Fundamental Limit of Selectivity in Photocatalytic Denitrification over Titania.

Abstract

Although photocatalytic decomposition of NO (deNO) into N2 and O2 is low-cost and non-polluting, it has a low NO conversion efficiency. Establishing the activity and selectivity trend among active sites is an important base to explore and improve the deNO processes. Because the experimental performances are determined by the reaction rate, it is worthwhile to investigate the kinetic limiting steps calculated by comparative microkinetic modeling. We found that, without illumination, N2 production is inactive over various TiO2 surfaces/sites, but photogenerated holes can break the scaling relation of the dark condition by weakening O2* adsorption, leading to a significant increase in deNO activity on defective titania surfaces. However, the low N2 selectivity can be attributed to the small strength of N2O adsorption. In contrast, the N2 selectivity is enhanced in Ti-modified zeolite because of a stronger N2O* adsorption. We demonstrate here that the reaction phase diagram analysis can clearly establish a global picture of reaction activity and selectivity over various catalytic sites. In combination with microkinetic modeling, it can effectively determine the kinetic limits, providing insights to improve the design of photocatalysts.