Effect of the metal-support interaction in Ag/CeO2 catalysts on their activity in ethanol oxidation

The interaction of silver with the surface of CeO2 in the Ag/CeO2 catalysts prepared by coprecipitation and impregnation techniques was studied by temperature-programmed reduction, X-ray diffraction, and high-resolution transmission electron microscopy. It was shown that coprecipitation technique led to formation of strong silver–support interaction and the epitaxy of silver particles (d111 = 2.35 Å) on the surface of CeO2 (d111 = 3.1 Å). This provided incresed catalytic activity in the oxidative dehydrogenation of ethanol at relatively low temperatures (a 15% conversion of ethanol with 100% selectivity for the formation of acetaldehyde was reached at 85°C). Above 130°C, the deep oxidation of ethanol to CO2 becomes the predominant direction of a catalytic reaction, and the Ag/CеО2 catalyst obtained by impregnation technique was most active in this region as a consequence of the weaker metal–support interaction

Stages of Filamentary Carbon Growth from Hydrocarbons on Nickelcontaining Catalysts and Causes of their Deactivation

Methane decomposition to carbon and hydrogen has been studied using the Ni/Al2O3, Ni-Cu/Al2O3 and Ni-Cu/MgO catalysts at 550 °C. The S-shaped kinetic curves of carbon formation from methane exhibit the following periods: induction, acceleration, stationary state and deactivation. The induction period is characterized by oversaturation of metal or alloy particles with carbon atoms and predominant formation of the graphite phase at the (111) faces of the catalyst particles. After formation of the graphite crystallization centers, the acceleration period is accompanied by the growth of graphite filaments and simultaneous reconstruction of the metal particles. After termination of the above processes, the carbon deposition rate becomes constant. Deactivation of the catalyst is caused by blocking of the front side of the metal particle with a carbon film. When the reaction temperature increases to 700 °C, deactivation of the nickel-containing catalyst follows a different mechanism. During the growth of the filamentary carbon, the metal particle becomes viscous-flowing. This fact allows for its partial capturing by the inner filament channel. As a result, the formed carbon filament has an internal channel filled either with metal or its alloy. Hydrogen addition to methane leads a decrease in the carbon formation rate on the catalyst and a change in the filamentary carbon morphology: now it contains a hollow channel

Study of the electrodes length influence on the trajectories of water droplets dispersed in oil and affected by non-uniform electric field

The paper presents the results of numerical modelling of the processes accompanying movement of drop viscous media (water) in oil under the influence of exterior forces of the electric and dynamic nature. Systematic calculations of influence on the electric field heterogeneity drops, created by a symmetric and asymmetrical configuration of electrodes are carried out both in inter electrode and behind electrode areas taking into account a complex operation of dielectrophoresis forces, buoyancies and drag, as well as the variability of electrode sizes. The analysis of drop movement trajectories shows that the asymmetrical configuration of electrodes can be applied for an electro-coalescence intensification of water-in-oil emulsion. Correctness of calculations of the mathematical model and numerical methods are confirmed by good results if compared with the available data of the other authors

Pt and Pd supported on glass fibers as effective combustion catalysts

Pd and Pt supported on glass fiber materials with developed porosity and high sp. surface areas were studied in total propane oxidn. The reaction was carried out in recycling reactor and the kinetic parameters were detd. under different reaction conditions in the temp. range 200-500 DegC. Pt catalysts were seen to be more active than Pd for the same metal loading on identical support. Catalytic activity was seen to depend on support compn. The highest activity was obsd. on Pt supported on glass fiber modified by titania, demonstrating the ignition temps. around 200 DegC. The catalyst surface morphol. and surface dispersion of active metal were characterized by high-resoln. electron microscopy. [on SciFinder (R)

Investigating the influence of silver state on electronic properties of Ag/Ag2O/TiO2 heterojunctions prepared by photodeposition

Two series of Ag-TiO2 photocatalysts were prepared with the use of photodeposition method with variation of the irradiation time and precursor concentration. The obtained photocatalysts were studied by XRD, Raman spectroscopy, HR TEM, UV–vis spectroscopy, and low-temperature N2 adsorption/desorption method. It has been consequently found that the optical properties of the resulting catalysts differ significantly: the positions of surface plasmon resonance peaks red-shifted with the increase in precursor concentration. At the same time, optical absorption of the samples increases with both precursor concentration and irradiation time. Photocatalytic activity for the obtained catalysts was evaluated in decolorization of Rhodamine B with the use of Xe arc lamp (250 W) with and without λ >420 nm light filter. Radical trap experiments have shown that the amount of both superoxide anions and hydroxyl radicals increased in full spectrum of the lamp, with the latter being absent in the reaction mixture during visible light photocatalysis. Comparison of different Ag-TiO2 catalysts has also been made, with 1-Ag-90 min sample being the most active in full spectrum, and 3-Ag-90 min demonstrating the highest conversion in visible light attributed to the increased generation of superoxide species on the surface of Ag clusters. Controversially, 1-Ag-45 min sample showed the lowest activity in full spectrum being surpassed even by unmodified TiO2, but reached the highest rate constant value in visible light. This effect can be related to advanced electronic interaction between Ag plasmonic nanoparticles and titania support in the presence of sensitizer compound, and the formation of Ag/Ag2O composite system on the surface of titania. Influence of the state of silver on photocatalytic activity and mechanism details is discussed with special attention to irradiation wavelengths

Pt/CeO2 and Pt/CeSnOx catalysts for low-temperature CO oxidation prepared by plasma-arc technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions

Structural Changes of Mo/ZSM-5 Catalysts During the Methane Dehydroaromatization

The structure changes of Mo/ZSM-5 catalysts with different Mo content (2 and 10 wt. % Mo) and Si/Al atomic ratio (17, 30 and 45) during the methane dehydroaromatization have been investigated by X-ray powder diffractometry, N2 adsorption and transmission electron microscopy. The treatment of Mo/ZSM-5 catalysts in reducing atmosphere (CH4 or H2) at about 700 oC promotes development of mesoporous system. The pores are open to the exterior of the zeolite grain and have an entrance diameter of ~ 4-10 nm. It is proposed that mesopore formation in Mo/ZSM-5 catalyst is connected with the dealumination of zeolite. The mesopore formation in the parent H-ZSM-5 zeolite by NaOH treatment does not improve the activity of /ZSM-5 catalyst

Deactivation and Regeneration of Mo/ZSM-5 Catalysts for Methane Dehydroaromatization

The methane dehydroaromatization (DHA) was studied over a series of impregnated Mo/ZSM-5 catalysts with different molybdenum contents (1-10 wt.%). It was shown that total methane conversion was decreased by 30% during 12 h of DHA reaction. The benzene formation rate was increased from 0.5 to 13.9 mol C6H6/(gMo·s) when the molybdenum content in the catalyst was lowered from 10 to 1 wt.%. The deactivated Mo/ZSM-5 catalysts were studied by a group of methods: N2 adsorption, XRD, TGDTA, HRTEM and XPS. The content and condensation degree (C/H ratio) of the carbonaceous deposits was found to increase with an increase of either of the following parameters: molybdenum content (1-10 wt.%), reaction temperature (720-780 °C), space velocity (405-1620 h-1), reaction time (0.5-20 h). The stability of Mo/ZSM-5 catalysts in reaction-regeneration cycles was better when the time on stream was shorter. The regeneration conditions of deactivated Mo/ZSM-5 catalysts providing their stable operation under multiple reaction-regeneration cycles have been selected

Pt/CeO2 and Pt/CeSnOx Catalysts for Low-Temperature CO Oxidation Prepared by Plasma-Arc Technique

We applied a method of plasma arc synthesis to study effects of modification of the fluorite phase of ceria by tin ions. By sputtering active components (Pt, Ce, Sn) together with carbon from a graphite electrode in a helium ambient we prepared samples of complex highly defective composite PtCeC and PtCeSnC oxide particles stabilized in a matrix of carbon. Subsequent high-temperature annealing of the samples in oxygen removes the carbon matrix and causes the formation of active catalysts Pt/CeOx and Pt/CeSnOx for CO oxidation. In the presence of Sn, X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) show formation of a mixed phase CeSnOx and stabilization of more dispersed species with a fluorite-type structure. These factors are essential for the observed high activity and thermic stability of the catalyst modified by Sn. X-Ray Photoelectron Spectroscopy (XPS) reveals the presence of both Pt2+ and Pt4+ ions in the catalyst Pt/CeOx, whereas only the state Pt2+ of platinum could be detected in the Sn-modified catalyst Pt/CeSnOx. Insertion of Sn ions into the Pt/CeOx lattice destabilizes/reduces Pt4+ cations in the Pt/CeSnOx catalyst and induces formation of strikingly high concentration (up to 50% at.) of lattice Ce3+ ions. Our DFT calculations corroborate destabilization of Pt4+ ions by incorporation of cationic Sn in Pt/CeOx. The presented results show that modification of the fluorite lattice of ceria by tin induces substantial amount of mobile reactive oxygen partly due to affecting geometric parameters of ceria by tin ions