Is there a single mechanism of catalytic rate oscillations on Pt?

Surface Science 183 (1987) L269-L278 North-Holland, Amsterdam

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SURFACE SCIENCE LETTERS IS T H E R E A S I N G L E M E C H A N I S M O F CATALYTIC RATE O S C I L L A T I O N S O N Pt? * S.B. S C H W A R T Z * * and L.D. S C H M I D T University of Minnesota, Minneapolis, MN 55455, USA Received 15 August 1986; accepted for publication 13 January 1987

Experiments are summarized which show that the N O + C O reaction on clean Pt(100) or on clean polycrystalline Pt can exhibit oscillations at all pressures between 10 - s and 1 Torr and that this is caused by the hex ~ 1 × 1 surface phase transition. It is suggested that m a n y of the observed examples of rate oscillations on Pt surfaces m a y arise from the adsorbate induced 1 × 1 ~ hex surface phase transition on Pt(100). This mechanism m a y also be operative on polycrystalline wires and foils and on supported Pt particles because in m a n y reactive gases all crystallographic orientations tend to facet into predominantly (100) crystal planes. Varying a m o u n t s of (100) facet formation m a y explain the lack of reproducibility of oscillations on all but Pt(100) at low pressures. Effects of reactor pressure and spatial variations are also discussed.

Most bimolecular reactions on Pt can probably be observed to exhibit oscillations under some conditions [1-14]. Table 1 lists some of the c o m m o n situations where oscillations have been observed repeatedly. They range from Pt(100) single crystals, to polycrystalline wires and foils to 100 A diameter Pt particles supported on A120 3 or SiO 2. Oscillations have been observed at pressures between 1 atmosphere and 10 - s Torr in situations from packed bed reactors to ultrahigh vacuum chambers. Temperature variations during oscillations between 100 K to less than 1 K have been noted. In contrast, oscillations have been more rarely observed on surfaces other than Pt. CO and H 2 oxidation on Ni, Pd, Rh, and Ir foils have been reported to oscillate near atmospheric pressure, but these systems are less reproducible [1-3]. There is thus a bewildering variety of systems and conditions which exhibit oscillations, and an almost equal number of models has been proposed to explain them [.1,2]. Several generic models of surface nonlinearities have been proposed to explain oscillations: The most common has been a surface Pt oxide [3-5] on which the activity is different than on the metal, although there is little evidence for surface oxide formation on Pt under these conditions except in the presence of Ca or Si contamination. Attractive a d s o r b a t e - a d * This work is partially supported by N S F under Grant No. DMR82126729. * * Current address: Sherwin Williams, Chicago, IL, USA.

0039-6028/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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S.B. Schwartz, LD. Schmidt / Catalytic rate oscillations on Pt

Table 1 Reaction

Surface

Pressure range

Ref.

CO + 02 ~ CO 2

Polycrystalline foil 1% Pt on A1203 Pt(100) Wire or foil Pt(100) Wire or foil Wire or foil Wire or foil

1 atm 1 atm - 10 5 Tort - 1 Torr - 10 -4 Torr 10-6-10 -8 Torr 1-10 2 Torr 1 atm 1 atm

[1-5] [1,2,6] [7,8] [9] [9] Present results [11] [1,2] [12]

Wire or foil

1 atm

[13,14]

NO + CO ---,N 2 + CO 2 NO+NH 3 ~ N 2 + H20 H 2 +02 ~ H20 NH 3 + 02 ~ N 2 + H20 Hydrocarbon + 02 ---,CO 2 + H 20

s o r b a t e i n t e r a c t i o n s have also b e e n p r o p o s e d [2], a l t h o u g h these p r e d i c t the p o s s i b i l i t y of an a d s o r b a t e p h a s e t r a n s i t i o n which c o u l d e l i m i n a t e oscillations [15]. Thus, n o consensus seems to have e m e r g e d as to the cause o r causes of these observations. W h i l e t e m p e r a t u r e a n d pressure v a r i a t i o n s are frequently involved, it is clear that there m u s t b e a s t r o n g n o n l i n e a r i t y in the r e a c t i o n kinetics w h i c h p r o v i d e s the d r i v i n g force for oscillations. W e suggest t h a t m a n y of these o b s e r v a t i o n s m a y have a c o m m o n n o n l i n e a r ity in the hex ~ 1 x 1 surface p h a s e t r a n s i t i o n on Pt(100). This, at first thought, seems i m p l a u s i b l e b e c a u s e of the variety of r e a c t i o n s a n d species involved a n d b e c a u s e oscillations also occur on p o l y c r y s t a l l i n e a n d on supp o r t e d Pt. W e shall show how the s a m e m e c h a n i s m m a y still b e responsible. C l e a n Pt(100) exhibits a h e x a g o n a l surface m e t a l a t o m layer o n t o p of the s q u a r e close p a c k e d b u l k structure u p to - 900 K [16]. H o w e v e r , a d s o r b a t e s such as CO, N O , N, H, a n d O destabilize the hex structure a n d p r o d u c e a reversion o f the surface Pt a t o m s to the 1 × 1 structure [16-19]. A d s o r p t i o n is m u c h s t r o n g e r o n this surface, a n d sticking coefficients are larger. Small coverages of a n y of these a d s o r b a t e s ( t y p i c a l l y - 0.05 m o n o l a y e r ) i n d u c e the hex ~ 1 × 1 t r a n s i t i o n r a p i d l y even b e l o w 300 K, b u t if the a d s o r b a t e s are r e m o v e d b y t i t r a t i o n with a n o t h e r gas (e.g. r e m o v a l of N O b y e x p o s u r e to H 2 ) [17], the 1 x 1 structure is stable i n d e f i n i t e l y o n the clean surface b e l o w 400 K. B e t w e e n 400 a n d 500 K the clean 1 × 1 surface reverts slowly to the stable hex structure. T h e two p h a s e s can coexist on Pt(100), a n d the kinetics a n d structures of these t r a n s f o r m a t i o n s have b e e n t h o r o u g h l y investigated [16-19]. Ertl a n d c o w o r k e r s [6,7] showed t h a t C O o x i d a t i o n 'exhibits oscillations o n Pt(100) at pressures of - 10 -5 Torr. By s c a n n i n g across the surface with w o r k f u n c t i o n a n d L E E D m e a s u r e m e n t s they were a b l e to show that the 1 × 1 surface regions c o i n c i d e d with the high rate p a r t of oscillation cycles while the hex surface c o i n c i d e d w i t h the low rate. T h e y were also able to show that

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u n d e r s o m e c o n d i t i o n s travelling waves of each t y p e of surface structure m o v e d over the crystal surface. T h e y w e r e able to s i m u l a t e oscillations q u a n t i t a t i v e l y using r e a s o n a b l e values of rate p a r a m e t e r s for this system. Lesley a n d S c h m i d t [20] o b s e r v e d strong nonlinearities in the N O + C O a n d N O + H 2 r e a c t i o n s on Pt(100). T h e r e a c t i o n was a u t o c a t a l y t i c in that no r e a c t i o n o c c u r r e d b e l o w 400 K, b u t the r e a c t i o n was o b s e r v e d to occur at a precise t e m p e r a t u r e in T P D (412 + 2 K for N O + C O a n d 405 _+ 2 K for N O + H 2 ) which was n e a r l y i n d e p e n d e n t of a d s o r b a t e coverage a n d c o m p o s i tion. W e have r e c e n t l y o b s e r v e d r e p r o d u c i b l e oscillations in the r e a c t i o n N O + C O ~ ½N 2 + C O z o n Pt(100) at pressures f r o m b e l o w 10 -7 to a b o v e 10 - 6 T o r r [21]. These o c c u r for PNo/Pco b e t w e e n 1 a n d 2 at t e m p e r a t u r e s b e t w e e n 400 a n d 460 K. Oscillations were also n o t e d in this r e a c t i o n on Pt(100) b y - S i n g h - B o p a r a i a n d K i n g [22] in a low p r e s s u r e m o l e c u l a r b e a m e x p e r i m e n t a n d o n p o l y c r y s t a l l i n e Pt b y A d l o c h , L i n t z a n d W e l s k e r [10] ( - 1 0 - 4 Torr) a n d b y K l e i n a n d

(0)

%A f

I

lmin

VV (b)

lOmin

i 0.4 Torr ~740 K

4 1.7 x 10-4 Torr 455 K

rR (

C

~

4251x 10-6K Torr

I

10 rain

I

2X 10-7Torr 425 K

(e)

t

~5x10 -9Torr 300 K

TIME Fig. 1. Plot of rate of N 2 formation versus time from the NO + CO reaction on Pt(100) at 2 × 10- 7 tO 10 - 9 Tort and on polycrystaUine Pt foils at 1 and 10 - 4 Torr. Curve (a) from ref. [9], (b) from ref. [10], (c) and (d) present results and ref. [21], (e) from ref. [22].

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S.B. Schwartz, L D . Schmidt / Catalytic rate oscillations on Pt

ld 5

i

i

i

i

i

i

i

f

i

i

i

PNO 1.1 -I ~ ~

p

hex

lo ~4

,

CO

= 1.5

5 x 10-6 Torr

/

1013

ld z

J

I

300

i

i

I

~

i

i

500

I

i

700

i

i

I

,00

T(K) Fig. 2. Steady state rate of N 2 production on Pt(100) at 3)<10 -6 Torr (upper curves) and at 1.8 x 10 -7 Tort (lower curves) for P r ~ o / P c o = 1.5. Two steady state branches are observed which are reversible until the temperatures indicated by the arrows. These branches correspond to the hex and the 1 × 1 surface Pt structures respectively.

Schmidt [9] ( - 1 Torr). Typical oscillation patterns observed in these experiments are shown in fig. 1. Fig. 2 shows a plot of steady state rate of N 2 formation versus temperature at two pressures. The surface in these experiments was a 1 cm z diameter disk of Pt oriented and polished to within 1 ° of (100). The crystal was mounted in a 50 liter U H V system equipped with AES, LEED, and a mass spectrometer. For detection of reaction products from the surface the poppet valve separating the chamber from the ion pump as d o s e d to obtain ~- --- 3 s. Gases were admitted to the system through leak valves to obtain the desired pressure. Rates were calculated from the mixed reactor equation as described elsewhere [9,23]. As the temperature is increased (fig. 2), reaction is first detectable at - 400 K, rises rapidly to - 500 K and then levels off at - 550 K. This upper branch is reversible in that the curve can be traced repeatedly by increasing or decreasing temperatures below 550 K. However, above ~ 550 K the rate is observed to drop discontinuously by a factor of - 3. It then traces the lower

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branch shown in fig. 2 for increasing and decreasing temperatures above - 500 K. Both branches can be retraced repeatedly as long as maximum and minimum temperatures are not exceeded. Above 550 K the upper branch is metastable in that the rate remains on that branch for times from seconds to minutes depending on temperature, and below - 500 K the lower branch is also metastable as indicated by the dashed lines in fig. 2. Oscillations were always observed by increasing the temperature above 500 K and then lowering it to 410-430 K. The reaction jumps to the upper branch and then "falls off" this curve as it tries to return to the lower branch. As shown in fig. 1, this produces large negative spikes followed by a positive overshoot with periods between 30 s and several minutes. We shall describe these results in detail later [21]. The data of figs. 1 and 2 show that the rate exhibits two stable steady state branches and that oscillations occur upon cooling to produce the transition to the upper branch. AES was used to measure C, O, N and contaminant coverages during reaction. This showed that the surface was saturated with CO belQw - 410 K and that in the upper branch the coverage falls to a few percent of a monolayer by 550 K. AES showed that the surface contained no more than a few percent of a monolayer at any temperature on thb lower branch. AES during oscillations also showed that the surface remained nearly clean (less than a few percent of saturation of any species) during the oscillations. LEED showed that the upper branch exhibits the (1 × 1) adsorbate covered structure, while the lower branch exhibits the hex structure [20]. LEED and AES measurements during oscillations were difficult to obtain quantitatively because of contamination from the electron beams at the small pumping speed necessary to measure CO 2 pressures. A small fraction of a monolayer of a species such as carbon totally suppresses the hex structure and oscillations. Oscillations damped out spontaneously in times from several minutes to several hours because of contamination. We conclude from these experiments that oscillations in this reaction are associated with the 1 × 1 ~ hex surface phase transition on Pt(100), analogous to those observed by Ertl and coworkers [7,8] in CO + 02 on this surface at higher pressures. It remains to be demonstrated how polycrystalline and supported Pt could exhibit oscillations if they are associated exclusively with properties of the (100) plane. In this section we argue that the (100) plane should be expected to predominate on Pt surfaces after extensive heat treatment in reactive gases. Wires and foils facet extensively upon heating in gases at pressures near one atmosphere [24-26]. These processes require heating for hours to clays in single gases such as 02 , but in reacting mixtures the process is accelerated and is evident within minutes after heating a smooth, freshly prepared wire or foil. Examination of heated foils showed that a common facet pattern exhibits facet

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S.B. Schwartz, L D . Schmidt / Catalytic rate oscillations on Pt

Fig. 3. (a) SEM micrograph of a single crystal Pt sphere following exposure to a N H 3 + 0 2 mixture at 1100 K for 5 h. The entire surface consists of regions and facets with orientations near (100). (b) T E M micrograph of - 200 A diameter Pt particles on planar amorphous SiO 2 after growth from a 10 ~, thick Pt film at 6 0 0 ° C in H 2 at 1 atm. All particles form cubes which shows that the entire Pt surface consists of (100) planes.

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planes at 90 o to each other to yield predominantly (100) crystal planes. Several years ago we quantified the facetting which occurs in reacting gases by examining Pt spheres after reaction in N H 3 oxidation, NH3 decomposition, hydrocarbon oxidation, and CO oxidation [24]. Fig. 3a shows a typical SEM micrograph of a surface after N H 3 oxidation for several hours at atmospheric pressure. The largest facets are within a few degrees of (100), and the entire surface consists of (100) and (111) facets. From the micrograph we estimate that (111) facets comprise no more than 25% of the total area of the sphere and that most of the surface consists of planes within - 5 o of (100). Similar results were observed with most gaseous reactions, and recent results using electrochemical etching [27] of Pt spheres in CI-, C N - , and SO42- electrolytes also show predominantly cubic shapes with (100) facets. We recently used TEM to determine shapes of 20 to 200 A crystallites of Pt [28] on planar amorphous SiO 2 and A1203. Upon converting a Pt film into particles by heating at 900 K, every particle forms either a cube or a rectangle as shown in fig.3b. These results show that small particles can form predominantly (> 90%) (100) crystal planes and that the (100) plane is perpendicular to the SiO 2 surface. Essentially identical results were obtained for Pt on "y-A1203 [28]. From additional micrographs in the references cited and from additional studies of facetting in high temperature environments [25], we conclude that the (100) crystal plane of Pt appears to be the most stable one in many situations on foils (1 mm ~ains), on spheres ( - 0 . 1 mm diameter) and on supported particles ( - 100 A diameter). This plane of Pt appears to form more often than any other in most gas or liquid environment. This strongly suggests that Pt(100) is in fact the thermodynamically stable surface under these conditions, and we have speculated that it may be associated with the very low surface free energy [28] of the hex (100) plane in high temperature, low adsorbate coverage environments. From the above we conclude that (1) oscillations are observed reproducibly and strongly on clean Pt(100) for two reactions involving CO, NO, and 02 and (2) extensive treatment of polycrystalline and supported Pt in gases frequently produces predominantly (100) planes. This suggests that all Pt surfaces which exhibit rate oscillations may in fact be predominantly (100) and that the (hex) ---, (1 × 1) surface phase transition may be nonlinearity responsible. A major problem in explaining all oscillation phenomena on surfaces is that of synchrony across the catalytic surface. Even on a single crystal, there will be small temperature variations which should alter the period of oscillations locally, and phases of cycles should be random. On a polycrystalline surface all grains will not be (100) and therefore should not oscillate spontaneously. On supported Pt each particle should have a different oscillation pattern because of geometrical and environment differences.

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Some mechanism is therefore required to synchronize oscillations over each region, crystal grain, or particle in order for oscillations to be observed on all three types of surfaces, because, in the absence of synchrony, each region, grain, or particle would oscillate independently to produce only multiple period low amplitude oscillations in reactor partial pressure which would probably appear as small fluctuations in rate or surface temperature when averaged over the entire surface. Synchronizing influences could be surface diffusion, surface temperature variations, and partial pressure variations. Surface diffusion appears totally incapable of synchronizing coverages over any macroscopic surface. Measured surface diffusion coefficients suggest that communication even o v e r 10 -4 cm distances would be difficult in most experimental situations. Surface temperature communication may be quite strong in oscillations on wires and filaments because these are usually operated adiabatically, reaction heat is large, and thermal conductivities are large. This mechanism is less plausible for supported catalysts because the thermal conductivities of porous oxides are quite low, although at low frequencies gas conduction may be sufficient for temperature synchrony. Temperature communications is impossible for single crystal surfaces at low pressure because heat generation rates are very low and these surfaces are nearly isothermal. The dominant synchronizing force in most experiments appears to be the variation in partial pressure which occurs during the period of oscillation. In most experiments this is at least a factor of two in the limiting reactant and in experiments on high area catalysts the reactor conversion frequently exceeds 90% in the active part of an oscillation cycle [1,2]. This mechanism appears to be the only one capable of synchronizing the entire surface of single crystals because temperature changes are small and oscillatory behavior is insensitive to small temperature perturbations. The overall picture we propose is that of all adsorbate species (CO, NO, O, H, N . . . . ) producing the active (1 x 1) surface on Pt(100) when their coverages exceed a small value, perhaps - 0.05 monolayer. Bimolecular reaction removes reactant species such that, at appropriate temperature, pressure, and gas composition, the total coverage falls to less than the 0.05 monolayers needed to stabilize the 1 x 1, and the surface therefore switches to the hex structure. This structure exhibits lower sticking coefficients and higher desorption rates, and the surface remains clean until the reactant pressures rise or temperatures fall enough to again produce the reactive 1 x 1 surface. In some situations the phase transition will produce steady state multiplicity as shown in fig. 2. At the pressures used in those experiments two steady states could be obtained over approximately a 100 K temperature interval. At appropriate temperature, pressure and compositions spontaneous switching occurs between the two states even at constant temperature to yield rate oscillations.

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A detailed model [1-5,8,15] of these processes will be exceedingly difficult to formulate and analyze, although Ertl and coworkers [7,8] were able to model qualitatively the oscillations they observed in C O oxidation. While m a n y of the adsorption and desorption parameters are k n o w n for each of these species, their influences on the surface phase transition are difficult to quantify because they occur at very low coverages and because the phase transition p r o b a b l y occurs as small patches or islands rather than being u n i f o r m over the surface. Also, while temperature variations can be made small in some situations, m a n y geometries such as supported catalysts require adiabatic operation. Temperature variations therefore provide a positive feedback which can destabilize steady states which might be stable in truly isothermal situations. The final p r o b l e m in modelling is spatial variations [8,15]. If synchrony is sufficiently strong, the systems might be describable through ordinary differential equations, although, in general, partial differential equations would be appropriate. The assumption of spatial uniformity is difficult to justify in a n y particular situation, because coupling between regions is a statistical problem of site distributions and geometries which should exhibit nucleation thresholds. Other mechanisms involving surface species such as carbon and impurities and their oxides m a y be operative in oscillations on Pt at high pressures and u n d e r ill-defined conditions. However, the frequency of observation of oscillations on Pt and its relative scarcity on other metals suggests that this c o m m o n mechanism m a y be operative in m a n y situations. While other metal surfaces also exhibit surface phase transitions induced by adsorbates, these seldom appear to occur on the most stable crystal planes.

References [1] [2] [3] [4] [5] [6] [7] [8]

M. Sheintuch and R.A. Schmitz, Catalysis Rev.-Sci. Eng. 15 (1977) 107. M.G. Slin'ko and M.M. Slin'ko, Catalysis Rev.-Sci. Eng. 17 (1978) 119. J.E. Turner, B.C. Sales, and M.B. Maple, Surface Sci. 103 (1981) 54; 114 (1982) 381. R.C. Yeates, J.E. Turner, A.G. Gellman and G.A. Somorjai, Surface Sci. 149 (1985) 175. V.A. Burrows, S. Sundaresan, Y.J. Chabal and S.B. Christman, Surface Sci. 160 (1985) 122. D.J. Kaul and E.E. Wolf, J. Catalysis 91 (1985) 216. G. Ertl, P.R. Norton and J. Rustig, Phys. Rev. Letters 49 (1982) 177. M.P. Cox, G. Ertl, R. Imbihl and J. Rustig, Surface Sci. 134 (1983) L517; J. Chem. Phys. 83 (1985) 1578. [9] R.L. Klein, PhD Thesis, University of Minnesota (1985); R.L. Klein and L.D. Schrnidt, J. Phys. Chem. 89 (1985) 4908; and to be published. [10] W.A. Adloch, J.G. Lintz and T. Weisker, Surface Sci. 103 (1981) 576. [11] C.G. Takoudis and L.D. Schmidt, J. Phys. Chem. 87 (1983) 958, 965. [12] M.F. Stephanopoulos, L.D. Schmidt and R. Caretta, J. Catalysis 64 (1980) 301. [13] I.L. Tsitovskaya, O.V. Altshuler and O.V. Crylov, Dokl. Akad. Nauk SSSR 212 (1973) 1400. [14] M.B. Cutlip and C.N. Kenney, Chem. Reaction Eng., ACS Symp. Ser. 65 (1978) 475.

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I.G. Kevrekidis, L.D. Schmidt and R. Aris, Surface Sci. 137 (1984) 151. P. Heilmann, K. Heinz and K. Mtiller, Surface Sci. 83 (1979) 487. H.P. Bonzel and G. Pirug, Surface Sci. 62 (1977) 45. P.A. Thiel, R.J. Behm, P.R. Norton and G. Ertl, J. Chem. Phys. 78 (1983) 7448; Surface Sci. 121 (1982) L553. M.A. Barteau, E.I. Ko and R.J. Madix, Surface Sci. 102 (1981) 99. M.W. Lesley and L.D. Schmidt, Surface Sci. 155 (1985) 215. S.B. Schwartz, PhD Thesis, University of Minnesota (1986), and to be published. S.P. Singh-Boparai and D.A. King, in: Proc. IVC-8, ICSS-4, ECOSS-3, Cannes, 1980. Le Vide, Les Couches Minces 201 (Suppl.) (1980). S.B. Schwartz, L.D. Schmidt and G.B. Fisher, J. Phys. Chem., to be published. M.F. Stephanopoulos, S. Wong and L.D. Schmidt, J. Catalysis 49 (1977) 51. M.F. Stephanopoulos and L.D. Schmidt, Progr. Surface Sci. 9 (1979) 83. R. McCabe, T. Pignet and L.D. Schmidt, J. Catalysis 32 (1974) 114. R. Caracciolo and L.D. Schmidt, J. Electrochem. Soc. 130 (1983) 603. T. Wang and L.D. Schmidt, Surface Sci. 163 (1985) 181.