Catalysis by very small Au clusters

Current Opinion in Solid State and Materials Science 11 (2007) 62–75

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Catalysis by very small Au clusters Steeve Chrétien, Steven K. Buratto, Horia Metiu * Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA

a r t i c l e

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Article history: Received 6 June 2008 Accepted 28 July 2008

Keywords: Catalysis Mass-selected clusters Small gold clusters CO oxidation

a b s t r a c t We review recent theoretical and experimental work on the catalytic properties of Au clusters that contain a few atoms and are supported on an oxide surface. The clusters are mass-selected and landed slowly on the oxide surface in ultra-high vacuum. STM measurements show that the clusters do not fragment and do not damage the surface when they are deposited nor do they coarsen after deposition. Their catalytic activity changes non-monotonically with the number of atoms and is sensitive to the nature of the support and to additives (hydroxyls, water, Na, Cl) present on the surface. Binary clusters (e.g. AunSr) can be more active than unary ones. Very recent work has managed to study catalysis by such clusters under realistic pressure conditions; their performance is very different from (and sometimes better than) that of large clusters. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Numerous experiments and calculations have established that the reactivity of small metallic clusters in gas-phase varies with the number of atoms and with the charge on the clusters. The chemistry of Men differs from that of Men1, or Men+1, or Meþ n , or Me n . It is therefore natural to assume that very small clusters supported on an oxide surface may have novel and interesting catalytic properties that are different from those of the larger clusters (over 1 nm) normally used in catalysis. One expects the following: (1) Changing the number of atoms in a small cluster will change its catalytic properties. (2) The small size limits the number of reactions that can take place on the cluster, which may increase selectivity. (3) A small cluster is likely to be more sensitive to the nature of the support than a large one. (4) The catalytic properties of a small cluster may change more drastically when it is alloyed. (5) Promoter or poisons can affect small clusters more dramatically. (6) The cluster may change its properties if exposed to charge donors or acceptors. (7) If the catalyst is an expensive metal or one that is in short supply, using small clusters is economically appealing. Overall, we expect that molecular clusters are more tunable catalysts, since their properties are sensitive to a change in the number of atoms, the composition of the cluster, the support or the additives. Before celebrating these possibilities, we need to be aware of the handicaps. (1) Under reaction conditions an ensemble of very small clusters is likely to coarsen and evolve into an ensemble of large clusters of non-uniform size distribution [1,2]. (2) The meth* Corresponding author. Tel.: +1 805 893 2256; fax: +1 805 893 4120. E-mail address: [email protected] (H. Metiu). 1359-0286/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2008.07.003

ods by which small mass-selected clusters are prepared are too expensive for practical use. These systems are interesting to the scientist. They are new and very little is known about their chemistry. They are amenable for theoretical studies due to the small size of the clusters involved. They may (or may not) provide some insights that can be extrapolated to the large clusters used in practical catalysts. In this article, we review work on CO oxidation by very small Au clusters. To distinguish them from the clusters usually used in catalysis, we call them ‘molecular clusters’. We have chosen Au because it is a new catalyst that is being studied extensively and because it illustrates the problems and the potential of catalysis by mass-selected clusters. 2. Experimental results 2.1. Generalities One can prepare mass-selected, ionic clusters and deposit them gently on a solid surface in ultra-high vacuum (UHV). They are neutralized very efficiently when they come in contact with the surface by either resonant charge transfer or by Auger neutralization [3]. The final system consists of neutral molecular clusters bound to the support surface, all having the same composition and number of atoms. This preparation method was introduced by Heiz, Sherwood, Cox, Kaldor and Yates [4] in 1995 who prepared Pt (n = 1, or 2, or 3) clusters on silica films. They showed that the chemistry of these clusters differs from that of large Pt clusters. Heiz pursued this line of experimental research very successfully [5–36], followed by the group of Anderson in Utah [37–44], that of Buratto [45–52] in Santa Barbara,

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and by Vajda and his colleagues at Argonne National Laboratory [53,54]. Heiz’s group probes the properties of the clusters by thermally programmed reaction (TPR). CO and O2 are adsorbed on the surface at low temperature. Then the temperature is increased linearly in time and the appearance of CO2 in the gas-phase is monitored. In addition, they measure the IR absorption spectrum of adsorbed CO. The vibrational frequency of CO depends on the size of the clusters to which it is bound, and if the clusters coarsen the vibrational frequency of CO changes. This may not tell us that Au2 clusters merged to make Au4 cluster, but it will detect the presence of large clusters. Anderson’s group monitors the surface properties with XPS and ion scattering spectroscopy (ISS). The ISS signal is proportional to the number of metal atoms of given mass that are exposed to an incident ion beam. ISS is most sensitive to the formation of large clusters that have ‘‘interior atoms” which no longer scatter ions, and whose formation makes the ISS signal decay. Anderson’s group performs kinetic measurements by adsorbing oxygen on the surface (with a long exposure, since the sticking coefficient is low) and then exposing the surface to a CO pulse and measuring the CO2 desorbed from the surface. The kinetics of a catalytic reaction depends on temperature and the partial pressure of the reactants, and the kinetics studied by TPR or pulsed molecular beam in UHV may differ from that taking place in practical catalysis. However, these methods are useful for comparing the activities of clusters of different sizes, under wellcontrolled conditions. A great advance was made by the Argonne group, which prepared supported mass-selected clusters and studied the kinetics of catalytic reactions in a high-pressure cell, under realistic conditions [53,54]. They have also managed to prepare supports that seem to prevent coarsening of the small clusters under reaction conditions [53,54].

the STM chamber, isolated Au atoms are observed on the surface. They do not coarsen whether scanning is performed at room temperature or at 600 K. The isolated Au atoms are adsorbed along the bridging oxygen rows of TiO2(1 1 0) and theory finds that they are located at oxygen vacancy sites (the number of Au atoms deposited is smaller than the number of vacancies on the surface). The kinetic energy of the clusters approaching the surface during deposition is larger than the energy needed for removing one atom from the cluster or for displacing surface atoms from their positions. However, when a cluster hits the surface, the impact energy is distributed over several atoms in the cluster and on the surface. Due to the interaction between the atoms, the collision energy spreads over more and more atoms. The natural tendency of the energy is to dissipate and it is improbable that enough energy will accumulate in one bond in the cluster to break it. Calculations show [55] that even low energy impact can displace surface atoms from their binding sites. If this happens, it is most likely that they will move on the surface and heal the damage (the damaged site is the best binding site). This opinion is reinforced by experiments in which high energy (100 eV) Au1 ions were deposited on the surface [47]. No damage of the TiO2(1 1 0) surface is observed even though some is bound to occur at these high incident energies. We assume that damage took place but the time between deposition and scanning is long enough to allow the damage to heal. The behavior of the Au atoms or of small clusters can be studied by STM even without a source of mass-selected clusters: small clusters are present on the surface if the Au coverage is low and coarsening does not take place [56–70]. Much useful STM work on the TiO2(1 1 0) surface [71–74] or on very thin oxide films on a conducting support [75–79] is also available.

2.2. STM work

An important question is whether the catalytic activity is strongly dependent on cluster size. The results obtained in Heiz’s group [13,17,28,31], for CO oxidation on Aun, n = 1–20, supported on MgO, are summarized in Fig. 1. This shows the temperature program reaction curves and the number of CO2 molecules produced per cluster as a function of the number of atoms in the cluster. No CO2 is detected for n = 1 and n = 2, comparatively little CO2 for n = 3–6, no CO2 for n = 7, and about one CO2 per cluster for n = 8. For larger n the yield has irregular oscillations with an ascending trend [13]. The most active is A18, which produces 2CO2 molecules per cluster. Anderson’s group studied [38,42] CO oxidation by Aun, n = 1–7, supported on rutile TiO2(1 1 0), and found no activity for Au2, a small jump in activity for Au3, a decay for Au4, very low activity for Au5, an increase for Au6, and a substantial increase for Au7. It is interesting that Au7 supported on MgO is inactive [13,17,28,31] while it has the highest activity on TiO2. Unfortunately, since the methods for probing kinetics in the two groups are so different, it is difficult to make a quantitative comparison between the activities of Aun on the two substrates. It is, however, clear that the substrate is important. The electron binding energy in the XPS spectra of the Aun clusters is the same regardless of n [38]. It is customary – but not always correct – to correlate the binding energy in XPS with the charge on the electron-emitting atom. According to this interpretation, the charge on the Au atoms is independent of n. The DFT calculations indicate that the charge on these small clusters changes with cluster size and location on the surface (see Section 4.5). The sticking coefficient of O2 is very low and exposures of 600 Langmuir were needed to adsorb enough O2 on the surface to detect it by ISS or to produce CO2 when the surface is exposed to CO pulses [38]. ISS measurements indicate that the added oxygen

The Santa Barbara group uses STM to examine the structure of mass-selected clusters after they are deposited on an oxide surface. By using STM measurements combined with DFT calculations, Buratto’s group addressed the following questions: (1) Do the clusters fragment during deposition? (2) Do they damage the surface of the support? (3) Do they coarsen? (4) Is there any relation between the gas-phase structure and the structure of the adsorbed clusters? They investigated these questions for mass-selected Au and Ag clusters deposited on rutile TiO2(1 1 0). While the STM images do not resolve the atoms of the cluster, the 5c-Ti rows and the bridging-oxygen rows on the oxide surface are visible. The measurements determine the size and the height of the cluster and its position with respect to the bridging-oxygen rows. They found that Au2 to Au7 clusters do not fracture and do not coarsen at room temperature when deposited on TiO2(1 1 0). By combining the STM measurements with DFT calculations, they proposed structures and binding sites for the isomers of Aun (n = 2–7) adsorbed on the TiO2(1 1 0) surface [46,49]. In general, the structures of the supported clusters differ from their structure in the gas-phase. The Au clusters have a propensity of binding to oxygen vacancies if these are present on the surface [46,49]. Many experiments evaporating Au atoms on rutile TiO2(1 1 0) at room temperature have shown that the atoms migrate on the surface and form large clusters. If such clusters are formed, they should affect the ISS signal in Anderson’s experiments and they do not [38,39,42]. Experiments performed at Santa Barbara show that the conflicting results are due to water contamination during sample transfer from the deposition chamber into the STM chamber. If the sample is heated to 600 K, to remove water prior to Au deposition, and is kept at this temperature during transfer into

2.3. CO oxidation by molecular Au clusters

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(a)

0.17%ML

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18

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Au 20 2

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0 0 2 4 6 8 10 12 14 1618 20 22

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Au8 Au7 Au6 Au2 100 200 300 400 500 600 700 800 900

Temperature / K Fig. 1. (a) Temperature programmed reaction experiments for the CO-oxidation on selected Aun clusters on defect-rich MgO(1 0 0) films. The model catalysts are saturated at 90 K with 13CO and 18O2 and the isotopomer 13C18O16O is detected with a mass spectrometer, as a function of temperature; (b) The reactivities for Aun expressed as the number of formed CO2 per cluster (from [36]).

shadows the Au (i.e. an increase in 18O ISS signal is accompanied by a decrease in the Au signal). ISS also established that CO adsorbs on Au except for Au5 and Au6, for which no Au-signal attenuation is observed upon CO adsorption. Anderson and co-workers concluded that the activation energy for CO desorption must exceed 0.9 eV (if a pre-exponential of 1013 is appropriate) because this will allow CO to live long enough on top of Au to produce an ISS signal. Heiz et al. [28] measured the activity for CO oxidation of AunSr for n = 1–7 and found that it is substantially more active than Aun. The Au7Sr cluster is very active (producing  0.8CO2 molecules per cluster) while Au7 shows no activity. Two-component clusters need further study! Significant progress has been made by the Argonne group, who performed high-pressure kinetics on mass-selected Au6–Au10 clusters deposited on alumina. The gold clusters were stabilized by depositing a protective layer of alumina on them. It was found that the clusters have high activity and selectivity for propylene epoxidation. Adding water to the feed improved selectivity. It is interesting that experiments with large Au clusters observed epoxidation only when the clusters were supported on titania and required the presence of H2 in the feed. Neither condition is required if the epoxidation is catalyzed by molecular clusters. The same group managed to show that Pt8–Pt10 clusters are two orders of magnitude more active than catalysts using large Pt particles, for the oxidative dehydrogenation of propane to propylene, and that Ag3 catalyzes propene epoxidation with good selectivity and activity, at temperatures below 100 °C. These studies provides some justification for the hope that the performance of the catalysts consisting of molecular clusters might exceed that of commercial catalysts, or that such clusters can be used to catalyze new chemistry. It appears that it is possible to prevent coarsening through a careful fabrication of the support. Unfortunately, it is not likely that industrial catalysts will be prepared by depositing mass-selected clusters on a support.

Another possibility is to use organometallic compounds and bind them chemically to the surface of an oxide or the inner pores of a zeolite. This has two advantages: (1) if the organometallic is thermally stable and its bond to the surface is strong, there will be very little coarsening; (2) these systems can be prepared in large quantities, on the surface of the highly dispersed supports used in practical catalysis. Excellent reviews of this interesting field are available [80–83] and we do not discuss it here. Before concluding this section, we mention another method of preparation, which is valuable for scientific studies [60–65,67,84– 87]. This uses supports consisting of very thin oxide films grown on an electrically conducting flat surface. In favorable cases, the film is crystalline and has a well-defined structure. Good STM images of the films are obtainable because the electrons tunnel through them into the conducting substrate. Small metal clusters deposited on them can be studied at low temperatures to obtain STM images and I–V curves. The metal atoms or clusters can also be moved with the tip to construct clusters of selected geometries and investigate their electronic properties individually [60,69,87–93].

3. Theory Supported molecular clusters have few atoms and it is possible to study their chemistry by using density functional theory (DFT). DFT has been successful for studying the adsorption and the reactivity of molecules on metal surfaces. Unfortunately we have reasons to suspect that GGA-DFT calculations for some quantities, for some oxides having narrow d- or f- bands, might be in error. We do not know for which oxides this might be true, nor do we know which quantities might be calculated erroneously. This issue is too involved to be reviewed properly here, and we recommend two excellent recent articles [94,95]. Given this situation, it is best to study trends, which depend on DFT being qualitatively correct.

S. Chrétien et al. / Current Opinion in Solid State and Materials Science 11 (2007) 62–75

Another problem is electron spin. In a theory that neglects the spin–orbit coupling, a reaction in which the total electronic spin of the reactants differs from that of the products is forbidden: its rate is equal to zero. If the spin–orbit interaction is taken into account, transitions between states with different total electron spin are allowed but are slow, since spin–orbit coupling energy is small. One can understand these constraints without invoking theory: the internal magnetic fields are much too weak to be able to ‘‘flip the electron spin” as the system evolves from reactants to products. This means that when one calculates the potential energy surface one should not allow a change in the electron spin polarization (i.e. the number of spins up minus the number of spin down) as the position of the nuclei changes along the reaction coordinate. One should calculate several potential energy surfaces, one for each possible spin polarization of the reactants. An interesting example is provided by the dissociation of O2 adsorbed near an oxygen vacancy on the TiO2(1 1 0) surface. DFT calculations that disregard the conservation of spin polarization [96–99] predict a very high activation energy for oxygen dissociation while experiments [73,100–103] show that the dissociation is facile at room temperature. Calculations which conserve spin [104] arrive a different conclusion. In Fig. 2 we show the four structures corresponding to four different minima on the potential energy surface, together with the energy level diagrams for transition between these states [104]. Since the vacancy is a triplet and O2 is a triplet, their joint spin state can be singlet, triplet or quintet. These states are degenerate when the O2 molecule is away from the surface. When O2 is brought on the surface the quintet state is repulsive and the singlet and the triplet states are bound. Depending on the dynamics of the adsorption process, the O2 molecule may bind in any of the states b–f. The dissociated state h can be reached from any of these states. For the triplet state all barriers for reaching h are small. The singlet can reach h rapidly only if O2 adsorbs in the state g; from the other states the barriers are rather high. The most important point of these calculations is that imposing spin conservation changes considerably the kinetics of oxygen evolution. A rigorous procedure for dealing correctly with the total spin of the electrons requires us to ensure that the many-body electron wave function is an eigenstate of the total spin operator squared. Unfortunately, we do not know how to impose this constraint within the density functional theory, since DFT does not provide the many-body electronic wave function. Some non-rigorous solutions to this problem, for surface science systems, have been proposed [105–107]. Our non-rigorous solution is to compute potential energy surfaces that conserve spin polarization [104,108]. The need for spin-conservation during a chemical reaction has been extensively discussed in work applying DFT to organometallic and radical chemistry [109–120]. Approximate theories can be tested by comparison with accurate experiments. Unfortunately, such comparisons are inconclusive for realistic catalysts, because we do not know the structure of their surfaces. DFT can also be tested by comparison with numerically exact calculations. Such calculations are not possible for the systems of interest to catalysis, because they contain too many electrons. The difficulties in performing accurate high-level calculations are illustrated by the recent efforts to determine whether the gas-phase Au8 cluster is planar or has a three-dimensional geometry [121–124]; the high-level, benchmark calculations had errors due to the limited basis set forced on them by the size of the system. In spite of the reservations expressed above it is reasonable to expect that most results of GGA-DFT are qualitatively correct. While the activation energies may not be accurate, the reaction mechanism is likely to be correct if the activation energy on the most likely pathway is substantially smaller than the activation energies on the discarded pathways.

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4. The activity of supported molecular Au clusters The discovery by Haruta that small Au clusters are good catalysts [70,125–133] has caused quite a bit of excitement. Many reasons for the activity of small clusters have been suggested and we review here some of them.

4.1. Orbital ‘‘roughness”, not low coordination Since the early work in Nørskov’s group, it has been generally accepted that small Au clusters are active because their surface has a large number of under-coordinated sites [134–141]. This concept has been refined (and modified) in a series of papers [142–146] that propose that the reactivity of a cluster is controlled by the shape of the frontier orbitals (HOMO or LUMO) and not by the coordination of the binding site. It is the ‘‘orbital roughness” of the surface, not the geometric roughness, that controls reactivity. However, often undercoordinated sites are also sites where the frontier orbitals have a favorable shape. In such cases, the two points of view lead to the same conclusions. There are however examples where the frontier orbital rules make correct predictions and rules based on undercoordination fail. To describe the orbital roughness rules we introduce the following nomenclature. We call SOMO a single occupied Kohn–Sham (KS) molecular orbital, LUMO1 the unoccupied KS orbital having the lowest energy and LUMO2 the unoccupied orbital having the second lowest energy. These orbitals control the binding site and the binding energy of those molecules (e.g. propene, propene oxide and CO) that bind to the clusters by donating electrons. We call an electron donor a molecule that binds more strongly to Auþ n than to Aun and more weakly to Au than to Au . n n Rule 1: A charge donor binds most strongly at a site where LUMO1 of the bare cluster protrudes farthest in the vacuum. This rule holds even if that site is not the lowest coordination site. The shape and the energy of SOMO are irrelevant. Rule 2: A given cluster can have several low-lying LUMOs which protrude in the vacuum at different sites on the cluster. If this is the case, an electron donor can bind at different sites, producing different isomers (same cluster, different adsorbate locations). The strongest bond is at the site where LUMO1 protrudes, the next strongest is where LUMO2 protrudes, etc. Exceptions are rare and occur only when the energies of LUMO1 and LUMO2 are very close to each other. Rule 3: A plot of the binding energy of the lowest energy isomer of AunC3H6 versus n, has the same shape as a plot of the energy of LUMO1 of the bare Aun cluster versus n. In other words, if we compare propene binding to two lowestenergy clusters having different numbers of Au atoms, the propene binds most strongly to the cluster that has the lowest LUMO1 energy. In many cases, LUMO1 protrudes most at low coordination sites on the cluster. However, this is not always true. An exception is shown in Fig. 3 where propene binds more strongly to the doubly coordinated site 1a than to a singly coordinates site in 1c. According to the orbital roughness rules, this happens because LUMO1 protrudes at the doubly coordinated site and LUMO2 protrudes at the singly coordinated site. Another instructive example is provided by the binding of propene to the clusters AgnAum (n + m = 4, m = 0, 1, 2, 3), which is illustrated in Fig. 4. We ask the following question: what will happen to the binding energy of the propene to site 1, if we

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(a)

(b)

(c)

(d)

(f)

(g)

(h)

(e)

Fig. 2. Some of the lowest energy structures corresponding to the molecular and dissociative O2 adsorption on a partially reduced rutile TiO2(1 1 0) surface. (a) Reaction profiles for oxygen evolution on TiO2(1 1 0). The upper diagram (in cyan) is for triplet, the lower diagram (in black) is for singlet. (b) O2 adsorbed on a 5c-Ti atom away from the vacancy site. (c) O2 adsorbed between two 5c-Ti atoms. (d) O2 adsorbed on a 5c-Ti atom located next to an oxygen vacancy. (e) O2 adsorbed between two Ti atoms at the vacancy site and a 5c-Ti atom adjacent to the oxygen vacancy. (f) O2 adsorbed at the vacancy site. (g) The lowest energy structure corresponding to the dissociative O2 adsorption for in the triplet state; one oxygen atom is healing the vacancy site while the other one is adsorbed over a 5c-Ti atom located next to the vacancy site. (h) The lowest energy structure corresponding to the dissociative O2 adsorption for the singlet state: one oxygen atom is healing the vacancy site while the other one is adsorbed between two 5c-Ti atoms and forming an oxygen molecule with an in-plane oxygen atom of TiO2. The relative energies (DE) are given with respect to O2 in the gas-phase and a bare, partially reduced TiO2 (1 1 0) surface. DEact are the activation energies. de is the dissociation energy. The energies are in eV and were obtained using the PW91 functional with a [3  2] supercell and a 12-layer slab. The bridging oxygen vacancy concentration is 16.7%. d[O–O] are the distances (in Å) between the two oxygen atoms of O2. Q[O2] is the total electronic charge on O2 obtained from a Bader analysis; negative numbers indicate electron gain upon adsorption (from [104]).

exchange the Ag atoms 2, 3 or 4, to which the propene is not bound, with Au atoms? Since Au is more electronegative than Ag, one would guess that replacing a Ag atom with Au will pull away electrons from the Ag atom to which the propene is bound. Since propene is an electron donor, removing some electrons from the Ag atom to which the propene is bound will make the propene-Ag bond stronger. Nothing of the sort happens when we replace the Ag atoms 2 or 3 (see Figs. 4c and 4d) with Au. Moreover, the energy of the LUMO1 of the bare Ag3Au and

Ag2Au2 clusters is the same as that of Au4. However, replacing Ag atom 4 (see Fig. 4e) has a substantial effect on both the energy of LUMO1 and on the binding energy of propene. The shape of the LUMO1, shown in Fig. 4b, and the rules proposed above allow us to understand why this is happening. The Ag atoms 2 and 3 participate marginally in LUMO1; replacing them with Au does not affect this orbital, and according to the rules, will not affect propene binding. Only the replacement of atom 4 affects the LUMO1 and the ability of the cluster to bind propene.

S. Chrétien et al. / Current Opinion in Solid State and Materials Science 11 (2007) 62–75

Fig. 3. (a, c) Two propene binding sites on the same Au cluster. De is the binding energy of propene. (b) The shape of the LUMO1 orbital and its energy. (d) The shape of LUMO2 orbital and its energy. Propene binds most strongly to the site where LUMO1 protrudes not to the site having the lowest coordination.

The rules for the adsorption of electron acceptors (i.e. oxygen) are very similar, except that HOMO is involved instead of LUMO. For example, oxygen binds to the sites where HOMO protrudes in the vacuum [145,146]. These rules also explain why no adsorption takes place on the flat faces of the Au clusters or on flat surfaces of single crystals of Au: the frontier orbitals do not have any protrusions on such sites [142–146]. On the other hand, one Au atom adsorbed on a Au(1 1 1) surface binds oxygen or propene because both its LUMO and HOMO are localized and stick out in the vacuum. The rules were also used to explain why the Au clusters bind to certain locations on the TiO2(1 1 0) surface. A surface modification that places electrons at the bottom of the conduction band will increase substantially the bond of Au1 or O2 to the 5c-Ti atoms on the surface of TiO2(1 1 0) [147]. This happens because these electrons populate an orbital that protrudes into the vacuum at the location of the 5c-Ti atoms and molecules such as Au1 and O2 use the electrons in this orbital to form a bond to the surface. It is not necessary that surfaces that bind molecules are also good catalysts. However, binding the reactants is a precondition for catalytic activity; bulk Au is not a catalyst because almost no molecule binds to it. This is why so many of the explanations for the activity of small clusters revolve around their ability to bind molecules.

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Fig. 4. (a) Propene bound on a Ag4 cluster at the site that makes the strongest bond. De is the binding energy of propene and eLUMO is the energy of LUMO1. (b) The shape of LUMO1 of Ag4. (c) Propene binding on the same site of a Ag3Au cluster. We replaced atom 2 with Au. The binding energy of propene and the energy of LUMO1 of the bare clusters are not changed by substitution with Au. (d) Similar to (c) but Ag 2 and 3 are replaced with Au. (d) Propene binding to the same site of Ag3Au but with Au replacing atom 4. Both the binding energy and the LUMO1 energy change.

small Au clusters placed on a Au(1 1 1) slab adsorb oxygen [145] or propene [142] even though the system has no gap. 4.3. Support-induced strain Distorting the geometry of a cluster when it is adsorbed on a support will change its reactivity, and this has been invoked as a possible cause for the activity of the small clusters [134]. It is undeniable that changing the geometry of the cluster changes its chemical properties, but this is not an essential factor in explaining the reactivity of Au. 4.4. The activated oxygen comes from the support Very often, in the catalytic oxidation by oxides, the surface provides the oxygen for the oxidation reaction (the Mars–van Krevelen mechanism). The role of the gas-phase oxygen is to reoxidize the surface. Experiments using 18O2 in the gas and a Au/Al2O3 catalyst to oxidize CO produce 18O16OC, indicating that in the case of supported Au, the oxidant is gas-phase oxygen [152]. 4.5. The oxygen vacancies activate the Au cluster

4.2. Metal–insulator transition It has been argued [148–151] that the catalytic activity of small Au clusters correlates with the presence of a band gap. This hypothesis is at odds with the observation that very small Au clusters, which are planar, do not adsorb molecules on their flat faces [142–144,146] even though they have a large band gap. Moreover

Experiments with negatively charged Au clusters in gas phase [153] have shown that they adsorb oxygen more strongly if they have an odd number of atoms. DFT calculations [146] generalized this rule and showed that Au clusters that contain an odd number of electrons adsorb O2 more strongly than do Au clusters that contain an even number of electrons, regardless of whether the

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clusters were negatively charged or were neutral. The adsorption of O2 on such clusters weakens the O–O bond. Addition of one electron per supercell in a Au(1 1 1) slab on whose surface one had adsorbed a small Au [145] cluster, also increases the ability of the system to bind O2 and to weaken the O–O bond. It is therefore reasonable to expect that the charge on a supported Au cluster plays a role in its activity. In a series of papers Heiz, Landman and their coworkers [13,28,35,36] have studied, by experiment and DFT calculations, CO oxidation by several Au clusters supported on a MgO(0 0 1) surface or on MgO(1 0 0) thin film. They found that the Au8 cluster catalyzes CO oxidation more efficiently when the MgO support has missing-oxygen vacancies. Calculations show [13,28,35,36] that the Au cluster takes electron density from the vacancy and the catalyst is a negatively charged cluster. The question is whether this observation is general and whether the oxidation reaction by Au clusters supported on a reduced (i.e. having oxygen vacancies) oxide surface is catalytic. DFT calculations [154] of the charge on the Aun (n = 1–7) adsorbed at an oxygen vacancy site on rutile TiO2(1 0 0) show that not all clusters are negatively charged. Some gain electron charge, some donate it, and some adsorb without exchanging charge with the support. The calculations done by Molina and Hammer [155] show that CO and O2 adsorption on a Au20 cluster does not depend on whether the cluster is located at an oxygen vacancy site or on a stoichiometric TiO2(1 1 0) surface. This probably happens because the electron density gained by the cluster is located under the cluster, at the cluster-oxide interface, and it is not accessible to the oxygen molecule. This seems to suggest that the presence of oxygen vacancies becomes less important as the cluster size increases. However, Goodman’s experiments [156–160] show that the presence of vacancies does matter for large Au islands that are twoatom layers high. Heiz et al. [17] have shown that the activity of Pd8 for CO oxidation is the same whether the MgO surface had vacancies or not, even though Pd is a fairly electronegative element. Recent calculations [161] have shown that when molecular Au clusters (n = 1–7) adsorbed at an oxygen vacancy site are exposed to O2, the oxygen molecule inserts itself in the vacancy. In Fig. 5 we show the evolution of O2 when it is adsorbed on a partially reduced TiO2(1 1 0) having a Au2 cluster on it [104]. One can see that the oxygen molecule pushes the Au2 cluster out of the oxygen vacancy site and later dissociates placing one O atom in the vacancy. This is not surprising since the energy of vacancy formation is very high and the vacancy has therefore a great affinity for oxygen. We find this behavior for all Aun clusters with n = 1–7. This means that if an oxidation reaction is run for a sufficiently long time, the vacancies will be annihilated. Therefore, Aun on reduced TiO2 is not a catalyst. 4.6. The catalyst is positively charged Au In a paper that stimulated a lot of research, Fu et al. [162] prepared a catalyst consisting of Au clusters on a high-area CeO2 support and dissolved the Au clusters in cyanide. The removal of the large Au cluster did not change the catalytic activity for water gas-shift. Before cluster removal, the XPS spectra had peaks typical of the presence of Au0 and Au3+; after the removal only the peaks corresponding to Au3+ were detected. Other studies made a similar suggestion [132,162–181]. The experiments cannot determine unambiguously what this ‘‘ionic” Au is. We review some of the opinions advanced so far. Wang et al. [182] performed calculations for a Au7 cluster supported on rutile TiO2(1 1 0). These led them to propose that a ‘‘positive Au” was observed in experiments (on different systems) because the Au cluster donated electrons to the support. CO adsorbs on such a cluster and its vibrational frequency is in the range of 2164–2174 cm1. Experimental measurements [183–186]

assign values in the range 2101–2110 cm1 for CO adsorbed on Au0 and 2168–2176 cm1 to CO adsorbed on positively charged Au. Thus a Au7 cluster that has transferred electrons to the substrate behaves like the positive Au identified by IR experiments on adsorbed CO. The XPS spectrum of this system has not been computed. It was also suggested that the ionic gold is a Au atom that substitutes the metal atom in the oxide support, to form a doped oxide [108,187–189]. The presence of the dopant weakens the bond of one of the neighboring oxygen atoms to the oxide, and makes it a better oxidant. As a result, CO adsorbs on the surface more readily to form a CO2 molecule (on TiO2(1 1 0) [108,187,189]) or a carbonate (on CeO2(1 1 1) [188]) which decomposes releasing CO2 in gas phase and leaving behind an oxygen vacancy. This adsorbs an O2 molecule at the vacancy site. The electron-rich vacancy transfers electrons into O2 and weakens the O–O bond making it react easily with CO to make a carbonate. This decomposes to release CO2 in the gas phase and close the catalytic cycle. There is quite a bit of experimental [121,169,174,190–220] and theoretical [108,188,221–232] work exploring the possibility that doped oxides are a new class of catalysts in which doping improves the oxidative power of an oxide. Finally, calculations [104,233] on very small Au clusters supported on TiO2 show that the oxygen breaks up the gold cluster to form an ‘‘oxide” (see for example Fig. 5). There is some evidence that such an oxide is formed on large Au clusters [234]. Moreover, Anderson’s group found that O2 does shadow the Au clusters in ISS experiments [38], which means that O2 is adsorbed on top of the clusters. Such an oxide may cause a shift in the XPS spectrum similar to that used to declare that Au is ionic. 4.7. Oxygen adsorption and activation: the role of coadsorption It seems that in most cases binding and activating the O2 molecule is the key to CO oxidation. Oxygen is a difficult molecule for DFT [235]. The binding energies of O2 to Au2 calculated by a more accurate method (CCSD(T)) [236] do not agree with the ones produced by DFT. Unfortunately, the large number of electrons in the system also makes the CCSD(T) results questionable: one is not sure that the basis set is converged. In addition, the ground state of O2 is a triplet and the spin conservation rules can play an important role in controlling its reactivity. The calculations reported here have, with one exception [104], ignored the role of spin conservation. This does not mean that they are wrong: often the DFT calculations may pick accidentally a spin-conserving pathway. It is likely that most of the conclusions we present below are qualitatively reliable. Oxygen does bind to small negatively charged Au clusters in gas phase [146,153,237,238]. Experiments found that for negatively charged clusters the bond of the oxygen to the cluster is strong if the number of Au atoms is even [153]. Calculations have generalized this observation and showed that regardless of whether the cluster is negative or neutral, O2 binds more strongly to small clusters having an odd number of electrons [146]. O2 does not bind to the flat faces of bulk Au [134,239] or to flat faces of small clusters [146], but it does bind to stepped faces of bulk Au [134,239] or to small clusters adsorbed on the surface of bulk Au [145]. The bond of O2 to large Au clusters (Au20 and Au34) is weak [240–242]. The observation [242–245] that the presence of a Au cluster can induce O2 to bind more strongly to the oxide has prompted Chretien and Metiu [147] to examine how the adsorption energy of O2 on the TiO2 surface is modified if another species (which they called a surface modifier) is pre-adsorbed on the surface. They studied the following modifiers: an oxygen vacancy in the bridging oxygen row, a hydrogen atom bound to a bridging oxygen row (to

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Fig. 5. O2 evolution on a partially reduced rutile TiO2(1 1 0) with Au2 preadsorbed on its surface, for the triplet state. The relative energies (DE) are given with respect to O2 in the gas-phase and Au2 adsorbed at its equilibrium position on a partially reduced TiO2(1 1 0) surface. DEact are the activation energies and de the dissociation energies. All energies are in eV and were calculated with the PW91 functional with a [3  2] supercell and a 12-layer slab (from [104]).

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form a hydroxyl), Na, K, Li, Au1, Au3, Au5. Here are their findings: (1) The presence of one of the modifiers enumerated above changes the binding energy of O2 to a 5c-Ti atom on the oxide surface from 0.15 eV to 1.0 eV. (2). The magnitude of the change is the same for all modifiers. (3) When it binds, the oxygen molecule takes 0.5 electron from the surface regardless of the modifier. (4) The bond strength of the O2 molecule to a 5c-Ti atom is the same regardless of the distance between the 5c-Ti atom and the modifier. (5) When the O2 molecule is adsorbed, there is no change in the charge of the modifier even though O2 becomes negatively charged. They proposed the following explanation for these observations: all modifiers they studied donate electrons to the conduction band, which consists of orbitals localized on the 5c-Ti atoms, having equal weight on each atom. These electrons use the empty 2p* orbitals of the O2 molecule to form the oxygen–oxide bond. The formation of this bond results in charge donation to the molecule. Since the electron transfer populates an anti-bonding orbital of O2, the O–O bond is weakened; as a result, the molecule is more reactive. Prior to O2 adsorption, the electron donated by the modifier can be on any of the 5c-Ti atoms with equal probability. Because of this, the O2 molecule can bind equally well to any 5c-Ti atom, regardless of its distance from the modifier. They performed several computer experiments to test this explanation. First, they calculated the binding energy and the charge of O2 when it binds to a stoichiometric (i.e. no modifier) TiO2 surface to which they added an electron in the conduction band. The oxygen bonded to this surface exactly as if there was a modifier on it. Second, they reasoned that because Au is a very electronegative atom the pre-adsorption of a modifier should affect its adsorption in the same way it affected the oxygen binding. They found this to be true: the strength of the bond of the Au atom to a 5c-Ti atom was increased substantially by the presence of a modifier and so was the charge taken by Au when it bonded to the surface. The magnitude of these effects is the same for each modifier. Third, they argued that O2 has two degenerate, low-lying, empty orbitals while Au has only one. Therefore, they expected that having two hydroxyls in unit cell or adding two electrons to the stoichiometric TiO2(1 1 0) should increase the binding energy of O2 (as compared to the value when one had one electron or one hydroxyl) and indeed the binding energy almost doubled. Fourth, they reasoned that according to their model doubling the number of hydroxyls or of the added electrons should have no effect on Au binding; the calculations showed this to be true. We emphasize that not all electron-donating modifiers change the binding energy of oxygen and the charge this takes from the surface by the same amount. An infinite Au strip having nine Au atoms in the unit cell (referred to as the Au9 strip) changes the binding energy of O2 to 1.11 eV but it transfers more charge than the modifiers (0.93 versus 0.5). This happens because, unlike the modifiers, the Au9 strip donates additional charge to the conduction band when the oxygen atom binds to a 5c-Ti atom on the surface. Finally, a strange thing happens when one Au atom is added on top of the Au9 strip (to turn it into a strip with ten Au atoms in the unit cell): the binding energy of O2 changes from 1.11 eV (when the Au9 strip is present) to 1.96 eV. Adding one atom can make a big difference! In the language of inorganic chemistry, all modifiers act as Lewis bases and, by donating electrons to the conduction band, they turn the modified 5c-Ti atoms on the oxide surface into Lewis bases. These bind well Lewis acids (such as O2 and Au). This point of view leads us to hypothesize that the adsorption energy of any Lewis acid will be enhanced by these modifiers. The calculations also suggest that placing electrons in the conduction band will cause the oxide to adsorb oxygen and activate it. The catalytic activity of an oxide nanowire can then be controlled by placing

the nanowire in a FET configuration and using a gate to pull electrons into the wire or push them out of it [246]. Coadsorption leads to another interesting effect, which is easy to miss if one does not look for it. The Au1, Au2, Au4, and Au6 clusters having the lowest energy do not donate charge to the oxide when adsorbed on the stoichiometric TiO2(1 1 0) surface. According to the mechanism proposed [147], their presence on the surface should not affect drastically oxygen adsorption. Calculations show that they do not. However, these clusters have higher energy isomers that donate electrons to the conduction band when adsorbed on clean (no adsorbed O2), stoichiometric TiO2(1 1 0). Because they have higher energy, we do not expect to see these isomers on the clean TiO2 surface. However, the electron-donating isomer, which has higher energy when bound to the clean TiO2 surface, becomes the more stable isomer when O2 is present on the surface (on a 5c-Ti site). The presence of oxygen changes which isomer is more stable. Let us denote by A the isomer that is stable in the absence of oxygen and by B the charge donating isomer. To change A into B we need to provide the isomerization energy. However, since B donates charge to the conduction band, this is used by O2 to make a stronger bond than the one it would have if the isomer A were on the surface. If this increase in the bond energy is larger than the isomerization energy, then B is more stable than A in the presence of oxygen. This is what happens for Au2 and Au4. A similar observation was made by Wang and Hammer [182] in their study of Au7 on TiO2(1 1 0). A two-dimensional structure of Au7 has the lowest energy if the surface is stoichiometric, but a three-dimensional structure is favored if the surface has a hydroxyl on the bridging oxygen and an oxygen atom bound to the 5c-Ti (the latter is formed by the dissociation of OH on 5c-Ti to produce an OH on the bridging oxygen and O on 5c-Ti). Molina, Rasmussen, and Hammer [242] noticed that CO does not bind to a Au atom adsorbed at an oxygen vacancy site, but it does if O2 is preadsorbed on the surface. The Au atom at the vacancy site is negatively charged which disfavors CO adsorption. When O2 is preadsorbed, it takes some charge away from the Au atom and this makes the bond of CO to it stronger. Finally, the fact that the structure of a cluster may change when the reactants are adsorbed was noted by Hakkinen et al. [28]. 4.8. The mechanism of CO oxidation by Au clusters The reaction mechanism for CO oxidation has been studied extensively [13,28,31,34–36,140,182,240–245,247–251] and good recent reviews [31,36,240,249,251] are available. Because of this, we make only a few remarks. It is difficult to use theory to determine the mechanism of CO oxidation for a number of reasons. (1) A cluster can have many geometries and binding sites that are close in energy and are likely to coexist on the surface. For example, Au5–Au7 on a stoichiometric [252] or a reduced [253] TiO2(1 1 0) surface each has two isomers whose structure and position on the surface are rather different, but whose binding energies are very close. (2) Under the influence of coadsorbates, the cluster may change its geometry (see Section 4.7). (3) CO and O2 can have a large number of binding sites. To determine the reaction mechanism one has to consider all these sites as a possible starting point. (4) The adsorption of oxygen and the oxidation of CO is promoted by the presence of water [252,254] or Na [255] and it is hindered if Cl [255] is present. Oxygen adsorption and activation is affected dramatically [147] by additives that donate electrons to TiO2 (it has not been determined whether this is true for the MgO support). In some cases, the adsorption of O2 on the cluster takes place only if CO is already adsorbed on it [240]. Finally, O2 does not adsorb on stoichiometric TiO2(1 1 0) but it will if certain Au clusters are present on the surface [147,242,243]. (5) Additional complications arise from the

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need to conserve spin, which require studying the reaction mechanism for several spin states. Therefore, theory must consider a very large number of possibilities and it is easy to overlook some of them. Often several mechanisms may take place simultaneously and one needs to study all of them. For example [36], Au8 supported on reduced MgO oxidizes CO through three competing mechanisms: A Langmuir–Hinshelwood mechanism with CO and O2 on the cluster, a Langmuir–Hinshelwood mechanism with O2 at the border and CO on the cluster, and a Eley–Rideal mechanism in which O2 is adsorbed on the cluster and CO from gas phase reacts directly with it. The existing calculations suggest that here is no universal mechanism that will hold for all clusters regardless of their size and support. This is not surprising. Small clusters are molecules and we expect them to change their structure and their chemistry as we add or remove one atom. We also expect them to be very sensitive to the nature of the support and react differently when bound to different sites on the support. The experiments and the calculations performed so far support this view. There are, however, some common features. (a) All calculations agree that activating oxygen is the key to CO oxidation. (b) The presence of low coordination sites on the cluster is essential (in our opinion, it is the shape of HOMO that matters, see IV.1). (c) In most cases, the dissociation of oxygen is not the favored oxidation pathway if the clusters are sufficiently large (i.e. more than 10 atoms). For small clusters the dissociation of oxygen and its reaction with the cluster (to make an ‘‘oxide”) are thermodynamically favorable [104,161]. (d) The calculations show [247,248,256,257] that for Au clusters supported on rutile the oxidation reaction takes place with CO on the cluster and O2 adsorbed at the border between oxide and Au. This was first suggested by Haruta [258] who observed that the activity of an oxidation catalyst seems to be proportional to the perimeter of the cluster-oxide interface not with the surface. Other experiments [132,259–261] supported this conjecture. Oxygen does not bind to Au or to a clean, stoichiometric titania, so one may wonder how does it get to the interface? Calculations show that O2 binds to a 5c-Ti atom on the surface when a Au cluster, or hydroxyls, or oxygen vacancies are present on the surface. From there it migrates to the border of the cluster.

5. Summary We can best understand the chemistry of small metal clusters if we stop thinking of them as tiny pieces of metal and accept that they are molecules. Moreover, they are radicals because they have broken bonds. If we view them in this way, it is not surprising that they are reactive, interact strongly with the support, have a large number of isomers, their chemical properties change with the number of atoms they contain, their reactivity is strongly affected by alloying and their activity is sensitive to additives. Very small gold clusters are to bulk gold what the radicals of benzene and naphthalene are to graphite. We are also learning that active oxides, such as titania, have unexpected properties. In particular adsorbing some molecules (which we call modifiers) on the surface can affect very strongly the adsorption energy of other molecules. Theory indicates that this happens through electron transfer from the modifier to the conduction band and then from the conduction band to the adsorbing oxygen molecule. Strictly speaking, the Kohn–Sham orbitals used for this interpretation are not observable quantities. Therefore, one should regard this mechanism as an explanation of why the adsorption energy calculated by DFT increases, rather than a description of reality. To test whether the model describes reality one should perform experiments that show that the modifiers that put electrons in the conduction band increase the conductivity of

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the oxide and that the conductivity does not have an Arrhenius temperature dependence (which would be indicative of electrons in the gap rather than in the conduction band). We are not aware of experiments of this kind. It is heartening to see that the experiments, in which the catalytic activity of molecular clusters is studied under realistic condition, are being performed. We are learning that coarsening can be suppressed, and that molecular clusters can be more active and more selective than the large clusters made from the same material. Ag3 performs the epoxidation of propene while large silver clusters burn it to CO2 and water; a mixture of Pt8 to P10 is more active for oxidative dehydrogenation of propane than large Pt clusters; and Au6-Au10 make propene epoxide while supported on alumina (while titania is required if large clusters are used) and without needing hydrogen. Acknowledgments We are grateful for the support from the National Science Foundation, from the Department of Energy, and from the Air Force Office of Scientific Research DURINT program and through Grant No. FAA 9550-06-1-0167. References [1] Mattsson TR, Mills G, Metiu H. A new method for simulating the late stages of island coarsening in thin film growth: the role of island diffusion and evaporation. J Chem Phys 1999;110:12151–60. [2] Mills G, Mattsson TR, Mollnitz L, Metiu H. Simulations of mobility and evaporation rate of adsorbate islands on solid surfaces. J Chem Phys 1999;111:8639–50. [3] Bozso F, Yates Jr JT, Arias J, Metiu H, Martin RM. A surface Penning ionization study of the CO/Ni(1 1 1) system. J Chem Phys 1983;78:4256–69. [4] Heiz U, Sherwood R, Cox DM, Kaldor A, Yates JT. CO chemisorption on monodispersed platinum clusters on SiO2 – detection of CO chemisorption on single platinum atoms. J Phys Chem 1995;99:8730–5. [5] Heiz U, Vanolli F, Trento L, Schneider WD. Chemical reactivity of size-selected supported clusters: an experimental setup. Rev Sci Instrum 1997;68:1986–94. [6] Heiz U, Vayloyan A, Schumacher E. A new cluster source for the generation of binary metal clusters. Rev Sci Instrum 1997;68:3718–22. [7] Vanolli F, Heiz U, Schneider WD. Vibrational coupling of CO adsorbed on monodispersed Ni11 clusters supported on magnesia. Chem Phys Lett 1997;277:527–31. [8] Vanolli F, Heiz U, Schneider WD. Thermal chemistry of Mn2(CO)10 deposited on MgO thin films. Surf Sci 1997;377:780–5. [9] Heiz U. Size-selected, supported clusters: the interaction of carbon monoxide with nickel clusters. Appl Phys A – Mater Sci Process 1998;67:621–6. [10] Heiz U, Vanolli F, Sanchez A, Schneider WD. Size-dependent molecular dissociation on mass-selected, supported metal clusters. J Am Chem Soc 1998;120:9668–71. [11] Heiz U, Sanchez A, Abbet S, Schneider WD. Catalytic oxidation of carbon monoxide on monodispersed platinum clusters: each atom counts. J Am Chem Soc 1999;121:3214–7. [12] Heiz U, Sanchez A, Abbet S, Schneider WD. The reactivity of gold and platinum metals in their cluster phase. Eur Phys J D 1999;9:35–9. [13] Sanchez A, Abbet S, Heiz U, Schneider WD, Hakkinen H, Barnett RN, et al. When gold is not noble: nanoscale gold catalysts. J Phys Chem A 1999;103:9573–8. [14] Abbet S, Sanchez A, Heiz U, Schneider WD, Ferrari AM, Pacchioni G, et al. Sizeeffects in the acetylene cyclotrimerization on supported size-selected Pdn clusters (1 6 n 6 30). Surf Sci 2000;454:984–9. [15] Abbet S, Sanchez A, Heiz U, Schneider WD, Ferrari AM, Pacchioni G, et al. Acetylene cyclotrimerization on supported size-selected Pdn clusters (1 6 n 6 30): one atom is enough! J Am Chem Soc 2000;122:3453–7. [16] Ferrari AM, Giordano L, Rosch N, Heiz U, Abbet S, Sanchez A, et al. Role of surface defects in the activation of supported metals: a quantum-chemical study of acetylene cyclotrimerization on Pd1/MgO. J Phys Chem B 2000;104:10612–7. [17] Heiz U, Sanchez A, Abbet S, Schneider WD. Tuning oxidation of carbon monoxide using nanoassembled model catalysts. Chem Phys 2000;262:189–200. [18] Heiz U, Schneider WD. Nanoassembled model catalysts. J Phys D – Appl Phys 2000;33:R85–R102. [19] Abbet S, Heiz U, Ferrari AM, Giordano L, Di Valentin C, Pacchioni G. Nanoassembled Pd catalysts on MgO thin films. Thin Solid Films 2001;400:37–42. [20] Abbet S, Heiz U, Hakkinen H, Landman U. CO oxidation on a single Pd atom supported on magnesia. Phys Rev Lett 2001;86:5950–3.

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