New strategies in the Raman study of the Cr/SiO2 Phillips catalyst:: Observation of molecular adducts on Cr(II) sites

We report the first Raman spectra of molecular adducts (CO and N-2) formed on Cr(II) sites dispersed on silica surface (Phillips catalyst for ethylene polymerization), thus obtaining indirect information about the metal anchored species. These results have been achieved using an ad hoe selected laser line (able to excite a ligand to Cr charge-transfer transition that does not relax in a radiative channel), and adopting as a support a silica aerogel behaving as an optically uniform medium in the region of work. These two combined strategies, never simultaneously applied before, allowed us to obtain great quality Raman spectra of surface species, demonstrating that Raman spectroscopy can have great sensitivity toward surface species present in small concentration. Raman spectroscopy thus becomes a powerful tool complementary to IR and UV-vis DRS spectroscopies for surface investigation. The improvement can be quantified by comparing published spectra of the oxidized Cr(Vl)/SiO2 system (the only available in the literature) with the more intense and much richer one obtained under the present experimental conditions, where new spectroscopic features (never observed before) are clearly observed.