Gas Adsorption Measurement Analysis of Surface Area and Porosity

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Why Gas Adsorption?

Gas adsorption involves the adherence of gas molecules onto the surface of a solid. This physical interaction, known as physisorption, forms the basis for characterizing materials with porous or high surface-area structures. By studying how much gas is adsorbed at different pressures, engineers and researchers can derive:

• Specific surface area (via BET method and more)
• Pore size distribution (via classical methods like BJH, or novel methods like GCMC or DFT models)
• Total pore volume

These metrics are vital across multiple applications:

• In adsorbents like activated carbon, surface area correlates with adsorption capacity
• In catalysis, higher surface area typically means more active sites for chemical reactions
• In batteries and fuel cells, pore volume and structure influence energy density and ion transport
• In pharmaceuticals, particle surface area affects dissolution rate and bioavailability

Methods and Standardization of Gas Adsorption Measurement

Core Technique: Volumetric Gas Adsorption

The most usual form of gas adsorption measurement is volumetric (manometric) physisorption. This method involves dosing a known volume of adsorptive gas (typically nitrogen or argon) into an evacuated sample cell and monitoring pressure changes as the gas adsorbs onto the sample's surface. The resulting adsorption isotherm—gas volume adsorbed vs. relative pressure (P/P₀)—forms the basis for analytical models.

Key steps:

• Degassing (Outgassing): Prior to analysis, samples are heated under vacuum to remove moisture and contaminants.
• Cryogenic cooling: Most measurements use liquid nitrogen (77 K) or liquid argon (87 K) to maintain stable, low temperatures ideal for physisorption.
• Equilibrium dosing: Gas is introduced incrementally until equilibrium is reached at each pressure step.

Microtrac's instruments support full automation of these steps with integrated degassing stations and precision dosing systems.

Standard Models for Data Analysis

BET Theory (ISO 9277)

The Brunauer–Emmett–Teller (BET) method is the gold standard for calculating specific surface area. It assumes multilayer adsorption and is applied to the linear region of the isotherm (typically P/P₀ = 0.05–0.30; except type I isotherm). BET surface area is calculated from the monolayer capacity using:

• Molecular cross-sectional area of the adsorbate (e.g. 0.162 nm² for N₂)
• Avogadro’s number
• Ideal gas law constants (such us molar volume of gas at STP)

Microtrac systems support both single-point and multi-point BET analysis as per ISO 9277 and ASTM D6556.

BJH Method (ISO 15901-2)

The Barrett–Joyner–Halenda (BJH) method is used to determine mesopore size distribution by analyzing the desorption branch of the isotherm. BJH applies the Kelvin equation to correlate pressure changes with pore diameters, assuming cylindrical pore geometry.

Ideal for:

• Silica gels
• Porous glass
• Mesoporous oxides

DFT, NLDFT & QSDFT

Density Functional Theory (DFT), non-local DFT (NLDFT), and quenched solid DFT (QSDFT) are advanced methods that model gas adsorption in porous materials based on statistical mechanics. Unlike BJH, which relies on certain assumptions about pore geometry and is limited in analyzing micropores, DFT-based methods can accurately account for a range of pore shapes and sizes, making them ideal for characterizing microporous materials such as activated carbon and metal-organic frameworks (MOFs).

• Provides high-resolution pore size distributions
• Applicable to a range of adsorbates and pore geometries
• Built-in libraries in Microtrac software enhance accuracy

Choice of Adsorbate Gases

The selection of gas affects sensitivity and pore accessibility:

• Nitrogen (N₂, 77 K): Standard for BET and mesopore analysis. Well-characterized and widely accepted.
• Argon (Ar, 87 K): Preferred for micropore analysis due to its non-polar nature and smoother adsorption behavior.
• Krypton (Kr, 77 K): Used for low surface area materials (<1 m²/g). Its low saturation pressure increases sensitivity for small sample surfaces.
• Carbon Dioxide (CO₂, 273 K or 298 K): Ideal for probing ultra-micropores (<0.7 nm), which are often inaccessible to nitrogen at 77 K due to kinetic restrictions and slower diffusion rates.

Microtrac’s BELSORP series supports all of these gases, enabling flexible, accurate analysis across materials.