Synthesis of highly ordered mesoporous CeO<Subscript>2</Subscript> and low temperature CO oxidation over Pd/mesoporous CeO<Subscript>2</Subscript>

Highly ordered mesoporous cerium dioxide (meso-CeO₂) was successfully synthesized using a facile solvent-free infiltration method from a mesoporous silica template, KIT-6. The meso-CeO₂ material, thus obtained, exhibited well-defined mesostructure and high surface area (153 m² g⁻¹). The physicochemical properties of meso-CeO₂ material and Pd-supported on meso-CeO₂ (Pd/meso-CeO₂) were characterized by electron microscopy, X-ray diffraction, N₂ adsorption–desorption, and temperature-programmed experiments. The Pd/meso-CeO₂ catalyst exhibited excellent catalytic activity for CO oxidation compared with those of other Pd/CeO₂ catalysts which were prepared using nanocrystalline CeO₂ and bulk-CeO₂ as the supports. Moreover, a hydrogen pretreatment of the Pd/meso-CeO₂ catalyst resulted in a remarkable increase of catalytic activity (T ₁₀₀ = 52 °C).     

Synergetic effect in PdAu/CeO<Subscript>2</Subscript> 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 Pd _x Ce_1?x O₂, 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 Pd_1?x CexO₂ solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.     

Co oxidation with oxygen in the presence of hydrogen on CoO/CeO<Subscript>2</Subscript> and CuO/CoO/CeO<Subscript>2</Subscript> catalysts

The catalytic activity of the CoO/CeO₂ and CuO/CoO/CeO₂ systems in selective CO oxidation in the presence of hydrogen at 20–450°C ([CuO] = 1.0–2.5%, [CoO] = 1.0–7.0%) is reported. The maximum CO conversion (X) decreases in the following order: CuO/CoO/CeO₂ (X = 98–99%, T = 140–170°C) > CoO/CeO₂ (X = 67–84%, T = 230–240°C) > CeO₂ (X = 34%, T = 350°C). TPD, TPR, and EPR experiments have demonstrated that the high activity of CuO/CoO/CeO₂ is due to the strong interaction of the supported copper and cobalt oxides with cerium dioxide, which yields Cu-Co-Ce-O clusters on the surface. The carbonyl group in the complexes Co^δ+-CO and Cu⁺-CO is oxidized by oxygen of the Cu-Co-Ce-O clusters at 140–160°C and by oxygen of the Co-Ce-O clusters at 240°C. The decrease in the activity of the catalysts at high temperatures is due to the fact that hydrogen reduces the clusters on which CO oxidation takes place, yielding Co⁰ and Cu⁰ particles, which are inactive in CO oxidation. The hydrogenation of CO into methane at high temperatures is due to the appearance of Co⁰ particles in the catalysts.     

Physicochemical and catalytic properties of systems based on CeO<Subscript>2</Subscript>

The effect of synthesis conditions, the nature of components, and the ratio between the components on the phase composition, the texture, and the redox and catalytic properties of the Ce-Zr-O, Ce-Zr-M₁-O (M₁ = Mn, Ni, Cu, Y, La, Pr, or Nd), N/Ce-Zr-O (N = Rh, Pd, or Pt), and Pd/Ce-Zr-M₂-O/Al₂O₃ (M₂ = Mg, Ca, Sr, Ba, Y, La, Pr, Nd, or Sm) was considered. A cubic solid solution with the fluorite structure was formed on the introduction of <50 mol % zirconium into CeO₂, and the stability of this solid solution depended on preparation procedure and treatment conditions. The presence of transition or rare earth elements in certain concentrations extended the range of compositions with the retained fluorite structure. The texture of the Ce-Zr-O system mainly depended on treatment temperature. An increase in this temperature resulted in a decrease in the specific surface area of the samples. The total pore volume varied over the range of 0.2–0.3 cm³/g and depended on the Ce/Zr ratio. The presence of transition or rare earth elements either increased the specific surface area of the system or made it more stable to thermal treatment. The introduction of the isovalent cation Zr⁴⁺ into CeO₂ increased the number of lattice defects both on the surface and in the bulk to increase the mobility of oxygen and facilitate its diffusion in the Ce_{1 − x} Zr _x O₂ lattice. The catalytic properties of the Ce-Zr-M₁-O or N/Ce-Zr-M₂-O systems were due to the presence of anion vacancies and the easy transitions Ce⁴⁺ ai Ce³⁺, M₁²ⁿ⁺ ai M₁ ⁿ⁺, and N ^δ+ → N ⁰ in the case of noble metals.     

CO oxidation over Au/CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> catalists: The effect of the support composition of the au-support interaction

The effect of the support composition on the Au-support interactions and its role in the creation of the activity of Au/CeO₂-ZrO₂ catalysts in CO oxidation has been studied. The CeO₂-ZrO₂ oxides and Au/CeO₂-ZrO₂ catalysts were synthesized, characterized by BET, XRD, HRTEM, AAS, TPR-H₂, and tested in CO oxidation. An approximate evaluation of the H₂ consumption for the surface reduction of the studied samples was estimated applying the model developed by Johnson and Mooi, which is based on the qualitative relationship between the amount of the capping oxygen and BET surface area. The sequence of the increasing percentage of O₂ atoms in the capping peak to the total Ce atoms follows the sequence of the decreasing Zr/Ce molar ratio in the sample. The activity of Au/CeO₂-ZrO₂ catalysts depends on the support composition and increases with the decrease in Zr/Ce molar ratio.     

Synthesis and antibacterial activity of Ag/CeO<Subscript>2</Subscript> hybrid architectures

Flower-like ceria (CeO₂) architectures consisting of well aligned nanosheets were first synthesized by a glycol solvothermal method. The size of CeO₂ architectures is about 5?μm in width and 10?μm in length, with the nanosheets thickness below 100?nm. Subsequently, the adsorbed Ag ions on the surface of CeO₂ were in situ reduced to form Ag nanoparticles (NPs), leading to the fabrication of Ag/CeO₂ hybrid architectures (HAs). The formed Ag NPs with sizes of 20–40?nm were uniformly loaded on the surface of the CeO₂ sheets. The antibacterial properties of Ag/CeO₂ HAs against Gram-negative E. coli and Gram-positive S. aureus were evaluated by minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and a filter paper inhibition zone method. The results demonstrated that Ag/CeO₂ HAs displayed excellent antibacterial activity toward S. aureus and E. coli, which were attributed to the synergistic antibacterial effect between Ag NPs and CeO₂ in HAs_. Here, CeO₂ nanoflowers as a new substrate could restrict Ag NPs aggregations and improve their antibacterial activities. Therefore, the resulted Ag/CeO₂ HAs would be considered as a promising antibacterial agent.

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Effect of the microstructure of the supported catalysts CuO/TiO<Subscript>2</Subscript> and CuO/(CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>) on their catalytic properties in carbon monoxide oxidation

The effect of the microstructure of titanium dioxide on the structure, thermal stability, and catalytic properties of supported CuO/TiO₂ and CuO/(CeO₂-TiO₂) catalysts in CO oxidation was studied. The formation of a nanocrystalline structure was found in the CuO/TiO₂ catalysts calcined at 500°C. This nanocrystalline structure consisted of aggregated fine anatase particles about 10 nm in size and interblock boundaries between them, in which Cu²⁺ ions were stabilized. Heat treatment of this catalyst at 700°C led to a change in its microstructure with the formation of fine CuO particles 2.5–3 nm in size, which were strongly bound to the surface of TiO₂ (anatase) with a regular well-ordered crystal structure. In the CuO/(CeO₂-TiO₂) catalysts, the nanocrystalline structure of anatase was thermally more stable than in the CuO/TiO₂ catalyst, and it persisted up to 700°C. The study of the catalytic properties of the resulting catalysts showed that the CuO/(CeO₂-TiO₂) catalysts with the nanocrystalline structure of anatase were characterized by the high-est activity in CO oxidation to CO₂.     

Effect of CeO<Subscript>2</Subscript> on the properties of the Pd/Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/cordierite catalyst in the conversion of CO,NO, and hydrocarbons

The effect of CeO₂ on the properties of the Pd/Co₃O₄-CeO₂/cordierite catalyst is a function of the method of its preparation. The catalyst obtained by the simultaneous deposition of cerium oxide and cobalt oxide showed high activity in the oxidation of CO (CO + O₂, CO + NO) and extensive oxidation of hexane (C₆H₁₄ + O₂). This behavior is due to the increased mobility of surface oxygen and increased dispersion of the catalyst components.     

ESR Study of the Formation of O<Stack><Subscript>2</Subscript><Superscript>−</Superscript></Stack> Radical Anions on Oxidized CeO<Subscript>2</Subscript> and CeO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript> Adsorbing a CO + O<Subscript>2</Subscript> Mixture

It is demonstrated by ESR measurements that O ₂ ⁻ (CO + O₂) radical anions result from CO + O₂ adsorption on the oxidized surface of CeO₂. These radical anions are stabilized in the coordination sphere of Ce⁴⁺ cations located in isolated and associated anionic vacancies. This reaction shows an activation behavior determined by CO adsorption. The variation of O ₂ ⁻ (CO + O₂) concentration with CO adsorption temperature suggests that surface carbonates and carboxylates participate in this reaction. In the (0.5– 10.0)%CeO₂/ZrO₂ system, O ₂ ⁻ forms on supported CeO₂ and is stabilized on Ce⁴⁺ and Zr⁴⁺ cations. The stability of O ₂ ⁻ -Ce⁴⁺ complexes is lower on supported CeO₂ than on unsupported CeO₂, indicating a strong interaction between the cerium cations and the support.__________Translated from Kinetika i Kataliz, Vol. 46, No. 3, 2005, pp. 423–429.Original Russian Text Copyright © 2005 by Il’ichev, Kuli-zade, Korchak.     

Selective oxidation of CO in the presence of hydrogen on CuO/CeO<Subscript>2</Subscript> catalysts modified with Fe and Ni oxides

The selective oxidation of CO in the presence of hydrogen on CuO/CeO₂ systems containing Fe and Ni oxides as promoters was studied. The catalysts containing 1–5 wt % CuO and 1–2.5 wt % Fe₂O₃ supported on CeO₂ and the CuO/CeO₂ systems containing 1–2.5 wt % NiO were synthesized, and their catalytic activity as a function of temperature was determined. It was found that the additives of Fe and Ni oxides increased the activity of the CuO/CeO₂ catalysts with a low concentration of CuO. In this case, the conversion of CO at 150°C approached 100%. At the same time, these additives had no effect on the activity of the CuO/CeO₂ systems at a CuO concentration of 5 wt % or higher, which exhibited an initially high activity in the above temperature region. The forms of CO adsorption and the amounts of active sites for CO adsorption and oxidation were studied using temperature-programmed desorption. It was found that the introduction of Fe and Ni additives in a certain preparation procedure facilitated the formation of an additional amount of active centers associated with CuO. Data on the temperature-programmed reduction of samples (the amount of absorbed hydrogen and the maximum temperature of hydrogen absorption) suggested the interaction of all catalyst components, and the magnitude of this interaction depended on the sample preparation procedure. With the use of Mössbauer spectroscopy, it was found that the procedure of iron oxide introduction into the CuO/CeO₂ system was responsible for the electron-ion interactions of catalyst components and the reaction mixture.     

Colloidal properties of CeO<Subscript>2</Subscript>-ZrO<Subscript>2</Subscript> hydrosols

CeO₂-ZrO₂ hydrosols are synthesized and the size, shape, phase composition, density, and electrophoretic mobility of particles are studied. The pH ranges of the stability of hydrosols and the thresholds of their fast coagulation in the presence of some electrolytes are determined. The nature of the aggregation stability of CeO₂-ZrO₂ hydrosols is discussed.     

Gold catalysts supported on ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for low-temperature CO oxidation

Gold catalysts with loadings ranging from 0.5 to 7.0 wt% on a ZnO/Al₂O₃ support were prepared by the deposition–precipitation method (Au/ZnO/Al₂O₃) with ammonium bicarbonate as the precipitation agent and were evaluated for performance in CO oxidation. These catalysts were characterized by inductively coupled plasma-atom emission spectrometry, temperature programmed reduction, and scanning transmission electron microscopy. The catalytic activity for CO oxidation was measured using a flow reactor under atmospheric pressure. Catalytic activity was found to be strongly dependent on the reduction property of oxygen adsorbed on the gold surface, which related to gold particle size. Higher catalytic activity was found when the gold particles had an average diameter of 3–5 nm; in this range, gold catalysts were more active than the Pt/ZnO/Al₂O₃ catalyst in CO oxidation. Au/ZnO/Al₂O₃ catalyst with small amount of ZnO is more active than Au/Al₂O₃ catalyst due to higher dispersion of gold particles.     

Performance and characterisation of CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript>-WO<Subscript>3</Subscript> catalysts for selective catalytic reduction of NO with NH<Subscript>3</Subscript>

Ce-Ti-W-O _x catalysts were prepared and applied to the NH₃-selective catalytic reduction (SCR) reaction. The experimental results showed that the Ce-Ti-W-O _x catalyst prepared by the hydrothermal method exhibited higher NO conversion than those synthesised via the sol-gel and impregnating methods, while the optimal content of WO₃ and molar ratio of Ce/Ti were 20 mass % and 4: 6, respectively. Under these conditions, the catalyst exhibited the highest level of catalytic activity (the NO conversion reached values higher than 90 %) across a wide temperature range of 225–450°C, with a range of gas hourly space velocity (GHSV) of 40000–140000 h^?1. The catalyst also exhibited good resistance to H₂O and SO₂. The influences of morphology, phase structure, and surface properties on the catalytic performance were investigated by N₂ adsorption-desorption measurement, XRD, XPS, H₂-TPR, and SEM. It was found that the high efficiency of NO removal was due to the large BET surface area, the amorphous surface species, the change to element valence states, and the strong interaction between Ce, Ti, and W.     

CeO<Subscript>2</Subscript>-catalyzed ozonation of phenol

Three different cerium citrate-based precursors were used for synthesizing CeO₂ through thermal treatment. Three morphological types of CeO₂ were obtained. Characterization of these oxides was carried out by XRD patterns, SEM microscopy, N₂ adsorption isotherms, Raman spectroscopy, zeta potential, and UV/Vis luminescence. Ozonation of phenol catalyzed by CeO₂ was studied as a representative reaction of environmental interest. The differences on the catalytic activity showed by these three oxides could be correlated to amounts of Ce³⁺ on CeO₂ surface and, consequently, to the demand for oxygen needed to burn each precursor.     

Properties of the surface layer and catalytic activity of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with Pt or Pd additives in the oxidation of hydrogen

The catalytic activity in the oxidation of hydrogen (in the gaseous state in the presence of excess oxygen) has been studied for samples of Pt(Pd)/Ta₂O_5−x, formed by reduction with hydrogen. The samples obtained had greater activity than the traditional catalysts Pt(Pd)/Al₂O₃. According to X-ray diffraction analysis and electron microscopic studies, Ta₂O_5−x becomes amorphous with the formation of more reduced non-stoichiometric oxygen-deficient tantalum oxides with a surface layer of catalyst. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 180–185, May–June, 2008.     

Carbon black oxidation mechanism in loose and tight contacts with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript> catalysts

The catalytic combustion of carbon black was investigated in the presence of CeO₂ and Al₂O₃. The influence of contact type between carbon particles and these oxides was examined by thermal analysis, the BET specific area, and EPR spectroscopy. For tight contact carbon black-catalyst mixtures, a new paramagnetic species is observed and can be considered as a fingerprint of the contact between the two solids. These new paramagnetic species increase the reactivity of the catalytic reaction of carbon black (CB) combustion and take part in the oxidation mechanism of CB. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 899–904. This article was submitted by the authors in English.     

Thermal synthesis of the CeO<Subscript>2</Subscript>-PrO<Subscript>2</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>pigments

Summary The synthesis of new compounds based on the CeO₂-PrO₂-Nd₂O₃system, which can be used as pigments for colouring of ceramic glazes, is investigated in our laboratory. The optimum conditions for the syntheses of these compounds have been estimated. The methods of thermal analysis provided first information about the temperature region of the formation of the pigments investigated. The synthesis of these compounds was followed by thermal analysis using STA 449/C Jupiter (Netzsch, Germany).     

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H<Subscript>2</Subscript>, CO,and CH<Subscript>4</Subscript>

The influence of hydrogen-containing molybdenum and tungsten bronzes on the catalytic activity of palladium composite catalysts for the oxidation of H₂, CO, and CH₄ was studied. It was found that the composite catalysts containing H _x MO₃ phases (M = W or Mo), which were formed by the reduction of MoO₃ and WO₃ oxides with hydrogen in the presence of deposited Pd, showed higher catalytic activity in the oxidation of small molecules (H₂, CO, and CH₄) with excess oxygen than the traditional Pd/Al₂O₃ deposited catalyst with the same content of the deposited metal. It was shown that the thermal stability of the H _x MO₃ phases was the limiting factor influencing the activity of these composite catalysts.