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Israel E. Wachs - One of the best experts on this subject based on the ideXlab platform.
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Catalysis science of supported Vanadium Oxide catalysts
Dalton transactions (Cambridge England : 2003), 2013Co-Authors: Israel E. WachsAbstract:Supported Vanadium Oxide catalysts contain a Vanadium Oxide phase deposited on a high surface area Oxide support (e.g., Al2O3, SiO2, TiO2, etc.) and have found extensive applications as oxidation catalysts in the chemical, petroleum and environmental industries. This review of supported Vanadium Oxide catalysts focuses on the fundamental aspects of this novel class of catalytic materials (molecular structures, electronic structures, surface chemistry and structure-reactivity relationships). The molecular and electronic structures of the supported Vanadium Oxide phases were determined by the application of modern in situ characterization techniques (Raman, IR, UV-vis, XANES, EXAFS, solid state (51)V NMR and isotopic oxygen exchange). The characterization studies revealed that the supported Vanadium Oxide phase consists of two-dimensional surface vanadia sites dispersed on the Oxide supports. Corresponding surface chemistry and reactivity studies demonstrated that the surface vanadia sites are the catalytic active sites for oxidation reactions by supported vanadia catalysts. Combination of characterization and reactivity studies demonstrate that the Oxide support controls the redox properties of the surface vanadia sites that can be varied by as much as a factor of ~10(3).
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Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification
Topics in Catalysis, 2002Co-Authors: Xingtao Gao, Israel E. WachsAbstract:Highly dispersed, multilayered surface metal Oxide catalysts (V2O5/MO x /SiO2, M = Ti(IV), Zr(IV) or Al(III)) were successfully synthesized by taking into account various factors that govern the maximum dispersion of metal Oxide species on silica. The characterization results revealed that the molecular structures of the surface Vanadium Oxide species on the modified supports are a strong function of environmental conditions. The surface Vanadium Oxide species under dehydrated conditions are predominantly isolated VO4 units, similar to the dehydrated V2O5/SiO2 catalysts. Upon hydration, the surface Vanadium Oxide species on the modified supports consist of polymerized VO5/VO6 units and/or less polymerized (VO3) n species, which depend on the vanadia content and the specific second metal Oxide loading. The surface V cations are found to preferentially interact with the surface metal (Ti, Zr or Al) Oxide species on silica. The V(V) cations in the dehydrated state appear to possess both oxygenated ligands of Si(IV)–O− and M–O−. Consequently, the reducibility and catalytic properties of the surface Vanadium Oxide species are significantly altered. The turnover frequencies of the surface VO4 species on these modified supports for methanol oxidation to redox products (predominantly formaldehyde) increase by more than an order of magnitude relative to the unmodified V2O5/SiO2 catalysts. These reactivity enhancements are associated with the substitution of Si(IV)–O− oxygenated ligands by less electronegative M–O− ligands in the O=V(–O–support)3 structure, which strongly suggests that the bridging V–O–support bonds play a key role in determining the reactivity of the surface Vanadium Oxide species on Oxide supports.
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Reactivity of supported Vanadium Oxide catalysts: The partial oxidation of methanol
Journal of Catalysis, 1994Co-Authors: Goutam Deo, Israel E. WachsAbstract:The partial oxidation of methanol was used to probe the reactivity of the surface Vanadium Oxide redox sites present in supported Vanadium Oxide catalysts. Formaldehyde was the main oxidation product for all the supported Vanadium Oxide catalysts operating under differential reactor conditions. The methanol oxidation turnover frequency (TOF) of the surface Vanadium Oxide phase varies by three orders of magnitude when the support is changed from ZrO2/TiO2 (100 s−1) to SiO2 (10−3). The TOF of the surface Vanadium Oxide phase supported on Nb2O5 (10−1 s−1) and Al2O3 (10−2 s−1) have intermediate values with the surface Vanadium Oxide phase on Nb2O5 having a TOF very close to ZrO2 or TiO2. The TOF of the surface Vanadium Oxide phase on all the Oxide supports is essentially independent of the Vanadium Oxide loading below monolayer coverage. The similar structures of the surface Vanadium Oxide phase on the different Oxide supports as well as the independence of the TOF with respect to Vanadium Oxide surface coverage suggests that a structural difference is not responsible for the difference in reactivity of the various supported Vanadium Oxide catalysts. Similar activation energies are observed for all the supported Vanadium Oxide catalysts (19.6 ± 2.3 kcal/mol) which correspond to the CH bond breaking of the surface methoxy species to form formaldehyde. The similar activation energies and different TOFs of the supported Vanadium Oxide catalysts with respect to the Oxide support imply that the specific Oxide support influences the Arrhenius pre-exponential factor. In situ Raman studies during methanol oxidation suggest that the pre-exponential factor is determined by the number of participating surface Vanadium Oxide sites. The importance of the activity per surface Vanadium Oxide site in determining the pre-exponential factor was not investigated and may also be significant. The TOF for methanol oxidation is not related to the terminal VO bond strength, but appear to be related to the reducibility (Tmax) of the supported Vanadium Oxide catalysts. It is proposed that the number of participating surface Vanadium Oxide sites is most probably related to the reducibility of the V0Support bond since this bridging bond strength controls the reducibility of the supported Vanadium Oxide catalysts.
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Effect of Additives on the Structure and Reactivity of the Surface Vanadium Oxide Phase in V2O5/TiO2 Catalysts
Journal of Catalysis, 1994Co-Authors: Goutam Deo, Israel E. WachsAbstract:Additives on a 1% V2O5/TiO2 catalyst exhibit two types of interactions with the surface Vanadium Oxide phase which are observed by Raman spectroscopy under dehydrated conditions and methanol oxidation. Under dehydration conditions, noninteracting additives (WO3, Nb2O5, and SiO2) coordinate directly to the Oxide support without significantly interacting with the surface Vanadium Oxide phase. Furthermore, the effect of the noninteracting additives on the surface Vanadium Oxide phase is independent of the order of preparation or precursor used. These noninteracting additives do not affect the methanol oxidation activity and selectivity. Interacting additives (K2O and P2O5), however, directly coordinate with the surface Vanadium Oxide phase. Addition of K2O progressively titrates the surface Vanadium Oxide sites, as observed from the changes in the structure and reactivity of the surface Vanadium Oxide phase. The effect of P2O5 on the surface Vanadium Oxide phase depends on the concentration and sequence of preparation. Addition of higher concentrations of P2O5 forms Vanadium phosphate compounds, and results in a change in methanol oxidation activity and selectivity. The addition of 1% V2O5 to a 5% P2O5/TiO2 sample, however, does not show any evidence of compound formation, but a part of the surface Vanadium Oxide phase appears to be titrated. Under ambient conditions, the additives change the pH at point of zero charge of the surface moisture layer which controls the structure of the surface Vanadium Oxide layer. Thus, depending on the nature of the additive, interacting or noninteracting, the dehydrated structure and reactivity toward methanol oxidation of the surface Vanadium Oxide phase are affected or remain essentially unchanged, respectively.
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Molecular Structure-Reactivity Relationships of Supported Vanadium Oxide Catalysts
Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis Proceedings of the ACS Symposium on Structure-Activity Relationships in He, 1991Co-Authors: Goutam Deo, Israel E. WachsAbstract:Abstract The molecular structure of the surface Vanadium Oxide species present on different Oxide supports (TiO 2 , γ-Al 2 O 3 , and SiO 2 ) were determined by laser Raman spectroscopy and 51 V solid state NMR under hydrated and dehydrated conditions. The structure of the Vanadium Oxide species changes with dehydration and a four coordinated Vanadium Oxide species with a short terminal bond was present on all Oxide supports considered. The reactivity of the supported Vanadium Oxide catalysts was determined via the methanol oxidation reaction. Correlation of the structure and reactivity data indicate the strength of the bridging, Vanadium-oxygen-support, bond to be controlling the activity of these supported Vanadium Oxide catalysts. The effect of promoters/impurities on 1% V 2 O 5 /TiO 2 catalyst depends on their acid/base nature. Basic promoters titrate the Vanadium Oxide site and destroy the Vanadium-oxygen-support bond of the parent 1% V 2 O 5 /TiO 2 . Acidic promoters/impurities coordinate to the support and do not show any appreciable change to the structure, reactivity, and the Vanadium-oxygen-support bond of the parent 1% V 2 O 5 /TiO 2 .
Li Qiang Mai - One of the best experts on this subject based on the ideXlab platform.
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Graphene decorated Vanadium Oxide nanowire aerogel for long-cycle-life magnesium battery cathodes
Nano Energy, 2015Co-Authors: Qin You An, Hyun Deog Yoo, Li Qiang Mai, Qiang Ru, Yifei Li, Shuo Chen, Yan YaoAbstract:We report graphene decorated hydrated Vanadium Oxide nanocomposite as an effective cathode material for long cycle-life Mg storage. Excellent electrochemical performance with specific capacity of 330mAhg-1at low rate and stable cycling of 200 cycles with 81% capacity retention at 1Ag-1was reported. Furthermore, the nanocomposite cathode shows a broad working temperature window from -30°C to 55°C with over 200mAhg-1capacity at 55°C (1.0Ag-1). The charge shielding effect of crystal water in the aerogel enhances the Mg2+insertion kinetics and the porous structure of aerogel allows easy access of electrolyte into the active material. The cycling performance, rate performance and broad temperature adaptability demonstrate that the graphene decorated Vanadium Oxide nanowire aerogel is a promising and attractive cathode material for practical Mg batteries.
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Vanadium Oxide nanowires for Li-ion batteries
Journal of Materials Research, 2011Co-Authors: Li Qiang Mai, Chunhua Han, Yanzhu LuoAbstract:Vanadium Oxide nanowires have gained increasing interest as the electrode materials for Li-ion batteries. This article presents the recent developments of Vanadium Oxide nanowire materials and devices in Li-ion batteries. First, we will describe synthesis and construction of Vanadium Oxide nanowires. Then, we mainly focus on the electrochemical performances of Vanadium Oxide nanowires, such as VO 2 , V 2 O 5 , hydrated Vanadium Oxides, LiV 3 O 8 , silver Vanadium Oxides, etc. Moreover, design and in situ characterization of the single nanowire electrochemical device are also discussed. The challenges and opportunities of Vanadium Oxide nanowire electrode materials will be discussed as a conclusion to push the fundamental and practical limitations of this kind of nanowire materials for Li-ion batteries.
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electrospun ultralong hierarchical Vanadium Oxide nanowires with high performance for lithium ion batteries
Nano Letters, 2010Co-Authors: Li Qiang Mai, Chunhua Han, Yanzhu Luo, Shiyong Zhao, Yunlong ZhaoAbstract:Ultralong hierarchical Vanadium Oxide nanowires with diameter of 100−200 nm and length up to several millimeters were synthesized using the low-cost starting materials by electrospinning combined with annealing. The hierarchical nanowires were constructed from attached Vanadium Oxide nanorods of diameter around 50 nm and length of 100 nm. The initial and 50th discharge capacities of the ultralong hierarchical Vanadium Oxide nanowire cathodes are up to 390 and 201 mAh/g when the lithium ion battery cycled between 1.75 and 4.0 V. When the battery was cycled between 2.0 and 4.0 V, the initial and 50th discharge capacities of the nanowire cathodes are 275 and 187 mAh/g. Compared with self-aggregated short nanorods synthesized by hydrothermal method, the ultralong hierarchical Vanadium Oxide nanowires exhibit much higher capacity. This is due to the fact that self-aggregation of the unique nanorod-in-nanowire structures have been greatly reduced because of the attachment of nanorods in the ultralong nanowires,...
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FTIR study of Vanadium Oxide nanotubes from lamellar structure
Journal of Materials Science, 2004Co-Authors: Wen Chen, Li Qiang Mai, Jun Feng Peng, Quanyao ZhuAbstract:Vanadium Oxides and compounds derived from them have attracted interest because of their potential use as cathode materials for rechargeable lithium batteries [1–3] and their important role in catalysis [4, 5]. Depending on the nature of the species present in the reaction medium, Vanadium Oxides may exhibit a wide range of morphologies (lamellar structures, molecular clusters, etc.) [6, 7]. Among them, the Vanadium Oxide nanotubes are unique because of their strongly anisotropic geometry, which is associated with interesting chemical and physical properties. Recently, Nesper and co-workers synthesized novel Vanadium Oxide nanotubes using Vanadium alkOxides as starting materials in a Chimia Douce route [7, 8]. However, Vanadium alkOxides are very expensive. Therefore, in the present work, Vanadium Oxide nanotubes have been prepared by reacting Vanadium Oxide, instead of Vanadium alkOxides, with a structure-directing agent followed by hydrothermal treatment. FTIR investigation was performed to study the structural changes of Vanadium Oxide before and after hydrothermal treatment to get a better understanding of nanotube formation. 10 mmol V2O5 (99.5%) and 10 mmol 1-hexadecylamine (ACROS CRGANICS Company) were mixed with 5 ml distilled water. After stirring for 1 h, to give an orange solution, 15 ml distilled water was added. The mixture was allowed to hydrolyze under vigorous stirring for 48 h, then a yellow composite of the organic template and the Vanadium Oxide component was obtained. The composite was then treated hydrothermally in a Teflon-lined autoclave with a stainless steel shell at 140 ◦C for 24 h and then 180 ◦C for 3 days. The obtained black product was washed with distilled water to remove the unreacted amine and decomposition product and finally dried at 70 ◦C in air atmosphere for 6 h. X-ray power diffraction (XRD) experiments were done on a D/MAX-III powder diffractometer with Cu Kα radiation (λ = 1.5406 A) and graphite monochrometer, with a scanning rate of 0.1 ◦/s. Scanning electron microscopy (SEM) images were collected on a JSM5610LV microscope operated at 20 kV. The transmission electron microscopy (TEM) images were obtained on a Jeol JEM-2010F microscope operated at 200 kV. The sample was deposited onto a perforated carbon foil supported on a copper grid. The Fourier transform infrared (FTIR) instrument used was a Nicolet 60-SXB spectrometer with a resolution of 4 cm−1. The XRD pattern of Vanadium Oxide nanotubes (Fig. 1) shows the low-angle reflection peaks, which are characteristic of the layered structure [7]. The peak with the highest intensity is located at d = 3.53 nm, and this corresponds to the distance between the VOx layers. The SEM images of the Vanadium Oxidehexadecylamine composite after aging for 48 h and the final products after hydrothermal treatment indicate that the Vanadium Oxide-hexadecylamine composite exhibits a well-ordered lamellar structure, but this is transformed into nanotubes after autoclave reaction, as shown in Fig. 2. The final products consist almost exclusively of Vanadium Oxide nanotubes. Vanadium Oxide nanotubes are frequently grown together in the form of bundles, but individual nanotubes with open ends can also be observed, which can be confirmed by the TEM investigations (see Fig. 3). The nanotube lengths range from 1 to 8 μm. Lengths and diameters of the nanotubes depend on the conditions of the preparation, such as different template molecules, concentration and reaction time [8]. The FTIR spectra for the Vanadium Oxidehexadecylamine composites and Vanadium Oxide nanotubes are represented in Fig. 4a and b, respectively. For comparison, FTIR measurement of pure hexadecylamine was made, as shown in Fig. 4c. This spectrum shows two sharp peaks between 3300 and 3500 cm−1, which can be associated with the NH2 vibration [9]. In the FTIR spectrum for the Vanadium Oxidehexadecylamine composites, the sharp peaks between 3300 and 3500 cm−1 disappear while the peaks at 2956 and 1589 cm−1 are observed. These are assigned to the stretching vibration and asymmetric bending vibration of the N H bonds in the NH3 group [9]. Therefore,
Jeanmarie Tarascon - One of the best experts on this subject based on the ideXlab platform.
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preparation of nanotextured vo2 b from Vanadium Oxide aerogels
Chemistry of Materials, 2006Co-Authors: E Baudrin, G Sudant, Dominique Larcher, Bruce Dunn, Jeanmarie TarasconAbstract:Vanadium Oxide aerogels were used as a precursor for preparing nanotextured VO2[B] by low-temperature heat treatment under vacuum. The VO2[B] material retains the fibrous morphology and high surface area of the aerogel. Evolution of the VO2[B] phase, as studied by FTIR and X-ray diffraction, indicates that the local structure of the Vanadium Oxide aerogel is close to that of VO2[B], in agreement with the bilayer-type structure previously proposed for Vanadium Oxide aerogels/xerogels. The electrochemical behavior of VO2[B] also bears similarity to that of Vanadium Oxide aerogels. Specific capacities for lithium as high as 500 mA·h/g are obtained for nanocrystalline VO2[B], and stable electrochemical response is obtained when cycled between 4 and 2.4 V vs Li+/Li0.
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Preparation of Nanotextured VO2[B] from Vanadium Oxide Aerogels
Chemistry of Materials, 2006Co-Authors: E Baudrin, G Sudant, Dominique Larcher, Bruce Dunn, Jeanmarie TarasconAbstract:Vanadium Oxide aerogels were used as a precursor for preparing nanotextured VO 2[B] by low-temperature heat treatment under vacuum. The VO 2[B] material retains the fibrous morphology and high surface area of the aerogel. Evolution of the VO 2[B] phase, as studied by FTIR and X-ray diffraction, indicates that the local structure of the Vanadium Oxide aerogel is close to that of VO 2[B], in agreement with the bilayer-type structure previously proposed for Vanadium Oxide aerogels/xerogels. The electrochemical behavior of VO 2[B] also bears similarity to that of Vanadium Oxide aerogels. Specific capacities for lithium as high as 500 mA·h/g are obtained for nanocrystalline VO 2[B], and stable electrochemical response is obtained when cycled between 4 and 2.4 V vs Li +/Li 0. © 2006 American Chemical Society
Elliot R. Bernstein - One of the best experts on this subject based on the ideXlab platform.
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Reactions of Neutral Vanadium Oxide Clusters with Methanol
The journal of physical chemistry. A, 2009Co-Authors: Feng Dong, Scott Heinbuch, Yan Xie, Jorge J. Rocca, Elliot R. BernsteinAbstract:Reactions of neutral Vanadium Oxide clusters with methanol and ethanol in a fast-flow reactor are investigated by time-of-flight mass spectrometry. Single-photon ionization through soft X-ray (46.9 nm, 26.5 eV) and vacuum ultraviolet (VUV, 118 nm, 10.5 eV) lasers is employed to detect both neutral cluster distributions and reaction products. In order to distinguish isomeric products generated in the reactions VmOn + CH3OH, partially deuterated methanol (CD3OH) is also used as a reactant in the experiments. Association products are observed for most Vanadium Oxide clusters in reaction with methanol. Products VOD, V2O3D, V3O6D, and V4O9D are observed for oxygen-deficient Vanadium Oxide clusters reacting with methanol, while oxygen-rich and the most stable clusters can extract more than one hydrogen atom (H/D) from CD3OH to form products VO2DH0,1, V2O4DH0,1, V2O5DH0,1, V3O7DH0,1, and V4O10DH0,1. Species VO2(CH3)2, VO3(CH3)2, V2O5(CH3)2, V3O7(CH3)2, and V3O8(CH3)2 are identified as some of the main products g...
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Density functional theory study of small Vanadium Oxide clusters.
The journal of physical chemistry. A, 2007Co-Authors: Elena Jakubikova, And Anthony K. Rappé, Elliot R. BernsteinAbstract:Density functional theory is employed to study structure and stability of small neutral Vanadium Oxide clusters in the gas phase. BPW91/LANL2DZ level of theory is used to obtain structures of VOy (y = 1−5), V2Oy (y = 2−7), V3Oy (y = 4−9), and V4Oy (y = 7−12) clusters. Enthalpies of growth and fragmentation reactions of the lowest energy isomers of Vanadium Oxide molecules are also obtained to study the stability of neutral Vanadium Oxide species under oxygen saturated gas-phase conditions. Our results suggest that cyclic and cage-like structures are preferred for the lowest energy isomers of neutral Vanadium Oxide clusters, and oxygen−oxygen bonds are present for oxygen-rich clusters. Clusters with an odd number of Vanadium atoms tend to have low spin ground states, while clusters with even number of Vanadium atoms have a variety of spin multiplicities for their ground electronic state. VO2, V2O5, V3O7, and V4O10 are predicted to be the most stable neutral clusters under the oxygen saturated conditions. T...
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Identification, structure, and spectroscopy of neutral Vanadium Oxide clusters.
The journal of physical chemistry. A, 2005Co-Authors: Yoshiyuki Matsuda, Elliot R. BernsteinAbstract:Neutral Vanadium Oxide clusters are studied by photoionization time-of-flight (TOF) mass spectroscopy, electronic spectroscopy, and density functional theory (DFT) calculations. Mass spectra of Vanadium Oxide clusters are observed by photoionization with lasers of three different wavelengths: 118, 193, and 355 nm. Mechanisms of 118 nm single photon ionization and 193 and 355 nm multiphoton ionization/fragmentation of Vanadium Oxide clusters are discussed on the basis of observed mass spectral patterns and line widths of the mass spectral features. Only the 118 nm laser light can ionize Vanadium Oxide neutral species by single photon ionization without fragmentation. The stable Vanadium Oxide neutral clusters under saturated oxygen growth conditions are found to be of the form (VO2)x(V2O5)y. Structures of the first few members of this series of clusters are determined through high level DFT calculations. Fragmentation of this series of clusters through 355 and 193 nm multiphoton ionization processes is di...
Goutam Deo - One of the best experts on this subject based on the ideXlab platform.
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Reactivity of supported Vanadium Oxide catalysts: The partial oxidation of methanol
Journal of Catalysis, 1994Co-Authors: Goutam Deo, Israel E. WachsAbstract:The partial oxidation of methanol was used to probe the reactivity of the surface Vanadium Oxide redox sites present in supported Vanadium Oxide catalysts. Formaldehyde was the main oxidation product for all the supported Vanadium Oxide catalysts operating under differential reactor conditions. The methanol oxidation turnover frequency (TOF) of the surface Vanadium Oxide phase varies by three orders of magnitude when the support is changed from ZrO2/TiO2 (100 s−1) to SiO2 (10−3). The TOF of the surface Vanadium Oxide phase supported on Nb2O5 (10−1 s−1) and Al2O3 (10−2 s−1) have intermediate values with the surface Vanadium Oxide phase on Nb2O5 having a TOF very close to ZrO2 or TiO2. The TOF of the surface Vanadium Oxide phase on all the Oxide supports is essentially independent of the Vanadium Oxide loading below monolayer coverage. The similar structures of the surface Vanadium Oxide phase on the different Oxide supports as well as the independence of the TOF with respect to Vanadium Oxide surface coverage suggests that a structural difference is not responsible for the difference in reactivity of the various supported Vanadium Oxide catalysts. Similar activation energies are observed for all the supported Vanadium Oxide catalysts (19.6 ± 2.3 kcal/mol) which correspond to the CH bond breaking of the surface methoxy species to form formaldehyde. The similar activation energies and different TOFs of the supported Vanadium Oxide catalysts with respect to the Oxide support imply that the specific Oxide support influences the Arrhenius pre-exponential factor. In situ Raman studies during methanol oxidation suggest that the pre-exponential factor is determined by the number of participating surface Vanadium Oxide sites. The importance of the activity per surface Vanadium Oxide site in determining the pre-exponential factor was not investigated and may also be significant. The TOF for methanol oxidation is not related to the terminal VO bond strength, but appear to be related to the reducibility (Tmax) of the supported Vanadium Oxide catalysts. It is proposed that the number of participating surface Vanadium Oxide sites is most probably related to the reducibility of the V0Support bond since this bridging bond strength controls the reducibility of the supported Vanadium Oxide catalysts.
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Effect of Additives on the Structure and Reactivity of the Surface Vanadium Oxide Phase in V2O5/TiO2 Catalysts
Journal of Catalysis, 1994Co-Authors: Goutam Deo, Israel E. WachsAbstract:Additives on a 1% V2O5/TiO2 catalyst exhibit two types of interactions with the surface Vanadium Oxide phase which are observed by Raman spectroscopy under dehydrated conditions and methanol oxidation. Under dehydration conditions, noninteracting additives (WO3, Nb2O5, and SiO2) coordinate directly to the Oxide support without significantly interacting with the surface Vanadium Oxide phase. Furthermore, the effect of the noninteracting additives on the surface Vanadium Oxide phase is independent of the order of preparation or precursor used. These noninteracting additives do not affect the methanol oxidation activity and selectivity. Interacting additives (K2O and P2O5), however, directly coordinate with the surface Vanadium Oxide phase. Addition of K2O progressively titrates the surface Vanadium Oxide sites, as observed from the changes in the structure and reactivity of the surface Vanadium Oxide phase. The effect of P2O5 on the surface Vanadium Oxide phase depends on the concentration and sequence of preparation. Addition of higher concentrations of P2O5 forms Vanadium phosphate compounds, and results in a change in methanol oxidation activity and selectivity. The addition of 1% V2O5 to a 5% P2O5/TiO2 sample, however, does not show any evidence of compound formation, but a part of the surface Vanadium Oxide phase appears to be titrated. Under ambient conditions, the additives change the pH at point of zero charge of the surface moisture layer which controls the structure of the surface Vanadium Oxide layer. Thus, depending on the nature of the additive, interacting or noninteracting, the dehydrated structure and reactivity toward methanol oxidation of the surface Vanadium Oxide phase are affected or remain essentially unchanged, respectively.
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Molecular Structure-Reactivity Relationships of Supported Vanadium Oxide Catalysts
Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis Proceedings of the ACS Symposium on Structure-Activity Relationships in He, 1991Co-Authors: Goutam Deo, Israel E. WachsAbstract:Abstract The molecular structure of the surface Vanadium Oxide species present on different Oxide supports (TiO 2 , γ-Al 2 O 3 , and SiO 2 ) were determined by laser Raman spectroscopy and 51 V solid state NMR under hydrated and dehydrated conditions. The structure of the Vanadium Oxide species changes with dehydration and a four coordinated Vanadium Oxide species with a short terminal bond was present on all Oxide supports considered. The reactivity of the supported Vanadium Oxide catalysts was determined via the methanol oxidation reaction. Correlation of the structure and reactivity data indicate the strength of the bridging, Vanadium-oxygen-support, bond to be controlling the activity of these supported Vanadium Oxide catalysts. The effect of promoters/impurities on 1% V 2 O 5 /TiO 2 catalyst depends on their acid/base nature. Basic promoters titrate the Vanadium Oxide site and destroy the Vanadium-oxygen-support bond of the parent 1% V 2 O 5 /TiO 2 . Acidic promoters/impurities coordinate to the support and do not show any appreciable change to the structure, reactivity, and the Vanadium-oxygen-support bond of the parent 1% V 2 O 5 /TiO 2 .
