Mechanistic Insights into the High Selectivity and Photocatalytic Activity of Brookite TiO<sub>2</sub> toward NO<i><sub>X</sub></i> Abatement
Muhammad Kamran, Tarek A. Kandiel, Safwat Abdel‐Azeim, Mohamed A. Morsy, Detlef W. Bahnemann
Abstract
TiO 2 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 TiO 2 for the NO x depollution from the air. Pure brookite and anatase/brookite mixtures have been synthesized and characterized. Interestingly, we found that brookite TiO 2 photocatalyst has a comparable photocatalytic activity to that of the benchmark TiO 2 P25, and more importantly, it has higher selectivity toward the conversion of NO x into nitrate ions. The selectivity of brookite TiO 2 derived from the TALH precursor is 4.6- and 3.5-fold higher than that of pure anatase and TiO 2 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 NO 2 and water adsorption on different anatase and brookite surfaces. The EPR results indicated that brookite TiO 2 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 NO 2, 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 NO 2 and water-dissociated molecules is crucial for the enhanced selectivity of DeNO x to nitrate ions on brookite TiO 2 .