Excited State Electronic Structure of Single-Site Vanadium Oxide Photocatalysts Supported on Mesoporous Silica

Evrard, Clint N.; Mahler, Andrew; Thompson, Lee M.

doi:10.1021/bk-2019-1331.ch015

Cited by 2 publications

(8 citation statements)

References 73 publications

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“…Previous work on the VO 4 /MCM-41 photocatalyst established the nature of the photoactivated catalytic center and is discussed here for completness . Analysis of the S 0 electronic structure revealed that the system was best characterized by vanadium +5 oxidation state.…”

Section: Resultsmentioning

confidence: 89%

“…Previous work on the VO 4 /MCM-41 photocatalyst established the nature of the photoactivated catalytic center and is discussed here for completness. 38 Analysis of the S 0 electronic structure Upon photoexcitation to the bright state, internal conversion 60−62 to S 1 is followed by geometric relaxation through stretching of the terminal V�O bond. The S 4 state crosses the S 1 state at V�O bond length of 1.62 Å (Figure 2).…”

Section: Resultsmentioning

confidence: 99%

“…The def2-TZVPD basis set was used for the vanadium center, while 6-31+G(d,p) was used for all other atoms (H, O, Si, C, and N). Initial coordinates were obtained from ref , in which a SiO 2 cluster was taken from a molecular dynamics snapshot . The structure was validated by comparing bond lengths, bond angles, and number and orientation of surface silanols within the 6 Å optimization region to structure found in the METASIL database. , The final structure on which calculations were performed consisted of a 68-atom cluster with valencies satisfied using hydrogen atoms (Figure ).…”

Section: Methodsmentioning

confidence: 99%

“…36,37 Additionally, the nature of the catalytic photoactive site as well as its formation has been analyzed both experimentally and theoretically, revealing that photon absorption oxidizes the terminal oxygen site and reduces the vanadium center, before undergoing intersystem crossing (ISC) to yield the reactive center. 11,38 However, to date the mechanism of the photocatalytic cycle has not been enumerated or validated. In particular, it is unclear whether the reaction proceeds by a radical mechanism, in which only the VO 4 site is involved and the amorphous silica acts as a support, or if the amorphous silica plays an important role in modifying activation barriers to control the selectivity.…”

Section: Introductionmentioning

confidence: 99%

“…In a particularly promising finding, Hu and co-workers used vanadium-doped mesoporous amorphous silica photocatalysts (VO 4 /MCM-41), which was demonstrated to give high methanol selectivity (88.4%) . The structure and mechanism by which vanadium-doped mesoporous amorphous silica photocatalysts function has been somewhat contested, focusing mainly on whether the VO 4 group is bound to the surface of mesoporous material, or whether it is incorporated into the amorphous framework. , Additionally, the nature of the catalytic photoactive site as well as its formation has been analyzed both experimentally and theoretically, revealing that photon absorption oxidizes the terminal oxygen site and reduces the vanadium center, before undergoing intersystem crossing (ISC) to yield the reactive center. , However, to date the mechanism of the photocatalytic cycle has not been enumerated or validated. In particular, it is unclear whether the reaction proceeds by a radical mechanism, in which only the VO 4 site is involved and the amorphous silica acts as a support, or if the amorphous silica plays an important role in modifying activation barriers to control the selectivity.…”

Section: Introductionmentioning

confidence: 99%

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Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Evrard

Thompson

2023

J. Phys. Chem. C

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Selective direct oxidation of methane to methanol is an important reaction for efficient utilization of natural gas. Photocatalytic routes for methane oxidation processes have advantages over traditional thermal catalysis as they can be more easily deployed at small-scale and remote off-grid sites, potentially reducing the rate of methane release and flaring. Hu and co-workers identified single-site vanadium-doped mesoporous amorphous silica photocatalysis which demonstrated methane conversion with high methanol selectivity (J. Photochem. Photobiol., A20132644855). Photocatalysts using amorphous SiO2 frameworks are significantly less studied than other heterogeneous photocatalysts. As a result, computational studies are important for elucidating mechanisms in order to gain fundamental understanding as to how amorphous SiO2-based photocatalysts can be used for partial oxidation of methane. This work uses density functional theory to establish the reaction barriers associated with the photocatalytic methane to methanol reaction combined with microkinetic modeling. The work is able to elucidate the role of terminal versus bridging oxygens at the photocatalytic center in enhancing photocatalytic efficiency. In addition, avoiding non-Mars–van Krevelen reaction pathways, in which the oxygen vacancy is filled before methanol is formed, is found to be important, owing to overstabilization of reaction intermediates leading to subsequent high energy reaction barriers. Finally, the amorphous SiO2 surface is found to modify reaction barriers by stabilizing intermediates through dispersion and preventing the photocatalytic center from forming the optimum coordination geometry.

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Section: Resultsmentioning

confidence: 89%

Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Evrard

Thompson

2023

J. Phys. Chem. C

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Reactivity of Group 5 and 6 Single-Site Photocatalysts for Partial Oxidation of Methane: Comparison of Chromium, Niobium, and Tungsten-Doped Mesoporous Amorphous Silica

Evrard

Thompson

2023

J. Phys. Chem. A

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Single-site transition-metal-doped photocatalysts can potentially be used for partial oxidation of methane (POM) at remote sites where natural gas is extracted and methane is often flared or released to the atmosphere. While there have been several investigations into the performance of vanadium, there has been no general survey of the performance of other metals. This work aims and examines Cr, Nb, and W metal oxide materials embedded in amorphous SiO 2 to determine the viability of each metal in catalyzing the POM. Photoexcited states are examined to determine the nature of the photoactivated species, and then the subsequent POM reaction mechanisms are elucidated. Using the calculated energies of reaction intermediates and transition states, the rate of methanol formation is evaluated through the use of a microkinetic model. The findings indicate that all three metals are potentially more suitable for catalyzing POM than vanadium but that niobium shows the most favorable energy profile.

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Excited State Electronic Structure of Single-Site Vanadium Oxide Photocatalysts Supported on Mesoporous Silica

Cited by 2 publications

References 73 publications

Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Mechanistic Origin of Selective Methane to Methanol Oxidation on Vanadium-Doped Mesoporous Amorphous Silica Photocatalyst

Reactivity of Group 5 and 6 Single-Site Photocatalysts for Partial Oxidation of Methane: Comparison of Chromium, Niobium, and Tungsten-Doped Mesoporous Amorphous Silica

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