VANADIA ON TITANIA PREPARED BY VAPOR-DEPOSITION OF VANADYL ALKOXIDE -INFLUENCE OF PREPARATION VARIABLES ON STRUCTURE AND ACTIVITY FOR THE SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE BY AMMONIA

Vanadyl-triisopropoxide (VOTIP) has been used for vapour deposition ou t of a carrier gas stream onto titania to prepare titania supported va nadia. The catalysts were characterized by means of temperature-progra mmed reduction (TPR), Raman- and V-51 nuclear magnetic resonance (NMR) spectroscopy and tested for the selective catalytic reduction of nitr ic oxide by ammonia. The influence of different preparation parameters on the structural and catalytic properties has been studied: (i) degr ee of dehydration (dehydroxylation) of the support; (ii) exposure to t he VOTIP vapour phase and effect of successive depositions; and, (iii) calcination. The degree of dehydration of the support, which can be c ontrolled by pretreatments, influences significantly the amount of dep osited alkoxide. The upper limit of vanadia deposition that can be ach ieved by the used vapour deposition conditions is markedly lower than the one that can be reached by immersion of the carrier into liquid VO TIP. 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 depos ited vanadia has been observed above a threshold coverage, at which th e formation of extended two-dimensional vanadia surface layers was rev ealed by Raman spectroscopy. Temperature-programmed desorption of ammo nia showed a significant dependence of the profiles on the vanadia loa ding. On the pure titania support two distinct desorption maxima occur ring at 405 K and at 520 K (broad) are observed. With increasing vanad ia loading the broad maximum decreases, and simultaneously new sites a re formed which correspond to ammonia weakly bound to Bronsted sites d esorbing at 350 K. The latter sites are considered to be most active f or selective catalytic reduction of nitric oxide.