Spatial coupling of autonomous kinetic oscillations in the catalytic CO oxidation on Pt(110)

Surface Science 215 (1989) L307-L315 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS SPATIAL COUPLING OF AUTONOMOUS KINETIC OSCILLATIONS I N T H E CATALYTIC C O O X I D A T I O N O N P t ( l l 0 ) R. I M B I H L , S. L A D A S * and G. E R T L Fritz-Haber-lnstitut der Max-Planck-Gesellschaft, Faradayweg 4 - 6, D-IO00 Berlin 33, Germany

Received 26 January 1989; accepted for publication 14 February 1989

Oscillations in the low-pressure oxidation of CO on Pt(110) were monitored simultaneously at different parts of the surface by means of two Kelvin probes. The oscillations in both regions were found to always exhibit the same frequency, however, in general with non-vanishing phase difference as well as different shapes and amplitudes. The results are interpreted in terms of spatial coupling through the gas phase between regions whose autonomous oscillatory properties are somewhat at variance to each other because of differences in the surface defect structure.

Kinetic oscillations in the catalytic CO oxidation at Pt single crystal surfaces under isothermal, low pressure ( 1 0 - 6 - 1 0 -3 Torr) conditions were demonstrated to be due to the occurrence of reversible structural transformations of the surfaces [1]. With Pt(100) coupling between various parts of the macroscopic ( - 1 cm 2) surface area is achieved via laterally propagating reaction waves [2], while with P t ( l l 0 ) synchronization occurs almost instantaneously via the gas phase through the small changes of the partial pressures associated with the reaction [3]. The formation of propagating reaction fronts on Pt(100) could be verified directly by means of the scanning L E E D technique [2,4], while the evidence for gas phase coupling with P t ( l l 0 ) was based on more indirect arguments, e.g. the narrow existence region for oscillations which makes this system sensitive to small pressure fluctuations [3]. Experiments with periodic partial pressure modulations demonstrated that the concomitant variations ( - 1% Pco) associated with the autonomous oscillations are in this case indeed sufficient to produce marked response of the system so that it becomes rather probable that the whole surface area oscillates in phase. Application of the scanning L E E D method to this system [3] was not very conclusive, since this technique requires large amplitude oscillations with relatively long periods (z>> 10 s), while with P t ( l l 0 ) these periods are * Alexander yon Humboldt Fellow. Permanent address: Department of Physics, University of Ioannina, Greece. 0039-6028/89/$03.50 © Elsevier Science Publishers B,V. (North-Holland Physics Publishing Division)

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R. Imbihl et aL / Kinetic oscillations in the CO oxidation on Pt(llO)

Kelvin probe I

_

0

/

shielding plate

ermocoupte To -wires

Kelvin probe If

I

5 mm

I

Fig. 1. Experimental set-up for measuring simultaneously the work function change at two different regions of the single crystal surface.

frequently shorter [6]. In addition, this oscillating system is easily distorted by the electron beam of the LEED method. The present paper describes a different approach for studying the communication between various parts of a Pt(ll0) surface: The occurrence of temporal oscillations in different regions was monitored by using two Kelvin probes for recording the work function changes accompanying the reaction. The results will clearly demonstrate that in this case gas phase coupling is the dominant mechanism of synchronization. On the other hand, it will, however, also become evident that even an apparently uniform single crystal surface may exhibit pronounced differences between various parts. The experimental set-up as well as details about the kinetic oscillations under consideration have been described previously [3-8]. The arrangement with two rings of oxidized Ta-wire serving as Kelvin probes is sketched in fig. 1. In order to prevent any electric field interactions between the two electrodes, a shielding plate was mounted in between them. An example of the two signals recorded during oscillations is reproduced in fig. 2 together with the overall reaction rate as monitored with a quadrupole mass spectrometer. Obviously the A~-signals from the two regions exhibit the same period, but differ in shape and amplitude. As outlined previously [3,7], the (overall) reaction rate varies in parallel to the (overall) change of the work function (which reflects essentially the oxygen coverage). In the present example the maximum of the reaction rate coincides with the A~-maximum of probe II, but exhibits a phase shift to the maximum of probe I. One has therefore to conclude that in this case region II is more representative for the integral behavior of the whole surface. On the other hand, the features of period doubling present in this A~trace do not manifest in the reaction rate, but are smeared out by contributions from the other parts of the surface. The differences between the A~-signals of the two regions are tentatively attributed to spatial differences in the defect structure of the single crystal surface which affect the adsorption properties and hence the oscillatory

R. Imbihl et aL / Kinetic oscillations in the CO oxidation on Pt(l l O)

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Pt (110) P02 = 8"lO-ST°rr PCO = 3.4.10 -5 Torr I

T

= 510K

I

I00 mV

I I

Pco2[a.u-]

t I

o

II

21o

l,o t Is]

I

80

Fig. 2. Example for work function oscillations recorded simultaneously at two different regions of the crystal surface as indicated in fig. 1. Together with the variations of the work function the changes in Pco2 were measured by a differentially pumped mass spectrometer.

behavior. The oscillations occur under conditions for which dissociative oxygen adsorption is rate-limiting, and this step is in turn sensitively affected by the presence of structural defects, which are an inevitable consequence of the sample preparation [9]. The presence of these structural defects was, on the other hand, found to affect the pressure conditions (at fixed temperature) for the occurrence of oscillations as well as their frequency [7]. The data of fig. 2 demonstrate, however, that the two regions do not oscillate independently from each other, but that an efficient coupling mechanism provides a constant frequency, albeit with nonzero phase shift between different regions. This behavior is very similar to the observations made with external small amplitude ( - 1%) modulation of one of the partial pressures [5] as will be discussed further below. The existence of slightly different conditions for oscillations in the two different regions manifests itself also upon continuously varying one of the

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R. Imbihl et al. / Kinetic oscillations in the CO oxidation on Pt(110)

Pt(110) pO2 =1.25x 10-4 Tort Pco ~ 2 xlO-6Torr T =370K Z~t#

r I lOOrnV

l

rr

I

0

i

,

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,

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100

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200

~

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tfs] Fig. 3. Example for strongly different oscillations measured at the two regions of the crystal surface as indicated in fig. 1 (T = 370 K, Po2 = 1.25 × 10 -4 Tort, ,°co - 2 × 10-6 Torr).

control parameters (Pco) across the existence range for oscillations. A typical situation near the edge of this existence range is illustrated by fig. 3: While the oscillations from region II are very regular and harmonic, those in region I are irregularly shaped but nevertheless exhibit the same frequency - obviously again a consequence of strong coupling between different parts of the surface. The results of a systematic study of crossing the existence range for oscillations are illustrated by figs. 4 and 5. At the chosen temperature (535 K) and the applied pressures the system is very stable and reproducible, since any interference with progressing facetting is negligible [10]. Fig. 4 shows the variation of the reaction rate R with Pco (at fixed Po2 = 8 × 10 -5 Torr). Beyond the maximum of R the rate is limited by oxygen adsorption (which in turn is inhibited by adsorbed CO), and oscillations are observed for CO pressures indicated by the shaded area in fig. 4. The upper part of this figure depicts the A@amplitudes of the oscillations recorded by both Kelvin probes at various points of this Pco-range. The corresponding time series are reproduced in fig. 5. If one follows the development of oscillations in the two different regions as Pco is varied, one realizes that coming from high Pco, region I starts to oscillate first (No. 1 in fig. 5), but as Pco is approaching the reaction rate maximum the oscillations in this region also die out first while region II is still

R. Imbihl et aL

/

Kinetic oscillations in the CO oxidation on Pt(l l O)

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Pt (110) P02 = 8"lO'5T°rr

.9

T

43--

o

=535K

~2 Kelvin probe II

Kelvin probe

I

cr

l

0

2

4

i

6

8

10

PC0 [10"5T°rr] Fig. 4. Differencesin the amplitude and existenceregion of oscillations at two different parts of the surface(see fig. 1) measured as Pco is varied stepwise across the oscillatoryregime. The lower section displays the dependenceof the reaction rate R (being proportional to Pco2),on Pco. maintaining its oscillatory behavior (No. 5 in fig. 5). Evidently, as indicated by the distribution of the A@amplitudes in fig. 5, the width of the existence regime for oscillations is about the same in both regions, but their maxima are slightly displaced from each other on the Pco-scale. Therefore only a single region is oscillating in the edge zone of the existence range for oscillations, but both regions oscillate with a large amplitude in the center of this range. The individual stages of the development of oscillations as Pco is varied show a number of interesting details as depicted in fig. 5. At high Pco (No. 1) the oscillations in region I exhibit roughly a triple period while the small-amplitude oscillations in region II follow phase-locked to the largest of the three oscillation peaks of each triple-period in region I. As Pco is decreased, the amplitude and the frequency in region II increases (No. 2), and now each simple period in region II corresponds to a double-period oscillation in region I. The phase relationship between the two Ac/)-traces displays a remarkable behavior as the A@maxima in region II are not in phase with the larger, but with the smaller peak of each double-period oscillation in region I. U p o n further decreasing Pco the amplitude of the oscillations in region II grows again, but also develops period-doubling. At this stage the oscillations in

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R. I m b i h l et al.

Kinetic oscillations in the C O oxidation on Pt(110)

P t (110)

T = 535K II Ii

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- t[s] Fig. 5. Time series of oscillationsbelonging to the A~-amplitudesshown in fig. 4. region I already start to become irregular with respect to the changes in their amplitude, but their phase and frequency remains locked to the oscillations in region II. It should be noted that the appearance of period-doubling in the oscillations of region II in No. 5 is also accompanied by a frequency change as the period in No. 5 is not twice the frequency of the oscillations in No. 4, but it increases only by a factor of 1.2. As Pco is decreased further (No. 4), the oscillations in both regions start to become quite irregular accompanied by a decrease in their amplitude. Frequency and phase still remain locked with respect to each other, and this relationship is finally lost only when the oscillations in region I assume the character of small random fluctuations (No. 5). If one compares the behavior of the oscillations near the high Pco and near

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the low Pco boundary of the existence range, one realizes that large fluctuations of the amplitude are only found towards the low Pco boundary, while the amplitude varies little at high Pco. This observation is consistent with previous experiments under similar conditions in which a well-defined transition from harmonic oscillations via a sequence of period-doublings to deterministic chaos was identified [6,8]. Although the present time series were not subject to the same mathematical analysis [8], it seems to be rather probable that e.g. the data of No. 4 are also chaotic in nature. If one attempts to rationalize the results obtained with two Kelvin probes in terms of different possible synchronization modes of the oscillating surface, the finding of phase-locked oscillations in the two regions clearly points towards gas phase coupling as the dominating mechanism. A coupling via CO diffusion with propagating reaction fronts as found on Pt(100) could hardly explain the fixed phase difference between different parts of the surface, as in that case irregularities would easily develop and arbitrary changes in the phase shift would be observed. As a coupling via the gas phase affects all parts of the surface practically without delay and in the same way one should expect a homogeneously reacting surface with the oscillations at various parts oscillating exactly in phase. However, since the various parts of the surface exhibit slightly different defect structures and hence oscillatory ranges (see fig. 4), these parts - if perfectly isolated from each other - are also expected to exhibit somewhat varying frequencies of their autonomous oscillations. It is mutual coupling of these oscillators through the gas phase which is responsible for the observed phase shifts. This conclusion is based on the previous experience with the oscillating Pt(ll0) surface being subject to external periodic modulation of one of the partial pressures [5]. The response of the system (oscillating with an autonomous frequency v0) to a periodic pressure modulation with frequency Vp and an amplitude A (of the order of 1%) was rationalized in a dynamic phase diagram which represents the conditions A, Vp/p0 under which the system follows the perturbation with the same frequency (harmonic entrainment) or with more complex behavior (subharmonic or superharmonic entrainment, quasiperiodic behavior). For the present context it suffices to mention that for a 1% amplitude of the O2-pressure modulation, ~,p may deviate by up to + 30% from 1'0 without leaving the region of harmonic entrainment. The system follows the perturbation over this range of ~,p with the same frequency, while the phase shift varies between 0 and ~r. In addition, the effect of period doubling (i.e. alternating small and large amplitudes) was observed under these conditions. These findings can easily explain the observations of the present study, if one takes into account that even without external partial pressure modulation the various oscillating parts of the surface may mutually force each other and cause the dominant frequency to represent the integral behavior of the system.

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There are several other consequences arising from the present results: The measurements displayed in figs. 4 and 5 demonstrate that the existence of surface non-uniformities enlarges the existence region for oscillations in parameter space. It is very likely that the existence region for oscillations of an ideal homogeneous surface will even be much narrower than the 10% of Pco reported before [6]. As it is however impossible to prepare a perfect single crystal surface, it will hardly be possible to determine the "intrinsic" width of the oscillatory region. The existence of non-vanishing phase differences between the oscillations occurring in different regions of the surface could create the impression of a propagating wave. Such a wave does not involve any mass transport on t h e s u r f a c e as shown to exist for Pt(100) as a consequence of reaction-diffusion coupling [2], but is in fact an optical illusion called kinematic wave [11]. A very nice example for the occurrence of standing and propagating wave oscillations, including antiphase oscillations, was recently found with the anodic dissolution reaction of nickel [12]. It will be an interesting task for future experimental work to verify such phenomena also with a system of the present type. The effects caused by slight structural differences between various parts of a nominally uniform single crystal surface are expected to become much more pronounced if the system is a priori heterogeneous. This is indicated by the results of recent experiments by Tsai et al. [13] on the CO oxidation at a polycrystalline Pt wire with partial pressures in the Torr region. By cutting the Pt wire in half which eliminated thermal coupling, but left the gas phase coupling intact, shape and frequency of the oscillations were changed as well as their existence region in parameter space. This last example demonstrates in the same way as the results presented above, that the coupling between chemical oscillators can strongly influence the behavior of an oscillating system and change the existence region for oscillations.

References [1] A listing of the publications on single crystal studies of kinetic oscillations can be found in ref. [3]. [2] M.P. Cox, G. Ertl and R. Imbihl, Phys. Rev. Letters 54 (1985) 1725. [3] M. Eiswirth, P. M~ller, K. Wetzl, R. Imbihl and G. Ertl, J. Chem. Phys. 90 (1989) 510. [4] R. Imbihl, M.P. Cox and G. Ertl, J. Chem. Phys. 84 (1986) 3519. [5] (a) M. Eiswirth and G. Ertl, Phys. Rev. Letters 60 (1988) 1526; (b) M. Eiswirth, P. MSller and G. Ertl, Surface Sci. 208 (1989) 13. [6] M. Eiswirth and G. Ertl, Surface Sci. 177 (1986) 90. [7] S. Ladas, R. Imbihl and G. Ertl, Surface Sci. 198 (1988) 42. [8] M. Eiswirth, K. Krischer and G. Ertl, Surface Sci. 202 (1988) 565.

R. Imbihl et al. / Kinetic oscillations in the CO oxidation on Pt(llO)

[9] R. Imbihl, M. Sander and G. Ertl, Surface Sci. 204 (1988) L701. [10] S. Ladas, R. Imbihl and G, Erfl, Surface Sci. 197 (1988) 153. [11] (a) N. Koppel and L.N. Howard, Science 180 (1973) 1171; (b) P. Ortoleva and J. Ross, J. Chem. Phys. 60 (1974) 5090. [12] O. Lev, M. Sheintuch, L.M. Pismen and H. Yamitsky, Nature 336 (1988) 458. [13] P.K. Tsai, M.B. Maple and R.K. Herz, J. Catalysis 113 (1988) 453.

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