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Notes

PULSE CHEMISORPTION

Previous issues of Altamira Notes have discussed different selective chemisorption techniques and how they may be used to determine the specific metal surface area of supported metal catalysts.   One additional technique commonly used for the same purpose is pulse chemisorption. This method is one of the simplest, most straightforward ways to measure adsorbate uptake by a metal surface; however, as with most other measurements in catalysis, interpretation of the results can be problematic if the nature of the catalyst system and the experiment itself are not well-understood. This Altamira Note discusses the principles of pulse chemisorption experiments as well as some common experimental observations

VOLUMETRIC CHEMISORPTION METHODS FOR METAL CRYSTALLITE SIZE DETERMINATION

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 OF ADSORBED SPECIES FROM CATALYST SURFACES

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.