Surface contribution to the interfacial chemistry of potassium modified oxide catalysts - Silica-alumina versus silica and alumina

The surface contribution to the interfacial chemistry involved in the preparation of potassium-modified metal oxide catalysts has been revealed by examining industrial 5 wt.% K+ silica-alumina versus silica and previously examined alumina. The investigation was conducted by means of potassium and nitrogen adsorption measurements, in situ IR spectroscopy, X-ray diffractometry and photoelectron spectroscopy, which allowed definition of the adsorptive and thermochemical events occurring during the impregnation, drying and calcination steps of the preparation. The oxide surface chemistry influences the adsorption amounts attained during the impregnation from aqueous K2CO3 solutions: non-specific and specific adsorption on silica (7 K+ nm(-2)) and only weaker specific adsorption on silica-alumina (1.1 K+ nm(-2)) related to the low content of very well dispersed silica. On both carriers, the presence of potassium leads to the lower amount and weaker stability of the surface hydroxy groups owing to synergetic effects with absorbed carbonate. During calcination, K+ greatly enhances the crystallization of silica into crystoballite at 973 K and tridymite at 1273 K, which is accompanied by a strong decrease in surface area, a better dispersion of K+ and an increase in surface basicity, by comparison with silica-alumina. No specific effect can be observed on silica-alumina during transformation of the boehmite form into the gamma-alumina-type structure; the presence of very well dispersed silica in alumina decreases the CO2 adsorption capacity observed on pure alumina.