Vanadia on titania prepared by vapour deposition of vanadyl alkoxide: Influence of preparation variables on structure and activity for the selective ca

Abstract

Vanadyl-triisopropoxide (VOTIP) has been used for vapour deposition out of a carrier gas stream onto titania to prepare titania supported vanadia. The catalysts were characterized by means of temperature-programmed reduction (TPR), Raman- and 51V nuclear magnetic resonance (NMR) spectroscopy and tested for the selective catalytic reduction of nitric oxide by ammonia. The influence of different preparation parameters on the structural and catalytic properties has been studied: (i) degree of dehydration (dehydroxylation) of the support; (ii) exposure to the VOTIP vapour phase and effect of successive depositions; and, (iii) calcination. The degree of dehydration of the support, which can be controlled by pretreatments, influences significantly the amount of deposited alkoxide. The upper limit of vanadia deposition that can be achieved by the used vapour deposition conditions is markedly lower than the one that can be reached by immersion of the carrier into liquid VOTIP. All preparation steps, including carrier pretreatment, deposition and calcination influence the structural and catalytic properties of the final catalysts. Highest intrinsic catalytic activity of the deposited vanadia has been observed above a threshold coverage, at which the formation of extended two-dimensional vanadia surface layers was revealed by Raman spectroscopy. Temperature-programmed desorption of ammonia showed a significant dependence of the profiles on the vanadia loading. On the pure titania support two distinct desorption maxima occurring at 405 K and at 520 K (broad) are observed. With increasing vanadia loading the broad maximum decreases, and simultaneously new sites are formed which correspond to ammonia weakly bound to Brønsted sites desorbing at 350 K. The latter sites are considered to be most active for selective catalytic reduction of nitric oxide.