Chemisorption of oxygen on the silver (111) surface

SURFACE

SCIENCE 43 (1974) 230-256 0 North-Holland

CHE~ISORPTION

OF OXYGEN

Publishing Co.

ON THE

SILVER (111) SURFACE* G. ROVIDA,

F. PRATESI,

M. MAGLIETTA

and E. FERRONI

Istituto di Chirnica Fisk-a, Universitri di Firenze, Firenze, Italy

Received 25

July 1973

The interaction of oxygen with the (I I I) surface of a silver single crystal is studied, mainly in the pressure range from 1Om3up to 1 torr and at temperatures from room up to 500°C. The experimental techniques employed were LEED, secondary electron spectroscopy, work function variation measurements, and desorption kinetics. Exposure to the high pressures was made with a sample isolation valve. The experimental procedures are examined in detail and critically discussed. The results obtained with the different techniques allow a correlation with many studies of other authors. The LEED technique indicates that in the range of pressures and temperatures examined, a surface superstructure is stable, having a unit mesh with sides four times greater than that of the silver (I I I) plane. The presence of this surface phase seems to be related to oxygen adsorbed in the dissociated form. On this assumption, an interpretation of the structure is proposed, which is based on a coincidence lattice formed by a (11 I) plane of AgzO on the (11I) plane of the metal. This interpretation is also in agreement with the thermodynamic data.

1. Introduction The preliminary

results

of a study

of the chemisorption

of oxygen

on

various faces of silver single crystals have been previously published*J). The purpose of such a study is to clarify the type of interaction of oxygen with the silver surface and particularly the structure of the eventual twodimensional phases formed, as well as their stability ranges, and to relate them to previous work on the adsorption on polycrystalline san~ples3~2~*). The final aim is to furnish a basis for the interpretation of the catalytic mechanism involved in the oxydation of ethylene. The main conclusion already drawn by us was the faceting, following oxygen chemisorption, of the ( 100) and (110) faces. The (I 11j face was found to be more stable and showed the formation of a surface phase consisting of a (4 x 4) superstructure. The (I 11) face was therefore chosen for a further investigation in which previous results are reexamined in the light of both new experimental data * Work supported

by the National Research Council of Italy (C.N.R.). 230

CHEMISORPTION

obtained published

by us and 22, 24).

of important

OF OXYGEN

231

ON SILVER

contributions

that

have

recently

been

In this paper, therefore, our results on the (111) surface are presented in detail and the hypotheses for their interpretation are fully discussed. The (111) face of silver has been studied with the LEED technique by several authors (see, for instance, refs. 25-27), without, however, observing its behavior following exposure to oxygen. LEED studies of the interaction of oxygen with this surface are, on the other hand, rare. The formation of surface phases of different periodicity than the metal has been pointed out in two previous papers. Mi.iller2s), exposing epitaxial films on mica to oxygen pressures of the order of 10m5 torr, observed the formation of a (3 x 3) structure and of u(J3 x 43) structure rotated through 30”. Melle et al.zg), using spherical crystals and exposing them to the air, found the formation of a surface phase on the (111) domains, even at high pressures, with an unit mesh seemingly close to that of the (,/3 x 43)-R 30” structure found by Miiller; with, however, about a 5% misfit. The studies of these authors are, however, supported neither by the analysis of the surface composition, nor by the observation of the desorption of oxygen from the surface. We felt it would be more interesting to work prevalently in a range of pressures greater than 10m3 torr, not only to approximate the actual conditions of technological catalysis, but also, and above all, because it has been demonstrated that only at such pressures can a coverage close to that of a monolayer be obtained in a period of several minutessllo). This has also been recently confirmed23). We felt it would be useful to describe in as much detail as possible the apparatus and the standard conditions that we adopted in this research [as done, for example, by Bradshaw et al. in a recent paperso) on the silver (110) surface], in that only in so doing is to be confronted non-equivocally with true in general for work on the surface tems such as this in which several types sorptive bonds can coexist.

it possible to allow the data acquired the data of other authors. This is of solids and even more so for sysof still insufficiently known chemi-

2. Experimental 2.1. APPARATUS AND TECHNIQUES Our apparatus is a standard Varian 240-LEED system, with three-grid optics. Normal residual pressures were less than 10e9 torr. The surface composition was established by energetic analysis of the secondary electrons with the LEED optics. The upper limits of the energy

232

G. ROVIDA

ET AL.

and the current of the primary beam were respectively about 1000 eV and IuA. The measurements of the work function variation were made with the beam-stop method using an auxiliary electron gun with a tungsten filament, placed near the sample. The use of this device allowed us, as will be seen later, to control the effect of electron bombardment on the stability of the chemisorbed layer. The main reason that rendered its use indispensable was, however, the lack of reproducibility observed when the measurements were made employing the oxide cathode of the LEED optics, which is very sensitive to oxygen. Notwithstanding the low monochromaticity of the electron beam of the auxiliary gun, the beam-stop curves allowed us to detect variations of about 20 meV, with notable reproducibility, even in time, due mostly to the simplicity of filament regeneration by a rapid high temperature flash. The desorption spectra of the gases from the sample were obtained by rapid heating of the sample and use of a quadrupole mass spectrometer, As concerns in particular the desorption of oxygen, it may be assumed that the following relation is valid in our apparatussr):

where A is the area of the sample, n the number of particles adsorbed per unit area, I/ the volume of the system, T its absolute temperature, p the partial pressure of the oxygen as given by the mass spectrometer, S the pumping speed of the oxygen by the whole system, and Ap the pressure variation of oxygen with respect to the residual value. We found a value of 8.2 set-’ for the factor S/V for the oxygen in our system. Since desorption is not very rapid, in our case the term dp/dt can be neglected. It follows that in our desorption spectra the increase of the pressure of oxygen is directly proportional to the rate of desorption, and that the area of the desorption peak is directly proportional to the amount of oxygen adsorbed. 2.2. SAMPLE PREPARATION AND HANDLING In previous studies crystals of various purity were used; in no case, however, was this greater than 99.9995% and the problem of segregation of impurities at the surface arose. In this investigation it was therefore decided to use a crystal of 6 N purity (supplied by the Metal Res. Corp.). Working on this crystal the validity of most previously obtained results was ascertained. Its dimensions were a diameter of 8 mm and a thickness of 2 mm. The precision of its orientation was + 1O. The surface was prepared by spark erosion and then by chemical etching to eliminate the altered surface

CH~MISORPTION

layer.

The

final

treatment

OF OXYGEN

preceding

233

ON SILVER

its introduction

into

the apparatus

consisted of a brief attack with 66% HNO,“5) followed by washing with distilled water and very pure acetone. Once introduced into the system, the crystal surface was cleaned by thermal treatment alone, without, that is, using the widely adopted ion bombardment to this end. In this connection, experiments conducted on various materials had indicated to us that small quantities of carbon, coming probably from impurities in the gaseous phase, can be incorporated in the sample during bombardment with argon ions. It must also be borne in mind that the Auger peak of carbon lies near that of transition, a circumstance which silver corresponding to the M,,,N,N,,, rendered its identification difficult when it is present in small quantities. Nonetheless this alone would not have constituted a sufficient reason to exclude the use of ion bombardment, in that measurements made with different techniques than Auger spectroscopy allowed us to ascertain that thermal treatments and ion bombardment conduct the crystal surface to the same final state, a state which it is therefore reasonable to consider as being that of the clean surface. In particular, the emission spectrum below 50 eV and the energy loss spectrum were found to be very sensitive to the state of the surface. The measurements of the variation of the work function were also quite sensitive to the condition of the surface. Aside from this, the motive that induced us to prefer thermal treatments was the necessity to clean the entire surface of the sample (and not only the surface exposed to the electron or ion beam), so that desorption spectra could be more meaningful. In fig. 1 is shown a series of Auger spectra obtained during the successive phases of cleaning. Initially S, Cl and C are found on the sample. After heating to 500°C there remain both S and C, although to a much smaller extent. After a treatment in oxygen at lo-’ torr at 500°C and successive heating in the ultrahigh vacuum, both of the latter disappear. Surface control techniques indicated that after the initial cleaning treatment it is possible to reobtain the clean surface by simply heating briefly to temperatures higher than 500°C in the ultrahigh vacuum. The clean silver surface was found to be rather stable in time in ultrahigh vacuum. It is known, in fact, that silver does not chemisorb any of the normal components of the residua1 gases at very low pressuresl5). The sample was clamped on a strip of tantalum or molybdenum which could be directly heated by electric current. Such a mounting allowed heating and cooling times which were sufficiently fast for the conduction of the experiment. The sample temperature was measured with a platinumplatinum/rhodium thermocouple. To avoid contaminating the sample, the thermocouple was clamped between it and the support, rather than being directly welded to it. In this way, however, the evaluation of the temperature

234

0. ROVIDA

ET AL.

Fig. 1. Auger spectra obtained during the cleaning procedure. (1) Untreated surface; (2) after heating in ultrahigh vacuum at 450°C for 3 min; (3) after heating in 10-I torr of oxygen and subsequent heating in ultrahigh vacuum at 500°C. EP 2 1000 eV; AV-- 5 V rms; primary current: curves I and 2, 2pA: curve 3, &IA.

of the sample is not precise. in particular, during a fast temperature rise, as happens during the desorption spectra, the measured temperature is greater than the actual. From our approximate evaluations, based on the variation of the LEED intensities with temperature, the maximum error for temperatures not greater than 300°C should not exceed 30°C if the temperature variation is not too rapid. For temperatures higher than 300°C the error can be of this order even in conditions of thermal equilibrium. Initially tantalum was used as the support material. We found with Auger spectroscopy that the tantalum segregated sulfur on its surface during heating and this sulfur, during the high temperature exposure to oxygen, gave rise, probably, to SO, which then reacted with the silver surface. This problem could almost be eliminated by repeated exposures of the tantalum at 500°C in IO-’ torr of oxygen, followed in each case by heating in ultrahigh vacuum. After such a treatment, exposures to oxygen at temperatures lower than 300°C revealed no trace of sulfur on the silver, within the limits of sensitivity of the Auger technique as employed by us. When the sulfur was present on the silver surface in appreciable quantities, increases in the work function even greater than 1 eV were noted, and the normal oxygen desorption peak was not observed. However, most of the experiments were repeated

CHEMISORPTION

with a molybdenum exposing

support,

to temperatures

OF OXYGEN

ON SILVER

which did not cause this problem.

greater than 300-35O”C,

small quantities

23.5

Actually, of sulfur

are found together with oxygen on the silver even with a molybdenum support. This could be due to heating of the whole sample holder, which is composed of several materials and whose surface composition is difficult to control. Exposing to temperatures greater than 45O”C, sulfur was generally not observed on the silver. Evidently the surface compounds formed on the silver by sulfur and oxygen are no longer stable at such temperatures. 2.3. EXPOSURE TO OXYGEN Exposure to oxygen at pressures greater than 10e3 torr was done by isolating the sample from the ultrahigh vacuum in a special valve. The interior of this valve was connected to an auxiliary apparatus in which it was possible to obtain the desired oxygen pressure, by means of a variable leak valve, and to pump off the oxygen at the end of the exposure. This apparatus was equipped with a sorption pump and was initially evacuated through the LEED chamber during the bake-out. Following this, the pumping of the oxygen was effected with the sorption pump. The oxygen pressure was evaluated by means of a thermocouple gauge calibrated over the range of lop3 to 1 torr. Once the oxygen had been pumped to a pressure below 10m3 torr, the sample isolation valve could be reopened and the residual oxygen rapidly pumped into the ultrahigh vacuum system. The isolation valve allowed the use of pressures even greater than I torr. In such a case it was possible to estimate the pressure by the slight leak found toward the ultrahigh vacuum. The nominal purity of the oxygen was 99.99x, with about 5 ppm of H,O and N,, the remaining impurities being noble gases. The composition of the residual gases in the auxiliary apparatus was not directly controlled during the experiments. The total residual pressure was always below lop3 torr, which is the limit of the gauge scale. Tests conducted by us, by opening the connection with the LEED chamber, indicated that the main components are the rare gases and oxygen, together with much smaller quantities of “normal” residual gases, such as N,, CO, COz and H20. The latter, when the valve is opened to the ultrahigh vacuum, remains in the apparatus for several minutes at a pressure of lO-9 torr since it desorbs only slowly from the surfaces which have been exposed to the high pressure. 3. Results 3.1. DESORPTION SPECTRA Following

exposure

to oxygen at pressures

up to 10e5 torr, directly in the

236

G.ROVIDA

ET AL.

LEED chamber, no appreciable oxygen desorption could be observed following heating of the sample up to 500°C. After exposure to oxygen in the isolation valve at pressures greater than 10P3 torr, it was on the other hand possible to reveal the desorption of the adsorbed oxygen. Following the return to ultrahigh vacuum, rapid heating of the sample and the support caused the desorption of various gases. In fig. 2 is reported a series of desorption curves as recorded with the mass spectrometer relative to the main molecular species. As already mentioned in the experimental part, the partial pressure of the H,O in the system is higher than those of the other components after exposure to high pressure in the isolation valve. As can be seen in fig. 2, only the oxygen shows a well-defined desorption peak. The other gases show mainly a fast desorption initially upon heating and, with the exception of CO,, do not show any important maxima. The small maxima in correspondence with the oxygen desorption peak could be due to the reduced pumping efficiency of the system caused by the appreciable increase of the oxygen pressure. The behavior of these gases would seem to indicate that they come from the sample support.

Fig. 2. numbers heating.

Desorption curves for the various gases, recorded by the ion spectrometer. The indicate the mass of the desorbing species. The arrow indicates the start of For water, the mass 17 has been used so as to record the curves with the same full-scale spectrometer range.

C~IEMISORPTION

The carbon

dioxide

shows

OF OXYGEN

a large initial

237

ON SILVER

maximum

which

is by itself

characteristic of desorption from a surface which is heterogeneous either with respect to composition or to temperature. Actually, trials run by us showed that such a CO, desorption is found with the support alone in the absence of sample. The only desorption curve which seems to be related uniquely to the sample is that relative to oxygen. Oxygen desorbs from the surface of the silver sample with a well-de~ned peak, whose maximum was found, in our experiments, around 280°C. if the error in the evaluation of the temperature is considered, in particular during the rapid rise, such a desorption kinetics is sufficiently in agreement with Kollen and Czanderna’s resultsas). It must also be recalled that our kinetics are much faster and that the law of temperature variation in our case is not well-defined. Moreover, the desorption maximum is a function of the initially adsorbed quantitys2). Approximate evaluations of the activation energy indicate values between 35 and 40 kcal/mole, which were obtained as follows. Since, as has already been discussed in section 2, the pumping rate of the oxygen by the system is very high in our case, the pressure of oxygen measured by the mass spectrometer during desorption may be considered to be directly proportional to the desorption rate. Limiting the analysis to the first points of the desorption peak, so as to consider a rather negligible fraction of the peak area, it may be considered that both in the case of a first order and a second order desorption kinetics the curve represents the variation of the desorption rate with temperature for an approximately constant degree of coverage. Plotting the data on an Arrhenius diagram good straight Iines were obtained from whose slope the activation energies were evaluated. Notwithstanding the approxinlations and the imprecision in the evaluation of the temperature, the agreement with Czanderna’s results suffices to indicate that the observed desorption process is substantially the same. Nonetheless, given the notable experimental limitations in our case, we considered the desorption curve mainly as an indication, on a relative scale, of the quantity of oxygen present on the sample. Besides the peak already described, the desorption curves obtained following exposure to oxygen at temperatures greater than about 200°C clearly show a wide rnaxilllurn around 5OO”C192). The area of this peak grows notably with an increase in the temperature of exposure to oxygen. The low temperature desorption peak is visible after exposures up to about 250°C. For higher exposure temperatures, only the high temperature peak is visible. It should be noted, however, that given the mode of exposure and in particular the usual method of cooling the sample during oxygen pumping, the amount of oxygen which desorbs in the low temperature peak following

238

G. ROVIDA

ET AL.

exposure to temperatures higher than 200°C can also be notably less than that present at equilibrium during exposure. After exposure to temperatures between 300 and 450°C the presence of another oxygen desorption maximum is often noted. It has not, however, been possible to clarify with certainty if this peak is related to the presence of sulfur or to the support material. Since these results are not well-reproducible and seem to be related to the characteristics of the sample support, we do not deem their illustration and discussion to be opportune at this point. Two typical oxygen desorption curves after exposure to respectively 150 and 500°C are reported in fig. 3. The area of the low temperature desorption peak was taken by us as a measure of the quantity of chemisorbed oxygen that remains on the surface even after having replaced the sample in ultrahigh vacuum. Exposure times greater than about 10 min, at least at temperatures greater than lOO”C, did not greatly increase the quantity of chemisorbed oxygen. Therefore our exposures were generally made for 10 min. The curves reported in fig. 4 indicate how the quantity of adsorbed oxygen varies with the pressure and the temperature of exposure to oxygen. In this graph the range of stability of Ag,O, as obtained from thermodynamic datasa), is also indicated, It follows that the oxide is not stable over most of the range of pressures and temperatures investigated by us. The adsorption of oxygen is maximum for temperatures

Fig. 3.

between

150

Desorption spectra of oxygen. (I) After exposure to IO-1 torr at 150°C for 5 min; (2) after exposure to the same pressure at 500°C for 3 min.

CHEMISORPTION

OF

OXYGEN

239

ON SILVER

0

-1

8 $ -2

-3 loo

200

300

t "C

Fig. 4. Full curves correspond to equal quantities of oxygen adsorbed as a function of pressure (in torr) and temperature of exposure; curves 1 to 3 correspond roughly to 0.3, 0.6 and 0.9, respectively, with respect to the maximum amount. The dashed curve delimits the region in which the (4 x 4) superstructure is observed. Dash-dotted line is equilibrium curve for the decomposition of the silver oxide.

and 200°C. This fact, also, is in agreement with the results of Czandernalo). The maximum quantity adsorbed at 1 torr is not much greater than that adsorbed at 10-l torr. 3.2.

VARIATIONS

OF THE

WORK

FUNCTION

After exposures at 1O-6-1O-5 torr small variations of the work function were noted, of the order of a tenth of an eV. More significant variations were observed after exposure at pressures higher than 10m3 torr. In fig. 5 are schematically reported the increases in work function, observed after treatment in oxygen. Let us first consider the results obtained after exposure at 150°C and lo- ’ torr. It has already been mentioned, in fact, that these are the most frequently used conditions, in that they correspond to the maximum chemisorption, as can be seen from the desorption curves, and, on the other hand, at this temperature there should be a negligible desorption during the return to zero pressure. The diagram shows that, beginning with a clean surface, obtained by

Fig. 5.

Work function variations observed after various surface treatments.

heating to 550°C in ultrahigh vacuum, exposure to oxygen under the above conditions causes an increase of the work function of about 0.25 eV. If the sample is rapidly heated to 350°C so as to desorb the oxygen of the first desorption peak, the work function does not return to the initial value, thus indicating that the structure or the initial surface composition has not yet been reestablished. Complete recovery of the surface characteristics is attained only after heating above 500°C in ultrahigh vacuum. This observation is to be related to the fact that the desorption curves show that some oxygen is still removed at high temperature. On the other hand, if the sample is exposed to oxygen at temperatures of the order of 500°C so as to exclude the presence of chemisorbed oxygen on the surface, the observed increase of the work function is even greater than that which remains after the desorption of oxygen which has been chemisorbed. If, after exposure to oxygen at high temperature, the sample is reexposed at 15O”C, there is a further increase of the work function which is of the order of that found by direct exposure. In conclusion, the main result of the work function variations following exposure to oxygen is an increase which is due to two contributions: one, of about 0.2 eV, is related to the first desorption peak and the other, whose importance varies mainly with the exposure temperature, is related to the high temperature desorption of oxygen. 3.3. ENERGY ANALYSIS OFTHE SECONDARY ELECTRONS 3.3.1. Auger spectra The Auger spectrum of the surface after reaction with oxygen is reported in fig. 6. The oxygen Auger peak at 525 eV is distinctly visible. This peak

CHEMISORPTION

decreases after heating temperature desorption

OF OXYGEN

241

ON SILVER

to 350°C (see curve 2 in fig. 6), that is, after the low of the oxygen. However, the peak disappears com-

pletely only after prolonged heating at above 500°C in ultrahigh vacuum. The oxygen Auger peak is also visible after exposure to oxygen at 500°C. As far as concerns the Gontribution to the Auger peak of the oxygen chemisorbed on the surface, there is a possibility that some of that oxygen is desorbed due to the action of the primary electron beam, especiahy if the relatively weak bond is taken into account. In this connection we ran some tests with the auxiliary gun used for the determination of the work function variations. A beam-stop curve was recorded after exposure to oxygen. Then,

200

400 E

600

IeV)

Fig. 6. Auger spectra of the surface after exposure to 10-l torr of oxygen at 150°C for 5 min (curve 1) and after desorption of oxygen by heating up to 350°C. Same conditions as for curve 3 in fig. I.

giving a positive potential to the sample, the electrons of the same beam were accelerated so as to bombard the same approximate surface as that explored in the beam-stop curves. The results indicate that for bombardments with electrons of an energy no greater than about a hundred eV, there is no variation of the work function, within the limits of reproducibility of our measurements. For higher energies there can be appreciable decreases, which indicate a possible desorption of the chemisorbed oxygen, as long as the observed variation is not related to a reconstruction of the adsorbed layer.

242

G. ROVIDA

ET AL.

No oxygen desorption, following electron bombardment, is measureable with the mass spectrometer. It must, however, be borne in mind that the area bombarded is about one square millimeter (or about one-hundredth of the area of the sample). Therefore, given that the desorption may be rather slow, an eventual desorption could be below the limits of sensitivity if measured as a variation of the partial pressure of oxygen, which is already relative high in the system after the exposure of the sample at high pressure. The doubt therefore remains that, during the Auger investigation, at least a part of the oxygen present on the surface can be desorbed. We can, however, say that the Auger oxygen peak does not show a notable variation in time. Moreover, the Auger spectra obtained with the defocalized electron beam, that is, in conditions such that the electrons are distributed over an area much greater with respect to the normal conditions, do not differ appreciably from spectra obtained with a non-defocalized beam. It is therefore probable that, if desorption and not reconstruction of the chemisorbed layer is concerned, only a small part of the oxygen is involved in desorption. An Auger peak of oxygen, of height smaller than that observed by us, was also reported in a study of the ( 110) face of silver 30). In this paper, too, some effect of the electrons on the adsorbed oxygen is reported. 3.3.2. Spectra of’ low energy secondary electrons As far as concerns the spectrum of the electrons having energies lower than 50 eV, it may be stated that it is very sensitive to oxygen exposurel). All the emission peaks decrease in intensity after exposure to oxygen, but no new structure appears. The most intense silver peak, at 13 eV, seems to be displaced a few tenths of an eV. This shift takes place simultaneously with a notable variation of the background, and its real existence can not be considered to be completely established. Exposure to oxygen at 500°C causes only a small decrease in intensity of the secondary emission peaks. Both the small magnitude of this phenomenon and the fact that new peaks do not also appear in this spectral region, would also seem to exclude the presence, on the surface, of impurities which could be responsible for the presence of the oxygen as revealed by the Auger spectrum. 3.3.3.

Energy loss spectra of the backscattered

Even the energy loss spectrum very sensitive to oxygen exposure. have been published in a previous this technique, too, shows two whether the sample is exposed to

electrons

of the slow electrons revealed itself to be Certain details of the observed variations paperss). Here it is important to note that different types of variation according to oxygen at 150°C or at 500°C. Only in the

(00)

beam

E teV)

Fig. 7. Intensity versus energy for different beams, Full curves: clean surface. Dashed curves: after exposure to 10-l torr of oxygen at 15OT for 5 min. For the (00) beam the curves were recorded at 5’ incidence, with reduced sensitivity.

244

(1. R”“lD.4 ET AL.

first case are notable

differences

in the peak intensities

and small shifts in

energy observed. 3.4. DIFFRACTION OF LOW ENERGY ELECTRONS The LEED diagram of the clean surface is in agreement with the observations of other authors. The intensity vs energy, or l/E curves, for the (00), (IO), and (01) beams are reported in fig. 7. Exposure to oxygen at pressures lower than 10e5 torr, directly in the LEED chamber, and for times up to thirty minutes, does not cause any visible variation in the LEED pattern, and the Z/E curves show only a slight decrease in the height of the peaks. Exposure to oxygen at pressures greater than 10m3 torr at from room temperature to about lOO”C, using the sample isolation valve, causes only a general weakening of the intensities of the LEED pattern and an increase of the continous background. From temperatures near IOO”C on, after exposure to oxygen, the LEED pattern shows a notable change, as can be seen in fig. 8. Besides the spots of

Fig. 8.

LEED

diagrams of silver (I I I) surface after exposure to IO 150°C for 10 min. (a) 16 V: (b) 48 V; (c) 119 V.

I tot-r of oxygen

at

CHEMISORPTION

the clean

silver,

which

remain

OF OXYGEN

the strongest,

245

ON SILVER

many

additional

spots

with

fractional indices, which are multiples of $, and whose intensities vary notably with the energy, also appear. In such conditions the I/E curves of the silver spots are noticeably changed (see fig. 7). They show mainly a lowering of the peaks [especially the (10) and (01) spots]. Moreover, several structural variations can be noted. This indicates the formation of a new surface structure following oxygen exposure. This structure shows a periodicity which is a multiple of that of silver. The additional spots can in fact be interpreted with an unit mesh of side four times greater than that of the (111) silver planes. Following the generally accepted notation, we will indicate such a structure as Ag (11 I)-(4 x 4)-O, or more briefly the (4 x 4) superstructure. At pressures between 10m3 and 1 torr this superstructure is visible following exposure at temperatures no greater than about 250°C. The range of temperatures and pressures of exposure at which the superstructure is formed is reported in fig. 4. This range is the zone delimited by the dashed curve. The tests were run by exposing the sample for ten minutes at each of the various temperatures and pressures. We also ran some tests in which the sample was exposed to a pressure of IO torr at about 150°C. Under these conditions, too, the (4 x 4) superstructure was observed. No other ordered bidimensional structure was observed by us with the LEED under the above cited experimental conditions. After heating to 350°C that is, after the desorption of the oxygen of the first peak, the LEED pattern again becomes that characteristic of the clean surface. In fig. 9 are reported two curves that show how the intensities of a LEED spot return to the characteristic value of the clean surface during heating at constant temperature. The intensities of the two curves are relative to the intensity of the clean surface at the same temperature, so as to eliminate the effect of the temperature on the intensities. As can be seen, for the LEED, when the oxygen is desorbed the structure returns to that of the initial surface (at least for exposures at relatively low temperatures), whereas the variations of the work function and the Auger spectra under the same conditions, as has already been discussed, indicate an incomplete return to the initial properties. Exposing to oxygen at high temperatures the LEED indicates only a slight decrease of all the intensities. During the LEED observation of the (4 x 4) structure it was found that, at least for primary energies greater than about 100 eV, the diffraction pattern of the superstructure weakens, thus indicating an effect of the electron beam on the adsorbed layer. This can be related to the discussion of section 3.3.1 concerning the Auger spectra.

246

G. ROVIDA

ET AL.

4

2 time

Fig. 9.

Intensity ture.

6 (mini

variation of the (00) beam at 52 V, during heating at constant The intensities are relative to that of the clean surface at the same temperature.

tempera-

Concerning the stability of the (4 x 4) superstructure in the ultrahigh vacuum, its LEED diagram is visible up to temperatures greater than 200°C. Above this temperature, however, the intensities of the superstructure spots rapidly decrease, in connection with the observed desorption of oxygen. After many adsorption and desorption cycles, the LEED shows the appearance of very weak spots, after oxygen adsorption, indicating some degree of faceting. This is observed earlier for crystals whose oriented surfaces were not well prepared. Since, after oxygen desorption, the spots of the facets again disappear, this faceting seems to be completely reversible.

4. Discussion Numerous previous studies of the oxygen-silver system led to contrasting results, which were difficult to frame into a unique picture of the adsorption mechanism and of the state of the adsorbed oxygen. More recent studies seem to indicate at least two states of the chemisorbed oxygen: one atomic (presumably O’-) and the other molecular (0, or else Oi-). Among the more reliable investigations in the past few years the most complete seem to be those of Czanderna10,22) and that of Kilty et al.29), which, moreover, are in good agreement with each other. Unfortunately, these papers, which

CHEMISORPTION

give quantitative

results

OF OXYGEN

on adsorption

ON SILVER

kineticsl0924),

247

on desorption

ki-

netics2s), and on the quantity adsorbed at various temperatureslo), concerned polycrystalline substances. Since our investigation is relative to phenomena studied on the oriented face of a single crystal, just how a comparison with the existing data can be made, is not immediately evident. Therefore, before considering the results, it would seem useful to briefly discuss our techniques and operating conditions. The desorption spectra obviously represent the less specific technique in that desorption can take place from the entire sample surface. From a macroscopic point of view our sample presented about two-thirds of a (111) surface and about one-third of a non-oriented surface (corresponding to the lateral surface of the slice). However, the oriented surfaces could also possess a certain roughness, the extent of which is difficult to evaluate with precision. Given the stability of the clean (111) face and on the basis of our preliminary resultsly s), one could be led to believe that the greatest portion of the two oriented faces of the crystal consisted of (111) planes. However, recent results obtained with the field emissione1p47) seem to indicate that even the (111) face is not completely stable following oxygen adsorption. Actually, as reported in section 3.4, (111) faces which have not been well prepared, and even others which have been but which have been subjected to several experimental cycles, show direct evidence of reversible faceting. It is therefore difficult to evaluate, even approximately, the percentage of the surface formed by (111) domains following oxygen adsorption. In any case, the desorption spectra constitute a good connection with the previous results and are therefore a valid basis for the interpretation of our results. The work function variations were measured on an oriented face, but, given the diameter (greater than 1 mm) of the zone that was explored, and considering the possibility of surface faceting, even this technique furnishes a mean result, which moreover does not consider the various patches to the same degrees6). With respect to the desorption spectra, at any rate, the results should themselves.

be largely

dominated

by the variations

of the (I 11) planes

Analysis of the secondary electron spectra furnishes mean results on the crystal surface. Thus this technique, too, is not capable of distinguishing eventual differences, on a microscopic scale, on the face under study. The only technique that furnishes specific results is therefore the LEED, notwithstanding that it, too, is influenced more or less appreciably by the irregularities (steps, facets, etc.) of the surface. Thus, although it is obviously advantageous to use several techniques simultaneously on the sample, a certain caution must be observed in correlating the results so obtained. Aside from the different sensitivity to the

248

G. ROVIDA

ET AL.

single surface domains, the various techniques can alter the adsorbed layer to a variable degree. As far as concerns our experimental conditions, it must be noted that the surface, after high pressure exposure to oxygen at the desired temperature, is studied in the ultrahigh vacuum environment of the LEED chamber at room temperature, that is, under conditions which are very different from those in which it is found during exposure. The adsorbed layer under study, which is in a condition of non equilibrium, can therefore possess a composition and a structure different from those that characterize it at equilibrium during the exposure. Such a drawback is not, however, avoidable since the surface study techniques employed do not allow one to work directly at such high pressures. Nonetheless, given the relative speed with which the sample is cooled during oxygen pumping, the quantity that remains adsorbed should not vary greatly. In fact, the greatest part of the adsorbed oxygen desorbs in appreciable quantity only above 150°C. Following exposures to higher temperature, up to about 250°C the 150°C temperature is reached in a few seconds, before the oxygen pressure falls below IO-” torr. For exposure to temperatures greater than 25O”C, the quantity that remains adsorbed can actually depend in the way in which the temperature and the pressure vary during cooling. Another problem to be discussed is the possibility of contamination of the surface during the high pressure exposure. We have already stated in section 2.2 that SO,, presumably formed by oxidation of the sulfur present in the support material, is a possible contaminant. Such contamination was rather well controlled by us, mostly due to the fact that Auger spectroscopy is particularly sensitive to sulfur. An approximate evaluation conducted by us, by considering the quantitative results reported by other authors”a, 35) to be applicable to silver, indicated that, under our conditions, a quantity of sulfur corresponding to l/20 of a monomolecular layer should be easily detectable. The range of exposure temperatures over which the quantity of sulfur on the surface could be reduced to below the limits of sensitivity of Auger spectroscopy extends from room temperature to about 3OO”C, and above 450°C. As has already been mentioned it is difficult to avoid the presence of small quantities of sulfur at intermediate temperatures since in order to maintain the sample at such temperatures, the entire sample holder (and not only the sample support) can reach temperatures that cause it to react with the oxygen. Our results in this intermediate temperature range were therefore not considered to be sufficiently reliable and will therefore not be discussed. Our observations indicate that, however, when exposure must be made at high pressure and at temperatures higher than the room temperature, a means of controlling the surface composition to verify if there is

CHEMISORPTION

contamination

due to interaction

OF OXYGEN

ON SILVER

of the support

materials

249

with the gaseous

atmosphere is particularly important. As concerns the possibility of contamination of the adsorbed layer by carbon-containing compounds, in particular CO and COZ, which are normal components of the residual gases, Auger spectroscopy, as has already been pointed out, is unfortunately of little help due to the superposition of the carbon and the silver peaks. Czanderna’s study37) of the adsorption of CO, on silver previously covered by oxygen would seem, however, to indicate that even during the chemisorption of oxygen in the isolation valve the partial pressure of CO, is too low to give an appreciable surface concentration. Moreover the CO, would desorb upon returning to ultrahigh vacuum since the sample temperature would still be sufficiently high. In agreement with this, the desorption spectra for the various gases seem to indicate in an evident manner that only oxygen remains adsorbed on the sample under our conditions. Let us now consider how the results obtained with the various techniques can be correlated both among themselves and with the results of other authors. We have seen that two desorptions can be distinguished: one at low temperature, between 200 and 300°C the other at high temperature, beyond 400°C. The low temperature desorption peak correspond to oxygen chemisorbed on the surface and seems to agree with Czanderna’s resultslO). According to the latter there would be three types of interaction of oxygen with silver in the range of temperatures and pressures investigated by us. Below about 100°C the surface would be only partially covered by immobile oxygen atoms. Moreover a certain quantity of undissociated oxygen could be adsorbed. Above 100°C the atomic oxygen would become mobile, thus rendering the adsorption of a further quantity of atomic oxygen possible. Such an interpretation would be in agreement with a recent proposal by Kilty et al.24). The atomic oxygen would desorb beyond 200°C as O,, with an activation energy of about 35 kcal/mole. A recent paper by Kollen and Czanderna22) indicates a second order kinetics for such a desorption. This constitutes further proof that the oxygen is adsorbed in the atomic state. Molecular oxygen, whose surface concentration is rather low, desorbs at lower temperatures. This type of oxygen most probably desorbs during the return to ultrahigh vacuum under our experimental conditions in that no oxygen desorption was noted as temperatures lower than 150°C in our spectra.The oxygen that desorbs in the low temperature peak must therefore correspond to atomic oxygen, as demonstrated by the activation energy and by the fact that the maximum of the adsorbed quantity is situated between I50 and 200 “C.

250

G. ROVIDA

It is our opinion that the work is found after exposure to oxygen with this oxygen. Such a value resultssf and could also be related

ET AL.

function variation of about 0.2 eV, which at low temperature (see fig. 5), is connected would be in agreement with Kummer’s to one of the two types of work function

variation, as a function of the quantity of adsorbed oxygen, found by other authorsso). These results are, however, relative to polycrystaiIine sampfes. It is, on the other hand, difficult to find a connection with Degeilh’s resuItss*f, although these were obtained on the (111) face of a silver single crystal. The high values found by this author (up to 1.5 eV) at pressures of lo-’ torr are difficult to explain, also since at low pressures the quantity of oxygen adsorbed should be less than that present on the surface in our case. Moreover, the greatest work function variation was observed after about onehalf hour. According to the recent results of Hall and Kingas), the sticking probability of oxygen on silver, if the high initial value at very low coverages is neglected, would be of the order of IO-‘. Therefore, under Degeilh’s conditions coverages greater than l/IO of a monolayer could not have been reached. On the other hand, the results of this author are not supported by an analysis of the surface composition and thus the presence of impurities on the surface cannot be excluded. Our Auger spectra clearly reveal the presence of oxygen. The transfer of this datum to a quantitative scale is found to be difficult for several reasons, such as the effect of electron bombardment on the adsorbed oxygen and the eventua1 contribution of oxygen which is found below the surface. Nonetheless, from a comparison with spectra obtained by us in similar cases we feel that the peak observed under our conditions is compatible with a surface concentration on the order of a monomolecular layer. This is confirmed by the results of Bradshaw et aLao) for silver oxide, which would show a peak about four times greater than ours. On the other hand the oxygen adsorbed on the surface causes variations, sometimes appreciable, both in the secondary emission spectrum below 50 eV and in the energy loss spectrum. Aside from the general decrease of the intensities, which is in good agreement with the presence of a chemisorbed surface layer which greatly diffuses the electrons at these low energies, the results cannot be interpreted precisely, mainly because the nature of the emission peaks of silver at such energies is not yet clearas). These variations however, are also consistent with a fairly high surface concentration. The LEED, after exposures to temperatures between room and about 100°C and to pressures between 10e3 and I torr, indicates a disordered structure of the surface, as shown by the decrease of the intensity of the spots and by the background increase. Actually, from the desorption spectra it results that the quantity that desorbs is very small with respect to that

CHEMISORPTION

observed

following

exposure

OF OXYGEN

to temperatures

ON SILVER

251

greater than 100°C (see fig. 4).

A possible explanation could be based on the model proposed by Czandernalo) and by Kilty et a1.24). According to these authors the oxygen, below about lOO”C, would be adsorbed in the form of immobile atoms. This would thus impede the formation of an ordered surface structure. Nonetheless it cannot be excluded that even under these conditions, after waiting for a sufficiently long time, the oxygen can slowly rearrange to give regular structures such as, for example, those previously observed by other authors2sY zg). The (4 x 4) superstructure is found for exposures to pressures from a few between about 100 units greater than 10m3 torr on, and at temperatures and 250°C. The range of exposures over which such a structure is observed, reported in fig. 4, is included in that range in which the desorption spectra show the 280°C peak. In the preceding paperl) the (4 x 4) structure was attributed to undissociated oxygen for various reasons, among which was the difficulty of explaining such a large mesh and the fact that no other structure with a smaller mesh had been observed by us. Since, however, the successive papers of Czanderna22) and Kilty et a1.24) gave weight to .the hypothesis that under our experimental conditions the oxygen is in a dissociated form, we now feel that any interpretation of the supErstructure must begin with this hypothesis. In this case the formation of the adsorbed layer could be due to two mechanisms: adsorption “on top” of oxygen atoms on the unaltered metal surface or else adsorption with rearrangement of the silver atoms and formation of a mixed surface layer. At first the formation of a pure layer seemed more favorable to us, even considering that the oxygen was in the atomic state. In fact, the slight variations of the I/E curve for the (00) spot following the formation of the (4 x 4) superstructure did not seem to indicate a complete rearrangement of the silver atoms. The (4 x 4) mesh could then be explained by a coincidence lattice between a plane of oxygen atoms and the first silver plane, as mentioned, for example, by Tucker for oxygen on rhodium40). However, the large variation of the intensity of the fractional index spots with the potential is difficult to reconcile with the assumption that the silver atoms remain on positions very close to those of the clean surface. Moreover, an examination of the Z/E curves for spots other than the (00) spot showed strong variations following adsorption. We therefore began to consider the formation of a mixed layer. This hypothesis is also in better agreement with the slight observed variation of the work function (although such variations cannot always be treated in a simple manner on the basis of the electronegativity difference between adsorbent and adsorbate alone). Many studies reported the formation of mixed layers on various metals, consisting of a true two-dimensional surface compound (see, for example,

G. ROWDA

252

ET AL.

refs. 41 and 42). In some cases, as shown by Bauer”“), complicated superstructures may be interpreted, on the basis of double diffraction, as due to the superposition of a thin layer of a three-dimensional compound, whose structure is known, on the surface plane of the metal. These considerations led us to attempt to interpret the (4 x 4) superstructure on the basis of the structure of Ag,O, which has a cuprite structure, whith cl=4.73 A. A surprising result was obtained when the (I I I) plane of the oxide was considered. As shown in fig. IO, the diagonal of the unit mesh of the (i I I) plane of Ag,O is exactly four times the edge of the unit mesh of the (lit) 0 .

.

.

_____*_____

. t

0 *

____ -..._

.

__I_

.

.

.

-Q \

.

.

i

Fig. 10. Schematic diagram of the lattices of silver (full circles; unit mesh a) and of AgpO (open circles; unit mesh b) (I 11) planes. The unit mesh c is that of the (4 ‘x:4) superstructure.

silver plane. The misfit is less than 0.5%. The 4 x 4 mesh would therefore be the periodic element in common between the (I 1 I) silver and silver oxide planes. Experimental evidence seems to exclude that epitactic crystals of the oxide are involved. In such a case there would simply exist a superposition of the oxide spots on those of the metal. Above all, as already noted, the bulk oxide is not stable under the conditions in which the formation of the (4 x 4) is noted. It must therefore be concluded that the superstructure is due to a coincidence lattice between the (11 I) silver plane and thin layer of atoms of silver and oxygen positioned as in the (1 I I) planes of Ag,O. Actually an oxide developed only in two dimensions would be involved. The formation of this two-dimensional compound, which evidently causes the complete

CHEMISORPTION

shifting

of the silver surface

atoms,

OF OXYGEN

would

ON SILVER

thus explain

253

the relatively

high

energy of adsorption activation of atomic oxygen above about 100°C [22-24 kcal/mole according to CzandernarO), 14 according to Kilty et alz4)]. Since the desorption activation energy is, according to these same authors, about 35 kcal/mole, there would be a heat of adsorption of 1l-21 kcal/mole. Such a value, considering the dispersion of the experimental data, is not in too great disagreement with the heat of formation of the oxide [14.7 kcal/ mole according to 0tto44)]. Another important correlation should also be mentioned. Allen45), studying the kinetics of decomposition of Ag,O found an activation energy of 36 kcal/mole, which is in surprising agreement with the results obtained for the desorption of oxygen from the surface of silver. Our experimental results do not allow us to establish how this surface layer is formed; that is, if a plane of silver atoms and a plane of oxygen atoms or else a plane of oxygen atoms between two planes of silver atoms is involved. The second hypothesis would perhaps be in better agreement with the slight variation of the work function. The (4 x 4) superstructure seems to be stable in ultrahigh vacuum up to temperatures at which oxygen begins to desorb. It is therefore very probable that it is stable in the presence of oxygen over the entire range reported in fig. 4. Oxygen which is also immobile in this temperature range would therefore be involved. The fact that, according to Czanderna, beyond 100°C atomic oxygen would become mobile, thus creating new sites for the adsorption of more atomic oxygen, can be related to our observation that up to 100°C the superstructure is not formed. The oxygen which is adsorbed at lower temperatures would therefore be adsorbed in a disordered manner and only at about 100°C would it acquire a sufficient mobility for the nucleation of ordered domains with the consequent possibility of further adsorption of oxygen atoms. As far as concerns desorption, the fact that the superstructure is stable up to the temperatures at which oxygen commences to desorb and the fact that the LEED intensities return directly to the values of the clean surface (see fig. 9), seems to indicate that the surface layer decomposes to give oxygen, which desorbs, and silver atoms, which are reinserted into the normal metal structure. Up to now the discussion has been based on the hypothesis that the oxygen responsible for the (4 x 4) superstructure is found in the atomic state. It should however be noted that the (4 x 4) structure could also be interpreted on the basis of the (111) planes of the superoxide, as indicated by various authors5T6) and also considered by Sachtler46), having an NaCl structure with a= 5.55 A. In fact, the edge of the(4 x 4) mesh would coincide, towithin

254

G. ROVIDA

ET AL,

less than about 2x, with three times the edge of the mesh of the (11 I) planes of this oxide. In such a case the oxygen would be present on this surface as an 0, group. It must however be noted that the composition, the exact structure, and the thermodynamic properties of this compound are not known. This hypothesis would appear to be less convincing than the preceding. We have seen from our desorption spectra that, besides the desorption of the chemisorbed oxygen, a high temperature desorption of oxygen, following exposure at temperatures above 15O”C, can be noted. A further work function increase, which can even reach a value of about 0.3 eV, would seem to be connected with this oxygen (see fig. 5). Following exposure at 450-500°C the Auger spectrum indicates the presence of oxygen in a quantity that is comparable with that found in the presence of the previously discussed chemisorbed oxygen. The LEED diagram shows no notable structural change with respect to clean silver. Only a slight decrease of all intensities can be noted. The low energy secondary emission spectrum and the energy loss spectrum are rather similar to the spectrum of the pure metal. These results can be interpreted either by the presence on the surface of crystallites of three-dimensional compound containing oxygen which covers only a small part of the surface or else by the incorporation of oxygen below the surface of the metal. The first hypothesis, if the presence of impurities, undetected by Auger analysis, that bind with the oxygen forming a stable oxide is excluded, cannot be sustained since no compound of silver with oxygen is stable at these temperatures and pressures. We are thus left with the hypothesis of diffusion of the oxygen below the sample surface. This is, in fact, the hypothesis already formulated by us in previous papers. On the other hand, a considerable diffusion of oxygen below the surface at the temperatures at which chemisorption takes place has been reported by Bagg and Bruce*) for polycrystalline films. Nonetheless, since this phenomenon does not appear to be specific to the (I 11) face, having been found by us also on other silver facesl), we will not discuss this problem here. Conclusions The interaction of oxygen with the (111) surface between between 10P3 and 1 torr and at temperatures about 3OO”C, leads to results which can be well framed by Czandernalosr5), for the chemisorption of oxygen confirmed by Kilty et aLz4). Under our experimental conditions it appears that in a dissociated form can be efficiently studied.

of silver at pressures room temperature and in the model proposed on silver, as recently only oxygen adsorbed

CHEMlSORPTION

Oxygen

adsorbed

below about

OF OXYGEN

ON SILVER

100°C seems to be in a disordered

255

state.

Beyond 100°C and up to about 250°C there is formation of a two-dimensional phase that is stable in ultrahigh vacuum until the oxygen begins to desorb. This surface phase has a unit mesh of edge four times greater than the (111) surface of silver. The oxygen present in this superstructure seems to be in an atomic state. On this basis the superstructure can be interpreted as a coincidence lattice between a (I 11) plane of Ag,O and the (I 11) plane of the metal. Such an interpretation would also be supported by the fact that the heat of adsorption for oxygen is close to the enthalpy of formation of the oxide and that the latter decomposes with an activation energy close to that for the desorption of atomic oxygen. Other modifications of the silver surface, which are not related to the previously described chemisorption, are attributed to the incorporation of oxygen below the surface of the metal.

1) G. Rovida, ~/?e~oine~ff,

E. Ferroni, M. Maglietta and F. Pratesi, in: Adso~pfion-D~sorpfj~ion Ed. F. Ricca (Academic Press, 1972) p. 4 17. F. Pratesi, M. Maglietta and E. Ferroni, J. Vacuum Sci. Technol. 9

2) G. Rovida, ( 1972) 796. 3) J. T. Kummer, J. Phys. Chem. 63 (1959) 460. 4) F. H. Buttner, E. R. Funk and H. Udin, J. Phys. Chem. 56 (1951) 657. 5) Yu. Ts. Vol and N. A. Shishakov, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk 4 (1962) 586. 6) S. Kagawa, H. Tokunaga and T. Seiyama, Kogyo Kagaku Zasshi 71fl968) 775. 7) V. E. Ostrovskii, I. R. Karpovich, N. V. Kul’kova and M. I. Temkin, Russian J. Phys. Chem. 37, (1963) 1407. 8) J. Bagg and L. Bruce, J. Catalysis 2 (1963) 92. 9) Ya. M. Fogel, B. T. Nadykto, V. 1. Shvachko and V. F. Rybalko, Russian J. Phys. Chem. 38 (1964) 1294. 10) A. W. Czanderna, J. Phys. Chem. 68 (1964) 2765. i 1) Y. L. Sandier and D. D. Durigon, J. Phys. Chem. 69 (1965) 4201. t2) A. W. Czanderna, 3. Phys. Chem. 70 (1966) 2120. 13) 1. G. Murgulescu and N. I. Ionescu, Rev. Roum. Chim. 11 (1967) 1395. 14) Y. L. Sandier, S. Z. Beer and D. D. Durigon, J. Phys. Chem. 70 (1966) 3881. 15) A. W. Czanderna, in: Clean Surface.r, Ed. G. Goldfinger (Dekker, New York, 1969) p. 133. 16) W. R. MC Donald and K. E. Hayes, J. Catalysis 18 (1970) 115. 17) J. Wood, J. Phys. Chem. 75 (1971) 2186. 18) D. Cismaru, E. Segal and S. Fatu, Rev. Roum. Chim. 16 (1971) 1655. 19) I. G. Murgulescu and N. I. Ionescu, Thin Solid Films 7 (1971) 355. 20) L. A. Rudnitskii, L. I. Shakovskaya, N. V.Kul’kova and M. I. Temkin, Dokl. Akad. Nauk SSSR 182 (1968) 1358. 21) M. M. P. Janssen, J. Moolhuysen and W. M. H. Sachtler, Surface Sci. 33 (1972) 624. 22) W. Kollen and A. W. Czanderna, J. Colloid Interface Sci. 38 (1972) 1.52. 23) P. G. Hall and D. A. King, Surface Sci. 36 (1973) 810. 24) P. A. Kilty, N. C. Rol and W. M. H. Sachtler, Fifth Intern. Congress on Catalysis, Miami 1972.

256 25) 26) 27) 28) 29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42) 43) 44) 45) 46) 47)

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ET AL.

H. E. Farnsworth, Phys. Rev. 40 (1932) 684. C. Corotte, P. Ducros and A. Mascall, Compt. Rend. (Paris) B267 (1968) 507. M. P. Seah, Surface Sci. 17 (1969) 181. K. Miiller, 2. Physik 195 (1966) 105. H. Melle, E. Menzel and J. Zaunert, Phys. Status Solidi (a) 1 (1970) 85. A. M. Bradshaw, A. Engelhardt and D. Menzel, Ber. Bunsenges. Physik. Chem. 76 (I 972) 500. R. S. Hansen and V. J. Mimeault, in: Experimental Methods in Catalytic Research, Ed. R. B. Anderson (Academic Press, 1968) p. 217. Handbook of Chemistry and Physics, 46th ed., p. D-26. F. Pratesi and G. Rovida, J. Electron Spectr. 1 (1972) 296. M. Perdereau, Compt. Rend. (Paris) B274 (1972) 448. R. W. Joyner, C. S. McKee and M. W. Roberts, Surface Sci. 27 (1971) 279. C. Herring and M. H. Nichols, Rev. Mod. Phys. 21 (1949) 185. A. W. Czanderna and J. R. Biegen, J. Vacuum Sci. Technol. 8 (1971) 594. R. Degeilh, Vide 139 (1969) 29. M. P. Seah, Surface Sci. 17 (1969) 132. C. W. Tucker, Jr, J. Appl. Phys. 37 (1966) 3013. J. L. Domange and J. Oudar, Surface Sci. 11 (1968) 124. M. Perdereau and J. Oudar, Surface Sci. 20 (1970) 80. E. Bauer, Surface Sci. 7 (1967) 351. E. M. Otto, J. Electrochem. Sot. 113 (1966) 643. J. A. Allen, Australian J. Chem. 13 (1960) 431. W. M. H. Sachtler, in: Catalysis Reviews, Vol. 4, Ed. H. Heinemann (Dekker, New York, 1971) p. 27. W. A. Schmidt, 0. Frank and A. W. Czanderna, J. Vacuum Sci. Technol., 19th National Symposium issue (1973).