Mechanistic Insights into the High Selectivity and Photocatalytic Activity of Brookite TiO2 toward NOX Abatement

TiO2 nanomaterials are promising photocatalysts for NOx depollution from air under sunlight irradiation. It commonly exists in three phases, i.e., anatase, rutile, and brookite. Anatase, rutile, and a mixture of them are heavily studied. Herein, we have tested the photocatalytic activity of the less studied brookite TiO2 for the NOx depollution from the air. Pure brookite and anatase/brookite mixtures have been synthesized and characterized. Interestingly, we found that brookite TiO2 photocatalyst has a comparable photocatalytic activity to that of the benchmark TiO2 P25, and more importantly, it has higher selectivity toward the conversion of NOx into nitrate ions. The selectivity of brookite TiO2 derived from the TALH precursor is 4.6- and 3.5-fold higher than that of pure anatase and TiO2 P25, respectively. Such impressive selectivity has been explained by measuring the content of physically and chemically adsorbed water, the relative density of defects, and the affinity of NO2 and water adsorption on different anatase and brookite surfaces. The EPR results indicated that brookite TiO2 exhibits a higher density of charge-trapping sites (defects) than anatase. The DFT calculation supported that these defects increase the tendency of brookite (001) surface toward the nondissociative adsorption of NO2, whereas the dissociative adsorption of water is preferred on the perfect surface. It seems that the coexistence of dual sites for the adsorption of molecular NO2 and water-dissociated molecules is crucial for the enhanced selectivity of DeNOx to nitrate ions on brookite TiO2.