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One of the most important properties which characterizes a supported metal catalyst is its specific metal surface area. This information may be gained by a number of different experimental techniques (1, 2) and is usually reported as the average metal crystallite diameter of the catalyst. This parameter provides a means of comparing catalysts prepared by different methods or in different laboratories. A previous Altamira Note (September 1989) discussed how surface area measurements may be obtained from temperature-programmed desorption data for supported metal catalysts. This Note will focus on how the same information is obtained from a more traditional chemisorption method known as volumetric or static chemisorption


Temperature-programmed desorption (TPD) of species adsorbed on the surfaces of metal oxides or supported metal catalysts is a technique commonly applied in the characterization of hetero-geneous catalysts. A typical TPD experiment consists of several steps:
    Pretreatment. The sample is first subjected to calcination, reduction or out-gassing usually at elevated temperature to remove water or impurities and to prepare the catalyst surface for the adsorption step.
    Adsorption. The sample is contacted with the molecule of interest in one of several different modes, including pulse adsorption, steady flow adsorption, or static non-flow adsorption. The adsorption process may be carried out to the extent that the surface is fully covered with adsorbing molecules or to some fraction of full coverage.
    Desorption. After the surface has been contacted with the adsorbing molecule to achieve the desired coverage, the temperature of the system is raised in a linear fashion while a constant flow of an inert gas passes over the catalyst.  Molecules leaving the catalyst surface are swept into this stream of inert gas and are carried to a detector which monitors the amount of gas and the temperature at which it desorbs.  Desorption into this gas stream occurs when an adsorbed species gains enough energy to overcome the activation energy barrier to the desorption process.

Chemisorption and Catalysis on Supported Metals

Chemisorption, the chemical bonding between gas-phase molecules and surface atoms, is the first and most important step in a catalytic reaction On supported metal catalysts, chemisorption takes place on small metal crystallites, which are typically anchored to a high surface area oxide material. These chemisorbed molecules then react with other adsorbed species or with gas-phase molecules to produce reaction products. Why is chemisorption so important? The rate of the catalytic reaction as well as its selectivity to desired products is directly related to the chemisorption properties of the supported metal catalyst.