On the propagation rate of the chemical waves observed during the course of CO oxidation on a Ag/Pt(110) composite surface

Article abstract of DOI:10.1021/jp0495491

We have analyzed the propagation rate of the chemical waves observed during the course of CO oxidation on a Ag/Pt(110) composite surface that were reported in our previous papers [Surf. Interface Anal. 2001, 32, 179; J. Phys. Chem. B 2002, 106, 5645]. In all cases, the propagation rate v can be adequately fitted as ?? = ??₀ + D₀/d, in which ??₀ and D₀ are constants, and d is the distance between the reaction front of the chemical wave and the boundary from which the chemical wave originates. We propose that the surface species responsible for the formation of the chemical wave comes from two paths: the adsorption of molecules in the gas phase on the surface and the migration from the adjacent surface with different catalytic activity. v₀ corresponds to the contribution from the surface species due to the adsorption, and D₀/d to that of the surface species that migrates from the adjacent surface. The rate equation clearly suggests that the observed chemical wave results from the coupling between adjacent surfaces with different catalytic activities during the course of heterogeneous catalysis. These results, together with our previous reports, provide a good fundamental understanding of spillover, an important phenomenon in heterogeneous catalysis.

Full text of DOI:10.1021/jp0495491

8390
J. Phys. Chem. B 2004, 108, 8390-8396
On the Propagation Rate of the Chemical Waves Observed during the Course of CO
Oxidation on a Ag/Pt(110) Composite Surface
Weixin Huang*,† and Xinhe Bao
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Dalian 116023, P.R. China
ReceiVed: February 1, 2004; In Final Form: April 14, 2004
We have analyzed the propagation rate of the chemical waves observed during the course of CO oxidation
on a Ag/Pt(110) composite surface that were reported in our previous papers [Surf. Interface Anal. 2001, 32,
179; J. Phys. Chem. B 2002, 106, 5645]. In all cases, the propagation rate ν can be adequately fitted as ν )
ν0 + D0/d, in which ν0 and D0 are constants, and d is the distance between the reaction front of the chemical
wave and the boundary from which the chemical wave originates. We propose that the surface species
responsible for the formation of the chemical wave comes from two paths: the adsorption of molecules in
the gas phase on the surface and the migration from the adjacent surface with different catalytic activity. ν0
corresponds to the contribution from the surface species due to the adsorption, and D0/d to that of the surface
species that migrates from the adjacent surface. The rate equation clearly suggests that the observed chemical
wave results from the coupling between adjacent surfaces with different catalytic activities during the course
of heterogeneous catalysis. These results, together with our previous reports, provide a good fundamental
understanding of spillover, an important phenomenon in heterogeneous catalysis.
1. Introduction
the course of the oscillatory CO oxidation on a Pt(100) surface,
the observed chemical wave propagates with a rate of 0.5-3
mm/min.16,17 However, the chemical wave propagates with a
rate of ∼60 mm/min during the course of the same oscillatory
reaction on the surface of a Pt polycrystal under atmospheric
pressure.18 To our knowledge, no detailed results have been
reported for the propagation rate of chemical waves resulting
from reaction coupling between adjacent surfaces.
The progress in surface imaging techniques has revealed the
formation and propagation of various sustainable chemical
waves during the course of the oscillatory heterogeneous
catalytic reactions on metal surfaces, such as CO oxidation,
hydrogen oxidation, and NO reduction on Pt and Rh single-
crystal surfaces.1 And the formation mechanisms of these
chemical waves are well understood.2,3
In our recent attempt to fundamentally understand spillover,
we fabricated a Ag/Pt(110) composite surface and employed
photoemission electron microscopy (PEEM) to study the kinetic
process of CO oxidation on such a composite surface.14-15,19
We observed the formation of chemical waves, which, depend-
ing on the reaction conditions, are attributed to the coupling
between the adjacent Pt surface and Ag surface through the
diffusion of adsorbed CO or oxygen atoms from strongly bound
surface sites (Pt) to weakly bound surface sites (Ag).14,15 In the
present paper, we made an analysis of the propagation rates of
the observed chemical waves, whose results not only strongly
support our previous conclusions, but also imply some general
features of the chemical waves resulting from the coupling
between adjacent surfaces.
Chemical waves have also been imaged during the course of
heterogeneous catalytic reactions with stable kinetics on the
metal surface consisting of multicomponents, whose individual
parts behave with different catalytic activities toward hetero-
geneous catalytic reactions. This type of chemical wave appears
to originate from the coupling between adjacent surfaces with
different catalytic activities. Due to their different activities, the
concentration gradient of the surface species is formed between
adjacent components, which can drive the diffusion of the
surface species on the surface. The diffusion of the surface
species is locally coupled with surface reaction, thus forming
the chemical wave on the surface. To some extent, this type of
chemical wave provides a fundamental understanding of spill-
over that is defined as the diffusion of surface species between
adjacent components of a catalyst, an important phenomenon
observed during the course of heterogeneous catalysis reactions.4
However, this type of chemical wave has not receive much
attention. The first example was reported by Ertl and co-
workers.5 Other reported systems include CO oxidation, hy-
drogen oxidation, and NO reduction on the polycrystal of Pt
and Pd,6-8 the Cu-modified Pt(111) surface,9 the microdesigned
Pt(110) single-crystal surface with active Pd, Rh boundaries,10-13
and the Ag/Pt(110) composite surface.14-15
2. Experimental Section
The experiments were performed in a stainless steel ultrahigh
vacuum (UHV) chamber equipped with photoemission electron
microscopy (PEEM), low-energy electron diffraction (LEED),
auger electron spectroscopy (AES), and mass spectroscopy.
Results reported here were acquired by PEEM. PEEM forms
an enlarged image of a sample surface irradiated by ultraviolet
(UV) light, whose intensity distribution is directly correlated
with the local work function of the sample surface.20 The
brighter area in a PEEM image generally possesses the lower
local work function. PEEM allows the real-time, in situ
observation of the kinetic processes on the mesoscopic surface
The propagation rates of chemical waves on solid surfaces
seem to be specific with the individual reaction system. During
* Address correspondence to this author.
† Current address: Fritz-Haber-Institut der Max-Planck-Gesellschaft,
Berlin 14195, Germany. E-mail: huang@fhi-berlin.mpg.de.
10.1021/jp0495491 CCC: $27.50 © 2004 American Chemical Society
Published on Web 05/25/2004

CO Oxidation on a Ag/Pt(110) Composite Surface
J. Phys. Chem. B, Vol. 108, No. 24, 2004 8391
Figure 1. The CO + O(a) reaction with PCO ) 5 × 10-7 mbar on the Ag/Pt(110) surface at 480 K: (a) PEEM images at different reaction times;
(b) temporal evolution of the brightness profile of the line (indicated in part a) with the same time series as in part a; and (c) positions and
propagation rates of the reaction front on Ag at different reaction times.
along with local work function changes, such as adsorption/
desorption, reaction, and diffusion.
diffusion of CO(a) from the Pt surface to the Ag surface, and
is formed by the local coupling of CO(a) diffusion with the
CO(a) + O(a) reaction.
The Ag/Pt(110) composite surface was prepared by direct
deposition of high-purity silver onto a clean Pt(110) substrate
at room temperature. The cleanliness of Pt(110) was confirmed
by AES and LEED. A mask was placed in front of Pt(110)
during deposition so that only part of the Pt(110) surface was
covered by silver. The freshly prepared Ag/Pt(110) sample was
annealed at 500 K for several hours to acquire a stable surface
structure under the reaction conditions (T ) 480 K). After
annealing, the surface consists of the Pt(110) surface, a AgPt
interface, and the Ag surface. The characterization of the
composite surface was discussed in detail elsewhere.19
Figure 1b presents the temporal evolution of the brightness
profile along the line indicated in Figure 1a. The CO + O(a)
reaction proceeds spatially homogeneously on the Pt(110)
surface while a chemical wave propagates on the AgPt interface
and the Ag surface. The chemical wave seems to initiate from
the boundary between adjacent surfaces. We consider the local
maximum of the brightness profile on the Ag surface to be the
reaction front of the chemical wave, which is covered by the
lowest coverage of CO(a) and O(a), and has the lowest local
work function, thus is brightest in the PEEM image.
On the basis of Figure 1b, we measured the positions of the
reaction front of the chemical wave on the Ag surface with
respect to the boundary between the AgPt interface and the Ag
surface at different reaction times, from which the propagation
rate of the chemical wave was derived (Figure 1c). The chemical
wave propagates with a rate around the order of 1 µm‚s-1, which
is far slower than the previous reported results.16-18 We attribute
the difference to the distinctly different experimental conditions
employed. The most interesting finding is that the propagation
rate of the chemical wave decreases with the distance (d)
increasing between its reaction front and the boundary. Since
the chemical wave is formed by the local coupling between CO-
(a) diffusion and the CO(a) + O(a) reaction, the propagation
rate of the chemical wave represents the reaction-coupled
diffusion rate of CO(a) on the Ag surface.
3. Results and Analysis
Two kinetic processes have been investigated on the Ag/Pt-
(110) composite surface at 480 K. One is CO adsorption on
the O(a)-covered surface, denoted as the CO + O(a) reaction;
the other is oxygen adsorption on the CO(a)-covered surface,
denoted as the CO(a) + O2 reaction. In both cases the chemical
waves were observed under certain reaction conditions.
3.1. CO + O(a) Reaction. The surface was first saturated
-4
by oxygen at PO ) 4.0 × 10
mbar, then exposed to CO
2
with different pressures.
3.1.1. PCO ) 5.0 × 10-7 mbar. When the O(a)-covered
surface was exposed to CO at a pressure of 5.0 × 10-7 mbar,
the formation of a chemical wave was imaged by PEEM, as
shown in Figure 1a (areas a, b, and c respectively corresponding
to Pt surface, AgPt interface, and Ag surface). As discussed in
our previous paper,15 the chemical wave originates from the
coupling between adjacent Pt and Ag surfaces through the
3.1.2. PCO ) 2.0 × 10-6 mbar. The formation of a chemical
wave was also observed when the CO pressure increased to
2.0 × 10-6 mbar (Figure 2a). By integrating the brightness

8392 J. Phys. Chem. B, Vol. 108, No. 24, 2004
Huang and Bao
Figure 2. The CO + O(a) reaction with PCO ) 2 × 10-6 mbar on the Ag/Pt(110) surface at 480 K: (a) PEEM images at different reaction times;
(b) temporal evolution of the brightness profile of the line (indicated in part a) with the same time series as shown in part a; and (c) positions and
propagation rates of the reaction front on Ag at different reaction times.
profiles of the line shown in Figure 2a and measuring the
positions of the reaction front at different reaction times, the
propagation rate of the chemical wave was again calculated on
the Ag surface. The results are presented in Figure 2, parts b
and c. Similar to the case of PCO ) 5.0 × 10-7 mbar, the
propagation rate decreases with the increasing distance (d)
between the reaction front and the boundary. But the chemical
wave propagates faster when the CO pressure increases from
5.0 × 10-7 to 2.0 × 10-6 mbar.
3.1.3. Analysis of the Propagation Rate. As shown above,
the propagation rate of the observed chemical wave, i.e., the
diffusion rate of CO(a), decreases with the increasing distance
(d) between the reaction front and the boundary between the
AgPt interface and the Ag surface. As an approximate analysis,
we plotted the curves of the propagation rate (in µm‚s-1) versus
the reciprocal of the distance d (in µm) (Figure 3). We found
that these data could be adequately linearly fitted as follows
PCO ) 5.0 × 10-7 mbar: ν ) 0.031 + 0.043/d
PCO ) 2.0 × 10-6 mbar: ν ) 0.049 + 0.14/d
These results clearly demonstrate that two different types of
CO(a) contribute to the diffusion process, i.e., the formation of
the chemical wave, which will be discussed in detail later.
Figure 3. Analysis of the propagation rates of the reaction fronts on
Ag during the course of CO + O(a) reaction.
3.2. CO(a) + O2 Reaction. The surface was first saturated
by CO at PCO ) 1.0 × 10-4 mbar, then exposed to oxygen
formation of the chemical wave to the coupling between the
adjacent Pt and Ag surfaces through the diffusion of O(a) from
Pt surface to Ag surface.14
We calculated the brightness profiles of the line indicated in
Figure 7 at different reaction times (Figure 4b). Similar to the
with different pressures.
3.2.1. PO ) 4.0 × 10-5 mbar. The chemical wave was
2
observed to form under this reaction condition. The correspond-
ing PEEM images are presented in Figure 4a. We attribute the

CO Oxidation on a Ag/Pt(110) Composite Surface
J. Phys. Chem. B, Vol. 108, No. 24, 2004 8393
Figure 4. The CO(a) + O2 reaction with PO2 ) 4 × 10-5 mbar on the Ag/Pt(110) surface at 480 K: (a) PEEM images at different reaction times;
(b) temporal evolution of the brightness profile of the line (indicated in part a) with the same time series as shown in part a; and (c) positions and
propagation rates of the reaction front on Ag at different reaction times.
CO + O(a) reaction, the CO(a) + O2 reaction proceeds spatially
homogeneously on the Pt(110) surface; however, a chemical
wave propagates on the AgPt interface and the Ag surface. By
measuring the positions of the reaction front of the chemical
wave on the Ag surface with respect to the boundary between
the AgPt interface and the Ag surface at different reaction times,
we calculated the propagation rate of the chemical wave on the
Ag surface, which was also found to decrease with increasing
distance (d) (Figure 4c). For the CO(a) + O2 reaction, the
propagation rate of the chemical wave actually reflects the
reaction-coupled diffusion rate of O(a).
that two kinds of O(a) contribute to the formation of the
chemical wave.
4. Discussion
4.1. Origin of the Chemical Waves. The observed chemical
waves apparently originate from the coupling between adjacent
Pt(110), AgPt interface, and Ag surface due to their different
activities. Figure 7 presents the brightness profiles of the
individual surface during the course of reactions, directly
revealing their different activities. For both the CO + O(a)
reaction and the CO(a) + O2 reaction, the Pt(110) surface is
most active, followed by the AgPt interface, and the Ag surface
is the least active. The similar shape between the brightness
profiles of the Pt(110) surface and AgPt interface implies that
Pt is the active site for the reactions on the AgPt interface,
although its activity is modified by the coexistence of Ag. This
is in consistence with our previous results.19 Increasing the
pressure of the reactants obviously accelerates the reaction rates
on all surfaces. On basis of the brightness of the starting
surfaces, it can been seen that the CO(a)-covered Pt(110) and
AgPt surfaces are brighter than the corresponding O(a)-covered
surfaces, showing that the increase of local work function caused
by CO adsorption is smaller than that caused by oxygen
adsorption. However, the increase of the work function induced
by CO adsorption seems to be slightly larger than that induced
by oxygen adsorption on the Ag surface.
3.2.2. PO ) 4.0 × 10-6 mbar. Decreasing oxygen pressure
2
to 4.0 × 10-6 mbar also gives rise to the formation of the
chemical wave (Figure 5a). We made a similar analysis of the
PEEM images and calculated the propagation rate of the
chemical wave on the Ag surface (Figure 5, parts b and c).
Although the propagation rate is about one order slower than
that for PO ) 4.0 × 10-5 mbar, it gives the same trend that the
2
propagation rate decreases with the increasing distance (d)
between the reaction front and the boundary.
3.2.3. Analysis of the Propagation Rate. Similar to the case
of the CO + O(a) reaction, we plotted the curves of the
propagation rate (in µm‚s-1) versus the reciprocal of the distance
d (in µm) (Figure 6) for the observed chemical waves. Again,
the data can be adequately linearly fitted as follows
PO ) 4.0 × 10-6 mbar: ν ) 0.00063 + 0.019/d
2
The different activities result in the large coverage difference
of surface species among the Pt(110) surface, AgPt interface,
and Ag surface during the course of surface reactions. For
example, the Pt(110) surface and Ag-Pt interface are saturated
with O(a) between 20 and 160 s while CO(a) + O2 reaction
PO ) 4.0 × 10-5 mbar: ν ) 0.0026 + 0.054/d
2
Consistent with previous results, these rate equations indicate

8394 J. Phys. Chem. B, Vol. 108, No. 24, 2004
Huang and Bao
Figure 5. The CO(a) + O2 reaction with PO2 ) 4 × 10-6 mbar on the Ag/Pt(110) surface at 480 K: (a) PEEM images at different reaction times;
(b) temporal evolution of the brightness profile of the line (indicated in part a) with the same time series as shown in part a; and (c) positions and
propagation rates of the reaction front on Ag at different reaction times.
4.2. Diffusion Species. As shown in Figure 7, the coverage
gradients of CO(a) and O(a) respectively form between the Pt
surface and the Ag surface during the course of the CO + O(a)
reaction and the CO(a) + O2 reaction. Therefore, we conclude
that the surface species responsible for the formation of chemical
waves are CO(a) and O(a) during the course of the CO + O(a)
reaction and the CO(a) + O2 reaction, respectively. This is
consistent with the calculated propagation rates of these chemical
waves. The chemical waves during the course of the CO + O(a)
reaction propagate systematically one or two orders faster than
those during the course of the CO(a) + O2 reaction, showing
that CO(a) is much more mobile than O(a) on metal surfaces.
The coupling between adjacent surfaces involves the migra-
tion of the surface species from the strongly adsorbed Pt surface
to the relatively weakly adsorbed Ag surface, which is an
enthalpy unfavorable process. Migration of surface species
generally is due to the chemical potential gradient between
different surface sites, to which both enthalpy (bond strength)
and concentration gradient of the migrating species contribute.
Our results imply that the coverage difference of the involved
surface species, created by the different catalytic activities of
the involved surfaces, is large enough to drive the surface species
to migrate from the strongly adsorbed Pt surface to the weakly
adsorbed Ag surface. On the other hand, the activation energy
for surface diffusion of adsorbed species over noble metal
Figure 6. Analysis of the propagation rates of the reaction fronts on
Ag during the course of the CO(a) + O2 reaction.
surfaces (e.g. Pd and Pt) is low, as compared with their heats
of desorption.21
4.3. The Model for the Formation of the Chemical Waves.
On the basis of the results, we propose a model for the formation
of the chemical waves, as illustrated in Figure 8. To be simple,
we assume that CO oxidation would not occur on the O(a)- or
dominates on the Ag surface, under which condition nearly no
O(a) stays on the Ag surface (Figure 7b, PO ) 4.0 × 10-5
2
mbar). The large coverage difference can result in the coupling
between adjacent surfaces, which initiates the chemical wave.

CO Oxidation on a Ag/Pt(110) Composite Surface
J. Phys. Chem. B, Vol. 108, No. 24, 2004 8395
Figure 7. Brightness profiles of different areas during the course of the CO + O(a) reaction and the CO(a) + O2 reaction on the Ag/Pt(110)
surface at 480 K.
reaction occurs between adjacent CO(a) and O(a), forming the
new reaction front and producing free sites on the Ag surface.
The diffusion/adsorption and surface reaction proceed alterna-
tively on the Ag surface, thus resulting in the chemical waves.
The most important point in our model is that diffusion and
adsorption compete for the filling of the free sites on the surface,
and both contribute to the formation of the chemical wave.
4.4. The Propagation Rate of the Chemical Waves. The
experimental results show that the propagation rate (ν) of the
chemical waves follows:
D0
ν ) ν0 +
d
where ν0 and D0 are constants, and d is the distance between
the reaction front and the boundary from which the chemical
wave initiates. With our model, the propagation rate of the
chemical waves can be rationally understood. We propose that
ν0 corresponds to the propagation rate of the chemical wave,
contributed by the surface species due to the adsorption of the
corresponding molecule in the gas phase on the Ag surface.
Under specific reaction conditions, ν0 is a constant determined
by both the partial pressure of the molecule and the sticking
coefficient of the molecule on the surface. Our results clearly
demonstrate that the corresponding chemical wave propagates
much faster when the pressure of CO or O2 is increased. And
D0/d corresponds to the propagation rate of the chemical wave
to which the surface species migrating from the adjacent Pt
surface to the Ag surface contributes. This item actually
Figure 8. Schematic illustration of the formation of the chemical wave
demonstrates a spillover process, defined as the diffusion of
on Ag during the course of the CO + O(a) reaction on the Ag/Pt(110)
surface species between adjacent components of a catalyst
surface. The dotted lines indicate the position of the reaction fronts at
during the course of heterogeneous catalysis reactions.4 It is
different times.
reasonable that the migration of the surface species depends on
CO(a)-covered Ag surface (this assumption is not correct). But
the surface reaction only affects the coverage difference between
the involved Pt and Ag surfaces, and does not affect our model.
Taking the CO + O(a) reaction as an example (Figure 8), the
chemical wave on the Ag surface forms as follows: (1) When
the O(a)-covered surface is exposed to CO, the Pt surface soon
switches from the O(a)-covered to the CO(a)-covered. But the
Ag surface still keeps O(a)-covered. (2) Coupling occurs
between the Pt surface and Ag surface in such a way that the
CO(a) on the Pt surface reacts with the adjacent O(a) on the
Ag surface on the boundary (the position of the reaction front),
giving rise to the free sites on both the Pt surface and the Ag
surface. (3) The free site on the Pt surface is quickly occupied
by CO(a). However, there are two methods for the filling of
the free site on the Ag surface: adsorption of CO from the gas
phase and the migration of CO(a) from the Pt surface. Then
the distance (d) between both sites involved. D0 accounts for
the coverage gradient of diffusing species between surface sites
involved because of their different activities toward the surface
reaction.
On the basis of the rate equation, the propagation rate (ν) of
a chemical wave is mainly contributed by D0/d within a very
small distance (d), which implies that the chemical waves on
metal surfaces are always triggered by the coupling between
the adjacent surfaces with different activities, consistent with
experimental observations that chemical waves on single crystals
originate from the defective sites.22 On the other hand, within
which range of d the surface migration between adjacent
surfaces is of considerable effect on the propagation rate of a
chemical wave after its formation is determined by the relative
values of ν0 and D0. In our case, because of the low sticking
coefficient of CO and oxygen on Ag, the migration process of

8396 J. Phys. Chem. B, Vol. 108, No. 24, 2004
Huang and Bao
the surface species is comparable with the adsorption process,
as reflected by the comparable values of ν0 and D0, therefore
the contribution from the surface migration, i.e., the coupling
between the Pt surface and Ag surface, can be reflected on the
chemical waves within several microns beyond the boundary
and observed by PEEM. The case is different for the chemical
waves observed on the much more active platinum surfaces.
Previous results have shown that the chemical wave propagates
at a constant rate during the course of the oscillatory CO
oxidation on the Pt surface, as revealed by PEEM.5,16-18 These
results, however, do not contradict with our model. With quite
high sticking coefficients of CO and oxygen on the platinum
surface, the surface species due to adsorption may dominate
the formation of the chemical wave beyond a very short distance
away from the boundary where the chemical wave initiates. If
the distance is beyond the spatial resolution of PEEM, then the
coupling effect cannot be observed by PEEM and the chemical
wave seems to propagate at a constant rate (ν ) ν0). For
example, the reaction front of O(a) (ν0) was reported to
propagate over several microns per second on the Pt(111)
surface,9 thus the coupling is only of considerable effect on the
propagation rate within several tens nanometer if we assume
D0 to be around 10-2 µm2‚s (taken from the present study).
Actually, Gorodetskii et al.’s results7 directly support our
viewpoint. They successfully employed a field ion microscope
(FIM) to observe the coupling between adjacent Pt(331) and
Pt(100) crystal planes on a platinum tip, i.e., the formation of
a chemical wave on Pt(100), through O(a) diffusion from Pt-
(331) to Pt(100) within the distance of several nanometers away
from the boundary during the course of NO reduction.7
Finally we would like to point out that the coupling between
adjacent surfaces is initiated by the concentration difference of
the surface species, therefore it would disappear when the
concentration difference between the involved surfaces could
not sustain the diffusion process with the proceeding of the
surface reaction.
responsible for the formation of the chemical wave comes from
two different methods. One is the adsorption of the molecules
in the gas phase on the surface; the other is the migration from
the adjacent surface. The latter actually reflects a spillover
process. Our work provides a rational surface science approach
to spillover, an important phenomenon in heterogeneous ca-
talysis.
Acknowledgment. The experiments were carried out in
Fritz-Haber-Institut der Max-Planck-Gesellschaft. Prof. G. Ertl
and Prof. H. H. Rotermund are gratefully acknowledged for their
support. The work was supported by the National Natural
Science Foundation of China through project 29525305.
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In summary, we have analyzed the propagation rate of the
chemical waves formed on the Ag surface during the course of
CO oxidation on a Ag/Pt(110) composite surface. The chemical
waves are triggered by the coupling between the adjacent Pt
surface and the Ag surface because of their different catalytic
activities. The propagation rate is found to be dependent on the
distance of the reaction front away from where the chemical
wave initiates. A model is proposed to account for the formation
of the chemical wave triggered by the coupling between adjacent
surfaces with different catalytic activities toward the surface
reaction. The most important point is that the surface species
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