Bringing Selectivity in H/D Exchange Reactions Catalyzed by Metal Nanoparticles through Modulation of the Metal and the Ligand Shell.

Ru and Rh nanoparticles catalyze the selective H/D exchange in phosphines using D2 as the deuterium source. The position of the deuterium incorporation is determined by the structure of the P-based substrates, while activity depends on the nature of the metal, the properties of the stabilizing agents, and the type of the substituent on phosphorus. The appropriate catalyst can thus be selected either for the exclusive H/D exchange in aromatic rings or also for alkyl substituents. The selectivity observed in each case provides relevant information on the coordination mode of the ligand. Density functional theory calculations provide insights into the H/D exchange mechanism and reveal a strong influence of the phosphine structure on the selectivity. The isotope exchange proceeds via C-H bond activation at nanoparticle edges. Phosphines with strong coordination through the phosphorus atom such as PPh3 or PPh2Me show preferred deuteration at ortho positions of aromatic rings and at the methyl substituents. This selectivity is observed because the corresponding C-H moieties can interact with the nanoparticle surface while the phosphine is P-coordinated, and the C-H activation results in stable metallacyclic intermediates. For weakly coordinating phosphines such as P(o-tolyl)3, the interaction with the nanoparticle can occur directly through phosphine substituents, and then, other deuteration patterns are observed.

Pt38 as a promising ethanol catalyst: a first principles study.

This first-principles study predicts Pt38 nanoparticles as a catalyst for ethanol reactions. Starting from the adsorption properties, we shed light on the effectiveness of Pt-based nanoclusters as ethanol catalysts. First, the ethanol adsorption on Pt38 shows that the most stable site positions the molecule with the oxygen anchored on top of an edge, whereas CH3 is oriented towards the facet and the molecule remains in trans-symmetry. The ethanol-oxygen adsorbed on top of a facet Pt-atom offers the least stable configuration and the longer Pt-O distance (2.318 Å), while the shorter Pt-O distance (2.237 Å) is found when ethanol is on top of an edge site and the molecule is vertically oriented with Gauche symmetry. A shorter Pt-O distance correlates with higher radial breathing of the nanoparticle after ethanol adsorption. Atomic charge redistribution is calculated on all the considered systems and cases. In any event, we show that the Pt-anchor receives a charge, whilst oxygen-ethanol donates electrons. Orbital analysis shows that Pt-anchors and ethanol-oxygen atoms primarily exchange p-charge. Energy barriers associated with the ethanol bond cleavage show that the C-C bond break is slightly more favourable on Pt38 than on an extended Pt(111). In addition, we find that the cleavage of the hydroxyl O-H ethanol bond shows a higher energy barrier while the removal of an H-atom from the CH3 group is easier. These three facts indicate that the Pt38 nanoparticle enhances ethanol catalysis and hence is a good candidate for ethanol-based fuel cells.

Structural characterisation of nanoalloys for (photo)catalytic applications with the Sapphire library.

A non-trivial interplay rules the relationship between the structure and the chemophysical properties of a nanoparticle. In this context, characterization experiments, molecular dynamics simulations and electronic structure calculations may allow the variables that determine a given property to be pinpointed. Conversely, a rigorous computational characterization of the geometry and chemical ordering of metallic nanoparticles and nanoalloys enables discrimination of which descriptors could be linked with their stability and performance. To this end, we introduce a modular and open-source library, Sapphire, which may classify the structural characteristics of a given nanoparticle through several structural analysis techniques and order parameters. A special focus is geared towards using geometrical descriptors to make predictions on a given nanoparticle's catalytic activity.

Bonding Nature between Noble Gases and Small Gold Clusters.

Noble gases are usually seen as utterly inert, likewise gold, which is typically conceived as the noblest of all metals. While one may expect that noble gases bind to gold via dispersion interactions only, strong bonds can be formed between noble gas atoms and small gold clusters. We combine mass spectrometry, infrared spectroscopy, and density functional theory calculations to address the bonding nature between Aun+ (n ≤ 4) clusters and Ar, Kr, and Xe. We unambiguously determine the geometries and quantitatively uncover the bonding nature in AunNgm+ (Ng = Ar, Kr, Xe) complexes. Each Au cluster can form covalent bonds with atop bound noble gas atoms, with strengths that increase with the noble gas atomic radius. This is demonstrated by calculated adsorption energies, Bader electron charges, and analysis of the electron density. The covalent bonding character, however, is limited to the atop-coordinated Ng atoms.

Exploring AuRh Nanoalloys: A Computational Perspective on the Formation and Physical Properties.

We studied the formation of AuRh nanoalloys (between 20-150 atoms) in the gas phase by means of Molecular Dynamics (MD) calculations, exploring three possible formation processes: one-by-one growth, coalescence, and nanodroplets annealing. As a general trend, we recover a predominance of Rh@Au core-shell ordering over other chemical configurations. We identify new structural motifs with enhanced thermal stabilities. The physical features of those selected systems were studied at the Density Functional Theory (DFT) level, revealing profound correlations between the nanoalloys morphology and properties. Surprisingly, the arrangement of the inner Rh core seems to play a dominant role on nanoclusters' physical features like the HOMO-LUMO gap and magnetic moment. Strong charge separations are recovered within the nanoalloys suggesting the existence of charge-transfer transitions.

The size-dependent influence of palladium doping on the structures of cationic gold clusters.

The physicochemical properties of small metal clusters strongly depend on their precise geometry. Determining such geometries, however, is challenging, particularly for clusters formed by multiple elements. In this work, we combine infrared multiple photon dissociation spectroscopy and density functional theory calculations to investigate the lowest-energy structures of Pd doped gold clusters, PdAu n-1 + (n ≤ 10). The high-quality experimental spectra allow for an unambiguous determination of the structures adopted by the clusters. Our results show that the Pd-Au interaction is so large that the structures of PdAu n-1 + and Au n + are very different. Pd doping induces a 2D to 3D transition at much smaller cluster sizes than for pure Au n + clusters. PdAu n-1 + clusters are three-dimensional from n = 4, whereas for Au n + this transition only takes place at n = 7. Despite the strong Au-Pd interaction, the Au n-1 + cluster geometries remain recognizable in PdAu n-1 + up to n = 7. This is particularly clear for PdAu6 +. In PdAu8 + and PdAu9 +, Pd triggers major rearrangements of the Au clusters, which adopt pyramidal shapes. For PdAu4 + we find a geometry that was not considered in previous studies, and the geometry found for PdAu8 + does not correspond to the lowest-energy structure predicted by DFT, suggesting kinetic trapping during formation. This work demonstrates that even with the continuous improvement of computational methods, unambiguous assignment of cluster geometries still requires a synergistic approach, combining experiment and computational modelling.

Data-driven simulation and characterisation of gold nanoparticle melting.

The simulation and analysis of the thermal stability of nanoparticles, a stepping stone towards their application in technological devices, require fast and accurate force fields, in conjunction with effective characterisation methods. In this work, we develop efficient, transferable, and interpretable machine learning force fields for gold nanoparticles based on data gathered from Density Functional Theory calculations. We use them to investigate the thermodynamic stability of gold nanoparticles of different sizes (1 to 6 nm), containing up to 6266 atoms, concerning a solid-liquid phase change through molecular dynamics simulations. We predict nanoparticle melting temperatures in good agreement with available experimental data. Furthermore, we characterize the solid-liquid phase change mechanism employing an unsupervised learning scheme to categorize local atomic environments. We thus provide a data-driven definition of liquid atomic arrangements in the inner and surface regions of a nanoparticle and employ it to show that melting initiates at the outer layers.

Born to be different: the formation process of Cu nanoparticles tunes the size trend of the activity for CO2 to CH4 conversion.

We investigate the impact of the formation process of Cu nanoparticles on the distribution of adsorption sites and hence on their activity. Using molecular dynamics, we model formation pathways characteristic of physical synthesis routes as the annealing of a liquid droplet, the growth proceeding via the addition of single atoms, and the coalescence of individual nanoparticles. Each formation process leads to different and characteristic size-dependent distributions of their adsorption sites, catalogued and monitored on-the-fly by means of a suitable geometrical descriptor. Annealed or coalesced nanoparticles present a rather homogeneous distribution in the kind and relative abundance of non-equivalent adsorption sites. Atom-by-atom grown nanoparticles, instead, exhibit a more marked occurrence of adsorption sites corresponding to adatoms and small islands on (111) and (100) facets. Regardless of the formation process, highly coordinated sites are more likely in larger nanoparticles, while the abundance of low-coordination sites depends on the formation process and on the nanoparticle size. Furthermore, we show how each characteristic distribution of adsorption sites reflects in different size trends for the Cu-nanoparticle activity, taking as an example the electro-reduction of CO2 into CH4. To this end, we employ a multi-scale method and observe that the faceted but highly defected structures obtained during the atom-by-atom growth become more and more active with increasing size, with a mild dependence on the original seed. In contrast, the activity of Cu-nanoparticles obtained by annealing decreases with their size, while coalesced nanoparticles' activity shows a non-monotonic behaviour.

A universal signature in the melting of metallic nanoparticles.

Predicting when phase changes occur in nanoparticles is fundamental for designing the next generation of devices suitable for catalysis, biomedicine, optics, chemical sensing and electronic circuits. The estimate of the temperature at which metallic nanoparticles become liquid is, however, a challenge and a standard definition is still missing. We discover a universal feature in the distribution of the atomic-pair distances that distinguishes the melting transition of monometallic nanoparticles. We analyse the solid-liquid change of several late-transition metals nanoparticles, i.e. Ni, Cu, Pd, Ag, Au and Pt, through classical molecular dynamics. We consider various initial shapes from 146 to 976 atoms, corresponding to the 1.5-4.1 nm size range, placing the nanoparticles in either a vacuum or embedded in a homogeneous environment, simulated by an implicit force-field. Regardless of the material, its initial shape, size and environment, the second peak in the pair-distance distribution function, expected at the bulk lattice distance, disappears when the nanoparticle melts. As the pair-distance distribution is a measurable quantity, the proposed criterion holds for both numerical and experimental investigations. For a more straightforward calculus of the melting temperature, we demonstrate that the cross-entropy between a reference solid pair-distance distribution function and the one of nanoparticles at increasing temperatures present a quasi-first order transition at the phase-change temperature.

Stability of cationic silver doped gold clusters and the subshell-closed electronic configuration of AgAu14.

Silver doping is a valuable route to modulate the structural, electronic, and optical properties of gold clusters. We combine photofragmentation experiments with density functional theory calculations to investigate the relative stability of cationic Ag doped Au clusters, AgAuN-1 + (N ≤ 40). The mass spectra of the clusters after photofragmentation reveal marked drops in the intensity of AgAu8 +, AgAu14 +, and AgAu34 +, indicating a higher relative stability of these sizes. This is confirmed by the calculated AgAuN-1 + (N ≤ 17) dissociation energies peaking for AgAu6 +, AgAu8 +, and AgAu14 +. While the stability of AgAu6 + and AgAu8 + can be explained by the accepted electronic shell model for metal clusters, density of states analysis shows that the geometry plays an important role in the higher relative stability of AgAu14 +. For this size, there is a degeneracy lifting of the 1D shell, which opens a relatively large HOMO-LUMO gap with a subshell-closed 1S21P41P21D6 electronic configuration.

Correlating Oxygen Reduction Reaction Activity and Structural Rearrangements in MgO-Supported Platinum Nanoparticles.

We develop a multi-scale approach towards the design of metallic nanoparticles with applications as catalysts in electrochemical reactions. The here discussed method exploits the relationship between nanoparticle architecture and electrochemical activity and is applied to study the catalytic properties of MgO(100)-supported Pt nanosystems undergoing solid-solid and solid-liquid transitions. We observe that a major increment in the activity is associated to the reconstruction of the interface layers, supporting the need for a full geometrical characterisation of such structures also when in-operando.

A genomic characterisation of monometallic nanoparticles.

Because size and shape can affect the chemo-physical properties of nanoparticles, we extend the use of geometrical descriptors to sequence a genome of monometallic nanoparticles. Selecting the generalised coordination number as a descriptor, the derived geometrical genome distinguishes, catalogues, and counts the variety of adsorption sites available on each isomer with a diameter up to 10 nm, therefore it depends on the nanoparticle size and shape. This procedure allows us to elucidate the effects of morphological diversity within a sample and those of thermally activated structural rearrangements among isomers on nanocatalyst activity. By screening the geometrical genome of archetypal shapes, we forecast Pt stellated twinned nanoparticles, elongated along their five-fold axis and with their shortest diameter of ∼2 nm, as optimal candidates for the electro-reduction of molecular oxygen at room temperature, in agreement with available experimental data.

Structural properties of sub-nanometer metallic clusters.

At the nanoscale, the investigation of structural features becomes fundamental as we can establish relationships between cluster geometries and their physicochemical properties. The peculiarity lies in the variety of shapes often unusual and far from any geometrical and crystallographic intuition clusters can assume. In this respect, we should treat and consider nanoparticles as a new form of matter. Nanoparticle structures depend on their size, chemical composition, ordering, as well as external conditions e.g. synthesis method, pressure, temperature, support. On top of that, at finite temperatures nanoparticles can fluctuate among different structures, opening new and exciting horizons for the design of optimal nanoparticles for advanced applications. This article aims to overview geometrical features of transition metal clusters and of their various rearrangements.