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The chemical properties of bimetallic surfaces: bonding between CO and Zn on Ru(001)
Surface Science Letters 289 (1993) L584-L590 North-Holland
surface s c i e n c e letters
Surface Science Letters
The chemical properties of bimetallic surfaces: bonding between CO and Zn on Ru(001) Jos6 A. Rodriguez Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA Received 19 January 1993; accepted for publication 15 February 1993
The adsorption of CO on Zn/Ru(001) surfaces has been studied using thermal desorption mass spectroscopy, core and valence level photoemission and Auger spectroscopy. At 80 K, CO does not adsorb on thick Zn films under ultra-high vacuum conditions. In contrast, adsorption of the molecule is observed on Zn atoms bonded to Ru(001). Bimetallic bonding induces a reduction of ~ 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of the Z n - C O bond. CO molecules adsorbed on Zn adatoms show an O(ls) binding energy close to 535.2 eV and a desorption temperature of ~ 125 K. Both properties indicate that ~--backbonding is very weak in the Z n - C O bond. For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the Ru-CO bond.
1. Introduction The formation of a heteronuclear metal-metal bond can enhance or reduce the ability of a metal to chemisorb CO. For Pd supported on early transition metals, the P d - C O bond is considerably weaker than that observed for Pd(100) [1-3]. The extreme case is CO/Pdt.0/Ta(ll0), which shows a reduction of 235 K in the CO desorption temperature and a weakening of ~ 15 kcal/mol in the P d - C O bond. The weakening in the P d CO bond is probably caused by a poor interaction between the Pd(4d) band and the CO(2~*) orbitals [3-5]. For Pd adatoms, photoemission studies show core and valence levels that are at higher binding energy than those of pure Pd [3,6,7]. A correlation between the magnitude of the positive shift in the Pd(3ds/2) level and the decrease in the work function of the metal substrates indicates that Pd is transferring charge to the substrates [3]. The charge transfer reduces electron-electron repulsion within the Pd atoms moving the Pd3d and 4d levels toward higher binding energy, increasing the Pd(4d)-CO(27r*) separation and reducing rr-backbonding [3-5].
Cu atoms supported on late transition metals exhibit a behavior contrary to that of Pd atoms on early transition metals [4,5,8]. The Cu adatoms bond CO stronger than Cu(100). C O / C u l 0 / Rh(100) and CO/Cut.0/Pt(111) show an increase of ~ 70 K in the CO desorption temperature [4,5]. The variations in the strength of the Cu-CO bond can be explained in terms of changes in the interaction between the CO(2Ir*) orbitals and the Cu valence bands [4,9]. For the Cu adatoms, the 2p levels and 3d band appear at lower binding energy than those corresponding to the surface and bulk atoms of Cu(100) [9,10]. This negative shift reduces the separation between the CO(2~r*) orbitals and the occupied Cu bands, increasing 1r-backbonding [4,9,11]. In the present work, the adsorption of CO on Zn/Ru(001) surfaces is examined using thermal desorption mass spectroscopy (TDS), core and valence level photoemission, and Auger spectroscopy. The effects of metal-metal bonding on the CO-chemisorption properties of Zn (3dl°4s 2 atomic configuration) are compared with those reported for Cu (3dl°4s 1 atomic configuration) and Pd (4d1°5s ° atomic configuration) adlayers.
0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
Previous TDS studies for Zn/Ru(001) [12] show desorption of the admetal at 460 (multilayer), 490 (second layer) and 720 K (first layer). The large separation ( ~ 250 K) between the desorption temperature of the monolayer and multilayer states indicates the existence of a very strong interaction between Zn and Ru. This is corroborated by results of photoemission spectroscopy [12], which show that the Zn atoms bonded to Ru(001) have 3s and 3d levels shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn. The large electronic perturbations in Zn/Ru(001) make this system ideal for studying the effects of bimetallic bonding on the chemical properties of a metal.
2. Experimental The ultrahigh vacuum system (base pressure < 5 × 10 -l° Torr) used in these studies was equipped with a quadrupole mass spectrometer for TDS, and a hemispherical electron-energy analyzer with multichannel detection for photoemission studies. The core and valence level spectra of section 3 were acquired using M g K a radiation. The Ru(001) crystal was cleaned following standard procedures [3,11,12]. The sample holder was capable of resistive heating to 1600 K and liquid nitrogen cooling to 80 K. The crystal temperature was monitored using a W - 5 % R e / W 26%Re thermocouple. A heating rate of ~ 3 K / s was used in the CO-TDS experiments. The mass spectrometer was housed in a differentially pumped liquid-nitrogen cooled jacket that has a small aperture in the front. With this arrangement, only CO molecules that desorbed from the Zn covered face of Ru(001) were detected during the TDS experiments. Zn deposition was performed by resistively heating a W filament wrapped with a high-purity wire of Zn. The metal doser was outgassed carefully prior to vapor deposition. During the adsorption of Zn, the Ru(001) sample was held at a temperature of ~ 80 K. Zn coverages were determined by TDS area analysis. In this work, adsor-
L585
bate coverages are reported with respect to the number of Ru(001) surface atoms (1.57 × 1015 atoms cm-2). One Zn adatom or CO admolecule per substrate surface atom corresponds to 0 = 1.0 ML. The saturation coverage of CO on Ru(001) is ~ 0.68 ML [9,13]. For the bimetallic surfaces, the composition is reported using the notation: Znx/Ru(001), where x = 0zn.
3. Results and discussion Fig. 1 shows CO-TDS spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K. For 0z~ < 1 ML, the Zn adatoms desorb from Ru(001) at temperatures above 650 K [12]. The lineshape of the CO-thermal desorption spectrum for clean Ru(001) (bottom of fig. 1) is in good agreement with results reported previously for CO/Ru(001) [13]. The CO-TDS spectra of C O / Z n / R u ( 0 0 1 ) surfaces can be separated in two regions. The first region includes desorption features at temperatures below 200 K and can be assigned to CO bonded to Zn adatoms (see below). The second region contains desorption peaks at temperatures between 250 and 550 K and
Ozn
CO-TDS
1.00
,g
I00
200
300
400
500
600
Temperature (K) Fig. 1. CO thermal desorption spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K.
1.586
,LA. Rodriguez / Bonding between ( ' 0 and Zn on Ru(O01)
corresponds to CO adsorbed on Ru sites that are not covered by Zn. No evidence for dissociation of CO on the Zn/Ru(001) surfaces was found. After heating the surfaces to 600 K during the TDS experiments, the only adsorbate detected in XPS was Zn. The CO-TDS and O(ls)-XPS spectra indicate that the amount of CO bonded to Zn atoms increases with Zn coverage up to 0zn ~ 0.4 ML. After this point, the amount of Zn-bonded CO decreases with increasing 0z,, and no adsorption of CO was observed on films with 0z. > 1 ML even after CO exposures as high as 250 L. Thus, the R u - Z n interaction enhances the strength of the Zn-CO bond. In fig. 1, the Zn-bonded CO desorbs at temperatures between 120 and 130 K. Assuming first-order desorption kinetics and a pre-exponential factor of 1013 s -1, a Redhead analysis [14] gives a dissociation energy of ~ 7.5 kcal/mol for the OC-Zn/Ru(001) bond. This binding energy is considerably smaller than values observed for the desorption of CO from Cu (10-15 kcal/mol [15,16]) and late transition metals (27-38 kcal/mol [13,17-19]). In fig. 1, the Zn atoms block the adsorption of CO on Ru sites approximately on a one-to-one basis. Adsorption of CO on Ru(001) was suppressed only when the Zn pre-coverage was equal or larger than one monolayer. In some aspects, the behavior observed for Zn as a site blocker is similar to that reported for noble metals [20-22], elements that tend to coalesce into two-dimensional (2D) islands leaving large regions of the substrate uncovered. The exact morphology of the Zn adlayer is unknown. Previous Zn-TDS studies for submonolayer coverages of Zn on Ru(001) show a desorption peak that shifts to higher temperature with increasing Zn coverage, indicating the existence of net attractive interactions between the adatoms [12]. At OZn < 0.4 ML, the Zn atoms could form small clusters on the surface. CO will adsorb on the edge atoms of these clusters. When 0zn increases above 0.4 ML, the clusters will coalesce into 2D islands reducing the number of Zn sites that are active for CO adsorption. This structural model explains the variation in the amount of Zn-bonded CO with 0zn.
The Zn adatoms induced a reduction in the desorption temperature of CO molecules bonded to Ru(001). For 0zn = 0.52 ML, the two characteristic CO desorption features of Ru(001) were shifted ~ 25 K toward lower temperature and an extra desorption peak appeared around 300 K. This new peak grew with increasing Zn coverage, reached its maximum intensity at 0z. = 0.4-0.5 ML, and disappeared near the completion of the first Zn monolayer. Such behavior suggests that the peak corresponds to the desorption of CO molecules bonded to Ru sites that are close to the periphery of small clusters of Zn. For these highly perturbed Ru sites, the bond with CO is ~ 12 kcal/mol weaker than that for CO on Ru(001). At 0zn--0.70 ML, the CO desorption peak at highest temperature appears around 450 K. Assuming first-order desorption kinetics with the pre-exponential factor of 1016 s-~ reported for low coverages of CO on Ru(001) [13], a Redhead analysis [14] yields a desorption activation energy of ~ 34 kcal/mol for this peak. A desorption energy of 38.3 kcal/mol has been observed for small coverages of CO on Ru(001) [13]. Thus, the chemical properties of a large fraction of the Ru sites are only weakly perturbed by the Ru-Zn interaction. Fig. 2 shows O(ls) XPS spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 (bottom) and 150 K (top). At 80 K, CO is adsorbed on Zn and Ru sites of Zn<0.s/Ru(001) surfaces and two well separate peaks are observed in the XPS spectra. The peak at higher binding energy corresponds to Zn-bonded CO and disappears after annealing the sample to temperatures above 130 K. This peak showed its maximum intensity at Zn coverages close to 0.4 ML. The peak at lower binding energy is due to emission of electrons from CO molecules adsorbed on Ru sites. The intensity and position of this peak decrease in a monotonic way with increasing Zn coverage. The total shift in the position of the low binding energy peak is ~ - 0.5 eV when 0z, varies from 0.0 to 0.8 ML. At Zn coverages above 1 ML, no O(ls) signal for adsorbed CO was detected in the XPS experiments. Fig. 3 illustrates the effects of CO adsorption on the Zn(2Pl/2)XPS spectra of Zn < o.s/Ru(001)
ZA. Rodriguez / Bonding betweenCO and Zn on Ru(O01)
L587
I o(10 xPs
A Zn(Zp,9 A1 XPS
I T= I , O . . ~ 0.84
r= 80 lr'
'
'
1 '0o •~
~
/
\
\
~
i.A7/2,. L 536
534 532 530 528 Binding Energy (eV)
1050 10'48 I046 I044 1042 1040
Fig. 2. O(ls) XPS spectra taken after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 (bottom panel) and 150 K (top panel). The electrons were excited using MgKa radiation.
surfaces. This set of experiments was carried out at 80 K. The Zn(2P3/2) level is not shown because it is obscured by overlapping with the signal for the Ru M45N2aV Auger transition [23]. For 0Zn < 0.4 ML, bonding between CO and Zn induces a shift of ~ 0.2 eV in the position of the Zn(2pl/2) ,
,
,
,
,
Binding Energy (eV) Fig. 3. Zn(2pl/2) XPS spectra taken before and after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K. The spectra were acquired using MgKa radiation.
level. At 0zn = 0.84 ML, the amount of CO adsorbed on Zn atoms is very small (see fig. 2) and almost no shift is observed in the Zn(2pl/2) spectrum after dosing CO. The CO-induced shift on
,
Zn LsM4sM45Auger
Valence Region
0
(.~
A+C°
°~
V~
14 lz 1'o S
i
4 ~ o
Binding Energy (eV)
996 988 990 992 994 996 998 Kinetic Energy (eIO
Fig. 4. Valence bands (part A) and Zn LaM45M45 Auger transitions (part B) for bare and CO-covered Zn/Ru(001) surfaces. The spectra were acquired at 80 K after vapor depositing Zn and dosing 50 L of CO. The electrons were excited by M g K a radiation.
L58~
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
the Zn(2pl/2) level disappeared after desorbing the Zn-bonded CO by annealing to 125 K. For Zn
Zn/Ru(O01) Cu(lO0)
Pt(llI)
Ru(O01)
[ ] O(ls) peak in CO ~]CO induced shift in metal care level • co desorpfion temperature
Fig. 5. TDS and XPS results for CO bonded to Zn atoms deposited on Ru(001) and for CO chemisorbed on a-top sites of Cu(100) [25-27,35], Pt(lll) [17,28,29] and Ru(001) 136]. The graph displays the CO-induced shift in the Zn(2pl/~), Cu(2P3/2) and Pt(4fT/2) levels. In the figure, the reference line represents 530.0 cV for the O(Is) peak position, 0.0 eV for the CO-induced shift in metal core level, and 100 K for the CO desorption temperature.
the CO-induced shift in a metal core level is proportional to the amount of rr-backdonation in the metal-CO bond [3,11,30], and (3) the O(ls) binding energy of CO should decrease when the electron density in the admolecule increases as a consequence of ~r-backdonation [33]. Thus, poor ~r-backdonation from Zn to CO leads to a weak adsorption bond on this metal and to a large O(ls) binding energy in the adsorbed molecule. The CO(27r*) orbitals are located at ~ 1.8 eV above the vacuum level [34]. Transition metals are good at rr-backdonation because they have a large density of states close to the Fermi level. When the d-band of the metal moves toward higher binding energy, ~r-backdonation becomes small (Cu case) or very weak (Zn case) depending on the separation between the d-band and the Fermi level. The effects of bimetallic bonding on the COadsorption properties of Zn are similar to those seen for Cu supported on late transition metals [4,5,8] and different from those found for Pd deposited on early transition metals [1-5]. However, in all the cases there is a correlation between the direction of the shift m the core and valence levels of the admetal and the changes in the strength
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
of the admetal-CO bond. For Pd adlayers, bimetallic bonding induces a shift toward higher binding energy in the Pd core and valence levels that leads to a weakening of the Pd-CO bond [1-5]. In the case of Cu and Zn overlayers, heteronuclear metal-metal bonding produces a shift toward lower binding energy in the core and valence levels of the admetal [9,10,12] that enhances the strength of the Cu-CO and Z n - C O bonds. The shifts in the core and valence levels of the admetal can be attributed to charge transfer within the bimetallic bond that decreases (Pd case) or increases (Cu or Zn case) the electron density of the admetal [3,8,12]. For CO chemisorption on Cu/Rh(100) [22], Cu/Ru(001) [21] and Zn/Ru(001), the enhancement in the strength of the Cu-CO or Z n - C O bond is accompanied by a weakening in the Rh-CO or R u - C O bond. If charge is transferred from Rh or Ru toward Cu or Zn, the ability of the admetal to donate electrons into the CO(2~-*) orbitals should increase, whereas that of the transitionmetal substrate should decrease.
4. Conclusions For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the R u - C O bond. The interaction between Zn and Ru enhances the ability of Zn to adsorb CO. Bimetallic bonding produces a shift toward the Fermi level in the position of the core and valence levels of Zn. This shift probably increases the strength of the Z n - C O bond. CO molecules adsorbed on Zn adatoms have an O(ls) binding energy close to 535.2 eV and a desorption temperature of ~ 125 K. Both properties indicate that 7r-backbonding is very weak in the Z n - C O bond.
Acknowledgments This work was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (DE-AC02-76CH00016), Of-
L589
rice of Basic Energy Sciences, Chemical Science Division.
References [1] B.E. Koel, R.J. Smith and P.J. Berlowitz, Surf. Sci. 231 (1990) 325. [2] P.J. Berlowitz and D.W. Goodman, Langmuir, 4 (1988) 1091. [3] R.A. Campbell, J.A. Rodriguez and D.W. Goodman, Phys. Rcv. B 46 (1992) 7077. [4] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Vac. Sci. Technol. A 9 (1991) 1698. [5] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 94 (1990) 6936. [6] M.W. Ruckman, V. Murgai and M. Strongin, Phys. Rev. B 34 (1986) 6759. [7] G.W. Graham, J. Vac. Sci. Technol. A 4 (1986) 760. [8] J.A. Rodriguez and D.W. Goodman, Science 257 (1992) 897. [9] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 95 (1991) 2477. [10] M.L. Shek, P.M. Stefan, I. Lindau and W.E. Spicer, Phys. Rev. B 27 (1983) 7277. [11] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 95 (1991) 5716. [12] J.A. Rodriguez and J. I-Irbek, J. Chem. Phys. 97 (1992) 9427. [13] H. Pfniir, P. Feulner and D. Menzel, J. Chem. Phys. 79 (1983) 4613. [14] P.A. Redhead, Vacuum, 12 (1962) 203. [15] C.M. Truong, J.A. Rodriguez and D.W. Goodman, Surf. Sci. Lett. 271 (1992) L385. [16] J.A. Rodriguez, W.D. Clendening and C.T. Campbell, J. Phys. Chem. 93 (1989) 5238. [17] G. Ertl, M. Neumann and K.M. Streit, Surf. Sci. 64 (1977) 393. [18] K. Christmann, O. Schober and (3. Ertl, J. Chem. Phys. 60 (1974) 4719. [19] B.A. Gurney, L.J. Richter, J.S. Villarubia and W. Ho, J. Chem. Phys. 87 (1987) 6710. [20] C.T. Campbell, Annual Rev. Phys. Chem. 41 (1990) 775. [21] J.E. Houston, C.H.F. Peden, D.S. Blair and D.W. Goodman, Surf. Sci. 167 (1986) 427. [22] X. Jiang and D.W. Goodman, Surf. Sci. 255 (1991) 1. [23] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin-EImer, Eden Praire, MN, 1978). [24] J.A. Rodriguez, to be published. [25] L.H. Dubois, B.R. Zegarski and H.S. Luftman, J. Chem. Phys. 87 (1987) 1367. [26] (a) P.R. Norton, R.L. Tapping and J.W. Goodale, Surf. Sci. 72 (1978) 33. (b) R.P. Messmer, S.H. Lamson and D.R. Salahub, Solid State Commun. 36 (1980) 265.
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[27] [28] [29] [30] [31]
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
(c) The O(ls) XPS spectrum of CO on Cu shows a main peak at 533.7 eV and two strong shake-up satellites [26a, b]. In fig. 5, we display the value for the main peak. W.F. Egelhoff, Phys. Rev. B 29 (1984) 4769. M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci. 133 (1983) 321. G. Apai, R.C. Baetzold, P.J. Jupiter, A.J. Viescas and I. Lindau, Surf. Sci. 134 (1983) 122. J.A Rodriguez, R.A. Campbell, J.S. Corneille and D.W. Goodman, Chem. Phys. Lett. 180 (1991) 139. K. Hermann, P.S. Bagus and C.J. Nelin, Phys. Rev. B 35 (1987) 9467.
[32] J. Rogozik and V. Dose, Surf. Sci. 176 (1986) L847. [33] W.F. Egelhoff, Surf. Sci. Rep. 6 (1987) 253. [34] (a) E. Lindholm and J. Li, J. Phys. Chem. 92 (1988) 1731. (b) M. Tronc, R. Azria and Y.J. LeCoat. Phys. B 13 (1980) 2327. [35] C.F. McConville, D.P. Woodruff, K.C. Prince, G. Paolucci, V. Chab, M. Surman and A.M. Bradshaw, Surf. Sci. 166 (1986) 221. [36] G. Michalk, W. Moritz, H. Pfniir and D. Menzel, Surf. Sci. 129 (1983) 92.
surface s c i e n c e letters
Surface Science Letters
The chemical properties of bimetallic surfaces: bonding between CO and Zn on Ru(001) Jos6 A. Rodriguez Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA Received 19 January 1993; accepted for publication 15 February 1993
The adsorption of CO on Zn/Ru(001) surfaces has been studied using thermal desorption mass spectroscopy, core and valence level photoemission and Auger spectroscopy. At 80 K, CO does not adsorb on thick Zn films under ultra-high vacuum conditions. In contrast, adsorption of the molecule is observed on Zn atoms bonded to Ru(001). Bimetallic bonding induces a reduction of ~ 0.5 eV in the binding energy of the core and valence levels of Zn. This shift probably increases the strength of the Z n - C O bond. CO molecules adsorbed on Zn adatoms show an O(ls) binding energy close to 535.2 eV and a desorption temperature of ~ 125 K. Both properties indicate that ~--backbonding is very weak in the Z n - C O bond. For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the Ru-CO bond.
1. Introduction The formation of a heteronuclear metal-metal bond can enhance or reduce the ability of a metal to chemisorb CO. For Pd supported on early transition metals, the P d - C O bond is considerably weaker than that observed for Pd(100) [1-3]. The extreme case is CO/Pdt.0/Ta(ll0), which shows a reduction of 235 K in the CO desorption temperature and a weakening of ~ 15 kcal/mol in the P d - C O bond. The weakening in the P d CO bond is probably caused by a poor interaction between the Pd(4d) band and the CO(2~*) orbitals [3-5]. For Pd adatoms, photoemission studies show core and valence levels that are at higher binding energy than those of pure Pd [3,6,7]. A correlation between the magnitude of the positive shift in the Pd(3ds/2) level and the decrease in the work function of the metal substrates indicates that Pd is transferring charge to the substrates [3]. The charge transfer reduces electron-electron repulsion within the Pd atoms moving the Pd3d and 4d levels toward higher binding energy, increasing the Pd(4d)-CO(27r*) separation and reducing rr-backbonding [3-5].
Cu atoms supported on late transition metals exhibit a behavior contrary to that of Pd atoms on early transition metals [4,5,8]. The Cu adatoms bond CO stronger than Cu(100). C O / C u l 0 / Rh(100) and CO/Cut.0/Pt(111) show an increase of ~ 70 K in the CO desorption temperature [4,5]. The variations in the strength of the Cu-CO bond can be explained in terms of changes in the interaction between the CO(2Ir*) orbitals and the Cu valence bands [4,9]. For the Cu adatoms, the 2p levels and 3d band appear at lower binding energy than those corresponding to the surface and bulk atoms of Cu(100) [9,10]. This negative shift reduces the separation between the CO(2~r*) orbitals and the occupied Cu bands, increasing 1r-backbonding [4,9,11]. In the present work, the adsorption of CO on Zn/Ru(001) surfaces is examined using thermal desorption mass spectroscopy (TDS), core and valence level photoemission, and Auger spectroscopy. The effects of metal-metal bonding on the CO-chemisorption properties of Zn (3dl°4s 2 atomic configuration) are compared with those reported for Cu (3dl°4s 1 atomic configuration) and Pd (4d1°5s ° atomic configuration) adlayers.
0039-6028/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
Previous TDS studies for Zn/Ru(001) [12] show desorption of the admetal at 460 (multilayer), 490 (second layer) and 720 K (first layer). The large separation ( ~ 250 K) between the desorption temperature of the monolayer and multilayer states indicates the existence of a very strong interaction between Zn and Ru. This is corroborated by results of photoemission spectroscopy [12], which show that the Zn atoms bonded to Ru(001) have 3s and 3d levels shifted 0.5 eV toward lower binding energy with respect to the corresponding levels of pure metallic Zn. The large electronic perturbations in Zn/Ru(001) make this system ideal for studying the effects of bimetallic bonding on the chemical properties of a metal.
2. Experimental The ultrahigh vacuum system (base pressure < 5 × 10 -l° Torr) used in these studies was equipped with a quadrupole mass spectrometer for TDS, and a hemispherical electron-energy analyzer with multichannel detection for photoemission studies. The core and valence level spectra of section 3 were acquired using M g K a radiation. The Ru(001) crystal was cleaned following standard procedures [3,11,12]. The sample holder was capable of resistive heating to 1600 K and liquid nitrogen cooling to 80 K. The crystal temperature was monitored using a W - 5 % R e / W 26%Re thermocouple. A heating rate of ~ 3 K / s was used in the CO-TDS experiments. The mass spectrometer was housed in a differentially pumped liquid-nitrogen cooled jacket that has a small aperture in the front. With this arrangement, only CO molecules that desorbed from the Zn covered face of Ru(001) were detected during the TDS experiments. Zn deposition was performed by resistively heating a W filament wrapped with a high-purity wire of Zn. The metal doser was outgassed carefully prior to vapor deposition. During the adsorption of Zn, the Ru(001) sample was held at a temperature of ~ 80 K. Zn coverages were determined by TDS area analysis. In this work, adsor-
L585
bate coverages are reported with respect to the number of Ru(001) surface atoms (1.57 × 1015 atoms cm-2). One Zn adatom or CO admolecule per substrate surface atom corresponds to 0 = 1.0 ML. The saturation coverage of CO on Ru(001) is ~ 0.68 ML [9,13]. For the bimetallic surfaces, the composition is reported using the notation: Znx/Ru(001), where x = 0zn.
3. Results and discussion Fig. 1 shows CO-TDS spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K. For 0z~ < 1 ML, the Zn adatoms desorb from Ru(001) at temperatures above 650 K [12]. The lineshape of the CO-thermal desorption spectrum for clean Ru(001) (bottom of fig. 1) is in good agreement with results reported previously for CO/Ru(001) [13]. The CO-TDS spectra of C O / Z n / R u ( 0 0 1 ) surfaces can be separated in two regions. The first region includes desorption features at temperatures below 200 K and can be assigned to CO bonded to Zn adatoms (see below). The second region contains desorption peaks at temperatures between 250 and 550 K and
Ozn
CO-TDS
1.00
,g
I00
200
300
400
500
600
Temperature (K) Fig. 1. CO thermal desorption spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K.
1.586
,LA. Rodriguez / Bonding between ( ' 0 and Zn on Ru(O01)
corresponds to CO adsorbed on Ru sites that are not covered by Zn. No evidence for dissociation of CO on the Zn/Ru(001) surfaces was found. After heating the surfaces to 600 K during the TDS experiments, the only adsorbate detected in XPS was Zn. The CO-TDS and O(ls)-XPS spectra indicate that the amount of CO bonded to Zn atoms increases with Zn coverage up to 0zn ~ 0.4 ML. After this point, the amount of Zn-bonded CO decreases with increasing 0z,, and no adsorption of CO was observed on films with 0z. > 1 ML even after CO exposures as high as 250 L. Thus, the R u - Z n interaction enhances the strength of the Zn-CO bond. In fig. 1, the Zn-bonded CO desorbs at temperatures between 120 and 130 K. Assuming first-order desorption kinetics and a pre-exponential factor of 1013 s -1, a Redhead analysis [14] gives a dissociation energy of ~ 7.5 kcal/mol for the OC-Zn/Ru(001) bond. This binding energy is considerably smaller than values observed for the desorption of CO from Cu (10-15 kcal/mol [15,16]) and late transition metals (27-38 kcal/mol [13,17-19]). In fig. 1, the Zn atoms block the adsorption of CO on Ru sites approximately on a one-to-one basis. Adsorption of CO on Ru(001) was suppressed only when the Zn pre-coverage was equal or larger than one monolayer. In some aspects, the behavior observed for Zn as a site blocker is similar to that reported for noble metals [20-22], elements that tend to coalesce into two-dimensional (2D) islands leaving large regions of the substrate uncovered. The exact morphology of the Zn adlayer is unknown. Previous Zn-TDS studies for submonolayer coverages of Zn on Ru(001) show a desorption peak that shifts to higher temperature with increasing Zn coverage, indicating the existence of net attractive interactions between the adatoms [12]. At OZn < 0.4 ML, the Zn atoms could form small clusters on the surface. CO will adsorb on the edge atoms of these clusters. When 0zn increases above 0.4 ML, the clusters will coalesce into 2D islands reducing the number of Zn sites that are active for CO adsorption. This structural model explains the variation in the amount of Zn-bonded CO with 0zn.
The Zn adatoms induced a reduction in the desorption temperature of CO molecules bonded to Ru(001). For 0zn = 0.52 ML, the two characteristic CO desorption features of Ru(001) were shifted ~ 25 K toward lower temperature and an extra desorption peak appeared around 300 K. This new peak grew with increasing Zn coverage, reached its maximum intensity at 0z. = 0.4-0.5 ML, and disappeared near the completion of the first Zn monolayer. Such behavior suggests that the peak corresponds to the desorption of CO molecules bonded to Ru sites that are close to the periphery of small clusters of Zn. For these highly perturbed Ru sites, the bond with CO is ~ 12 kcal/mol weaker than that for CO on Ru(001). At 0zn--0.70 ML, the CO desorption peak at highest temperature appears around 450 K. Assuming first-order desorption kinetics with the pre-exponential factor of 1016 s-~ reported for low coverages of CO on Ru(001) [13], a Redhead analysis [14] yields a desorption activation energy of ~ 34 kcal/mol for this peak. A desorption energy of 38.3 kcal/mol has been observed for small coverages of CO on Ru(001) [13]. Thus, the chemical properties of a large fraction of the Ru sites are only weakly perturbed by the Ru-Zn interaction. Fig. 2 shows O(ls) XPS spectra acquired after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 (bottom) and 150 K (top). At 80 K, CO is adsorbed on Zn and Ru sites of Zn<0.s/Ru(001) surfaces and two well separate peaks are observed in the XPS spectra. The peak at higher binding energy corresponds to Zn-bonded CO and disappears after annealing the sample to temperatures above 130 K. This peak showed its maximum intensity at Zn coverages close to 0.4 ML. The peak at lower binding energy is due to emission of electrons from CO molecules adsorbed on Ru sites. The intensity and position of this peak decrease in a monotonic way with increasing Zn coverage. The total shift in the position of the low binding energy peak is ~ - 0.5 eV when 0z, varies from 0.0 to 0.8 ML. At Zn coverages above 1 ML, no O(ls) signal for adsorbed CO was detected in the XPS experiments. Fig. 3 illustrates the effects of CO adsorption on the Zn(2Pl/2)XPS spectra of Zn < o.s/Ru(001)
ZA. Rodriguez / Bonding betweenCO and Zn on Ru(O01)
L587
I o(10 xPs
A Zn(Zp,9 A1 XPS
I T= I , O . . ~ 0.84
r= 80 lr'
'
'
1 '0o •~
~
/
\
\
~
i.A7/2,. L 536
534 532 530 528 Binding Energy (eV)
1050 10'48 I046 I044 1042 1040
Fig. 2. O(ls) XPS spectra taken after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 (bottom panel) and 150 K (top panel). The electrons were excited using MgKa radiation.
surfaces. This set of experiments was carried out at 80 K. The Zn(2P3/2) level is not shown because it is obscured by overlapping with the signal for the Ru M45N2aV Auger transition [23]. For 0Zn < 0.4 ML, bonding between CO and Zn induces a shift of ~ 0.2 eV in the position of the Zn(2pl/2) ,
,
,
,
,
Binding Energy (eV) Fig. 3. Zn(2pl/2) XPS spectra taken before and after dosing 50 L of CO to Zn/Ru(001) surfaces at 80 K. The spectra were acquired using MgKa radiation.
level. At 0zn = 0.84 ML, the amount of CO adsorbed on Zn atoms is very small (see fig. 2) and almost no shift is observed in the Zn(2pl/2) spectrum after dosing CO. The CO-induced shift on
,
Zn LsM4sM45Auger
Valence Region
0
(.~
A+C°
°~
V~
14 lz 1'o S
i
4 ~ o
Binding Energy (eV)
996 988 990 992 994 996 998 Kinetic Energy (eIO
Fig. 4. Valence bands (part A) and Zn LaM45M45 Auger transitions (part B) for bare and CO-covered Zn/Ru(001) surfaces. The spectra were acquired at 80 K after vapor depositing Zn and dosing 50 L of CO. The electrons were excited by M g K a radiation.
L58~
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
the Zn(2pl/2) level disappeared after desorbing the Zn-bonded CO by annealing to 125 K. For Zn
Zn/Ru(O01) Cu(lO0)
Pt(llI)
Ru(O01)
[ ] O(ls) peak in CO ~]CO induced shift in metal care level • co desorpfion temperature
Fig. 5. TDS and XPS results for CO bonded to Zn atoms deposited on Ru(001) and for CO chemisorbed on a-top sites of Cu(100) [25-27,35], Pt(lll) [17,28,29] and Ru(001) 136]. The graph displays the CO-induced shift in the Zn(2pl/~), Cu(2P3/2) and Pt(4fT/2) levels. In the figure, the reference line represents 530.0 cV for the O(Is) peak position, 0.0 eV for the CO-induced shift in metal core level, and 100 K for the CO desorption temperature.
the CO-induced shift in a metal core level is proportional to the amount of rr-backdonation in the metal-CO bond [3,11,30], and (3) the O(ls) binding energy of CO should decrease when the electron density in the admolecule increases as a consequence of ~r-backdonation [33]. Thus, poor ~r-backdonation from Zn to CO leads to a weak adsorption bond on this metal and to a large O(ls) binding energy in the adsorbed molecule. The CO(27r*) orbitals are located at ~ 1.8 eV above the vacuum level [34]. Transition metals are good at rr-backdonation because they have a large density of states close to the Fermi level. When the d-band of the metal moves toward higher binding energy, ~r-backdonation becomes small (Cu case) or very weak (Zn case) depending on the separation between the d-band and the Fermi level. The effects of bimetallic bonding on the COadsorption properties of Zn are similar to those seen for Cu supported on late transition metals [4,5,8] and different from those found for Pd deposited on early transition metals [1-5]. However, in all the cases there is a correlation between the direction of the shift m the core and valence levels of the admetal and the changes in the strength
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
of the admetal-CO bond. For Pd adlayers, bimetallic bonding induces a shift toward higher binding energy in the Pd core and valence levels that leads to a weakening of the Pd-CO bond [1-5]. In the case of Cu and Zn overlayers, heteronuclear metal-metal bonding produces a shift toward lower binding energy in the core and valence levels of the admetal [9,10,12] that enhances the strength of the Cu-CO and Z n - C O bonds. The shifts in the core and valence levels of the admetal can be attributed to charge transfer within the bimetallic bond that decreases (Pd case) or increases (Cu or Zn case) the electron density of the admetal [3,8,12]. For CO chemisorption on Cu/Rh(100) [22], Cu/Ru(001) [21] and Zn/Ru(001), the enhancement in the strength of the Cu-CO or Z n - C O bond is accompanied by a weakening in the Rh-CO or R u - C O bond. If charge is transferred from Rh or Ru toward Cu or Zn, the ability of the admetal to donate electrons into the CO(2~-*) orbitals should increase, whereas that of the transitionmetal substrate should decrease.
4. Conclusions For CO chemisorption on Zn/Ru(001) surfaces, Zn blocks Ru sites approximately on a one-to-one basis and induces a weakening of the R u - C O bond. The interaction between Zn and Ru enhances the ability of Zn to adsorb CO. Bimetallic bonding produces a shift toward the Fermi level in the position of the core and valence levels of Zn. This shift probably increases the strength of the Z n - C O bond. CO molecules adsorbed on Zn adatoms have an O(ls) binding energy close to 535.2 eV and a desorption temperature of ~ 125 K. Both properties indicate that 7r-backbonding is very weak in the Z n - C O bond.
Acknowledgments This work was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (DE-AC02-76CH00016), Of-
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rice of Basic Energy Sciences, Chemical Science Division.
References [1] B.E. Koel, R.J. Smith and P.J. Berlowitz, Surf. Sci. 231 (1990) 325. [2] P.J. Berlowitz and D.W. Goodman, Langmuir, 4 (1988) 1091. [3] R.A. Campbell, J.A. Rodriguez and D.W. Goodman, Phys. Rcv. B 46 (1992) 7077. [4] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Vac. Sci. Technol. A 9 (1991) 1698. [5] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 94 (1990) 6936. [6] M.W. Ruckman, V. Murgai and M. Strongin, Phys. Rev. B 34 (1986) 6759. [7] G.W. Graham, J. Vac. Sci. Technol. A 4 (1986) 760. [8] J.A. Rodriguez and D.W. Goodman, Science 257 (1992) 897. [9] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 95 (1991) 2477. [10] M.L. Shek, P.M. Stefan, I. Lindau and W.E. Spicer, Phys. Rev. B 27 (1983) 7277. [11] J.A. Rodriguez, R.A. Campbell and D.W. Goodman, J. Phys. Chem. 95 (1991) 5716. [12] J.A. Rodriguez and J. I-Irbek, J. Chem. Phys. 97 (1992) 9427. [13] H. Pfniir, P. Feulner and D. Menzel, J. Chem. Phys. 79 (1983) 4613. [14] P.A. Redhead, Vacuum, 12 (1962) 203. [15] C.M. Truong, J.A. Rodriguez and D.W. Goodman, Surf. Sci. Lett. 271 (1992) L385. [16] J.A. Rodriguez, W.D. Clendening and C.T. Campbell, J. Phys. Chem. 93 (1989) 5238. [17] G. Ertl, M. Neumann and K.M. Streit, Surf. Sci. 64 (1977) 393. [18] K. Christmann, O. Schober and (3. Ertl, J. Chem. Phys. 60 (1974) 4719. [19] B.A. Gurney, L.J. Richter, J.S. Villarubia and W. Ho, J. Chem. Phys. 87 (1987) 6710. [20] C.T. Campbell, Annual Rev. Phys. Chem. 41 (1990) 775. [21] J.E. Houston, C.H.F. Peden, D.S. Blair and D.W. Goodman, Surf. Sci. 167 (1986) 427. [22] X. Jiang and D.W. Goodman, Surf. Sci. 255 (1991) 1. [23] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin-EImer, Eden Praire, MN, 1978). [24] J.A. Rodriguez, to be published. [25] L.H. Dubois, B.R. Zegarski and H.S. Luftman, J. Chem. Phys. 87 (1987) 1367. [26] (a) P.R. Norton, R.L. Tapping and J.W. Goodale, Surf. Sci. 72 (1978) 33. (b) R.P. Messmer, S.H. Lamson and D.R. Salahub, Solid State Commun. 36 (1980) 265.
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[27] [28] [29] [30] [31]
J.A. Rodriguez / Bonding between CO and Zn on Ru(O01)
(c) The O(ls) XPS spectrum of CO on Cu shows a main peak at 533.7 eV and two strong shake-up satellites [26a, b]. In fig. 5, we display the value for the main peak. W.F. Egelhoff, Phys. Rev. B 29 (1984) 4769. M. Kiskinova, G. Pirug and H.P. Bonzel, Surf. Sci. 133 (1983) 321. G. Apai, R.C. Baetzold, P.J. Jupiter, A.J. Viescas and I. Lindau, Surf. Sci. 134 (1983) 122. J.A Rodriguez, R.A. Campbell, J.S. Corneille and D.W. Goodman, Chem. Phys. Lett. 180 (1991) 139. K. Hermann, P.S. Bagus and C.J. Nelin, Phys. Rev. B 35 (1987) 9467.
[32] J. Rogozik and V. Dose, Surf. Sci. 176 (1986) L847. [33] W.F. Egelhoff, Surf. Sci. Rep. 6 (1987) 253. [34] (a) E. Lindholm and J. Li, J. Phys. Chem. 92 (1988) 1731. (b) M. Tronc, R. Azria and Y.J. LeCoat. Phys. B 13 (1980) 2327. [35] C.F. McConville, D.P. Woodruff, K.C. Prince, G. Paolucci, V. Chab, M. Surman and A.M. Bradshaw, Surf. Sci. 166 (1986) 221. [36] G. Michalk, W. Moritz, H. Pfniir and D. Menzel, Surf. Sci. 129 (1983) 92.