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The interaction of CO and O2 with the (111) surface of Pt3Ti
121
Surface Science 177 (1986) 121-138 North-Holland, Amsterdam
THE INTERACTION SURFACE OF Pt,Ti J. PAUL,
OF CO AND 0, WITH THE (111)
S.D. CAMERON,
D.J. DWYER
Corporcrte Resecrrch Science L.uhomiories, East, Atmandule, NJ 08801, USA Received
29 April 1986; accepted
EXXON
for publication
and F.M. HOFFMANN
Research and Engineering
Compuny,
Route 22
17 June 1986
The electronic properties of clean and partly oxidized Pt,Ti(lll) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt?Ti(lll) is quasi-pure platinum. Scarcely any (0 = 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(ll1) and a p(2 X 2) “reconstruction” of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm-l which coincides with infrared absorption spectra of TiO,. Flashing with CO shifts the band to 500 cm -‘. Correlated with this shift we observe (i) CO, desorption at a temperature well above that observed for Pt(lll)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same ~(4x2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates “islands of Ti02”. segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti3+ species. The vibrational shift, 650 to 500 cm-‘. can be understood by the dipole active bands of a triatomic O-Ti4+ -0 vibrator compared to a diatomic Ti3+-0 vibrator.
1. Introduction The chemisorption properties of intermetallic compounds have been studied in hopes of understanding the role of electronic modifications with relation to their catalytic activity. From a surface science point of view, we characterize the adsorption potential of those material by discriminating between “ensemble” and “electronic” effects. Work on Ni,Ti suggests that the covalent interaction and the effective charge transfer between the two elements are responsible for its facile dissociation of CO [l]. CO chemisorption on single 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
122
J. Pml et ~1. / Interaction
of CO and OL with Pt,Tl(lll)
crystals of various Pt-Ni alloys shows a slightly weaker metal-CO bond compared to the pure metals due to the preferential segregation of Pt to the surface [2]. In this communication, we report the chemisorption properties of a Pt ,Ti(lll) single crystal surface. With a variety of spectroscopies we will show that although XPS shows large core level shifts this has little effect on the crystal’s chemisorption properties. As with the Pt-Ni alloys, Pt,Ti(lll) has a Pt enriched surface whose oxide, presumably TiO,, segregates to the surface. This suppresses CO chemisorption much in the same way as TiO, overlayers on Pt do [3]. We observed the chemical reduction of the oxide by CO in a shift of the Ti2p core level, a depletion of surface oxygen, and carbon dioxide desorption at an anomalously high temperature. In addition, vibrational spectroscopy characterized the reduction by a striking 150 cm- ’ shift of the main Ti-0 adsorption band to lower energy.
2. Experimental Experiments were performed in two separate ultra-high vacuum (UHV) chambers, routinely operated at a background pressure of lo-” Torr. Both chambers accommodated low energy electron diffraction (LEED, Varian 981) and multiplexed mass spectrometers (UT1 1OOC) with drift tubes to selectively sample desorption products from the front side of the crystal. These two spectroscopies enabled us to in situ correlate experiments performed in the two chambers. Surface composition was verified by XPS (chamber I) or Auger electron spectroscopy (AES, chamber II). In addition the first chamber contained facilities for low energy ion scattering (ISS) and the second chamber a double pass electron energy loss spectrometer (Leybold). XPS data were obtained with Ka radiation (1253.6 eV) and photoelectrons collected with a hemispherical electron energy analyzer (50 eV pass energy) which enabled us to rotate the crystal in order to change the azimuthal angle and thus the depth profile of sampled electrons. He + ion spectra (500 eV, 10 nA) were collected in a constant relative resolution mode (SE/E = 4). Finally, the EELS spectrometer was operated at 40 cm- ’ resolution and 2 eV primary beam energy. The crystallization procedure and the crystal mount was described by Bardi and Ross [4]. In short, a single crystal slice was gold brazed to a tantalum foil which served as a resistive heater 141. The crystal was sputtered (300 K, 10 PA Art, 1 kV, 5 min) and annealed (1100 K, 5 min) before each set of experiments. This procedure gave symmetric LEED spots with low background intensity. No significant signal from residual carbon could be detected by either XPS or Auger (272 eV). Note however the difficulty to separate a minor carbon signal from adjacent platinum Auger peaks [5]. Carbon monoxide was adsorbed at 100 K either via back-filling of the chamber for LEED and EELS studies or via a beam doser or multi-channel
.I. Paul et al. / Interaction of CO and 0, with Pf,Ti(III)
123
array l-4 cm from the sample for thermal desorption studies (TDS). Typical heating rates were 15-25 K s-i. The crystal was also dosed with CO at 370 and 420 K in order to examine if any of the major TDS features was due to carbon/oxygen recombination. To develop a surface oxide, the sample was heated to 650-1000 K in front of a doser in an effective background of 10-8-10-7 Torr of oxygen. In case of ISS the sample would be re-oxidized briefly to replace sputtered oxygen.
3. Results and discussion 3.1. Pt,Ti/CO TDS, EELS, and XPS observations show that CO binds reversibly and associatively to the (111) surface of Pt,Ti and that the outermost atomic layer after annealing resembles quasi-pure platinum with a few “defect” sites (t9 = 0.01) where dissociation occurs. A 4 kcal/mol decrease in the heat of adsorption, a 30-40 cm-’ decrease of C-O stretching frequencies, and a 0.2 eV shift of the 0 1s is peaks compared to Pt(ll1) are the only evidence for an electronic contribution to the chemisorption bond due to alloy formation. These shifts are reasonable effects of a second layer enriched in a different metal (titanium) and similar in magnitude to shifts observed for the platinum enriched surface of Pt,Ni,_, alloys compared to Pt(lll) [2] or for a-closely packed copper overlayer on Ru(0001) compared to Cu(ll1) [6]. 3.1.1. Discussion
The CO thermal desorption spectra of the Pt,Ti(lll) surface exhibit features which are remarkably similar to those of the Pt(ll1) surface [7,8]. In fig. la we see these features fill as a function of coverage. Saturation with CO at 100 K (5 L) changed the p(2 x 2) LEED pattern of the clean surface (fig. 2a and ref. [4]) to a diffuse c(4 X 2) pattern which sharpens upon annealing to 250 K (fig. 2b). The annealed CO overlayer exhibits vibrational losses characteristic of terminally (2060 cm-‘) and multiply coordinated (1810 cm-‘) molecules (fig. 3a, top). Annealing to 325 K resulted in the desorption of the multiply coordinated species leaving only on-top bonded molecules (fig. 3b, top). This second annealing changed the LEED pattern back to a less sharp c(4 x 2) structure. We can, as shown in figs. 3a and 3b, unambiguously correlate CO molecules vibrating at 2050-2060 cm-’ with the desorption peak at around 380 K and the loss at 1810 cm-’ with the peak at 320 K. TDS (fig. la) as well as EELS show that these two sites fill subsequently. Finally, fig. 4 displays the C 1s and 0 1s features in XPS spectra as a function of CO coverage. We have not observed any surface bound CO molecules except those that correlate with the desorption peaks at 320 and 380 K.
J. Paul et al. / Interaction of CO and O2 with Pt,Ti(lIl)
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Fig. 2. Schematic representations of LEED images of (a) the bare (111) surface of Pt,Ti and (b) the surface saturated with CO at 100 K and annealed to 250 K (sharp pattern) or 325 K (dim pattern). (c) The LEED pattern correlated with exclusively on-top bonded CO molecules on the Pt(ll1) surface [8,24].
As previously mentioned, the CO TDS spectra from Pt,Ti(lll) are very similar to those of Pt(lll), however the Pt,Ti spectra exhibit a 50 K rigid shift towards lower temperature compared to pure platinum [S]. This converts to a 4 kcal/mol decrease in the adsorption energy assuming the same frequency factor as for Pt(ll1) [8,9]. Correlated with this decrease in adsorption energy we observe a 30 cm-’ decrease of the metal-carbon vibrations and a 40 cm-’ decrease of the carbon-oxygen vibrations, again compared to Pt(ll1) [8]. The shifts of the carbon-oxygen vibrations are opposite those expected for a transition metal surface. The Pt,Ti(lll) surface thus follows a pattern which distinguishes the interaction of CO with noble metals (Cu, Ag, Au, and Pt) as compared to transition metals [lo]. CO chemisorption on noble metals uniquely gives negative workfunction shifts [7,11], a shake-up below the CO40 peak in valence band photoemission spectra [7,12,13], and a marked asymmetry of the 01s peak in core level spectra [7,14,15]. Furthermore, on-top positions on these metals exhibit higher desorption temperatures [6,7,16] and higher vibrational frequencies than multiply coordinated sites [17,18]. The minor CO desorption peak around 440 K (fig. la) has also been observed for Pt(ll1) and interpreted as originating either from residual defect sites [19,20] or from the rim of the crystal [8]. The small diameter of the Pt,Ti crystal does not allow us to fully discriminate against the latter possibility. Pt(ll1) crystals are typically annealed at 1400 K to reduce the number of defects. However, due to the gold-brazing technique we were hesitant to
Fig. 1. TDS spectra saturation
of Pt,Ti/CO at (a) increasing CO coverages coverage at increasing oxygen pre-exposures.
on the bare surface and (b) CO Heating rate 25 K s-l.
126
J. Paul et crl. / Interuction
of CO
Electron Energy Loss (cm 1)
--
0
and 0,
with Pt,Ti(llI)
Desorption Temperature (K)
1 1000
2000
Electron Energy Loss (cm -1)
200
400
600
Desorption Temperature (K)
Fig. 3. EELS spectra of CO adsorbed onto the clean or partly oxidized (111) surface of PtsTi. Each surface was saturated with CO at 100 K (5 L) and annealed either to 250 K (a) or to 325 K (b). After the first annealing (250 K) we observed a sharp ~(4x2) LEED pattern which became dim after a second annealing (325 K). Note that metal-carbon vibrations are shadowed by metal-oxygen bands for the partly oxidized surface. To the right of each vibrational spectrum is a CO desorption spectrum of that particular overlayer. Heating rate 15 K s-‘.
anneal our sample above 1200 K in fear of having gold melt and wet the Pt 3Ti surface or even break the mount. The lower annealing temperature presumably leads to a higher number of defect sites.
J. Paul et al. / Interaction
103 cps
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127
of CO and 0, with Pt,Ti(lll)
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Binding
Energy (eV)
Fig. 4. C Is and 0 1s core level spectra for Pt ;Ti(ll l)/CO as a function of CO exposure. Substrate temperature 100 K.
The weakly bound CO TDS feature at 220 K (fig. la) diverges from recent Pt(ll1) TDS spectra [8] but was observed in an earlier study [7]. It may originate from sites unresolved by other spectroscopies and populated only after a high dose at low temperature or may simply be the result of desorption from the sample holder. This peak is significantly altered by oxygen adsorption which suggests the former interpretation as discussed in section 3.3.1. A very weak high temperature TDS feature likely to be a CO recombination peak is seen at 900 K (fig. la) with no change in intensity over the complete coverage range until saturation. Dosing the sample at elevated temperatures also failed to change the intensity of this peak. Although neither EELS nor XPS nor AES showed any evidence of atomic carbon/oxygen present on the surface, this CO recombination feature still puts an upper limit on the bare crystal’s ability to dissociate CO and thus possibly on the number of titanium sites since at least unmodified titanium metal is known to readily dissociate the molecule [21]. A possible CO recombination peak has been reported for polycrystalline platinum at 650 K [20], i.e. at a significantly lower temperature compared to the peak in fig. la. Our LEED observations are in full accordance with CO adsorption on Pt(ll1). The sharp c(4 X 2) structure have a measured fractional coverage 0 of 0.47 [22] and is believed to possess equal occupancy of on-top and bridge sites [23]. Up to a coverage of about 0.17, CO is exclusively linearly bonded at on-top sites [8]. This coverage corresponds for Pt(ll1) to a LEED pattern consisting of the spots marked with filled circles in fig. 2c. Schweizer et al. suggested a real-space model with clustered molecules, fi x &R30”-(4 x 4) for this pattern [24]. However, since in our case the bare Pt ,Ti(lll) crystal
128
J. Paul et ul. / Interactton
of CO and 0,
with Pt ,Ti(lIl)
itself gives a p(2 x 2) pattern (fig. 2a) the overlapping image will be that of a c(4 x 2) structure (fig. 2b). Our interpretation of the p(2 x 2) image of the bare surface is a “reconstructed” quasi-pure platinum surface layer rather than a termination of the bulk lattice with stoichiometric numbers of platinum and titanium atoms present at the surface [4]. The second layer is presumably enriched in titanium. Such a surface-sandwich segregation has been observed for Pt xNi, _x alloys by means of LEED 1-V measurements [25] and also thermodynamically justified taking only nearest-neighbor interactions into account [26]. One final LEED observation is that the bare Pt,Ti(lll) surface gives a pattern of C,, symmetry. This reveals scattering against a second atomic layer and informs about the growth of oxide islands as discussed in the following section. As with LEED, carbon and oxygen XPS spectra (fig. 4) are very similar to previous work with CO on Pt(ll1) [7,27]. The two 01s peaks are rigidly shifted 0.2 + 0.1 eV to higher binding energy compared to literature data for Pt(lll)/CO [7,27]. Within the limits of our detection the two features are observed at all CO coverages and the width of the Cls level remains constant. We do, however hesitate to assign either of the two features to a multielectron excitation since their relative intensities change both with coverage and temperature on Pt(ll1) 171. For the bare surface we also observe a 0.7 eV shift to higher binding energies of the Pt 4f levels compared to platinum metal [3,28]. Bertolini et al. have made a similar observation of a 0.5 eV shift for the [29]. We also observe a 1.7 eV shift of platinum enriched surface of Pt,,Ni, the Ti 2p level towards higher binding energy compared to the pure metal [28]. Interpretation of all shifts are complicated due to extra-atomic relaxation in the final states but can invoke rehybridization of particularly the titanium atoms. The relative shifts of the Pt 4f and Ti 2p levels may indicate a charge transfer to the more electronegative platinum atoms. 3.2. Pt,Ti Independent evidence for platinum enrichment at the surface is obtained from angularly resolved XPS and ion scattering. By monitoring the ratio of the Ti2p (binding energy, E, = 456 eV) and Pt 4f (Et, = 72 eV) core levels at different take-off angles, we observe a depletion of titanium at the surface. An emission angle of 10” reduces the escape depth of electrons with a kinetic energy of 1.2 - E, keV (Mg Ka) by about l/5 compared to normal emission. At this angle a 40% reduction of the titanium-to-platinum ratio was observed which strongly suggests a platinum enriched surface disregarding any effect of matrix elements. Ion scattering data show an increase in the Ti/Pt ratio with higher ion energies again indicating a surface depleted of titanium (table 1). If we correct the ratios in table 1 for variations in cross-sections as measured by Brineu et al. [30], we would indeed say that the titanium concentration in the
J. Paul et al. / Interaction Table 1 Titanium
relative
platinum
ISS intensity
of CO and 0, with Pt,Ti(lll)
for clean Pt,Ti(lll)
Eo (v)
Ti/Pt
500 1000 1500
0.020 0.085 0.202
as a function
129
of ion energy
second layer is higher than on the surface or in the bulk. However, due to the uncertainty in the cross-sections for ion scattering we can from these data alone only conclude that the titanium concentration increases from the surface to the bulk. 3.3. Pt,Ti/ O2 The oxidation of Pt,Ti deviates significantly from literature data on the chemisorption of oxygen on Pt(ll1). He+ ion scattering as well as XPS and AES clearly show that oxygen adsorption segregates titanium to the surface. LEED, TDS, and EELS reveal that the oxide, presumably TiO,, initially grows in the form of islands with no influence on remaining areas of the crystal. Finally, following high exposures at elevated temperatures no bare areas are left on the sample which then shows the characteristics of evaporated TiO, overlayers. Our results at large agree with the detailed study of Bardi and Ross [311. 3.3. I. Discussion: mild oxidation Under mild conditions (650 K in lo-’ Torr 0,) sub-monolayer coverages of titanium oxide were easily produced. AES shows that titanium segregates to the surface even under these conditions (fig. 5 and ref. [31]) and that the stoichiometry of the surface oxide is similar to TiO, [31,32]. EELS reveals one dominant oxygen induced loss at around 650 cm-’ (fig. 6. bottom). This position does not coincide with any known frequency of atomic (480 cm- ‘) or molecular (700 and 870 cm-‘) oxygen on Pt(lll) nor with an observed “oxide” band (760 cm-‘) on the same surface [33,34]. Instead we recall the dominant infrared active absorption band of TiO, which is centered just at his energy [35]. We were not able to resolve the small isotopic shift (1602 + ‘a 0,) of this peak which suggests atomic oxygen coordinated to a light metal (Ti) rather than a heavy one (Pt). Obviously we cannot completely exclude a mixed metal oxide with platinum atoms “substituted” at some titanium sites in a TiO, lattice. CO reduction to CO, (section 3.3) suggests the presence of some platinum bound oxygen atoms. We have not been able to identify a corresponding Pt-0 vibration presumably because of the high intensity of the nearby band at 650 cm-‘.
130
J. Paul et ul. / Interaction of CO and O2 with Pt,Ti(lIl)
3.5
3.0
2.0
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1
I
I
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0
1
2
3
4
5
0503/pt237 Fig. 5. Surface segregation of titanium as a result of oxidation in O2 at 650 K. Ti 387/Pt 237 and 0503/Pt237 give titanium (387 ev) and oxygen (503 eV) Auger intensities relative to the platinum peak at 237 eV. E, = 3 keV. The lines represent (i) a least squares fit to the data points and (ii) the titanium/oxygen ratio of bulk TiO, [32].
The LEED image remained essentially constant under these mild oxidation conditions but with a slightly enhanced background intensity [31]. We note the preserved C,, symmetry of the pattern which shows that the oxidation does not randomize the orientation of remaining, unoxidized areas. A thorough comparison of the structure of these areas and the initially flat unoxidized crystal would require LEED I-V measurements not performed in this study. Such measurements are the more motivated for this system since atomic oxygen is known to produce a p(2 X 2) structure on Pt(ll1) [33,34,36], thus overlapping the pattern of the bare Pt,Ti(lll) crystal.
.J. Paul et al. / Interaction
ofC0
and O2 with Pt,Tl(lIl)
131
3 x 3L CO, Flashed
3 x 3L CO, Flashed
I
0
1
1000 Electron Energy Loss (cm - 1)
Fig. 6. EELS spectra of oxygen bound to Pt,Ti(lll). The primary 0, adsorption gave the spectrum at the bottom of the figure. Subsequent flashing with CO and readsorption of oxygen cycled the peak intensity back and forth. As discussed in section 3.3 we interpret this as a difference between hexacoordinated Ti4+ and pentacoordinated Ti3+ at the surfaces of “islands” of titanium oxide. The ratio between relative Auger signals indicate the portion of titanium atoms participating in the redox process.
Chemisorbed carbon monoxide on the partially oxidized surface produced a c(4 X 2) pattern on remaining “open” areas on the crystal. Futhermore, CO desorption occurred at the same temperatures as on the clean surface (fig. lb and refs. [31,37]) and C-O vibrational frequencies were unshifted compared to the unoxidized sample (figs. 3a and 3b, bottom). These observations exclude any anomalous electronic influence but may indicate a low coverage of atomic of bridge oxygen on “open” areas as observed in the selective suppression bonded species (fig. 3b, bottom and ref. [38]). Possibly related to this we note the absence of any desorption at 220 K for the partly oxidized surface (fig. lb). Further studies are needed to elucidate the role of oxygen site blocking in
132
J. Paul et al. / Interuction
of CO und 0, with Pt,Ti(lll)
preventing the formation of a compressed phase correlated to these weakly bound species. Conclusively we do observe indirect evidence of small quantities of atomic oxygen on remaining “open” areas but the “oxide islands” appear only to physically cover areas of the sample without any significant electronic spillover. A similar influence was observed for TiO, produced by evaporation of the metal onto an oxidized platinum foil [3]. The interaction between chemisorbed CO and the partially oxidized surface of Pt sTi will be further discussed in section 3.4. 3.3.2. Discussion: extensive oxidation Extensive oxidation (1000 K in 10e7 Torr 0,) completely covered the surface by titanium oxide. He + ion scattering showed a seven-fold increase in the Ti scattering peak coupled with the appearance and growth of the oxygen signal and complete loss of the platinum signal (fig. 7). This corresponded to a 40% increase of the Ti2p signal and a 25% reduction of the Pt 4f signal in XPS. We calculate an overlayer thickness of > 6 A assuming a mean free path of > 20 A for 1 keV electrons penetrating the oxide. Whereas mild oxidation I
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Fig. 7. Low energy ISS spectra showing the migration of titanium oxide to the surface as a result of oxidation. (A) Clean Pt;Ti(lll). (B) After 0, exposure (1050 K, lo-’ Torr, 10 min). (C) After additional 0, exposure (1050 K. lo- 5 Torr, 20 min).
J. Puul et al. / Interaction of CO und O2 with Pr,Ti(ll
133
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452.0
450.0
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Fig. 8. XPS Ti2p spectra following mild oxidation and CO reduction. (A) Clean Pt,Ti(lll). (B) After O2 exposure (650 K, 300 s, lo-’ Torr). (C) After subsequent saturation with CO at 100 K and flashing to 650 K. The widths of the peaks indicate altered contributions from Ti4+ and Ti’+ (cf. fig. 9).
only showed a slight broadening of the Tie 2p peak of metallic titanium (fig. 8) extensive oxidation produced entirely new features assigned as Ti3+ and Ti4+ ions (fig. 9 and ref. [3]). Note the strong suppression of the Tie signal under these conditions. By going to low take-off angles we observed an increase in the Ti3+ signal (fig. 9) indicating a higher portion of defects or uncoordinated ions in the surface region. A fully coordinated oxide was never observed even under these elevated conditions. It has been observed that for titania on Pt the titanium oxide species which are in contact with the Pt metal are not easily oxidized from Ti3+ to Ti4+ [39]. 3.4. Pt,TiO,/CO EELS, AES, TDS, and XPS each provide independent proof that CO reduces a portion of the surface segregated oxide to form CO, and surface coordinated Ti3+ ions. This again points at the similarity between the present system and the intentionally produced TiO, oxide on the surface of a Pt foil [3]. Similar partial reductions have been observed for oxidized polycrystalline
134
J. Pnul et al. / Interaction 1253.6
eV -Y
of CO
and O2 with Pt,TI(l
I I)
Analyzer
A
Tib4 459.40 ev
1000
475.0
456.90eV
K, 10 ~8 torr O2
470.0
465.0 6. Energy
Fig. 9. Angular
460.0
455.0
450.0
eV
resolved XPS of a thick multi-valent oxide on Pt;Ti(lll). ~=90°.(B)Thickoxide,B=90”.(C)Thickoxide.8=100.
Pt,Ti after annealing in hydrogen [31] and after Art sputtering or electron bombardment
for single [40].
(A) Clean
crystalline
Pt;Ti(lll),
TiO,(llO)
3.4.1. Discussion EELS at sub-monolayer coverages of the oxide provides the most dramatic effect of the reduction as the peak at 650 cm-’ shifts to 500 cm-’ following CO desorption (fig. 6). Consecutive exposures to dioxygen and carbon monoxide cycles the maximum back and forth. We have verified that this is not caused by changes in scattering or detection angles. Oxidized titanium species have been reported to reversibly segregate into bulk platinum [41]. We believe that this cannot explain the observed shifts since all our measurements were done at 100 K and the sample never annealed above the onset temperature of the reported transformation. We conclude that the peak is a superposition of
J. Paul et al. / Interaction
of CO and O2 with Pt,Ti(l
II)
135
two bands and that the apparent shift actually is the result of an altered relative strength of two intensities. One consistent way to assign these bands is to parameter&e the two equal stretch coordinates of a triatomic 0-Ti4+-0 vibrator so as to fit the asymmetric and thus most intense band to the peak at 650 cm-‘. When we reduce the hexacoordinated Ti4+ species to pentacoordinated Ti3+ ions our one-dimensional model will change from 0-Ti4+-0 to Ti3+-0. Since the Ti-0 bond is very close to ionic, the potential will scale with the net charge of the interacting species. Utilizing the previous but now by 25% reduced force constant we calculate the Ti3+-0 vibration to 504 cm-‘. Obviously this is a first order model which considers only vibrations normal to the surface but we believe that it grips the essentials of a reduction occurring at the surface of TiO,“islands”. The altered intensity between the bands of the reduced and oxidized samples is understandable since a “surface bound” sixth ligand of Ti4’ presumably gives a higher effective dynamical dipole moment than a half embedded species coordinated to Ti3+. Associated with the above shift is a 15% reduction of the AES oxygen signal at a constant Pt/Ti ratio. Moreover, when a detailed analysis of the Ti 2p XPS signal before and after reduction is applied (fig. 8) a significant 0.1 eV narrowing is observed. This result is consistent with a previous study which clearly demonstrated that chemisorbed CO will reduce TiO, in contact with platinum metal [3]. Obviously the low intensity and the lower binding energy
F m44 x10
m30 x156
III11 200
1
400 Desorption
600
200
400
600
Temperature (K)
Fig. 10. Multiplexed TDS spectra following saturation with CO on the partly oxidized (111) surface of Pt,Ti. We associate each mass with a labelled CO or CO, molecule: C’60’RO( m46), C’60,(m44), C’sO(m30), and C’60(m28). Unity intensity refers to the CO desorption peak for clean Pt,Ti(lll). Heating rate 25 K s-l.
136
J. Pm1 et (11./ Intermtion
of CO und O2 wth Pt ,Ti(ll I)
of the Tie signal for these evaporated films compared to the present system helped to contrast the oxidized and reduced forms. Our final observations utilized isotopic labelling and multiplexed mass spectroscopy. Fig. 10 shows the desorption of CO and CO, from Pt,TiO, following saturation with CO at 100 K. We again observe the overall suppression of CO chemisorption on a partly oxidized surface. Moreover we note a CO, peak at around 300 K which is typical for CO oxidation on Pt(ll1) [42,43]. The CO, peak at around 450 K cannot be explained by background contribution and is assigned to the reduction of TiO,. Finally a very weak m30(i2C’s0) peak around 550 K for Pt3Ti’X0_J12C160 indicates atomic exchange with the oxide and thus presumably dissociation and recombination of a small fraction of the CO molecules,
4. Summary and outlook This work on the interaction between CO and 0, and the (111) surface of Pt 3Ti addresses two problems of some relevance. The first is the chemical properties of a bimetallic system and the range and influence of surface segregation. The second problem is the intensely focused interaction between TiO, and Pt. The chemisorption pattern of CO as well as XPS and ISS measurements on the bare surface show a surface layer enriched in platinum and a second layer presumably titanium enriched. As a result of this surface-sandwich segregation we observe a weakened chemisorption bond compared to CO adsorbed onto Pt(ll1). The magnitude of the shifts (50 K, 30-40 cm-‘, 0.2 eV) are typical for the influence of a metal substrate on the chemical properties of a monolayer thick metallic film. The cause of the altered interaction in terms of a perturbed free-electron density or rehybridation of localized orbitals is, however, unknown. We have no evidence for extensive CO dissociation on the clean Pt,Ti(lll) surface. Atomic carbon/oxygen following CO adsorption were not detected nor have we observed any chemisorbed CO with a stretch frequency below 1810 cm- ‘. A recombination peak observed in TDS sets 0 = 0.01 as an upper limit for the dissociation of CO on the (111) surface of Pt,Ti. Oxygen adsorption at elevated temperatures (> 650 K) causes segregation of titanium to the surface at a fraction corresponding to the formation of TiO, with some Ti3+ “defects”. The oxide apparently grows as “islands” with no electronic influence on remaining bare areas of the crystal. CO adsorption on these inhomogeneous and partly oxidized samples causes the reduction of about l/6 of the Ti4+ ions to Ti3+. This partial reduction is accompanied by CO, desorption at around 450 K and a marked 150 cm ’ shift of the major vibrational bands in EELS spectra. We understand this shift
J. Paul ei al. / Interaction
137
of CO and O2 with Pt,Ti(llI)
by a change from hexacoordinated Ti4+ to pentacoordinated Ti3+ at the surface of the “oxide islands”. One may speculate about the role of bare metallic areas where CO can be readily adsorbed in the vicinity of these “oxide islands” for the efficiency of the reduction. We have no evidence for any anomalously coordinated molecules decorating the islands. Consequently, a reaction scenario would be that CO adsorbed on metallic areas becomes available at the oxide following annealing and surface migration. Future in situ measurements could possibly reveal if anomalous adsorption sites are available exclusively in the reducing environments of high H, and CO pressures.
Note added in proof A recent study on Ti(OOOl)/Pt/CO ditional information [44].
and
Ti(OOOl)/TiO,/CO
provides
ad-
Acknowledgement We acknowledge Dr. P.N. Ross at the Department of Chemistry, of California for providing us with the Pt,Ti(lll) crystal.
University
References [l] T.E. Fischer, S.R. Keleman and R.S. Polizzotti, J. Catalysis 69 (1981) 345. [2] J.C. Bertolini, B. Tardy, M. Abon, J. Billy, P. Delichere and I. Massardier, Surface Sci. 135 (1983) 117. [3] D.J. Dwyer, SD. Cameron and J. Gland, Surface Sci. 159 (1985) 430. [4] U. Bardi and P.N. Ross, Surface Sci. 146 (1984) L555. [5] F.P. Netzer and J.A.D. Matthew, J. Electron Spectrosc. Related Phenomena 16 (1979) 359. [6] J. Paul and F.M. Hoffmann, Surface Sci. 172 (1986) 151. [7] P.R. Norton, J.W. Goodale and E.B. Selkirk, Surface Sci. 83 (1979) 189. [8] H. Steininger, S. LehwaId and H. Ibach, Surface Sci. 123 (1982) 264. [9] P.A. Redhead, Vacuum 12 (1962) 203. [lo] Due to the giant number of references we limit ourselves to data for Cu(ll1) and Pt(ll1). [ll] J. Paul, S.A. Lindgren and L. Wallden, Solid State Commun. 40 (1981) 395. [12] J.N. Miller, D.T. Ling, I. Lindau, P.M. Stefan and W.E. Spicer, Phys. Rev. Letters 38 (1977) 1419. [13] H.J. Freund, W. Eberhardt, D. Heskett and E.W. Plummer, Phys. Rev. Letters 50 (1983) 768. [14] M. Kiskinova, G. Pirug and H.P. Bonzel, Surface Sci. 133 (1983) 623. [15] Y. Jugnet and Tran Minh Due, Chem. Phys. Letters 58 (1978) 243. [16] P. Hollins and J. Pritchard, Surface Sci. 89 (1979) 486. [17] B.E. Hayden, K. Kretzschmar and A.M. Bradshaw. Surface Sci. 155 (1985) 553. [18] B.E. Hayden and A.M. Bradshaw, Surface Sci. 125 (1983) 787.
138
J. Paul et al. / Interaction
of CO and 0, with Pt,Ti(lll)
[19] H. Hopster and H. Ibach, Surface Sci. 77 (1978) 109. [20] D.M. Collins and W.E. Spicer, Surface Sci. 69 (1977) 85. [21] Photoemission spectra show modification of the Ti electronic structure compared to pure titanium (SD. Cameron and D.J. Dwyer, Surface Sci 176 (1986) L857 and in preparation). The significance of this with respect to CO dissociation is, however, yet unclear. (221 P.R. Norton, J.A. Davies and T.E. Jackman, Surface Sci. 122 (1982) L593. [23] D.F. Ogletree, M. Van Hove and G.A. Somotjai, Surface Sci. 173 (1986) 351. [24] E. Schweizer, H. Conrad and A.M. Bradshaw, to be published. [25] Y. Gauthier, Y. Joly, R. Baudoing and J. Rundgren, Phys. Rev. B31 (1985) 6216. [26] M. Hillert and J. Rundgren, Phys. Rev. B32 (1985) 640. [27] M. K&nova, G. Pirug and H.P. Bonzel, Surface Sci. 133 (1983) 623. [28] G.N. Derry and P.N. Ross, Solid State Commun. 52 (1984) 151, and references therein. [29] J.C. Bertolini, J. Massardier, P. Delichere, B. Tardy, B. Imelik, Y. Jugnet, Tran Minh Due, L. De Temmerman, C. Creemers, H. Van Hove and A. Neyens, Surface Sci. 119 (1982) 95. [30] J.S. Brine”, D.A. d’Avignon, E.A. Meyers, P.T. Deng and D.W. Behnken, Surface Interface Anal. 6 (1984) 295. [31] U. Bardi and P.N. Ross, J. Vacuum Sci. Technol. A2 (1984) 1461. [32] J.S. Solomon and W.L. Baun, Surface Sci. 51 (1975) 228. [33] J.L. Gland, B.A. Sexton and G.B. Fisher, Surface Sci. 95 (1980) 587. [34] H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 1. (351 An Infrared Spectroscopy Atlas for the Coatings Industry (Federations of Societies for Coatings Technology, Philadelphia, 1980) p. 526. [36] K. Kern, R. David, R.L. Palmer, G. Comsa, Jin He and T.S. Rahman, Phys. Rev. Letters 56 (1986) 2064. [37] U. Bardi, G.A. Somojai and P.N. Ross, J. Catalysis 85 (1984) 272. [38] J.L. Gland and E.B. Kollin, Surface Sci. 151 (1985) 260. [39] C.S. Ko and R.J. Gorte, Surface Sci. 161 (1985) 597. [40] P.A. Cox, R.G. Egdell, S. Eriksen and W.R. Flavell, J. Electron Spectrosc. Related Phenomena 39 (1986) 117. [41] C.S. Ko and R.J. Gorte, J. Catalysis 90 (1984) 59. [42] T. Matsushima, Surface Sci. 127 (1983) 403. [43] J.L. Gland and E.B. Kollin, J. Chem. Phys. 78 (1983) 963. [44] C. Ocal and S. Ferrer, J. Chem. Phys. 84 (1986) 6474.
Surface Science 177 (1986) 121-138 North-Holland, Amsterdam
THE INTERACTION SURFACE OF Pt,Ti J. PAUL,
OF CO AND 0, WITH THE (111)
S.D. CAMERON,
D.J. DWYER
Corporcrte Resecrrch Science L.uhomiories, East, Atmandule, NJ 08801, USA Received
29 April 1986; accepted
EXXON
for publication
and F.M. HOFFMANN
Research and Engineering
Compuny,
Route 22
17 June 1986
The electronic properties of clean and partly oxidized Pt,Ti(lll) surfaces have been studied utilizing carbon monoxide both as a probe and as a reducing agent. Vibrational frequencies and desorption profiles of chemisorbed CO as well as ion scattering and angular resolved X-ray photoelectron spectroscopy (XPS) suggest that the first atomic layer of annealed Pt?Ti(lll) is quasi-pure platinum. Scarcely any (0 = 0.01) dissociation of CO was observed. Minor shifts of vibrational frequencies and desorption temperatures compared to Pt(ll1) and a p(2 X 2) “reconstruction” of the clean surface reveal some influence of the bulk. Auger spectroscopy, XPS, and ion scattering all show an increased titanium signal as a result of oxidation. Surface bound atomic oxygen gives a vibrational band around 650 cm-l which coincides with infrared absorption spectra of TiO,. Flashing with CO shifts the band to 500 cm -‘. Correlated with this shift we observe (i) CO, desorption at a temperature well above that observed for Pt(lll)/O, (ii) an altered Ti XPS signal, and (iii) a reduced oxygen concentration. Subsequently adsorbed CO molecules vibrate at the same frequencies as on the bare surface, give the same ~(4x2) LEED pattern, and desorb at the same temperatures but with reduced intensity, in all proving that the surface oxide only acts as a site-blocker with respect to the metal surface. Our current understanding of these observations is that oxygen creates “islands of Ti02”. segregated to the surface but with no electronic influence on remaining areas of the platinum enriched metal surface. The hexacoordinated Ti4+ ions on the surface of these islands are reduced by CO to pentacoordinated Ti3+ species. The vibrational shift, 650 to 500 cm-‘. can be understood by the dipole active bands of a triatomic O-Ti4+ -0 vibrator compared to a diatomic Ti3+-0 vibrator.
1. Introduction The chemisorption properties of intermetallic compounds have been studied in hopes of understanding the role of electronic modifications with relation to their catalytic activity. From a surface science point of view, we characterize the adsorption potential of those material by discriminating between “ensemble” and “electronic” effects. Work on Ni,Ti suggests that the covalent interaction and the effective charge transfer between the two elements are responsible for its facile dissociation of CO [l]. CO chemisorption on single 0039-6028/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
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J. Pml et ~1. / Interaction
of CO and OL with Pt,Tl(lll)
crystals of various Pt-Ni alloys shows a slightly weaker metal-CO bond compared to the pure metals due to the preferential segregation of Pt to the surface [2]. In this communication, we report the chemisorption properties of a Pt ,Ti(lll) single crystal surface. With a variety of spectroscopies we will show that although XPS shows large core level shifts this has little effect on the crystal’s chemisorption properties. As with the Pt-Ni alloys, Pt,Ti(lll) has a Pt enriched surface whose oxide, presumably TiO,, segregates to the surface. This suppresses CO chemisorption much in the same way as TiO, overlayers on Pt do [3]. We observed the chemical reduction of the oxide by CO in a shift of the Ti2p core level, a depletion of surface oxygen, and carbon dioxide desorption at an anomalously high temperature. In addition, vibrational spectroscopy characterized the reduction by a striking 150 cm- ’ shift of the main Ti-0 adsorption band to lower energy.
2. Experimental Experiments were performed in two separate ultra-high vacuum (UHV) chambers, routinely operated at a background pressure of lo-” Torr. Both chambers accommodated low energy electron diffraction (LEED, Varian 981) and multiplexed mass spectrometers (UT1 1OOC) with drift tubes to selectively sample desorption products from the front side of the crystal. These two spectroscopies enabled us to in situ correlate experiments performed in the two chambers. Surface composition was verified by XPS (chamber I) or Auger electron spectroscopy (AES, chamber II). In addition the first chamber contained facilities for low energy ion scattering (ISS) and the second chamber a double pass electron energy loss spectrometer (Leybold). XPS data were obtained with Ka radiation (1253.6 eV) and photoelectrons collected with a hemispherical electron energy analyzer (50 eV pass energy) which enabled us to rotate the crystal in order to change the azimuthal angle and thus the depth profile of sampled electrons. He + ion spectra (500 eV, 10 nA) were collected in a constant relative resolution mode (SE/E = 4). Finally, the EELS spectrometer was operated at 40 cm- ’ resolution and 2 eV primary beam energy. The crystallization procedure and the crystal mount was described by Bardi and Ross [4]. In short, a single crystal slice was gold brazed to a tantalum foil which served as a resistive heater 141. The crystal was sputtered (300 K, 10 PA Art, 1 kV, 5 min) and annealed (1100 K, 5 min) before each set of experiments. This procedure gave symmetric LEED spots with low background intensity. No significant signal from residual carbon could be detected by either XPS or Auger (272 eV). Note however the difficulty to separate a minor carbon signal from adjacent platinum Auger peaks [5]. Carbon monoxide was adsorbed at 100 K either via back-filling of the chamber for LEED and EELS studies or via a beam doser or multi-channel
.I. Paul et al. / Interaction of CO and 0, with Pf,Ti(III)
123
array l-4 cm from the sample for thermal desorption studies (TDS). Typical heating rates were 15-25 K s-i. The crystal was also dosed with CO at 370 and 420 K in order to examine if any of the major TDS features was due to carbon/oxygen recombination. To develop a surface oxide, the sample was heated to 650-1000 K in front of a doser in an effective background of 10-8-10-7 Torr of oxygen. In case of ISS the sample would be re-oxidized briefly to replace sputtered oxygen.
3. Results and discussion 3.1. Pt,Ti/CO TDS, EELS, and XPS observations show that CO binds reversibly and associatively to the (111) surface of Pt,Ti and that the outermost atomic layer after annealing resembles quasi-pure platinum with a few “defect” sites (t9 = 0.01) where dissociation occurs. A 4 kcal/mol decrease in the heat of adsorption, a 30-40 cm-’ decrease of C-O stretching frequencies, and a 0.2 eV shift of the 0 1s is peaks compared to Pt(ll1) are the only evidence for an electronic contribution to the chemisorption bond due to alloy formation. These shifts are reasonable effects of a second layer enriched in a different metal (titanium) and similar in magnitude to shifts observed for the platinum enriched surface of Pt,Ni,_, alloys compared to Pt(lll) [2] or for a-closely packed copper overlayer on Ru(0001) compared to Cu(ll1) [6]. 3.1.1. Discussion
The CO thermal desorption spectra of the Pt,Ti(lll) surface exhibit features which are remarkably similar to those of the Pt(ll1) surface [7,8]. In fig. la we see these features fill as a function of coverage. Saturation with CO at 100 K (5 L) changed the p(2 x 2) LEED pattern of the clean surface (fig. 2a and ref. [4]) to a diffuse c(4 X 2) pattern which sharpens upon annealing to 250 K (fig. 2b). The annealed CO overlayer exhibits vibrational losses characteristic of terminally (2060 cm-‘) and multiply coordinated (1810 cm-‘) molecules (fig. 3a, top). Annealing to 325 K resulted in the desorption of the multiply coordinated species leaving only on-top bonded molecules (fig. 3b, top). This second annealing changed the LEED pattern back to a less sharp c(4 x 2) structure. We can, as shown in figs. 3a and 3b, unambiguously correlate CO molecules vibrating at 2050-2060 cm-’ with the desorption peak at around 380 K and the loss at 1810 cm-’ with the peak at 320 K. TDS (fig. la) as well as EELS show that these two sites fill subsequently. Finally, fig. 4 displays the C 1s and 0 1s features in XPS spectra as a function of CO coverage. We have not observed any surface bound CO molecules except those that correlate with the desorption peaks at 320 and 380 K.
J. Paul et al. / Interaction of CO and O2 with Pt,Ti(lIl)
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200
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700
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300
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Fig. 2. Schematic representations of LEED images of (a) the bare (111) surface of Pt,Ti and (b) the surface saturated with CO at 100 K and annealed to 250 K (sharp pattern) or 325 K (dim pattern). (c) The LEED pattern correlated with exclusively on-top bonded CO molecules on the Pt(ll1) surface [8,24].
As previously mentioned, the CO TDS spectra from Pt,Ti(lll) are very similar to those of Pt(lll), however the Pt,Ti spectra exhibit a 50 K rigid shift towards lower temperature compared to pure platinum [S]. This converts to a 4 kcal/mol decrease in the adsorption energy assuming the same frequency factor as for Pt(ll1) [8,9]. Correlated with this decrease in adsorption energy we observe a 30 cm-’ decrease of the metal-carbon vibrations and a 40 cm-’ decrease of the carbon-oxygen vibrations, again compared to Pt(ll1) [8]. The shifts of the carbon-oxygen vibrations are opposite those expected for a transition metal surface. The Pt,Ti(lll) surface thus follows a pattern which distinguishes the interaction of CO with noble metals (Cu, Ag, Au, and Pt) as compared to transition metals [lo]. CO chemisorption on noble metals uniquely gives negative workfunction shifts [7,11], a shake-up below the CO40 peak in valence band photoemission spectra [7,12,13], and a marked asymmetry of the 01s peak in core level spectra [7,14,15]. Furthermore, on-top positions on these metals exhibit higher desorption temperatures [6,7,16] and higher vibrational frequencies than multiply coordinated sites [17,18]. The minor CO desorption peak around 440 K (fig. la) has also been observed for Pt(ll1) and interpreted as originating either from residual defect sites [19,20] or from the rim of the crystal [8]. The small diameter of the Pt,Ti crystal does not allow us to fully discriminate against the latter possibility. Pt(ll1) crystals are typically annealed at 1400 K to reduce the number of defects. However, due to the gold-brazing technique we were hesitant to
Fig. 1. TDS spectra saturation
of Pt,Ti/CO at (a) increasing CO coverages coverage at increasing oxygen pre-exposures.
on the bare surface and (b) CO Heating rate 25 K s-l.
126
J. Paul et crl. / Interuction
of CO
Electron Energy Loss (cm 1)
--
0
and 0,
with Pt,Ti(llI)
Desorption Temperature (K)
1 1000
2000
Electron Energy Loss (cm -1)
200
400
600
Desorption Temperature (K)
Fig. 3. EELS spectra of CO adsorbed onto the clean or partly oxidized (111) surface of PtsTi. Each surface was saturated with CO at 100 K (5 L) and annealed either to 250 K (a) or to 325 K (b). After the first annealing (250 K) we observed a sharp ~(4x2) LEED pattern which became dim after a second annealing (325 K). Note that metal-carbon vibrations are shadowed by metal-oxygen bands for the partly oxidized surface. To the right of each vibrational spectrum is a CO desorption spectrum of that particular overlayer. Heating rate 15 K s-‘.
anneal our sample above 1200 K in fear of having gold melt and wet the Pt 3Ti surface or even break the mount. The lower annealing temperature presumably leads to a higher number of defect sites.
J. Paul et al. / Interaction
103 cps
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127
of CO and 0, with Pt,Ti(lll)
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Binding
Energy (eV)
Fig. 4. C Is and 0 1s core level spectra for Pt ;Ti(ll l)/CO as a function of CO exposure. Substrate temperature 100 K.
The weakly bound CO TDS feature at 220 K (fig. la) diverges from recent Pt(ll1) TDS spectra [8] but was observed in an earlier study [7]. It may originate from sites unresolved by other spectroscopies and populated only after a high dose at low temperature or may simply be the result of desorption from the sample holder. This peak is significantly altered by oxygen adsorption which suggests the former interpretation as discussed in section 3.3.1. A very weak high temperature TDS feature likely to be a CO recombination peak is seen at 900 K (fig. la) with no change in intensity over the complete coverage range until saturation. Dosing the sample at elevated temperatures also failed to change the intensity of this peak. Although neither EELS nor XPS nor AES showed any evidence of atomic carbon/oxygen present on the surface, this CO recombination feature still puts an upper limit on the bare crystal’s ability to dissociate CO and thus possibly on the number of titanium sites since at least unmodified titanium metal is known to readily dissociate the molecule [21]. A possible CO recombination peak has been reported for polycrystalline platinum at 650 K [20], i.e. at a significantly lower temperature compared to the peak in fig. la. Our LEED observations are in full accordance with CO adsorption on Pt(ll1). The sharp c(4 X 2) structure have a measured fractional coverage 0 of 0.47 [22] and is believed to possess equal occupancy of on-top and bridge sites [23]. Up to a coverage of about 0.17, CO is exclusively linearly bonded at on-top sites [8]. This coverage corresponds for Pt(ll1) to a LEED pattern consisting of the spots marked with filled circles in fig. 2c. Schweizer et al. suggested a real-space model with clustered molecules, fi x &R30”-(4 x 4) for this pattern [24]. However, since in our case the bare Pt ,Ti(lll) crystal
128
J. Paul et ul. / Interactton
of CO and 0,
with Pt ,Ti(lIl)
itself gives a p(2 x 2) pattern (fig. 2a) the overlapping image will be that of a c(4 x 2) structure (fig. 2b). Our interpretation of the p(2 x 2) image of the bare surface is a “reconstructed” quasi-pure platinum surface layer rather than a termination of the bulk lattice with stoichiometric numbers of platinum and titanium atoms present at the surface [4]. The second layer is presumably enriched in titanium. Such a surface-sandwich segregation has been observed for Pt xNi, _x alloys by means of LEED 1-V measurements [25] and also thermodynamically justified taking only nearest-neighbor interactions into account [26]. One final LEED observation is that the bare Pt,Ti(lll) surface gives a pattern of C,, symmetry. This reveals scattering against a second atomic layer and informs about the growth of oxide islands as discussed in the following section. As with LEED, carbon and oxygen XPS spectra (fig. 4) are very similar to previous work with CO on Pt(ll1) [7,27]. The two 01s peaks are rigidly shifted 0.2 + 0.1 eV to higher binding energy compared to literature data for Pt(lll)/CO [7,27]. Within the limits of our detection the two features are observed at all CO coverages and the width of the Cls level remains constant. We do, however hesitate to assign either of the two features to a multielectron excitation since their relative intensities change both with coverage and temperature on Pt(ll1) 171. For the bare surface we also observe a 0.7 eV shift to higher binding energies of the Pt 4f levels compared to platinum metal [3,28]. Bertolini et al. have made a similar observation of a 0.5 eV shift for the [29]. We also observe a 1.7 eV shift of platinum enriched surface of Pt,,Ni, the Ti 2p level towards higher binding energy compared to the pure metal [28]. Interpretation of all shifts are complicated due to extra-atomic relaxation in the final states but can invoke rehybridization of particularly the titanium atoms. The relative shifts of the Pt 4f and Ti 2p levels may indicate a charge transfer to the more electronegative platinum atoms. 3.2. Pt,Ti Independent evidence for platinum enrichment at the surface is obtained from angularly resolved XPS and ion scattering. By monitoring the ratio of the Ti2p (binding energy, E, = 456 eV) and Pt 4f (Et, = 72 eV) core levels at different take-off angles, we observe a depletion of titanium at the surface. An emission angle of 10” reduces the escape depth of electrons with a kinetic energy of 1.2 - E, keV (Mg Ka) by about l/5 compared to normal emission. At this angle a 40% reduction of the titanium-to-platinum ratio was observed which strongly suggests a platinum enriched surface disregarding any effect of matrix elements. Ion scattering data show an increase in the Ti/Pt ratio with higher ion energies again indicating a surface depleted of titanium (table 1). If we correct the ratios in table 1 for variations in cross-sections as measured by Brineu et al. [30], we would indeed say that the titanium concentration in the
J. Paul et al. / Interaction Table 1 Titanium
relative
platinum
ISS intensity
of CO and 0, with Pt,Ti(lll)
for clean Pt,Ti(lll)
Eo (v)
Ti/Pt
500 1000 1500
0.020 0.085 0.202
as a function
129
of ion energy
second layer is higher than on the surface or in the bulk. However, due to the uncertainty in the cross-sections for ion scattering we can from these data alone only conclude that the titanium concentration increases from the surface to the bulk. 3.3. Pt,Ti/ O2 The oxidation of Pt,Ti deviates significantly from literature data on the chemisorption of oxygen on Pt(ll1). He+ ion scattering as well as XPS and AES clearly show that oxygen adsorption segregates titanium to the surface. LEED, TDS, and EELS reveal that the oxide, presumably TiO,, initially grows in the form of islands with no influence on remaining areas of the crystal. Finally, following high exposures at elevated temperatures no bare areas are left on the sample which then shows the characteristics of evaporated TiO, overlayers. Our results at large agree with the detailed study of Bardi and Ross [311. 3.3. I. Discussion: mild oxidation Under mild conditions (650 K in lo-’ Torr 0,) sub-monolayer coverages of titanium oxide were easily produced. AES shows that titanium segregates to the surface even under these conditions (fig. 5 and ref. [31]) and that the stoichiometry of the surface oxide is similar to TiO, [31,32]. EELS reveals one dominant oxygen induced loss at around 650 cm-’ (fig. 6. bottom). This position does not coincide with any known frequency of atomic (480 cm- ‘) or molecular (700 and 870 cm-‘) oxygen on Pt(lll) nor with an observed “oxide” band (760 cm-‘) on the same surface [33,34]. Instead we recall the dominant infrared active absorption band of TiO, which is centered just at his energy [35]. We were not able to resolve the small isotopic shift (1602 + ‘a 0,) of this peak which suggests atomic oxygen coordinated to a light metal (Ti) rather than a heavy one (Pt). Obviously we cannot completely exclude a mixed metal oxide with platinum atoms “substituted” at some titanium sites in a TiO, lattice. CO reduction to CO, (section 3.3) suggests the presence of some platinum bound oxygen atoms. We have not been able to identify a corresponding Pt-0 vibration presumably because of the high intensity of the nearby band at 650 cm-‘.
130
J. Paul et ul. / Interaction of CO and O2 with Pt,Ti(lIl)
3.5
3.0
2.0
I
1
I
I
I
0
1
2
3
4
5
0503/pt237 Fig. 5. Surface segregation of titanium as a result of oxidation in O2 at 650 K. Ti 387/Pt 237 and 0503/Pt237 give titanium (387 ev) and oxygen (503 eV) Auger intensities relative to the platinum peak at 237 eV. E, = 3 keV. The lines represent (i) a least squares fit to the data points and (ii) the titanium/oxygen ratio of bulk TiO, [32].
The LEED image remained essentially constant under these mild oxidation conditions but with a slightly enhanced background intensity [31]. We note the preserved C,, symmetry of the pattern which shows that the oxidation does not randomize the orientation of remaining, unoxidized areas. A thorough comparison of the structure of these areas and the initially flat unoxidized crystal would require LEED I-V measurements not performed in this study. Such measurements are the more motivated for this system since atomic oxygen is known to produce a p(2 X 2) structure on Pt(ll1) [33,34,36], thus overlapping the pattern of the bare Pt,Ti(lll) crystal.
.J. Paul et al. / Interaction
ofC0
and O2 with Pt,Tl(lIl)
131
3 x 3L CO, Flashed
3 x 3L CO, Flashed
I
0
1
1000 Electron Energy Loss (cm - 1)
Fig. 6. EELS spectra of oxygen bound to Pt,Ti(lll). The primary 0, adsorption gave the spectrum at the bottom of the figure. Subsequent flashing with CO and readsorption of oxygen cycled the peak intensity back and forth. As discussed in section 3.3 we interpret this as a difference between hexacoordinated Ti4+ and pentacoordinated Ti3+ at the surfaces of “islands” of titanium oxide. The ratio between relative Auger signals indicate the portion of titanium atoms participating in the redox process.
Chemisorbed carbon monoxide on the partially oxidized surface produced a c(4 X 2) pattern on remaining “open” areas on the crystal. Futhermore, CO desorption occurred at the same temperatures as on the clean surface (fig. lb and refs. [31,37]) and C-O vibrational frequencies were unshifted compared to the unoxidized sample (figs. 3a and 3b, bottom). These observations exclude any anomalous electronic influence but may indicate a low coverage of atomic of bridge oxygen on “open” areas as observed in the selective suppression bonded species (fig. 3b, bottom and ref. [38]). Possibly related to this we note the absence of any desorption at 220 K for the partly oxidized surface (fig. lb). Further studies are needed to elucidate the role of oxygen site blocking in
132
J. Paul et al. / Interuction
of CO und 0, with Pt,Ti(lll)
preventing the formation of a compressed phase correlated to these weakly bound species. Conclusively we do observe indirect evidence of small quantities of atomic oxygen on remaining “open” areas but the “oxide islands” appear only to physically cover areas of the sample without any significant electronic spillover. A similar influence was observed for TiO, produced by evaporation of the metal onto an oxidized platinum foil [3]. The interaction between chemisorbed CO and the partially oxidized surface of Pt sTi will be further discussed in section 3.4. 3.3.2. Discussion: extensive oxidation Extensive oxidation (1000 K in 10e7 Torr 0,) completely covered the surface by titanium oxide. He + ion scattering showed a seven-fold increase in the Ti scattering peak coupled with the appearance and growth of the oxygen signal and complete loss of the platinum signal (fig. 7). This corresponded to a 40% increase of the Ti2p signal and a 25% reduction of the Pt 4f signal in XPS. We calculate an overlayer thickness of > 6 A assuming a mean free path of > 20 A for 1 keV electrons penetrating the oxide. Whereas mild oxidation I
I
A.
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Pt
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<
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Fig. 7. Low energy ISS spectra showing the migration of titanium oxide to the surface as a result of oxidation. (A) Clean Pt;Ti(lll). (B) After 0, exposure (1050 K, lo-’ Torr, 10 min). (C) After additional 0, exposure (1050 K. lo- 5 Torr, 20 min).
J. Puul et al. / Interaction of CO und O2 with Pr,Ti(ll
133
I)
Ti (2P) 455.50 eV
6=15 FWHM=1.79
eV
3.2 -
2.4 FWHM = 1.87
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.
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454.0
452.0
450.0
Binding Energy eV
Fig. 8. XPS Ti2p spectra following mild oxidation and CO reduction. (A) Clean Pt,Ti(lll). (B) After O2 exposure (650 K, 300 s, lo-’ Torr). (C) After subsequent saturation with CO at 100 K and flashing to 650 K. The widths of the peaks indicate altered contributions from Ti4+ and Ti’+ (cf. fig. 9).
only showed a slight broadening of the Tie 2p peak of metallic titanium (fig. 8) extensive oxidation produced entirely new features assigned as Ti3+ and Ti4+ ions (fig. 9 and ref. [3]). Note the strong suppression of the Tie signal under these conditions. By going to low take-off angles we observed an increase in the Ti3+ signal (fig. 9) indicating a higher portion of defects or uncoordinated ions in the surface region. A fully coordinated oxide was never observed even under these elevated conditions. It has been observed that for titania on Pt the titanium oxide species which are in contact with the Pt metal are not easily oxidized from Ti3+ to Ti4+ [39]. 3.4. Pt,TiO,/CO EELS, AES, TDS, and XPS each provide independent proof that CO reduces a portion of the surface segregated oxide to form CO, and surface coordinated Ti3+ ions. This again points at the similarity between the present system and the intentionally produced TiO, oxide on the surface of a Pt foil [3]. Similar partial reductions have been observed for oxidized polycrystalline
134
J. Pnul et al. / Interaction 1253.6
eV -Y
of CO
and O2 with Pt,TI(l
I I)
Analyzer
A
Tib4 459.40 ev
1000
475.0
456.90eV
K, 10 ~8 torr O2
470.0
465.0 6. Energy
Fig. 9. Angular
460.0
455.0
450.0
eV
resolved XPS of a thick multi-valent oxide on Pt;Ti(lll). ~=90°.(B)Thickoxide,B=90”.(C)Thickoxide.8=100.
Pt,Ti after annealing in hydrogen [31] and after Art sputtering or electron bombardment
for single [40].
(A) Clean
crystalline
Pt;Ti(lll),
TiO,(llO)
3.4.1. Discussion EELS at sub-monolayer coverages of the oxide provides the most dramatic effect of the reduction as the peak at 650 cm-’ shifts to 500 cm-’ following CO desorption (fig. 6). Consecutive exposures to dioxygen and carbon monoxide cycles the maximum back and forth. We have verified that this is not caused by changes in scattering or detection angles. Oxidized titanium species have been reported to reversibly segregate into bulk platinum [41]. We believe that this cannot explain the observed shifts since all our measurements were done at 100 K and the sample never annealed above the onset temperature of the reported transformation. We conclude that the peak is a superposition of
J. Paul et al. / Interaction
of CO and O2 with Pt,Ti(l
II)
135
two bands and that the apparent shift actually is the result of an altered relative strength of two intensities. One consistent way to assign these bands is to parameter&e the two equal stretch coordinates of a triatomic 0-Ti4+-0 vibrator so as to fit the asymmetric and thus most intense band to the peak at 650 cm-‘. When we reduce the hexacoordinated Ti4+ species to pentacoordinated Ti3+ ions our one-dimensional model will change from 0-Ti4+-0 to Ti3+-0. Since the Ti-0 bond is very close to ionic, the potential will scale with the net charge of the interacting species. Utilizing the previous but now by 25% reduced force constant we calculate the Ti3+-0 vibration to 504 cm-‘. Obviously this is a first order model which considers only vibrations normal to the surface but we believe that it grips the essentials of a reduction occurring at the surface of TiO,“islands”. The altered intensity between the bands of the reduced and oxidized samples is understandable since a “surface bound” sixth ligand of Ti4’ presumably gives a higher effective dynamical dipole moment than a half embedded species coordinated to Ti3+. Associated with the above shift is a 15% reduction of the AES oxygen signal at a constant Pt/Ti ratio. Moreover, when a detailed analysis of the Ti 2p XPS signal before and after reduction is applied (fig. 8) a significant 0.1 eV narrowing is observed. This result is consistent with a previous study which clearly demonstrated that chemisorbed CO will reduce TiO, in contact with platinum metal [3]. Obviously the low intensity and the lower binding energy
F m44 x10
m30 x156
III11 200
1
400 Desorption
600
200
400
600
Temperature (K)
Fig. 10. Multiplexed TDS spectra following saturation with CO on the partly oxidized (111) surface of Pt,Ti. We associate each mass with a labelled CO or CO, molecule: C’60’RO( m46), C’60,(m44), C’sO(m30), and C’60(m28). Unity intensity refers to the CO desorption peak for clean Pt,Ti(lll). Heating rate 25 K s-l.
136
J. Pm1 et (11./ Intermtion
of CO und O2 wth Pt ,Ti(ll I)
of the Tie signal for these evaporated films compared to the present system helped to contrast the oxidized and reduced forms. Our final observations utilized isotopic labelling and multiplexed mass spectroscopy. Fig. 10 shows the desorption of CO and CO, from Pt,TiO, following saturation with CO at 100 K. We again observe the overall suppression of CO chemisorption on a partly oxidized surface. Moreover we note a CO, peak at around 300 K which is typical for CO oxidation on Pt(ll1) [42,43]. The CO, peak at around 450 K cannot be explained by background contribution and is assigned to the reduction of TiO,. Finally a very weak m30(i2C’s0) peak around 550 K for Pt3Ti’X0_J12C160 indicates atomic exchange with the oxide and thus presumably dissociation and recombination of a small fraction of the CO molecules,
4. Summary and outlook This work on the interaction between CO and 0, and the (111) surface of Pt 3Ti addresses two problems of some relevance. The first is the chemical properties of a bimetallic system and the range and influence of surface segregation. The second problem is the intensely focused interaction between TiO, and Pt. The chemisorption pattern of CO as well as XPS and ISS measurements on the bare surface show a surface layer enriched in platinum and a second layer presumably titanium enriched. As a result of this surface-sandwich segregation we observe a weakened chemisorption bond compared to CO adsorbed onto Pt(ll1). The magnitude of the shifts (50 K, 30-40 cm-‘, 0.2 eV) are typical for the influence of a metal substrate on the chemical properties of a monolayer thick metallic film. The cause of the altered interaction in terms of a perturbed free-electron density or rehybridation of localized orbitals is, however, unknown. We have no evidence for extensive CO dissociation on the clean Pt,Ti(lll) surface. Atomic carbon/oxygen following CO adsorption were not detected nor have we observed any chemisorbed CO with a stretch frequency below 1810 cm- ‘. A recombination peak observed in TDS sets 0 = 0.01 as an upper limit for the dissociation of CO on the (111) surface of Pt,Ti. Oxygen adsorption at elevated temperatures (> 650 K) causes segregation of titanium to the surface at a fraction corresponding to the formation of TiO, with some Ti3+ “defects”. The oxide apparently grows as “islands” with no electronic influence on remaining bare areas of the crystal. CO adsorption on these inhomogeneous and partly oxidized samples causes the reduction of about l/6 of the Ti4+ ions to Ti3+. This partial reduction is accompanied by CO, desorption at around 450 K and a marked 150 cm ’ shift of the major vibrational bands in EELS spectra. We understand this shift
J. Paul ei al. / Interaction
137
of CO and O2 with Pt,Ti(llI)
by a change from hexacoordinated Ti4+ to pentacoordinated Ti3+ at the surface of the “oxide islands”. One may speculate about the role of bare metallic areas where CO can be readily adsorbed in the vicinity of these “oxide islands” for the efficiency of the reduction. We have no evidence for any anomalously coordinated molecules decorating the islands. Consequently, a reaction scenario would be that CO adsorbed on metallic areas becomes available at the oxide following annealing and surface migration. Future in situ measurements could possibly reveal if anomalous adsorption sites are available exclusively in the reducing environments of high H, and CO pressures.
Note added in proof A recent study on Ti(OOOl)/Pt/CO ditional information [44].
and
Ti(OOOl)/TiO,/CO
provides
ad-
Acknowledgement We acknowledge Dr. P.N. Ross at the Department of Chemistry, of California for providing us with the Pt,Ti(lll) crystal.
University
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J. Paul et al. / Interaction
of CO and 0, with Pt,Ti(lll)
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