cage originally occupied by the O atom. The
net effect is a complete O/CO site exchange.
However, the barrier of the first step is >50%
higher than the experimentalE 2 value, which,
given the high quality of the Arrhenius plot, we
consider a substantial difference. Thus, we also
rule out this mechanism.
In the sequential mechanism in which a CO
molecule moved first, the initializing step could,
for example, be a move of a CO to the top posi-
tion 1 (Fig. 4A, orange arrow). The related barrier
is 0.30 eV (Fig. 4B, orange line), and the energy
of the O/CO configuration increased by 0.16 eV
(Fig. 4B, solid black line). The same barriers and
energy shifts were found when the CO initially
moved to one of the other neighboring top sites.
The increase in energy was caused by the mutual
repulsions of the CO molecules on adjacent top
sites, but this effect was relatively small. This
repulsion also led to an outward tilt of the CO
axes in the range of 5° to 9°, depending on the
particular configuration. When, in the following
step, the O atom moved to the indicated fcc site
(Fig. 4A, black arrow), the barrier was consider-
ably reduced to 0.62 eV (Fig. 4B, solid black line).
The subsequent transfer of the O atom to hcp
site 2 only had a barrier of 0.29 eV, and the con-
secutive rearrangements of the CO molecules,
such as along the blue arrows in Fig. 4A, were
again fast. Once the O atom moved to the hcp 2
site, it was much more likely for the CO mol-
ecules to rearrange than for the O atom to jump
back. If we assume that the initial CO shift to a
neighboring top site is a fast preequilibrium, the
activation energy for the full process is the sum
of the initial energy shift of 0.16 eV and the
barrier for the O atom to jump to the fcc site of
0.62 eV, giving 0.78 eV. This value is the lowest
from all mechanisms tested, and it was in rea-
sonable agreement with the experimentalE 2
value of 0.63 eV.
The sequential mechanism in which a CO
molecule moved first followed by the movement
of the O atom described the STM data best. It
resembles the ring-exchange mechanism in 3D
solids, in which the tracer atom and several matrix
atoms move in a ringlike fashion ( 12 ), but the
classical ring exchange is a concerted mechanism.
We ruled out a concerted movement here be-
cause, in this case, the barriers of all elementary
steps would add, giving unreasonably high activa-
tion energies. The mechanism is sequential, and
it is better described as a local density fluctuation
of the CO layer that occasionally opens a door,
allowing the O atom to escape from its cage.
Once the O atom has escaped, the locally dis-
ordered CO molecules quickly reorder, closing
the door and preventing the O atom from im-
mediately jumping back, which completes the
exchange.
Such a diffusion mechanism is not known in
3D solids and has also not been reported for the
diffusion of particles in close-packed surfaces.
That such a mechanism occurred here can be
understood by the CO molecules—despite form-
ing an ordered overlayer—not being very densely
packed, so that density fluctuations could occur
with low activation energy. The mechanism is
efficient, allowing the O atom to move at a sur-
prising speed. For example, the frequency of the
exchange jumps at 300 K of 4.7 s−^1 [normalized
by 6/4 to account for the reduced number of four
directions in which the O atom can exchange
with CO (Fig. 3E, inset)] is only by a factor of
3.5 lower than the hopping frequency of 16.6 s−^1
of an O atom on the empty Ru surface (only
measured at room temperature, so that no bar-
rier is available) ( 24 ). The O atom could thus
travel on the crowded surface almost as fast as on
the empty one. Additional effects of a weakened
O–Ru bond by the coadsorbed CO have been
investigated but played a minor role (supple-
mentary materials).
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mayplayageneralrolefordiffusiononcatalyst
surfaces. Strongly bound particles such as O, N,
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as the distribution of the particles on the ad-
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disturbed by the reaction, this condition is only
fulfilled if the surface mobility is high enough to
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ACKNOWLEDGMENTS
A.-K.H. and J.Win. thank F. Kreuzer, T. Gisicius, and R. Hiermaier,
LMU Munich, for expert precision mechanics support.Funding:
S.S. and A.G. acknowledge support by the Baden-Württemberg
Foundation through the project MSMEE within the High-
Performance Computing II program and computer time through
bwHPC and the German Research Foundation (DFG) through grant
INST 40/467-1 FUGG.Author contributions:A.-K.H. performed
the STM measurements, developed the data recording and analysis
software, and evaluated the data. A.-K.H. and J.Win. wrote the
manuscript and designed Figs. 1 to 3. J.Win. supervised the project
and developed the mathematical diffusion model. S.S. and A.G.
performed the DFT calculations and designed Fig. 4. P.K.M. and
D.C.L. provided the atom-tracking algorithm. R.S. wrote a first
version of the data recording software. J.Wie. built the I/V
converter for the high-speed STM. All authors reviewed and
commented on the manuscript.Competing interests:The authors
declare no competing interests.Data and materials availability:
The particle tracking algorithm is available under https://gitlab.
com/phme/wavelet-tracking. All data are available in the main text
or the supplementary materials.SUPPLEMENTARY MATERIALS
http://www.sciencemag.org/content/363/6428/715/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S3
Table S1
References ( 25 – 34 )
Movies S1 and S2
13 September 2018; accepted 26 December 2018
10.1126/science.aav4143Henßet al.,Science 363 , 715–718 (2019) 15 February 2019 4of4
Fig. 4. Paths and energy diagram of the exchange jump of an O atom with CO.(A) Unit cell
used for the DFT calculations. The black arrows indicate the path along which the O atom moves;
the orange and blue arrows are paths taken by two CO molecules with possible intermediate
positions 1 and 2. (B) Energy profile when the O atom moves first (dotted black line), and when a
CO molecule moves first and then the O atom (solid black line).
RESEARCH | REPORT
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