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.