Combined use of carbon dioxide (CO2) at 273.15 K / 0 °C with Ar at 87 K or N2 at 77 K is considered a standard methodology for pore size analysis of microporous carbons. Both Ar and N2 suffer from restricted diffusion into very small micropores (<0.45 nm). Because of the slightly smaller kinetic diameter and the higher temperature of the analysis, CO2 at 273 K can access pores down to 0.35 nm. Furthermore, a typical micropore run utilizing CO2 can be completed in a few hours, compared to over 40 hours utilizing Ar or N2. However, because of the high saturation pressure of CO2 at 273 K, low pressure (up to atmospheric pressure) experiments are limited to analysis of pores <1.5 nm. CO2 adsorption is therefore considered complementary to Ar or N2 adsorption for smaller pores. The pore size distributions resulting from the different tests can be combined to obtain the distribution over the full micro- and mesopore size range.
Choosing the right gas for surface area and pore size analysis
A material’s surface area and pore size distribution often correlates directly with its application proficiency and success. Small changes in these properties can lead to completely different behavior in particular applications. Therefore, determining surface area and pore size is crucial for optimizing material properties. For nanoporous materials this can be done with gas adsorption.
Historically, nitrogen gas (N2) at liquid nitrogen temperature (77 K / -196.15 °C) was the preferred choice for gas adsorption studies for the structural characterization of materials. While N2 is still the most commonly used gas found in the literature other gases are now recommended by the International Union of Pure and Applied Chemistry (IUPAC) because they offer certain benefits.
Figure 1: Different adsorption gases are employed for different applications.
Nitrogen at 77 K / -196.15 °C
N2 adsorption at liquid nitrogen temperature (77 K / -196.15 °C) sees wide usage due to the ease of accessibility of liquid nitrogen in most laboratories and a long history of use as the traditional adsorptive. However, it is now recognized that nitrogen has some limitations in its use, primarily due to N2’s quadrupole moment (caused by higher electron density on the ends of the N2 molecule), which leads to specific interactions with functional groups or exposed ions on material surfaces. These interactions between the N2 molecule and the material’s surface functionality can shift the pore filling pressure, leading to inaccuracies in pore size calculation. In addition, the possibility of different orientations of the N2 molecule on the surface of polar materials leads to uncertainty in the cross sectional area used for BET surface area calculations. Per IUPAC, this uncertainty in the assumed cross-sectional area can result in a reported N2 BET surface area which differs from the true surface area of the material by as much as 20 %.
In spite of these difficulties, nitrogen can readily be used in the pore size analysis of mesopores (2 nm to 50 nm), where surface interactions do not play a large role in pore filling processes, and in the analysis of microporous materials (<2 nm) with non-polar surfaces like many activated carbons. Although there can be significant uncertainty in the assumed cross-sectional area of N2, N2 BET surface areas can be highly reproducible and remain useful for benchmarking and inter-laboratory comparisons.
Figure 2: Historically, nitrogen gas was the most common choice for gas adsorption studies.
Argon at 87 K / -186.15 °C
IUPAC recommends the use of argon (Ar) gas at liquid argon temperature (87 K / -186.15 °C) for pore size and surface area characterization. This is especially true for microporous, polar materials such as zeolites, metal oxides, and metal-organic frameworks (MOFs). There are some reasons for that:
First, Ar is monoatomic and, therefore, has a consistent orientation on the surface of the material. This allows for unambiguity in Ar’s cross-sectional area used for surface area calculations. Also, Ar lacks a dipole or quadrupole moment, thus it eliminates possible interactions with any surface functional groups or exposed ions present in these materials. As a result of having no specific interactions with the material’s surface chemistry, there is a direct correlation between argon pore filling pressures and the pore size of the material.
Second, the micropore filling step utilizing Ar at 87 K shifts to higher relative pressures than N2 at 77 K (illustrated in Figure 3). This can have a large impact on the speed of analysis. In addition to these elevated pressures, the analysis is performed at a slightly warmer temperature, which improves the kinetics of adsorption. In summation, these factors can lead to a reduction of analysis time of up to 50 % for Ar at 87 K compared to N2 at 77 K.
Figure 3: N2 (77 K) and Ar (87 K) adsorption isotherms on a faujasite zeolite, plotted with a logarithmic x-axis to highlight the differences in the low pressure region. The micropore filling step in the N2 isotherm is shifted to lower P/P0 because of specific interactions between N2 and the polar sites of the zeolite.
Carbon dioxide at 273.15 K / 0 °C
Krypton at 77 K and 87 K (-196.15 °C and -186.15 °C)
For materials with low surface areas (typically below 0.5 m2), analysis by krypton (Kr) at 77 K / liquid Kr at 77 K, Kr has enhanced sensitivity for low surface areas. Krypton at 77 K is exclusively used for surface area assessment. At 87 K, Kr still has increased sensitivity and is recommended for pore size analysis of low volume samples such as thin films, although adsorption is limited to filling of pores <10 nm.
Conclusion
| Adsorptive | Temperature | Purpose/benefit | |||
| Surface area | Micropore size distribution | Mesopore size distribution | Total pore volume | ||
| Nitrogen | 77 K | ✓ | not recommended | ✓ | ✓ |
| Argon | 87 K | ✓ | IUPAC recommendation | ✓ | ✓ |
| Carbon dioxide | 273 K | – | ✓ (carbons) | – | – |
| Krypton | 77 K | ✓ (low surface area) | – | – | – |
| Krypton | 87 K | ✓ (thin films) | ✓ (thin films) | ✓ (up to 10 nm) | – |
Table 1: Gases recommended for physisorption characterization of common materials.
The choice of analysis gas for pore size and surface area measurements is critical to obtain the most useful and accurate information about a material and often guides material selection for application. A summary of the IUPAC recommended gases for different common materials is given in Table 1. The choice of gas depends highly on the surface chemistry and functional groups present in the sample and is also guided by the pore size range and surface area of the material. Gas sorption instruments such as Autosorb-iQ, Quadrasorb, NovaTouch, and Autoflow are capable of measurements using some or all of the gases mentioned above. Temperature control accessories such as CryoSync, which uses commonly available cryogens, and CryoCooler, which is cryogen-free, can help obtain the appropriate analysis conditions.