Defective Nickle–Iron Layered Double Hydroxide for Enhanced Photocatalytic NO Oxidation with Significant Alleviation of NO2 Production

Photocatalysis offers a sustainable avenue for the oxidative removal of low concentrations of NOx from the atmosphere. Layered double hydroxides (LDHs) are promising candidate photocatalysts owing to their unique layered and tunable chemical structures, and the abundant hydroxide (OH−) moieties on their surfaces that are hydroxyl radical (•OH) precursors. However, inferior charge separation and limited active sites on an LDH hinder its practical applications. Herein, we developed a facile N2H4-driven etching (et) approach that introduces dual Ni2+ and OH− vacancies (Niv and OHv) into NiFe-LDH nanosheets (referred to as NiFe-LDH-et) that facilitate improved charge-carrier separation and the formation of active Lewis acidic sites (Fe3+ and Ni2+ exposed at OHv). In contrast to inert pristine LDH, NiFe-LDH-et actively removes NO when illuminated with visible light. Specifically, Ni76Fe24-LDH-et etched in 1.50 mmol·L−1 N2H4 solution removes 32.8% of the NO from continuously flowing air (NO-feed concentration: ∼500 parts per billion (ppb)) when illuminated with visible light, thereby outperforming most reported catalysts. Experimental and theoretical data reveal that the dual vacancies promote the production of reactive oxygen species (•O2− and •OH) and the adsorption of NO on the LDH. In-situ spectroscopy revealed that NO is preferentially adsorbed at Lewis acidic sites, particularly exposed Fe3+ sites, and then converted into NO+ that is subsequently oxidized to NO3− without the formation of any of the more toxic NO2 intermediate, thereby alleviating risks associated with its production and emission.