Significance and Use

Pore volume distribution curves obtained from nitrogen sorption isotherms provide one of the best means of characterizing the pore structure in porous catalysts, provided that the limitations of the method are kept in mind. Used in conjunction with the BET treatment for surface area determination (5), these methods provide an indispensable means for studying the structure associated with pores usually important in catalysts. This practice is particularly useful in studying changes in a series of closely related samples caused by treatments, such as heat, compression, or extrusion often used in catalyst manufacturing. Pore volume distribution curves can often provide valuable information during mechanistic studies dealing with catalyst deactivation.

Scope

This practice covers the calculation of pore size distributions for catalysts and catalyst carriers from nitrogen desorption isotherms. The computational procedure is particularly useful for determining how the pore volume is distributed in catalyst samples containing pores whose sizes range from approximately 1.5 to 100 nm (15 to 1000 Å) in radius. It should be used with caution when applied to isotherms for samples containing pores both within this size range and pores larger than 100 nm (1000 Å) in radius. In such instances the isotherms rise steeply near P/Po  = 1 and the total pore volume cannot be well defined. The calculations should begin at a point on the isotherm near saturation preferably in a region near P/Po  = 0.99, establishing an upper limit on the pore size distribution range to be studied. Simplifications are necessary regarding pore shape. A cylindrical pore model is assumed, and the method treats the pores as non-intersecting, open-ended capillaries which are assumed to function independently of each other during the adsorption or desorption of nitrogen.

Summary of Practice

The pore size distribution is determined by analyzing the desorption data of the nitrogen isotherm. The nitrogen uptake is caused by the multilayer adsorption of a film of nitrogen on the pore walls and by capillary condensation of the nitrogen in the “inner core” regions of the pores. The relative pressure at which filling of the core occurs for a given pore size by capillary condensation is predicted from the Kelvin equation (1). During desorption, thinning of the multilayer film adsorbed on the pore walls occurs in pores which have previously lost their capillary condensate. Corrections for film thinning are determined by a procedure involving the surface area and radius of the film which becomes exposed as desorption proceeds. In principle, the computational procedure can be applied to either the adsorption branch or desorption branch of the nitrogen isotherm. Unless the presence of ink-bottle shaped pores is suggested by an abrupt closure of the desorption branch on the adsorption branch, the distribution curve derived from the desorption data is preferred, and is described in this procedure. The computational method is essentially the procedure developed by Barrett, Joyner, and Halenda (2), except for the incorporation of a few simplifications.

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