Mitsugu Okamoto1 and Zenji Hagiwara1
1Central Research Laboratory, Osaka Oxygen Industries, Ltd., Moriyama
The overall process of hydrogen absorption was divided into such three processes that chemical reaction on the surface between metal and hydrogen, transformation from solution to metal hydride, and diffusion of hydrogen atom (ion) in metal. The authors discussed the rate of reaction by noticing the shape of its curves which represents the reaction in progress.
The results obtained were summarized as follows:
(1) From the hydrogen sorption curves, it was obvious that the rate limiting process changed from surface reaction control to diffusion. The reaction ratio corresponding to the transition from surface reaction to diffusion was 30∼40% and 40∼50% in absorption and desorption, respectively. The rate limiting step in magnesium was shifted to diffusion control after a short stay of about 17% in surface reaction control, because the activation treatment of magnesium was difficult.
(2) The unreacted core model for the reaction rate between spherical particles and gases could be fairly well applied to estimate the controling step of hydriding and dehydriding reactions; ie. at the initial stage surface reaction controls the rate and at the final stage diffusion controls it.
(3) When the hydrogen pressure was higher than 1.7×104 Pa under the present experimental condition, the surface reaction in hydrogen absorbing process followed the first order rate law, but when it was lower than 1.7×104 Pa, the surface reaction might be suggested to be the second order.
(4) The hydrogen absorbing process in metals is considered a series-type reaction by which each process takes place one after another continuously. The overall reaction is controlled by the latest step. It is possible that various defects in metals may give rise to bending in the Arrhenius plot due to the trapping effect of hydrogen.
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