Unbalancing the Hydrogen Bond Accepting and Donating Effects of Water for Photoelectrocatalytic Ammonia Oxidation Reaction

The ammonia oxidation reaction (AOR) is the key reaction in ammonia splitting for hydrogen production and wastewater treatment, in which N-H cleavage and N-N formation are the key steps. In this work, we demonstrate that the hydrogen-bond accepting effect from H₂O to NH₃ plays the pivotal role in accelerating the N-H cleavage, while the hydrogen-bond donating effect from H₂O to NH₃ inhibits the N-N formation of NH₃ nucleophilic attack. Alkali metal cations are further applied to unbalance the hydrogen-bond accepting and -donating effects, among which the large-size cations assist in breaking the hydrogen bonds among H₂O-H₂O and H₂O-NH₃. The resulting isolated H₂O and NH₃ molecules promote the accepting effect and suppress the donating effect, which enhance the activity of photoelectrocatalytic AOR by 8 times on BiVO₄ photoanodes. This study illustrates the molecular-level understanding of the hydrogen-bonding effect in AOR mechanisms and to develop efficient AOR systems.