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

TiO₂ nanomaterials are promising photocatalysts for NO_x 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 NO_x 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 NO_x 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.