An XPS investigation of the chemisorption of oxygen on the iridium (111) surface

Surface Science 61 (1976) 25-36 Q North-Holland Publishing Company

AN XPS INVESTIGATION OF THE CHEMISORPTION OF OXYGEN ON THE

IRIDIUM (111) SURFACE

P.A. ZHDAN, G.K. BORESKOV and A.I. BORONIN

Institute of Catalysis, Academy of Sciences, Novosibirsk, USSR

W.F. EGELHOFF, Jr. and W.H. WEINBERG *

division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, ~aliforn~ 91125, USA

Received 23 June 1976

Using X-ray photoelectron spectroscopy (XPS) to monitor the concentration of adsorbed oxygen on Ir(11 l), it was found that at temperatures well below the onset of thermal desorption, the adsorbed oxygen disappears from the surface. A pressure of approximately lo-’ Torr of 0, over the crystal surface ensures that the adsorbed oxygen is not removed via reaction with background CO. The implication is that oxygen is diffusing into the bulk of the crystal. Support for this idea comes from the observation of a small, broad peak in the X-ray photoelectron spectrum of the “clean” surface at the same binding energy as the oxygen signal and the difficulty in removing the oxygen which contributes to this peak. Evidently, this peak derives from oxygen .dissolved in the Ir lattice. The saturation coverage of oxygen is estimated to be 7.9 X 1014 atoms/cm2 by comparison with XPS data for a CO saturated surface. This coverage is consistent with the interpretation that the (2X2) oxygen LEED pattern arises from three independent domains of (2x1) oxygen superstructures rotated 120” with respect to each other.

1. Introduction

The surface chemistry of Ni [l-7], Pd [8-141, and Pt [ 15-271 has been exten- sively investigated in recent years in response to the widespread industrial use of these metals as catalysts. However, in spite of the fact that II has very similar chemical properties, very few studies have been published concerning the surface chemistry of Ir single crystals [28-331. Studies of the CO oxidation reaction on (11l)Ir [32] and (1lO)Ir [33] single crystals have been reported. The chemisorption of oxygen and the CO oxidation reaction have been investigated on a recrystallized Ir

* Alfred P. Sloan Foundation Fellow.

25

26 P.A. Zhdan et al. / XPS investigation of chhemisorption of oxygen on Ir(llI)

foil [29], (evidently exposing largely (110) oriented microcrystallites at the surface [34]). Iridium is known to form a stable oxide with the stoichiometry IrO, in a

rutile structure [35]. This oxide decomposes at approximately 1380 K in one atmo- sphere of 0, [36], but it has been reported that this oxide cannot be formed by heating the pure metal in 0, and that this is apparently due to an impervious oxide coating that prevents complete oxidation 1361. In studies of the vaporization of iridium in air, the desorbing species have been reported to be IrOz molecules with a heat of vaporization of 85 + 7 kcal/mole [37].

The work reported in this paper represents one part of a series of investigations that we are conducting concerning the surface chemistry of various crystallographic orientations of Ir single crystals ]34,38-413. In this paper, we consider chemisorption of oxygen on the (111) surface of Ir. These results are of direct relevance to questions concerning the initial stages of the oxidation of Ir.

In our previous mass spectrometric study of the chemisorption of oxygen on (lll)Ir 1391, several points were noted which support and elucidate the XPS results reported here. It was found that during the heating of the crystal in the thermal desorption of oxygen, a part of the oxygen which was chemisorbed on the surface at 325 K diffused into the crystal at a temperature between 325 and 700K. After diffusion into the crystal, the oxygen in this near-surface state did not desorb until the crystal reached 1270 K. However, a major fraction of the oxygen adsorbed on the surface at 325 K desorbed as molecular oxygen between 800 and 1150 K with a coverage dependent activation energy for desorption of 65-10 0 k&/mole. The oxygen which diffused into the crystal was found to be highly unreactive toward CO at temperatures which oxygen adsorbed on the surface is extremely reactive with CO (e.g., 600 K). In other work, we found that after diffusion of large amounts of oxygen into the crystal (e.g., 10 min in 2 X 10m7 Torr 0, at 700 K), the surface exhibited somewhat altered properties in the transient kinetics of the CO oxidation reaction [4 1] . Thus, further characterization of this diffusion of oxygen into the crystal was warranted on the basis of its possible relevance both to oxidation catalysis as well as to the incipient stages of corrosion of noble metals.

2. Experimental

The X-ray photoelectron spectra (XPS) reported in this paper were obtained with a Vacuum Generators ESCA-3 spectrometer. After bakeout, the base pressure in the instrument was I X lo-‘* Torr. The design of the spectrometer includes a sample preparation chamber and a separate chamber in which the XPS experiments are conducted. These two chambers are pumped by separate oil diffusion pumps. The sample under study is mounted on the end of a cylindrical probe which may be retracted into the sample preparation chamber or advanced into the XPS chamber while maintaining the UHV environment.

The sample preparation chamber was equipped with two Vacuum Generators

P.A. Zhdan ct al. / XPS investigation of chemisorption of oxygen on Ir(lll) 21

model 20 ion gauges, one in the chamber and the other at the end of the pumping line immediately above a diffusion pump. This chamber also had an electron bombardment gun for sample heating, a Varian ion gun (operated at 21.1A of 600 eV Xe’ ions incident on the crystal), a Vacuum Generators Micromass Two mass spectrometer, and independent leak valves to admit the desired gases.

The Ir( 111) crystal (the diameter and thickness of which were seven and 0.6 mm, respectively) was cut and polished by well established procedures as described

in ref. [38]. The crystal was mounted on the end of the cylindrical probe by spot welding it to a Ta foil, 0.3 mm in thickness, which was mechanically clamped to the probe. This ensured almost complete temperature equivalence between the crystal and the end of the probe. This is important because the crystal temperature (for T < 900 K) was monitored by a chromel-alumel thermocouple embedded in the end of the probe. A careful estimate of the temperature difference led US to make minor corrections to the thermocouple reading at the highest temperatures used during the XPS measurements. For lower temperatures, this correction factor (proportional to 9 due to radiative losses from the crystal) rapidly became insignifi-

cant. The crystal was cleaned by repeated cycles of heating at approximately 1100 K

in lo-’ Torr of 0, followed by extensive Xe+ bombardment. Temperatures above 1000 K were measured externally with an optical pyrometer. For cleaning purposes, the crystal was heated by electron bombardment. For kinetic studies, the crystal was rnaintained at constant temperatures by a resistive heating element mounted in the end of the probe.

After the repeated cleaning cycles had removed carbon from the near-surface

region of the crystal, the crystal could be cleaned subsequently by heating for a few minutes in lo-’ Torr 0, at 1100 K followed by heating for 30 set to 1600 K in UHV. The crystal and probe are then transferred immediately to the XPS chamber for the kinetic studies.

When the probe is fully inserted in the XPS chamber, a metal collar presses against a viton O-ring to isolate the XPS chamber from the sample preparation chamber. For the kinetic studies reported here, it was necessary to mount the crystal so that it would be in position for the XPS experiment before the collar reached its seating position on the viton O-ring. This arrangement permitted the introduction of CO and 0, through the leak valves in the sample preparation chamber into the XPS chamber for the adsorption studies. The total pressure of these gases is monitored by an ion gauge in the XPS chamber which is located just above the XPS chamber diffusion pump. Our estimate, based on a similar situation in the sample preparation chamber where there are two ion gauges, is that the gas pressure over the crystal is a factor of two larger than that measured at the gauge. Thus, the pres- sures we report in this paper are a factor of two larger than the actual gauge read- ings. In a similar spectrometer, Menzel et al. [42] have reported an analogous pressure correction factor of up to 50%. Therefore, some caution is advised with respect to the exact value of the pressure over the crystal.

28 P.A. Zhdan et al. / XPS investigation of chemisorption of oxygen on Ir(lI1)

(e)

Cd)

I 5c

(b)

T z 612 K

T = 710 K

I I I I I / I, I, I I

526 520 530 532 534 536

BINDING ENERGY. eV

Fig. 1. Equilibrium XPS data for the oxygen 1s region of the spectrum for various surface con-

ditions. The spectrum of the cleanest surface routinely attainable is presented in (a). Spectra

(b), (c) and (d) illustrate the decrease in the oxygen 1s peak intensity when the crystal is heated

in lo-’ Torr Oz. Spectrum (e) demonstrates that after heavy oxidation and subsequent removal

by CO of reactive oxygen from the surface, the oxygen 1s binding energy is shifted from that of

oxygen adsorbed on the surface. In spectra (b) through (e) the smoothed “clean” surface spec-

trum has been included to illustrate better the changes occurring.

P.A. Zhdan et al. / XPS investigation of chemisorption of oxygen on Ii-(11 1) 29

When XPS data were taken at a constant temperature in lop7 Torr of Oa, the crystal was always allowed five to ten minutes after each temperature change to ensure that thermal and kinetic equilibrium were attained before the XPS measurement was begun. The XPS measurement sequences consisted of a scan of the photoelectron spectrum of the Ir 4f levels (binding energy approximately 60 eV), then a scan of the carbon 1s levels (binding energy approximately 285 eV), and finally a scan of the oxygen 1s levels (binding energy approximately 530 eV). Generally, the scans of the 4f levels were terminated after only two scans (to ensure the accuracy

of the spectrometer calibration), the scans of the carbon region were terminated

after 10 to 20 scans (to ensure the absence of unwanted contamination by carbon), and 30 scans were made through the oxygen region. The counting time was 0.2 sec- ond per point per scan. Thus, each point in the spectra of fig. 1 represents a total counting time of six seconds. The counts obtained in each scan were summed with those of previous scans and stored in an on-line PDP-8 computer. The entire elapsed time for collection of data at each temperature was typically 10 to 15 min. This ensures that the measured oxygen concentration was indeed a steady-state value.

In this work, all reported electron binding energies are referenced to the observed Fermi level. The Ir 4f levels were used only as a check that the spectrometer calibration ‘was unchanged. Since the XPS analyzer collected electrons at -60” to the crystal surface normal, the binding energy of the Ir 4f electrons is certainly independent of adsorbate coverage [43].

In all XPS data reported here, the pass energy of the analyzer was set at 20 eV (a retarding potential is applied to the crystal) to give an analyzer resolution of 0.4 eV. This produced the optimum compromise between signal intensity and spectral resolution. The observed linewidths in the spectra are considerably broader than the analyzer resolution due to the approximately 1 eV linewidth of the Al Ka radia- tion. The X-ray source was always operated at 10 keV and 40 mA for a total power dissipation at the anode of 400 W.

3. Results

In fig. 1, we present XPS data for the oxygen 1 s region of binding energy measured under various conditions. The spectrum of the “clean” surface is shown in fig. la. This spectrum shows a broad peak of low intensity centered near a binding energy of 530 eV. Since this peak evidently represents residual oxygen on the surface, we made extensive efforts to reduce its intensity. High temperature annealing to 1600 K proved to be the most successful treatment. However, fig. la illustrates the minumum intensity of this peak which we were routinely able to obtain. We attribute this feature to the presence of oxygen dissolved in the Ir lattice since this type of oxygen has very different properties from those of oxygen adsorbed on the surface. This dissolved oxygen was found to be very unreactive toward either Ha or CO under conditions which readily remove oxygen adsorbed on the surface. This

30 P.A. Zhdan et al. / XPS investigation of chemisorptiorl of oxyRen on Ir(lll)

was demonstrated by the observation that the spectrum of fig. la was unaffected by heating the crystal to 1100 K for 10 min in lop7 Torr of either CO or HZ. Lower temperatures were also tested with a similar lack of effect.

We have verified that the peak in fig. la is not an intrinsic Ir line. Excitations corresponding to all the core electronic energy levels of Ir are accounted for in other parts of the spectrum. We identified all the intense, XPS induced, Auger electron transitions in the spectrum by recording comparison spectra with a Mg Ka X-ray excitation source. This established that the peak at 530 eV in fig. la was not an Auger transition. The only other possible sort of intrinsic Ir line would be an inelastic loss peak. However, this possibility is very unlikely since the nearest intrinsic Ir line has a binding energy 3.5 eV less than the feature in fig. la. Furthermore, an almost identical feature has been observed in XPS data on a “clean” Pt foil, and we consider this to be final confirmation that we are not viewing an intrinsic metal

line in fig. la [25]. The spectra in figs. lb-d illustrate the changes which take place in the equilib-

rium concentration of adsorbed oxygen when the crystal is heated in 1 X lop7 Torr 0,. In order to determine the area under the peak due to oxygen adsorbed on the surface, the smoothed “clean” surface spectrum has been included as a background. In fig. lb, and to some extent in fig. lc, a tail of inelastically scattered or “shake- up” electrons is evident on the high binding energy side of the oxygen 1s peak. It makes little difference to our results concerning surface coverage whether we include or neglect this inelastic tail.

In all the spectra that we recorded in 0, with a crystal temperature below 650 K, e.g., figs. lb and lc, the oxygen 1s peak maximum occurred at a binding energy 529.8 * 0.2 eV greater than the observed Fermi level. However, at higher temperatures the peak maximum shifted to a greater binding energy as illustrated in fig. Id. This shift of approximately 1 eV indicates a considerable perturbation in the character of the oxygen species present near the surface [44]. This behavior was clarified by fig. le which represents the spectrum after the following treatment. After recording the spectrum shown in fig. Id, the crystal was cooled to 490 K, a temperature at which the reaction between CO and oxygen adsorbed on the crystal surface proceeds at a rate of approximately one CO, molecule formed for each incident CO molecule [41]. At 490 K, the crystal was exposed for five minutes to lop7 Torr CO. The CO was then evacuated from the vacuum system, and the crystal was heated to 612 K in the ambient vacuum of approximately 2 X 10-l’ Torr. This is suffi- cient to remove all adsorbed CO, as we confirmed in separate XPS studies. At 612 K, the spectrum shown in fig. le was recorded. This spectrum demonstrates that exposure of the surface at high temperatures to O2 produces a type of oxygen which is highly unreactive with respect to CO and exhibits a 1s binding energy significantly different from that of oxygen adsorbed on the crystal surface.

A plot of the amount of oxygen adsorbed on the Ir(ll1) surface (proportional to the intensity of the peak at a binding energy of 529.8 eV) as a function of crystal temperature is shown in fig. 2. The spectrum of fig. Id at 710 K was not

P.A. Zhdan et al. / XPS investigation of chemisorption of oxygen on Ir(il1) 31

included in fig. 2 since the oxygen peak at this temperature is no longer centered at

529.8 eV. The absolute coverage values indicated in fig. 2 were obtained by comparison both with XPS and LEED data for CO on Ir (111). At saturation coverage of CO at room temperature, the Ir(ll1) surface exhibits a (243 X 2d3)R30° LEED pattern indicating an absolute coverage of 9.1 X lOi CO molecules/cm~ (32,38,41]. Within experimental error (rms noise), the XPS oxygen Is ~tensity is the same for the oxygen saturated surface as for the CO saturated surface. The (2X2) LEED pattern of the oxygen is consistent with two possible values for the saturation coverage [39]. If the periodicity of the adsorbed oxygen superstructure is twice that of the substrate, the coverage would be 3.9 X lOI oxygen atoms/cm2, assuming one oxygen atom per unit cell. However, the (2X2) LEED pattern could be the result of three independent domains of (2X1) superstructures rotated 120’ with respect to each other. If islands of each of the three domains were present, all half order spots in the (2X2) LEED pattern would be accounted for. The indepen-

dent domains of (2X1) structure would give a ~turation coverage of 7.9 X lOI oxygen atoms/cm*, assuming one oxygen atom per unit cell. This coverage is quite close to that predicted by LEED for saturation coverage of CO. Since the XPS oxygen 1s peak intensities for the two cases are equal within experimental error, we feel that the balance of the evidence favors an oxygen coverage at saturation of 7.9 X 1014 atoms/cm*.

We believe that the decrease in the oxygen coverage above 600 K shown in fig. 2

9- O(ls) 61~~1~~ ENERGY 529.8eV -

O- t I I 300 400 500 600 700

TEMPERATURE. K

Fig. 2. A plot of the equilibrium concentration of adsorbed oxygen on the surface as a function of crystal temperature in lo-“ Torr O-2. Results of spectra Id and le are not included since they represent formation of a near-surface oxide. See text for method of calibration of the concentration.

32 PA. Zlrdan et al. / A’PS investigation of chemisorption of oxygen on Ir(l1 I)

cannot be explained by a chemical reaction with background CO. In order to pre- vent this possibility an 0, pressure of 10M7 Torr was maintained over the crystal while the data presented in fig. 2 were being recorded. In order for the decrease to be due to reaction with background CO, we estimate that a CO pressure greater than 1 X lo-* Torr would have been required (recognizing the sticking probability for oxygen is an order of magnitude smaller than for CO, and the reaction probability is unity [38-411). However, under such conditions the background pressure of CO must have been less than 1 X 10W9 Torr. A more reasonable interpretation of the data shown in fig. 2 is the diffusion of oxygen into the bulk of the lr.

After the spectrum shown in fig. le was recorded, the lr crystal was cooled to 475 K in a continuous flow of 1 X 10m7 Torr 0,. Although the resulting XPS spectrum of the surface showed the oxygen Is peak to be slightly asymmetric due to the feature seen in fig. le, the 1s peak maximum was nevertheless at 529.8 eV with exactly the same intensity as that of fig. lb. This data point was not plotted in fig. 2, but it demonstrates the reliability of the results in fig. 2.

4. Discussion

It is our opinion that there are many opportunities for important research in the

area of the initial stages of oxidation of the catalytically active Group VIII metals. For example, diffusion of oxygen into the microcrystallites of supported metal catalysts may well be a common occurrence [4.5]. One of the more interesting facts to emerge from the present work is that the greatest problem in obtaining an atom- ically clean lr surface is the complete removal of oxygen. Previous work on various

metallic single crystal surfaces generally leads one to believe that carbon is the most ubiquitous contaminant. This is certainly not the case with the Ir( 111) surface, and preliminary indications are that this is also not the case for the lr( 110) surface [34]. It is interesting to note that in a study of iridium by field ion microscopy, the main bulk impurity was reported to be oxygen dissolved in the lattice at a concentration of one oxygen atom per 2100 Ir atoms [46]. A limited amount of work has been published on various Pt surfaces which indicates that stable near-surface oxides can form in high vacuum studies and can substantially alter the surface properties [17,47].

The intensity of the residual oxygen XPS peak on the “clean” Ir( 111) surface, as well as that observed on a “clean” Pt foil [25], does not indicate so great a concentration of oxygen near the surface as might be thought by cursory inspection of the XPS data. It must be remembered that XPS is only a near-surface probe in the sense that the escape depth for photoelectrons with 950 eV kinetic energy is at least 10 A and perhaps as large as 30 A [48,49]. If the residual oxygen peak on the “clean” surface is taken to represent, for example, 1 X lOI atoms/cm*, we may calculate the concentration of oxygen atoms in this near-surface region to be one per 71 Ir atoms for a 10 A escape depth, or one per 214 Ir atoms for a 30 A escape

P.A. Zhhdan et al. / XPS investigation of chemisorption of oxygen on Ir(l I I) 33

depth. Certainly such a small concentration, assuming it is evenly dispersed, does

not constitute a stoichiometric oxide of Ir. Furthermore, we believe that we may exclude the possibility that this residual oxygen is present in the form of oxide nuclei. Any IrO, present at the surface would have decomposed or desorbed during the extensive heating to 1600 K [36,37].

It is of further interest to reflect on the minimum treatment that would be necessary to establish this oxygen concentration throughout the entire bluk of the crystal. We have previously estimated the sticking probability of 0, on Ir(ll1) to be 0.09 until saturation coverage is almost reached. This value is consistent with published values for the sticking probability of 0, on various Pt surfaces [21]. If we assume that there exists a crystal temperature at which every oxygen atom entering a chemisorbed state on the surface will eventually diffuse into the bulk, that the sticking probability of 0, is 0.1, and that there is no significant concentration of oxygen in the bulk initially, then knowing the crystal thickness to be one mm, we may easily calculate the time necessary to reach a concentration of one oxygen atom per 100 Ir atoms. In a background pressure of 1 X lo-’ Torr O,, this time is 5.5 days. In contrast to this situation, the metal crystallites in a typical alumina supported industrial catalyst have diameters of only a few tens of Angstroms and could saturate with oxygen instantaneously when calcined in air. It is noteworthy that results almost identical to those presented in fig. 2 have been reported for the Pt (100) surface [ 501.

Ours is certainly not the first investigation to delineate both the importance of diffusion of oxygen into a Group VIII single crystal lattice and the large exposures to oxygen needed to achieve saturation. May and Germer found quite similar behavior on a Ni(ll0) surface [l]. They estimated, using LEED, that only when the bulk reached a concentration of one oxygen atom per 2000 Ni atoms was there an appreciable equilibrium concentration of oxygen on the surface [ 11. Nickel is, of course, well known for its property of being oxidized easily to NiO, so perhaps their results were not too unexpected. It is significantly more surprising to find a non-stoichiometric oxide in the Ir(ll1) near-surface region.

It is interesting to note that the increased intensity of the oxygen signal following heavy oxidation of the surface (fig. le) exhibits a binding energy different both from that of oxygen adsorbed on the surface and that of the residual peak of the “clean” surface. Since this increased intensity can be removed by heating the crystal to 1600 K for a few seconds, it may be that this peak represents, in part, the formation of a near-surface oxide state different in character from the oxygen dissolved in the lattice non-stoichiometrically. The surprising feature of the oxide peak in fig. le is its binding energy of approximately 531.5 eV. It has generally been found in XPS studies that the binding energy for oxygen present in molecularly bound adstates (e.g., CO, Ha0 and CO,) is larger than that corresponding to atomi- cally bound oxygen. This is generally though to be caused by a larger screening of the oxygen core hole when the oxygen is in intimate contact with the quasi-free electrons at the metal surface [24,25,27,44,51]. A simple extension of this argu-

34 P.A. Zhdan et al. ,J XPS investigation of chemisorption of oxygen on b-(11 I)

ment would lead one to expect that the 1 s binding energy for oxygen in this unreactive near-surface oxide state should be even less than for oxygen adsorbed on the surface. Since the complexities of electronic interactions at metal surfaces have yet to be understood fully, we can only suggest why the simplest model does not explain our data. One possibility, based on differences in the electronic reorganiza- tion energy, is that the electrons in the metal and at the metal surface are more polarizabie than the electrons in the near-surface oxide of fig. le. A related possibility based on initial state effects, is that the oxygen acquires a smaller net negative charge in the near-surface oxide than it does when adsorbed on the surface. Still another possibility is that when oxygen is adsorbed on the crystal surface, the oxygen nucleus lies within the electrostatic potential barrier of the surface SO that the Is electron loses less kinetic energy in surmounting this barrier than would be the case for oxygen in a near-surface oxide. Finally, it is possible that the additional oxygen in fig. Ie is present as “adsorbed” IrO,. Hopefully, future studies will resolve

these issues.

5. Conclusions

Our major conclusions may be summarized as follows. (1) At crystal temperatures 2600 K, dissociatively adsorbed oxygen on Ir( 111) dif- fuses into the bulk to form a non-stoichiometric oxide state. (2) It is exceedingly difficult to remove this non-stoichiometric oxide state com- pletely from the near-surface region since it is apparently impervious to reaction with CO or II,, and it is stable at 1600 K. (3) Extensive oxidation of the surface (more than IO min at approximately 700 K in lo-’ Torr 0,) produces a near-surface oxide state which exhibits a 1s binding energy larger than either oxygen adsorbed on the crystal surface or oxygen in the non-stoichiometric state. (4) Oxygen in this near-surface oxide is unreactive toward CO at a crystal temperature at which oxygen adsorbed on the crystal surface is very reactive. (5) The saturation coverage of oxygen on the room temperature surface produces an apparent (2X2) I&ED pattern via the formation of three domains of (2X1) structures which are rotated 120” with respect to each other. This structure corresponds to a saturation coverage of 7.9 X lOI oxygen atoms/cm’.

Acknowledgement

This work represents one phase of the Joint US-USSR Program in Chemical Ca- talysis and was supported by the National Science Foundation under Grant Number GP-41807.

P.A. Zhdan et al. / XPS investigation of chemisorption of oxygen on Ir(ll1) 35

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Documents

An XPS investigation of the chemisorption of oxygen on the iridium (111) surface