Redox Isotherms for Vanadia Supported on Zirconia

Shah, Parag R.; Vohs, John M.; Gorte, Raymond J.

doi:10.1007/s10562-008-9539-9

Cited by 12 publications

(8 citation statements)

References 38 publications

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“…This 61% transformation of V 5+ → V 4+ after cyclohexane reduction is similar to that of our previous study of 2 ML VO x /α-TiO 2 (110), for which H 2 was used as the reductant and where we saw 100% transformation . It is not surprising to observe this partial reduction of V 5+ to V 4+ in an ambient condition because of the coexistence of multiple chemical states of V. This has similarly been reported for V on silica, alumina, and zirconia, , which, like titania, are also difficult to reduce.…”

supporting

confidence: 89%

Catalysts Transform While Molecules React: An Atomic-Scale View

Feng

et al. 2013

J. Phys. Chem. Lett.

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We explore how the atomic-scale structural and chemical properties of an oxide-supported monolayer (ML) catalyst are related to catalytic behavior. This case study is for vanadium oxide deposited on a rutile α-TiO2(110) single-crystal surface by atomic layer deposition (ALD) undergoing a redox reaction cycle in the oxidative dehydrogenation (ODH) of cyclohexane. For measurements that require a greater effective surface area, we include a comparative set of ALD-processed rutile powder samples. In situ single-crystal X-ray standing wave (XSW) analysis shows a reversible vanadium oxide structural change through the redox cycle. Ex situ X-ray photoelectron spectroscopy (XPS) shows that V cations are 5+ in the oxidized state and primarily 4+ in the reduced state for both the (110) single-crystal surface and the multifaceted surfaces of the powder sample. In situ diffuse reflectance infrared Fourier transform spectroscopy, which could only achieve a measurable signal level from the powder sample, indicates that these structural and chemical state changes are associated with the change of the V═O vanadyl group. Catalytic tests on the powder-supported VOx revealed benzene as the major product. This study not only provides atomic-scale models for cyclohexane molecules interacting with V sites on the rutile surface but also demonstrates a general strategy for linking the processing, structure, properties, and performance of oxide-supported catalysts.

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confidence: 89%

Catalysts Transform While Molecules React: An Atomic-Scale View

Feng

et al. 2013

J. Phys. Chem. Lett.

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“…Shah et al studied redox isotherms of vanadia supported on zirconia for vanadia loadings of 2.5 and 5 wt %, correponding to 2.9, and 5.8 V/nm 2 , respectively. Since the fully vanadia loaded surfaces host about 8 V/nm 2 (Table ), these samples correspond to Θ = 0.36 and 0.72, respectively, and fall in between our Θ = 0.25, 0.5, and 1.0 models.…”

Section: Resultsmentioning

confidence: 99%

“…Since the fully vanadia loaded surfaces host about 8 V/nm 2 (Table ), these samples correspond to Θ = 0.36 and 0.72, respectively, and fall in between our Θ = 0.25, 0.5, and 1.0 models. The numbers given in ref are free energies and refer to one O 2 molecule. With additional data for the corresponding enthalpies from ref and assuming that the entropy change is the same for the different materials (−212 J/molK), we arrive at the following enthalpies of reduction per 1/2 O 2 molecule (eq ) from refs and −225 kJ/mol (2.5%VZr), −129 kJ/mol (5%VZr), −113 kJ/mol (ZrV 2 O 7 ), −143 kJ/mol (V 2 O 5 ), which can be compared with the calculated DFT(PW91) data in Table , that is, about −390 to −410 kJ/mol for small (isolated and dimeric) species, about −150 to −190 kJ/mol for polymeric species, and −186 kJ/mol for V 2 O 5 (ref ).…”

Section: Resultsmentioning

confidence: 99%

“…The numbers given in ref are free energies and refer to one O 2 molecule. With additional data for the corresponding enthalpies from ref and assuming that the entropy change is the same for the different materials (−212 J/molK), we arrive at the following enthalpies of reduction per 1/2 O 2 molecule (eq ) from refs and −225 kJ/mol (2.5%VZr), −129 kJ/mol (5%VZr), −113 kJ/mol (ZrV 2 O 7 ), −143 kJ/mol (V 2 O 5 ), which can be compared with the calculated DFT(PW91) data in Table , that is, about −390 to −410 kJ/mol for small (isolated and dimeric) species, about −150 to −190 kJ/mol for polymeric species, and −186 kJ/mol for V 2 O 5 (ref ). Although the PW91 results in Table are overestimated as we already know from previous studies for V 2 O 5 , , they are in reasonable agreement with the trend in the above experimental data.…”

Section: Resultsmentioning

confidence: 99%

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Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

2009

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The surface structure of vanadia supported on zirconia is examined by density functional theory with periodic boundary conditions. Adsorption of V 2 O 5 and H 2 O on the (101) and (001) surfaces of tetragonal zirconia is studied for different loadings up to (V 2 O 5 ) 4 or V 2 O 5 • 6H 2 O per eight Zr surface sites (monolayer coverage). The considered surface species include vanadia monomers, dimers, and polymers on water-free surfaces as well as different combinations of coadsorbed vanadia, hydroxyls, and water. Calculated vibrational spectra do not show unique features that could be used for identifying certain surface species. Statistical thermodynamics is used to evaluate the relative stability of different surface structures as function of vanadia loading, water partial pressure, and temperature (surface phase diagrams). The presence of water has a strong influence on the surface species. Under water free conditions and for Θ(V 2 O 5 ) ) 0.25 vanadia is present as a monomeric species on the (001) surface and as dimeric species on the (101) surface. For water partial pressures typical of oxidative dehydrogenation of propane (10 -2 bar, 775 K), hydrolysis of the dimeric species on the (101) surface is observed. The presence of water also affects the reactivity in partial oxidations as characterized by oxygen defect formation or hydrogen attachment energies. At Θ(V 2 O 5 ) ) 0.25 the presence of water stabilizes less reactive species, whereas at high vanadia coverage more reactive species are present.

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“…S4 †), the rst neighbor structure around V in the OX sample is best reproduced by 2.2 V-O bonds of 1.62 Å and 1.7 V-O bonds of 1.82 Å (Table 1), suggesting a total coordination number N ¼ 4. The small Debye-Waller factor, s 2 (<10 À3 Å2 compared to 5.7 Â 10 À3 from V 2 O 5 analysis), may be indicative of somewhat underestimated value of N, given the strong correlation between s 2 and N. However, the large H/S ratio of $0.55 for the OX VO X indicates a dominant low symmetry coordination conguration around V. Previous studies 14,[37][38][39][40] have suggested that oxide-supported VO X in a V 5+ oxidation state has a VO 4 tetrahedral structure with one V]O bond, one V-O-M support bond (M represents the support metal cation) and two V-O-V bonds. Although EXAFS studies 30,[41][42][43][44] of vanadium oxides on different supports suggested that the VO 4 unit has one V]O bond $1.6 Å and multiple V-O bonds $1.8 Å, our study shows that the OX contains VO 4 tetrahedral units with two short V-O bonds.…”

Section: Resultsmentioning

confidence: 87%