Dispersion, measurement with hydrogen

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

The crystal structure of Pd. h Y zeolite was determined before and after hydrogen reduction at different temperatures. When the zeolite is evacuated at 600°C, Pd2+ ions are mainly found to occupy SI sites within the sodalite cages. Hydrogen adsorption at 25° C results in a complete withdrawal of Pd2+from SI sites. This displacement out of cation sites is attributed to the reduction Pd2+ — Pd(0) consistent with hydrogen volumetric measurements. Reduced palladium remains atomically dispersed inside the sodalite cages up to about 200° C. Between 200 and 800° C, Pd 0) atoms migrate toward the outer surface of the zeolite where they agglomerate into 20-A diameter crystallites. 

From the relative area of the Pt spectrum that is sensitive to the double resonance, the fraction of Pt atoms that are bonded to a CO can be found. This fraction was 40% for a sample with a dispersion of 0.26 measured by hydrogen adsorption [0.22 from transmission electron microscopy (TEM)] (Fig. 40a), and it was 81% for a sample with dispersion of 0.76. No clean-surface Pt, NMR spectra have been published for these samples, but it was stated that the change in Pt lineshape upon CO chemisorption is similar to that caused by hydrogen chemisorption (see Section VI.C). 

Fig. 4 The components of the surface energy measured on hydrogenated carbon overcoated thin film magnetic recording media (A) The dispersive component of the surface energy for PFPE Z and Zdol (B) the polar component of the surface energy for PFPE Zdol with molecular weight (M-w) of 1100 (A) 1600 (O), and 3100 ( ) g/mol and (C) the disjoining pressure as a function of film thickness for PFPE Zdol (Mw is 3100 g/mol).

The physical and chemical nature of the rhenium in platinum-rhenium catalysts has been considered by a number of investigators. Johnson and Leroy (63) concluded that the rhenium is present as a highly dispersed oxide at typical reforming conditions. They studied a series of alumina-supported platinum-rhenium catalysts with platinum contents ranging from 0.31 to 0.66 wt% and rhenium contents ranging from 0.20 to 1.18 wt%. Their conclusions were based on measurements of hydrogen consumption during reduction of the catalysts at 482°C and on X-ray diffraction studies of the metal component of the catalyst after the alumina had been leached from the catalyst by treatment with a solution of fluoboric acid. 

---