Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping

Aug 29, 2018 - Piero Ferrari, Jan Vanbuel, Ewald Janssens, and Peter Lievens*. Laboratory of Solid State Physics and Magnetism, Department of Physics ...
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Tuning the Reactivity of Small Metal Clusters by Heteroatom Doping Piero Ferrari, Jan Vanbuel, Ewald Janssens, and Peter Lievens*

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Laboratory of Solid State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200d, Box 2414, 3001 Leuven, Belgium CONSPECTUS: The reactivity of small metallic clusters, nanoparticles composed of a countable number of atoms (typically up to ∼100 atoms), has attracted much attention due to the fascinating properties these objects possess toward a variety of molecules. Cluster reactivity often is significantly different from the homologous bulk, with gold as prototypical example. Bulk gold is the noblest of all metals, whereas small gold clusters react with carbon monoxide, molecular oxygen, and hydrocarbons, among others. Furthermore, cluster reactivity is strongly size and composition dependent, allowing a wide range of tuning possibilities. The study of cluster reactivity usually follows two routes of investigation. In the first, research aims for fundamental understanding of mechanisms, mainly driven by curiosity. One consequence of the inherent small size of a cluster is that atoms can arrange themselves very differently from the crystallographic structure of the homologous bulk. In addition, quantum confinement effects dominate the electronic structure of a cluster with atom-like electronic shells instead of the electronic bands in bulk. These features result in a very rich and size-dependent interaction of a cluster with small molecules, governed by a fine interplay between the geometry and the electronic structure of the system. An alternative research approach uses the investigation of chemical reactions of isolated small clusters in the gas phase as model systems for the reactions taking place in more complex systems. This offers several advantages compared to more conventional methods and techniques used to study such complex systems. First, clusters can be produced under welldefined conditions, with control over size, composition, and charge state. Second, clusters in the gas phase solely interact with the molecule(s) chosen by the researcher, since contaminations are limited by the high vacuum conditions of the experiments. Third, due to the small number of atoms involved, detailed quantum chemical calculations can be performed on the systems under investigation. Thus, even though gas phase clusters differ significantly in size and in environmental conditions from those encountered, for example, in industrial catalysis, they can be used to unravel the complicated nature of a metal−molecule chemical bonding process. In this Account, both routes of investigation are discussed. The nature of the interaction between small gas phase clusters with diverse molecules is described, stressing the broader relevance of these studies. Particular emphasis is given to the effect of heteroatom doping. By adding a different element to a cluster, its geometric and electronic structure is modified, thereby altering its reactivity. Specifically, the effect of varying size and composition of doped gold, platinum, and aluminum clusters on their reactivity toward diverse molecules, relevant for catalytic applications, is discussed. Most studies presented here combine experiments based on mass spectrometric techniques with density functional theory calculations, allowing a deep understanding of the reaction mechanisms at a molecular level.

1. INTRODUCTION The size reduction of matter down to the nanometer scale leads to the emergence of a wide variety of new phenomena, making small metallic clusters, systems composed of a countable number of metal atoms, very attractive objects, in particular, for their reactivity properties. Early studies on small monometallic and heteroatom doped clusters in the gas phase identified the possibility of tuning reactivity by changing particle size and composition. For example, Whetten et al. showed that small iron clusters are reactive toward O2 but unreactive with methane.1 Kaya and co-workers investigated the adsorption of H2 on bimetallic ConVm+ clusters, showing a composition dependent cluster−H2 interaction.2 Castleman and collaborators studied the reactivity of single niobium- and vanadium-doped aluminum clusters toward O2, proving that specific cluster species are unreactive.3 Other studies have © XXXX American Chemical Society

shown that elements inert in the bulk, such as gold, become reactive at the nanoscale,4,5 suggesting the possibility of using clusters as catalysts, with size-dependent and reaction specific properties.6,7 Besides, the modification of composition provides another dimension of tuning. Adding a dopant atom to a monometallic cluster can significantly alter its geometry and electronic structure by changing the number of delocalized valence electrons and inducing electron charge transfers, if elements with a different electronegativity are selected.8 During recent years, cluster research has followed two routes of investigation. In the first, small clusters are investigated from a fundamental point of view, aiming to achieve enhanced Received: August 29, 2018

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Accounts of Chemical Research 2.4. Reaction Cell

understanding on how material properties emerge from the atom up to larger scales, with emphasis on the size dependence of this development.9 In the second, small clusters are used as model systems to understand chemical processes taking place in larger and more complex systems.10 The simplicity of a very small group of atoms is exploited in this process, allowing the possibility of combining size-selective mass spectrometric experiments with quantum chemical calculations.11 Extensive discussions on the reactivity of mono- and bimetallic gas phase clusters studied with both approaches were presented in recent reviews.12−14 In this Account, we discuss investigations aimed at understanding the effect of doping on physical and chemical properties, and in particular on their reactivity, for a selected set of metallic clusters: doped platinum clusters, silver- and palladium-doped gold clusters, and transition metal doped aluminum clusters.

One of the most easily implementable methods to study gas phase reactivity is a reaction cell, which is placed directly in the cluster flight path but, in contrast to the flow tube reactor approach, after the supersonic expansion has taken place.21 By variation of the low pressure of the reagent in the reaction cell, the clusters undergo a limited number of collisions with the reagent gas. Because of the limited number of collisions in a reaction cell, the products do not thermalize and dissociation competes with the forward reaction rate, allowing for quantification of cluster−molecule reaction rates.22 Similar to the flow tube reactor, one of the main advantages of reaction cells is that they can also be used to study the reactivity of neutral clusters.23 A drawback of this approach is the lack of thermal stabilization, hampering the study of a sequence of multiple reaction steps. 2.5. Collision Induced Dissociation Experiments

2. MASS SPECTROMETRIC METHODS Gas phase clusters can be produced by a variety of techniques, such as laser ablation, magnetron sputtering, arc discharge, pick-up, etc.15 Most sources produce a distribution of cluster sizes, necessitating the use of mass spectrometric techniques to study individual clusters. This section contains an overview of possible ways of introducing reactants to a cluster beam, which, in combination with mass spectrometric methods, is used in the study of cluster reactivity in the gas phase.

Collision induced dissociation (CID) probes the kinetic energy dependence of a certain ion−molecule reaction. After formation, the clusters are decelerated to a specific kinetic energy and guided into a reaction cell. The reactant and product intensities are then measured as a function of the ion’s kinetic energy. CID has the advantage that it can be used to measure reaction activation barriers as well as bond dissociation energies (BDEs).24 A drawback of this technique is that only charged clusters can be studied.

2.1. Reactant Added to the Carrier Gas

3. REACTIVITY OF DOPED PLATINUM CLUSTERS Platinum nanoparticles are an important component of protonexchange membrane fuel cells (PEMFCs). Deposited at the surface of the PEMFC’s anode, Pt NPs catalyze the oxidation reaction under which a PEMFC operates, enhancing its performance.25 A major drawback of PEMFCs, however, is the effect of CO poisoning, in which CO molecules, present either as a contaminating agent or as a subproduct of the oxidation reaction itself, are adsorbed at the NP’s surface, thereby decreasing the cell’s performance.26 A tested possibility to overcome this limitation is the use of Pt-alloy NPs, although the complex mechanism behind the alloy-induced tolerance to the CO poisoning is still under debate.27 In the Blyholder model,29 the interaction between a Pt surface and a CO molecule is understood as a bidirectional transfer of electronic charge from the 5σ MO of CO to empty d states of Pt and a back-donation from occupied d states of Pt to the unoccupied 2π MO of CO (Figure 1a). In a more precise picture, however, this bonding involves hybridization of the MOs of the free CO and the d band of the metallic surface.30 This idea is represented in a simplified way in Figure 1a for the case of a single Pt atom interacting with a CO molecule. This picture can also be applied to understand the local adsorption of CO on transition and coinage metal clusters,31 as depicted in Figure 1b for the Pt19+−CO complex.28 Gas-phase experiments on small and doped Pt clusters are an ideal model system to investigate the complex interaction of a Pt-alloy NP and CO in PEMFCs. For this purpose, mass spectrometric experiments were conducted on a series of doped Pt clusters, with a selection of dopants (M = Nb, Mo, Sn, and Ag) based on tested Pt-alloy NPs in PEMFCs.28,8 By use of the collision cell approach, desorption rates of CO (kD) characterize the bonding strength of CO adsorbed on pure and doped Pt clusters. An example of a typical mass spectrum of

Several cluster production methods rely on the plasma creation of a target material. A carrier gas is used to cool the plasma, initiate cluster formation, and transport the clusters throughout a molecular beam setup. By addition of the reagent(s) to the carrier gas, the reaction occurs simultaneously with cluster formation.16 The reaction rate can be controlled by the concentration of the reagent. Disadvantages of this technique are that the reaction conditions are not well-defined and the possibility of activation or dissociation of the reagent(s) in the plasma, complicating the identification of the intrinsic cluster− reagent interaction. The main advantage of this approach is its simplicity. 2.2. Ion Trap Experiments

A widely used experimental method for studying the reactivity of metal clusters is the introduction of the reagent in an ion trap, where clusters are stored by applying electromagnetic fields. This offers excellent control of reaction conditions such as reaction time, temperature, and pressure.17 With this technique, however, neutral species cannot be investigated, and there is a limit in the investigation of reactions that are too fast. 2.3. Flow Tube Reactor

To obtain a directed beam, many cluster sources make use of a supersonic expansion after cluster formation.18 After the expansion, the density of the carrier gas is strongly diluted, and the velocity of the molecules are aligned so that there are no or at maximum a few collisions between the carrier gas and the clusters before they reach the detector. Flow tube reactors are usually placed right after the source but before the supersonic expansion has taken place.19,20 Given the sufficiently high density of the carrier gas and the more randomized velocity before the expansion, the heat of formation of the products is taken away by collisions with the carrier gas. By variation of the pressure of the reactant in the tube reactor, the rate of complexation can be controlled. B

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to-size effects and a size-dependent trend, which is mainly related to heat capacity changes,28 Nb and Mo doping increase kD, suggesting a decrease in Eads upon doping. In clear contrast, Sn and Ag doping do not alter kD above the uncertainties of the experiment. Complementary DFT calculations were essential for understanding the experimental observations. With the 19-atom cluster as a representative system, DFT calculations on MPt18+ were conducted showing remarkable agreement with the experiments. In Figure 3a, the ground state structures of Pt19+,

Figure 1. (a) Representation of the metal surface−CO interaction by calculating the Pt−CO system: (left) electronic bands of the metal; (right) 2π, 5σ, 1π, and 4σ molecular orbitals of the free CO; (middle) hybridized 2π′, 5σ′, 1π′, and 4σ′ orbitals of the Pt−CO system. (b) Hybridized 2π′, 5σ′, 1π′, and 4σ′ orbitals of the CO−Pt19+ system. Adapted with permission from ref 28. Copyright 2016 Wiley.

Ptn+ and NbPtn−1+ clusters interacting with CO is presented in Figure 2a. Inspection of this mass spectrum shows that the

Figure 3. (a) Bader charge distributions on MPt18+ clusters (M = Pt, Nb, Sn). Positively (negatively) charged atoms are colored with various shades of red (blue). The labeled charge indicates the value (in e) of the corresponding dopant atom. (b) Densities of states of Pt19+−CO and NbPt18+−CO projected on the CO molecular orbitals. The brown line shows total DOS. Dashed vertical lines indicate the position of free CO MOs. Adapted with permission from ref 28. Copyright 2016 Wiley.

NbPt18+ and SnPt18+ clusters are shown with the corresponding Eads energies listed. Upon Sn doping, the Eads = 2.38 eV calculated for Pt19+ is hardly reduced to 2.21 eV, whereas a larger decrease to 1.81 eV is induced by Nd doping. Similarly, while Ag doping only slightly affects Eads (2.26 eV), Mo doping decreases this energy down to 1.83 eV. Thus, calculated energies are in line with the observation that kD is increased upon Nb and Mo doping, while they are basically unaffected by Sn and Ag. The explanation of this effect lies in the clusters’ electronic structure, as the Blyholder interpretation suggests. The different colors in Figure 3a denote the electronic charge of each atom. In Pt19+, most atoms carry a positive charge, whereas upon Nb doping a strong electronic charge transfer takes place, from Nb to Pt, providing each Pt atom with a partially negative electronic charge, thereby reducing Eads. This electron transfer is weaker for Sn and Ag doping, with a concomitant Eads similar as for pure Pt. An analysis of the partial density of states of Pt19+−CO and NbPt18+−CO shows a strong stabilization of the 5σ MO of CO after adsorption. This stabilization, however, is higher in Pt19+, since the partially positive Pt atoms can easily accept electrons from CO. Experimental confirmation of the Mo to Pt electron charge

Figure 2. (a) Mass spectrum of Ptn+ and NbPtn−1+ after the interaction with CO at the reaction cell. The numbers (l,n,m) represent NblPtn+−(CO)m. (b) Ratio of CO dissociation rates of the M doped and monometallic Pt clusters. Adapted with permission from ref 28. Copyright 2016 Wiley.

intensity ratios of the cluster−(CO)2 complexes (red arrows) to that of the bare clusters with the same composition are significantly lower for the doped clusters than for the pure platinum clusters. This suggests a decreased adsorption energy of CO (Eads) upon Nb doping. The kD rates, providing quantitative information on this process, are given in Figure 2b. The figure shows the doped to monometallic cluster kD ratios for the investigated Nb, Mo, Sn, and Ag dopants. Despite sizeC

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Figure 4. Reaction probabilities of CO adsorption for (a) Aun (black), Aun−1Ag (red) and Aun−2Ag2 (blue). Adapted with permission from ref 35. Copyright 2011 American Chemical Society. (b) Aun (black), Aun−1V (red), and Aun−2V2 (blue). Adapted with permission from ref 36. Copyright 2012 Royal Society of Chemistry. (c) Aun+ (black) and PdAun−1+ (red).

transfer in MoPtn−1+ clusters was obtained by IR photodissociation experiments.8

that the d-states of Au and Ag clusters are well below the Fermi level. DOS plots of Au4+, Ag4+, and Pd4+ clusters are shown in Figure 5, as representative cases. An important difference

4. REACTIVITY OF DOPED GOLD CLUSTERS The discovery that gold, inert in the bulk phase, becomes reactive as a nanoparticle below a certain critical size,4 triggered a large amount of research on gold clusters.32,33 Because the Au atom is heavy, relativistic effects are important in gold clusters,34 with the consequence that atomic Au 3d and 4s orbitals are strongly hybridized in their MOs near the Fermi level. Therefore, d electrons play a determining role in the chemistry of Au clusters. The reactivity of Au clusters toward a wide variety of molecules has attracted much attention. For example, the interaction with O2, N2, and H2,37 CH4,11 C3H6,23 and CO38,39 has been studied. In the latter case, a very strong influence of charge state was found, with adsorption energies following the trend cations > neutrals > anions. This trend suggests that, within the Blyholder model, 5σ′ electron charge donation is the main Au−CO bonding mechanism, an assumption supported by the fact that COs adsorbed on Au clusters have C−O stretching frequencies higher than those in the free CO. Doping strongly influences the electronic structure of a cluster and, thus, its interaction with CO. By use of the collision cell approach, the effect of Ag doping on the reactivity of neutral gold clusters toward CO was investigated (Figure 4a).35 The probability that a cluster adsorbs CO and that the formed complex survives on the time scale of the experiment decreases if one and two Ag atoms are included in the clusters. The size-dependent odd−even pattern of the reaction probability, however, is not affected by Ag doping. A second investigated case is V doping (Figure 4b).36 Here, the effect of doping is very size-dependent, strongly altering the odd−even pattern of the reaction probability. A third case, presented in Figure 4c, is Pd doping of cationic gold clusters. With a few exceptions, Pd increases the reaction probability. Therefore, depending on the dopant atom, the reactivity of the Au clusters can be affected quite differently. V doping is complicated and requires a size-to-size analysis.40 The comparison of Ag and Pd doping, however, is interesting because they affect the gold clusters’ reactivity in opposite ways. A key feature here is the electronic structure of the pure Au, Ag, and Pd clusters. Au and Ag atoms have a filled valence d-shell and one valence s electron. Thus, it is expected

Figure 5. Total density of states of Au4+, Ag4+, and Pd4+, with respect to their corresponding Fermi level. Arrows indicate the center of the clusters’ d-states.

between Au4+ and Ag4+ is that, because of the relativistic character of Au, its d-states are higher in energy and more hybridized with the s-states. Pd ([Kr]5d10) behaves differently; the d-states of Pd4+ are right below EF. Therefore, an enhanced d character in the DOS of PdAu3+ is expected near EF as compared to Au4+, opposite from the case of AgAu3+, where a diminished d contribution is expected. To illustrate the effect of doping on the CO reactivity, Au4+ −CO, PdAu3 + −CO and AgAu3 + −CO clusters are analyzed. Figure 6a exemplifies the Au4+−CO interaction, via the cluster’s PDOS. The hybridized 5σ′ MO is strongly stabilized, whereas orbitals with 2π′ character are located well above EF. The ground state structures of the three clusters are shown in Figure 6b. Adsorption energies are 1.73, 1.42, and 1.85 eV, respectively, in line with the experimental observations. The orbitals of Au4+−CO with the strongest CO contribution are those plotted in Figure 6a. There are, however, several additional states with CO contributions in the DOS between −15 eV and −11 eV, where the Au d-states are found (Figure 6c, top). Most prominent features in this region are labeled based on their bonding (B) or antibonding (A) character with respect to the metal−CO system. Three of these states are of bonding character. D

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Figure 6. (a) Projections of the density of states of Au4+−CO on atomic Au d and s states and on orbitals of CO. Orbitals with major contribution of CO are plotted and labeled in terms of the free CO MOs 4σ′, 5σ′, 1π′, and 2π′. (b) Ground-state structures of Au4+−CO, Au3Ag+−CO, and Au3Pd+−CO. (c) Density of states of Au4+−CO, Au3Ag+−CO, and Au3Pd+−CO, zoomed in on the energy range of the Au 5d states. Most prominent peaks are labeled in terms of their bonding (B) or antibonding (A) character with respect to the cluster−CO interaction.

Figure 7. (a) Fraction of hydrogenated AlnVH2m+ clusters as function of the number of Al atoms n. (b) Experimental infrared dissociation spectra of Al10VH2+ and IR absorption spectrum of the calculated lowest energy configuration. Adapted with permission from ref 47. Copyright 2017 Wiley.

In AgAu3+−CO, the 5σ′ MO is only slightly less stabilized than in Au4+−CO. A major difference, however, is found within the cluster’s d-states (Figure 6c, bottom). Opposite to the case of Au4+−CO, the states with larger CO contribution are antibonding with respect to the metal−CO interaction, thus lowering Eads. This demonstrated that, upon Ag doping, orbital hybridization is less efficient. This effect is a consequence of the diminished d character of the states near EF in AgAu3+, being less appropriate to hybridize with the MOs of CO. Considering Pd doping, again the energies of the 4σ′, 5σ′, and 1π′ states in Au3Pd+−CO are not much different from those in Au4+−CO. However, the hybridized orbitals within the cluster’s d-states are different (Figure 6c, middle). In this case, more states of bonding character are observed, showing

that upon Pd doping the cluster’s d-states are more appropriate to hybridize with CO.

5. REACTIVITY OF DOPED ALUMINUM CLUSTERS The reactivity of aluminum clusters has a long and rich history, starting from the early days of cluster science.41,42 Of particular interest is the reactivity of aluminum clusters toward hydrogen. Aluminum is one of the lightest and most abundant metals and, therefore, an ideal material for hydrogen storage applications. Unfortunately, like bulk aluminum, most aluminum clusters have large activation barriers toward H2 activation or dissociation, due to Pauli repulsion between the delocalized orbitals of the clusters and the σ bonding orbital of H2.42 Experimentally, it was shown that titanium doping improves the hydrogenation kinetics of aluminum surfaces.43 E

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Accounts of Chemical Research Theoretical calculations on both surfaces and clusters predicted that transition metal doping lowers the activation barriers,44 due to favorable overlap of the transition metal dorbitals and the antibonding σ* orbital of hydrogen. Figure 7a shows the fractional distribution of hydrogencluster complexes as a function of size, with hydrogen injected via a flow tube type of reactor. This fraction is defined as [Al nVH 2m+]frac =

I(Al nVH 2m+) 2

∑i = 0 I(Al nVH 2i +)

, with I being the intensity in

mass spectra. With the exception of Al2V+, most small clusters (n < 10) are unreactive toward hydrogen. Beyond n = 10, clusters become reactive but adsorb mainly a single hydrogen molecule. For n > 16, reactivity decreases again. The latter observation hints at cage formation,45,46 which prevents the hydrogen from directly interacting with the vanadium dopant. A powerful technique for determining whether the hydrogen binds dissociatively or molecularly is IR spectroscopy, combined with DFT calculations. This is exemplified in Figure 7b, where the IR dissociation spectrum of Al10VH2+ is shown (top), together with the calculated IR spectra of the lowestenergy isomer (bottom). There are three main modes in these spectra, two within the 1000−1500 cm−1 range and one around 1900 cm−1. The first two correspond to the antisymmetric and symmetric stretching modes of a hydrogen atom bound in a bridging position between the vanadium dopant and an aluminum atom. The vibrational band around 1900 cm−1 is the stretching mode of a hydrogen atom bound on-top of an aluminum atom. All spectra for the reactive species (n ≥ 10) show a feature around 1800−1900 cm−1, which implies that the hydrogen binds dissociatively. Reactivity trends for the smaller aluminum clusters can be explained by calculating reaction pathways for the dissociative adsorption of hydrogen. Figure 8 shows the pathways for Al2V+, Al4V+ and Al10V+. For n = 2, the reaction occurs almost barrierlessly, and for n = 10, there is a barrier of only 0.2 eV, which can easily be overcome by the thermal energy of the cluster. For n = 4, on the other hand, there are two considerable barriers of approximately 1.5 eV along the reaction path, which impedes the hydrogenation of the cluster and hence explains its hydrogen fraction (Figure 7a). By comparison of the HOMO of the cluster−hydrogen complexes before dissociation, it was found that the presence or absence of an activation barrier is related to the occupation of cluster orbitals that have a large spatial overlap with the antibonding orbital of hydrogen.47 Small clusters of rhodium have been found to adsorb an unexpectedly large amount of hydrogen.48 Ideally, one would like to combine the high hydrogen adsorption capacity of small rhodium clusters with a lighter material to reduce the hydrogen weight percentage, which would make it suitable for mobile applications. As a model system, we studied AlnRh2+ (n = 10−13) clusters. Figure 9 shows experimental and calculated IR spectra of AlnRh2+ (n = 10−13) clusters with one adsorbed hydrogen molecule, either molecularly or dissociatively. The IR spectra of Al12Rh2+ and Al13Rh2+ show two bands, one around 800 cm−1 and one around 1600 cm−1. For Al10Rh2+ and Al11Rh2+, in contrast, only one band is seen around 1900 cm−1. This resonance strongly suggests hydrogen spillover from the rhodium dopant to the aluminum atoms. DFT calculations confirm that it is energetically more favorable for Al10Rh2+ and Al11Rh2+ to adsorb the hydrogen dissociatively. For Al12Rh2H2+ and Al13Rh2H2+ on the other hand, the observed resonances

Figure 8. Hydrogenation pathways of Al2V+, Al4V+, and Al10V+. Adapted with permission from ref 47. Copyright 2017 Wiley.

correspond to the symmetric and asymmetric stretching modes of molecularly bound hydrogen. Additional calculations of the hydrogen binding energy suggest that the molecular adsorption for n = 12 and 13 is not kinetically but thermodynamically controlled, that is, due to a higher binding energy of the molecularly adsorbed hydrogen−cluster complex. In Figure 10, the HOMOs of the bare AlnRh2+ (n = 10−13) are shown on the left, whereas those of the clusters with hydrogen bound molecularly are on the right. Inspection of these states shows that the hydrogen molecule initially forms a strongly bound Kubas complex with the Al11−13Rh2+ clusters50 but not with Al10Rh2+. There seems to be a competition between molecular and dissociative adsorption for Al11−13Rh2+, with small energy differences of the order of 0.1 eV between the two types of adsorption.

6. CONCLUDING REMARKS The conditions under which research on isolated metallic clusters is conducted are distinct from those encountered in surface chemistry and catalysis. In this account, the three reported examples explain how the investigation of sizeselected clusters in the gas phase is useful for elucidating the nature of local interactions relevant to catalytic chemistry. By combining detailed quantum chemical calculations with gasphase experiments under well-defined conditions including the particles’ composition, size, and charge state, chemical mechanisms that are difficult to probe in experiments on larger and complex systems can be scrutinized at the molecular level. First, the local interaction of carbon monoxide with platinum clusters doped with several (transition) metals shows the relevance of electronic charge transfer from the dopant to the platinum cluster. For the doped clusters with a F

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Figure 9. Experimental infrared dissociation spectra of AlnRh2+ (n = 10−13) and computed IR spectra of the corresponding lowest energy isomers. Adapted with permission from ref 49. Copyright 2018 Springer.

Figure 10. Highest occupied molecular orbitals of AlnRh2+ (n = 10−13) clusters. Bare clusters are shown on the left, whereas hydrogenated clusters are presented on the right. Adapted with permission from ref 49. Copyright 2018 Springer.

combination of ab initio calculations with gas phase experiments on few-atom particles provide key pieces of information for understanding local chemical processes taking place in much more complex systems. Even though those studies are of fundamental nature, driven by the curiosity of understanding the physical and chemical properties of matter at subnanometer scales, our findings can be relevant in designing novel applications, bridging the gap from fundamental research to applied science.

larger electron transfer, adsorption energies of CO are lower. This effect is relevant for understanding the enhanced performance of fuel cells, which make use of platinum-alloy based nanoparticles for catalytic purposes. Next, the interaction of carbon monoxide on a series of doped gold clusters shows that the CO bonding strength on gold clusters can be tuned by doping. This tunability stems from the opposite effect different dopant atoms have on the electronic structure of gold clusters, in particular on their d-states, which are the main levels involved in the interaction with CO. Finally, upon doping with transition metal atoms, the interaction of molecular hydrogen with aluminum clusters is enhanced. Even though pure aluminum clusters are generally inert toward H2, singly and doubly transition metal doped aluminum clusters do interact with hydrogen. In addition, the combination of gas-phase IR experiments and theoretical calculations presented here allowed the unambiguous identification of the hydrogen binding geometry. A better understanding of the different hydrogen binding motifs can aid in the design of novel hydrogen-storage materials. The studies presented in this Account concern very different cluster systems, but they all bear witness to the fact that the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ewald Janssens: 0000-0002-5945-1194 Peter Lievens: 0000-0001-6570-0559 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.accounts.8b00437 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research Biographies

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Piero Ferrari received his M.Sc. degree in physics at the Pontifical Catholic University of Chile in 2013. In 2017, he obtained his Ph.D. degree at KU Leuven, Belgium. Since 2017, he works as a postdoctoral researcher, grant awarded by the Research Foundation − Flanders (FWO). Jan Vanbuel received his M.Sc. in Physics from KU Leuven in 2015. Since 2015, he is working as a doctoral researcher at the KU Leuven with a Ph.D. fellowship from the Research Foundation − Flanders (FWO). Ewald Janssens obtained his M.Sc. in Physics from KU Leuven in 2000 and his Ph.D. in 2004. He was postdoctoral researcher at the Free University in Berlin and with the Research Foundation − Flanders (FWO). In 2014, he became Associate Professor, and since 2018, he is Professor at the Department of Physics and Astronomy of the KU Leuven. Peter Lievens obtained his Ph.D. in 1991 in Physics at KU Leuven. He was Fellow at CERN, Geneva, Switzerland, and postdoctoral researcher with the Research Foundation − Flanders (FWO), Belgium. He was appointed in 2001 as Research Professor at the Department of Physics and Astronomy, Faculty of Science, of the KU Leuven, since 2007 as Full Professor in Experimental Physics at KU Leuven, from 2009 until 2017 as Dean of the Faculty of Science, and since 2017 as Vice Rector.

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ACKNOWLEDGMENTS This work was supported by the Research FoundationFlanders (FWO/G0B41.15N) and the KU Leuven Research Council (GOA/14/007 and C14/18/073). P.F. and J.V. acknowledge the FWO for a postdoctoral and a doctoral grant, respectively.

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REFERENCES

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DOI: 10.1021/acs.accounts.8b00437 Acc. Chem. Res. XXXX, XXX, XXX−XXX