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...

Thermally stable Au single-atoms supported by monolayered CuO grown at Cu(110) have been successfully prepared. The charge transfer from the CuO support to single Au atoms is confirmed to play a key role in tuning the activity for CO oxidation. Initially, the negatively charged Au single-atom is active for CO oxidation with its adjacent lattice O atom depleted to generate an O vacancy in the CuO monolayer. Afterward, the Au single-atom is neutralized, preventing further CO reaction. The produced O vacancy can be healed by exposure to O at 400 K and accordingly the reaction activity is restored.

Previous observations of methyl formate (HCOOCH 3 ) during the photo-oxidation of methanol (CH 3 OH) on TiO 2 catalysts suggested that photocatalysis on TiO 2 could be used to build up complex molecules from a single precursor. We have investigated the mechanism of HCOOCH 3 formation by irradiating a CH 3 OH-adsorbed TiO 2 (110) surface with 400 nm light at low surface temperatures. Through the detection of volatile products after irradiation by temperature programmed desorption, we have found, as previously reported [Phillips et al. J. Am. Chem. Soc. 2013, 135, 574−577] that HCOOCH 3 is formed by the cross-coupling reaction of CH 3 O and CH 2 O, which are products of the first and second dissociation steps, respectively, in the stepwise photocatalysis of CH 3 OH on TiO 2 (110). Unlike the previous study, we have observed the photocatalytic production of HCOOCH 3 without preoxidation of the surface, and we have concluded that the final reaction step to produce HCOOCH 3 (i.e., the cross-coupling reaction of CH 2 O with CH 3 O) does not involve a transient HCO intermediate.

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 ...

Photocatalysis of methanol (CH3OH) on anatase (A)-TiO2(101) has been investigated using temperature programmed desorption (TPD) method with 266 nm light at low surface temperatures. Experimental results show that CH3OH adsorbs on the A-TiO2(101) surface predominantly in molecular form, with only a small amount of CH3OH in dissociated form. Photocatalytic products, formaldehyde (CH2O) and methyl formate (HCOOCH3), have been detected under 266 nm light irradiation. In addition to H2O formation, H2 product is also observed by TPD spectroscopy. Experimental results indicate that H2 product is formed via thermal recombination of H-atoms on the BBO sites from photocatalysis of CH3OH on the A-TiO2(101) surface, and H2 production on the A-TiO2(101) surface is significantly more efficient than that on the rutile (R)-TiO2(110) surface.