Encapsulation of small metal particles in solid organic matrices

The methods of synthesis of macroscopic amounts of size-selected clusters with desired properties, and most importantly, the possibility of their controlled assembly into new materials with novel properties are currently of great interest. The interaction of metal atoms and small clusters with solid organic matrices may lead to the stabilization of reactive particles up to room temperatures. Thus obtained nanoscale heterogeneous materials offer an area of intriguing technological promise. In this review, we discuss recent developments in the encapsulation of small metallic particles in different organic solid matrices: organic monomeric compounds, polymers and carbon derivatives.     

Enhanced raman spectra of molecules adsorbed on small metal particles

The Raman spectra of benzene adsorbed on small nickel and platinum particles have been measured. The spectra are strongly enhanced, compared with those wavelengths, where no enhancement has been observed. The spectra of benzene adsorbed on platinum and nickel particles are rather similar. The most intensive vibrational band is the totally symmetric breathing mode, which dominates the spectrum. Besides the Raman active vibrations, some other weak vibrational modes are observed, which may be assigned to normally infrared and inactive vibrational transitions. The enhanced Raman effect seems to be dependent on the particle size and shape. Therefore it is important to investigate the behaviour of small metallic particles. First results of pyridine bonded to Ag₂ and Ag₃ are reported.     

Shape effect and quantum size effect on small metal particles

We examine the small-energy cross section behaviour for the processe+H^?→H*+2e. The excitation-detachment function appears approximately constant above the classical threshold, with a weak undulatory component superimposed. The energy distribution function for the outgoing electrons is found almost uniform within our classical model.     

Facile filling of metal particles in small carbon nanotubes for catalysis

A versatile wet chemistry method is developed for filling of subnanometer sized metal particles in carbon nanotubes with a diameter smaller than 1.5 nm. As an example, we showed that a confined bi-component Pd-V catalyst exhibit a higher benzene hydroxylation activity compared with that within multi-walled carbon nanotubes.     

Systems of small metal particles: Optical properties and their structure dependences

Experiments on small particles usually require samples containing large numbers of particles. The properties of such samples are determined both by the properties of the individual particle and by collective effects, if particles are packed closely together. Collective optical effects strongly depend on the topography of the samples. It is shown that they can be classified according to the effective local electromagnetic field. Recent experiments and calculations are presented for optical extinction spectra in the spectral region of plasmon polariton excitations, which clearly show the different behaviour of effective medium-like samples and of samples containing particle aggregates.     

Early stages of some small metal particles: A pulse radiolysis study

Recent advances in the pulsing system of the 4 MeV van de Graaff accelerator ELBENA are described, which now allow the generation of stable timed pulses with nanosecond duration. The reaction of tetrachloroplatinate with the hydrated electron, the t-butanol radical and the methanol radical results, via the Cl⁻→H₂O ligand exchange, in different complexes which absorb at 310 and 360 nm, respectively, or in a chain reaction.     

Formation of small rhodium metal particles on the surface of a carbon support

A rhodium catalyst supported on a Sibunit graphitized carbon carrier was studied by in situ XAFS spectroscopy. A comparative study of the reduction of rhodium was performed for the following two samples: Rh/C(120) dried at 120°C and Rh/C(350) calcined at 350°C. EXAFS data showed an absence of carbon atoms within the nearest environment of rhodium atoms in the Rh/C(120) uncalcined sample, which implies the absence of direct interaction between rhodium and the carbon support. In the course of the reduction of this sample (200°C), coarse particles with small metal cores were initially formed. These metal particles rapidly agglomerated upon the complete reduction of rhodium (350°C). These reduction of the Rh/C(350) calcined sample at 100–500°C resulted in the formation of small metal particles early in the reduction (100°C). The high dispersity of these particles was retained as the temperature of treatment in hydrogen was increased to 500°C due to metal-support interaction. The conversion of benzene into cyclohexane on the Rh/C(350) catalyst containing small rhodium particles was much higher at the same temperature of hydrogenation.Translated from Kinetika i Kataliz, Vol. 46, No. 1, 2005, pp. 122–130. Original Russian Text Copyright © 2005 by Stakheev, Tkachenko, Klementev, Grünert, Bragina, Mashkovskii, Kustov.     

Dissociation of small metal particles induced by surface plasmon excitation with laser light

This paper reports the dissociation of supported metal clusters after illumination with low intensity visible laser light. The process is non-thermal and exhibits a resonant dependence on the laser frequency. This indicates that a surface plasmon excitation preceeds the dissociation.     

The size dependent shift of the surface plasmon absorption band of small spherical metal particles

It has been previously established that the surface plasmon of small spherical silver particles, which are embedded in a noble gas matrix, shifts to higher energies (blue shift) as the mean diameterD of the particles decreases (100 Å>D>20 Å). This blue shift has also been found for supported silver particles, and quite recently we observed it by elastic light scattering in the gas phase. This latter experiment proves unambiguously that the blue shift in small silver particles is not induced by interactions with the environment, the presence of which is clearly recognized in less inert matrices such as O₂ or CO. From self-consistent calculations of the surface response of planar jellium surfaces one would expect a red shift, which is also directly confirmed by a few calculations for selected jellium spheres. The contradiction between the observed blue shift for small particles and the predicted red shift for jellium spheres disappears, if one accounts for thed-electrons of silver in a very simple approximation.     

On the structure and stability of small metal particles: high-resolution UHV electron microscope study

Structures of gold clusters consisting of several to several tens of atoms and of small particles of 20–30 Å are seen by high-resolution UHV electron microscopy. The small particles have multiply twinned structures and show orientational fluctuation on a graphitized carbon substrate at room temperature, but no structural change from twinned to the fcc structure occurred during the observation with electron beam of 10–15 Amp/cm² on the specimen. Their structure changes at substrate temperature of 500°C. Small clusters show configurational change at room temperature from 5-fold arrangement to fcc one. These fluctuating motions are concluded to be due to thermal effect by estimating the temperature rise of the particles due to the electron beam to be less than 20–50°C from the melting temperature of lead particles.     

Morphology-dependent nanocatalysis: metal particles

The rapid development of materials science now enables tailoring of metal and metal oxide particles with tunable size and shape at the nanometre level. As a result, nanocatalysis is undergoing an explosive growth, and it has been seen that the size and shape of a catalyst particle tremendously affects the reaction performance. The size effect of metal nanoparticles has been interpreted in terms of the variation in geometric and electronic properties that governs the adsorption and activation of the reactants as well as the desorption of the products. At the same time, it has been verified that the morphology of a catalyst particle, determined by the exposed crystal planes, also considerably affects the catalytic behavior. This is termed as morphology-dependent nanocatalysis: a catalyst particle with an anisotropic shape alters the reaction performance by selectively exposing specific crystal facets. This perspective article initially surveys the recent progress on morphology-dependent nanocatalysis of precious metal particles to emphasise the chemical nature of the morphology effect. Then, the fabrication of transition metal particles with controllable size/morphology is examined, and their shape is correlated with their catalytic properties, with the aim to clarify the structure-reactivity relationship. Finally, the future outlook presents our personal perspectives on the concept of morphology-dependent nanocatalysis of metal particles, which is a rapidly growing topic in heterogeneous catalysis.     

Ignition of highly reactive metal particles

It was shown that, with increasing reaction rate constant above a certain limit, the bulk ignition mechanism changes to a surface mechanism. In this case, surface layers of a particle undergo local heating and a reaction front forms and propagates into the particle. The time of the frontal burnout of the particle is shorter than the time of its bulk burnout, during which the temperature field relaxes instantaneously.     

Monitoring adsorption on small particles

A review on the application of photoelectron emission from small particles suspended in a gas to monitor the adsorption properties and to detect adsorbates on the particles is given. The relation between surface coverage with an adsorbate and the shift in photothreshold and photoelectric yield is demonstrated by two examples: The adsorption of O₂ on silver particles and of perylene on carbon particles. A third example concerns the detection of polyaromatic hydrocarbons (PAH) on particles arising from combustions.     

Optical absorption of small metallic particles

In an extremely small metallic particle in which electron energies take discrete values, a light is absorbed by the surface plasmon which is not decayed by the surface scattering. The usually used relation between polarizability of a particle and electrical susceptibility does not hold.     

Chemisorption and reactions of small molecules on small gold particles

The activity of supported gold particles for a number of oxidations and hydrogenations starts to increase dramatically as the size falls below ~3 nm. This is accompanied by an increased propensity to chemisorption, especially of oxygen and hydrogen. The explanation for these phenomena has to be sought in kinetic analysis that connects catalytic activity with the strength and extent of chemisorption of the reactants, the latter depending on the electronic structure of the gold atoms constituting the active centre. Examination of the changes to the utilisation of electrons as particle size is decreased points to loss of metallic character at about 3 nm, as energy bands are replaced by levels, and a band gap appears. Detailed consideration of the Arrhenius parameters (E and ln A) for CO oxidation points clearly to a step-change in activity at the point where metallic character is lost, as opposed to there being a monotonic dependence of rate on a physical property such as the fraction of atoms at corners or edges of particles. The deplorable scarcity of kinetic information on other reactions makes extension of this analysis difficult, but non-metallic behaviour is an unavoidable property of very small gold particles, and therefore cannot be ignored when seeking to explain their exceptional activity.     

Topological segregation in small bimetallic particles

The spatial distribution of the two components at the surface of small RhM/Al₂O₃ (M=Sn, Pb, Ge) bimetallic aggregates has been investigated by using hydrogenolysis of model alkanes, IR spectroscopy of adsorbed CO and quantum chemical calculations. The addition to a Rh aggregate of a small amount of Sn or Pb inhibits selectively the sites of low coordination, and this demonstrates that there is a topological segregation to these sites; Ge atoms appear to be randomly distributed at the surface. Quantum chemical calculations of the relative stabilities of Rh₁₂M model clusters support these conclusions.     

Synthesis and characterisation of small ZnS particles

Small ZnS particles, prepared at room temperature in an alcoholic medium using a zinc salt and thioacetamide as sulphur source, have been characterised using a suite of techniques which includes XRD, TEM and Zn K-edge EXAFS. The investigation suggests that aggregates of small sphalerite particles (cubic lattice), with average size of 3.5 nm and well-defined morphology are obtained and the particle size appears not to change with increase in the reaction time from 2 to 24 h. Zn K-edge EXAFS experiments were performed at 10 K, in order to reduce thermal disorder and the refinement of the EXAFS data resulted in very small second shell coordination numbers with respect to the bulk samples. The result is in good agreement with SEM and XRD data about the presence of nanosized particles, having a large number of surface atoms with low second shell coordination number.     

The structure of small penta-twinned gold particles

The structural feathers of penta-twinned gold particles (size between 2 and 6 nm) generated by gas evaporation have been investigated by high resolution TEM. The structural characteristic of penta-twinned particles is different from that of quasi-crystals that the five coherent or incoherent twin boundaries separating the twin oriented segments do not join up along a common edge. The lattice parameter is reduced by 4–5% in comparison to that of bulk gold. The formation of the penta-twinned particles is proposed to occur by particle collision. The particles were observed to be crystalline at ambient temperature.     

Reactivity of small transition metal clusters

A new experiment for measuring the reactivity of neutral metal clusters is presented. A low pressure reaction cell is used to measure the sticking ofO ₂ andD ₂ gas on small transition metal clusters ofCu, Fe, Co andNi. The experiment yields absolute values for the sticking, at a controlled number of cluster/gas collisions, facilitating comparison with theoretical calculations and other experiments. The most striking result of these preliminary measurements is the difference between oxygen sticking onCo _N and onCu _N , with the sticking onCu _N showing a clear correlation to the electronic shell model, while the sticking onCo _N only exhibits a sharp increase with size, reaching sticking probability=1.0 forN>25.