CHARACTERIZATION OF H4PMO11VO40 HETEROPOLYACID ON SIO2 AND SIC SUPPORTS

Heteropolyacid H4PMo11VO40 has been deposited in different amounts on the surface of SiO2 (Aerosil) and SiC supports. The samples were chara cterized by surface potential, ion scattering spectroscopy (ISS) and X -ray photoelectron spectroscopy (XPS) techniques. The thermal stabilit y of the acid increases at a low acid content corresponding to ca. 0.1 -0.75 theoretical monolayers for the SiO2 support and up to 2.5 monola yers for the SiC support. At higher acid content it is lower (SiO2) or the same (SIC) as that of the unsupported acid. On the basis of the X PS and ISS data, a model of the acid dispersion has been proposed. The deposited phase forms blocks which cover maximally 20% of the SiO2 an d 50-60% of the SiC surface, even when the acid content is equal (SiO2 ) or exceeds (SiC) the theoretical monolayer coverage.