Properties of NiO–ThO2 Mixed Oxides From Hydroxides and Their Hydrogen Reducibility Behaviour

Some physico-chemical properties of NiO–ThO₂ mixed oxides of various compositions have been investigated. The presence of strongly bound constitutional water in the hydrogel of reactive forms of thorium dioxide, determined by their origin (thermal decomposition of mixed hydroxides) caused the different reduction behaviour as compared with other mixed oxide systems containing the only, thermodynamically less stabile reducible component. The significant effects of the thermal treatment in oxygen atmosphere, pre-irradiation by the gamma rays or accelerated electrons under various conditions (in air or in water suspension) as well as of surface chemical activation with a platinum complex on the reactivity of mixed oxides or reoxidized samples during their hydrogen reduction have been also proved. This revised version was published online in August 2006 with corrections to the Cover Date.     

Investigation of the Cu-Zr-Y oxides activity in the carbon black catalytic oxidation by differential thermal analysis and temperature programmed reduction

Different copper/zirconium-yttrium catalysts have been tested in carbon black oxidation reaction. Supported mainly on differential thermal analysis and temperature programmed reduction, two different mechanisms have been proposed to explain the catalytic results. In the absence of copper, it has been shown that Zr³⁺ ions and associated anionic vacancies are responsible to the catalytic enhancement observed in the mixed oxides, oxygen species being activated on these sites. Among mixed zirconia-yttria solids, ZrO₂-5 mol%Y₂O₃ is the most active catalyst. Copper impregnation on these oxides leads to the formation of different copper species. Small particles of CuO in low interaction with the support, induce a catalytic improvement due to the highest reducibility of these species. Moreover, in order to be more efficient, CuO species should have some interactions with the support, since impregnated samples are more active than the simple mechanical mixtures.     

Electron donor properties and catalytic activity of manganese ferrospinels

The electron donating properties of manganese ferrospinels of various compositions (MnFe₂O₄, Mn_1.2Fe_1.8O₄, Mn₂FeO₄ and Mn_2.5Fe_0.5O₄) were studied from the adsorption of electron acceptors of various electron affinity values from acetonitrile as solvent. The limit of electron transfer from the oxide surface is from 1.77 to 2.40 eV in terms of the electron affinity of the electron acceptor. The data have been correlated with the catalytic activity of these oxides towards autoxidation of sulfites. Both weak and strong electron donor sites catalyze the reaction.     

Dependence of the Physicochemical and Catalytic Properties of Ce<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>O<Subscript>2</Subscript> Oxide on the Means of Synthesis

The effect the means of synthesis have on the texture, phase composition, redox properties, and catalytic activity of binary oxide systems with the composition Ce_0.5Zr_0.5O₂ are studied. The obtained samples are characterized via BET, SEM, DTA, XRD, and Raman spectroscopy. A comparative analysis is performed of the physicochemical properties of biomorphic systems Ce_0.5Zr_0.5O₂ obtained using wood sawdust and cellulose as templates and the properties of binary oxides of the same composition obtained by template-free means. The catalytic properties of the obtained oxide systems Ce_0.5Zr_0.5O₂ are studied in the reaction of carbon black oxidation. It is shown that the texture of the oxide depends on the means of synthesis. When biotemplates are used, fragile porous systems form from thin binary oxide plates containing micro-, meso-, and macropores. Oxide obtained via coprecipitation consists of dense agglomerates with pores around 30 Å in size. In supercritical water, nanoparticles of metal oxide form that are loosely agglomerated. The intermediate spaces between them act as pores more than 100 Å in size. A system of single-phase pseudocubic modification is obtained using a cellulose template. The crystal lattices of all the obtained systems contain a great many defects. It is shown that the system prepared via synthesis in supercritical water has the best oxygen-exchange properties. A comparative analysis is performed of the effect the physicochemical properties of the samples have on their activity in the catalytic oxidation of carbon black.     

Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO<Subscript>2</Subscript> and gas sensitivity

The data on active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO₂ are reviewed. Their interrelation to the gas sensitivity of the materials toward the main air pollutants, viz., CO, NO₂, NH₃, and H₂S, is analyzed. The influence of the synthesis conditions, microstructure parameters, content of dopant impurities, and the presence of catalytic modifiers on the concentration of various active sites on the oxide surface is considered. Relationships between the concentration of the surface sites and sensitivity of the oxides to gases with various chemical properties are revealed. The active sites responsible for the formation of a sensory signal upon the selective interaction with molecules of the detected gases are determined.     

Physicochemical studies on the TiO2-silica gel system

The physicochemical properties (crystal structure, surface acidity, surface area, catalytic activity and electrical conductivity) of TiO₂-silica gel mixed oxides have been investigated. A series of mixed oxides of various compositions in the range of 0–100% for each component were prepared by calcining the mixed oxides in air at temperatures of 115, 300, 600 and 1000°C. The results obtained have been discussed and correlated.     

Characterization and catalytic behavior of La2 - x (Sr, Th)xCuO4 ± λ in reaction NO+CO

Two mixed oxide systems La₂-_xSr_xCuO_{4± λ} (0.0⩽x⩽1. 0) and La_2-xTh_xCuO_{4± λ} (O. O⩽x⩽ 0.4) with K₂NiF₄ structure were prepared by varyingx values. Their crystal structures were studied by means of XRD and IR spectra. The average valence of Cu ion at B site, nonstoichiometric oxygen (λ) and the chemical composition in the bulk and on the surface of the catalysts were measured by means of chemical analysis and XPS. The catalytic behavior in reaction CO+NO was investigated under the regular change of average valence of Cu ion at B site and nonstoichiometric oxygen (λ). Meanwhile, the adsorption and activation of the small molecules NO and the mixture of NO+CO over the mixed oxide catalysts were studied by means of MS-TPD. The catalytic mechanism of reaction NO+CO over these oxide catalysts were proposed; and it has been found that, at lower temperatures the activation of NO is the rate determining step and the catalytic activity is related to the lower valent metallic ion and its concentration, while at higher temperatures the adsorption of NO is the rate determining step and the catalytic activity is related to the oxygen vacancy and its concentration. Project supported by the National Natural Science Foundation of China.     

A Rapid Microwave-Induced Solution Combustion Synthesis of Ceria-Based Mixed Oxides for Catalytic Applications

We have been exploring the utilization of a simple and fast microwave-induced solution combustion synthesis technique for the preparation of various ceria-based mixed oxides for different catalytic applications. In our comprehensive investigation, CeO₂–SiO₂ (MWCS), CeO₂–TiO₂ (MWCT), CeO₂–ZrO₂ (MWCZ) and CeO₂–Al₂O₃ (MWCA) mixed oxides were synthesized by solution combustion synthesis method using microwave dielectric heating and employed for CO and soot oxidation applications. The intricate relationship between ceria and other supporting oxides has been explored with the help of various analytical techniques namely, X-ray diffraction (XRD), temperature programmed reduction/oxidation (TPR/TPO), temperature programmed desorption (TPD) of ammonia and CO₂, Raman spectroscopy (RS), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), BET surface area and thermogravimetry analysis (TGA) methods. XRD results revealed amorphous nature of the material in case of ceria-silica mixed oxide and formation of a specific cubic fluorite type Ce_0.5Zr_0.5O₂ solid solution in the case of ceria-zirconia mixed oxide. Ceria-titania and ceria-alumina mixed oxides exhibited diffraction lines only due to crystalline ceria. Zirconia-based mixed oxide exhibited a lower reduction temperature and better redox properties compared to other samples. TPD of ammonia and CO₂ results revealed superior acid–base properties for MWCS mixed oxide. TGA measurements indicated a complete combustion in all preparations. RS results suggested defective structure of mixed oxides resulting in the formation of oxygen vacancies. XPS results revealed that ceria-zirconia mixed oxide contained more Ce³⁺ compared to other oxides. Among all the mixed oxides, the MWCZ sample exhibited a higher oxygen storage capacity, and better CO and soot oxidation activities. All these interesting findings have been elaborated in this publication.     

Amorphous Oxide Nanostructures for Advanced Electrocatalysis

Amorphous oxides have attracted special attention as advanced electrocatalysts owing to their unique local structural flexibility and attractive electrocatalytic properties. With abundant randomly oriented bonds and surface-exposed defects (e.g., oxygen vacancies) as active catalytic sites, the adsorption/desorption of reactants can be optimized, leading to superior catalytic activities. Amorphous oxide materials have found wide electrocatalytic applications ranging from hydrogen evolution and oxygen evolution to oxygen reduction, CO₂ electroreduction and nitrogen electroreduction. The amorphous oxide electrocatalysts even outperform their crystalline counterparts in terms of electrocatalytic activity and stability. Despite of the merits and achievements for amorphous oxide electrocatalysts, there are still issues and challenges existing for amorphous oxide electrocatalysts. There are rarely reviews specifically focusing on amorphous oxide electrocatalysts and therefore it is imperative to have a comprehensive overview of the research progress and to better understand the achievements and issues with amorphous oxide electrocatalysts. In this minireview, several general preparation methods for amorphous oxides are first introduced. Then, the achievements in amorphous oxides for several important electrocatalytic reactions are summarized. Finally, the challenges and perspectives for the development of amorphous oxide electrocatalysts are outlined.     

Recent progress in catalytic NO decomposition

Recent progress in catalytic direct NO decomposition is overviewed, focusing on metal oxide-based catalysts. Since the discovery of the Cu-ZSM-5 catalyst in the early 1990s, various kinds of catalytic materials such as perovskites, C-type cubic rare earth oxides, and alkaline earth based oxides have been reported to effectively catalyze direct NO decomposition. Although the activities of conventional catalysts are poor in the presence of coexisting O₂ and CO₂, some of the catalysts reviewed in this article possess significant tolerance toward these coexisting gases. The active sites for direct NO decomposition are different depending on the types of metal oxide-based catalysts. In the case of perovskite type oxides, oxide anion vacancies act as catalytically active sites on which NO molecules are adsorbed. C-type cubic rare earth oxides contain oxide anion vacancies with large cavity space, enabling easy access of NO molecules and their subsequent adsorption. Surface basic sites on alkaline earth based oxides participate in NO decomposition as active sites on which NO molecules are adsorbed as NO₂⁻ species. The reaction mechanisms of direct NO decomposition are also discussed.