VANADIA CATALYSTS FOR THE SELECTIVE CATALYTIC REDUCTION OF NO BY NH3 PREPARED BY VAPOR-DEPOSITION OF VANADYL ALKOXIDE ONTO VARIOUS SUPPORTS

Chemical vapour deposition of vanadium-oxy-triisopropoxide has been us ed to prepare vanadia submonolayers on TiO2, SiO2, Al2O3, ZrO2, and me chanical mixtures of the latter three oxides with TiO2. The catalysts were tested for the selective catalytic reduction (SCR) of NO by NH3. Vanadia supported on titania and, to a lesser extent, vanadia on zirco nia exhibit high intrinsic activity and high selectivity to N2 compare d to vanadia supported on alumina or silica. Vanadia on silica showed a non-linear temperature dependence in the Arrhenius diagram and consi derable production of undesired N2O. The stability of the deposited va nadia towards reduction in hydrogen was significantly influenced by th e support material and increased in the sequence ZrO2 < TiO2 < Al2O3 < SiO2. The catalytic behaviour of the catalysts prepared by deposition of vanadia onto mechanical mixtures of these oxides with titania conf irmed the high suitability of the latter as a support for vanadia base d SCR catalysts. The pure titania support showed marked NO conversion and significant formation of undesired N2O. Upon deposition of vanadia this undesired reaction was suppressed leading to highly selective SC R catalysts. Raman spectroscopy showed for the vanadia deposited from the vapour phase similar structural features as known for correspondin g catalyst prepared by grafting in the liquid phase. Temperature progr ammed desorption and reaction of ammonia revealed significant differen ces in the ammonia adsorption behaviours of the pure supports and the vanadia loaded catalysts. The distribution of the Lewis- and Bronsted- bound ammonia on the supported vanadia catalysts was found to be influ enced by the support. the vanadia loading, and particularly by the pre treatment to which the catalyst was subjected prior to the ammonia des orption measurements. Ammonia bound to V-OH sites is suggested to be m ost active in SCR of NO.