The TiO2(110) surface, prepared with different densities of anion vacancy defect sites, has been investigated by studies of O2 photodesorption at 105 K. Two separate O2 photodesorption processes, α1 and α2, are detected and are postulated to be due to the presence of two different types of defect sites produced by annealing the crystal in vacuum before O2 adsorption. The measured photodesorption cross sections for these two states are Q α 1 -O 2 = (3.1 ± 0.2) × 10-16 cm2 and Q α 2 -O 2 = (4.3 ± 0.3) × 10-17 cm2 for 3.96 ± 0.07 eV photons. Both O2 states photooxidize CO chemisorbed on the reduced TiO2(110) surface. While the yield of the CO2 photoproduct increases with increasing crystal annealing temperature (and increasing defect coverage), the yield of α1-O2 and α2-O2 passes through a maximum at an annealing temperature of ∼600 K.

The adsorption and photooxidation of CH3CN chemisorbed on TiO2 has been investigated using infrared spectroscopy. It was found that CH3CN forms an ice layer on TiO2 at 126K and above 126K CH3CN diffusion into TiO2 occurs. At 200K, the adsorption of CH3CN on both isolated Ti−OH groups and on the Ti4+ Lewis acid site was observed. Under UV irradiation (350 ± 50 nm; hυ = 3.10−4.13 eV), CH3CN was oxidized in the presence of O2 to form H2O, CO2, surface CO3 2- and an intermediate partially oxidized isocyanate species, NCO, which was identified by various isotopic experiments. Photooxidation of the CH3 moiety in adsorbed CH3CN occurs more readily does the CN moiety in CH3CN. Lattice oxygen of TiO2 was found to preferentially form adsorbed NCO, compared to oxygen from adsorbed O2. A mechanism of the formation of NCO species was postulated.

The photodecomposition of chemisorbed NO has been studied using ultraviolet radiation of (3.96 ( 0.07) eV. Both the TiO 2 (110) single-crystal substrate and high area compressed TiO 2 powders have been investigated. A primary photoproduct is N 2 O gas, which desorbs immediately upon irradiation of the TiO 2 (110) surface. Following this process, the photoproduction of NO gas is observed to reach a maximum rate and then to decline. The cross section for the initial photodepletion of NO is about 1 × 10 -15 cm 2 , corresponding to a quantum efficiency near unity. In contrast, the quantum efficiencies of gas-phase N 2 O and NO photoproduction from chemisorbed NO on TiO 2 are only in the range 10 -2 -10 -4 , indicating that NO photodecomposition primarily yields an intermediate photoproduct (N 2 O) which is captured on the crystal surface at 118 K. Studies of the infrared spectral behavior of NO on powdered and compressed high area TiO 2 powders during photolysis confirm that much of the N 2 O photoproduct is retained on the surface. Furthermore, the infrared studies indicate that the penetration of ultraviolet light into the powder occurs to a depth of order 0.003 cm, which is very large compared to the light attenuation length known for individual TiO 2 crystals (200 Å). This effect is thought to be due to light transport effects at the particle boundaries in the compressed powder, and this effect therefore is favorable for photoprocesses using powders. Evidence for sub-bandgap excitation of chemisorbed NO, leading to N 2 O production is presented.

The adsorption and photochemistry of N 2 O chemisorbed on TiO 2 powder has been investigated. At 157 K, nitrous oxide is molecularly adsorbed on the reduced titanium dioxide. Adsorption on the TiO 2 surface takes place through both the N and O ends of the nitrous oxide molecule. The photochemistry of the adsorbed N 2 O molecules was studied at 157 K by use of UV irradiation in the range 2.1-5.0 eV. The photoactivity of the adsorbed molecule is dependent on the N 2 O coverage. At low coverages, the adsorbed N 2 O is photodepleted to produce adsorbed nitrogen. Larger coverages of N 2 O are not photodepleted to N 2 due to the site blocking in the adsorption process. N-bonded species show a higher photoreactivity than the O-bonded species. Photoformation of NO was not observed at any N 2 O coverage.

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