INSIGHTS | PERSPECTIVES
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tical adsorption sites. How surface diffusion
proceeds in this more complex case is the
central topic of the report by Henß et al.
As an example, they chose the diffusion of
oxygen atoms on a ruthenium surface cov-
ered by carbon monoxide, which is an im-
portant model system with relevance to the
catalytic oxidation of CO. On the hexagonal
surface lattice of the basal plane of Ru, oxy-
gen atoms occupy a hollow site, situated be-
tween three neighboring Ru atoms, whereas
CO is located on top of a Ru surface atom.
Because of this mismatch, three symmetrical
oxygen-binding sites exist within the cage
formed by the surrounding CO (see the fig-
ure, bottom). Jumps between the three po-
sitions within the cage are not impeded by
the CO coadsorbates. They thus occur much
more frequently than jumps to the other
neighboring binding sites on the Ru surface,
which require relocation of a CO molecule.
Nevertheless, the latter jumps also occur at a
surprisingly high rate—nearly as high as on
the CO-free Ru surface.
Henß et al. explain this observation with
a mechanism in which first one CO steps
aside, moving closer to the neighboring CO
adsorbates. This opens a door in the cage by
which the oxygen atom can escape to an ad-
jacent site, followed by CO rearrangement to
restore the coadsorbate lattice (see the fig-
ure, bottom). Density functional calculations
show this door opening to be energetically
preferred both to a mechanism in which the
oxygen atom jumps first and triggers the nec-
essary CO displacement and to a concerted
ringlike exchange of O and CO adsorbates.
In this scenario, the coadsorbates do not
necessarily represent static obstacles to
adsorbate diffusion, as implied in the site
blocking concept. Rather, natural dynamic
fluctuations in the density of the coadsorbate
lattice can enable efficient transport path-
ways for the embedded diffusing species. One
might expect the surrounding coadsorbate
layer to nevertheless still lead to a strong
reduction in the diffusion rate. The reason
why this is not the case in the system stud-
ied by Henß et al. is probably that although
the CO adlayer is ordered, its density is only
50% of the saturation value. This facilitates
the fluctuations required for this mechanism.
Future studies should clarify the effect of ad-
layer density and in-plane order. In addition
to the dynamic effects highlighted by Henß
et al., coadsorbed layers may also weaken or
modify the adsorbate’s binding to the surface
and thus lower the energy barriers that result
from the adsorbate-surface interaction or
even change the diffusion mechanism.
Henß et al.’s findings show that the influ-
ence of coadsorbates on adsorbate diffusion
are complex and do not necessarily result in
a reduction of the surface mobility. This has
important consequences for understanding
real-world interface processes. In the field
of interface reactions, such as in heteroge-
neous catalysis, high mobility ensures rapid
intermixing of the species on the surface. As
pointed out by Henß et al., this rapid inter-
mixing is of substantial relevance for macro-
scopic reaction kinetics. Fast diffusion is also
important for mineralization, technological
deposition, and organic self-assembly. With
better understanding of the way by which
coadsorbates control surface transport, these
processes may be fine-tuned through the tar-
geted development of suitable additives. j
REFERENCES
- A.-K. Henß et al., Science 363 , 715 (2019).
- G. Antczak, G. Ehrlich, Surface Diffusion: Metals, Metal
Atoms, and Clusters (Cambridge Univ. Press, 2010). - J. V. Barth, Surf. Sci. Rep. 40 , 75 (2000).
- M.-T. Nguyen, P. N. Phong, J. Phys. Chem. A 121 , 5520
(2017). - M.-F. Hsieh, D.-S. Lin, S.-F. Tsay, Phys. Rev. B 80 , 045304
(2009). - B. Rahn et al., Angew. Chem. Int. Ed. 57 , 6065 (2018).
10.1126/science.aaw4900
Clean surface
On a clean surface, an
adsorbate can move
around freely.
Adsorbate exchange
When other adsorbates are
present that bind to the same
positions, adsorbates move
by swapping places with
neighboring coadsorbates.
Within cage
When the adsorbate
binds to a diferent
position than the
coadsorbate, it will
rapidly change
between equivalent
sites within a cage
of surrounding
coadsorbates.
Cage opening
Density fuctuations
allow it to break out
of this cage.
Difusion then can be
almost as rapid as on
the clean surface.
Movement assisted by density fuctuations
1
2
(^34)
GEOPHYSICS
Earth’s rugged
lower mantle
Seismic data reveal
kilometer-scale topography of
the lower-mantle boundary
By Christine Houser
T
o know a planet is to know its bound-
aries, where rapid changes in state
and/or composition occur. The rock-
atmosphere boundary is the one we
surface dwellers are most familiar
with, but other boundaries lie hid-
den deep within Earth; for example, the
crust-mantle boundary is a change from
more silicon-rich rock to denser, more
magnesium-rich rock. The transport of
heat and rock between the upper and
lower mantle largely determines the evolu-
tion of our planet, but little is known about
this boundary at small scales. On page 736
of this issue, Wu et al. ( 1 ) report seismic-
array data that suggest the existence of 1-
to 3-km ripples along the top of the lower
mantle. Such a structure can only be main-
tained across boundaries with distinct
chemistry, indicating that portions of the
lower mantle may contain distinct relics
from the planet’s earliest history.
Popular representations of the inner
Earth depict the mantle as a yellow or red
substance that appears to ooze across the
planet. Much to the contrary, the upper
mantle is a solid rock that is mostly green-
ish in color. It largely consists of a 60/40
blend of olivine (also known as peridot)
and pyroxene. The properties of these min-
erals change substantially as temperatures
and pressures rise with depth. Beginning
at ~1800 K and 400 km depth (22 GPa),
changes in the olivine mineral structure
increase the mantle’s rigidity and density,
leading to seismic wave reflections; at a
similar depth, pyroxene gradually changes
to garnet phases. This region is referred to
as the mantle transition zone.
At ~1900 K and 660 km depth (26 GPa),
the garnet and olivine phases rapidly con-
vert to perovskite [now called bridgmanite
( 2 )], magnesium-iron oxide, and calcium
perovskite, which persist all the way to the
mantle-core boundary. This rapid change in
mineralogy is accompanied by an ~30-fold
Earth-Life Science Institute, Tokyo Institute of Technology,
Ookayama, Meguro-ku, Tokyo 152-8550, Japan.
Email: [email protected]
696 15 FEBRUARY 2019 • VOL 363 ISSUE 6428
Diffusion on surfaces
Coadsorbates normally hinder diffusion of other
adsorbate atoms or molecules. As Henß et al. show,
density fluctuations in the adsorbate layer still can
enable rapid surface diffusion.
Published by AAAS
on February 14, 2019^
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