the interaction of cu and s2 with aluminum and alumina surfaces: a comparative study

ELSEVIER Surface Science 380 (1997) 397-407

surface science

The interaction of Cu and S 2 with aluminum and alumina surfaces: a comparative study

Jos6 A. Rodriguez *, Mark Kuhn, Jan Hrbek Department of Chemistry, Brookhaven National Laboratory, Upton, N Y 11973, USA

Received 21 October 1996; accepted for publication 12 December 1996

Abstract

The bonding interactions between Cu and A1 are much stronger than those between Cu and Al203. Cu atoms supported on alumina show a narrow 3d band with a centroid shifted ~0.35 eV with respect to that of the 3d band in bulk metallic Cu. In contrast, Cu atoms deposited on aluminum exhibit shifts of 1.3 1.6 eV in the centroid of the 3d band. Similar differences are observed when comparing the behavior of Ag and Pt overlayers on alumina and aluminum. The d band shifts on the oxide substrate are in the order of 0.3 0.4 eV, whereas on the metal substrate they vary from 0.8 to 2.0 eV. These trends are explained in terms of a simple model that takes into account changes in the energy of the Al(3s,3p) bands when going from metallic aluminum to alumina. The sticking coefficient of $2 on alumina surfaces is at least one order of magnitude smaller than on aluminum, a difference that also reflects variations in the position of the Al(3s,3p) bands. Submonolayer coverages of Cu do not produce significant changes in the electronic properties of A1203. In contrast, the deposition of small amounts of sulfur (~0.1 ML) induces a substantial reduction (0.4 0.5 eV ) in the binding energies of the O KVV, O I s and AI 2p features of alumina. This is consistent with a transfer of electrons from alumina into the S atoms that produces a transformation similar to a change from n-type to p-type semiconductors. The reactivity of Cu/A1203 surfaces toward sulfur is much larger than that of pure A1203 surfaces. Cu clusters supported on alumina react with $2 to form CuSx compounds that decompose at temperatures between 850 and 1100 K. © 1997 Elsevier Science B.V.

Keywords: Aluminum; Aluminum oxide; Copper; Metal insulator interfaces; Metal-metal interfaces; Photoelectron spectroscopy; Sulfur: Sulphides

1. Introduction

Metal/oxide interfaces play an important role in a large number of technological applications (catal- ysis, microelectronics, gas sensors, metal/ceramic composites, crystal growth, etc.) [1-3]. In many technologies economic and operational issues are strongly affected by the successful control of the metal/oxide interface [ 1 ]. This requires a detailed

* Corresponding author. Fax: + 1 516 344 5815; e-mail: [email protected]

0039-6028/97/$17.00 Published by Elsevier Science B.V. PH S0039-6028(97)00032-0

knowledge of the factors that determine bonding interactions between a metal and an oxide, and how these interactions affect the structural and chemical properties of the interface. In recent years very useful information about the behavior of metal/oxide systems has been obtained through experiments that combine the modern techniques of surface science with the in situ preparation of the metal overlayer and oxide substrate [1 4].

In this article, we investigate (and compare) the interactions of copper and sulfur with aluminum and alumina films supported on Mo( l l0) .

398 J.A. Rodriguez et al. / SurJace Science 380 (1997) 397-407

Cu/AI203 surfaces are active catalysts for the synthesis of methanol from H2/CO/CO2 mixtures and the water-gas shift reaction (CO+H20-- ,H2+ CO2) [5]. These Cu/AI203 catalysts are very sensi- tive to sulfur poisoning [5]. With respect to copper, sulfur exhibits a much bigger electronegativity [6] and a larger thermochemical reactivity toward oxygen and aluminum [7]. Thus by studying the behavior of the S2/A1203 and Cu/AI203 systems one is examining the response of the oxide to different types of adsorbates.

No work has been reported examining the adsorption of S 2 o n A1203 or Cu/AI203 surfaces. High-resolution electron-energy loss spectroscopy (HREELS) and Auger electron spectroscopy (AES) have been used to investigate the interaction of Cu with ultrathin (4-8 A) films of A1203 prepared by the oxidation of an AI(l l 1) surface [4]. The deposited Cu atoms do not interact strongly with alumina and, therefore, no substantial changes are observed in the vibrational modes of the oxide [4]. At 300 K, the initial sticking probability of Cu on ultrathin (20-25 ~,) films of A1203 supported on Mo( l l0 ) is ~0.85 [8]. Studies using thermal desorption spectroscopy (TDS) show that copper desorbs from the A1203/Mo(l10) substrate at temperatures above 900 K. As the Cu coverages increases, the complete Cu desorption peak shifts toward higher temperature. Such a phenomenon, which is typical of the desorption of small metal particles (or clusters) [2,9], suggests that copper is not wetting the alumina at high temperatures [8]. This is corrobo- rated by the results of recent studies of low-energy ion scattering spectroscopy (ISS) for Cu/ A1203/Re(001) systems [10], which indicate that Cu grows as three-dimensional (3D) clusters on alumina films at both 80 and 300 K.

Here we study the behavior of a series of Cu/A1203/Mo (110), S/A1203/Mo (110) and S/Cu/A1203/Mo(ll0 ) systems using core- and valence-level photoemission. Cu clusters supported on alumina exhibit a partially developed metallic band structure. Large differences are found in the electronic perturbations that copper and sulfur induce on alumina. The interactions between $2 and Cu/AI203 are much stronger than those between $2 and A 1 2 0 3 .

2. Experimental

The experiments described here were performed in a UHV chamber (base pressure < 3 × 10-X°Torr) equipped with instrumentation for TDS, ISS, LEED and photoemission. The photoemission spectra were acquired using MgK~ radiation and a hemispherical electron energy ana- lyzer with multichannel detection. The binding- energy scale in the spectra was calibrated by using the Cu 2p3/2 and Mo 3d5/2 peaks of pure Cu and Mo, which were set at binding energies of 932.5 and 227.9 eV [ 11 ], respectively.

Ultrathin (~ 20 ,~) films of A1203 were grown on Mo( l l0 ) following a methodology described in previous works [8,12,13]. Aluminum atoms (generated by heating a W filament wrapped with a high-purity wire of A1 ) were dosed to the molyb- denum substrate at 700-800 K in a background 02 pressure of 10 5_10-6 Torr, with subsequent annealing to 1200 K to improve the crystalline quality of the alumina films. Pure films of polycrystalline aluminum were prepared by depositing aluminum on clean Mo( l l0 ) at 100 K [14].

Cu was vapor-deposited on the A1203/Mo (110) and A1/Mo(ll0) systems at temperatures between 100 and 350 K. The Cu doser consisted of a ultra pure Cu wire wrapped around a W filament. The atomic flux from the doser was calibrated following the procedure described in Ref. [8], comparing Cu-TDS spectra for Cu/Mo (110) and Cu/A1203/Mo(ll0 ) systems. In this work, the amount of Cu deposited is reported with respect to the number of Mo(110) surface atoms. One equivalent monolayer of the admetal corres- ponds to the deposition of 1.43 × 1015 Cu atoms c m - 2.

A1, A1203, and Cu/A1203 surfaces were exposed to $2 gas generated by the decomposition of Ag2S in a solid-state electrochemical cell: Pt/Ag/ AgI/Ag2S/Pt [14]. For small doses of $2 the coverage of sulfur on the surface was determined by measuring the area under the S 2p peaks which was scaled to absolute units by comparing to the corresponding area for 0.9 ML of S on Mo(110) [141.

J.A. Rodriguez et al. / Surface Science 380 (1997) 397 407 399

3. Results

3.1. Reaction Q~ S 2 with aluminum and alumina

Fig. 1 displays A1 and S 2p XPS spectra taken after exposing a polycrystalline aluminum film (0A1~4 ML) to $2 gas at 550 K. The uptake of sulfur is substantially larger than a monolayer, and the presence of an A1 2p peak around 75.5 eV indicates the formation of A1Sx compounds [14]. In general, we found that A1 films are very reactive toward S 2 at temperatures between 300 and 900 K. As the thickness of the AISx overlayer increases, the dissociative sticking coefficient of $2 decreases. For films with 0AI>4 ML a complete sulfidation

ilJ

AI 2p: SIAl film

80 ] I I I ' I

78 76 74 72 70

Binding Energy (eV)

f,

S 2p: SIAl film

I I I I I

168 166 164 162 160

Binding Energy (eV)

Fig. 1. AI 2p (top) and S 2p (bottom) XPS spectra acquired after dosing $2 gas to a polycrystalline aluminum film (0Aj ~4 ML) at 550 K. In the bottom of the figure, for comparison, we include the S2p spectrum for 0.9 ML of S on Mo(llO) [14].

of the aluminum present in the system was difficult to achieve. On the other hand, for films with 0A1<3 ML, all the aluminum was easily trans- formed into AISx. The formation of A1Sx species induced shifts of 2.4 2.8 eV in the A1 2p features. The S 2p spectra of the AISx overlayers exhibited only a broad peak (for an example, see Fig. 1) centered at 162.8-162.1 eV. This behavior was observed for films with a large range of A1 coverages (2-15 M L). The line shape of the S 2p spectra indicated that there were at least two different types of S atoms in the AISx overlayers. The S atoms bonded to A1 exhibited S 2p3, 2 binding energies larger than those seen for S atoms bonded to Mo( l l0 ) [14].

The sticking coefficient of $2 on A1203 surfaces is very small at temperatures between 300 and 700 K. Fig. 2 shows At2p, O l s and S 2p XPS spectra acquired before and after dosing a large amount of $2 to an ultrathin (~20 A) film of A1203 at 310 K. The dose of $2 was similar to that used in the experiments of Fig. 1 for a S2/A1 system, but on the alumina surface the amount of sulfur deposited was ~0.1 ML. Dosing of S 2 at 550 and 700 K produced even smaller coverages of sulfur (0.08 and 0.06 ML, respectively) on the A1203 film. These results indicate that the sticking coefficient of S 2 o n AlzO 3 surfaces is at least one order of magnitude smaller than on AI surfaces.

In Fig. 2, the deposition of small amounts of S ( ~ 0.1 ML) induces big negative shifts (0.4-0.5 eV) in the binding energies of the A1 2p and O ls features of alumina. An identical behavior was seen for the O KVV Auger transition of the oxide (not shown). These binding-energy shifts may be a consequence of changes in the Fermi level within the band gap of the system due to a transfer of electrons from alumina into the S atoms. Similar binding-energy shifts are observed when doping semiconductors and switching from n-type to p-type materials [1,18], To induce this transformation only small concentrations of the doping agent are necessary [18].

After heating the S/A1203 system in Fig. 2 to 700 K, ~45% of the sulfur was removed from the surface and there was a positive binding-energy shift (~0 .2eV) in the A12p, O Is and OKVV features of the oxide. Complete removal of the

400 ZA. Rodriguez et al. / Surface Science 380 (1997) 397 407

-=e

=5

AI 2p: $IA1203 ~ }

/

, , , , - 4 80 78 76 74 72

v

0 ls: SIAl20 ~

i i i i

536 534 532 530 528

S 2P: S/AI'zOa ~ 1

f \c,,o.=~l /

168 166 164 162 160 Binding Energy (eV)

Fig. 2. Al 2p (top), O ls (center) and S 2p (bottom) XPS spectra recorded before and after exposing an ultrathin (~20 A) film of A1203 to $2 gas at 310 K, with subsequent annealing to 1200 K,

v >- I-

Z LLI t-" Z

12 10 8 6 4 2 0 -2

BINDING ENERGY (eV)

5

938 936 934 932 930 928 926

BINDING ENERGY (eV)

Fig. 3. Valence band and Cu2p3/2 photoemission spectra acquired after depositing Cu on an ultrathin (~ 20 A) film of alumina at 320 K. For the valence spectra the coverages of Cu were 0.6, 1.0, 1.3, 2.0, 3.1 and 9.4 ML. In the Cu 2p3/2 spectra, the Cu coverages were 0.2, 0.6, 1.3, 3.1 and 9 . 4 M L The electrons were excited using Mg K~ radiation.

sulfur was achieved by annealing to 1200K. At this point, the sample showed A1 2p, O ls and O KVV peak positions almost identical (+0.03 eV) to those seen before the deposition of sulfur. During the annealing of the S/A1203 surface (300-700 K, 700-1200 K), we were not able to detect the desorption of S or Sz. This may be due to the fact that the amount of sulfur initially present on the oxide was small (~0.1 ML), and it desorbed in a very large range of temperature.

3.2. Interaction of Cu with aluminum and alumina

In the top of Fig. 3 are shown XPS valence- band spectra acquired after depositing Cu on an ultrathin ( ,-~ 20 A) film of A1203 at 320 K. In He I and II UPS spectra reported in the literature for

Cu/A12Oa/AI( 111 ) [ 15, 16 ] the Cu valence features appear in a background dominated by the alumina bands. This is not the case in Fig. 3. Since the valence region was examined using Mg Ks radiation the intensity of electron emissions from the alumina bands is very weak due to a low cross- section [13] and the valence spectra are dominated by electron emissions from the Cu bands. For a surface with 0.6 ML of Cu, emissions from the Cu 3d band appear from 2.5 to 5 eV, with a band centroid located around 3.5 eV. This is different from the band structure of metallic Cu [17], where the 3d band extends from 1.5 to 6eV and is dominated by a strong peak at ~2.8 eV. As the Cu coverage increases in Fig. 3, the band structure of the Cu adlayer evolves toward that of bulk Cu. There is a clear rise in the density of states in the

J.A. Rodriguez et al. / Sur/dce Science 380 (1997) 397-407 40l

3.0-2.0 eV region, and the full-width at half-maxi- mum of the 3d band increases from 1.9 eV at 0.6 ML, to 2.2 eV at 2.0 ML, and finally to 2.6 eV at 9.4 ML.

The Cu 3d binding-energy shifts found for Cu on Al20 3 (_<0.4 eV) are much smaller than those seen for the deposition of Cu on A1 surfaces [19,23]. For example, after depositing 1-2 ML of Cu on AI(100) [19] or A I ( l l l ) [23], the Cu3 d band is centered at ~4.5 eV and the emission of electrons in the 3 0 eV region is very weak. In a similar way for the deposition of Cu (0.3 1.1 ML) on polycrystalline A1 at 100 K we found shifts of 1.3-1.6eV in the centroid of the C u 3 d band. Thus, the electronic interactions between Cu and AI are much stronger than those between Cu and A 1 2 0 3 .

The bottom panel of Fig. 3 displays Cu 2p3/2 XPS spectra for a series of Cu/A1203/Mo(l l0) systems. A surface with 0.2 ML of Cu exhibits a Cu2p3,. 2 peak position that is shifted ,--0.85 eV toward higher binding energy with respect to that of metallic Cu (932.5 eV [11]). As the Cu coverage increases, there is a monotonic decrease in the Cu 2p3/2 binding energy until the value for pure Cu is reached at 0cu=9.4 ML. Similar negative binding-energy shifts have been observed in recent studies dealing with the electronic properties of Cu/AIzOa/Re(001) systems [10]. These shifts mainly come from changes in final-state relaxation energy [10]. In a "bulk" metal the screening of the photoemission core-hole is more effective than in an oxide or small metal particles [18]. Therefore when the Cu coverage increases in the Cu/alumina systems there is an "apparent" shift toward lower binding energy in the Cu 2p levels.

Cu atoms deposited on polycrystalline aluminum at 100 K show Cu 2p3/2 binding-energy shifts considerably larger than those seen for Cu on A1203 (see Fig. 4), supporting the idea that the Cu~-~A! interactions are much stronger than the Cu~-~AI203 interactions. The graph at the bottom of Fig. 4 compares Cu 2p3/2 signal intensities measured after depositing Cu on A1 at 100 K, and on A1203 at 100 and 320 K. For similar coverages of Cu, the largest Cu 2p3/2 intensities are observed on the metal substrate at 100 K, and the lowest on the oxide at 320 K. On Al at 100 K, Cu is expected

934.2

934 0

933.8

n 933.6 pq

.~ 933.4 ~ J r~

935.2 c ~

933.0

c ) 932.8

932 6

932 4 0.0

i i i I I i

' - Cu/Al(100 K)

/A1203(100 K)

~..._~Cu/AI203 (320 K)

Cu meta l

I I I I I I

05 1.0 1.5 2.0 25 30 5.5

Cu coverage (ML)

20

16

',~ 1 2

~J .9

8 c o

e~ c q

o 4

i [ I t

/c Cu/AI(I oo K)

/ /

/

/

oa(32o ~)

I I I

0.5 1.0 I .5

Cu coverage (ML)

0 - - L

O0 2 0

Fig. 4. Cu 2p3;2 binding energies and intensities measured after depositing Cu on polycrystalline a luminum at 100 K (O sym- bols), and alumina at 100 K (Q) and 320 K (V) . The dashed line in the top panel of the figure denotes the Cu 2p3~2 binding energy for bulk metallic copper. The solid lines connecting the points are drawn to guide the eye.

to grow in a quasi layer-by-layer fashion [23]. The trends for the Cu 2p3/2 intensities in Fig. 4 indicate that Cu is not wetting the oxide support. The admetal probably forms 3D clusters on top of alumina, with the clusters at 320 K being larger than the ones at 100 K. Some authors, on the basis of AES intensity plots as a function of Cu depos-

402 J.A. Rodriguez et al. / Surface Science 380 (1997) 397-407

ition time (or coverage), have proposed that Cu forms a uniform monolayer o n A1203

(Stranski-Krastanov growth mode) at 300 [15] and 95 K [4]. The data in Fig. 4 are not consistent with this behavior. There is no clear change in the slope of the Cu-2p-intensity versus Cu-coverage curve for C u / A I 2 0 3 around 1 ML, and for a given Cu coverage the Cu 2p3/2 intensity on A1203 is much smaller than on A1 where there is a quasi layer-by-layer growth mode. Our XPS results are consistent with the results of recent ISS [10] and SEXAFS [20] experiments which indicate that Cu grows as 3D particles o n A1203 via a Volmer-Weber growth mode.

Fig. 5 illustrates the effects of Cu on the O ls features of alumina. The deposition of Cu leads

v >.-

Z ILl I,-- Z

536 534 532 530 528

BINDING ENERGY (eV)

Z W I-- Z

82 80 78 76 74 72 70

BINDING ENERGY (eV)

Fig. 5. O ls, A12p, and Cu 3p XPS spectra taken after depositing Cu on an ultrathin ( ~20 ,~) alumina film at 320 K. For the O ls spectra, the coverages of Cu are 0.2, 1.3, 3.1 and 9.4 ML (from top to bottom). The copper coverages in the {AI 2p+Cu 3p} spectra are 9.4, 3.1, 1.3 and 0.2 ML (from top to bottom).

to a decrease in the O ls intensity and small (< 0.3 eV) negative binding-energy shifts. For systems with submonolayer coverages of Cu, the shifts in the O ls peak position were negligible (<0.1 eV). Thus it appears that Cu is neither transferring electrons into the surface states of alumina [21] nor forming CuOx species [22]. This is consistent with the behavior observed in Cu/AI203/Re(001) systems [10], where the Cu Auger parameter indicates that there is no oxidation of Cu. The small negative shifts observed in Fig. 5 for the O 1 s features can be attributed to an increase in final-state relaxation energy within the escape depth probed by XPS [18]. Similar binding- energy shifts were seen for the AI 2p features, but the analysis of these data is complicated when the Cu coverages are large (> 1.5 ML), because there is significant overlapping between the A1 2p and Cu 3p signals (bottom panel in Fig. 5).

3.3. Reaction of S 2 with Cu/AI203 surfaces

After examining the interaction of $2 with a series of C u / A 1 2 0 3 surfaces (0cu< 1.5 ML) at 300 and 700 K, we found that these systems have a much larger reactivity toward $2 than pure alumina. The uptake of sulfur depended strongly on the amount of copper present on the surface, and we were able to generate S/Cu/AI203 systems with a S/Cu atomic ratio close to 0.9. Part of the sulfur adsorbed on the C u / A I 2 0 3 systems was bonded to the oxide and modified its electronic properties. Fig. 6 shows A1 2p, O ls and S 2p spectra taken after exposing an alumina surface with 0.5 ML of Cu to S 2 at 315K. Upon the adsorption of ~0.4 ML of sulfur there is a negative binding- energy shift of ,-~0.5 eV in the A12p and O ls features of the oxide. A similar result was found after exposing pure alumina to $2 (see Fig. 2). Heating of the S/Cu/A1203 system to 700 K induced desorption of part of the sulfur and a drastic change in the line shape of the S2p spectrum. After annealing to 1200 K only a very small amount (~ 0.03 ML) of sulfur remained on the surface. At this point no Cu was left on the oxide.

Fig. 7 displays Cu 2p3/2 XPS and Cu L3VV Auger spectra acquired before and after dosing

J.A. Rodriguez et al. Surface Science 380 (1997) 397-407 403

g

AI 2p: S / C u / A I 2 0 3 ~

' I I I ~ I

80 78 76 74 72

= 0 lS: S/Cu/AI203

536 534 532 530 528

S 2p: S/Cu/AI~O 3 ~ = _ ~ ' ~ ..CA.-- -~, (x o.5)

g

a n n e a l 1 2 [ . I u K

I I ' ) I

168 166 164 162 160 Binding Energy (eV)

Fig. 6. AI 2p (top), O ls (center) and S 2p (bottom) XPS spectra acquired after dosing $2 gas to a Cu/AI203 surface (0c~=0.5 ML) at 315 K, with subsequent annealing to 700 and 1200 K. In the bottom of the figure, for comparison, we also include the S 2p spectrum for 0.9 ML of S on Mo(ll0) [14].

S 2 to Cu/A1203 surfaces with Cu coverages of 0,7 (dotted traces) and 1.3 ML (solid traces). The adsorption of sulfur produces a reduction of ~ 0.4 eV in the Cu 2p3/2 peak position. Since this binding-energy shift is similar to those induced by sulfur on the A l2p and O ls levels, it can be attributed to a change in the Fermi level within the band gap of the system. On the other hand, the formation of CuSx species can produce also negative shifts in the Cu2p3/2 level (a Cuso,d+Sz.gas ~ CuSsolia transformation leads to a reduction of -~0.3 eV in the binding energy of the Cu 2p3j2 peak [24-26]), The Cu L3VV spectra in Fig. 7 exhibit clear negative kinetic-energy shifts (0.4-0.7 eV) upon the deposition of sulfur. This behavior indicates [24-26] that CuS~ compounds are formed when $2 reacts with Cu clusters sup-

Cu 2p ] ~ S on Cu/AI203

nos ~ / \ \

T I ~ T - -

936 934 g32 930 Binding Energy (eV)

Cu Auger j ~ ]

914 916 918 920 922 Kinetic Energy (eV)

Fig. 7. Cu2p32 (top panel) and Cu L3VV Auger spectra (bottom panel) for Cu/A1203 and S/Cu/AI203 surfaces. The solid traces correspond to surfaces with 1.3 ML of Cu, while the dotted traces come from surfaces with 0.7 ME of Cu. The spectra were acquired using Mg K~ radiation.

ported on alumina. Furthermore, a comparison of valence-band spectra for Cu/AI203 and S/Cu/A1203 surfaces showed narrower (10-20%) Cu 3d bands in the systems with sulfur, which is also consistent with the formation of CuSx compounds [26],

Previous studies indicate that Cu desorbs from alumina at temperatures between 950 and 1100 K [8]. Fig. 8 shows Cu-TDS, mass 63, and S2-TDS, mass 64, spectra for a S/Cu/A1203 system with 1.1 M L of S and 1.3 M L of Cu. The desorption of Cu occurs at temperatures that are similar to those seen for the desorption of the metal from Cu/AI203 surfaces, Evolution of $2 into gas phase takes place at 350-500 K and 850-1150 K. XPS spectra taken after heating to 1250 K revealed the

404 J.A. Rodriguez et al. / Surface Science 380 (1997) 397-407

.o m

E o

5

S / G u / a l u m i n a - - mass 63

.... mass 64

i I i i

4 0 0 6 0 0 800 1 0 0 0

Temperature (K)

i

1200

Fig. 8. Cu- and S2-TDS spectra (mass 63 and mass 64, respectively) acquired during the heating of a S/Cu/Alz03 surface {0cu=l.3 ML, 0s=l.1 ML}. In the first step, $2 was dosed to a Cu/AI203 surface at 315 K. Heating ra te=5 K s -1.

absence of Cu and ~0.06 ML of S on the oxide. This small amount of sulfur was removed by annealing to 1400 K, but the heating also induced decomposition of part of the A1203 film. During the heating of the S/Cu/AI203 system from 300 to 1250 K, we did not find any significant evidence for the desorption of A1- or O-containing species.

4. Discussion

4.1. Interaction of Cu with aluminum and alumina

when dealing with oxides [18]. The problem of charging can be amended by working with ultrathin oxide films [2,3]. On oxides, electronegative and electropositive adsorbates can induce large core-level shifts by changing the position of the Fermi level [1,3, 18]. Cu, Ag and Pt do not induce significant changes in the electronic properties of alumina at submonolayer coverages [ 10,13,29]. Thus, no reference-level changes are expected for the Cu/Al203, Ag/A1203 and Pt/A1203 systems under these conditions. To avoid problems associated with variations in final-state relaxation energy, we will focus our attention on the valence spectra of the Cu, Ag and Pt overlayers.

Fig. 9 compares shifts found in the centroid of the d band for monolayers of Cu, Ag [13, 19] and Pt [28,29] on aluminum and alumina. The shifts on the oxide substrate are in the order of 0.3-0.4 eV, and arise from the fact that small metal clusters have a band structure different from that of bulk metals. On aluminum the d band shifts are relatively large (0.8-2.0 eV ) and reflect changes in the d population of the admetals induced by bimetallic bonding [30-33]. Ab initio SCF calculations for Cu/A19, Ag/A19 and Pt/A19 clusters (mod- eling the bonding of Cu, Ag and Pt to hollow sites of Al( l l 1)) show a reduction in the d population of the admetals (Cu, 0.09e [30]; Ag, 0.04e [30]; Pt, 0.31e [31]). This is consistent with the results

The experiments presented above indicate that the Cu*-->A1203 interactions are weak, in agreement with the results of previous studies [4,8,10,20]. Weak admetal,--,oxide interactions have also been observed for the deposition of Zn [13], Rh [27], Ag [13] and Pt [27] on alumina. In all these systems, the admetal grows forming 3D islands on the oxide.

In Cu/alumina surfaces the presence of oxygen (or O-A1 bonds) prevents strong Cu~--~AI interactions and one sees electronic perturbations in the admetal that are much smaller than those seen after depositing Cu on clean aluminum surfaces. Similar trends are observed for Ag and Pt overlayers. In general a quantitative comparison of core- level shifts on metal and oxide surfaces is difficult due to changes in reference level and final-state relaxation and the possibility of charging effects

2.5

~ 2.0

= 1 . 5 ] "~ 1.0

~ 0 . 5

0.~

A h n ' n i n l m l

\

Ca Ag Pt

Alumina

Cu Ag Pt

Fig. 9. Shift in the centroid of the valence d band of a monolayer of Cu, Ag or Pt deposited on aluminum [19,28] and alumina surfaces (Fig. 3 and Refs. [13,29]). The shifts are reported with respect to the centroid of the corresponding d bands of the pure metals.

J.A. Rodriguez et al. / Surlace Science 380 ( 19971 397 407 4(15

of L-edge XANES measurements for bulk Ag-Al and Pt-Al alloys [32].

In order to explain the differences in Fig. 9 between aluminum and alumina, plus the weak admetal.-+oxide interactions, we must examine the band structure of these systems. Fig. 10 shows the energy range covered by the valence bands of A1203, Cu, Ag, Pt and AI. The values for alumina come from band-structure calculations [34], while the position of the Cu(3d,4s), Ag(4d,5s), Pt(5d,6s) and Al(3s,3p) bands was estimated [35] using work functions [36] and photoemission data [17,19,28,37]. In alumina the O2p band is fully occupied, the A1 3s,p band is empty, and there are unoccupied surface states ("SS" in Fig. 10) within the band gap [34,38]. For aluminum the 3p band is very broad and almost empty [37]. The formation of AI O bonds and AI203 produces a very large reduction in the stability of the Al(3s,p) bands.

By comparing the position of the bands in Fig. 10, one can see that the unoccupied and occupied states of the admetals match much better with the bands of aluminum than with the bands of alumina. This leads to strong admetal~--~A1 and

-6

-111

-14

AlzO s Cu o - . L . . . . . . "

AI 3s,p

-2

-4

I

-12

Ag Pt

5 s

4d 6s,5d

AI

a

i i i

i a i i

7 3s

Fig. 10. Energy posi t ions for the bands o f a l um in a [34] copper [ 19, 36], silver [ 19, 36], p l a t inum [28, 36] and a l u m i n u m [36, 37]. The energies are repor ted with respect to the v a c u u m level (see textt . The empty and occupied s tates are indicated by dashed and solid lines, respectively. T he label "SS" refers to surface s tates in a lumina [34].

weak admetal~A1203 bonding interactions. For example the proximity of the empty 3p states of aluminum to the occupied d states of the admetals makes possible a hybridization of these states, stabilizing in this way the admetal-aluminum bonds and producing a reduction in the d population of the admetals [30,31]. On alumina, the attractive Al(3s,p)-admetal(d,s) band interactions are weak due to a mismatch in energy, and one can expect [39] strong closed-shell repulsive interactions between the O 2p band and the d bands of the admetals. These phenomena produce weak adsorption bonds and small electronic perturbations in the C u / A I 2 0 3 , Ag/AI203 and Pt/Al203 systems.

4.2. Reaction of S 2 with AI. A I 2 0 3 and C u/A[20 3

suFfaees

Sulfur forms compounds with oxygen and aluminum that have heats of formation much more exothermic than the heats of formation of copper oxides and aluminates [7]. In spite of this, the sticking coefficient of $2 on alumina at room temperature (<0.1) is not bigger than that of Cu (~ 0.85 [8]). In Table 1 are listed experimental and calculated (INDO) ionization potentials for $2 [40]. The ground ayes- state of $2 arises from the configuration: 2 2 " ) 2 2 2 10"g 10"u~ag 2O'u 3Crg 1 rc 417Zg4 3 a u2 4ag2

2 2 4 2 4Cru5%2r~u2~g. The 2~g orbitals are only half occupied and appear at an energy of --- -9 .4 eV with respect to the vacuum level [40,41 ]. These orbitals, which are S-S anti-bonding, control the reactivity

Table 1 Calcula ted and observed vertical ionizat ion potent ials for 82(3~g )a

Final ionic state Observed IPleV } Calcula ted IP(eV t

2Fig 9.41 9.40 417 u 11.82 11.(13 ZFlu( 1 ) 12.33 I 1.79 %2g 13.20 I I.':18 2~ u - 12.09 2Ag 13.'15 2y~g 14.62 14.28 211u(2) 14.16 21-1u( 3 ) 15.58 15.50

" F r o m Ref. [40]

406 J.A. Rodriguez et al. / Surface Science 380 (1997) 397 407

of the $2 molecule. A comparison of the results in Table 1 and Fig. 10 shows that the occupied states of $2 are well separated from the empty bands of aluminum and alumina, making difficult sulfur~substrate electron-donor interactions. The lowest unoccupied molecular orbitals of $2 (2Zrg'S) are much more stable than occupied states in the 3p band of aluminum. This favors an Al~S2(27zg) charge transfer, the breaking of the S-S bond, and the formation of AISx compounds. In the case of 52/A1203, the 2r~g orbitals of the molecule are not much more stable than occupied states in the valence O 2p band of the oxide. This leads to relatively weak A1203~S2(2rCg) electron- donor interactions, and the sticking coefficient and reactivity of $2 on surfaces of alumina are low. For Cu/AI203 surfaces the supported Cu clusters (or particles) provide a large number of states that can be very efficient for donating electrons to $2 and, therefore, the rate of sulfidation of these systems is larger than that of A1203 surfaces.

The adsorption of ~ 0.1 ML of sulfur (an electronegative element [6]) induced a substantial reduction (0.4-0.5 eV) in the binding energy of the A12p, O ls and O KVV features of alumina. On the other hand, in a previous study [13] we found that small amounts of cesium (an electropositive element [6]) produced a large increase (0.9-1.1 eV) in the binding energy of the core levels and Auger features of alumina. These binding-energy shifts are probably a consequence of changes of the Fermi level within the band gap of the system [1, 18]: sulfur withdraws electrons from alumina (equivalent to p-type doping of a semicon- ductor), whereas cesium donates electrons to the oxide (equivalent to n-type doping of a semicon- ductor). The electronic perturbations observed in the S/AI20 3 and Cs/AI20 3 surfaces contrast with the behavior found for Cu/A1203 surfaces where submonolayer coverages of Cu do not produce significant changes in the electronic properties of the oxide.

Cu/Al203 surfaces are active catalysts for the synthesis of methanol from H2/CO/CO 2 mixtures and the water-gas shift reaction (CO+HzO--*COz+H2) [5]. One major problem associated with the use of these catalysts is sulfur poisoning [5]. Our results indicate that Cu clusters supported on alumina react with sulfur forming

CuSx compounds. Some of these CuSx species are stable up to very high temperatures (800-1000 K). These temperatures are much bigger than those typically used for methanol synthesis and the water-gas shift reaction (450-650 K [5]).

5. Conclusions

(1) Copper does not wet the surface of alumina well, and forms 3D clusters on this oxide. The deposition of copper induces very minor perturbations in the electronic properties of alumina. Small Cu clusters supported on alumina show a narrow 3d band with a centroid shifted ~0.35 eV with respect to that of the 3d band in bulk metallic Cu. In contrast, Cu atoms deposited on aluminum exhibit shifts of 1.3-1.6 eV in the centroid of the 3d band. The bonding interactions between Cu and A1 are stronger than those between Cu and A1203. These differences reflect changes in the energy of the Al(3s,3p) bands due to the formation of AI-O bonds.

(2) At room temperature the sticking coefficient of $2 on alumina surfaces is at least one order of magnitude smaller than on aluminum, a phenomenon that also reflects changes in the position of the Al(3s,3p) bands. The deposition of small amounts of sulfur ( --~ 0.1 ML) induces a substantial reduction (0.4-0.5 eV) in the binding energies of the O KVV, O ls and A1 2p features of alumina. This is consistent with a transfer of electrons from alumina to the S atoms that produces a transformation similar to a change from n-type to p-type semiconductors.

(3) The reactivity of Cu/AI203 surfaces toward sulfur is much larger than that of pure A1203 surfaces. Cu clusters supported on alumina react with $2 to form CuSx species (0.1 <x<0.9) . TDS spectra for S/Cu/A1203 surfaces show the desorption of $2 at temperatures from 350 to 600 K and from 850 to 1100 K, plus desorption of Cu from 950 to 1100 K.

Acknowledgements

This work was carried out at Brookhaven National Laboratory under Contract

J.A. Rodriguez et aL / Surlace Science 380 f 1997) 397 407 407

DE-AC02-76CH00016 with the US Department of Energy, Office of Basic Energy Sciences, Chemical Science Division.

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the interaction of cu and s2 with aluminum and alumina surfaces: a comparative study

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