The Complete 51 V MAS NMR Spectrum of Surface Vanadia Nanoparticles on Anatase (TiO 2 ): Vanadia Surface Structure of a DeNO x Catalyst Ulla Gro Nielsen, ²,§ Nan-Yu Topsøe, ‡ Michael Brorson, ‡ Jørgen Skibsted, ² and Hans J. Jakobsen* ,² Contribution from the Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, UniVersity of Aarhus, DK-8000 Aarhus C, Denmark, and Haldor Topsøe A/S, NymølleVej 55, DK-2800 Lyngby, Denmark Received February 10, 2003; Revised Manuscript Received January 5, 2004; E-mail: hja@chem.au.dk Abstract: The first observations of the complete manifold of spinning sidebands (ssbs) including both the central and satellite transitions in 51 V MAS NMR spectra of surface vanadia nanoparticles on titania in DeNO x catalysts are presented. 51 V quadrupole coupling and chemical shift anisotropy parameters for the dominating vanadia structure are determined from 51 V MAS NMR spectra recorded at 9.4 and 14.1 T. Based on correlations previously established between 51 V NMR parameters and crystal structure data for inorganic vanadates, the NMR data are consistent with vanadium in a distorted octahedral oxygen coordination environment for the so-called strongly bonded vanadia species on the surface. The investigation includes two vanadia-titania model catalysts and six industrial-type DeNO x catalysts. 1. Introduction The combustion of fossil fuels and other organic materials results in the formation of harmful nitrogen oxides (NO x ) which greatly contribute to the formation of smog and acid rain. The reddish-brown smog in urban areas, caused by the photochemi- cal oxidation of NO to NO 2 , and the damage to forest areas have revealed the immense environmental problems the world is facing today. As a result, increasingly stringent local governmental and global political restrictions are imposed on the emission of NO x from power plants, various industries, and vehicles. Thus, one of the challenges to research in heteroge- neous catalysis has been the development of efficient catalysts for removal of NO x from the flue or exhaust gases by the selective catalytic reduction (SCR) process where NO x is reduced by ammonia to form dinitrogen and water. This has motivated the need for further understanding the relationships between chemical structure and catalytic performance of heterogeneous catalysts. Vanadia supported on the surface of anatase, a titanium dioxide (TiO 2 ) polymorph, is the commonly employed DeNO x catalyst today. 1 Tungsten trioxide (WO 3 ) is often added to enhance the thermal stability and catalytic performance. 2 Various proposals (see references in ref 1) have previously been made regarding the mechanism of the DeNO x reaction based on studies of structure-activity relationships of the catalysts. In particular, combined FTIR and activity studies 3-6 have shown that both V-OH and VdO groups on the titania surface are required for the reaction, which is initiated by adsorption of ammonia on Brønsted acid sites (V-OH), followed by activation of ammonia via reaction with redox sites (VdO). The activated ammonia then reacts with weakly adsorbed NO to yield dinitrogen and water. Furthermore, recent theoretical studies have provided insight into the nature of adsorption and surface reaction of ammonia with nitric oxide involving these surface sites. 7-9 Nevertheless, despite various characterizations of the vanadia-titania catalyst system by different methods, 10 a controversy still exists regarding the exact structure around vanadium for the strongly bonded vanadia nanoparticles on the titania surface. 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