Atomic Cluster Generation with an Atmospheric Pressure Spark Discharge Generator

A new method for generating metal clusters in the gas phase is described and characterized in this work. The method is based on material evaporation by spark ablation at atmospheric pressure. The characterization of atomic clusters was done by measuring their electrical mobility. The measured mobilities were compared with values found in literature in order to identify the cluster species. We show that silver clusters consisting from one up to 25 atoms can be produced in helium at atmospheric pressure. In addition, the effect of oxygen concentration on the resulting cluster mobility distribution was investigated. Results show that at higher oxygen level, the mobility distribution is dominated by the abundance of stable clusters (i.e., magic number clusters). This can be attributed to an oxidation etching effect.

Copyright 2015 American Association for Aerosol Research     

On-line characterization by EXAFS of tin promoted platinum graphite catalysts in the aqueous phase

The structure of tin promoted graphite supported platinum catalysts has been studied with extended X-ray absorption fine structure spectroscopy (EXAFS). A newly developed EXAFS cell allows on-line characterization avoiding contact to ambient or drying. Hereto catalyst samples are transferred from a slurry reactor to the EXAFS cell forming a “bed” of catalyst particles in the EXAFS cell. The cell design was based on considerations concerning possible mass transport limitations while performing reactions in the liquid phase. The structures of the tin promoted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous phase and a hydrogen gas treatment at 573 K at both the Pt L_III and the Sn K-edge. After preparation, under aqueous hydrogen, reduced platinum can be detected with three coordinations: Pt-Pt, Pt-C and Pt-Sn. Tin appears to be partly oxidic showing a Sn-O and a Sn-Pt coordination. A treatment with aqueous oxygen or exposure to ambient leads to oxidized platinum and tin. At the Pt L_III-edge only a Pt-Pt and Pt-O coordination for platinum are detected. At the Sn K-edge tin has only a Sn-O coordination. An aqueous treatment with hydrogen at 363 K reduces platinum showing, however, different coordination numbers for the Pt-Pt and Pt-Sn coordination. Tin only shows a Sn-O coordination. A treatment with hydrogen at 573 K reduces both the platinum and the tin. Platinum shows a Pt-Pt, Pt-C and Pt-Sn coordination. Tin shows a Sn-Pt and Sn-O coordination indicating tin deposition on the platinum, tin being bonded via oxygen to the graphite support. Reductive treatments in the aqueous phase appear to reduce platinum and only the tin deposited on the platinum. The effects of drying and consecutive reductive treatments could only be studied since the developed EXAFS cell allowed catalyst preparation and treatments avoiding contact to ambient.     

Infrared spectroscopy of carbon monoxide adsorbed on Pt/L zeolite

A series of 0.6 wt% Pt/MBaL zeolites, where M is Li, Na, K, Rb or Cs, were prepared and characterized by transmission electron microscopy, chemisorption, and infrared spectroscopy of adsorbed carbon monoxide. Greater than 90% of the exposed platinum in the samples is associated with small clusters, less than 7 Å across, inside the zeolite channels. The remaining fraction of exposed platinum is on 100–500 Å crystallites outside the channels. Adsorption of carbon monoxide on the platinum at 25 °C produces a broad infrared band whose maximum shifts from 2065 to 2025 cm^–1 as the alkali cations in the zeolite are changed from Li to Cs. This shift is indicative of electron transfer between the cations and the platinum clusters. Heating the Pt/L catalysts to 225 °C produces new infrared bands at 2020–2015, 1975, and 1935–1920 cm^–1. The appearance of these low-frequency bands strongly suggests that the CO-covered platinum clusters change their structure during heating. We propose that the new structure is one in which the carbon monoxide molecules insert into spaces between the framework atoms of the L zeolite.     

Density functional theory (DFT) and microcalorimetric investigations of CO adsorption on Pt clusters

Microcalorimetric measurements were conducted at 573 K of CO adsorption on Pt clusters supported in L-zeolite. The measured heat of CO adsorption is 175 kJ/mol, and the heat decreases to 90 kJ/mol near saturation coverage. Quantum chemical calculations were performed using density functional theory to study the interaction of CO with 10-atom Pt clusters. The heat of CO adsorption on atop-sites is calculated to be 209 kJ/mol, while a lower heat of 142 kJ/mol is calculated for CO on bridge-sites. These values decrease to 197 and 102 kJ/mol for population of two atop-sites and two bridge-sites, respectively, on the same Pt₁₀ cluster. The heat of adsorption decreases to 157 kJ/mol when six CO molecules adsorb on six atop-sites of the cluster. The calculated initial heat of CO adsorption on Pt₁₀ clusters is in agreement with experimental and theoretical values reported for CO adsorption on Pt single-crystal surfaces. The higher heat of CO adsorption at atop-sites may be caused by more σ-donation from CO to sp orbitals of Pt for atop-sites. The heat of CO adsorption on bridge-sites becomes higher on negatively charged platinum clusters. The calculated C-O stretching frequencies for charged and neutral platinum clusters agree with experimental data. This revised version was published online in August 2006 with corrections to the Cover Date.     

NaPn（n=1～9）团簇结构和稳定性的研究

在前人所研究的磷团簇基础上设计出NaPn（n=1～9）团簇的各种结构模型,并在B3LYP／6—311G（d）水平上对它们进行优化和频率分析,得到NaPn（n=1—9）每种团簇的各种稳定结构,并对每种团簇的最稳定结构进行二阶能量有限差分分析。结果表明：与纯磷团簇相比,NaPn（n=1～9）团簇的对称性与稳定性都有所降低;n=3,6时NaPn（n=1～9）团簇的最稳定结构存在幻数结构,它们在所有NaPn（n=1～9）团簇中最稳定。     

Computer investigation of the structure of Si73 clusters surrounded by hydrogen

The stabilization of the structure of Si₇₃ clusters that are surrounded by 60 hydrogen atoms and subjected to seventeenfold stepwise heating from 35 to 1560 K (in steps of ∼90 K) is investigated using the molecular dynamics method. The analysis is performed for clusters of three types, i.e., a particle assembled from an icosahedron and a fullerene, a nanocrystal, and a particle with a random atomic packing. In all cases, an increase in the temperature in the course of heating is accompanied by evaporation of a Si atom from the clusters and an increase in the size of silicon particles. The temperature of detachment of Si atoms from clusters is lowest for the cluster with a random atomic packing and highest for the nanocrystal. The nanoassembled particle has the most stable number (close to four) of Si-Si bonds per atom over the entire temperature range 35 ≤ T ≤ 1560 K. For each type of Si₇₃ clusters, the mean length of the Si-Si bond decreases with an increase in the temperature. According to the radial distribution functions, the Si₇₃ clusters have different structures even at the temperature T = 1560 K. The distributions of bond angles reflect the presence of fourfold symmetry elements in the nanoassembled cluster and the nanocrystal. The relative depth of “penetration” of hydrogen atoms into the cluster is largest for the nanocrystal and smallest for the nanoassembled nanoparticle. The largest number of hydrogen atoms is “adsorbed” on the particle with a random atomic packing. Original Russian Text ? A.E. Galashev, I.A. Izmodenov, 2008, published in Fizika i Khimiya Stekla.     

Electrocatalytic activity mapping of model fuel cell catalyst films using scanning electrochemical microscopy

Scanning electrochemical microscopy has been employed to spatially map the electrocatalytic activity of model proton exchange membrane fuel cell (PEMFC) catalyst films towards the hydrogen oxidation reaction (the PEMFC anode reaction). The catalyst films were composed of platinum-loaded carbon nanoparticles, similar to those typically used in PEMFCs. The electrochemical characterisation was correlated with a detailed physical characterisation using dynamic light scattering, transmission electron microscopy and field-emission scanning electron microscopy. The nanoparticles were found to be reasonably mono-dispersed, with a tendency to agglomerate into porous bead-type structures when spun-cast. The number of carbon nanoparticles with little or no platinum was surprisingly higher than would be expected based on the platinum-carbon mass ratio. Furthermore, the platinum-rich carbon particles tended to agglomerate and the clusters formed were non-uniformly distributed. This morphology was reflected in a high degree of heterogeneity in the film activity towards the hydrogen oxidation reaction.     

Sonoelectrochemical (20 kHz) production of platinum nanoparticles from aqueous solutions

The present work describes the production of platinum nanoparticles from aqueous chloroplatinic solutions in the presence of low-frequency high-power ultrasound (20 kHz) on titanium alloy electrodes. The production of this new type of Pt nanoparticles was performed galvanostatically at (298 ± 1) K using a newly designed experimental set-up and ‘sonoelectrode’ producing ultrasonic pulses triggered and followed immediately by short applied current pulses. From galvanostatic studies, it was shown that Pt mean grain size ranging from 11 to 15 nm was produced. Morphological and structural studies of the produced nanoparticles were performed by TEM, SEM, XRD and SAED and showed that Pt nanoaggregates were predominantly formed, with no redissolution of the nanoaggregates. Globular clusters had a mean size ranging between 100 and 200 nm which in turn aggregated and built complex structures.     

Application of high-field,high-resolution 13C CP/MAS n.m.r. spectroscopy to the structural analysis of Yallourn coal

High-field, high-resolution ¹³C cross-polarization and magic angle spinning nuclear magnetic resonance experiments are reported on Yallourn brown coal and on products obtained from the coal by heat treatment. The spectral resolution at 75 MHz is much improved over that previously reported for lower frequencies. A comparison of experimental and simulated spectra allowed the distribution of carbon types in Yallourn coal to be determined; the carbon aromaticity is 42%, and 30% of the carbon is present in methylene groups. The method was also used to determine structural changes during pyrolysis. At 773 K, aliphatic carbon structures are lost, but higher temperatures are necessary to decompose phenolic structures.     

The effect of preparation variables on the dispersion of supported platinum catalysts

The effects of several preparation variables on the dispersion of supported platinum catalysts were examined in this study. Supported catalysts were prepared using two different platinum salts, four high surface area support materials, and two aqueous deposition techniques. The catalysts were characterised by hydrogen chemisorption, oxygen chemisorption, and the hydrogen titration reaction. In addition, X-ray diffraction measurements were conducted on all catalysts. Results from this study show that one of the platinum salts, chloroplatinic acid, always gave an equally or more highly dispersed catalyst than the other salt, tetraammine platinum (II) nitrate. The incipient wetness preparation technique produced equal or better dispersions than did the excess water method, with the possible exception of the silica-alumina support. The highest dispersions were attained with alumina, and the lowest with carbon. In some samples, a lack of good agreement existed between X-ray diffraction and chemisorption measurements of crystallite size which indicated broad or bimodal particle size distributions. However, these X-ray data were qualitatively helpful in understanding the unusual chemisorption behaviour of Pt/C catalysts, one of which had a dispersion higher than any previous reported Pt/C catalyst prepared by aqueous impregnation techniques. Experiments were also conducted which showed that either a 1 h or a 12 h treatment in hydrogen at 723 K was sufficient to reduce the platinum salt, and 1 h or 12 h evacuations at 698 K gave similar results. Finally, it was found that approximately one-half of the hydrogen monolayer on supported Pt could be removed by evacuating for 1 h at 300 K.     

Reactions between germanium and platinum in sub/supercritical water

Reactions between platinum and germanium in sub/supercritical water were carried out by exposing a platinum plate to an aqueous solution at 20-40 MPa and 553-713 K. The surface of the platinum plate was found to be covered with a film of fine crystals of platinum-germanium compounds, Pt₃Ge, Pt₃Ge₂, Pt₂Ge and PtGe, depending upon the pressures and temperatures of the environment.     

In situ FTIR and UV/VIS spectroscopic study of the generation of anionic Pt-carbonyl complexes within a faujasite matrix

The temporal generation of anionic platinum-carbonyl complexes in platinum ionexchanged zeolites X and Y by reductive carbonylation at 10⁵ Pa and 363 K is monitored by in situ UV/VIS and FTIR spectroscopy. A monomer [Pt₃(CO)₆]^2–, exhibiting bands at 318 and 456 nm in the UV/VIS spectra and at 1790 and 2025 cm^–1 in the FTIR spectra, is the only platinum/species formed in NaX. The monomer as well as oligomers are generated in NaY, where the formation of the latter species is due to the stronger acidity in the NaY as compared to NaX. The decomposition of the complexes results in the generation of Pt clusters of the size 1 nm.     

Catalytic Carbon Monoxide Oxidation Using Bio-Templated Platinum Clusters

The catalytic properties of highly dispersed, bacterial surface layer supported nanoscale platinum clusters immobilized at alumina particles are studied with respect to carbon monoxide oxidation. Compared to samples prepared from platinum impregnated alumina, the templated metal clusters are catalytically active at lower temperatures. The catalytic behaviour of the samples is discussed with respect to their cluster morphology studied by TEM.     

The hydrogenolysis of methylcyclopentane on platinum model catalysts: Particle size effect due to a reaction occurring at the phase boundary metal-support

The hydrogenolysis of methylcyclopentane at 520 K on platinum model catalysts was studied and the mechanism of this reaction investigated. Catalysts of low metal dispersion (<0.15) were prepared by high-vacuum evaporation of platinum onto thin films of amorphous alumina. After annealing at 770 K in air and hydrogen treatment at 520 K the mean size of the Pt particles was about 10 nm as determined by electron microscopy. On these catalysts the reaction products are mainly 2-methylpentane and 3-methylpentane and only 9–14% n-hexane is formed, as is to be expected for large Pt particles. On top of these catalysts a thin layer of Al₂O₃ (0.3 nm mean thickness) was then deposited in order to reduce the surface area of platinum in relation to the phase boundary platinum/alumina. Thereafter a decrease of the catalytic activity, as well as a shift in the product distribution, was observed. The n-hexane content was significantly enhanced (up to 22%) and the ratio of the three products was comparable to that usually obtained with Pt catalysts of higher dispersion. This result supports a reaction model which consists of two parallel reactions (i) occurring on the platinum surface and producing mainly 2-methylpentane and 3-methylpentane and (ii) occurring on the phase boundary platinum/support and producing additional n-hexane.     

Restructuring of Hydrogenation Metal Catalysts Under the Influence of CO and H2

CO and H₂ structure and reactivity on single-crystal transition metal surfaces (platinum, rhodium, and palladium) were examined by surface-sensitive techniques including scanning tunneling microscopy (STM) and sum frequency generation (SFG) in high-pressure surface science studies. The studies indicated that ordered CO structures not observed in ultrahigh vacuum (UHV) can form at high pressure (10^-6–10³ torr). In addition, CO and H₂ induce metal atom mobility and restructure the surface. On platinum, CO dissociates at high temperature (≥ 500 K), and a platinum carbonyl precursor is implicated. Concerning catalytic reactions, structure sensitive CO dissociation plays an important role in the ignition of CO oxidation, whereas CO poisons olefin hydrogenation, which becomes CO desorption limited. Lastly, solid-state hydrogen atoms are more active for hydrogenation than surface hydrogen atoms. These results suggest that spatially and temporally resolved techniques would permit molecular studies of reaction intermediates of CO and H₂ in the future.     

单层氮化硼空位缺陷的形成与稳定性的研究

本文利用密度泛函理论方法研究了单层氮化硼中多原子空位缺陷的结构和稳定性。研究发现缺陷多为对称结构,并且存在幻数现象。通过对不同尺寸缺陷离解能的回归分析,发现对于尺寸较大的缺陷,周围的氮原子数越多,缺陷越稳定,该结论很好的解释了实验中观察到的现象。     

Variation, with Temperature, of the Cracking Velocity of Glass

The limiting velocity of fracture for a soda-lime glass was measured as a function of temperature over the range of 80° and 475°K. The value of the fracture velocity was obtained by electronically measuring the time necessary for the crack to travel between two platinum lines fired into the surface of the glass specimen. The value of the fracture velocity for this glass was found to be 1485 m. per second at 300°K. with the velocity decreasing slightly with increasing temperature over the range studied. The results indicate a temperature coefficient on the order of –0.15 m. per second per °K.     

Oxygen reduction of pre-synthesized organically capped platinum nanoparticles assembled in mixed Langmuir-Blodgett films: Evolutions with the platinum amount and leveling after fatty acid removal

Mixed Langmuir-Blodgett (LB) films based on crown derivatized platinum nanoparticles (NPs) and fatty acid showed a direct electrochemical activity toward oxygen reduction reaction (ORR) in acidic medium. This paper reports on the original and strong effects observed in the electrochemical response of these two-phase systems towards oxygen reduction by varying the Pt amount in the mixed LB films. The easy manipulation of pre-synthesized organically capped nanoparticles, combined with the features of the Langmuir-Blodgett technique allows good control of the platinum loading in the films. Pt loadings are set by changing the number of deposited monolayers and/or their nanoparticle density. Loading variations induce strong modifications in the voltammograms and are considered in terms of the relative weight of the two phases: platinum nanoparticles and fatty acids. We point out the strong influence of the environment of platinum nanoparticles towards their electrocatalytical behaviour. Various effects, such as electrical resistance of the nanostructures are considered. Evolution of the peak current density ratios from O₂ and [Fe(CN)₆]^3−/4− probes as a function of platinum loading in mixed films could suggest a possible change in the dominating number of electrons exchanged in the ORR. The effect of the fatty acid phase is definitely demonstrated by the leveling of the electrochemical responses after its elimination from the thin film structures. The electrochemical response is then likely governed by only the organic crown of the particles, which controls the ultimate local environment of the platinum particles. In such a situation, the optimized platinum loading is found to be 2.4 μg cm⁻².     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

Further Studies of the Crystallization of a Lithium Silicate Glass

Rates of crystallization of Li₂O · 2SiO₂, from an Li₂O · 4SiO₂ glass as a function of platinum concentration at 600° and 650° C were determined. At both temperatures a maximum rate was obtained in the region of 0.005% Pt. Calculations of the activation energies for the crystallization process showed that 0.005%, Pt lowered it to less than half its original value of 120 kcal per mole. Electron microscopy of this glass quenched from 1450OC revealed a granular structure 250 A in size which was believed to be due to lithium-rich clusters in the glass, svpporting the concepts of other investigators. In glasses containing 0.025%, an additional phase identified as silica O was found. Explanations for the effectiveness of platinum as a nucleating agent in this glass are offered.