- Email: [email protected]
Interaction of CO with the ordered Ni3Al(111) surface
s u r f a c e science ELSEVIER
Surface Science 331-333 (1995) 18-22
Interaction of CO with the
ordered Ni3AI(111) surface
J. Kandler, B. Eltester, H. Busse, G.R. Castro, K. Wandelt * lnstitut ftir Physikalische und Theoretische Chemie, Universitiit Bonn, Wegelerstrasse 12, 53115 Bonn, Germany
Received 17 August 1994; acceptedfor publication5 September1994
Abstract The interaction of CO with the structurally and chemically ordered (2 X 2) Ni3AI(lll) surface was studied with TDS, UPS and LEED. At low coverages, a CO-induced (2 X 2)-LEED structure and the same activation energy of CO desorption as well as the same (5~r/lrr)-(4o') splitting of chemisorbed CO as found on Ni(lll) suggest (a) an initial CO adsorption on the Ni-trimer sites in the alloy surface and (b) no significant electronic influence from the adjacent AI atoms. With increasing coverage, the (2 X 2) CO-pattern fades away and two more chemisorbed CO states are detected suggesting a less-ordered CO layer and the population of inequivalent (probably "mixed") adsorption sites. Keywords: Alloys; Aluminum; Carbon monoxide; Chemisorption; Low index single crystal surfaces; Nickel; Photoelectron emission;
Thermal desorption; Thermal desorption spectroscopy;Visible and ultravioletphotoelectron spectroscopy
1. Introduction The adsorption properties of binary alloy surfaces may be distinctly different from those of the pure component surfaces. The origin for this deviation lies simply in the fact that each surface lattice site may be occupied by one out of two different atomic species. The distribution of these two kinds of atoms ultimately determines both the number of one- or two-component adsorption sites of given coordination (geometrical "ensemble" effect) and, thereby, the mutual electronical interaction between (like and unlike) surface atoms (electronic "ligand" effect between unlike atoms) [1]. Even though it may be conceptually useful to distinguish between these two aspects it is not straightforward to separate both effects experimentally. The most promising approach
* Corresponding author.
is to keep the distribution of both components constant and to vary the nature of one of the two species. In this respect ordered binary alloy surfaces compared to the respective pure component surfaces provide ideal model systems. As one example out of a series of systematic experiments in the present paper we report about the adsorptive properties of an ordered N i 3 A I ( l l l ) surface towards CO as revealed by UV-photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). The results are compared with those of CO adsorption on N i ( l l l ) .
2. Experimental The experiments were carried out in a UHV chamber (base pressure < 10 - l ° Torr) equipped with facilities for LEED, AES, TDS and UPS. The angle integrated UPS spectra were excited with He I
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00146-8
19
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
(21.22 eV) and He II (40.8 eV) radiation. The sample, a disk cut parallel to the (111) plane of an ordered Ni3AI single crystal, was mounted between two parallel tungsten wires connected to two electrically insulated copper blocks. The latter were connected to a power supply as well as to a closed-loop helium refrigerator. The sample temperature was controlled by means of a chromel-alumel thermocouple. A cle an, single-crystalline and ordered Ni 3ml(111) surface could be obtained after many cycles of sputtering and annealing under the following conditions. For sputtering the surface was bombarded with a beam of Ar ÷ ions of 5 / z A current and 2 keV energy for 30 min. Subsequently the sample was heated for 20 min at 820 K followed by annealing for 40 min at 720 K. This temperature program was necessary because above 750 K aluminum segregates to the surface [2]. After the first basic preparation of the surface 2-3 such cycles sufficed to restore the cleanliness as well as the structural and chemical order of the surface before each new adsorption experiment, because after CO desorption the surface was always contaminated with oxygen. AES was used to check the composition and the cleanliness of the surface. However, UPS (He II) spectra turned out to be more sensitive towards traces of oxygen than AES. LEED pictures registered with a CCD-camera confirmed the order of the surface. CO was dosed through a capillary pointing to the surface and using a He : CO -- 100 : 1 gas mixture in order to avoid contamination of the whole chamber and, in particular, the cold parts. During exposure the sample was kept at temperatures between 50 and 170 K depending on the kind of experiments. TDS spectra were registered with a heating rate /3 = 3.1 K / s . The comparative UPS spectra from N i ( l l l ) shown in this paper were monitored in the same apparatus under identical conditions.
3. Results and discussion The Ni3ml alloy possesses Cu3Au structure, with four A1 atoms at the corners and six Ni atoms at the face centers of the cubic unit cell. The clean and well-annealed (111) surface (see previous section) is characterized by a (2 X 2) LEED pattern as dis-
LEED o •
0
(llt)-surface .
o
•
• •
•
0 •
o
•
• •
•
0 •
o
O
Ni
0
A1
(b) Fig. 1. (a) LEED pattern and (b) ideal surface structure of the structurally and chemicallyorderedNi3AI(lll) surface. played in Fig. la. This finding suggests that the Ni3AI(lll) surface exhibits the same ordered structure as an ideal (111) bulk plane of the ordered alloy single crystal shown in Fig. lb. As a consequence the AI atoms in the surface are isolated from each other forming a (2 X 2) structure within a Ni matrix. Owing to the fuzziness of the LEED spots, however, some remnant disorder at the surface can not be excluded. In some sense this NiaAI(lll) surface is complementary to the previously, studied Cu3Pt(lll) surface [3-5]. While in the latter one the more active species towards CO adsorption, i.e. Pt, was present in the form of a (2 X 2) structure of single atoms within the matrix of the less active component, i.e. Cu, the situation is just reversed in the former surface. In the Ni3AI(lll) surface the more active species, i.e. Ni, exists in the form of trimers within the (2 X 2) structure of the less active AI atoms. (CO does not adsorb on A I ( l l l ) at room temperature [6].) The interaction of CO with the single center Pt-atoms within the Cu matrix was strongly modified from that with pure Pt(lll). The present results are to show, how the physical and adsorptive properties of the Ni surface trimers within the Ni3AI(lll) surface (see Fig. lb) are modified from those of the pure N i ( l l l ) surface. Fig. 2 first displays the UPS (He II) valence band spectra of N i ( l l l ) and Ni3AI(lll). Even though their overall appearance looks very similar, the difference spectrum accentuates the shift of Ni(3d)-density of states (Ni(3d)-DOS) to slightly higher binding energy (peak at 0.9 eV) in the Ni3AI(lll) sample compared to Ni(lll). This is in perfect agreement with theoretical band structure calculations [7-9] and
20
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
0,9
UPS(hv=40.8eV)
CO/NioAI(IlI) TDS 370
a) T,d = 1 5 0 K
[
~
~
b)
a)Ni3AI(III) b) Ni(lll) c)=a)-b)
6,5
c)
/
L
J\
c)
.
. . . . .
manifests some alloy induced electronic modification. The extra intensities at 6.3 and 6.5 eV, respectively, are due to d-band satellites typical for pure nickel and nickel-based alloys [10]. Both features by the way, i.e. the Ni(3d)-DOS and the d-band satellite, follow the trend as to shift to higher binding energy with decreasing Ni content in the NiA1 alloy. This becomes apparent from theoretical band structure calculations [7-9] and a comparison of the spectra from Fig. 2 with that of NiAI(ll0) [11]. The work function of the N i ( l l l ) and the Ni3Al(lll) surface are 5.40 eV [12] and 5.30 eV, respectively. Fig. 3 displays a complete series of TDS spectra of CO from the Ni3AI(lll) surface. The nearly
tel. CO-coverage[ML]: 368
441
1.00 0.g3 0.86 0.85 0.77
.~
0.75
>"
0.64 0.62 0.60 0.54
-~
0.44
0.30 0.23 0.16
0.11 0.02 . . . . .
i
. . . . .
200
i
375 i
. . . . .
300
453
. . . . .
400 temperature[K]
. . . . .
i
. . . .
300
,
I
. . . . .
400
I
. . . . .
500
i
. . . . .
600
|emperature [ K ]
Fig. 2. He(II) excited valence band spectra of (a) ordered NiaAI(lll) and (b) Ni(lll). The difference spectrum (c) accentuates the shift of (3d)-density of states to slightly higher binding energy in the alloy.
T,d = 170K
i
200
binding energy[eV]
CO/Ni3AI(III) TDS
dosis[L]: 2.0
441
T,e = 2 9 5 K Tad = 3 7 5 K
i
. . . . .
500
~ . . . . .
600
Fig. 3. Series of TDS spectra for increasing coverages of CO on ordered Ni3AI(lll). Note the successive population of three desorption states near 450, 370 and 290 K.
Fig. 4. TDS spectra registered after saturating an ordered Ni3AI(lll) surface with CO at 375 K (a), 295 K (b) and 150 K
(c). successive population of three desorption states (I, II, III) near 450, 370 and 290 K as a function of CO coverage is clearly visible. With respect to saturation coverage (final spectrum in Fig. 3 set equal to 1 ML) the second desorption state first emerges at a relative coverage (determined from the total area under the TDS spectra) of 0.15 ML while the third state starts to appear above 0.5 ML. Fig. 4 shows three desorption traces registered after saturating the Ni3AI(lll) surface with CO at sample temperatures of 375 K (a), 295 K (b) and 150 K (c), respectively, and cooling down to the common start at 170 K. This figure is to emphasize the existence of three different desorption states with no noticeable equilibration between them. All three states are assigned to chemisorbed CO as suggested by the corresponding UPS data (see below) and by the fact that two more physisorbed states are found to desorb at 60 and 84 K (not shown here) [13]. Up to 0.5 ML, i.e. predominant population of state I, CO adsorption at 100 K causes an enhancement of the (2 × 2) LEED structure. Above 0.5 ML this (2 × 2) structure fades away probably due to the formation of a denser, lessordered CO overlayer comprising the population of inequivalent (e.g. " m i x e d " ) surface sites [13]. State I desorbs very much at the same temperature as moderate CO coverages from pure Ni(lll). It is therefore safely assigned to CO adsorbed on the Ni trimers and shall thus be discussed first in more detail as a probe for the adsorptive properties of Ni sites within the Ni3AI(lll) surface compared to Ni sites within the pure N i ( l l l ) surface.
J. Kandler et aL /Surface Science 331-333 (1995) 18-22 Table 1 Kinetic parameters for the desorption of small coverages of CO from Ni3AI(lll)
Ni3Al(111) Ni(111)
e~os(0 ~ o)
,(0 --, o)
/3
(kJ/mol)
(s- 1)
(K/s)
138+_10 127 + 10
2.0×1015 5 × 10 la
3.1 7.5
The data for N i ( l l l ) are obtained by the same analysis after digitalization of the spectra from Ref. [14], figure 2; fl = heating rate.
The kinetic parameters, namely the activation energy Edes and the frequency factor v for desorption of low CO coverages from Ni3AI(lll) and N i ( l l l ) are compiled in Table 1. These numbers are obtained from the best simulation of our measured TDS spectra of state I on Ni3AI(lll) and published TDS spectra [14] of C O / N i ( l l l ) using the PolanyiWigner equation assuming first-order desorption kinetics. The relatively large error for Edes results from the analysis of several independent desorption experiments with Ni3AI(lll). Within one and the same TDS series the error was only _+2 kJ/mol. In view of the different origin of the data and the slightly deviating registration conditions (e.g. /3) we consider Ed¢s indistinguishable for both surfaces. The bond strength of CO molecules with the Ni trimers in the Ni3AI(lll) surface is the same as with the
UPS after adsorption of CO Hel I(hv=40,8eV)
I/\\
8.1
10.9
fie = 2.8 eV
NIaAI(lll)
/
J binding energy EF~[eV]
Fig. 5. UPS (He II) spectra of an ordered Ni3AI(lll) and a N i ( l l l ) surface, both covered with approximately 0.5 ML CO, which on the alloy surface corresponds to CO state I characterized in Figs. 3 and 4. Note the same energetic separation between the (50"/1~') and (4o') peaks of chemisorbed CO.
21
pure N i ( l l l ) surface. In particular, no weakening due to the presence of the Ai atoms is detectable. This surprising result is supported by photoemission measurements. Fig. 5 compares UPS (He II) valence band spectra of the N i ( l l l ) and the Ni3AI(lll) surface covered with approximately 0.5 ML CO. Since the alloy surface was exposed to CO at 375 K only the CO state I is populated. On both surfaces the typical features of chemisorbed CO appear at the same electron binding energies, namely at 8.1 eV (5o'/17r) and 10.9 eV (40.). A deviating energy separation between these two CO-induced peaks on different substrates has been discussed in terms of a metal-dependent bond strength. On pure metal surfaces the 50.-40- separation decreases significantly with increasing CO adsorption energy [15,16]. No difference is observed on the present two surfaces in agreement with the unchanged desorption energy.
4. Conclusions The (111) surface of an ordered Ni3AI single crystal can be prepared to exhibit the same lateral structural and chemical order as a corresponding bulk plane, i.e. the AI atoms form a (2 × 2) sublattice and are embedded between Ni trimers. At low coverages CO is supposed to chemisorb on the latter ones as concluded from the same values for the desorption energy as well as the energetic (50"/ 17r)-40" separation as found on Ni(lll). The rather minor difference in the valence electronic structure between the clean Ni3AI(lll) and the N i ( l l l ) surface has obviously no significant influence on the local adsorptive properties of the Ni-trimer sites as compared to the CO sites on the N i ( l l l ) surface. In contrast, the interaction of CO with the single Pt atoms in the C u 3 P t / l l l ) surface was strongly weakened (corresponding to a lowering of the desorption temperature by ~ 100 K) compared to P t ( l l l ) [3]. At higher coverages additional CO desorption states suggest the population of inequivalent (possibly " m i x e d " ) surface sites and/or the formation of CO compression structures on Ni3AI(lll) [13]. More insight into this assignment and, in particular, the exact CO site on the Ni trimers is anticipated from forthcoming HREELS studies.
22
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
References [1] V. Ponec, Adv. Catal. 32 (1983) 149. [2] D. Sondericker, F. Jona and P.M. Marcus, Phys. Rev. B 34 (1986) 6770. [3] G.R. Castro, U. Schneider, H. Busse, T. Janssens and K. Wandelt, Surf. Sci. 269/270 (1992) 321. [4] R. Linke, U. Schneider, H. Busse, C. Becker, U. Schr6der, G.R. Castro and K. Wandelt, Surf. Sci. 307-309 (1994) 407. [5] C. Becker, U. Schr6der, G.R. Castro, U. Schneider, H. Busse, R. Linke and K. Wandelt, Surf. Sci. 307-309 (1994) 412. [6] K. Jacobi, C. Astaldi, P. Geng and M. Bartolo, Surf. Sci. 223 (1989) 569.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Nautiyal and S. Auluck, Phys. Rev. B 45 (1992) 13930. C. Calandra and F. Manghi, Phys. Rev. B 45 (1992) 5819. G. Cubiotti, private communication. J.G. Fuggle, F.U. Hillebrecht, R. Zeller, Z. Zolnierek and P. Bennett, Phys. Rev. B 27 (1982) 2145. H. Isern Herrera, PhD Thesis, Univ. Caracas, 1990. J. H61zl and F.K. Schulte, in: Springer Tracts in Modern Physics, Vol. 85 (Springer, Berlin, 1979) p. 4. J. Kandler, Diplom Thesis, Univ. Bonn, 1994, and to be published. F.P. Netzer and T.E. Madey, J. Chem. Phys. 76 (1982) 710. G. Ertl and J. Kiippers, Low Energy Electrons and Surface Science (VCH, Weinheim, 1985). R. Miranda, K. Wandelt, D. Rieger and R.D. Schnell, Surf. Sci. 139 (1984) 430.
Surface Science 331-333 (1995) 18-22
Interaction of CO with the
ordered Ni3AI(111) surface
J. Kandler, B. Eltester, H. Busse, G.R. Castro, K. Wandelt * lnstitut ftir Physikalische und Theoretische Chemie, Universitiit Bonn, Wegelerstrasse 12, 53115 Bonn, Germany
Received 17 August 1994; acceptedfor publication5 September1994
Abstract The interaction of CO with the structurally and chemically ordered (2 X 2) Ni3AI(lll) surface was studied with TDS, UPS and LEED. At low coverages, a CO-induced (2 X 2)-LEED structure and the same activation energy of CO desorption as well as the same (5~r/lrr)-(4o') splitting of chemisorbed CO as found on Ni(lll) suggest (a) an initial CO adsorption on the Ni-trimer sites in the alloy surface and (b) no significant electronic influence from the adjacent AI atoms. With increasing coverage, the (2 X 2) CO-pattern fades away and two more chemisorbed CO states are detected suggesting a less-ordered CO layer and the population of inequivalent (probably "mixed") adsorption sites. Keywords: Alloys; Aluminum; Carbon monoxide; Chemisorption; Low index single crystal surfaces; Nickel; Photoelectron emission;
Thermal desorption; Thermal desorption spectroscopy;Visible and ultravioletphotoelectron spectroscopy
1. Introduction The adsorption properties of binary alloy surfaces may be distinctly different from those of the pure component surfaces. The origin for this deviation lies simply in the fact that each surface lattice site may be occupied by one out of two different atomic species. The distribution of these two kinds of atoms ultimately determines both the number of one- or two-component adsorption sites of given coordination (geometrical "ensemble" effect) and, thereby, the mutual electronical interaction between (like and unlike) surface atoms (electronic "ligand" effect between unlike atoms) [1]. Even though it may be conceptually useful to distinguish between these two aspects it is not straightforward to separate both effects experimentally. The most promising approach
* Corresponding author.
is to keep the distribution of both components constant and to vary the nature of one of the two species. In this respect ordered binary alloy surfaces compared to the respective pure component surfaces provide ideal model systems. As one example out of a series of systematic experiments in the present paper we report about the adsorptive properties of an ordered N i 3 A I ( l l l ) surface towards CO as revealed by UV-photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). The results are compared with those of CO adsorption on N i ( l l l ) .
2. Experimental The experiments were carried out in a UHV chamber (base pressure < 10 - l ° Torr) equipped with facilities for LEED, AES, TDS and UPS. The angle integrated UPS spectra were excited with He I
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00146-8
19
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
(21.22 eV) and He II (40.8 eV) radiation. The sample, a disk cut parallel to the (111) plane of an ordered Ni3AI single crystal, was mounted between two parallel tungsten wires connected to two electrically insulated copper blocks. The latter were connected to a power supply as well as to a closed-loop helium refrigerator. The sample temperature was controlled by means of a chromel-alumel thermocouple. A cle an, single-crystalline and ordered Ni 3ml(111) surface could be obtained after many cycles of sputtering and annealing under the following conditions. For sputtering the surface was bombarded with a beam of Ar ÷ ions of 5 / z A current and 2 keV energy for 30 min. Subsequently the sample was heated for 20 min at 820 K followed by annealing for 40 min at 720 K. This temperature program was necessary because above 750 K aluminum segregates to the surface [2]. After the first basic preparation of the surface 2-3 such cycles sufficed to restore the cleanliness as well as the structural and chemical order of the surface before each new adsorption experiment, because after CO desorption the surface was always contaminated with oxygen. AES was used to check the composition and the cleanliness of the surface. However, UPS (He II) spectra turned out to be more sensitive towards traces of oxygen than AES. LEED pictures registered with a CCD-camera confirmed the order of the surface. CO was dosed through a capillary pointing to the surface and using a He : CO -- 100 : 1 gas mixture in order to avoid contamination of the whole chamber and, in particular, the cold parts. During exposure the sample was kept at temperatures between 50 and 170 K depending on the kind of experiments. TDS spectra were registered with a heating rate /3 = 3.1 K / s . The comparative UPS spectra from N i ( l l l ) shown in this paper were monitored in the same apparatus under identical conditions.
3. Results and discussion The Ni3ml alloy possesses Cu3Au structure, with four A1 atoms at the corners and six Ni atoms at the face centers of the cubic unit cell. The clean and well-annealed (111) surface (see previous section) is characterized by a (2 X 2) LEED pattern as dis-
LEED o •
0
(llt)-surface .
o
•
• •
•
0 •
o
•
• •
•
0 •
o
O
Ni
0
A1
(b) Fig. 1. (a) LEED pattern and (b) ideal surface structure of the structurally and chemicallyorderedNi3AI(lll) surface. played in Fig. la. This finding suggests that the Ni3AI(lll) surface exhibits the same ordered structure as an ideal (111) bulk plane of the ordered alloy single crystal shown in Fig. lb. As a consequence the AI atoms in the surface are isolated from each other forming a (2 X 2) structure within a Ni matrix. Owing to the fuzziness of the LEED spots, however, some remnant disorder at the surface can not be excluded. In some sense this NiaAI(lll) surface is complementary to the previously, studied Cu3Pt(lll) surface [3-5]. While in the latter one the more active species towards CO adsorption, i.e. Pt, was present in the form of a (2 X 2) structure of single atoms within the matrix of the less active component, i.e. Cu, the situation is just reversed in the former surface. In the Ni3AI(lll) surface the more active species, i.e. Ni, exists in the form of trimers within the (2 X 2) structure of the less active AI atoms. (CO does not adsorb on A I ( l l l ) at room temperature [6].) The interaction of CO with the single center Pt-atoms within the Cu matrix was strongly modified from that with pure Pt(lll). The present results are to show, how the physical and adsorptive properties of the Ni surface trimers within the Ni3AI(lll) surface (see Fig. lb) are modified from those of the pure N i ( l l l ) surface. Fig. 2 first displays the UPS (He II) valence band spectra of N i ( l l l ) and Ni3AI(lll). Even though their overall appearance looks very similar, the difference spectrum accentuates the shift of Ni(3d)-density of states (Ni(3d)-DOS) to slightly higher binding energy (peak at 0.9 eV) in the Ni3AI(lll) sample compared to Ni(lll). This is in perfect agreement with theoretical band structure calculations [7-9] and
20
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
0,9
UPS(hv=40.8eV)
CO/NioAI(IlI) TDS 370
a) T,d = 1 5 0 K
[
~
~
b)
a)Ni3AI(III) b) Ni(lll) c)=a)-b)
6,5
c)
/
L
J\
c)
.
. . . . .
manifests some alloy induced electronic modification. The extra intensities at 6.3 and 6.5 eV, respectively, are due to d-band satellites typical for pure nickel and nickel-based alloys [10]. Both features by the way, i.e. the Ni(3d)-DOS and the d-band satellite, follow the trend as to shift to higher binding energy with decreasing Ni content in the NiA1 alloy. This becomes apparent from theoretical band structure calculations [7-9] and a comparison of the spectra from Fig. 2 with that of NiAI(ll0) [11]. The work function of the N i ( l l l ) and the Ni3Al(lll) surface are 5.40 eV [12] and 5.30 eV, respectively. Fig. 3 displays a complete series of TDS spectra of CO from the Ni3AI(lll) surface. The nearly
tel. CO-coverage[ML]: 368
441
1.00 0.g3 0.86 0.85 0.77
.~
0.75
>"
0.64 0.62 0.60 0.54
-~
0.44
0.30 0.23 0.16
0.11 0.02 . . . . .
i
. . . . .
200
i
375 i
. . . . .
300
453
. . . . .
400 temperature[K]
. . . . .
i
. . . .
300
,
I
. . . . .
400
I
. . . . .
500
i
. . . . .
600
|emperature [ K ]
Fig. 2. He(II) excited valence band spectra of (a) ordered NiaAI(lll) and (b) Ni(lll). The difference spectrum (c) accentuates the shift of (3d)-density of states to slightly higher binding energy in the alloy.
T,d = 170K
i
200
binding energy[eV]
CO/Ni3AI(III) TDS
dosis[L]: 2.0
441
T,e = 2 9 5 K Tad = 3 7 5 K
i
. . . . .
500
~ . . . . .
600
Fig. 3. Series of TDS spectra for increasing coverages of CO on ordered Ni3AI(lll). Note the successive population of three desorption states near 450, 370 and 290 K.
Fig. 4. TDS spectra registered after saturating an ordered Ni3AI(lll) surface with CO at 375 K (a), 295 K (b) and 150 K
(c). successive population of three desorption states (I, II, III) near 450, 370 and 290 K as a function of CO coverage is clearly visible. With respect to saturation coverage (final spectrum in Fig. 3 set equal to 1 ML) the second desorption state first emerges at a relative coverage (determined from the total area under the TDS spectra) of 0.15 ML while the third state starts to appear above 0.5 ML. Fig. 4 shows three desorption traces registered after saturating the Ni3AI(lll) surface with CO at sample temperatures of 375 K (a), 295 K (b) and 150 K (c), respectively, and cooling down to the common start at 170 K. This figure is to emphasize the existence of three different desorption states with no noticeable equilibration between them. All three states are assigned to chemisorbed CO as suggested by the corresponding UPS data (see below) and by the fact that two more physisorbed states are found to desorb at 60 and 84 K (not shown here) [13]. Up to 0.5 ML, i.e. predominant population of state I, CO adsorption at 100 K causes an enhancement of the (2 × 2) LEED structure. Above 0.5 ML this (2 × 2) structure fades away probably due to the formation of a denser, lessordered CO overlayer comprising the population of inequivalent (e.g. " m i x e d " ) surface sites [13]. State I desorbs very much at the same temperature as moderate CO coverages from pure Ni(lll). It is therefore safely assigned to CO adsorbed on the Ni trimers and shall thus be discussed first in more detail as a probe for the adsorptive properties of Ni sites within the Ni3AI(lll) surface compared to Ni sites within the pure N i ( l l l ) surface.
J. Kandler et aL /Surface Science 331-333 (1995) 18-22 Table 1 Kinetic parameters for the desorption of small coverages of CO from Ni3AI(lll)
Ni3Al(111) Ni(111)
e~os(0 ~ o)
,(0 --, o)
/3
(kJ/mol)
(s- 1)
(K/s)
138+_10 127 + 10
2.0×1015 5 × 10 la
3.1 7.5
The data for N i ( l l l ) are obtained by the same analysis after digitalization of the spectra from Ref. [14], figure 2; fl = heating rate.
The kinetic parameters, namely the activation energy Edes and the frequency factor v for desorption of low CO coverages from Ni3AI(lll) and N i ( l l l ) are compiled in Table 1. These numbers are obtained from the best simulation of our measured TDS spectra of state I on Ni3AI(lll) and published TDS spectra [14] of C O / N i ( l l l ) using the PolanyiWigner equation assuming first-order desorption kinetics. The relatively large error for Edes results from the analysis of several independent desorption experiments with Ni3AI(lll). Within one and the same TDS series the error was only _+2 kJ/mol. In view of the different origin of the data and the slightly deviating registration conditions (e.g. /3) we consider Ed¢s indistinguishable for both surfaces. The bond strength of CO molecules with the Ni trimers in the Ni3AI(lll) surface is the same as with the
UPS after adsorption of CO Hel I(hv=40,8eV)
I/\\
8.1
10.9
fie = 2.8 eV
NIaAI(lll)
/
J binding energy EF~[eV]
Fig. 5. UPS (He II) spectra of an ordered Ni3AI(lll) and a N i ( l l l ) surface, both covered with approximately 0.5 ML CO, which on the alloy surface corresponds to CO state I characterized in Figs. 3 and 4. Note the same energetic separation between the (50"/1~') and (4o') peaks of chemisorbed CO.
21
pure N i ( l l l ) surface. In particular, no weakening due to the presence of the Ai atoms is detectable. This surprising result is supported by photoemission measurements. Fig. 5 compares UPS (He II) valence band spectra of the N i ( l l l ) and the Ni3AI(lll) surface covered with approximately 0.5 ML CO. Since the alloy surface was exposed to CO at 375 K only the CO state I is populated. On both surfaces the typical features of chemisorbed CO appear at the same electron binding energies, namely at 8.1 eV (5o'/17r) and 10.9 eV (40.). A deviating energy separation between these two CO-induced peaks on different substrates has been discussed in terms of a metal-dependent bond strength. On pure metal surfaces the 50.-40- separation decreases significantly with increasing CO adsorption energy [15,16]. No difference is observed on the present two surfaces in agreement with the unchanged desorption energy.
4. Conclusions The (111) surface of an ordered Ni3AI single crystal can be prepared to exhibit the same lateral structural and chemical order as a corresponding bulk plane, i.e. the AI atoms form a (2 × 2) sublattice and are embedded between Ni trimers. At low coverages CO is supposed to chemisorb on the latter ones as concluded from the same values for the desorption energy as well as the energetic (50"/ 17r)-40" separation as found on Ni(lll). The rather minor difference in the valence electronic structure between the clean Ni3AI(lll) and the N i ( l l l ) surface has obviously no significant influence on the local adsorptive properties of the Ni-trimer sites as compared to the CO sites on the N i ( l l l ) surface. In contrast, the interaction of CO with the single Pt atoms in the C u 3 P t / l l l ) surface was strongly weakened (corresponding to a lowering of the desorption temperature by ~ 100 K) compared to P t ( l l l ) [3]. At higher coverages additional CO desorption states suggest the population of inequivalent (possibly " m i x e d " ) surface sites and/or the formation of CO compression structures on Ni3AI(lll) [13]. More insight into this assignment and, in particular, the exact CO site on the Ni trimers is anticipated from forthcoming HREELS studies.
22
J. Kandler et al. / Surface Science 331-333 (1995) 18-22
References [1] V. Ponec, Adv. Catal. 32 (1983) 149. [2] D. Sondericker, F. Jona and P.M. Marcus, Phys. Rev. B 34 (1986) 6770. [3] G.R. Castro, U. Schneider, H. Busse, T. Janssens and K. Wandelt, Surf. Sci. 269/270 (1992) 321. [4] R. Linke, U. Schneider, H. Busse, C. Becker, U. Schr6der, G.R. Castro and K. Wandelt, Surf. Sci. 307-309 (1994) 407. [5] C. Becker, U. Schr6der, G.R. Castro, U. Schneider, H. Busse, R. Linke and K. Wandelt, Surf. Sci. 307-309 (1994) 412. [6] K. Jacobi, C. Astaldi, P. Geng and M. Bartolo, Surf. Sci. 223 (1989) 569.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Nautiyal and S. Auluck, Phys. Rev. B 45 (1992) 13930. C. Calandra and F. Manghi, Phys. Rev. B 45 (1992) 5819. G. Cubiotti, private communication. J.G. Fuggle, F.U. Hillebrecht, R. Zeller, Z. Zolnierek and P. Bennett, Phys. Rev. B 27 (1982) 2145. H. Isern Herrera, PhD Thesis, Univ. Caracas, 1990. J. H61zl and F.K. Schulte, in: Springer Tracts in Modern Physics, Vol. 85 (Springer, Berlin, 1979) p. 4. J. Kandler, Diplom Thesis, Univ. Bonn, 1994, and to be published. F.P. Netzer and T.E. Madey, J. Chem. Phys. 76 (1982) 710. G. Ertl and J. Kiippers, Low Energy Electrons and Surface Science (VCH, Weinheim, 1985). R. Miranda, K. Wandelt, D. Rieger and R.D. Schnell, Surf. Sci. 139 (1984) 430.