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The binding of vapor on silica pre-heated at 200, 560, and 820 ¡C was studied between 160 and Cr(acac) 3 240 ¡C in a Ðxed bed Ñow reactor by determining the Cr and carbon concentrations, recording FTIR spectra and analyzing the thermal stability of the supported Cr complexes (thermogravimetric analysis and temperature programmed desorption). Saturation of silica with the Cr complex was observed at all reaction temperatures. The saturation density varied from 0.3 to 0.8 Cr nm~2 with variation in the reaction temperature and the pre-treatment temperature of silica (200, 560 or 800 ¡C), evidently determined by the number of available sites on silica and by the size of the Cr complex. At 160 ¡C, both associatively adsorbed and reactedwere concluded to exist. The amount of reacted increased with increase in Cr(acac)3 Cr(acac) 3 reaction temperature, and at 200 ¡C the samples appeared to contain mainly species formed in SiOÈCr(acac) 2 the reaction with isolated OH groups. Samples prepared at slightly higher temperature of 220È240 ¡C also contained partially decomposed acac ligands, denoted Cr acetate species. The ligands of the surface complex could be removed by air treatment with concurrent oxidation of Cr3`to Cr6`surface species. The oxidation was almost complete in the samples prepared at 200È240 ¡C, whereas clustering of Cr3`to occurred in Cr 2 O 3 the samples prepared at 160 ¡C.