This package provides detailed characterisation of the nitrogen-accessible surface area and pore structure of a sample using a 20-point nitrogen adsorption isotherm. Measurements are collected across a wider range of relative pressures than in our five-point BET package (P360), allowing the adsorption behaviour and pore structure of the sample to be evaluated in greater detail.

 

The package includes the complete nitrogen adsorption and desorption isotherm. The two branches show how nitrogen enters and leaves the biochar pore network as pressure is increased and subsequently reduced. Differences between the adsorption and desorption branches, known as hysteresis, can provide useful information on mesopore structure, pore connectivity, pore blocking and the presence of narrow pore entrances or ink-bottle-shaped pores.

 

The isotherm and pore-size-distribution plots are available as interactive charts through the Celignis Database and are also included in the analytical PDF report.

 

BET Surface-Area Analysis

The Brunauer–Emmett–Teller (BET) method is used to calculate the nitrogen-accessible specific surface area of the biochar in m²/g. This represents the apparent internal and external surface available for nitrogen adsorption and is commonly used when comparing biochars produced from different feedstocks or under different pyrolysis and activation conditions.

 

The BET analysis also provides the regression parameters used in the calculation, including the number of isotherm datapoints, correlation coefficient, C constant, slope and intercept. These values help assess the mathematical quality and characteristics of the BET fit.

 

The BET C constant provides an indication of the strength of interaction between nitrogen and the biochar surface during formation of the first adsorbed layer. The correlation coefficient indicates how closely the selected data follow the linearised BET model, while the slope and intercept are used to calculate the monolayer adsorption capacity, C constant and specific surface area.

 

The package also reports the total nitrogen-accessible pore volume and an equivalent average pore diameter calculated from the measured pore volume and BET surface area. The average pore diameter is a whole-sample calculated value and should not be interpreted as the most common pore size in the biochar.

 

BJH Pore-Size Analysis

The Barrett–Joyner–Halenda (BJH) method is used primarily to evaluate the mesopore structure of the biochar. Mesopores are conventionally defined as pores approximately 2–50 nm wide.

 

The BJH pore-size-distribution plot displays pore volume in cc/g against pore width in nm. It shows how the measured mesopore volume is distributed across different pore sizes and identifies the principal BJH pore-width mode, reported as the main cluster of pores.

 

The following BJH results are provided:

- cumulative BJH surface area;

- cumulative BJH pore volume; and

- modal BJH pore width.

 

BJH analysis is widely used for mesoporous materials, but it is not suitable for accurately characterising narrow micropores below approximately 2 nm. It can also be affected by pore blocking, network effects and the assumptions used to describe capillary condensation. The BJH results should therefore be interpreted principally as a description of the mesoporous portion of the biochar pore network.

 

QSDFT Pore-Size Analysis

Quenched Solid Density Functional Theory, or QSDFT, provides a molecular-level interpretation of nitrogen adsorption within pores of different widths. For biochar and other carbonaceous materials, QSDFT is particularly useful because it can characterise micropores as well as mesopores.

 

Micropores, which are below approximately 2 nm in width, often contribute a large proportion of a biochar’s total surface area even though they may account for a relatively small volume. These narrow pores are not adequately described by the BJH method.

 

The QSDFT pore-size-distribution plot displays pore volume in cc/g against pore width in nm. It identifies the principal pore-width mode and shows the distribution of pore volume across the modelled micropore and mesopore ranges.

 

The following QSDFT results are provided:

- cumulative QSDFT surface area;

- cumulative QSDFT pore volume;

- modal QSDFT pore width;

- lower confidence limit; and

- QSDFT fitting error.

 

The lower confidence limit represents the smallest pore width that can be determined reliably from the measured nitrogen isotherm and the selected QSDFT 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 analysis and may extend into smaller pores that cannot be confidently distinguished.

 

The QSDFT fitting error indicates how closely the theoretical isotherm reconstructed from the calculated pore-size distribution matches the experimentally measured nitrogen isotherm. A lower fitting error indicates closer mathematical agreement, although the result remains dependent on the selected carbon pore model and assumed pore geometry.

 

Why the Methods Give Different Results

BET, BJH and QSDFT evaluate different aspects of the same nitrogen adsorption dataset and should not be expected to produce identical values.

- BET provides the overall apparent nitrogen-accessible specific surface area and related regression parameters.

- BJH principally describes mesopore surface area, pore volume and pore-size distribution.

- QSDFT provides a model-based assessment of both micropores and mesopores and is generally more suitable for evaluating the narrow pores commonly found in biochar.

 

A biochar may therefore have a high BET surface area, a relatively low BJH pore volume and a strong QSDFT micropore peak. This would indicate that much of the biochar’s surface area is contained within narrow micropores rather than larger mesopores.

 

Applications to Biochar

Surface area and pore-size distribution can influence a range of biochar properties and applications, including:

- adsorption of organic compounds and contaminants;

- retention of nutrients and dissolved ions;

- water-holding behaviour;

- accessibility to microorganisms;

- performance as a catalyst or catalyst support;

- gas adsorption and storage;

- effectiveness in filtration and remediation applications; and

- the effects of feedstock, pyrolysis temperature and activation conditions.

 

This package is recommended where a detailed understanding of biochar pore structure is required. The 20-point isotherm provides substantially more information than a basic five-point BET analysis and allows the surface area, total pore volume, mesopore distribution and micropore distribution to be assessed together.

Constituents Determined

For every constituent determined via wet-chemical analysis each sample is analysed in duplicate and we provide the compositional data for each replicate along with the average value and the standard deviation between the duplicates. Our analytical protocols typically provide results with a high level of precision, as detailed here.

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".

Click here to place an order for determining Surface Area and Pore-Size Distribution.

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