PdCu Electrocatalysts for Selective and Stable Nitrate Reduction to Nitrogen

Abstract

Electrocatalytic conversion of nitrate in waste can enable efficient waste remediation (NO3- to N3) or waste valorization (NO3- to NH4+) depending on the selectivity of the catalyst. Palladium and copper electrocatalysts typically exhibit exceptional nitrate and nitrite binding properties, allowing for effective destruction of nitrate. However, rational steering of selectivity through material design remains a critical challenge for PdCu electrocatalyst. Here, we use the electrochemical underpotential deposition method to synthesize palladium nanocube electrocataysts with controlled copper surface coverage (e.g. partial and full copper coatings). We then examine the potential for NO3- destruction (conversion) and catalyst selectivity. We identify that partial copper-coated Pd nanocubes effectively facilitate the reduction of 95% of NO3-. Partial surface coverage of copper also allows exposure of Pd(100) surface facets, allowing selective reduction of NO3- to N3 with 89% selectivity over 20 consecutive cycles (80 hours). Complete copper-covered Pd nanocubes effectively facilitate the reduction of 98.8% of NO3-. Complete coverage of copper also prevented exposure of Pd surfaces (100), promoting selective reduction of NO2- to NH4+ with a 70% selectivity over 20 consecutive cycles (80 hours). Density functional theory (DFT) calculations show that NO3- and NO2- can easily be reduced to NO* on the Cu surface (100). The adsorbed NO* then migrates favorably from the Cu(100) surface to the Pd(100) surface, where NO* is hydrogenated to form an NOH* intermediate that readily dissociates to generate N*. N* can then be coupled with NO* on the surface of Pd (100) with high NO* coverage to form N2O*, which is the precursor intermediate for N2 formation.