Behavior of titania-supported vanadia and tungsta SCR catalysts at high temperatures in reactant streams: Tungsten and vanadium oxide and hydroxide vapor pressure reduction by surficial stabilization

摘要

In order to meet impending regulations for soot and NO_x reduction from mobile diesel engines, advanced emission control systems may require SCR catalysts with substantially improved thermal durability. State of the art vanadia-based SCR catalysts are composed of vanadia, tungsta, and possibly silica, present at relatively low mass fraction and supported on high-surface-area titania. Concern over the possible emission of metals such as vanadia from diesel vehicles fitted with vanadia-based catalysts limits their potential utility. Vanadia and tungsta oxide and hydroxide vapor pressures over the bulk oxides under conditions relevant for the simulated lifetime exposure of catalysts in the mobile SCR application were estimated on the basis of literature data, and the vapor pressures can be consequential. For the bulk tungsta and vanadia, the most volatile component is WO2(OH)2, formed from the reaction of tungsta with water, followed by V4O_(10) and then VO(OH)3, also a reaction product. An experimental method was developed to measure the vapor-phase transport (a manifestation of vapor pressure) of such inorganic components over real catalysts in representative gas streams by collection on high-surface-area alumina at exposure temperature. In the absence of water at 750 °C, only V4O_(10) was anticipated as the volatile species. However, no V was observed downstream of the catalyst, so that the vapor pressure of titania-supported vanadia was strongly suppressed relative to the bulk oxide. In the presence of water, the results depended on the support. In the case of a support that underwent substantial loss of surface area during exposure, amounts of W were collected consistent with equilibrium vaporization as WO2(OH)2. However, the amounts of V collected were below the amount expected based on equilibrium vaporization as either V4O_(10) or VO(OH)3. Thus, the reaction of titania-supported vanadia with water at high temperatures was also suppressed. The vapor pressures of the vanadia and tungsta, and the extent of their reaction with water, can be reduced by varying degrees by reducing loss of the surface area of the titania support during exposure and by minimizing the mass fractions of the surficial oxides. New, highly stable titania supports with optimized compositions were found to virtually eliminate the loss of vanadia and tungsta.