INTERFACIAL CHEMISTRY IN THE PREPARATION OF CATALYTIC POTASSIUM-MODIFIED ALUMINAS

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 Al2O3 via, 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.