Mixed gels of vanadia and silica: Structural properties and catalytic behavior in selective reduction of nitric oxide with ammonia

Abstract

Mixed gel catalysts of vanadia and silica containing 0.1, 1, 10, 20, and 50 wt% of vanadia were prepared from alkoxides of the constituents. The catalysts were investigated with regard to their physical and chemical properties, and their activity in the selective catalytic reduction (SCR) of nitric oxide. The textural properties (grain morphology, BET surface area, specific pore volume, pore size distribution) were found to depend on the composition of the mixed gels and varied most strongly in the composition range 1–20 wt% of vanadia. Bulk structural and chemical properties were investigated by means of X-ray diffraction (XRD), high-resolution electron microscopy (HREM), electron spin resonance, and temperature-programmed reduction and oxidation. XRD and HREM revealed that silica was existing in all catalysts as an amorphous phase only. Vanadia showed more complex behavior. At low concentrations (less than 10 wt%) it consisted of well-dispersed vanadium oxide species, whereas at higher concentrations small macrocrystalline domains were found in the amorphous V 2O 5 matrix. Highest specific activities (molecules of NO converted to N 2 per V 5+ site per second) were measured for the catalysts containing 1 and 10 wt% of vanadia. Catalysts with higher vanadia contents exhibited a significant decrease in the activation energy around 430 K which is attributed to agglomeration of the well-dispersed vanadium oxide species under SCR conditions. The stability of the well-dispersed vanadium oxide species under SCR conditions was found to decrease with increasing temperature and vanadia content of the catalyst. In all mixed gel catalysts the well-dispersed vanadium oxide species were more stable than those in silica-supported vanadia layers.