Gas Adsorption

At Micromeritics, we provide precision instruments tailored for accurate gas adsorption analysis, essential for applications in material science, catalysis, and other advanced fields. This page offers insights into the principles of gas adsorption, the innovative technologies we employ, and how our instruments can enhance your research and industrial applications.

What is Gas Adsorption?

Gas adsorption refers to the process by which gas molecules adhere to the surface of a material. Understanding the types of gas adsorption is vital for engineering and scientific applications. Physical adsorption (physisorption) and chemical adsorption (chemisorption) are two fundamental mechanisms through which gases interact with material surfaces.

Physisorption involves the weak bonding of gas molecules, primarily through van der Waals forces, which are reversible and occur at a range of temperatures

This technique is utilized in various applications to determine BET surface area and porosity.

Chemisorption involves stronger chemical bonds forming between the gas molecules and the surface atoms or molecules of the material.

This process typically results in irreversible adsorption and plays a crucial role in catalyst characterization, surface modification, and understanding reaction kinetics.

Why Physisorption?

Micromeritics instruments are finely calibrated to measure pressure and temperature which are used to determine the volume of gas adsorbed onto the sample. Data is collected in the form of isotherms, typically from low pressure (~0.00001 torr) to saturation pressure (~760 torr). The pressure range is determined based on the information desired.

The data obtained from physisorption experiments is used to detemine the specific surface area (BET), porosity, and the adsorption capacity of the material.

Applications:

Artificial Bone

Adsorbents

Battery Anodes & Cathodes

Catalysts

Ceramics

COFs

Metal & Polymer Powders for Additive Manufacturing

MOFs

Pharmaceuticals

Zeolites

BET Surface Area

The amount of gas adsorbed on the materials surface can be used to calculate the surface area. Surface area is a measure of the exposed surface of a solid sample on the molecular scale.
BET (Brunauer, Emmet, and Teller) theory is the most popular model used to determine the specific surface area.

Typically, BET analysis is performed using nitrogen gas (N2) as the adsorbate due to its high affinity for solid surfaces. The gas is introduced at low pressures and molecules begin adsorbing to the surface, as gas pressure increases the monolayer is formed, followed by multilayer adsorption (we have image showing this process). The amount adsorbed is determined to calculate the surface area using the BET equation. For low surface area materials, krypton is commonly used as an alternative adsorbate. Due to its lower vapor pressure (2.5 mmHg) compared to N2 (760 mmHg) at 77.35 K, Kr analyses involve a greater pressure change during the adsorption step at the same relative pressure, resulting in greater accuracy.

The BET surface area of a material is calculated from the monolayer capacity which is the volume of the first single layer of gas molecules or atoms adsorbed on the surface.

The BET equation is linearized to conveniently calculate the monolayer capacity from the slope and y-intercept of the BET transform plot, which must achieve a sufficiently high correlation coefficient for a valid BET calculation, which is typically 0.999.

Porosity

Gas adsorption enables the characterization of a material’s porosity, revealing insights into its structure and properties. As gas pressure increases, pores within the material begin to fill. This process starts with smaller pores and progresses to larger ones until all are saturated. Overall, gas adsorption is applicable to pores ranging from ~0.35 nm to ~400 nm in diameter. Once details of the isotherm curve are accurately expressed as a series of pressure vs quantity adsorbed, a number of different methods (theories or models) can be applied to determine the pore size distribution.

Classification	Size	Typical Calculation Models
Micropore	<2 nm	Density Functional Theory (DFT) M-P Method Dubinin Plots (D-R, D-A) Horvath Kawazoe (H-K) t-plot (total micropore area)
Mesopore	2-5 nm	Barrett, Joyner, and Halenda (BJH) Density Functional Theory (DFT) Dollimore-Heal (DH)
Macropore	> 50 nm	Barrett, Joyner, and Halenda (BJH) Density Functional Theory (DFT) Dollimore-Heal (DH)
*Special Considerations	>400 nm	For pores exceeding 400 nm, other techniques such as mercury intrusion porosimetry (link to page) are employed. This technique offers insights into larger pores, typically starting from 3 nm up to 1100 µm

Our Solutions

Instruments

3Flex

Advanced gas adsorption system
Highest performance micropore analysis
Vapor analysis

ASAP 2020 Plus

High resolution surface area and porosity analyzer
Independent preparation and analysis instrument in a single cabinet
Ideal for research, development, and quality control applications

TriStar II Plus

Highest-throughput automated BET surface area analysis
Three-sample parallel measurements maximize productivity
Independent sample port transducers, a dedicated vacuum system, and a computer-controlled servo valve provide the shortest available measurement time for multi-sample analysis
Available Krypton configuration for low surface area materials

Gemini

Unique differential measurement design
Fastest individual surface area measurement
High accuracy for low surface area using N2 gas

Services

We provide a comprehensive range of characterization services whether it is the analysis of a single sample, a complex method development or validation, new product assessments, or addressing large-scale manufacturing projects.

Available options

Multipoint surface area using nitrogen gas (ISO 9277)
Multipoint surface area using krypton gas (ISO 9277)
Multipoint surface area and STSA using nitrogen gas (ASTM D6556)
Multipoint surface area using Argon

Multipoint surface area using Co2
40-point nitrogen adsorption isotherm (20 A to 3000 A)
40-pt Nitrogen adsorption and 40-pt desorption isotherm (20 A to 3000 A)
High-resolution micropore analysis plus mesopore isotherm (4 A to 3000 A)

Special CO2 isotherms at O °C
Adsorption isotherms at user defined conditions (specialty gases)
High-pressure isotherms using hydrogen, nitrogen, oxygen, methane, or other gases

FAQ

What are other methods for characterizing a materials porosity?

Mercury Intrusion
Capillary Flow

What is the difference between Physisorption and Chemisorption

Physisorption and chemisorption are the primary types of gas adsorption.
The differences are highlighted in the table below:

Physisorption (Physical Adsorption)	Chemisorption (Chemical Adsorption)
Non-selective	Selective
Weak Interactions (van der Waals)	Strong Interactions (chemical bonds)
Lower Energy	Higher Energy
Reversible	Irreversible & Reversible