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

Abstract

Photocatalysis offers a sustainable means for the oxidative removal of low concentrations of NO_x (NO, NO₂, N₂O, N₂O₅, etc.) from the atmosphere. Layered double hydroxides (LDHs) are promising candidate photocatalysts owing to their unique layered and tunable chemical structures and abundant surface hydroxide (OH⁻) moieties, which are hydroxyl radical (OH) precursors. However, the practical applications of LDHs are limited by their poor charge-separation ability and insufficient active sites. Herein, we developed a facile N₂H₄-driven etching approach to introduce dual Ni²⁺ and OH⁻ vacancies (Ni_v and OH_v, respectively) into NiFe-LDH nanosheets (hereafter referred to as NiFe-LDH-et) to facilitate improved charge-carrier separation and active Lewis acidic site (Fe³⁺ and Ni²⁺ exposed at OH_v) formation. In contrast to inert pristine LDH, NiFe-LDH-et actively removed NO under visible-light illumination. Specifically, Ni₇₆Fe₂₄-LDH-et etched with 1.50 mmol·L⁻¹ N₂H₄ solution removed 32.8% of the NO in continuously flowing air (NO feed concentration: ∼500 parts per billion (ppb)) under visible-light illumination, thereby outperforming most reported catalysts. Experimental and theoretical data revealed that the dual vacancies promoted the production of reactive oxygen species (O₂⁻ and OH) and the adsorption of NO on the LDH. In situ spectroscopy demonstrated that NO was preferentially adsorbed at Lewis acidic sites, particularly exposed Fe³⁺ sites, converted into NO⁺, and subsequently oxidized to NO₃⁻ without the notable formation of the more toxic intermediate NO₂, thereby alleviating risks associated with its production and emission.