E. M. Slavinskaya

Catalytic Purification of Exhaust Gases Over Pd–Rh Alloy Catalysts

Three-way catalysts with low content of Pd–Rh alloy particles used as active components were synthesized and studied. Monometallic and bimetallic mixed catalysts with corresponding precious metals loading were chosen as reference samples. The catalytic activity was tested in oxidation of carbon monoxide, hydrocarbons, and in nitrogen oxides reduction. The stability of the samples was estimated by prompt thermal aging in situ technique. Ethane hydrogenolysis testing reaction was used to determine the surface concentration of Pd and Rh, and to confirm the alloy formation. Photoluminescence and electron paramagnetic resonance spectroscopy were applied to clarify the possible reasons of deactivation and to elucidate the mechanism of stabilization. It was shown that Pd–Rh alloyed catalyst is characterized by comparable activity and enhanced stability. While the Pd and Rh particles of monometallic samples were found to interact with support at high temperatures resulting in sintering and bulk diffusion, the particles of alloy type kept their initial state of dispersion and catalytic activity.

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.

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.

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.

Silver nanoparticles obtained by laser ablation as the active component of Ag/SiO2 catalysts for CO oxidation

Composite systems comprising a macroporous SiO2 support and silver nanoparticles preliminarily prepared by laser ablation in various liquids were synthesized and studied. Investigation of the catalytic properties of the synthesized composite catalysts demonstrated that, irrespective of particle size, non-interacting metallic silver particles in SiO2 matrix are not active toward the low temperature oxidation of CO, their activity appearing only at temperatures above 300xa0°C. A combination of X-ray photoelectron spectroscopy and transmission electron microscopy allowed revealing that the activation redox treatment (O2-500xa0°C/H2-200xa0°C) of such particles in macroporous SiO2 matrix leads to the structures resembling surface silver silicates AgSiOx, which are active in CO oxidation at temperatures above 200xa0°C.

Dependence of the properties of Ce-Zr-Y-La-M-O systems on synthetic conditions and on the nature of the transition metal M (Mn, Fe, Co)

The effects of synthetic conditions, component ratios, and the nature of the transition metal on the physicochemical and catalytic properties of Ce-Zr-Y-La-M-O (M = Mn, Fe, Co) systems are studied. The Ce-Zr-Y-La-M-O samples precipitated at ∼23°C and calcined at 600°C are single-phase and are solid solutions with a fluorite structure, which persists upon calcination at 1150°C. The Ce-Zr-Y-La-Fe(Co)-O samples precipitated at 70°C and calcined at 1150°C consist of two solid solutions, one cubic, and the other tetragonal. The specific surface area (Ssp) of the samples precipitated at ∼23°C and calcined at 600°C increases in the order Ce-Zr-Y-La-O < Ce-Zr-Y-La-Mn-O < Ce-Zr-Y-La-Co-O ≈ Ce-Zr-Y-La-Fe-O. The specific surface area of the samples precipitated at 70°C is independent of M and is ∼110 m2/g. Calcination at 1150°C reduces Ssp approximately by two orders of magnitude. The TPR of the unpromoted systems in H2 proceeds in two steps at 600–650 and 750–840°C. The introduction of M decreases the reduction temperatures and gives rise to a lower temperature peak between 150 and 300°C. The most effective promoter is cobalt. The fluorite-type catalysts containing no noble metal are active in NO reduction (XNO ≈ 100%) at Treact = 400–450°C. The cobalt-containing catalysts are the most active in the oxidation of CO (Xmax = 28%) and hydrocarbons (Xmax = 4.3%).

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.

Deposition of silver nanoparticles into porous system of sol–gel silica monoliths and properties of silver/porous silica composites

Porous monolithic gels based on silica with pore size from 16xa0nm to 3–5xa0μm have been synthesized using sol–gel technology. Parameters of porous structure are determined by the components molar ratio in the reaction mixture. The reduction processes of silver ions by formamide in the synthesized porous gel were studied. It has been shown that at the initial stage of the reaction, silver particles with size up to 10xa0nm are formed in the absence of any stabilizers. The composites Ag/SiO2 were synthesized by means of the threefold impregnation of porous monoliths using the solution of silver nitrate in the mixture of methanol and formamide. Their catalytic activity in the CO oxidation was studied. It was discovered that after activation in oxygen and hydrogen the samples display a low temperature activity, which depends on the number of Si–O-nonbridging oxygen groups on the surface of silica porous monoliths.

Synthesis and catalytic activity of porous blocked Ag/SiO2 composites in low-temperature carbon monoxide oxidation

The Ag/SiO2 composites were synthesized based on porous blocked silica with a pore size of 30–50 nm and a specific surface area of 99 m2/g. Silver particles were introduced into the pores of the support by its impregnation with a solution of an ammonium complex of silver followed by reduction with hydrogen. The liquid-phase reduction of silver ions in pores was performed in the absence of stabilizing agents with the use of ethylene glycol (a polyol method) or formamide as a reducing agent. The methods used in the preparation of composites made it possible to vary the particle size of silver. The greatest size that is almost comparable with the pore size was achieved with the use of formamide. The catalytic activity of the Ag/SiO2 composites was studied in the reaction of CO oxidation. It was found that the catalysts obtained upon the reduction of Ag+ ions by formamide exhibited considerable low-temperature activity. A necessary condition for the manifestation of low-temperature activity is redox treatment, in the course of which the particle size of silver considerably decreases.