Sergey A. Gurevich

Efficiency improvement of AlGaInN LEDs advanced by ray-tracing analysis

A modified ray-tracing model is applied to analyze the dependence of external quantum efficiency and far-field radiation pattern of AlGaInN light-emitting diodes (LEDs) on various device parameters. Considerable increase (about 1.5 times) in the efficiency is predicted for LEDs with highly reflective p-contact as compared to those with the semitransparent p-contact. It is shown that the LEDs efficiency degrades with the increasing of the optical loss in AlGaInN epitaxial layers as well as with increasing of the LED chip area. The ways of improving the external efficiency of large-area (and thus, powerful) LEDs are discussed including utilization of a textured (light-scattering) backside of sapphire substrate or a diffusive interface between epitaxial layers and p-contact.

High catalytic activity and stability of palladium nanoparticles prepared by the laser electrodispersion method in chlorobenzene hydrodechlorination

Palladium nanoparticles deposited on thermally oxidized silicon and on the carbon support Sibunit by the laser electrodispersion method are extremely active in the gas-phase hydrodechlorination of chlorobenzene at 100–200°C. High conversion of chlorobenzene (above 90%) has been achieved with catalysts with an unusually low metal content (from 10−4 to 10−3 wt %). The cyclohexane-to-benzene ratio in the reaction products depends on the process duration, palladium content, and support nature. According to X-ray photoelectron spectroscopy (XPS) data, palladium in the catalysts retains its metallic state over a long time under the reaction conditions. Possible causes of the high catalytic activity (105 mol (mol Pd)−1 h−1) of the palladium nanoparticles and their stability to chlorination are discussed.

Laser electrodispersion as a new chlorine-free method for the production of highly effective metal-containing supported catalysts*

Laser electrodispersion (LED) of metals is a promising technique for the preparation of heterogeneous catalysts as an alternative to wet impregnation of supports with the corresponding salt solutions. The LED technique can be used to deposit highly active chloride- and nitrate-free metal nanoparticles onto carbon or oxide supports. We report preparation and properties of new Ni-, Pd-, and Au-containing alumina-supported catalysts with low metal loadings (10–3–10–4 % mass) and their comparison with the previously studied carbon (Sibunit) supported systems. The catalysts demonstrate high stability and extremely high specific catalytic activity (by 2–3 orders of magnitude higher than for traditional catalysts) in the gas-phase hydrodechlorination (HDC) of chlorobenzene (CB).

SURFACE DENSITY OF PARTICLES IN THE DESIGN OF NANOSTRUCTURED CATALYSTS

A discussion is given for theoretical and experimental data confirming the correlation between the catalytic properties of supported catalysts based on metal nanoparticles, the existence of contacts between the particles, and the possibility of charge transfer between them. Laser electrodispersion was used to prepare model systems.

Laser electrodispersion method for the preparation of self-assembled metal catalysts

Abstract Laser electrodispersion (LED) method makes possible to fabricate dense nanostructured catalysts with unique catalytic properties. In contrast to earlier laser ablation techniques, where nanoparticles were synthesized from vaporized matter, LED is based on the cascade fission of liquid metallic drops. Fabricated catalysts consist of ensembles of nanoparticles that are uniform in size and shape, amorphous and stable to coagulation. The catalytic activity of these self-assembled Pt, Ni, Pd, Au and Cu catalysts with extremely low metal content ( -3 mass.%) in hydrogenation and hydrodechlorination is several orders of magnitude higher compared to that for separated metal clusters, highly loaded metal films and supported catalysts prepared by usual methods.

Catalysis of carbon monoxide oxidation with oxygen in the presence of palladium nanowires and nanoparticles

A new synthesis method based on the coagulation of metal nanoparticles, introduced by laser ablation into superfluid helium, inside of quantized vortices has been used for the fabrication of nanoweb consisting of interconnected palladium wires of a 4 nm diameter. It has been found that at temperatures above 523 K, the Pd nanoweb effectively catalyzes the oxidation of CO with molecular oxygen. Temperature cycling leads to a shift of Pd nanoweb activity to lower temperatures. The catalytic action of the Pd nanoweb has been compared to that of Pd nanoparticles with a diameter of about 2 nm prepared by laser electrodispersion.

Interaction of amorphous and crystalline nickel nanoparticles with hydrogen

Nickel nanoparticles were deposited on the surface of highly oriented pyrolytical graphite by laser electrodispersion and precipitation from a solution of reverse micelles. The particles were studied using scanning tunneling microscopy and spectroscopy as well as Auger spectroscopy. The elemental composition of the nanoparticles and specific features of the electronic structure of their surface were determined. Adsorption of molecular hydrogen was also studied.

Peculiarities of the structure and catalytic behavior of nanostructured Ni catalysts prepared by laser electrodispersion

The peculiarities of the structure and catalytic behavior of nickel nanoparticles deposited onto an Al2O3 surface by laser electrodispersion (LED) with subsequent activation in carbon monoxide atmosphere has been considered. The reduction of these nanoparticles by in situ treatment in a catalytic cell in Ar + 5% Н2 atmosphere at 150–450°C has been studied using X-ray photoelectron spectroscopy (XPS). It is shown that formation of metal nickel starts by reduction in hydrogen at 300°C. A comparison of catalytic activity of the Ni/Al2O3 systems in the catalytic oxidation of CO is carried out. It is found that the preliminary treatment of Ni/Al2O3 sample by carbon monoxide leads to an increase in the catalyst efficiency and decrease in the reaction temperature by 50–100°C.

Chlorobenzene hydrodechlorination on bimetallic catalysts prepared by laser electrodispersion of NiPd alloy

Abstract NiPd bimetallic systems were for the first time synthesized by laser electrodispersion (LED) of the Ni77Pd23 alloy target followed by the deposition of produced bimetallic particles on a TEM copper grid and alumina granules. Selective area energy-dispersive analysis confirms the bimetallic nature of NiPd particles deposited on a TEM copper grid. Their mean size is 1.0 nm according to TEM. XPS data demonstrate that under deposition on alumina granules (total metal content of 0.005 wt.%), nickel in bimetallic particles nearly completely oxidizes to Ni2+ species predominantly in the form of aluminate. At the same time major part of palladium (84%) exists in Pd0 but oxidizes to Pd2+ (80%) during 6 months storage in air. Both metals are deposited on the external surface of alumina granules and localized in the same areas. In situ reduction of both metals by H2 in the catalytic cell of XPS spectrometer is hindered. Nickel is not reduced even at 450°C, confirming the formation of NiAlOx, whereas palladium is reduced at higher temperatures compared to a similar monometallic catalyst. Nevertheless, NiPd/Al2O3 catalyst is more efficient in gas-phase chlorobenzene hydrodechlorination at 150–350°C than Ni/Al2O3 and even Pd/Al2O3, and much more stable. The difference may be caused by the formation of new active sites due to the contact between Pd0 and NiAlOx-modified support, and the protective action of spinel reacting with HCl by-product.