Methyl Formate Production on TiO<sub>2</sub>(110), Initiated by Methanol Photocatalysis at 400 nm

Guo, Qing; Xu, Chenbiao; Yang, Wenshao; Ren, Zefeng; Ma, Zhibo; Dai, Dongxu; Minton, Timothy K.

doi:10.1021/jp401613s

Cited by 101 publications

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“…The laser flux was thus 9. 49 were observable but not significant. TPD measurements were carried out with a heating rate of 2 K/s.…”

Section: ■ Experimental Detailsmentioning

confidence: 71%

“…Because the increase in surface temperature during laser irradiation is quite small (only about 5 K), both dissociation steps must be photoinduced and not thermally activated. CH 2 O may be desorbed by light; 49 thus, monitoring the CH 2 O product yield is not the best approach to measure the product formation rate, as the CH 2 O production rate would not be linked directly with the CH 3 OH depletion rate. Another major product from CH 3 OH dissociation on TiO 2 (110) is atomic hydrogen on the BBO sites, resulting from steps (1) and (2).…”

Section: ■ Resultsmentioning

confidence: 99%

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

Yang

Ren

et al. 2013

J. Am. Chem. Soc.

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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...

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“…The laser flux was thus 9. 49 were observable but not significant. TPD measurements were carried out with a heating rate of 2 K/s.…”

Section: ■ Experimental Detailsmentioning

confidence: 71%

Section: ■ Resultsmentioning

confidence: 99%

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

Yang

Ren

et al. 2013

J. Am. Chem. Soc.

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“…A previous study in our lab implies that multiphoton effects are not important at this photon flux. 53 The laser irradiation area on the surface was the elliptical projection of a 6 mm diameter laser beam with an angle of ∼30°between the surface parallel and laser direction. The TiO 2 (110) surface was annealed in vacuum at 850 K for 20 min between TPD experiments to flatten and clean the surface.…”

Section: −12mentioning

confidence: 99%

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

Yang

Guo

et al. 2013

J. Phys. Chem. Lett.

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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 ...

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“…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.…”

Section: * S Supporting Informationmentioning

confidence: 99%

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

et al. 2013

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It is well established that adding methanol to water could significantly enhance H 2 production by TiO 2 . Recently, we have found that methanol can be photocatalytically dissociated on TiO 2 (110) at 400 nm via a stepwise mechanism. However, how molecular hydrogen can be formed from the photocatalyzed methanol/ TiO 2 (110) surface is still not clear. In this work, we have investigated deuterium formation from photocatalysis of the fully deuterated methanol (CD 3 OD) on TiO 2 (110) at 400 nm using a temperature programmed desorption (TPD) T iO 2 has attracted enormous interest in heterogeneous catalysis, photocatalysis, solar energy devices, etc. 1−8 Photocatalytic water splitting by TiO 2 is especially attractive because of its potential application in clean hydrogen production. 9 A previous study found that pure TiO 2 is not active for hydrogen production from pure water. 10 Adding methanol to pure water, however, can dramatically enhance hydrogen production. 11 Because of the apparently crucial role in hydrogen production, the photochemistry of methanol has been extensively investigated on single crystal TiO 2 surfaces 12−31 and TiO 2 powders. 32−35 Although investigations on powder TiO 2 with methanol steam 32−35 and a water−methanol mixture 11 show that hydrogen can be produced from methanol by reaction,the detailed mechanism of gaseous hydrogen formation from methanol photocatalysis on TiO 2 remains unknown. In a recent study, 28 we have shown that the elementary photocatalytic dissociation of CH 3 OH on TiO 2 (110) without any other coadsorbed species occurs in a stepwise mechanism in which the O−H dissociation proceeds first and is then followed by C−H dissociation to form formaldehyde (CH 2 O) with only methanol adsorption on TiO 2 (110),where Ti 5C refers to a five-coordinated Ti 4+ (Ti 5C ) site, and H BBO refers to an H atom adsorbed on a bridge-bonded oxygen (BBO) site on the TiO 2 (110) surface. From our experiment, we have found that both dissociation steps are photoinitiated. This means that at low temperature photocatalytic dissociation products from CH 3 OH, i.e., CH 2 O and H atoms on BBO sites, are all left on the TiO 2 surface after laser irradiation, whereas Henderson and co-workers found that molecular CH 3 OH is not photoactive on TiO 2 (110) using a Hg lamp as the surface photocatalysis source. 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. Further oxidation of CH 3 OH on TiO 2 (110) to form methyl formate has also been observed in three different laboratories. 36−38 However, the important question of how hydrogen molecules are formed from the photocatalysis of methanol on TiO 2 (110) remains unanswered. In order to understand the mechanism of hydrogen formation, the photocatalytic chemistry of CD ...

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Methyl Formate Production on TiO₂(110), Initiated by Methanol Photocatalysis at 400 nm

Cited by 101 publications

References 54 publications

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

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

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

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

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Methyl Formate Production on TiO2(110), Initiated by Methanol Photocatalysis at 400 nm

Cited by 101 publications

References 54 publications

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

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

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

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

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Methyl Formate Production on TiO₂(110), Initiated by Methanol Photocatalysis at 400 nm

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

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

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

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