JOURNAL OF CATALYSIS 134, 492-505 (1992) The Effects of Structure on the Catalytic Activity and Selectivity of V2Os/mio 2 for the Reduction of NO by NH3 GREGORY T. WENT, LI-JEN LEU, RICHARD R. ROSIN, AND ALEXIS T. BELL Center for Advanced Materials, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California Received May 29, 1991; revised October 24, 1991 An investigation of the effect of catalyst structure on the activity and selectivity of TiO 2 (anatase)- supported V205 for the selective catalytic reduction of NO by NH 3 has been carried out. The structure of the catalyst and the adsorbed species present on the surface was characterized by in situ laser Raman spectroscopy (LRS), and the interaction of NH3 was investigated using temperature- programmed desorption (TPD). At vanadia loadings corresponding to less than a theoretical mono- layer, the vanadia is present in the form of monomeric vanadyl and polymeric vanadate species. When the vanadia coverage exceeds a monolayer, crystallites of V205 form at the expense of the polymeric species. Analysis of the catalytic activity shows that the specific activity of the polymeric vanadates species is about 10 times greater than that of the monomeric vanadyl species. Monomeric species produce N2 as the principle reaction product, independent of the presence or absence of 02 in the feed, whereas polymeric vanadates species produce both N 2 and N20, with the selectivity to N2 decreasing with increasing concentrations of 02 in the feed. LRS experiments reveal that in the absence of 02 in the feed stream, the catalyst undergoes reduction but that in the presence of 02, the catalyst remains in a nearly fully oxidized state. TPD experiments indicate that a crucial step in the catalytic reduction of NO is the activation of adsorbed NH 3 to produce NHx (x = 0 - 2) species. The removal of H atoms from adsorbed NH 3 occurs via reaction with V=O groups present in clusters of monomeric vanadyl species and in polymeric vanadate species, the latter being more reactive than the former. The observations reported in this study are interpreted in terms of a reaction mechanism, which accounts for the effects of catalyst structure and oxidation state on the observed properties. © 1992 AcademicPress, Inc. INTRODUCTION Titania-supported vanadia is a highly ac- tive catalyst for the selective catalytic re- duction of NO by NH 3 in the presence of 02 (1). Previous research has shown that the preferred phase of TiO2 is anatase (2-4) and that the activity per gram of catalysts in- creases with increasing vanadia loading up to the point where the surface of the support is covered by a theoretical monolayer of vanadia (2-5). Spectroscopic studies of sub- monolayer coverages of vanadia on TiO2 have shown that the dispersed vanadia is present as a combination of monomeric va- nadyl and polymeric vanadate species (6-11), with the distribution of these two structures varying with the loading of va- nadia (11). When the vanadia loading is 0021-9517/92 $3.00 Copyright © 1992 by AcademicPress, Inc. All rightsof reproductionin any form reserved. raised above the dispersive capacity of the support, crystallites of V2O 5 are observed (5, 8-12), and the activity of the catalyst declines (2, 3, 5). For a fixed vanadia load- ing, the activity has been found to increase upon addition of O2 to the gas phase (2, 4, 13-16). IR, XPS, and ESR studies suggest that the role of Oz is to maintain the vanadia in a highly oxidized state (4, 13, 16). Efforts to explain the mechanism of NO reduction by NH3 have been made on the basis of isotopic tracer experiments (17-20) and spectroscopic studies of the stable adsorbed species (13, 16, 21-24). Using mixtures of ~SNH3 and ~4NO, it has been observed that 15N14N and 15NI4NO are produced selec- tively, suggesting that the reduction of NO occurs via reaction of NO with an NH x spe- cies on the catalyst surface (13, 15-24). The 492