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CHARACTERIZATION OF MICROPOROUS MATERIALS By Carlos A. Leon y Leon D.

All solid powders, and in particular those with apparent surface areas > 400 m2/g, are bound to contain micropores. The importance of micropores lies in that materials containing micropores generally have most of their apparent surface area residing in them. Hence, all applications of materials (adsorbents, catalysts) that depend on their accessible surface area must deal with the possible effects arising from the presence of micropores in the materials.

MEASURING ACIDITY IN ZEOLITES USING TPD

Temperature-programmed desorption (TPD) of basic molecules from the surface of zeolites has been extensively used to measure their acid properties. The TPD experiment consists of sorbing a base molecule on the material of interest and, while flushing the surface with an inert gas, linearly ramping the temperature and measuring the desorption of the base. By quantitatively measuring the amount of base desorbed and noting the temperature(s) of desorption, information can be obtained on both the intrinsic and extrinsic acid properties in a single experiment (See Figure 1).

EFFECT OF SOME EXPERIMENTAL PARAMETERS ON TPR PROFILES

Temperature-programmed reduction (TPR) techniques can yield direct information on the reducibility of catalysts and catalyst precursors and is an excellent technique for characterizing a variety of catalysts. The technique consists of exposing the sample to a flowing mixture of a reducing agent, such as hydrogen, in an inert gas while linearly ramping the temperature. The rate of consumption of the reducing agent is monitored and related to the rate of reduction of the sample. Figure 1 shows the TPR profile obtained for a 10% NiO/Si02 catalyst using a 10% H2/Ar mixture at a flow rate of 30 ml/min and a linear heating rate of 20 K/min. Such a signal gives information concerning the ease of reducibility (temperature at maximum) as well as the extent of reducibility (signal area) of the material being studied. An excellent comprehensive description of this technique is found in the book "TemperatureProgrammed Reduction for Solid Materials Characterization" by A. Jones and B.D. McNicol (Marcel-Dekker, Inc., 1986).