turnover number.