Micropore Size Analysis of Porous Carbons Using CO2 Adsorption at 273.15 K (0 °C)

Activated carbons in the form of fibers, pellets, granules, and powders are used in air filters, water purification systems, for removing organic impurities from various solutions, as well as in a variety of medical, environmental, and agricultural applications. These highly microporous, high surface area materials are often characterized by their apparent BET surface area and their pore size distribution (PSD). Measuring the PSD of microporous samples using nitrogen or argon adsorption is a slow process, requiring very low vacuum levels that require an expensive turbomolecular pump. Alternatively the PSD can be determined quickly using carbon dioxide at 0°C, at higher pressures than using nitrogen or argon, thus increasing throughput and reducing the equipment cost.

Why Measure Pore Size Distribution of Porous Carbons?

The pore size distribution (PSD) is a key factor in characterizing porous materials. The PSD analysis can be useful in developing new porous materials for specific applications as well as for testing of existing products. Traditionally, the PSD of a porous solid is evaluated from the analysis of nitrogen or argon adsorption isotherms measured at 77.4 K or 87.3 K, respectively. It is well-known, however, that at such cryogenic temperatures, diffusion of adsorptive molecules into carbon micropores is very slow. Moreover, it was pointed out that diffusional limitations at this temperature might influence adsorption in ultra-micropores (pores smaller than 7 Å (0.7 nm)).

For porous carbons, usually containing a wide distribution of pore sizes including ultra-micropores, these diffusional limitations lead to time consuming measurements and may cause under-equilibration of measured adsorption isotherms, giving erroneous results for the PSD. It has long been recognized that problems of this type can be eliminated by using CO2 adsorption at 273.15 K (0°C).

Which Instrument is Used?

Although it is possible to perform these measurements on the Autosorb iQ and the Quadrasorb evo, the low cost and ease of use of the NOVAtouch makes this a preferred choice for this technique. The 0°C temperature can be maintained using a wide-mouth dewar, filled with an ice / water mixture; however, more stable temperature control can be maintained using a circulating chiller and jacketed vessel (recirculating dewar) filled with a suitable coolant (usually an antifreeze / water mixture). In either case the liquid level is maintained using a water level float switch.

Which Samples are Tested?

Activated carbon fiber, ACF-10 (Nippon Kynol, Japan) and BPL granular activated carbon (Calgon Carbon, USA) were chosen as representative materials for this report. This method can be reliably applied to a wide variety of nanoporous carbons; provided the material under study does not have a significant number of surface functional groups, which can interact with the quadrupole moment of CO2 and affect the results.

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This information has been sourced, reviewed and adapted from materials provided by Quantachrome Instruments.

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