Hydrogen permeation mechanism

Strength needed for the application in chemical processes and for tolerating the down-up pressure difference, which is the hydrogen permeation mechanism driving force. 

Fig. 8.38 Hydrogen permeation current as a function of applied potential, showing effect of change in reaction mechanism below —1200 mV (SCE) (after Obuzor )...

The electrochemical hydrogen permeation technique has proved to be a valuable tool in the study of these reaction mechanisms. This is mainly due to the ability to estimate the amount of an intermediate (Hads) in the reaction scheme. Such studies have been presented, for example, by Devanathan and Stachurski, by Bockris et and by Iyer et The applicability of the Volmer-Tafel reaction scheme can be evaluated by considering the kinetic expressions for reactions (22) and (23), together with equilibrium in the absorption process (25)... 

The main problem with the integration of inert membranes in fixed bed reactors is the membrane itself. It has to be sufficiently stable against thermal and mechanical stress, inexpensive and of high selectivity and permeability. At present, Pd-based alloys are the only sufficiently temperature stable and permselective membranes for hydrogen permeation. Since permeation through homogeneous membranes is a rather slow process, large membrane areas are required for reasonable production quantities. 

Hydrogen permeation tests on ferritic stainless steels indicated that hydrogen can diffuse through the alloys, though the permeation was drastically decreased by formation of chromia scale on the alloys. - The mechanisms by which the presence of hydrogen or protons at the air side affects the oxide scale structure and growth are not clearly understood at this time. Several mechanisms have been proposed to tentatively explain the observed anomalous oxidation behavior. ... 

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