In physical gas adsorption, an inert gas, typically nitrogen, adsorbs onto solid material surfaces. This adsorption occurs on the outer surface and, in porous materials, within internal pore surfaces. BET analysis, a well-known method, measures surface area by gas adsorption. Often, it is simply called BET surface area measurement. Nitrogen forms an adsorption isotherm, or BET isotherm, at liquid nitrogen temperatures (77 K). This isotherm is measurable on both porous and non-porous materials.
Non-porous materials produce a type II isotherm, which enables BET surface area calculation. Conversely, porous materials yield type I or type IV isotherms, depending on pore types. These materials need specific handling for accurate BET surface area assessment. For materials with very small micropores, like zeolites or activated carbons, argon or carbon dioxide adsorption often offers more precise measurements than nitrogen. Samples with very low BET surface areas benefit from krypton gas adsorption. Krypton measures a limited portion of the isotherm yet provides accurate BET surface area data.
Determining Specific Surface Area and Pore Analysis via BET and Gas Adsorption
To determine specific surface area, the principle of monolayer gas molecule formation on a solid surface applies. The BET model, using a linearized BET equation, converts the experimental adsorption isotherm into a BET plot. This plot calculates the monolayer volume. By combining this volume with the gas molecule’s cross-sectional area, the specific surface area (in m²/g) is calculated.
Beyond BET analysis, gas adsorption also measures pore volume and pore size distribution. Capillary condensation determines pore size distribution, as condensation pressure varies with pore size. Larger pores condense gas at higher pressures, while smaller pores fill at lower partial pressures.
Before measurement, samples undergo pre-treatment at elevated temperatures in a vacuum or flowing gas. This process removes contaminants. Adjusting pre-treatment conditions is crucial, as incorrect temperatures can alter BET surface area results.
Techniques for Adsorption Analysis: Flow and Volumetric Approaches
Two main techniques apply in adsorption analysis:
Flow Technique: This method uses a TCD detector to measure adsorbed gas amounts, providing specific BET surface area or total pore volume. A Qsurf M3 analyzer conducts flow measurements, producing a single BET area or total pore volume value in the report.
Volumetric Technique: This method relies on pressure measurements within an enclosed volume, capturing multiple adsorption and desorption points to generate a full isotherm. It provides insights into BET surface area, pore volume, and pore size distribution.
Instruments like the Quantachrome Autosorb-6B (for Nâ‚‚ or COâ‚‚), Micromeritics TriStar 3000 (Nâ‚‚), Micromeritics Gemini, or Micromeritics ASAP 2020 (Ar or Kr) perform volumetric measurements. Reports may include a single BET surface area value or a detailed analysis with isotherm, specific BET surface area, total pore volume, and pore size distribution, offering more detailed information than basic BET analysis.