DOI: 10.1002/chem.201304586

Concept

& Model Catalysis

Platinum Group Metal Clusters: From Gas-Phase Structures and Reactivities towards Model Catalysts Dan J. Harding[b] and Andr Fielicke*[a] Dedicated to Dr. David M. Rayner on the occasion of his 65th birthday

Chem. Eur. J. 2014, 20, 3258 – 3267

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Concept Abstract: Transition-metal clusters have long been proposed as model systems to study heterogeneous catalysts. In this Concept article we show how advanced spectroscopic techniques can be used to determine the structures of gas-phase transition-metal clusters and their complexes with small molecules. Combined with computational studies, this can help to develop an understanding of the reactivity of these catalytic models.

Introduction Late transition metals (TM), and the members of the platinum group in particular (Ru, Rh, Pd (4d); Os, Ir, Pt (5d)), are widely used in heterogeneous catalysis for a range of economically and environmentally important processes. Examples for largescale catalytic applications of platinum group metals include car-exhaust treatment, the Ostwald process, that is, the oxidation of ammonia to nitric oxide, or the reforming of hydrocarbons into high-octane fuels.[1] Real catalysts are often very complex systems, made up of multiple components and phases, covering a wide range of scales, from nanometer-sized metal particles to macroscopic support structures. Consequently, separating the influences of the different parts is challenging, and has led to a broad range of attempts to find simplified model systems, which, nevertheless, retain the relevant features of the real catalysts. ‘Classical’ surface science is the largest and best known of these approaches and has provided detailed information about, and understanding of, the atomic scale processes occurring during reactions on transition-metal surfaces, leading to the elucidation of detailed reaction mechanisms.[2] The use of well characterized metal surfaces has even allowed the quantum state dependence of gas–surface reactions to be investigated, for example probing the effects of vibrational excitation and molecular orientation on the reaction probabilities of methane on platinum surfaces.[3] A recent development is the investigation of supported nanosized clusters[4] and the size dependence of their (catalytic) properties, which can be achieved by either size selecting clusters prior to deposition on well characterized surfaces[5–7] or by investigating individual clusters.[8] [a] A. Fielicke Institut fr Optik und Atomare Physik Technische Universitt Berlin, Hardenbergstr. 36 10623 Berlin (Germany) E-mail: [email protected] [b] D. J. Harding Institut fr Physikalische Chemie Georg-August Universitt, Tammannstr. 6 37077 Gçttingen (Germany) and Department of Dynamics at Surfaces Max-Planck-Institut fr biophysikalische Chemie Am Faßberg 11, 37077 Gçttingen (Germany) Chem. Eur. J. 2014, 20, 3258 – 3267

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Computational studies can provide atomic-scale insights into the processes occurring during surface reactions[9] but platinum group metals have proven challenging to treat theoretically. There are several causes of these difficulties, including strong correlation of the d-electrons, the possibility of multiple, competing, low-energy open-shell electronic states and the effects of relativity on the electronic structure. Even the seemingly simple task of predicting the correct adsorption site for CO, that is, top, bridge or hollow, on Pt(111) is beyond some of the more common density functional theory (DFT) methods.[10] Since their discovery, atomic clusters have generated significant interest both because of their unusual intrinsic properties and through a desire to use them as simple model systems to provide deeper insight into more complex systems, particularly heterogeneous catalysts.[11] Gas-phase studies on isolated clusters provide a means to work with well-defined and, in principle, well characterizable systems. By combining molecular beams, mass spectrometry, and/or different spectroscopic techniques detailed information about reaction pathways can be obtained as a function of cluster size, composition and charge state. The relatively small size of the systems also makes them attractive to treat theoretically, although they present many of the challenges associated with the bulk metals. Gas-phase cluster studies can complement both traditional surface science approaches and the investigations of supported clusters on surfaces,[5, 6, 12, 13] helping to separate intrinsic cluster properties and the effects induced by their interaction with the support. Recently, Lang and Bernhardt have described in detail many of the justifications for using cluster models and the differences between these and real catalyst systems.[14] Given this interest in catalysis, gas-phase cluster reactivity studies have considered a number of catalytically relevant reactions on clusters including dehydrogenation of hydrocarbons,[16] reduction of NOx,[17–20] or dehydrogenation and formation of NH3.[21] It is worth noting that there are two distinct regimes under which reactivity experiments can be carried out, which may lead to different products or kinetics. At low pressure, under single collision conditions, the total energy of the reacting cluster-molecule complex is, apart from radiative processes, constant. The release of the initial adsorption energy can lead to substantial heating of the complex, allowing reactions with significant barriers to occur. Reactions can also be performed under thermalized conditions at higher pressure, here collisions with a buffer gas are used to maintain the cluster temperature at a fixed value. Under these conditions, measurements of equilibrium constants as a function of temperature can be used to estimate the binding energies of adsorbates on the clusters.[22] There has also been considerable interest in the possibility of multi-step catalytic cycles on clusters, and reactions such as the oxidation of CO to CO2 on platinum,[15, 23] rhodium,[24] and palladium[25] clusters have been demonstrated. Figure 1 shows part of the cycle determined by Balaj et al. for CO oxidation on Ptn + . Under single collision conditions the long-lived intermediate species include a range of PtnCOm + and PtnOm + complexes. These studies are all based on purely mass spectromet-

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Concept

Figure 1. Part of the catalytic cycle of CO oxidation on Pt7 + proposed by Balaj et al., adapted from ref. [15].

ric techniques, which means that while they provide information about the composition of the clusters and their complexes at various stages of the reaction they are generally not able to provide information about their structures. Many of the properties of small transition clusters have been found to vary widely as a function of their size. These range from the relatively straightforwardly understood odd-even variation in the ionization energies and electron affinities of gold clusters[26] due to electronic shell closings to much more complex examples, which remain poorly understood, such as the variation in the magnetic moments of late TM clusters.[27] Large variations have been observed in the reaction rates of clusters with small molecules, in several systems these have been found to vary by several orders of magnitude for the addition of a single atom.[28–30] Some systems even have biexponential reaction kinetics which suggests the presence of two different forms, isomers or electronic states, of a given cluster size with very different reactivity[19, 28, 29, 31] This, in turn, suggests that part of the size-dependent variation in reactivity is due to variations in the geometric and/or electronic structures of the clusters as a function of size. This idea is reinforced by the similarities in size-dependence of reaction rates with different reactants, for example, Rhn + reacting with CH4 or N2O[19, 31] have local reactivity minima for n = 19, 28 while in both cases Rh12 + exhibits biexponential kinetics. Figure 2 shows examples of the size-dependent rates of reaction for two of the systems we will discuss in more detail, Ptn  + CH4 and Rhn  + N2O, demonstrating the significant size- and structure-dependence. For some systems, in particular the chemisorption of H2 on Fe, Co, and Ni clusters, the reactivity appears to be strongly correlated with the electronic structure, as measured by the ionization energies (IE) or electron affinities (EA).[29, 32, 33] At present, similar IE or EA data is not available for other late TMs. In addition to the information that can be extracted directly from reactivity studies, they are also important to screen systems where spectroscopic information may help to determine the underlying causes of the differences in reactivity, for example, large changes in reactivity for adjacent cluster sizes or biexponential kinetics. Here, we will concentrate on the spectroscopic methods that have been developed to investigate clusters and their extension to reactive intermediate species. Though the focus of the article is on Pt-group clusters we will also discuss some examples where experiments have been performed on other types of cluster, particularly early transitionmetal clusters, as many of the methods may usefully be applied to the Pt-group clusters.

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Figure 2. Absolute rate constants for the reactions of Ptn  + CH4 and Rhn  + N2O. Data are taken from Achatz et al.[30] and Harding et al.[19] Two points are shown for clusters sizes with biexponential kinetics.

Structural Probes One of the first steps towards developing a detailed, molecular, understanding of the properties of clusters is the determination of their structures. In the last ten years significant advances have been made in this direction using several methods, each with their own strengths. Trapped ion electron diffraction provides essentially direct structural information[34] but works best for strong electron scatterers, that is, large clusters and heavy nuclei. Combined with DFT calculations the structures of a range of Cu, Ag, and Au clusters[35] have been determined. In a comprehensive study of 55 atom clusters of most of the 3d and 4d transition metals strong correlations between cluster and bulk structures have been demonstrated.[36] Anion photoelectron (PE) spectroscopy can provide detailed information about the electronic structures of the anion and neutral states and, in some cases, the vibrational modes in the cluster,[37] but experimental interpretation is made more challenging by the need to have information about both the anion and the neutral and, possibly, multiple excited electronic states. The geometric structures of many gold[38] and copper[39] cluster anions have been determined by this combination of PE spectroscopy and electronic structure calculations. Although PE spectra have been measured for many late TM clusters[40–42] no direct comparisons were made with theory. Given the complexity of the electronic structures of Pt-group metals, it is not clear that the theoretical methods available when these experiments were performed could have provided useful

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Concept information, but revisiting these systems with beyond-DFT methods may prove beneficial. Very recently, PE spectra of small Rh cluster anions have been reported together with DFT calculations[43] which suggest that the structures are similar to those of the cationic clusters (discussed in the next section). Vibrational spectra, measured using several different techniques, can be used to probe the structure of charged or neutral clusters. Asmis has recently described some of the possible variations which have been applied to metal oxide clusters.[44] In many cases, the frequencies of observed vibrational bands can provide direct information about the chemical species present in a cluster. Determination of the detailed cluster structure almost always requires comparison with theoretical predictions but, generally, only the electronic ground states of the different possible isomers need to be known. The first infrared (IR) spectra of gas-phase metal cluster complexes[45] were measured using infrared multiple photon dissociation (IR-MPD). In IR-MPD the sequential absorption of multiple photons is used to raise the internal energy of the cluster/ molecule to a point where rapid fragmentation occurs. This fragmentation can be observed using mass spectrometric methods, and allows IR-MPD to be a sensitive method even for systems with low number density. These early spectra were measured with line tunable CO2 lasers, as these were the only IR sources capable of providing sufficient power to drive the multiple photon processes. However, CO2 lasers have a rather limited tuning range around 1000 cm 1. The application of IR free electron lasers significantly increased the range over which IR-MPD could be used to cover the full far-IR and mid-IR range[46] while new table top IR laser sources and cryogenic ion traps have allowed single photon infrared predissociation (IR-PD) spectra of weakly bound complexes, tagged with H2 or He, to be measured down to below 1000 cm 1.[47] Both PE and IR spectroscopy have been used to investigate the binding of small molecules on clusters, which can provide direct information about reaction pathways and constraints for theory. Alongside the experimental work there have also been a large number of theoretical investigations of the structures of clusters and their interactions and reactions with small molecules. Platinum group metals have been of particular interest but have proved to be particularly challenging for theory. Examples include the conflicting reports of planar and three dimensional structures of small platinum clusters[48] and the sensitivity of the favored low-energy structures of ruthenium[49] and rhodium[50–52] clusters to the choice of functional in DFT calculations. In the first case there is no obvious dependence on the choice of method while in the second it seems clear that pure functionals favor different structures to hybrid functionals, which include a portion of exact exchange. The combination of experiment and theory in all three approaches therefore provides a powerful means to test the theoretical methods used to describe the clusters.

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Far-IR Spectroscopy of Metal Clusters The determination of the geometric structure of gas-phase metal clusters by vibrational spectroscopy faces several challenges, including the low number density and size distribution of clusters in a molecular beam and the low frequencies and low oscillator strengths of their vibrational transitions. For charged clusters, some of these problems can be overcome by mass selecting and trapping the clusters, in an ion trap or in a rare gas matrix, allowing an increase in the number density and the interaction time. Action spectroscopy can provide a sensitive alternative to direct absorption spectroscopy for complex, dilute samples. Here, instead of measuring the effect of the sample on the light, the effect of the light on the sample is measured. The most widely used form of action spectroscopy so far applied to metal clusters is IR-MPD.[53] The high binding energy of transition-metal clusters and the low frequencies at which they have IR active modes means that, in most cases, the messenger technique[54] must be used. Here, the dissociation of a weakly bound complex of the cluster with a rare gas atom is used to monitor the absorption of IR light. Other variants of action spectroscopy are also possible; for anions, resonance enhanced multiple photon electron detachment (IRREMPED)[55] and for neutrals resonance enhanced multiple photon ionization (IR-REMPI)[56] or infrared-ultraviolet two color ionization (IR/UV-2CI).[57] In all three techniques, the charge of the clusters is changed following absorption of IR light. The vibrational fundamentals of metal clusters are typically found below 400 cm 1, that is, in the far-IR, where free electron lasers are the only intense and tunable laser sources. Using IR-MPD of messenger complexes, several of the late transition metals have been investigated, including Co, Rh, Pt, and they illustrate many of the challenges posed by late TM

Figure 3. Experimental set-up for the formation of metal cluster complexes and their IR spectroscopy. Clusters are formed by laser ablation and quenching of the atomic plasma with a short rare gas pulse. Reactants are added to the reactor directly attached to the source via a pulsed valve. The temperature of the reactor can be controlled from 80 K to about 400 K. Alternatively, ligand exchange can be performed in the crossed beam arrangement starting with rare gas (Ar) complexes that are initially formed in the cold cluster source. The mass distribution of the clusters (complexes) is then analyzed using time-of-flight mass spectrometry. Changes induced in the distribution by irradiation with the intense and monochromatic IR laser beam can be analyzed as a function of IR wavelength to obtain the size-specific IR spectra.

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Concept clusters. The small cobalt clusters have relatively feature rich experimental IR spectra but these spectra are highly dependent on the number of argon messenger atoms.[58] Additionally, the Co clusters themselves appear to present a real challenge to DFT, either to determine the correct energetic ordering of the isomers or to calculate the force constants, demonstrated by the rather poor agreement between the calculated and experimental spectra. The comparison suggest the clusters favor polytetrahedral structures. Rhodium clusters present a slightly different challenge, as the choice of density functional leads to the prediction of qualitatively different low-energy structures (see frontispiece, top part, for possible structures of eight-atom clusters).[50–52, 59–63] A number of theoretical investigations, all using pure density functionals, had predicted relatively open isomers with cubic geometric motifs for rhodium clusters. Comparison of calculated spectra for these isomers provided a poor match to experimental IR-MPD spectra. Calculations using a hybrid functional, which includes a portion of Hartree–Fock exact exchange, favor polytetrahedral isomers[50–52] and these provide a much better match to the experiment,[51] particularly in the case of Rh8 + shown in Figure 4. Similar predictions of cubic isomers have also been made for other Pt-group metals including ruthenium, iridium, and osmium.[60, 64] At this point, it appears that magnetic clusters, for example, of Co or Rh, present a particular challenge for DFT, both in determining the correct low-energy isomers and in describing the bonding in the cluster, exemplified by the relatively poor agreement between calculated and experimental spectra when compared to early TM clusters such as Nb or Ta.[65, 66] There have been a relatively large number of theoretical investigations of the structures of small platinum clusters,[48] roughly half have predicted that small clusters should have planar structures with the other reports finding that three dimensional isomers are favored. The high atomic number of Pt means that relativistic effects may become important and lead to changes in the properties of the clusters. Spin-orbit coupling (SOC) is one of the mechanisms by which relativistic effects can manifest themselves and it has been suggested that SOC may lead to changes in the isomers favored by Pt clusters. However, different groups have reported that the inclusion of SOC leads to the stabilization of both planar and 3D isomers. Comparison of IR-MPD measurements and calculations found good agreement for tetrahedral and trigonal bipyramidal isomers for Pt4 + (shown in Figure 4) and Pt5 + respectively, showing that, at least for the cations, 3D structures are favored at the smallest possible sizes.[67] The predictions of planar structures appears to be due the small energy gaps between the different isomers, and planar structures can be stabilized by adsorbates, vide infra. Somewhat surprisingly, these three systems, as well as the early TMs V, Nb, Ta,[65, 66, 68] all appear to favor similar geometric motifs, that is, polytetrahedral packing, shown in Figure 5. Consequently the local geometry around individual atoms does not change dramatically as a function of cluster size.

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Figure 4. Examples of IR-MPD spectra of transition-metal clusters measured using the messenger atom technique with Ar. a) Comparison of the experimental spectrum of Rh8 + with calculated spectra of the lowest energy isomers at the PBE and PBE0 level. b) Comparison of the spectra calculated for different isomers of Pt4Ar4 + with experiment.

Figure 5. Favored geometric motifs identified for four- to eight-atom clusters of several transition metals by the combination of IR-MPD spectroscopy and DFT calculations.

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Concept Stable Adsorbates Spectroscopic investigation of the products or stable intermediates formed in the gas-phase reactions of clusters allows the types of species formed to be precisely determined. In many reactivity experiments metal clusters with adsorbed light atoms, that is, H, C, N, or O, are, or are thought to be, the products of dissociative adsorption of small molecules, making them important both as intermediates in catalytic cycles and in determining the overall thermodynamics of cluster reactions. Ambiguity can arise because mass spectrometry is only sensitive to the mass-to-charge ratio of a complex and not to its structure, making it difficult to determine the state of molecules like carbon monoxide, CO, or nitric oxide, NO, where molecular or dissociative adsorption is possible. In some cases, the products of consecutive reactions[17] or the fragmentation of the metal cluster[18] can help to determine the adsorption state but spectroscopic methods can provide much more detailed information. One of the systems that has been studied in most detail is the adsorption of carbon monoxide on TM clusters.[41, 69] The binding of CO, via a s-donation/p-back donation mechanism, makes the C O bond strength and frequency extremely sensitive to the degree of electron donation into the CO p* orbital. The C-O stretch frequency therefore provides direct information about the electron density at the binding site and the binding mode of the molecule, that is, atop, bridge, or hollow, while the absence of a CO stretch is evidence for dissociative adsorption. CO adsorption on cationic, neutral and anionic rhodium clusters has been investigated and, in common with the other late TM, CO is adsorbed molecularly, favoring atop sites on most cluster sizes similar to the bulk metal.[70] The frequency shifts as a function of cluster charge and size are consistent with the charge delocalization that is expected for a conducting sphere.[71] The CO stretch can also act as a probe for co-adsorbate induced changes in electronic structure of the cluster.[72] These studies provide a direct link to surface science, where measurements of the CO stretch frequency have been used as a probe for the properties of surface sites.[73] Other late TM clusters have also been found to have similar behavior to their bulk counterparts with CO, with 3d-metals favoring atop sites, the 4d-metals having a mix of binding sites including atop, bridge and hollow, and the 5d-metals return to atop binding due to relativistic effects on their bonding.[74] The activation of N2, which binds to metals in the same way as CO, can also be probed by its vibrational properties. For example, it has been concluded that N2 is molecularly adsorbed on small anionic W clusters, due to the observation of vibrational structure in their PE spectra.[75] N2 activation is the ratelimiting step in ammonia synthesis and surface studies have shown that N2 dissociation on Ru is a highly structure-sensitive process. IR-MPD studies have been performed for N2 bound to neutral Ru clusters containing 5 to 16 Ru atoms.[76] The observed N N stretch frequencies, in the range of 2110 to 2201 cm 1, suggest the presence of s-bonded chemisorbed N2 oriented perpendicularly to the cluster surface. Interestingly, for the complexes of Ru8 and Ru9 larger deviations from the Chem. Eur. J. 2014, 20, 3258 – 3267

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trends in n(N-N) are observed, similar to the case for CO adsorbates.[77] This may be due to special geometric or electronic structures and unusual cubic structures have been predicted for these sizes.[78] For both the CO and N2 complexes, the molecularly bound ligand itself is used as the messenger in the IR photodissociation spectroscopy and the molecule must therefore be intact and relatively weakly bound. High resolution electronic spectroscopy has been performed on small carbide and oxide complexes[79] providing detailed information about both their geometric structures and electronic states, but it appears that larger clusters do not have the longlived Rydberg states needed as intermediates for these techniques. The complementary IR dissociation spectroscopy of such species is only possible when using rare gas atom messengers, as in the case of the far-IR spectroscopy of bare metal clusters. The studies on TanO + clusters[66] and comparison to the bare Tan + demonstrate the potential of such experiments. The binding motif of the oxygen atoms has been determined and found to differ from those on extended surfaces and, by extending measurements into the far-IR to probe the metalmetal bonds, the distortions induced in the underlying cluster by the oxygen atoms can also be investigated. In TanO + these changes are rather small, but much more significant structural changes have been found for small platinum cluster complexes with O or C,[80, 81] PtnO2,4 + or PtnC + . The oxide complexes are important intermediates for catalytic oxidation of CO or H2.[82] Molecular oxygen dissociates on platinum clusters and the resulting O atoms prefer bridge sites on the edges of the clusters. It is notable that platinum cluster cations are able to dissociate O2 while gold clusters are able to activate O2 and form superoxo species,[83] which may help to explain the differences in the oxidation catalysis of the clusters. Carbon atoms, coming from the complete dehydrogenation of methane, drive the formation of a distorted triangular structure with C in the centre.[81] In both cases, planar structures are stabilized. Figure 6 shows examples of the structures of the bare clusters and their complexes for Pt3 + and Pt3C + , Pt5 + and Pt5O4 + , and Ta6 + and Ta6O + , demonstrating the greater effect the adatom has on the structure of platinum clusters.

Figure 6. Examples of the effects of adatom complex formation; Pt3 + and Pt3C + , Pt5 + and Pt5O4 + , and Ta6 + and Ta6O + . The structures of platinum clusters are strongly affected by the presence of O or C, while tantalum clusters are only slightly distorted.

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Concept Nb6C has also been investigated,[55] one of very few anionic TM cluster systems which have been characterized using IR spectroscopy, here the C atom prefers a 4-fold hollow site.

Reactive Intermediates While the bare clusters are the starting point for reactions, stable adsorbate complexes can be either end points or intermediates for reactions occurring following further activation and/or a second collision with another gas-phase molecule, but they do not provide information about the activated complex itself. Calculating reaction pathways and barriers is one of the most important tasks for theory to help determine the underlying causes of the wide variations in cluster reactivity but it is also one of the most challenging. The difficulties stem from two distinct causes. First, the need for extremely accurate calculation of the relative energies of the stationary points along the reaction path, in turn requiring accurate treatment of electron correlation, and second, identification of the important low-energy path(s) in system with large numbers of degrees of freedom. Experimental methods which can probe the intermediates along (multi-step) reaction paths provide an important means to test theory. While the structures of transition states are still inaccessible, details about the local potential energy surfaces around the important activated species can be obtained. The experimental challenge is to form these metastable complexes so that they can be investigated. For some systems, such species can be stabilized when the reaction occurs under thermalized rather than single collisions conditions. This can be straightforwardly achieved by adding the reactant into the thermalization channel of the cluster source, where the total pressure is a few tens of mbar (mainly He), see Figure 3. The reaction temperature can be controlled and varied by cooling or heating (parts of) the cluster source. This, for instance, allows RhnN2O + complexes to be formed[84, 85] while under single collision conditions RhnO + is the only product observed in the reaction of Rhn + with N2O. Some systems appear to be too reactive to be prepared in this way, an example are PtnCH4 + complexes, where adding very small amounts of CH4 in the reaction channel leads to very efficient formation of PtnC + species. An alternative is to use a ligand exchange reaction between PtnArm + and CH4 in a crossed molecular beams arrangement (Figure 3). This gives products of the type PtnCH4Ar1,2 + , where desorption of the Ar efficiently carries away the heat of CH4 adsorption. Spectroscopic investigation using IR-MPD can then provide details about the structure of the cluster complex, including the state of the adsorbed molecule, information about its binding site on the cluster, and the degree of bond activation. Figure 7 shows example spectra for PtnCH4Arm + complexes.[86] Comparison with calculated spectra shows methane to be molecularly adsorbed under these conditions, in a bidentate configuration different to that observed on bulk platinum surfaces. The dissociation of the complex, the finger print of IR absorption, can provide more information than the IR spectrum Chem. Eur. J. 2014, 20, 3258 – 3267

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Figure 7. Experimental and calculated (DFT: TPSS/def2-TZVP) IR spectra of complexes of molecular CH4 with cationic platinum clusters.

alone. By monitoring the intensity growth for potential product ion masses, changes in the reaction pathways can be observed as a function of cluster size and composition. In that way one finds for the PtnCH4Arm + complex with n = 4 loss of the intact CH4 molecule is the dominant reaction channel upon IR excitation whereas for n = 5 dehydrogenation is favored, shown by the growth in the Pt5CH2 + channel. Similar experiments on RhnO0,1N2O + complexes found N2O to bind via the N-end and that on many cluster sizes N2O could be efficiently dissociated by pumping the N2O ligand or the Rh-O modes,[84] leading to the loss of N2 from the complex. The Rh5N2O + complex was found to be an exception, where loss of the intact N2O molecule was the favored dissociation channel. The addition of an oxygen atom to the cluster leads to a change in the dissociation channel, with loss of N2 again being favored for Rh5ON2O + .[85] For both the rhodium and platinum clusters the changes in dissociation channels correlate well with the reactivity observed for these system under single collision conditions, where Pt4 + and Rh5 + were found to be particularly unreactive (cf. Figure 2). The changes in the dissociation channels provide information about the relative heights of the free energy barriers for the different pathways as a function of size, and suggest that even small changes in either the adsorption energy or the reaction barrier height can lead to changes in the favored reaction pathway and, therefore, large changes in the rate of reaction. DFT calculations suggest the root of the changes in the Rh5O0,1 + are an increase in the binding energy of N2O to the oxide cluster rather than a significant change in the N-O dissociation barrier. For the Ptn + + CH4 reactions, independent DFT calculations had predicted that the desorption

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Concept energy from Pt4 + should be lower than the barrier height to dehydrogenation[87] while on Pt3 + the inverse was predicted, see Figure 8.[88] The qualitatively good agreement between the

Figure 8. Schematic potential energy curves for the desorption vs. dehydrogenation processes in PtnCH4 + . IR excitation induces dehydrogenation in the cases of Pt3 + and Pt5 + (lower curve), while for Pt4 + the main reaction channel is loss of intact CH4 (upper curve).

predicted barriers and the observed dissociation channels then gives some confidence that here the DFT calculations capture the most important physics in these systems and can help to explain the properties of a wider range of sizes and other similar reactions. Experiments using IR excitation to induce reactions need not be limited to the unimolecular dissociation of adsorbed molecules. Complexes with two different adsorbed molecules can also be formed, either by using an appropriate reaction gas mixture,[89] where the partial pressure of the different gasses are matched to their relative sticking probabilities on the clusters or by using separate inlets, which gives more flexibility to individually tune the partial pressures. Recently, the formation and desorption of CO2 from PtnO2CO + complexes has been observed.[90] These O2/CO co-adsorbates are so far unobserved intermediates in the catalytic CO oxidation cycle, but, by forming them under thermalized conditions, they can be stabilized and their structures determined. From the IR spectrum it is clear that they initially contain CO moieties and bridge-bound O atoms, though the Pt core is significantly distorted compared to the pure Pt clusters. Following IR pumping of the CO stretch, depletion of the PtnO2CO + complex was observed along with corresponding growth in the monoxide complex, a signature for the formation and desorption of CO2.

Conclusion We have shown here how the use of a combination of vibrational spectroscopy, mass spectrometry, and electronic structure theory can provide detailed information about the structures of late transition-metal clusters and their interaction with small molecules. These techniques are applicable for a wide range of cluster complexes and open up new possibilities to explain the wide range of size-dependent reactivities which had been previously observed for these systems. In many cases the smallest clusters, containing only a few atoms, already show many similarities in their chemical properChem. Eur. J. 2014, 20, 3258 – 3267

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ties to much larger metal particles or even bulk systems. This is particularly surprising given that the binding of atoms and molecules on small clusters is often different to that observed on larger systems. The similar geometric motifs found for different sized clusters suggest that the geometry per se is not the most important factor in affecting their reactivity, but that it is determined by their electronic structure, that is, HOMO– LUMO gap, IE, EA, and so forth. The ultimate goal would, of course, be not only explaining the observed reactivities but using the mechanistic insights obtained for the cluster systems to design real catalysts with tailored properties. Gas-phase transition metal clusters offer a wide variability in their structural and electronic properties and the possibility to tune them by varying their charge state and by accurately controlling the size and composition atom by atom.

Acknowledgements We gratefully acknowledge the support of the Stichting voor Fundamental Onderzoek der Materie (FOM) for providing FELIX beamtime and thank the FELIX staff, particularly Dr. A.F.G (Lex) van der Meer and Dr. Britta Redlich, for their skilful assistance. We thank Gerard Meijer for his continued support. This work is supported by the Max Planck Society, the Cluster of Excellence “Unifying Concepts in Catalysis” coordinated by the Technical University Berlin and funded by the Deutsche Forschungsgemeinschaft (DFG) and through the DFG research grant FI 893/ 3. DJH thanks the Alexander von Humboldt Foundation for a fellowship. Keywords: clusters · gas-phase chemistry · heterogeneous catalysis · mass spectrometry · reactivity [1] G. A. Somorjai, Y. Li, Introduction to surface science and catalysis, 2nd ed., Wiley, 2010. [2] G. Ertl, Angew. Chem. 2008, 120, 3578 – 3590; Angew. Chem. Int. Ed. 2008, 47, 3524 – 3535. [3] a) L. Juurlink, D. Killelea, A. Utz, Prog. Surf. Sci. 2009, 84, 69 – 134; b) B. L. Yoder, R. Bisson, R. D. Beck, Science 2010, 329, 553 – 556. [4] H.-J. Freund, Surf. Sci. 2002, 500, 271 – 299. [5] B. Yoon, H. Hkkinen, U. Landman, A. S. Wçtz, J. Antonietti, S. Abbet, K. Judai, U. Heiz, Science 2005, 307, 403 – 407. [6] W. E. Kaden, T. Wu, W. A. Kunkel, S. L. Anderson, Science 2009, 326, 826 – 829. [7] S. Bonanni, K. Aı¨t-Mansour, W. Harbich, H. Brune, J. Am. Chem. Soc. 2012, 134, 3445 – 3450. [8] X. Lin, B. Yang, H.-M. Benia, P. Myrach, M. Yulikov, A. Aumer, M. A. Brown, M. Sterrer, O. Bondarchuk, E. Kieseritzky, J. Rocker, T. Risse, H.-J. Gao, N. Nilius, H.-J. Freund, J. Am. Chem. Soc. 2010, 132, 7745 – 7749. [9] a) G.-J. Kroes, Phys. Chem. Chem. Phys. 2012, 14, 14966 – 14981; b) J. K. Nørskov, F. Abild-Pedersen, F. Studt, T. Bligaard, Proc. Natl. Acad. Sci. USA 2011, 108, 937 – 943. [10] P. J. Feibelman, B. Hammer, J. K. Nørskov, F. Wagner, M. Scheffler, R. Stumpf, R. Watwe, J. Dumesic, J. Phys. Chem. B 2001, 105, 4018 – 4025. [11] a) E. L. Muetterties, Science 1977, 196, 839 – 848; b) T. G. Dietz, M. A. Duncan, D. E. Powers, R. E. Smalley, J. Chem. Phys. 1981, 74, 6511 – 6512. [12] T. Schalow, B. Brandt, D. E. Starr, M. Laurin, S. K. Shaikhutdinov, S. Schauermann, J. Libuda, H.-J. Freund, Angew. Chem. 2006, 118, 3775 – 3780; Angew. Chem. Int. Ed. 2006, 45, 3693 – 3697. [13] H.-J. Freund, Chem. Eur. J. 2010, 16, 9384 – 9397. [14] S. M. Lang, T. M. Bernhardt, Phys. Chem. Chem. Phys. 2012, 14, 9255 – 9269.

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