Langmuir 1994,10,3937-3939
3937
Dependence of the Strength of Interaction of Carbon Monoxide with Transition Metal Clusters on the Cluster Size A. K. Santra, Samrat Ghosh, and C. N. R. Rao* Solid State and Structural Chemistry Unit and Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India Received June 20, 1994. In Final Form: August 24, 1994@ The interaction of CO with Cu, Pd, and Ni at different coverages of the metals on solid substrates has been investigated by He I1 and core-levelspectroscopies,after the nature ofvariation ofthe metal core-level binding energies with the coverage or the cluster size is established. The separation between the ( I n 5a) and 4a levels of CO increases with a decrease in the size of the metal clusters, accompanied by an increase in the desorption temperature. In the case of Cu, the intramolecular shakeup satellite of CO disappears on small clusters. More importantly, CO dissociates on small Ni clusters, clearly confirming that metal-CO interaction strength increases with a decrease in the cluster size.
+
Size-dependent changes in the electronic structure and other properties of metal clusters have attracted much attention.1-8 On the basis of high-energy spectroscopy studies a s well as scanning tunneling conductance measurements, we have recently shown that it is likely that a transition from the metallic to the nonmetallic state occurs with a decrease in the cluster ~ i z e . Such ~ , ~ a major change in electronic structure would be expected to be accompanied by significant changes in the surface reactivity of clusters. Encouraged by preliminary studies in this direction: we have investigated the surface reactivity of clusters of Cu, Pd, and Ni with respect to carbon monoxide. In this letter, we show how the interaction of CO with metal clusters is markedly affected by the cluster size, the small clusters favoring stronger chemisorption, even leading to dissociation. In order to study the interaction of CO with the metal clusters, we deposited Cu, Ni, or Pd metal clusters a t room temperature under ultrahigh-vacuum (UHV)conditions in the preparation chamber (base pressure ca. 2 x Torr) of the electron spectrometer (VG ESCA 3 MK 11)by means of resistive evaporation of high-purity metals (>99.9%)wound around a thoroughly degassed tungsten filament. Amorphized graphite (obtained by Ar+ ion bombardment of a graphite surface) and A1203 were used as substrates. The A1203 films were grown in situ on pure (299.9%)Al foils by heating at 700 K for 2 h in oxygen (1 atm). The thickness of the A 1 2 0 3 layers was estimated to be -20 A using the equation I =IO exp(-dlA), where I and IOare the intensities of the AIo signal before and after the oxidation, respectively, A is the escape depthgin the oxide, and d is the thickness of the oxide layer. It is knownlOJ1 that diffusion of Cu, Ni, or Pd through such a A1203 layer
* To whom correspondence should be addressed at Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012,India. Abstract published inAdvunce ACSAbstruct, October 1,1994. (1)Wertheim, G.K. Phase Transitions 1990,24-26,203. (2)Wertheim, G.IC;Dicenzo, S. B. Phys. Rev. B 1968,37,844. (3)Mason, M.G. In Clusters Model for Surface and Bulk Phenomena; Pacchionic, G., et al., Eds.; Plenum Press: New York, 1992. (4)Martin, T. P.; Bergmann, T.; Lange, T. J.Phys. Chem. 1991,95, 6421. (5)El-Sayed, M. A.J. Phys. Chem. 1991,95,3898. (6)Vijayakrishnan, V.; Chainani, A.; Sarma, D. D.; Rao, C. N. R. J. Phys. Chem. 1992,96,8679. (7)Rao, C.N. R.; Vijayakrishnan, V.; Aiyer, H. N.; Kulkami, G. U.; Subbanna, G. N.J. Phys. Chem. 1993,97,11157. (8) Rao, C. N. R.; Vijayakrishnan, V.; Santra, A. K.; Prins, M. W. J. Angew. Chem., Int. Ed. Engl. 1992,31,1062. (9)Penn, R. D. J. Electron Spectros. Relut. Phenom. 1976,9,29. @
0743-7463l94I2410-3937$04.50lO
occurs only when the oxide layer thickness is less than 1 nm. No such interaction occurs through a n oxide layer of thickness of 1.5 nm and above, as in the present study. In order to ensure that our results are not vitiated by the diffusion of the deposited metal into the AZO3layer, the substrate was not heated above room temperature. Al Ka (1486.6 eV) and He I1 (40.8 eV) radiations were employed for XPS and UPS measurements, respectively. The coverages of the deposited metal, 8, were estimated by the method of Seah and Dench.12 The coverages 8 I 1monolayer are in the region of nanometric clusters as found independently by microscopic measurements of metal c l ~ s t e r s . ~Accordingly, J~ in the case of Ni clusters, the mean cluster diameter is less than 1nm when 8 = 0.6 and around 2 nm when 8 = 1.9 monolayer. In the case of Pd deposited on Al2O3, we could not calculate the 8 values since the Pd (3~312)signal overlaps with the 0 (1s) signal of the substrate. The coverage in this case is therefore expressed in terms of the normalized Pd intensity, M n o r ) , which is the relative intensity of the deposited metal with respect to the bulk Pd metal, recorded under the same operating conditions. The smallest coverage of Pd (Ipdnor) = 0.04) obtained is comparable to the smallest coverage (8Pd 0.3) on amorphized graphite. The mean cluster diameter of Pd clusters is -1 nm when 8 = 0.3 and -3 nm when 8 = 2.1 monolayer. The clusters were exposed to purified CO in the preparation chamber (1langmuir = Torrs). In Figure 1we have shown the variation in the shift of the core-level binding energies, AE,of Cu, Ni, and Pd, relative to the bulk metals, with the metal coverage. We see that the Cu(2p3d, Ni(2p32), and Pd(3dsn) binding energies increase relative to the bulk metal values (933.1, 852.9, and 335.1 eV, respectively) as the coverage or the cluster size decreases. The shifts observed in the smallest clusters are in the range 0.8-1.3 eV on amorphized graphite and in the 1.4- 1.9 eVrange onAl2O3. The larger core-level shifts observed on A1203 compared to the graphite substrate are due to Fermi level pinning1*in the former case. After the Fermi-level alignment (-0.6 eV) is corrected for, the observed core-level shifts on the two substrates become comparable. The increase in the corelevel binding energy of metals with a decrease in cluster
-
(10)Sarapatka, T. J. J. Phys. Chem. 1993,97,11274. (11)Sarapatka, T. J. Chem. Phys. Lett. 1993,212,37. (12)Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979,1,2. (13)Aiyer, H.N.; Vijayakrishnan, V.; Subbanna, G. N.; Rao, C. N. R.Surf. Sci. 1994,313,392. (14)Kendelewicz,T.;Petra, W. G.;Pan, S. A.; Williams,M. D.; Lindau, I.; Spicer, W. E. Appl. Phys. Lett. 1984,44, 113.
0 1994 American Chemical Society
Letters
3938 Langmuir, Vol. 10,No. 11, 1994 CU/AI,O~ o Cula-Gra
1.2
A D
NilA1203 Nila-Gro
>
-
0.41
0.0
,
/
.. . . ..* ,
I
0.2 0.4
I
1
1
I
co
100OL
1
0.6 0.8
1.0
lpd(nor)
Figure 1. (a) Variation in the shift of the core-level (2pa) binding energiesof Cu and Ni clustersrelative to the bulk metals with the metal coverage (e) on amorphized graphite and A 1 2 0 3 substrates. (b) Variation in the shift of the core-level( 3 b ) binding energy of Pd clusters deposited on A 1 2 0 3 relative to the bulk metal with coverage as quantifiedbyIpd(n0r). Inset shows the variation in the shift of the P d ( 3 h ) binding energy with the Pd coverage, 8, on amorphized graphite.
10
EF
(b)
(9)
size is only partly due to the final state effect and has a significant contribution from the metal to nonmetal transition occurring as the cluster size de~reases.~fjJJ~ We suggest that the smallest clusters studied by us are likely to be in the nonmetallic regime. It is also to be noted that the core-levelbinding energy (measured at 300 K) did not change after the surface was warmed to 500 K or cooled to 80 K, indicating the near absence of change in particle size. We present the He I1 difference spectra of CO adsorbed on Cu a t different coverages in Figure 2. We see that the shakeup satellitels around 13.6 eV (due to excitation to the 2n* orbital accompanying 4a ionization) found on the surfaces of bulk Cu a s well as a t high Cu coverages (Oc, 5.2) disappears a t small Cu coverages (Oc, 1.1). Furthermore, the separation between the (50 In)and 40 levels increases to 3.4 eV a t small Cu coverages compared to 3.1 eV on the surface of bulk Cu. The disappearance of the shakeup satellite and the increase in the 4a - (5a In) separation16 with the decrease in Cu coverage or cluster size is a manifestation of the enhanced Cu-CO interaction. In the inset ofFigure 2 we show the He I1 difference spectra of CO adsorbed on Ni clusters a t different Ni coverages (&). We observe a similar increase in the 4a - (5a In) separation (3.7 eV) in the case of small Ni clusters relative to the bulk Ni surface (3.1 eV). More interestingly, in the case of the smallest Ni cluster (8Ni 0.6) studied by us, adsorption of CO at 80 K followed by warming to 300 K gives rise to a feature around 7.0 eV in the UP difference spectrum suggesting possible dissociation of CO. We examine this aspect in greater detail later. In Figure 3 we show the COS) spectra of CO adsorbed a t two different Cu coverages. When OC, = 6.9, the C(ls) feature disappears completely around 250 K due to
-
+
-
+
+
-
(15)Mariani, C.; Middlemann, H. U.;Iwan,M.;Horn, K Chem.Phys. Lett. 1982.93, 308. (16)Broden, G.; Rhodin, T. N.; Brucker, C.; Benbow, R.; Hurych, Z. Surf. Sci. 1976, 59, 593.
15
BE (eV) Figure 2. He I1 difference spectra of CO adsorbed on various coverages of Cu clusters deposited on amorphized graphite substrate. Inset shows the He I1 difference spectra of CO adsorbed on various coverages of Ni clusters depositedOn A 1 2 0 3 substrate. e:,
[C)
ecu= 0.7
6.9
;...'(.....,-....,,......
(,.,,
I...
*.,
((,i ( j
I pa (nor) = 0.06
.....:....,.,...,'......'...*'.
/
.i:
Clean
Clean
Clean
I
283
286
289
283
BE(eV)
286
289
283
BE(eVI
286
289
BE(eV)
-
-
Figure 3. Change in C(1s) spectra of CO adsorbed on Cu or Pd clusters with temperature (a)for Bcu 6.9, (b) for Bcu 0.7, and (c) for Zpd(nor) 0.06.
-
-
desorption (Figure 3a), just as on the surface of bulk Cu. However, a t a smaller Cu coverage (Oc, 0.7))the C(1s) feature disappears only around 300 K (Figure 3b), indicating a n increase in the CO desorption temperature. We find a similar increase in the desorption temperature (500 K)on the surfaces of small Pd clusters (Ipd(nor) 0.061, while CO desorbs around 450 K from the surfaces of larger clusters or of bulk Pd (Figure 312). The results we have obtained with Ni clusters are much more interesting. At a high Ni coverage (ONi 6.6))just as in the case of bulk Ni, CO desorbs at 450 K as shown 1.91, the in Figure 4a. At medium coverage (ONi desorption temperature increases to 500 K (Figure 4b). When the cluster size is very small (ONi 0.61,we find a drastic change in the C(ls) spectrum of adsorbed CO at 300 Kcompared to that at 80 K. At 80 K, the C( 1s)feature is around 285.9 eV due to chemisorbed CO. After the surface is warmed to 300 K, there is a shift in the C(1s) feature to 284.2 eV, characteristic of carbidic species, suggesting dissociation. We recall that the signature of dissociation was found in the UPS difference spectrum of CO on small Ni clusters as well (inset of Figure 2). We consider the observation of increased CO desorption temperature in small clusters of Cu, Pd, or Ni and the
-
-
-
-
Letters
Langmuir, Vol. 10,No. 11, 1994 3939
.\.\,...... ....... ....c
._i...\.\,......L ..e.-
:.
...i: . ... . . . 5$,: .,.:.' ....... ......,-. ,-. ('
..' .
450K
.:, ;:>.:,
.......... ....,.(. .
........-.....
..
7 7
.r..
203
3300K 00~
L.i
. .. .,......... ,(..-'"
.\.-..I"
9"')K 300K ..:"i ......".., 1000Lt8OK) h..".,.
..........................
v.....".>.......,.i.'
Clean
Clean
4
283
286
289
BE Lev)
283
289
206 BE (eV)
-
-
Figure 4. Change in C(ls) spectra of CO adsorbed on Ni clusters with temperature (a)for e N i 6.6, (b)for @Ni 1.9,and (c) for
ONi
- 0.6.
clusters, while Doering et aLZ1observed particle size dependent decomposition of CO on Ni clusters on the basis ofthermal desorption measurements. Thermal desorption studies have also shown a greater tendency for CO dissociation on smaller Pd particles, on repeated cycling.23 Our observations of a n increase in the desorption temperature and occurrence of dissociation reported here clearly establish that the cluster size indeed has important ramifications. Noting that on bulk Ni, CO chemisorbs moderately strongly and that atomic Ni forms Ni(C0)4,24 the dissociation ofCO on small clusters (ONl 0.6) suggests enhancement of chemical reactivity in the nanometric (nonmetallic) regime of clusters. This could be due to the Ni(3d) level in small clusters coming close to the antibonding level of the C0(2n*)level of CO.
occurrence of dissociation in the case of very small Ni clusters to be significant. There have been reports of such increased metal-CO interaction with a decrease in cluster size from some of the earlier s t ~ d i e s . ~ JThus, ~ - ~ ~Gillet et aLZofind large sticking coefficients of CO on small Pd (17)Baetzold, R. C. J.Am. Chem. SOC.1991,103,6116.
-
(18)Baetzold, R. C. Znorg. Chem. 1992,21,2189. (19)Grunze, M. Chem. Phys. Lett. 1978,58,409. (20)Gillet, E.;Channakhone, S.; Matolin, V.; Gillet, M. Surf. Sci. 1986,1521153,603. (21)Doering, D. L.;Dickinson, J. T.; Poppa, H. J. Cutul. 1982,73, 91. (22)Doering, D. L.;Poppa, H.; Dickinson,J. T. Catul. 1982,73,104. (23)Stara, I.; Matolin, V. Surf. Sci. 1994,313,99. (24)Block, J. H.;Kruse, N. Ann. Quim. Ser. A 1988,84,292.
