The nucleation, growth, and stability of oxide-supported metal clusters W.T. Wallace, B.K. Min, and D.W. Goodman* Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012 The optimization of oxide-supported metal clusters as heterogeneous catalysts requires a detailed understanding of the metal cluster–oxide interface. Model catalysts, prepared by deposition of a catalytically active metal onto a thin film oxide support, closely mimic real-world catalysts, yet are amenable to study using surface sensitive techniques. Surface science methods applied to model catalysts, combined with the use of in situ high-pressure reaction studies, have provided a wealth of information about cluster structure and reactivity. STM capabilities for imaging individual particles under reaction temperatures and pressures offer a new approach for studying supported cluster catalysts on a particle-by-particle basis. This article describes recent work in our laboratories using variable temperature STM to investigate the role of the support and its defects in the nucleation and stabilization of metal clusters. KEY WORDS: metal clusters; metal-oxide surfaces; nucleation; bimetallic clusters; gold; silver; palladium; defects. 1. Introduction The extensive use of metal clusters dispersed on metal-oxide supports as catalysts has motivated a plethora of studies [1–11]. An atomic level understand- ing, however, is a formidable challenge given the complexity of technical catalysts and the conditions under which the catalysts typically operate. Numerous relevant studies have been carried out using single crystal metal surfaces as model catalysts, facilitating the use of ultrahigh vacuum surface analytical techniques [12, 13]. However, single crystals are incapable of modeling certain properties of supported metal clusters, e.g. those related to limited dimensions, where the cluster size and the oxide support are important. Also, ultrahigh vacuum environments preclude studies of the influence of reactant environments on surface interme- diates, composition, and morphology. To more realis- tically model technical catalysts, simplified planar supported cluster catalysts have been synthesized by nucleating metal clusters onto a thin oxide film grown on a refractory metal substrate [13]. These thin oxide films are structurally and electronically similar to the corresponding bulk oxides, yet are thin enough to permit the use of electron spectroscopic techniques [14]. Support defects are important as reaction and metal cluster nucleation sites. For example, for small, size- selected clusters, Heiz and coworkers have reported that oxygen vacancies (point defects) on MgO are essential for the nucleation of active Au clusters [15, 16]. Furthermore, theoretical studies have shown that elec- tron transfer from defects to the cluster facilitate CO oxidation. Also, Heiz and Pacchioni, in studies of acetylene polymerization on Pd, have suggested that trapped electrons at oxygen vacancies more efficiently activate single Pd atoms for reaction than do low- coordinated oxygen atoms [17]. Jennison and Bogicevic [18, 19] and a brief review discussing oxygen vacancies by Pacchioni [20], summarize recent relevant theoretical studies related to the metal cluster-oxide interface. The role of defects in the nucleation and stability of larger clusters has been studied recently in some detail for several systems [9, 11, 21–26]. For copper and vanadium on thin film alumina, Wiltner et al. found that metal clusters, rather than preferentially decorat- ing steps or defects, nucleate at one of two superstruc- tures [22]. The growth of bimetallic CoAPd clusters on alumina was studied by Freund and coworkers who noted that Pd clusters tend to nucleate at domain boundaries (line defects) of an alumina film [11]. Co clusters, on the other hand, nucleate at point defects at room temperature and at line defects at higher tem- peratures. When deposited subsequently to Co, Pd nucleates only on the Co particles, forming a core-shell structure. Due to a significant metal-support interac- tion, Co deposited subsequently to Pd decorates primarily Pd clusters and point defects. In this review, we discuss recent work from our laboratory addressing the role of defects in cluster nucleation and as reactive sites. 2. Metal–oxide surfaces 2.1. SiO 2 film production The recipe for the synthesis of the SiO 2 thin films used in these studies has been described in detail [27– 31]. In order to achieve a high quality, ordered thin film, a Mo(112) single crystal was first oxidized in 1 · 10 )7 Torr O 2 at 800 K for 5 min to produce a p(2 · 3)-O structure. The SiO 2 film was prepared sufficiently thick to produce the structural and elec- *To whom correspondence should be addressed. E-mail: [email protected]Topics in Catalysis Vol. 34, Nos. 1–4, May 2005 (Ó 2005) 17 DOI: 10.1007/s11244-005-3786-4 1022-5528/05/0500–0017/0 Ó 2005 Springer Science+Business Media, Inc.
Transcript
The nucleation, growth, and stability of oxide-supported metal clusters
W.T. Wallace, B.K. Min, and D.W. Goodman*
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012
The optimization of oxide-supported metal clusters as heterogeneous catalysts requires a detailed understanding of the metal
cluster–oxide interface. Model catalysts, prepared by deposition of a catalytically active metal onto a thin film oxide support,
closely mimic real-world catalysts, yet are amenable to study using surface sensitive techniques. Surface science methods applied to
model catalysts, combined with the use of in situ high-pressure reaction studies, have provided a wealth of information about
cluster structure and reactivity. STM capabilities for imaging individual particles under reaction temperatures and pressures offer a
new approach for studying supported cluster catalysts on a particle-by-particle basis. This article describes recent work in our
laboratories using variable temperature STM to investigate the role of the support and its defects in the nucleation and stabilization
The extensive use of metal clusters dispersed onmetal-oxide supports as catalysts has motivated aplethora of studies [1–11]. An atomic level understand-ing, however, is a formidable challenge given thecomplexity of technical catalysts and the conditionsunder which the catalysts typically operate. Numerousrelevant studies have been carried out using singlecrystal metal surfaces as model catalysts, facilitating theuse of ultrahigh vacuum surface analytical techniques[12, 13]. However, single crystals are incapable ofmodeling certain properties of supported metal clusters,e.g. those related to limited dimensions, where thecluster size and the oxide support are important. Also,ultrahigh vacuum environments preclude studies of theinfluence of reactant environments on surface interme-diates, composition, and morphology. To more realis-tically model technical catalysts, simplified planarsupported cluster catalysts have been synthesized bynucleating metal clusters onto a thin oxide film grownon a refractory metal substrate [13]. These thin oxidefilms are structurally and electronically similar to thecorresponding bulk oxides, yet are thin enough topermit the use of electron spectroscopic techniques [14].
Support defects are important as reaction and metalcluster nucleation sites. For example, for small, size-selected clusters, Heiz and coworkers have reported thatoxygen vacancies (point defects) on MgO are essentialfor the nucleation of active Au clusters [15, 16].Furthermore, theoretical studies have shown that elec-tron transfer from defects to the cluster facilitate COoxidation. Also, Heiz and Pacchioni, in studies ofacetylene polymerization on Pd, have suggested thattrapped electrons at oxygen vacancies more efficiently
activate single Pd atoms for reaction than do low-coordinated oxygen atoms [17]. Jennison and Bogicevic[18, 19] and a brief review discussing oxygen vacanciesby Pacchioni [20], summarize recent relevant theoreticalstudies related to the metal cluster-oxide interface.
The role of defects in the nucleation and stability oflarger clusters has been studied recently in some detailfor several systems [9, 11, 21–26]. For copper andvanadium on thin film alumina, Wiltner et al. foundthat metal clusters, rather than preferentially decorat-ing steps or defects, nucleate at one of two superstruc-tures [22]. The growth of bimetallic CoAPd clusters onalumina was studied by Freund and coworkers whonoted that Pd clusters tend to nucleate at domainboundaries (line defects) of an alumina film [11]. Coclusters, on the other hand, nucleate at point defects atroom temperature and at line defects at higher tem-peratures. When deposited subsequently to Co, Pdnucleates only on the Co particles, forming a core-shellstructure. Due to a significant metal-support interac-tion, Co deposited subsequently to Pd decoratesprimarily Pd clusters and point defects. In this review,we discuss recent work from our laboratory addressingthe role of defects in cluster nucleation and as reactivesites.
2. Metal–oxide surfaces
2.1. SiO2 film production
The recipe for the synthesis of the SiO2 thin filmsused in these studies has been described in detail [27–31]. In order to achieve a high quality, ordered thinfilm, a Mo(112) single crystal was first oxidized in1 · 10)7 Torr O2 at �800 K for 5 min to produce ap(2 · 3)-O structure. The SiO2 film was preparedsufficiently thick to produce the structural and elec-
Topics in Catalysis Vol. 34, Nos. 1–4, May 2005 (� 2005) 17DOI: 10.1007/s11244-005-3786-4
1022-5528/05/0500–0017/0 � 2005 Springer Science+Business Media, Inc.
tronic properties of bulk SiO2, yet thin enough toallow the use of the full array of surface spectroscopiesas well as STM. The Si-covered surface was thenoxidized and annealed in O2 (1 · 10)7 Torr) at�1150 K for 30 min. This step forms a highly-orderedSiO2 thin film that exhibits an atomically resolved
STM image and a sharp LEED pattern with c(2 · 2)periodicity (figures 1 and 2). The formation of astoichiometric film was confirmed by the absence ofSi0 or Si2+ AES features.
2.2. TiO2 surfaces
The STM and electron spectroscopic measurementsdescribed herein were carried out on TiO2(110) singlecrystals, although TiO2 thin films have been synthesizedin our laboratories [32–34]. The TiO2(110) surface wascleaned by several cycles of Ar+ sputtering and annealsto 700–1000 K. An image of a TiO2(110) surface(50 · 50 nm) before metal deposition is shown infigure 3(a); an atomically resolved image is shown infigure 3(b). As seen in the enlarged image, the TiO2
surface consists of numerous extended defects, namelystep edges with very small terraces. The bond distancesderived from the high-resolution STM image agree withthose of a (1 · 1) TiO2(110) unit cell.
3. Au
During the last decade following the pioneering workof Haruta and coworkers, numerous studies of Au as acatalyst have been reported [35–37]. In their studies,Haruta and coworkers found that small Au clusters(<5 nm) highly dispersed on reducible metal-oxidesupports are highly active for a variety of catalyticreactions at or below room temperature, including theoxidation of various hydrocarbons and carbon monox-ide. The need to understand the microscopic underpin-nings of these surprising results have led to a number ofstudies on high surface area Au catalysts [37–48], modelcatalysts [2, 5, 9, 21, 49–58], size-selected clustercatalysts [16, 59–61], and Au clusters in the gas-phase[62–68]. Although much remains to be understoodregarding the electronic and structural effects of theAu clusters, a key limitation to their use as practicalcatalysts is the tendency of the clusters to sinter at theelevated temperatures and reactant gas pressures typi-cally used for practical applications [8, 53]. Because thisagglomeration causes the catalysts to quickly lose theiractivity, methods for stabilization of the clusters areessential for their practical utilization. A step towardthis goal is the determination of the preferential clusternucleation sites on the support and the strength of thecluster–oxide interaction. These latter two issues are theprincipal motivations for our recent studies on thenucleation and thermal behavior of Au clusters on TiO2
and SiO2.
3.1. Au cluster nucleation on TiO2(110)
Initial studies on the growth and behavior of Auclusters on TiO2(110) surfaces were carried out using a
Figure 1. STM image (100 · 100 nm) of a bare SiO2 thin film
prepared using the methods described in the text. The tunneling
parameters are: Us ¼ )1.7 V and I ¼ 0.18 nA. Wide terraces with low
point defect densities can be seen under these conditions.
Figure 2. LEED pattern obtained from a sample such as that shown
in figure 1. A sharp c(2 · 2) array can be seen. The sharpness of the
lattice spots is a strong indicator of both the long range and short
range (point defect density) order of the thin film.
W.T. Wallace et al./Oxide-supported metal clusters18
room temperature STM (Omicron) [69] that providesbefore-and-after ‘‘snapshots’’ of the surface. Theseexperiments provide statistical distributions of clustersizes and locations on the surface. However, a recentdevelopment in our laboratory has made it possible tostudy individual clusters up to 1000 K and reactive gaspressures near one atmosphere [2, 57]. Using a modifiedcommercial RHK STM, individual particles have beenimaged using a STM tip-shadowing technique. Thismethodology, illustrated in figure 4, uses a collimateddoser to deposit metals of interest onto a surface withthe tip in its scanning position. The tip shadowprecludes metal deposition onto the surface behind thetip, thereby leaving a reference silhouette.
This STM apparatus has been used to study in detailthe coverage-dependent growth behavior of Au on
TiO2(110). Figure 5 shows a series of STM imagesfollowing the deposition of Au at various coverages. Asin figure 3, the image in figure 5(a) shows a clean TiO2
surface with numerous defects visible as steps separatingrelatively small terraces. With the deposition of Au atlow coverages, clusters preferentially nucleate at the stepedges, as noted previously (figure 5(b)) [69]. Withfurther Au deposition, cluster nucleation falls off atextended defects and initiates on the terraces (figures 5(cand d)). During the early stages of Au deposition, theaverage cluster diameter grows quickly, reaching �90%of saturation at �0.2 ML.
To understand the role of defects on Au clusternucleation, further experiments were carried out on aTiO2 surface containing more extensive step edges andterraces. A series of images of this surface with variousAu coverages is shown in figure 6 with identical areashighlighted. It is apparent that the growth mode is muchlike that of the more defective surface, with initialgrowth at the step edges. Subsequent growth on theterraces initiates prior to saturation of the step edge,highlighting the importance of point defects on theterraces. In this respect, Besenbacher and coworkershave shown that Au clusters nucleate at oxygen vacan-cies on TiO2 [54]. This is also apparent from thequenching of the Ti3+ states in the UPS spectra withincreasing metal deposition [70]. The annealing temper-ature and time play a key role in determining the densityof these defects.
Using our in situ STM instrument to image individ-ual clusters, the growth behavior of clusters formed atstep edges can be separated from those formed on theterraces. The STM images show that preferentialgrowth of clusters at step edges results in a higherdensity and a larger diameter compared to those
Figure 3. (a) STM image (50 · 50 nm) of a TiO2(110) surface prior to the deposition of any metal. This area of the surface shows an extremely
high step density with small terraces. (b) 3-D STM image (6 · 6 nm) of the TiO2 (110) surface showing atomic resolution. The measured distances
between adjacent atoms confirm the presence of a (1 · 1) TiO2(110) unit cell.
Figure 4. A cartoon showing the ‘‘shadowing’’ method used to
monitor the behavior of individual particles during high-pressure gas
exposure or annealing. During metal deposition, the STM tip is placed
in front of the doser, resulting in the creation of a silhouette of the tip
on the surface. This silhouette provides a marker that allows one to
find the same spot on the surface, even when tip drift occurs.
W.T. Wallace et al./Oxide-supported metal clusters 19
formed on the terraces. It is noteworthy that these datashow that the metal-support interaction is sufficientlyweak to allow cluster nucleation at the more stronglybound sites.
3.2. Au cluster nucleation on SiO2 and TiOx–SiO2 thinfilms
Recently, we have studied the nucleation behaviorof Au clusters on thin, crystalline silica films (�1 ML)[71] and on mixed titania–silica thin films [72]. Todate, there is no consensus regarding the structure ofthe as-grown SiO2 thin film, although several possibil-ities have been suggested [31, 72, 73]. Generally,however, three major defects are expected to bepresent in the films: extended defects (steps and kinks),line defects (antiphase domain boundaries), and pointdefects (oxygen vacancies). Au deposited at roomtemperature preferentially nucleates on line defects, asseen in the STM images of figure 7. Using SPA–LEED, Freund and coworkers have proposed thepresence of line defects based on the broadening ofsuperlattice spots [31].
Compared to reducible oxide supports, SiO2 isknown to have a relatively weak metal-support inter-
action with noble metals [74]. In this case, sintering,due to increased temperatures or pressures, is antici-pated to be more pronounced for metals supported onSiO2. For Au clusters supported on SiO2 this is indeedthe case, in that thermally-induced sintering has beenobserved. As mentioned above, room temperaturedeposition of Au generally leads to cluster nucleationat line defects, whereas for an anneal to 700–850 K,decoration of line defects no longer occurs. A dramaticdecrease in cluster density and an increase in clustersize accompany the diffusion of clusters to the stepedges as seen in figure 8.
A dramatic change in the nucleation properties of Auon the silica surface can be produced by the addition ofTi. A preparation method for a mixed-oxide has beenproposed in which Ti atoms replace Si atoms in the silicanetwork. This method involves the deposition of smallamounts of Ti (<10% in relation to the SiO2 surface),oxidation at 950 K, and annealing in vacuum at 1150 K[72]. In fact, examples of substitutional Ti have beendemonstrated in ‘‘real world’’ mixed titania–silica cat-alysts [75]. The Ti atoms become part of the silicastructure by forming heterogeneous defects. By depos-iting Au on a low-Ti content TiOx–SiO2 surface, thenumber density of Au clusters on both terraces and step
Figure 5. STM images (100 · 100 nm) of a TiO2(110) surface under different Au depositions at room temperature using the shadowing technique
to observe the same area. (a) clean TiO2 surface. (b) after 0.21 ML Au deposition. (c) after 0.34 ML Au deposition. (d) after 0.59 ML Au
deposition. Au clusters initially nucleate at step edges. However, larger depositions lower the preference for step edge nucleation.
W.T. Wallace et al./Oxide-supported metal clusters20
edges increases, consistent with an increase in thenucleation site density. The Ti ‘‘defects’’ therefore actas nucleation sites for cluster growth. This effectbecomes more obvious upon deposition of a smallcoverage of Au (0.04 ML). STM images provide acomparison of Ti defects (figure 9(a)) and Au clustersnucleated on these defects (figure 9(b)). In addition tothe bright contrast due to Ti defects, a somewhat highercontrast is observed due to the presence of Au clusterson these defects. On the reduced TiO2(110) surface, asmentioned above, Au clusters nucleate at oxygen vacan-cies, and the bonding between the clusters and thecoordinatively unsaturated Ti atoms has been describedas covalent [54]. Even though the Ti species in our mixed
oxide film are not reduced, if they adopt the samebonding scheme as silicon, they only form four bonds,leaving dangling bonds that are still free to interact withgold.
Increasing the amount of Ti eventually leads tosaturation of the point defects, at which point TiOx
islands nucleate. These islands serve as platforms uponwhich Au clusters nucleate and grow. These islandsdecorated with Au clusters are evident in the 3-D STMimage of figure 10.
Although the addition of small amounts of Ti(<10%) to the SiO2 surface leads to an increase in thecluster nucleation probability, it does not affect thetendency for the clusters to sinter under reaction
Figure 6. STM images (100 · 100 nm) of the same area of a TiO2(110) surface under Au coverages of: (a) 0.17 ML (b) 0.34 ML (c) 0.51 ML (d)
0.69 ML (e) 0.86 ML and (e) 1.3 ML. The circles in each image show the same area of the surface.
W.T. Wallace et al./Oxide-supported metal clusters 21
conditions, as is evident in figure 11(a) before, and infigure 11(b) after an 850 K anneal. However, sinterresistant properties are observed when the Ti coverage isincreased to 17% (figure 12(a)); no apparent morpho-logical changes are seen following an anneal to 850 K
(figure 12(b)). The histogram of figure 13 shows the Aucluster density after an 850 K anneal and after reactionin CO + O2 relative to the cluster density followingnucleation at room temperature for the Ti-free, 8%, and17% TiO2–SiO2 surfaces, respectively. An approxi-mately 70% decrease in the Au cluster density isobserved on the Ti-free surface due to the thermally-induced sintering effect. However, the extent of sinteringof Au clusters is attenuated significantly with an increasein the Ti coverage, with no apparent sintering for the17% Ti-covered SiO2 surface. The sinter resistantproperties of the 17% Ti-covered SiO2 surface werefurther tested by exposing the sample to CO oxidationreaction conditions (CO:O2 ¼ 2:1, 60 Torr, �370 K,and 120 min). Reaction leads to no significant change inthe cluster density as noted in figure 13. Therefore, it isapparent that the mixed-oxide serves as a sinter resistantsupport for thermal treatment and high-pressure gasexposure.
The origin of the sinter resistant properties for thetitania-silica mixed oxide surface is not understood andis currently under investigation. However, the forma-tion of 2-D and/or 3-D TiOx islands are likelyimportant, possibly in physically confining the Auclusters, since silica surfaces with atomically substi-tuted Ti do not show significantly altered sinteringproperties.
4. Palladium
4.1. Pd cluster nucleation on SiO2 thin films
Supported Pd clusters are widely used as catalystsfor a variety of reactions, yet many questions remainunanswered regarding the role of the support inaltering the catalytic properties of Pd. [3, 76]. Sekizawaet al. found that Pd supported on SnO2 and ZrO2
exhibit excellent activity for the low temperatureoxidation of methane. The structure of the supportplays an important role, for example, in that mono-clinic ZrO2 shows higher activity compared withtetragonal ZrO2. For Pd/SnO2 catalysts, the prepara-tion method has a significant effect on catalytic activity[4]. For silica supports, the extent of silicide formationbetween the support and the cluster, as palladium-silicide, is very important during high temperaturereduction of SiO2-supported Pd clusters. Such com-pound formation has been shown to dramaticallyincrease the selectivity of this catalyst for the isomer-ization of neopentane [77]. Recent work in ourlaboratory has focused on understanding the nucle-ation and stability of Pd clusters on SiO2 in order tounderstand the strength of the interaction, critical fordetermining properties such as sintering tendency,compound formation, and cluster encapsulation.
Thin silica films were prepared for these studies bytwo different methods. The first, described above and
Figure 7. STM image (200 · 200 nm) of Au clusters (0.4 ML) depos-
ited on a crystalline SiO2 thin film at room temperature. The clusters
preferentially nucleate at the step edges and line defects under these
conditions.
Figure 8. STM image (100 · 100 nm) of the same surface described in
figure 7 after annealing at 850 K. Indicating the relatively weak
interaction between the clusters and the surface, the cluster density is
dramatically decreased, and the average cluster diameter has increased
due to sintering.
W.T. Wallace et al./Oxide-supported metal clusters22
illustrated in figures 1 and 14(d), produces a film withlong range (as observed via LEED) and short range (viaSTM) order, together with a bulk-like silica band gap[78]. This particular film will hereafter be referred to as alow-defect film. The second method of film growthconsists of depositing elemental Si onto a Mo(112)substrate and oxidizing and annealing (at the sametemperatures described above) without first producing a(2 · 3)-O reconstructed Mo surface. An STM image of asurface prepared using this method is shown in fig-ure 14(a). As can easily be seen, this surface has muchless short-range uniformity compared to the less defec-tive film and will be referred to hereafter as a high-defectfilm.
The differences in short-range order are highlightedeven further with the deposition of Pd onto the two
Figure 9. 3-D STM images (12 · 12 nm) of (a) TiOx(8%)-SiO2; (b) Au(0.04 ML)/TiOx(8%)-SiO2; The areas of bright contrast in (a) indicate
atomically substituted Ti in the silica lattice. In (b), Au clusters nucleate on top of the Ti defects in (a).
TiOx(17%)-SiO2 showing that TiOx islands also play a role as
nucleation sites for Au nano-clusters.
W.T. Wallace et al./Oxide-supported metal clusters 23
films. Figures 14 (b and e) show the nucleation of Pdon the high- and low-defect surfaces, respectively. Onthe high-defect SiO2 surface, Pd clusters withdiameters of 2–3 nm are distributed relatively homo-geneously. This is in contrast to the low-defectsurface, where the Pd cluster density dramaticallydecreases and the cluster diameters are on the order of3–6 nm.
Figures 14 (c and f) show STM images acquiredafter annealing the Pd/SiO2 samples at 1000 K. Nosignificant changes in the cluster sizes and shapes areobserved on either of the SiO2 thin films uponannealing to 700 K. However, as shown in Figures 14(c and f), a 300 K increase in the annealing tempera-ture has a dramatic effect on cluster density andmorphology. On the more defective film (figure 14(c)),the clusters distribute uniformly on the surface. On theother hand, the clusters on the low-defect surface(figure 14(f)) assume a rectangular shape with adecrease in height and density. These changes inmorphology on the low-defect surface are accompaniedby a 20% loss of Pd estimated from the STM images.Since Pd does not desorb from silica at 1100 K [79],
Figure 11. STM images (100 · 100 nm) of Au(0.4 ML) clusters: (a) nucleated on a TiOx(8%)-SiO2 thin film at room temperature and (b) clusters
of (a) annealed to 850 K. While the addition of small amounts of Ti aid in the nucleation of Au, the clusters still undergo thermal sintering.
Figure 12. STM images (100 · 100 nm) of Au (0.4 ML) clusters (a) nucleated on a TiOx(17%)-SiO2 thin film at room temperature and (b)
clusters of (a) annealed to 850 K. At this Ti coverage, the clusters do not undergo sintering under high temperature annealing conditions.
Figure 13. A histogram of Au cluster density after the indicated
treatment normalized to the cluster density after nucleation at room
temperature. The Au coverage in each of the experiments was 0.4 ML
Au. The first three columns compare the cluster density on Ti-free
SiO2, TiOx(8%)-SiO2, and TiOx(17%)-SiO2 thin films, respectively,
after a 850 K anneal. The fourth column shows the normalized Au
cluster density of a TiOx(17%)-SiO2 thin film after a CO oxidation
W.T. Wallace et al./Oxide-supported metal clusters24
interdiffusion of Pd into the film likely accounts forthis loss.
Compared to cluster sintering on the low-defect film,the effects of annealing on the sintering of clusters onthe high-defect film are much more dramatic. Tounderstand the stability of Pd/SiO2 (high-defect), step-wise annealing and Auger analyses were carried out. Infigure 15, the AES intensity ratios of Pd/Mo (solid
squares) and O/Mo (hollow squares) are shown. As inthe STM experiments, no changes in these AES ratiosare apparent at 700 K. However, at �750 K andcontinuing to �1050 K, a decrease in the Pd/Mo ratiois evident. Even accounting for uncertainty in the AESmeasurements, it is clear that there is a decrease in thePd/Mo ratio at much lower temperatures than for theO/Mo ratio. The O/Mo ratio remains unchanged until1050 K, above which it decreases, accompanied by adecrease in the Si intensity. Since the onset of SiO2
decomposition is approximately 1200 K [30, 80], theresults cannot be explained simply by Si desorption. Amore likely explanation for the oxygen and siliconevolution above 1050 K is Pd-induced film decompo-sition. This decomposition may lead to the formationof volatile Pd-silicide or possibly SiO/Si that desorbconcurrently with Pd. Formation of these products isconsistent with the absence of silicide features in theAES data.
Point defects on SiO2 thin films have been sug-gested as the primary route for Pd interdiffusion. Inorder to test this hypothesis, electron-bombardmentwas used to create a high-defect surface of the typeshown in figure 14(d). Following this bombard-ment, elemental silicon was observed on the surfaceby AES. Palladium was deposited, the surface thenannealed to 1000 K, and finally elemental silicon
Figure 14. STM images of: (a) a clean SiO2 film produced through methods leading to a more defective surface (note the pits in the image), (b)
film (a) after deposition of �2 ML Pd at room temperature, (c) film (b) after annealing to 1000 K, (d) a clean SiO2 film produced through
methods leading to a less defective surface, (e) film (d) after deposition of �2 ML Pd at room temperature, and (f) film (e) after annealing to
1000 K. (a) and (b) are 200 · 200 nm images; all others are 100 · 100 nm.
Figure 15. Auger intensity ratios of (a) Pd/Mo, and (b) O/Mo plotted
as a function of annealing temperature. Loss of Pd signal begins at
~750 K, while the O signal is stable up to ~1050 K.
W.T. Wallace et al./Oxide-supported metal clusters 25
deposited. Subsequent to annealing there was noevidence for Pd-silicides even though Pd and Si areknown to mix at these temperatures [81, 82]. Theseresults suggest that Pd and Si nucleate separately onthe silica surface.
E-beam bombardment of the Pd/SiO2 (high-defect)surface annealed to 1000 K (figure 14(c)) producedpoint defects, which, upon annealing to 1000 K,yielded a palladium-rich [83] silicide. Deposition ofelemental silicon and an anneal to 1000 K leads to theformation of a silicon-rich silicide. From these resultsone concludes that an ‘‘activated’’ Pd/SiO2 surface, ora high-defect surface annealed to 1000 K, is requiredfor low-temperature silicide formation and that pointdefects (whether due to deposited Si or to e-beambombardment) aid in the formation of silicides below1000 K.
The ‘‘activated’’ or high-defect surface was studiedfurther by STM. Images obtained of the Pd/SiO2 (moredefective) surface after annealing at 300, 800, and950 K are shown in figures 16(a–c), respectively. Below750 K, there is no change in the cluster morphology ordensity. However, as can be seen in figure 16(b),annealing above 750 K leads to a decrease in thecluster density and an increase in the cluster diameter.Annealing to an even higher temperature lowers thecluster density and increases their diameter. Fig-ure 16(d) shows the change in the average clusterheight with an increase in annealing temperature.These results imply that Pd sintering and interdiffusionoccur simultaneously.
5. Bimetallic clusters
5.1. Ag/Au clusters on TiO2(110)
Compared to single-metal component catalysts,mixed-metal catalysts are often superior. For example,Besenbacher et al. have shown that alloying Au withnickel yields a more stable catalyst for steam reforming[84]. Behm and coworkers have reported that depositionof Pt onto Ru(0001) produces a better catalyst for theoxidation of CO compared to either of the monometalliccounterparts [85]. Similar results have been reported forsupported bimetallic catalysts, which exhibit catalyticproperties superior to either of the corresponding singlemetal components [86, 87].
Critical uncertainties regarding the characterizationof bimetallic clusters have limited significantly ourunderstanding of their property-function relationship.First, it is extremely challenging to control the clustersize, shape, and composition of specific clusters. Sec-ondly, it is difficult to distinguish whether the twometals are homogeneously alloyed or whether the metalsconsist of a heterogeneous core-shell structure. Devel-opment of the ‘‘shadowing’’ technique described abovehas allowed cluster growth and sintering to be trackedon a particle-by-particle basis. For example, studieshave been carried out addressing the growth of Au–Agclusters on a Ag-precovered titania surface as a functionof Au coverage. Ag nucleates on titania as uniform,thermally stable clusters [69, 88, 89]. The growth of Agclusters is generally self-limiting, in marked contrast tothe behavior of Au on this surface.
Figure 16. 3-D STM images (34 · 34 nm) of a Pd/SiO2(more defective) surface after annealing at (a) 300 K (b) 800 K, and (c) 950 K. Below
750 K, there is no change in the cluster morphology or density. Annealing above 750 K leads to a decrease in the cluster density and an increase in
the cluster diameter. (d) Average cluster height plotted as a function of annealing temperature.
W.T. Wallace et al./Oxide-supported metal clusters26
Using the shadowing technique, 0.08 ML of Agwas deposited on a TiO2(110) surface. This coverageof Ag is sufficient to effectively saturate the step sites.A series of STM images obtained at various Aucoverages subsequently deposited is shown in fig-ures 17(a–f). As can be seen, increasing the Aucoverage leads to an increase in particle size andnumber density. Under the lowest Au coverages,however, there is no evidence of new cluster forma-tion. Histograms of the cluster diameters derived fromthe corresponding STM images (not shown) confirm
that the average particle diameter shifts to largervalues, indicating preferential addition of Au toexisting Ag particles upon Au deposition.
In order to determine the effect of available stepsites for Au nucleation, 0.033 ML of Ag was depositedonto the TiO2 surface. This coverage is insufficient tosaturate the step edges. As can be seen in figure 17,relatively large coverages of Au were necessary for thenucleation of pure Au clusters. Two factors couldaccount for this behavior. First, Ag and Au can forman alloy. Secondly, the Ag clusters can simply serve as
Figure 17. STM images (100 · 100 nm) of a TiO2 (110) surface precovered with 0.08 ML Ag after deposition of (a) no Au (b) 0.17 ML Au (c)
0.34 ML Au (d) 0.85 ML Au (e) 1.53 ML Au, and (f) 2.04 ML Au. Under the lowest Au coverages, almost no new particles form; higher
coverages lead to an increase in particle size and number density.
W.T. Wallace et al./Oxide-supported metal clusters 27
preferred nucleation sites. On the 0.033 ML Ag cov-ered surface, a small amount of Au (0.17 ML) leads tothe formation of new clusters at the step edges, as canbe seen in the images of figure 18. With increasing Aucoverages, however, the behavior begins to mimic thatof figure 17. Figure 19 shows a series of plots thatindicate that the growth of Au clusters on the stepedges is highly accelerated compared to the terraces.There seems to be little difference in the growthbehavior with respect to cluster height or diameter.
However, there is a clear difference in the clusterdensity at the various sites.
These studies show that Au clusters prefer tonucleate at step sites. Blocking these sites with Agclusters leads to competitive nucleation and growth atthe step sites and on pre-existing Ag clusters. However,it is still not understood as to why the Ag clusters actas active nucleation sites for Au cluster growth andwhether these bimetallic clusters are of the alloy orcore-shell type.
Figure 18. STM images (100 · 100 nm) of a TiO2 (110) surface precovered with 0.033 ML Ag after deposition of (a) no Au (b) 0.17 ML Au (c)
0.34 ML Au (d) 0.51 ML Au (e) 0.85 ML Au, and (f) 1.53 ML Au. The areas indicated by white circles show the growth of Au clusters at low
coverages, in contrast to the surface in figure 17, showing the preferential nucleation of Au at step sites.
W.T. Wallace et al./Oxide-supported metal clusters28
6. Conclusions
While supported metal clusters are essential toheterogeneous catalysis, many questions remain regard-ing the effect of cluster size on reactivity and the cluster-support interaction in maintaining cluster stability. Inorder to better understand these issues, surface science
techniques have been applied to ‘‘model’’ catalystssynthesized to mimic commercial catalysts. One of themajor thrusts has been the study of the nucleation,growth, and stability of the clusters on various supports.We have focused on recent studies in our laboratoryconcerning the interactions between Au, Ag, and Pdwith metal oxide surfaces. These studies have reinforcedthe importance of surface defects in heterogeneouscatalysis, with extended and point defects showing alarge effect on the nucleation and stability of clustersunder reaction conditions. A better understanding ofthese processes gained through the use of modelcatalysts may very well lead to the development of moreefficient and stable nanostructured, technical catalysts.
Acknowledgments
We acknowledge with pleasure the support of thiswork by U.S Department of Energy, Office of BasicEnergy Science, Division of Chemical Sciences, RobertA. Welch Foundation and the Texas Advanced Tech-nology Program under Grant No. 010366-0022-2001.
References
[1] M. Bowker, P. Stone, R. Bennett and N. Perkins, Surf. Sci. 497
(2002) 155.
[2] A. Kolmakov and D.W. Goodman, Surf. Sci. 490 (2001) L597.
[3] G. Prevot, O. Meerson, L. Piccolo and C.R. Henry, J. Phys.:
Condens. Matter 14 (2002) 4251.
[4] K. Sekizawa, H. Widjaja, S. Maeda, Y. Ozawa and K. Eguchi,
Catal. Today 59 (2000) 69.
[5] S.K. Shaikhutdinov, R. Meyer, M. Naschitzki, M. Baumer and
H.-J. Freund, Catal. Lett. 86 (2003) 211.
[6] S.K. Shaikhutdinov, R. Meyer, D. Lahav, M. Baumer, T. Kluner
and H.-J. Freund, Phys. Rev. Lett. 91 (2003) 076102.
[7] S.J. Tauster, S.C. Fung and R.L. Garten, J. Am. Chem. Soc. 100
(1978) 170.
[8] M. Valden, S. Pak, X. Lai and D.W. Goodman, Catal. Lett. 56
(1998) 7.
[9] C. Xu, W.S. Oh, G. Liu, D.Y. Kim and D.W. Goodman, J. Vac.
Sci. Technol. A 15 (1997) 1261.
[10] X. Lai and D.W. Goodman, J. Mol. Catal. A 162 (2000) 33.
[11] A.F. Carlsson, M. Naschitzki, M. Baumer and H.-J. Freund,
