FT-IR study of CO and NO adsorption on a VSiBEA zeolite

A VSiBEA zeolite containing 2.1 wt.% V has been prepared by a two-step post-synthesis method. which consists of first creating vacant T-sites by dealumination of the BEA zeolite with nitric acid and then impregnation of the SiBEA zeolite with an aqueous solution of ammonium metavanadate (NH4VO3), used as V5+ precursor. The sample has been studied by XRD analysis, FT-IR, TPR and diffuse reflectance UV-Visible spectroscopy. The disappearance of the IR band near 3520 cm(-1) after impregnation of NH4VO3 indicates that a specific reaction has occurred between the vanadium precursor and silanol groups of vacant T-sites. As a result, tetrahedral V5+ ions connected with OH groups are incorporated in the framework. In addition, weakly bound extraframework V5+ species (tetrahedral and octahedral) exists. CO and NO were used as IR probes for establishing the state of reduced vanadium sites. It was found that NO was a better probe for this purpose, but CO provided valuable complementary information on the coordination state of the reduced vanadium sites. As expected, adsorption of both, CO and NO, was negligible on non-reduced sample. However, sample reduced with hydrogen at 673 K is characterised by a high concentration of V4+ and V3+ sites produced by the reduction of the extraframework sites. NO was found to form dinitrosyls with both V4+ and V3+ cations. The V4+(NO)(2) species are characterised by v(s) at 1902 cm(-1) and v(as), at 1755 cm(-1). The dinitrosyls of V3+ were registered at 1830 (vs) and 1687 cm(-1) (v(as)). Reduction at 773 K led to increase in amount of the V3+ species and formation of framework V4+ cations (respective dinitrosyl bands at 1900 and 1729 cm(-1)). The spectra of CO adsorbed on the reduced sample are complex. Evidence of tricarbonyl species was found. It was concluded that CO is a better probe than NO for testing the coordination state of V4+ and V3+ sites. (c) 2005 Elsevier Inc. All rights reserved.