Journal ArticleDOI

Complex reactions on a convertible catalyst surface: A study of the S-O-Cu system

01 Dec 2018-Surface Science (North-Holland)-Vol. 678, pp 228-233

AbstractThe interaction of clean and partially oxidized Cu(110) with sulphur was studied by scanning tunneling microscopy and density functional theory calculations in the low-coverage range. On the clean Cu surface individual S atoms adsorb in the troughs between the Cu atom rows. Hollow sites are preferred, but long-bridge sites are occasionally occupied as well. The majority of adsorbed S, however, seems to be involved in the formation of highly mobile CuxSy clusters of various sizes. The clusters preferentially attach to steps thus changing the step morphology completely. Some of the clusters form aggregates on the terraces. On the partially oxidized surface similar clusters form and cause long-range mass transport to steps. Additionally, nanowires form in [001] direction on and along the surface oxide stress domains. These nanowires have a complex composition, exhibit different corrugations and appear sometimes as three-dimensional needles. Occasionally they flip their direction by 90°, but doing so they partially decompose. Finally, annealing of the S-O-Cu surface leads to consumption of the surface oxide stripes indicating loss of oxygen presumably via SO2 formation. Simultaneously, linear sulphur chains suspended between the [001] –O-Cu-O- chains form in [ 1 1 ¯ 0 ] direction. The surprising multitude of processes and products even at low-pressure, low-temperature conditions in the comparatively simple S-O-Cu system highlights the difficulty of controlling reactivity and selectivity on such convertible catalyst surfaces.

Summary (2 min read)

Introduction:

• Copper catalyzes a number of industrially important chemical reactions.
• It is used for instance as desulfurization catalyst [1, 2], in the low-temperature stage of the water gas shift reaction[3, 4] and in catalytic CO2 reduction [5, 6].
• The catalytic activity of copper can in part be attributed to its electronic structure [7], but morphology, e.g. number density and type of under-coordinated sites etc. is an important descriptor as well[8].
• The Sulfur-Cu interaction has been addressed in several previous surface science studies, both experimentally [9-20] and theoretically [21-25].
• The authors expected this to simplify the surface reactions in comparison to SO2 or H2S exposure, since disproportionation reactions and hydroxyl or water formation should be suppressed in this case.

Experiment and Theory:

• The experiments have been carried out in a UHV system featuring a preparation chamber and a separate cryostate chamber housing the STM.
• In the following, the authors give the sulfur dose in atoms per cm2.
• The calculations were performed with plane-wave density functional theory (DFT) using the Vienna ab initio simulation package (VASP) [27, 28].
• For bulk optimization, the lattice parameters were obtained by minimizing the total energy of the unit cell using a conjugated gradient algorithm to relax the ions and considering a set of 4×4×4 Monkhorst−Pack k-points to sample the Brillouin Zone. Cu(110) surfaces were modelled with slabs of five layer thicknesses.
• A kinetic energy cutoff of 400 eV was employed for all the calculations.

Results and Discussion:

• Three major changes occur at the Cu(110) surface upon S dosing.
• More importantly, the 110 chains formed between the oxide stripes after SO2 exposure have a twofold periodicity and form occasionally a local p(2×2) configuration.
• The 110 chains observed here are also located in the troughs between the top-layer copper rows and they are also mobile to some extent (see Fig. 3C, where the 110 chain seems to change position while the image is recorded).
• Notably, the 110 chains form only in the presence of the Cu-O-Cu chains in the Cu surface oxide and the surface oxide stripes are consumed during the formation of the chains.
• We first followed the suggestion of Alemozafar et al. [15, 17] trying to construct the chains from Cu and SO3 building units or from SO3 alone.the authors.the authors.

Summary:

• Even under UHV conditions and moderate temperatures the interaction of Cu(110) with S gives rise to a variety of restructuring processes.
• Long-bridge and hollow sites are both occupied with a slight preference for the latter.
• Sx species terminated by Cu+ atoms within –O-Cu-O- chains.
• Each of the observed structures might result in different reaction paths in a catalytic process.
• This work was supported by the Office of Basic Energy Sciences (BES), Division of Materials Sciences and Engineering, of the U.S. Department of Energy (DOE) under contract no.

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Recent Work
Title
Complex reactions on a convertible catalyst surface: A study of the S-O-Cu system
https://escholarship.org/uc/item/1ph7w7k7
Authors
Dürrbeck, S
Shi, XR
et al.
Publication Date
2018-12-01
DOI
10.1016/j.susc.2018.03.010
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

1
Complex Reactions on a Convertible Catalyst Surface:
A Study of the S-O-Cu System
S. Dürrbeck
1,2
, X.-R. Shi
3
2,4
, J. Redinger
5
, E. Bertel
1*
, M. Salmeron
2*
1
Institute of Physical Chemistry, University of Innsbruck, Austria
2
Lawrence
Berkeley Laboratory, Berkeley CA
3
College of Materials Engineering, Shanghai University of Engineering Science, Shanghai
201620, P. R. China
4
Present address: Tbilisi, Georgia
5
Department of Applied Physics, Vienna University of Technology, Austria
The interaction of clean and partially oxidized Cu(110) with sulphur was studied by
scanning tunneling microscopy and density functional theory calculations in the low-
coverage range. On the clean Cu surface individual S atoms adsorb in the troughs between
the Cu atom rows. Hollow sites are preferred, but long-bridge sites are occasionally
occupied as well. The majority of adsorbed S, however, seems to be involved in the
formation of highly mobile Cu
x
S
y
clusters of various sizes. The clusters preferentially
attach to steps thus changing the step morphology completely. Some of the clusters form
aggregates on the terraces. On the partially oxidized surface similar clusters form and
cause long-range mass transport to steps. Additionally, nanowires form in [001] direction
on and along the surface oxide stress domains. These nanowires have a complex
composition, exhibit different corrugations and appear sometimes as three-dimensional
needles. Occasionally they flip their direction by 90°, but doing so they partially
decompose. Finally, annealing of the S-O-Cu surface leads to consumption of the surface
oxide stripes indicating loss of oxygen presumably via SO
2
formation. Simultaneously,
linear sulphur chains suspended between the [001] O-Cu-O- chains form in
110


direction. The surprising multitude of processes and products even at low-pressure, low-
temperature conditions in the comparatively simple S-O-Cu system highlights the
difficulty of controlling reactivity and selectivity on such convertible catalyst surfaces.
__________________
*)
Authors, to whom correspondence should be addressed.

2
Introduction:
Copper catalyzes a number of industrially important chemical reactions. It is used for
instance as desulfurization catalyst [1, 2], in the low-temperature stage of the water gas
shift reaction[3, 4] and in (electro)catalytic CO
2
reduction [5, 6]. The catalytic activity of
copper can in part be attributed to its electronic structure [7], but morphology, e.g.
number density and type of under-coordinated sites etc. is an important descriptor as
well[8]. The Sulfur-Cu interaction has been addressed in several previous surface science
studies, both experimentally [9-20] and theoretically [21-25]. Interest in this system
arises not only from the use of Cu as desulfurization catalyst, but also from the S poisoning
of the water gas shift reaction. S induced corrosion of Cu plays an important role in the
deterioration of cultural artefacts and is relevant for nuclear waste disposal, since the
radioactive waste is sometimes confined in copper containers [25]. In the present study
we revisit the interaction of Cu(110) and partially oxidized Cu(110) with Sulfur by
scanning tunneling microscopy (STM). In contrast to the majority of previous
investigations we dose pure S onto Cu(110). We expected this to simplify the surface
reactions in comparison to SO
2
or H
2
S exposure, since disproportionation reactions and
hydroxyl or water formation should be suppressed in this case. Surprisingly, an extremely
complex variety of surface reactions is observed with mobile cluster formation, long-
distance mass transport, and most notably formation of different minority species, which
will be very difficult to identify by spectroscopic methods and yet may be important
reaction intermediates in catalytic processes.
Experiment and Theory:
The experiments have been carried out in a UHV system featuring a preparation chamber
and a separate cryostate chamber housing the STM. The preparation chamber was
equipped with a solid-state electrolysis cell for sulfur dosing [26]. Exposures to both, S
and O
2
, were carried out at room temperature, whereas the STM images were recorded at
77 K. In the following, we give the sulfur dose in atoms per cm
2
. Global sulfur coverages
are of little significance since we observe immediate attack of the S at Cu step edges,
mobile clusters and three-dimensional (3D) growth, i.e. extremely inhomogeneous local
S coverage. Long-range ordered overlayers formed only after large exposures, but the

3
present investigation is focused on the lower coverage range, were the Cu surface is still
well defined and a variety of structures is observed.
The calculations were performed with plane-wave density functional theory (DFT) using the
Vienna ab initio simulation package (VASP) [27, 28]. Potentials within the projector
augmented wave method (PAW) [29] and the generalized gradient approximation (GGA) with
the Perdew-Wang 91 functional were used [30]. For bulk optimization, the lattice parameters
were obtained by minimizing the total energy of the unit cell using a conjugated gradient
algorithm to relax the ions and considering a set of 4×4×4 Monkhorst−Pack k-points to sample
the Brillouin Zone. Cu(110) surfaces were modelled with slabs of five layer thicknesses. During
optimization, the top three layers together with the adsorbed species were allowed to relax, and
a set of 1×2×1 Monkhorst-Pack k-points was used for the (4×4) surface unit cell. A kinetic
energy cutoff of 400 eV was employed for all the calculations.
Results and Discussion:
Three major changes occur at the Cu(110) surface upon S dosing. Bright islands appear
with frizzy edges. This indicates the presence of a mobile species at room temperature,
which tends to form islands. The islands have different sizes, variable shapes and seem to
be mobile even at the measuring temperature of 77 K. From Fig. 1A one can conclude that
they move preferentially along the
110


direction.
Secondly, dark rings with bright centres in trough sites are observed. Comparison with
contours of constant local density of states (LDOS) obtained from a DFT calculation of the
(2×2)-S/Cu(110) system (inset in Fig. 1) identifies these features as atomic S species.
For atomic S, the hollow site (H) is the most stable one [9, 21], but the long-bridge site
(LB) is only slightly higher in energy. In Fig. 1B a grid is locally superimposed with the
gridpoints centered at the Cu surface atoms. Although the individual Cu atoms are not
resolved in Fig. 1B, the STM length scale can be accurately calibrated from the Cu row
distances and cross-checked with the dimensions of the Cu-O stripes of the partially
oxidised surface. This leaves only the lateral positioning of the grid for adjustment. Here,

4
the grid has been aligned in such a way that the majority of the sulphur atoms is located
in the more stable H site. Quite clearly, however, some occupied LB sites are found as well.
Figure 1: A: Clean Cu(110) surface exposed to 3.1210
13
atoms/cm
2
(the actual coverage
presumably being significantly smaller). Two species are observed: dark rings around
bright centers and bright islands with somewhat frizzy edges. (2020 nm²; U
bias
:
-25 mV,
I
t
: 50 pA). B: Same preparation as A, but annealed to 373 K. The grid in the upper part
marks the positions of the Cu atoms on the Cu(110) surface (7.37.3 nm²; U
bias
: 30 mV,
I
t
: 150 pA). The inset shows a DFT simulation of the STM contrast for a (4×4)-S/Cu(110)
adsorption layer with tip-sample distance at 2 Å. C: Large-scale image of the same
preparation as B. (280280 nm²; U
bias
: 500 meV, I
t
: 50 pA). D: Clean Cu(110) surface
exposed to 2.1810
14
atoms/cm
2
(140140 nm²; U
bias
: 50 mV, I
t
: 50 pA).
The third change brought about by S exposure is particularly evident, if the exposure is
increased significantly beyond the one for Figs. 1A to 1C. Fig. 1D shows the surface
morphology after an exposure of 2.18×10
14
atoms/cm
2
. The steps are no longer parallel

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• ...and Heegemann et al.(31) Sulfur coverage (θS) was taken as the ratio of adsorbed S atoms to the number of Ag atoms in the surface plane, and was determined by counting individual S atoms in a given area....

[...]

• ...Specifically, it has been observed that both oxygen and sulfur can strongly accelerate coarsening on many coinage metal surfaces.(26-31) While M3S3 is a strong candidate on the (111) surfaces, MS2 and MO2 are reasonable candidates for (110) and (100) surfaces....

[...]

• ...The sulfur source was an in situ electrochemical evaporator following the design by Wagner,29 which has been characterized in detail by Detry et al.30 and Heegemann et al.31 Sulfur coverage (θS) was taken as the ratio of adsorbed S atoms to the number of Ag atoms in the surface plane, and was determined by counting individual S atoms in a given area....

[...]

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Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special metric'' and a special preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

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• ...Potentials within the projector augmented wave method (PAW) [29] and the generalized gradient approximation (GGA) with the Perdew-Wang 91 functional were used [30]....

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• ...augmented wave method (PAW) [29] and the generalized gradient approximation (GGA) with the Perdew-Wang 91 functional were used [30]....

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Frequently Asked Questions (1)
Q1. What are the contributions in "Complex reactions on a convertible catalyst surface: a study of the s-o-cu system" ?

In this paper, the authors revisited the interaction of pure S onto Cu ( 110 ) by scanning tunneling microscopy ( STM ) and found an extremely complex variety of surface reactions with mobile cluster formation, long distance mass transport, and most notably formation of different minority species.