The Adsorption and Absorption of Hydrogen by Metals

BY R. BURCH

The reversible interaction of hydrogen with metals is an essential part of many catalysed reactions, is used to determine specific surface areas, and offers the prospect of using metal hydrides for hydrogen storage. A number of comprehensive reviews dealing with various aspects of H in metals have appeared and reference is made to these at appropriate points in the text. It is not our intention, therefore, to attempt an in-depth survey of the interaction of H with metals. Instead, we shall trace the progress of a H atom from its initial state in a gaseous molecule through a variety of intermediate adsorbed states to its final absorbed state.

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.

1 Adsorption of Hydrogen

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(111) 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(111) Single Crystal Surfaces. — Somorjai and co-workers investigated the adsorption of hydrogen on Pt(111) at elevated temperatures and reported that hydrogen adsorbed readily only on to stepped Pt(111) 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.

Thermal desorption spectra (TDS) of hydrogen preadsorbed at 78 K show three peaks (β1, β2, and β3) with Tmax at 140, 230, and 310 K. Preadsorption at 150 K gives two peaks in the desorption spectrum. Typical spectra are shown in Figures 1(a) and 1(b). The total surface coverage is about a mono layer (θ = 1). Christmann et al. have calculated heats of desorption (Ed) from TDS and from adsorption isotherms and obtained values of 30 and 40 kJ mol-1 for the β2 and β3 states, respectively. In contrast, from spectra having al most identical Tmax McCabe and Schmidt 4 calculate values of 53 and 75 kJ mol-1 for β2 and β3, and 33 kJ mol-1 for β1. However, they report a strong composition dependence of Ed over the range 0.03 < θ < 0.25, so that Ed ≈ 42 kJ mol-1 at θ = 0.25. The discrepancies at low coverage (θ < 0.25) may be due to differences between the surfaces examined (the high value of Ed at θ= 0.03 may reflect preferential adsorption at surface defects), or because of errors in the calculation of Ed. King has pointed out a possible error when deriving Ed from TDS if weakly bound intermediate states exist. Schwartz et al. also describe how errors can arise if the wrong order of reaction is assumed. By utilizing the entire desorption spectrum of H/Ti they show that the order of the desorption reaction is 1.5, yielding a value for Ed of 88 kJ mol-1. Analysis using only Tmax and assuming a second-order reaction gave Ed = 210 kJ mol-1.

McCabe and Schmidt note that the β3 state for H/Pt(111) could equally well be described by second-order kinetics or by first-order kinetics with a variable Ed. However, H2/D2 exchange experiments confirm that β2 and β3 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 β3-state which only desorbs when θ ≤ 0.5.

Work function data at 150 K show that after a small positive maximum (2 meV), attributed to preferential adsorption of Hδ- at surface steps, the work function decreases continuously with H coverage (Hδ+) to -230 meV. Neither a change in polarity nor even a discontinuity in the dipole moment is observed when the β2 state begins to fill after completion of the β3 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.

The distribution of H atoms on Pt(111) has been considered. Christmann et al. reject the possibility that the β2 and β3 states correspond to different geometric locations. Toya had previously proposed two types of adsorbed H, an 'r-adatom' situated above a single metal atom and located outside the electronic surface of the metal, and an 's-adatom' situated at a surface interstitial site. However, this is not consistent with the observed smooth decrease in work function up to θ = 1.0. Christmann et al. suggest instead that repulsive interactions between H atoms produce an ordered structure in which H desorbs from the β3 state by the recombination of two H atoms when neighbouring sites are empty, while H desorbs from the β2 state when neighbouring sites are occupied.

It is difficult to determine whether H atoms are adsorbed above or between metal atoms. Flores et al. have presented a model for H/Pt(111) which is consistent with the data described above, and also with n.m.r., i.r., and neutron inelastic scattering data, all of which point to H bound to more than one Pt atom. In this model the H — H repulsions originate from the interaction of the screening potentials around the hydrogens. Their model (Figure 2) is interesting with regard to absorption (see later) because it shows that as θ -> 1.0 both forms of hydrogen (β2 and β3) become equivalent. Only when hydrogen is being adsorbed or desorbed are β2 and β3...