REPORT
◥SURFACE SCIENCE
Density fluctuations as door-opener
for diffusion on crowded surfaces
Ann-Kathrin Henß^1 , Sung Sakong^2 , Philipp K. Messer^1 , Joachim Wiechers^3 ,
Rolf Schuster^4 , Don C. Lamb^1 , Axel Groß^2 , Joost Wintterlin^1 *
How particles can move on a catalyst surface that, under the conditions of an industrial
process, is highly covered by adsorbates and where most adsorption sites are occupied has
remained an open question. We have studied the diffusion of O atoms on a fully CO-covered
Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling
microscopy combined with density functional theory calculations. Atomically resolved
trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the
clean surface. This finding can be explained by a“door-opening”mechanism in which local
density fluctuations in the CO layer intermittently create diffusion pathways on which the
O atoms can move with low activation energy.
W
e investigated the question of how
atoms can move on a“crowded surface,”
the situation on solid catalysts under
operation conditions. Surface diffusion,
which determines the mixing of the
reactant particles on the catalyst surface and
their lateral transport—for example, to defects
acting as“active sites”—is generally regarded as
extremely fast. This assumption is based on the
low hopping barriers that the adsorbed particles
can easily overcome at the applied elevated
temperatures ( 1 ). Surface diffusion is thus usually
not even included in the kinetics of catalytic
reactions.
On the other hand, at the high pressures of
industrial processes, catalyst surfaces can be
highly covered by molecules adsorbing from
the gas phase, reaction intermediates, and by-
products. Whether the picture of an extremely
mobile layer still holds for such crowded sur-
faces on which most adsorption sites are oc-
cupied is an open question. Measurements of
macroscopic surface diffusion coefficients usually
show a decrease with increasing coverages of
the adsorbates ( 2 – 6 ) so that one may expect just
the opposite: complete immobility at saturation.
However, such macroscopic data are difficult
to interpret with respect to atomic processes;
simple site-blocking usually seems insufficient,
and more complex effects—such as repulsive or
attractive interactions between the particles,
phase formation, and trapping at defects—play
a role. For atomic diffusion mechanisms on
crowded catalyst surfaces, one thus usually
relies on kinetic Monte Carlo simulations ( 7 , 8 ).
Some experimental, atomic-scale information
of how particles may move when embedded in a
close-packed two-dimensional (2D) layer has
been obtained with scanning tunneling micros-
copy (STM). For In and Pd atoms embedded in
the first layer of a Cu(100) surface, these“tracer
atoms”moved by means of a vacancy mechanism
( 9 – 11 ), which is also the most prominent diffu-
sion mechanism in 3D solids ( 12 ). The tracer
atom can only jump to a neighboring lattice
site when one of the mobile vacancies, whichexist in a solid at a finite temperature, comes
near the tracer atom, so that it can exchange sites
with the vacancy. For Cl atoms on an H-covered
Si(100) surface, a direct exchange of the tracer
atom with neighboring H atoms was suggested
( 13 ). Such a direct exchange mechanism has also
been postulated for 3D solids but is usually not
observed because of the high activation energies
involved. On Cl- and Br-covered Cu(100) electrodes
in an electrolyte solution, S tracer atoms were
proposed to move by means of two other mech-
anisms known from 3D solids: a ring-exchange
mechanism or a mechanism resembling the so-
called interstitialcy mechanism ( 14 ). In both
cases, the dynamics were strongly affected by
the electrical potential and thus specific to the
electrochemical environment.
Here, we show that in an adsorbate layer on
a catalyst, which is chemically and structurally
quite different from these systems, diffusion may
follow a different mechanism. Using a combined
high-speed/variable-temperature STM that achieves
imaging rates of up to 50 frames s−^1 ,weinvestigated
the dynamics of individual O atoms on a fully
CO-covered Ru(0001) surface. This system was
chosen because the CO oxidation on Pt group
metals is a well-studied model for catalysis ( 15 ).
Because (metallic) Ru is the least active of these
metals ( 16 ), diffusion could be studied without
competing CO 2 formation. By means of statistical
analysis of movies that consist of a large number
of STM images over an extended range of tem-
peratures and complementary density func-
tional theory (DFT) calculations, we obtained a
complete description of the atomic mechanism.
In a“door-opening”mechanism, local densityRESEARCH
Henßet al.,Science 363 , 715–718 (2019) 15 February 2019 1of4
(^1) Department of Chemistry, Ludwig-Maximilians-Universität
München, Germany.^2 Institute of Theoretical Chemistry,
University of Ulm, Ulm, Germany.^3 Dionex Softron GmbH,
Germering, Germany.^4 Institute of Physical Chemistry,
Karlsruher Institut für Technologie, Karlsruhe, Germany.
*Corresponding author. Email: [email protected].
Fig. 1. Hopping of O in a CO cage.(AtoC) Three consecutive STM images of the position of a
single O atom embedded in the layer of CO molecules on Ru(0001). The bright dot is the
O atom, and the hexagonal pattern is theð
ffiffiffi
3
p
ffiffiffi
3
p
ÞR30° structure of CO molecules (dark dots);
the“x”marks að
ffiffiffi
3
p
ffiffiffi
3
p
ÞR30° lattice point (300 K, 10 frames s−^1 , tunneling voltageVt=–0.22 V,
tunneling currentIt= 10 nA). (DtoF) Corresponding structure models. The O atom is indicated
in red, the CO molecules in blue, and the Ru atoms in gray.
on February 14, 2019^
http://science.sciencemag.org/
Downloaded from