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Molinari JE, Nakka L, Kim T, Wachs IE. Dynamic Surface Structures and Reactivity of Vanadium-Containing Molybdophosphoric Acid (H3+xPMo12–xVxO40) Keggin Catalysts during Methanol Oxidation and Dehydration. ACS Catal 2011. [DOI: 10.1021/cs2001362] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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77
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Wachs IE, Hardcastle FD, Chan SS. Characterization of Supported Metal Oxides by Laser Raman Spectroscopy: Supported Vanadium Oxide on Al2O3 and TiO2. ACTA ACUST UNITED AC 2011. [DOI: 10.1557/proc-111-353] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022]
Abstract
AbstractThe interaction of supported vanadium oxide with Al2O3 and TiO2 substrates is examined with Raman spectroscopy. The Raman spectra of the supported vanadium oxide reveal that the strong interaction of the vanadium oxide with the Al2O3 and TiO2 supports results in the formation of an atomically dispersed surface vanadium oxide phase as well as supported crystalline V2O5. The relative concentrations of the atomically dispersed surface vanadium oxide and crystalline V2O5 depend on the vanadium oxide loading and the surface area of the oxide support.
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Merzlikin SV, Tolkachev NN, Briand LE, Strunskus T, Wöll C, Wachs IE, Grünert W. Anomalous surface compositions of stoichiometric mixed oxide compounds. Angew Chem Int Ed Engl 2011; 49:8037-41. [PMID: 20845336 DOI: 10.1002/anie.201001804] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
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79
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Routray K, Zhou W, Kiely CJ, Wachs IE. Catalysis Science of Methanol Oxidation over Iron Vanadate Catalysts: Nature of the Catalytic Active Sites. ACS Catal 2010. [DOI: 10.1021/cs1000569] [Citation(s) in RCA: 113] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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80
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Zhou W, Doura KF, Watanabe M, Herzing AA, Okunishi E, Ross-Medgaarden EI, Wachs IE, Kiely CJ. Aberration-corrected Analytical Microscopy Characterization of Double-Supported WO3/TiO2/SiO2 Solid Acid Catalysts. ChemCatChem 2010. [DOI: 10.1002/cctc.201000273] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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81
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Soultanidis N, Zhou W, Psarras AC, Gonzalez AJ, Iliopoulou EF, Kiely CJ, Wachs IE, Wong MS. Relatingn-Pentane Isomerization Activity to the Tungsten Surface Density of WOx/ZrO2. J Am Chem Soc 2010; 132:13462-71. [DOI: 10.1021/ja105519y] [Citation(s) in RCA: 86] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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82
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Merzlikin SV, Tolkachev NN, Briand LE, Strunskus T, Wöll C, Wachs IE, Grünert W. Anomale Oberflächenzusammensetzung stöchiometrischer Mischoxid-Verbindungen. Angew Chem Int Ed Engl 2010. [DOI: 10.1002/ange.201001804] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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83
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Molinari JE, Wachs IE. Presence of Surface Vanadium Peroxo-oxo Umbrella Structures in Supported Vanadium Oxide Catalysts: Fact or Fiction? J Am Chem Soc 2010; 132:12559-61. [DOI: 10.1021/ja105392g] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023]
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84
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Wachs IE, Roberts CA. Monitoring surface metal oxide catalytic active sites with Raman spectroscopy. Chem Soc Rev 2010; 39:5002-17. [DOI: 10.1039/c0cs00145g] [Citation(s) in RCA: 233] [Impact Index Per Article: 16.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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85
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Zhou W, Ross-Medgaarden EI, Knowles WV, Wong MS, Wachs IE, Kiely CJ. Identification of active Zr–WOx clusters on a ZrO2 support for solid acid catalysts. Nat Chem 2009; 1:722-8. [DOI: 10.1038/nchem.433] [Citation(s) in RCA: 133] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2009] [Accepted: 10/01/2009] [Indexed: 11/09/2022]
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86
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Nakka L, Molinari JE, Wachs IE. Surface and Bulk Aspects of Mixed Oxide Catalytic Nanoparticles: Oxidation and Dehydration of CH3OH by Polyoxometallates. J Am Chem Soc 2009; 131:15544-54. [DOI: 10.1021/ja904957d] [Citation(s) in RCA: 78] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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87
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Ross-Medgaarden EI, Wachs IE, Knowles WV, Burrows A, Kiely CJ, Wong MS. Tuning the Electronic and Molecular Structures of Catalytic Active Sites with Titania Nanoligands. J Am Chem Soc 2008; 131:680-7. [DOI: 10.1021/ja711456c] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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90
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Liu J, Zhao Z, Xu C, Duan A, Jiang G, Gao J, Lin W, Wachs IE. In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts. ACTA ACUST UNITED AC 2008. [DOI: 10.1007/s11426-008-0027-2] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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91
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Liu J, Zhao Z, Liang P, Xu C, Duan A, Jiang G, Lin W, Wachs IE. Study on the Reaction Mechanism for Soot Oxidation Over TiO2 or ZrO2-supported Vanadium Oxide Catalysts by Means of In-situ UV-Raman. Catal Letters 2007. [DOI: 10.1007/s10562-007-9267-6] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
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92
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93
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Wachs IE, Kim T, Ross EI. Catalysis science of the solid acidity of model supported tungsten oxide catalysts. Catal Today 2006. [DOI: 10.1016/j.cattod.2006.02.085] [Citation(s) in RCA: 134] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
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94
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Wachs IE. Environmental Catalysis Edited by Vicki H. Grassian (University of Iowa). CRC Press (an imprint of Taylor and Francis Group): Boca Raton, FL. 2005. xx + 702 pp. $169.95. ISBN 1-57444-462-X. J Am Chem Soc 2006. [DOI: 10.1021/ja059877u] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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95
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Tian H, Ross EI, Wachs IE. Quantitative Determination of the Speciation of Surface Vanadium Oxides and Their Catalytic Activity. J Phys Chem B 2006; 110:9593-600. [PMID: 16686507 DOI: 10.1021/jp055767y] [Citation(s) in RCA: 177] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
A quantitative method based on UV-vis diffuse reflectance spectroscopy (DRS) was developed that allows determination of the fraction of monomeric and polymeric VO(x) species that are present in vanadate materials. This new quantitative method allows determination of the distribution of monomeric and polymeric surface VO(x) species present in dehydrated supported V(2)O(5)/SiO(2), V(2)O(5)/Al(2)O(3), and V(2)O(5)/ZrO(2) catalysts below monolayer surface coverage when V(2)O(5) nanoparticles are not present. Isolated surface VO(x) species are exclusively present at low surface vanadia coverage on all the dehydrated oxide supports. However, polymeric surface VO(x) species are also present on the dehydrated Al(2)O(3) and ZrO(2) supports at intermediate surface coverage and the polymeric chains are the dominant surface vanadia species at monolayer surface coverage. The propane oxidative dehydrogenation (ODH) turnover frequency (TOF) values are essentially indistinguishable for the isolated and polymeric surface VO(x) species on the same oxide support, and are also not affected by the Brønsted acidity or reducibility of the surface VO(x) species. The propane ODH TOF, however, varies by more than an order of magnitude with the specific oxide support (ZrO(2) > Al(2)O(3) >> SiO(2)) for both the isolated and polymeric surface VO(x) species. These new findings reveal that the support cation is a potent ligand that directly influences the reactivity of the bridging V-O-support bond, the catalytic active site, by controlling its basic character with the support electronegativity. These new fundamental insights about polymerization extent of surface vanadia species on SiO(2), Al(2)O(3), and ZrO(2) are also applicable to other supported vanadia catalysts (e.g., CeO(2), TiO(2), Nb(2)O(5)) as well as other supported metal oxide (e.g., CrO(3), MoO(3), WO(3)) catalyst systems.
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Tian H, Wachs IE, Briand LE. Comparison of UV and Visible Raman Spectroscopy of Bulk Metal Molybdate and Metal Vanadate Catalysts. J Phys Chem B 2005; 109:23491-9. [PMID: 16375323 DOI: 10.1021/jp053879j] [Citation(s) in RCA: 122] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The visible (532 and 442 nm) and UV (325 nm) Raman spectra of bulk mixed metal oxides (metal molybdates and metal vanadates) were compared on the same spectrometer, for the first time, to allow examination of how varying the excitation energy from visible to UV affects the resulting Raman spectra. The quality of the Raman spectra was found to be a strong function of the absorption properties of the bulk mixed oxide. For bulk mixed metal oxides that absorb weakly in the visible and UV regions, both the visible and UV Raman spectra were of high quality and exhibit identical vibrational bands, but with slightly different relative intensities. For bulk mixed metal oxides that absorb strongly in the UV and visible regions and/or strongly in the UV and weakly in the visible regions, the visible Raman spectra are much richer in structural information and of higher resolution than the corresponding UV Raman spectra. This is a consequence of the strong UV absorption that significantly reduces the sampling volume and number of scatterers giving rise to the Raman signal. The shallower escape depth of UV Raman, however, was not sufficient to detect vibrations from the surface metal oxide species that are present on the outermost surface layer of these crystalline mixed metal oxide phases as previously suggested. It was also demonstrated that there is no sample damage by the more energetic UV excitation when very low laser powers and fast detectors are employed, thus avoiding the need of complicated fluidized bed sample arrangements sometimes used for UV Raman investigations. The current comparative Raman investigation carefully documents, for the first time, the advantages and disadvantages of applying different excitation energies in collecting Raman spectra of bulk mixed metal oxide materials.
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Resini C, Montanari T, Busca G, Jehng JM, Wachs IE. Comparison of alcohol and alkane oxidative dehydrogenation reactions over supported vanadium oxide catalysts: in situ infrared, Raman and UV–vis spectroscopic studies of surface alkoxide intermediates and of their surface chemistry. Catal Today 2005. [DOI: 10.1016/j.cattod.2004.09.029] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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98
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Wachs IE, Jehng JM, Ueda W. Determination of the Chemical Nature of Active Surface Sites Present on Bulk Mixed Metal Oxide Catalysts. J Phys Chem B 2004; 109:2275-84. [PMID: 16851220 DOI: 10.1021/jp048839e] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
CH3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH3OH-TPSR spectra peak temperature, Tp, for model supported metal oxides and bulk, pure metal oxides was found to be sensitive to the specific surface metal oxide as well as its oxidation state. The catalytic activity of the surface metal oxide sites was found to decrease upon reduction of these sites and the most active surface sites were the fully oxidized surface cations. The surface V5+ sites were found to be more active than the surface Mo6+ sites, which in turn were significantly more active than the surface Nb5+ and Te4+ sites. Furthermore, the reaction products formed also reflected the chemical nature of surface active sites. Surface redox sites are able to liberate oxygen and yield H2CO, while surface acidic sites are not able to liberate oxygen, contain either H+ or oxygen vacancies, and produce CH3OCH3. Surface V5+, Mo6+, and Te4+ sites behave as redox sites, and surface Nb5+ sites are Lewis acid sites. This experimental information was used to determine the chemical nature of the different surface cations in bulk Mo-V-Te-Nb-Ox mixed oxide catalysts (Mo(0.6)V(1.5)Ox, Mo(1.0)V(0.5)Te(0.16)Ox, Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox). The bulk Mo(0.6)V(1.5)Ox and Mo(1.0)V(0.5)Te(0.16)Ox mixed oxide catalytic characteristics were dominated by the catalytic properties of the surface V5+ redox sites. The surface enrichment of these bulk mixed oxide by surface V5+ is related to its high mobility, V5+ possesses the lowest Tammann temperature among the different oxide cations, and the lower surface free energy associated with the surface termination of V=O bonds. The quaternary bulk Mo(1.0)V(0.3)Te(0.16)Nb(0.12)Ox mixed oxide possessed both surface redox and acidic sites. The surface redox sites reflect the characteristics of surface V5+ and the surface acidic sites reflect the properties normally associated with supported Mo6+. The major roles of Nb5+ and Te4+ appear to be that of ligand promoters for the more active surface V and Mo sites. These reactivity trends for CH3OH ODH parallel the reactivity trends of propane ODH because of their similar rate-determining step involving cleavage of a C-H bond. This novel CH3OH-TPSR spectroscopic method is a universal method that has also been successfully applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.
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Malleswara Rao TV, Deo G, Jehng JM, Wachs IE. In situ UV-vis-NIR diffuse reflectance and Raman spectroscopy and catalytic activity studies of propane oxidative dehydrogenation over supported CrO3/ZrO2 catalysts. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2004; 20:7159-7165. [PMID: 15301500 DOI: 10.1021/la049590v] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
The molecular structures, oxidation states, and reactivity of 3 and 6% CrO3/ZrO2 catalysts prepared by incipient wetness impregnation were examined under different conditions. The in situ Raman spectroscopic studies under dehydrated conditions reveal that the 3 and 6% CrO3/ZrO2 catalysts possess equal amounts of monochromate and polychromate species. Consequently, monolayer coverage on this ZrO2 support is about 3% CrO3. The 6% CrO3/ZrO2 possesses an additional Raman band due to Cr2O3 crystals corresponding to the remaining 3% CrO3. Furthermore, during reaction conditions the polychromate species is preferentially reduced, the monochromate species are slightly affected, and the Cr2O3 crystals are not affected. The in situ UV-vis-NIR diffuse reflectance spectroscopy results reveal that under steady-state reaction conditions the extent of reduction and edge energy position of surface Cr6+ cations increase with an increase in reduction environment for the 3 and 6% CrO3/ZrO2 samples. Propane oxidative dehydrogenation (ODH) studies reveal that the catalytic activity expressed in moles of propane converted per gram catalyst per second is similar for the two catalysts, which is consistent with equal amounts of molecularly dispersed chromia present. The turnover frequency for the 6% CrO3/ZrO2 catalyst is, however, smaller than that for the 3% CrO3/ZrO2 sample due to the presence of Cr2O3 crystals, which are relatively inactive for propane ODH. For this catalytic system and for the experimental conditions used, propene, CO, and CO2 are primary products. Furthermore, the 33-39% propene selectivity is not affected by the C3H8/O2 ratio for both catalysts. Structure-reactivity studies suggest that the molecularly dispersed species are present in equal amounts in the 3 and 6% CrO3/ZrO2 samples as Cr6+ monochromate and polychromate species are the most effective catalytic active sites taking part in the propane ODH reaction.
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Briand LE, Tkachenko OP, Guraya M, Gao X, Wachs IE, Grünert W. Surface-Analytical Studies of Supported Vanadium Oxide Monolayer Catalysts. J Phys Chem B 2004. [DOI: 10.1021/jp037675j] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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