J. Phys. Chem. 1996, 100, 381-389
381
Interaction of Zinc with Transition-Metal Surfaces: Electronic and Chemical Perturbations Induced by Bimetallic Bonding Jose´ A. Rodriguez* and Mark Kuhn Department of Chemistry, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: August 2, 1995; In Final Form: October 3, 1995X
The addition of zinc can induce significant changes in the chemical and catalytic properties of a transitionmetal surface. The properties of a series of bimetallic surfaces that combine Zn with Rh or group 10 elements (TM ) Ni, Pd, or Pt) have been examined using thermal desorption mass spectroscopy, core- and valencelevel photoemission, CO chemisorption, and ab initio self-consistent-field calculations. The deposition of Zn on Rh(111) or polycrystalline surfaces of group 10 metals leads to the formation of alloys. These surface alloys decompose at high temperatures: 600-800 K, ZnNi; 650-850 K, ZnRh; 750-950 K, ZnPt; 8501000 K, ZnPd. In the alloys, the core levels and valence d band of the transition metals exhibit positive binding energy shifts, while negative shifts are observed for the Zn 2p levels and 3d band. For CO/ZnRh and CO/ZnTM surfaces, the CO adsorption bond is weaker than for CO/Rh and CO/TM surfaces: 1-2 kcal/ mol on ZnNi; 4-5 kcal/mol on ZnRh; 4-8 kcal/mol on ZnPt; and 12-16 kcal/mol on ZnPd. For the ZnRh and ZnTM systems, a very good correlation exists between the strength of the bimetallic bond and changes in the electronic and chemical properties of the metals. In these systems, there is an important redistribution of charge that shifts d electrons from the transition metal toward Zn, producing an accumulation of electrons around the bimetallic bonds. The larger this shift of d electrons, the stronger the bimetallic bond, and the bigger the changes in the band structure and chemical properties of the transition metal.
I. Introduction Catalysts that combine two different metals are frequently used in reaction processes that are at the core of the chemical and petrochemical industries.1 This fact has motivated an extensive effort to investigate phenomena that accompany the formation of heteronuclear metal-metal bonds on surfaces.1-5 The addition of zinc to a transition-metal surface can induce important changes in the chemical properties of the surface, producing in many cases a catalyst that has superior activity and/or selectivity.5-8 In principle, these changes can be the result of electronic perturbations induced by metal-metal bonding (“ligand effect”2,9) or a consequence of a reduction in the number of active sites present on the transition-metal surface (“ensemble effect”2,9). A good knowledge of the factors that control the interaction between zinc and transition metals can have a significant impact in the design of new catalysts. In metallic zinc (3d104s2-x4px electronic configuration10) the 3d band is very stable, appearing at ∼10 eV below the Fermi level.10,11 This fact makes the d-d bonding interactions negligible in systems that contain zinc and a transition metal. In general, the chemical bonds between Zn and transition metals are considerably weaker than the typical bonds between two transition metals.12 In spite of this, zinc is able to induce substantial changes in the properties of a transition metal.5-8,13-15 For example, in PdZn surfaces,13 the Pd atoms exhibit electronic and chemical perturbations that are as large as those found for Pd bonded to early-transition metals and much bigger than those seen when Pd is bonded to late-transition metals. The formation of Pd-Zn bonds produces a large depletion in the density of Pd 4d states around the Fermi level and a positive binding energy shift of ∼1 eV in the centroid of the Pd 4d band.13,14 This is accompanied by a significant reduction in the CO chemisorption ability of Pd, with a weakening of 12-16 kcal/ mol in the strength of the Pd-CO adsorption bond.13 * Author to whom all correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0381$12.00/0
Zinc has a 4s valence band that is almost fully occupied, while the 4p band is almost empty.10 Thus, depending on the electronegativity of the transition metal, zinc can behave as an electron donor or an electron acceptor when present in a bimetallic surface. The redistribution of charge around zinc affects the chemical properties of this element.13,16-18 For example, in the case of Zn/Ru(001) surfaces, the complex nature of the bimetallic bond enhances the ability of Zn to adsorb CO,17 and at the same time decreases its reactivity toward O2.18 A general picture for the chemical bond of Zn in bimetallic surfaces can only be obtained through systematic studies combining the results of experimental and theoretical techniques. In this work, we examine the behavior of a series of bimetallic surfaces that contain Zn and Rh or a group 10 element (Ni, Pd, Pt). The electronic properties of these systems are investigated using photoelectron spectroscopy and ab initio self-consistentfield (SCF) calculations. CO chemisorption is used to probe possible changes in the chemical properties of the bonded metals. Our results show a good correlation between the electronic perturbations of a Zn adatom and its bonding energy on a metal surface. Bimetallic bonds that display a large stability usually involve a redistribution of charge around the metal centers. The larger the redistribution of electrons, the bigger the change in the chemical properties of the bonded metals. II. Experimental and Theoretical Methods II.1. Photoemission and Thermal Desorption Studies. The experimental setup used for the thermal desorption and photoemission studies has been described previously.13 The ultrahigh vacuum chamber (base pressure ∼2 × 10-10 Torr) was equipped with a Mg KR X-ray source, a hemispherical electron energy analyzer with multichannel detection, and a quadrupole mass spectrometer. The mass spectrometer was housed in a differentially pumped liquid nitrogen cooled jacket that has a small aperture in the front. In this arrangement, the © 1996 American Chemical Society
382 J. Phys. Chem., Vol. 100, No. 1, 1996 mass spectrometer detected only species that desorbed from the “front” face of the sample during the thermal desorption experiments. The sample was mounted in a manipulator capable of electron beam heating to 2400 K, resistive heating to 1550 K, and liquid nitrogen cooling to 80 K. Temperatures were monitored with a W-5% Re/W-26% Re thermocouple spot welded to the sample edge. In a first set of experiments, Zn was vapor-deposited on a Rh(111) crystal. Zn coverages were monitored by taking thermal desorption spectra after the photoemission or CO chemisorption studies. In a second set of experiments, NiZn surfaces were prepared by vapor-depositing Zn on polycrystalline Ni or Ni on polycrystalline Zn. The polycrystalline films of Ni and Zn were supported on a Mo(110) crystal. The rate of evaporation of Zn and Ni from the metal dosers was calibrated by depositing these metals on clean Mo(110) at 300 K and taking thermal desorption spectra.19,20 (Zn and Ni do not alloy with Mo(110) at room temperature.19,20) Submonolayer coverages of Zn or Ni were also determined by measuring the areas under the Zn 3d or Ni 2p3/2 XPS peaks, which were scaled to absolute coverages by comparing against the corresponding XPS areas for Zn/Mo(110) or Ni/Mo(110).19b,20 In this work, the amount of admetal dosed is reported with respect to the number of surface atoms in Rh(111), Zn/Rh surfaces, or Mo(110), Ni/ Zn, and Zn/Ni surfaces. One equivalent monolayer (ML) of the admetal corresponds to the deposition of 1.6 × 1015 atoms/ cm2 in Zn/Rh(111) or 1.4 × 1015 atoms/cm2 in Ni/Zn and Zn/ Ni. II.2. Molecular Orbital Calculations. The electronic properties of a series of clusters that contain Zn and a group 10 metal, Zn3TM10 and Zn4TM4 (TM ) Ni, Pd, or Pt), were investigated at the ab initio SCF level. The molecular orbital (MO) calculations were performed using the HONDO program.21 Since the systems under consideration contain a large number of heavy atoms, the use of all-electron wave functions is not practical from a computational viewpoint. The nonempirical effective core potentials (ECP’s) of Wadt and Hay22 were used to describe the inner shells of Zn and the group 10 metals. The ECP’s for Pd and Pt include mass velocity and Darwin relativistic effects.22 Gaussian basis sets obtained through a (3s3p4d/2s1p2d) contraction scheme were used to describe the outer nd and (n + 1)s,p atomic orbitals of Pd and Pt.23,24 The 4s, 4p, and 3d atomic orbitals of Ni and Zn were expressed in terms of (3s3p5d/2s1p2d) Gaussian basis sets.15,25 For each bimetallic cluster, we examined the properties of several possible electronic states with different orbital occupancies and spin multiplicities. The properties reported in section III correspond to those observed for the ground state of each cluster. To estimate the atomic charges and orbital populations in the bimetallic systems, we used a Mulliken population analysis.26 Due to the limitations in this type of analysis,27 the charges must not be interpreted in quantitative terms, and we will focus our attention only on qualitative trends. III. Results III.1. Properties of Zn/Rh(111) Surfaces: Photoemission and CO Chemisorption Studies. Zinc and rhodium can form bulk alloys.28 Results of thermal desorption mass spectroscopy (TDS) for a series of Zn/Rh(111) surfaces are shown in Figure 1. The Zn atoms that were bonded to Rh desorb in a broad feature between 650 and 900 K. This behavior is consistent with the formation of an alloy at submonolayer coverages of Zn.13,15 Under these conditions, the Zn T Zn interactions are limited, and the Zn adatoms are forced to interact strongly with the Rh substrate. The desorption peaks at low temperature
Rodriguez and Kuhn
Figure 1. Zinc thermal desorption spectra acquired after depositing Zn on Rh(111) at 300 K. Heating rate ) 5 K/s.
Figure 2. XPS spectra for the valence region (A) and Zn 2p1/2 level (B) of Zn/Rh(111) surfaces. Zinc was vapor-deposited at 300 K, and the surfaces were annealed to ∼550 K before taking the spectra.
(450-550 K) have been observed in previous TDS studies for Zn/Ru(001) and Zn/Mo(110) surfaces (alloying does not occur in these systems12,20) and correspond to Zn atoms that were unaffected (Zn multilayer, 480 K) or weakly perturbed (second layer of Zn, 520 K) by the transition-metal substrate. The top of Figure 2 shows photoemission spectra for the valence region of Zn/Rh(111) as a function of admetal coverage. The spectrum for the Zn multilayer agrees well with those reported in the literature for polycrystalline Zn.11,29 Electron emissions in the range between 8 and 12 eV correspond to excitation of the Zn 3d levels, while the occupied states with Zn 4s and 4p character are localized between 8 and 0 eV and have a small cross section in photoemission.11,29 Previous XPS studies for rhodium show a valence 4d band that extends from 7 to 0 eV.30 In Figure 2A, the Zn/Rh(111) surfaces with submonolayer coverages of Zn exhibit a Zn 3d band shifted 0.4-0.6 eV toward lower binding energy with respect to that of pure Zn. Similar binding energy shifts are observed in the
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J. Phys. Chem., Vol. 100, No. 1, 1996 383
Figure 3. Zn 2p1/2 peak position and peak area for a Zn0.64/Rh(111) surface as a function of annealing temperature. A 0.64 ML dose of zinc was vapor-deposited at 80 K, and the system was annealed to the indicated temperatures for a period of 1 min. For pure metallic Zn, the Zn 2p1/2 level appears at 1044.9 eV.31
corresponding Zn 2p1/2 XPS spectra (see Figure 2B). The Rh 3d and 4d XPS spectra of the Zn/Rh(111) surfaces showed a decrease in the intensity of the signal without significant changes in peak position or line shape. Our photoemission experiments did not have the energy resolution that is required to separate the signals of zinc-bonded and pure rhodium. In a series of experiments, we investigated the effects of annealing on the Zn 3d and 2p1/2 features of Zn/Rh(111) surfaces. Figure 3 shows how the intensity and position of the Zn 2p1/2 peak of a Zn0.64/Rh(111) surface vary with temperature. At temperatures above 400 K, one can see a significant decrease in the binding energy of the Zn 2p1/2 peak. This binding energy shift may be the result of extensive alloying of Zn and Rh at these relatively high temperatures. The data for the intensity of the Zn 2p1/2 signal (b symbols) show almost no variation up to the onset for Zn desorption around 600 K. Thus, even if alloying is occurring, the Zn atoms always remain close to the surface of the system. At submonolayer coverages of Zn, the large difference between the surface free energies of Zn (0.94 J m-2 32) and Rh (2.83 J m-2 32) probably prevents the migration of Zn deep into the bulk of the sample. The formation of Rh-Zn bonds affects the chemical reactivity of Rh toward CO. Figure 4 shows CO thermal desorption spectra acquired after adsorbing the molecule on clean Rh(111) and a Zn/Rh(111) surface (θZn ) 0.35 ML) at 300 K. Before dosing CO, the bimetallic system was annealed to 700 K to induce the formation of a homogeneous surface alloy. (At this point, the Zn/Rh(111) surface exhibited a Zn 3d binding energy ∼0.6 eV smaller than that of pure metallic Zn.) In each experiment, the CO coverage was determined by measuring the area under the CO TDS peak and comparing it to the corresponding area for the CO saturation coverage on Rh(111)33 The CO desorption temperatures observed on clean Rh(111) (Figure 4A) are in good agreement with those seen in previous studies.33a,34 At 300 K, there is no adsorption of CO on Zn.13,17 After dosing CO to the Zn0.35/Rh(111) surface, one finds desorption temperatures that are 50-70 K smaller than those seen for CO/ Rh(111). For example, at θCO ≈ 0.1 ML, the CO desorption temperatures are: 410 K for Zn0.35/Rh(111) and 480 K for Rh(111). This corresponds to a weakening of ∼4 kcal/ mol in the strength of the Rh-CO bond.35 On clean Rh(111), the initial sticking coefficient of CO is close to 0.8.33a By comparing the CO exposures necessary to produce a CO coverage of 0.1 ML on Rh(111) (∼0.1 langmuir) and Zn0.35/
Figure 4. CO thermal desorption spectra acquired after dosing CO to Rh(111) and Zn/Rh(111) surfaces at ∼300 K. Heating rate ) 5 K/s.
Rh(111) (∼5.0 langmuir), one can conclude that there is a decrease of more than 1 order of magnitude in the sticking coefficient of CO when going from pure Rh to the bimetallic surface. The presence of Zn leads to a large reduction in the reactivity of the Rh surface atoms toward CO. III.2. Interaction between Zn and Group 10 Metals. III.2.1. Photoemission and CO Chemisorption Studies for ZnTM (TM ) Ni, Pd, or Pt) Surfaces. The phase diagrams for the {Zn + TM} systems indicate that Zn and the group 10 metals are very miscible, being able to form alloys with a large range of relative compositions.28,36 Figure 5 displays Zn 3d and 2p1/2 XPS spectra acquired after depositing zinc on polycrystalline Ni at 300 K (solid traces) and subsequent annealing to 550 K (dotted traces). Bonding between Zn and Ni induces shifts (0.4-0.6 eV) toward lower binding energy in the core and valence levels of zinc. At 300 K, a system with 0.6 ML of Zn exhibits a Zn 3d binding energy ∼0.45 eV smaller than that of pure metallic Zn. Annealing to 550 K leads to penetration of Zn into the bulk of Ni, decreasing the intensity and binding energy of the Zn 3d and 2p1/2 features. In previous studies examining the bonding of Zn atoms with Pd and Pt surfaces,13,15 we also observed negative binding energy shifts for the Zn 3d and 2p peaks, but these shifts were larger (0.2-0.3 eV) than those seen for the deposition of Zn on Ni. In the Ni 3d and 2p3/2 XPS spectra for the bimetallic systems in Figure 5, it is difficult to separate the contributions coming from Zn-bonded Ni and pure (bulk) Ni. A more clear picture of the effects of bimetallic bonding on the electronic properties of Ni can be obtained by examining the deposition of Ni on Zn. Figure 6 shows Ni 3d and 2p3/2 XPS spectra taken after depositing Ni on polycrystalline Zn at 80 K (solid traces). For Ni(100), the surface atoms have a Ni 2p3/2 binding energy ∼0.45 eV smaller than that of the bulk atoms.37 In Figure 6B, the formation of bonds between Ni and Zn shifts the 2p3/2 level of the Ni surface atoms toward higher binding energy, close to the position for bulk Ni. In Figure 6A, the systems with 0.7 and 1.1 ML of Ni exhibit a clear narrowing of the Ni 3d band,
384 J. Phys. Chem., Vol. 100, No. 1, 1996
Rodriguez and Kuhn
Figure 7. Valence photoemission spectra acquired after dosing a monolayer of Pt (A), or Pd (B), to polycrystalline Zn at 80 K.13,15 For comparison, we also include the corresponding spectra for pure Pt, Pd, and Zn.
Figure 5. Valence and Zn 2p1/2 XPS spectra for the deposition of zinc on polycrystalline nickel. The solid traces correspond to spectra taken after dosing Zn at 300 K. The spectra in the dotted traces were acquired after annealing a system with 0.6 ML of Zn from 300 to 550 K.
Figure 6. Valence and Ni 2p3/2 XPS spectra for the deposition of nickel on polycrystalline zinc. Most of the spectra (solid traces) were recorded after vapor depositing Ni at 80 K. The dotted traces correspond to spectra taken after annealing a system with 2.1 ML of Ni from 80 to 300 K. In part B, the b symbol denotes the binding energy observed for the surface atoms of Ni(100).37
with a depletion in the density of Ni 3d states around the Fermi level. When the Ni coverage is raised from 1.1 to 2.1 ML, one can see a broadening of the Ni 3d band as a result of an increase in the Ni T Ni interactions. This broadening disappears after annealing the sample to 300 K (dotted traces in Figure 6). The heating probably provides the energy necessary for the migration of Ni from the surface into the bulk of Zn, diminishing in this way the Ni T Ni interactions and producing a decrease in the intensity of the XPS features for Ni. Figure 7 shows valence photoemission spectra acquired after depositing a monolayer of Pt or Pd on polycrystalline Zn at 80
Figure 8. Zn thermal desorption spectra acquired after vapor-depositing zinc on Ni (A) and Pt (B) surfaces. Heating rate ) 5 K/s. The TDS data for the Zn/Pt systems were taken from ref 15.
K.13,15 In these systems, bimetallic bonding induces a substantial increase in the binding energy for the centroid of the group 10 metal d band. Supported Pd displays very large electronic perturbations, showing a band structure that is similar to that of a noble metal.13,14 By comparing the photoemission data in Figures 6A and 7, it is clear that among the group 10 metals Pd shows the strongest electronic perturbations, while Ni exhibits the weakest (Ni < Pt < Pd). In the top of Figure 8, we can see Zn thermal desorption spectra acquired after dosing zinc to polycrystalline nickel at 300 K. The breaking of the Zn-Ni bonds, ZnNisolid f Zngas + Nisolid, occurs at temperatures between 600 and 800 K. From
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J. Phys. Chem., Vol. 100, No. 1, 1996 385
Figure 10. Clusters used to study the interaction between group 10 metals (dark circles) and Zn (empty circles). Clusters I and II model adsorption on fcc hollow sites of Zn(001). In these clusters, the atoms are arranged in three layers. The notation (a:b:c) specifies how many atoms are in each layer.
Figure 9. CO thermal desorption spectra taken after dosing CO to Ni and NiZn surfaces at 300 K. Heating rate ) 5 K/s.
a thermochemical viewpoint, the Zn-Ni bonds are much stronger than Zn-Zn bonds (decomposition temperature 460520 K12,20) but weaker than Zn-Pt (decomposition temperature 750-950 K15) or Zn-Pd bonds (decomposition temperature 850-1000 K.13) Figure 9 shows CO thermal desorption spectra taken after exposing polycrystalline Ni and a thick ZnNi alloy to CO at 300 K. The alloy was prepared by dosing Ni to a Zn multilayer at 300 K, with subsequent annealing to 575 K to induce homogeneous mixing. The atomic fraction of Zn in the alloy, θZn/(θZn + θNi), was ∼0.45. The alloy displayed a binding energy shift of ∼-0.5 eV in the Zn 3d features, with a Ni 3d band that was relatively narrow and did not have a large density of states around the Fermi level. The line shape of the Ni 3d band was very similar to that seen in Figure 6A for the Ni 3d band of a Ni/Zn system annealed to 300 K (dotted traces). In Figure 9, the initial sticking coefficient of CO on the bimetallic surface is ∼5 times smaller than that on metallic Ni. At the same time, CO molecules bonded to the NiZn surface show desorption temperatures that are 20-30 K smaller than those seen on pure Ni. This corresponds to a weakening of 1-2 kcal/ mol38 in the strength of the Ni-CO bond. In previous studies dealing with the adsorption of CO on PdZn and PtZn surfaces, we found that bimetallic bonding induced a large reduction in the strength of the TM-CO bond: 12-16 kcal/mol for Pd,13 and 4-8 kcal/mol for Pt.15 III.2.2. Ab Initio SCF Calculations for ZnTM Clusters. The experimental results presented above show large variations in the magnitude of the electronic and chemical perturbations induced by Zn on group 10 metals. The perturbations found for Zn-bonded Pd or Pt are among the largest seen in bimetallic surfaces. On the other hand, Zn-bonded Ni exhibits relatively small perturbations that are comparable to those observed after depositing Zn on Mo,20 Ru,17 and Rh surfaces. Thus, by understanding the nature of the bond between Zn and group 10 metals, one can get a general idea on how Zn can affect the properties of a transition metal.
Figure 11. Energies calculated for the occupied orbitals of the Zn8, Ni4Zn4, Ni8, Pt4Zn4, and Pt8 clusters. The dark and hatched rectangles display the energy range covered by those MO’s which showed a significant amount (g5%) of Zn(3d), Ni(3d), or Pt(5d) character. The energies are reported with respect to the vacuum level.
In order to obtain a good description of the Zn T TM interactions at an atomic orbital level, we investigated the electronic properties of the clusters shown in Figure 10 using ab initio SCF calculations. Clusters I and II model the adsorption of TM atoms on hollow sites of the Zn(001) surface. Cluster III was used to examine the bonding interactions in ZnTM alloys. In these clusters, the Zn-TM nearest-neighbor distances were set equal to values found in 1:1 ZnTM alloys:28,36 2.52 Å for Zn-Ni, 2.65 Å for Zn-Pd, and 2.66 Å for Zn-Pt. Figures 11 and 12 show calculated energies for the occupied molecular orbitals (MO’s) of a series of bimetallic and monometallic clusters. One can see the energy range covered by the valence “bands” of Zn and the group 10 metals. Our calculations do not include effects of final-state relaxation present in photoemission processes. Thus, a quantitative comparison between eigenvalue spectra and photoemission spectra is not possible, and we will focus our attention on qualitative trends. The results of the MO calculations indicate that the binding energy shifts seen in the photoemission spectra of the ZnTM surfaces reflect changes in the initial state of these systems. For
386 J. Phys. Chem., Vol. 100, No. 1, 1996
Rodriguez and Kuhn TABLE 2: Valence Atomic Orbital Populations (electrons) of Cluster III TM d
s
p
3d
4s
4p
charge
Ni4Zn4 Ni8
8.82 8.88
0.88 0.89
0.26 0.23
0.04 0.00
9.99
1.62
0.43
-0.04
Pd4Zn4 Pd8
9.33 9.74
0.51 0.23
0.09 0.03
0.07 0.00
9.99
1.69
0.39
-0.07
Pt4Zn4a Pt8a
9.06 9.37
0.89 0.51
0.16 0.12
-0.11 0.00
9.99
1.67
0.23
0.11
1.56
0.44
0.00
Zn8a a
Figure 12. Calculated MO energies for a series of TM3/Zn10 and TM3/ TM10 clusters (TM ) Ni or Pt). Only occupied MO’s are included, and the energies are reported with respect to the vacuum level. The hatched rectangles display the energy range covered by the MO’s which showed significant (g8%) contributions from the d orbitals of the Ni or Pt adatoms.
TABLE 1: Ni, Pd, and Pt Valence Populations (electrons) of Clusters I and IIa d
s
p
Ni/Zn9 Ni/Ni9 Ni3/Zn10 Ni3/Ni10
8.78 8.88 8.83 8.91
0.95 0.92 0.92 0.95
0.21 0.16 0.17 0.12
charge 0.06 0.04 0.08 0.02
Pd/Zn9 Pd/Pd9 Pd3/Zn10 Pd3/Pd10
9.31 9.72 9.38 9.76
0.44 0.18 0.41 0.20
0.09 0.02 0.07 0.02
0.16 0.08 0.14 0.02
Pt/Zn9 Pt/Pt9 Pt3/Zn10 Pt3/Pt10
9.14 9.39 9.19 9.41
0.73 0.51 0.71 0.47
0.16 0.04 0.15 0.09
-0.03 0.06 -0.05 0.03
a The listed values are for one Ni, Pd, or Pt adatom (in cluster II all the adatoms are equivalent).
example in Figure 11, when going from a Zn8 cluster to a Ni4Zn4 or Pt4Zn4 cluster, one can see a destabilization of the Zn 3d levels that is also observed in the photoemission spectra for the ZnNi (Figure 5) and ZnPt15 surface alloys. The trends observed in the SCF calculations and XPS spectra for the d levels of the group 10 metals are also very similar: bimetallic bonding leads to a stabilization of the TM(d) orbitals. In Figure 12, a change from a Pt to a Zn substrate is accompanied by a large decrease in the width of the 5d “band” of the Pt adatoms and a shift of the “band” centroid toward deeper energy. In the Pt3/Zn10 cluster, there is a clear lack of Pt 5d character in the MO’s that are close to the Fermi level (HOMO) of the system. For Ni3/Zn10 and Ni4Zn4, the changes in the Ni d “band” are not as large as those seen for the Pt d “band” in Pt3/Zn10 and Pt4Zn4. Tables 1 and 2 list the calculated charges and atomic orbital populations for the clusters in Figure 10. The bonding interactions between Zn and the group 10 metals are complex. In general, the net charge transfer within the Zn-TM bonds is small, but the metal-metal interactions induce a redistribution of charge around the metal centers that changes the relative populations of the d, s, and p orbitals. In the TM4Zn4 clusters, the 4s electron population of a Zn atom is bigger than that of an atom in a Zn8 cluster, whereas the 4p electron population is smaller. For the group 10 metals, there is an increase in the electron population of the s,p orbitals and a depletion in the
Zn charge
10.0
From ref 15.
population of the d orbitals. Previous studies indicate that these metals can act as d-electron donors and s,p-electron acceptors when forming compounds.39 The MO calculations for the TMnZnm clusters indicate that Pt behaves as a much better electron acceptor than Pd or Ni. This is consistent with differences in the electron affinities of the group 10 metals (Pt . Ni > Pd40). In the TMnZnm clusters, the tendency of a group 10 metal to lose d electrons increases in the following order: Ni < Pt < Pd. This sequence agrees well with the relative occupancy of the d shell in the isolated elements: Ni, d8s2 < Pt, d9s1 < Pd, d10s0.40 L2,3-edge XANES (X-ray absorption nearedge structure) measurements for a large series of bulk compounds have shown that Pd exhibits a bigger tendency to lose d electrons than Pt.39e For the bimetallic systems in Tables 1 and 2, the change in the relative populations of the TM(d) and TM(s,p) orbitals is equivalent to a d f s,p rehybridization. This rehybridization produces a redistribution of charge in which d electrons “move” from the TM centers into the region around the bimetallic bonds. IV. Discussion IV.1. Electronic Interactions between Zn and Transition Metals. The photoemission results for the ZnNi, ZnPd, and ZnPt surfaces illustrate how Zn can alter the electronic properties of a transition metal. In these systems, bimetallic bonding produces positive binding energy shifts in the core levels and valence d band of the group 10 metals. The ab initio SCF calculations indicate that these shifts are accompanied by a reduction in the valence d population of the group 10 metals. Figure 13 displays a series of properties observed for the ZnTM surfaces. By comparing the trends in Figure 13A,B a clear correlation is seen between the change in the d population of a group 10 metal and its valence band shift. Pd shows the biggest drop in the d population and the largest d-band shift. On the other hand, Ni exhibits the weakest electronic perturbations. In the ZnTM surfaces, the positive binding energy shifts in the core and valence levels of the group 10 metals reflect the effects of a decrease in the d population of these elements. From the viewpoint of changes in electron-electron repulsion, a reduction in the TM(d) population is much more important than an increase in the TM(s,p) populations.41,42 For example, in atomic Pt, a change of electronic configuration from 5d96s1 to 5d86s2 increases the stability of the 4f and 5d levels by ∼4 eV.42b In the group 10 metals, a nd f (n + 1)s,p rehybridization moves electrons from an inner and localized d shell into outer and larger s,p shells, reducing in this way the Coulombic repulsion experienced by the core electrons and the electrons that remain in the d shell.41,42 The bigger the d f s,p electron transfer, the larger the stabilization in the core levels and valence d band of the group 10 metal. For the ZnTM systems, the Zn 3d band and 2p core levels appear at lower binding energy than for bulk Zn. To explain
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Figure 13. Properties of ZnTM surfaces (TM ) Ni, Pd, or Pt). (A) Experimental binding energy shift for the first peak (or maximum) in the valence band of a group 10 metal supported on polycrystalline zinc (θTM ≈ 1 ML). (B) Difference between the d populations of a TM adatom in TM3/Zn10 and TM3/TM10 clusters (from Table 1). (C) Desorption temperature for a zinc monolayer from Ni (Figure 8), Pd,13 and Pt15 surfaces. (D) Decrease in the desorption temperature of CO from Ni (Figure 9), Pd,13 and Pt15 atoms bonded to zinc.
these negative shifts, we must consider changes in the Zn orbital populations induced by bimetallic bonding and how the electrons of the group 10 element that are present in the metal-metal bond affect the Zn 2p and 3d electrons. For the ZnTM clusters in Table 2, the Zn 4s electron population is larger than for the pure Zn8 and Zn10 clusters, whereas the Zn 4p population is smaller. These changes in the relative populations of the Zn 4s and 4p orbitals contribute to the negative binding energy shifts seen for the Zn 2p and 3d levels in the bimetallic surfaces.15 (For atomic Zn a 4s14p1 f 4s24p0 rehybridization decreases the stability of the Zn 2p and 3d levels by 1.4 and 1.6 eV.43) Another contribution to these shifts comes from the “movement” of d electrons from the group 10 metals toward the region around the bimetallic bonds. The electron density around a Zn-TM bond is larger than that near a Zn-Zn bond. This extra electron density is “felt” by the Zn 2p and 3d electrons. The net result is an increase in electron-electron repulsion that helps to shift the Zn 2p and 3d levels toward lower binding energy. Figure 14 compares the shifts found for the 2p1/2 level of Zn on Rh (Figure 2) and Ni (Figure 5) with shifts observed on Ag,16 Cu,16 Au,12 Mo,20 Ru,12 Pt,15 and Pd13 surfaces. Similar trends can be seen after plotting the corresponding shifts found for the Zn 3d band. Zn atoms bonded to Rh and Ni display electronic perturbations that are bigger than those reported for Zn bonded to noble metals and smaller than electronic perturbations found for Zn bonded to Pt and Pd. In general, the changes in the electronic properties of Zn increase when the occupancy of the valence band of the metal substrate rises. IV.2. Electronic Perturbations and the Stability of Zn Transition-Metal Bonds. The results in Figure 13 for the ZnTM surfaces show a clear correlation between the electronic perturbations in a group 10 metal and the strength of the ZnTM bond. The bigger the TM(d) f TM(s,p) rehybridization and shift of TM d electrons toward the “metal-metal interface”, the larger the stabilization of the TM d band and the stronger the Zn-TM bond. Among the group 10 metals, Pd exhibits the highest ability to form strong bonds with Zn, while Ni shows the lowest. We found that the Zn 2p levels and 3d band are also sensitive to changes in the strength of the zinc-transition metal bond. The electrons in the 3d band of Zn are too stable to be directly involved in metal-metal bonding, but they can respond to
Figure 14. (A) Zn desorption temperatures for a monolayer of Zn supported on several metal substrates. (B) Difference in Zn 2p1/2 binding energy between 1 ML of supported Zn and the bulk atoms of metallic Zn as a function of metal substrate. The data for Zn on Ag, Cu, Au, Mo, Ru, Pt, and Pd surfaces were taken from refs 12, 13, 15, 16, and 20.
changes in the electron density around Zn induced by the formation of bimetallic bonds. Figure 14 compares the desorption temperatures and Zn 2p1/2 binding energy shifts observed for the deposition of a Zn monolayer on several substrates.12,13,15,16,20 The strength of the bonding interactions between Zn and the metal substrate increases when the fraction of empty states in the valence band of the metal substrate decreases. The overall change in the Zn adsorption energy is close to 20 kcal/mol. Following the results of the ab initio SCF results for the ZnTM surfaces, one can expect that the metal substrates “move” electrons toward the region around the Znsubstrate bond: the larger the accumulation of electrons in the “metal-metal interface”, the stronger the bimetallic bond and the larger the destabilization of the Zn 2p levels. In a {zinc + transition metal} surface, the electronic perturbations induced by the formation of bimetallic bonds can be viewed as the result of an optimization of the bonding capabilities of the metal centers. IV.3. CO Chemisorption on ZnRh and ZnTM Surfaces. The results in section III for the adsorption of CO on ZnRh and ZnNi surfaces indicate that Zn reduces the reactivity of the transition metals toward CO. For CO/Zn/Rh(111), one finds CO desorption temperatures that are 50-70 K smaller than those seen for CO/Rh(111). In the case of CO/Zn/Ni, the CO desorption temperatures are only 20-30 K smaller than those of polycrystalline Ni. The stronger the bonding interactions between Zn and the transition metal (Zn-Rh > Zn-Ni, see Figures 1 and 8), the larger the weakening in the CO-transition metal bond. This pattern of behavior becomes very obvious after examining the data in Figure 13C,D for the ZnTM surfaces. In these systems, Pd shows the strongest Zn-TM bond and the weakest TM-CO bond. The opposite is valid for Ni. The bonding mechanism between CO and a metal involves electron transfer from the CO(5σ) orbital into the empty bands of the metal, σ donation, and electron transfer from the occupied bands of the metal into the CO(2π*) orbitals, π back-donation.44,45 From a thermochemical viewpoint, the π back-
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donation is energetically more important than the σ donation.45 For metals, the valence d orbitals are much better for bonding interactions with CO than the s,p orbitals.45b,46 In the formalism of crystal-orbital perturbation theory,47 the heat of adsorption of CO on a transition-metal atom, QCO, should be roughly proportional to the occupancy of the metal d band, Nd, and inversely proportional to the separation between the centroid of the metal d band and the CO(2π*) orbitals, namely
QCO ∝ Nd/(ECO(2π) - EM(d)) The ab initio SCF calculations for the TM/Zn systems show a shift of d electrons from the group 10 metals toward the region around the bimetallic bonds (or “metal-metal interface”). These electrons are not available for π-back-donation toward CO. In general, the interaction between a group 10 element and Zn shifts the TM(d) orbitals toward higher binding energy (away from the CO(2π*) orbitals,48) and reduces the electron population of these d orbitals by a TM(d) f TM(s,p) rehybridization. The combination of these phenomena leads to a reduction in the TM(d) T CO(2π*) bonding interactions. The bigger the changes in the electron population and stability of the group 10 metal d band (Ni < Pt < Pd, Figure 13A,B), the larger the weakening in the TM-CO bond (Ni < Pt < Pd, Figure 13D). From the CO chemisorption studies discussed in this section, it is clear that zinc can induce significant changes in the chemical properties of late-transition metals. For surfaces that combine zinc and transition metals with electron-rich d bands, the bonding interactions between the metals are so strong that the band structure of the transition metal is substantially modified, introducing in this way the possibility for new and unique chemical properties. IV.4. The Interaction of Group 10 Metals with Zinc and Other Transition-Metal Surfaces: A Comparison. Previous studies have examined the properties of group 10 metal films supported on surfaces of transition metals in detail.2,3,49-59 It is worthwhile to compare the changes in the electronic and chemical properties induced by zinc on group 10 metals with changes observed after bonding these elements to other transition metals. In bimetallic surfaces that contain two transition metals, the d T d orbital interactions play a significant role in the bonding and chemical properties of the system. On the other hand, the effects of the TM(d) T Zn(3d) interactions on the bonding and chemical properties of the ZnTM surfaces are negligible. Nevertheless, Zn induces electronic and chemical perturbations on the group 10 metals that are comparable to those found when these elements are in contact with surfaces of early-transition metals. The photoemission and CO-TDS results in Figure 15 illustrate this phenomenon, showing very similar trends for the deposition of the group 10 metals on Zn and W. The ab initio SCF calculations for the TM/Zn(001) systems (Table 1) show a shift of d electrons from the group 10 metals toward the Zn substrate. For the deposition of Ni, Pd, or Pt on W(110),55 a decrease in the work function of the system suggests that there is also a net shift (or polarization) of electrons from the group 10 metals toward the W substrate. This picture is supported and confirmed by the results of ab initio (or “first principles”) calculations based on local-density functional theory60,61 or the Hartree-Fock method.62 For the adsorption of CO on TM/W(110) surfaces, the results of theoretical calculations58a,60,63 and experimental measurements56,57 show a decrease in the amount of π-back-donation from the metals to CO that reduces the strength of the TM-CO bond. An identical phenomenon is occurring in the CO/ZnTM systems.
Figure 15. Properties of TM/Zn and TM/W surfaces (TM ) Ni, Pd, or Pd; θTM ) 1 ML). (A) Experimental shift in the first peak (or maximum) of the group 10 metal d band.12-15,50,51 (B) Decrease in the desorption temperature of CO from the group 10 metal.12,15,52-54
Zn and W induce perturbations in the properties of the group 10 metals that are much larger than those seen for the interaction of group 10 elements with late-transition metals.49i,56,57 For example, in the case of CO/Pd/Ru,56 the Pd CO desorption temperature decreases by only 120 K, against 180 K for Pd/ W52 and 220 K for Pd/Zn.13 The key to the large perturbations produced by Zn and W, or other early-transition metals,49,59 is probably in the fact that these substrates have almost empty valence bands. Therefore, they can be very effective at inducing a shift of d electrons from the group 10 metals toward them. V. Conclusions 1. Zn atoms deposited on Rh(111) desorb at 650-850 K (first layer, surface alloy), 520 K (second layer), and 480 K (multilayer). The strong Zn T Rh interaction leads to shifts of 0.3-0.5 eV in the Zn 2p and 3d levels. The ZnRh surfaces exhibit CO desorption temperatures that are 50-70 K smaller than that of Rh(111). This corresponds to a weakening of ∼4 kcal/mol in the strength of the Rh-CO bond. 2. ZnNi surface alloys decompose at temperatures between 600 and 800 K, with Zn desorbing into gas phase and Ni remaining solid. In the ZnNi alloys, the Ni 2p levels and 3d band exhibit positive binding energy shifts (0.2-0.5 eV), while negative binding energy shifts are observed for the Zn 2p levels and 3d band (0.3-0.6 eV). On the ZnNi surfaces CO desorbs at temperatures that are 20-30 K smaller than those seen on polycrystalline Ni. This reflects a weakening of 1-2 kcal/mol in the strength of the Ni-CO bond. 3. For overlayers of group 10 metals on zinc, there is a very good correlation between the changes in the electronic and chemical properties induced by bimetallic bonding. Among the group 10 metals, Pd shows the strongest perturbations, while Ni exhibits the weakest (Ni < Pt < Pd). In a TM/Zn system, there is an important redistribution of charge that shifts d electrons from around the group 10 metal into the “metal-metal interface”, producing an accumulation of electrons around the bimetallic bonds. This redistribution of charge affects the stability of the core levels and valence d band of the group 10 metal. The larger the movement of d electrons from the group 10 metal toward the “metal-metal interface”, the stronger the bimetallic bond, and the lower the ability of the group 10 metal to bond CO through π-back-donation. 4. In general, for surfaces that contain zinc and a transition metal, the strength of the interactions between the metals
Interaction of Zinc with Transition-Metal Surfaces increases when the fraction of empty states in the valence band of the transition metal decreases. In these systems, bimetallic bonding can induce electronic and chemical perturbations that are comparable to those seen in surfaces that combine two transition metals. Acknowledgment. This work was carried out at Brookhaven National Laboratory under contract DE-AC02-76CH00016 with the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Science Division. References and Notes (1) (a) Sinfelt, J. H. Bimetallic Catalysts; Wiley: New York, 1983. (b) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2d ed.; McGraw-Hill: New York, 1991. (2) Campbell, C. T. Annu. ReV. Phys. Chem. 1990, 41, 775. (3) Bauer, E. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984; Vol. 3. (4) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: New York, 1994. (5) Clarke, J. K. A. Chem. ReV. 1975, 75, 291. (6) (a) Ichikawa, M.; Lang, A.; Shriver, D. F.; Sachtler, W. M. H. J. Am. Chem. Soc. 1985, 107, 7216. (b) Jen, H. W.; Zheng, Y.; Shriver, D. F.; Sachtler, W. M. H. J. Catal. 1989, 116, 361. (7) (a) Boccuzzi, F.; Chiorino, A.; Ghiotti, G. Surf. Sci. 1989, 209, 77. (b) Boccuzzi, F.; Chiorino, A.; Ghiotti, G.; Pinna, F.; Strukul, G.; Tessari, R. J. Catal. 1990, 126, 381. (8) (a) Green, B. E.; Sass, C. S.; Germinario, L. T.; Wehner, P.; Gustafson, B. L. J. Catal. 1993, 140, 406. (b) Potochnik, S. J.; Falconer, J. L. J. Catal. 1994, 147, 101. (9) Sachtler, W. M. H. Faraday Discuss. Chem. Soc. 1981, 72, 7. (10) (a) Himpsel, F. J.; Eastman, D. E.; Koch, E.; Williams, A. R. Phys. ReV. B 1980, 22, 4604. (b) Miyazaki, E.; Tsukada, M.; Adachi, H. Surf. Sci. 1983, 131, L390. (11) Ley, L.; Kowalczyk, S. P.; McFeely, F. R.; Pollack R. A.; Shirley, D. A. Phys. ReV. B 1973, 8, 2392. (12) Rodriguez, J. A.; Hrbek, J. J. Chem. Phys. 1992, 97, 9427. (13) Rodriguez, J. A. J. Phys. Chem. 1994, 98, 5758. (14) Fansana, A.; Braicovich, L. Surf. Sci. 1982, 120, 239. (15) Rodriguez, J. A.; Kuhn, M. J. Chem. Phys. 1995, 102, 4279. (16) Rodriguez, J. A.; Hrbek, J. Surf. Sci. 1994, 312, 345. (17) Rodriguez, J. A. Surf. Sci. 1993, 289, L584. (18) Rodriguez, J. A. J. Phys. Chem. 1993, 97, 6509. (19) (a) Tikhov, M.; Bauer, E. Surf. Sci. 1990, 232, 73. (b) Campbell, R.; Rodriguez, J. A.; Goodman, D. W. Surf. Sci. 1991, 256, 272. (20) Kuhn M.; Rodriguez, J. A. Surf. Sci. 1995, 336, 1. (21) Dupuis, M.; Chin, S.; Marquez, A. in RelatiVistic and Electron Correlation Effects in Molecules and Clusters; Malli, G. L., Ed.; NATO ASI Series; Plenum: New York, 1992. (22) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (23) Rodriguez, J. A. Surf. Sci. 1994, 318, 253. (24) Rodriguez, J. A.; Kuhn, M. J. Phys. Chem. 1994, 98, 11251. (25) Kuhn, M.; Rodriguez, J. A. Surf. Sci. Submitted for publication. (26) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1841. (27) Szabo, A.; Ostlund, N. S. Modern Quantum Chemistry; McGrawHill: New York, 1989. (28) Hansen, M. Constitution of Binary Alloys, 2d ed; McGraw-Hill: New York, 1958. (29) Andrews, P. T.; Hisscott, L. A. J. Phys. F 1975, 5, 1568. (30) Baer, Y.; Heden, P. F.; Hedman, J.; Klasson, M.; Nordling, C.; Siegbahn, K. Solid State Commun. 1970, 8, 517. (31) Williams, G. P. Electron Binding Energies of the Elements, version II; National Synchrotron Light Source, Brookhaven National Laboratory, 1992. (32) Mezey, L. Z.; Giber, J. Jpn. J. Appl. Phys. 1982, 21, 1569. (33) (a) Thiel, P. A.; Williams, E.; Yates, J. T.; Weinberg, W. H. Surf. Sci. 1979, 84, 54. (b) DeLousie, L. A.; White, E. J.; Winograd, N. Surf. Sci. 1984, 147, 252. (34) Bowker, M.; Guo, Q.; Li, Y.; Joyner, R. W. Catal. Lett. 1993, 18, 119. (35) (a) The weakening in the strength of the Rh-CO bond was calculated by comparing the activation energies for desorption of CO from clean Rh(111) and the rhodium-zinc surface. The activation energies for desorption of CO were estimated using a Redhead analysis for first-order desorption kinetics,35b with a preexponential factor of 1013 s-1, a heating rate of 5 K/s, and desorption temperatures of 410 and 480 K. (b) Redhead, P. A. Vacuum 1962, 12, 203. (36) Elliot, R. Constitution of Binary Alloys: First Supplement; McGrawHill: New York, 1965. (37) Egelhoff, W. F. Phys. ReV. Lett. 1983, 50, 587.
J. Phys. Chem., Vol. 100, No. 1, 1996 389 (38) Values estimated using a Redhead analysis for first-order desorption kinetics,35b with a preexponential factor of 1013 s-1, a heating rate of 5 K/s, and the CO desorption temperatures seen in Figure 9 for Ni and NiZn surfaces. (39) (a) Apai, G.; Baetzold, R. C.; Jupiter, P. J.; Viescas, A. J.; Lindau, I. Surf. Sci. 1983, 134, 122. (b) Horsley, J. A. J. Chem. Phys. 1982, 76, 1451. (c) Hay, P. J. J. Am. Chem. Soc. 1981, 103, 1390. (d) Jeon, Y.; Qi, B.; Lu, F.; Croft, M. Phys. ReV. B 1989, 40, 1538. (e) Jeon, Y.; Chen, J.; Croft, M. Phys. ReV. B 1994, 50, 6555. (f) Watson, R. E.; Bennett, L. H. Phys. ReV. B 1977, 15, 5136. (g) Sham, T. K. Phys. ReV. B 1985, 31, 1903. (40) Emsley, J. The Elements; Clarendon Press: Oxford, 1990; pp 125, 137, 141, and 231. (41) Egelhoff, W. F. Surf. Sci. Rep. 1987, 6, 253. (42) (a) Clementi, E.; Roetti, C. At. Data Nucl. Data Tables 1974, 14, 177. (b) McLean, A. D.; McLean, R. S. At. Data Nucl. Data Tables 1981, 26, 197. (43) (a) The calculations for atomic Zn in [Ar]3d104s24p0 and [Ar]3d104s14p1 electronic configurations were carried out using the HONDO program.21 The Zn basis set consisted of (14s,11p,6d) primitive Gaussian functions. It was taken from the (14s,9s,5d) basis of ref 43b augmented by two additional p functions (R ) 0.243 683 and 0.071 654) and by one d function (R ) 0.17).43c (b) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. (c) Bauschlicher, C. W.; Langhoff, S. R. Chem. Phys. Lett. 1986, 126, 163. (44) Bhyholder, G. J. Phys. Chem. 1964, 68, 2772; 1975, 79, 756. (45) (a) Hermann, K.; Bagus, P. S.; Nelin, C. J. Phys. ReV. B 1987, 35, 9467. (b) Davidson, E. R.; Kunze, K. L.; Machado, F. B.; Chakravorty, S. J. Acc. Chem. Res. 1993, 26, 628. (46) For CO on transition-metal surfaces, the valence d orbitals are actively involved in bonding, and the heat of adsorption of the molecule varies between 90 and 20 kcal/mol. On the other hand, for CO on noble metals, the participation of the valence d orbitals in bonding is weak, and the heat of adsorption of CO is relatively small (
