VANADIA SUPPORTED ON TITANIA PREPARED BY GRAFTING - INFLUENCE OF SUPPORT PROPERTIES ON THE STRUCTURE AND ACTIVITY OF VANADIA FOR THE SELECTIVE CATALYTIC REDUCTION OF NO BY NH3

The selective catalytic reduction (SCR) of NO by NH3 has been investigated over vanadia/titania catalysts prepared by grafting of vanadyl alkoxides onto three titania supports which differed in their morphologies (surface area, phase composition) and concentrations of impurities. Neutron activation analysis revealed the presence of chlorine and potassium as major impurities in the titania carriers. The supported catalysts were prepared by the reaction of VO(OR)3 (R = i-C3H7, i-C4H9) with support surface hydroxyl groups. Attention was paid to the influence of various preparation parameters (vanadia precursor, support pretreatment, impurities) and their relation to the amount, reducibility and activity of the vanadia deposited. Thermal analysis, X-ray diffraction and Raman spectroscopy revealed that pre-conditioning the titania supports before the grafting, if not carried out under mild conditions, results in partial transformation of the anatase phase to rutile. Potassium impurities appear to stabilize the anatase phase with regard to this transformation. For titania containing potassium as impurity, the rutilization occurs at a significantly higher temperature than observed with titania containing chlorine as impurity. Raman spectroscopy showed that within the series of titania supports investigated, the vanadia loading is the most important factor that determines the structure of the immobilized vanadia surface species. Small clusters and ribbons prevail at low coverages (approximately 2mumol V5+/m2). At higher coverages, the catalytically most active two-dimensional vanadia layers are prevailing. The maximum level of NO turnovers is achieved at coverages exceeding approximately 3mumol V5+/m2. This behaviour was observed for vanadia deposited on all three titania carriers, irrespective of the largely different BET surface areas and contaminant levels of the supports.