Photoinduced Decomposition of Formaldehyde on a TiO₂(110) Surface, Assisted by Bridge-Bonded Oxygen Atoms

Abstract

ABSTRACT:We have investigated the photoinduced decomposition of formaldehyde (CH 2 O) on TiO 2 (110) at 400 nm using temperature-programmed desorption. Formate (HCOO), methyl radicals (CH 3 ), and ethylene (C 2 H 4 ) have been detected, while no evidence of polymerization of CH 2 O was found. The initial step in the decomposition of CH 2 O on TiO 2 (110) is the formation of a dioxymethylene intermediate in which the carbonyl O atom of CH 2 O is bound both to a Ti atom on the five-fold-coordinated lattice site (Ti 5C ) and to a nearby bridgebonded oxygen (BBO) atom. During 400 nm irradiation, the dioxymethylene intermediate can transfer methylene to the bridging oxygen row and break the C−O bond, thus leaving the original carbonyl O atom on the Ti 5C site. After this transfer of methylene, several pathways to products are available. Thus we have found that BBO atoms are intimately involved in the photoinduced decomposition of CH 2 O on TiO 2 (110). SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis T iO 2 has been investigated as a potential energy-efficient catalyst for photo-oxidation.1−8 The development of efficient catalysts from TiO 2 can be facilitated by an understanding of the site-specific surface dynamical processes of adsorbed molecules and reaction intermediates. Much work has been done to study the photocatalysis of organic compounds on TiO 2 , for example, the degradation of CH 2 O 9 and the development of a TiO 2 -based gas sensor.10 However, most of the studies are on powders of TiO 2 .11−13 To clarify the role of different kinds of active sites and gain a better understanding of photocatalytic reactions, single crystals 14−20 and thin films 12,13 of TiO 2 have been used as model surfaces for studying photo-oxidation and other photoinduced processes. Among the rutile TiO 2 surfaces, the (110) single-crystal surface has been especially widely used in fundamental studies of photocatalytic reactions as well as for studies of adsorbate− surface interactions, surface reconstructions, defects, and many other phenomena.It has been demonstrated that surface Ti 3+ defect sites, 21−24 bulk Ti 3+ defect sites, 25 and surface Ti 4+ sites 26,27 on TiO 2 (110) play important roles in molecular adsorption, thermal reactions, and photocatalytic reactions. There are also some suggestions in the literature that lattice oxygen plays a role in photooxidation reactions on TiO 2 mainly because of its presence in products or intermediates (as determined through isotopic labeling studies).28−32 However, a direct mechanistic experimental study of how lattice oxygen is involved in photochemical reactions on TiO 2 has not been reported. In this work, we have conducted a temperature-programmed desorption (TPD) investigation of the mechanism of the photoinduced decomposition of CH 2 O on a TiO 2 (110) surface and have identified the important role of the bridge-bonded oxygen (BBO) atoms in the first step toward the final reaction products: formate, C 2 H 4 , and CH 3. Figure 1 shows TPD spectra collected at ...

Elementary photocatalytic chemistry on TiO2surfaces

“…Yates and coworkers 225 The most well-studied aldehydes on r-TiO 2 (110) in terms of photochemistry is formaldehyde (CH 2 O). [233][234][235] Using the highly sensitive TPD method, Xu and co-workers 233 (110) was very low, which was similar to the results obtained by Xu and coworkers. 233 With increasing bulk reduction of the crystal, the photochemical activity on the reduced surface was severely diminished.…”

Excitation Wavelength Dependence of Photocatalyzed Oxidation of Methanol on TiO2(110)

“…The depletion of the high temperature portion is attributed to the interaction between bridging hydroxyls which are produced in the photoinduced process and the molecularly adsorbed methanol molecules in a previous study [28]. [37,38]. These results unambiguously show that methanol is photocatalytically oxidized into formaldehyde.…”

Highly sensitive and selective room-temperature formaldehyde sensors using hollow TiO2 microspheres

“…Fig. 4(a) shows the time-dependent response of the hollow TiO 2 microspheres based sensor to different concentrations of formaldehyde, a carcinogen causing chemical and usually emitted from newly furnished household furniture [14,15]. The sensors show excellent sensitivity to low concentrations of formaldehyde down to ∼124 ppb at room temperature driven by microwatts-powered UV photon energy.…”

“…It has been widely studied for water splitting, solar cells and decontamination of some volatile organic compounds (VOCs) . In particular, the decomposition process of pollutant volatile gas vapors such as formaldehyde under UV illumination had been proposed to be intimately related with oxidation of chemicals by the adsorbed ionized oxygen and hydroxyls on TiO 2 surface [14,15]. It is known that ad-/de-sorptions of ionized oxygen at the oxide surface would induce a change of the electron concentrations within the subsurface (i.e.…”