Chemisorption, in regard to our laboratory applications, refers to the chemical adsorption and desorption phenomena by which gas or vapor molecules bond to or are liberated from the solid surface of sample material. Chemical and physical adsorption differ mainly in the way the adsorptive bonds to the solid surface, the selectivity of this process making it particularly useful as an analytical technique.
Because the chemisorptive (gas or vapor) adsorbs only on surfaces having particular chemical properties and does so only under certain thermal conditions, it can be used to probe a surface for a specific molecular species. The most widely used application of this aspect of chemisorption is to determine the surface area only of specific adsorption sites on a heterogeneous surface, for example, the active metal surface residing on a supporting substrate. This typically is expressed in terms of active metal surface area, metal dispersion or percent metal.
When the active surface area analysis just described is performed at one specific temperature, it is called isothermal chemisorption. Two techniques are employed in our laboratory to obtain this information, the static volumetric technique and the dynamic (flowing gas) technique. The former technique is performed by the Micromeritics ASAP 2020-C, which operates over a range of pressures. The dynamic chemisorption technique is performed using either the Micromeritics AutoChem or Chemisorb
series of instruments. These instruments utilize pulse chemisorption in a dynamic flow system to titrate the active surface.
Temperature-programmed analyses are used to study chemisorption bonds under controlled conditions of varying thermal energy. Perhaps the most illustrative example of a temperature-controlled reaction study is the temperature-programmed desorption (TPD) analysis in which the temperature of the sample is increased until the thermal energy is sufficient to break the chemisorption bond. By monitoring the temperatures at which molecules are liberated and determining the volume of gas desorbed at each temperature, the strength and number of adsorption sites can be determined. Other common temperature controlled studies include TPR (reduction) which measures the quantity of reducible species in the sample, while TPO (oxidation) is used to quantify the oxidizable sites. These reactions become even more useful when the effluent is analyzed by a mass spectrometer to qualify the products of reaction or the molecular species being liberated. When different temperatures are studied, calculations can be made to determine the heat of desorption.
Available Chemisorption Tests:
- Volumetric Chemisorption analysis
- Dynamic or pulse Chemisorption analysis
- Pulse Chemisorption using liquid vapors
Other Chemisorption Experiments:
- Temperature-Programmed Reduction (TPR)
- Temperature-Programmed Desorption (TPD)
- Temperature-Programmed Oxidation (TPO)
- Mass Spectrometry analysis (must be combined with Temperature Program study or TGA)
- Heat of Desorption, first order Kinetics
- Isosteric Heat of Adsorption
To assist customers with interpretation of their results we offer the following technical documents and explanations. If you still have questions, please do not hesitate to contact Micromeritics Analytical Services at 770-662-3630.