The chemisorption of carbon monoxide on iridium and platinum studied by UV- and X-Ray photoelectron spectroscopy

Surface Science 71 (1978) 267-278 0 North-Holland Publishing Company

THE CHEMISORPTION OF CARBON MONOXIDE ON IRIDIUM AND PLAINT STUDIED BY UV- AND X-RAY PHOTOELECTRON SPECTROSCOPY P.A. ZHDAN, G.K. BORESKOV, A.I. BORONIN and A.P. SCHEPELJN Institute

of Catalysis of the Academy of Sciences of the USSR, Novosibirsk 630090

USSR

and

W.F. EGELHOFF,

Jr. * and W.H. WEINBERG t

revision of Chemistry and Chemical ~n~.~ee~~g, Californ~ 91125 USA

Caljfo~~a Institute of Tec~~o~o~,

Pasadena,

Received 2 May 1977; manuscript received in final form 1 August 1977

The adsorption of CO on Ir(ll1) has been found to be a non-activated process with an adsorption probability of approximately unity, independent of coverage up to a coverage to a (43 X 1/3)R30” LEED structure, i.e., one third of a monolayer based on the number of II substrate atoms. Furthermore, CO has been found to adsorb nondissociatively on fr(l11) at temperatures up to 533 K. An analysis of the photoemission cross sections for the orbitals of adsorbed CO indicates that, upon adsorption, the 5a orbital of CO loses the appreciable 2s character it possesses in gaseous CO. This interpretation suggests a method for determining the nature of the chemisorption bond.

1. Introduction Due to the importance of CO oxidation catalysis from the point of view of environmental concern and CO reduction in the methanation reaction, and FischerTropsch synthesis, much work has been devoted to understanding the chemisorption of CO on various metal surfaces [I]. One of the newest experimental techniques to be applied to this problem is ultraviolet photoelectron spectroscopy (UPS). This technique permits the observation of the valence electronic energy levels of the adsorbate and gives insight into the nature of the chemisorption bond [ 11. Previous UPS results for CO adsorbed both on Ir(lO0) and Ir(ll1) have indicated that the character of the electronic structure of the CO-metal bond may be

* Present address: Physics Department, General Motors Research Laboratories, Warren, Michigan 48090, USA. t Camille and Henry Dreyfus Foundation Teacher-Scholar and Alfred P. Sloan Foundation Fellow. 267

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P. A. Zhdan et al. / Chernisorption

of‘C0 on Ir and Pt

somewhat different for Ir surfaces in comparison to the other noble metals [2]. Upon chemisorption of CO on Ni [3 1, Pt [4], Pd [ 51 and Ru [6], it is apparent that the 5a and 17~ molecular orbitals have almost coincident binding energies as opposed to the 2.5 eV separation between these orbitals in gaseous CO. On Ir surfaces, however, these two peaks are much more distinctly resolved, and the apparent separation is at least 1 eV [2,7]. The purpose of this present study is to investigate the character of these orbitals more throughly in order to gain a better understanding of the differences between Ir and these other transition metals insofar as CO chemisorption is concerned.

2. Experimental All the experiments reported here were carried out in a Vacuum Generators ESCA-3 UHV spectrometer which was equipped both with an Al Ko and a Mg Ka X-ray source and with a dc resonance lamp for He I and II photon excitation. The Ir(ll1) crystal was oriented, cut, polished and cleaned by well established procedures as discussed in ref. [8]. The crystal was mounted so that it would be exposed to gases continuously during the recording of the photoelectron spectra. A more complete discussion of our experimental procedures and of the instrument has been published previously [9].

3. Results Previous work on Ru(OOl), Ni(lOO), and Ni(ll1) has established that there is a photon energy dependence of the relative intensities of the peaks in photoelectron spectra of adsorbed CO [6,10]. At hv = 40.8 eV (He II), the peak due to the overlapping 5a and In orbitals of CO adsorbed on Ni and Ru has a larger intensity than the peak due to the 4a orbitals [6,10]. However, for hv = 1486.6 eV (Al Ka) the intensity ratio for (In + 5~) : 40 is approximately 1 : 1.5 [6,10]. In fig. 1, we present the corresponding data for Ir(ll1). In figs. la and lb, the XPS results for the valence band of the clean surface and the surface saturated with CO (50 L), respectively, are presented. Fig. lc is the difference spectrum, i.e. lb minus la. Fig. Id is a representation of the relative peak intensities that would obtain if the ionization cross sections of the CO orbitals were unchanged upon adsorption [ Ill. It must be pointed out that the ordering of the CO electronic states, viz., 50, lrr, 4u, is in accordance with the scheme which is generally accepted at this time [7,10,12], and contrary to the In, 5u, 40 proposal scheme which has been suggested very recently [13]. Figs. le and If show the He II (hv = 40.8 eV) UPS results for the clean surface and the surface saturated with CO (50 L), respectively. Fig. lg is the difference spectrum, If minus le. On Ir(l1 I), the 50 and In orbitals give rise to two distinguishable peaks in the He II spectrum. Their aggregate intensity is considerably

P.A. Zhdan et al. / Chemisorption

of CO on Ir and Pt

269

Ir (111)

N(E

Fig. 1. XPS (hv = 1486.6 eV) data for the clean Ir(ll1) surface (a), the CO saturated surface (b), the difference spectrum (c), and the XPS ratio of intensities predicted by gas phase CO (d). UPS (hv = 40.8 eV) data for the clean Ir(ll1) surface (e), the CO saturated surface (f), and the difference spectrum (g). larger than that of the 4a peak for He II excitation. However, for the Al KCYpeak radiation, the only new peak observed upon CO saturation of the surface is the 4a peak. This demonstrates a very large difference in the energy dependence of the photoionization cross section of these orbitals between CO adsorption on Ir( 111) in comparison to Ni(lOO) and (111) and Ru(OO1). A possible interpretation of this observation is that it reflects different atomic orbital character in the molecular orbitals of CO adsorbed on Ir(ll1) compared to Ru(001) and Ni(lOO) and (111). This possibility will be explored further in the next section. Fig. 2 presents the difference spectra (spectrum with adsorbed CO minus clean surface spectrum) for the He II spectra of Ir( 111) at room temperature after various exposures to CO. An interesting feature of these spectra is the change in the relative intensity of the 50 and In peaks. At an exposure of 1 L (1 Langmuir f 1 L s 10e6 Torr-set), fig. 2a, the 50 peak has a slightly larger intensity than the In peak, However, by 5 L, fig. 2b, the In peak is slightly more intense than the 5~; and at CO saturation (in 10m8 Torr CO), Fig. 2c, the 1~ peak is considerably more intense. It

270

P.A. Zhdan et al. / Chemisorption

hv=40BeV

of CO on Ir and Pt

1

8.5

In

10-e

TorrCO

1

1

E,

2

4

6

BINDING

8

IO

ENERGY,

I2

I4

16

eV

Fig. 2. The difference spectra for various CO exposures, (a) 1 L, (b) 5 L, and (c) in equilibrium with lo-’ Torr CO for the room temperature Ir(ll1) surface with He II radiation.

has been determined previously that exposures on the order of 2 L produce a (d3 Xd3)R30” LEED structure for this gas-surface system corresponding to l/3 monolayer coverage, and at higher exposures a (243 X 2d3)R30° LEED structure develops corresponding to 7/12 monolayer coverage [8]. As the exposure exceeds the optimum for the (43 Xd3)R30° LEED structure, it was found previously that the CO overlayer undergoes a continuous compression with the adsorbate molecules moving out of registry with the substrate atoms [8]. This phenomenon evidently correlates with the change in the relative intensities of the In and 50 peaks. A representation of the intensities of the CO peaks in the He II difference spectra as a function of exposure is presented in fig. 3. It was reported previously that the probability of adsorption of CO on Ir(ll1) is given by Se (1 - 0)’ where Se is the initial probability of adsorption and 8 is the fraction of saturation coverage [8]. Furthermore, it was found that the initial probability of adsorption is very nearly unity [8]. This behavior is also reflected in fig. 3 where an initially rapid increase in intensity of the 5a, In, and 40 peaks with CO exposure is followed by a region (near saturation coverage) where the increase in intensity is quite small. For each of the three peaks, a linear extrapolation of the two regions gives a crossing point which should correspond approximately to completion of the (d3 X~/3)R30” LEED structure. The average of these three intersection points occurs at 1.5 L. Since it is known from LEED that the surface coverage for this structure is 5 X 1014 molecules/cm*, an adsorption probability of 0.98 may be computed, in agree-

P.A. Zhdan et al. / Chemisorption

0

I

2 3 EXPOSURE,

of CO on Ir and Pt

271

4 5 LANGMUIRS

Fig. 3. The intensities of the CO peaks from the difference spectra as a function of exposure for He II radiation. The two regimes of adsorption probability (approximately, below and above 1.5 L) are extrapolated by straight lines to deduce their intersection.

ment with previous thermal desorption results which indicate a value of very nearly unity [8]. The He I clean surface spectrum and the difference spectra for three different CO exposures are presented in fig. 4. Consistent with the trends of figs. 1 and 2, the 40 peak is very weak relative to the In and 50 peaks at this lower photon energy. The reversal in the relative intensities of the In and 50 peaks with increased coverage is apparent in fig. 4, but to a lesser degree than in fig. 2. Difference spectea for the He II excitation with the crystal held at vatious temperatures in an ambient of lop8 Torr CO are shown in fig. 5. It is apparent that at a given surface coverage the individual spectra are indistinguishable from those of fig. 2, indicating that there is no temperature dependence of the molecular orbital energy levels. This may be taken as strong evidence that even at 533 K, CO does not dissociate at an appreciable rate on h-(111). This is in contrast to Ir( 110) for which dissociative adsorption of CO in this temperature range has been reported [14]. Also noteworthy in fig. 5 is that the In and 50 peaks exhibit a reversal of intensity with coverage exactly as in fig. 2. In any attempt to understand the adsorption kinetics of a particular gas-surface system, the temperature dependence of the adsorption probability is a very important parameter. In fig. 6, data are presented to establish this dependence for CO on Ir(ll1). The intensity of the He II difference spectra at 8.5 eV below EF (the maximum in the In peak) is plotted as a function of CO exposure for four temperatures: 293,323,423 and 473 K. It is immediately apparent that the initial slope at

272

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of CO on Ir and h

hv=Zl.ZeV

ONCE

, E,

2

4

I

6 BINDING

/

8

1

I

IO ENERGY,

12

14

eV

I$?. 4. The difference spectra for various CO exposures, (a) 1 L. (b) 5 L, and (c) in equilibrium with lo@ Torr CO for the room temperature Ir(l11) surface with Hc I radiation.

hl’ = 40.0ev

PC.= 10.‘Tom

533

AN(E)

Fig. 5. The He II difference spectra at the indicated surface temperatures.

for the Ir(ll1)

surface

K

473

K

293

K

in equilibrium

with

lo-’

Torr CO

P.A. Zhdan et al. / Chemisorption

of CO on Ir and Pt

213

low exposure is independent of temperature. This established that adsorption in this low coverage range is essentially nonactivated. As discussed above, the slope of the plot in this low coverage regime leads to an adsorption probability of very nearly unity. In order to illustrate the sensitivity of the adsorbed CO spectra to the surface crystallography, fig. 7 shows UPS difference spectra for various CO exposures on a clean, polycrystalline Ir foil. The foil was cleaned by the same standard procedures as in the case of the Ir( 111) and yet does not expose a perfect (111) plane [ 151. The difference spectra for CO adsorption also are not the same as for adsorption on the (111) surface. First, it should be noted that the 50 level does not give rise to such a distinctive peak on the Ir foil; it appears only as a weak shoulder. Second, and perhaps more significant, the entire spectrum of CO energy levels is shifted with respect to the Fermi level in comparison to the Ir( 111) data. Both the In and 40 peaks are 0.5-0.6 eV nearer the Fermi level on the foil than on the Ir(ll1) surface. Third, and perhaps most significant, is the observation that the 50 and In peaks do not exhibit the coverage dependent reversal of relative intensity that is obsevred on the (111) surface. This further supports the contention that this reversal is related to the compression of the CO superstructure and the consequent loss of registry that occurs on Ir(ll1) as the CO coverage exceeds l/3 monolayer, i.e.,

I

I

1

I

I

I

hv 40.8 eV =

Ir(lll)

293 K

r

z5 z

323

K

423

K

473

K

k.f c k z iii is 5-

Fig. 6. The intensity ation at the indicated

of the In peak (8.5 eV below EF) as a function surface temperatures.

of exposure

for He II radi-

274

AN(E)

2

4

6

BINDING Pig. 7. The difference temperature Ir foil.

spectra

for Ile II radiation

8

IO

ENERGY,

I2

I4

eL

for the indicated

c\posures

of CO to a room

that coverage which corresponds to the (43 Xd3)K30” LEED structure. On the surface of the Ir foil, this compression may very well not occur due to the microscopic surface crystallography. In order to provide additional support for the idea there is a fundamental difference in CO chemisorption on different transition metals, fig. 8 present the UPS difference spectra of a clean, polycrystalline Pt foil for CO adsorption. As might be expected, the difference spectra for CO adsorption on the two foils are somewhat different. On the Pt foil, the 50 and In levels apparently have coincident binding energies at high surface coverages since only a single nearly symmetrical peak is observed which is centered at -8.6 eV. This observation is characteristic of the adsorption of molecular CO on all Group VIII metals investigated thus far with the exception of Ir [3-61. Another difference between the energy levels of CO on Ir and Pt foils is that on Pt the levels appear to have a binding energy approximately 0.660.9 eV larger with respect for the Fermi level. However, this discrepancy is resolved partially when it is recalled that the average work function of Pt is approximately 0.5 eV smaller than that of Ir [4]. When properly referenced to the vacuum level, rather than the metallic Fermi levels, the binding energies of the CO electronic energy levels are approximately the same on both metals.

P.A. %h&z et al. / C~~rnisor~iion ofC0 7

(

I

I

1

hv = 40.8eV Polycrystollme

\

I

on Ir and Pt

215

I

,

Pt

AN(E)

1

’

E,

>

2

I

I

6 BINDING 4

Fig. 8. The difference spectra for He temperature Pt foil.

II

!

I

I

8 10 12 ENERGY, eV

14

radiation for the indicated exposures of CO to a room

4. Discussion The results of fig. 1 indicate a dramatic dependence on photon energy of the photoionization cross section of the 4a orbital of adsorbed CO relative to the In and 5% orbitals. Furthermore, this change in relative cross sections was not observed in similar measurements for CO adsorption on Ru(OOl), Ni(l1 l), and Ni(l00) [6,10]. A possible explanation for this difference in CO bonding characteristics on Ir compared to Ru and Ni may be formulated in terms of the atomic orbital composition of the molecular orbitals of adsorbed CO. It is well established that the cross section for ionization of 2p orbitals of the first row elements decreases much more rapidly with increasing photon energy than for the 2s orbitals [ 161. For example, with Al KCXradiation, gas phase CO exhibits a ratio of intensities 50 : In : 40 of 2 : 1 : 5; whereas for He II radiation, the ratio 50 : In : 4a is 0.70 : 1 : 0.44 [ 111. Molecular orbital calculations show the 5u orbital to be partly 2s and 2p in character, the In orbital to be entirely 2p and the 4a to be essentially 2s in character [17]. In compa~ng these ratios, it should be remembered that the la orbital contains four electrons and the 50 and 40 orbitals two electrons each. Within this

framework, it is clear that if the 50 orbital lost all of its 2s character upon adsorption, the 50 : 4a intensity ratio in XPS would be on the order of 1 : 10 since the In : 40 ratio is I : 5 with four electrons in the In orbitals and two electrons in the 5~ orbital. This is consistent with the absence of a 50 peak in fig. la. If this interpretation of the relative cross sections is correct, it would imply that for CO on Ru and Ni, the 50 orbital (nominally the 2pa bonding molecular orbital) would have an appreciable 2s component to account for the observed peak in the X-ray photoelectron spectrum. Certainly more theoretical work is indicated concerning this point since it suggests a potentially useful method for determining the composition of the molecular orbitals of chemisorption bonds. The results presented in figs. 2-5 characterize the reversal in the relative intensity of the In and So peaks which occurs as a function of CO coverage on Ir(ll1). While this reversal may be due to complex changes in the molecular orbitals of CO and their photoionization cross sections, there is at least one very simple interpretation which is also quite plausible. it is well established for gaseous CO that the lrr orbital is predominantly localized on the oxygen atom, and the 50 oribital is predominantly localized on the carbon atom. At low coverages of CO, where the 1~ and 50 peaks have approximately the same intensity, it is easy to imagine that the 5a electrons can escape into the vacuum without a significant reduction in intensity by inelastic scattering with the oxygen atoms since the experimental geometry coilects electrons at 45” from the surface normal. (Here we assume that the CO bond axis is normal to the surface with the carbon end of the molecule attached to the surface (41.) However, at saturation coverage when the CO molecules are compressed into a closely packed structure on the surface, the photoelectron intensity from the 5a orbitals may well be reduced upon passing through the layer of oxygen atoms created by adjacent CO molecules. This interpretation may also be applicable to the absence of a reversal in the 5o-ln intensities on the Ir foil surface where continuous compression of the CO overlayer may well be absent due to different surface crystallography. In any case, this is an unusual phenomenon worth further investigation on other surfaces. The data of fig. 6 provide a useful insight into the dynamics of CO adsorption on Ir(l 1 I) at the microscopic level. As mentioned above, the parallel slopes of intensity as a function of exposure at low exposures for all temperatures indicate that the adsorption probabiIity is independent of temperature (T< 533 K) in this coverage range leading to formation of the (43 Xd3)lUO” LEED structure. This fact, together with the calculated adsorption probability of unity, established that any activation energy for chemisorption of CO must be negligibly small. The contrast between the CO difference spectra of fig. 2 for Ir(ll1) and those of fig. 7 for polycrystalline Ir is noteworthy. The binding energy of CO molecular orbitals is approximately 0.5-0.6 eV larger for Ir(ll1) than for polycrystalline Ir. However, if this were to be explained on the basis of different surface work functions (under the assumption that the binding energies are the same with respect to the true vacuum level), it would imply that the polycrystalline Ir surface has a work

P.A. Zhdan et al. / Chemisorption

of CO on Ir and Pt

211

function 0.5-0.6 eV larger than the Ir(ll1) surface. Since this is not likely, it may be concluded that the different surface crystallographies produce different orbital energies in the CO chemisorption bond. In contrast, the apparently different binding energies for the CO molecular orbitals on polycrystalline Ir and polycrystalline Pt do not necessarily imply true binding energy differences. The average work function of Ir is approximately 0.5 eV larger than that of Pt [4]. Thus, the fact that the CO orbital energies are 0.60.9 eV greater on Pt in fig. 8 than they are on Ir in fig. 7 may be predominantly the effect of the larger Ir work function with a smaller contribution (0.1-0.4 eV) due to a genuine difference in CO orbital energy levels with respect to the true vacuum level. 5. Summary Our major conclusions may be summarized as follows: (1) At a pressure of lo-’ Torr, CO adsorbs nondissociatively on Ir(ll1) below 533 K. With increasing coverage, there is a reversal in the relative intensity of the 5a and In peaks, which are separated by >l eV in the UV photoelectron spectra, with the lrr peak becoming dominant at higher surface coverage. (2) The 5a and In orbitals of adsorbed CO have a much lower intensity in the X-ray photoelectron spectra than the 40 orbital. This may be due to a much larger 2s component in the 4a orbital than in the 5u and In orbitals. The contrast of these results with those for CO adsorbed on Ru and Ni may suggest a method of determining the atomic orbital composition of the molecular orbitals involved in the chemisorption bond. (3) The adsorption of CO on Ir( 111) is a non-activated process with an initial probability of adsorption of approximately unity. (4) The bonding of CO on Ir(ll1) is dissimilar to that on polycrystalline Ir. On polycrystalline Ir, the 50 and In orbitals are nearly coincident in energy, both the 50 and lrr orbitals occur LO.3 eV nearer the Fermi level, and there is no reversal in the relative intensity of the 50 and In orbitals with coverage.

Acknowledgment This work represents one phase of the joint US-USSR Program in Chemical Catalysis and was supported by the National Science Foundation under Grant Number CHE74-09019. References [ 1] C.R. Brundle, J. Vacuum Sci. Technol. 11 (1974) 212; and references therein. [2] P.A. Zhdan, G.K. Boreskov, A.I. Boronin, W.F. Egelhoff, Jr. and W.H. Weinberg, Phys. Letters 44 (1976) 528.

Chem.

[3] P.J. Page and P.M. Williams, ITaraday Disc. Chem. Sot. 5X (1975). [4] P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567; G. Apai, P.S. Wehner. R.S. Williams, J. Stdhr and D.A. Shirley, Phys. Rev. Letters 37 (1976) 1497; R.J. Smith, J. Anderson and G.J. Lapeyre, Phys. Rev. Letters 37 (1976) 1081. [S] J. Kippers, H. Conrad. G. Ertl and F.F. Latta. Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 225. 161 J.C. IYuggle, T.L. Madey, M. Steinkilberg and D. Mcnzel, Surface Sci. 52 (1975) 521. [7] G. Brodkn and T.N. Rhodin, Solid State Commun. 18 (1976) 105; G. Brodin, T.N. Rhodin, C. Brucker, R. Bcnbow and Z. llurych, Surface Sci. 59 (1976) 593. [Sj C.M. Comrie and W.H. Weinberg, J. Chem. Phys. 64 (1976) 250; 1. Vacuum Sci. Technol. 13 (1976) 264; W.11. Weinberg, C.M. Comrie and R.M. Lambcrt, J. Catalysis 41 (1976) 489. 191 P.A. Zhdan, G.K. Boreskov, W.1:. Egclhoff, Jr. and W.H. Weinberg, Surface Sci. 6 I (I 976) 377. [lo] P.R. Norton, R.L. Tapping and J.W. Goodale. Chem. Phys. Letters 41 (1976) 247. [ 111 K. Siegbahn et al.. ESCA Applied to Free Molecules (North-Ilolland, Amsterdam, 1969); L.S. Cederbaum, W. Domcke, W. von Niessen and W. Brenig, Z. Physik 821 (1975) 381. 1121 C’.R. Brundle and A.1:. Carley, I:araday Disc. Chem. Sot. 60 (1975); P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567. 1131 D.R. Lloyd, C.M. Quinn and N.V. Richardson, Solid State Commun. 20 (1976) 409. 1141 K. Christmann and G. Ertl, Z. Naturforsch. 28a (1973) 1144. [ 151 Recent results (V.P. Ivanov, G.K. Borcskov, V.I. Savchenko, W.F. Egelhoff, Jr. and W.H. Weinberg, to be published) indicate that the polycrystallinc Ir ribbon of Ageev and lonov [Zh. Tekh. Fiz. 41 (1971) 21961 recrystallized to expose the (110) plane. [ 161 A good example is: C.J. Allan, V. Gelius, D.A. Allison, G. Johansson, H. Siegbahn and K. Siegbahn, J. Electron Spectr. I (1972) 131, and references therein. 117) P.J. Ransil, Rev. Mod. Phys. 32 (1960) 245.