Structural, Surface and Catalytic Properties of Nano-Sized Ceria Catalysts

Well-dispersed uniform spheres of crystalline CeO 2 were prepared by calcining precursor particles obtained by heating ammonium cerium nitrate for 4 h. These spherical substrates were examined using XRD and TEM methods, and by nitrogen adsorption studies at −196 °C. Subsequently, such cerium oxide particles prepared by calcination at 400–600 °C were used as catalysts for the conversion of isopropanol at 250–450 °C, using a flow method. The results obtained showed that increasing the heating temperature of the system investigated from 400 °C to 600 °C stimulated the formation of a well-crystallized CeO 2 phase having a crystallite size varying between 10 and 20 nm. Both the surface area and catalytic activity of cerium oxide were found to decrease on increasing the calcination temperature. All solids investigated behaved as dehydrogenation catalysts which were selective towards the formation of acetone. The heat treatment did not alter the mechanism of dehydrogenation of isopropanol, but changed the concentration of active sites involved in the catalyzed reaction without altering their energetic nature

Influence of Precursor Compounds on the Structural and Catalytic Properties of Cobalt-Based Catalysts

The effects of different cobalt precursor compounds on the structural and catalytic properties of cobalt metal and Co 3 O 4 catalysts have been investigated. The techniques employed for characterizing the different solids were XRD, ESR and TEM methods, together with their effectiveness as catalysts in the decomposition of H 2 O 2 at 30–50 °C. The results obtained indicate that the investigated catalysts contained clusters or very small particles of cobalt metal or Co 3 O 4 phases. Cobalt metal and Co 3 O 4 catalysts based on cobalt chloride exhibited higher catalytic activities than other catalysts derived from cobalt nitrate and sulphate salts, respectively, due to their decreased particle sizes. The activation energies of the catalytic reaction over the as-prepared catalysts revealed that the different precursor compounds did not modify the energetic nature of the active sites involved in the catalyzed reaction but changed their concentrations

Effect of LiO Doping on the Surface and Catalytic Properties of the FeO–NiO/AlO System

Ferric–nickel/aluminium mixed oxide solids have the formula Fe 2 O 3 –0.42NiO/Al 2 O 3 were treated with Li 2 O (0.75–3 mol%) and heated in air for 4 h at 500°C and 800°C, respectively. The effects of this treatment on the surface characteristics of these solids and their catalytic properties in relation to CO oxidation by O 2 have been investigated. The results reveal that Li 2 O doping at 0.75 mol% concentration resulted in an increase of 24% and 18%, respectively, in the value of the specific surface areas, SBET, of the solids precalcined at 500°C and 800°C, while the addition of 3 mol% Li 2 O led to a slight decrease of ca. 10% in the SBET value of the same solids. In contrast, irrespective of whether the doping process involved solids precalcined at 500°C or 800°C, a significant decrease of 37% and 78%, respectively, was observed in the catalytic activity of these materials. This decrease in catalytic activity was not accompanied by any appreciable change in the magnitude of the activation energy for the catalytic reaction, i.e. Li 2 O doping brings about a decrease in the concentration of catalytically active sites without changing their energetic nature

Surface and Catalytic Properties of NiO–FeO Solids Supported on AlO

A series of NiO–Fe 2 O 3 catalysts supported on γ-Al 2 O 3 was prepared. The effect of the NiO and Fe 2 O 3 contents and the precalcination temperature on the surface and catalytic properties of the various solids has been investigated. The surface characteristics, viz. SBET, V p and r, were determined using N 2 adsorption conducted at –196°C. The catalytic activities of the various solids were studied using the oxidation of CO by O 2 at temperatures in the range between 150°C and 400°C. The prepared solids were preheated in air at various temperatures between 400°C and 1000°C. The results obtained revealed that the SBET values of the different solids decrease progressively on increasing the precalcination temperature above 400°C due to sintering. The specific surface areas were also found to decrease on increasing both the NiO and Fe 2 O 3 contents. The catalytic activities, expressed as reaction rate constant (k) and reaction rate constant per unit area (k), were found to decrease on increasing the precalcination temperature in the range 400–1000°C. Furthermore, the amounts of NiO and Fe 2 O 3 in the different solids modified their catalytic activities in different manners

Novel Preparation and Physicochemical Characterization of a Nanocrystalline Cobalt Ferrite System

Well-defined CoFe 2 O 4 nanoparticles with an average grain size of about 50 nm were successfully synthesized by a combustion method employing different ratios of fuel to cations within the range 0–2.67. The as-prepared powders were characterized and investigated by TG, XRD, IR, SEM, TEM and VSM methods, and via isopropanol conversion at 250–450 °C. The results showed that increasing the ratio between fuel and cations stimulated the formation of cobalt ferrite due to the increasing flame temperature. Changing the fuel/cation ratio brought about modifications in the structure, surface activity, selectivity and magnetic properties of the investigated solids. A fuel/cation ratio of 2 led to the formation of catalyst having a high activity (49%) and magnetization (77.54 emu/g). All the solids investigated behaved as dehydrogenation catalysts leading to the formation of acetone as the major product. The fuel/cation ratio did not alter the mechanism of dehydrogenation of isopropanol, but increased the concentration of active sites involved in the catalyzed reaction

Decomposition of HO on Pure and ZnO-Treated CoO/AlO Solids

The effects of Co 3 O 4 loading, precalcination temperature and ZnO treatment on the catalytic properties of the Co 3 O 4 /Al 2 O 3 system were investigated. The amounts of Co 3 O 4 were varied between 5.57 wt% and 32.0 wt% and the resulting solids subjected to heat treatment at temperatures in the range 400–600°C. The amounts of ZnO were varied between 0.36 wt% and 2.12 wt%. The results obtained indicated that ZnO treatment of Co 3 O 4 /Al 2 O 3 solids followed by precalcination at 400°C resulted in a progressive decrease in the particle size of the Co 3 O 4 crystallites in the resulting samples. The catalytic activity of such solids towards H 2 O 2 decomposition decreased progressively as the precalcination temperature employed was increased in the range 400–600°C. The relationship between the catalytic activity expressed as a plot of the reaction rate constant, k, versus the amount of Co 3 O 4 in the samples showed a progressive increase in the range 5.6–17.7 wt% followed by an abrupt increase when the extent of loading exceeded this limit. Treatment with ZnO effected a measurable increase (42%) in the specific surface area (S BET ) of the treated solids. However, such treatment also resulted in a considerable increase in the value of the reaction rate constant for the catalyzed reaction. Thus, the maximum increase in the value of k 20°C due to doping with 2.12 wt% ZnO attained a value of 543% while the corresponding increase in the value of the reaction rate constant per unit surface area, k̄ 20°C , was 331%. Precalcination at 400–600°C of Co 3 O 4 /Al 2 O 3 solids subjected to ZnO treatment did not modify the mechanism whereby the catalytically active constituents (surface cobalt species) were involved in the reaction although their concentration was altered without affecting their energetic nature