Different Roles of Water in Photocatalytic DeNOx Mechanisms on TiO₂: Basis for Engineering Nitrate Selectivity?

The nitrate selectivity of TiO 2 has important consequences for its efficiency as a NOx depollution photocatalyst. Most emphasis is typically given to photocatalyst activity, a measure of the rate at which NOx concentrations are reduced, but a reduction in NOx concentration (mainly NO + NO 2 ) is not necessarily a reduction in atmospheric NO 2 concentration because the catalytic process itself generates NO 2 . With NO 2 being considerably more toxic than NO, more emphasis on nitrate selectivity, a measure of the NOx conversion to nitrate, and how to maximize it, should be given in engineering photocatalytic systems for improved urban air quality. This study, on the importance of adsorbed water in the photocatalytic oxidation of NOx, has identified important correlations which differentiate the role that water plays in the oxidation of NO and NO 2 . This observation is significant and offers insights into controlling nitrate selectivity on TiO 2 and the potential for increased effectiveness in environmental photocatalyst applications.