Photocatalytic hydrogen production from liquid methanol and water

Kawai, Tomoji; Sakata, Tadayoshi

doi:10.1039/c39800000694

Cited by 383 publications

(270 citation statements)

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“…Previous studies 11,33 have demonstrated that noble-metalmodified TiO 2 can indeed enhance the efficiency of hydrogen production with a methanol−water mixture considerably. This implies that hydrogen migrations from TiO 2 to a noble metal surface could facilitate hydrogen production.…”

Section: * S Supporting Informationmentioning

confidence: 97%

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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Section: * S Supporting Informationmentioning

confidence: 97%

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

et al. 2013

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Section: ■ Introductionmentioning

confidence: 99%

“…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.…”

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confidence: 99%

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

et al. 2012

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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...

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“…44 It has been reported that methanol is an effective sacrificial agent for hydrogen production. 46,47 Clearly, the photochemistry of methanol on TiO 2 plays a significant role in this process. However, the mechanism of photocatalytic chemistry of methanol and water on TiO 2 surfaces remains unclear.…”

Section: Photocatalysis Of Methanol On Tio 2 (110)mentioning

confidence: 99%

Surface photochemistry probed by two-photon photoemission spectroscopy

Zhou

Ren

et al. 2012

Energy Environ. Sci.

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Two-photon photoemission (2PPE) has been widely used in the study of electronic structure and dynamics of unoccupied electronic states on different types of surfaces and interfaces. Since 2PPE probes electronically excited states, it should be sensitive to surface excited electronic structure changes that accompany surface chemical reactions. Therefore, this method could potentially be used to study the kinetics and dynamics of surface chemical reactions as well as surface photocatalysis. In this article, we briefly review recent progress made in the study of surface photochemistry and photocatalysis using the time-dependent 2PPE (TD-2PPE) method. A few examples are given to demonstrate the application of this method in probing surface photochemistry and photocatalysis, particularly photocatalysis of methanol on TiO 2 surfaces. Since many problems associated with surface photochemistry and surface photocatalysis are related to energy and environmental issues, the 2PPE technique could have important applications in the study of the fundamental problems in energy and environmental sciences.

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Photocatalytic hydrogen production from liquid methanol and water

Cited by 383 publications

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

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

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

Surface photochemistry probed by two-photon photoemission spectroscopy

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Photocatalytic hydrogen production from liquid methanol and water

Cited by 383 publications

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

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

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

Surface photochemistry probed by two-photon photoemission spectroscopy

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

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

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