Publication date: 15th December 2025
Electrocatalytic nitrate reduction (NO3RR) is of great interest for the simultaneous treatment of a hazardous waste and sustainable production of valuable ammonia. However, selectivity requires steering across a complex reaction pathway, involving the transfer of eight electrons and nine/ten protons (considering ammonium at lower pH).[1] We have studied a range of first row transition metals, which differ greatly in the overall Faradaic efficiency (FE) towards NO3RR (which competes with water reduction to H2) and N-product selectivity.[2] To understand the link between catalyst electronic structure and selectivity, we interrogate the ability of catalysts to adsorb and dissociate NO using ambient pressure X-ray spectroscopy (AP-XPS). Catalysts with a high d-band center are capable of cleaving the NO bond, having high selectivity to ammonia, whereas metals with low d-band center bind NO too weakly and have appreciable nitrite formation.[3]
Amongst first row transition metals, cobalt is unique in it’s high overall FE for NO3RR and its high selectivity to NH3. To better understand the active state of the electrocatalyst, we perform operando X-ray absorption spectroscopy (XAS). Nitrate oxidizes cobalt nanoparticles extensively at the open circuit potential, however the electrocatalyst is reduced throughout the bulk during NO3RR at more reducing potentials. Differences in oxidation at open circuit with electrolyte composition indicate the unique role this autocatalytic process may play in defining the active catalyst state.
This material is based upon work supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, & Biosciences Division, Catalysis program under Award numbers DE-SC0022970 and DE-SC0024865. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. This research used resources of the Advanced Light Source, a U.S. DOE Office of Science User Facility under contract no. DE-AC02-05CH11231.
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