J. Phys. Chem. 1994, 98, 12059-12066
12059
Interaction of Sulfur with Bimetallic Surfaces: Coadsorption of Sulfur and Noble Metals on Ru(001) Mark Kuhn and Jose A. Rodriguez* Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973 Received: June 22, 1994; In Final Form: September 9, 1994@
The coadsorption of S with Cu or Ag on Ru(001) has been investigated using TDS, XPS, XAES, and CO chemisorption. At 300 K, copper and silver atoms in contact with Ru(OO1) react with S2 to form noble-metal sulfides. The Cu-S and Ag-S bonds in these surface compounds break at high temperatures ('800 K) producing sulfur and noble-metal adatoms that compete for the ruthenium electrons. This competition leads to a weakening of 5-6 kcallmol in the strength of the Ru-Cu and Ru-Ag bonds. A sulfur adatom produces long-range perturbations on the surface, diminishing the ability for bimetallic bonding of several (5- 10) adjacent ruthenium atoms. At 8, = 0.2 ML (ML = monolayer), all the ruthenium sites show a strong weakening in their bonding interactions with copper or silver adatoms. Photoemission experiments examining the interaction of S2 with copper and silver multilayers at 300 K show the formation of thick films of Cu2S and Ag2S at a fast rate. The decomposition uathwavs for these films are similar: evolution of S2 into gas phase, with the noble metal remaining solid. For Ag;S films the decomposition process starts around SOCK, whereas Cu2S films are stable up to 950 K.
I. Introduction Studies examining the dehydrogenation of cyclohexane on Cu/Ru(OOl) show that the presence of copper increases the production of benzene and reduces the rate of C-C hydrogenolysis on the ruthenium surface.' Frequently, the addition of a noble metal to a transition-metal surface produces dramatic changes in the catalytic activity or selectivity of the surface for reactions that lead to the conversion of hydrocarbon^.'-^ Bimetallic catalysts that combine noble and transition metals are common in the chemical industry. One major problem associated with the use of these catalysts is sulfur poisoning.6 This work is part of a research program aimed at gaining a better understanding of the effects of S on the structural, electronic, and chemical properties of bimetallic surfaces. Here, we investigate the coadsorption of sulfur and noble metals on Ru(001). Sulfur forms very stable compounds with ruthenium (RuS2, AHf = -47 kcallmol 7). In contrast, compounds that contain sulfur and a noble metal exhibit a moderate ( C U ~ SAHf , = -19 kcal/mo17) or low (Ag2S, AHf = -8 kcal/ mol 7, thermochemical stability. In principle, sulfur can affect the properties of a noble metal on Ru(001) by making direct bonds with the noble metal or by modifying the electronic properties of the ruthenium substrate. Previous photoemission experiment^^,^ and molecular-orbital calculations9indicate that the Ru noble metal interaction makes the noble metal "electron rich", favoring an electrophilic attack on the noble metal by sulfur. Our studies for the S/Cu/Ru(OOl) and SlAgl Ru(001) surfaces show the formation of CuS, and AgS, compounds. The properties of these sulfides were examined using thermal desorption mass spectroscopy (TDS), core-level photoemission, and CO chemisorption.
-
11. Experimental Section
The experiments were carried out in a conventionalultrahighvacuum system (base pressure -3 x Torr) equipped with a quadrupole mass spectrometer for TDS and a hemispherical
* To whom all correspondence should be addressed. @Abstractpublished in Advance ACS Absrracrs, October 15, 1994. 0022-365419412098-12059$04.50/0
electron energy analyzer with multichannel detection for photoemission studies. The photoemission spectra reported in section 3 were acquired using unmonochromatized Mg K a radiation. In the photoemission experiments, electron detection was perpendicular to the surface, and the X-ray source was at an incident angle of 60" from the surface normal. The bindingenergy scale in the photoemission spectra was calibrated by using the Cu 2~312and Ag 3d512 peaks of pure Cu and pure Ag, which were set at binding energies of 932.5 and 368.0 eV,l0 respectively. The Ru(001) crystal was mounted in a manipulator capable of resistive heating to 1650 K and liquid nitrogen cooling to 80 K. A W-5%Re/W-26%Re thermocouple was spot welded to the edge of the sample for temperature measurements. The typical cleaning procedure involved successive cycles of oxidation between 600 and 1400 K in an 0 2 background pressure of 5x Torr, followed by annealing of the crystal to 1650 K. Surface cleanliness was verified by means of X-ray photoelectron spectroscopy (XPS) and by obtaining the typical thermal desorption spectra for oxygen on clean Ru(001). During the adsorption of Cu, Ag, and S, the sample was held at a temperature of 300-350 K. The noble metals were vapordeposited on Ru(001) and S/Ru(OOl) surfaces from resistively heated W filaments wrapped with high-purity wires of Cu or Ag. These metal dosers were outgassed carefully prior to vapor deposition. Cu and Ag coverages were determined by TDS area analysis.",l2 A solid-state electrochemical cell (Pt/Ag/AgIl Ag2S/Pt13,14) was used to vapor-deposit sulfur on the Ru(001) and NMRu(001) surfaces (NM = Cu or Ag). When a voltage was applied across the cell, sulfur evolved as S, clusters (predominantly S2) and was adsorbed as atomic sulfur on the ~amp1e.l~ The coverages of atomic sulfur were determined using the methodology described in ref14. In this work, coverages are reported with respect to the number of Ru(001) surface atoms (1.57 x IOl5 atoms cm-2). One adatom per substrate surface atom corresponds to 8 = 1 ML (ML = monolayer). In some cases, the relative composition of an overlayer is reported using the notation: NM,S, where x is the noble-metallsulfur atomic ratio of the film. 0 1994 American Chemical Society
12060 J. Phys. Chem., Vol. 98, No. 46, 1994
Kuhn and Rodriguez
8
-mass 63, Cu
SICulRu(001)
mass64 S,
I
,
I
1
I
400
600
800
1000
1200
Temperature (K)
increasingS 938
1
I
I
I
936
934
932
930
928
Figure 2. Cu- and S2-TDS spectra acquired during the annealing of a Cu1.3S film to 600,800, and 1250 K. The film was prepared by dosing sulfur to a Cu multilayer (&, = 4.55) at 300 K (see Figure 1). Heating rate = 5 Ws.
Binding Energy (eV)
Cu 2p,,:
S/Cu/Ru(001)
Cu L 3 W Auger A
ecu=4 55
1/ I
I
I
I
I
I
I
I
I
I
914 916 918 920 922
168 166 164 162 160
Kinetic Energy (eV)
Binding Energy (eV)
\------I
I
Figure 1. Cu 2p3n (A), Cu L3VV (B), and S 2p (C) spectra acquired after dosing sulfur to a thick copper f i i (0, = 4.55). The experiments were carried out at 300 K.
111. Results
1250 K
I
I
I
I
I
I
938
936
934
932
930
928
Binding Energy (eV)
111.1. Interaction between S and Cu on Ru(100). This section is divided in two parts. First, we will examine the adsorption of sulfur on thick copper films supported on Ru(001). This will be followed by studies examining the coadsorption of small amounts of Cu and S (0s Ocu 5 1 ML) on top of the Ru substrate. Figure 1 shows Cu 2~312XPS and Cu L3VV Auger spectra acquired after dosing sulfur to a copper multilayer. Bonding between Cu and S produces significant changes in the electronic properties of C U . ~ ~After , ' ~ comparing the results of Figure 1 with XPS and Auger data reported for copper ~ u l f i d e s , ' ~one J~ can get an idea of the type of compound formed during the reaction of sulfur with the copper film. The formation of Cu2S from Cu is accompanied by a small binding-energy shift of -+0.2 eV in the Cu 2p3/2 peak,15J6band a large binding-energy shift of -+1.4 eV in the Cu L3VV transition.16 An analysis of photoemission data reported in the literature for bulk sulfides shows that a Cu2S 0.5 S2 2 CuS transformation induces a reduction of -0.6 eV in the binding energy of the Cu 2~312and Cu L3VV feature^.'^^^^ The trends seen in Figure 1, A and B, are similar to those observed when the S K U atomic ratio of bulk sulfides is varied from 0 to The photoemission results in Figure 1C show a clear change in the line shape of the S 2p features when the S/Cu atomic ratio in the film is increased above 0.5. At the same time, there is a positive shift of -0.6 eV in the peak maximum. Similar differences are found when comparing the S 2p spectra of Cu2S and C U S . ~ ~ ~
+
+
-
l.15916
I
,
I
914 916 918 920 922
168 166 164 162 160
Kinetic Energy (eV)
Binding Energy (eV)
'
Figure 3. Cu 2p3n (A), Cu L3VV (B) and S 2p (C) spectra taken after annealing the Cu1.3S film of Figures 1 and 2 to 600,800, and 1250 K.
The spectra reported at the bottom of Figure 1, A and B, and at the top of Figure 1C correspond to a copper film "saturated" with sulfur. At this point the Cu/S atomic ratio was close to 1.3, and the sticking coefficient of sulfur on the film was extremely small at 300 K. Figure 2 displays Cu-TDS, mass 63, and S2-TDS, mass 64,spectra acquired during the thermal decomposition of this Cu1.3S film. The heating of the sample was interrupted at 600,800, and 1250 K, and the photoemission spectra shown in Figure 3 were acquired. In Figure 2, an increase in temperature from 300 to 800 K produces desorption of a significant amount of Sz. (Between 300 and 800 K, the signal for desorption of Cu, mass 63, is an artifact caused by the onset of the signal for desorption of S 2 , mass 64.) Photoemission spectra taken after annealing the sample to 800 K (see Figure 3) revealed that, at this point, a film of Cu2S was
J. Phys. Chem., Vol. 98, No. 46, 1994 12061
Interaction of Sulfur with Bimetallic Surfaces Cu 2p,
Cu/S/Ru(OOl)
8
,+.L; :
b
T.300
K
100
1 06 0 76
0 20
0.08 0 26 0 09
938
936
934
932
930
928
Binding Energy (eV)
t
914
I
I
I
I
916
918
920
922
I
I
900
Kinetic Energy (eV) 213312 (A) and Cu L3VV (B) spectra acquired after depositing small amounts of Cu on a Ru(001) surface partially covered by 0.5 ML of sulfur. The dotted curves correspond to the spectra of a Cu monolayer on clean Ru(001). The experiments were performed at 300 K.
present on top of the Ru substrate. This film decomposed at temperatures between 900 and 1100 K, producing evolution of S2 and Cu into gas phase (see Figure 2). After the crystal was heated to 1250 K, only a small amount of S remained on the Ru(001) surface (see Figure 3). In general, we found that thick Cu overlayers are very reactive toward sulfur, being able to form sulfides with a large range of relative compositions at 300 K. Thick C u d films showed a large thermal stability, decomposing at temperatures in the range between 900 and 1100 K. On the other hand, thick Cu
