Mesoporosity mercury porosimetry

A closed porosity, for example (Fig. 7.20), will show up very easily in electron microscopy, whereas it will not be directly detected by mercury porosimetry if it is only accessible via micro or mesoporosity. 

Whereas the nitrogen isotherm provides quite reliable data for PSD with a mean size of about 20 nm or less, it is much less reliable for jx>res larger than this. Mercury porosimetry is the alternate technique for characterizing the sizes of the larger mesoporosity and dates back to 1921 and developed later by Ritter and Drake in 1945. TTie physical phenomenon of the mercury porosimeter is that mercury essentially does not wet surfaces of solids, it has a contact angle (0) with solids of about 140° and hence an applied force (pressure (/>)) is needed to push mercury into tubes, or pores with size Tp, as shown below ... 

When calcium is used in the preparation of activated carbons it is found that the adsorption capacity increases in both carbon series with the extent of burn-off. However changes in the porosity of the activated carbons with burn-off differ considerably in the presence of calcium, mainly for CO, activation. Figure 1 (a and b) for carbon A and Figure 2 (a and b) for carbon B show the remarkable effect of the catalyzed carbon-CO, activation. The adsorption isotherms shapes are very different from those found for the uncatalyzed activation. Isotherms are a combination of type I and II in contrast to the well defined type I isotherms obtained for the uncatalyzed CO, activation. Carbon A2 and B2 behave differently (Figure 1 b and 2b) probably due to their different initial porosity and calcium contents. In any case, catalytic activation in CO, gives rise to a noticeable development of mesoporosity and, as a result, a much wider pore size distribution. Mercury porosimetry. Figure 3 (a and b), show the very different pore size distributions obtained by catalytic activation with calcium mesoporosity development is very noticeable in agreement with the N, adsorption data. 

Mercury porosimetry is a convenient method to characterise macropores in cokes, covering a wide range of mesoporosity and approaching the microporosity, 7.5 pm to 3.75 nm (at a pressure of 200 MPa). There are, however, several serious limitations (ref. 21), to mercury porosimetry. There is mercury contamination, advancing or retreating of mercury over the solid surface, and a measurement of pore entrance radii which may be smaller than the main body of the pore. 

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