An Investigation of the Reduction and Reoxidation of Isolated Vanadate Sites Supported on MCM-48
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An Investigation of the Reduction and Reoxidation of Isolated Vanadate Sites Supported on MCM-48 Jason L. Bronkema Æ Alexis T. Bell
Received: 25 October 2007 / Accepted: 3 December 2007 / Published online: 20 December 2007 Ó Springer Science+Business Media, LLC 2007
Abstract The reduction and subsequent reoxidation of isolated vanadate species supported on silica was investigated using temperature-programmed reduction and oxidation, along with in-situ XANES and Raman spectroscopy. Approximately 70–80% of the vanadium was reduced to V3+ after reduction in H2 at temperatures up to 923 K. Upon reduction, the vanadyl oxygen was removed and the three remaining V–O bonds are lengthened by 0.2 ˚ . The vanadate species are rapidly reoxidized when A exposed to O2, with the amount of oxygen uptake matching well with the amount removed during reduction. In-situ Raman spectroscopy during reoxidation in 18O2 showed that significant scrambling occurs between gas phase oxygen and surface oxygen species during the reoxidation of the vanadate species. Keywords
Vanadate Silica Reduction Oxidation
1 Introduction Supported vanadium oxide catalysts have been shown to be active for the partial oxidation of methane and methanol to formaldehyde, the oxidative dehydrogenation of light alkanes, and the selective reduction of NOx. Interest in understanding the mechanism and kinetics of these reactions has motivated studies of the structure of the vanadate J. L. Bronkema A. T. Bell Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, USA J. L. Bronkema A. T. Bell (&) Department of Chemical Engineering, University of California, Berkeley, CA 94720-1462, USA e-mail: [email protected]; [email protected]
species and of the dynamics of reduction and oxidation of the supported vanadia. The structure of vanadia on the surface of metal oxide supports such as silica, alumina, titania, and zirconia has been studied using infrared and Raman spectroscopy [1–3], UV-Visible spectroscopy [3, 4], 51V NMR [5], XANES, and EXAFS [6]. Application of these techniques has shown that depending on the surface density of vanadium, vanadia can be present as isolated monovanadate species, two-dimensional polyvanadate species, and bulk V2O5. At very low surface densities (less than about 0.2 V/nm2), vanadium is present exclusively as isolated vanadate species. As the surface density of vanadium increases polyvanadate species are formed. Finally, as monolayer coverage by polyvanadate species is approached, nanoparticles of V2O5 are observed. Several investigators have examined the reduction of supported vanadate species. Kanervo et al. [7]. have reported that samples containing mainly isolated monovanadate species could be reduced by 50% from V5+ to V3+; however, at higher vanadium loadings, they obtained reductions of 62%. A broad TPR peak was observed in the range of 600–800 K, with the maximum for samples containing exclusively monovanadate species occurring at 750 K. Work by Koranne et al. on V/SiO2 and V/Al2O3 showed
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