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Catalytic activity of supported nanometer-sized metal clusters
Applied Surface Science 164 Ž2000. 252–259 www.elsevier.nlrlocaterapsusc
Catalytic activity of supported nanometer-sized metal clusters Claude R. Henry ) CRMC2 1-CNRS, Case 913, Campus de Luminy, F-13288 Marseille Cedex 09, France
Abstract The understanding of the reactivity of supported nanometer-sized particles, which are used in heterogeneous catalysis, can be approached in two ways: by studying the reactivity of size selected and soft-landed small metal clusters containing 2 to 50 atoms Ž‘molecular’ approach. or by studying the reactivity of extended single crystal surfaces Ž‘surface science’ approach.. We discuss the advantages and the limitations of these two approaches. We show in particular that often these two approaches cannot explain quantitatively the reaction kinetics for supported clusters a few nanometers in diameter. The peculiarity of these nanometer-sized supported clusters is due to their intrinsic heterogeneities: the presence of different types of facets, the presence of edges and the presence of the support. Taking account the recent experimental results on model supported catalysts, we show how the role of these heterogeneities can be investigated. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Heterogeneous catalysis; Reaction kinetics; Model catalysts
1. Introduction Real Ži.e. industrial. catalysts are made of nanometer-sized transition-metal clusters dispersed on an oxide powder. Real catalysts have a very complex structure and their characterization is very difficult. Fundamental studies in heterogeneous catalysis are more easily performed on model catalysts. The model catalysts are of two types. The first ones are made of clean well-ordered surfaces of metal single crystals. These model catalysts are characterized precisely in using the large set of surface
)
Tel.: q33-6-6292-2832; fax: q33-4-918916. E-mail address: [email protected] ŽC.R. Henry.. 1 Associated to the Universities of Aix-Marseille II and III.
science techniques w1x. Such studies are necessary to investigate the mechanism of catalytic reactions at the atomic level. However, single crystal studies cannot take into account for two characteristics of real catalysts, which are the finite size of the metal particles and the presence of a support. To investigate the possible role of these two particularities of real catalysts, it is necessary to use the second type of model catalysts: the supported model catalysts. They are obtained by growing under UHV metal clusters on clean ordered oxide surfaces w2x. The supported model catalysts are also characterized by surface science techniques while the size distribution and the morphology of the metal clusters are more conveniently obtained by microscopy techniques like transmission electron microscopy ŽTEM. scanning tunneling microscopy ŽSTM. and atomic force microscopy ŽAFM.. By changing the growth parame-
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 4 4 - 5
C.R. Henry r Applied Surface Science 164 (2000) 252–259
ters Žvapor flux, deposition time, substrate temperature., it is possible to control the number density of clusters and their size w2,3x. Their shape is more difficult to control. However, by epitaxial growth on oxide single crystals, it is possible to get, at high growth temperature, a single shape, which is close to the equilibrium shape w4x. In this paper, we will discuss the reactivity of nanometer Ž1–10 nm.-sized supported clusters emphasizing the peculiarity of this size range comparing on the one side few atoms clusters and on the other side extended surfaces. Indeed, one can try to understand the catalytic properties by looking on clusters containing a small number of atoms Žtypically 2 to about 30., which behave more like molecules than bulk metal: it is the ‘molecular’ approach. On the contrary, one can use extended surfaces of various orientations to understand the reactivity of nanometer sized clusters: it is the ‘surface science’ approach.
2. Molecular approach Small metal clusters are generally produced in a free jet by laser evaporation w5x. They can be sizeselected by mass spectrometry. Their electronic properties are studied by photoemission spectroscopy. It has been shown, for example by Taylor et al. w6x, that the electronic properties of small clusters change non-monotonously with the number of atoms. From a number of atoms of about 30, the band structure appears clearly and the electronic properties evolve smoothly towards bulk ones. The chemical properties have been investigated on free clusters by chemisorption experiments. The sticking probability of molecules follows the discrete evolution of the electronic properties for small clusters and a smooth continuous evolution for large clusters w7x. Recently, it has been possible to soft-land small size-selected clusters on extended surfaces in UHV conditions. The most advanced group is actually the group of Heiz and Schneider in Lausanne. They have deposited small clusters of Ni, Pt, Pd and Au on MgO thin films and studied their reactivity by infrared spectroscopy and thermal programmed reaction ŽTPR. w8x. They have particularly investigated
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the CO oxidation from a preadsorbed layer of oxygen and CO. Fig. 1 displays the number of CO 2 produced per atom on Pt clusters containing 4 to 20 atoms w9x. Again, it appears that the catalytic properties depend discontinuously on the exact number of atoms inside the cluster. As the reactivity of small clusters is a non-monotonous function of the cluster size, it cannot be extrapolated to nanometer-sized clusters. However, the question of a possible application of the small clusters in heterogeneous catalysis remains. It is clear that it is not reasonable to use size-selected soft-landed clusters to make industrial catalysts for the sake of time consumption and price. Meanwhile, it is possible to make powder catalysts with metal clusters containing few atoms. Recently, Gates w10x has shown that it is possible to make catalysts based on iridium clusters containing 4 Žtetrahedron. or 6 Žoctahedron. clusters. These catalysts are prepared by impregnation of an oxide powder Žsilica, alumina or magnesia. with organometallic precursors of Ir4ŽCO.12 or Ir6 ŽCO.16 followed by a gentle decarbonylation under helium gas. EXAFS measurements have shown that the tetrahedral or octahedral structures of the metal clusters are preserved. The activity of these small cluster-based
Fig. 1. Oxidation of CO on size selected Pt clusters supported on MgO thin films: Number of CO 2 molecules produced per Pt atom as a function of the number of atoms in the Pt clusters. ŽAdapted from Heiz et al. w9x..
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C.R. Henry r Applied Surface Science 164 (2000) 252–259
catalysts has been tested on the reaction of toluene hydrogenation w10,11x. Fig. 2 shows the Turn-Over Frequency ŽTOF. as a function of cluster size for Ir4and Ir6-based catalysts as well as for conventional Ir catalysts. Three observations can be made from this figure. First, we see that the catalysts based on few atom clusters are less active than conventional catalysts. Second, Ir4 clusters are more active Žon MgO. than Ir6 clusters. Third, tetrahedral Ir4 clusters are less active on MgO than on Al 2 O 3 support. The second point is a direct consequence of the non-monotonous variation of the reactivity of small clusters with the number of atoms. The second and the third points have probably the same explanation: the metal–support interaction. Indeed, the electronic properties of small clusters are more affected by the bonding to the support than large clusters Žexcept for two-dimensional clusters.. We will see later that, in general, the adsorption energy increases and the dissociation barrier decreases Ž for adsorbed molecules. when the coordination number of the atom binding the molecule decreases. Then, if the rate limiting step for the reaction is an adsorption or a dissociation step, we expect an increase of the
reactivity by decreasing the cluster size. Then, small free clusters are expected to be more active than larger ones. Now if a small cluster is adsorbed on a support, the bonds created are equivalent to an increase in the coordination, then supported cluster are less active than free clusters. This decrease will be more important if the metal–support interaction is stronger. Indeed, magnesia is considered as a much stronger interacting support than alumina. If we do not expect to get very active catalysts with small size-selected supported clusters, it is still possible that for some reaction the selectivity is much better than with conventional catalysts.
3. Surface science approach The surface science studies on catalysis have allowed to discover the mechanism of several reactions like the CO oxidation w12x, the ammonia synthesis w13x and more recently, the methanol synthesis w14x. A central question remains: is it possible, from a kinetic model obtained from data on extended surfaces at low pressure, to extrapolate the reaction kinetics on real catalysts in reaction conditions? In the case of ammonia synthesis, the answer is yes. The kinetic model established by Stoltze and Norskov w13x is able to reproduced experimental TOF on a very large range of pressures and for extended surfaces as well of industrial catalysts. This result was indeed the first quantitative Žwithout any fitting parameter. extrapolation of single crystal data to real catalysis. In fact, the result is not surprising because industrial catalysts for ammonia synthesis use large iron particles Ž20–40 nm., which behave like bulk single crystals. For CO oxidation w15–18x and methanol synthesis w19x, the direct extrapolation of single crystal data to nanometer-sized clusters failed. Nanometer-sized supported clusters have special properties that we will examine in the next sections.
4. Supported model catalysts Fig. 2. Reactivity of Ir clusters towards toluene hydrogenation. TOF as a function of the cluster size. The data labeled Ir4 and Ir6 correspond to tetrahedron and octahedron clusters containing four and six atoms, respectively. ŽAdapted from Gates w10x and Alexeev and Gates w11x..
To illustrate the specificity of the catalytic properties of nanometer-sized supported clusters, we take the now well-documented case of CO oxidation. The
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Langmuir–Hinshelwood mechanism for this reaction on Pt-group metals is being well established from the work of Engel and Ertl w20x. Several groups have tried to apply this model and the data obtained on extended surfaces to Pd clusters supported on Al 2 O 3 Ž0001. w15,16x and MgO Ž100. w18x but small clusters exhibited a higher reactivity than extended surfaces at temperatures where the reaction rate is maximum. Fig. 3a shows the activity for CO oxidation as a function of temperature for Pd clusters of 2.8, 6.9 and 13 nm supported on MgO Ž100. w18x. The activity Žper surface atom and per second. is in all cases higher than on extended Pd surfaces. This activity increase is due to two effects. The first one is the capture by the Pd clusters of the CO molecules physisorbed on the support increasing the CO coverage. In the temperature range, where the activity is higher, the reaction rate is proportional to the CO coverage. Then due to the extra-coverage provided by the capture of CO adsorbed on the support, the activity increases. The effect of the capture of CO physisorbed on the support by the clusters has been quantitatively studied on the PdrMgOŽ100. system w21x. The CO flux coming from the support increases when the cluster size or the number density of clusters decreases. After correction from this effect, the activity of the measured activity of the small clusters Ž2.8 and 6.9 nm. is still larger than for large clusters or extended surfaces Žsee Fig. 3a.. This difference is due to the second effect, which corresponds to a higher coverage of CO on the low coordinated edge sites of the Pd clusters. This second effect is more clearly seen in transient experiments. On Fig. 3b, we see the CO 2 production when a pulse of CO Žfrom a molecular beam. impinges on the PdrMgO model catalyst, an isotropic oxygen pressure being kept constant during the experiment w22x. At low temperatures Žbelow 2008C., after closing of the CO beam, a second peak of CO 2 is produced. It becomes larger when the cluster size decreases and its maximum shifts towards longer delays when the substrate temperature or the oxygen pressure decreases. The origin of this peak is due to the stronger adsorption of CO on the particle edges Žand probably also on defect sites.. At low temperature, the edges are saturated by CO and no reaction occurs on these sites. When the CO beam is closed, CO adsorbed on the facets reacts with adsorbed oxygen or desorbs
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Fig. 3. Oxidation of CO on a PdrMgOŽ100. model catalyst. Ža. Steady-state turnover number as a function of surface temperature for different cluster sizes and PCO s 3.5=10y5 Pa Žmolecular beam. and PO 2 s1.3=10y5 Pa Žisotropic pressure.; The continuous, dashed and dotted lines correspond to the correction of the TON on an extended surface of Pd Ždashed–dotted line. from the capture of CO physisorbed on the MgO substrate Žfrom Ref. w18x.. Žb. CO 2 transients, at different temperatures, for 4-nm Pd clusters during a CO pulse and a constant isotropic pressure of oxygen Ž PO s6.6=10y6 Pa. Žfrom Ref. w23x.. 2
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rapidly. Then, CO on the edges starts to diffuse on the facets and reacts immediately. Oxygen can now be adsorbed on the free edge sites and the reaction rate increases. It decreases again until the consumption of all the CO previously adsorbed on the edges. A kinetic model based on this mechanism gives a good qualitative agreement with the evolution of this secondary CO 2 peak and indicates that the rate-determining step is the diffusion from the edges to the facets w23x. In this example, we have seen that the reaction kinetic is different on the nanometer-sized supported clusters than on extended surfaces. These differences are not due to a different electronic structure of the clusters Žwe consider here the electronic structure of the whole cluster, the local electronic structure can change from one surface site to another one as for extended surfaces. but to the intrinsic heterogeneities of these systems: edges, support.
5. Discussion We have seen that the reactivity of small clusters Ž2–40 atoms. varies discontinuously with the number of atoms as their electronic properties. They behave more like molecules rather than bulk crystals. On the contrary, nanometer-sized clusters, containing 50 to 10 5 atoms, have an electronic structure close to bulk crystals. However, their catalytic activity is often different than predicted from measurements on extended surfaces that leads to various size effects well known in heterogeneous catalysis w24x. What could be the specificity of the supported nanometersized particles? First, consider the structure of the clusters. It is known that the most stable structure of small clusters is often different than bulk one. However, for Pt group metals the fcc structure is the most stable already at about 50 atoms w25x. However, the lattice parameter generally decreases Žby a few percents. with cluster size due to surface stress w26x. In the case of supported clusters, the lattice can be expanded Žor contracted. if they grow pseudomorphically on the substrate lattice. In the case of Pd on MgOŽ100., 2-nm clusters have their lattice dilated by 8% w27x. These variations of the interatomic distances induce changes in the adsorption energies and
dissociation barriers. Mavrikadis et al. w28x have shown that the adsorption energy increases linearly with the dilatation of the lattice. This increase of the adsorption energy results from the shift-up of the valence band when the coordination of the atoms decreases. The same effect is at the origin of the increase of the adsorption energy Žand the decrease of the dissociation barriers. when the crystal plane becomes more open w29x. In the case of Pd clusters, Mottet et al. w30x have calculated that starting from a nine-fold coordinated Pd atom on a Ž111. facet and going toward a seven-fold coordinated edge atom and a six-fold coordinated vertex atom, the gravity center of the d-band Žlocal density of states. ,shifted by 0.22 and 0.45 eV that corresponds, from Hammer and Norskov w31x calculations, to an increase of the adsorption energy of CO of 3.9 and 8.1 kcalrmol, respectively. Experimentally, adsorption of CO, at low coverage, on Pd clusters has shown a large size effect w32x. Below 5 nm, the adsorption energy increases from around 30 up to 38 kcalrmol at cluster size of 2.2 nm in good agreement with the theoretical prediction. This illustrated clearly that the geometrical factor cannot be dissociated from the electronic factor Žhere, we consider the local electronic structure.. The second learning of this results is that the key parameter to understand the reactivity of the clusters is the position of the d-band ŽLDOS. at the adsorption site Žnot the density of state at the Fermi level as often advanced.. Another difference between an extended surface and a nanometer-sized cluster is the presence of different facets, which can have different reactivities. However, the reaction rate is not simply the average rate between the different facets. In fact, the facets are generally not isolated because communications between them does exist by surface diffusion. Then the steady state coverages of adsorbed reactants can be very different than for extended surfaces. Zhdanov and Kasemo w33x have studied recently, by Monte Carlo simulation, a generic Langmuir– Hinshelwood reaction on nanometer-sized clusters. Their simulations have shown that the reaction window is very different than those on extended planes corresponding to the exposed facets and that it cannot be obtained by averaging on the relative areas of the different facets. The presence of edges can lead to a further complication because they can present
C.R. Henry r Applied Surface Science 164 (2000) 252–259
diffusion barriers higher than on smooth facets w23x. We have seen that the morphology of the clusters is of great importance for reactivity. The morphology of nanometer-sized clusters prepared Žor annealed. at high temperature is close to the equilibrium shape. Recently, it has been shown that Pd particles of 10 nm have the equilibrium shape of a bulk crystal: a truncated Wulf-polyhedron exposing Ž111. and Ž100. facets w34x. However, the adsorption of oxygen dramatically changes the equilibrium shape because of the stronger adsorption on the Ž100. facets than on the Ž111. facets w34x. Already at coverages of 0.1 ML, the equilibrium shape can be affected significantly then during a chemical reaction the morphology of the clusters can be very different than under UHV. The cluster–support interfacial energy can also change with the atmosphere composition if the support is easily reducible. This effect has been introduced to explain the disagreement of the kinetic model for the methanol synthesis extrapolated from single crystal data w19x. It becomes therefore necessary in the future to develop new tools to study in situ the morphology of supported metal clusters. Several groups are developing STM and AFM, working from UHV up to 1 atm in reaction conditions. The last important difference between supported catalysts and extended surfaces is the presence of the support. We have already seen that the electronic properties of very tiny clusters Žcontaining few atoms. are modified by the support. However, this effect can be also active for large clusters if they are sufficiently thin. A remarkable example of the metal– support interaction is catalytic gold. Haruta w35x has shown that gold clusters around 3 nm in diameter supported on particular supports ŽTiO 2 , Fe 2 O 3 , Co 3 O4 . are very active for CO oxidation at low temperature while on more conventional supports like SiO 2 and Al 2 O 3 , they are practically inactive. The origin of this surprising behavior is still under debate but it is clear that the support plays an important role. In the first study of a supported gold model catalysts, Valden et al. w36x have recently shown, by STM that active gold clusters are in fact very flat. The maximum of reactivity is observed for 3-nm clusters being two monolayers high. Scanning Tunneling Spectroscopy shows the appearance of a conduction gap between 0.3 and 0.6 eV for these clusters. Their extraordinary activity could be due to
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the confinement of the electrons in the quasi 2D clusters w36x. However, other explanations like reaction at the clusterrsupport boundary, role of special sites, charges on the clusters . . . are still under debate. The presence of the supports has other important consequences. The shape of the clusters is partially controlled by the metal–support interaction through the epitaxial relationships and the truncation of the Wulf polyhedron Žfor equilibrium shapes.. In the case where a misfit exists between the lattices of the metal and of the support, strain can be present at least near the interface or in the whole clusters for smaller ones. In the case of strain, the equilibrium shape is different than for a crystal with no misfit Žsee paper by Ref. w37x.. As the strain relief depends on cluster size, the equilibrium shape can change with cluster size below the apparition of interfacial dislocations w38x. The support can also take a part in the reaction as a additional source of reactant. As we have seen in Section 4, reactants physisorbed on the support can diffuse towards the metal clusters increasing the adsorption rate, then the coverage. The reverse process is also observed especially for hydrogen, which is dissociated on the particles and diffuses as atomic species on the substrate w39x. Another specific effect of the support is known as Strong Metal Support Interaction ŽSMSI. effect, which is observed for some metals on TiO 2 w40x. It corresponds to the diffusion, during annealing under hydrogen atmosphere, of some support species ŽTi suboxides. on the cluster surface leading to a strong decrease of the adsorption capacity. We have seen that nanometer-sized supported metal clusters behave differently than few atoms clusters and than extended surfaces. The differences with extended surfaces are due to the intrinsic heterogeneities of the supported clusters systems: presence of different facets, presence of low coordinated edge sites, presence of the support. All these heterogeneities have an effect on the reactivity, which is generally a function of cluster size. To investigate these effects in detail, it is necessary to be able to fabricate uniform collections of metal clusters having all the same size and a unique shape. In addition, to study the effect of the spillover of the reactants on the support, it is useful to have a constant distance between the clusters. There are several ways to
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prepare a regular array of clusters. The first way is to use scanning probe techniques to fabricate metal clusters at preselected places. Electron-beam lithography has already been used to prepare arrays of Pt clusters of 50 nm separated by 200 nm on an oxidized silicon wafer w41x. By this technique, the actual resolution limit is more than 10 nm; therefore, it is too large to study size effects. However, spillover effects can be studied. Much smaller clusters can be fabricated by an STM tip. Engelmann et al. w42x have recently obtained electrochemical fabrication with an STM of an array of 10 4 Cu clusters, 0.7 nm in height and separated by 11 nm on a gold surface. The fabrication of this array was very fast: 17 min but to extend this array to 1 mm2 , several years should be necessary. Thus, STM Žor AFM. nanofabrication is too slow to make model catalysts. The second way to prepare an array of clusters is to nucleate the metal clusters on a regularly nanostructured surface. Several methods have been used to prepare nanostructured metal surfaces w43x. Reconstruction of the AuŽ111. surface gives a regular pattern that has been used to grow metal clusters but the distance between clusters cannot be changed. Another more flexible method is to create a dislocation network by deposition of a thin metal film on a substrate. Hexagonal arrays of defects are produced and the lattice parameter is changed by changing the nature of the metals w43x. Metal clusters have been grown on this kind of patterned surfaces. Nucleation occurs very rapidly on the defects then the size distribution is much narrower than for homogeneous nucleation. However, for model catalysts, metallic supports are not very interesting but much less is known on the reconstruction of oxide surfaces. Actually, we are testing the possibility to create a nanostructured MgO surface w44x. For this purpose, we grow epitaxially thin films of MgOŽ111.. Calculations predict that this unstable surface may decrease its surface energy by spontaneous creation of an array of nanopyramids w45x. The lattice parameter should be a function of the thickness of the MgO film. References w1x G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994.
w2x C.R. Henry, Surf. Sci. Rep. 31 Ž1998. 231. w3x M. Baumer, H.J. Freund, Prog. Surf. Sci. 61 Ž1999. 127. ¨ w4x C.R. Henry, C. Chapon, C. Duriez, S. Giorgio, Surf. Sci. 253 Ž1991. 177. w5x W.A. De Heer, Rev. Mod. Phys. 65 Ž1993. 611. w6x K.J. Taylor, C.L. Pettiette-Hall, O. Chesnowsky, R.E. Smalley, Phys. Rev. B 96 Ž1992. 3319. w7x L. Holmgren, H. Gronbeck, M. Andersson, A. Rosen, Phys. ¨ Rev. B 53 Ž1996. 16644. w8x U. Heiz, W.D. Schneider, in: K.H. Meiwes-Broer ŽEd.., Metal Clusters and Dots, Springer-Verlag, Heidelberg, 2000, p. 237. w9x U. Heiz, A. Sanchez, S. Abbet, W.D. Schneider, J. Am. Chem. Soc. 121 Ž1999. 3214. w10x B.C. Gates, Chem. Rev. 95 Ž1995. 511. w11x O. Alexeev, B.C. Gates, J. Catal. 176 Ž1998. 310. w12x T. Engel, G. Ertl, J. Chem. Phys. 69 Ž1978. 1267. w13x P. Stoltze, J.K. Norskov, Phys. Rev. Lett. 55 Ž1985. 127. w14x T.S. Askgaard, J.K. Norskov, C.V. Ovesen, P.J. Stoltze, J. Catal. 156 Ž1995. 229. w15x S. Ladas, H. Poppa, M. Boudart, Surf. Sci. 102 Ž1981. 151. w16x F. Rumpf, H. Poppa, M. Boudart, Langmuir 4 Ž1988. 722. w17x C.R. Henry, Surf. Sci. 223 Ž1989. 519. w18x C. Becker, C.R. Henry, Surf. Sci. 352–354 Ž1996. 407. w19x C.V. Ovesen, B.S. Clausen, J. Schiotz, P. Stoltze, H. Topsoe, J.K. Norskov, J. Catal. 168 Ž1997. 133. w20x T. Engel, G. Ertl, Adv. Catal. 28 Ž1979. 1. w21x C.R. Henry, C. Chapon, C. Duriez, J. Chem. Phys. 95 Ž1991. 700. w22x C. Becker, C.R. Henry, Catal. Lett. 43 Ž1997. 55. w23x L. Piccolo, C. Becker, C.R. Henry, Eur. Phys. J. D 9 Ž1999. 415. w24x M. Che, C.O. Benett, Adv. Catal. 36 Ž1989. 55. w25x C. Mottet, G. Treglia, B. Legrand, Surf. Sci. 383 Ž1997. ´ L719. w26x C.R. Henry, Cryst. Res. Technol. 33 Ž1998. 1119. w27x S. Giorgio, C.R. Henry, C. Chapon, J.M. Penisson, J. Cryst. Growth 100 Ž1990. 254. w28x M. Mavrikadis, B. Hammer, J.K. Norskov, Phys. Rev. Lett. 76 Ž1998. 2819. w29x B. Hammer, O.H. Nielsen, J.K. Norskov, Catal. Lett. 46 Ž1997. 31. w30x C. Mottet, G. Treglia, B. Legrand, Surf. Sci. 352–354 Ž1996. ´ 675. w31x B. Hammer, J.K. Norskov, in: R.M. Lambert, G. Pacchioni ŽEds.., Chimisorption, Reactivity of Clusters, Thin Films, NATO ASI series E, Kluwer Academic Publishing, Dordrecht, 1997, p. 27. w32x C.R. Henry, C. Chapon, C. Goyhenex, R. Monot, Surf. Sci. 272 Ž1992. 283. w33x V.P. Zhdanov, B. Kasemo, Surf. Sci. 405 Ž1998. 27. w34x H. Graoui, S. Giorgio, C.R. Henry, Surf. Sci. 417 Ž1998. 350. w35x M. Haruta, Catal. Today 36 Ž1997. 153. w36x M. Valden, X. Lai, D.W. Goodman, Science 282 Ž1998. 1647.
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P. Muller, R. Kern, in this issue. ¨ S. Giorgio, H. Garoui, C.R. Henry, submitted for publication. M. Eriksson, L.G. Petersson, Surf. Sci. 311 Ž1994. 139. G.L. Haller, D.E. Resasco, Adv. Catal. 36 Ž1989. 173. M.Y. Yang, D.H. Gracias, P.W. Jacobs, G.A. Somorjai, Langmuir 14 Ž1998. 1458.
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w42x G.E. Engelmann, J.C. Ziegler, D.M. Kolb, Surf. Sci. 401 Ž1998. L420. w43x H. Brune, Surf. Sci. Rep. 31 Ž1998. 121. w44x S. Ferrero, A. Piednoir, C.R. Henry, to be published. w45x A. Pojani, F. Finocchi, J. Goniakowski, C. Noguera, Surf. Sci. 387 Ž1997. 354.
Catalytic activity of supported nanometer-sized metal clusters Claude R. Henry ) CRMC2 1-CNRS, Case 913, Campus de Luminy, F-13288 Marseille Cedex 09, France
Abstract The understanding of the reactivity of supported nanometer-sized particles, which are used in heterogeneous catalysis, can be approached in two ways: by studying the reactivity of size selected and soft-landed small metal clusters containing 2 to 50 atoms Ž‘molecular’ approach. or by studying the reactivity of extended single crystal surfaces Ž‘surface science’ approach.. We discuss the advantages and the limitations of these two approaches. We show in particular that often these two approaches cannot explain quantitatively the reaction kinetics for supported clusters a few nanometers in diameter. The peculiarity of these nanometer-sized supported clusters is due to their intrinsic heterogeneities: the presence of different types of facets, the presence of edges and the presence of the support. Taking account the recent experimental results on model supported catalysts, we show how the role of these heterogeneities can be investigated. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Clusters; Heterogeneous catalysis; Reaction kinetics; Model catalysts
1. Introduction Real Ži.e. industrial. catalysts are made of nanometer-sized transition-metal clusters dispersed on an oxide powder. Real catalysts have a very complex structure and their characterization is very difficult. Fundamental studies in heterogeneous catalysis are more easily performed on model catalysts. The model catalysts are of two types. The first ones are made of clean well-ordered surfaces of metal single crystals. These model catalysts are characterized precisely in using the large set of surface
)
Tel.: q33-6-6292-2832; fax: q33-4-918916. E-mail address: [email protected] ŽC.R. Henry.. 1 Associated to the Universities of Aix-Marseille II and III.
science techniques w1x. Such studies are necessary to investigate the mechanism of catalytic reactions at the atomic level. However, single crystal studies cannot take into account for two characteristics of real catalysts, which are the finite size of the metal particles and the presence of a support. To investigate the possible role of these two particularities of real catalysts, it is necessary to use the second type of model catalysts: the supported model catalysts. They are obtained by growing under UHV metal clusters on clean ordered oxide surfaces w2x. The supported model catalysts are also characterized by surface science techniques while the size distribution and the morphology of the metal clusters are more conveniently obtained by microscopy techniques like transmission electron microscopy ŽTEM. scanning tunneling microscopy ŽSTM. and atomic force microscopy ŽAFM.. By changing the growth parame-
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 3 4 4 - 5
C.R. Henry r Applied Surface Science 164 (2000) 252–259
ters Žvapor flux, deposition time, substrate temperature., it is possible to control the number density of clusters and their size w2,3x. Their shape is more difficult to control. However, by epitaxial growth on oxide single crystals, it is possible to get, at high growth temperature, a single shape, which is close to the equilibrium shape w4x. In this paper, we will discuss the reactivity of nanometer Ž1–10 nm.-sized supported clusters emphasizing the peculiarity of this size range comparing on the one side few atoms clusters and on the other side extended surfaces. Indeed, one can try to understand the catalytic properties by looking on clusters containing a small number of atoms Žtypically 2 to about 30., which behave more like molecules than bulk metal: it is the ‘molecular’ approach. On the contrary, one can use extended surfaces of various orientations to understand the reactivity of nanometer sized clusters: it is the ‘surface science’ approach.
2. Molecular approach Small metal clusters are generally produced in a free jet by laser evaporation w5x. They can be sizeselected by mass spectrometry. Their electronic properties are studied by photoemission spectroscopy. It has been shown, for example by Taylor et al. w6x, that the electronic properties of small clusters change non-monotonously with the number of atoms. From a number of atoms of about 30, the band structure appears clearly and the electronic properties evolve smoothly towards bulk ones. The chemical properties have been investigated on free clusters by chemisorption experiments. The sticking probability of molecules follows the discrete evolution of the electronic properties for small clusters and a smooth continuous evolution for large clusters w7x. Recently, it has been possible to soft-land small size-selected clusters on extended surfaces in UHV conditions. The most advanced group is actually the group of Heiz and Schneider in Lausanne. They have deposited small clusters of Ni, Pt, Pd and Au on MgO thin films and studied their reactivity by infrared spectroscopy and thermal programmed reaction ŽTPR. w8x. They have particularly investigated
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the CO oxidation from a preadsorbed layer of oxygen and CO. Fig. 1 displays the number of CO 2 produced per atom on Pt clusters containing 4 to 20 atoms w9x. Again, it appears that the catalytic properties depend discontinuously on the exact number of atoms inside the cluster. As the reactivity of small clusters is a non-monotonous function of the cluster size, it cannot be extrapolated to nanometer-sized clusters. However, the question of a possible application of the small clusters in heterogeneous catalysis remains. It is clear that it is not reasonable to use size-selected soft-landed clusters to make industrial catalysts for the sake of time consumption and price. Meanwhile, it is possible to make powder catalysts with metal clusters containing few atoms. Recently, Gates w10x has shown that it is possible to make catalysts based on iridium clusters containing 4 Žtetrahedron. or 6 Žoctahedron. clusters. These catalysts are prepared by impregnation of an oxide powder Žsilica, alumina or magnesia. with organometallic precursors of Ir4ŽCO.12 or Ir6 ŽCO.16 followed by a gentle decarbonylation under helium gas. EXAFS measurements have shown that the tetrahedral or octahedral structures of the metal clusters are preserved. The activity of these small cluster-based
Fig. 1. Oxidation of CO on size selected Pt clusters supported on MgO thin films: Number of CO 2 molecules produced per Pt atom as a function of the number of atoms in the Pt clusters. ŽAdapted from Heiz et al. w9x..
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catalysts has been tested on the reaction of toluene hydrogenation w10,11x. Fig. 2 shows the Turn-Over Frequency ŽTOF. as a function of cluster size for Ir4and Ir6-based catalysts as well as for conventional Ir catalysts. Three observations can be made from this figure. First, we see that the catalysts based on few atom clusters are less active than conventional catalysts. Second, Ir4 clusters are more active Žon MgO. than Ir6 clusters. Third, tetrahedral Ir4 clusters are less active on MgO than on Al 2 O 3 support. The second point is a direct consequence of the non-monotonous variation of the reactivity of small clusters with the number of atoms. The second and the third points have probably the same explanation: the metal–support interaction. Indeed, the electronic properties of small clusters are more affected by the bonding to the support than large clusters Žexcept for two-dimensional clusters.. We will see later that, in general, the adsorption energy increases and the dissociation barrier decreases Ž for adsorbed molecules. when the coordination number of the atom binding the molecule decreases. Then, if the rate limiting step for the reaction is an adsorption or a dissociation step, we expect an increase of the
reactivity by decreasing the cluster size. Then, small free clusters are expected to be more active than larger ones. Now if a small cluster is adsorbed on a support, the bonds created are equivalent to an increase in the coordination, then supported cluster are less active than free clusters. This decrease will be more important if the metal–support interaction is stronger. Indeed, magnesia is considered as a much stronger interacting support than alumina. If we do not expect to get very active catalysts with small size-selected supported clusters, it is still possible that for some reaction the selectivity is much better than with conventional catalysts.
3. Surface science approach The surface science studies on catalysis have allowed to discover the mechanism of several reactions like the CO oxidation w12x, the ammonia synthesis w13x and more recently, the methanol synthesis w14x. A central question remains: is it possible, from a kinetic model obtained from data on extended surfaces at low pressure, to extrapolate the reaction kinetics on real catalysts in reaction conditions? In the case of ammonia synthesis, the answer is yes. The kinetic model established by Stoltze and Norskov w13x is able to reproduced experimental TOF on a very large range of pressures and for extended surfaces as well of industrial catalysts. This result was indeed the first quantitative Žwithout any fitting parameter. extrapolation of single crystal data to real catalysis. In fact, the result is not surprising because industrial catalysts for ammonia synthesis use large iron particles Ž20–40 nm., which behave like bulk single crystals. For CO oxidation w15–18x and methanol synthesis w19x, the direct extrapolation of single crystal data to nanometer-sized clusters failed. Nanometer-sized supported clusters have special properties that we will examine in the next sections.
4. Supported model catalysts Fig. 2. Reactivity of Ir clusters towards toluene hydrogenation. TOF as a function of the cluster size. The data labeled Ir4 and Ir6 correspond to tetrahedron and octahedron clusters containing four and six atoms, respectively. ŽAdapted from Gates w10x and Alexeev and Gates w11x..
To illustrate the specificity of the catalytic properties of nanometer-sized supported clusters, we take the now well-documented case of CO oxidation. The
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Langmuir–Hinshelwood mechanism for this reaction on Pt-group metals is being well established from the work of Engel and Ertl w20x. Several groups have tried to apply this model and the data obtained on extended surfaces to Pd clusters supported on Al 2 O 3 Ž0001. w15,16x and MgO Ž100. w18x but small clusters exhibited a higher reactivity than extended surfaces at temperatures where the reaction rate is maximum. Fig. 3a shows the activity for CO oxidation as a function of temperature for Pd clusters of 2.8, 6.9 and 13 nm supported on MgO Ž100. w18x. The activity Žper surface atom and per second. is in all cases higher than on extended Pd surfaces. This activity increase is due to two effects. The first one is the capture by the Pd clusters of the CO molecules physisorbed on the support increasing the CO coverage. In the temperature range, where the activity is higher, the reaction rate is proportional to the CO coverage. Then due to the extra-coverage provided by the capture of CO adsorbed on the support, the activity increases. The effect of the capture of CO physisorbed on the support by the clusters has been quantitatively studied on the PdrMgOŽ100. system w21x. The CO flux coming from the support increases when the cluster size or the number density of clusters decreases. After correction from this effect, the activity of the measured activity of the small clusters Ž2.8 and 6.9 nm. is still larger than for large clusters or extended surfaces Žsee Fig. 3a.. This difference is due to the second effect, which corresponds to a higher coverage of CO on the low coordinated edge sites of the Pd clusters. This second effect is more clearly seen in transient experiments. On Fig. 3b, we see the CO 2 production when a pulse of CO Žfrom a molecular beam. impinges on the PdrMgO model catalyst, an isotropic oxygen pressure being kept constant during the experiment w22x. At low temperatures Žbelow 2008C., after closing of the CO beam, a second peak of CO 2 is produced. It becomes larger when the cluster size decreases and its maximum shifts towards longer delays when the substrate temperature or the oxygen pressure decreases. The origin of this peak is due to the stronger adsorption of CO on the particle edges Žand probably also on defect sites.. At low temperature, the edges are saturated by CO and no reaction occurs on these sites. When the CO beam is closed, CO adsorbed on the facets reacts with adsorbed oxygen or desorbs
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Fig. 3. Oxidation of CO on a PdrMgOŽ100. model catalyst. Ža. Steady-state turnover number as a function of surface temperature for different cluster sizes and PCO s 3.5=10y5 Pa Žmolecular beam. and PO 2 s1.3=10y5 Pa Žisotropic pressure.; The continuous, dashed and dotted lines correspond to the correction of the TON on an extended surface of Pd Ždashed–dotted line. from the capture of CO physisorbed on the MgO substrate Žfrom Ref. w18x.. Žb. CO 2 transients, at different temperatures, for 4-nm Pd clusters during a CO pulse and a constant isotropic pressure of oxygen Ž PO s6.6=10y6 Pa. Žfrom Ref. w23x.. 2
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rapidly. Then, CO on the edges starts to diffuse on the facets and reacts immediately. Oxygen can now be adsorbed on the free edge sites and the reaction rate increases. It decreases again until the consumption of all the CO previously adsorbed on the edges. A kinetic model based on this mechanism gives a good qualitative agreement with the evolution of this secondary CO 2 peak and indicates that the rate-determining step is the diffusion from the edges to the facets w23x. In this example, we have seen that the reaction kinetic is different on the nanometer-sized supported clusters than on extended surfaces. These differences are not due to a different electronic structure of the clusters Žwe consider here the electronic structure of the whole cluster, the local electronic structure can change from one surface site to another one as for extended surfaces. but to the intrinsic heterogeneities of these systems: edges, support.
5. Discussion We have seen that the reactivity of small clusters Ž2–40 atoms. varies discontinuously with the number of atoms as their electronic properties. They behave more like molecules rather than bulk crystals. On the contrary, nanometer-sized clusters, containing 50 to 10 5 atoms, have an electronic structure close to bulk crystals. However, their catalytic activity is often different than predicted from measurements on extended surfaces that leads to various size effects well known in heterogeneous catalysis w24x. What could be the specificity of the supported nanometersized particles? First, consider the structure of the clusters. It is known that the most stable structure of small clusters is often different than bulk one. However, for Pt group metals the fcc structure is the most stable already at about 50 atoms w25x. However, the lattice parameter generally decreases Žby a few percents. with cluster size due to surface stress w26x. In the case of supported clusters, the lattice can be expanded Žor contracted. if they grow pseudomorphically on the substrate lattice. In the case of Pd on MgOŽ100., 2-nm clusters have their lattice dilated by 8% w27x. These variations of the interatomic distances induce changes in the adsorption energies and
dissociation barriers. Mavrikadis et al. w28x have shown that the adsorption energy increases linearly with the dilatation of the lattice. This increase of the adsorption energy results from the shift-up of the valence band when the coordination of the atoms decreases. The same effect is at the origin of the increase of the adsorption energy Žand the decrease of the dissociation barriers. when the crystal plane becomes more open w29x. In the case of Pd clusters, Mottet et al. w30x have calculated that starting from a nine-fold coordinated Pd atom on a Ž111. facet and going toward a seven-fold coordinated edge atom and a six-fold coordinated vertex atom, the gravity center of the d-band Žlocal density of states. ,shifted by 0.22 and 0.45 eV that corresponds, from Hammer and Norskov w31x calculations, to an increase of the adsorption energy of CO of 3.9 and 8.1 kcalrmol, respectively. Experimentally, adsorption of CO, at low coverage, on Pd clusters has shown a large size effect w32x. Below 5 nm, the adsorption energy increases from around 30 up to 38 kcalrmol at cluster size of 2.2 nm in good agreement with the theoretical prediction. This illustrated clearly that the geometrical factor cannot be dissociated from the electronic factor Žhere, we consider the local electronic structure.. The second learning of this results is that the key parameter to understand the reactivity of the clusters is the position of the d-band ŽLDOS. at the adsorption site Žnot the density of state at the Fermi level as often advanced.. Another difference between an extended surface and a nanometer-sized cluster is the presence of different facets, which can have different reactivities. However, the reaction rate is not simply the average rate between the different facets. In fact, the facets are generally not isolated because communications between them does exist by surface diffusion. Then the steady state coverages of adsorbed reactants can be very different than for extended surfaces. Zhdanov and Kasemo w33x have studied recently, by Monte Carlo simulation, a generic Langmuir– Hinshelwood reaction on nanometer-sized clusters. Their simulations have shown that the reaction window is very different than those on extended planes corresponding to the exposed facets and that it cannot be obtained by averaging on the relative areas of the different facets. The presence of edges can lead to a further complication because they can present
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diffusion barriers higher than on smooth facets w23x. We have seen that the morphology of the clusters is of great importance for reactivity. The morphology of nanometer-sized clusters prepared Žor annealed. at high temperature is close to the equilibrium shape. Recently, it has been shown that Pd particles of 10 nm have the equilibrium shape of a bulk crystal: a truncated Wulf-polyhedron exposing Ž111. and Ž100. facets w34x. However, the adsorption of oxygen dramatically changes the equilibrium shape because of the stronger adsorption on the Ž100. facets than on the Ž111. facets w34x. Already at coverages of 0.1 ML, the equilibrium shape can be affected significantly then during a chemical reaction the morphology of the clusters can be very different than under UHV. The cluster–support interfacial energy can also change with the atmosphere composition if the support is easily reducible. This effect has been introduced to explain the disagreement of the kinetic model for the methanol synthesis extrapolated from single crystal data w19x. It becomes therefore necessary in the future to develop new tools to study in situ the morphology of supported metal clusters. Several groups are developing STM and AFM, working from UHV up to 1 atm in reaction conditions. The last important difference between supported catalysts and extended surfaces is the presence of the support. We have already seen that the electronic properties of very tiny clusters Žcontaining few atoms. are modified by the support. However, this effect can be also active for large clusters if they are sufficiently thin. A remarkable example of the metal– support interaction is catalytic gold. Haruta w35x has shown that gold clusters around 3 nm in diameter supported on particular supports ŽTiO 2 , Fe 2 O 3 , Co 3 O4 . are very active for CO oxidation at low temperature while on more conventional supports like SiO 2 and Al 2 O 3 , they are practically inactive. The origin of this surprising behavior is still under debate but it is clear that the support plays an important role. In the first study of a supported gold model catalysts, Valden et al. w36x have recently shown, by STM that active gold clusters are in fact very flat. The maximum of reactivity is observed for 3-nm clusters being two monolayers high. Scanning Tunneling Spectroscopy shows the appearance of a conduction gap between 0.3 and 0.6 eV for these clusters. Their extraordinary activity could be due to
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the confinement of the electrons in the quasi 2D clusters w36x. However, other explanations like reaction at the clusterrsupport boundary, role of special sites, charges on the clusters . . . are still under debate. The presence of the supports has other important consequences. The shape of the clusters is partially controlled by the metal–support interaction through the epitaxial relationships and the truncation of the Wulf polyhedron Žfor equilibrium shapes.. In the case where a misfit exists between the lattices of the metal and of the support, strain can be present at least near the interface or in the whole clusters for smaller ones. In the case of strain, the equilibrium shape is different than for a crystal with no misfit Žsee paper by Ref. w37x.. As the strain relief depends on cluster size, the equilibrium shape can change with cluster size below the apparition of interfacial dislocations w38x. The support can also take a part in the reaction as a additional source of reactant. As we have seen in Section 4, reactants physisorbed on the support can diffuse towards the metal clusters increasing the adsorption rate, then the coverage. The reverse process is also observed especially for hydrogen, which is dissociated on the particles and diffuses as atomic species on the substrate w39x. Another specific effect of the support is known as Strong Metal Support Interaction ŽSMSI. effect, which is observed for some metals on TiO 2 w40x. It corresponds to the diffusion, during annealing under hydrogen atmosphere, of some support species ŽTi suboxides. on the cluster surface leading to a strong decrease of the adsorption capacity. We have seen that nanometer-sized supported metal clusters behave differently than few atoms clusters and than extended surfaces. The differences with extended surfaces are due to the intrinsic heterogeneities of the supported clusters systems: presence of different facets, presence of low coordinated edge sites, presence of the support. All these heterogeneities have an effect on the reactivity, which is generally a function of cluster size. To investigate these effects in detail, it is necessary to be able to fabricate uniform collections of metal clusters having all the same size and a unique shape. In addition, to study the effect of the spillover of the reactants on the support, it is useful to have a constant distance between the clusters. There are several ways to
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prepare a regular array of clusters. The first way is to use scanning probe techniques to fabricate metal clusters at preselected places. Electron-beam lithography has already been used to prepare arrays of Pt clusters of 50 nm separated by 200 nm on an oxidized silicon wafer w41x. By this technique, the actual resolution limit is more than 10 nm; therefore, it is too large to study size effects. However, spillover effects can be studied. Much smaller clusters can be fabricated by an STM tip. Engelmann et al. w42x have recently obtained electrochemical fabrication with an STM of an array of 10 4 Cu clusters, 0.7 nm in height and separated by 11 nm on a gold surface. The fabrication of this array was very fast: 17 min but to extend this array to 1 mm2 , several years should be necessary. Thus, STM Žor AFM. nanofabrication is too slow to make model catalysts. The second way to prepare an array of clusters is to nucleate the metal clusters on a regularly nanostructured surface. Several methods have been used to prepare nanostructured metal surfaces w43x. Reconstruction of the AuŽ111. surface gives a regular pattern that has been used to grow metal clusters but the distance between clusters cannot be changed. Another more flexible method is to create a dislocation network by deposition of a thin metal film on a substrate. Hexagonal arrays of defects are produced and the lattice parameter is changed by changing the nature of the metals w43x. Metal clusters have been grown on this kind of patterned surfaces. Nucleation occurs very rapidly on the defects then the size distribution is much narrower than for homogeneous nucleation. However, for model catalysts, metallic supports are not very interesting but much less is known on the reconstruction of oxide surfaces. Actually, we are testing the possibility to create a nanostructured MgO surface w44x. For this purpose, we grow epitaxially thin films of MgOŽ111.. Calculations predict that this unstable surface may decrease its surface energy by spontaneous creation of an array of nanopyramids w45x. The lattice parameter should be a function of the thickness of the MgO film. References w1x G.A. Somorjai, Introduction to Surface Chemistry and Catalysis, Wiley, New York, 1994.
w2x C.R. Henry, Surf. Sci. Rep. 31 Ž1998. 231. w3x M. Baumer, H.J. Freund, Prog. Surf. Sci. 61 Ž1999. 127. ¨ w4x C.R. Henry, C. Chapon, C. Duriez, S. Giorgio, Surf. Sci. 253 Ž1991. 177. w5x W.A. De Heer, Rev. Mod. Phys. 65 Ž1993. 611. w6x K.J. Taylor, C.L. Pettiette-Hall, O. Chesnowsky, R.E. Smalley, Phys. Rev. B 96 Ž1992. 3319. w7x L. Holmgren, H. Gronbeck, M. Andersson, A. Rosen, Phys. ¨ Rev. B 53 Ž1996. 16644. w8x U. Heiz, W.D. Schneider, in: K.H. Meiwes-Broer ŽEd.., Metal Clusters and Dots, Springer-Verlag, Heidelberg, 2000, p. 237. w9x U. Heiz, A. Sanchez, S. Abbet, W.D. Schneider, J. Am. Chem. Soc. 121 Ž1999. 3214. w10x B.C. Gates, Chem. Rev. 95 Ž1995. 511. w11x O. Alexeev, B.C. Gates, J. Catal. 176 Ž1998. 310. w12x T. Engel, G. Ertl, J. Chem. Phys. 69 Ž1978. 1267. w13x P. Stoltze, J.K. Norskov, Phys. Rev. Lett. 55 Ž1985. 127. w14x T.S. Askgaard, J.K. Norskov, C.V. Ovesen, P.J. Stoltze, J. Catal. 156 Ž1995. 229. w15x S. Ladas, H. Poppa, M. Boudart, Surf. Sci. 102 Ž1981. 151. w16x F. Rumpf, H. Poppa, M. Boudart, Langmuir 4 Ž1988. 722. w17x C.R. Henry, Surf. Sci. 223 Ž1989. 519. w18x C. Becker, C.R. Henry, Surf. Sci. 352–354 Ž1996. 407. w19x C.V. Ovesen, B.S. Clausen, J. Schiotz, P. Stoltze, H. Topsoe, J.K. Norskov, J. Catal. 168 Ž1997. 133. w20x T. Engel, G. Ertl, Adv. Catal. 28 Ž1979. 1. w21x C.R. Henry, C. Chapon, C. Duriez, J. Chem. Phys. 95 Ž1991. 700. w22x C. Becker, C.R. Henry, Catal. Lett. 43 Ž1997. 55. w23x L. Piccolo, C. Becker, C.R. Henry, Eur. Phys. J. D 9 Ž1999. 415. w24x M. Che, C.O. Benett, Adv. Catal. 36 Ž1989. 55. w25x C. Mottet, G. Treglia, B. Legrand, Surf. Sci. 383 Ž1997. ´ L719. w26x C.R. Henry, Cryst. Res. Technol. 33 Ž1998. 1119. w27x S. Giorgio, C.R. Henry, C. Chapon, J.M. Penisson, J. Cryst. Growth 100 Ž1990. 254. w28x M. Mavrikadis, B. Hammer, J.K. Norskov, Phys. Rev. Lett. 76 Ž1998. 2819. w29x B. Hammer, O.H. Nielsen, J.K. Norskov, Catal. Lett. 46 Ž1997. 31. w30x C. Mottet, G. Treglia, B. Legrand, Surf. Sci. 352–354 Ž1996. ´ 675. w31x B. Hammer, J.K. Norskov, in: R.M. Lambert, G. Pacchioni ŽEds.., Chimisorption, Reactivity of Clusters, Thin Films, NATO ASI series E, Kluwer Academic Publishing, Dordrecht, 1997, p. 27. w32x C.R. Henry, C. Chapon, C. Goyhenex, R. Monot, Surf. Sci. 272 Ž1992. 283. w33x V.P. Zhdanov, B. Kasemo, Surf. Sci. 405 Ž1998. 27. w34x H. Graoui, S. Giorgio, C.R. Henry, Surf. Sci. 417 Ž1998. 350. w35x M. Haruta, Catal. Today 36 Ž1997. 153. w36x M. Valden, X. Lai, D.W. Goodman, Science 282 Ž1998. 1647.
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