Titanium Dioxide Nanoparticle Surface Reactivity with Atmospheric Gases, CO2, SO2, and NO2: Roles of Surface Hydroxyl Groups and Adsorbed Water in the Formation and Stability of Adsorbed Products

The reactivity of O-H groups on titanium dioxide nanoparticle surfaces with gas-phase carbon dioxide, sulfur dioxide, and nitrogen dioxide is compared. Carbon dioxide, sulfur doxide, and nitrogen dioxide react with ca 5, 50, and nearly 100%, respectively, of all hydroxyl groups on the surface at 298 K. As shown here, the surface reactivity of O-H groups with these three triatomic gases differs considerably due to different reaction mechanisms for adsorption and surface chemistry. In addition to investigate O-H group reactivity, the role of adsorbed water in the stability of different surface species that form from adsorption of carbon dioxide, nitrogen dioxide, and sulfur dioxide on hydroxylated TiO2 nanoparticles is probed as a function of relative humidity as is quantitative measurements of water uptake on TiO2 nanoparticles before and after surface reaction. These water uptake studies provide insights into the stability of adsorbed species on oxide surfaces under atmospherically relevant conditions as well as changes in particle hygroscopicity and adsorbed water dynamics following reaction with these three atmospheric gases.