Pd Au Clusters Research Articles

Coalescence-induced shape transformations in Pd and AuPd clusters.

The coalescence of pure Pd and Pd-rich AuPd nanoparticles is studied by molecular dynamics simulations. The collision of icosahedral nanoparticles with face-centered-cubic truncated octahedral ones is considered. These structures are considered because they have been experimentally shown to be the most stable for AuPd nanoalloys and for pure Pd clusters. The final structures of the coalescence process are determined depending on the composition of the initial colliding units, which heavily influences the structure of the final outcome. The key mechanisms at the atomic level of coalescence processes are analysed, revealing the peculiar types of collective atomic rearrangements that enable the transformation of an icosahedron into a sequence of close packed planes and, conversely, the transformation of a series of close packed planes into icosahedral shells.

Carbon bowl-confined subnanometric palladium-gold clusters for formic acid dehydrogenation and hexavalent chromium reduction

Formic acid (FA), a high-value product of CO2 hydrogenation and biomass conversion, is considered a promising liquid organic hydrogen carrier for its high hydrogen content, easy accessibility, and relative stability. The development of an efficient heterogeneous catalyst toward FA dehydrogenation and Cr(VI) reduction by FA is needed to boost its sluggish kinetics but still remains a challenge. Herein, uniformly dispersed subnanometric PdAu alloy clusters (i.e., 0.9 nm) were successfully prepared and confined by amine-functionalized carbon bowls (ACB). By virtue of the tiny size and abundant active sites of PdAu clusters, the promotional effect of surface amine groups, and electronic interaction between subnanometric PdAu clusters and support, this as-prepared PdAu/ACB catalyst exhibits superior catalytic property for additive-free FA dehydrogenation (turnover frequency, 10597 h−1 at 323 K) and Cr(VI) reduction (rate constant, 0.47 min−1 at 298 K) under mild conditions, higher than most of the catalysts reported so far. This study offers insight into the design of efficient and durable catalysts for various catalytic applications in energy and environment.

The effect of rutile TiO[formula omitted](110) surface on the physicochemical properties of subnanometer Au–Pd clusters: A DFT study

Bimetallic clusters are of particular interest in several applications due to the unique properties they exhibit as a result of the synergistic effect of their metals. In addition, the substrate can alter the structure and stability of these clusters by means of cluster–support interactions. In this work, we study the physicochemical properties of Au–Pd/TiO2(110) supported clusters via density functional theory (DFT) calculations. The effect of surface TiO2(110) on the structural and energetic properties of Au–Pd clusters is relevant. The cohesion energy depends on both metal–metal and metal–support interactions. Au and Au-rich bimetallic clusters change their gas phase configuration upon adsorption. Some clusters do not fit well on the surface, resulting in decreased stability. By contrast, pure Pd and Pd-rich bimetallic clusters have the highest adsorption energies and charge polarization due to the cluster–surface interaction. These clusters exhibit elongation in the metal–metal bonds and redshifts in their vibrations. The Bader charge analysis shows that Au–Pd clusters donate charge to the surface, thus becoming positively charged. Finally, the TiO2(110) surface induces stiffness in the clusters, since the supported clusters do not exhibit low-frequency vibrations like their gas-phase analogs.

Catalytic Oxidation of Benzyl Alcohol to Benzaldehyde on Au8 and Au6Pd2 Clusters: A DFT Study on the Reaction Mechanism

Density functional theory calculations were performed to investigate the reaction mechanism of the aerobic oxidation of benzyl alcohol to benzaldehyde catalyzed by Au and Au–Pd clusters. Two consecutive reaction mechanisms were examined with Au8 and Au6Pd2 clusters: (1) the oxidation of benzyl alcohol with dissociated O atoms on metal clusters generating benzaldehyde and H2O; and (2) oxidation with adsorbed oxygen molecules generating benzaldehyde and H2O2. The calculations show that the aerobic oxidation of benzyl alcohol energetically prefers to proceed in the former mechanism, which agrees with the experimental observation. We demonstrate that the role of Au centers around the activation of molecular oxygen to peroxide-like species, which are capable of the H–abstraction of benzyl alcohol. The roles of Pd in the Au6Pd2 cluster are: (1) increasing the electron distribution to neighboring Au atoms, which facilitates the activation of O2; and (2) stabilizing the adsorption complex and transition states by the interaction between positively charged Pd atoms and the π-bond of benzyl alcohol, both of which are the origin of the lower energy barriers than those of Au8.

Fabrication of Pd–Au Clusters by In Situ Spontaneous Reduction of Reductive Layered Double Hydroxides

In this work, layered double hydroxides (LDHs) including the variable valence Co2+ ions (CoAl-LDHs) is discovered to be capable of serving as the support and the reducing agent for the fabrication of well-distributed PdAu nanoparticles because of the low potential for reduction. Specifically, the active metal precursors undergo the redox process, in which Co2+ is oxidized to Co3+, while active metal ions are successfully reduced to bimetallic PdAu nanoparticles with the dispersion of 73%. Furthermore, the obtained catalysts are evaluated in the selective oxidation of benzyl alcohol as the probing reaction to explore the catalytic behavior. As for intrinsic activity, PdAu/CoAl-LDHs derived by the above in situ spontaneous reduction exhibits activation energy of 62.4 kJ mol−1, much lower than that of PdAu/CoAl-LDHs prepared by sol-immobilization. When the reaction time reaches to 4 h, the benzaldehyde selectivity over the former catalyst is up to 94%, originating from the surface electron starvation of active metals that promots the hydroxyl groups reacting with O2 to produce benzaldehyde.

Theoretical Analysis of Au-Pd Nanoalloy Clusters: A DFT Study

The study of bimetallic nanoalloy clusters has attracted a lot of attention now a days due to its unique physico-chemical properties. Among several bimetallic clusters, the compound formed between gold and palladium is of high importance due to its interesting catalytic properties. Density Functional Theory (DFT) is highly dominating method to compute the electronic properties of materials. In this article, we have reported the physical and chemical properties of AunPd (n=1-6) nanoalloy clusters in terms of Conceptual Density Functional Theory (CDFT) based descriptors. The CDFT based descriptors viz. HOMO-LUMO energy gap, hardness, softness, electronegativity andelectrophilicity index of AunPd nanoalloy clusters have been calculated. The result exhibits that energy gap of Au-Pd clusters maintain linear relationship with hardness values and inverse relationship with softness values. The HOMO-LUMO energy gap also display odd-even oscillation behavior, in which cluster containing total number of even atoms are more stable as compared to their neighbor cluster with total number of odd atoms.

Molecular and Dissociative Adsorption of Oxygen on Au-Pd Bimetallic Clusters: Role of Composition and Spin State of the Cluster.

Utilization of molecular oxygen as an oxidizing agent in industrially important reactions is the ultimate goal to design environmentally benign processes under ambient conditions. However, the high thermal stability and a large O–O dissociation barrier in O2 molecule pose a great challenge toward its successful application in the oxidative chemistry. To achieve this goal, different catalysts based on monometallic and bimetallic clusters have been developed over the years to promote binding and dissociation of molecular oxygen. The successful design of efficient metal cluster catalysis needs an in-depth knowledge of synergistic effects between different metal atoms and intrinsic catalytic mechanisms for O2 adsorption and dissociation. Here, we present a systematic theoretical investigation of reaction pathways for O2 adsorption and dissociation on Au8, Pd8, and Au8–nPdn (n = 1–7) nanoclusters in different spin states. The density functional calculations point out that the O2 dissociation barriers can be significantly reduced with the help of certain bimetallic clusters along specific spin channels. Our results particularly indicate that Au5Pd3 and Au1Pd7 show very large O2 binding energies of 1.76 and 1.69 eV, respectively. The enhanced O2 binding subsequently leads to low activation barriers of 0.98 and 1.19 eV along the doublet and quartet spin channels, respectively, without the involvement of any spin flip-over for O2 dissociation. Furthermore, the computed O2 dissociation barriers are significantly low as compared to the already reported barriers (1.95–3.65 eV) on monometallic and bimetallic Au–Ag clusters. The results provide key mechanistic insights into the interaction and dissociation of molecular oxygen with Au–Pd clusters, which can prove informative for the design of efficient catalysts for oxidative chemistry involving molecular oxygen as a reactant.

Molecular Beam Study of the CO Adsorption on a Regular Array of PdAu Clusters on Alumina

The adsorption kinetics of CO on PdAu bimetallic clusters, containing 140 $\pm$ 12 atoms and a composition varying between 0% and 55% of Pd atoms, is investigated by a pulsed molecular beam method (MBRS). The clusters are grown on a nanostructured ultrathin film of alumina on Ni3Al (111) playing the role of a template which gives a hexagonal array of bimetallic clusters having a sharp size distribution and a uniform composition. The surface concentration calculated, assuming segregation of gold to the surface, varies between 0 and 90% of Au atoms on the surface. From the adsorption-desorption kinetics of CO, the lifetime of CO is measured at various temperatures. At low coverage, plotting the CO lifetime in an Arrhenius diagram one obtains the adsorption energy of CO. When the surface concentration of Au increases, the adsorption energy of CO on the PdAu clusters decreases. This evolution of the adsorption energy is discussed, from previous studies, in term of ligand and ensemble effects. We find that the ensemble effect plays a dominant role in the observed decrease of the adsorption energy of CO.

Structural stability of binary $$\hbox {Pd}_{34-n}\hbox {M}_{n}$$ Pd 34 - n M n ( $$\hbox {M}=\hbox {Cu}$$ M = Cu , Ag, Au) clusters

We have investigated the structural properties, stability and magnetic properties of the $$\hbox {Pd}_{34-n}\hbox {M}_{n}$$ ( $$\hbox {M} = \hbox {Cu}$$ , Ag and Au) clusters series. The basin hopping Monte Carlo sampling method was employed to find low-energy candidates to the ground-state structures. Further density functional theory structural refinements were carried out in order to improve the physical property determinations. Stable compositions were identified, and structural and magnetic analyses were performed. Stability parameters, such as the excess energy, showed that binary particles are energetically favored over pure clusters for every composition. Structural descriptors showed that Pd–Ag and Pd–Au clusters exhibit core–shell-like arrangements, while Pd–Cu clusters present disordered mixing.

Calixarene-Based {Ni18} Coordination Wheel: Highly Efficient Electrocatalyst for the Glucose Oxidation and Template for the Homogenous Cluster Fabrication.

Catalyst plays a very important role in the exploration of new energy. To obtain a highly efficient electrocatalyst for the glucose oxidation and tiny metal nanocluster catalysts, a calixarene-based {Ni18} coordination wheel with sulfur atoms on the cavity surface was designed, synthesized, and used as the porous template. Contributing from the active sites of nickel cations, the as-synthesized coordination wheels can efficiently catalyze the electrochemical oxidation of glucose with the onset and peak potentials of 0.3 and 0.46 V in alkaline medium, and the catalysis does not depend on the atmosphere (N2, air, or O2), which indicates that the coordination wheel will be a promising electrocatalyst candidate for the compartmentless glucose-air fuel cell. Meanwhile, benefiting from its confined cavity and inner sulfur surface, such a coordination wheel can serve as a general template for the fabrication and encapsulation of tiny metal nanoclusters of Au, Pd, Ir, Ru, Rh, Pt, and AuPd. In electrochemical examinations, the bimetallic AuPd clusters confined in the coordination wheel show higher current density than commercial Pt/C toward hydrogen evolution reaction (HER). The present study shows that the designed coordination wheel can be used as not only a type of novel catalyst itself but also a class of templates for metal cluster catalysts.

Application of a parallel genetic algorithm to the global optimization of medium-sized Au\u2013Pd sub-nanometre clusters

To contribute to the discussion of the high activity and reactivity of Au–Pd system, we have adopted the BPGA-DFT approach to study the structural and energetic properties of medium-sized Au–Pd sub-nanometre clusters with 11–18 atoms. We have examined the structural behaviour and stability as a function of cluster size and composition. The study suggests 2D–3D crossover points for pure Au clusters at 14 and 16 atoms, whereas pure Pd clusters are all found to be 3D. For Au–Pd nanoalloys, the role of cluster size and the influence of doping were found to be extensive and non-monotonic in altering cluster structures. Various stability criteria (e.g. binding energies, second differences in energy, and mixing energies) are used to evaluate the energetics, structures, and tendency of segregation in sub-nanometre Au–Pd clusters. HOMO–LUMO gaps were calculated to give additional information on cluster stability and a systematic homotop search was used to evaluate the energies of the generated global minima of mono-substituted clusters and the preferred doping sites, as well as confirming the validity of the BPGA-DFT approach.

PdAu alloyed clusters supported by carbon nanosheets as efficient electrocatalysts for oxygen reduction

PdAu alloyed clusters supported on carbon nanosheets were prepared and employed as efficient electrocatalysts for oxygen reduction reaction (ORR). PdAu clusters protected by glutathione were synthesized, while the structure and composition of the alloys were tuned through the variation of Pd-to-Au ratio. The PdAu clusters were loaded into carbon nanosheets and calcined at elevated temperature. The protecting ligands were completely removed after calcination, and uniform hybrid materials were formed. The as-prepared nanocomposites were characterized by transmission electron microscopy (TEM), scanning tunneling electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) as well as other techniques. The composites demonstrated effective ORR activity in alkaline media. Among a series of samples, the composite with a metal mass loading of 30% and the ratio of Pd-to-Au (1:2) exhibited the highest activity, and its performance is comparable to that of commercial Pt/C, superior to PdAu clusters, carbon nanosheets as well as other supported alloyed samples, in terms of onset potential, diffusion limited current density as well as number of electron transfer. Notably, the long-term stability of the composite is markedly higher than Pt/C. The strategy can be extended for the preparation of other supported bi-metallic nanoclusters with controllable composition and optimized electrocatalytic activity for fuel cell applications.

Displacement-reduction routes to PtPd clusters and mechanistic inferences for the synthesis of other bimetallic compositions

Bimetallic PtPd clusters (2.1–2.9nm) dispersed on SiO2 and uniform in composition and size were prepared using colloidal methods with reagents containing only C, O, H, and N atoms. These synthetic protocols extend galvanic displacement-reduction (GDR) processes previously used to prepare AuPd and AuPt clusters. Such processes exploit the different redox potentials of two elements to encourage their deposition within the same cluster. The size, composition, and formation mechanism of PtPd clusters were probed using transmission electron microscopy, UV–visible spectroscopy, energy-dispersive X-ray spectroscopy, and high-angle annular dark-field imaging. Taken together with previous data for AuPd and AuPt systems, these findings highlight key general features, properties, and protocols required to form uniform bimetallic clusters. Exothermic alloys, such as PtPd and AuPd, form predominantly via selective GDR routes; in contrast, alternate routes become significant for endothermic alloys, such as AuPt. Bimetallic clusters grow via GDR processes (PtPd, AuPd) at rates proportional to the surface area of each cluster; therefore, compositional uniformity is dictated by the size distribution of the seed clusters. The rate of GDR processes reflects the difference in reduction potentials of the two components, as shown by more facile formation of AuPd than PtPd clusters. These considerations and experimental evidence provide useful guidance for conditions and protocols likely to succeed for other bimetallic pairs. Low-temperature (≤423K) reductive treatments (in H2 or EtOH) successfully removed all synthetic detritus from Pd and PtPd clusters dispersed on SiO2, without significant coalescence. Such removal strategies are more challenging for Pd than for Pt clusters because of stronger Pd-polymer bonds and the greater sintering tendency of Pd clusters.

Synthesis of stable monodisperse AuPd, AuPt, and PdPt bimetallic clusters encapsulated within LTA-zeolites

AuPd, AuPt, and PdPt bimetallic clusters uniform in size and composition were prepared using hydrothermal assembly of LTA crystals around cationic precursors stabilized by protecting mercaptosilane ligands. The sulfur moiety in these bifunctional ligands forms adducts that prevent premature reduction or precipitation of metal precursors during crystallization. The silane groups can form bridges with silicate oligomers as they form, thus enforcing homogeneous distributions of precursors throughout crystals and ensuring that subsequent reductive treatments lead to the two elements residing within small and nearly monodisperse clusters. Their confinement within LTA crystals, evident from microscopy and titrations with large poisons, renders them stable against sintering during thermal treatments at high temperatures (820–870K). Infrared spectra of chemisorbed CO show that bimetallic surfaces are free of synthetic debris after thermal treatments; these spectra also indicate that intracluster segregation occurs upon CO chemisorption, a demonstration of the presence of the two elements within the same clusters. The number and type of atoms coordinated to a given absorber atom, determined from the fine structure in X-ray absorption spectra, are consistent with bimetallic structures of uniform composition. The rates of ethanol oxidative dehydrogenation on these bimetallic clusters were essentially unaffected by exposure to dibenzothiophene, a large poison that suppresses rates on unconfined clusters, indicating that bimetallic clusters are protected within the confines of LTA crystals. These synthetic protocols seem generally applicable to other bimetallic compositions and zeolites, for which the monometallic counterparts have been successfully encapsulated within several microporous frameworks using ligand-stabilized precursors and hydrothermal crystallization methods.

Synthesis of Bimetallic AuPt Clusters with Clean Surfaces via Sequential Displacement-Reduction Processes

We report the synthesis of bimetallic AuPt nanoparticles (3.3–4.3 nm) of uniform size and composition using colloidal methods and reagents containing only C, H, O, and N. These clusters were dispersed onto SiO2 and treated at low temperatures in the presence of reductants to remove all surface residues without concomitant agglomeration, thus leading to bimetallic structures suitable for mechanistic inquiries into bimetallic effects on surface reactivity. Synthesis protocols exploit and generalize galvanic displacement-reduction (GDR) processes previously used to prepare AuPd clusters; these routes promote bimetallic mixing but become more challenging for systems (e.g., AuPt) with smaller reduction potential differences and less favorable mixing enthalpies than AuPd. These hurdles are addressed here through procedural modifications that inhibit the formation of large Au-rich clusters, which compromise size and compositional uniformity. In doing so, we extend GDR techniques to endothermic alloys with elemen...

Selective hydrogenation of the C[double bond, length as m-dash]C bond in α,β-unsaturated aldehydes and ketones over ultra-small Pd-Au clusters.

Pd-Au clusters of 1.8 nm preferentially catalyzed the hydrogenation of the C[double bond, length as m-dash]C bond in α,β-unsaturated aldehydes and ketones with a selectivity of ∼99%, primarily because of the unique electronic properties and isolated Pd ensembles on the ultra-small alloyed particles.

Design and Synthesis of Silica Supported Nanoporous Gold-Palladium Bimetallic Catalyst for Alkyl Benzene Oxidation

Gold palladium (Au-Pd) bimetallic catalysts are very promising for various reactions including oxidative catalysis. Mesoporous silica supported Au-Pd catalysts have large surface area, controlled hydrophobi and file-cities and are thus highly efficient for the oxidation of alkyl benzene to selective products. Alkyl Benzene oxidation is important for the productions of drugs, perfumes, polymers, insecticides and pesticides. Unfortunately, the efficient oxidation of alkyl benzene has been remaining a challenging task due to lack of suitable catalysts. Functionalized mesoporous silica with ordered surface and uniform porosity is an exciting template for the synthesis of supported catalysts. Metal precursors could be co-impregnated on the highly ordered surface of ammine functionalized silica. This method facilitates great dispersion of the Au-Pd clusters with tiny and controlled particle size. Here, we described a two-step adsorption –reduction method along with various ratios of metal loadings for the synthesis of silica supported nanoporous Au-Pd catalyst for alkyl benzene oxidation.

In Situ Generated H2O2 over Supported Pd–Au Clusters in Hybrid Titania Nanocrystallites

Abstract Bimetallic Au–Pd clusters were successfully elaborated by selective deposition over citrate-functionalized TiO2 nanocrystallites. Anisotropic gold nanoparticles are obtained by adjusting the Pd/Au ratio. An original method to generate “in situ” H2O2 from formic acid and O2 is discussed and evaluated in the selective oxidation of benzyl alcohol as a model reaction.

Tuning the catalytic activity of Au–Pd nanoalloys in CO oxidation via composition

Using a combination of analytic-potential and first-principles density functional theory (DFT) calculations, composition effects on energetics, adsorption energies, and catalytic activity of Au–Pd nanoalloys are investigated, selecting CO oxidation to CO2 as a prototypical reaction and 55-atom Au–Pd clusters with Mackay icosahedral structure as template systems. It is first shown that the Au54Pd1, Au43Pd12, Au42Pd13, and Au12Pd43 nanoalloys with highly symmetric structures are well separated from other nanoalloys due to their special relative stability at the empirical potential and/or DFT levels, with CO adsorption energies on top of Au atoms found to be weakly dependent on composition in a range around 50:50 Au:Pd ratio. The explicit calculation of reaction energy barriers for the CO oxidation process then shows that these are sensitive to the composition of nanoalloys, where Au-rich clusters possess a high catalytic activity and the Au43Pd12 cluster is predicted to have the highest catalytic activity among the clusters here considered. Our results highlight a non-monotonous behavior of the catalytic activities of Au–Pd nanoalloys on composition that is of fundamental interest for the design of new catalysts, but also the subtleties of a delicate interplay of structural and electronic effects in determining reaction energetics, which are difficult to summarize into few, simple prescriptions.

Mechanistic Evidence for Sequential Displacement–Reduction Routes in the Synthesis of Pd–Au Clusters with Uniform Size and Clean Surfaces

Bimetallic Pd–Au clusters with (Pd/Au)at compositions of 0.5, 1.0, and 2.0 narrowly distributed in size were prepared using colloidal methods with reagents containing only C, H, and O atoms, specifically polyvinyl alcohol (PVA) as protecting species and ethanol as the organic reductant. Synthesis protocols involved contacting a solution of Au precursors with nearly monodisperse Pd clusters. The formation of Pd–Au clusters was inferred from the monotonic growth of clusters with increasing Au content and confirmed by the in situ detection of Au plasmon bands in their UV–visible spectra during synthesis. Specifically, transmission electron microscopy (TEM) showed that growth rates were proportional to the surface area of the clusters, and rigorous deconvolution and background subtraction allowed for determination of the intensity and energy of Au-derived plasmon bands. This feature emerged during initial contact between Au precursors and Pd clusters apparently because Au3+ species deposit as Au0 using Pd0 as the reductant in a fast galvanic displacement process consistent with their respective redox potentials. The plasmon band ultimately disappeared as a result of the subsequent slower reduction of the displaced Pd2+ species by ethanol and of their deposition onto the bimetallic clusters. Such displacement–reduction pathways are consistent with the thermodynamic redox tendencies of Au, Pd, and ethanol and lead to the conclusion that such triads (two metals and an organic reductant) can be chosen from thermodynamic data and applied generally to the synthesis of bimetallic clusters with other compositions. These bimetallic clusters were dispersed on mesoporous γ-Al2O3 supports, and PVA was removed by treatment in ozone at near-ambient temperature without any detectable changes in cluster size. The absence of strongly bound heteroatoms, ubiquitous in many other colloidal synthesis protocols, led to Al2O3-dispersed clusters with chemisorption uptakes consistent with their TEM-derived cluster size, thus demonstrating that cluster surfaces are accessible and free of synthetic debris. The infrared spectra of chemisorbed CO indicated that both Pd and Au were present at such clean surfaces but that any core–shell intracluster structure conferred by synthesis was rapidly destroyed by adsorption of catalytically relevant species, even at ambient temperature; this merely reflects the thermodynamic tendency and kinetic ability of an element to segregate and to decrease surface energies when it binds an adsorbate more strongly than another element in bimetallic particles.