Surface Area and Pore-Size Distribution Ultimate
This package provides our most comprehensive characterisation of the surface area and pore structure of biochar. It combines two separate gas-adsorption analyses:
- a 40-point nitrogen adsorption and desorption isotherm; and
- a separate 40-point carbon-dioxide adsorption and desorption isotherm.
The nitrogen analysis provides detailed information on overall nitrogen-accessible surface area, total pore volume, mesoporosity and the wider micropore-to-mesopore distribution. The carbon-dioxide analysis provides complementary information on narrow micropores that may be difficult to characterise fully using nitrogen at liquid-nitrogen temperature.
Using both gases provides a more complete assessment of the biochar pore network than either experiment alone. This is particularly valuable for activated biochars and other highly microporous carbon materials, where a substantial proportion of the surface area may be contained within extremely narrow pores.
This package includes interactive nitrogen and carbon-dioxide isotherm charts through the Celignis Database, together with BJH, QSDFT and carbon-dioxide DFT pore-size-distribution plots. The charts are also included in our PDF reports.
Nitrogen Adsorption and Desorption Analysis
The nitrogen experiment uses a 40-point adsorption and desorption isotherm to evaluate the wider pore structure of the biochar. The adsorption branch shows how nitrogen progressively enters and fills the accessible pores as relative pressure increases. The desorption branch shows how nitrogen leaves the pore network as pressure is reduced.
Differences between the two branches, known as hysteresis, can provide information on:
- mesopore structure;
- pore connectivity;
- restricted pore entrances;
- pore blocking;
- ink-bottle-type pores; and
- the mechanisms by which pores fill and empty.
The 40-point isotherm provides a detailed experimental dataset for BET, BJH and QSDFT calculations and gives improved definition of pore-filling behaviour compared with analyses based on fewer datapoints.
Nitrogen BET Surface-Area Analysis
The Brunauer–Emmett–Teller method is used to calculate the apparent nitrogen-accessible specific surface area of the biochar in m²/g. BET surface area is widely used to compare biochars produced from different feedstocks, pyrolysis temperatures, residence times and activation treatments. It represents the internal and external surface accessible to nitrogen under the analysis conditions.
The nitrogen BET analysis also reports:
- the number of BET isotherm datapoints;
- the BET correlation coefficient;
- the BET C constant;
- the BET slope;
- the BET intercept;
- the total nitrogen-accessible pore volume; and
- an equivalent average pore diameter.
The correlation coefficient indicates how closely the selected transformed adsorption data follow the linear BET model. The C constant provides an indication of the relative strength of first-layer nitrogen adsorption, while the slope and intercept are the regression coefficients used to calculate the monolayer adsorption capacity, C constant and BET surface area.
The reported average pore diameter is calculated from the total pore volume and BET surface area. It is a whole-sample equivalent value and should not be interpreted as the most frequently occurring pore width.
Nitrogen BJH Mesopore Analysis
The Barrett–Joyner–Halenda method is used primarily to evaluate the mesoporous portion of the biochar pore network. Mesopores are conventionally defined as pores approximately 2–50 nm wide.
The BJH pore-size-distribution plot presents pore volume in cc/g against pore width in nm. It shows how the BJH-assigned pore volume is distributed across the mesopore range and identifies the principal BJH pore-width mode.
The BJH results include:
- cumulative BJH surface area;
- cumulative BJH pore volume; and
- modal BJH pore width, reported as the main cluster of pores.
BJH analysis is useful for describing mesoporosity but is not suitable for accurately characterising narrow micropores. It may also be influenced by pore blocking, network effects and the assumptions used to describe capillary condensation and pore geometry.
Nitrogen QSDFT Micro- and Mesopore Analysis
Quenched Solid Density Functional Theory provides a molecular-level model of nitrogen adsorption within carbon pores of different widths. QSDFT is particularly useful for biochar because it can evaluate micropores as well as mesopores. Micropores can contribute a very large surface area while accounting for a relatively small proportion of the total pore volume.
The nitrogen QSDFT plot presents pore volume in cc/g against pore width in nm. The results include:
- cumulative QSDFT surface area;
- cumulative QSDFT pore volume;
- modal QSDFT pore width;
- lower confidence limit; and
- QSDFT fitting error.
The modal pore width identifies the strongest feature in the calculated pore-size distribution. The lower confidence limit represents the smallest pore width that can be determined reliably from the measured isotherm and selected model.
Where the modal pore width is equal to the lower confidence limit, the dominant pore feature occurs at the lower reliable resolution of the nitrogen analysis. In such cases, narrower pores may be present but cannot be confidently differentiated using the nitrogen dataset alone.
The QSDFT fitting error indicates how closely the theoretical adsorption isotherm reconstructed from the calculated pore distribution matches the experimentally measured nitrogen isotherm.
Why Carbon Dioxide is Also Used
Nitrogen adsorption is normally undertaken at approximately 77 K. At this low temperature, diffusion of nitrogen into extremely narrow pores can be slow or restricted.
Carbon-dioxide adsorption is undertaken at a higher temperature, normally approximately 273 K. Under these conditions, carbon dioxide can access narrow micropores more readily and can provide valuable information on ultramicroporosity that may not be fully resolved by nitrogen adsorption.
This does not mean that carbon dioxide provides a replacement for nitrogen analysis. The gases characterise different and overlapping parts of the pore network:
- nitrogen provides broader characterisation across the micropore and mesopore ranges;
- carbon dioxide provides greater sensitivity to narrow micropores and ultramicropores;
- the combined analyses provide a more complete representation of the biochar pore structure.
Carbon-Dioxide Adsorption and Desorption Isotherm
The carbon-dioxide isotherm shows the amount of carbon dioxide adsorbed by the biochar as pressure increases and the amount remaining adsorbed as pressure is subsequently reduced. The 40-point carbon-dioxide isotherm provides a detailed dataset for evaluating adsorption within narrow pores. The adsorption and desorption branches can also indicate whether the adsorption process is fully reversible over the measured pressure range.
The carbon-dioxide isotherm is provided as an interactive chart through the Celignis Database and is included in the analytical PDF report.
Carbon-Dioxide BET Results
The carbon-dioxide dataset is also processed using the BET calculation, providing the following CO?-specific parameters:
- BET isotherm datapoints;
- BET surface area;
- total pore volume;
- equivalent average pore diameter;
- BET C constant;
- BET correlation coefficient;
- BET slope; and
- BET intercept.
These parameters have the same mathematical roles as the corresponding nitrogen BET values but are derived from the carbon-dioxide isotherm.
The carbon-dioxide BET surface area should not be treated as directly interchangeable with the nitrogen BET surface area. The two gases differ in molecular properties, measurement temperature, accessible pore-size range and adsorption behaviour. Differences between the values can therefore provide information on which parts of the pore network are accessible to each gas.
The carbon-dioxide average pore diameter is an equivalent value calculated from the reported pore volume and surface area. It should not be interpreted as the principal or modal pore width.
Carbon-Dioxide DFT Analysis
Density Functional Theory is used to derive a model-based pore-size distribution from the carbon-dioxide adsorption data.
The carbon-dioxide DFT plot presents pore volume in cc/g against pore width in nm. It is particularly useful for examining the narrow micropores that may contribute significantly to the performance of highly porous biochars.
The carbon-dioxide DFT results include:
- cumulative DFT surface area;
- cumulative DFT pore volume;
- modal DFT pore width;
- lower confidence limit; and
- DFT fitting error.
The cumulative DFT surface area represents the internal pore-wall area assigned by the selected carbon-dioxide pore model. The cumulative pore volume represents the total internal void volume assigned across the modelled pore-width range.
The modal DFT pore width identifies the strongest feature in the carbon-dioxide pore-size distribution. The lower confidence limit represents the smallest pore width that can be determined reliably from the experiment and selected DFT model.
The fitting error describes the agreement between the measured carbon-dioxide isotherm and the theoretical isotherm reconstructed from the calculated pore-size distribution. A lower fitting error indicates closer mathematical agreement.
The carbon-dioxide DFT analysis must use a model specifically appropriate to carbon dioxide at the analysis temperature and to carbonaceous pores. The reported results remain dependent on the selected model and its assumptions concerning pore geometry and surface structure.
Interpreting the Nitrogen and Carbon-Dioxide Results Together
The nitrogen and carbon-dioxide results should be viewed as complementary rather than as duplicate measurements.
A biochar may, for example, show:
- a high nitrogen BET surface area;
- a strong nitrogen QSDFT micropore feature;
- a relatively modest BJH mesopore volume; and
- substantial additional narrow-pore surface area detected by carbon dioxide.
This would indicate that much of the biochar’s accessible surface is located within micropores, including pores that carbon dioxide can access more readily than nitrogen.
Alternatively, similar nitrogen and carbon-dioxide results may indicate that the relevant pore network is readily accessible to both gases.
The values should not normally be added together because the two methods may include overlapping portions of the pore structure. Their principal value lies in comparing accessibility and pore-size distributions across complementary measurement ranges.
Advantages of the Ultimate Package
P368 provides:
- a 40-point nitrogen adsorption and desorption isotherm;
- a separate 40-point carbon-dioxide adsorption and desorption isotherm;
- BET surface-area and regression parameters for both gases;
- total pore-volume and equivalent average-pore-diameter results;
- nitrogen BJH mesopore analysis;
- nitrogen QSDFT micro- and mesopore analysis;
- carbon-dioxide DFT narrow-micropore analysis;
- interactive online isotherm and pore-distribution charts; and
- detailed PDF reporting.
The combination of high-density nitrogen and carbon-dioxide isotherms is particularly useful where small differences between biochars are important or where the material is expected to contain a complex, highly microporous or multimodal pore structure.
Applications to Biochar
Detailed nitrogen and carbon-dioxide pore characterisation can support the evaluation of biochar for applications including:
- contaminant adsorption and environmental remediation;
- gas adsorption and separation;
- carbon-dioxide capture;
- water treatment and filtration;
- activated-carbon-type applications;
- nutrient retention and soil amendment;
- catalyst and catalyst-support applications;
- electrochemical and energy-storage materials;
- comparison of physical or chemical activation treatments;
- assessment of feedstock and pyrolysis conditions; and
- research into relationships between pore accessibility and biochar performance.
P368 is recommended where the most complete available assessment of biochar surface area and pore structure is required. It is particularly suitable for research, product development, activation studies and applications in which narrow micropores are expected to make an important contribution to performance.
Constituents Determined
Examples of the data reports generated can be viewed on the Celignis Database. Please log on to the guest account using email "test@celignis.com" and password "celignis".
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Sample Weight Requirements
Under nornal conditions there is a minimum requirement of 1g of sample for this analysis package with a recommended weight of 5+g of sample.However, it may be possible for us to undertake the anaysis with lower quantities than the minimum specified above. Please email us at info@celignis.com to find out.
Equipment Used for Surface Area and Pore-Size Distribution Ultimate
