Support effect in metal-organic framework-derived copper-based electrocatalysts facilitating the reduction of nitrate to ammonia

Metal-organic framework (MOF)-derived copper supported by ceria/carbon, zirconia/carbon, and carbon are synthesized by thermally carbonizing a copper-installed cerium-based MOF, a copper-installed zirconium-based MOF, and a copper-based MOF constructed from the same organic building block, respectively. These materials are characterized by X-ray diffraction, electron microscopes, Raman spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectrometry, two-probe conductivity measurements, and nitrogen adsorption-desorption analysis. Modified electrodes of these electrocatalysts with the same areal loading of copper are prepared in order to investigate the effect of underlying supports on the resulting electrocatalytic activity of copper for the reduction of nitrate. Electrolytic experiments with these modified electrodes are conducted in neutral aqueous solutions containing 0.5 M of nitrate at various applied potentials, and product analysis is performed. The overall reaction rate, Faradaic efficiency of each product, selectivity toward the production of ammonia against the formation of nitrite, and the turnover frequency for ammonia production normalized by the amount of electrochemically addressable copper sites are thus quantified at every electrolytic condition. The copper supported by ceria/carbon exhibits the highest selectivity toward ammonia production against the formation of nitrite among all the three materials; a selectivity of 73.4 % at -1.29 V vs. standard hydrogen electrode is achieved, which is much higher than those achieved by the zirconia/carbon-supported copper (36.0 %) and carbon-supported copper (47.2 %). Density functional theory (DFT) computational studies are performed to probe the reason for such a difference in the electrocatalytic activity of copper caused by the underlying support. Findings here suggest the importance of selecting the underlying support upon the design of Cu-based electrocatalysts used for nitrate reduction.