Pd8(PDip)6: Cubic, Unsaturated, Zerovalent

Atomically precise nanoclusters hold promise for supramolecular assembly and (opto)electronic- as well as magnetic materials. Herein, this work reports that treating palladium(0) precursors with a triphosphirane affords strongly colored Pd8(PDip)6 that is fully characterized by mass spectrometry, heteronuclear and Cross-Polarization Magic-Angle Spinning (CP-MAS) NMR-, infrared (IR), UV-vis, and X-ray photoelectron (XP) spectroscopies, single-crystal X-Ray diffraction (sc-XRD), mass spectrometry, and cyclovoltammetry (CV). This coordinatively unsaturated 104-electron Pd(0) cluster features a cubic Pd8-core, µ4-capping phosphinidene ligands, and is air-stable. Quantum chemical calculations provide insight to the cluster's electronic structure and suggest 5s/4d orbital mixing as well as minor Pd─P covalency. Trapping experiments reveal that cluster growth proceeds via insertion of Pd(0) into the triphosphirane. The unsaturated cluster senses ethylene and binds isocyanides, which triggers the rearrangement to a tetrahedral structure with a reduced frontier orbital energy gap. These experiments demonstrate facile cluster manipulation and highlight non-destructive cluster rearrangement as is required for supramolecular assembly.

Data-driven stabilization of NimPdn-m nanoalloys: a study using density functional theory and data mining approaches

Green hydrogen, generated through the electrolysis of water, is a viable alternative to fossil fuels, although its adoption is hindered by the high costs associated with the catalysts. Among a wide variety of potential materials, binary nickel-palladium (NiPd) systems have garnered significant attention, particularly at the nanoscale, for their efficacious roles in catalyzing hydrogen and oxygen evolution reactions. However, our atom-level understanding of the descriptors that drive their energetic stability at the nanoscale remains largely incomplete. Here, we investigate by density functional theory calculations the descriptors that drives the stability of the NimPdn-m clusters for different sizes (n = 13, 27, 41) and compositions. To achieve our goals, a large number of trial configurations were generated and selected using data mining algorithms (k-means, t-SNE) and genetic algorithms, while the most important physical-chemical descriptors were identified using Spearman correlation analysis. We have found that core-shell formation, with the smaller Ni atoms lying in the center of the particle, plays a major role in the stabilization of the nanoalloys, and this effect causes the alloys to assume a icosahedral-fragment configuration (as the unary nickel cluster) instead of a fcc fragment (as the unary palladium cluster). However, the core-shell formation in this alloy is unique in that Pd poor compositions exhibit scattered Pd atoms on the surface. As the palladium content increases, this gives rise to the complete Pd shell. This stabilization mechanism is quantitatively supported by the different correlations observed in the number of Ni-Ni and Pd-Pd bonds with energy, in which the latter tends to decrease alloy stability. Furthermore, a notable trend is the correlation between the coordination number of Ni atoms with alloy stabilization, while the coordination of Pd atoms shows an inverse correlation.

Exploring the synergistic effect of palladium nanoparticles and highly dispersed transition metals on carbon nitride/super-activated carbon composites for boosting electrocatalytic activity

In the present work, multifunctional electrocatalysts formed by palladium nanoparticles (Pd NPs) loaded on Fe or Cu-containing composite supports, based on carbon nitride (C3N4) and super-activated carbon with a high porosity development (SBET 3180 m2/g, VDR 1.57 cm3/g, and VT 1.65 cm3/g), were synthesised. The presence of Fe or Cu sites favoured the formation of Pd NPs with small average particle size and a very narrow size distribution, which agreed with Density Functional Theory (DFT) calculations showing that the interaction of Pd clusters with C3N4 flakes is weaker than with Cu- or Fe-C3N4 sites. The electroactivity was also dependent on the composition and, as suggested by preliminary DFT calculations, the Pd-Cu catalyst showed lower overpotential for hydrogen evolution reaction (HER) while bifunctional oxygen reduction reaction/ oxygen evolution reaction (ORR/OER) behaviour was superior in Pd-Fe sample. The Pd-Fe electrocatalyst was studied in a zinc-air battery (ZAB) for 10 h, showing a performance similar to a commercial Pt/C + RuO2 catalyst with a high content of precious metal. This study demonstrates the synergistic effect between Pd species and transition metals and shows that transition metals anchored on C3N4-based composite materials promote the electroactivity of Pd NPs in HER, ORR and OER due to the interaction between both species.

A zero-valent palladium cluster-organic framework

Acquiring spatial control of nanoscopic metal clusters is central to their function as efficient multi-electron catalysts. However, dispersing metal clusters on surfaces or in porous hosts is accompanied by an intrinsic heterogeneity that hampers detailed understanding of the chemical structure and its relation to reactivities. Tethering pre-assembled molecular metal clusters into polymeric, crystalline 2D or 3D networks constitutes an unproven approach to realizing ordered arrays of chemically well-defined metal clusters. Herein, we report the facile synthesis of a {Pd3} cluster-based organometallic framework from a molecular triangulo-Pd3(CNXyl)6 (Xyl = xylyl; Pd3) cluster under chemically mild conditions. The formally zero-valent Pd3 cluster readily engages in a complete ligand exchange when exposed to a similar, ditopic isocyanide ligand, resulting in polymerization into a 2D coordination network (Pd3-MOF). The structure of Pd3-MOF could be unambiguously determined by continuous rotation 3D electron diffraction (3D-ED) experiments to a resolution of ~1.0 Å (>99% completeness), showcasing the applicability of 3D-ED to nanocrystalline, organometallic polymers. Pd3-MOF displays Pd03 cluster nodes, which possess significant thermal and aerobic stability, and activity towards hydrogenation catalysis. Importantly, the realization of Pd3-MOF paves the way for the exploitation of metal clusters as building blocks for rigidly interlocked metal nanoparticles at the molecular limit.

A DFT perspective on organometallic lanthanide chemistry

Computational studies of the coordination chemistry and bonding of lanthanides have grown in recent decades as the need for understanding the distinct physical, optical, and magnetic properties of these compounds increased. Density functional theory (DFT) methods offer a favorable balance of computational cost and accuracy in lanthanide chemistry and have helped to advance the discovery of novel oxidation states and electronic configurations. This Frontier article examines the scope and limitations of DFT in interpreting structural and spectroscopic data of low-valent lanthanide complexes, elucidating periodic trends, and predicting their properties and reactivity, presented through selected examples.

DAPD Set of Pd-Containing Diatomic Molecules: Accurate Molecular Properties and the Great Lengths to Obtain Them

In the present study, we obtained reliable bond energy, bond length, and zero-point vibrational frequency for a set of diatomic Pd species (the DAPD set). It includes PdH, Pd2, and PdX (X = B, C, N, O, F, Al, Si, P, S, and Cl). Our highest-level protocol (W4X-L) represents scalar and spin-orbit relativistic, valence- and inner-valence correlated, extrapolated CCSDTQ(5) energy. The DAPD set of molecules is challenging for computational chemistry methods in different manners; for Pd2, the spin-orbit contribution to the bond energy is fairly large, whereas for PdC and PdSi, the post-CCSD(T) correlation components are considerable. The diverse range of requirements represents a significant challenge for lower-level methods. While density functional theory (DFT) methods generally yield good agreements for bond lengths and vibrational frequencies, large deviations are found for bond energies. In general, hybrid DFT methods are more accurate than nonhybrid functionals, but the agreement in individual cases varies. This illustrates the critical role that new high-quality reference data would play in the continual development of lower-cost methods.

TURBOMOLE: Today and Tomorrow

TURBOMOLE is a highly optimized software suite for large-scale quantum-chemical and materials science simulations of molecules, clusters, extended systems, and periodic solids. TURBOMOLE uses Gaussian basis sets and has been designed with robust and fast quantum-chemical applications in mind, ranging from homogeneous and heterogeneous catalysis to inorganic and organic chemistry and various types of spectroscopy, light-matter interactions, and biochemistry. This Perspective briefly surveys TURBOMOLE's functionality and highlights recent developments that have taken place between 2020 and 2023, comprising new electronic structure methods for molecules and solids, previously unavailable molecular properties, embedding, and molecular dynamics approaches. Select features under development are reviewed to illustrate the continuous growth of the program suite, including nuclear electronic orbital methods, Hartree-Fock-based adiabatic connection models, simplified time-dependent density functional theory, relativistic effects and magnetic properties, and multiscale modeling of optical properties.

A database of low-energy atomically precise nanoclusters

The chemical and structural properties of atomically precise nanoclusters are of great interest in numerous applications, but the structures of the clusters can be computationally expensive to predict. In this work, we present the largest database of cluster structures and properties determined using ab-initio methods to date. We report the methodologies used to discover low-energy clusters as well as the energies, relaxed structures, and physical properties (such as relative stability, HOMO-LUMO gap among others) for 63,015 clusters across 55 elements. We have identified clusters for 593 out of 1595 cluster systems (element-size pairs) explored by literature that have energies lower than those reported in literature by at least 1 meV/atom. We have also identified clusters for 1320 systems for which we were unable to find previous low-energy structures in the literature. Patterns in the data reveal insights into the chemical and structural relationships among the elements at the nanoscale. We describe how the database can be accessed for future studies and the development of nanocluster-based technologies.

Toward a new definition of surface energy for late transition metals

Surface energy is a top-importance stability descriptor of transition metal-based catalysts. Here, we combined density functional theory (DFT) calculations and a tiling scheme measuring surface areas of metal structures to develop a simple computational model predicting the average surface energy of metal structures independently of their shape. The metals considered are W, Ru, Co, Ir, Ni, Pd, Pt, Cu, Ag and Au. Lorentzian trends derived from the DFT data proved effective at predicting the surface energy of metallic surfaces but not for metal clusters. We used machine-learning protocols to build an algorithm that improves the Lorentzian trend's accuracy and is able to predict the surface energies of metal surfaces of any crystal structure, i.e., face-centred cubic, hexagonal close-packed, and body-centred cubic, but also of nanostructures and sub-nanometer clusters. The machine-learning neural network takes easy-to-compute geometric features to predict metallic moieties surface energies with a mean absolute error of 0.091 J m^-2 and an R² score of 0.97.

Estudio teórico de la oxidación de CO con O2 usando catalizadores de Au-Pd y Au-Pt

En el presente estudio se realizaron cálculos con base en la Teoría del Funcional de la Densidad Electrónica (DFT) con la aproximación B3PW91/LANL2DZ para optimizar los sistemas monometálicos y bimetálicos Au9, Au8Pd, Au8Pt, AuPd8, AuPt8, Pd9 y Pt9. Los materiales fueron teóricamente evaluados como catalizadores para la oxidación de monóxido de carbono (CO) y se determinó el sistema más favorable para la adsorción de esta molécula. La sustitución de átomos de Pt y Pd por átomos de Au en los nonámeros generó un cambio en la estructura tridimensional del sistema. El análisis de reactividad global mostró que el clúster más reactivo es 𝑃𝑡9, seguido por 𝐴𝑢𝑃𝑡8. Los índices de Fukui identificaron los sitios más susceptibles para un ataque nucleofílico de ambos clústeres. La adsorción de CO generó una cascada de oxidación que liberó ~4,5 eV, indicando que la reacción es altamente exotérmica y exergónica. Los clústeres 𝐴𝑢𝑃𝑡8 y 𝑃𝑡9 mostraron los valores más bajos de energía de activación de la etapa determinante del mecanismo. En general, la sustitución de un átomo de platino (o paladio) por un átomo de oro no afecta la reactividad de los nonámeros y, por tanto, se infiere que el clúster 𝐴𝑢𝑃𝑡8 podría ser un catalizador promisorio en la oxidación de CO.

Thermodynamic stability of Pd-Ru alloy nanoparticles: combination of density functional theory calculations, supervised learning, and Wang-Landau sampling

Solid-solution alloy nanoparticles (NPs) comprising Pd and Ru, which are immiscible in the bulk state, have been synthesised and show excellent catalytic performance. To date, most studies have evaluated the stability of alloy NPs at 0 K only. Because the thermodynamic stability of Pd-Ru alloy NPs may differ from that of the alloy in the bulk state, the stable configuration of the NPs must be evaluated under a finite temperature. Such stability evaluations are critical for developing the durable NPs as catalysts. Therefore, the thermodynamic stability of Pd-Ru alloy NPs was analysed using density functional theory (DFT), supervised learning (SL), and Wang-Landau sampling. We calculated the excess energy of Pd-Ru alloy NPs, which depends on their composition, structure, NP size, adatom type, and defects, and applied SL to all models. The excess energies of the Pd-Ru alloy NPs expressed by structural information, such as the surface-to-volume ratio, correlated with those calculated using DFT. Wang-Landau sampling based on the energy estimated by SL gave the thermodynamic stability of Pd-Ru alloy NPs with a stable configuration under a finite temperature. The solid-solution atomic configuration was subdivided into partially mixed configurations in the surface layer or in the core of the NPs, which is different from the bulk state. The partially mixed configuration was determined by the overall composition and surface properties. The findings from the combined method could contribute to a better understanding of the alloy-NP stability and their application in catalysis.

Palladium clusters, free and supported on surfaces, and their applications in hydrogen storage

Palladium is a late transition metal element in the 4d row of the periodic table. Palladium nanoparticles show efficient catalytic activity and selectivity in a number of chemical reactions. In this paper, we review the structural and electronic properties of palladium nanoclusters, both isolated and deposited on the surface of different substrates. Careful experiments and extensive calculations have been performed for small Pd clusters which provide ample information on their properties. Work on large Pd clusters is less abundant and more difficult to perform and interpret. Cluster deposition is a method to modify material surfaces for different applications, and we report the known results for the deposition of Pd clusters on the surfaces of a number of interesting substrates: carbonaceous substrates like graphene and some layered novel materials related to graphene, metal oxide substrates, silicon and silicon-related substrates and metallic alloy substrates. Emphasis is placed on revealing how the structural, electronic and magnetic properties change when the clusters are deposited on the substrate surfaces. Some examples of chemical reactions catalyzed by supported Pd clusters and nanoparticles are reported. An issue discussed in detail is the influence of Pd on the storage of hydrogen in porous materials. Experimental work shows that the amount of stored hydrogen increases when the absorbing material is doped with Pd atoms, clusters and nanoparticles, and a spillover mechanism from the metal particle to the substrate is usually accepted as the explanation. To shed light on this issue, a critical analysis based on density functional simulations of the mechanisms of hydrogen spillover in perfect and defective graphene doped with palladium clusters is presented.

Hyperfine Coupling Constants in Local Exact Two-Component Theory

We present a highly efficient implementation of the electron-nucleus hyperfine coupling matrix within the one-electron exact two-component (X2C) theory. The complete derivative of the X2C Hamiltonian is formed, that is, the derivatives of the unitary decoupling transformation are considered. This requires the solution of the response and Sylvester equations, consequently increasing the computational costs. Therefore, we apply the diagonal local approximation to the unitary decoupling transformation (DLU). The finite nucleus model is employed for both the scalar potential and the vector potential. Two-electron picture-change effects are modeled with the (modified) screened nuclear spin-orbit approach. Our implementation is fully integral direct and OpenMP-parallelized. An extensive benchmark study regarding the Hamiltonian, the basis set, and the density functional approximation is carried out for a set of 12-17 transition-metal compounds. The error introduced by DLU is negligible, and the DLU-X2C Hamiltonian accurately reproduces its four-component "fully" relativistic parent results. Functionals with a large amount of Hartree-Fock exchange such as CAM-QTP-02 and ωB97X-D are generally favorable. The pure density functional r²SCAN performs remarkably and even outperforms the common hybrid functionals TPSSh and CAM-B3LYP. Fully uncontracted basis sets or contracted quadruple-ζ bases are required for accurate results. The capability of our implementation is demonstrated for [Pt(C₆Cl₅)₄]^- with more than 4700 primitive basis functions and four rare-earth single-molecule magnets: [La(OAr*)₃]^-, [Lu(NR₂)₃]^-, [Lu(OAr*)₃]^-, and [TbPc₂]^-. Here, the results with the spin-orbit DLU-X2C Hamiltonian are in an excellent agreement with the experimental findings of all Pt, La, Lu, and Tb molecules.

Theoretical Investigation of the Size Effect on the Oxygen Adsorption Energy of Coinage Metal Nanoparticles

This study evaluates the finite size effect on the oxygen adsorption energy of coinage metal (Cu, Ag and Au) cuboctahedral nanoparticles in the size range of 13 to 1415 atoms (0.7–3.5 nm in diameter). Trends in particle size effects are well described with single point calculations, in which the metal atoms are frozen in their bulk position and the oxygen atom is added in a location determined from periodic surface calculations. This is shown explicitly for Cu nanoparticles, for which full geometry optimization only leads to a constant offset between relaxed and unrelaxed adsorption energies that is independent of particle size. With increasing cluster size, the adsorption energy converges systematically to the limit of the (211) extended surface. The 55-atomic cluster is an outlier for all of the coinage metals and all three materials show similar behavior with respect to particle size.

Graphic Abstract

On the ligand-free palladium cluster catalysed Suzuki-Miyaura reaction

C-C cross coupling reactions have been widely used for developing synthesis protocols for pharmaceuticals and agricultural products in the past few decades. Of all the reported C-C cross coupling reactions, the Suzuki-Miyaura reaction is preferred because of its mild reaction conditions, the commercial availability of associated reagents and the ease of removal of boron containing by-products. Recently, Corma and co-workers [Leyva-Perez et al., Angew. Chem., 2013, 125, 11768] reported water-stabilized three- and four-atom Pd clusters as highly active catalytic species for C-C coupling reactions. The present work focuses on developing detailed mechanistic insights into the Suzuki-Miyaura reaction with Pd₃ and Pd₄ clusters utilizing density functional theory calculations. The role of the base in the reaction was analysed in this study, which was found to lower the activation barriers of transmetalation over both Pd₃ and Pd₄. Free energy landscapes for Suzuki-Miyaura coupling of bromobenzene and phenylboronic acid over Pd₃ and Pd₄ clusters were developed. The highest free energy barriers of 34.7 and 30.4 kcal mol^-1 were observed for the oxidative addition over Pd₃ and Pd₄, respectively, indicating the oxidative addition as the rate limiting step. Detailed energetics conclusively proved the active nature of small-atom Pd clusters for catalyzing the Suzuki-Miyaura reaction.

Towards a morphology of cobalt nanoparticles: size and strain effects

Cobalt nanoparticles with diameters of 8 nm have recently shown promising performance for biomedical applications. However, it is still unclear how the shape of cobalt clusters changes with size when reaching the nanoparticle range. In the present work, density functional theory calculations have been employed to compare the stabilities of two non-crystalline (icosahedron and decahedron) shapes, and three crystalline motifs (hcp, fcc, and bcc) for magic numbered cobalt clusters with up to 1500 atoms, based on the changes in the cohesive energies, coordination numbers, and nearest-neighbour distances arising from varying geometries. Obtained trends were extrapolated to a 10⁴ size range, and an icosahedral shape was predicted for clusters up to 5500 atoms. Larger sized clusters adopt hcp stacking, in correspondence with the bulk phase. To explain the crystalline/non-crystalline crossovers, the contributions of the elastic strain density and twin boundary from the specimen surfaces to the cohesive energy of different motifs were evaluated. These results are expected to aid the design and synthesis of cobalt nanoparticles for applications ranging from catalysis to biomedical treatments.

On the influence of exact exchange on transition metal superatoms

The electronic structure of A7C (A = Hg, Pd, V, Cr, Mn, Fe, Ni, Cu; C = 0, ±1, ±2) clusters has been determined using density functional theory methods. The A7C (A = Hg, Pd, Cr, Cu; C = 0, ±1, ±2) clusters all conform to the existing superatomic model, with a sufficiently stabilised local structure to prevent perturbation upon the introduction of exact exchange to the exchange correlation functional. For the A7C (A = Mn, Fe, Ni; C = 0, ±1, ±2) clusters the incorporation of exact exchange separates the atomic s- and d-electrons, leading to a net increase in the number of superatomic electrons. Conversely the incorporation of exact exchange into the exchange correlation functional decreases the number of superatomic electrons for the V7C (C = 0, ±1, ±2) clusters, owing to the radial extension of the d-orbitals influencing their ability to contribute into superatomic shells.

Density functional theory study of palladium cluster adsorption on a graphene support

The geometric, thermodynamic and electronic properties of Pd–graphene nanocomposites are comprehensively studied through quantum mechanical methods. Geometries of these clusters are optimized with the well-calibrated Minnesota functional M06-2X. The adsorption energies calculated at the M06-2X/LANL2DZ level show better agreement with those calculated from MP2/ANO-RCC-VDZP. Two different representative models for graphene, coronene and hexabenzocoronene, are used. The adsorption energies analysis reveals that the interaction energies increase with the size of the adsorbed cluster. However, for Pd_n/hexabenzocoronene, the interaction energies show a sudden drop at Pd₈/hexabenzocoronene. The difference in behavior between the interaction energies of Pd_n/hexabenzocoronene and Pd_n/coronene is attributed to the edge effect present in coronene. The electronic properties, including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), Fermi level, molecular electrostatic potential (MEP), dipole moment, vertical ionization potential (VIP), vertical electron affinity (VEA), chemical hardness (η), softness (S) and chemical potential (μ) are studied. The VIP and VEA reveal that Pd_n/coronene clusters are stable in nature with the least reactivity. The HOMO–LUMO energy gaps are reduced with the increase in cluster size. The electronic properties show irregular trends, where the most favorable electronic properties are obtained for Pd₇/coronene and Pd₁₀/coronene.

The geometric, thermodynamic and electronic properties of Pd–graphene nanocomposites are comprehensively studied through quantum mechanical methods.

A quantum chemical study of hydrogen adsorption on carbon-supported palladium clusters

A key step for achieving better insight into catalytic hydrogenation reactions is to understand in detail the process of hydrogen adsorption on the catalyst. The present article focuses on hydrogen adsorption on carbon-supported palladium clusters, which are nowadays one of the most common catalysts in industrial applications. Density functional theory is applied to study Pd₆ and Pd₂₁ clusters to reveal the influence of the carbon support material on the properties of the catalyst as well as on the mechanisms and energetics of the hydrogen adsorption. In general, a stepwise hydrogen adsorption process is observed consisting of molecular adsorption followed by dissociative chemisorption. The carbon support material does not noticeably affect the reaction mechanisms, but has a large influence on energy barriers and preferential adsorption sites. Our comparison of Pd₆ and Pd₂₁ systems reveals that small clusters, such as Pd₆, are able to model some but not all important properties of palladium nanoparticles and, therefore, it is essential to also study larger cluster sizes.

A Global Optimizer for Nanoclusters

We have developed an algorithm to automatically build the global minimum and other low-energy minima of nanoclusters. This method is implemented in PyAR (https://github.com/anooplab/pyar) program. The global optimization in PyAR involves two parts, generation of several trial geometries and gradient-based local optimization of the trial geometries. While generating the trial geometries, a Tabu list is used for storing the information of the already used trial geometries to avoid using the similar trial geometries. In this recursive algorithm, an n-sized cluster is built from the geometries of n−1 clusters. The overall procedure automatically generates many unique minimum energy geometries of clusters with size from 2 up to n using this evolutionary growth strategy. We have used our strategy on some of the well-studied clusters such as Pd, Pt, Au, and Al homometallic clusters, Ru-Pt and Au-Pt binary clusters, and Ag-Au-Pt ternary cluster. We have analyzed some of the popular parameters to characterize the clusters, such as relative energy, singlet-triplet energy difference, binding energy, second-order energy difference, and mixing energy, and compared with the reported properties.

Dimensionality Expansion of a Butterfly Shaped Pd4 Framework: Constructing Edge-Sharing Pd6 Tetrahedra

The construction of well-defined transition-metal clusters has attracted substantial attention due to their unique chemical and/or physical properties. Metal clusters with 1D or 2D structures are now accessible by template-synthesis methods, in which multiple metal atoms are arranged with the aid of template molecules and their 1D or 2D structures. However, the rational synthesis of 3D clusters remains challenging, mostly due to a lack of appropriate template molecules. Herein, we report the rational synthesis of a 2D butterfly shaped Pd₄ framework (2) and 3D edge-sharing Pd₆ tetrahedra (5) by treatment of easily available organosilicon compounds with Pd(CNtBu)₂ . The diphenylsilylene moiety thereby serves as the key component to generate the butterfly structure of the Pd₄ clusters in 2. A dimensionality expansion, induced by two Cl atoms, of two butterfly shaped Pd₄ subunits supported by two diphenylsilylene moieties afforded the edge-sharing tetrahedral architecture of the Pd₆ cluster in 5.

Thermodynamically accessible titanium clusters TiN, N = 2–32

We have performed a genetic algorithm search on the tight-binding interatomic potential energy surface (PES) for small Ti_N (N = 2–32) clusters.

Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO₂(111). Starting from stable Pd _n clusters (n = 1-21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pd _n clusters on the CeO₂(111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.

Effects of Palladium(II) Chlorocomplex Speciation on the Controlled Interaction with a Polyaniline Film in Acid

In this study, we determined the Pd(II) chlorocomplex species that has the most favorable interaction with an electropolymerized and protonated polyaniline (PANI) film. This study was completed with the intent to use this species to electrochemically build atomic palladium clusters in the PANI matrix. Varying amounts of NaCl were added to a K₂PdCl₄/HClO₄ solution to result in three species studied: PdCl₂(H₂O)₂, PdCl₃(H₂O)^-, and PdCl₄^2-. UV-vis spectroscopy was used to confirm the speciation, and Raman spectroscopy, X-ray photoelectron spectroscopy, and cyclic voltammograms were used to probe the interaction between the Pd species and PANI. It was determined that PdCl₃(H₂O)^- most effectively interacts with PANI as a result of the charge balance between the anion and the protonated nitrogen-containing groups in the polymer. It has been also found that some fraction of inserted Pd(II) cannot be reduced to Pd(0).

Evolution of the structural, energetic, and electronic properties of the 3d, 4d, and 5d transition-metal clusters (30 TMn systems for n = 2–15): a density functional theory investigation

Subnanometric transition-metal (TM) clusters have attracted great attention due to their unexpected physical and chemical properties, leastwise compared to their bulk counterparts.

Tunable interaction between metal clusters and graphene

Using first principles calculations we find that the interaction between small transition metal clusters and graphene follows the d-band model.

Development of Hierarchical Polymer@Pd Nanowire-Network: Synthesis and Application as Highly Active Recyclable Catalyst and Printable Conductive Ink

A facile one-pot approach for preparing hierarchical nanowire-networks of hollow polymer@Pd nanospheres is reported. First, polymer@Pd hollow nanospheres were produced through metal-complexation-induced phase separation with functionalized graft copolymers and subsequent self-assembly of PdNPs. The nanospheres hierarchically assembled into the nanowire-network upon drying. The Pd nanowire-network served as an active catalyst for Mizoroki-Heck and Suzuki-Miyaura coupling reactions. As low as 500 μmol % Pd was sufficient for quantitative reactions, and the origin of the high activity is ascribed to the highly active sites originating from high-index facets, kinks, and coalesced structures. The catalyst can be recycled via simple filtration and washing, maintaining its high activity owing to the micrometer-sized hierarchical structure of the nanomaterial. The polymer@Pd nanosphere also served as a printable conductive ink for a translucent grid pattern with excellent horizontal conductivity (7.5×10(5) S m(-1)).

Insights into the geometries, electronic and magnetic properties of neutral and charged palladium clusters

We performed an unbiased structure search for low-lying energetic minima of neutral and charged palladium Pd_n^Q (n = 2–20, Q = 0, + 1 and –1) clusters using CALYPSO method in combination with density functional theory (DFT) calculations. The main candidates for the lowest energy neutral, cationic and anionic clusters are identified, and several new candidate structures for the cationic and anionic ground states are obtained. It is found that the ground state structures of small palladium clusters are more sensitive to the charge states. For the medium size Pd_n^0/+/– (n = 16–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at n = 14 for the neutral and cationic clusters. In contrast, for the anionic counterparts, the structural transition occurs at Pd₁₃^–. The photoelectron spectra (PES) of palladium clusters are simulated based on the time-dependent density functional theory (TD-DFT) method and compared with the experimental data. The good agreement between the experimental PES and simulated spectra provides us unequivocal structural information to fully solve the global minimum structures, allowing for new molecular insights into the chemical interactions in the Pd cages.

Global optimization of small bimetallic Pd–Co binary nanoalloy clusters: a genetic algorithm approach at the DFT level

The global optimisation of small bimetallic PdCo binary nanoalloys are systematically investigated using the Birmingham Cluster Genetic Algorithm (BCGA).

Size-dependent properties of transition metal clusters: from molecules to crystals and surfaces--computational studies with the program ParaGauss

In the so-called scalable regime the size-dependent behavior of the physical and chemical properties of transition metal clusters is described by scaling relationships. For most quantities this scalable regime is reached for cluster sizes between a few tens and a few hundreds of atoms, hence for systems for which an accurate treatment by density functional theory is still feasible. Thus, by invoking scaling relations one is able to obtain properties of very large nanoparticles and even the bulk limit from the results of a series of smaller cluster models. In this invited review we illustrate this strategy by exploiting results from computational studies that mostly were carried out with the density functional theory software ParaGauss. We address mainly the size-dependent behavior of the properties of transition metal clusters. To this end, we first present benchmark studies probing various approximations that are used in such density functional calculations. Subsequently we show how physical insight may be gained by exploring less understood types of systems. These applications range from bare clusters to nanoislands and nanoalloys to adsorption complexes.

Extending DFT-based genetic algorithms by atom-to-place re-assignment via perturbation theory: a systematic and unbiased approach to structures of mixed-metallic clusters

Energy surfaces of metal clusters usually show a large variety of local minima. For homo-metallic species the energetically lowest can be found reliably with genetic algorithms, in combination with density functional theory without system-specific parameters. For mixed-metallic clusters this is much more difficult, as for a given arrangement of nuclei one has to find additionally the best of many possibilities of assigning different metal types to the individual positions. In the framework of electronic structure methods this second issue is treatable at comparably low cost at least for elements with similar atomic number by means of first-order perturbation theory, as shown previously [F. Weigend, C. Schrodt, and R. Ahlrichs, J. Chem. Phys. 121, 10380 (2004)]. In the present contribution the extension of a genetic algorithm with the re-assignment of atom types to atom sites is proposed and tested for the search of the global minima of PtHf12 and [LaPb7Bi7](4-). For both cases the (putative) global minimum is reliably found with the extended technique, which is not the case for the "pure" genetic algorithm.

Chelation technology: a promising green approach for resource management and waste minimization

Green chemical engineering recognises the concept of developing innovative environmentally benign technologies to protect human health and ecosystems. In order to explore this concept for minimizing industrial waste and for reducing the environmental impact of hazardous chemicals, new greener approaches need to be adopted for the extraction of heavy metals from industrial waste. In this review, a range of conventional processes and new green approaches employed for metal extraction are discussed in brief. Chelation technology, a modern research trend, has shown its potential to develop sustainable technology for metal extraction from various metal-contaminated sites. However, the interaction mechanism of ligands with metals and the ecotoxicological risk associated with the increased bioavailability of heavy metals due to the formation of metal-chelant complexes is still not sufficiently explicated in the literature. Therefore, a need was felt to provide a comprehensive state-of-the-art review of all aspects associated with chelation technology to promote this process as a green chemical engineering approach. This article elucidates the mechanism and thermodynamics associated with metal-ligand complexation in order to have a better understanding of the metal extraction process. The effects of various process parameters on the formation and stability of complexes have been elaborately discussed with respect to optimizing the chelation efficiency. The non-biodegradable attribute of ligands is another important aspect which is currently of concern. Therefore, biotechnological approaches and computational tools have been assessed in this review to illustrate the possibility of ligand degradation, which will help the readers to look for new environmentally safe mobilizing agents. In addition, emerging trends and opportunities in the field of chelation technology have been summarized and the diverse applicability of chelation technology in metal extraction from contaminated sites has also been reviewed.

Theoretical study of catalytic oxidation of CO on free PdxO2+ (x = 4–6) clusters: size dependent comparison of combustion

A DFT study has been adopted to explore the catalytic CO oxidation on a free Pd_xO₂⁺ (x = 4–6) cluster.