Significance and Use

The test method has two main functions: first, it provides data useful for establishing the pore size distribution of catalyst materials, which in turn may influence their performance; and second, it serves as a laboratory test which may be used to study porosity changes that may occur during the manufacture and evaluation of catalysts.

Scope

This test method covers the determination of nitrogen adsorption and desorption isotherms of catalysts and catalyst carriers at the boiling point of liquid nitrogen.2 A static volumetric measuring system is used to obtain sufficient equilibrium adsorption points on each branch of the isotherm to adequately define the adsorption and desorption branches of the isotherm. Thirty points evenly spread over the isotherm is considered to be the minimum number of points that will adequately define the isotherm.

Summary of Test Method

The sample is heated and evacuated to remove adsorbed vapors from the surface. The nitrogen adsorption branch of the isotherm is determined by evacuating the sample, cooling the sample to the boiling point of liquid nitrogen (;77.3 K), and subsequently adding stepwise, known amounts of nitrogen gas to the sample in such amounts that the form of the adsorption isotherm is adequately defined and the saturation pressure of nitrogen is reached. Each additional dose of nitrogen is introduced to the sample only after the foregoing dose of nitrogen has reached adsorption equilibrium with the sample. By definition, equilibrium is reached if the change in gas pressure is no greater than 0.1 torr over a 5 min interval. The desorption isotherm is determined by desorbing nitrogen from the saturated sample in a stepwise mode with the same precautions taken to ensure desorption equilibration as applied under adsorption conditions. It is essential that the experimental points be distributed over the isotherm in such a manner as to correctly identify and define the isotherm. If the additions or withdrawals of nitrogen are too large, the temporarily too-high nitrogen gas pressure during adsorption or too-low gas pressure during desorption, may result in so-called scanning effects within the hysteresis loop of the adsorption-desorption branches of the isotherm. The occurrence of scanning may result in too-high equilibrium values for the adsorption isotherm and too-low values for the desorption isotherm.

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