Zn-Al layered double hydroxides as efficient photocatalysts for NOx abatement

In this study, we report that layered double hydroxides (LDH) exhibited high photocatalytic activities in degrading NOx gases for the first time. ZnAl-CO3 LDHs with a 1.5-3.0 Zn/Al ratio were prepared by a coprecipitation method both with and without hydrothermal treatment. Syntheses were carried out with high and low metal concentrations, the latter being the most favorable in obtaining pure LDHs in the whole Zn/Al ratio range. The samples were characterized by different techniques such as PXRD, FT-IR, ICP mass, TGA, SBET, SEM and Diffuse reflectance (DR). The LDH particles grew as well-defined hexagonal nanolayers, whose size and crystallization depended on the synthetic procedure and the Zn/Al ratio. Those samples with lower crystallinity exhibit the highest specific surface area values (> 50 m(2).g(-1)). The ZnAl-CO3 LDHs were UV light responsive with band-gap values close to 3.5 eV. The LDH photocatalysts show a high performance towards the photochemical oxidation process of NO gas, with efficiencies of around 55%. Remarkably, the ZnAl-CO3 photocatalysts exhibit an impressive selectivity towards the deNOx process, avoiding the emission of the toxic NO2 gas into the atmosphere. Interestingly, these promising deNOx results are repeated when working for a long irradiation period or with the highest concentration of NO in polluted atmospheres.