STRUCTURAL INVESTIGATION ON THE STOICHIOMETRY OF BETA-PDHX IN PD/SIO2 CATALYSTS AS A FUNCTION OF METAL DISPERSION

Structural investigations of dispersed Pd/SiO2 catalysts were carried out with XRD and SAXS techniques, supported by TPR and CO chemisorption measurements. The stoichiometry of Pd hydride, beta-PdHx, was determined by measuring the shift of Pd 111 XRD reflection in the presence of hydrogen. It was confirmed that x decreases when the metal dispersion increases. This behaviour could be quantified up to about 0.45 of Pd dispersion since the angular position of XRD peaks cannot be determined with the necessary precision at higher dispersions. TPR data, obtained up to dispersions of about 0.6, confirms such behaviour. The beta-PdHx stoichiometry versus Pd dispersion relationship substantially agrees with that found with other techniques by Boudart and Hwang.