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Chemisorption

Dateline: 08/03/98

By Alan Bruzel

Atoms and molecules use several adsorptive mechanisms for binding to surfaces. Adsorption is a general term describing physical, not chemical, binding of gaseous, liquid, or solid adsorbates to an adsorbent surface. Most adsorptive phenomena are classified under the heading of physisorption. In this process, adsorbates may freely move on the adsorbent surface. Repeated addition of adsorbate produces an adsorbate monolayer; eventually the adsorbate entities stack atop one another. The weak electrostatic forces involved in physisorption include van der Waal's forces and dipole interactions. Temperatures less than room temperature are sufficient for these processes to take place.

For certain combinations of adsorbate and adsorbent, higher temperatures are required to construct either stronger electrostatic interactions or actual chemical bonds. The adsorbate's capture of a free electron from the adsorbent surface results in bond formation. This adsorptive process, known as chemisorption, occurs on such surfaces as activated carbon, cerium oxide, platinum, titania-silica, and zeolite. The importance of understanding chemisorptive events at the molecular level has spurred the investigation of various model systems. One such system employs carbon monoxide as the chemisorbate and metal surfaces such as copper, gold, nickel, palladium, platinum, ruthenium, and silver as the adsorbents. Rather than remaining content with simple gaseous molecules, this field of study has expanded to include chemisorption of organic molecules such as porphyrins, uracil, and cytosine on gold surfaces. Chemisorption also allows buckminsterfullerene (C₆₀) to coat the metallic tips used in scanning tunneling microscopes, resulting in better performance of the microscope.

The chemisorptive capacity of an adsorbent surface is determined by quantitation of the surface's active sites. The surface of a chemisorptive metal is first reduced with hydrogen gas at high temperature. A precise amount of adsorbate is then dispensed onto the surface. The adsorbate is usually a gas such as ammonia, carbon monoxide, hydrogen, nitrous oxide, or oxygen. Automatic measurement of the small differences in gas pressures before and after chemisorption permits calculation of the extent of adsorbate binding. Another approach utilizes a pyroelectric sensing technique to measure the chemisorptive heat of reaction of carbon monoxide's binding to a platinum surface. Measurement of 10^-42 moles of carbon monoxide (an 18ľK temperature change) has been reported.

Practical applications of chemisorption are found in the aerospace industry as well as in earthbound environmental cleanup operations. Carbon dioxide lasers produce carbon monoxide as a by-product, reducing the efficiency of the laser. A platinum surface catalyzing the ambient temperature oxidation of carbon monoxide to carbon dioxide is under development for use in weather satellites. Another implementation of chemisorption is an outgrowth of the Montreal Protocol that bans chlorofluorocarbons (CFCs) by the year 2000. Hydrofluorocarbons (HFCs) will still be allowed. (See this site's article on Chlorofluorocarbons and Ozone.) A chemical trick is needed to hydrodechlorinate the CFCs into HFCs. The chemisorptive palladium surface can act as a site where chlorine atoms of CFCs are replaced with hydrogen atoms, thereby forming the permissible HFCs. A further innovative use of chemisorption in pollution control employs activated carbon impregnated with sulfur atoms. Mercury, present as a contaminant in gases passing through this material, is removed by chemisorption onto the sulfur. This treatment could be used to remove mercury and other impurities from exhaust gases generated during manufacturing processes.

Recommended Web resources for additional information:

Adsorption and Thin Film Growth Mechanisms
Lectures from John A. Venables including chemisorption.

Center for Atomic-scale Materials Physics
Includes studies of adsorbate-induced restructuring of metal surfaces.

Structure and Dynamics at Surfaces
Research and publications of David King, Cambridge University.

Ultra High Vacuum Scanning Tunneling Microscopy
Examination of chemisorptive bonding of C₆₀ to silicon surfaces.

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