Examination of Factors Influencing Promotion of H Absorption into Metals by Site‐Blocking Elements

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Abstract
Renewed interest in absorption of H and D into transition metals has been stimulated by current work on metal‐hydride anodes for rechargeable batteries and by the intense interest in recently studied anomalous phenomena associated with absorption of D into Pd. One principal problem in this field is how site‐blocking elements (SBE), also called “catalyst poisons” (P), co‐adsorbed with H on transition metal H hosts, usually substantially promote the absorption of H into a number of such metals, yet such poisons are known to inhibit cathodic evolution and diminish surface coverage by H, the intermediate in the latter process and the source of the absorbed H. These effects of catalyst poisons in promoting H absorption into host metals are examined in terms of effects of competitive adsorption between the poison and H at the surface. Conventional ideas about these effects are inconsistent with information on diminution of H coverage caused by competitive adsorption of poisons. Consideration of various possibilities indicates that the enhancement of H absorption by SBEs can arise only on account of direct interaction effects and long‐range indirect electronic charge‐density effects. Numerical simulation calculations are given on the competitive chemisorption of H vis à vis a poison, P, in relation to the configurational contribution to the Gibbs energy of the adlayer, . Numerically derived, three‐dimensional diagrams are presented to illustrate these effects. The role of lateral interaction effects among adsorbed H and P species in the thermodynamics and kinetics of H absorption is also examined.

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