Optimizing divalent metal composition in layered double hydroxides for sustainable NOx photocatalytic removal

The presence of nitrogen oxides (NOx) in urban areas has adverse effects on both human health and the environment, but efficient and affordable photocatalytic elimination of NOx represents a challenge. In this study, we investigated a series of Aqueous Miscible Organic Solvent Treatment Layered Double Hydroxides (AMO-LDHs), Znx(Mg3−x)Al-CO3 AMO-LDHs (0 ≤ x ≤ 3), for the photocatalytic removal of gaseous NOx from air (DeNOx process). Considering that zinc is an “endangered element” where projected demand is far greater than known reserves, the aim was to identify a sustainable, low-cost material with promising photocatalytic properties by modifying the Mg/Zn ratio. The LDHs were prepared using a simple co-precipitation method followed by Aqueous Miscible Organic Solvent Treatment (AMOST). All the samples exhibited uniform metal distribution without impurities. Incorporation of Zn into the AMO-LDH structure increased the nanosheet thickness and reduced their aggregation, resulting in slight morphological changes. It also influenced the positions of the valence and conduction bands, thereby affecting the photochemical mechanism. Among the studied compositions, Zn0.5Mg2.5Al AMO-LDH showed the most promising performance for NOx removal, offering NOx removal efficiencies of 51 %, low NO₂ formation with outstanding selectivity (up to 93 %) and significantly enhanced efficiency compared to Mg3Al AMO-LDH or commercial ZnO. Furthermore, this sample exhibited exceptional stability and reusability, maintaining its photocatalytic efficiency throughout the repeated cycles, highlighting its potential for practical DeNOx application.