(Invited) Iron Catalyst Facilitated Synthesis of Ammonia from N2 and Water at High Efficiency By Electrolysis

Abstract

An electrosynthetic process to produce ammonia that avoids the greenhouse gas carbon dioxide is presented here. The electrocatalytic process combines water and nitrogen directly to generate ammonia at high coulombic efficiency. Catalyst enhancements were studied and improved on from our early advances,1-5by varying particle size, catalyst calcination preparation conditions, variation of catalyst carbon conductive matrix support, type and amounts of additives for better carbon coating of the catalyst, etc. Two sets of iron oxide catalysts were probed, nano-sized iron oxides and micron sized nano-oxides formed by calcination of micron sized iron metal particles. A 2µm nano-graphite carbon as the catalyst coating yields higher rates of ammonia than larger sized carbons or iron oxides without the coating. The catalysts from iron metal were made by first mixing iron powder (sized either 1-3 µm, 6-10µm, ~74µm,~149µm, ~841µm). Subsequently, the catalyst was converted to iron oxide coated partially with nano-graphiteby calcination (heating in air at a series of increasing stepped temperatures). Ammonia syntheses were investigated in two configurations (1) with free catalyst suspended in the electrolyte, and (2) with the catalyst confined to the cathode. The ammonia electrosynthesis was conducted between nickel and Monel electrodes in concentrated Na0.5K0.5OH electrolyte. The coulombic efficiency of the electrosynthesis, ηNH3, is measured by comparing the applied charge (the integrated constant current) to the 3 Faraday efficiency per mole of measured synthesized NH3.The highest performance (efficiency and ammonia rate of production) at 100°C was achieved by the catalyst synthesized with 1-3µm iron and prepared with nano-graphite coating. At 100 °C for catalysts prepared respectively with either 1-3µm, 6-10µm, ~74µm,~149µm, ~841µm iron powder, the synthesis coulombic efficiencies ηNH3 are 47%, 39%, 34%, 31%, 25%respectively and maximum ammonia formation rates are 4.7×10−9, 3.8×10−9, 3.4×10−9, 3.0×10−9, 2.4×10−9mol·(s·cm2)−1respectively. Lower temperature and/or confining the catalyst to the cathode will permit future cell variations such as polymer membrane separations in the synthesis cell. The synthesis is also effective at a lower temperature of 60°C, the formation rate using the 1-3µmiron with nano-graphite prepared catalyst is 2.2 mol·(s·cm2) −1with 21% ηNH3. Smaller sized catalysts can be effective, but are a challenge to confine in a stable configuration at the cathode. Rather than dispersing the catalyst in the electrolyte, we also introduce here a configuration that confines the catalyst to the cathode region. This consists ofsealed tea-bag shape Monel mesh cathode separated into 8 individual layers confining equal amounts of catalyst within each layer. At 60°C, the formation rate is 2.3×10−9mol·(s·cm2)−1with 23.7% ηNH3. The catalyst is stablized for ammonia synthesis ata current density of 3 mA/cm2. (1) Licht, S. Cui, B. Wang, Li, F.-F. Lau, J. Lui., S. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3.Science2014, 345, 637-640. (2) Li, F.-F. Licht, S. Advances in understanding the mechanism and improved stability of the synthesis of ammonia from air and water in hydroxide suspensions of nanoscale Fe2O3. Inorg. Chem., 2014, 53, 10042-10044. (3) Cui, B. Zhang, J. Liu, S.,Liu, X., Xiang, W. Liu, L., Xin, H., Lefler, M. J. Licht, S. Electrochemical synthesis of ammonia directly from N2and water over iron-based catalysts supported on activated carbon. Green Chem., 2017, 19, 298-304. (4) Peng, P., Li, F.-F., Liu, X., Ren, J. Stuart, J., Lefler, M., Johnson, M., Vicini, J., Licht, S.. Chemical transformation of Fe, air & water to ammonia: variation of reaction rate with temperature, pressure, alkalinity, and iron. ChemRxiv.org,posted 31.07.2018. (5) Li, F.-F., Singhal, R., Ren, J., Johnson, Lefler, M., Licht, Solar electrochemical thermal process (STEP ammonia: Optimization of the electrolysis conditions. ChemRxiv.org,posted 25.06.2018. Illustration: The ammonia synthesis from water and nitrogen, avoiding the emission of carbon dioxide, and illustrated using a multi-layer cathode configuration via the mechanism: Cathode: Fe2O3+ 3H2O + 6e−→ 2Fe + 6OH− Anode: 6OH−→ 3／2 O2(g) + 3H2O + 6e− Chemical: 2Fe + N2+ 3H2O → 2NH3+ Fe2O3 Net: N2+ 3H2O → 2NH3+ 3／2 O2 Figure 1