Total Surface Area and Pore-size Distribution

1 Total Surface Area and Pore-size Distribution 

The total surface area is calculated from the amount of physical adsorption of nitrogen at 77 K. During the thirties Brunauer, Emmett, and Teller [1,2] presented a theory dealing with the multilayer adsorption of gases on solids. They assumed that the first layer of gas molecules is adsorbed more strongly than subsequent layers, and that the heat of adsorption of subsequent layers is constant. They also assumed the absence of lateral interaction between adsorbed molecules. On the basis of these much criticized assumptions they derived an adsorption isotherm, which describes the experimentally determined adsorption isotherms excellently. From the adsorption isotherm a value corresponding to the volume of the adsorbed monolayer is calculated. With physical adsorption the amount of gas adsorbed is usually plotted as a function of the relative pressure, that is the pressure 

Brunauer, Deming, Deming, and Teller [3] later distinguished five different physical adsorption isotherms. The Type I adsorption isotherm is characteristic of chemisorption, for which the first layer is adsorbed much more strongly than subsequent layers. The type II isotherm is characteristic of the multilayer adsorption exhibited with physical adsorption near the boiling point of the adsorbate. Type III isotherms are obtained for multilayer physical adsorption with condensation of the adsorbate in narrow pores whereas Type IV isotherms are obtained when the first layer is adsorbed with a lower heat than the heat of condensation of the adsorbate. Finally, Type V isotherms are characteristic of adsorption according to Type IV on an adsorbent with narrow pores. 

The volume of (liquid) nitrogen taken up at saturation measures the pore volume of the catalyst, Vp j. A mean pore diameter can be calculated from the pore volume and the BET surface area, 5bet- If presence of cylindrical pores is assumed, the mean pore diameter, d, is d = 4(Vpoj/5bet)- 

When capillary condensation of nitrogen in pores present in the catalyst proceeds, there is a positive deviation from the f-plot because at a given relative pressure more nitrogen is taken up than is adsorbed on a flat surface. Because the curvature of the meniscus of the capillary condensed nitrogen is different during adsorption and desorption, the sorption isotherm usually exhibits hysteresis, unless only open slit-shaped pores are present. The shape of the hysteresis loop indicates 

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The so-caUed surface texture of a sohd catalyst is adequately described in terms of the total surface area and pore size distribution, both of which are conveniently measured by gas absorption techniques familiar by now to all catalytic chemists 23). Wheeler (24), among... 

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