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Surface adsorption and the velocity of chemical action at gas-solid interfaces

Published online by Cambridge University Press:  24 October 2008

F. Hurn Constable
F. Hurn Constable
Affiliation:
Fellow of St John's College
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Extract

The relation between the reaction velocity of catalytic dehydrogenation and the rate of passage of alcohol vapour over the catalyst has been found by experiment. At low rates of flow the concentration of the aldehyde and hydrogen in the effluent gas, and therefore over the catalyst, is comparable with that of the reactant. The whole of the surface is thus not covered with the reactant, and the adsorbed reaction products cause marked decrease in the reaction velocity.

When the concentration of the reaction products becomes sufficiently small, that is, when the rate of passage of the alcohol vapour exceeds some limit, the velocity is approximately independent of the rate of flow of reactant. With very high speeds the reaction velocity tends to fall again, but this is probably due to the difficulty of keeping the temperature of the catalyst constant.

The formula previously deduced connecting the partial pressure of the reactant in a mixture with the fractional reaction velocity has been verified for mixtures of alcohol vapour with water, acetone and benzene; and the composition of the adsorbed gas film with the fractional partial pressure of the gases for mixtures of carbon monoxide with oxygen on platinum.

The effect of mixture of vapours on the temperature coefficient of reaction is considered, and equations deduced to show the alteration produced. If the heat of desorption of the reactant is greater than that of the diluent, then the temperature coefficient is diminished, if they are equal there is no alteration, and if less the temperature coefficient is increased. Experiments show that water present in alcohol has little effect on the temperature coefficient of the reaction.

Type
Research Article
Information
Mathematical Proceedings of the Cambridge Philosophical Society , Volume 23 , Issue 5 , January 1927 , pp. 593 - 606
Copyright
Copyright © Cambridge Philosophical Society 1927

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References

* I.e., that do not form complexes with the reactant on the surface.Google Scholar

† Constable, , Proc. Camb. Phil. Soc., vol. 23, p. 172 (1926).CrossRefGoogle Scholar

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* Mr Palmer agrees that this source of error was present in his experiments, for he observed the formation of considerable amounts of nickel hydroxide. Slight variations in the activity of the catalyst would be very serious in experiments of this type and constitute an additional source of error.Google Scholar

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