Strong Photon Energy Dependence of the Photocatalytic Dissociation Rate of Methanol on TiO₂(110)

Abstract

Photocatalytic dissociation of methanol (CH 3 OH) on a TiO 2 (110) surface has been studied by temperature programmed desorption (TPD) at 355 and 266 nm. Primary dissociation products, CH 2 O and H atoms, have been detected. The dependence of the reactant and product TPD signals on irradiation time has been measured, allowing the photocatalytic reaction rate of CH 3 OH at both wavelengths to be directly determined. The initial dissociation rate of CH 3 OH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm, suggesting that CH 3 OH photocatalysis is strongly dependent on photon energy. This experimental result raises doubt about the widely accepted photocatalysis model on TiO 2 , which assumes that the excess potential energy of charge carriers is lost to the lattice via strong coupling with phonon modes by very fast thermalization and the reaction of the adsorbate is thus only dependent on the number of electron−hole pairs created by photoexcitation. ■ INTRODUCTIONSince the discovery of the photocatalytic splitting of water (H 2 O) on TiO 2 electrodes by Fujishima and Honda in 1972, 1 tremendous research efforts have gone into understanding the fundamental processes and in enhancing the photocatalytic efficiency of TiO 2 . 2 These efforts have been driven by the potential benefits to renewable energy, energy storage, and environmental cleanup. 3,4 In particular, TiO 2 has attracted much attention because of its applications in heterogeneous catalysis, photocatalysis, solar energy devices, etc. 5−14In an ideal photocatalyst, all photon energy invested in the generation of charge carriers would be available for redox chemistry. The higher the potential energy of the electron (or hole) the more reductive (or oxidative) capacity there is.15 But charge carrier thermalization is rapid. For example, Gundlach and co-workers used two photon photoemission (2PPE) to track thermalization following electron injection from two dyes adsorbed on rutile TiO 2 (110). 16,17 Fast initial decay of the 2PPE signal resulting from thermalization of the injected electron occurred on the 10 fs time scale. This result suggests that excess potential energy is lost to the lattice via strong coupling with phonon modes, thus reducing the potential advantage gained by the specificity in the absorption event. Additional evidence for the rapid thermalization of electrons comes from photoemission spectra of Ag clusters on rutile TiO 2 (110) 18 as a function of injection electron energy and from photoluminescence spectra of rutile TiO 2 (110) 19,20 with 3.35 eV photon irradiation. In both these studies, a constant energy of the emitted photons was observed (at and below the band gap energy3.05 eV), independent of the energy of the exciting electron or photon. Furthermore, experimental investigations indicate that the photodesorption yield and translational energy of O 2 from an O 2 -adsorbed TiO 2 (110) surface are independent of the excitation photon energy above 3.4 eV, but instead depend on the photon flux. 21...

Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges

“…To further explore the possible charge/energy transfer processes in CH 3 OH photolysis, Xu et al measured the initial reaction rates of CH 3 OH photolysis on R‐TiO 2 (110) at 355 and 266 nm using the yield of dissociated H b atoms by detecting the recombinative desorption of the H 2 O product at a high temperature ( Figure ). The initial rate of CH 3 OH photolysis (or H 2 O formation) at 266 nm is much faster than at 355 nm by approximately two orders of magnitude, namely, the initial rate is strongly dependent on the irradiation wavelength (or photon energy).…”

“…The unfilled squares indicate the rise times at 90% of the asymptotic values of the fits for both 355 and 266 nm photolysis. Reproduced with permission . Copyright 2013, American Chemical Society.…”

Fundamentals of TiO2 Photocatalysis. Consequences for Some Environmental Applications

“…10.4) [11,12], presumably because these events can sometimes be faster than thermalization [13]. A transfer to adsorbates of the phonon energy, which would be increased by thermalization, has also been suggested [12] and thought to possibly be capable of producing chemical changes.…”

“…10.4) [11,12], presumably because these events can sometimes be faster than thermalization [13]. A transfer to adsorbates of the phonon energy, which would be increased by thermalization, has also been suggested [12] and thought to possibly be capable of producing chemical changes. A strong interaction of adsorbates with the TiO 2 surface, such as in the cases of 2-hydroxybenzoic acid [11] (with respect to aliphatic alcohols with long chains) [12], should obviously be favorable to these transfers of charge or energy.…”

Coverage dependence of the level alignment for methanol on TiO2(110)

“…In particular, model surface science experiments have been conducted on the single crystal rutile TiO 2 (110) surface, under ultrahigh vacuum (UHV) and controlled coverage conditions. Experimentalists have characterized the electronic structure using ultraviolet, Xray, and two photon photoemission spectroscopy (UPS, XPS, and 2PP) [8][9][10][11][12][13][14][15], scanning tunnelling microscopy (STM) [14][15][16][17], and reaction products using temperature programmed desorption (TPD) [10,[18][19][20][21][22]. These experiments have shown that CH 3 OH can be photocatalytically dissociated on TiO 2 (110) to form CH 3 O and surface OH species [14,21], and selectively photooxidized to CH 2 O [19] and HCOOCH 3 [10,20].…”