Method of Adsorption Amount Measurement Typical measurement methods explained

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

Volumetric adsorption apparatus

The apparatus for the volumetric method consists of pressure sensors, valves, and a vacuum system as seen in the figure below. 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 pre-treatment to achieve the high throughput measurement of surface area and porosity. However, a disadvantage of the volumetric method is the not ideal gas behavior at high density and thermal transpiration for low pressure measurements.

Pulse adsorption method, Flow method

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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