Mercury porosimetry (Hg porosimetry) is a method for characterizing the pore structure of solids. It allows the determination of parameters such as pore size distribution, pore volume, and specific pore surface area. In this method, liquid mercury (Hg), which is naturally hydrophobic, is forced into the pores of a material under high pressure.
The method is based on the fact that mercury, due to its high surface tension, does not penetrate pores on its own. To force the mercury into the pores, external pressure must be applied. The relationship between the applied pressure and the diameter of the pores is described by the Washburn equation:
- Pore diameter
- Surface tension of mercury (~480 mN m-1)
- Contact angle of mercury with the solid surface (~140°)
- Applied pressure
The equation shows that with increasing pressure, increasingly smaller pores can be filled. The pore size distribution can therefore be calculated from the measured pressure during the filling process. In contrast to BET analysis, this allows for the differentiation between macropores (> 50 nm) and mesopores (3–50 nm). This is important for the selectivity of the catalyst, as the pore size determines the diffusion rate of reactants into it. For example, a microporous catalyst (< 3 nm) may have a high total surface area (BET) and dispersion (chemisorption), but if the molecules are too large to diffuse into the pores, this catalyst is ineffective.
