NSF-DFG Echem: Electrochemically enhanced low-temperature catalytic ammonia synthesis

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As an alternative to centralized Haber-Bosch, small-scale distributed ammonia synthesis has great value. Avoiding very high pressures reduces high capital cost, and electrochemical promotion potentially enables carbon-free ammonia. Foregoing research was based on polarized protonic-ceramic electrochemical cells, with either steam electrolysis or methane reforming on the anode, a proton-conducting ceramic membrane, and ammonia synthesis on the cathode. Because equilibrium ammonia synthesis rates decrease greatly as temperature increases, low-temperature (e.g., T< 450 °C) catalysis is needed. However, even with the best catalysts, synthesis is greatly reduced by kinetic limitations below about 500 °C. Protons likely play a significant role in catalysis. Practical protonic-ceramic electrochemical cells usually operate between 500 and 700 °C. Electrochemical cells do produce ammonia, but at low rates. The proposed approach here is different. We will study the direct electrochemical activation of a novel catalyst support to increase synthesis rates greatly at low temperatures where the process is kinetically limited by N2 activation. This objective of the joint project of Karlsruhe Institute of Technology (KIT) and Colorado School of Mines (CSM) is to develop and demonstrate electrochemical enhancement that enables low-temperature and low-pressure ammonia synthesis. Nanophase Ru is dispersed on a proton-conducting BCZY support. Directly polarizing the catalyst structure with an electric field decreases the kinetically limited barrier for N2 activation. Although the proposed research is scientifically fundamental, it has great technology potential for cost-effective distributed production of ammonia. The research focuses on postulating, modeling, and validating proposed chemical behaviors. The electrical field is expected to reduce rate-limiting N2 dissociation barriers via two synergistic mechanisms:1. Electrical fields affect the proton-conducting BCZY support, enabling H2 dissociation to form protons that can activate gas-phase N2, directly forming desired surface adsorbates such as NH(BCZY).2. Fields in the range of 0.1 to 1.0 V/Å on dispersed nano-Ru also reduce the nitrogen activation barrier. Based on our validated reaction mechanisms for Ba-promoted Ru/YSZ, simulations show that reducing the N2 dissociation energy by 10 kJ mol-1 will increase the ammonia formation rate by an order of magnitude.Achieving the proposed objectives relies on the combined, complementary, and unique expertise of the partners in the context of heterogeneous catalysis, materials synthesis, characterization and process demonstration (KIT) and physically based modeling of the electrochemistry, charged-defect transport, and catalysis (CSM).