Stepwise Photocatalytic Dissociation of Methanol and Water on TiO₂(110)

Abstract

ABSTRACT:We have investigated the photocatalysis of partially deuterated methanol (CD 3 OH) and H 2 O on TiO 2 (110) at 400 nm using a newly developed photocatalysis apparatus in combination with theoretical calculations. Photocatalyzed products, CD 2 O on Ti 5c sites, and H and D atoms on bridge-bonded oxygen (BBO) sites from CD 3 OH have been clearly detected, while no evidence of H 2 O photocatalysis was found. The experimental results show that dissociation of CD 3 OH on TiO 2 (110) occurs in a stepwise manner in which the O−H dissociation proceeds first and is then followed by C−D dissociation. Theoretical calculations indicate that the high reverse barrier to C−D recombination and the facile desorption of CD 2 O make photocatalytic methanol dissociation on TiO 2 (110) proceed efficiently. Theoretical results also reveal that the reverse reactions, i.e, O−H recombination after H 2 O photocatalytic dissociation on TiO 2 (110), may occur easily, thus inhibiting efficient photocatalytic water splitting. ■ INTRODUCTIONTitanium dioxide has been extensively investigated as a catalyst or photocatalyst, 1−11 particularly in applications involving photodegradation of organic molecules and water splitting, 5,12,13 which have important implications in environmental remediation and clean energy. Pure TiO 2 is apparently not photocatalytically active for splitting water to produce hydrogen, 14 but the addition of methanol to water can dramatically enhance the photocatalytic activity for hydrogen production. 15 Therefore, understanding the key differences between the photocatalytic chemistry of methanol and water on a model TiO 2 surface at the molecular level may provide valuable insight into the dynamics of photocatalysis that would enhance efforts for developing new and efficient photocatalysts for water splitting.Theoretical and experimental studies often focus on TiO 2 (110) as a model surface, 6,16 with the methanol/ TiO 2 (110) system serving as a model for photocatalysis on TiO 2 . 17−20 Henderson and co-workers 18 conducted a temperature-programmed desorption (TPD) study of CH 3 OH on TiO 2 (110) and concluded that the majority of the CH 3 OH molecules are adsorbed in molecular form. This conclusion is consistent with a scanning tunneling microscopy study by Dohnalek et al. 10 that showed that methanol molecules are adsorbed molecularly on the Ti 5c sites and are dissociated only at bridge-bonded oxygen (BBO) vacancy sites. The photocatalysis of CH 3 OH on TiO 2 (110) was investigated in a twophoton photoemission (2PPE) experiment, which inferred the presence of an excited electronic state on the surface. 20,21 Zhou et al. attributed this surface state to a photocatalytic dissociated state of methanol using a time-dependent 2PPE (TD-2PPE) technique. 22 They also used a combination of photoexcitation with STM and found that 400 nm light could induce dissociation of methanol on the surface, and they assigned the dissociated state as methoxy (CH 3 O) on a Ti 5c site and a hydrogen atom on a BBO site. Shen and Hend...

Suppression of Photoinduced BBO Defects Generation on TiO2(110) by Water

“…The "valley" between the first and second layer states remains well defined, implying that the regular Ti 4+ rows are nearly not affected by UV irradiation. The first layer peak shifts to lower temperature and sharpens after irradiation, that is due to the increased intensity of H atoms on bridging oxygen rows [24]. The increased 450 K peak intensity indicates that controllable creation of defects on TiO 2 (110)-(1×1) by UV light is feasible.…”

Molecular Hydrogen Formation from Photocatalysis of Methanol on TiO2(110)

“…where Ti 5C refers to a five-coordinated 26 In our experiment, 28,36 we used a femtosecond laser source that has considerably higher photon flux than the Hg lamp used in ref 26, in addition to the highly sensitive mass spectrometric detector with a vacuum background of 1 × 10 −12 Torr. We believe this makes our experiment much more sensitive in detecting TPD products.…”

“…It is interesting to point out that oxygen vacancy will be created on the surface after recombination desorption of D 2 O from hydroxyl groups on BBO rows. 28,29 In order to determine if the molecular D 2 product could be formed, TPD spectra at m/z = 4 (D 2 + ) were measured after 400 nm laser irradiation. Figure 1B shows TPD spectra for four different laser irradiation periods, corresponding to four different m/z = 4 (D 2 + ) sources.…”

Structural Dependence of Competitive Adsorption of Water and Methanol on TiO2 Surfaces

“…[6][7][8] Water and methanol are respectively the reactant and sacrificial agent in the photocatalytic water reduction reaction catalyzed by TiO 2 , thus their individual adsorptions on various TiO 2 single crystals have been studied on TiO 2 single crystal surfaces using various techniques including scanning tunneling microscopy, thermal desorption spectra (TDS), X-ray photoelectron spectroscopy (XPS) and theoretical calculations. [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] Since water and methanol co-exist as the photocatalytic reactants, the competitive adsorption of water and methanol on TiO 2 surfaces must play an important role in the photocatalytic activity. However, only limited results have been reported on the competitive adsorption of water and methanol on TiO 2 single crystal surfaces.…”