I. G. Danilova

Low-temperature oxidation of carbon monoxide on Pd(Pt)/CeO2 catalysts prepared from complex salts

Catalysts containing cerium oxide as a support and platinum and palladium as active components for the low-temperature oxidation of carbon monoxide were studied. The catalysts were synthesized in accordance with original procedures with the use of palladium and platinum complex salts. Regardless of preparation procedure, the samples prepared with the use of only platinum precursors did not exhibit activity at a low temperature because only metal and oxide (PtO, PtO2) nanoparticles were formed on the surface of CeO2. Unlike platinum, palladium can be dispersed on the surface of CeO2 to a maximum extent up to an almost an ionic (atomic) state, and it forms mixed surface phases with cerium oxide. In a mixed palladium-platinum catalyst, the ability of platinum to undergo dispersion under the action of palladium also increased; as a result, a combined surface phase with the formula PdxPtyCeO2 − δ, which exhibits catalytic activity at low temperatures, was formed.

Effect of preparation procedure on the properties of CeO2

The effect of preparation procedure on the physicochemical and catalytic properties of CeO2 was studied. Differences in the electronic and structural characteristics of CeO2 depending on preparation procedure and treatment temperature were found using X-ray diffraction analysis, transmission electron microscopy, UV-visible electronic spectroscopy, and X-ray photoelectron spectroscopy. With the use of the temperature-programmed reaction with CO, it was demonstrated that CeO2 samples with a high concentration of point defects—oxygen vacancies caused by the presence of Ce3+—were characterized by an increased mobility of bulk oxygen. The samples of CeO2 with a high concentration of structural defects—micropores of size 1–2 nm and stepwise vicinal faces in crystallites—exhibited a high catalytic activity in the reaction of CO oxidation.

Synthesis and physicochemical characterization of palladium-cerium oxide catalysts for the low-temperature oxidation of carbon monoxide

The effect of CeO2 preparation procedure on the electronic and structural states of the active component of Pd/CeO2 catalysts and their activity in the low-temperature reaction of CO oxidation was studied. The following two nonequivalent states of palladium were detected in the catalysts having low-temperature activity using XPS and IR spectroscopy: Pd0(Pdδ+) as the constituent of a palladium-reduced interaction phase and Pd2+ as the constituent of a palladium-oxidized interaction phase PdxCeO2 −δ. It was found that the procedure used for preparing a CeO2 support considerably affected the formation of these phases and quantitative ratios between them. It was demonstrated that the palladium-oxidized interaction phase was responsible for low-temperature activity, whereas the palladium-reduced interaction phase was responsible for activity in the region of medium and high temperatures.

Low-temperature oxidation of carbon monoxide over (Mn1 − xMx)O2 (M = Co, Pd) catalysts

Abstract(Mn1 − xMx)O2 (M = Co, Pd) materials synthesized under hydrothermal conditions and dried at 80°C have been characterized by X-ray diffraction, diffuse reflectance spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, and adsorption and have been tested in CO oxidation under CO + O2 TPR conditions and under isothermal conditions at room temperature in the absence and presence of water vapor. The synthesized materials have the tunnel structure of cryptomelane irrespective of the promoter nature and content. Their specific surface area is 110–120 m2/g. MnO2 is morphologically uniform, and the introduction of cobalt or palladium into this oxide disrupts its uniformity and causes the formation of more or less crystallized aggregates varying in size. The (Mn,Pd)O2 composition contains Pd metal, which is in contact with the MnO2-based oxide phase. The average size of the palladium particles is no larger than 12 nm. The initial activity of the materials in CO oxidation, which was estimated in terms of the 10% CO conversion temperature, increases in the following order: MnO2 (100°C) < (Mn,Co)O2 (98°C) < (Mn,Co,Pd)O2 (23°C) < (Mn,Pd)O2 (−12°C). The high activity of (Mn,Pd)O2 is due to its surface containing palladium in two states, namely, oxidized palladium (interaction phase) palladium metal (clusters). The latter are mainly dispersed in the MnO2 matrix. This catalyst is effective in CO oxidation even at room temperature when there is no water vapor in the reaction mixture, but it is inactive in the presence of water vapor. Water vapor causes partial reduction of Mn4+ ions and an increase in the proportion of palladium metal clusters.

Hydroisomerization of benzene-containing gasoline fractions on a Pt/SO42−-ZrO2-Al2O3 catalyst: II. Effect of chemical composition on acidic and hydrogenating and the occurrence of model isomerization reactions

The acidic and hydrogenating of Pt/SO42−-ZrO2-Al2O3 samples containing from 18.8 to 67.8 wt % Al2O3 as a support constituent were studied by the IR spectroscopy of adsorbed CO and pyridine, and the model reactions of n-heptane and cyclohexane isomerization on these catalysts were examined. The total catalyst activity in the conversion of n-heptane decreased with the concentration of Al2O3; this manifested itself in an increase in the temperature of 50% n-heptane conversion from 112 to 266°C and in an increase in the selectivity of isomerization to 94.2%. In this case, the maximum yield of isoheptanes was 47.1 wt %, which was reached on a sample whose support contained 67.8 wt % Al2O3. A maximum yield (69.6 wt %) and selectivity (93.7%) for methylcyclopentane formation from cyclohexane were also reached on the above catalyst sample. This can be explained by lower concentrations of Lewis and Brønsted acid sites in the Pt/SO42−-ZrO2-Al2O3 system, as compared with those in Pt/SO42−-ZrO2. The experimental results allowed us to make a preliminary conclusion that the Pt/SO42−-ZrO2-Al2O3 catalyst whose support contains 67.8 wt % Al2O3 is promising for use in the selective hydroisomerization of benzene-containing gasoline fractions in the thermodynamically favorable process temperature range of 250–300°C.

Synergetic effect in PdAu/CeO2 catalysts for the low-temperature oxidation of CO

Gold-palladium catalysts supported on cerium oxide were synthesized with the double complex salts. X-ray photoelectron spectroscopy (XPS) and other physicochemical methods (TEM, TPR) were used to demonstrate that synthesis of highly active palladium catalysts requires the oxidative treatment stimulating the formation of a catalytically active surface solid solution PdxCe1−xO2, which is responsible for the lowtemperature activity (LTA) in the reaction CO + O2. In the case of gold catalysts, active sites for the lowtemperature oxidation of CO are represented by gold nanoparticles and its cationic interface species. Simultaneous deposition of two metals increases the catalyst LTA due to interaction of both gold and palladium with the support surface to form a Pd1−xCexO2 solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.

Hydroisomerization of benzene-containing gasoline fractions on a Pt/SO42−-ZrO2-Al2O3 catalyst: III. The hydrogenating properties of the catalyst

The properties and state of platinum in Pt/SO42−-ZrO2-Al2O3 catalysts with various alumina contents have been investigated in benzene hydrogenation as a model reaction using IR spectroscopy, temperature-programmed reduction, and H2 chemisorption. As the Al2O3 content is raised, the hydrogenating activity of the catalyst increases, which is due to the increasing proportion of metallic platinum on the surface.

Kinetics of Free Radical Generation in the Catalytic Oxidation of Methanol

The formation of free radicals over the surface of platinum-containing catalysts in the methanol oxidation reaction depending on the temperature, the composition of the reaction mixture, and the procedure used for introducing platinum was studied by the matrix isolation method technique. The nature and transformations of surface intermediates depending on the temperature and the presence of oxygen in the gas phase were studied by Fourier transform IR spectroscopy. The main surface intermediate was the methoxy group. The following three types of these groups were stabilized in alumina-based catalysts: (I) CH3O–Aloct (νs(С–Н) = 2806 cm–1), (II) CH3O–Altetr (νs(С–Н) = 2825 cm–1), and (III) CH3O < (Al)2 (νS(С–Н) = 2845 cm–1, δаs(С–Н) = 1460 cm–1, δs(С–Н) = 1440 cm–1, r|| (СН3) = 1185 cm–1, and ν(С–О) = 1095 cm–1). At the same time, isolated methoxy groups (νas(С–Н) = 2997 cm–1, νas(С–Н) = 2959 cm–1, νs(С–Н) = 2857 cm–1, and δ(СН3) = 1450 cm–1) and hydrogen-bonded groups (ν(О–Н) = 3400–3550 cm–1), which resulted from chemisorption at siloxane bridges, were stabilized in silica gel–based catalysts. It was found that CH3O• and CH3OO• radicals were formed only over the surfaces of pure supports (SiO2 and Al2O3) and their mechanical mixtures with platinum. The total concentration of radicals was described by an extremal function of the composition of reactants, whereas the relative concentration depends on the nature of the support. This is conceivably due to the effect of coordinatively unsaturated cations of the support, which are formed by dehydroxylation in the course of catalyst pretreatment. An increase in the rate of formation of gas-phase radicals on mixed catalysts was explained by special properties of the platinum/support interface region, at which surface intermediates were formed in superequilibrium concentrations under reaction conditions.

Role of the support in the formation of the properties of a Pd/Al2O3 catalyst for the low-temperature oxidation of carbon monoxide

The Pd/Al2O3 catalysts were prepared by the impregnation of aluminum hydroxide, which was synthesized by precipitation in the presence of polyvinyl alcohol, with a solution of palladium nitrate and were heat-treated at different temperatures. The resulting samples were characterized by X-ray diffraction, electron microscopy, diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy and were tested in CO oxidation in two modes: in a temperature-programmed reaction and under isothermal conditions at 20°C in the absence and in the presence of water vapor. The activity of the catalysts in the former mode was almost independent of support preparation conditions, but it was different in the latter mode. The catalyst whose support was obtained in the presence of polyvinyl alcohol and treated at 300°C in an atmosphere of nitrogen exhibited the highest activity in CO oxidation at 20°C. In the absence of water vapor from the reaction mixture, the initial conversion of CO reached 40% and then decreased. In the presence of water vapor, a continuous increase in the conversion of CO to 88% was observed, and the activity was stabilized at this level. The smallest size of palladium metal nanoparticles, the nearly monolayer carbon surface coverage, and the presence of OH groups, which are formed upon the dissociation of water present in the reaction mixture, facilitate an increase in activity.

Catalytic and Physicochemical Properties of Oxidative Condensation Products in the Oxidative Dehydrogenation of Propane by Sulfur Dioxide on SiO2

The effect of the deposition of oxidative condensation products in the reaction of oxidative propane dehydrogenation in the presence of SO2 on the catalytic, acid–base, and texture characteristics of silica was studied. It was found that the oxidative condensation products exhibited high catalytic activity in this reaction. The carbonization of silica from 0 to ∼40 wt % was accompanied by an increase in the yield of propylene from 3.4 to 46 mol % (640°C; a C3H8/SO2/He + N2 mixture, 10 : 10 : 80 vol %). Further accumulation of condensation products resulted in a considerable decrease in the pore volume and radius; this imposed diffusion limitations on both propane conversion and selectivity to propane conversion products. The nature of active and deactivated condensation products was studied by DRIFT spectroscopy, diffuse-reflectance UV–VIS spectroscopy, EPR spectroscopy, XPS, thermal analysis, and electron microscopy.