Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure

Density functional theory is applied to molecular models and embedded cluster models for vanadia on three different crystalline silica supports that span the range of local structures found in amorphous silica. For monomeric vanadyl sites, three different types of vibrational modes exist. Contrary to previous assignments, the V-O-Si in-phase modes occur at highest frequencies, 1086-1020 cm(-1), whereas the vanadyl modes are found between 1047-1013 cm(-1). The V-O-Si out-of-phase modes have the lowest frequencies in the 962-873 cm(-1) range. For dimeric and polymeric sites, the frequencies are similar, but an additional V-O-V mode is found between 870 and 770 cm(-1) and not in the 940-900 cm(-1) range as assumed earlier. The vibrational frequencies of cluster and embedded cluster models are in general agreement, except that the embedded clusters predict a strong mixing of vanadia modes with modes of the silica support. This mixing changes the normal modes but has only a small effect on the frequencies. Vanadia oil very special support models, like a periodic SiO2 slab consisting of hexagonal prisms, shows distinctly different frequencies with 50 cm(-1) blue-shifted V-O-Si frequencies.