Surface area from intrusion curves

The surface area of all pores and voids filled up to pressure P can be obtained from mercury intrusion data. 

pores in the radius range dr which take up volume dV will possess area dS as given by 

The area of pores filled up to radius r is given by integration of the preceding equation which is, assuming constancy of surface tension and wetting angle, given by 

The surface area calculated by the above method is modelless and assumes no specific pore geometry. 

For various pressure units with V in cubic centimeters, 6 = 140°, and y = 480 erg cm equation (11.5) becomes 

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Figure 11.7 is a cumulative pore-volume intrusion curve which shows the summation of volume intruded into the pores and interparticle voids plotted versus the applied pressure. Recognizing that the increase in interfacial area from equation (10.22) is effectively the pore and void surface area S, this equation can be rewritten as... 

Nitrogen adsorption/condensation is used for the determination of specific surface areas (relative pressure < 0.3) and pore size distributions in the pore size range of 1 to 100 nm (relative pressure > 0.3). As with mercury porosimetry, surface area and PSD information are obtained from the same instrument. Typically, the desorption branch of the isotherm is used (which corresponds to the porosimetry intrusion curve). However, if the isotherm does not plateau at high relative pressure, the calculated PSD will be in error. For PSD s, nitrogen condensation suffers from many of the same disadvantages as porosimetry such as network/percolation effects and pore shape effects. In addition, adsorption/condensation analysis can be quite time consuming with analysis times greater than 1 day for PSD s with reasonable resolution. 

The specific surface area of vanadium pentoxide coated membrane was 1.1 m /g, and the average pore diameter of it, measured by nitrogen adsorption, was 30A. Macro pores identified by mercury intrusion did not exist, and the porosity was approximately 9%. From results of isotherm adsortion-desorption curve,as shown in Fig.2, V2O5-coated membrane has slit-shaped pore structure. Hence the pores mostly exist between the V2O5 layers, and the surface of those are nearly non-porous. 

In Table 1 the results obtained from the textural characterization of the supports and catalysts by nitrogen adsorption and mercury intrusion porosimetry are presented. In the table the values of surface area obtained from the gas adsorption results, using the BET method for which the linear portion was usually located in the relative pressure range of 0.05 to 0.3 Sbet [9], and those from the intrusion curve of the porosimetry analysis, using a nonintersecting cylindrical pore model Sng [10], are shown. The pore volume Vp is that recorded at the liighest intrusion pressure reached during the porosimetry analysis, and as such represents the pore volume of pores between ca. SOpm to 3mn pore radius. The pore radii were taken from the maxima of the curves of pore size distribution. 

The catalysts prepared on these supports had similar properties to those corresponding to the support used. From the results presented in Table 1 the modifications in the properties which were caused by the introduction of the active phase on the supports may be seen. Thus, on the introduction of the active phases on the acid washed sepiolite, SLCuNi, the BET surface area was reduced, probably due to blocking of the narrower mesopores. Due to the deposition of the active phases on the pore walls the areas calculated from the intrusion curves of... 

The surface areas calculated from the porosimetry intrusion curves (Hg Area), presented in column 4 were calculated using a cylindrical nonintersecting pore model, and only represent the surface area of pores down to 3.5 nm pore radius [10]. Discrepancies between the surface area results obtained from this method and that of gas adsorption, presented in column 5, were due to the presence of pores of less than 3.5 nm which remained undetected. These differencies were greatest with the lower heat treatments but as the thermal treatment was increased the shifts in the pore size distributions to wider mesopores and eventually to macropores, brought the two measurements into closer agreement. 

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