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Chemical Adsorption - Chemisorption Diagram

Chemical adsorption, or chemisorption, is a process resulting from a chemical bond between adsorbate molecules and specific surface locations on a material, known as active sites. This interaction is much stronger than physical adsorption, or physisorption, which takes place on all surfaces if temperature and pressure conditions are favorable. Chemisorption only occurs on clean active sites and, unlike physisorption, ceases when the adsorbate can no longer make direct contact with the surface, making chemisorption a single layer process.

Chemisorption measurement techniques are useful for evaluating physical and chemical properties of materials that are critical for process / reaction performance. Primarily, chemisorption is used to evaluate the number of available active sites to increase the rate of, or catalyze, chemical reactions. Other properties can include the (reduction or oxidation) temperature at which catalysts become active, strength of specific types of active sites, or ability of materials to perform after reduction/oxidation cycles.

Chemisorption measurements are important for characterization of catalysts used in several industries including oil and gas (e.g. petroleum refining, syngas conversions, biofuel production, fuel cells), petrochemicals and fine chemicals (e.g. hydrogen production, polymers and plastics production), environmental (e.g. automotive catalytic converters, green chemistry),  and many others.

Analyses can be performed using either static or dynamic flow methods. Either method of chemisorption, conducted at a temperature of interest, can determine the number of accessible active sites, active surface area, degree of dispersion, and active particle (crystallite) size.  Pulse chemisorption is commonly used to probe strong active sites only, while the static technique can distinguish between strong or weak active sites.

Since industrial catalytic applications often involve changes in reaction temperature, non-isothermal methods are also available including Temperature-Programmed Reduction (TPR), Temperature Programmed Oxidation (TPO), and Temperature Programmed Desorption (TPD). TPR measurements are used to evaluate the reducible sites, such as metal oxides, while TPO measurements are used to evaluate the oxidative sites, such as metals or carbonaceous deposits on catalysts.  Temperature Programmed Desorption (TPD) is used to evaluate the number, relative strength and heterogeneity of the active sites, such as solid or supported acid catalysts.

PTL offers the following chemisorption analyses which are highly customizable to client specific parameters:

Chemisorption Tests:

  • Static (Volumetric) Chemisorption analysis
  • Dynamic or Pulse Chemisorption analysis
  • Pulse Chemisorption using liquid vapors

Temperature-Programmed Studies:

  • Temperature-Programmed Reduction (TPR)
  • Temperature-Programmed Desorption (TPD)
  • Temperature-Programmed Oxidation (TPO)

Other Chemisorption Experiments:

  • Heat of Desorption, first order Kinetics
  • Isosteric Heat of Adsorption


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