An XPS and UPS investigation of the chemisorption of CO on Ir(111).

Volume 44, number

CHEMICAL

3

AN XI’S AND UPS INVESTIGATION P.A. ZHDAN, Institute

C.K. BORESKOV,

of Catulym,

Novosrhrrsk,

PlIYSICS

LETTLRS

OF THE CHEMISORPTION

15 Dcccmbcr

1976

OF CO ON Ir( 111) .

A-1. BARONIN

USSR

and W.F. EGELHOFF

Jr. and W.H. WEINBERG *

Division of Chnrstry and Chcnlical Engincmng. Pasadena, Calrfornio YI 125, US.4 Rcccwed

Calrfornia Instttutc

of Technology.

21 June 1976

The chcmisorptlon of CO on Ir( 111) hds been investigated both by XPS and UPS. It has been demonstrated that X15 is a useful technique for the dctermin.ltion of isosteric heclts of adsorption. The shdpc of the chemisorbed CO ln-So pwks in the UPS data for Ir(l II) i\ different from that prtwiously published for Ir(100). This may well be indicative of zm mflucncc of the geometric structure of the surface upon the electronic structure of the chemisorption bond.

I. Introduction The interaction of CO with various surfaces of Pt [l-4], Pd [S-8] and Ni [9-l I] single crystals has been investigated extensively in recent years by various ultrahigh vacuum (UHV) probes such as low-energy electron diffraction (LEED), Auger electron spectroscopy, thermal dcsorption mass spcctrometry, contact potential diffcrcncc measurements, and both X-ray and UVphotoelectron spectroscopy (XPS and UPS, respectively). In VICWof the many similarities in catalytic properties between thcsc metals and iridium, it is surprising that so few UHV studies of the interaction of CO with iridium single crystal surfaces have been carried out, The CO oxidation reaction has been studied by mass spectrometry over a recrystallized Ir foil [ 12 ] (evidently, largely (110) oriented crystallites were exposed [13]), over the Ir(ll0) surface 1141, and over the Ir(ll1) surface [15-l 73 . LEED, Auger spectroscopic, and thermal dcsorption mass spectrometric invcstigations of the adsorption of CO have been reported both for (110)lr [14] and (11 l)Ir [15,10,18] surfaces. The CO oxidation reaction has been studied by XPS over * Alfred P. Sloan Foundation 528

Wlow.

the Ir(l1 I) surface [17], and the adsorption of CO on (lOO)Ir [19] and (1ll)lr [15] has been studied by UPS. The present work is part of a continuing scrics of investigations which we arc carrying out on various surfaces of single crystal iridium. An initial aim of this work is to attain a microscopic understanding of the CO oxidation reaction on various surfaces of iridium [13,16--18,201.

2. Experimental The work reported here was conducted in a Vacuum Generators ESCA3 UIN spectrometer equipped both with an Al Ka X-ray source and with a dc resonance lamp. The Ir(ll1) crystal was oricntcd, cut, polished and cleaned by standard procedures as discussed prcviously in ref. [18]. The crystal was mounted so that it could be exposed continuously to the desired gases while recording photoelectron spectra. The instrument and our experimental techniques have been described fully in previous publications [17,20].

15 December 1976

CIiEMICAL PHYSICS LETTERS

Volume 44, number 3 3. Results and discussion

In fig. la, we present the XPS data for the oxygen 1s region of the spectrum of the “cleanest” surface of (11l)lr which we were able to obtain routinely. In previous work [20], WC have attributed the broad peak of low intensity at 530 eV binding energy (with respect to the iridium Fermi level) to oxygen dissolved in the Ir lattice. There is ample evidence that this peak is due neither to oxygen adsorbed on the surface nor to an intrinsic Ir emission line [20]. We have estimated that this peak represents a concentration on the order of one oxygen atom per 100 iridium atoms present in the near-surface region. Extensive Xc* bombardment (e.g., one hour at 2 PA and GO0 eV) did not reduce further the intensity of this peak.

In figs. lb-ld, the crystal was heated to the indicated temperatures, and the spectra were recorded in an ambient of 6 X 10SS torr CO. In each of these spectra, the smoothed “clean” surface spectrum has been included explicitly for a better perception of the intensity of the CO peak. There appears to be intensity on the high binding energy side of each peak which is due to inelastic losses. This phenomenon has been rcportcd previously, but it is still not entirely cIear how to correct for the effect [21]. WC have attempted to make the necessary adjustment by including in each spcctrum a dashed horizontal line which ensures that the CO peak is as nearly symmetrical as possibIe. This horizontal line serves as the baseline in the integrated intensities which we report below. In fig. 2, the CO coverage (determined from fig. 1) has been plotted as a function of crystal temperature.

The crystal temperature in the spectrum of fig. Ib is below the onset of desorption of CO. Therefore we T - 520”

K I

5000

I

5?6

I

I

1

COUNTS

I

I

I

,

8

528

530

532

534

536

BINDING

ENERGY,

eV

Fig. 1. The XPS data for the oxygen 1s region of the spectrum for various surface coverages of CO. The spectrum of the “clean” surface, which contains some near-qurfacc oulde, is presented in (a). Spectra (b)-(d) illustrate the amount of adsorbed CO in equilibrium with a pressure of 6 X 10-a torr CO at the indicat-

300

400 TE~~PERATuRE

~si%l~

600

,%

cd crystal temperatures. The smoothed “clean” surface spcctrum has been included in (b)-(d) to indicate more clearly the

Fig. 2. A plot of the equihbrium surface coverage of CO in a 6 X IO-’ torr CO ambient as Q function of crystal tempcrature. The absolute calibration of the ordinate is discussed in

baseline for the CO peak.

the text.

529

Volume 44, number 3

CIUMICAL PHYSICS LETTERS

assume that the peak intensity corrcsponds to a covcrage of9.1 X lOi4 molcculcslcm* as implied by the (2fi X 24) R30” LEED pattern observed for saturationcovcragcofCOon(lIl)Ir[l5,16,18].Thisassumption forms the basis for the quantitative scale on the ordinate of fig. 2. The CO coverage for the higher tcmperatures was dctcrmincd by the intensities of the CO peak relative to that of fig. 1b. The data in fig. 2 for the three higher tcmpcratures rcprcscnt cquihbrium surfact coverages of CO at those tcmpcratures. Since WCknow the pressure, temperature and equihbrium covcragc for the condrtions of CO chemisorption on (I I1)Ir shown in fig. 2, WConly need to know the CO sticking probabihty in order to calculate the isosteric heat of adsorption at each coverage. From the measured relationshrp bctwecn surface coverage of CO and gas exposure in ref. [I 8 ] , it is possible to calculate the sticking probability (S) for each of the three surface coverages shown as data points in fig. 2. At temperatures of 423 K, 465 K and 520 K, we find sticking probabilitrcs of 0,065,0.5 and 0.87, respectively. Using these values, it is straightforward to calculate the rate of adsorption. Since at cquihbrmm the rate of adsorption is equal to the rate of desorption, we can calculate the activation crlergy of desorption using the appropriate first order dcsorptron rate equation, namely, R = vu [CO] exp(-E.Jkn

cmm2 s- * ,

where R is the rate of dcsorption m units of molecular flux. v. is the frequency factor or preexponential factor of the desorption rate coefficient [previously found to be 2.4 X lOI s-r for CO dcsorption from Ir(li1) at a coverage of 5.2 X 1Ol4 molecules/cm*], [CO] is the concentration of adsorbed CO in molecules/cm2, and f$, is the activation energy to dcsorption. The activation energy for desorption will differ only ncgligrbly from the isostcric heat of adsorption (due to the internal energy cf the desorbing gas molecules). The calculated values (using v. = 2.4 X 1Or4 s-l) for the isosteric heat of adsorption are 33.2 kcal/mole at a CO coverage of 7.1 X lOI cmB2 34 . I kcal/moIe at a CO coverage of4.0 X 1014 crne2: and 36.9 kcal/ mole at a CO coverage of 2.1 X 101” cmm2. The largest source of error in these determinations lies in the measurement of the CO pressure. However, a factor of two error in the pressure, which we believe is possible but not probable, would introduce errors of only f 0.5 530

15 Deccmbcr 1976

kcal/molc in the calculated val~~csof the heat of adsorption_ These values for the heat of adsorption are in good agreement with those previously detcrmmcd in two different UHV systems in our laboratories [ 16,181. We prcvrously found an isostcric heat of adsorption of 35 ? I kcal/molc at a coverage of 5.2 X 1014 molecules/ cm* [ 16,181. The present work would indicate at the same covcragc a value of 33.8 kcal/molc. The shift to a lower heat of adsorption at higher surface coverage is complctcly consrstent with the observation that the threshold for thermal dcsorption of CO occurs at a lower tcmpcraturc with increasing covcragc [I 5,16,i 81. Thcsc results are also in very good agrcemcnt wrth the mass spectrometry measurements of Kuppcrs and Plaggc who found activation energies for desorption of 34. kcal/moIc at 5.2 X 1014 molecules/cm* and 32 kcal/moIe at 9.1 X iO14 molecules/cm2 for CO on (I I I)Ir [ISJ. The consistency of these results clearly demonstratcs the usefulness of XPS as a tcchniquc for dctermining heats of adsorption. XPS has the clear advantage in this rcspcct that the concentrations of the adsorbed species are measured directly. In contrast, the thermal dcsorption technique is subject to error from adsorption on the edge of the crystal which exhrbits crystallographic orientations different from that under consideration. This could well be a serious disadvantage for the thermal dcsorption technique in any attempt to determine the heat of adsorption m the limit of low coverage. The XPS technique also has the distinct advantage over Auger spectroscopy that it mcasurcs the adsorbate coverage without sigmficant likelihood of electron impact effects such as desorption or dissociation. A typical KPS pbotocurrcnt is 0.05 iuA/cm* ;1s opposed to a typical Auger clcctron gun incident current of 1 E.cA/mm2. In fig. 3, we present the He II (Izv = 40.8 eV) photoelectron spectra of (a) the clean Ir(l I 1) surface, of (b) the surface following exposure to 9 X IO-6 torr s of CO, and (c) the plot of the differcncc bctwcen ‘(a) and (b). The binding energy 1s referenced to the Ir Fermi level in these spectra. The adiabatic ionization potentials of gas phase molecular CO arc illustrated at the top of fig. 3. They were refercnccd to the true vacuum energy zero by making the rc~zuous assumption that the Ir Fermi level can be rcfercnced to true vacuum by the simple addition of the clean surface work function. Although the aim of comparing directly

Volume 44, number 3

CHEhIICAL

GAS PHASE IR(III)

CO

+ 9L co

I a’ i’ I _

r-e

!

w-S

PHYSICS LETTERS

!

l--

B

1:~. 3. Tbc UPS (1~ = 40.8 cV) d.lt,l for tbc followm8: (a) the “clcm” Ir( 111) ~rface; (b) the surface followmg c\posure to 9 X 10e6 torr 5 CO; and (c) the dlffcrcnce belwccn the two spectra (a) and (b). At the top, the adiab,ttlc ionkation potent1al\ of &USphase CO drc indicated. and dashed Imc~ connect them to the peaks in the spectrunl of ddsorbcd CO to which they qparcntly correspond.

adsorbed state spectra to gas phase spectra prcscnts a very complex problem, we belicvc this simple method can give an approximate solution. The dashed lines at the top of fig. 3. indicate the apparent shifts in peak positions upon chcmisorption. The difference spectrum shown 111fig. 3c is similar to those for CO chemisorbcd on other Group VIII transition metals [2,8,9,15,22-251. It is generally bclieved that the peak with the largest binding energy (peak A at -11.3 eV in fig. 3c rcprescnts the 40 molecular orbital of associatively chcmisorbed CO. The other prominint peak (peak B at --8.6 eV in fig. 3c) is gcncrally thought to represent both the Su and the In molecular orbitals of CO, i.e., there is an accidental neardcgcncracy between these two moIccular orbitals of the chemisorbcd molecule. This would imply a dccrease in the So- 171energy separation of approximately 2 cV as compared to the gas phase UPS result. This increase is attributed tc; the stabilization of the lone-pair

15 Dcccmbcr 1976

50 orbital by electron donation to the m&al. Surprisingly, the 30 molecular orbital, predominantly oxygen 2s in character, has not yet been reported in either UPS or XPS studies. The 20 and Lo orbitaIs are, of course, the carbon and oxygen Is orbit&, respectively. An interesting feature in fig. 3c is f.hc asymmetry in peak B. This implies that the So and 1~ orbitals do not have the same binding energy but arc separated by up to I eV. Ilowevcr, this observation must be interpreted with caution since for each peak we ace obscrvmg an envelope of the vibrational excitations in the ionized CO molecule. This envelope might involve only the vibrational ground state (u = 0) for the 50 orbital but might be spread over the v = 0 to v = 5 vibrations for the Isr orbital in analogy with the results for gaseous CO [26]. It will be necessary to obtain estimates of the Franck-Condon factors governing the transition from the ground state to the ionized state of chemisorbed CO before this point can be clarified complctcly. Hopefully, future theoretical work can illuminate this matter. One of the more interesting aspects of fig. 3c is that it does not agree very closely with UPS results obtained previously for CO chemisorbed on (100)Ir [ 191. BrodCn and Rhodin have found that the He II UPS spectrum of CO on Ir( 100) yields a So- 15f peak which is approximately symmetrical with only d very small shoulder on the low binding energy side. On Ir(11 I), we find a much more pronounced shoulder on the low binding energy side of the peak. The significance of the contrast betwee their results and ours is that the electronic energy levels of the CO molecule arc perhaps affected differently by the adsorption on the two different crystallographic orientations of Ir. In view of the probabiblc importance of the geometric structure of a surface in defcrmining its catalytic properticp, we consider it significant that the electronic structure of a molccuIar adsorbate can be influenced by the geometric structure of the surface. However, at the present time the theories of photoemission and chemisorption are nut sufficientIy advanced to permit a detailed description of such differences in electronic structure.

Acknowledgement This work represents one phase of the Joint USUSSR Program in Chemical Catalysis and was support531

Volume 44, number

3

ed by the National Science Foundation Number GP41807.

CllChlICAL

PIIYSICS LETI-ERS

under Grant

References [I]

H.P. Bonzcl and R. Ku, Surface Sci. 33 (1972) 91; J. Vacuum Sci. Technol. 9 (1972) 663; H.P. Bonzcl and J.J. Burton, Surfxc Sci. 52 (1975) 223. [ 21 II-P. Bonzcl and TX. Fischer, Surface Sci. 5 1 (1975) 213. [3] R.L. Palmer, J. Vacuum Sci. Tcchnol. 12 (1975) 1403. [4] R. Ducros and R.P. Mcrrtll, Surface Scr 55 (1976) 227. [S] J.C. Tracy and P.W. P&nberg, J. Chcm. Pbys. 51 (1969) 4852. (61 G. Err1 and J. Koch, in: Adsorptton-dcsorption phenomena, ed. I’. Ricc.r (Academic Press, New York, 1972) p. 345. [7] Il. Conrad, G. Ertl. J. Koch and E.E. Latta, Surface Sci. 43 (1974) 462. (81 J. Kilppers. II. Conrad, G. Ertl and E E. Latta, Japan J. Appl. Phys. Suppl. 2, Part 2 (1974) p. 225. 191 D.E. Eastman and J.K. Cashion, Phys. Rev. Lcttcrs 27 (1970) 1520; G 11. Bcckcr hnd H.D. Hagstrum. J. Vacuum SCI. Tcchnol. 10 (1973) 31. [IO] J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. [ 111 K. Christmann, 0. Schobcr and G. Ertl, J. Chcm. Phys. 60 (1974) 4719. [ 121 N.V. Agccv and N-1. Ionov, Kinctic.t i Kataliz 14 (1973) 687.

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1976

[13] V.P. 1vanov.C.K. Borcskov,V.L.Tataurov. V.I. Savchenko, W.F. ESclhoff Jr. and W.H. Weinberg, Dokl. Akad. Nauk USSR, to be published [ 141 K. Chri\tmann and G. Ertl, 2. Ndturforsch. 28.1 (1973) 1144. [ 151 J. Kuppers and A. Plaggc, J. Vacuum Set. Technol. 13 (1976) 259. [ 161 V.P. Ivanov, G.K. Borcskov, V.I. Savchcnko,W.F. Egclhoff Jr. and W.H. Weinberg. Surface Sm. 161 (1976) 207. I171 P.A. Zhdan, G.K. Boreskov, W.F. Lgclhoff Jr. and W.H Wcmberg, Surface SCI. (1976), to be published. [181 C.M. Comric and W.H. Weinberg, J. Vacuum Sci. Technol. 13 (1976) 264; J. Chcm. Phys. 64 (1976) 250. I191 G. Broddn and T.N. Rhodm, Faraday Discussions 60 (1975); Sohd State Commun. 18 (1976) 105. P.A. Zhd,m, G.K. Borcskov, A I. Bcronm, W.F. Egelhoff m Jr. and W.H. Wcmbcrg, Surface Sm. 61 (1976) 25 WI J.T. Yates Jr., N.E. Lrrckson, S.D. Worlcy and T.E. Madey, In: The physical basis for heterogeneous cataIysis, cd\. E. Drauglis and R-F. Jaffcc (Plenum Press, New York, 1975) p. 75 1221 P.R. Norton and P.J. Richards, Surface Sci. 49 (1975) 567. 1231 P.R. Norton and R L. Tapping. Chcm. Phys. Letters 38 (1976) 207. ]24] J.C. I-ugglc, ICI.Stemktlbcrg and D. Mcnzcl. Chcm. Phys. I1 (1975) 307. 1251 T. Gustafsson, E.W. Plummcr, D.E. Eastman and J.L. Frecouf, Sohd State Commun. 17 (1975) 391. 1261 D.W. Turner, C. Baker, A.D. Baker and CR. Brundlc, Molecular photoclcctron spectroscopy (%ley-Intcrscicncc, New York, 1970) p. 49