Phase transformation in CeO2–Co3O4 binary oxide under reduction and calcination pretreatments

The CeO₂–Co₃O₄ binary oxide was prepared by impregnation of the high surface area Co₃O₄ support (S.A. = 100m² g⁻¹) with cerium nitrate (20 wt% cerium loading on Co₃O₄). Pretreatment of CeO₂–Co₃O₄ binary oxide was divided both methods: reduction (under 200 and 400 °C, assigned as CeO₂–Co₃O₄–R200 and CeO₂–Co₃O₄–R400 and calcination (under 350 and 550 °C, assigned as CeO₂–Co₃O₄–C350 and CeO₂–Co₃O₄–C550). The binary oxides were investigated by means of X-ray diffraction (XRD), nitrogen adsorption at −196 °C, infrared (IR), transmission electron microscopy (TEM), diffuse reflectance spectroscopy (DRS) and temperature programmed reduction (TPR). The results showed that the binary oxides pretreatment under low-temperatures possessed larger surface area. The cobalt phase of binary oxides also was transferred upon the treating temperature, i.e., the CeO₂–Co₃O₄–R200 binary oxide exhibited higher surface area (S.A. = 109m² g⁻¹) and the main phase was CeO₂,Co₃O₄ and CoO. While, the CeO₂–Co₃O₄–R400 binary oxide exhibited lower surface area (S.A. = 40m² g⁻¹) and the main phase was CeO₂, CoO and Co. Apparently, the optimized pretreatment of CeO₂–Co₃O₄ binary oxide can control both the phases and surface area.     

Preparation and characterization of nanostructured Ce0.9Cu0.1O2−δ solid solution with high surface area and its application for low temperature CO oxidation

Nanostructured Ce_0.9Cu_0.1O_2−δ solid solution with high surface area was prepared by improved citrate sol–gel method with incorporation of thermal treatment under N₂. The sample was characterized by TG–DSC, BET nitrogen adsorption, XRD and H₂-TPR. Its catalytic activity for CO oxidation was tested. It was found that the improved method offered catalysts with higher surface area and smaller crystallite size, which led to higher catalytic activity for low temperature CO oxidation. H₂-TPR measurement indicated that there were three CuO species in the Ce_0.9Cu_0.1O_2−δ solid solutions: finely dispersed CuO species on the surface of CeO₂, partial Cu²⁺ penetrated into CeO₂ lattice and bulk CuO phase. The finely dispersed CuO species was regarded as the active site for the low temperature CO oxidation.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Deficiency of O2 molecules enhances ionic conductivity in Cr-doped O-Ce-O for solid oxide fuel cell applications

Production of high performance low-energy loss solid oxide fuel cells (SOFCs) is a challenge and is the global demand of the current market. We have focused on to develop SOFCs that can be operated at 600–800 °C with better ionic conductivity when compared to the conventional SOFCs functioning at 1000–800 °C. Bulk cerium oxide (CeO₂)-based solid electrolyte lessens ionic conductivity at room temperature, thus nanocrystalline CeO₂ has been used to improve the conductivity and to control the temperature. The transition metal-doped CeO₂ (Ce_(1−X)Cr_(X)O₂) nanocrystalline is used to increases the deficiency of oxygen molecules which in turn enhances ionic conductivity in electrolyte material for SOFC applications. The structural and morphological characterization have been done using XRD, RAMAN and FESEM, while electrical and magnetic characterization at room temperature was analysed using vibrating sample magnetometer, impedance spectroscopy and cyclic voltammetery shows better ionic conductivity in Cr doped CeO₂ in comparison with pure nanocrystalline CeO_2.     

Synthesis and characterization of Cu-doped ceria nanopowders

Nanopowdered solid solution Ce_1−xCu_xO_2−γ samples (0 ≤ x ≤ 0.15) were synthesized by self-propagating room temperature synthesis (SPRT). Raman spectroscopy and XRD at room temperature were used to study the vibration properties of these materials as well as the Cu solubility in ceria lattice. The solubility limit of Cu²⁺ in CeO₂ lattice was found to be lower than published in the literature. Results show that obtained powders with low dopant concentration are solid solutions with a fluorite-type crystal structure. However, with Cu content higher than 7.5 mass%, the phase separation was observed and two oxide phases, CeO₂ and CuO, coexist. All powders were nanometric in size with high specific surface area.     

Role of vanadium sites in NO and O2 adsorption processes over VOx/CeO2-ZrO2 catalysts – EPR and IR studies

The interaction of NO and O₂ with 5 mol.% of vanadia deposited on Ce_0.10Zr_0.90O₂ and Ce_0.69Zr_0.31O₂ supports by wet impregnation was studied by means of EPR and IR. The supports were structurally characterized by XRD and Raman spectroscopy. Influence of the phase composition of the support on vanadium speciation as well as on surface architecture of the oxovanadium entities was discussed. The NO forms adsorbed on vanadium-containing systems were compared to those observed on bare CeO₂-ZrO₂ supports. The main products appearing on the catalysts surface during the consecutive reaction with NO and O₂ were identified and their thermal evolution was observed. Changes in vanadium speciation accompanying redox processes related to NO and O₂ activation were also observed and discussed.     

Comparison of CuO/Ce0.8Zr0.2O2 and CuO/CeO2 Catalysts for Low-temperature CO Oxidation

CuO/Ce_0.8Zr_0.2O₂ and CuO/CeO₂ catalysts were prepared via a impregnation method characterized by using FT-Raman, XRD, XPS and H₂-TPR technologies. The catalytic activity of the samples for low-temperature CO oxidation was investigated by means of a microreactor-GC system. The influence of the calcination temperature and different supports on the catalytic activity was studied.     

Catalytic Efficiency of Ceria–Zirconia and Ceria–Hafnia Nanocomposite Oxides for Soot Oxidation

The catalytic oxidation of soot particulates has been investigated over CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ nanocomposite oxides. These oxides were synthesized by a modified precipitation method employing dilute aqueous ammonia solution. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and BET surface area methods. The soot oxidation has been evaluated by a thermogravimetric method under ‘tight contact’ conditions. The XRD results revealed formation of cubic CeO₂, Ce_0.75Zr_0.25O₂ and Ce_0.8Hf_0.2O₂ phases in case of CeO₂, CeO₂–ZrO₂ and CeO₂–HfO₂ samples, respectively. TEM studies confirm the nanosized nature of the catalysts. Raman measurements suggest the presence of oxygen vacancies, lattice defects and oxide ion displacement from normal ceria lattice positions. UV-Vis DRS studies show presence of charge transfer transitions Ce³⁺←O^2? and Ce⁴⁺←O^2? respectively. The catalytic activity studies suggest that the oxidation of soot could be enhanced by incorporation of Zr⁴⁺ and Hf⁴⁺ into the CeO₂ lattice. The CeO₂–HfO₂ combination catalyst exhibited better activity than the CeO₂–ZrO₂. The observed high activity has been related to the nanosized nature of the composite oxides and the oxygen vacancy created in the crystal lattice.     

Highly active and coking resistant Ni/CeO2-ZrO2 catalyst for partial oxidation of methane

Nickel catalysts over the CeO₂-ZrO₂ solid solution were successfully prepared by the co-precipitation method for partial oxidation of methane. The structures of the catalysts were systematically examined by N₂ adsorption/desorption, CO chemisorption, X-ray diffraction (XRD) and H₂-TPR techniques. The catalytic performance and carbon deposition were investigated for partial oxidation of methane as well. The results showed that the Ni/CeO₂-ZrO₂ catalysts had a large BET area and fine Ni dispersion. By the co-precipitation method, Ni and CeO₂-ZrO₂ solid solution had strong interaction confirmed by the H₂-TPR analysis. The Ni/CeO₂-ZrO₂ catalysts showed high activity and stability and the Ni/Ce_0.25Zr_0.75O₂ exhibited the best activity and coking resistance among these catalysts. The catalytic activities and coking resistant behaviors of catalysts were affected by the surface and structural properties of the catalysts.     

XANES studies of the reduction behavior of (Ce1-yZry)O2 and Rh/(Ce1-yZry)O2

Using X-ray absorption near-edge spectroscopy (XANES) at the Ce L_III edge, we have measured the extent of reduction of Rh-loaded and Rh-free, mixed Ce-Zr oxides under hydrogen as a function of temperature. The high surface area, mixed oxides were synthesized by sol-gel techniques and hypercritical drying. Using a simple spectrum subtraction method, the degree of reduction has been measured and compared with previous results for CeO₂ and (Ce_0.5Zr_0.5)O₂. Addition of Zr lowers the temperature of reduction and increases the extent of Ce reduction. Rh catalyzes the reduction process at low temperatures but does not substantially affect the extent of reduction achieved at high temperature. A synergism between Rh and Zr is found which leads to very high reducibility in the range of 400–600 K. This revised version was published online in August 2006 with corrections to the Cover Date.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

Preparation and characterization of 10 mol% Gd doped CeO2 (GDC) electrolyte for SOFC applications

Ceria-based materials are prospective electrolytes for low and intermediate temperature solid oxide fuel cells. In the present work, fully dense CeO₂ ceramics doped with 10 mol% gadolinium (Gd_0.1Ce_0.9O_1.95, GDC) have been prepared with a Pechini method. Characterization studies were realized with thermo-gravimetric analysis (TGA), differential thermal analysis (DTA), mass spectroscopy (MS), high temperature FT-IR (HT-FTIR) and X-ray diffraction analysis (XRD). A single-phase with a fluorite type structure was found to form at a relatively low calcination temperature of 500 °C. Dense GDC pellets having 98% of the relative density were obtained at sintering temperature of 1400 °C/6 h, which gave significantly higher total ionic conductivity of 3.4×10⁻² S cm⁻¹ at 500 °C in air. The present work showed that the Pechini method is a relatively low-temperature preparation technique to synthesize Gd_0.1Ce_0.9O_1.95 powders that provided high sinterability and good ionic conductivity.     

Controlled Hydrogenation of Acetophenone Over Pt/CeO2–MO x (M = Si, Ti, Al, and Zr) Catalysts

Vapour phase selective hydrogenation of acetophenone has been performed over a series of Pt/CeO₂–MO _x (MO _x  = SiO₂, Al₂O₃, TiO₂, and ZrO₂) catalysts. The controlled hydrogenation was carried out in the 453–533 K temperature range at normal atmospheric pressure. The ceria-based mixed oxides were prepared through a co-precipitation or deposition-precipitation route. Platinum was deposited by a wet impregnation method. The obtained catalysts were calcined at 773 K and characterized by means of X-ray diffraction, Raman spectroscopy, BET surface area, temperature programmed reduction, temperature programmed desorption, thermogravimetry, and scanning electron microscopy. XRD analyses suggest that CeO₂–SiO₂ and CeO₂–Al₂O₃ primarily consist of CeO₂ nanoparticles dispersed over the amorphous silica or alumina surface. In the case of CeO₂–TiO₂, presence of segregated nanocrystalline CeO₂ and TiO₂-anatase phase were noted. Formation of cubic Ce_0.75Zr_0.25O₂ solid solution was observed in the case of CeO₂–ZrO₂. No peaks pertaining to platinum could be detected from XRD profiles. Formation of zirconia rich tetragonal phase (Ce_0.4Zr_0.6O₂) was observed in the case of Pt/CeO₂–ZrO₂ sample. Raman measurements revealed the fluorite structure of ceria and presence of oxygen vacancies in all samples. TPR results suggest that the presence of Pt facilitates the reduction of ceria. The catalytic performance of Pt-based catalysts was found to depend strongly on the nature of the support oxide employed. Among various catalysts investigated, the Pt/CeO₂–SiO₂ catalyst exhibited better product yields.     

CeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript>/ZSM-5 sorbents for high-temperature H<Subscript>2</Subscript>S removal

Performance of CeO₂-La₂O₃/ZSM-5 sorbents for sulfur removal was examined at temperature ranging from 500 oC to 700 oC. The sulfur capacity of 5Ce5La/ZSM-5 was much bigger than that of CeO₂/ZSM-5. H₂ had a negative impact on the sulfidation; however, CO had little influence on sulfur removal. The characterization results showed that CeO₂ and La₂O₃ were well dispersed on ZSM-5 because of the intimate admixing of La₂O₃ and CeO₂, the major sulfidation products were Ce₂O₂S and La₂O₂S, the XRD and SEM results revealed that ZSM-5 structure could remain intact during preparation and sulfidation process, the H₂-TPR showed that the reducibility of CeO₂ can be remarkably enhanced by addition of La.     

The catalytic activity for CO oxidation of CuO supported on Ce0.8Zr0.2O2 prepared via citrate method

CuO/Ce_0.8Zr_0.2O₂ catalysts were prepared by citrate method and used for carbon monoxide oxidation. The samples were characterized by XRD, XPS, BET and ICP-AES techniques. The catalytic properties of the catalysts were studied by using a microreactor-GC system. XRD analysis showed Ce_0.8Zr_0.2O₂ was cubic, fluorite structure for all the catalysts. The XPS indicated the valence of Ce atom was +4 and there were reduced copper species presented in the CuO/Ce_0.8Zr_0.2O₂ catalyst. The results showed that the CuO loadings, calcination temperature and calcination time affected the catalytic activity of the catalysts for low-temperature CO oxidation. For comparison, the catalytic activities of CuO/CeO₂ catalysts calcined at different temperatures were also studied. The results indicated that CuO/Ce_0.8Zr_0.2O₂ catalyst had better thermal resistance than CuO/CeO₂ catalyst and had inferior activity than the CuO/CeO₂ catalyst when they were both calcined at 600 °C.     

Catalytic performances of NiO–CeO2 for the reforming of methane with CO2 and O2

Ce_1−xNi_xO₂ oxides with x varying from 0.1 to 0.5 were prepared by precipitation and characterized by XRD, TPR and XPS techniques. Three kinds of Ni phases co-exist in Ce_1−xNi_xO₂ catalysts: (1) aggregated NiO on the support CeO₂, (2) highly dispersed NiO with a strong interaction with CeO₂ and (3) Ni atoms incorporated into the CeO₂ lattice that forms the solid solution. For the reforming of methane with CO₂ and O₂, the Ce_1−xNi_xO₂ oxides prepared by precipitation are superior catalysts, which had a high catalytic activity and thermal stability, especially Ce_0.8Ni_0.2O₂. The catalytic activity is affected by the Ni dispersion on the surface of the catalyst. The Ni–CeO₂ solid solution formed by the Ni species incorporated into CeO₂ promotes the capability of coking resistance.     

Synergic enhancement effect of zirconium oxide,cerium oxide and iron or cobalt oxide on the formation of isobutene from CO and H2

Fe₂O₃ or CoO modified CeO₂-ZrO₂ catalysts lead to four times higher yield of hydrocarbons than ZrO₂ with C₄ hydrocarbons selectivity of more than 50% and isobutene selectivity of more than 80%. XRD and XPS measurements suggested that the interaction of Fe or Co oxide with CeO₂ causes the higher Ce³⁺ concentration with their higher oxidation state. Their combination with ZrO₂ synergistically causes the active and selective formation of isobutene.     

Unique polyhedron CeO2 nanostructures for superior formaldehyde gas-sensing performances

Highly exposed surface area CeO₂ polyhedral nanostructures were successfully prepared via a two-step hydrothermal route for gas-sensing applications. The surface chemistry and formation of polyhedral nanostructures was attributed to the interaction between polyvinylpyrrolidone and ammonium bicarbonate surfactants with parent ceria. The synthesized polyhedron CeO₂ structures were characterized using XRD, XPS, BET, SEM, EDS and TEM, respectively. The polyhedrons exhibited a high specific surface area 98.76?m²/g. For gas-sensing applications, the CeO₂ polyhedrons were exposed to different gases at various temperatures, from a low to high concentration range (1–150?ppm). At an optimal temperature 220?°C, superior gas-sensing response towards formaldehyde was observed than other target gases. The enhanced sensor response was attributed to multifaceted polyhedral nanostructures. The polyhedral structure based sensors have great potential in industrial sensing applications.     

Processing of ThO2/CeO2 Ceramic Fuel

The paper describes an effective procedure for mixing and conditioning ThO₂ and CeO₂ powders so they are suited for pressing and sintering into high‐density (Th_0.9,Ce_0.1)O₂ ceramic pellets – this material being a “pilot” for (Th,Pu)O₂ fuels. Wet ball milling with an organic dispersant aided the powder dispersing process by reducing the agglomeration of very small oxide particles. Homogeneous elemental distributions were seen within the calcined powder mixture. Heat treatments were applied to the calcined, mixed ThO₂/CeO₂ mix to study phase and surface area transformations. Solid solution formation commences at around 1300°C and goes to completion at a temperature of 1500°C. We also report the effect of a granulation strategy that can be applied to the production of high quality, mixed ThO₂ nuclear fuel ceramics. Sized granules of blended ThO₂/CeO₂ powder were produced from precompacted disks of this material that were subsequently heat treated. This had a positive effect on die filling and compaction into green pellets, as well as on final sintered (Th,Ce)O₂ pellet density. The microstructure of the sintered (Th,Ce)O₂ ceramic was characterized using SEM‐based electron back‐scatter diffraction from which a uniform density and grain size were readily apparent. XRD results showed that a single phase Th_0.9Ce_0.1O₂, fuel ceramic had been produced. Its density was ~94% TD.     

Role of CeO2–ZrO2 in diesel soot oxidation and thermal stability of potassium catalyst

Potassium nitrate catalysts supported on different oxides (CeO₂, Ce_0.5Zr_0.5O₂ and ZrO₂) were prepared for diesel soot combustion. The ageing treatment was performed at 800 °C for 24 h and the catalytic activity was evaluated by a temperature-programmed oxidation technique. The results demonstrated that, compared with CeO₂ and ZrO₂, Ce_0.5Zr_0.5O₂ presented good redox properties, a high surface area and available potassium-holding capacity at an elevated temperature. For aged K/Ce_0.5Zr_0.5O₂, the combustion temperature of soot particle was 359 °C under tight contact conditions and 455 °C under loose contact conditions. Thus, ceria–zirconia mixed oxides were considered as good candidate supports for diesel soot oxidation catalysis.