Size Effect in Ethylene Oxidation over Silver Nanoparticles Supported on HOPG

A preparation procedure for a model catalyst – stable silver nanoparticles with a narrow size distribution supported on highly oriented pyrolytic graphite (Ag/HOPG) – is presented. This procedure is carried out under ultra high vacuum conditions and consists of three stages: 1 – surface defect formation by soft Ar+-treatment; 2 – silver deposition; 3 – defect annealing by heating at T = 300 °C. The analysis of oxygen forms on a silver surface was carried out by in-situ X-ray photoelectron spectroscopy (XPS). The simultaneous presence of two oxygen species (electrophilic and nucleophilic) on the silver surface was found to be necessary for high activity of the model catalyst in ethylene epoxidation. An alternative explanation of the size effect in ethylene oxidation has been suggested: size dependence of the reaction rate may result from the existence of a ring-shaped zone at the edge of silver particles in which the surface concentration of nucleophilic oxygen is not uniform. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3514

Structure Features of Bismuth Films Doped with Tellurium

The influence of doping degree on the structural characteristics of bismuth films doped with tellurium in the concentration range 0.005-0.150 at. % Te and the thickness range 0.3-0.7 µm is studied at present article. Authors have established that an increase of the doping degree with tellurium in bismuth films leads to a significant decreasing of the growth figures. The weak influence of annealing on the crystallite size of bismuth films doped with tellurium indicates their high temporal stability of the structure

Synthesis of symmetrical chromeno[2,3-b]chromenes on the basis of о-quinone methides and 1,1-bis(morpholino)ethene

[Figure not available: see fulltext.] The reaction of 1,1-bis(morpholino)ethene with precursors of о-quinone methides derived from Mannich bases of phenols and 2-naphthols provided a series of chromeno[2,3-b]chromenes. The cascade process included a hetero-Diels–Alder reaction between о-quinone methide and ketenaminal, elimination of secondary amine, and [4+2] cycloaddition between о-quinone methide and the chromene intermediate. © 2018, Springer Science+Business Media, LLC, part of Springer Nature

Oxidative rearrangement of 4H-chromenes to 2-aroylbenzofurans in the presence of selenium dioxide

[Figure not available: see fulltext.] A new method is proposed for the preparation of 2-aroylbenzofurans and 2-aroylnaphtho[2,1-b]furans via oxidative rearrangement of 2,4-diaryl-4Н-chromenes and 1,3-diaryl-1Н-benzo[f]chromenes in the presence of selenium dioxide. © 2017, Springer Science+Business Media, LLC, part of Springer Nature

Oxidative rearrangement of 4H-chromenes to 2-aroylbenzofurans in the presence of selenium dioxide

[Figure not available: see fulltext.] A new method is proposed for the preparation of 2-aroylbenzofurans and 2-aroylnaphtho[2,1-b]furans via oxidative rearrangement of 2,4-diaryl-4Н-chromenes and 1,3-diaryl-1Н-benzo[f]chromenes in the presence of selenium dioxide. © 2017, Springer Science+Business Media, LLC, part of Springer Nature

Synthesis of symmetrical chromeno[2,3-b]chromenes on the basis of о-quinone methides and 1,1-bis(morpholino)ethene

[Figure not available: see fulltext.] The reaction of 1,1-bis(morpholino)ethene with precursors of о-quinone methides derived from Mannich bases of phenols and 2-naphthols provided a series of chromeno[2,3-b]chromenes. The cascade process included a hetero-Diels–Alder reaction between о-quinone methide and ketenaminal, elimination of secondary amine, and [4+2] cycloaddition between о-quinone methide and the chromene intermediate. © 2018, Springer Science+Business Media, LLC, part of Springer Nature

Preparation of Ag HOPG model catalysts with a variable particle size and an in situ xps study of their catalytic properties in ethylene oxidation

The preparation of model silver catalysts supported on highly oriented pyrolytic graphite is described, and the effect of the Ag particle size on the catalytic ethylene oxidation into ethylene oxide, studied by in situ XPS and mass spectrometry, is considered. For a mean particle diameter of 8 nm, the adsorbed oxygen species characterized by an O 1s binding energy of 530.8 ± 0.2 eV (electrophilic oxygen) forms on the silver surface exposed to the ethylene–oxygen reaction mixture. Larger silver particles with a mean diameter of 40 nm additionally contain the adsorbed oxygen species characterized by an O 1s binding energy of 529.2 ±0.2 eV (nucleophilic oxygen). The presence of both oxygen species on the surface of the larger particles ensures the formation of ethylene oxide, while the sample with the smaller silver particles is inactive in the epoxidation reaction. The O 1s signal at 530.8 eV is partly due to oxygen dissolved in the subsurface layers of silver

The silver oxygen system in catalysis new insights by near ambient pressure X ray photoelectron spectroscopy

We addressed the interaction of oxygen with silver by synchrotron based near ambient pressure X-ray photoelectron spectroscopy at temperatures relevant for industrial oxidation reactions performed with silver catalysts. For silver single crystals, polycrystalline foils and powders in equilibrium with gas phase O2, we observed the dynamics of the formation of five different atomic oxygen species with relative abundances depending on the temperature and time. Correlation of their formation kinetics with spectroscopic features and thermal stability indicates that these are distinct species with different electronic structures, which might relate to the different roles of silver in oxidation reactions