Learn how BET and t-range selection affects carbon black surface area accuracy, with NSA and STSA data across 11 standard carbon black grades.
Compare N2, Ar, and CO2 adsorbates to improve the characterization of nanoporous materials. Learn how to achieve more accurate and complete micropore analysis.
Accurate pore-structure characterization is a core requirement for developing and qualifying nanoporous materials. As application demands grow in adsorption, catalysis, and separations, laboratories need measurement strategies that deliver complete and reliable structural data across the full micropore and mesopore range. Selecting the right gas adsorbate is central to achieving that goal, and no single probe molecule covers all cases.Gas adsorbate characterization is the process of using probe molecules, such as nitrogen, argon, or carbon dioxide, to measure the pore structure of a material through gas adsorption. By analyzing how a gas adsorbs and desorbs across a range of relative pressures, laboratories can determine surface area, pore volume, and pore size distribution. These measurements are critical for materials such as zeolites, activated carbons, and metal-organic frameworks (MOFs), where pore architecture directly controls performance.
Nanoporous materials such as zeolites, activated carbons, and MOFs feature abundant microporosity across a wide range of pore sizes, including ultramicropores below 0.7 nm and larger micropores approaching 2 nm. Resolving this full range with a single adsorbate is not possible, and several technical limitations compound the problem:No single probe molecule accesses the complete relative-pressure range required to characterize both ultramicropores and larger micropores in one measurementNitrogen at 77 K carries a quadrupole moment that can produce orientation-specific interactions on polar or ion-containing surfaces, biasing pore size distribution resultsDiffusional limitations at cryogenic temperatures slow equilibration in ultramicropores, extending measurement times and risking kinetic artifactsLaboratories managing high sample volumes face throughput constraints when each measurement must be repeated or cross-validated due to probe-specific limitationsThe problem is not the physisorption technique itself. It is the gap between what a single adsorbate condition can deliver and what complete pore characterization requires.Pore filling during gas physisorption is governed by the adsorbate's molecular properties and the temperature at which the measurement is performed. Ultramicropores fill at very low relative pressures through overlapping dispersion fields, a process known as primary micropore filling. As pore width increases, adsorbate-adsorbate interactions dominate and filling shifts to higher relative pressures. The choice of probe molecule determines which part of that pressure window is accessible, how fast equilibrium is reached, and whether surface chemistry interferes with the measurement. These factors affect:
The pore size range that can be resolved in a given measurementThe accuracy of pore size distribution models such as Horvath-Kawazoe-Saito-Foley (HK-SF), which are calibrated to specific adsorbate and temperature conditionsThe reliability of surface area and micropore volume values on polar or functionalized surfacesThe total measurement time required to achieve equilibrium in ultramicropore-rich materialsFor laboratories developing or qualifying materials where pore structure governs performance, these are not minor calibration considerations. They directly affect whether the data produced is sufficient for material design or quality control decisions.The AMI Micro 300C static volumetric analyzer provides the measurement platform required to run N2, Ar, and CO2 under all relevant conditions with the precision and vacuum performance that multi-adsorbate characterization demands. The system is equipped with a turbo molecular pump capable of reaching vacuum levels up to 10-8 Pa. This level of vacuum performance is necessary to access the ultra-low pressure regime where primary micropore filling occurs, and to minimize residual gas contamination that would otherwise distort isotherm baselines. Temperature control is provided by auxiliary cryogenic devices for Ar at 87 K and CO2 at 195 K, and by a precision water bath for CO2 at 273 K. Key measurement capabilities include BET surface area, total and micropore pore volume, and pore size distributions derived using HK-SF and related models. The system captures full adsorption and desorption isotherms from P/P0 values as low as 10-7 to near saturation, providing complete coverage of the micropore and mesopore ranges. For laboratories requiring complementary data, the Micro 300C integrates seamlessly with the RuboSorp MPA for high-pressure volumetric measurements and the RuboSorp MSB for high-resolution gravimetric kinetics.
To evaluate how different probe molecules perform across representative material types, three nanoporous materials were analyzed: a zeolite, an activated carbon, and a MOF. Each was measured under all four adsorbate conditions. Sample Preparation All samples were degassed under vacuum prior to analysis to remove pre-adsorbed species and ensure that measurements reflected the true pore structure of each material. Measurement Approach The analysis included:
Full adsorption and desorption isotherms using N2 at 77 K, Ar at 87 K, CO2 at 195 K, and CO2 at 273 KN2 and Ar measured across P/P0 ranges from 10-7 to near 1, capturing primary and secondary micropore filling and mesopore capillary condensationCO2 at 273 K measured up to P/P0 approximately 3x10-2, targeting ultranarrow micropores below approximately 1 nmCO2 at 195 K extended to P/P0 approaching 1, enabling characterization of pores in the 1 to 2 nm range and total pore volume determinationMicropore pore size distributions derived using HK-SF models for consistency across all probe conditionsFor all three materials, both N2 and Ar isotherms exhibited Type-I behavior, confirming dominant microporosity and strong adsorbent-adsorbate interactions. In the low-pressure region, Ar initiates micropore filling at a higher relative pressure (approximately 10-5) compared to N2 (approximately 10-7). Despite this difference in filling pressure, HK-SF micropore pore size distributions derived from N2 and Ar agreed closely across all three materials. Ar at 87 K recovered the same structural information as N2 at 77 K while offering two practical advantages:
Faster equilibration due to reduced diffusional resistance at the higher measurement temperatureElimination of quadrupole-induced orientation effects on polar surfaces, which can bias N2 uptake and distort apparent pore size distributionsFor materials containing polar functional groups or exposed ions, such as zeolites with exchangeable cations or functionalized MOFs, Ar at 87 K is the preferred probe for micropore characterization. For purely carbonaceous materials, N2 at 77 K remains reliable, though complementary AR measurements add confidence when sub-nanometer features are critical.CO2 at 273 K: Resolving Ultranarrow Micropores At ambient temperature, CO2 has substantially higher diffusivity than cryogenic probes. This makes it the preferred adsorbate for characterizing pores below approximately 1 nm, where N2 and Ar can exhibit kinetic limitations that slow equilibration and introduce artifacts. For activated carbon, CO2 at 273 K resolved micropores down to approximately 0.4 nm within a P/P0 window limited to 3x10-2 at 100 kPa. This narrower window means that CO2 at 273 K is a targeted tool for ultramicropore analysis rather than a complete characterization solution. Total pore volume and characterization of larger micropores in the 1 to 2 nm range are outside its accessible pressure range. CO2 at 195 K: Broadening the Analysis Range Reducing the CO2 measurement temperature to 195 K extends the analysis window to P/P0 approaching 1, enabling both ultramicropore characterization below 1 nm and coverage of larger micropores in the 1 to 2 nm range. Total pore volume can also be determined at this condition, which CO2 at 273 K cannot provide. CO2 at 195 K isotherms showed strong Type-I behavior across all three materials, with saturation capacities exceeding those at 273 K. Pore size distributions aligned closely with those derived from N2 at 77 K and Ar at 87 K across the sub-2 nm regime, confirming consistency between probe conditions. One limitation applies to polar surfaces: CO2 has a larger quadrupole moment than N2, and its interactions on polar or ion-containing surfaces can reflect specific surface chemistry rather than pure pore geometry. For such materials, CO2-based pore size distributions should be cross-validated against Ar at 87 K results.
Zeolite N2 at 77 K and Ar at 87 K produced nearly identical pore size distributions for the zeolite, with close agreement in both micropore diameter and volume. Given the zeolite's polar, ion-exchanged framework, Ar provides a more physically meaningful measurement by eliminating orientation effects. CO2 at 195 K confirmed the micropore population independently and aligned well with both cryogenic probes. Activated Carbon Activated carbon presented the widest range of micropore sizes among the three materials. CO2 at 273 K resolved the finest ultramicropores below 0.4 nm with high resolution and fast kinetics. CO2 at 195 K extended coverage to the full micropore range and provided total pore volume data. N2 and Ar produced consistent pore size distributions for the broader micropore population. Complete characterization of activated carbon benefits from combining all four conditions. MOF The MOF showed the highest total pore volume of the three materials and a narrow micropore pore size distribution concentrated below 1.5 nm. N2 and Ar isotherms and pore size distributions matched closely. CO2 at 195 K confirmed the ultramicropore population with strong Type-I behavior. The MOF's organic linker framework may present coordination sites with polar character, making Ar at 87 K a useful independent cross-check on N2-derived surface areas.
A practical advantage of the AMI Micro 300C platform is the ability to process multiple samples in parallel. Three independently operating analysis ports allow simultaneous measurements under different probe conditions or across different sample types, reducing total experiment time without introducing cross-contamination or measurement interference between ports. Unlike single-port systems that require sequential analysis, this configuration supports higher sample throughput while maintaining the stable vacuum and temperature conditions required for consistent isotherm collection at ultra-low pressures.
In laboratory settings, improvements in characterization capability must translate into practical workflow gains to be meaningful. The multi-adsorbate approach demonstrated here provides several direct operational benefits:Combining N2, Ar, and CO2 in a complementary measurement strategy eliminates the need to repeat analyses due to probe-specific limitations on polar surfacesFaster equilibration with Ar at 87 K compared to N2 at 77 K reduces total measurement time per sampleCO2 at 195 K and 273 K resolves pore features below 1 nm that cryogenic probes cannot access kinetically, removing the need for separate instrument platforms to complete ultramicropore analysisInternally consistent pore size distribution data across probe conditions reduces uncertainty in material comparison and quality control workflowsFor teams evaluating platform options from AMI Instruments, these workflow considerations are as relevant as the analytical specifications, since they determine how efficiently complete characterization data can be generated across a range of material types.
A multi-adsorbate approach becomes most important when working with materials whose performance depends on precise pore structure across the full micropore range. This applies particularly to:Zeolites and ion-exchanged frameworks where N2 quadrupole interactions can bias surface area and pore size dataActivated carbons with significant ultramicropore populations below 0.7 nm where cryogenic probes face diffusional limitationsMOFs with narrow micropore distributions where independent cross-validation between probe conditions is necessary to confirm structural assignmentsAny material development or quality control program where both ultramicropore and larger micropore characterization are required within the same analysis workflowLaboratories working across these material types benefit from an instrument platform and measurement strategy capable of covering all four adsorbate conditions with the vacuum performance and temperature control required for accurate data at each.No single gas adsorbate provides a complete picture of pore structure in nanoporous materials. Nitrogen at 77 K establishes the foundation for surface area and broad micropore characterization. Argon at 87 K removes quadrupole artifacts on polar surfaces and accelerates equilibration. Carbon dioxide at 273 K delivers high-resolution access to the finest ultramicropores. Carbon dioxide at 195 K extends that coverage to the full micropore range and enables total pore volume determination. Used in combination on a platform with the vacuum performance and temperature control required to run all four conditions accurately, this multi-probe approach resolves the pore structure of zeolites, activated carbons, and MOFs with consistency and completeness that single-adsorbate methods cannot match. The result is characterization data that supports reliable material comparison, confident design decisions, and efficient laboratory workflows.
(1) Thommes, M. and Schlumberger, C. Characterization of nanoporous materials. Annu. Rev. Chem. Biomol. Eng. 2021, 12, 137-162. (2) Kianfar, E. and Sayadi, H. Recent advances in properties and applications of nanoporous materials and porous carbons. Carbon Lett. 2022, 32, 1646-1669. (3) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez-Reinoso, F.; Rouquerol, J.; Sing, K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1050-1069. (4) Beda, A.; Vaulot, C.; Ghimbeu, C. M. Hard carbon porosity revealed by the adsorption of multiple gas probe molecules (N₂, Ar, CO₂, O₂ and H₂). J. Mater. Chem. A, 2021, 9, 937-943. (5) Kim, K. H. and Kim, M. H. Adsorption of CO2, CO, H2, and N2 on zeolites, activated carons, and metal-organic frameworks with different surface nonuniformities. Sustainability, 2023, 15, 11574.
Pore-structure characterization is the analysis of a material's internal void network, including pore size, pore volume, and surface area. For nanoporous materials, this typically involves gas physisorption measurements using probe molecules such as nitrogen, argon, or carbon dioxide.Typical examples of nanoporous solids are zeolites, activated carbon, metal–organic frameworks, ceramics, silicates, aerogels, pillared materials, various polymers, and inorganic porous hybrid materials.Different probe molecules access different parts of the pore size range and respond differently to surface chemistry. Nitrogen at 77 K provides broad micropore and mesopore coverage. Argon at 87 K eliminates quadrupole effects on polar surfaces. Carbon dioxide at 273 K resolves the finest ultramicropores. Using them together produces a more complete and reliable picture of pore structure than any single probe can provide.Multiple gas adsorbates are used to characterize nanoporous materials because no single probe molecule can fully capture the entire pore size range. Nitrogen, argon, and carbon dioxide each provide unique insights into different pore regions, allowing laboratories to achieve more accurate and complete pore-structure characterization.
There is no single best gas adsorbate for all nanoporous materials. Nitrogen is commonly used for general surface area and pore analysis, argon is preferred for polar surfaces due to its neutral behavior, and carbon dioxide is ideal for analyzing ultramicropores below 1 nm. Combining these adsorbates delivers the most reliable and comprehensive results.
Learn how BET and t-range selection affects carbon black surface area accuracy, with NSA and STSA data across 11 standard carbon black grades.
Explore how a MOF-based direct air capture CO2 adsorbent performs across pore characterization humid breakthrough testing with AMI Instruments.
Learn how static and dynamic BET methods compare for measuring silicon nitride surface area, with precise results and throughput insights from AMI Instruments.
Learn how pretreatment method, temperature, time, and storage affect amorphous silica surface area and pore volume in nitrogen physisorption.