Activation Energy for the Desorption of Carbon Dioxide from Oxygen-Covered Silver

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Abstract
The thermal desorption of carbon dioxide from an oxygen-covered silver powder was studied from 25 ° to 150 °C by applying a nominal heating rate of 2 °C/min and measuring the mass lost with a vacuum microbalance. A maximum rate of desorption was reached at 49 °C and the desorption was completed at about 100 °C. The temperature at which the maximum rate of desorption occurred was independent of the initial coverage of carbon dioxide to within 2 °C. However, the data could not be simply explained by a fit to the first order Wigner–Polanyi desorption equation, in which it is assumed that the frequency factor (ν) and the desorption energy (E) are independent of coverage and temperature. Beyond the maximum, a slow decrease in the experimental rate of desorption, which was found in contrast to the usual sharp cutoff for a first order process, was attributed to desorbing oxygen, readsorption of carbon dioxide, or a variation of E at low coverages. Parallel studies with a residual gas analyzer showed that small quantities of oxygen were desorbed in addition to the carbon dioxide. The experimental desorption curve could be fit to the Wigner–Polanyi equation by assuming that the desorption consisted of all the adsorbed carbon dioxide and a small amount of oxygen. The desorption parameters for carbon dioxide which were obtained ranged from E = 20 to 24 kcal/mol and ν = 1011 to 1013 sec−1.

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