Sulfur Dioxide Adsorption on TiO2 Nanoparticles: Influence of Particle Size, Coadsorbates, Sample Pretreatment, and Light on Surface Speciation and Surface Coverage

The adsorption of sulfur dioxide (SO2) on titanium dioxide (TiO2) nanoparticle surfaces at 296 K under a wide range of conditions has been investigated. X-ray photoelectron spectroscopy is used to investigate the surface speciation and surface coverage of sulfur-containing products on ca. 4 nm TiO2 anatase particles that remain on the surface following adsorption of SO2. The effects of various environmental conditions of relative humidity, molecular oxygen, and broadband UV/vis irradiation as well as sample pretreatment were found to impact the speciation of adsorbed SO2 as well as the saturation coverage. In particular, in the absence of light, the majority surface species upon SO2 adsorption is found to be adsorbed sulfite. Broadband UV/vis irradiation during sulfur dioxide adsorption leads to an increase (nearly 2-fold) in the amount of adsorbed sulfur species, as compared to experiments with no light, and results in the formation of adsorbed sulfate. The formation of sulfate was quantitative in the presence of molecular oxygen. New surface species including chemisorbed molecular SO2 were observed on samples that have been reduced in vacuum through argon ion sputtering. The total amount of adsorbed sulfur was impacted by surface hydroxyl group coverage and molecularly adsorbed water layer. Additionally, comparison of sulfur dioxide adsorption on 4 versus 32 nm sized anatase nanoparticles showed that surface saturation coverages of adsorbed sulfite on the 4 nm particles was almost twice that of 32 nm particles as measured by the S2p:Ti2p peak area ratios, thus showing an increase in the inherent adsorption capacity of the smaller particles. Proposed adsorption sites and mechanisms to account for the observed experimental data are discussed.