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Spatiotemporal pattern formation in catalytic reactions on single crystal surfaces
Physica A 188 (1992) 34-46 North-Holland
PHYSICA/1
Spatiotemporal pattern formation in catalytic reactions on single crystal surfaces R. I m b i h l Fritz-Haber-lnstitut der Max-Planck-Gesellscha[t, Faradayweg 4-6, W-IO00 Berlin 33, Germany
Two oscillatory surface reactions, namely catalytic CO oxidation and the catalytic reduction of NO with various agents (CO, H,, NH3), have been studied on Pt single crystal surfaces at low pressure (p < 10 ~Tort) using spatially resolved techniques. Both Turing structures as well as chemical wave patterns with varying degrees of complexity were observed. The facetting of Pt(ll()) in catalytic CO oxidation is discussed as an example for the experimental realization of a Turing structure. A detailed microscopic picture is available for the formation of the facet pattern as demonstrated by a Monte Carlo simulation. The catalytic reduction of NO with CO and NH~ on Pt(100) serves as an example for the transition from ordered to turbulent spatiotemporal patterns. The spatial coherence in these patterns is determined by the competition between long-range synchronization via gas-phase coupling and short-range coupling via the surface diffusion of a mobile adsorbate.
1. Introduction
The study of oscillatory reactions on single crystal surfaces, which was started about a decade ago, has led to the discovery of a variety of new phenomena in the field of nonlinear dynamics and provided detailed insights i n t o t h e m i c r o s c o p i c m e c h a n i s m of t h e o s c i l l a t i o n s # l [ 2 , 3 ] . B a s e d o n t h e s e r e s u l t s , m a t h e m a t i c a l m o d e l s c o u l d b e f o r m u l a t e d w h o s e e s s e n t i a l steps w e r e b a s e d o n e x p e r i m e n t a l facts. T h e a r e a w h e r e t h e u s e of single s u r f a c e s t u r n e d o u t to b e m o s t f r u i t f u l p r o v e d to b e t h e s t u d y o f t h e v a r i o u s a s p e c t s of s p a t i a l self-organization. Here new phenomena appeared which have no counterpart in fluid p h a s e c h e m i s t r y . B o t h T u r i n g s t r u c t u r e s as well as c h e m i c a l w a v e p a t t e r n s w i t h v a r y i n g d e g r e e s o f c o m p l e x i t y h a v e b e e n f o u n d in s u r f a c e r e a c t i o n s [ 3 - 9 ] . T h e r e g u l a r facet pattern which develops during catalytic CO oxidation on a Pt(ll0) s u r f a c e has b e e n i d e n t i f i e d as a T u r i n g s t r u c t u r e [6]. T h i s t y p e o f d i s s i p a t i v e s t r u c t u r e has for a l o n g t i m e o n l y b e e n d i s c u s s e d t h e o r e t i c a l l y a n d t h e f a c e t t e d P t ( l l 0 ) s u r f a c e r e p r e s e n t s t h e first e x a m p l e w h e r e a c l e a r - c u t e x p e r i m e n t a l #~ For a review of single crystal studies of oscillatory surface reactions see ref. [1]. 0378-4371/92/$05.00 © 1992- Elsevier Science Publishers B.V. All rights reserved
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
35
realization of a Turing structure could l~e achieved #2. The formation of the facet pattern, whose length scale is microscopic (100-200 ,~), can be described by a detailed microscopic mechanism as will be demonstrated in a Monte Carlo simulation [7]. In addition to catalytic CO oxidation, another oscillatory reaction system that has been thoroughly investigated in the past years is catalytic NO reduction with CO, H 2 or N H 3 on a Pt(100) surface [8, 9, 11-15]. Similar to P t ( l l 0 ) / C O + 0 2 these reaction systems exhibit a variety of spatiotemporal patterns, which could be investigated with the newly developed photoemission electron microscope ( P E E M ) [4, 5, 8, 9]. Of particular interest is that these patterns exhibit well-defined transitions from ordered to turbulent behavior and therefore offer the possibility to gain insights into the mechanism leading to spatiotemporal chaos.
2. Turing structures in Pt(ll0)/CO 2.1.
+ 0 2
K i n e t i c oscillations
It has been well established that catalytic CO oxidation proceeds via the L a n g m u i r - H i n s h e l w o o d ( L H ) mechanism, incorporating the following steps: CO -~- :~ ~-- GOad ,
0 2 + 2 " ~ 2Oad ,
GOad ~- Oad ~ 2 * + C O 2
(* denotes a vacant adsorption site). Kinetic oscillations in this reaction can arise if the metal substrate on which the reaction takes place exhibits an adsorbate-driven surface phase transition (SPT). This is the case with Pt(100) and Pt(110) surfaces, both of which display a reconstruction in their clean state that can be reversibly lifted upon C O adsorption [1, 2]. The principle of the mechanism shall be illustrated with the SPT of Pt(110) which is depicted in a ball model in fig. 1. The left-hand side of fig. 1 displays the bulk-like 1 x 1 structure of the Pt(110) surface, which, however, is not stable in its clean state but undergoes reconstruction into the 1 x 2 "missing row" structure shown in the right side. The name "missing row" structure has its origin in the building principle that every second of the close-packed [110J-rows has to be removed in order to create the 1 x 2 structure displayed in fig. 1. This transition can be reversed by adsorbing CO, which lifts the 1 × 2 reconstruction and causes the surface atoms #'2In fluid-phase chemistry Turing structures were found as described in ref. [10].
36
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
Pt 1110) CO covered
lxl SO2 ==0.5-0.6
cleon
=
=
lx2 SO2 ==0.3-0£
Fig. 1. Ball model illustrating the formation of a step with (100) orientation in the CO-induced 1 x 1 ~ 1 x 2 phase transition of a Pt(ll0) surface. In the ball model the 1 x 2 "missing row" structure shown on the right has been created by removing every second row in the front half of the 1 x 1 surface (left) and then placing the additional balls on top of the rear half of the model surface. to m o v e b a c k into t h e i r 1 x 1 positions. T h u s an a d s o r b a t e - d r i v e n 1 x 1 ~ 1 × 2 S P T exists which is c o n t r o l l e d b y a critical C O c o v e r a g e OCO,crit ~-0.2. W i t h the 1 x 1 ~ 1 x 2 S P T also the o x y g e n sticking coefficient So2 c h a n g e s a n d since this p r o p e r t y is r a t e - l i m i t i n g at high P c o , kinetic oscillations can arise. A n o s c i l l a t o r y cycle m a y l o o k like this. Starting with a C O c o v e r e d 1 x 1 s u r f a c e , So2 a n d , h e n c e the r e a c t i o n r a t e , will be high. A c c o r d i n g l y , the high r e a c t i o n r a t e will cause Oco to d e c r e a s e a n d b e l o w •CO,crit the 1 x 2 p h a s e will a p p e a r . B u t on this surface So2 is low a n d c o n s e q u e n t l y C O a d s o r p t i o n will d o m i n a t e until the C O c o v e r a g e is high e n o u g h to r e e s t a b l i s h the initial situation of a CO covered 1 x 1 phase. T h e o s c i l l a t o r y cycle d e p i c t e d a b o v e has b e e n verified b y in-situ L E E D ( = l o w e n e r g y e l e c t r o n diffraction) e x p e r i m e n t s which d e m o n s t r a t e d t h a t t h e o s c i l l a t i o n s in the r e a c t i o n rate are in fact a c c o m p a n i e d by p e r i o d i c s t r u c t u r a l t r a n s f o r m a t i o n s via t h e 1 x 1 ~ - 1 x 2 S P T [1, 2]. B u t b e s i d e s limit cycle b e h a v i o r , t h e 1 x 1 ~---1 x 2 S P T also p r o d u c e s a n o t h e r t y p e of d i s s i p a t i v e s t r u c t u r e which will b e p r e s e n t e d in the following section.
2.2. Facetting
If o n e uses t h e ball m o d e l o f P t ( l l 0 ) d i s p l a y e d in fig. 1 a n d tries to r e a r r a n g e
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
37
the atoms of the 1 x 1 surface such that a 1 x 2 structure is produced, then this can be accomplished by taking out every second of the close-packed [ll0]-rows in the front half and putting them on top of the 1 x 1 layer in the rear half. But as shown in fig. 1, this rearrangement opens up a new terrace layer thereby creating a step with (100) orientation. This simple example demonstrates that the mass transport of Pt atoms that is associated with the 1 × 1 ~---1 x 2 SPT necessarily creates atomic steps and therefore causes a certain roughening of the surface. This may lead to the following consequences. As one exposes a P t ( l l 0 ) surface to catalytic CO oxidation under conditions very similar to those where kinetic oscillations occur (but at lower T) facetting may occur [16, 17]. The facets which are formed are of microscopic dimension ( t y p i c a l l y ~ 100,~) as judged from L E E D where facetting gives rise to a splitting of the integral order beams [16]. The facetting of a surface due to the interaction with a strong adsorbate such as oxygen is nothing unusual but in the present case there are several observations that contradict the possibility of simple adsorbate-driven restructuring of the surface. First the surface facets only under reaction conditions but not if the sample is treated with one adsorbate alone. Furthermore the facets which are formed are thermally unstable and simple heating above ~500-600 K is sufficient to restore the flat surface. This reordering process also slowly removes the facets if the gas flow in a facetting experiment is stopped. Apparently an on-going reaction is necessary to stabilize the facets. Moreover, these only form under reaction conditions where the 1 x 1 ~-1 x 2 SPT can proceed, e.g., if the CO coverage is close t o •CO,crit ~ 0.2. Associated with the facetting of P t ( l l 0 ) is an increase in catalytic activity, which in the experiment shows up as a continuous increase of the reaction rate accompanying the facetting process [16]. All of the above observations suggest that the facetting of P t ( l l 0 ) in catalytic CO oxidation represents a dissipative structure. Further evidence for this assignment was gained in a quantitative characterization of the facetted surface using spot profile analysis with a high resolution L E E D instrument ( S P A L E E D ) [6]. This investigation revealed that the facets on P t ( l l 0 ) form a regular pattern in which facets of uniform orientation and size are arranged in the shape of a symmetric sawtooth as indicated in fig. 2a. This sawtooth-like pattern exhibits a well-defined lateral periodicity of ~ 2 0 0 , ~ in the [ll0]direction corresponding to 74 lattice units. The sides of the sawtooth are formed by facets of (340) and (430) orientation of which a ball model is displayed in fig. 2b. The diagram depicts the structural elements consisting of (110) terrace units and (100) step units that build up the facet. By arranging these two structural elements in a regular sequence all orientations of the [001]-zone can be constructed. These orientations are formed exclusively in the facetting of P t ( l l 0 ) . The steepness of the
38
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces @ [110]
I
,
I
I
I-
. [1io]
x = 2oo~I
c( = 8.1 ° ~rms = 3.0 o t o m i c
®
Ioyers
l [1To] i
[ooq
Fig.'2. Model of the facetted Pt(110) surface. (a) Geometrical model showing the sawtooth-like arrangement of the facets as derived from LEED measurements. Also indicated in the diagram is the lateral periodicity of 200 A in the [ll0]-direction. The Pt(110) surface was facetted by exposing the surface at T = 480 K to catalytic CO oxidation in the 10 ~mbar range for ~40 min (after ref. [6]). (b) Ball model of the (430) facet illustrating the structural elements of which the facet consists. facets d e p e n d s o n the r e a c t i o n c o n d i t i o n s with (210) r e p r e s e n t i n g the limiting case [16]. O n this o r i e n t a t i o n , (100) step a n d (110) terrace units a l t e r n a t e . T h e i n c r e a s e in catalytic activity which a c c o m p a n i e s facetting is caused by the p r e s e n c e of (100) steps since these sites exhibit a higher oxygen a d s o r p t i o n rate t h a n a flat (110) terrace. T h e increase of So2 at step sites can be a t t r i b u t e d to a local l o w e r i n g of the work f u n c t i o n , which facilitates the e l e c t r o n t r a n s f e r f r o m the m e t a l to a n i n c o m i n g O2-molecule.
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
39
The findings that the facets form a periodic pattern and that they only develop under reaction conditions suggest their interpretation as a dissipative structure of the Turing type. The facet pattern is, of course, somewhat different from the original idea of a Turing structure, since what is modulated here is not a chemical concentration, but the surface structure. But this point is considered to be of minor importance. As will be shown below, facetting develops because the homogeneous state of the flat surface is unstable with respect to a perturbation by diffusing Pt atoms. The origin of facetting is therefore an instability against diffusion and, accordingly, the facetted surface can be classified as a Turing structure. 2.3. S i m u l a t i o n o f f a c e t t i n g
As has been demonstrated with the ball model displayed in fig. 1, the 1 x 1~---1 x 2 SPT can be considered to represent the elementary step of facetting since this mechanism produces steps whose accumulation may lead to the development of facets. Following this idea a simulation based on a Monte Carlo (MC) algorithm was conducted in which we tried to reproduce the formation of facets during catalytic CO oxidation [7]. We proceed in the same way as one would envision it with the ball model of fig. 1. We let the surface reaction take place on a 200 x 5 substrate lattice. We interrupt the reaction periodically in order to rearrange the substrate structure according to the local distribution of adsorbed CO. In the SPT, which proceeds locally, steps can either be created or annihilated following the rule of mass conservation. The simulation encompasses the elementary steps of the L H mechanism, the properties of the CO-induced 1 x 1 ~-1 × 2 SPT, which is controlled by a critical CO coverage OCO,cr~t = 0.2, the enhancement of oxygen adsorption at step sites, and thermally activated surface diffusion of Pt atoms. We assigned probabilities to the individual steps which were derived from experimentally determined constants. If the model surface is exposed to a constant flow of CO and O 2 comparable to the experimental condition, the initially flat surface roughens as demonstrated by the surface profiles displayed in fig. 3. The roughening occurs as a consequence of local CO coverage fluctuations, which cause the mass transport of Pt atoms each time the surface is forced to undergo the SPT. As the reaction progresses facets develop with a sawtooth-like pattern similar to the one derived from experiment (see fig. 2). The pattern remains practically stationary above 4000 MC cycles. In order to demonstrate that the facets are in fact only stabilized by the on-going reaction, the flow of the gases was stopped in the simulation after 4000 cycles. Thermal reordering leads then to the restoration of the flat surface as demonstrated by the profile recorded after 6000 cycles.
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
40
[110]
T
Number of Cycles
[~To] -
-
6000
~
~ooo
3000 ='
15oo 3oo II
II
1
50
II
lOO Row Number
U
150
II
2 OO
Fig. 3. Monte Carlo simulation showing the development of a regular facet pattern as a P t ( l l 0 ) surface is exposed to catalytic CO oxidation. After stopping the gas flow following 4000 cycles, the flat surface is restored in a thermal annealing process. Shown in the diagram are profiles of the surface in the [ll0)-direction (after ref. [7]).
The simulation described above could reproduce practically all essential features of the experiment. The lateral periodicity of 40 lattice units in the [ll0]-direction is not very far from the experimental value of --70 lattice units. Even the time scale is comparable since the 4000 cycles in the simulation correspond to - 4 0 0 s in real time, which is roughly the period during which facetting develops in the experiment. In addition the simulation could also correctly reproduce the region in Pco parameter space where facetting occurs as well as the increase in catalytic activity. The simulation thus confirms the interpretation of the facetted P t ( l l 0 ) surface as a Turing structure, and it provides a detailed microscopic picture of how the interplay between the LH reaction and the phase transition can give rise to a facetting process.
3. From ordered behavior to chemical turbulence: the catalytic reduction of
NO on Pt(100)
3.1. Short-range versus long-range coupling Unlike oscillatory reactions in a fluid phase or in heterogeneous catalysis at high pressure (p - 1 atm), spatial coupling is conceptually very simple in the oscillation studies which were performed under low pressure ( p < 10 3 Torr) on single crystal surfaces [18]. The reaction is then practically isothermal and
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
41
since the U H V chamber is operated as a gradient free flow reactor, transport limitations play no role. Spatial coupling of the different local oscillators under these conditions is provided by two basic mechanisms, surface diffusion via a mobile adsorbate and a coupling via partial pressure changes in the gas phase. The latter simply arises due to mass balance in the reaction since the oscillations in the product rate have to be accompanied by corresponding variations in the educt partial pressures. Typically amplitudes on the order of 1% of the educt partial pressures are observed. But while surface diffusion is short range in nature, the partial pressure changes in the gas phase affect all parts of the surface in the same way and practically without any measurable delay. The two coupling modes compete in their influence as one can see by constructing two limiting cases. If gas phase coupling were to dominate, perfect long-range synchronization could be achieved and a homogeneously oscillating surface would result. Quite in contrast, with surface diffusion being the only coupling mode, no long-range synchronization will exist. In this case, the surface will still be oscillating locally but over macroscopic distances spatiotemporal pattern which are irregular or turbulent will prevail. Realization of the two extreme cases sketched above and transitions between them have been observed in the catalytic reduction of NO on a Pt(100) surface. These examples will be presented in the following section. 3.2.
O r d e r e d a n d t u r b u l e n t spatial p a t t e r n s
The catalytic reduction of NO on a Pt(100) surface has been investigated with CO, H 2 and N H 3 as reducing agent [11-15]. All three systems exhibit similar dynamical behavior whose origin presumably lies in the requirement of vacant sites for the rate-limiting step of NO dissociation: NO + *
-->
Nad
+ Oad .
Since more vacant sites are liberated in the subsequent product forming steps than are consumed by the above step, autocatalytic behavior results. The autocatalysis manifests itself in the occurrence of extremely narrow product peaks if a coadsorbed layer of the reactants NO + X (X = CO, H 2, NH3) is heated up in temperature programmed reaction experiments. This effect is also referred to as a "surface explosion". The same autocatalytic mechanism presumably is also the driving force for the kinetic oscillations which are found under similar conditions in all three systems. The adsorbate-induced 1 x 1 ~ - h e x phase transition which exists on Pt(100) due to the N O / C O - i n d u c e d lifting of the quasihexagonal ( " h e x " )
42
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
reconstruction on this surface, is considered to be of minor importance for the oscillatory mechanism. This conclusion could be verified experimentally for the N O + C O reaction for which it it was supported by mathematical modeling [11, 14]. For the N O + H 2 reaction and also for the N O + N H 3 reaction which is under consideration here, the picture has to be modified slightly since in these systems the phase transition appears to be essential for the oscillations [9, 12, 13]. The N O + N H 3 reaction yields N~_ and H~O as reaction products according to 3NO + 2 N H 3 ~ 2.5N~ + 3 H 2 0 . Kinetic oscillations in the NO + N H 3 reaction on Pt(100) can be found in the cooling branch of a heating/cooling cycle where they appear in a t e m p e r a t u r e interval of 15 K width during the lifting of the hex reconstruction by N O adsorption [13]. Spatial pattern formation on the catalyst surface during the reaction can be followed by means of the photoemission electron microscope ( P E E M ) , which allows one to image the laterally varying adsorbate concentration with high spatial (~1 I~m) and temporal (~-50ms) resolution. This instrument is based on the principle that the n u m b e r of photoelectrons emitted from a sample illuminated with a suitable photo energy ( 5 - 6 eV) depends on the local work function, which in turn is altered by the presence of an adsorbate [4]. A schematic overview of the spatial patterns which have been observed with P E E M during the cooling period is given in fig. 4 [9]. During the oscillations at high t e m p e r a t u r e the intensity in the imaged area changes homogeneously in phase with the macroscopic rate oscillations. As judged from the P E E M intensity, these oscillations take place on the reconstructed Pt(100) surface with the adsorbate coverage always remaining low. Evidently gas phase coupling is efficient enough here to ensure a homogeneously oscillating surface. As the t e m p e r a t u r e is lowered to ~450 K the influence of N O adsorption Surface structure:
lxl
t
hex
Reaction rate :
I II I I II I I ] I/ItI I I I I Pattern :
irregular spirats & patterns target (turbulence) p a t t e r n s
Dominant coupling mode : [
~30
island for marion
surface diffusion
homogeneous intensity changes gas phase coupling
I
~0
I
[
~50
~60
---*T [K]
Fig. 4. Schematic overview of the temperature ranges in which macroscopic rate oscillations and different types of spatial patterns are observed in the NO + NH3 reaction on Pt(100). (Reaction conditions: PNo = 1.1 x 10 ~ mbar, PNH~= 1.6 X 10 * mbar, pumping rate ~60 (/s) (after ref. [9}).
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
43
Fig. 5. PEEM images (diameter 450 ixm) showing spirals coexisting with target patterns in the NO + NH 3 reaction on Pt(100). The experimental conditions were for (a) Pro = 2.1 x 10-~ mbar, PyH3 = 5.6 x 10 6 mbar, T = 455 K and for (b) PNo = 2.3 x 10 ~mbar, PNH3= 2.8 x 1 0 - 6 mbar and T = 450 K (after ref. [9]).
b e c o m e s stronger and fluctuating adsorbate islands of typically to 10 to 50 ~xm d i a m e t e r a p p e a r on the surface. While these islands continuously nucleate, grow, and then dissolve again, the area s u r r o u n d i n g t h e m is still oscillating h o m o g e n e o u s l y . U p o n further lowering of the t e m p e r a t u r e , the fluctuating islands turn into stable structures which cover most of the imaged area. O n e o b s e r v e s target patterns and rotating spirals (to ~ 1.2 min ~), b o t h of which are d e p i c t e d in the P E E M images displayed in fig. 5. T h e bright area in these areas can be attributed to a N H x (x = 1 - 3) species a d s o r b e d on a 1 x 1 substrate, while the dark area can be assigned to N O , which is partially dissociated into a t o m i c o x y g e n and nitrogen on the 1 x 1 substrate. W i t h these stable structures some long-range synchronization still persists as o n e still observes m a c r o s c o p i c rate oscillations. R e a c t i o n fronts p r o p a g a t e on the area s u r r o u n d i n g the spirals and target patterns, but do not interfere with t h e m . A small t e m p e r a t u r e decrease is now sufficient to cause the m a c r o s c o p i c rate oscillations to disappear. In P E E M the regular patterns turn into irregular patterns, o f which o n e typical e x a m p l e is displayed in fig. 6a. In these patterns the surface is still oscillating locally with a large amplitude as one can d e m o n s t r a t e by integrating the intensity in a small area of ~ 3 0 x 30 txm 2 size and by following its time d e p e n d e n c e . T h e c o r r e s p o n d i n g time series displayed in fig. 6b shows irregular oscillations with a strongly fluctuating amplitude. A p p a r e n t l y the transition f r o m m a c r o s c o p i c rate oscillations to the stationary r e a c t i o n rate is caused by the b r e a k - d o w n of long-range synchronization via gas
44
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
Pt(lO0) / NO+ NH3 T=421K PNH3: PNO : 2 : 1
~6 C
c
4
0
500
1000
1500
t [s] (a)
(b)
Fig. 6. Chemical turbulence in the N O + N H 3 reaction on Pt(100). (a) P E E M image (diameter 450 ixm) showing an irregular spatiotemporal pattern recorded at T = 432 K and with PNo = 1.3 x 10 6 m b a r and PNH3 = 2.1 x 10 -6 mbar (after ref. [9]). (b) Time series demonstrating irregular local oscillations in turbulent spatiotemporal patterns of the type displayed in (a). The time series was obtained by integrating the intensity of the video frames in a small selected area of ~ 3 0 × 30 I~m 2 (after ref. [9]).
phase coupling. Since the different surface parts in the turbulent patterns oscillate unsynchronized, macroscopically a stationary reaction rate results. Experimentally it is difficult to follow the transition from synchronized to turbulent behavior but the present data suggest that this transition takes place discontinuously. The results obtained with the NO + NH 3 reaction can be complemented by the PEEM observations made with the NO + CO reaction on Pt(100) [8]. Here one finds turbulent spatiotemporal patterns which macroscopically correspond to a stationary reaction rate. A small temperature jump of -~1-3 K suffices, however, to excite large amplitude rate oscillations whose amplitude decays in a small number of cycles [11]. The effect of the temperature pulse is to synchronize the oscillations by transforming the turbulent pattern into a target pattern. The decaying amplitude just reflects then the progressive loss in spatial
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
45
coherence as the initially ordered pattern becomes turbulent again [8]. The NO + CO reaction on Pt(100) thus represents an example how macroscopic rate oscillations can be excited by an external synchronization pulse. The filtering properties of the NO + CO reaction on Pt(100) have been investigated further by perturbing the systems randomly with small amplitude temperature fluctuations [15]. The response behavior of the reaction system can be described as a narrow bandpass amplifier which only selects frequencies close to the natural frequency of the unperturbed oscillator.
4. Concluding remarks Examples have been presented for the formation of Turing structures and spatiotemporal patterns in catalytic reactions on single crystal surfaces. The regular facet patterns which form in catalytic CO oxidation on Pt(ll0) with a lateral periodicity of 100 to 1000 A, could be identified as Turing structures. All essential experimental observations could be simulated with a relatively simple model based solely on the experimentally determined properties of the surface reaction and the phase transition. Various forms of spatiotemporal patterns were observed in the catalytic reduction of NO with CO or NH 3 on a Pt(100) surface. The degree of order in these patterns is determined by the competition between long-range synchronization via gas-phase coupling and short-range interaction through surface diffusion. One finds transitions from synchronized to turbulent behavior which macroscopically correspond to a transition from rate oscillations to a stationary reaction rate. The study of these transitions and the characterization of spatiotemporal chaos will be the aim of future research.
References [1] (a) R. Imbihl, in: Optimal Structures in Heterogeneous Reaction Systems, P.J. Plath, ed., Springer Series in Synergetics, vol. 44 (Springer, Berlin, 1990); (b) G. Ertl, Adv. Catal. 37 (1991) 213. [2] R.M. Eiswirth, K. Krischer and G. Ertl, Appl. Phys. A 51 (1990) 79. [3] M.P. Cox, G. Ertl and R. Imbihl, Phys. Rev. Lett. 54 (1985) 1725. [4] S. Jakubith, H.H. Rotermund, W. Engel, A. von Oertzen and G. Ertl, Phys. Rev. Lett. 65 (1990) 3013. [5] H.H. Rotermund, S. Jakubith, A. von Oertzen and G. Ertl, Phys. Rev. Lett. 66 (1991) 3083. [6] J. Falta, R. Imbihl and M. Henzler, Phys. Rev. Lett. 64 (1990) 1409. [7] R. Imbihl, A.E. Reynolds and D. Kaletta, Phys. Rev. Lett. 67 (1991) 275. [8] G. Veser and R. Imbihl, J. Chem. Phys., in press. [9] G. Veser, F. Esch and R. Imbihl, Catal. Lett. 13 (1992) 371.
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R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
[10] (a) V. Castets, E. Dulos, J. Boissonade and P. De Kepper, Phys. Rev. Lett. 64 (1990) 2953; (b) Q. Ouyang and H.L. Swinney, Nature 352 (1991) 610. [11] T. Fink, J.-P. Dath, R. Imbihl and G. Ertl, J. Chem. Phys. 95 (1991) 2109. [12] M. Slinko, T. Fink, T. L6her, H.H. Madden, S.J. Lombardo, R. Imbihl and G. Ertl, Surf. Sci. 264 (1992) 157. [13] S.J. Lombardo, F. Esch and R. Imbihl, Surf. Sci., in press. [14] R. Imbihl, T. Fink and K. Krischer, J. Chem. Phys. 96 (1992) 6236. [15] J.-P. Dath, T. Fink, R. Imbihl and G. Ertl, J. Chem. Phys. 96 (1992) 1582. [16] S. Ladas, R. Imbihl and G. Ertl, Surf. Sci. 1997 (1988) 153. [17] S. Ladas, R. Imbihl and G. Ertl, Surf. Sci. 198 (1988) 42. [18] M. Eiswirth, P. M611er, K. Wetzl, R. Imbihl and G. Ertl, J. Chem. Phys. 90 (1989) 510.
PHYSICA/1
Spatiotemporal pattern formation in catalytic reactions on single crystal surfaces R. I m b i h l Fritz-Haber-lnstitut der Max-Planck-Gesellscha[t, Faradayweg 4-6, W-IO00 Berlin 33, Germany
Two oscillatory surface reactions, namely catalytic CO oxidation and the catalytic reduction of NO with various agents (CO, H,, NH3), have been studied on Pt single crystal surfaces at low pressure (p < 10 ~Tort) using spatially resolved techniques. Both Turing structures as well as chemical wave patterns with varying degrees of complexity were observed. The facetting of Pt(ll()) in catalytic CO oxidation is discussed as an example for the experimental realization of a Turing structure. A detailed microscopic picture is available for the formation of the facet pattern as demonstrated by a Monte Carlo simulation. The catalytic reduction of NO with CO and NH~ on Pt(100) serves as an example for the transition from ordered to turbulent spatiotemporal patterns. The spatial coherence in these patterns is determined by the competition between long-range synchronization via gas-phase coupling and short-range coupling via the surface diffusion of a mobile adsorbate.
1. Introduction
The study of oscillatory reactions on single crystal surfaces, which was started about a decade ago, has led to the discovery of a variety of new phenomena in the field of nonlinear dynamics and provided detailed insights i n t o t h e m i c r o s c o p i c m e c h a n i s m of t h e o s c i l l a t i o n s # l [ 2 , 3 ] . B a s e d o n t h e s e r e s u l t s , m a t h e m a t i c a l m o d e l s c o u l d b e f o r m u l a t e d w h o s e e s s e n t i a l steps w e r e b a s e d o n e x p e r i m e n t a l facts. T h e a r e a w h e r e t h e u s e of single s u r f a c e s t u r n e d o u t to b e m o s t f r u i t f u l p r o v e d to b e t h e s t u d y o f t h e v a r i o u s a s p e c t s of s p a t i a l self-organization. Here new phenomena appeared which have no counterpart in fluid p h a s e c h e m i s t r y . B o t h T u r i n g s t r u c t u r e s as well as c h e m i c a l w a v e p a t t e r n s w i t h v a r y i n g d e g r e e s o f c o m p l e x i t y h a v e b e e n f o u n d in s u r f a c e r e a c t i o n s [ 3 - 9 ] . T h e r e g u l a r facet pattern which develops during catalytic CO oxidation on a Pt(ll0) s u r f a c e has b e e n i d e n t i f i e d as a T u r i n g s t r u c t u r e [6]. T h i s t y p e o f d i s s i p a t i v e s t r u c t u r e has for a l o n g t i m e o n l y b e e n d i s c u s s e d t h e o r e t i c a l l y a n d t h e f a c e t t e d P t ( l l 0 ) s u r f a c e r e p r e s e n t s t h e first e x a m p l e w h e r e a c l e a r - c u t e x p e r i m e n t a l #~ For a review of single crystal studies of oscillatory surface reactions see ref. [1]. 0378-4371/92/$05.00 © 1992- Elsevier Science Publishers B.V. All rights reserved
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
35
realization of a Turing structure could l~e achieved #2. The formation of the facet pattern, whose length scale is microscopic (100-200 ,~), can be described by a detailed microscopic mechanism as will be demonstrated in a Monte Carlo simulation [7]. In addition to catalytic CO oxidation, another oscillatory reaction system that has been thoroughly investigated in the past years is catalytic NO reduction with CO, H 2 or N H 3 on a Pt(100) surface [8, 9, 11-15]. Similar to P t ( l l 0 ) / C O + 0 2 these reaction systems exhibit a variety of spatiotemporal patterns, which could be investigated with the newly developed photoemission electron microscope ( P E E M ) [4, 5, 8, 9]. Of particular interest is that these patterns exhibit well-defined transitions from ordered to turbulent behavior and therefore offer the possibility to gain insights into the mechanism leading to spatiotemporal chaos.
2. Turing structures in Pt(ll0)/CO 2.1.
+ 0 2
K i n e t i c oscillations
It has been well established that catalytic CO oxidation proceeds via the L a n g m u i r - H i n s h e l w o o d ( L H ) mechanism, incorporating the following steps: CO -~- :~ ~-- GOad ,
0 2 + 2 " ~ 2Oad ,
GOad ~- Oad ~ 2 * + C O 2
(* denotes a vacant adsorption site). Kinetic oscillations in this reaction can arise if the metal substrate on which the reaction takes place exhibits an adsorbate-driven surface phase transition (SPT). This is the case with Pt(100) and Pt(110) surfaces, both of which display a reconstruction in their clean state that can be reversibly lifted upon C O adsorption [1, 2]. The principle of the mechanism shall be illustrated with the SPT of Pt(110) which is depicted in a ball model in fig. 1. The left-hand side of fig. 1 displays the bulk-like 1 x 1 structure of the Pt(110) surface, which, however, is not stable in its clean state but undergoes reconstruction into the 1 x 2 "missing row" structure shown in the right side. The name "missing row" structure has its origin in the building principle that every second of the close-packed [110J-rows has to be removed in order to create the 1 x 2 structure displayed in fig. 1. This transition can be reversed by adsorbing CO, which lifts the 1 × 2 reconstruction and causes the surface atoms #'2In fluid-phase chemistry Turing structures were found as described in ref. [10].
36
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
Pt 1110) CO covered
lxl SO2 ==0.5-0.6
cleon
=
=
lx2 SO2 ==0.3-0£
Fig. 1. Ball model illustrating the formation of a step with (100) orientation in the CO-induced 1 x 1 ~ 1 x 2 phase transition of a Pt(ll0) surface. In the ball model the 1 x 2 "missing row" structure shown on the right has been created by removing every second row in the front half of the 1 x 1 surface (left) and then placing the additional balls on top of the rear half of the model surface. to m o v e b a c k into t h e i r 1 x 1 positions. T h u s an a d s o r b a t e - d r i v e n 1 x 1 ~ 1 × 2 S P T exists which is c o n t r o l l e d b y a critical C O c o v e r a g e OCO,crit ~-0.2. W i t h the 1 x 1 ~ 1 x 2 S P T also the o x y g e n sticking coefficient So2 c h a n g e s a n d since this p r o p e r t y is r a t e - l i m i t i n g at high P c o , kinetic oscillations can arise. A n o s c i l l a t o r y cycle m a y l o o k like this. Starting with a C O c o v e r e d 1 x 1 s u r f a c e , So2 a n d , h e n c e the r e a c t i o n r a t e , will be high. A c c o r d i n g l y , the high r e a c t i o n r a t e will cause Oco to d e c r e a s e a n d b e l o w •CO,crit the 1 x 2 p h a s e will a p p e a r . B u t on this surface So2 is low a n d c o n s e q u e n t l y C O a d s o r p t i o n will d o m i n a t e until the C O c o v e r a g e is high e n o u g h to r e e s t a b l i s h the initial situation of a CO covered 1 x 1 phase. T h e o s c i l l a t o r y cycle d e p i c t e d a b o v e has b e e n verified b y in-situ L E E D ( = l o w e n e r g y e l e c t r o n diffraction) e x p e r i m e n t s which d e m o n s t r a t e d t h a t t h e o s c i l l a t i o n s in the r e a c t i o n rate are in fact a c c o m p a n i e d by p e r i o d i c s t r u c t u r a l t r a n s f o r m a t i o n s via t h e 1 x 1 ~ - 1 x 2 S P T [1, 2]. B u t b e s i d e s limit cycle b e h a v i o r , t h e 1 x 1 ~---1 x 2 S P T also p r o d u c e s a n o t h e r t y p e of d i s s i p a t i v e s t r u c t u r e which will b e p r e s e n t e d in the following section.
2.2. Facetting
If o n e uses t h e ball m o d e l o f P t ( l l 0 ) d i s p l a y e d in fig. 1 a n d tries to r e a r r a n g e
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
37
the atoms of the 1 x 1 surface such that a 1 x 2 structure is produced, then this can be accomplished by taking out every second of the close-packed [ll0]-rows in the front half and putting them on top of the 1 x 1 layer in the rear half. But as shown in fig. 1, this rearrangement opens up a new terrace layer thereby creating a step with (100) orientation. This simple example demonstrates that the mass transport of Pt atoms that is associated with the 1 × 1 ~---1 x 2 SPT necessarily creates atomic steps and therefore causes a certain roughening of the surface. This may lead to the following consequences. As one exposes a P t ( l l 0 ) surface to catalytic CO oxidation under conditions very similar to those where kinetic oscillations occur (but at lower T) facetting may occur [16, 17]. The facets which are formed are of microscopic dimension ( t y p i c a l l y ~ 100,~) as judged from L E E D where facetting gives rise to a splitting of the integral order beams [16]. The facetting of a surface due to the interaction with a strong adsorbate such as oxygen is nothing unusual but in the present case there are several observations that contradict the possibility of simple adsorbate-driven restructuring of the surface. First the surface facets only under reaction conditions but not if the sample is treated with one adsorbate alone. Furthermore the facets which are formed are thermally unstable and simple heating above ~500-600 K is sufficient to restore the flat surface. This reordering process also slowly removes the facets if the gas flow in a facetting experiment is stopped. Apparently an on-going reaction is necessary to stabilize the facets. Moreover, these only form under reaction conditions where the 1 x 1 ~-1 x 2 SPT can proceed, e.g., if the CO coverage is close t o •CO,crit ~ 0.2. Associated with the facetting of P t ( l l 0 ) is an increase in catalytic activity, which in the experiment shows up as a continuous increase of the reaction rate accompanying the facetting process [16]. All of the above observations suggest that the facetting of P t ( l l 0 ) in catalytic CO oxidation represents a dissipative structure. Further evidence for this assignment was gained in a quantitative characterization of the facetted surface using spot profile analysis with a high resolution L E E D instrument ( S P A L E E D ) [6]. This investigation revealed that the facets on P t ( l l 0 ) form a regular pattern in which facets of uniform orientation and size are arranged in the shape of a symmetric sawtooth as indicated in fig. 2a. This sawtooth-like pattern exhibits a well-defined lateral periodicity of ~ 2 0 0 , ~ in the [ll0]direction corresponding to 74 lattice units. The sides of the sawtooth are formed by facets of (340) and (430) orientation of which a ball model is displayed in fig. 2b. The diagram depicts the structural elements consisting of (110) terrace units and (100) step units that build up the facet. By arranging these two structural elements in a regular sequence all orientations of the [001]-zone can be constructed. These orientations are formed exclusively in the facetting of P t ( l l 0 ) . The steepness of the
38
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces @ [110]
I
,
I
I
I-
. [1io]
x = 2oo~I
c( = 8.1 ° ~rms = 3.0 o t o m i c
®
Ioyers
l [1To] i
[ooq
Fig.'2. Model of the facetted Pt(110) surface. (a) Geometrical model showing the sawtooth-like arrangement of the facets as derived from LEED measurements. Also indicated in the diagram is the lateral periodicity of 200 A in the [ll0]-direction. The Pt(110) surface was facetted by exposing the surface at T = 480 K to catalytic CO oxidation in the 10 ~mbar range for ~40 min (after ref. [6]). (b) Ball model of the (430) facet illustrating the structural elements of which the facet consists. facets d e p e n d s o n the r e a c t i o n c o n d i t i o n s with (210) r e p r e s e n t i n g the limiting case [16]. O n this o r i e n t a t i o n , (100) step a n d (110) terrace units a l t e r n a t e . T h e i n c r e a s e in catalytic activity which a c c o m p a n i e s facetting is caused by the p r e s e n c e of (100) steps since these sites exhibit a higher oxygen a d s o r p t i o n rate t h a n a flat (110) terrace. T h e increase of So2 at step sites can be a t t r i b u t e d to a local l o w e r i n g of the work f u n c t i o n , which facilitates the e l e c t r o n t r a n s f e r f r o m the m e t a l to a n i n c o m i n g O2-molecule.
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
39
The findings that the facets form a periodic pattern and that they only develop under reaction conditions suggest their interpretation as a dissipative structure of the Turing type. The facet pattern is, of course, somewhat different from the original idea of a Turing structure, since what is modulated here is not a chemical concentration, but the surface structure. But this point is considered to be of minor importance. As will be shown below, facetting develops because the homogeneous state of the flat surface is unstable with respect to a perturbation by diffusing Pt atoms. The origin of facetting is therefore an instability against diffusion and, accordingly, the facetted surface can be classified as a Turing structure. 2.3. S i m u l a t i o n o f f a c e t t i n g
As has been demonstrated with the ball model displayed in fig. 1, the 1 x 1~---1 x 2 SPT can be considered to represent the elementary step of facetting since this mechanism produces steps whose accumulation may lead to the development of facets. Following this idea a simulation based on a Monte Carlo (MC) algorithm was conducted in which we tried to reproduce the formation of facets during catalytic CO oxidation [7]. We proceed in the same way as one would envision it with the ball model of fig. 1. We let the surface reaction take place on a 200 x 5 substrate lattice. We interrupt the reaction periodically in order to rearrange the substrate structure according to the local distribution of adsorbed CO. In the SPT, which proceeds locally, steps can either be created or annihilated following the rule of mass conservation. The simulation encompasses the elementary steps of the L H mechanism, the properties of the CO-induced 1 x 1 ~-1 × 2 SPT, which is controlled by a critical CO coverage OCO,cr~t = 0.2, the enhancement of oxygen adsorption at step sites, and thermally activated surface diffusion of Pt atoms. We assigned probabilities to the individual steps which were derived from experimentally determined constants. If the model surface is exposed to a constant flow of CO and O 2 comparable to the experimental condition, the initially flat surface roughens as demonstrated by the surface profiles displayed in fig. 3. The roughening occurs as a consequence of local CO coverage fluctuations, which cause the mass transport of Pt atoms each time the surface is forced to undergo the SPT. As the reaction progresses facets develop with a sawtooth-like pattern similar to the one derived from experiment (see fig. 2). The pattern remains practically stationary above 4000 MC cycles. In order to demonstrate that the facets are in fact only stabilized by the on-going reaction, the flow of the gases was stopped in the simulation after 4000 cycles. Thermal reordering leads then to the restoration of the flat surface as demonstrated by the profile recorded after 6000 cycles.
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
40
[110]
T
Number of Cycles
[~To] -
-
6000
~
~ooo
3000 ='
15oo 3oo II
II
1
50
II
lOO Row Number
U
150
II
2 OO
Fig. 3. Monte Carlo simulation showing the development of a regular facet pattern as a P t ( l l 0 ) surface is exposed to catalytic CO oxidation. After stopping the gas flow following 4000 cycles, the flat surface is restored in a thermal annealing process. Shown in the diagram are profiles of the surface in the [ll0)-direction (after ref. [7]).
The simulation described above could reproduce practically all essential features of the experiment. The lateral periodicity of 40 lattice units in the [ll0]-direction is not very far from the experimental value of --70 lattice units. Even the time scale is comparable since the 4000 cycles in the simulation correspond to - 4 0 0 s in real time, which is roughly the period during which facetting develops in the experiment. In addition the simulation could also correctly reproduce the region in Pco parameter space where facetting occurs as well as the increase in catalytic activity. The simulation thus confirms the interpretation of the facetted P t ( l l 0 ) surface as a Turing structure, and it provides a detailed microscopic picture of how the interplay between the LH reaction and the phase transition can give rise to a facetting process.
3. From ordered behavior to chemical turbulence: the catalytic reduction of
NO on Pt(100)
3.1. Short-range versus long-range coupling Unlike oscillatory reactions in a fluid phase or in heterogeneous catalysis at high pressure (p - 1 atm), spatial coupling is conceptually very simple in the oscillation studies which were performed under low pressure ( p < 10 3 Torr) on single crystal surfaces [18]. The reaction is then practically isothermal and
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
41
since the U H V chamber is operated as a gradient free flow reactor, transport limitations play no role. Spatial coupling of the different local oscillators under these conditions is provided by two basic mechanisms, surface diffusion via a mobile adsorbate and a coupling via partial pressure changes in the gas phase. The latter simply arises due to mass balance in the reaction since the oscillations in the product rate have to be accompanied by corresponding variations in the educt partial pressures. Typically amplitudes on the order of 1% of the educt partial pressures are observed. But while surface diffusion is short range in nature, the partial pressure changes in the gas phase affect all parts of the surface in the same way and practically without any measurable delay. The two coupling modes compete in their influence as one can see by constructing two limiting cases. If gas phase coupling were to dominate, perfect long-range synchronization could be achieved and a homogeneously oscillating surface would result. Quite in contrast, with surface diffusion being the only coupling mode, no long-range synchronization will exist. In this case, the surface will still be oscillating locally but over macroscopic distances spatiotemporal pattern which are irregular or turbulent will prevail. Realization of the two extreme cases sketched above and transitions between them have been observed in the catalytic reduction of NO on a Pt(100) surface. These examples will be presented in the following section. 3.2.
O r d e r e d a n d t u r b u l e n t spatial p a t t e r n s
The catalytic reduction of NO on a Pt(100) surface has been investigated with CO, H 2 and N H 3 as reducing agent [11-15]. All three systems exhibit similar dynamical behavior whose origin presumably lies in the requirement of vacant sites for the rate-limiting step of NO dissociation: NO + *
-->
Nad
+ Oad .
Since more vacant sites are liberated in the subsequent product forming steps than are consumed by the above step, autocatalytic behavior results. The autocatalysis manifests itself in the occurrence of extremely narrow product peaks if a coadsorbed layer of the reactants NO + X (X = CO, H 2, NH3) is heated up in temperature programmed reaction experiments. This effect is also referred to as a "surface explosion". The same autocatalytic mechanism presumably is also the driving force for the kinetic oscillations which are found under similar conditions in all three systems. The adsorbate-induced 1 x 1 ~ - h e x phase transition which exists on Pt(100) due to the N O / C O - i n d u c e d lifting of the quasihexagonal ( " h e x " )
42
R. Imbihl / Patterns in catalytic reactions on single crystal surfaces
reconstruction on this surface, is considered to be of minor importance for the oscillatory mechanism. This conclusion could be verified experimentally for the N O + C O reaction for which it it was supported by mathematical modeling [11, 14]. For the N O + H 2 reaction and also for the N O + N H 3 reaction which is under consideration here, the picture has to be modified slightly since in these systems the phase transition appears to be essential for the oscillations [9, 12, 13]. The N O + N H 3 reaction yields N~_ and H~O as reaction products according to 3NO + 2 N H 3 ~ 2.5N~ + 3 H 2 0 . Kinetic oscillations in the NO + N H 3 reaction on Pt(100) can be found in the cooling branch of a heating/cooling cycle where they appear in a t e m p e r a t u r e interval of 15 K width during the lifting of the hex reconstruction by N O adsorption [13]. Spatial pattern formation on the catalyst surface during the reaction can be followed by means of the photoemission electron microscope ( P E E M ) , which allows one to image the laterally varying adsorbate concentration with high spatial (~1 I~m) and temporal (~-50ms) resolution. This instrument is based on the principle that the n u m b e r of photoelectrons emitted from a sample illuminated with a suitable photo energy ( 5 - 6 eV) depends on the local work function, which in turn is altered by the presence of an adsorbate [4]. A schematic overview of the spatial patterns which have been observed with P E E M during the cooling period is given in fig. 4 [9]. During the oscillations at high t e m p e r a t u r e the intensity in the imaged area changes homogeneously in phase with the macroscopic rate oscillations. As judged from the P E E M intensity, these oscillations take place on the reconstructed Pt(100) surface with the adsorbate coverage always remaining low. Evidently gas phase coupling is efficient enough here to ensure a homogeneously oscillating surface. As the t e m p e r a t u r e is lowered to ~450 K the influence of N O adsorption Surface structure:
lxl
t
hex
Reaction rate :
I II I I II I I ] I/ItI I I I I Pattern :
irregular spirats & patterns target (turbulence) p a t t e r n s
Dominant coupling mode : [
~30
island for marion
surface diffusion
homogeneous intensity changes gas phase coupling
I
~0
I
[
~50
~60
---*T [K]
Fig. 4. Schematic overview of the temperature ranges in which macroscopic rate oscillations and different types of spatial patterns are observed in the NO + NH3 reaction on Pt(100). (Reaction conditions: PNo = 1.1 x 10 ~ mbar, PNH~= 1.6 X 10 * mbar, pumping rate ~60 (/s) (after ref. [9}).
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
43
Fig. 5. PEEM images (diameter 450 ixm) showing spirals coexisting with target patterns in the NO + NH 3 reaction on Pt(100). The experimental conditions were for (a) Pro = 2.1 x 10-~ mbar, PyH3 = 5.6 x 10 6 mbar, T = 455 K and for (b) PNo = 2.3 x 10 ~mbar, PNH3= 2.8 x 1 0 - 6 mbar and T = 450 K (after ref. [9]).
b e c o m e s stronger and fluctuating adsorbate islands of typically to 10 to 50 ~xm d i a m e t e r a p p e a r on the surface. While these islands continuously nucleate, grow, and then dissolve again, the area s u r r o u n d i n g t h e m is still oscillating h o m o g e n e o u s l y . U p o n further lowering of the t e m p e r a t u r e , the fluctuating islands turn into stable structures which cover most of the imaged area. O n e o b s e r v e s target patterns and rotating spirals (to ~ 1.2 min ~), b o t h of which are d e p i c t e d in the P E E M images displayed in fig. 5. T h e bright area in these areas can be attributed to a N H x (x = 1 - 3) species a d s o r b e d on a 1 x 1 substrate, while the dark area can be assigned to N O , which is partially dissociated into a t o m i c o x y g e n and nitrogen on the 1 x 1 substrate. W i t h these stable structures some long-range synchronization still persists as o n e still observes m a c r o s c o p i c rate oscillations. R e a c t i o n fronts p r o p a g a t e on the area s u r r o u n d i n g the spirals and target patterns, but do not interfere with t h e m . A small t e m p e r a t u r e decrease is now sufficient to cause the m a c r o s c o p i c rate oscillations to disappear. In P E E M the regular patterns turn into irregular patterns, o f which o n e typical e x a m p l e is displayed in fig. 6a. In these patterns the surface is still oscillating locally with a large amplitude as one can d e m o n s t r a t e by integrating the intensity in a small area of ~ 3 0 x 30 txm 2 size and by following its time d e p e n d e n c e . T h e c o r r e s p o n d i n g time series displayed in fig. 6b shows irregular oscillations with a strongly fluctuating amplitude. A p p a r e n t l y the transition f r o m m a c r o s c o p i c rate oscillations to the stationary r e a c t i o n rate is caused by the b r e a k - d o w n of long-range synchronization via gas
44
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
Pt(lO0) / NO+ NH3 T=421K PNH3: PNO : 2 : 1
~6 C
c
4
0
500
1000
1500
t [s] (a)
(b)
Fig. 6. Chemical turbulence in the N O + N H 3 reaction on Pt(100). (a) P E E M image (diameter 450 ixm) showing an irregular spatiotemporal pattern recorded at T = 432 K and with PNo = 1.3 x 10 6 m b a r and PNH3 = 2.1 x 10 -6 mbar (after ref. [9]). (b) Time series demonstrating irregular local oscillations in turbulent spatiotemporal patterns of the type displayed in (a). The time series was obtained by integrating the intensity of the video frames in a small selected area of ~ 3 0 × 30 I~m 2 (after ref. [9]).
phase coupling. Since the different surface parts in the turbulent patterns oscillate unsynchronized, macroscopically a stationary reaction rate results. Experimentally it is difficult to follow the transition from synchronized to turbulent behavior but the present data suggest that this transition takes place discontinuously. The results obtained with the NO + NH 3 reaction can be complemented by the PEEM observations made with the NO + CO reaction on Pt(100) [8]. Here one finds turbulent spatiotemporal patterns which macroscopically correspond to a stationary reaction rate. A small temperature jump of -~1-3 K suffices, however, to excite large amplitude rate oscillations whose amplitude decays in a small number of cycles [11]. The effect of the temperature pulse is to synchronize the oscillations by transforming the turbulent pattern into a target pattern. The decaying amplitude just reflects then the progressive loss in spatial
R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
45
coherence as the initially ordered pattern becomes turbulent again [8]. The NO + CO reaction on Pt(100) thus represents an example how macroscopic rate oscillations can be excited by an external synchronization pulse. The filtering properties of the NO + CO reaction on Pt(100) have been investigated further by perturbing the systems randomly with small amplitude temperature fluctuations [15]. The response behavior of the reaction system can be described as a narrow bandpass amplifier which only selects frequencies close to the natural frequency of the unperturbed oscillator.
4. Concluding remarks Examples have been presented for the formation of Turing structures and spatiotemporal patterns in catalytic reactions on single crystal surfaces. The regular facet patterns which form in catalytic CO oxidation on Pt(ll0) with a lateral periodicity of 100 to 1000 A, could be identified as Turing structures. All essential experimental observations could be simulated with a relatively simple model based solely on the experimentally determined properties of the surface reaction and the phase transition. Various forms of spatiotemporal patterns were observed in the catalytic reduction of NO with CO or NH 3 on a Pt(100) surface. The degree of order in these patterns is determined by the competition between long-range synchronization via gas-phase coupling and short-range interaction through surface diffusion. One finds transitions from synchronized to turbulent behavior which macroscopically correspond to a transition from rate oscillations to a stationary reaction rate. The study of these transitions and the characterization of spatiotemporal chaos will be the aim of future research.
References [1] (a) R. Imbihl, in: Optimal Structures in Heterogeneous Reaction Systems, P.J. Plath, ed., Springer Series in Synergetics, vol. 44 (Springer, Berlin, 1990); (b) G. Ertl, Adv. Catal. 37 (1991) 213. [2] R.M. Eiswirth, K. Krischer and G. Ertl, Appl. Phys. A 51 (1990) 79. [3] M.P. Cox, G. Ertl and R. Imbihl, Phys. Rev. Lett. 54 (1985) 1725. [4] S. Jakubith, H.H. Rotermund, W. Engel, A. von Oertzen and G. Ertl, Phys. Rev. Lett. 65 (1990) 3013. [5] H.H. Rotermund, S. Jakubith, A. von Oertzen and G. Ertl, Phys. Rev. Lett. 66 (1991) 3083. [6] J. Falta, R. Imbihl and M. Henzler, Phys. Rev. Lett. 64 (1990) 1409. [7] R. Imbihl, A.E. Reynolds and D. Kaletta, Phys. Rev. Lett. 67 (1991) 275. [8] G. Veser and R. Imbihl, J. Chem. Phys., in press. [9] G. Veser, F. Esch and R. Imbihl, Catal. Lett. 13 (1992) 371.
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R. lmbihl / Patterns in catalytic reactions on single crystal surfaces
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