N. S. Telegina

Platinum nanoparticle size effect on specific catalytic activity in n-alkane deep oxidation: Dependence on the chain length of the paraffin

The specific activity of 0.8% Pt/Al2O3 catalysts in the deep oxidation of C1–C6n-alkanes increases with an increase in the Pt particle size from 1 to 3–4 nm. Further coarsening of the particles insignificantly changes the specific activity. The size effect was studied for a series of catalysts containing platinum nanoparticles 1 to 11 nm in diameter. The specific catalytic activity variation range depends on the size of the reacting hydrocarbon molecules. As the platinum particle size increases, the specific catalytic activity increases 3–4 times for the oxidation of CH4 and C2H6 and by a factor of 20–30 for the oxidation of n-C4H10 and n-C6H14.

Specific features of the catalytic behavior of supported palladium nanoparticles in heterogeneous catalytic reactions

Specific features of the catalytic behavior of supported palladium nanoparticles were analyzed in terms of both the size of the particles and their interaction with the support. The influence of these factors on the activity and selectivity of palladium nanoparticles in carbon-carbon bond hydrogenolysis, hydrogenation of aromatic compounds, olefins, and acetylenes, hydrodechlorination, as well as complete oxidation of organic compounds was discussed. It was shown that the optimal nanoparticle size depends on the type of the reaction and also such factors as the nature of interaction between the nanoparticles and support, absorptivity of the substrates and catalytic reaction products, and electronic and crystal structure of the nanoparticles.

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.

Effect of electron irradiation on the catalytic properties of supported Pd catalysts

The effect of electron irradiation on the properties of the systems 1% Pd/C, 1% Pd/Al2O3, and 1% Pd/TiO2 is studied in gas-phase and liquid-phase toluene hydrogenation. An increase in the irradiation dose to 120–900 Mrad increases the catalytic activity by a factor of 2–8 relative to that of the original system. XPS data for the Pd/C catalyst suggest that, after irradiation with high-energy electrons, the metal particles are stabilized on the surface of the carbon support, their degree of dispersion is increased, and their sintering is suppressed. These inferences are consistent with the observed changes in catalytic properties.

Propane oxidation with chemically bound oxygen on Pt/TiO2/Al2O3 and Pt/CeO2/Al2O3 catalysts

Possible mechanisms are suggested for propane oxidation on Pt/TiO2/Al2O3 and Pt/CeO2/Al2O3 catalysts in the cyclic reactant supply mode. As compared to the steady-state process, the process conducted as catalyst oxidation-reduction cycles results in a very different product composition: it is more selective toward partial oxidation products and yields much smaller amounts of complete oxidation products. It is established by isothermal and temperature-programmed oxygen desorption that, under the reaction conditions examined, the oxygen desorbed from the catalyst surface into the gas phase makes a negligible contribution to propane oxidation. It is proved by XPS that propane oxidation is due to the chemically bound oxygen of the catalyst. The hypothetical mechanism of the process includes propane activation on Pt followed by the transfer of the activated species to the oxygen-storing component (TiO2 or CeO2), where the intermediates are oxidized by chemically bound oxygen.

Study of the formation and stability of the Pd and Pt metallic nanoparticles on carbon support

Catalysts containing Pd and Pt on a Sibunit carbon support were studied by the temperature-programmed reduction, in situ X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy (XAFS). The reduction of Pd and Pt species in samples 2%Pd/C and 2%Pt/C calcined in an air flow at 370°C was studied. Reduction of the 2%Pd/C sample begins at 50—60 °C and is completed at 250—300°C. Particles of various dispersion are formed during reduction. Long-distance peaks observed in the EXAFS spectra point to the presence of a fraction of relatively large crystallites. The average Pd—Pd coordination number (∼5) at 200 °C gives evidence that a number of very small Pd nanoparticles, oligomeric clusters, is present. Reduction at T > 200°C results in sintering of a small fraction of the Pd particles. Reduction of Pt in 2%Pt/C sample begins at 120—150 °C and is completed at 300—350°C. The sintering-resistant monodispersed Pt particles are formed under these conditions.

New catalyst supports and catalytic systems on the basis of metallic gauzes coated with II–IV group element oxides

Abstract Preparation of catalysts and catalyst supports on the basis of metal gauzes by electrophoretic deposition was studied. Samples of stainless-steel gauzes with the thickness of wire of 50 m coated with the A1 2 O 3 , ZrO 2 , SiO 2 and zeolite layers from 1 to 10 m were obtained. The properties of the prepared coatings were studied by SEM and XPS. Additionally, the catalytic properties of obtained by EPD zeolite catalyst were studied in the reaction of partial oxidation of benzene to phenol with N 2 0.

Mn-Ce/beta "bifunctional" catalyst for the selective catalytic reduction of nitrogen oxides with ammonia

The properties of the Mn–Ce/Beta zeolite catalyst in the selective catalytic reduction (SCR) of NOx have been investigated. The introduction of Ce leads to a marked increase in the NOx conversion at 100–250°C. The data of this study are consistent with the “bifunctional” pathway of SCR suggested for Mn/Beta, which consists of two stages—NO oxidation to NO2 over the oxide component and “fast” SCR over the zeolite. The increased activity of Mn–Ce/Beta at the NO-to-NO2 oxidation stage is due to the formation of MnCeOx mixed oxides enhancing the mobility of lattice oxygen. The determining role is played by the activity of the zeolite component in the “fast” SCR reaction.

Development of the bifunctional catalyst Mn—Fe—Beta for selective catalytic reduction of nitrogen oxides*

The catalytic properties of the Mn-Fe-Beta system with Mn contents in the range 0.1–16 wt.% were studied in the selective catalytic reduction (SCR) of NOx with ammonia. The catalyst structure was investigated using IR spectra of adsorbed NO, temperature-programmed reduction with hydrogen (H2-TPR), X-ray diffraction analysis, and ESR. The use of manganese as a promoter substantially increases the activity of iron-containing catalysts in the SCR of NOx with ammonia. At low contents (<2 wt.%), Mn exists in the cation form and the catalytic activity of the Mn-Fe-Beta system does not increase. At a higher content of Mn, clusters MnOx begin to form, which are highly active in the oxidation of NO to NO2 and the low-temperature catalytic activity of the Mn-Fe-Beta system increases. The observed increase in the low-temperature catalytic activity in the process of SCR of NOx with ammonia is related to a change in the reaction route. The MnOx clusters favor the oxidation of NO and the iron cations facilitate the reaction of “fast” SCR.

Oscillations of surface composition in Pt-Co alloys induced by CO chemisorption

The influence of carbon monoxide adsorption on surface composition of cobalt alloys, containing 30 and 1 at.% platinum was studied by low energy ion scattering spectroscopy and X-ray photoelectron spectroscopy. Composition oscillations in Co/Pt ratio across the several outermost layers and caused by CO chemisorbed at 400 K were found. XPS indicated the presence of both molecular and dissociatively adsorbed CO. A possible explanation of the surface process which cause these oscillations is proposed.