The lack of coordination above the surface leads to adsorbed H atoms closer
to the underlying metal atoms than in the bulk, so distances of jumps from the
adsorption sites to the nearest subsurface sites are shorter than the nearest neighbor
distances in the bulk (a /
√
3).
On the (111) face, each hcp surface site is located just above a subsurface
tetrahedral site T
+
12
(located between the first and the second metal layer, below
three surface atoms), from which three octahedral subsurface sites O
12
may be
reached. Each fcc surface site is just above such subsurface octahedral site O
12
,
from which, via three T
+
12
or three T
–
12
sites (below one surface atom), six O
12
sites
and three O
23
sites (below the second layer) may be reached, which makes nine
possibilities of jumps under the surface.
On the (100) face (if it is not reconstructed), an H atom adsorbed in a
fourfold hollow site (at a position close to that of an octahedral site) may jump to
four tetrahedral subsurface sites T
12
, from each of which two octahedral sites O
2
(at the second layer level, below surface atoms)may be reached.
On the (110) face (the more open face of the three low-index faces on fcc
metals) the adsorbed H atoms are alternatively adsorbed on each side of the
close-packed [110] rows, in the pseudo-threefold fcc sites close to the bridge sites
[82]. They may jump to one subsurface site O
2
at the second layer level, under the
bridge sites. On the (110) faces of Ni and Pd, above a coverage of one, hydrogen
induces reconstruction into a (110)-(1 × 2) structure, probably of the pairing-row
type [8,15,82]. This opens the surface more and allows accommodation of H on
the second metal layer, leading to a coverage of 1.5 H per first layer metal atom,
and makes it easier to reach subsurface sites [8,83,84].
Energetic Aspects
Gas Phase
Energetic analysis using temperature-programmed desorption (TPD), also called
thermal desorption spectroscopy (TDS), on a well-defined surface makes it possible
to determine the structure sensitivity and the heights of the energy barriers of the
surface processes. It was shown that the role of the substrate surface orientation in
the population of subsurface sites is crucial. On the reconstructed (110) faces of Ni
and Pd or on more open faces [8,83,84], subsurface sites are populated at
temperatures as low as 100 K and low H
2
pressures (10
–6
Pa), whereas higher
temperatures and pressures are necessary with the more densely packed planes [8].
The overall energetics of the hydrogen-metal reactions are illustrated in the
schematic drawing of Figure 8a showing the one-dimensional potential energy
versus distance curves of the various H states at the metal-gas interface. Two H
adsorbed states (corresponding to the hollow and on-top sites) are represented.
Actually, the energy levels indicate the chemical potentials of these states, referred to
the standard chemical potential of ½H
2(g)
. The standard free energy differences
between the various states are indicated. The diagram show that at 1 atm
nonactivated adsorption occurs spontaneously in the deep energy wells of the hollow
sites, whereas adsorption is activated in the shallow wells of the on-top sites, which
need a higher pressure to be filled. The diagram shows a case of endothermic H
absorption, with a high activation energy barrier for the transition between strongly
bonded H
ads
and H
diss
. The depths, of the wells and the height of the energy barrier,
Surface Effects on Hydrogen Entry into Metals 71
Copyright © 2002 Marcel Dekker, Inc.