Vibrational properties of Cr-II Centers on reduced Phillips catalysts highlighted by resonant Raman spectroscopy

It is shown that the Cr(II) surface species, present on the reduced Phillips catalyst, are characterized by two well defined vibrational modes at nu(low)=568 cm-1 and nu(high)=1009 cm-1 which are detected, for the first time, by resonant Raman spectroscopy. These vibrational features are perturbed by the adsorption of probe molecules (N2 and CO), shifting downwards (nlow=542 cm_1) and upwards (nhigh=1048 cm-1). Furthermore, the probes adsorption changes the optical spectrum of the Cr(II)/SiO2 and influences the attainment of pre-resonant Raman conditions. The emersion of the Cr(II) modes from the support background proves that resonant Raman spectroscopy is a site-sensitive technique, also in the presence of a strong admixture of sites and support modes. The results shown for the Phillips catalyst have a more general validity, since they demonstrate that the Raman technique, under resonant and pre-resonant conditions, affords precious and direct information on the structure of the sites playing a role in catalysis.