Fundamental Limit of Selectivity in Photocatalytic Denitrification over Titania.

Although photocatalytic decomposition of NO (deNO) into N 2 and O 2 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, N 2 production is inactive over various TiO 2 surfaces/sites, but photogenerated holes can break the scaling relation of the dark condition by weakening O 2 * adsorption, leading to a significant increase in deNO activity on defective titania surfaces. However, the low N 2 selectivity can be attributed to the small strength of N 2 O adsorption. In contrast, the N 2 selectivity is enhanced in Ti-modified zeolite because of a stronger N 2 O* 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.