Interfacial chemistry in the preparation of catalytic potassium-modified aluminas

Abstract

The generation of 6 wt.% potassium-modified alumina (K-Al₂O₃) from K₂CO₃-impregnated alumina (Al₂O₃/K₂CO₃) has been explored by means of surface and bulk analytical techniques, so as to characterize the interfacial synthetic events involved in the various preparative steps, namely: (i) wet impregnation, (ii) the drying process and (iii) high-temperature calcination. K⁺(aq) adsorption measurements indicated that 50% of the potassium content in the impregnating solution is loaded onto Al₂O₃via, essentially, a specific adsorption mechanism, involving a type of cation exchange with isolated Al—OH groups and formation of chemically bound Al—OK surface groups. In situ infrared spectroscopic observations supported the adsorption measurements, revealing that after the impregnated material had been dried at 393 K, most of the isolated Al—OH groups are eliminated and, simultaneously, KAl(CO₃)(OH₂)-like surface species are formed. Calcination products up to 1273 K were examined using ex situ X-ray diffraction and photoelectron spectroscopy. The high-temperature treatment was found to activate various solid/solid interfacial interactions leading to low-temperature decomposition of surface carbonate and hydroxy groups, and formation of highly dispersed K-Al₂O₃. The results further indicate that the presence of K⁺ ions has no significant bearing on the accessible area of the alumina and on the thermal stability and structure of the bulk.