Surface Of Metal Clusters Research Articles

Photo-induced image current

We study the possibility of generation of photo-induced image currents at a distance from the surface of nano-sized metal clusters by using the time-dependent perturbation theory. We reveal that the wave function of an electron excited to the image state is localized outside the surface and the current flows in a spherical shell whose radius is a few times the radius of the sphere. Circularly polarized light has been applied to a perfect icosahedral metal cluster Li13 whose optimization is achieved by molecular dynamic simulation and band structure is obtained by the DFT method and by the solution of the radial Schrödinger equation. To the best of our knowledge, despite the great effort put into determining their characteristics, image electrons have not been the subject of studies on photo-induced current.

Unique Dynamics of Water Confined on the Surface of Metal Oxide Clusters.

Water confined on metal oxide surface plays significant roles in heterogeneous catalysis. Heteropolyacid, a 1.2 nm-metal oxide cluster with well-defined structure, is applied as a model to understand the dynamics of water on its surface. The surface water strongly associates with heteropolyacid cluster and form the so-called 'pseudoliquid phase' where catalytic reactions are conducted. Broadband dielectric spectroscopy and differential scanning calorimetry have been applied to probe the dynamics of water in this pseudoliquid phase. A supercooling phase transition of water below its normal melting temperature and a dipolar glassy relaxation behaviour due to the hindered dynamics of water have been observed. The rich dynamic behavior on the surface of such well-defined metal clusters provide new perspectives to understand the properties of surface water and their relation to catalytic performance of heteropolyacid.

Adsorption, Dissociation, and Dehydrogenation of Water Monomer and Water Dimer on the Smallest 3D Aluminum Particle. The O-H Dissociation Barrier Disappears for the Dimer.

We present a detailed mechanistic study on the interaction and reaction of water monomer and water dimer with the smallest 3D aluminum particle (Al6) by employing density functional and explicitly correlated coupled cluster CCSD(T)-F12 theories. Water adsorption, dissociation, and dehydrogenation are considered. For the monomer reaction, where core-valence correlation and an extrapolation to the complete basis set limit is allowed for, our coupled cluster calculations predict the O-H dissociation barrier of about 2 kcal/mol. For the dimer reaction, two distinct reaction paths are identified, initiated by forming separate dimer complexes wherein (H2O)2 adsorbs mainly via the oxygen atom of the donor H2O molecule. The key O-H dissociation transition states of the dimer reaction involve a concerted migration of two H atoms resulting in the dissociation of the donor molecule and formation of the OH-water complex adsorbed on the metal cluster's surface. The most remarkable feature of both dimer reaction energy profiles is the lack of the overall energy barrier for the (rate-determining) O-H dissociation. The hydrogen bond acceptor molecule is suggested to have an extra catalytic effect on the O-H dissociation barrier of the hydrogen bond donor molecule by removing this barrier. A similar effect on the dehydrogenation step is indicated.

Assessment of BOMD simulations for the ground-state structure determination of transition metal clusters in the nanometer scale

ABSTRACTState-of-the art calculations have been performed for bimetallic transition metal clusters such as Pd19M19 (M=Co and Ni) by employing the linear combination of Gaussian-type orbitals density functional theory (LCGTO-DFT) approach. Structures and energy properties were calculated for these clusters. For each cluster, several dozens of isomers were studied to determine the lowest energy structures. Initial structures for the geometry optimisation were taken along Born–Oppenheimer molecular dynamics (BOMD) trajectories,considering several spin multiplicities. All structures were fully optimised without any symmetry restriction. The optimised structures were characterised by vibrational analysis. Lowest energy structures, relative stability energy, harmonic frequencies, binding energies, vertical ionisation potential, vertical electron affinity and spin density plots are reported. The obtained results are compared with data from the literature. The ground-state structure topology is the same for both clusters. The Pd atoms decorate the surface of the core formed by either Co or Ni atoms. This work demonstrates the importance of using ab initio BOMD simulations to fully explore the potential energy surface of large transition metal clusters.

Enantiospecific adsorption of cysteine on a chiral Au34 cluster

The interaction of biological molecules like chiral amino acids with chiral metal clusters is becoming an interesting and active field of research because of its potential impact in, for example, chiral molecular recognition phenomena. In particular, the enantiospecific adsorption (EA) of cysteine (Cys) on a chiral Au55 cluster was theoretically predicted a few years ago. In this work, we present theoretical results, based on density functional theory, of the EA of non-zwitterionic cysteine interacting with the C3-Au34 chiral cluster, which has been experimentally detected in gas phase, using trapped ion electron diffraction. Our results show that, indeed, the adsorption energy of the amino acid depends on which enantiomers participate in the formation Cys-Au34 chiral complex. EA was obtained in the adsorption modes where both the thiol, and the thiol-amino functional groups of Cys are adsorbed on low-coordinated sites of the metal cluster surface. Similarly to what was obtained for the Cys-Au55 chiral complex, in the present work, it is found that the EA is originated from the different strength and location of the bond between the COOH functional group and surface Au atoms of the Au34 chiral cluster. Calculations of the vibrational spectrum for the different Cys-Au34 diastereomeric complexes predict the existence of a vibro-enantiospecific effect, indicating that the vibrational frequencies of the adsorbed amino acid depend on its handedness.

Catalytic Consequences of the Thermodynamic Activities at Metal Cluster Surfaces and Their Periodic Reactivity Trend for Methanol Oxidation

AbstractThe periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First‐order rate coefficients are a single‐valued function of the O2‐to‐CH3OH ratio, because this ratio, together with the rate constants for O2 and CH3OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH3OH activation rate constants on oxygen‐covered Ag, Pt, and Pd and on RuO2 clusters vary with the metal–oxygen binding strength in a classical volcano‐type relation, because the oxygen‐binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH3OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO2>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH3OH activation.

Catalytic consequences of the thermodynamic activities at metal cluster surfaces and their periodic reactivity trend for methanol oxidation.

The periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First-order rate coefficients are a single-valued function of the O2 -to-CH3OH ratio, because this ratio, together with the rate constants for O2 and CH3OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH3OH activation rate constants on oxygen-covered Ag, Pt, and Pd and on RuO2 clusters vary with the metal-oxygen binding strength in a classical volcano-type relation, because the oxygen-binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH3OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO2>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH3OH activation.

Geometric structure of silver clusters with and without adsorbed Cl and Hg

We report density functional theory calculations, with the TPSS functional, for the clusters Agn (n=3–20), AgnCl2 (n=3–10, and n=12) and AgnHgm (n=5, 6 and m=1–4). We performed unbiased global optimization for all of these species by Tabu Search in Descriptor Space. The Agn global minima geometries that we find confirm the earlier PBE functional results of Yang et al. (M. Yang, K.A. Jackson, J. Jellinek, First-principles study of intermediate size silver clusters: shape evolution and its impact on cluster properties, J. Chem. Phys. 125 (2006) 144308 (8pp.)). We find special stability associated with Ag6, Ag7, and Ag8 which we explain by their closed electronic shell (n=6, 8) or closed atomic shell (n=7). The Ag16 cluster stands out in that it is particularly unstable, it has a small HOMO–LUMO gap, and it has a high-coordination binding site (the site of the missing thirteenth atom of an icosahedron). This may explain the experimental observations (L. Lian, S.A. Mitchell, P.A. Hackett, D.M. Rayner, Size dependence of mobility on the surface of metal clusters: ammonia on Ag10 and Ag16, J. Chem. Phys. 104 (1996) 5338–5344) that NH3 binds strongly to Ag16 and does not diffuse on it. We calculate the Cl—Agn binding energy to be roughly 3.0eV in most cases, and the Hg—Agn to be roughly 0.25eV. Binding of two Cl atoms severely disrupts the Agn geometric structure in some cases (n=5, 6, 7), but leaves the Agn structures essentially intact in other cases (n=4 and 9). Binding to Hg atoms has little effect on the geometry of Ag5 but a big effect on that of Ag6.

Infrared driven CO oxidation reactions on isolated platinum cluster oxides, PtnOm+

This collaboration has recently shown that infrared excitation can drive decomposition reactions of molecules on the surface of gas-phase transition metal clusters. We describe here a significant extension of this work to the study of bimolecular reactions initiated in a similar manner. Specifically, we have observed the infrared activated CO oxidation reaction (CO(ads) + O(ads) --> CO2(g)) on isolated platinum oxide cations, Pt(n)O(m)+. Small platinum cluster oxides Pt(n)O(m)+ (n = 3-7, m = 2, 4), have been decorated with CO molecules and subjected to multiple photon infrared excitation in the range 400-2200 cm(-1) using the Free Electron Laser for Infrared eXperiments (FELIX). The Pt(n)O(m)CO+ clusters have been characterised by infrared multiple photon dissociation spectroscopy using messenger atom tagging. Evidence is observed for isomers involving both dissociatively and molecularly adsorbed oxygen on the cluster surface. Further information is obtained on the evolution of the cluster structure with number of platinum atoms and CO coverage. In separate experiments, Pt(n)O(m)CO+ clusters have been subjected to infrared heating via the CO stretch around 2100 cm(-1). On all clusters investigated, the CO oxidation reaction, indicated by CO2 loss and production of Pt(n)O(m) = 1+, is found to compete effectively with the CO desorption channel. The experimental observations are compared with the results of preliminary DFT calculations in order to identify both cluster structures and plausible mechanisms for the surface reaction.

Selectivity of chemisorbed oxygen in C–H bond activation and CO oxidation and kinetic consequences for CH 4–O 2 catalysis on Pt and Rh clusters

Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O * reactions with CO and CH 4, during CH 4–O 2 catalysis on Pt and Rh clusters. CO 2 and H 2O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O *) are depleted from cluster surfaces. Oxygen selectivities, measured using 12CO– 13CH 4–O 2 reactants, increase with O 2/CO ratio and O * coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O * reacts much faster with CO than with CH 4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O * to produce CO 2 at any residence time required for detectable extents of CH 4 conversion. O * selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH 4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8 nm diameter, 201 Pt atoms) at low O * coverages, which are prevalent at low O 2 pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O * atoms (on *–O * site pairs). CH 4 oxidation occurs via kinetically relevant C–H bond activation on *– * site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH 4, forming a three-centered HC 3–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH 3 and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH 3 * binding energies. Ensemble-averaged O * selectivities increase linearly with O 2/CO ratios, which define the O * coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O 2 dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33 nm) at all temperatures (573–1273 K) relevant for CH 4–O 2 reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O * selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in 12CO– 13CH 4–O 2 mixtures, consistent with lower CO/CO 2 ratios measured by varying the residence time and O 2/CH 4 ratio independently in CH 4–O 2 reactions. These mechanistic assessments and theoretical treatments for O * selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH 4 to CO (and H 2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H 2) are predominantly formed upon complete O 2 depletion from the sequential reforming steps.

Structure and reactivity of Au–Rh bimetallic clusters on titanate nanowires, nanotubes and TiO 2(1 1 0)

Au and Rh clusters, as well as Au–Rh bimetallic nanoparticles were prepared on titanate nanowires, nanotubes and on TiO 2(1 1 0). They were characterized by X-ray photoelectron spectroscopy (XPS), low energy ion scattering spectroscopy (LEIS) and Fourier transform infrared spectroscopy (FTIR). By performing careful LEIS experiments, it was found that for appropriate Au and Rh coverage, a thin Au layer almost completely covers the Rh nanoparticles, a Rh core–Au shell structure was detected. The formation of this structure was not affected by alkali (K) adatoms. LEIS and FTIR measurements disclosed that adsorbed CO at 300 K causes the segregation of Rh atoms to the surface of metal clusters in order to bind to CO. Upon CO adsorption on Rh/titanate nanostructures the IR stretching frequencies characteristic of the twin form were dominant, whereas bimetallic nanosystems featured a pronounced linear stretching vibration as well. In spite of this structure adsorbed CO is detectable during the ethanol adsorption on gold–rhodium bimetallic cluster and the ethanol decomposition rate is twice higher than on Au/TiO 2.

Exploring the accuracy of relative molecular energies with local correlation theory

Local coupled-cluster singles–doubles theory (LCCSD) is a theorist’s attempt to captureelectron–electron correlation in a fast amount of time and with chemical accuracy. Manyof the difficult computational hurdles have been navigated over the last twentyyears, including how to construct a linear scaling algorithm and how to producesmooth potential energy surfaces. Nevertheless, there remains the question ofjust how accurate a local correlation model can be, and what are the chemicallimits within which local models are largely applicable. Here, we investigate howaccurately can LCCSD approximate full CCSD for cases of atomization energies,isomerization energies, conformational energies, barrier heights and electron affinities. Ourconclusion is that LCCSD computes relative energies that are correct to within1–2 kcal mol−1 of the CCSD energy using relatively aggressive cutoffs and over a broad range of differentmolecular environments—alkane isomers, dipeptide conformations, Diels–Alder transitionstates and electron attachment in charge delocalized systems. These findings should pushthe reach of local correlation applications into new research terrain, includingmolecules on metal cluster surfaces or perhaps even metal–molecule–metal clusters.

In situ XAFS analysis of Pd–Pt catalysts during hydrotreatment of model oil

Supported Pd–Pt catalysts are efficient for hydrodesulfurization (HDS) and hydrodearomatization (HDA) reactions of diesel fuel and their activity varied with the kinds of supports. Concerning HDA, alumina supported catalysts showed four times higher TOF (turn over frequency) than silica supported one. In order to elucidate the difference in activity, the structural analysis of the active phase was performed. After reduction pretreatment, relatively uniform and large metallic alloy Pd–Pt particles were formed on SiO 2, whereas, Pd and Pt atoms formed rather segregated particles on Al 2O 3. Subsequent X-ray absorption of fine structure (XAFS) analysis under HDS conditions showed no contribution of sulfur for SiO 2 supported catalyst, whereas, formation of sulfided metal species was observed in XAFS spectra for the Al 2O 3 supported catalyst. It is suggested that on Pd–Pt/SiO 2, thin sulfide layer on the metal cluster surface blocked the active sites and lowered the HDA activity. Presence of partially sulfided phase originated from rather segregated structure like Pd–Pt/Al 2O 3 is thought to be requisite for high HDA activity.

Surface and bulk melting of small metal clusters

We present an analytical solution to the two-parabola Landau model, applied to melting of metal particles with sizes in the nanoscale range. The results provide an analytical understanding of the recently observed pseudo-crystalline phase of nanoscale Sn particles. Liquid skin formation as a precursor of melting is found to occur only for particles with radii greater than an explicitly given critical radius. The size dependences of the melting temperature, and of the latent heat, have been calculated, and a quantitative agreement is found with the experiment on tin particles.

Interaction of Nitrogen Oxide and Carbon Molecules Adsorbed on Ni Clusters Formed on the MgO(111) Thin Film Surface

The joint adsorption of CO and NO molecules on the surface of nanodimensional nickel clusters formed on the Mo(110) face of a thin MgO(111) magnesium oxide film under conditions of ultrahigh vacuum is investigated by methods of infrared reflection and thermal desorption spectroscopy. It is revealed that the adsorption of the NO molecules changes significantly the state of the adsorbed CO molecules. Based on an analysis of thermal desorption and IR spectra, a conclusion can be drawn that the adsorption of the NO molecules stimulates the migration of the CO molecules from the metal cluster surface to the boundary between the cluster and oxide accompanied by a decrease in the tilt angle of the molecular axis to the surface.

A novel dynamic kinetic model of oxygen isotopic exchange on a supported metal catalyst

A time-resolved kinetic analysis has been developed for modeling experimental results of 18 O / 16 O isotopic exchange over oxide-supported metal catalysts. Model is based on two very important points: (1) the parallel calculation of surface and bulk diffusion and (2) the implication of certain O species such as superoxides. The model includes adsorption–desorption processes on metal clusters and oxygen spillover from the metal to the surface of support and vice versa. Different mechanisms of exchange were also taken into account via mononuclear (O atoms, O − , OH) or binuclear (superoxides) oxygen species. A refined model taking into account surface diffusion, direct exchange on surface of support by binuclear oxygen species and bulk diffusion was also developed. Kinetic (reaction rates and diffusion coefficients) as well as thermodynamic parameters (activation energies) were derived by fitting theoretical and experimental curves of 18 O 2 , 18 O 16 O and 16 O 2 gas phase concentrations versus time. The experimental results of Pt/CeZrO 2 catalyst samples obtained in the 200–450 °C range of temperatures are examined. The refined model provides a very good fitting of the kinetic curves recorded with ceria–zirconia-supported catalysts. Moreover, values of diffusion coefficients and activation energies are in good agreement with already published values found by other methods. For a better understanding of all the steps of exchange, the kinetics of 18 O and 16 O distribution on the surface of metal clusters and on the surface of support are calculated and analyzed. On the basis of this model, a computer code is developed for analysis and calculations of kinetic and thermodynamic parameters of automotive catalysts.

A novel dynamic kinetic model of oxygen isotopic exchange on a supported metal catalyst

A time-resolved kinetic analysis has been developed for modeling experimental results of 18O/16O isotopic exchange over oxide-supported metal catalysts. Model is based on two very important points: (1) the parallel calculation of surface and bulk diffusion and (2) the implication of certain O species such as superoxides. The model includes adsorption–desorption processes on metal clusters and oxygen spillover from the metal to the surface of support and vice versa. Different mechanisms of exchange were also taken into account via mononuclear (O atoms, O−, OH) or binuclear (superoxides) oxygen species. A refined model taking into account surface diffusion, direct exchange on surface of support by binuclear oxygen species and bulk diffusion was also developed. Kinetic (reaction rates and diffusion coefficients) as well as thermodynamic parameters (activation energies) were derived by fitting theoretical and experimental curves of 18O2, 18O16O and 16O2 gas phase concentrations versus time. The experimental results of Pt/CeZrO2 catalyst samples obtained in the 200–450°C range of temperatures are examined. The refined model provides a very good fitting of the kinetic curves recorded with ceria–zirconia-supported catalysts. Moreover, values of diffusion coefficients and activation energies are in good agreement with already published values found by other methods. For a better understanding of all the steps of exchange, the kinetics of 18O and 16O distribution on the surface of metal clusters and on the surface of support are calculated and analyzed. On the basis of this model, a computer code is developed for analysis and calculations of kinetic and thermodynamic parameters of automotive catalysts.

Chloroform vapour sensor based on copper/polyaniline nanocomposite

Chemically synthesised copper/polyaniline (PANi) nanocomposite has been utilised as a chloroform sensor for ppm level vapour concentration. The response in terms of increase in dc electric resistance on exposure to chloroform vapours has been observed. The FT–IR spectra of nanocomposite on exposure to chloroform show remarkable modifications in the far IR region indicating the interaction of chloroform with metal cluster. It is believed that the sensing mechanism mainly involves adsorption–desorption of chloroform at metal cluster surfaces.

Modeling the CO Oxidation Reaction in a Thin Granular Catalyst Layer

A new mathematical model is proposed for the CO oxidation reaction in a thin granular catalyst layer. It describes processes on various spatial-temporal scales, including several levels of the reaction system. The model takes into account the flow of the reagents through the catalyst layer, diffusion in granule pores, reaction on the surface of metal clusters, the thermal effect of the reaction, and also heat and mass transport in the layer. The goal of the simulation is to elucidate the effect of various factors on system dynamics, including the rate of diffusion in the catalyst pores, the reagent flow rate, the temperature, the period of kinetic oscillations on the palladium surface, and other factors. The simulation identifies the conditions that are essential for satisfactory description of dynamical behavior on each level. It also establishes the role of each condition in the synchronization of oscillations and in the appearance of the experimentally observed chaotic modes.

Femtosecond time-resolved second-harmonic generation at the surface of alkali metal clusters

Optical second-harmonic generation (SHG) at the surface of sodium and potassium clusters was studied using fs laser pulses. The size dependence of the SHG intensity can be explained by resonant enhancement by surface plasmon–polariton excitation. At different cluster sizes the second-order interferometric autocorrelation of a 25 fs pulse was recorded using the clusters as nonlinear medium. Compared to a reference autocorrelation, the trace exhibits no broadening within the experimental error of 1 fs. Taking into account inhomogeneous line broadening due to the particle size distribution, the single cluster plasmon dephasing time is found to be τ≤15 fs. A lower limit of τ≥1 fs is obtained from the width of the linear extinction spectrum.