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Seminar on Adsorption : Adsorption

Method of Adsorption Amount Measurement

Typical measurement methods of adsorption isotherm are volumetric, gravimetric, pulse adsorption and dynamic method. To measure the surface area and porosity, the volumetric method is mainly used.

The apparatus for volumetric method consists of pressure sensors, valves, and vacuum system as in the figure. The inner volume is calibrated before the measurement. The adsorption amount is calculated by subtracting the number of remaining molecules at the adsorption equilibrium from the number of introduced molecules by using the ideal gas equation. The volumetric method has the advantage of separating the apparatus only for sample pretreatment to achieve the high throughput measurement of surface area and porosity. However, as the disadvantage, the volumetric method needs to account for the non-ideal gas behavior for high density gas and thermal transpiration for the low pressure measurement.

Volumetric adsorption apparatus

The gravimetric apparatus consists of the balance to measure the adsorption amount and pressure sensor to measure the equilibrium pressure. The resolution is up to 0.1µg. It is useful for measuring high pressure adsorption and water vapor adsorption. To calculate the adsorption amount, it is necessary to correct for the buoyancy during the measurement. In case of the low temperature measurement such as the one using liquid nitrogen (77K), it is difficult to maintain the temperature stability due to the balance design (no physical contact between the sample and sample cell). So the gravimetric method is commonly used for above room temperature measurement. The magnetic suspension balance manufactured by Rubotherm is possible to locate the balance outside of the measuring atmosphere so that the high temperature and high pressure measurement can be carried out accurately.

Gravimetric adsorption

The pulse adsorption apparatus uses the chemisorption of H2 or CO on rare metals such as Pt and Pd to evaluate the active surface area. As the detector, TCD (Thermal Conductivity Detector) is commonly used, which is a kind of gas chromatography. Before the adsorption measurement, the sample needs to have the pretreatment to clean the surface by oxidation and reduction. Then it flows the carrier gas such as He and doses the adsorptive gas as pulses.
The dynamic adsorption apparatus is used in BET 1-point method and TPD/R/O. It also uses the TCD detector to detect the change in the gas concentration after going through the sample. The dynamic method can measure the adsorption amount quickly compared to other methods and is useful for the quality control.


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