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. XPS is used to investigate the surface speciation and surface coverage of sulfur-contg. 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, mol. oxygen, and broadband UV/vis irradn. as well as sample pretreatment were found to impact the speciation of adsorbed SO2 as well as the satn. coverage. In particular, in the absence of light, the majority surface species upon SO2 adsorption is found to be adsorbed sulfite. Broadband UV/vis irradn. during sulfur dioxide adsorption leads to an increase (nearly 2-fold) in the amt. of adsorbed sulfur species, as compared to expts. with no light, and results in the formation of adsorbed sulfate. The formation of sulfate was quant. in the presence of mol. oxygen. New surface species including chemisorbed mol. SO2 were obsd. on samples that have been reduced in vacuum through argon ion sputtering. The total amt. of adsorbed sulfur was impacted by surface hydroxyl group coverage and molecularly adsorbed water layer. Addnl., comparison of sulfur dioxide adsorption on 4 vs. 32 nm sized anatase nanoparticles showed that surface satn. 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 obsd. exptl. data are discussed. [on SciFinder(R)]