Platinum catalysts supported, hydrogen chemisorption

Standard Test Methodfor Surface Area of Catalysts. (D3663—78) Standard Test Method for Hydrogen Chemisorption on Supported Platinum on Alumina Catalysts. (D3908-80) American Society for Testing and Materials (ASTM), Philadelphia, PA. 

Catalysis of cyclohexene hydrogenation has been studied extensively both in the vapour and liquid phases on platinum ", palladium and other metallic surfaces. Here the kinetics of the cyclohexene hydrogenation on platinum have been considered lu terms of the specific activities of samples of silica-supported platinum, previously characterised by hydrogen chemisorption. Particular attention has been paid to the structure sensitivity-insensitivity of the reaction and how this varies as carbonaceous overlayers are built up on the catalysts with increasing reaction time. 

Pt NMR has been used to study platinum particles embedded in a zeolite and to compare their characteristics with the more common oxide-supported platinum catalysts (Tong etal. 1993). Spin-lattice relaxation measurements indicated that a measurable fraction of the platinum in a zeolite-Y sample is not in a metallic environment, but it was not clear whether the loss of metallic signal refiected very small particle sizes or was due to interactions with the framework and its counter-ions. The latter possibility was supported by the observation that part of the metallic signal was restored by chemisorption of hydrogen (Tong et al. 1993). 

When the platinum-iridium clusters are still more highly dispersed, and are supported on an alumina carrier instead of silica, the results shown in the right-hand sections of Figure 4.30 are obtained (48). The metal dispersion of the clusters as determined by hydrogen chemisorption is 0.93. If the clusters were spherical, the average diameter calculated from the chemisorption data would be about 12 A. Again the clusters are too small to give a satisfactory X-ray diffraction pattern. As with the previous two catalysts, the values... 

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 process took place on Ni/Cr203 between 373 and 525 K and must require the dissociative chemisorption of the methanes followed by random recombination not surprisingly, deactivation was found at all temperatures." The reaction between normal and deuterated n-butane has also been studied on a number of supported platinum catalysts." Little or no exchange was seen between light methylcyclo-hexane and deuterated n-octane (or the reverse) over Pt/Si02 at 755 K either in the presence or absence of hydrogen or deuterium because of rapid conversion to other molecules reaction did however take place between them at lower temperatures, and on Pt/Al203 it occurred both on the metal and the support. ... 

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