Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

491 Surface Science 122 (1982) 491-504 Norm-Holland ~blis~ng Company ADSORPTION OF SULFUR DIOXIDE AND THE INTERACTION OF COADSORBED OXYGEN AND SULFU...

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491

Surface Science 122 (1982) 491-504 Norm-Holland ~blis~ng Company

ADSORPTION OF SULFUR DIOXIDE AND THE INTERACTION OF COADSORBED OXYGEN AND SULFUR ON Pt(ll1) St. ASTEGGER

and E. BECHTOLD

Institut fiir Physikalische Chemie, Universifiit Innsbruck, A -6020 Innsbruck, Austria Received

14 July 1982; accepted

for publication

30 August

1982

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt( t 11) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 x IO’” S atoms cm- ’ ((6 x fi)R30° structure) oxygen exposures of 5 x 10-s Torr s do not result in the adsorption of oxygen nor in the formation of SO,. At concentrations lower than 3.8 x lOI S stems cm-* ((2x 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO, and 0,. With decreasing sulfur concentration the amount of desorbing 0, increases and that of SO, passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 X 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO, is adsorbed with high sticking probability and can be desorbed nearly compIetely as SO, with desorption maxima occurring at 400,480 and 580 K. The adsorbed SO, is highly sensitive to hydrogen. Small H, doses remove most of the oxygen and Ieave adsorbed sulfur on the surface. After adsorption of SO, on an oxygen predosed surface small amounts of SO, were desorbed in addition to SO, and 0, during heating. Preadsorbed oxygen produces variations of the SOs peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.

1. Introduction The interaction of sulfur and of sulfur compounds with platinum has attracted interest because adsorbed sulfur can poison or modify the activity of platinum catalysts [I]. In particular the reaction of sulfur dioxide with platinum in an oxidizing atmosphere has become increasingly important since platinum catalysts are used for cleaning exhaust gases. Beside this practical aspect the system sulfur-oxygen-platinum deserves attention from a molecular point of view. It represents a relative simple system of a general type of reaction and the body of literature on the adsorption of oxygen [2,3] and of sulfur [4-61 on platinum can provide basic information for understanding their interaction on platinum. Several papers report on the adsorption of SO* and on the oxidation of adsorbed sulfur on platinum. Bonzel and Ku [7] found that adsorbed sulfur 0039-6028/82/00~-0000/$02.75

0 1982 North-Holland

492

St. Astegger, E. Bechtold / Adwrption

of sulfurdioxide on Pt(Il1)

reacts on Pt( 110) with oxygen from the gas phase in a Langmuir-Hinshelwood type reaction to SO2 via adsorbed oxygen atoms, whereas Berthier, Perdereau and Oudar [8] concluded from their measurements on the oxidation of adsorbed sulfur on Pt(ll1) that the reaction occurs via adsorbed 0, molecules. Wu and Bums [9] report that after adsorption of SO, on a polycrystalline platinum foil thermal desorption yields SO,, but from Pt( 110) Ku and Wynblatt [lo] recovered only a few percent of the adsorbed SO, during thermal desorption. Hence drastic changes of the prevailing processes seem to occur when the crystallographic orientation of the platinum surface or the reaction conditions are varied. During the present work the interaction of oxygen and sulfur on Pt( 111) has been studied by thermal desorption of the mixed adlayer formed from 0, and S,. Complementary information was obtained from the adsorption and desorption of SO, on the clean and on the oxygen precovered surface. Connection to continuous oxidation experiments of adsorbed sulfur with 0, from the gas phase was accomplished by studying the adsorption of oxygen on the sulfur precovered surface.

2. Experimental The experiments were performed in a stainless steel UHV chamber which was pumped to a base pressure of 2 X 10-t’ Torr by an ion pump and a Ti sublimation pump. The apparatus was equipped with a line-of-sight mass spectrometer (ionizer-sample distance 2 cm) and a LEED and retarding field AES system. The high pumping speed in combination with the spatial arrangement of the sample with respect to the mass spectrometer (MS) insured that the MS signals induced by heating were directly proportional to the desorption rates. O,, SO, and H, were dosed by leak valves. A mokcular beam of S, molecules was produced inside the vacuum chamber in a solid state electrochemical cell Pt/Ag/AgJ/Ag,S/Pt [5]. A (111) oriented platinum disc was suspended by 2 tantalum wires for resistive heating. Its temperature was regulated by a programmable temperature controller using a Pt/PtRh temperature sensor which was spot welded to the back of the platinum crystal. Cleaning of the crystal surface was accomplished by the usual methods and controlled by LEED and AES.

3. Results 3.1. Adsorption

and desorption of oxygen and of sulfur

The adsorption of oxygen and of sulfur on Pt(ll1) surfaces has been described in various papers. During the present study a few adsorption and

St. Astegger,

400

600

800

E. Bechtold / Adsorption

low

493

of sulfur dioxide on Pt(lll)

M 64

----

M32

-

4006008001000 T/K

Fig. 1. Desorption spectra following adsorption Exposures: 0.6, 3, 10, 60 X 10e6 Torr s.

T/K of 0,

on clean Pt(

111).Heating rate 24 K s- ‘.

Fig. 2. Desorption spectra following adsorption of 0, on Pt( 111)precovered with sulfur. Heating rate 50 K SC’. Before each subsequent desorption run the surface was exposed to 6 x 10m5 Torr s oxygen. The series was started with a sulfur concentration of 3.8 X lOI S atoms cm-*. Shown are the 1, 5, 9 and 13 desorption experiment.

desorption experiments with each of these gases have been carried out in order to obtain a base for the subsequent coadsorption and reaction experiments. Fig. 1 shows a series of 0, flash desorption peaks which were obtained with different initial oxygen surface concentrations. In agreement with reported desorption spectra (e.g. refs. [2,11,12]) the peak maxima are shifted to higher temperatures with decreasing surface concentration. The desorption energies which were derived from the desorption traces on the basis of second order desorption kinetics decrease from about 213 to 175 kJ mole“ with increasing concentration [2]. The oxygen covered Pt(ll1) surface exhibited a (2 x 2) LEED pattern which corresponds to atomically adsorbed oxygen. Subsurface oxygen or platinum oxides [3,13] have not been observed under the conditions

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E. Bechtold / Adsorption

of suljur dioxide on Pt(l I I)

of the present work, whereas molecularly adsorbed oxygen is not stable under these conditions [3]. Elemental sulfur is adsorbed dissociatively on the Pt( 111) surface. With increasing surface concentration at first a (2 X 2) structure (full coverage at 3.8 X 1014 S atoms cme2 ) and then a (6 x fi)R30° structure (full coverage at 5 X lOI S atoms cme2) are formed [4,5]. The adsorption of S, occurs with high sticking probability via a precursor state [5]. With an initial concentration of 5 X 1014 S atoms cm-’ the onset of desorption occurs at about 850 K, with an initial concentration of 3.8 X lOI S atoms/cm2 at about 1050 K. Complete desorption is achieved at 1400 K [5]. The reaction and coadsorption experiments described in the following sections were performed at temperatures below 1050 K and with low sulfur ~ncentrations. Under these conditions desorption of elemental sulfur can be neglected. The desorption of elemental sulfur from a Pt( 111) surface will be described in more detail in a forthcoming paper [ 141. 3.2. Interaction

of oxygen with preadsorbed

sulfur

The experiments described in this section were performed as follows: At first the clean Pt( 111) surface was exposed to an S, beam at 320 K. A defined surface concentration was obtained either by applying an appropriate sulfur dose or by saturating the surface completely, followed by partial thermal desorption. Then the surface was exposed to Oz at 320 K. Subsequently the platinum crystal covered with the mixed oxygen sulfur adlayer was heated with constant rate up to 1050 K while monitoring the desorbing species at mass and 80 (SOT). After numbers 16 (O+), 32 (02, S*), 48 (SO+), 64 (SO:) cooling to 320 K the crystal was exposed to oxygen again and the flash desorption experiment was repeated. These procedures were continued until the preadsorbed sulfur was completely removed from the surface. -The series of experiments shown in fig. 2 were started with a sulfur concentration corresponding to the complete (2 X 2) sulfur structure (3.8 X 1014 S atoms cm- ‘). P rior to every desorption run the surface was exposed to 6 x lo- 5 Torr s 0,. In the temperature range from 320 to 1050 K SO, and 0, were desorbed whereas the formation of SO, was not observed under the conditions of these experiments. The small mass 32 peak at 480 K is due to fragmentation of SO,. The oxygen desorption peaks occur at constant temperature when identical oxygen exposures were applied before each desorption experiment although the amount of desorbed oxygen increases from fig. 2( 1) to 2( 133, i.e. with decreasing sulfur conc~tration. When the 0, exposure is changed in the case of coadsorbed sulfur, the oxygen peaks are shifted in the same manner as on the sulfur free surface. At the start of every desorption experiment shown in fig. 2 the Pt(ll1) surface exhibited a (2 x 2) LEED pattern which is caused in the first experi-

St. Astegger,

E. Bechtold / Adsorption

of sulfur dioxide on Pt(1 I I)

Fig. 3. Desorption yields of SO,, 0, and their sum as a function exposure in each case: 3 X IOF5 Torr s.

of the sulfur surface

495

concentra-

tion. Oxygen

ment (fig. 2(l)) by the pure sulfur adlayer, in the following experiments (up to fig. 2(13)) by a mixed sulfur oxygen adlayer and finally in the last experiment by the pure oxygen adlayer. From experiments of the type shown in fig. 2 the amounts of SO, and 0, which are desorbed per oxygen dose and desorption run were determined as a function of the sulfur surface concentration present at the beginning of each desorption run (fig. 3). The calibration factor for the SO, signal of the mass spectrometer was obtained from the ratio of the sum of the SO, peak areas of the whole oxidation series and of the known initial sulfur surface concentration. The calibration of the 0, signal will be described in section 3.3. In fig. 3 also the total amount of oxygen desorbing in each desorption run either as 0, or as SO, is given. The ratio of the desorption yields of SO, and 0, obtained after oxygen dosing on the sulfur precovered surface depends on the pretreatment of the sulfur adlayer. The series of experiments shown in fig. 2 was started with the sulfur concentration corresponding to the completed (2 X 2) adlayer. In another series of experiments sulfur was dosed up to a surface concentration less than 3.8 X lOi S atoms cme2 ((2 x 2) S structure) at 320 K followed by an oxygen exposure of 5 x low5 Torr s. During the subsequent desorption experiment more sulfur dioxide and less oxygen were desorbed than during the corresponding desorption experiment of the series shown in fig. 2 at the same sulfur con~ntration in each case (e.g., 1.0 X lOi SO* molecules crne2 as compared to

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of suljiir dioxide on Pt(ll I)

b

C

----

M 64

i

5_ i b 1 a

A

500

700

900

1100

T/

300

K

500

700 T/K

Fig. 4. Desorption spectra following adsorption of S, on Pt( 111) precovered with oxygen. Heating rate 80 K SC’; (a) after an oxygen exposure of 3x 10e5 Torr s followed by exposure to S,; (b) after an oxygen exposure of 3 x 10-s Torr s. Fig. 5. Sulfur dioxide desorption spectra after various exposures. Heating rate 45 K SC’; (a) after dosage of 1.3~ lOI S atoms cm-* on the clean Pt( 111) surface followed by exposure to 3 X 10m5 Torr s 0,; (b) after exposing the clean surface to 3x 10e5 Torr s 0, followed by the same sulfur exposure as in experiment (a): (c) after exposing the clean surface to 1, 2.2 and 4X lo-’ Torr s so,.

of 1.3 X lOI S atoms 0.5 X lOI SO, molecules cm-* at a sulfur concentration cm-*). Furthermore, the oxygen peak was shifted to a higher temperature than observed on the sulfur free surface after the same oxygen exposure. Similar but less pronounced effects occur when sulfur adlayers with concentrations of less than 3.8 x lOI S atoms cm-* were prepared by partial thermal desorption of a full coverage adlayer. At an initial sulfur concentration of 5 X lOI S atoms cm-* ((6 X fi)R30” structure) repeated exposures to 6 x 1O-5 Torr s 0, at 320 K do not result in detectable oxygen adsorption or reaction. In the intermediate concentration

St. Astegger, E. Bechtold / Adwrption of sulfur dioxide on Pt(lI1)

497

range from 3.8 to 5 X 1014 S atoms cm-* where the surface is covered with domains of the (2 x 2) and the (6 x fi)R30” structure, the reaction with oxygen starts very slowly. 3.3. Interaction

of S, with predosed

oxygen

In the experiments shown in fig. 4 the clean Pt(ll1) surface was exposed to 3 x 1O-5 Torr s 0, followed by an exposure to sulfur. The resulting mixed adlayer had a stoichiometric deficit of adsorbed sulfur of about 25% with respect to the formation of SO*. The desorption spectrum obtained from the mixed adlayer shows SO, at the same temperatures as observed after the reverse dosing sequence (fig. 4a). The oxygen peak is shifted to a higher temperature as compared to that observed after the same oxygen dose but without subsequent sulfur dosing. After a further oxygen dose only 0, was desorbed (fig. 4b) which shows that the adsorbed sulfur reacted completely to SO* during the first experiment. The ratio of the intensities of the SO, desorption peaks at 480 and 580 K depends on the oxygen surface concentration. Increasing oxygen precoverage enhances the 580 K peak at the expense of the 480 K peak, when equal sulfur doses are applied after oxygen dosing. If a slight excess of sulfur is dosed on the oxygen precovered surface then only SO2 is desorbed during the subsequent desorption experiments since the adsorbed oxygen reacts to SO2 completely. The ratio of the calibration factors for the mass spectrometer detection of SO, and 0, can be obtained by balancing the areas of the oxygen and sulfur dioxide peaks in fig. 4a and of the area of the oxygen peak which is obtained after the same oxygen exposure but without subsequent sulfur dosing (fig. 4b). This ratio and the SO, calibration factor (section 3.2) yield the 0, calibration. According to these titration experiments a 3 X lop5 Torr s 0, dose applied to the clean Pt(ll1) surface results in a surface concentration of about 4.2 X lOI 0 atoms cm-* which, however, is not the maximal attainable concentration. As has been shown previously [ 1 l] the saturation concentration is obtained only after very high exposures. The calibration of the oxygen detection relies on the assumption that during S, dosing at 320 K oxygen is not removed from the surface neither as 0, nor as SO,. Experiments with different S, doses support this assumption. The accuracy of this calibration is limited by the well known intricacies of the oxygen adsorption on platinum especially by the influence of clean off reactions and by the low sticking probability at higher oxygen surface concentrations. 3.4. Adsorption

and desorption of SO,

Desorption spectra measured after SO, doses from 1 X 10e6 to 4 x 10e6 Torr s are shown in fig. 5c. Only SO, was desorbed during heating up to 1050

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St. Astegger,

E. Bechtold / Adsorption

SO,exposure/

1d’Torr

of surfur dioxide on Pt(lll)

s

Fig. 6. Desorption yield of SO2 (8) and amount of sulfur remaining on the surface after desorption of SO, (7) versus SO, exposure.

K. After every desorption experiment about 3 x 1013 S atoms cm-* were left on the surface, nearly independent on the previous SO, exposure (fig. 6). This residual amount of sulfur was determined by subsequent oxygen titration. Fig. 6 shows the uptake curve of SO, and the amount of sulfur remaining on the surface after the desorption of SO*. In order to study background pressure effects the SO, covered Pt(ll1) surface (3.8 X 10 l4 SO, molecules cm -*) was exposed to hydrogen. After exposure to 5 x lo-’ Torr s H, the subsequent desorption experiment yielded only about 10% of the adsorbed SO*. The remaining 90% of the sulfur atoms which were present in the original SO2 adlayer could be recovered nearly completely as SO, molecules during subsequent oxygen titration experiments. Thus small H, doses remove most of the oxygen from the SO, adlayer but leave the sulfur content practically unaffected. In view of this result it seems that the oxygen deficiency of the SO, adlayer observed after SO, dosing (fig. 6) is caused mainly by clean off reactios with residual gases. Furthermore, the adsorption and desorption of SO, has been studied on the oxygen precovered Pt( 111) surface (fig. 7). The desorption yields SO, and 0, and a small amount of SO, at 580 K. The temperatures of the SO, peaks are only slightly influenced by the presence of adsorbed oxygen, but the ratio of the peak intensities is changed. The 580 K peak is enhanced whereas the 480 K peak is diminished with respect to the peaks observed without previous oxygen adsorption.

St. Astegger,

E. Bechtold / Adsorption of sulfur dioxide on Pt(l1 I)

499

4. Discussion 4.1. Interaction

of sulfur and oxygen on the Pt(lll)

surface

The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur concentration. The (0 x I/j-)R30° adlayer (5 X lOI S atoms cme2) is not affected by repeated exposures to 3 X lop5 Torr s oxygen at 320 K. At an initial concentration of 3.8 x lOI S atoms cm- 2 ((2 x 2) structure) the adsorption of oxygen and the formation of SO, start slowly, but they both increase as the sulfur concentration decreases (fig. 3). Thus sulfur free platinum surface regions, i.e. defects or holes in the (2 X 2) sulfur adlayer, are a necessary prerequisite for the adsorption of oxygen and the formation of SO, under the conditions of the experiments described in section 3.2. On the perfect (2 X 2) S adlayer dissociative adsorption of 0, does not occur although the structure is rather open. This compares with the conclusion of Oudar [l] and Bonzel and Ku [7] for Pt( 1 lo), that adsorbate vacancies are necessary for oxygen dissociation. After the adsorption of sulfur on Pt( 111) half order LEED spots are well developed even at 1 X lOI S atoms cmp2 (corresponding to about l/4 of the full (2 x 2) coverage) showing that the sulfur atoms are aggregated preferentially in domains with (2 x 2) structure. Oxygen exposure of such a partially sulfur covered surface improves the intensity and the sharpness of the half order spots. This observation is compatible with the assumption that oxygen is adsorbed on the free surface between the sulfur covered islands, and that the LEED pattern results from the superposition of the intensities of the two half order patterns from the sulfur and oxygen covered surface regions. The improved sharpness of the half order spots indicates ordering in the sulfur covered domains induced by the oxygen adsorption. According to this model the interaction of preadsorbed sulfur with oxygen starts at the boundaries of the sulfur covered regions. Since the adsorbed oxygen and sulfur atoms are mainly arranged in domains, the further progress of the reaction requires that at least one of the reactants becomes mobile. From reported data on the surface mobility of oxygen [ 151 and of sulfur [7,16] on platinum one may derive that the diffusion coefficients of both adsorbed reactants attain values > lo-l3 cm2 s-’ during heating to the observed desorption temperatures of SO,. Thus mean displacements of the adsorbed species > lo-’ cm may occur during a desorption experiment. Under the conditions of the experiments of figs. 2 and 3 one part of the oxygen adsorbed on the bare surface reacts to SO, and the other part, which increases with decreasing sulfur concentration, desorbs as 0,. The ratio of the amounts of desorbing SO, and 0, depends on the history of the sulfur adlayer. When the series of oxidation experiments are started with the full (2 X 2) S structure coverage, then the major amount is desorbed as O,,

500

St. Astegger, E. Be&old / Adsorption of st&w dioxide on Pr(Illj

b ____.

M 64

M 30 _*_._._ M32

So2

I’\

/‘i

J :

1

1

I

:

---

1. ._._.

a

/’

I

I

1 ./-. \

L_

~ so3-.‘---------

-.,._.)

fl i i

I I I

I

I I

’ Ib’\

I i

\ \ \

\

I

I .J

so2

\_fl\

\ \

:

‘A-___-_---__

I

400

600

800 TfK

Fig. 7. Desorption spectra obtained: (a) after adsorption of SO, on clean Pt(lll) (exposure 6 X 10e6 Tom s); (b) after adsorption of SO, on Pt( 1 II) precovered with oxygen (oxygen exposure 3 X lo-’ Torr s, sulfur dioxide exposure 6 X 10m6 Torr s). Heating rate 60 K SC’.

except at the beginning of the series (figs. 2 and 3). But in the case of a sulfur adlayer, which was prepared by S, dosing at 320 K up to a concentration less than the (2 X 2) saturation concentration, the SO, reaction yield is much higher (section 3.2). This influence of the pretreatment of the sulfur adlayer seems to be caused by the different size and number of sulfur covered regions formed under the different conditions during the preparation of the sulfur adlayers. With a full (2 x 2) structure coverage at the beginning of the oxidation experiments the reaction seems to start from relatively few centres yielding large and separate oxygen and sulfur covered regions in the course of the oxidation. This parallels findings of Bonzel and Ku for the oxidation of adsorbed sulfur on Pt( 110) [7]. Thermal desorption following adsorption of S, on the oxygen precovered surface yields SO, at the same temperature as does desorption after the reverse dosing sequence of the reactants. But a greater amount of SO2 can be formed

St. Astegger, E. Bechtold / Adsorption of surfur dioxide on Pt(l1 I)

501

per oxygen exposure than in the case of preadsorbed sulfur. The increased SO, formation is due to the more uniform distribution of adsorbed sulfur and oxygen on the surface since adsorption of S, takes place also on the oxygen covered surface, as can be seen from the rate of S, uptake by the oxygen precovered surface. After dosing a slight sulfur excess nearly all of the adsorbed oxygen reacts to SO,. The importance of the distribution of the adsorbed species for the various surface processes (desorption or reaction) becomes evident also from the characteristic features of the oxygen desorption from the sulfur precovered surface (section 4.3). Another characteristic feature of the SO, desorption traces is particularly pronounced when sulfur is dosed on the oxygen precovered surface, but it is also observable after the reverse dosing sequence: Increasing oxygen surface concentration enhances the 580 K SO, peak at the expense of the 480 K peak, although the binding energy of the adsorbed oxygen atoms decreases with increasing oxygen concentration [2,11]. The enhancement of the 580 K peak occurs also when SO, instead of sulfur is adsorbed on an oxygen precovered surface. In that case in addition small amounts of SO, are desorbed at 580 K (fig. 7). Altogether these observations seem to indicate the formation of an adsorbed species containing sulfur and oxygen which is stabilized by adsorbed oxygen. 4.2. Adsorption

and desorption of SO, on the Pt(lll)

surface

On the clean Pt( 111) surface SO, is adsorbed with a high sticking probability (= 0.5) which remains nearly constant up to a high relative coverage indicating precursor type kinetics (fig. 6). At surface concentrations above about 3.8 x lOI SO, molecules cme2 which corresponds to the sulfur concentration in the complete (2 x 2) adlayer the sticking coefficient decreases to a very low value. Recently Kohler und Wassmuth [ 181 investigated the adsorption of SO, on Pt( 111) by means of Auger electron spectroscopy. They found a slightly lower saturation concentration but the rate of adsorption was only about 1% of that observed during the present work. The reason for this large discrepancy is not evident at the present time. Following adsorption of SO, two desorption maxima occur at the same temperatures (480 and 580 K) as after dosing sulfur and oxygen. In addition a third peak appears at 400 K with a shoulder at 385 K (fig. 5). It is the most prominent peak at higher coverages. With increasing SO, dosage the high temperature peak at 580 K saturates at first (fig. 5). The maxima at 480 and 400 K increase approximately simultaneously whereas the 380 K shoulder appears near saturation of the surface with SO,. The observation that two SO, desorption maxima occur at 480 and 580 K irrespective of the type of dosing (sulfur dioxide or sulfur and oxygen) suggest

502

St. Astegger, E. Bechtold / Adsorption

of sulfiir dioxrde on Pt(l I I)

identical rate determining steps in each case which implies in turn that identical adsorbed species are formed either already during dosing or during heating to the desorption temperature. The sensitivity of adsorbed SO, to hydrogen, which compares with that of adsorbed oxygen suggest at least partial dissociative adsorption of SO,. The 400 K SO, peak, however, which obtains a noticeable intensity only after SO, dosing could originate from molecularly adsorbed SO,. A rough estimate of the energetic feasibility of complete dissociation of SO, to adsorbed sulfur and oxygen may be obtained from the desorption energies of sulfur and of oxygen from the pure adlayers. The desorption energy of 0, from the dissociatively adsorbed state is in the range of 175-213 kJ mole-’ depending on the coverage [2,11]. For low surface concentrations the binding energy of adsorbed sulfur atoms has been determined from thermal desorption experiments to be about 375 kJ/mole [ 141. These values and the heat of formation of gaseous SO, molecules and S atoms [ 171 yield for the energy of the dissociative adsorption of SO, - 30 to - 70 kJ mole-’ depending on the oxygen coverage. Thus dissociative adsorption of SO, on Pt(ll1) appears energetically feasible, but the desorption energies of SO, which may be estimated from the desorption temperatures are higher than the absolute value of these calculated adsorption energies for the dissociative adsorption: With the assumption of first order desorption kinetics and of a “normal” preexponential of lOI s- ’ desorption energies from 100 to 150 kJ mole-’ may be estimated from the observed peak temperatures. This method should yield desorption energies which are correct within *20% also for second order processes at low coverages [19]. The differences of these experimental desorption energies and of the calculated energies for dissociative adsorption do not rule out dissociative adsorption, since some of the energy values used are known only approximately. Moreover entropy effects and additional activation barriers were not considered. These considerations do not take into account the formation of adsorbed sulfur and oxygen containing species, e.g. adsorbed SO, whose occurrence is suggested by the stoichiometry of the formation of SO, from the elements, since the reaction will probably not occur in one step. Furthermore several experimental observations as the dependence of the relative intensities of the SO, desorption maxima on the concentration of coadsorbed oxygen and the formation of SO, (fig. 7) seem to indicate the presence of more complex sulfur and oxygen containing species. 4.3. Adsorption

and desorption of oxygen on the sulfur precovered

Pt(l1 I) surface

In the course of the oxidation of the (2 x 2) S adlayer the total amount of oxygen (desorbing as 0, or SO,) which is adsorbed per oxygen exposure increaes in proportion to the sulfur free surface fraction. This result was

St. Astegger, E. Bechtold / Aclsorption of sulfur dioxide on Pt(I I I)

503

obtained with 3 x lop5 Torr s 0, exposures per desorption run as in the experiments shown in fig. 3 as well as with 2 x 10e5 and 6 X lop5 Torr s 0, exposures. According to ref. [ 1 l] and to fig. 1 these oxygen exposures do not saturate a sulfur free Pt( 111) surface. Thus one may conclude that under the conditions of these experiments (i) oxygen adorption occurs only on the sulfur free surface and (ii) the local sticking probability of oxygen on the platinum surface between the sulfur covered regions corresponds approximately to the sticking probability on a completely sulfur free surface. The oxygen desorption peaks shown in fig. 2 occur at the same temperature as on the sulfur free surface after the same oxygen exposure independent of the amount of desorbing oxygen. According to the proposed model this may be rationalized as follows: Oxygen adsorption takes place on the sulfur free surface between the (2 x 2) S regions. If the domains are large the local oxygen concentration is not strongly decreased by the formation of SO, at the domain boundaries. Thus the 0, desorption peak whose shape and peak temperature is determined by the local and not by the mean surface oxygen concentration occurs at practically constant temperature although the amount of desorbed oxygen varies during the oxidation series (fig. 2). On the sulfur free surface, however, the oxygen desorption peaks are shifted to higher temperatures with decreasing amount of adsorbed oxygen (fig. 1). These arguments hold when the oxidation experiments are started with the full (2 X 2) S concentration, which seems to produce large separate oxygen and sulfur covered regions (section 4.1). Under conditions, however, where the reaction yield of SO, is higher due to the better intermixing of the reactants, the oxygen peaks are shifted to higher temperatures (section 3.2) which in terms of the proposed model is caused by the reduction of the local oxygen concentration as a consequence of the enhanced formation of SO,. These simple considerations of site blocking and of the spatial distribution of the adsorbed species on the surface can explain the observed effects qualitatively, but it should be emphasized that additional effects resulting from direct or indirect interaction of the adsorbed species are not excluded by these experiments.

5. Summary From the present work the following main results can be extracted: (1) On a sulfur precovered Pt(ll1) surface the adsorption of 0, and the formation of SO, requires sulfur free surface, i.e. defects or holes in the (2 X 2) S adlayer. (2) The amount of SOI formed from coadsorbed sulfur and oxygen and the desorption temperature of oxygen depend on the degree of intermixing of the adsorbed reactants.

504

St. Astegger,

E. Bechtold / Adsorption

of sulfur dioxide on Pt(lll)

(3) Two SO2 desorption peaks occur at the same temperatures independent of the type of dosing, i.e. SO, or sulfur plus oxygen. In the case of SO, dosage additional maxima occur at lower temperatures. (4) SO, is adsorbed with high sticking probability. The saturation concentration is about 4 X 1014 molecules cm-* whereby more than 90% of the adsorbed SO, can be desorbed as SO,. (5) Adsorbed SO, is highly sensitive towards hydrogen. Small hydrogen exposures remove the oxygen from the adsorbed SO, nearly completely. but leave the sulfur content unaffected. (6) Desorption following adsorption of SO, on the oxygen precovered Pt( 111) surface yields SO,, 0, and a small amount of SO,. Changes in the relative intensities of the various SO, desorption peaks give evidence for the stabilization of an adsorbed complex by adsorbed oxygen.

Acknowledgements Support by the Fonds zur Forderung der wissenschaftlichen Forschung, Austria, is gratefully acknowledged. The authors are indebted to Dr. R. Abermann for valuable discussions and for experimental advice and to Dr. H. Leonhard for the construction of the temperature controller.

References Ill J. Oudar, Catalysis Rev. 22 (1980) 171. 121CT. Campbell, G. Ertl. H. Kuipers and J. Segner, Surface Sci. 107 (1981) 220, and references therein. 131 J.L. Gland, B.H. Sexton and G.B. Fisher, Surface Sci. 95 (1980) 587. [41 Y. Berthier, M. Perdereau and J. Oudar, Surface Sci. 36 (1973) 225. K.H. Meister, E. Bechtold and K. Hayek, Surface Sci. 49 (1975) 161. 151 H. Heegemann, 161 T.E. Fisher and S.S. Kelemen, Surface Sci. 69 (1977) 1. [71 H.P. Bonzel and R. Ku, J. Chem. Phys. 59 (1973) 1641. [81 Y. Berthier, M. Perdereau and J. Oudar, Surface Sci. 44 (1974) 281. 191 O.K.T. Wu and R.P. Burns, Surface Interface Analysis 3 (1981) 29. [JOI R.C. Ku and P. Wynblatt, Appl. Surface Sci. 8 (1981) 250. [Ill K. Schwaha and E. Bechtold, Surface Sci. 65 (1977) 277. 1121 F.P. Netzer and R.A. Wille, Surface Sci. 74 (1978) 547. [I31 H. Niehus and G. Comsa, Surface Sci. 93 (1980) L147. [I41 H. Leonhard and E. Bechtold, to be published. I151 R. Lewis and R. Gomer, Surface Sci. 12 (1968) 157. [I61 E. Bechtold and J.H. Block, Z. Physik. Chem. (NF) (1974) 135. [I71 CRC Handbook, 60th ed. (CRC Press, 1980). 1181 U. Kohler and H.-W. Wassmuth, Surface Sci. 117 (1982) 668. 1191 T.N. Rhodin and D.L. Adams, in: Treatise on Solid State Chemistry, Vol. 6A, Ed. N.B. Hannay (Plenum, New York, 1976) ch. 5.