An active surface science model for the Phillips ethylene polymerization catalyst has been prepared by
impregnating aqueous CrO3 on a flat silicium(100) substrate covered by amorphous silica. Using a combination
of X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and Rutherford backscattering
spectrometry, we studied the effect of calcination on the state of the supported chromium. Depending on the
calcination temperature and the initial Cr loading of the catalyst, two processes are observed. The impregnated
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/nm2 for a calcination temperature of
450 °C. Superficial, not anchored, chromate slowly desorbs from the flat silica surface. Under crowded
conditions a portion of the surface chromates also desorb if the calcination temperature is increased, while
low Cr loadings (>1 Cr/nm2) are stable up to the highest calcination temperature in our experiments
(730 °C). The silica-bound surface chromates are monochromates exclusivly, independent of the initial loading
or calcination temperature.
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