Bonding of supported chromium during thermal activation of the CrOx/SiO2 (Phillips) ethylene polymerization catalyst

An active surface science model for the Phillips ethylene polymerization ca talyst has been prepared by impregnating aqueous CrO3 on a flat silicium(10 0) substrate covered by amorphous silica. Using a combination of X-ray phot oelectron spectroscopy, secondary ion mass spectrometry, and Rutherford bac kscattering spectrometry, we studied the effect of calcination on the state of the supported chromium. Depending on the calcination temperature and th e initial Cr loading;of the catalyst, two processes are observed. The impre gnated chromate anchors to the silica surface in an esterification reaction with the surface silanol groups of the support. The saturation coverage of these surface chromates is 2.4 Cr/nm(2) for a calcination temperature of 4 50 degreesC. Superficial, not anchored, chromate slowly desorbs from the fl at silica surface. Under crowded conditions a portion oi the surface chroma tes also desorb if the calcination temperature is increased, while low Cr l oadings (>1 Cr/nm(2)) are stable up to the highest calcination temperature in our experiments (730 degreesC). The silica-bound surface chromates are m onochromates exclusivly, independent of the initial loading or calcination temperature.