Electronic Structure of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix: Implications for Catalysis and Photocatalysis

Kim, Tran; Hanning-Lee, Mark A.; Biswas, Abhijit; Stiegman, A. E.; Scott, Gary W.

doi:10.1021/ja00114a026

Cited by 73 publications

(83 citation statements)

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“…Furthermore, it Since the a 1 and e modes couple to the phosphorescent state, the emission spectrum should shed light on the vibronic structure in the present system. In the assignments published so far, explaining the various separations of the vibrational fine structure have failed since they have considered only the stretching of V = O t bond (a 1 (4) mode) [7,14,15] or attributed the progression to a stretching of the V-O-Si ligands [9].…”

Section: Resultsmentioning

confidence: 99%

“…The terminal oxygen reacts with a substrate in the photo-excited T 1 state [2,3,7]. Spectroscopy and molecular orbital calculation have revealed that the HOMO of the ground state consists of p orbitals of the basal oxygens, and LUMO's are doubly degenerate orbitals which originated from d orbitals of vanadium [8,9]. The first triplet state is assigned to the excitation from the HOMO to the LUMO [9].…”

Section: Introductionmentioning

confidence: 99%

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Active Center Induced by Vibronic Interactions in V2O5/SiO2

et al. 2009

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Active Center Induced by Vibronic Interactions in V2O5/SiO2

et al. 2009

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“…The difference between gas and liquid phase can be explained by the structure of the tantalum ethoxide precursor. Most simple tantalum alkoxides adopt binuclear structures (Ta 2 (OR) 8 (µ-OR) 2 ), with two bridging alkoxide ligands. Evaporation of the alkoxide occurs only after dissociation to monomers (Ta(OR) 5 ).…”

Section: Discussionmentioning

confidence: 99%

Supported Tantalum Oxide and Supported Vanadia-tantala Mixed Oxides: Structural Characterization and Surface Properties

et al. 2001

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Silica-, alumina-, and zirconia-supported tantalum oxide and supported vanadia-tantala mixed oxide catalysts were investigated by XRD, UV-vis-DR, FTIR, and FT-Raman spectroscopy to determine the nature of the surface tantala species. Two types of surface TaO x species were identified. On the SiO 2 support, at low coverages tetrahedral TaO 4 species exist, with a TadO bond and three bridging Ta-O-support bonds. At higher coverages, octahedral TaO 6 species occur. On Al 2 O 3 and ZrO 2 both surface species exist already at considerably low Ta loading. The acidic properties of the catalysts were investigated by an FTIR study of adsorbed pyridine. Supported tantala is clearly more Lewis acidic in comparison to vanadia. The oxidation of methanol was used to probe the catalytic performance of the supported oxide species and from the activity and product distribution it is concluded that supported tantala predominantly shows acidic over redox behavior.

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“…The latter is of importance in order to better understand vanadium oxide-support effects. For these reasons, the molecular structures of supported VO x on a variety of support materials, under various conditions and over a broad range of vanadium oxide loadings have been the subject of many spectroscopic studies including IR [8][9][10][11][12][13][14][15], Raman [15][16][17][18][19][20][21][22][23][24], UV-vis-NIR [25][26][27][28][29] and EXAFS [30][31][32][33][34][35][36].…”

Section: Introductionmentioning

confidence: 99%

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

Keller

Visser

Soulimani

et al. 2007

Vibrational Spectroscopy

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The effect of hydration on the molecular structure of silica-supported vanadium oxide catalysts with loadings of 1-16 wt.% V has been systematically investigated by infrared, Raman, UV-vis and EXAFS spectroscopy. IR and Raman spectra recorded during hydration revealed the formation of V-OH groups, characterized by a band at 3660 cm À1. Hydroxylation was found to start instantaneously upon exposure to traces of water, reflecting a very high sensitivity of the supported vanadium oxide catalysts for H 2 O. Further hydration resulted in the appearance of a V-O-V vibration band located around 700 cm À1 pointing to the formation of di-or polymeric species. EXAFS analysis at 77 K indicated structural changes as the oxygen coordination changed from four to five. Moreover, a VÁ Á ÁV contribution was detected for the hydrated species. The IR, Raman and UV-vis data suggested a pyramidal anchoring of the dehydrated VO x species, whereas, the EXAFS data pointed to the presence of single V-O-Si bonded VO x species. This difference is attributed to water condensation effects at 77 K during EXAFS acquisition, resulting in a partial re-hydroxylation of the dehydrated samples, as confirmed by complementary IR and Raman analysis. as well as literature led to a reaction scheme in which a monomeric VO x species anchored by three Si-O-V bonds to the silica support (pyramidal-type structure) is transformed into a monomeric VO x species anchored by one Si-O-V bond (umbrella-type structure) by partial hydration of the catalyst material. This results in the formation of both V-O-H and Si-O-H bonds. At higher water pressures, larger vanadium oxide clusters are formed due to full hydration of the catalyst surface and a de-attachment of the vanadium oxide from the support surface. The results of this study provide evidence, that an umbrella-type structure (i.e., Si-O-V O(OH) 2 ) could be present under catalytic conditions where H 2 O is a reaction product (e.g., partial oxidation of methanol to formaldehyde and oxidative dehydrogenation of alkanes). In other words, both the pyramidal ((Si-O) 3 -V O) and the umbrella (Si-O-V O(OH) 2 ) model can exist at a support surface, their relative ratio depending on the hydration degree of the catalyst material. This study also illustrates that a corroborative characterization requires the use of multiple spectroscopic techniques applied at the same samples under almost identical measuring conditions. #

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Electronic Structure of Discrete Pseudotetrahedral Oxovanadium Centers Dispersed in a Silica Xerogel Matrix: Implications for Catalysis and Photocatalysis

Cited by 73 publications

References 0 publications

Active Center Induced by Vibronic Interactions in V2O5/SiO2

Active Center Induced by Vibronic Interactions in V2O5/SiO2

Supported Tantalum Oxide and Supported Vanadia-tantala Mixed Oxides: Structural Characterization and Surface Properties

Hydration effects on the molecular structure of silica-supported vanadium oxide catalysts: A combined IR, Raman, UV–vis and EXAFS study

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