Basic operating principles of the Sorptomatic 1990

The Sorptomatic 1990 is based on the static volumetric principle to characterize solid samples by the technique of gas adsorption. It is designed to perform both physisorption and chemisorption enabling determination of the total specific surface area, porosity and the specific surface area of metals present and their dispersion over the surface.

It features very precise saturation pressure monitoring in real time by means of coolant temperature measurement and has a powerful turbomolecular vacuum pump to assure the correct vacuum degree in the sample holder during the pre-treatment and before the analysis start. Two pressure transducers working in different ranges are connected to the sample holder to measure the equilibrium pressure. The automatic calibration routine and automatic change of scale assure that the pressure is measured using the best resolution and accuracy. Being completely made of stainless steel, it can use a wide range of gases (inert and corrosive) to enlarge the investigation possibilities on the solid state, matching the increasing necessities of the researchers. A microprocessor controls and monitors the analysis and the sample pre-treatment (with a choice of 3 temperature programs), is connected to the personal computer by a RS232 link. Through the software it is possible to monitor all of the instrument parameters, open/close valves, change gases, evacuate the tubing, calibrate the pressure transducers, perform the automatic leak test, pre-treat the samples, etc. Once the experiment is started, the microprocessor can control the injection pressure and volume via a feedback loop of sample adsorption rate and hence can optimise the number of equilibrium points that are collected. In addition to the sample being analysed two further samples can be pre-treated simultaneously using different gases, temperature and flow conditions. The final degassing before the analysis can be performed in a flow of inert gas or under very high vacuum conditions to clean completely the activated sample surface and to perform reliable and reproducible measurements.

Adsorption

Adsorption is defined as the concentration of gas molecules near the surface of a solid material. The adsorbed gas is called adsorbate and the solid where adsorption takes place is known as the adsorbent. Adsorption is a physical phenomenon (usually called physisorption) that occurs at any environmental condition (pressure and temperature) but only at very low temperature it becomes measurable. Thus physisorption experiments are performed at very low temperature, usually at the boiling temperature of liquid nitrogen at atmospheric pressure.

Adsorption takes place because of the presence of an intrinsic surface energy. When a material is exposed to a gas, an attractive force acts between the exposed surface of the solid and the gas molecules. The result of these forces is characterized as physical (or Van der Waals) adsorption, in contrast to the stronger chemical attractions associated with chemisorption. The surface area of a solid includes both the external surface and the internal surface of the pores.

Due to the weak bonds involved between gas molecules and the surface (less than 15 KJ/mole), adsorption is a reversible phenomenon. Gas physisorption is considered non-selective, thus filling the surface step by step (or layer by layer) depending on the available solid surface and the relative pressure. Filling the first layer enables the measurement of the surface area of the material, because the amount of gas adsorbed when the mono-layer is saturated is proportional to the entire surface area of the sample. The complete adsorption/desorption analysis is called an adsorption isotherm. The six IUPAC standard adsorption isotherms are shown below, they differ because the systems demonstrate different gas/solid interactions.


The Type I isotherm is typical of microporous solids and chemisorption isotherms. Type II is shown by finely divided non-porous solids. Type III and type V are typical of vapor adsorption (i.e. water vapor on hydrophobic materials). Type VI and V feature a hysteresis loop generated by the capillary condensation of the adsorbate in the mesopores of the solid. Finally, the rare type VI step-like isotherm is shown by nitrogen adsorbed on special carbon.

Once the isotherm is obtained, a number of calculation models can be applied to different regions of the adsorption isotherm to evaluate the specific surface area (i.e. BET, Dubinin, Langmuir, etc.) or the micro and mesopore volume and size distributions (i.e. BJH, DH, H&K, S&F, etc.).

Static volumetric gas adsorption

Static volumetric gas adsorption requires a high vacuum pumping system, able to generate a good vacuum over the sample of at least 10-4 torr. The system features stainless steel plumbing with high vacuum fittings to ensure precise results as the experiment is carried out starting from high vacuum and increasing step by step the pressure up to the adsorbate saturation pressure. A schematic of the instrument is shown below.
The principle behind this method consists of introducing consecutive known amounts of adsorbate to the sample holder, which is kept at liquid nitrogen temperature (77 K). Adsorption of the injected gas onto the sample causes the pressure to slowly decrease until an equilibrium pressure is established in the manifold. The injection system of the Sorptomatic 1990 consists of a calibrated piston, where both the pressure and the injection volume can be automatically varied by the system according to the adsorption rate and the required resolution. The piston method is advantageous over other methods as it does not increasing the manifold dead volume while the system is waiting for pressure equilibration. A small dead volume over the sample makes the instrument very sensitive to the amount of gas adsorbed. The equilibrium pressure is measured by a transducer chosen according to the pressure range where adsorption is established during the experiment. The raw experimental data are the equilibrium pressures and the amount of gas adsorbed for each step. The gas uptake is calculated directly from the equilibrium pressure values but a dead volume calibration has to be performed before or after the measurement by a “blank run” (that is an analysis using an inert gas not adsorbed on the sample in the analytical conditions, most commonly used is helium).

The static volumetric method is very precise and is considered as a very accurate technique to evaluate surface area and pore size in the region of micro and mesopores. However it is not advisable whenever a fast measurement of surface area is required, because this method involves long analysis time that are required to produce highly accurate and reliable results.

Porosity

Many solid and powder materials both natural (stones, soils, minerals, etc.) and manufactured (catalysts, cement, pharmaceuticals, metal oxides, ceramics, carbons, zeolites, etc.) contain a certain void volume of empty space. This is distributed within the solid mass in the form of pores, cavities, and cracks of various shapes and sizes. The total sum of the void volume is called the porosity.

The type and nature of porosity in natural materials depend on their formation (for instance rocks can be of igneous, sedimentary or metamorphic origin) while in man-made materials depend on their manufacturing and generally it can be controlled. Porosity strongly determines important physical properties of materials such as durability, mechanical strength, permeability, adsorption properties, etc. The knowledge of pore structure is an important step in characterizing materials, predicting their behavior.

There are two main and important typologies of pores: closed and open pores. Closed pores are completely isolated from the external surface, not allowing the access of external fluids in neither liquid nor gaseous phase. Closed pores influence parameters like density, mechanical and thermal properties. Open pores are connected to the external surface and are therefore accessible to fluids, depending on the pore nature/size and the nature of fluid. Open pores can be further divided in dead-end or interconnected pores. Further classification is related to the pore shape, whenever is possible to determine it.

The characterization of solids in terms of porosity consists in determining the following parameters:

Pore size

Pore dimensions cover a very wide range. Pores are classified according to three main groups depending on the access size:

Micropores: less than 2 nm diameter
Mesopores: between 2 and 50 nm diameter
Macropores: larger than 50 nm diameter

Specific pore volume and porosity

The internal void space in a porous material can be measured. It is generally expressed as a void volume (in cc or ml) divided by a mass unit (g).

Pore size distribution

It is generally represented as the relative abundance of the pore volume (as a percentage or a derivative) as a function of the pore size.

Bulk density

Bulk density (or envelope density) is calculated by the ratio between the dry sample mass and the external sample volume.

Percentage porosity

The percentage porosity is represented by ratio between the total pore volume and the external (envelope) sample volume multiplied by 100.

Specific surface area

The surface area of a solid material is the total surface of the sample that is in contact with the external environment. It is expressed as square meters per gram of dry sample. This parameter is strongly related to the pore size and the pore volume i.e. the larger the pore volume the larger the surface area and the smaller the pore size the higher the surface area. The surface area results from the contribution of the internal surface area of the pores plus the external surface area of the solid or the particles (in case of powders). Whenever a significant porosity is present, the fraction of the external surface area to the total surface area is small.

 
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