Nanoscale Control of Metal Clusters on Templating Supports

Chapter 8

Nanoscale Control of Metal Clusters on Templating Supports Erik Vesselli*,† and Maria Peressi*,†,{ *

University of Trieste, Trieste, Italy CNR-IOM, Trieste, Italy { National Interuniversity Consortium of Materials Science and Technology (INSTM), Research Unit of Trieste, Trieste, Italy †

Chapter Outline 8.1 Introduction 8.1.1 Metal Nanoclusters for Model Catalytic Studies: Finite Size Effects 8.1.2 The Role of the Support 8.1.3 Interaction With the Gas Phase 8.2 Investigating Metal Nanoclusters: Experimental and Theoretical Approaches 8.2.1 Experimental Approaches 8.2.2 Theoretical Approaches

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8.3 Clusters Growth and Characterization 8.3.1 Ultra-Thin Oxide Supports 8.3.2 Graphene Supports 8.4 Reactivity 8.4.1 Thermal Stability 8.4.2 Activity and Chemical Stability 8.4.3 Pressure Gap 8.5 Perspectives Acknowledgments References

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INTRODUCTION

The physics and chemistry of metal clusters at surfaces are being tackled since decades, and great insight has already been reached by means of both computational and experimental approaches. A large amount of scientific papers have been written, including books [1,2] and scientific reviews [3–11]. Past and present research efforts in the field originate from at least two different needs. Starting from applicative devices like catalysts where active-metal Studies in Surface Science and Catalysis, Vol. 177. http://dx.doi.org/10.1016/B978-0-12-805090-3.00008-5 Copyright © 2017 Elsevier B.V. All rights reserved.

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nanoparticles are dispersed onto a supporting, high-surface-area matrix, a top-down insight is tackled in order to understand the underlying electronic, magnetic, and chemical properties. On the other side, a bottom-up approach would yield unprecedented design and engineering capabilities on the basis of fundamental information obtained by means of atomic-level experimental and theoretical methods. The whole of this thanks to the peculiar properties showing up when bulk metals are size-reduced down to nanometer particles or even to few atoms clusters. A famous example and case study is that of gold [12]. Up to here, there are no points of novelty, so calling for a justification in order to deliver a new book chapter about this subject, thus getting to the ultimate scope of the present contribution. Indeed, whereas the synthesis of supported metallic nanoclusters and their characterization with atomic-level detail have reached great insight under controlled environments (ultrahigh-vacuum conditions—UHV—and single-crystal supports), large gaps are present, separating model from applicative conditions. In particular, the structural and chemical stability of the clusters is strongly influenced by the chemical environment (pressure gap) yielding sintering, deactivation, and poisoning of the systems. Therefore, the present contribution aims at providing a perspective with respect to the state-of-the-art status in order to bridge present limits toward a deep and detailed comprehension of supported metal cluster systems.

8.1.1 Metal Nanoclusters for Model Catalytic Studies: Finite Size Effects The case study of gold that is usually inert but becomes catalytically active when structured into nanoparticles [12,13] is only an example of a common trend: metal clusters of limited size, for example, less than 4 nm, have unique chemical and physical properties that are unlike the corresponding bulk metal. These changes include discrete electronic structure, modified physical structure, and altered chemical reactivity. Furthermore, both the reactivity and the selectivity can depend on the particle size. Studies over different supported metal catalysts have shown that there is an activity enhancement for smaller clusters relative to larger clusters, until a critical size is reached [3]. Although various explanations have been raised to account for all these findings, including steric and electronic effects, yet much remains to be understood. Some key factors are evident. Presenting a large surface-to-volume ratio, metal nanoclusters offer a larger exposed surface with respect to single-crystal surfaces. The size dependence of reactivity properties can be related to the relative abundance of one facet with respect to another. A crucial role in the enhanced activity of nanoclusters is played by the significant number of low-coordination sites (edges and corners), which are particularly active for catalysis and might favor reaction paths with lower reaction barriers; in contrast to intrinsically rigid regular single-crystal surfaces, they offer a high

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degree of flexibility. Indeed, it has been proved, for instance, in the case of Pd nanoparticles that a dramatic increase of the overall reaction rate arises from the modification of only a fraction of edge and corner sites [11]. Experimental and theoretical findings suggest a special role of the perimeter sites of supported nanoparticles, thus, at the cluster-support interface, which enables molecules from the gas phase to interact simultaneously with the nanocluster and the support [11]. Electronic properties also play a crucial role in defining the catalytic reactivity of small metal clusters. In general, for small metallic nanoclusters, the continuous band structure characteristic of an infinite solid breaks down to discrete and separated levels, with separations increasing with decreasing the nanocluster size (Kubo effect [14]). The band gap can thus vary dramatically with the cluster size: this is, for instance, the case of oxide-supported Au clusters, where the onset of catalytic activity correlates with a metal-tononmetal transition, whose fingerprint is the opening of the gap [3]. The valence band of supported metal clusters can be calculated and analyzed in details in terms of atomic projected density of states (DOS) and probed by scanning tunneling spectroscopy (STS), X-ray photoelectron spectroscopy (XPS), or Ultraviolet Photoelectron Spectroscopy (UPS) [5]. Its evolution with cluster size can be different: when the size is reduced, the band becomes typically sharper, with a decrease of the DOS at the Fermi level and an overall shift toward higher binding energy. The narrowing of the valence band can be ascribed to a localization of the electronic states arising from the decrease of the mean coordination of the atoms in the clusters. The shift of the valence band, which generally follows the one of the core levels, does not depend only on the average atomic coordination but also on the charge state of the nanocluster that in turn depends on its size and on the support. The shift of the valence band has very important consequences for the chemisorption properties.

8.1.2

The Role of the Support

The catalyst’s support is not a simple “inert” mechanical template for the metal particles since it can stabilize them, modify their geometric and electronic properties, and consequently affect their reactivity [14]. Such modification seems not very important for particles larger than 2–4 nm, but as the cluster size decreases, the interaction with the substrate becomes more and more significant. One of the factors determining the shape and the stability of nanoclusters is indeed the adhesion energy of the metal to the support. Lattice mismatch between the substrate and the deposited metal may induce a strain propagating within the nanocluster up to its surface. Defects on the support may act as active sites for seeding and nucleation. Particular interest is devoted to supports that can act as a deposition template for the growth of regular arrays

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of well-defined equally sized and shaped nanoclusters and that can prevent their sintering, which would yield deactivation of the catalytic devices. Suppression of sintering of particles can be achieved, for example, by using supports that strongly bind the clusters or by pinning the clusters into the support. The supported clusters can show no evidence of agglomeration over a wide temperature range. Among the most common supports, the major interest is deserved by oxides surfaces or ultrathin oxide films on metals and by carbon derivatives (amorphous carbon, graphite, and graphene) as novel and promising materials. With respect to bulk surfaces, thin films supported on metal substrates have the advantage of tunability. The mismatch between the metal and the oxide film induces a strain that can indeed influence its morphology and defectivity. Moreover, charge transfer through ultrathin oxide films, which is possible by quantum tunneling, can propagate to the adsorbed nanoclusters [14]. The charge transfer can induce the self-organization of atoms into assembled structures and can change considerably their catalytic properties. Supported nanoclusters can withdraw electrons from the surface and become negatively charged. In addition to charge transfer, chemical exchange can also occur between clusters and support with the diffusion of adsorbed species from the catalyst particles to the substrate (spillover) or vice versa (reverse-spillover) [5,15]. Ultrathin oxide films are optimal matrices for the growth or regular arrays of nanoclusters: among them, it is worth to mention MgO, tin oxide, and alumina (Al2O3). The surface structure of the alumina film grown onto a Ni3Al (111) substrate shows a periodicity related to the presence of oxygen vacancies that act as preferential nucleation and anchoring sites for equidistant, regularly distributed metallic clusters [16–25]. Graphene forms ordered epitaxial moir
e structures on several metallic surfaces. Apart from Ni (111), which is almost lattice matched, this is the case for graphene on Ir (111) and on the (0001) surface of several transition metals with wurtzite structure such as Ru [26–39].

8.1.3 Interaction With the Gas Phase Chemisorption is the simplest surface reaction and the first step in heterogeneous catalytic reactions. Perhaps carbon monoxide is the most commonly studied adsorbate, together with few other simple molecular gases, such as oxygen and hydrogen. Although not exhaustive, review papers report much of the effort on the subject [4,5,40–42]. More complex reaction frameworks have also been considered, including CO dissociation, oxidation, hydrogenation or NO reduction by CO [14,38,43–51], and more in general water-gas shift processes and hydrocarbons’ conversion mechanisms [5,52,53]. Long exposure of supported nanoclusters to gas-phase species may affect not only their capacity of adsorption and catalytic activity but also

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their stability [54]. For instance, the observed decrease of the adsorption capacity for CO and NO in some cases has been explained by particle restructuring, sintering, and by partial dissociation of CO and NO. The resulting carbon or nitrogen atoms strongly bind to the nanoclusters and become a poison with respect to further adsorption of molecules from the gas phase [5]. The presence of reactant gases under realistic conditions can also affect nanocluster size and distribution. The restructuring process may be reversible [55] or not [54], depending on the system. Restructuring combined with metal mobility may result in agglomeration of nanoclusters inducing a rapid deactivation process. In conclusion of this short and necessarily partial introduction, it is also interesting to report the possibility of preparing ordered arrays of size-selected and isomorphic nanoclusters either fully oxidized or in a metal-core/oxideshell configuration, as proved by spectroscopic and theoretical characterization. The method is exemplified in the case of Ni nanoclusters but is generally applicable to a wide range of metallic systems [56].

8.2 INVESTIGATING METAL NANOCLUSTERS: EXPERIMENTAL AND THEORETICAL APPROACHES 8.2.1

Experimental Approaches

The growth of ordered arrays of metal nanoparticles on supporting templates under UHV conditions can be achieved by means of two different approaches. Chemical vapor deposition (CVD) methods can be exploited, so that metal atoms are deposited on a templating surface from a hot atomic gas-vapor phase. The nucleation and growth of the nanoparticles are determined by the seeding properties of the support, while the deposition rate and the support temperature can be tuned to properly optimize the process. Otherwise, the metal clusters can be preformed in the gas phase by means of a cluster source and subsequently deposited (or landed) on the surface. The eventual lateral ordering of the clusters is determined by the nature of the templating, supporting substrate. In the following, we will analyze in detail the two cluster growth and deposition approaches and the experimental characterization methodologies.

8.2.1.1 Chemical Vapor Deposition CVD of single metal atoms at a surface is the easiest and most straightforward method to grow nanoparticles under UHV conditions. Very low fluxes and consequently very low surface coverage can be attained. For materials with sufficiently high resistivity and melting temperature, resistive heating of a pure metal wire or filament is sufficient, whereas for other metals, for example, copper, gold, and silver, evaporation from a hot crucible is instead

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preferable. Atomic fluxes as low as a few 0.01 ML/min can be obtained under very low background pressure conditions (<10 10 mbar), thus assuring almost no contamination issues and yielding even single ad-atoms and dimers at the surface [57,58]. In this case, the cluster growth and size distribution is entirely governed by the metal-metal and metal-support interaction. Important kinetic effects can influence the process upon landing at the surface [59], due to the ad-atom diffusion mechanisms, the energy dissipation, and the corrugation of the potential energy surface (PES) of the support, in combination with the surface temperature during growth. Different size distributions for the clusters and different degrees of ordering on selected growth templates can be attained, depending on the above conditions, and have been widely investigated on model supports like ultrathin oxide films [9,10,16–25] or a single epitaxial graphene sheet [26–39]. Another technique that is similar to CVD is the dissociative deposition of an organometallic precursor, for example, metal carbonyls. It can be exploited to grow thin metallic films [60] and also metal clusters, as successfully demonstrated in the case of Rh/Al2O3/Ni3Al (111) [22].

8.2.1.2 Cluster Sources Sputtered gas-phase metal clusters are instead energy analyzed and mass filtered in a cluster source and deposited on a surface under vacuum in this case. The first realizations of this approach date more than 30 years ago [8,61]. This methodology yielded the development of effective tools to produce size-controlled metal cluster beams (Ag, Pt, etc.) up to the end of the 1990s [62,63], when a detailed description of a cluster source that exploited laser ablation was reported [64]. Several technical papers have appeared (see, e.g., references. [65–69]), with description and characterization of devices in which the metal vapor phase can be generated by direct heating of a crucible, sputtering, laser ablation, magnetron sputtering, and other approaches. However, exhaustive reviews summarize in great detail the methodology [6,8]. The very first step in a cluster source is the cluster production process that requires thermodynamic nonequilibrium: clusters of high-melting point materials are mainly produced with laser vaporization or plasma-based approaches in which the material is brought into the gas phase in a partially ionized form. The vaporization methods include laser ablation from a rotating target (allowing also for alloy materials), ion sputtering, magnetron sputtering (a plasma is ignited in the presence of a rare gas close to the target), and arc discharge (high currents at variance with magnetron sputtering, where high voltage discharge arcs are employed). After evaporation of the metal into an aggregation chamber filled with several mbar of an inert gas that can be either pulsed or continuously streamed at low pressure, and upon reaching supersaturation and thermalization, the single atoms nucleate to clusters and subsequently

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expand into the next vacuum chamber forming a subsonic beam through differentially pumped stages. Clusters produced with the above approaches show a wide size distribution and need for mass selection through different techniques, depending whether the clusters are neutral or charged. In the former case, differences in the momenta can be exploited after scattering with an atomic beam, or aerodynamic focusing can be also employed. In the case of charged particles (a consistent fraction is already charged upon extraction, but an ionizer can be used), instead, mass separation is easier. The choice between the available techniques depends on the required mass resolution (up to 1 amu) and on the mass range. The highest mass resolution can be achieved using time-of-flight spectrometers, but for most applications, a radio-frequency quadrupole spectrometer with typical relative mass resolutions of 10 4 is sufficient. Not only another mass selection approach relies on dispersion of the charged clusters in an electric filed, but also Wien filters and cyclotron resonance mass selection methods can be used [6]. The final step is the cluster deposition at the surface in vacuum conditions. The important parameter is the impinging kinetic energy. Namely, soft landing is intended when the kinetic energy per atom is lower than the binding energy of the cluster’s constituents. This is extremely important when the cluster properties should be maintained upon adsorption on a supporting surface, avoiding coalescence, fragmentation, or implantation.

8.2.1.3 Characterization Techniques Experimental models of supported nanoclusters grown under controlled conditions can be investigated by exploiting a large variety of approaches, generally belonging to the surface science methods. A detailed description of the techniques is surely out of the scope of the present essay and would include microscopies and spectroscopies, diffraction approaches, mechanical, photon, and electronic probes, and laboratory and synchrotron radiation sources. We will instead address that, whereas for supported, dispersed 3D catalysts transmission electron microscopy is the tool of choice to determine cluster distribution, size, and structure, also in situ [70], cluster imaging is a challenge for 2D systems, that is, clusters deposited on films or graphene supported on single-crystal surfaces. Scanning probe microscopies may provide atomic-scale resolution, but generally, ultrahigh-vacuum conditions are needed [71], even if success in selected cases at near-ambient pressure was also reported [72]. Fostered by the need for insight under catalyst’s working conditions, the state-of-the-art developments focus on experimental methods capable to operate under reaction environments near or that may be compared with applicative frameworks. A set of spectroscopies are receiving progressively increasing attention in this sense. In particular, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) can be now performed

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up to pressures of the order of 101 mbar, both with conventional or synchrotron radiation sources, thus providing information about the electronic configuration of the sample under close-to-realistic catalytic conditions [73]. Complementary, structural information may be obtained, for example, by means of standard X-ray absorption fine structure analyses or by surface X-ray diffraction [74]. By modulating the polarization of the laser probe, recent developments of infrared absorption spectroscopy (IRAS) have extended the pressure range of its applicability, making now possible to investigate vibrational modes of molecules adsorbed at the surface up to near-ambient conditions [75]. This provides a tool to investigate the properties of metallic nanoclusters by means of an indirect approach that involves characterization of adsorption sites and reactivity by means of simple molecular probes like carbon monoxide. However, the most significant step forward in this direction is represented by the diffusion of nonlinear optical spectroscopy approaches, namely, sum-frequency generation laser techniques [75,76]. In the latter case, important selection rules provide exclusive access to the information stemming from interfaces, thus bridging the pressure limitation of most techniques due to the predominant contribution to the signal from the gas phase or due to the mean free path of the spectroscopic probe. In particular, infrared-visible sum-frequency generation (IR-Vis SFG) in its doubleresonance configuration can be regarded as a vibronic spectroscopy, providing experimental information about both vibrational properties of the adsorbates and electronic properties of the surface in an extended pressure range [77]. As a future perspective and as already foreseen by Goodman [3], these latter photon-in/photon-out techniques are expected to dominate in probing catalytic surfaces with increasing complexity under realistic conditions.

8.2.2 Theoretical Approaches The investigation of equilibrium and dynamic properties of nanoclusters is extremely interesting and challenging from a theoretical point of view. When dealing with systems containing up to a thousand atoms, microscopic approaches and numerical simulations are feasible and more appropriate rather than analytic or semianalytic treatments that could be useful in some cases for larger size, bulk-like systems. The major challenge for microscopic approaches is related to the complexity of the PES of nanoclusters. The configurational energy of a cluster with N atoms, within the Born-Oppenheimer approximation, is a function of atomic coordinates, corresponding to a rather complex energy landscape. Finding the most stable structures for a given cluster size is a very difficult task already for nanoclusters containing a few tens of atoms, requiring the determination of the PES minima and in particular of the lowest, global minimum. Further complications arise in the case of nonhomogeneous nanoparticles or nanoalloys, where additional degrees of freedom are related to the

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composition. Bimetallic nanoalloys may show a core shell, fully segregated Janus, random or ordered alloyed patterns, and evaluating their phase diagrams is extremely complex. In any case, both for homogeneous and heterogeneous nanoparticles, chemical or physical intuition may be used for an initial guess, but not to determine the most stable structures, which are often highly asymmetrical and nontrivial. The search for the global minimum can be approached by global optimization methods with a proper sampling of the PES. On the other hand, a thorough exploration of the PES is important in the investigation of finite-temperature equilibrium properties to make statistical averages over different configurations. Although theoretical studies have been mostly focused on free nanoclusters in the gas phase, supported nanoclusters are the subject of a lively and recent investigation, concerning their nucleation, growth mechanism, evolution with temperature, diffusion and sintering phenomena on one side, and detailed shape, electronic, and reactivity properties for a given size on the other.

8.2.2.1 Ab Initio Approaches Several approaches can be used to address specific problems. Ab initio methods make use of electrons, nuclei, and their interactions as basic ingredients: they do not require any empirical input and are therefore able to properly describe the interatomic interactions in very different environments. The density functional theory (DFT), focusing on the electron density, has been the major breakthrough allowing the application of ab initio methods to systems of increasing complexity [78–81]. Ab initio approaches are suitable to determine PESs at different spin multiplicities, to distinguish between similar and competing configurations of gas phase as well as supported nanoclusters, to describe even small local modulations of the electronic structure of the support, and to predict the chemical activity of nanoclusters, being able to fully account for bonds formation and breaking [9,14,16,17,28,82–84]. Particular care has to be paid when dealing with weakly bonded systems. For instance, the binding of graphene to Ir (111), a promising template for the growth of ordered nanocluster arrays, has been studied using DFT both with the local density approximation (LDA) and the generalized gradient approximation for the exchange-correlation functional and in most cases, with the inclusion of van der Waals corrections [27,85–87]. All the different approximations predict a globally weak bonding between physisorption and weak chemisorption, but the predicted result crucially depends on the details of the calculations that need tight convergence requirements: the mean graphene/ ˚. substrate distance ranges from 3.0 to 3.8 A Beside information on energetics, equilibrium geometries, and many details on the electronic structure, ab initio methods can also give access to energy barriers, thus allowing studying kinetics and not only thermodynamics.

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A successful tool for this purpose is the so-called nudged-elastic-band (NEB) approach [88]. The investigation of individual metal ad-atoms adsorbed on a substrate can already provide useful insight on the growth mode and stability of one metal relative to another. For instance, some hint concerning a 2D rather than a 3D growth mode can be given by the ratio between the adsorption energy (Eads) and the bulk binding energy (Eb) of the metals. 3D morphology could be favored for nanoclusters based on metals with a strong bulk binding energy. The calculation of diffusion barriers can also explain the different behaviors of one metal with respect to another in the nucleation process [16,17]. Several examples on both experimental and computational studies of the growth morphology and properties of metallic nanoclusters on graphene are reviewed in Refs. [14,84,89]. DFT calculations of core-level spectra compared with XPS measurements complement and validate other DFT studies, normally reporting equilibrium geometries, STM simulated images, and DOS analysis [32]. Core-level shift (CLS) modulations, splitting, and variations can be related to specific bonding geometries, allowing distinguishing atoms within the nanoclusters (edge atoms with respect to terrace atoms): this suggests the possibility of using CLSs as tool for studying adsorption of molecules on nanoclusters and chemical reactivity. Recent effort has been devoted to simulate the interaction of supported nanoclusters with small molecules, such as CO. Although there is clear evidence of important effects on the stability of the clusters, shape change, and mobility [54], DFT studies do not fully clarify the mechanism that induces diffusion and coalescence. In general, DFT is a powerful tool to investigate reactivity on supported clusters, in combination with mechanism-based rate equations to study different possible reaction channels. DFT can give access to equilibrium geometries and energetics of reactants and possible products as well as of transition states, with the estimate of energy barriers through the NEB approach. Of course, chemical and physical intuition and suggestions from experiment may help in limiting the exploration of the large possible scenario, much wider than the one corresponding to heterogeneous catalysis on extended surfaces: in fact, the high coverage of reactants that can be achieved on the curved nanocluster surfaces may considerably exceed that achievable on single-crystal surfaces, facilitating also the metal restructuring and opening other reaction channels [82]. In case of supported nanoclusters, periodic boundary conditions are used to describe the system under investigation: the surface is described using a slab geometry, i.e., a periodically repeated cell containing a slab to simulate the infinite surface in two directions, with some vacuum space in the third direction. The size of the cell, which has to be large enough to avoid

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fictitious interactions between repeated images, determines the computational workload. Notwithstanding the increasing availability of high-performance computing resources, ab initio approaches are still limited either to the comparison of few different configurations of nanoclusters of the order of one hundred atoms or to the full optimization of clusters of even smaller sizes (well below 50 atoms).

8.2.2.2 Atomistic Approaches and Interatomic Potentials In any case, a complete exploration of the PES by means of ab initio methods is presently unaffordable even for nanoclusters with few tens of atoms. To overcome this problem, a hierarchical approach can be used, starting from simpler methods based on interatomic potentials as a first step in the study. These models are less computationally intensive, thus allowing a much more complete sampling of the PES. The possible cluster structures identified in this way can be the starting point for further ab initio calculations. A variety of empirical interatomic potentials have been extensively developed for atomistic simulations to describe cohesion in metals and alloys. These potentials are normally fitted for an element to reproduce its fundamental bulk properties, such as experimental cohesive energy, lattice constant, elastic modulus, and melting point. Important examples are the Gupta potential [90] and the embedded-atom method that include somehow many-body effects to overcome the limitations of the pair-wise potentials [91]. The Gupta potential in particular has been widely used because of its simplicity (it has only few adjustable parameters) and possibility of extension to alloys [7,92]. Refined formulations [93,94] derived from tight-binding (TB) electronic structure theory and in particular in the so-called “second-moment approximation” (SMTB) have been successfully and widely used to investigate nanoclusters. Simple interatomic potentials must be used with care, since their accuracy is strongly system- and environment-dependent and may fail in the description of systems where the crystalline symmetries are usually broken and in particular at the nanoscale. In other words, they are normally not transferable and may give an oversimplified description of nanoclusters, missing important configurations or distinction among similar isomers or predicting wrong shapes. They are therefore unsuitable when the precise structure of the nanocluster is important. In order to overcome the limits of ab initio methods on one side and of simple interatomic potentials on the other when studying nanoparticles, a large effort has been recently devoted to the development of more efficient and accurate atomistic models. One possibility is to start from SMTB potentials and develop new parametrizations, considering more data than standard bulk experimental ones. For instance, in dealing with nanoalloys, to correctly

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parameterize the heterogeneous interactions, it could be important to consider also solubility energies of impurities or other configurations. A possible strategy is to use DFT to complement experimental data with additional ones or, in the extreme case, to fit all parameters from DFT results, making the potential a DFT-based one [95]. Atomistic potentials of this type are promising for nanoclusters in the size range between 102 and 103 atoms, where DFT calculations are prohibitive, and thermodynamic modeling is not fully reliable. Being much less computationally expensive than ab initio calculations, atomistic models allow a satisfactory exploration of the energy landscape. Once validated against available experimental data and further DFT data not used in the fitting, they can be used not only to search for the most stable configurations of nanoparticles but also to simulate more complex phenomena such as phase transitions, nucleation, and growth on realistic time scales.

8.2.2.3 Nanoclusters Growth and Dynamical Processes Interatomic potentials can be successfully used in connection with Monte Carlo (MC) or molecular dynamics methods and with packages like large-scale atomic/molecular massively parallel simulator (LAMMPS) [96]. Kinetic MC allows the simulation of kinetic processes involved, such as direct assembly on nanostructured templates, including cluster nucleation and progressive growth. The different sites of the substrate can be accounted for by a modulation of the adsorption energy. Different possible elementary events and their probability of occurrence determine the evolution, including diffusion, adsorption/desorption processes, and 2D or 3D growth mechanisms [97]. The direct assembly of nanoclusters on metal-supported graphene has been studied using atomistic models [84]. Alternatively, simpler stochastic lattice-gas models can be implemented, thus tracking only the position and size of the nanoclusters but not their detailed structure. Needless to say, although the combination of surface science techniques with accurate atomistic and quantum-mechanical numerical simulations has allowed understanding and, to a large extent, predicting the properties of metal clusters on templating supports, a gap between theory/modeling and experiments still exists, especially in the description of the interaction with an ambient pressure gas phase and in general with a realistic environment. Increasing computational resources may help in closing such a gap, but further improvement and development in theory and modeling, as well as in global minimum searching algorithms, are necessary. More reliable numerical simulations could guide the design of systems, size, and composition to achieve specific desired properties.

8.3 CLUSTERS GROWTH AND CHARACTERIZATION Here, we will focus on the idea of a template-assisted cluster growth process, consisting in the deposition of a metal vapor on a substrate that contains a

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regular array of point defects. Nucleation may preferentially occur at these distributed defects, depending on the shape of the 2D PES for metal ad-atom diffusion and adsorption. The latter energy profile will determine the energetics of the final cluster distribution and the kinetics of the growth process, in combination with the metal-metal lateral interaction. When all seeding defects are occupied, metal clusters will grow uniformly because the spatial distribution of ad-atoms is the same around every seeding cluster [98]. The first attempts to exploit a templating array of defects involved dislocation networks between an ultrathin metal film and a supporting single crystal having a slightly different lattice constant, elbow sites at reconstructed (111) terminations, or vicinal surfaces of the Au (111) direction [98]. Recently, ultrathin oxide films and graphene single layers on single-crystal surfaces have been shown to be optimal supports for ordered metal cluster growth. It has to be mentioned that the influence of the substrate periodicity (e.g., of the moir
e pattern of graphene supported on a single-crystal surface) on the organization of metal clusters has been recently reported also in the case of preformed, size-selected, and soft-landed nanoparticles [37], thus opening the way to other possible approaches with respect to CVD.

8.3.1

Ultra-Thin Oxide Supports

The growth and characterization of metallic nanoparticles on ultrathin oxide films represent bridging steps of the well-known material gap between surface science and applicative heterogeneous catalysis. A series of reviews appeared in order to summarize the efforts to investigate methods and systems to obtain model, ordered arrays of size-selected and oxide-supported metal particles [9,10,18,21,99]. The main point is the template: indeed, an ordered surface oxide with a periodic arrangement of defects is the optimal support to yield nucleation of a 2D lattice of clusters. Among a large variety of oxide films that have been investigated so far, including MgO, Al2O3, NiO, FexOy, CoO, TiO2, ZrO2, Cr2O3 [10], and ceria [100], intense focus has been shed on the ultrathin alumina film that is obtained by high-temperature oxidation of a Ni3Al (111) single-crystal surface [25,101]. The latter process yields indeed the formation of a (√67  √67)R12.2° superstructure with a periodic array of defects that can act as seeding centers for cluster nucleation, depending on the metal [19,20,24,25]. The “dot” structure of this ultrathin oxide film ˚ ) of holes in the oxide extending down consists in a hexagonal lattice (41.5 A to the underlying alloy-metal surface (Fig. 8.1, left panel). A second, ordered set of defects is also present, namely, the “network” structure [17]. In general, when defects are present on a templating support, diffusing hot metal ad-atoms may be trapped upon CVD at these surface sites, thus forming nuclei for the subsequent growth step (heterogeneous nucleation, Fig. 8.1, central panel). In the case where, instead, the stable nucleus is generated by aggregation of several ad-atoms on regular and nonpreferential sites, then, we have a

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FIG. 8.1 Left panels: top and side view of the structural model of the Al2O3/Ni3Al (111); a portion of the underlying alloy is shown in inset in the top view; the corners of the sketched unit cell correspond to the “dot” or “hole” structure. Right lower panel: side view of a Cu13 nanocluster anchored in a hole through a 2-atom seed. Right upper panel: calculated adsorption energies of different atoms in the hole, reported in connection with their covalent radius. Results from DFT calculations.

random seeding process (homogenous nucleation). The minimum number of ad-atoms that a single diffusing atom must find in order to generate a seed is called critical cluster size. The behavior is different for each ad-metal, depending on both the metal-oxide and the metal-metal interactions [16,17], and is effectively determined by both energetic and kinetic arguments, thus making the characterization of these systems quite demanding. In the case of the Al2O3/Ni3Al (111) ultrathin film, different degrees of order and different nucleation sites are found for Pd, Cu, Ag, Au, Mn (dot structure), and V clusters (network structure) [19]. Pd is found to be very stable [25], so that it can be exploited as a seed to promote nucleation at the dot structure sites of other metals, for example, Ni, Fe, and Co [23,24,56]. A tentative rationale has been recently proposed in order to unravel the mechanisms at the basis of the cluster formation process and at the origin of their effective stability, focusing on Ni, Fe, Co, Pd, Cu, Au, and Ag (Fig. 8.1, right panel) [16,17]. As expected, it is found that nucleation and growth of the nanoparticles are governed by adsorption energies and diffusion barriers that are different at each nonequivalent site of the large oxide surface unit cell. It is obtained that the actual PES for adsorption and diffusion is described by a delicate interplay among oxygen affinity, atomic radii (covalent radius), and electronic configuration (d-levels) of the ad-metal, thus making the picture quite complex and far from a straightforward comprehension in the optics of predictive capabilities. Long-range, dispersive, and electrostatic interactions between ad-atoms/ clusters and the support are of special importance [21]. Indeed, the influence

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of the metal underneath the oxide film is relevant. The preferential interaction mechanism between metallic ad-atoms and an ionic material, for example, Al2O3 and MgO, is the polarization in the oxide Madelung field, resulting in an electrostatic attraction that is usually stronger at anionic than at cationic surface sites. Oxygen ions are therefore preferred binding sites, the binding energy increasing with the polarizability of the adsorbed metal. Therefore, larger atoms like Au should bind stronger than smaller, less polarizable atoms like Cu and Ag. However, this contribution is partly balanced by a reduced Pauli repulsion experienced by small ad-species, allowing a shorter bond length with the surface. For noble and seminoble metals, polarization in the oxide Madelung field is the predominant interaction mechanism, while transition metal atoms yield the formation of covalent bonds, leading to a substantial increase of the adsorption energy. In the latter case, metal d-states and oxygen 2p orbitals participate in a hybridization process, usually with no charge transfer. For noble metals, since d-states are filled, this contribution is absent in the bond formation process. For oxide materials with a more covalent character, for example, V2O3, polarization is instead less important and the adsorption mechanism involves the formation of chemical bonds, involving a consistent charge transfer. As seen above in the case of the Al2O3/Ni3Al(111) film, a larger variety of binding sites is offered when defects are present at the oxide termination, since defects often contain excess charge in localized states available for more stable bonding on the basis of the above considerations.

8.3.2

Graphene Supports

Single-layer graphene sheets with long-range order can be obtained by reactive decomposition of hydrocarbons at high temperature (>1000 K) on several single-crystal metal surfaces. STM imaging showed already in 1992 that the graphene layer formed a moir
e pattern due to the lattice mismatch with respect to the underlying Pt (111) metal surface [102], and the idea of

FIG. 8.2 STM imaging at 77 K of ordered arrays of Pt clusters grown at room temperature by chemical vapor deposition of Pt on the moir
e pattern of a graphene sheet on Ir (111) for different Pt loadings: (A) 0.05 ML, (B) 0.74 ML, and (C) 1.48 ML.

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˚ , depending on the subexploiting the induced lateral periodicity (20–30 A strate) to seed cluster growth was recently made reality. For example (Fig. 8.2), well-ordered arrays of metal clusters have been grown by CVD on graphene on the (111) termination of Ir (Au, Co, Fe, Ir, Ni, Pt, Re, Rh, and W) and Pt (Pt), Rh (Ni) and on the (0001) termination of Ru (Au, Co, Pd, Pt, Rh, and Ru) [98]. Very detailed microscopy investigations on the graphene/Ir (111) system shed light on the templating effect of the graphene moir
e [27–29]. The lattice mismatch between the carbon sheet and the metallic support yields fcc, hcp, and on-top regions that show different adsorption properties (Fig. 8.3). Metals most stably absorb in the hcp part of the graphene layer, but a second shallower potential energy minimum is found also in the fcc-type regions [27]. In the case of Ir, by tuning the cluster growth parameters, especially the substrate temperature in the 300–400 K range, different Ir cluster morphologies can be obtained (from 2D islands to 3D nanoparticles), thus indicating the predominating role of kinetics in the process. Indeed, diffusion of ad-atoms or even larger assemblies like dimers or small clusters contributes to the assembly mechanism for many similar systems [38]. Moreover, the progressive growth of cluster seeds yields a deformation of the graphene layer and can even pin it to the underlying metal surface, thus further steering the

FIG. 8.3 Lower panel: Top view of the unit cell of the graphene moire` structure on Ir (111). The different registry of graphene with respect to the substrate can be recognized, corresponding to the hcp, fcc, and top regions. Upper panel: side view of an Ir3 nanocluster, showing the pinning effect on graphene.

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process, as predicted by ab initio calculations [28] (and also Fig. 8.3, top panel) and observed by means of X-ray diffraction [34]. This is observed by means of high-energy-resolution photoelectron spectroscopy studies for Pt and Rh clusters on graphene on Ir (111) [26,32], where the large CLS of the C 1s components from graphene is related with the rehybridization from sp2 to sp3 of C atoms under the particles and therefore in contact with two metals. This effect was evidenced in the case of adsorption and diffusion of metal ad-atoms on graphene on 6H-SiC (0001), where a local deformation of the graphene sheet has been evidenced even upon adsorption of a single atom for a large collection of both common and exotic ad-metals [33]. Together with the metal-graphene bonding mechanism, the metal-metal interaction also plays an important role. By changing the metal of the nanoparticles, different ordering and size distributions of the clusters are obtained, for example, in the case of highly cohesive elements like Pt, W, and Re on graphene on Ir (111) [29]. For low cohesive energy metals, instead, like for Au, Fe, and Ni, no superlattice is obtained at room temperature. However, a seeding method can be used. Since such metals wet high-cohesive-energy metals due to their lower surface free energy, their nucleation site can be defined by small seeds of a high-cohesive-energy metal on which the actual low-cohesive-energy metal cluster can grow. This method has been successfully applied on graphene [29], but similar evidence has been obtained also for cluster nucleation on ultrathin oxide layers like alumina as discussed above in the case of Pd/Al2O3/Ni3Al (111) [25]. The cluster-graphene interaction can be indirectly tuned also by appropriately choosing the graphene support. Indeed, the scenario can change significantly when going from a weakly interacting graphene sheet like in the former case of Ir (111), to a stronger interaction like in the case of Ru (0001) [39]. Here, thanks to the interplay between the strength of the metal-C bond, formed upon sp3 rehybridization of the C atoms, and the moir
e-induced modulation of the binding energy, a disparate cluster nucleation and growth behavior is observed. Small Rh, Pt, Ru, Pd, Co, and Au clusters are found to nucleate in the fcc regions of graphene on Ru (0001), but while Rh, Pt, and Ru form a mixture of singleand multilayer clusters and remain dispersed, Pd and Co rapidly grow 3D and Au forms 2D islands [30,31,36,39]. In an advanced view, the different relative orientations of graphene domains with respect to the underlying substrate can induce an additional modulation of the adsorption-site properties, like in the case of Mn clusters [35].

8.4

REACTIVITY

One of the most important goals of surface science studies related to heterogeneous catalysis is to achieve an atomic-level understanding of structureactivity relations. This is of particular interest in the case of supported nanoclusters in model systems that mimic realistic supported catalysts, since

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a number of peculiar effects take a role when downsizing particles to nanometer and subnanometer level. Indeed, quantum-finite size, electronic, and geometric properties blow up in a variety of effects that affect the systems’ behavior from both the stability and the reactivity points of view.

8.4.1 Thermal Stability Temperature plays an important role in the determination of the actual cluster size and distribution, as we have already discussed, due both energetic and kinetic effects implicit in the cluster formation process upon diffusion and aggregation of hot metal ad-atoms on the oxide or graphene templating support [19]. After completion of the formation process, the temperature stability is an essential characteristic of the system, in order to guarantee the possibility of raising the surface temperature to promote chemical catalytic reactions without inducing structural and activity changes of the catalyst. Temperature stability is strongly related to cluster adhesion energy to the substrate and to the cohesion energy of the constituent metal [19]. In the case of the Al2O3/ Ni3Al (111) system, Pd seeding at the alumina defective sites can strongly improve the stability in this sense [23]. In the case of Rh clusters, gentle annealing up to 473 K further sharpens the cluster size distribution, thus improving the lattice quality with respect to the deposition at room temperature. Instead, higher temperatures (up to 873 K) yield evidence for coalescence and sintering [22]. High-temperature stability was instead observed for Pt clusters grown on mono- and bilayer graphene on SiC (0001), showing no significant agglomeration or atom detachment events up to 1000 K [103]. Interestingly, this happens at variance with respect to graphene on Ir (111), thus evidencing the role of the metal that supports the graphene sheet. In the latter case, indeed, Pt clusters are found to be stable only up to 400 K [29]. An extensive STS investigation of clusters’ stability at graphene/Ir (111) evidences a different behavior, depending on the metal [29]. While Au is the less stable (sintering occurs already at 200 K), Re is stable up to room temperature, W, Pt, W, and Ir-seeded Au clusters resist up to 400 K. The most stable metal is Ir, yielding nanoparticles that remain unchanged up to almost 800 K, possibly due to a pinning mechanism [28]. Calculations, supported by suggestions from STM images, indicate that the most favored region for nucleation and growth is the hcp region, labeled in this way, by convention and by the relative position of the center of the C hexagon w.r.t. the substrate sites. On the basis of LDA investigations, Feibelman [28] confirms a pinning effect in this case for flat small Ir dots, formed by few (up to 9) atoms. The proposed idea, based on the electronic structure analysis, is that there is a rehybridization of the carbon atoms from the sp2 bonding, characteristic of free-standing graphene, to diamond-like sp3 beneath the adsorbed metal clusters. The graphene/substrate registry in the hcp region is adequate for such rehybridization, and this could explain why that region is more attractive than other regions. Two-layer

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clusters, that is, 3D rather than 2D, become energetically favorable for sizes larger than 26 atoms [104]. Time-lapse STM imaging of the Pt clusters on graphene on Ir (111) at constant temperature yields some insight into the coalescence mechanisms [29]. The decay of the cluster lattice occurs due to the thermal motion of the particles that fluctuate around their equilibrium positions at the binding sites of the moir
e lattice. The magnitude of the fluctuations depends on the cluster size and on the internal cluster structure (isomer type). Upon encountering, two clusters immediately merge the dynamics depending on the cluster’s size. For few-atom clusters, thermal fluctuations may be sufficient to overcome the activation barrier for diffusion to an adjacent site. If the latter is already occupied, the two particles coalesce and reshape. This mechanism is active for Pt particles of 4–5 atoms already at 390 K. Instead, if the clusters are large (130 atoms), close to the coalescence threshold, they fluctuate around their potential energy minima. Since the mass is large, the amplitude of the fluctuations is significantly smaller than in the former case. However, since the particles have only a small edge of separation, these small fluctuations are already sufficient. When two adjacent clusters get in touch at 470 K they sinter: reshaping does not occur completely, and the resulting cluster spans two adjacent sites.

8.4.2

Activity and Chemical Stability

Novel and unprecedented chemical and reactivity properties of supported nanostructured systems are the dream of a bottom-up approach that exploits features typical of nanoclusters. The most widely known case study involves gold. Being almost an unreactive catalyst, Au exhibits extraordinary high activities in the form of dispersed ultrafine particles supported on metal oxides toward low-temperature catalytic combustion, partial oxidation of hydrocarbons, hydrogenation of unsaturated hydrocarbons, and reduction of nitrogen oxides [43]. This is related in the specific case to a metal-tononmetal transition as the cluster size is decreased below 3.5  1.0 nm2, corresponding to about 300 atoms, when Au is supported on TiO2 (110), similarly to Pd. However, the observed effects are not only due to the electronic configuration (DOS and band gap opening) but also related to the geometry with respect to adsorption of the reactant gases. To investigate this aspect, carbon monoxide is the most widely employed probe that allows for a characterization of both structure and reactivity of the supported nanoparticles. In most cases it is found, as we shall see in the following text, that the effective activity of the supported clusters is determined by the interplay among support, finite size, cluster reconstruction, and local coverage of the adsorbates, thus linking the properties to the chemical environment (gas pressure and surface temperature). Adsorption of CO on Cu/Al2O3/Ni3Al (111) is partially dissociative even at liquid nitrogen temperature for Cu clusters

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smaller than 200 atoms [82]. Carbidic carbon accumulates via the Boudouard process since breaking of the CdO bond becomes energetically favored. The low-coordinated nature of Cu atoms in the clusters allows a high CO saturation coverage (1 ML), much higher than in the case of a flat Cu surface. Upon CO saturation (see Fig. 8.4), distortion of the cluster geometry is induced, making CdO dissociation almost spontaneous and incorporation of C atoms in the cluster bulk energetically more convenient as the cluster size becomes smaller [82]. A similar behavior has been observed also for Rh nanoclusters supported on Al2O3/NiAl (100), where a size-dependent (1.0–3.0 nm) rate of CO dissociation was measured, in parallel with an increasing density of adsorbed CO molecules for smaller particles [44,105]. The intraisland restructuring of Rh clusters upon adsorption of CO was discussed also in this case [44]. A large investigation effort has been dedicated to palladium particles grown on the same support, in the view of extending the insight from ultrahigh-vacuum to near-ambient pressure conditions

FIG. 8.4 Adsorption and dissociation of CO is energetically favored and occurs already at 77 K on a small Cu cluster supported by an ultrathin alumina film due to concurrent coverage, finite-size, and support effects as explained by ab initio simulations (bottom figures). The experimental evidence from the evolution of the C 1s core levels is observed during the CO uptake (upper-left panel) and also upon annealing (upper-right panel).

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(up to 200 mbar) by exploiting novel experimental techniques such as PM-IRAS and IR-Vis SFG spectroscopy [40,45–48,106]. While at low pressure (10 7 mbar), the CO site-occupancy distribution (on-top and bridge coordination) depends on the Pd cluster’s morphology and on the temperature of the surface, under adsorption conditions closer to ambient, the CO pressure actually governs the equilibrium, thus again questioning about the transferability of ultrahigh-vacuum results to applicative conditions. As already mentioned, Pd particles can be exploited as nucleation seeds fur successive cluster growth: in the case of Ni/Pd/Al2O3/Ni3Al (111), the well-ordered Ni nanoparticles are found to be very reactive toward oxidation, thus yielding NiO clusters already at 200°C after a 200 L exposure to gas-phase oxygen [56]. Nanosized iron clusters show a similar behavior when raising the carbon monoxide pressure. Upon disproportionation of CO on a 55-atom Fe particle [107], the first step is found to be the CdO bond scission, yielding C and O atomic species at the cluster’s surface. The latter remain adsorbed, while the former move into the cluster’s bulk, presumably as the initial step toward the formation of a carbidic phase. A parallel discussion is dedicated to graphene as a template and a support to the clusters. Au islands grown on graphene/Ru (0001) interestingly conform to the corrugation of the underlying graphene and display commensurate moir
e patterns. These Au islands are found to be active toward CO oxidation even at cryogenic temperature (85 K), while graphene neither adsorbs CO [49]. Both the negative charge at the Au defective atoms and the presence of undercoordinated sites play a fundamental role. This was particularly put in evidence by a spectroscopic study of CO adsorption on Pt/graphene/Rh (111), where by means of high-energy resolution core-level photoelectron spectroscopy, several C 1s components were identified, originating from nonequivalent CO species adsorbed at on-top, bridge, and several different step defective sites [50]. The interplay between adsorption and the presence of low-coordinated sites strongly affects the chemical stability of the supported clusters. This aspect is relevant in the optics of a stable catalytic activity of the system that is related with restructuring, sintering, and poisoning processes yielding deactivation. Disruption of the metal clusters into single ad-atoms forming mobile surface carbonyls has been observed for Pt, Ir, and Rh particles on TiO2 when exposed to carbon monoxide, decay of Au and Co islands on Au (111) is promoted by the presence of S, Ostwald ripening of Pd clusters on SiO2 occurs in the presence of hydrogen, Pt particles on Al2O3 decay through mobile PtxOy species [54]. The size of the clusters plays a fundamental role: for Pt/graphene/Ir (111), clusters containing fewer than 10 atoms are unstable upon CO adsorption and sinter through Smoluchowski ripening, while larger particles remain immobile with a substantial 3D restructuring [54]. These effects are expected to be even more important when entering the world of ambient pressure conditions. Higher pressures yield higher surface coverage values and a significant contribution of the gas-phase chemical potential, thus opening to

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important pressure gap issues related with the cluster structural and chemical stability.

8.4.3 Pressure Gap Evident influence of the gas-phase pressure was reported even for single-crystal bulk-like terminations. This is the case, for example, of a stepped platinum surface that breaks up upon exposure to a near-ambient pressure (0.1 Torr) of carbon monoxide [108]. Extensive and reversible restructuring occurs yielding the spontaneous formation of Pt nanoclusters driven by the CO binding to Pt atoms and by the CO-CO lateral repulsion. Similarly, intense, CO-induced surface restructuring is observed also at a Cu (111) surface [109] and is expected to determine the actual surface reactivity. Interestingly, etching and roughening of copper electrodes strongly influences the chemical activity and selectivity in the electrocatalytic conversion of carbon dioxide [110]. If the pressure gap driving force yields such effects at bulk terminations, the consequences are expected to be even more dramatic in the case of nanoparticles where a large variety of low-coordinated metal sites is exposed. Indeed, restructuring of the clusters can occur already under vacuum conditions, as previously discussed [54]. Moreover, at near-ambient pressure, the interaction of the gas phase with the supporting substrate can neither be neglected. As an example, alumina films become OH terminated through hydroxylation in the presence of water and offer hydrogen-bonding capabilities, while alumina films can also strongly rearrange from a structural point of view through cooperative reaction mechanisms originating from defective sites [99]. Being their growth limited by kinetics related to diffusion and segregation of oxygen and metal atoms at the film-metal interface, it is natural consequence that the gas-phase composition influences the film structure, morphology, and stoichiometry. Single-layer graphene grown at single-crystal terminations is instead generally unreactive and can passivate the underlying metal surface, being stable up to ambient pressure [111,112]. However, the presence of defects in the graphene honeycomb or an uncomplete graphene sheet open up the way toward intercalation of gas-phase molecules that can adsorb at the metal surface underneath the graphene sheet. This is the case, for example, of graphene flakes on Ir (111) exposed to oxygen [113] or carbon monoxide [114], but a similar behavior was observed also in the case of strongly interacting graphene, like that grown on nickel [115–117].

8.5 PERSPECTIVES In summary, we have surveyed the properties and characteristics of metallic nanoclusters supported on both oxide and graphene templates, discussing their

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geometric and electronic structure and their chemical reactivity and stability with reference to simple chemical reactions. These systems allow partial bridging of the material gap issues related to surface science and act as optimal models to yield novel atomic-level insight into the heterogeneous catalysis mechanisms at the basis of many applicative relevant synthesis and conversion reactions, hopefully providing novel and predictive findings. However, when bridging the pressure gap by means of modern techniques allowing for experimental investigations in situ and operando, experimental evidence for chemical instability was reported, yielding sintering, ripening, reconstruction, passivation, and deactivation of the nanoparticles that may consequently lose their distinctive properties. Due to this reason, a stiffer anchoring and a geometric confinement of the metallic reactive centers should be provided. Supporting templates for an ordered metal cluster growth like oxide films and graphene at single-crystal surfaces allow obtaining a regular 2D lattice of size-distributed nanoparticles, but seeding and anchoring at defective or peculiar sites of the support is not sufficient. An idea to bypass this point is to mimic Nature. As an example, we will refer to a reaction that has recently been addressed as one of the challenges in chemistry for 2016 and beyond—CO2 conversion [118]. Nature performs catalytic conversion of CO2 for the synthesis of energy vectors and organic compounds by exploiting nanosized or single-atom photosystem and enzymatic catalytic centers, where metals (Mn, Cu, Ni, Fe…) are supported by C, S, or N linkers. Here, conversion and transport of oxygen, carbon monoxide, and carbon dioxide occur at near-ambient partial pressure of the reactants [119–122]. Interestingly, cooperative adsorption mechanisms have been observed in heme proteic groups, and recent evidence for cooperative interactions between adsorbates has been provided under very constrained model conditions (T < 77 K) [123,124]. Model biomimetic single-atom catalysts (SACs) may be obtained from 2D self-assembled monolayers of metallorganic bricks, for example, metal phthalocyanines and porphyrins [125–127]. This approach would represent the ultimate downsizing of metal clusters (see Fig. 8.5). The structural, chemical, and thermal stability of the abovementioned metallorganic molecules has already been proved upon adsorption on single-crystal surfaces [128], graphene [112,129–131] and ultrathin oxide films [132], and their effective, applicative activity is already known for catalysis and electrocatalysis [133]. Within the framework of a desired, thorough, atomic-level description, adsorption of small molecules like CO and NO at the SAC’s metal center under cryogenic conditions has already been observed by both microscopy [126,127] and spectroscopy approaches [134,135]. However, due to the low-binding energies of the reactants at the central metal site, investigations for an in situ and operando characterization at the atomic level at ambient conditions has still to come, therefore representing an interesting and very promising perspective in this field.

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FIG. 8.5 STM imaging of a well-ordered Fe-Pcs monolayer (B) obtained by chemical vapor deposition at room temperature of the molecules on a single graphene sheet (A) grown on Ir (111).

ACKNOWLEDGMENTS We acknowledge support from Italian Ministry of University and Research (MIUR) through Futuro in Ricerca FIRB 2010 project RBFR10J4H7, Consortium for Physics of Trieste, and Directorate General for the Country Promotion of the Italian Ministry of Foreign Affairs and International Cooperation (MAECI) through the executive program with Argentina. We are grateful to G. Comelli, C. Dri, Z. Feng, and N. Podda for the STM images.

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