Characterization of Cr/SiO2 catalysts and ethylene polymerization by XPS

Cr/SiO2 catalysts with 1 or 3 wt.% Cr loadings and different chromium precursors were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). A method to determine chromium species in the sample was developed through the decomposition of the Cr 2p XPS spectrum in Cr6+ and Cr3+ standard spectra. The results of the binding energy from the Cr 2p region and of the distribution of chromium species allowed to evaluate the dynamic photo-reduction of the surface chromium species during XPS analysis. Photo-reduction of surface Cr6+ to Cr3+ species was verified for all samples supported in silica, depending on the precursor and chromium content. Bulk CrO3 and Cr2O3 standards did not reveal variation in the binding energy of Cr 2p3/2, but a physical mixture of CrO3 with SiO2 presented photo-reduction. The behavior of this mixture resembled to the catalysts and suggests the participation of the surface hydroxyls of silica in the photo-reduction process. XPS intensity measurements for assessing dispersion of chromium oxide were used to compare the calcined and reduced catalysts to different chromium precursors. Polyethylene chains were detected by in situ XPS, while oligomerization products were not observed.