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The desorption of oxygen from silver powder

Authors
Journal
Journal of Colloid and Interface Science
0021-9797
Publisher
Elsevier
Publication Date
Volume
38
Issue
1
Identifiers
DOI: 10.1016/0021-9797(72)90230-5
Disciplines
  • Chemistry

Abstract

Abstract Desorption of oxygen from silver powder has been studied with a microbalance during and after using a thermochemical surface preparation technique. The powder was exposed cyclically to outgassing, oxygen adsorption at 5 Torr, outgassing and carbon monoxide reduction at 5 Torr (OAOR cycling) at a constant temperature chosen from the range 300–400°C. The microbalance was used to measure the rate and amount of oxygen desorbed from the powder at various stages of cycling by the techniques of thermal desorption and isothermal desorption. The data were analyzed for E and v by assuming that the rate of desorption follows the relation: − dθ s dt = v nθ s n exp − E RT , where n is the order of the process and is either one or two, E is the activation energy of desorption, v is the frequency factor, and θ s is the fractional coverage of a chemisorbed state. For a surface cycled at 325–400°C, the desorption parameters E and υ changed during the first five cycles and then remained constant with additional OAOR cycling. The desorption of oxygen was found to be a second-order process and E nearly independent of coverage when oxygen was adsorbed at 154°C and 5 Torr. The same result was obtained when oxygen was adsorbed at the cycling temperature and 5 Torr, and then cooled to 154°C in the ambient oxygen. The values of E and υ obtained are: E = 34.5 ± 3.5 kcal/mole and v = 2 × 10 11 to 2 × 10 14 sec −1 from thermal desorption data and E = 41.8 ± 3.5 kcal/mole and v = 3 × 10 13 to 5 × 10 16 sec −1 from isothermal desorption data. It can be concluded from these results that the adsorbed state at 154°C consists of weakly localized adsorbed oxygen atoms that recombine prior to desorption. For a sample cycled at 300°C, the amount of oxygen desorbed was insufficient to determine the parameters with any meaning even after extended cycling.

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