Abstract
The energetics and kinetics of hydrogen adsorption on Re(1010)
and Pd(210) have been investigated by means of several
experimental techniques, although investigations by thermal
desorption
spectroscopy yielded the most valuable information. Similar to other
hydrogen
adsorption systems involving transition metal surfaces, a high
sticking
coefficient was observed for small surface coverages. For higher
coverages,
a
drastic decrease of the sticking coefficient was observed. These
observations
can be explained with the existence of a precursor state. The
hydrogen-metal bond (hydrogen adsorbs as a atomic species in that investigated
temperature range above 100 K)
was found to be slightly stronger for Re(1010)
and Pd(210) have been investigated by means of several
experimental techniques, although investigations by thermal
0)
than for Pd(210), but the binding energy of both systems (2.7-2.9 eV)
was
within the
range usually observed for hydrogen-metal-systems (e.g., H/Pd(110)
[9] and H/Ru(1010) [91]).
A crystallographic analysis of the Re(1010)/c(2x2)-3H phase was
performed
by LEED I-V studies. The structure of this phase resembles that of the
c(2x2)-3H structure found for the system H/Ru(1010). In both phases,
hydrogen
populates a quasi-threefold adsorption site and an unusual bridge site.
A contraction (5%) of the spacing between the first and second layer
( ?d12)
exhibited by the clean surface is lifted by hydrogen adsorption. Again,
similar effects have previously been observed for many other
hydrogen-metal
adsorption systems. Although the radius (Bohr radius: 0.53Å) of
hydrogen
is small in comparison with the hcp-radius of rhenium metal (1.4Å),
the
hydrogen atoms have a strong effect on the structure of the substrate
extending
at least down to the third layer of rhenium atoms. This behaviour can be
explained with the size of the rhenium d-orbitals which extend far into
the vacuum. The 1s-orbital of the adsorbed hydrogen overlaps strongly
with
the 5d-orbitals of the rhenium causing a significant perturbation of the
metal-metal bonds.
No evidence for subsurface-hydrogen was observed for the system
H/Re(1010).
On the other hand, evidence for substrate penetration by hydrogen was
obtained
for the system H/Pd(210). The Pd(210)-surface exhibits pore-like
openings
allowing the uptake of hydrogen without additional restructuring of the
surface atoms, as necessary, e.g., for Pd(110) [9]. This puts the
(210)-surface of Pd into very special position among the more ``open``
Pd-surfaces. To investigate the influence of this special morphology, it
would be useful to examine crystallographically similar systems which do
not absorb hydrogen because of a greater activation barrier. For
example,
the Ni (111) surface does not allow any hydrogen penetration, but
perhaps
penetration of the Ni(210)-surface would be possible.
Another important point was the observation that it was
necessary
to populate three highly coordinated adsorption sites per unit mesh of
Pd(210) before subsurface hydrogen was formed. This suggests that
metal-metal
bonds have to be weakened by chemisorption to decrease the cohesive
energy
of the substrate lattice and allow the filling of interstitial sites
with
hydrogen atoms.
Finally, it should be mentioned that adsorption of molecular hydrogen
was observed at very low sample temperatures (approximately 50 K). That last
item will probably be a very rich field
of future investigations. |
