The CrOx/SiO2/Si(100) catalyst - A surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic suppor t forms an attractive way to make supported catalyst suitable for surface s cience characterization. Here we show how this approach has been applied to the Phillips (CrOx / SiO2) ethylene polymerization catalyst. The model cat alyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X-ray Ph otoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) w e can draw a molecular level of the activated catalyst that features exclus ively monochromate species, which are anchored to the silica support via es ter bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing cal cination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attri bute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated act ive site. Diluting the amount of chromium to 200 Cr-atoms/nm(2) of silica s urface enables the visualisation of polyethylene produced by a single activ e site.