The Adsorption and Absorption of Hydrogen by Metals

Problems of current interest to which we draw attention in this review are (1) the nature of adsorbed hydrogen, (2) the possibility of weak adsorption in excess of a monolayer and its influence on surface area determinations, (3) the adsorption/ absorption transition and the mechanism of absorption, and (4) the selectivity of H for special sites in alloys and the structural modifications in alloys caused by H. Finally, we shall comment briefly on the extent to which existing theoretical models can account for some of these features. 

Many different forms of adsorbed hydrogen have been observed or postulated. These include molecular hydrogen, positively and negatively charged H atoms, weakly and strongly bound states, etc. We consider first the adsorption of H on Pt(l 11) surfaces in the knowledge that similar adsorption occurs on other Pt surfaces, and in the expectation that Pt is representative of face-centred-cubic metals in general. 

Pt(lll) Single Crystal Surfaces.—Somorjai and co-workers investigated the adsorption of hydrogen on Pt(lll) at elevated temperatures and reported that hydrogen adsorbed readily only on to stepped Pt(lll) surfaces. This is not correct. Hydrogen adsorption on any clean Pt surface is rapid even at low temperatures (initial sticking coefficient 0.1). The discrepancy arises because the comparatively small heat of adsorption of hydrogen on Pt means that in experiments in vacuo above room temperature the equilibrium amount of adsorbed H is very small.  

McCabe and Schmidt note that the P state for H/Pt(lll) could equally well be described by second-order kinetics or by first-order kinetics with a variable E. However, H2/D2 exchange experiments confirm that P2 and p- are both atomic states. Adsorbed atomic hydrogen could still desorb as molecular hydrogen with first-order kinetics if only a single surface site is involved in the desorption. This seems to be the case for H on Pd at high surface coverages (see later), but is unlikely to be important for the 03-state which only desorbs when 0 0.5. 

Work function data at 150 K show that after a small positive maximum (2 meV), attributed to preferential adsorption of at surface steps, the work function decreases continuously with H coverage (H ) to — 230 meV. Neither a change in polarity nor even a discontinuity in the dipole moment is observed when the y 2 state begins to fill after completion of the state. The sign of the work function change is opposite to that found with Ni or Pd. However, in all three cases the dipole moment is small, indicating essentially covalent bonding. 

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Burch, R. (1979) The Adsorption and Absorption of Hydrogen by Metals, in Chemical Physics of Solids and Their Surfaces, vol. 9, Chemical Society, London,... 

The adsorption (chemisorption) of hydrogen on clean metal surfaces is almost always accompanied by absorption of hydrogen into the interior of the structure. This absorption is a slow activated process and has in the past been mistaken for activated adsorption of hydrogen on the surface. 

The particular importance of surface effects in hydrogen adsorption and absorption by metals, for getters, permanent magnets, in catalytic reactions, battery electrode reaction, H embrittlement and plasma-waU interaction in fusion stems from two facts The first relates to the surface itself The sharp discontinuity of matter with electric charges and potentials of electrons and atom cores at the surface together with the loss of periodicity in the direction orthogonal to the surface leads to... 

Absorption of a gas into the bulk of the material involves adsorption, possibly molecular dissociation, then diffusion into the material. The process of injecting gas into a surface is called charging . Diffusion of gases, particularly hydrogen, into metals can be enhanced by exposure to a hydrogen plasma and low energy ion bombardment. Reasons for the rapid absorption of hydrogen into surfaces include ... 

The present Section, which provides an outline of selected relevant topics in electrochemistry, is intended primarily as an introduction to aqueous corrosion for those readers whose basic training has not involved a study of electrochemistry. The scope of electrochemistry is enormous and cannot be treated adequately here, but there are now a number of excellent books on the subject, and it is hoped that this outline will serve to stimulate further study. The topics selected are as follows a) the nature of the electrified interface between the metal and the solution, (b) adsorption, (c) transfer of charge across the interface under equilibrium and non-equilibrium conditions, d) overpotential and the rate of an electrode reaction and (e) the hydrogen evolution reaction and hydrogen absorption by ferrous alloys. For reasons of space a number of important topics, such as the electrochemistry of electrolyte solutions, have been omitted. 

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