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Electron- and Hole-Mediated Reactions in UV-Irradiated O 2 Adsorbed on Reduced Rutile TiO 2 (110)

2010; American Chemical Society; Volume: 115; Issue: 1 Linguagem: Inglês

10.1021/jp108909p

1932-7455

Nikolay G. Petrik, Greg A. Kimmel,

Catalytic Processes in Materials Science

The ultraviolet (UV) photon-stimulated reactions in oxygen adsorbed on reduced TiO2(110) at low temperatures (<100 K) are studied. When a single O2 is chemisorbed in each bridging oxygen vacancy, only ∼14% of the O2 desorbs after prolonged UV irradiation. For the remaining O2 on the surface after irradiation, about one-half dissociates, and the other one-half is left in a nondissociated state that is inactive for hole-mediated photodesorption. For the maximum coverage of chemisorbed oxygen, the fraction of O2 that photodesorbs increases substantially, but is still only ∼40%. However, when physisorbed oxygen is also present, ∼70% of the initially chemisorbed O2 photodesorbs. On the basis of the experimental results, we propose that both hole- and electron-mediated reactions with O2 chemisorbed on TiO2(110) are important. Hole-mediated reactions lead to O2 photodesorption, while electron-mediated reactions lead to O2 dissociation. The electron-mediated reactions explain the low total photodesorption yield when no physisorbed O2 is present. For a fixed amount of chemisorbed 18O2, its PSD yield increases substantially if 16O2 is subsequently chemisorbed, indicating that the hole-mediated O2 photodesorption probability depends on the charge state of the chemisorbed O2; it decreases for more negatively charged O2. Because the charge state of the chemisorbed O2 depends on the total oxygen coverage, the coverage influences the photodesorption process. A simple model based on the oxygen coverage and the charge of the chemisorbed oxygen, which accounts for the observations, is presented. In the model, O2 chemisorbs as either O2− or O22− depending on the oxygen coverage. O2− (O22−) reacting with a hole leads to O20 desorption with a high (low) probability. O22− plus an electron typically leads to O2 dissociation, while O2− + e− does not lead to dissociation.