N2adsorption

N2 adsorption is a widely used laboratory technique to characterize the surface area and porosity of solid materials by measuring the adsorption and desorption of nitrogen gas at cryogenic temperature, typically 77 Kelvin. The sample is degassed to remove prior adsorbates, then exposed to nitrogen over a range of relative pressures, and the adsorption-desorption isotherms are recorded. Data are analyzed to yield the Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution by methods such as BJH or density-functional theory. Outputs commonly include surface area in square meters per gram (m2/g), total pore volume, and a pore-size distribution profile.

The method aids porosity classification via isotherm shapes: Type I indicates microporosity, while Type IV and V with hysteresis reflect mesoporosity and capillary condensation within pores. For accurate interpretation, appropriate pressure ranges are used for BET analysis, and desorption data are often employed for pore-size calculations. Limitations include reduced sensitivity for ultramicropores inaccessible to nitrogen at 77 K, potential chemical interactions with the sample, and the necessity of thorough degassing. In cases of very narrow pores, carbon dioxide adsorption at 273 K may be used as an alternative probe.

Applications span catalysts, adsorbents such as activated carbons, zeolites, and metal-organic frameworks, as well as various polymers and oxides. The technique is standardized by organizations such as IUPAC, ISO, and ASTM, with routine reporting of BET surface area, pore volume, and pore-size distribution. Advanced analyses may employ nonlocal DFT methods to improve pore-size resolution.