The adsorption of nitrogen dioxide on gamma aluminium oxide (gamma-Al(2)O(3)) and alpha iron oxide (alpha-Fe(2)O(3)) particle surfaces under various conditions of relative humidity, presence of molecular oxygen and UV light has been investigated. X-Ray photoelectron spectroscopy (XPS) is used to monitor the different surface species that form under these environmental conditions. Adsorption of NO(2) on aluminum oxide particle surfaces results primarily in the formation of surface nitrate, NO(3)(-) with an oxidation state of +5, as indicated by a peak with binding energy of 407.3 eV in the N1s region. An additional minority species, sensitive to the presence of relative humidity and molecular oxygen, is also observed in the N1s region with lower binding energy of 405.9 eV. This peak is assigned to a surface species in the +4 oxidation state. When irradiated with UV light, other species form on the surface. These surface-bound photochemical products all have lower binding energy, between 400 and 402 eV, indicating reduced nitrogen species in the range of N oxidations states spanning +1 to -1. Co-adsorbed water decreases the amount of these reduced surface-bound products while the presence of molecular oxygen completely suppresses the formation of all reduced nitrogen species on aluminum oxide particle surfaces. For NO(2) on iron oxide particle surfaces, photoreduction is enhanced relative to gamma-Al(2)O(3) and surface bound photoreduced species are observed under all environmental conditions. Complementing the experimental data, N1s core electron binding energies (CEBEs) were calculated using DFT for a number of nitrogen-containing species in the gas phase and adsorbed on an Al(8)O(12) cluster. A range of CEBEs is calculated for various nitrogen species in different adsorption modes and oxidation states. These calculated values are discussed in light of the peaks observed in the XPS N1s region and the possible species that form following NO(2) adsorption and photoreaction on metal oxide particle surfaces under different conditions of relative humidity, presence of molecular oxygen and UV light.

The adsorption, thermal, and UV reactions of ethanol over a TiO 2 (110) single-crystal surface have been studied in the presence and the absence of molecular oxygen. Adsorption of ethanol is dissociative at room temperature and gives rise to two C1s peaks of equal intensities (at 285.2 and 286.5 eV) attributed to -CH 3 and -CH 2 O-groups, respectively. The surface coverage at saturation (of the dissociative adsorption mode at 300 K) is close to 0.5 with respect to Ti atoms. Thermal annealing resulted in the disappearance of the C1s signal attributed to both groups (-CH 3 and -CH 2 O-), with negligible oxidation of the ethoxide groups. The decrease of both peaks is not symmetric, it is attributed to water desorption consuming bridging surface oxygen followed by migration of ethoxide species into these defects in the process of healing surface oxygen atoms. Exposures to UV irradiation (3.2 eV) of the ethoxide covered surface in the presence of oxygen at 300 K resulted in considerable decrease of the ethoxide C1s peaks with irradiation time and the formation of a carboxylate peak at about 290 eV. This XPS C1s signal, attributed to both CH 3 COO(a) and HCOO(a) species, is most likely due to oxidation by the photoactive O 2or O 2 2species, formed by capture of the photoexited electrons at the conduction band (Ti 3d). The dependence of the rate of ethoxide decomposition on the O 2 pressure follows the expected Langmuir-Hinshelwood kinetics. The photoreaction cross section was estimated from the decay of the XPS C1s signal (starting from surface saturation) and was found equal to ca. 2 × 10 -18 cm -2 at 1 × 10 -6 Torr of O 2 . This figure compares well with that obtained for acetate decomposition under similar conditions on this same surface.

The adsorption of sulfur dioxide (SO 2 ) on titanium dioxide (TiO 2 ) 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 TiO 2 anatase particles that remain on the surface following adsorption of SO 2 . 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 SO 2 as well as the saturation coverage. In particular, in the absence of light, the majority surface species upon SO 2 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 SO 2 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.