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

Abstract

Post-irradiation temperature-programmed desorption (TPD) has been used to study the photocatalyzed oxidation of methanol on TiO 2 (110) surface under the irradiation of 360, 380 and 400 nm light. The photocatalytic process initiated by ultraviolet light of different wavelength are similar. Methanol has been photocatalytically converted into formaldehyde, and the released hydrogen atoms transfer to the neighboring twofold coordinated oxygen to form bridging hydroxyls. The reaction rate, however, is strongly wavelength dependent. The reaction rate under 360 nm light irradiation is 4.8 times of that in the case of 400 nm exposure, consistent with a previous femtosecond time-resolved absorption measurement on TiO 2 which shows the faster charge carrier recombination in the near-band-gap than the overband-gap excitation. So far, the underlying factors which govern the excitation wavelength dependence of photocatalytic activity of TiO 2 and other photocatalysts remain unclear, and future studies are needed to address this important issue.

What Controls Photocatalytic Water Oxidation on Rutile TiO2(110) under Ultra-High-Vacuum Conditions?

“…The equivalent path for the photoinduced oxidation of CH 3 OH ads at a Ti cus site to CH 2 O ads •– is exothermic by ∼1.9 eV . This difference explains why the CH 2 O ads •– is a stable detectable species, ,− whereas the OH ads • /O ads •– products located at ∼2.38 eV/∼2.40 eV are transient species which may only be detected with time-resolved spectroscopy techniques.…”

Excitonic Interfacial Proton-Coupled Electron Transfer Mechanism in the Photocatalytic Oxidation of Methanol to Formaldehyde on TiO2(110)

“…From this perspective, the limiting factor for the photocatalytic efficiency of the first step, which is probably responsible for the low experimental yields, ,, is the localization of the photogenerated exciton to give this particular species and not the remaining, unreactive excitons. In contrast, charge carrier recombination of this exciton before O–H dissociation does not appear to be so critical in spite of the short distance between the h and e , since the probability of nonradiative recombination is low given the high energy gap between the exciton state and the neutral ground state.…”

“…TiO 2 based heterogeneous photocatalysis and photoelectrocatalysis have a very important potential role for the production of fuels and chemicals and environmental remediation. − Because of its stability, the rutile TiO 2 (110) surface is an ideal substrate for model surface science studies where H 2 O or CH 3 OH are adsorbed under ultrahigh vacuum (UHV) conditions. A series of photochemical studies on these systems have been conducted using different techniques in combination with laser UV irradiation. − These studies under well-defined reaction conditions aim at providing insight into the elementary steps that compose the photochemical mechanism and understanding the fundamental principles of photocatalysis. This fundamental knowledge will benefit the improvement or development of technologies based on heterogeneous photocatalysis and photoelectrocatalysis.…”

“…In particular, H 2 O photocatalysis on TiO 2 (110) , has been studied because of its relation to photoelectrocatalytic H 2 O splitting on titania . CH 3 OH photocatalysis on TiO 2 (110) − has been investigated because of its important role in the enhancement of photoelectrocatalytic H 2 production, and because of its relation with the selective photooxidation of alcohols by TiO 2 based photocatalysts. , Finally, CH 3 OH photocatalysis provides an interesting contrast to H 2 O photocatalysis on titania.…”

“…Photocatalytic O–H bond dissociation is also relevant in the context of methanol oxidation to formaldehyde CH 2 O. ,,− This reaction involves the cleavage of the O–H bond (structures 1A and 1B ) and one C–H bond (structures 1B ′ and 1C ), resulting in the CH 2 O species adsorbed at a Ti cus site and two O br H groups. In the most widely accepted stepwise mechanism the O–H bond dissociation precedes the C–H bond dissociation.…”