Room-Temperature Electrocatalytic Synthesis of NH3 from H2O and N2 in a Gas–Liquid–Solid Three-Phase Reactor

Fe2O3-CNT samples are studied for the room-temperature electrocatalytic synthesis of NH3 from H2O and N2 in a gas–liquid–solid three-phase reactor. A 30 wt % iron-oxide loading was found to be optimal. The performances greatly depend on the cell design, where the possibility of ammonia crossover through the membrane has to be inhibited. The reaction conditions also play a significant role. The effect of electrolyte (type, pH, concentration) was investigated in terms of current density, rate of ammonia formation, and Faradaic efficiency in continuous tests up to 24 h of time on stream. A complex effect of the applied voltage was observed. An excellent stability was found for an applied voltage of −1.0 V vs Ag/AgCl. At higher negative applied voltages, the ammonia formation rate and Faradaic selectivity are higher, but with a change of the catalytic performances, although the current densities remain constant for at least 24 h. This effect is interpreted in terms of reduction of the iron-oxide species above a negative voltage threshold, which enhances the side reaction of H+/e– recombination to generate H2 rather than their use to reduce activated N2 species, possibly located at the interface between iron-oxide and functionalized CNTs.