Characterization of supported rhenium oxide catalysts : e†ect of loading, support and additives Brishti Mitra,a Xingtao Gao,b Israel E. Wachs,b A. M. Hirtc and Goutam Deo*a a Department of Chemical Engineering, Indian Institute of T echnology, Kanpur 208 016, India. E-mail : goutam=iitk.ac.in b Zettlemoyer Center for Surface Studies and Department of Chemical Engineering, L ehigh University, Bethlehem, PA 18015, USA c Materials Research L aboratory, Inc., 290 North Bridge Street, Struthers, OH 44471, USA Received 12th September 2000, Accepted 15th January 2001 First published as an Advance Article on the web 20th February 2001 The nature of the surface rhenium oxide species present in supported rhenium oxide catalysts was determined as a function of the oxide support, rhenium oxide loading and secondary metal oxide additives. The catalysts were prepared using the incipient wetness impregnation method with dilute aqueous perrhenic acid as the precursor and characterized using Raman, FTIR and X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR) experiments. The Raman and XPS studies reveal that the surface rhenium oxide species are well dispersed on the and supports. Under dehydrated conditions, Al 2 O 3 TiO 2 Raman and IR studies reveal that the surface rhenium oxide species is independent of oxide support (Al 2 O 3 and and it appears that the same isolated rhenium oxide species is formed for all loadings. The TiO 2 ) structural details of this dehydrated isolated rhenium oxide species are, however, not clear. TPR measurements suggest that the reducibility of the surface rhenium oxide species depends strongly on the speciÐc oxide support : the surface rhenium oxide species on is more reducible than the surface rhenium oxide species TiO 2 on The di†erence in reducibility is related to the bridging ReÈOÈsupport bond strength. The secondary Al 2 O 3 . metal oxide additives, sodium and vanadium, can be categorized as interacting and non-interacting, respectively. The non-interacting additive (vanadium oxide) coordinates directly to the oxide supports without signiÐcantly a†ecting the structure and reducibility of the surface rhenium oxide species. The interacting additive (sodium oxide), however, directly coordinates with the surface rhenium oxide species and changes its structure and hydrogen reducibility. Introduction Supported metal oxide systems constitute a very important class of heterogeneous catalysts. These catalysts are formed when a metal oxide is deposited on the surface of a second metal oxide substrate that usually possesses a high surface area.1h3 The deposited metal oxide component is considered to be the active phase of the catalyst and some of the reported active oxides are those of vanadium, rhenium, chromium, molybdenum, niobium, tungsten, etc. The typical high surface area metal oxide supports used are alumina, titania, silica, zir- conia, niobia, etc. These supported metal oxide catalysts Ðnd numerous applications in the petrochemical industry and for pollution control.4 Supported rhenium oxide catalysts belong to this group of supported metal oxide catalysts. In such catalysts the active rhenium oxide component forms a two-dimensional overlayer on oxide supports. The importance of these catalysts lies in their extensive use for the metathesis of oleÐns.5 h7 OleÐn metathesis or oleÐn disproportionation reactions are indus- trially very important. These reactions provide an alternative way of producing useful compounds that cannot be synthe- sized from readily available starting materials by standard methods.8,9 Tungsten oxide and molybdenum oxide catalysts are also employed for metathesis of oleÐns, but rhenium oxide is superior under mild reaction conditions, viz., room tem- perature and atmospheric pressure. These conditions help reduce side reactions such as isomerization and poly- merization, yielding exclusively the primary metathesis pro- ducts.9 Rhenium based catalysts have also been examined for hydrodesulfurization,10,11 hydrodenitrogenation,12 selective hydrogenation,13 selective catalytic reduction of and NO x 14 partial oxidation of methanol.15 The catalytic activity of the supported rhenium oxide catalysts for metathesis is enhanced by the presence of other metal oxides additives, such as V 2 O 5 , and and by the use of mixed oxide supports MoO 3 WO 3 ,16,17 such as and Addition of SiO 2 É Al 2 O 3 Al 2 O 3 É B 2 O 3 .17h19 (M \ Sn or Pb; R \ alkyl) also helps in improving the MR 4 activity of supported rhenium oxide catalysts.17h20 The industrial importance of the supported rhenium oxide catalysts has motivated fundamental studies about the nature and function of the active sites in such catalysts, namely the surface rhenium oxide species present. Knowledge of the surface composition and the local structure of the catalyst at the molecular level will help in understanding better the role played by the surface species in the catalytic reactions. The molecular-level information can then be used to assist in the design of improved catalysts. The surface rhenium oxide over- layer in the supported rhenium oxide catalysts has been char- acterized by extended X-ray absorption Ðne structure (EXAFS),21 X-ray absorption near edge spectroscopy (XANES),21,22 Fourier transform infrared (FTIR) spectros- copy,23 h25 UVÈvisible spectrophotometry23 and Raman spec- troscopy.20,22h24,26,27 Of these characterization techniques, Raman spectroscopy is best suited for the study of supported metal oxide catalysts and, consequently, supported rhenium 1144 Phys. Chem. Chem. Phys., 2001, 3, 1144È1152 DOI : 10.1039/b007381o This journal is The Owner Societies 2001 ( Published on 20 February 2001. Downloaded by Lehigh University on 20/07/2014 23:12:49. 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