Photochemistry of NO Chemisorbed on TiO₂(110) and TiO₂ Powders

Abstract

The photodecomposition of chemisorbed NO has been studied using ultraviolet radiation of (3.96 ± 0.07) eV. Both the TiO2 (110) single-crystal substrate and high area compressed TiO2 powders have been investigated. A primary photoproduct is N2O gas, which desorbs immediately upon irradiation of the TiO2 (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 cm2, corresponding to a quantum efficiency near unity. In contrast, the quantum efficiencies of gas-phase N2O and NO photoproduction from chemisorbed NO on TiO2 are only in the range 10-2−10-4, indicating that NO photodecomposition primarily yields an intermediate photoproduct (N2O) which is captured on the crystal surface at 118 K. Studies of the infrared spectral behavior of NO on powdered and compressed high area TiO2 powders during photolysis confirm that much of the N2O 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 TiO2 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 N2O production is presented.