CO Photooxidation on TiO₂(110)

Abstract

The photooxidation of chemisorbed CO on TiO2(110) was investigated. Molecular O2, chemisorbed at 105 K at anion vacancy defect sites on TiO2(110), is shown to undergo excitation leading to either photodesorption of O2 or photooxidation of coadsorbed CO to produce CO2. The yield of photoproduced CO2 and photodesorbed O2 follows the same excitation curve versus photon energy with a threshold at the TiO2 band gap, 3.1 eV. The decay time for photoproduced CO2 is essentially identical with the characteristic decay time for photodesorbed α-O2, corresponding to a photodesorption cross section = 8 × 10-17 cm2 for 3.94 eV photons. This indicates that both O2 photodesorption and CO2 photoproduction are related to the same α-O2 species. Compared to O2 photodesorption, CO2 photoproduction is a minor process. Lattice oxygen in TiO2 is not chemically involved in CO2 formation during ultraviolet irradiation.

Evidence for photochemical production of reactive oxygen species in desert soils

“…3) and at a near 1:1 molar rate ratio with the O 2 Á À that is generated (in the 1st hour) by sunlight-exposed desiccated soils (Fig. 2a), in agreement with the mechanism for TiO 2 photocatalysis 15,36 . The same mechanism also could apply for the desert soils (Fig.…”

“…2a). This near 1:1 molar O 2 Á À /OH ratio is in agreement with the mechanism of TiO 2 photocatalysis 15,36 , which assumes equimolar Á OH and O 2 Á À aqueously generated; the latter also being photogenerated in desiccated soils by the same mechanism (Fig. 4).…”

“…The same mechanism might also apply for the albite-bearing control site soil. Also, metal superoxides and peroxides may be continuously photogenerated in all soils because TiO 2 , Feoxides and Fe-Ti-oxides-present in the tested soils and typical of desert soils 45 -are known to be involved in the photogeneration process of O 2 Á À (refs [13][14][15]17,36 …”

“…Moreover, the photo-catalytic generation of Á OH by aqueous desert soils could provide a light-based mechanism for their reactivity with organics. This would involve the photo-catalytic ejection of an electron from metal oxides (to generate O 2 Á À by dissolved O 2 ) and the subsequent abstraction of an electron by the resulting h þ VB hole from H 2 O or OH À (resulting also from H 2 O via reaction with a bridging-oxygen atom 15 ). Then, Á OH (and H 2 O 2 to a lesser degree) could oxidize soil organics to CO 2 and NOy.…”

“…Photogenerated ROS have been identified in laboratory studies on mineral oxides (mainly TiO 2 ) exposed to ultraviolet radiation: atomic oxygen radical anion (O Á À ), ozone radical anion (O 3 Á À , and possibly O 4 Á À ), O 2 Á À and Á OH [13][14][15] . The postulated process in these studies involves ultraviolet-induction of electron mobilization from the oxide surface and its subsequent capture by O 2 to form oxygen radical surface adsorbates 16 .…”