Fundamental Theoretical Investigations of the Photocatalytic deNOx Reactions over TiO2 Surfaces

Abstract

Emissions of nitrogen oxides (NOx) have a serious impact on our environment and human health. Photocatalytic denitrification (deNOx) on titania attracted much attention due to it is low-cost and nonpolluting nature, but in reality undesired nitrite and nitrate were produced, instead of harmless N2. Establishing the global activity and selectivity trend among active sites is an important base to explore and improve the deNOx processes. Herein, I have investigated the reaction mechanisms of NO2 and H2O on anatase TiO2(101) and the direct decomposition of NO on various TiO2 surfaces. A polaron-corrected GGA functional (GGA + Lany-Zunger) was applied to improve the description of electronic states in photo-assisted process and a reaction phase diagram (RPD) was applied to understand the (quasi) activity trend over different active sites. In the deNO2 process, it was found that the perfect surface exhibits high activity while the activity on defective surfaces is limited by the sluggish recombinative desorption. A photogenerated hole can weaken the OH* adsorption energies and circumvent the scaling relation of the dark reaction, ultimately significantly increasing the denitrification activity. In the deNO process, it was found that without illumination N2 production is inactive over various TiO2 surfaces/sites, and photo-generated holes can break the scaling relation of the dark condition by weakening O2* adsorption, leading to a significant increase in deNO activity of defective titania surfaces. The low N2 selectivity in experiments was further investigated by microkinetic modeling. It was found that the weak adsorption of N2O led to the low N2 selectivity on titania surfaces. On the contrary, the N2 selectivity of Ti-modified zeolite was enhanced due to stronger N2O* adsorption. I show here that the reaction phase diagram (RPD) analysis can clearly establish a global picture of reaction activity and selectivity over various catalytic sites. The reactivity can be tuned by illumination-induced localized charge. Combined with microkinetic modeling, it can effectively determine the kinetic limits. This study provides insights for improving the design of photocatalysts.