Adsorption in mesoporous materials

Porous solids represent a broad class of materials that finds many applications in a variety of different fields, ranging from catalysis to oil extraction, from mixture separation to pollution control. They all share a complicated pore connectivity and a broad distribution of pore sizes, whose mean value ranges from nanometer to micron size, depending on the fabrication method. The key parameters of these materials are the exposed surface area and the mean pore diameter. They are usually deduced through the measurement of adsorption isotherms.
These curves represent the mass of a gas adsorbed onto the porous matrix as a function of the equilibrium vapor pressure of the surrounding vapor (typically N2 or Ar at liquid nitrogen temperature). In the case of mesoporous materials (i.e. mean pore diameter comprised between 10 and 200 nm) [1], they typically exhibit two main features: i) a sharp increase in the amount of adsorbed gas well below the liquid-vapor coexistence pressure P0 of the bulk adsorbate, which is explained in terms of capillary condensation in the small pores, (ii) an hysteresis loop between the adsorption (gas is added to the sample cell) and the desorption (gas is removed from the sample cell) branches. Condensation occurs at a pressure Pads larger than the pressure of evaporation Pdes. As D increases, Pads moves closer to P0. From the shape of this loop it is possible to derive information about the pore size distribution and connectivity.
Despite its commonplace occurrence, the origin of the hysteresis phenomenon is still a matter of debate. Various contradictory explanations have been so far proposed, none of them gaining a general consensus yet [1]. To find out the essential factors affecting the hysteresis, we have started a systematic adsorption study using nanoporous anodic aluminum oxide (AAO) [2]. This material presents many important advantages: i) the pores are straight and strictly separated, i.e., no junctions and no micropores penetrating through the pore walls are present; ii) their mean diameter can be easily tailored in an ample interval (10-200 nm); iii) the length of the pores can be greater than hundreds of microns; iv) the pore size distribution is narrow (the ratio between the standard deviation and the mean pore diameter amounts to about 8%); v) tailoring of the pore shape can be done.
The adsorption isotherms are measured with a torsional microbalance [3]. The porous samples are attached to the extremity of a hardened steel rod and driven to the torsional resonant frequency of the system by means of a piezoelectric crystal acting onto the extremity of a stainless steel arm hard soldered to the torsion rod. The oscillations are detected by a similar piezo mounted in a symmetric way with respect to the other one. As the sample is exposed to a vapor, the resonance frequency decreases because of an increase in the total moment of inertia I. Assuming that under equilibrium conditions a homogeneous film covers the substrate, the relative change of I is then proportional to the film mass. Therefore, apart from a multiplicative constant, the mass of the adsorbed film is proportional to the relative frequency shift. References [1,5-6] contain the principal results obtained till now.
 

1. L. Bruschi and G. Mistura, "Adsorption Within and On Regularly Patterned Substrates", J. Low Temp. Phys. 157, 206-220 (2009).
2. W. Lee, K. Schwirn, M. Steinhart, E. Pippel, R. Scholz and U. Goesele, "Structural engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium", Nature Nanotechnol. 3, 234 (2008).
3. L. Bruschi, A. Carlin and G. Mistura, "Wetting on a linear wedge", Phys. Rev. Lett. 89, 166101 (2002).
4. L. Bruschi, G. Fois, G. Mistura, K. Sklarek, R. Hillebrand, M. Steinhart and U. Gœsele, "Adsorption hysteresis in self-ordered nanoporous alumina", Langmuir, 24, 12945 (2008).
5. L. Bruschi, G. Mistura, L. Liu, W. Lee, U. Gœsele and B. Coasne, "Capillary Condensation and Evaporation in Alumina Nanopores with Controlled Modulations", Langmuir, 26, 11894-11898 (2010).
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