Kinetic analysis of effects of adsorbed S-species on H sorption into Pd from UPD and OPD H

Studies of the effects of sulfide poison on H sorption into Pd can be conducted without the complications of processes involved in cathodic H2 evolution since UPD H can be generated as the initial chemisorbed source of H below the H2 reversible potential, unlike the situation at e.g. Fe, mildsteel, Ni, Ti, etc. By means of a previously developed kinetic fitting procedure, the effects of S and HS ions on the rates of individual steps in the HER under OPD conditions resulting in H sorption, are compared with the situation involving only sorption from UPD states of H, i.e. at electrode potentials positive to that of the HER reversible potential. It is shown that the expected 1:1 ratio correspondence between the H adsorption and H permeation rates arises almost quantitatively over the UPD region for which, in both the presence and absence of sulfide species, electrodeposition of H is found to be the rate-determining step in the overall H permeation process. Over the OPD potential region, sorption of H into Pd arises both from the OPD and the UPD H.