Correlation of surface acidity with electrical conductivity of silica-supported heteropoly compounds studied by the complex-impedance method

Abstract

A.c. electrical measurements and impedance analysis have been carried out to characterize silica-supported heteropoly compounds (HPC), viz. 12-molybdophosphoric acid (H3PMo12O40), 12-tungstophosphoric acid (H3PW12O40) and their sodium, potassium, and caesium salts under various conditions of humidity. The absorption of water was found to enhance the electrical conductivity of silica-supported HPC. In H3PMo12O40 and its salts supported on silica, complex-impedance plots showed one semicircular arc due to adsorbed water. For H3PW12O40 and its salts supported on silica, the plots showed two arcs; these are ascribed to fast and slow relaxation processes of orientation polarization for conductive species, supposedly protons in occuluded and adsorbed water, respectively. As the relative humidity was increased, the conductivity increased logarithmically, showing humidity-sensing characteristics. The conduction behaviour was affected by the loaded amount of HPC and the substitution of cations in HPC. The activation energy for electrical conduction of low-loaded heteropoly acid (HPA) on silica was lower than that for bulk HPA. It changed both with the amount of loaded HPA and with the substitution for hydrogen ions in the loaded HPA.The acidity function, H0, of the silica-supported HPC has been examined also in connection with their electrical behaviour. The electrical conductivity increased with increasing acidity. A linear relationship was found between the electrical conductivity and the acidity function.