Cytotoxic activity of greener synthesis of cerium oxide nanoparticles using carrageenan towards a WEHI 164 cancer cell line

Carrageenan hydrogel as a “greener” and a vegetable-based stabilizing agent has the potential for many biosyntheses of different nanoparticles by sol-gel method. Herein, we describe for the first time an economic and eco-friendly preparation of cerium oxide nanoparticles (CeO₂-NPs) using carrageenan. When carrageenan hydrogel comes in contact with a cerium nitrate solution, cerium ions anchor themselves to the –SO₃^- groups into the carrageenan and after the gelation process, have fewer opportunities escaping from the polymeric network. The CeO₂-NPs were well-prepared and successfully characterized by PXRD, FTIR, FESEM, UV–Vis, and TGA-DTA. The calcined CeO₂-NPs showed strong UV absorption (λ_max = 328?nm) with the calculated band gap of 2.69?eV. The results obtained from FESEM images indicate that CeO₂-NPs obtained at 600?°C ranges from 18 to 60?nm and have a mean diameter of ~34?nm. The in vitro cytotoxicity study on WEHI 164 cell line has mentioned low toxic and non-toxic CeO₂-NPs in a range of concentrations (0.97–250?μg/ml), thus, we reckon that the greener synthesized CeO₂-NPs will have persistent utilization in various fields of medical applications.     

Defect induced magnetic transition in Co doped CeO2 sputtered thin films

Cobalt-doped cerium dioxide thin films exhibit room temperature ferromagnetism due to high oxygen mobility in doped CeO₂ lattice. CeO₂ is an excellent doping matrix as there is a possibility of it losing oxygen while retaining its structure. This leads to increased oxygen mobility within the fluorite CeO₂ lattice, leading to formation of Ce³⁺ and Ce⁴⁺ species. Magnetic ceria materials are important in several applications from magnetic data storage devices to magnetically recoverable catalysts. In this paper, the room temperature ferromagnetism of rf sputtered Co doped CeO₂ thin films is reported whereas undoped CeO₂ thin films exhibit paramagnetic behavior. The ferromagnetic properties of the Co doped films were explained based on oxygen vacancies created by Co ions in Ce sites. This is further supported by X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and Raman. Change in surface morphology due to Co doping of the samples were analyzed using atomic force microscopy (AFM).     

Biosynthesis,characterization and cytotoxic activity of CeO2 nanoparticles

Cerium oxide nanoparticles (CeO₂-NPs) have been biosynthesized using the aqueous extract of aerial parts of Prosopis farcta. This plant contains phenolic compounds that function as reducing and capping agents. Biosynthesized particles have been characterized through the use of UV–vis, PXRD, TEM, FESEM, EDX, Raman and FTIR analysis. The UV–vis spectrum clearly showed absorption peak at 317?nm, which indicated the formation of spherical CeO₂-NPs. It displayed in the FESEM and TEM images that the biosynthesized particles are uniformly and spherically shaped with a size of about 30?nm, while the EDX has clearly shown that only the elements Ce and O are present in the biosynthesized sample. Moreover, the Raman spectrum of synthesized nanoparticles has displayed a Ce-O stretching band at a measurement of 459?cm^?1. The cytotoxic activity of biosynthesized nanoparticles against the HT-29 cancer cell line was studied using the MTT assay. All the concentrations of CeO₂-NPs (0–800?µg/ml) showed non-toxic effects and thus, it can be suggested that these nanoparticles have the potential of being utilized in various fields of medicine, such as drug delivery.     

Morphology-controlled therapeutic activity of cerium oxide nanoparticles for mild traumatic brain injury

The design and optimization of nanostructures with unique morphologies and properties are at the forefront of biomedical nanotechnology. Cerium oxides are widely used to investigate the effect of morphology on performance. However, elucidating the morphology–activity relationship of cerium oxide nanocrystals in biomedical applications remains challenging. Herein, the therapeutic effects of cerium oxide nanoparticles with different morphologies: cerium oxide nanorods with two different aspect ratios (CeO_x NRs_A and CeO_x NRs_B), cerium oxide nanopolyhedra (CeO_x NPs), and cerium oxide nanocubes (CeO_x NCs) are investigated in in vivo and in vitro mild traumatic brain injury (TBI) models. Cerium oxide nanoparticles inhibit oxidative stress and inflammation after mild TBI, alleviating cognitive impairment; furthermore, the therapeutic effect is significantly affected by their morphology. Owing to the higher Ce³⁺/Ce⁴⁺ ratio, exposure of more active crystal surfaces, and greater number of exposed oxygen vacancies, CeO_x NRs show better activity than CeO_x NPs and CeO_x NCs for mild TBI. Among the two investigated types of cerium oxide nanorods, CeO_x NRs_A, with a higher Ce³⁺/Ce⁴⁺ ratio on the surface, appear to spread better than CeO_x NRs_B in the injured lesions. The factors causing morphology-controlled biomedical performance, such as Ce³⁺/Ce⁴⁺ molar ratio, surface area, and aspect ratio, are discussed.     

Effect of CeO2 on the Glass Structure of Sodium Germanate Glasses

Germanate glasses have potential applications as optical fibers. Materials doped with rare earth ions are good candidates for optical, lasing, and magnetic applications. Based on the ternary system, CeO₂–Na₂O–GeO₂ a series of six glasses were fabricated using powder fusion, and varying the Na₂O content from 0 to 45 mol%, and a CeO₂ content constant at 3 mol%. The glasses were analyzed by FT‐IR, Raman and X‐ray photoelectron (XPS) spectroscopies to obtain information about the glass structure, cerium oxidation's state and how it is introduced in the glass network. FT‐IR and Raman spectra revealed the presence of GeO₆ and GeO₄ groups as well as Q² and Q³ units in the glasses with alkali low content. XPS spectra analysis revealed that the cerium ions were reduced from Ce⁴⁺ to Ce³⁺. The nonbonding to total oxygen ratio was estimated from the curve fitting of the O 1s core level spectra. Density and elastic parameters showed a nonlineal tendency in the change of the physical properties as a function of Na₂O content. Finally, photoluminescence spectroscopy confirmed the presence of Ce³⁺ ions. The characteristic 4f → 5d electronic transitions at 360 nm were detected, when a 280 nm excitation line of pulsed laser was used as excitation source.     

Ce Dispersed Al-MCM-41: A Photocombinate for Phenol Degradation Activity

Al-MCM-41 supported ceria samples of various cerium content (0.2, 0.3, 0.5, 0.8 and 3.0 wt.%) were prepared by impregnation and are characterized by BET, XRD, UV–VIS DRS, XPS and ESR techniques. At lower cerium contents the DRS clearly shows a blue shift of 50 nm in the absorbtion edge of CeO₂ indicating the size quantization and high dispersion of cerium particles. The XPS analyses of Ce–Al-MCM-41 samples revealed the interaction of cerium with Al-MCM-41 in Ce³⁺ state and at higher loadings in Ce^3+/4+ states. ESR studies also further substantiated this observation. These catalysts when subjected to photo degradation activity of phenol, 0.3 wt.% Ce–Al-MCM-41 is showing maximum degradation among all the catalysts tested. The present study highlights that cerium at lower loadings is in +3 oxidation state and is dispersed highly showing good photocatalytic activity in comparison with pure ceria.     

XPS and EIS studies of sputtered Al–Ce films formed on AA6061 aluminum alloy in 3.5% NaCl solution

X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of films at different deposition parameters of sputtered Al-Ce coatings on AA6061 aluminum alloys. By means of electrochemical impedance spectroscopy (EIS) measurements, the protective character of these coatings was studied for 21 days of exposure in a 3.5 wt% NaCl solution and an attempt was made to establish the relationship between film thickness and chemical composition (Al/Ce, Ce³/Ce⁴⁺ ratios) of the surface before and after the electrochemical characterization. XPS studies revealed the presence of the Al^o, Al₂O₃, CeO₂ and Ce₂O₃ compounds, confirming that the sputtered Al-Ce films were deposited in the metallic form and thereafter were superficially oxidized under ambient conditions. The Al–Ce bonds were overlapped with the signal of cerium oxides. The transport phenomena in the oxide film or controlled diffusion process are strongly dependent on the deposition parameters and exposure time in the aggressive medium. It was also found that in the deposited samples at p₄P₂₀₀t₃₀₀, the film was still present after 21 days of exposure, although with visible cracks and erosion areas; however, the Ce³/Ce⁴⁺ ratio almost remained constant before and after the electrochemical characterization, which explained the barrier properties of these samples as compared with others at different deposition parameters.     

Study of cerium species in molten Li2CO3–Na2CO3 in the conditions used in molten carbonate fuel cells. Part I: Thermodynamic, chemical and surface properties

The chemical and surface behaviour of cerium oxide, a candidate material for MCFC applications is analysed in Li₂CO₃–Na₂CO₃ carbonate eutectic in reducing (H₂:CO₂:H₂O:CO) and oxidizing (O₂:CO₂) atmospheres. The electrochemical stability domains of cerium species are established at different temperature on the basis of thermochemical calculations. CeO₂ is the stable species whatever the acidity level in both the anode and cathode conditions; nevertheless, a partial solubility of Ce₂O₃ in CeO₂ can be predicted. The solubility of cerium and cerium oxide samples, determined by absorption spectrophotometry, is about 5 × 10^–4 mol kg^–1 in cathodic conditions and 3 × 10^–4 mol kg^–1 in anodic conditions. X-ray diffraction (XRD) confirmed the presence of CeO₂ at the surface of the samples. Incorporation of sodium species in the CeO₂ lattice is likely; the presence of Ce(III) in long endurance tests was detected by X-ray photoelectron spectroscopy (XPS).     

Activity of oxygen reduction reaction on small amount of amorphous CeOx promoted Pt cathode for fuel cell application

Pt on ceria (CeO_x) particles supported on carbon black (CB) were synthesized using the combined process of hot precipitation and impregnation methods. During 30 cycles of cyclic voltammetry pre-treatment in the potential ranging from −0.2 to 1.3 V (V vs. Ag/AgCl), it was observed that a small amount of CeO_x, which consisted of the interface region between Pt and CeO_x, remained on Pt particles. Other free CeO_x particles were dissolved into H₂SO₄ aqueous solution. To develop the Pt-CeO_x/CB catalyst, the surface chemical states, the net chemical composition, morphology and electrochemical behavior in H₂SO₄ aqueous solution were characterized. Our microanalysis and electrochemical analysis indicate that the active CeO₂ with high specific surface area provides the continuous amorphous cerium oxide (Ce³⁺, Ce⁴⁺) layer with pores on the surface of Pt particles. It is concluded that the amorphous cerium oxide layer on Pt inhibits the oxidation of Pt surface and contributes to enhancement of the activity on Pt cathode. The single cell performance was also improved using the Pt-CeO_x/CB cathode. Based on all data, it is expected that the design based on characterization of the interface between Pt and small amount of amorphous cerium oxide layer could help in preparation of more active Pt catalyst.     

Solar Thermal Hydrogen Production from Water over Modified CeO2 Materials

A series of cerium oxide based materials for hydrogen production from water was studied by temperature-programmed reduction, X-ray diffraction and X-ray photoelectron spectroscopy (XPS) in addition to thermal reactions with water vapour. The addition of uranium ions into CeO₂, making mixed oxides Ce _x U_1?x O₂, resulted in noticeable modification of the reduction properties of CeO₂; with the main observations being the decrease in reduction temperature and the increase of hydrogen consumption when compared to CeO₂ alone. XPS U4f of the as prepared Ce_0.5U_0.5O₂ showed the presence of large amounts of U⁶⁺ cations at 380.9 eV in addition to the U⁴⁺ cations at 379.9 eV; the ratio U⁴⁺ to U⁶⁺ cations was found equal to 0.35. XPS Ce3d showed, on the contrary, considerable amount of Ce³⁺ cations with an estimated ratio of Ce³⁺ to Ce⁴⁺ = ca. 0.5. Ar-ions sputtering results in decreasing the U⁶⁺ contribution and a dramatic increase of the Ce³⁺ contribution. The decrease of U⁶⁺ cations was, however, not mirrored by the increase in Ce³⁺ cations. After five minutes of Ar ions sputtering (1 kV, 10 mA) the surface and near surface Ce3d line shapes looked closer to those of Ce₂O₃ with prominent Ce3d_5/2 and Ce3d_3/2 lines at 885.6 and 904.0 eV attributed to v′ and u′, respectively. The Ce _x U_1?x O₂ series was tested for hydrogen production from water (where x = 0, 0.25, 0.5, 0.75 and 1). All uranium containing oxides had higher activity than CeO₂ or UO₂ alone. Ce_0.75U_0.25O₂ was found to have the highest activity in the studied series; about one order of magnitude higher than that of CeO₂ alone at the same temperature. The reason for the enhanced activity is linked to the ease by which oxygen ions are removed from the oxide materials.     

Impact of physicochemical properties of cerium oxide nanoparticles on their toxicity effects

The advancing production and application of cerium oxide nanoparticles (CeO₂-NPs) in recent years have raised scientists concern about their toxicity. Numerous investigations have been performed to study the toxicity of CeO₂-NPs although their results are sometimes contradictory. In this review, we display the most important factors that are effective in CeO₂-NPs toxicity. The studies are classified based on the target that is selected for toxicity assessment (cytotoxicity, respiratory toxicity, hepatotoxicity, neurotoxicity, dermal toxicity, phytotoxicity, and environmental toxicity). Various representative examples are presented in each class. It seems to be difficult to achieve a comprehensive view and a deterministic conclusion since several parameters are involved in interpreting the results. In order to reach repeatable and comparable results, it is necessary to design a standard protocol to study the toxicity of CeO₂-NPs. Physicochemical properties of nanoparticles, experiment set up, and toxicity assessment methods are some parameters that should be considered in this standardization.     

Bio-template synthesis of CeO2 ultrathin nanosheets for highly selective and sensitive detection of ppb-level p-xylene vapor

Highly selective of carcinogenic and flammable p-xylene vapor and its sensing detection through metal oxides-based sensors has recently attracted much attention. In this work, mesoporous CeO₂ nanosheets were synthesized by simple cerium nitrate impregnation and air calcination using rose petals as bio-template. The effect of calcination temperature on its microstructure, Ce³⁺/Ce⁴⁺ mole ratio, as well as sensing performance was investigated. The CeO₂-650 ultrathin nanosheets calcined at 650 °C are assembled by cross-linking nanoparticles with small size, which possess homogeneous mesoporous distribution and relatively large specific surface area. At 217 °C, the sensor fabricated from CeO₂-650 ultrathin nanosheets shows short response time (T_res = 5 s), high selectivity and response (S = 22.1) towards 100 ppm p-xylene vapor, and its limit of detection (30 ppb) is the lowest among reported sensors based on pure metal oxides. The good sensing performance mainly originate from the synergistic effect of intrinsic features of mesoporous CeO₂-650 ultrathin nanosheets, surface adsorbed oxygen control, oxygen vacancy defects induced by Ce³⁺ and biotemplate imprinting. Therefore, mesoporous CeO₂-650 ultrathin nanosheets could be utilized as candidate for the detection of trace p-xylene vapor.     

Influence of DC magnetron sputtering reaction gas on structural and optical characteristics of Ce-oxide thin films

The influence of the reaction gas composition during the DC magnetron sputtering process on the structural, chemical and optical properties of Ce-oxide thin films was investigated. X-ray diffraction (XRD) studies confirmed that all thin films exhibited a polycrystalline character with cubic fluorite structure for cerium dioxide. X-ray photoelectron spectroscopy (XPS) analyses revealed that cerium is present in two oxidation states, namely as CeO₂ and Ce₂O₃, at the surface of the films prepared at oxygen/argon flow ratios between 0% and 7%, whereas the films are completely oxidized into CeO₂ as the aforementioned ratio increases beyond 14%. Various optical parameters for the thin films (including an optical band gap in the range of 2.25–3.1?eV) were derived from the UV–Vis reflectance. A significant change in the band gap was observed as oxygen/argon flow ratio was raised from 7% to 14% and this finding is consistent with the high-resolution XPS analysis of Ce 3d that reports a mixture of Ce₂O₃ and CeO₂ in the films. Density functional theory (DFT+U) implemented in the Cambridge Serial Total Energy Package (CASTEP) was carried out to simulate the optical constants of CeO₂ clusters at ground state. The computed electronic density of states (DOSs) of the optimized unit cell of CeO₂ yields a band gap that agrees well with the experimentally measured optical band gap. The simulated and measured absorption coefficient (α) exhibited a similar trend and, to some extent, have similar values in the wavelength range from 100 to 2500?nm. The combined results of this study demonstrate good correlation between the theoretical and experimental findings.     

Study of cerium species in molten Li2CO3–Na2CO3 in the conditions used in molten carbonate fuel cells. Part II: Potentiometric and voltammetric behaviour

The electrochemical behaviour of metallic cerium and cerium dioxide is analysed in molten Li₂CO₃–Na₂CO₃ under the anodic and cathodic conditions used for the molten carbonate fuel cells (MCFC). Chronopotentiometric measurements on metallic cerium rods or foils show the influence of CeO₂ growth and the deviation from its stoichiometric composition. A study of the voltammetric characteristics of dissolved cerium at a gold electrode, of metallic cerium and cerium oxide is carried out in both oxidising and reducing MCFC atmospheres. Experimental evidence is given for the existence of a solid–solid cerium system: Ce₂O₃(s)|CeO₂(s) and a system relative to dissolved cerium species Ce₂O₃(l)|CeO₂(l). These systems are in agreement with thermodynamic predictions. Other more complex phenomena involving interactions with alkali species are discussed.     

The influence of chelating agents on cerium oxide decorated on graphite synthesized by the hydrothermal route

Morphology features of cerium oxide nanoparticles, such as size and agglomeration, are important as a coating that improves corrosion resistance and as reinforcement in mechanical applications. In this work, the influence of two heat treatments (160° and 190 °C) in combination with three different chelating agents in the preparation of CeO₂ and CeO₂ decorated on graphite (CeO₂_Gr) nanoparticles is studied. The novelty of this work is that CeO₂_Gr was successfully prepared using the hydrothermal method. All the samples evaluated by X-ray diffraction exhibit a single fluorite-type structure in the cubic phase and Fm3m space group. The spherical harmonics method using the Fullprof Suite program was used to determine the average crystallite sizes, which were 9 nm for CeO₂ and 7 nm for CeO₂_Gr. Transmission electron micrographs for the prepared samples with citric acid showed non-agglomerate particles with homogeneous particle sizes and a quasi-spherical shape distribution. Raman spectra show a band centre at 600 cm^-1 associated with the presence of Frenkel-type oxygen vacancies that induced the reduction of Ce⁴⁺ to Ce³⁺. The analysis of X-ray photoelectron spectra corroborates the coexistence of Ce³⁺ and Ce⁴⁺ species for CeO₂ and CeO₂_Gr nanoparticles. This work forms new perspectives in the development of CeO₂ decorated on graphite prepared by the hydrothermal method to obtain composites not only for sensing applications and wastewater treatment but also for corrosion resistance and reinforcement materials.     

Passivity of OC404 steel modified electrochemically with CeO2-Ce2O3 layers in sulfuric acid media

The effect of cerium oxides film, formed electrochemically on OC404 stainless steel (SS), upon the corrosion behavior of steel in 0.1N H₂SO₄ was investigated. The modification of the steel surface by deposition of cerium oxides films was found to improve the steel corrosion resistance. A linear dependence between the stationary corrosion potential of the cerium oxides/SS system and the cerium concentration in the oxide film was established. The shift of the corrosion potential in the positive direction was found to depend on the proceeding of a depolarizing cathode reaction of CeO₂ reduction (instead of the hydrogen depolarizing reaction) occurring on the cathodic zones, formed by this oxide. On the basis of XPS analyses of the samples, subjected to real corrosion under the conditions of self-dissolution, a pronounced drop of the surface concentration of CeO₂ was established. This is a proof of the occurrence of an effective cathode process of CeO₂ reduction to Ce₂O₃, which was then dissolved in H₂SO₄. Data were obtained (XPS) on the composition and structure of the surface film (SEM) after electrodeposition of cerium oxides and after corrosion in the sulfuric acid medium under consideration for time intervals ranging from 50 up to 1000 h. The ICP-AES studies acquired data on the quantity of dissolved elements, forming the passive layer. After exposure to the corrosive medium, the deposited layer showed enrichment in oxides of chromium and aluminium. The passive film on stainless steel, modified in this way, proved to be more stable to the effect of aggressive sulfuric acid medium, compared to the case of natural passive film.     

Nanostructured quaternary Ni1-xCuxFe2-yCeyO4 complex system: Cerium content and copper substitution dependence of cation distribution and magnetic-electric properties in spinel ferrites

Quaternary Ni_1-xCu_xFe_2-yCe_yO₄ complex nano-ferrites system with different cerium content ratio and copper substitution degree were synthesized via co-precipitation wet chemical technique. The newly obtained nanoparticles, with general formula Ni_1-xCu_xFe_2-yCe_yO₄ (where x = 0.0, 0.3, 0.6 and y = 0.00, 0.03, 0.05, 0.08 and 0.10) were heated up to 600 °C to stabilize the specific crystalline spinel structure. The limit of cerium content was quantitively determined to be around 0.08 and up to 0.10. Furthermore, the powders were pelletized in a 13 mm wide pellets and thermally treated at 950 °C. The thermal treatment affected even more the phases segregation process, as CeO₂ was identified in the sample with lowest degree of cerium insertion – 0.03. Also, a difference in color and size of pelletized samples was noticed after the 950 °C thermal treatment. The Rietveld refinement, crystal structure confirmation, morphology magnetic and electrical properties of samples have been deeply studied. The cation distribution carried out from Rietveld refinement confirms the occupancy of (Fe³⁺) on tetrahedral sites and [Ni²⁺], [Cu²⁺], [Fe³⁺] and [Ce²⁺] on octahedral sites in the crystal lattice. Preliminary information regarding the cation distribution in spinel structures were suggested by FTIR spectral results, precisely in the 650-520 cm^?1 region, as a consequence of peak shape and lack of shiftiness of M^Td – O bond. Spherical-shaped quaternary nano-ferrites of 17–28 nm were determined from FE-SEM analysis and the samples composition was confirmed by EDX analysis. Hysteresis loops shows modifications in coercivity, magnetization and magnetic remanence with Ni²⁺ and Cu²⁺ ions doping in Ni_1-xCu_xFe_2-yCe_yO₄ complex systems with typical ferrimagnetic behavior. Dielectric measurements were employed in order to determine the electrical permittivity, dielectric losses and conductivity values in a 10 Hz – 1 MHz frequency range.     

Preparation of oxygen vacancy-controllable CeO2 by electrotransformation of a CeCl3 solution and its oxidation mechanism

CeO₂ was successfully prepared by electrotransformation of CeCl₃ solution in previous work. In this work, the oxidation mechanism of Ce³⁺ was determined by cyclic voltammetry and a potential-pH diagram, which indicated that Ce³⁺ was oxidized by oxygen during the electrotransformation, and the important role of the oxygen content in solution was confirmed. Thus, the influence of atmosphere (air/argon/oxygen) on CeO₂ preparation was investigated by adjusting the gas type and flow rate. XRD patterns demonstrated the cubic fluorite structure and grain size of CeO₂. With an increase in the gas flow rate, the grain size decreased, but the particle size increased. This result indicated that the smaller grains contained more lattice defects and higher surface energy, so the grains preferred to agglomerate together, forming larger particles. XPS spectra proved that Ce³⁺ and Ce⁴⁺ both existed in the CeO₂ samples. Raman scattering revealed that more oxygen vacancies could be produced with an argon atmosphere and high gas flow.     

Defect generation and morphology transformation mechanism of CeO2 particles prepared by molten salt method

Ceria is widely used in industrial fields due to its unique chemical properties. In this work, a series of CeO₂ particles with controllable morphology, size, and defect concentration were obtained by a simple molten salt method. The adjustment of temperature and molten salt concentration has a considerable effect on the morphology and particle size of the final CeO₂ particles while prolonging the holding time has little effect. Ion doping and reducing atmosphere calcination were used to regulate the defect concentration to improve the chemical activity of CeO₂ particles. SEM results show that the morphology of CeO₂ particles transforms from sphere to octahedron under the two treatments. The Rietveld refinement results and the XPS spectra indicate that increasing calcination temperature, reducing atmosphere calcination and ion doping are beneficial to improving the oxygen vacancies and Ce³⁺ concentration of CeO₂ samples, which are the reason for enhancing the photocatalytic activity of the samples. Moreover, the oriented attachment, agglomeration and merging of crystals formed by the decomposition of cerium precursors are the key to the growth of CeO₂ particles. Aggregates with exposed low-energy planes merge directly to form particles of various morphologies to maintain their own low energy.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.