Pd Nanoparticle Size-Dependent H^* Coverage for Cu-Catalyzed Nitrate Electro-Reduction to Ammonia in Neutral...

Electrochemical conversion of nitrate (NO₃ ^-) to ammonia (NH₃) is an effective approach to reduce nitrate pollutants in the environment and also a promising low-temperature, low-pressure method for ammonia synthesis. However, adequate H^* intermediates are highly expected for NO₃ ^- hydrogenation, while suppressing competitive hydrogen evolution. Herein, the effect of H^* coverage on the NO₃RR for ammonia synthesis by Cu electrocatalysts is investigated. The H^* coverage can be adjusted by changing Pd nanoparticle sizes. The optimized Pd@Cu with an average Pd size of 2.88 nm shows the best activity for NO₃RR, achieving a maximum Faradaic efficiency of 97% (at -0.8 V vs RHE) and an NH₃ yield of 21 mg h^-1 cm^{- 2}, from an industrial wastewater level of 500 ppm NO₃ ^-. In situ electrochemical experiments indicate that Pd particles with 2.88 nm can promote NO₃ ^- hydrogenation to NH₃ via well-modulated coverage of adsorbed H^* species. Coupling the anodic glycerol oxidation reaction, ammonium and formate are successfully obtained as value-added products in a membrane electrode assembly electrolyzer. This work provides a feasible strategy for obtaining size-dependent H^* intermediates for hydrogenation.