Redox Isotherms for Vanadia Supported on Zirconia
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Redox Isotherms for Vanadia Supported on Zirconia Parag R. Shah Æ John M. Vohs Æ Raymond J. Gorte
Received: 7 May 2008 / Accepted: 3 June 2008 / Published online: 24 June 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Redox isotherms were measured for zirconiasupported vanadia between 10-2 and 10-28 atm at 748 K for two vanadia loadings, 2.9 and 5.8 V/nm2, corresponding to isolated VO4 species and monolayer, polymeric vanadates. The catalyst with isolated VO4 species, which is expected to have predominantly V–O–Zr linkages, had a redox isotherm that showed a well-defined step corresponding to one oxygen per V. By contrast, the redox isotherm for the catalyst with polymeric vanadates changed more gradually with PO2 and the change in the oxygen stoichiometry corresponded to 0.85 O/V. Comparison of these results to the redox isotherms for bulk vanadates suggests that oxidation of the isolated vanadates proceeds by a direct transition from V+3 $ V+5, while transitions from V+3 $ V+4 and V+4 $ V+5 are possible with the polyvanadates. Rate measurements for methanol and propane oxidation over the two supported vanadia catalysts and several bulk vanadates showed that specific rates for each reaction were similar on all of the samples, suggesting that that the V–O bond strength does not affect the rate determining step of these reactions. Keywords Supported catalyst Vanadia Vanadium oxide Cerium vanadate Magnesium vanadate Zirconium vanadate Zirconia Coulometric Titration Methanol oxidation Formaldehyde Oxidation Redox Equilibrium Partial oxidation
P. R. Shah J. M. Vohs R. J. Gorte (&) Department of Chemical & Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA 19104, USA e-mail: [email protected]
1 Introduction Vanadia-based catalysts are used for a number of partial oxidation reactions including the oxidation of methanol to formaldehyde [1–4], the oxidation of o-xylene to phthalic anhydride [5, 6], and the oxidative dehydrogenation (ODH) of propane [7–9] and butane [10, 11] to produce alkenes. An important aspect of these catalysts is that the active form generally consists of highly dispersed vanadia supported on a second high-surface-area oxide such as TiO2, ZrO2, or Al2O3 [12]. As shown in Fig. 1, the vanadia can either be present in the form of isolated VO4 species which have a tetrahedral geometry containing a single V=O bond and three V–O–S bonds (S = support cation), or polyvanadates that contain V–O–V bonds in addition to the V=O and V–O–S bonds [12, 13]. Since the oxidation reactions mentioned above are generally thought to proceed via a Mars-van Krevelen mechanism in which oxygen is removed and inserted into the oxide lattice, the redox properties of the catalyst are important and may affect catalytic properties. Indeed, the identity of the support has been reported to have a large effect on reactivity [2, 14], suggesting that the V–O–S bonds may play a role in the reactions. There are many studies in the literature in which the redox properties of
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