Gas physisorption is an important tool for the characterization of porous solids and fine powders. The initial IUPAC report on gas physisorption, published in 1985, was dedicated to the determination of surface area and porosity by physical adsorption measurements and was broadly accepted and referred to in the scientific community. These recommendations served as the basis for many ASTM and ISO standards. Since 1985, major advances have been made in porous materials characterization including the development of nanoporous materials with uniform, tailored pore structures such as M41S, SBA, KIT, and CMK, among others, the development of computer simulation and density functional theory (DFT) methods to study adsorption of fluids in pores, and the development of high resolution experimental protocols and other experimental advances.
These advances necessitated the updating and extension of the original 1985 recommendations to provide authoritative, state of the art guidance on gas physisorption. Consequently in 2015, IUPAC published new recommendations for physisorption. This report focuses on important aspects of physical adsorption characterization and gives recommendations concerning adsorption methodology and evaluation of adsorption data, including surface area and micro- and mesopore size. The IUPAC classification of isotherms and hysteresis loops has been extended to include new characteristic types. The classification of pore sizes into micropores (pores of width ≤ 2nm), mesopores (pores of width 2-50 nm), and macropores (pores of width > 50 nm) remains unchanged, but with the addition of the term nanopore, which embraces the above three categories, but with an upper limit of 100 nm.
For surface area, pore size, and porosity characterization with nitrogen, argon, and krypton at cryogenic temperatures, IUPAC recommends a manometric (volumetric) measurement technique. Anton Paar’s manometric adsorption instruments, including the Autosorb iQ, Quadrasorb, and NOVAtouch, are in full accordance with the IUPAC recommendations. In particular, the Autosorb iQ-XR model contains the necessary pressure transducers (0.1, 10, and 1000 torr) to tackle any task associated with surface area (including ultra-low surface area) and micro- and mesopore analysis. The Autosorb iQ allows one to measure adsorption isotherms with the highest accuracy and resolution over the complete pressure range required for full micro to mesopore size characterization.
The first step prior to an adsorption experiment is the pre-treatment (outgassing) of the adsorbent to remove all pre-adsorbed species from the surface. IUPAC recommends that for microporous materials, outgassing under vacuum (pressures < 1 Pa (1x10-5 bar), achievable by turbo molecular pump), should be applied. With sensitive samples where powder elutriation could be a problem, a sample controlled heating procedure and lower crossover pressure is recommended.
While vacuum outgassing is recommended for microporous materials, for non-microporous materials, flushing the adsorbent with inert, dry gas (nitrogen or helium) at elevated temperature may be sufficient to prepare samples for manometric measurements.
Sample Cell Void Volume Determination
In order to perform accurate volumetric adsorption experiments, a reliable procedure to determine the void volume is required. The amount of gas adsorbed at a given equilibrium pressure is defined as the difference between the amount of gas admitted and the amount of gas required to fill the space around the adsorbent, i.e., the void volume. The standard procedure uses helium for this task, assuming the adsorption of helium can be neglected; however, this may be problematic for nanoporous solids with very narrow micropores because of possible helium entrapment. In this case, an alternative measurement procedure must be applied.
Anton Paar’s manometric gas sorption instruments are flexible and allow one to determine the void volume with or without helium. In line with the IUPAC recommendation, the instruments feature an automated routine which allows the user to pull a vacuum on the sample after the void volume determination with helium at room temperature for a user defined extended amount of time. However, in the case of a sample with narrow micropores, where helium entrapment might occur, it may be of advantage to use instead the NOVA mode (NO Void Analysis). NOVA mode is in line with IUPAC recommendations, where a multipoint void volume determination of an empty sample cell with adsorptive prior to the isotherm measurement is carried out and recorded.
Choice of Adsorptive
The choice of adsorptive for characterization applications is of the utmost importance for accurate and comprehensive surface area and pore structural analysis. For many years, nitrogen adsorption at 77 K was generally accepted as the standard adsorptive for both micropore and mesopore size analysis, but for several reasons it is now accepted that nitrogen is not a satisfactory adsorptive for assessing the micropore size distribution. The quadrupole of the nitrogen molecule is responsible for specific interaction with a variety of surface functional groups and exposed ions. This influences the orientation of the adsorbed nitrogen molecule on the surface, which affects the reliability of the surface area determination, and strongly affects the micropore filling pressure and shifts the pore filling range to very low pressures (< P/P0 = 10-5). In addition to experimental challenges associated with the adsorption kinetics in this ultra-low pressure range, the major problem is that the pore filling pressure in now not correlated with the pore size in a direct way.
In contrast to nitrogen, argon does not exhibit specific interactions with surface functional groups. Consequently, argon at 87 K fills micropores of dimensions 0.5 nm at significantly higher relative pressures compared to N2 at 77 K, leading to accelerated diffusion and faster equilibration time – allowing for reliable pore size and structure analysis. Ar (87 K) has been recommended by IUPAC, particularly for micropore size analysis.
In the case of nanoporous carbons, which exhibit narrow micropores that are inaccessible for argon and nitrogen due to kinetic restrictions at cryogenic temperatures, IUPAC recommends CO2 adsorption at 273 K. Finally, in order to evaluate low surface area materials, krypton at 77 K is the recommended adsorptive.
Anton Paar instruments are equipped to perform adsorption experiments using all of the characterization gases and temperatures recommended by IUPAC. In order to readily achieve 87 K, even in the absence of liquid argon, devices such as the CryoSync or CryoCooler allow one to easily achieve these temperatures using liquid nitrogen or a cryogen-free system, respectively.
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This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.
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