Pd/CO average chemisorption stoichiometry in highly dispersed supported Pd/γ-Al2O3 catalysts

Following our previous papers regarding Pd/C and Pd/SiO2 catalysts, the determination of the Pd/CO average chemisorption stoichiometry from experimental data has been extended to the Pd/alumina system. To this purpose, the Pd crystallite size determined by the well-established line-broadening (LB) method, using X-ray powder diffraction (XRPD) associated with the Fourier analysis of suitable best-fitted peak profiles, and the chemisorbed CO volume, determined by the pulse flow method, were employed. HRTEM was also used to check the structural features of the Pd particles, which proved to be monodomains of cubooctahedral shape. A high-dispersion 5 wt % Pd/alumina catalyst was prepared by impregnation and thermally treated at different temperatures up to 1073 K. Even at this high temperature a good Pd dispersion was retained. A suitable subtraction of the X-ray scattering, due to the support, was done to obtain the real profile of the supported metal, from which the surface-weighted average particle diameters were calculated. Using these values and the corresponding CO chemisorbed volumes, a Pd/CO average chemisorption stoichiometry close to 2 was obtained. On the basis also of our previous work regarding Pd/C and Pd/SiO2 catalysts, it can be concluded that for all supported Pd catalysts such stoichiometry is close to 2, independent of support nature and Pd dispersion, when chemisorbed volumes are measured by the routinely used pulse flow method.