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Surface Science 601 (2007) 772–780

Spatio-temporal dynamics of the NO + NH3 reactionon polycrystalline platinum

Noah McMillan a, Christopher Snively a,b, Jochen Lauterbach a,*

a Department of Chemical Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, United Statesb Department of Materials Science and Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716, United States

Received 20 July 2006; accepted for publication 2 November 2006Available online 27 November 2006

Abstract

Spontaneous reaction rate oscillations and spatio-temporal patterns have been observed by mass spectrometry and photoemissionelectron microscopy (PEEM) during the reduction of NO by NH3 on polycrystalline platinum at 1 · 10�4 Torr and temperatures from460–520 K. The appearance of both oscillations and patterns was found to be strongly dependent on the gas phase composition and thetemperature. In addition, the overall dynamics of the catalyst were found to be dominated by the nonlinear behavior of Pt(100) typegrains, while other types of grains did not participate. In contrast to previous studies, a large number of complex multimodal oscillationswere observed, particularly as the coupling between the surface and the gas phase was increased. The appearance of these complex oscil-lations demonstrates the importance of gas phase coupling to understanding catalytic reactions, even in high vacuum systems.� 2006 Elsevier B.V. All rights reserved.

Keywords: Electron microscopy; Polycrystalline surfaces; Platinum; Surface chemical reaction

1. Introduction

Selective catalytic reduction (SCR) is widely used for thereduction of NOx in the presence of O2 from stationarysources, particularly in power generating facilities andnitric oxide plants [1]. Mobile SCR systems for transporta-tion applications are being introduced in Europe in orderto meet more stringent air pollution control regulations,however this technology is not yet available in the UnitedStates [2,3]. Although mechanistically simpler than SCR,the reduction of NO by NH3 has been studied frequentlyas a model reaction for SCR. It has been found to exhibitconsiderable dynamic complexity and has been used togain insight into the industrial process.

The NO + NH3 reaction follows two predominant path-ways [4]. The first pathway is the complete reduction of NOto N2:

0039-6028/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.susc.2006.11.005

* Corresponding author. Tel.: +1 302 831 6327; fax: +1 302 831 3009.E-mail address: lauterba@udel.edu (J. Lauterbach).

6NO + 4NH3! 5N2 + 6H2O ð1Þ

The second pathway is the partial reduction of NO to N2O,a strong greenhouse gas:

8NO + 2NH3! 5N2O + 3H2O ð2Þ

When carried out over platinum catalysts, this reaction sys-tem gives rise to nonlinear behavior, such as reaction rateoscillations and spatio-temporal pattern formation. Partic-ularly intriguing is the fact that, in contrast to most oscil-lating reactions, nonlinear behavior is seen not only inthe catalyst activity, but also in the selectivity [5,6].

Self-sustained reaction rate oscillations were first re-ported for the reaction of NO and NH3 over a Pt wire atpressures between 0.2 and 1.0 Torr and temperatures be-tween 473 and 1473 K [6]. Oscillations were also reportedat atmospheric pressures and temperatures between 603and 673 K on a polycrystalline platinum foil [7]. Studiesof the same reaction over Pt(100) single crystals at ultrahigh vacuum (UHV) conditions have shown similar behav-ior in which oscillations were observed near 1 · 10�6 Torr

N. McMillan et al. / Surface Science 601 (2007) 772–780 773

and temperatures from 420 to 485 K [5,8]. Spatio-temporalpatterns such as spirals and targets have also been observedon Pt(100) single crystals by photoemission electronmicroscopy (PEEM) at these conditions [9].

Although the mechanism accounting for this behavior isnot fully understood, the adsorbate-induced reconstructionmechanism, previously used successfully to explain non-linear phenomena during CO oxidation on Pt(110) andPt(100) [10,11], has also been proposed as a model forNO reduction by NH3 on Pt(10 0) [12]. The Pt(100) surfaceexhibits a surface reconstruction that can be lifted by NO[13,14], but not by NH3 [15]. At low coverages of NO,the bulk-terminated 1 · 1 surface becomes unstable andreconstructs to the hex phase. The overall production rateof N2 is low on the hex reconstructed surface because thedissociation of NO and NH3 is slow on the hex surfacecompared to the 1 · 1 surface [15,16]. Accumulation ofNO on the hex surface lifts the reconstruction, and thereaction rate on the 1 · 1 surface increases. If a sufficientamount of NO is removed by reaction with NH3, the1 · 1 surface again becomes unstable and reverts to thereconstructed surface, completing the cycle. Repetition ofthis cycle accounts for the observed reaction rate oscilla-tions in this system, and spatio-temporal patterns arisedue to coupling across the surface due to surface diffusion.

Because oxygen is always present in combustion ex-haust, it can also be added to the reactant mixture in orderto approach more realistic conditions. In fact, the ability toreduce NO in the presence of oxygen is a key feature ofSCR [1]. Again, this process occurs primarily by two reac-tions, producing N2 and N2O, respectively:

4NOþ 4NH3 þO2 ! 4N2 þ 6H2O ð3Þ4NOþ 4NH3 þ 3O2 ! 4N2Oþ 6H2O ð4Þ

This paper presents new examples of pattern formationand complex oscillations for the reaction of NO and NH3

on a polycrystalline platinum foil at 1 · 10�4 Torr, withand without O2. Oscillations in the products N2, H2O,and N2O were observed simultaneously with several typesof spatio-temporal patterns. The relationship between pat-tern formation and reaction rate oscillations is discussed.Finally, unlike previous reports of oscillations on Pt(10 0)single crystals, we report the appearance of multimodeoscillations in this system which suggests the importanceof gas phase coupling between different Pt(100) grains onthe polycrystalline surface.

2. Experimental

All experiments were performed in a stainless steel UHVchamber with a base pressure below 1 · 10�9 Torr. Thechamber was continuously pumped by a turbomolecularpump backed by a mechanical pump and was equippedwith an ion gun for sample cleaning. The PEEM (StaibInstruments) was mounted directly onto the chamber andwas differentially pumped by a second turbomolecular

pump. This differential pumping arrangement allowed thePEEM to be operated with pressures in the main chamberas high as �5 · 10�4 Torr. As has been described previ-ously [17–19], PEEM allows the imaging of surfaces basedon local differences in work function. Photoelectrons areexcited from the sample surface by illumination with aUV light source with peak intensity between 200 and240 nm. The photoelectrons are focused through a seriesof electrostatic lenses and projected onto a phosphorousscreen. The image is captured by a CCD camera andrecorded directly to a DVD recorder. The overall reactionrate within the vacuum chamber was monitored by aquadrupole mass spectrometer (Hiden HAL-201) thatwas attached to the back of the PEEM and was also differ-entially pumped.

The sample used for these experiments was a polishedpolycrystalline platinum foil 1 cm on each side and0.5 mm thick. The sample was mounted vertically in frontof the PEEM on a manipulator with an x–y–z translationstage and a feedthrough, rotatable in the horizontal plane.The sample was mounted by means of two 0.5 mm-thickTa wires spot-welded to the back of the sample, allowingthe sample to be resistively heated. A type-K thermocouplewas also spot-welded to the sample in order to monitor thetemperature. Before performing reaction experiments, thesample was cleaned by repeated cycles of annealing at1100 K, oxidizing at 950 K under 1 · 10�6 Torr of O2,and Ar-ion sputtering with 5 lA of sputtering currentand 1 · 10�4 Torr of Ar.

Reactant gases were metered by mass flow controllers(Brooks 5850E) and mixed before being introduced intothe chamber through a leak valve. Pressure in the vacuumchamber was measured by a Pirani gauge and was con-trolled by manual adjustment of the leak valve. The gasesused were NO (Praxair, 99.8%), NH3 (BOC), and O2

(BOC, 99.9999%). The gas inlet system was pumped con-tinuously by a mechanical pump, which maintained thepressure near 1 · 10�1 Torr.

3. Results and discussion

Fig. 1 shows the reaction rate hysteresis curve for thereaction of NO and NH3 on a polycrystalline platinum foilat 3 · 10�4 Torr and NO/NH3 = 0.5. This curve is qualita-tively similar to results previously reported for this reactionon Pt(100) [8]. The shape of the curve can be explained asfollows. At temperatures below 420 K, the surface is pre-dominantly covered by NO and the reaction rate is low.Increasing the temperature above 470 K causes a rapid in-crease in the reaction rate due to the autocatalytic freeingof surface sites upon the dissociation and reaction of NOon the 1 · 1 surface. As the temperature is increased be-yond 570 K, the reaction rate decreases due to thermaldesorption of surface species, and the surface reconstructsto the hex phase due to the low coverage of NO. Whenthe temperature is lowered, the reaction rate increasesdue to increasing coverage, however, the rate remains

Fig. 1. Hysteresis curve for NO + NH3 on platinum foil, measured atconstant gas phase composition (NO/NH3 = 0.5) and constant pressure of1 · 10�4 Torr. The rectangle indicates the range of temperatures whereoscillations and pattern formation are observed.

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lower than that on the heating branch. Between 520 and470 K, the reaction rate abruptly increases resulting in acrossover with the heating branch. A similar crossoverwas previously attributed to the hex to 1 · 1 phase transi-tion on the Pt(10 0) single crystal surface, which is inducedby adsorption of NO to the hex surface [8]. Cooling below470 K returns the surface to a low reactive state in whichthe surface is covered by NO.

In nonlinear systems hysteresis in the reaction rate is of-ten accompanied by oscillations and pattern formation[20]. As in previous studies on Pt(100) single crystals [9],reaction rate oscillations and pattern formation were ob-served on polycrystalline platinum in the hysteretic region,

Fig. 2. Phase diagram for the NO + NH3 system

as indicated in Fig. 1. In the current study, both patternformation and reaction rate oscillations were observed overa wide range of temperature (460–520 K) and gas phasecomposition (NO/NH3 = 0.2–0.8).

Fig. 2 shows the parameter space over which spatio-temporal patterns were observed at a total pressure of1 · 10�4 Torr. Several different types of self-sustainedpatterns were observed in different areas of the pressure–composition parameter space. These patterns were qualita-tively classified as three types: island growth, turbulence,and wavetrains. These three types occurred simultaneouslywith global reaction rate oscillations and this could be seenvisually as a uniform darkening of the surface periodicallyinterrupted the pattern formation. This uniform darkening,followed by the resumption of pattern formation wasobserved for each of the pattern types.

An example of island growth is shown in Fig. 3a. Thedarker, high work function areas indicate a high coverageof NO* and O*, while the lighter, low work function areasare predominantly covered by NH�3 and the products ofammonia decomposition [9]. Initially, several NH3 islandsnucleate on the dark NO-covered surface. The bright is-lands grow until the entire surface is covered by NH3. Thistransformation is quickly followed by a uniform darkeningof the entire surface, indicating the adsorption and dissoci-ation of NO. The entire cycle takes about 30 s. At hightemperatures (>490 K), a small number of NH3 islandsnucleate and these grow to sizes of 50–100 lm during theoscillatory period. At lower temperatures, a much largernumber of NH3 islands nucleate, but they only grow to10–20 lm in diameter, before being extinguished by NOadsorption. This type of island growth gives the surface aspeckled appearance.

A second type of pattern is turbulence, as shown inFig. 3b. This type consists of a series of alternating brightand dark waves with no apparent long-range structure.

on polycrystalline platinum at 1 · 10�4 Torr.

Fig. 3. Several examples of pattern formation. Each image shows patterning on a single grain which is outlined by white lines. (a) Island growth; 512.2 K;1.6 · 10�4 Torr; NO/NH3 = 0.6. (b) Turbulence; 461.3 K; 1.6 · 10�4 Torr; NO/NH3 = 0.6. (c) Bistability; 436.7 K; 1.6 · 10�4 Torr; NO/NH3 = 0.6.

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The turbulent patterns are interrupted periodically by abrightening and then uniform darkening of the entire grain,due to the adsorption of NO. After the surface becomescompletely NO-covered, the turbulent patterns appearagain. Within the turbulence, spatio-temporal structuressimilar to target patterns and spirals can be observed, how-ever these structures are usually unstable and short-lived.In addition, the turbulent patterns give rise to wavetrainsat certain conditions. As in the case of the turbulent pat-terns, the periods of wavetrain propagation are regularlyinterrupted by a uniform brightening and darkening ofthe entire grain.

At temperatures below 460 K, the system exhibits bista-bility, an example of which is shown in Fig. 3c. In the bista-ble region, both the NO and NH3 covered surfaces areobserved, but neither self-sustained pattern formation noroscillations occur.

The pattern formation described above does not involveevery grain on the surface. Instead, only about 20–30% ofthe grains participate. We propose that only the Pt(10 0)grains, or possibly some higher index stepped grains, par-

ticipate in the observed nonlinear behavior on polycrystal-line platinum. This is supported by comparing ourpolycrystalline sample to the results of previous single crys-tal studies that show there is a large (�1 eV) work functiondifference between the NH3-covered and NO-covered sur-faces [9]. Although we cannot report absolute work func-tions directly from PEEM images, the difference inbrightness between two surfaces is proportional to the dif-ference in work function. In Fig. 4, only two of the grainsvisible on the polycrystalline foil exhibit a large change inbrightness between NO- and NH3-covered conditions,and these are exactly the grains that participate in the non-linear behavior in subsequent experiments. In addition,brightness measurements on our polycrystalline sampleunder CO- and O-covered conditions were compared toliterature values of the work functions on various singlecrystal surfaces under these conditions. Again, the grainsthat exhibit nonlinear behavior during the NO + NH3

reaction are consistent with the (100) orientation. Theconclusion that only the (100) grains participate in patternformation is further supported by the fact that neither

Fig. 4. PEEM images of an area of polycrystalline platinum under (a) NO- and (b) NH3-covered conditions. The grains with the largest change inbrightness were determined to be Pt(100) grains. Image diameter = 500 lm.

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pattern formation nor oscillations during the NO + NH3

reaction have been previously reported on platinum singlecrystals other than Pt(100) [1].

Although only certain grains exhibit pattern formation,the active grains are synchronized across the entire poly-crystalline surface. Fig. 5 shows simultaneous averagebrightness measurements for several grains exhibiting pat-tern formation during reaction conditions. The brightnessprofiles show synchronization of the grains in terms of theiroverall period of oscillation which occurs due to couplingthrough the gas phase. Despite the synchronization, thedifferent profiles are not identical. For example, Grains 2and 3 lag Grain 1 by 2–3 s in the transition from the min-imum brightness to maximum. These differences betweengrains are attributed to slight differences in rates of adsorp-tion or reaction due to differences in steps or defect concen-trations from grain to grain.

The changes in local work function observed by PEEMcorrespond to changes in the global reaction rate as mea-sured by mass spectrometry. In contrast to previous obser-vations on Pt(100) at lower pressures [9], reaction rate

Fig. 5. Average brightness of several simultaneously patterning-forminggrains; T = 492 K; P = 1.6 · 10�4 Torr, NO/NH3 = 0.63.

oscillations were observed simultaneously in every case thatpattern formation was observed. Fig. 6 shows the relation-ship between the two phenomena. Pattern formation is rep-resented by the PEEM images and average brightness ofthe patterned grain; the global reaction rate is shown bythe mass spectrometry signals of N2 and H2O. The surfaceof the oscillating Pt(100) grain is brightest when the reac-tion rate is at a minimum. By the surface reconstructionmodel, this state corresponds to an NH3-covered surfacein the hex phase. In about 10 s, the surface darkens, indi-cating the adsorption of NO and the lifting of the recon-struction. This accompanies an increase in the reactionrate due to the increased rate of NO and NH3 decomposi-tion on the 1 · 1 surface. NH3 islands nucleate and grow onthe 1 · 1 surface which slowly reconstructs to the hexphase. Within 25 s, the reaction rate again reaches a mini-mum and the brightness of the grain is at a maximum, indi-cating that the grain is once more predominantly NH3

covered and much of the grain has reconstructed to thehex phase. This sequence is compatible with the adsor-bate-induced reconstruction mechanism utilized in theLFI model [12].

Many of the oscillations observed in these experiments,particularly those in the area of the parameter space (indi-cated in Fig. 2) where turbulent patterns are observed,exhibited complex multi-peaked behavior. Similar oscilla-tions have been reported previously, but the mechanismfor such oscillations has not been fully explained [5]. Asan example of this behavior, consider a series of experi-ments where the sample is cooled from 494 to 472 K at con-stant gas phase composition (NO/NH3 = 0.5) and constantpressure (1.6 · 10�4 Torr). The nitrogen (mass = 28 U)mass spectrometry signals are shown in Fig. 7. At 494 K,the oscillation is single-peaked with a period of 28 s. Thisasymmetric, single-peaked oscillation is typical of the oscil-lations that are observed in the island growth region of theparameter space. Cooling the sample to 492 K, the largeoscillation continues, now with a period of 30.5 s. In addi-tion, a second peak can be easily distinguished with nearlyhalf the amplitude of the large oscillation and a period of15.2 s. In comparison with Fig. 2, the appearance of thehigher frequency oscillation coincides with the appearance

Fig. 6. The correspondence between oscillations and spatio-temporal pattern formation. Data was collected at 1.6 · 10�4 Torr, NO/NH3 = 0.5 and 477 K.The width of each image is 200 lm.

Fig. 7. Development of multipeak oscillations at low pressure. The curvesare mass spectrometry signals for nitrogen. Data was collected at 1.6 ·10�4 Torr, NO/NH3 = 0.5 and (a) 494 K, (b) 492 K, (c) 489 K, (d) 477 K,(e) 472 K.

N. McMillan et al. / Surface Science 601 (2007) 772–780 777

of more complex spatio-temporal patterns such as targetpatterns and wavetrains. Further cooling gives rise to a

three-peaked oscillation at 489 K, where the period of thehigher frequency oscillation is 10.2 s. Cooling further de-stroys the complex oscillations as well as the highly-ordered spatio-temporal patterns. At 477 K, the oscillationis again single peaked with a long, sloping shoulder on thetrailing edge. At 472 K, the oscillation is still single-peaked,similar in appearance to the oscillations observed at 494 K,but now the amplitude of the oscillations is half thatobserved at the higher temperature. The amplitude of theoscillations continues to decrease as the sample is cooledinto the bistable region. This example demonstrates thatincreasing amplitude and complexity of the reaction rateoscillations accompany increasing complexity in the simul-taneous spatio-temporal patterns.

Even more complex oscillations are observed when thecoupling between the surface and the gas phase is increasedby partially closing a gate valve in front of the main turbo-pump, thereby reducing the pumping speed and increasingthe gas phase residence time. Based on the volume of thereactor and the pumping speed of the main turbopump,it is estimated that the residence time of the chamber withthe gate valve fully open is �0.1 s. Partially closing the gatevalve obviously increases the residence time, however it was

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not possible to quantitatively determine the magnitude ofthis change. The total pressure inside the reactor can stillbe adjusted freely by opening or closing the leak valve.Under these conditions of greater coupling through thegas phase, the overall period of oscillations (t1) is signifi-cantly longer than before, typically around 90 s. Multiplehigher frequency oscillations can occur within this period,with the time between small peaks (t2) varying from 6 to14 s. As summarized in Table 1, oscillations containing 3,6, 7, 10, 11, and 14 high frequency peaks have beenobserved within a single period at various conditions oftemperature and pressure. In addition to the temperatureand pressure, two periods (t1 and t2), corresponding tothe periods of the lowest frequency and highest frequencyexcursions, are reported for each oscillation.

A typical example of the complex oscillations is shownin Fig. 8, along with the Fourier spectrum of the nitrogenmass spectrometry signal. The largest peak in the Fourierspectrum (b) corresponds to an oscillation with a periodof 6.6 s. Another peak in the Fourier spectrum (a) repre-sents a period of 94.3 s, and corresponds exactly with thefrequency of the large amplitude excursions. The 13 subse-quent peaks in the Fourier spectrum occur at integer mul-tiples of (a). Oscillations in the N2O signal are also evidentin Fig. 8, which are exactly out of phase with the oscilla-tions in N2 and H2O, in agreement with previous reports[5,6]. In effect, therefore, the system oscillates between astate where selectivity to total reduction is at a maximum

Table 1Summary of observations of complex oscillations

Number of peaks s1 (s) s2 (s) T (K) NO/NH3 P (Torr)

6 79.4 13.2 501.5 0.5 3.50 · 10�4

7 87.7 11.2 506.6 0.5 3.60 · 10�4

10 82.0 8.4 495.9 0.5 3.40 · 10�4

11 73.0 6.8 499.4 0.5 3.50 · 10�4

14 94.3 6.5 500.5 0.5 3.60 · 10�4

14 94.3 6.6 491.6 0.5 3.60 · 10�4

Fig. 8. An example of a 14-peaked complex oscillation. T = 481 K, NO/NHshows a peak corresponding to the low frequency, large amplitude oscillation

(when N2 is a maximum) and a state where selectivity to to-tal reduction is at a minimum (when N2O is a maximum).

The oscillations in selectivity are qualitatively consistentwith the surface reconstruction model for the reactionalready discussed. Previous studies have suggested thatthe formation of N2O is favored by a shortage of vacancieson the surface [1,21]. On the high reactive (1 · 1) surface,dissociation of NO and the reaction to form N2 proceedseasily. The fast desorption of N2 creates additional vacantsites that further accelerate the dissociation of NO. Whenthe surface reconstructs to the hex phase, dissociation ofNO no longer occurs and the reaction to form N2O isfavored. It has been proposed previously that intentionaloperation of certain reactions in an oscillatory regimecould be used to improve selectivity beyond that achievableby steady state operation [22,23], however, we have foundno evidence of an overall improvement in selectivity duringoscillatory behavior in the NO + NH3 system.

The multi-peaked oscillations similar to those shown inFig. 8 have also been observed to evolve into chaotic oscil-lations. The Fourier spectrum of the N2 signal during oneexample of chaotic oscillations is shown in Fig. 9, whereseveral frequencies are distinguishable. The highest fre-quency peaks appear in a band centered near 0.15 Hz, cor-responding to a period near 6.9 s. The width of this bandreflects the chaotic character of the high frequency oscilla-tions. Unlike this high frequency band, the lowest fre-quency peak in the spectrum (0.013 Hz) is very sharp andcorresponds to an oscillation with a period of 75 s. Thesharpness of this peak and its two harmonics suggest thatthe corresponding oscillation is not related to period dou-blings of the higher frequency processes and instead arisesfrom some independent mechanism that has not been pre-viously explained.

The effect of oxygen on the reaction dynamics providesadditional insight into the nature of the multimode oscilla-tions. Initially, multimode oscillations are observed duringthe reaction of NO and NH3 as shown in Fig. 10. The

3 = 0.5, P = 3.6 · 10�4 Torr. The Fourier spectrum of the mass 28 signals (a) as well as 13 higher frequency peaks.

Fig. 10. The effect of oxygen on the system is demonstrated by a series ofexperiments at 500 K and 3.5 · 10�4 Torr. Curve (a) is the initial six-peaked oscillation with NO/NH3 = 1/2. In (b), O2 is added to the gasphase such that NO/NH3/O2 = 3/6/1. In (c), the O2 is turned off and themulitpeak oscillations return.

Fig. 9. Fourier spectrum of the nitrogen signal during chaotic oscillationsoccurring at 500 K, NO/NH3 = 0.5, and 3.5 · 10�4 Torr.

N. McMillan et al. / Surface Science 601 (2007) 772–780 779

oscillations are six-peaked with periods of 82 s and 14 s.These oscillations are accompanied by wavetrain patterns.When 10% O2 is added to the gas phase, the oscillations inthe NO + NH3 reaction continue, but the oscillationschange in character. Although the low frequency oscilla-tions continue unchanged, the higher frequency oscillationsare almost completely destroyed. Removing oxygen fromthe system causes the fine structure oscillations to reappear.The fact that the addition of oxygen has a disproportionateeffect on the high frequency oscillations again suggests thatthe low and high frequency oscillations arise from differentmechanisms. The specific role of oxygen in the reactionmechanism remains unclear, however recent work suggests

that oxygen may facilitate hydrogen abstraction from ad-sorbed NH3 [24].

While the surface reconstruction model does provide amechanism for simple, single-peaked oscillations, it cannoteasily account for the appearance of the multimodal oscil-lations of the type described above. Multimodal oscilla-tions can arise due to the simultaneous influence of morethan one oscillatory mechanism. For example, multimodaloscillations observed during CO oxidation on Pt(110) wereattributed to the surface reconstruction mechanism cou-pled with strong surface faceting [10]. Other than thesurface reconstruction mechanism, a vacancy model hasbeen proposed by Nieuwenhuys as a possible oscillatorymechanism for NO + NH3 on Pt(100) [5]. In this model,changes in the number of surface vacancies provide thenecessary feedback to produce oscillations. A shortage offree surface sites is assumed to inhibit the dissociation ofNO*. If sites become free due to desorption and NO disso-ciation can occur, the succeeding steps in the reactionmechanism establish an autocatalytic feedback loop inwhich the reaction increases the number of free surfacesites. The greater number of free surface sites favors thedissociation of NO, which, in turn further increases thenumber of free surface sites. This autocatalytic mechanismeventually depletes the surface of reactants, leading to a de-crease in reaction rate that allows the surface to be replen-ished. Coupling the surface reconstruction mechanism withthe vacancy mechanism could give rise to multimodaloscillations, but such oscillations have not been observedon Pt(100) single crystal samples. Nieuwenhuys did ob-serve double and triple-peaked oscillations, but this onlyoccurred after the addition of oxygen to the system [5].

Another feasible explanation for multimode oscillationsis the coupling of more than one nonlinear oscillator [20].This is a good description of a polycrystalline platinumfoil, because each oscillating (100) type grain can be con-sidered a separate oscillator and each of these oscillatorsis coupled only through the gas phase. Slight differencesin adsorption probabilities or surface reaction rates fromgrain to grain could account for the multimodal behavior.This could explain the fact that multimode oscillations areobserved on polycrystalline platinum, but not on platinumsingle crystals. It is significant that the complexity of mul-timode oscillations increases greatly as the pumping speedof the system is reduced, therefore increasing the gas phasecoupling between various grains of the surface.

This coupling through the gas phase occurs due to thefinite residence time of molecules in the chamber and theexchange of molecules between the gas phase and the oscil-lating surface. The oscillating surface reaction rate causestemporal changes in the gas phase composition which inturn affect the rates of adsorption of species onto the cata-lyst surface. This feedback results in coupling between thegas phase composition and the reaction rate oscillations ofeach grain on the polycrystalline surface. The strength ofthis coupling is a function of operating conditions, inparticular the residence time of gas phase molecules in

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the vaccum chamber. Although the effect of the reactionrate on the gas phase composition is relatively small at fastpumping speeds, as the pumping speed is reduced and thegas phase residence time increases, the surface reaction ratehas a larger effect on the gas phase composition and thecoupling through the gas phase is greater.

4. Conclusion

The results presented have shown the appearance ofoscillations and, for the first time, spatio-temporal patternson polycrystalline platinum during the NO + NH3 reac-tion. The nonlinear behavior on polycrystalline platinumwas found to be due primarily to the effect of (100) typegrains, which are synchronized across the sample surfacedue to coupling through the gas phase. The appearanceof a wide variety of multimode oscillations was observed,particularly with increased surface/gas phase coupling.While the surface reconstruction model alone is sufficientto explain simple oscillations in this reaction system, a sin-gle mechanism cannot account for these complex oscilla-tions. Our results indicate that coupling through the gasphase of independently oscillating Pt(100) grains is themost likely explanation for this complex behavior.

Acknowledgement

Acknowledgment is made to the donors of the AmericanChemical Society Petroleum Research Fund for support ofthis research.

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