ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
If activated chemisorption occurs directly from the gas phase and also from a precursor state on the surface, then the importance of internal molecular excitations as compared with translational motion in promoting chemisorption can be determined by rate measurements under two different conditions: (a) When the surface and the gas are kept isothermal, the net activation energy is the same whether translational or internal motions contribute to activation, and is given by the difference Ec−Ed between the barrier Ec to the conversion of the precursor into the chemisorbed state and the barrier Ed to the desorption of the precursor back into the gas phase. (b) When the surface is kept cold and only the gas is heated, the activation energy for chemisorption directly from the gas phase is measured. The result will be equal to that in (a) only if the translational energy of the incident molecules can be utilized effectively in passing over the barrier. If internal motions must be excited, however, a higher activation energy, approaching in value Ec , will be measured in experiments with the gas heated. A special apparatus has been built for doing adsorption studies of the type (b), on freshly evaporated metal films maintained at a temperature different from that of the surrounding gas. Measurements of chemisorption on rhodium films at 0 °C, with CH4 at temmperatures from 35 to 91 °C, yield an activation energy of 11.1 kcal/mol, as compared with a value of 5.0 kcal/mol determined in previous isothermal experiments. The difference between the activation energies found for CH4 in experiments of type (a) and type (b) is in excellent agreement with the desorption energy Ed of precursor molecules established previously. This is expected if internal molecular motions, such as vibrations, rather than translation are most effective in promoting the passage of methane over the barrier to chemisorption.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.452886
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