Catalytic oxidation of CO on Pt(111): The influence of surface defects and composition on the reaction dynamics

505

Surface Science 138 (1984) 505-523 North-Holland, Amsterdam

CATALYTIC OXIDATION OF CO ON Pt(ll1): THE INFLUENCE SURFACE DEFECTS AND COMPOSITION ON THE REACTION DYNAMICS J. SEGNER

*, C.T. CAMPBELL

Instiiut /iir Physikalische Received

Chemie,

14 July 1983; accepted

**, G. DOYEN

Uniuersrriit Miinchen, for publication

OF

and G. ERTL

D - 8000 Miinchen 2, Fed. Rep. of Germany

1 November

1983

Angular distributions of CO,, I(y,), formed by catalytic reaction between CO and 0, on a stepped Pt(ll1) surface, have been measured by means of molecular beam techniques as a function The of CO and 0 coverages, in the presence of “subsurface oxide”, and at varying temperatures. resulting data can well be fitted by I(y,) = a cos y, +(la) cos’y, with a being an adjustable parameter which varies with the surface conditions. The cos yr part is ascribed to particles which were completely accommodated with their translational energy to the surface temperature before leaving the surface, while the cos’y, channel corresponds to molecules carrying excess translational energy from crossing the activation barrier of the surface reaction. The fraction of thermally accommodated particles increases by the presence of steps or “subsurface oxide”, and decreases with 0 and CO coverage as well as with increasing temperature. The effect of coverage can qualitatively be attributed to the varying participation of defect sites in product formation, while the influence of surface temperature is well reproduced in the framework of a general theoretical model. Scattering experiments with CO, revealed that the trapping probability for this molecule (in the temperature range interesting here) is of the order of 0.5, thus confirming the conclusion of incomplete thermal accommodation of a CO, molecule interacting with a Pt(ll1) surface.

1. Introduction In a chemical reaction, transformation of energy occurs through elementary excitations of the reacting systems. With gas phase reactions, these involve variations of the translational energy and of the populations of quantized internal (i.e. rotational, vibrational and electronic) levels which can most directly be studied by crossed-beam experiments. In the case of gas/surface reactions an additional complicating factor arises from the existence of quasicontinuous phononic (and electronic) excitations of the solid whose participation can so far only be treated as a more or less efficient heat bath and which * Present address: Department ** Present address: Chemistry Mexico 87545, USA.

of Chemistry, University of Toronto, Toronto, Ontario, Canada. Division, Los Alamos National Laboratory, Los Alamos. New

0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

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are not generally accessible to direct experimental analysis - the predominant source of information is offered by the gas particles coming off the surface. From this point of view, systems in which the reaction products are not in thermal equilibrium with the solid are of particular interest. Experiments of this type have been performed with associative hydrogen desorption from metal surfaces [1,2] where the measured angular and velocity distributions indicated incomplete accommodation of the translational energy. These effects were attributed to the existence of a particular activation barrier for the surface recombination of adsorbed hydrogen atoms, which for example leads to a deviation of the angular distribution from the cosine (Knudsen) law for complete thermal accommodation [3]. Measurements on the associative desorption of N, from Fe concentrated on the internal (vibrational) energy which was found to be considerably higher than the thermal energy of the solid, which effect was likewise attributed to the existence of an activation barrier [4]. Extensive studies in this respect have concered the catalytic oxidation of carbon monoxide, CO + l/20, + CO,, on Pt surfaces. It was found that the CO, molecules formed leave the surface with angular distributions which are strongly peaked in the direction of the surface normal [5-81, whereby the detailed shape of the angular distribution was influenced by the microscopic roughness of the surface [5]. Time-of-flight measurements with a polycrystalline Pt foil [9] as well as with a Pt(ll1) single crystal surface [lo] revealed that CO, molecules detected along the direction of the surface normal were translationally “hot” as compared with the value 2kT, which would correspond to thermal equilibrium with the solid at temperature T,. In addition, excess energy was found also to be stored in the vibrational [11,12] as well as rotational degrees of freedom [ll]. Catalytic oxidation of CO on Pt surfaces has already been extensively studied [6,13], and the mechanism of this reaction appears to be well established. Product formation occurs by a Langmuir-Hinshelwood reaction between adsorbed 0 atoms and adsorbed CO molecules. (For T < 200 K, a different reaction channel via adsorbed oxygen molecules becomes accessible [8], which is of no importance in the present context.) In previous publications we already reported on the determination of the kinetic parameters of the surface reaction [6] as well as of the adsorption and desorption of the reacting molecules [14,15] at a (stepped) Pt(ll1) surface by using molecular beam techniques. The present investigation concentrated on the influence of surface defects and of the concentrations of adsorbed reactants (including the non-reactive “subsurface oxide”) on the degree of accommodation of the produced CO, molecules. This information was obtained by measuring the angular distributions of the formed CO,. According to a recent theory [16] deviations from the simple cosine law can directly be related with the mean translational energy perpendicular to the surface, (E, ). These data were supplemented by

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measurements of angular distributions of CO, molecular beams scattered at the surface. The results demonstrate the very sensitive influence of the surface properties on the degree of translational energy transfer.

2. Experimental The experiments were performed with a molecular beam apparatus which has been described in detail elsewhere [17]. In brief, it consists of a 3-stage, differentially pumped beam source and of an UHV chamber. The nozzle beam is collimated by several apertures before entering the UHV chamber and can also be modulated by a chopper. The diameter of the final aperture can be varied between 0.5 and 6 mm which then controls the beam diameter at the sample surface. For the experiments concerning CO oxidation, a wide beam diameter was used in order to obtain uniform intensity over the whole surface; scattering of CO, was performed with a narrow beam in order to improve the angular resolution. The effective beam pressure at the surface is of the order of 10e6 Torr, while the background pressure remains well below lo-” Torr. Cooling of the double-wall chamber by liquid nitrogen proved to be particularly effective in suppressing the CO, background pressure. The quadrupole mass spectrometer has a 1 mm wide slit aperture at a distance of 4 cm from the sample and can be rotated in the scattering plane (as defined by the molecular beam orientation and the surface normal). Angles of incidence, yi, and angles of detection, y,, refer to the surface normal. The angular resolution varied between 8.5” (if the beam diameter is equal to the diameter of the crystal, 6 mm) and 2.9’ (beam diameter 1 mm). Preparation and cleaning of the Pt surface has been described elsewhere [ 141. The surface structure was analyzed by means of LEED, He scattering and by UPS of adsorbed Xe (PAX) [18]. The LEED pattern exhibited sharp splitted spots of a stepped Pt(ll1) surface with low background intensity. Analysis of the spot splitting revealed the existence of about 5% monoatomic steps with (111) orientation almost perpendicular to the scattering plane; the short-hand notation for the surface is therefore Pt[(lll) x 20(111)]. The cleanliness of the surface was probed by AES, He reflection as well as through the CO, product angular distribution measurements. As will be discussed in detail later, this latter property will be sensitively affected by the presence of the so called Pt subsurface oxide. Complete removal of this phase could be achieved by heating to 1300 K. CO and 0, were introduced either through the beam or by filling the background. Since the reaction proceeds through the Langmuir-Hinshelwood mechanism, the angular distribution of the formed CO, is independent of the angles of incidence of the reactants [6]. A CO, beam was produced by expansion of the highly purified gas (99.9998% at p = 300 Torr) through a 70

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pm diameter nozzle at room temperature. Under these conditions rotational relaxation is almost complete and on the basis of ref. [19] a velocity ratio Au/u = 0.04 and a mean translational energy in the beam direction (E)/k = 980 K was evaluated. The concentration of (CO,), clusters is still negligible under these conditions [20].

3. Results 3.1. CO, product angular distributions 3.1. I. Transient experiments In order to study the influence of the CO and 0 coverage on the CO, angular distribution a series of titration-type, transient experiments was performed. With these measurements the surface was first precovered by oxygen to 19,= 0.25 (giving rise to a 2 x 2 LEED pattern), and then at t = 0 suddenly a CO beam (I,., = 3.1 x lOI molecules/cm. s, y, = 45”) was switched on by removing a flag from the beam path. The evolution of CO, was recorded as a function of time at a fixed angle of detection y,, and then identical experiments were repeated at various yr. These measurements yield CO, angular distributhe total (i.e. angle tions at various times of titration, t,. By measuring integrated) CO, production rate (with the QMS rotated behind the sample) and by recording the intensity of scattered CO as a function of time, the 0 and CO coverages could be determined at any t, as described in detail elsewhere [6]. Typical CO, angular distributions at T, = 467 K for two 0 coverages are reproduced in fig. 1. For T, > 430 K and 19,> 0.05 the CO coverage is always

-20

0

20

LO -

60

80 yr

Fig. 1. CO, product angular distributions for T, = 467 K, I,, = 3.1 X lOI crnm2 s-’ at two oxygen cover+ges: (0) 0, = 0.05; (x) 0, = 0.25. The full lines are analytical expressions I( y,) = 0.43 cos’y, +0.57 cos y, (for 0, = 0.05), and I(y,) = 0.71 cos ‘y, +0.29 cos yr (for 0, = 0.25). The dashed line is the cos5y, function for comparison.

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negligibly small [6], so that these data are representative for &o = 0. The variation of the angular distribution has therefore solely to be attributed to the differing 0,. As first qualitative result it is noticed that the half-width (FWHM) decreases with increasing 0 coverage. Any attempts to fit the experimental angular distributions by a CO&Y, relation (width d as adjustable parameter) failed. If d is adjusted to the data at small yr, the experimental intensities at higher y, always exceed those from the fit curve. A similar conclusion was already reached by Palmer and his co-workers [21] in their earlier study. Matsushima [8] described the angular distribution of (non-isothermal) CO, production on a Pt(ll1) surface by COS~~‘~~ and on a polycrystalline Pt surface by about cos3yr, however with appreciable scatter of the experimental data. The angular distribution of desorbing CO, (from its physisorbed state), on the other hand, was found to follow nicely a cos y, distribution. Comsa and David [22] had developed a model for desorption through two channels which proved to provide a satisfactory description for the data of the present work. Following this model the angular distributions are fitted by a sum of the kind

I&

y,) = a cos yr + (1 - u) cosdyr.

(11

The parameter a varied with the actual experimental conditions (T,, 0,, &.o). The exponent of the second term, on the other hand, could always be kept constant, d = 7. Thus a was the only adjustable parameter, whose proper choice yielded always agreement with the experimental data. Integration of eq. (1) over the half-space in front of the surface yields the fraction of molecules leaving the surface with a cosdyr angular distribution F,=2(1-a)/(2-a-tad).

(2)

At this point an experimental problem has to be discussed: the QMS measures the gas density rather than the flux. As long as the velocity distribution remains unchanged, both quantities are proportional to each other. A peaked angular distribution is, however, usually associated with an increased mean velocity [5,9,16] while, on the other hand, a cosine distribution reflects molecules whose mean translational energy is given by 2kTs [16,23,24]. Therefore the measured density angular distributions are expected not to be identical with the flux angular distributions. The maximum error introduced in this way was estimated to be less than + 15%. In view of the more qualitative conclusions drawn from the results no attempts for further correction were made, but instead the measured intensities were set equal to the corresponding beam fluxes. Fig. 2 shows F, (as derived from the experimental angular distributions through eq. (2) with d = 7) as a function of 0 coverage, do, at various surface temperatures. For T, > 550 K, the CO coverage, Bco, remains always below

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J. Segner et al. /

I

/ 0.05

01 -

Catalytic

I

,

Lu 015

oxidation

of CO on Pt(l I I)

I

02

80

Fig. 2. Relative flux of CO, molecules into the cos’y, channel 0,. y, = 45”. I,, = 3.1 X lOI cmm2 s-‘.

F,, as a

function of oxygen coverage,

2 x 10-4, so that the data reflect the influence of only 0, and T,. (For 8, < 0.05 the reaction rate is very small [6] causing considerable experimental error.) For T, < 550 K the situation is more complicated: At high 0, the reaction is limited by CO adsorption and the steady-state 0co is still fairly low. With decreasing do, however, the reaction rate decreases and tic, continuously approaches the steady-state value corresponding to the CO adsorption/desorption equilibrium [6]. At T, = 470 K, for example, under the applied CO beam flux, 0c, increases from 5 10e3 at 0, > 0.05 to about 0.05 when all the oxygen has been reacted off. At this temperature F, decreases with decreasing 8, from its initial value of about 0.2 to a minimum of 0.1 at 0, = 0.05 and then starts to increase again. This effect demonstrates that there is an additional effect of &, on F,. At temperatures below - 400 K the situation becomes rather complex since now considerable accummulation of CO on the surface during the titration experiment takes place. Fig. 3 shows the variation of f3o and f&o with time during the titration experiments at T, = 280 and 390 K. Comparison for 390 K with the data of fig. 2 demonstrates that the minimum in F, occurs at 0, = 0.13 of the total coverage, 6, + Bco, when eco starts to increase. The minimum coincides, however, not with the minimum of F, - it has to be concluded that adsorbed CO exerts a more pronounced effect on the CO, angular distribution than O,,. These qualitative conclusions are confirmed by the data for 280 K. F7

J. Segner et al. / Catalytic oxidation of CO on Pt(lll)

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o*T=393K AbT:282K

Fig. 3. Variation of the oxygen and CO coverages, @, and &.., during the course of the titration experiments.

increases continuously during the titration experiment. Inspection of fig. 3 demonstrates that now a considerabIe CO coverage is built up at the surface which obviously overcompensates the loss of Oad, and the total coverage increases monotonically.

t

0&)iAI r(o,au)

02

315

01

305

Fig. 4. Flux of CO2 in the cos’y, channel, F., (full circle), rate of CO, formation (open circles) and CO coverage (triangles) under steady-state conditions as a function of surface temperature, T,. The oxygen coverage was always 8, < 0.02. Beam composition: I,, = 3.04X 1013 cme2 s-l, Zo, = 1.75 X 1Ol4 cm-’ s-l. The full and broken lines in the ~~-tem~rature range of F, are resulting from a theoretical treatment as outlined in the text.

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d. Segner et al. / Cafdytrc oxrdution of CO on Pt(l I I)

Comparison of the angular distributions extrapolated to t = 0, 8,, = 0.25, 8,., = 0, at different surface temperatures indicates an influence of T, as well: F, increases with increasing T<, 3. f.2. Steady-state reaction The influence of T, on F, has been studied in more detail under steady-state conditions. The surface was exposed to a mixed beam at yi = 45” with I co = 3.04 x 10” molecules/cm? . s and IO, = 1.75 X 10” molecules/cm’. s. CO, angular distributions were recorded after steady state was reached. The corresponding d,, was determined by suddenly switching off the beam and simultaneously recording a fast thermal desorption spectrum. Calibration was achieved by comparison with the data after saturation at T,, = 300 K where d,., = 0.5 is generally accepted [25]. For T,, x 500 K the steady-state d,.,, is relatively high. After switching off the beam the adsorbed oxygen atoms are completely reacted off, and their concentration could therefore be determined by integrating the CO, flux over time. It turned out that under these conditions 8,) was always < 0.02. Similar low oxygen coverages were also present at 7; > 500 K, which was concluded from more indirect observations and from knowledge of the overall reaction kinetics [6]. Fig. 4 shows the variation of F, with the surface temperature up to T, = 750 K. With decreasing T, also F, decreases continuously to a minimum at - 580 K, below which temperature it increases steeply. In the temperature range above 580 K the reaction proceeds at a practically clean surface, and therefore the data reflect the effect of the surface temperature alone. Below 580 K the CO coverage increases considerably, which effect is then essentially responsible for the continuous increase of F7. Note that 8,, = 0.05 causes an increase of F7 to about 0.4! At surface temperatures above 750 K, the reaction rates were so small that too large experimental errors were introduced. In addition, the socalled “subsurface oxide” is rapidly formed at these elevated temperatures [26] so that the data would no longer be representative for the clean Pt surface. In addition fig. 4 contains data on the variation of the CO, production rate which equals just twice the rate of 0, adsorption under these conditions. Here one notes two effects: A sharp increase in these rates with temperature up to about 525 K. and then a gradual decrease going to even higher temperatures. The latter effect occurs in a region where d,,, is very small and just reflects the well-known temperature dependence of the initial sticking coefficient for dissociative oxygen chemisorption [15]. The sharp decrease below 525 K has to be attributed to the buildup of CO coverage. It is quite interesting to note that the oxygen sticking coefficient is decreased by a factor of at least 6 from its value at zero CO coverage by the presence of d,, = 0.1. Previous measurements with the same surface [14] indicated that the 5% step and defect sites are,

due to their favourable energetics, occupied phasizes the dominant role played by defect more detail in the discussion section.

first by C%,. This again emsites which will be outlined in

3.1.3. The influence of i’sub.wrface” oxide The “subsurface oxide” phase is formed from chemisorbed oxygen atoms at T, > 700 K, or at even lower temperatures under the influence of a high 0, partial pressure [26]. This species is rather unreactive towards CO (or Hz) and can only be removed by Ar’ sputtering or by annealing to temperatures above 1250 K [26,27]. In order to study the influence of this phase on the CO, reaction dynamics the sample was first treated by 1 X lop6 Torr O2 at 800 K for 30 min. After cooling to 580 K steady-state CO, formation with a mixed O,/CO beam (as in the preceding section) was studied. Afterwards the sample was repeatedly flashed to 1300 K until the CO, angular distributions did not change any more. Typical experimental rest&s are reproduced in fig. 5. The presence of the subsurface oxide broadens the CO, angular distributions considerably. After 5 min annealing at 1300 K the concentration of this oxide was below the detection limit of AES (about 2% of a monolayer); nevertheless, the angular distribution was still quite different from that from the oxide-free surface (curve A). The overall kinetics, on the other hand, were only slightly affected. These results demonstrate the sensitivity of the energy transfer (= dynamics) if compared with the energetics (2 kinetics) themselves.

Fig. 5. The influence of “subsurface oxide” on the CO, product angular distributions at 7; = 580 K: (0) after exposure of the surface to po, = 10e6 Torr for 30 min at T, = 800 K; (0) subsequent measurement, after intermediate heating to 1300 K for 5 min in vacua; (A) after severaf heating cycles the angular dist~butions become again identical to those from a clean Pt surface.

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3.2. Scattering

of CO,

Scattering of a CO, molecular beam at the clean Pt surface leads to angular distributions of the type reproduced in fig. 6. These distributions are characterized by a lobe with maximum intensity near the specular direction which is typical for direct-inelastic scattering, as for example also reported for Ar and Xe scattering at metal surfaces [28]. In addition there appear molecules even at negative polar angles whose distribution can be fitted by a cosine function. These molecules are tentatively identified with particles which had undergone trapping/desorption, i.e. they were adsorbed at the surface long enough to completely accommodate and are then desorbed again after a finite surface lifetime. This can be qualitatively proven even without time-of-flight measurements. As long as the sticking coefficient is constant (which in fact is the case under the chosen conditions up to T, = 650 K as will be shown later) the CO, intensity in the cosine part should decrease proportional to (T,) ~ ‘I* if the mean kinetic energy of those molecules is determined by the surface temperature. (This effect is again due to property of the QMS to record the density rather than the flux.) Fig. 7 reproduces the CO, signal intensity at y, = - 19” as a function of q over a wide temperature range which indeed for 290 < T, < 650 K follows closely the predicted law and confirms the existence of a trapping/desorption channel. This conclusion is also in agreement with recent findings by Matsushima [7,8] according to whom physisorbed CO, molecules desorb with a cosine angular distribution. Above Ts = 650 K, the CO, intensity in the cosine part no longer follows the (r,)_ t/* dependence.

I co2 t

-i -20

0 --

20 40 60 8; yr [degree]

Fig. 6. Scattering of CO, with (E)/k = 980 K, yi = 40’ at T, = 900 K (0) and 7” = 295 K (X ). The dashed and dotted lines represent a cos y, distribution fitted to the experimental data at negative polar angles.

J. Segner et al. / Catalytic oxidation of CO on Pt(IIl)

1,

a

300

LOO

500

600

700

900 IKI

800 -

515

-k

Fig. 7. Variation of the CO, signal intensity at y, = - 19” (cosine part) with surface temperature. The dotted line is the function T,- ‘I* The intensity scale is normalized to Ice, = 1 at 320 K.

If an adsorption energy of 5 kcal/mol [29] and a normal frequency factor for desorption of lOI s-r are assumed, then the mean surface residence time of CO, molecule at 650 K is estimated to be around 3 X 10-” s. This time is already of the order of magnitude of surface vibration periods and makes it plausible why under these conditions “sticking” (i.e. complete thermal accommodation at the surface) starts to loose its meaning. Since CO, is completely scattered back from the surface (i.e. there is no detectable dissociation), the total flux has to be constant and independent of T,. The flux into the direct-inelastic channel is expected to exhibit a velocity distribution which is only weakly affected by T, (due to the incomplete energy exchange) [30], and indeed integration over the specular lobe (assuming rotational symmetry [31] and after subtraction of the cosine part) yields almost constant intensity. The FWHM of the inelastic lobe increases slightly from 40” at 290 K to 44” at 700 K. By assuming (‘direct-inelastic) = lubeam)

and

(G,>

= (4kT,/~co,)“*

for the average particle velocities in both channels, an estimate for the sticking coefficient, s = 0.5, results. That means a CO, molecule striking the Pt surface undergoes with about equal probability direct-inelastic scattering or trapping/desorption. Fig. 8 shows angular distributions for q = 520 K at three different angles of incidence, corresponding to a variation of the normal component of the kinetic energy of the impinging CO, molecules (E, ), between 0.02 and 0.03 eV. In this range the FWHM of the direct-inelastic part as well as its intensity remain constant, i.e. also the sticking coefficient is not changing. Earlier measurements by Cardillo et al. [32], however, indicated no trapping/desorption channel for CO, molecules incident with considerably higher kinetic energy (E, ) = 0.14

I

cc2

I/

T, =520K

I

-LO -20 0 --

I

20 &O 60 80 yr

li

[degree]

Fig. 8. Angular distributions for CO, scattering at T, = 520 K and for three different incidence (y, = - 18’. - 30”, - 60”). The specular directions are indicated by arrows.

angles

of

eV. This means that the sticking coefficient is markedly lower at higher (E, > which would be in agreement with general experience (241. As has been outlined above, the FWHM of the dir~t-inelastic lobe is almost independent of 7;. It is also not changing with (E,): even for (E, ) = 0.14 a value of 42” has been reported [32]. The deviation of the angle of maximum intensity from the specular angle, n = y,,,,, - yr,specr depends, however, on the angle of incidence and on 7 and can rather satisfactorily be described by the simple “hard cube” model [33]. This model has frequently been found to provide a correct description of the trends observed with scattering of noble gases [34]. It fails, however, to explain the constancy of the intensity and FWHM of the direct-inelastic scattering channel in the present investigation.

4. Discussion The mechanism of the platinum catalyzed CO oxidation (above 200 K) is well established, and its energetics are illustrated by fig. 9. The numbers were derived for a slightly stepped Pt(ll1) surface (as used in the present work) and hold for the limit of very small coverages [6]. With increasing coverage the adsorption energies for the reactants [14,15] as well as the activation energy for the surfac,: reaction, E:n, decrease. For 19,= 0.25, Sc, = 0, for example, E&, = 11 kcaI/mol [6]; it decreases even to - 1 kcal/mol if also @co is increased [13]. The adsorption energy for CO, is not exactly known, but is around 5 kcal/mol [8,29]. Inspection of fig. 9 demonstrates that the energy level of the transition state is about 30 kcal/mol above that of the gaseous CO,

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co* f 0, ____-------------3 transition

stpe

62

Fig. 9. Potential

diagram

illustrating

AH= 67,6 kcal/mol

the course of the reaction

CO+

lo,

+ CO, at low coverages

[61. molecule in its ground state: This excess energy can be either transferred to the solid (which would be equivalent to intermediate trapping the CO,,,, state) or is carried away by the product molecules. Previous investigations had in fact already demonstrated that energy accommodation is incomplete, and the present data suggest that indeed both channels are operating. Experiments with polycrystalline Pt revealed that the CO, molecules formed “hot” [11,12], whereby particularly the are vibrationally and rotationally asymmetric stretch and the bending modes exhibited appreciable populations of higher levels - in agreement with the plausible bent structure of the transition state [35]. Measured angular distributions are generally described by cosdyr laws. Palmer and Smith [5] could fit their data for Pt-poly with exponents 4 G d G 6, whereby a smoother surface (as probed by He reflection) was found to exhibit a higher d value. Matsushima [8] described his data for Pt(ll1) with d = 9 + 1 and for Pt-poly with d = 3, as already mentioned. Becker et al. [9] reported also d = 2-3 for Pt-poly. Experiments of the same group with a smooth Pt(ll1) surface at TY= 700 K could, on the other hand, very well be fitted by a cos7yr distribution [lo]. These latter results are of particular relevance for the data presented in this paper: Under comparable conditions (0 = 0.1, q = 700 K) the angular distribution recorded with our sample (containing about 5% step atoms) can be described with about equal fractions of molecules coming off the surface with cos y, and cos7yr distributions. This demonstrates the pronounced effect of surface defects. A cos y, angular distributions (Knudsen law) of molecules leaving a surface is observed if the translation energy of these particles are in thermal equilibrium with the solid and its mean value normal to the surface is given by (E, ) = 2kT, [3]. Sharper-than-cosine distributions necessarily reflect higher (E, ) values. According to a theoretical treatment of this problem [16], for a cosdyr distribution (E,)

= f(3 + d)kT,

(3)

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results. This relation yielded good agreement with experimental results for which both angular and velocity distributions were measured. Analysis of the present data by eq. (1) i.e. as a sum of a cos7yr and a cos y, function, is of course somewhat arbitrary, but is, on the other hand justified by the good fit to all experimental data by varying only one adjustable parameter which governs the relative participation of both channels. As already mentioned, the model underlying equ. (1) is identical to that proposed by Comsa and David [22] for associative desorption of diatomic molecules (Hz). For interpretation, these authors considered the reverse process which they called “activated adsorption through holes”. This term should express that part of the molecules striking the surface was dissociatively adsorbed by surmounting an activation barrier, while the rest passed “through holes” without thermal activation. This somewhat artificial picture can certainly not be applied for the present system since there is no indication for dissociative CO, adsorption (“through holes”) even if the translation energy of the impinging molecules is increased to 0.14 eV [32]. Instead it is concluded that the transition state remains essentially the same (even if EL” is changing, see fig. 9), but that a certain fraction a: of the molecules formed is intermediately trapped in the CO,,,, potential well, while the fraction F, = 1 - a leaves the surface “directly”, i.e. with excess energy. This conclusion is supported by the experiments of section 3.2: About 50% of the molecules striking the surface is accommodated (A sticking), while the rest has not lost its memory of the initial momentum (A direct-inelastic scattering). This result makes at least qualitatively plausible that a single encounter of a CO, molecule with the surface does not cause trapping in the adsorption well with unit probability. The combination of eqs. (1) to (3) implies a bimodal distribution of the velocities which (together with the angular dependence of the velocity distribution) is, e.g., in agreement with the careful measurements of Comsa et al. [36] for the system H,/Pd. For the CO, formation on Pt-poly at T, = 770 K, Becker et al. [9] reported for yr = 0 on (E)/k = 3650 K, while eq. (3) predicts 3850 K for d = 7, which value is regarded to represent the upper limits. For y, = 45”, (E)/k = 2140 K was found which is in qualitative agreement with the predicted increased contribution from the cos yr part at higher angles. The essential conclusion of the present work is that (Y (i.e. the fraction of product molecules intermediately thermally accommodated) is sensitively affected by the state of the surface. The qualitative findings can be summarized as follows: (1) (Yis increased by the presence of surface defects; (2) (Ydecreases with increasing surface temperature; (3) (Yincreases in the presence of the “subsurface oxide”; (4) (Ydecreases with increasing 0 coverage; (5) (Y decreases with increasing CO coverage, even more pronounced than under the influence of adsorbed oxygen.

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It is felt that these effects can be made plausible in the following way: (1) Energy transfer to the solid occurs through excitation of surface phonons whose frequency increases with increasing force constant. The latter will be highest for the most densely packed plane (which exhibits the lowest surface free energy due to the maximum number of nearest neighbors of an individual surface atom). The introduction of non-(111) sites (in particular of surface defects) will therefore improve the CO, solid energy transfer. This effect is probably superimposed by a second, different phenomenon. There is general experience that the adsorption energy at surface defects is higher than on densely-packed terrace sites. This holds even for weakly adsorbed Xe atoms [37] and can therefore also be assumed for CO,. A deeper adsorption well is, however, generally associated with a higher sticking coefficient, i.e. more efficient gas/solid translational energy transfer, most probably because the gradient of the gas/solid interaction potential (= force on the molecule by the surface) increases. (2) The probability for transfer of energy from a molecule to the solid decreases generally with increasing substrate temperature. This is also reflected in a general decrease of the sticking coefficient with T, in the absence of an activation barrier. Simply speaking, at higher T, there is a higher probability for a molecule in front of a surface to be pushed into the gas phase by the vibrating lattice. An additional effect comes into play by the varying participation of step (defect) and low-index (terrace) sites in the reaction, depending on the surface temperature: As will be outlined in more detail below, at low total coverages CO, formation will be essentially restricted to step sites at lower temperatures, but with increasing temperature the probability for reaction on terrace sites increases. This effect will of course be associated with a decrease of LY.The sticking coefficient for dissociative oxygen adsorption is much higher at steps than on terrace sites which parallels an enhanced adsorption energy at step sites [15,38]. In addition, the mobility of adsorbed oxygen atoms is small [39] if compared with that for adsorbed CO [40]. At low oxygen coverages (and not too high temperatures) the reaction will therefore be mainly restricted to the defect sites where the 0 atoms are located and to which the CO molecules are diffusing. This preferential binding of 0 and CO to defect sites will be governed by a Boltzmann factor, if the adsorbed reactants are in equilibrium with the solid. The probability P,(q) for occupation of terrace sites can then be written in the form:

J’,(T) = [1+(WN,) ew(ACdkT,)]-‘.

(4)

AE,, is the difference between the binding energy at defects and on the terraces. N, is the number of available terrace sites, N, the number of available

J. Segner

520

er al. /

Catulytic

oxidation

of CO on Pt(III)

defect sites. Eq. (4) is only valid for low coverage, i.e., for substrate tures i?, 12 580 K for the experiments displayed in fig. 4. The branching ratio has the form:

F,(T) =

R,(T) R,(T) +R,(T,) 11- J’,(Th’f’,(irl) ’

where R, is the rate constant ansatz for R, is:

for a cosdyr distribution.

R, = vd exp( - E;,/kT,). Inserting &(T,)

tempera-

The generally

accepted

(6)

eqs. (4) and (6) into eq. (5) yields: = [I +(N,v,/N,v,)

exp(A/Q)l

-‘,

(7)

where A = A E,, + E& (terraces)

- E&, (defects)

is the difference between the heights of the saddle points for the cos7yr channel and the cos y, channel. and v, have not been measured Eq. (7) is general, but AE,,, E&(terraces) and therefore eq. (7) cannot be evaluated straightforwardly. One could, however, check, if the experimentally observed temperature dependence is compatible with existing theories, when plausible numbers are inserted for the unmeasured quantities. We do this for the renowned Van Willigen model [41,22] and for a formalism developed recently by one of the authors [16]. Consistency is required with the experimental observation that the cos7yr shape of the angular distribution for reaction at terrace sites is: of the activation energies (i) independent of coverage, i.e., independent involved; (ii) independent of the substrate temperature. Point (i) can perhaps be rationalized in the Van Willigen picture, if one assumes that after reaction (i.e., after passing the saddle point or transition state) the CO, molecule loses a certain amount of its kinetic energy to the solid and then desorbs with the remaining excess kinetic energy. In order to satisfy point (i) one could then postulate that the height of the saddle point remains approximately constant and that only the adsorption energies of the reactants decrease an the terraces and with increasing coverage. This means that A is zero. For A = 0 and assuming no temperature dependence of v, and v,, one would from eq. (7) predict no or only a weak temperature dependence of the branching ratio - in contradiction to the experimental findings (cf. fig. 4). For A = 6 kcal/mol and v, = v7, a qualitative fit to the data can be obtained. Point (ii) cannot be understood in the Van Willigen model, which predicts a

J. Segner et al. / Catalytic oxidation

of CO on Pt(lllJ

521

strong temperature dependence of the angular distribution shape, because the analytic expression contains the factor exp[ - (E,/kT,)tan* y,] [41]. This term would lead to a broadening of the distribution with increasing substrate temperature, which is definitely not observed. Summarizing this investigation of the Eyring-Van Willigen picture, one would conclude that it contains enough unknown parameters for a qualitative fit, but that there are still great difficulties in understanding the experimental findings mentioned as points (i) and (ii) above. At the present state of the art, a more modest approach is perhaps more appropriate - dispensing with a detailed microscopic understanding as aimed at in the Van Willigen model. A previously developed theory [16] establishes relationships between the angular distribution of the reaction flux and various other measurable quantities, without assuming anything about the mechanism involved. Points (i) and (ii) are satisfied a priori, because the theory starts from the existence of two concurrent channels having angular distributions cos y, and cos7yr, respectively, independent of other external parameters. The relationships to be used have been derived for a desorption process from a perfectly flat surface insuring parallel translational invariance [16]. Unpublished work has, however, revealed that they are more general and depend only on the assumption that the parallel velocities of the product particles are Maxwell distributed with (E,,) = 2kT,. In particular this can be shown to be valid for both a flat surface and reactions originating from extremely localized states on the surface where transfer of parallel momentum is important. For a cosdyr distribution, the frequency factor is predicted to have the following temperature dependence: Vd =

CdTdj2.

The constant Cd includes among other quantities the transition matrix element, which would contain information about the microscopic reaction mechanism. Eq. (8) holds for both the cos7yr channel and the cos y, channel. The temperature dependence given by eq. (8) can be understood physically in the following way: A forwardly peaked angular distribution requires a microscopic transition probability which increases with increasing perpendicular kinetic energy of the product particle. This microscopic transition probability has to be multiplied by the Boltzmann factor and then to be integrated over all kinetic energies in order to obtain the total yield. For increasing T, the Boltzmann factor will probe the microscopic transition probability at higher kinetic energies. The sharper the increase of the latter is with increasing energies the more pronounced is the increase of the total yield with temperature. Inserting eq. (8) into eq. (7) yields:

&CT,)= [l +(TBFJ3 ew(A/kT)]-‘,

522

J. Segner ei al. / Catalytic oxidation of CO on Pt(l1 I)

with

T,=(N,c,/N,~,)'?

(10)

The parameter TB might be called “branching temperature”, because it is the temperature at which both channels would contribute equal intensities for A = 0. TB has been fitted to the experimental data. Assuming A = 0 and T, = 1060 K, F,(T,) has been plotted as a full curve in fig. 4. Even better agreement with experiment can be obtained if one assumes that the saddle point for the cos y, channel (reaction at defects) is lower by 1.5 kcal/mol than the saddle point for the cos7yr channel (reaction on the flat terraces). This is demonstrated by the dashed curve in fig. 4, which corresponds to the parameters Ts = 730 K and A = 1.5 kcal/mol. When judging the usefulness of this latter theoretical treatment, one should also note that by means of eq. (3) it predicts the correct mean translational energies for both channels without resorting to microscopic pictures. (3) The primary effect of the formation of a “subsurface oxide” will consist in an enhancement of the microscopic roughness and therefore an increase of (Y is expected in a similar way as if surface defects (e.g. steps) are introduced. Apart from this the phonon spectrum may be altered, whose consequence is, however, difficult to predict without more detailed discussion. (4) Following the discussion under (2) the reaction will proceed mainly at the step (defect) sites as long as 0, (and &,) is low. With increasing 0, (and still low Qo) oxygen atoms will more and more also occupy flat terrace sites from where they are reacted off. As a consequence (Ywill decrease, as observed. (5) The heat of adsorption of CO at steps is about 4 kcal/mol higher than at terrace sites. Therefore adsorbed CO molecules will preferentially occupy step sites where they inhibit dissociatioe oxygen adsorption [6]. (This is not so for the opposite sequence, i.e. a surface already saturated with O,, can still adsorb CO [6]. Therefore even at higher 8, the reaction at steps will not be suppressed.) The oxygen molecules have to dissociate at terrace sites, and since the 0 atoms exhibit a relatively low surface mobility they will be reacted off to a large extent before they can reach the defect sites. This explains why increasing the CO coverage has a more pronounced effect on cx than a higher oxygen coverage.

Acknowledgements

Technical assistance by H. Kuipers in the early stages of the experiments as well as financial support by Deutsche Forschungsgemeinschaft (SFB 128) is gratefully acknowledged. C.T.C. thanks the A. von Humboldt Foundation for granting a fellowship.

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