Platinum-alumina catalyst surface area

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

Fig. 6. Platinum crystallite size distribution for 2.5% (w/w) platinum/alumina catalyst. Full line, number distribution broken line, surface area distribution. After G. R. Wilson and W. K. Hall, J. Catal. 17, 190 (1970).

The maximum amount of platinum fixed on alumina from HjPtCU acid, without any significant attack of the support, differs widely according to various authors [16, 34, 35, 73, 95, 108, 110], varying between 2 and about 8 wt %. Most of the values obtained at saturation of y-aluminas with surface areas ranging between 150 and 250 m2g-1 are normally between 2 and about 4wt% [16, 34, 95, 110]. The platinum content of industrial catalysts is much lower, varying between 0.2 and 0.6 wt%. 

For the overall performance of potential catalysts in practical application additional factors, such as number of active sites, physical form, and porosity must also be taken into account. The classical commercial iron catalyst is an unsupported catalyst. First of all iron is a cheap material and secondly by the incorporation of alumina a surface area similar to that attained in highly dispersed supported catalysts can be obtained. Of course, for an expensive material such as the platinum group metals, the use of a support material is the only viable option. The properties of the supported catalyst will be influenced by several factors [172]... 

Bead catalysts were based on typical supports such as Y, 5 or 0 alumina with surface areas of about 100 m /g. Alumina was easily impregnated with the platinum and palladium and the resulting catalysts were stable at temperatures up to 1000-1100°C before the phase change to a alumina took place. Alumina can be stabilized to a limited extent by the addition of other refiactory oxides but will still sinter at high temperature. 

To add surface area, the supports are uniformly coated with a slurry of gamma-alumina and recalcined under moderate conditions. The wash coat acts to accept the active metals, typically low levels of platinum and palladium, in a conventional impregnation process. In the United States in passenger car apphcations the spherical catalyst is used almost exclusively, and methods have been developed to replace the catalyst without removing the converter shell when vehicle inspection reveals that emission standards are not met. 

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. 

In order to increase the contact of a catalyst with hydrogen and the compounds to be hydrogenated platinum (or other metals) is (are) precipitated on materials having large surface areas such as activated charcoal, silica gel, alumina, calcium carbonate, barium sulfate and others. Such supported catalysts are prepared by hydrogenation of solutions of the metal salts, e.g. chloroplatinic acid, in aqueous suspensions of activated charcoal or other solid substrates [28. Supported catalysts which usually contain 5, 10 or 30 weight percent of platinum are very active, and frequently pyrophoric. 

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