Activity and deactivation of sulphated TiO2- and ZrO2-based V, Cu, and Fe oxide catalysts for NO abatement in alkali containing flue gases

Vanadia, copper and iron oxide catalysts supported on conventional TiO2, ZrO2, and sulphated-TiO2 and ZrO2 have been prepared. These catalysts were characterized by elemental analysis, N2-BET, XRD, and NH3-TPD methods. The influence of potassium oxide additives on the acidity and activity in NO selective catalytic reduction (SCR) with ammonia was studied. The absolute activity of the samples does not vary significantly depending on the nature of the active metal and the acidic properties of the support used, seem to be influenced mainly by the concentration of active metal. Loading of the catalysts with potassium leads to a considerable decrease of their catalytic activity. In the case of the traditional carriers (TiO2, ZrO2), the poisoning of the catalyst with small amounts of potassium oxide (K/metal ratio <0.5) leads to almost complete deactivation of the catalysts. The use of transition metals which reveal mainly Lewis acidity (Fe, Cu), slightly increase the resistance towards alkali poisoning in comparison with V-based catalyst. For the sulphated systems, strongly acidic surface sulphate groups represent attractive sites for hosting potassium oxide at temperatures below 400 °C. The resistance of corresponding catalysts towards poisoning correlates well with the strength of acid sites of the support used. The highest resistance was observed for the catalyst based on sulphated-ZrO2. At temperatures above 375–400 °C, potassium additives become more mobile and are no longer bonded by the sulphated groups of the carrier and potassium migration leads to its preferential localization at the active sites responsible for the SCR activity, which is followed by considerable decrease of the activity.