Ni and Mo atom pairs as single sites on N-doped graphitic carbon for urea formation by simultaneous CO2 and NO3− reduction with pulsed electrocatalysis

Abstract

Ni and Mo atom pairs as single sites supported on N-doped graphitic carbon was prepared by pyrolysis of a mixture of Ni(NO3)2, (NH4)6Mo7O24, glucose, and melamine at 800 °C and subsequent washing with HCl. Coulombic association between Ni2+ and Mo7O246− is key for the formation of the Ni-Mo pairs (distance: 0.23 nm), whose presence was determined by atomic resolution aberration-corrected STEM and EXAFS. The dual NiMo-DASC exhibits better performance for urea formation by simultaneous electrochemical CO2 and NO3− reduction reactions than the Ni- or Mo-single atom catalysts on N-doped graphitic carbon prepared analogously at similar total metal loadings and surface areas. Using pulsed electrochemical reduction of −0.5 V vs. RHE for NO3RR and −0.7 V vs. RHE to promote CO2RR, urea was formed with a faradaic efficiency of 31.8% and a yield of 11.3 mmol h−1 g−1. The sources of C and N were confirmed by isotopic 13C and 15N labelling experiments using NMR spectroscopy. In situ surface enhanced IR spectroscopy shows the appearance of adsorbed *CO (1937 cm−1), *NH species (1636 cm−1) and C-N (1597 cm−1) vibration bands. DFT calculations of the Ni-Mo pair on N-doped graphene model predict a distance of 0.22 nm between the two metal atoms and suggest that the synergistic effect is derived from co-the adsorption of CO2, preferentially on the Ni atom, and NO3− on the Mo atom, with the crucial C-N bond formation occurring between neighbor CO (on Ni) and NH (on Mo), thereby showing the synergistic effect arising from the presence of Ni and Mo at the catalytic site.