CeO2 Phase Research Articles

Optimizing the Microstructure of SnO2-CeO2 Binary Oxide Supported Palladium Catalysts for Efficient and Stable Methane Combustion.

The preparation of palladium-based catalysts with both high catalytic activity and hydrothermal stability currently appears as a critical topic in methane combustion. Herein, we propose a facile strategy to boost the performance of SnO2-CeO2 binary oxide supported palladium catalysts by tuning the composition of supports. The coexistence of SnO2 and CeO2 phases in an appropriate ratio is favorable for the formation of both PdxCe1-xO2-δ and PdxSn1-xO2-δ solid solutions due to the reduced crystallite size. This unique microstructure could enhance the metal-support interaction to stabilize the active PdO phase and promote its reoxidation, meanwhile generating more oxygen vacancies to improve the reducibility of PdO. On account of the facilitated conversion of PdO ↔ Pd, coupled with the low-temperature dissociation of methane promoted by abundant active oxygen species, the Pd/5Sn5Ce catalyst exhibits a superior catalytic activity with a T99 of ca. 360 °C, a robust stability under both dry and wet conditions, and an excellent thermal stability during heating-cooling light-off tests.

Broad sintering temperature range and stable microwave dielectric properties of Mg2TiO4–CeO2 composite ceramics

There has been extensive research on microwave dielectric materials considering their application in 5G and 6G communication technologies. In this study, the sintering temperature range of Mg2TiO4–CeO2 (MT-C) ceramics was broadened using a composite of CeO2 and Mg2TiO4 ceramics, and their microwave dielectric performance was stabilized. Low-loss MT-C composite ceramics were prepared using the solid-phase reaction method, and their microwave dielectric properties, microscopic morphologies, and phase structures were investigated. The proposed MT-C ceramics contained Mg2TiO4 and CeO2 phases; their average grain size was maintained at 2–4 μm in the sintering temperature range of 1275–1425 °C, and the samples were uniformly dense without porosity. The cross-distribution of Mg2TiO4 and CeO2 grains in the samples inhibited the growth of ceramic grains, providing uniform and dense surfaces. The dielectric loss of MT-C ceramics remained constant in the temperature range of 1300–1425 °C at 9 × 10−4 (8.45 ≤ f ≤ 8.75 GHz). As opposed to the base material, MT-C ceramics are advantageous owing to their wide sintering temperature range and the stable microwave dielectric properties, and there are suitable substrate materials for further industrial applications.

Cerium-Copper Oxides Synthesized in a Multi-Inlet Vortex Reactor as Effective Nanocatalysts for CO and Ethene Oxidation Reactions

In this study, a set of CuCeOx catalysts was prepared via the coprecipitation method using a Multi-Inlet Vortex Reactor: the Cu wt.% content is 5, 10, 20, 30 and 60. Moreover, pure CeO2 and CuO were synthesized for comparison purposes. The physico-chemical properties of this set of samples were investigated by complementary techniques, e.g., XRD, N2 physisorption at −196 °C, Scanning Electron Microscopy, XPS, FT-IR, Raman spectroscopy and H2-TPR. Then, the CuCeOx catalysts were tested for the CO and ethene oxidation reactions. As a whole, all the prepared samples presented good catalytic performances towards the CO oxidation reaction (1000 ppm CO, 10 vol.% O2/N2): the most promising catalyst was the 20%CuCeOx (complete CO conversion at 125 °C), which exhibited a long-term thermal stability. Similarly, the oxidative activity of the catalysts were evaluated using a gaseous mixture containing 500 ppm C2H4, 10 vol.% O2/N2. Accordingly, for the ethene oxidation reaction, the 20%CuCeOx catalyst evidenced the best catalytic properties. The elevated catalytic activity towards CO and ethene oxidation was mainly ascribed to synergistic interactions between CeO2 and CuO phases, as well as to the high amount of surface-chemisorbed oxygen species and structural defects.

Degradation of CeO2 and TiO2 co-stabilized ZrO2 by the V2O5 molten salts and Na2SO4 + V2O5 molten salts

In this study, (Ce0.15Ti0.05)Zr0.80O2 (CTZ) with the tetragonal (t) phase was fabricated by solid-state reaction, and the destabilization resistance of CTZ ceramic was investigated in V2O5, and Na2SO4 + V2O5 molten salts, respectively. After CTZ was corroded in V2O5 molten salt at 700 °C for 2 h, ZrV2O7 and CeVO4 were formed, and the t phase was the main crystalline phase on the CTZ surface. After CTZ was corroded in V2O5 molten salt at 800 °C or 900 °C for 2 h, the amount of CeVO4 increased significantly, and CTZ surface underwent a more corrosion degradation than at 700 °C. After CTZ was corroded in Na2SO4 + V2O5 molten salts at 700 °C or 800 °C for 2 h, the hot corroded products were m-ZrO2 and CeVO4. However, after CTZ was corroded in Na2SO4 + V2O5 molten salts at 800 °C for 10 h or 900 °C for 2 h or 10 h, a new phase of CeO2 was formed due to the mineralization effect. The destabilization of CTZ ceramic under hot corrosion was mainly caused by the chemical interaction and phase transformation.

From Khoi-San indigenous knowledge to bioengineered CeO2 nanocrystals to exceptional UV-blocking green nanocosmetics

Single phase CeO2 nanocrystals were bio-synthesized using Hoodia gordonii natural extract as an effective chelating agent. The nanocrystals with an average diameter of 〈Ø〉 ~ 5–26 nm with 4+ electronic valence of Ce displayed a remarkable UV selectivity and an exceptional photostability. The diffuse reflectivity profile of such CeO2 exhibited a unique UV selectivity, in a form of a Heaviside function-like type profile in the solar spectrum. While the UV reflectivity is significantly low; within the range of 0.7%, it reaches 63% in the VIS and NIR. Their relative Reactive Oxygen Species (ROS) production was found to be < 1 within a wide range of concentration (0.5–1000 μg/ml). This exceptional photostability conjugated to a sound UV selectivity opens a potential horizon to a novel family of green nano-cosmetics by green nano-processing.

MnII/III and CeIII/IV Units Supported on an Octahedral Molecular Nanoparticle of CeO2.

The preparation of three new heterometallic clusters [Ce6Mn12O17(O2CPh)26] (1), [Ce10Mn14O24(O2CPh)32] (2), and [Ce23Mn20O48(OH)2(tbb)46(H2O)4](NO3)2 (3; tbb- = 4-tBu-benzoate) is reported. They all possess unprecedented structures with a common feature being the presence of an octahedral CeIV-oxo core: a Ce6 in 1, two edge-fused Ce6 giving a Ce10 bioctahedron in 2, or a larger Ce19 octahedron in 3. Complex 1 is the first Ce6 cluster with a central μ6-O2-. 2 and the cation of 3 are molecular nanoparticles of CeO2 (ceria) because they possess the fluorite structure of bulk ceria and are thus ultrasmall ceria nanoparticles in molecular form. The {Ce19O32} octahedral subunit of the cation of 3 had been predicted from density functional theory studies to be one of the stable fragments of the CeO2 lattice, but has never been previously synthesized in molecular chemistry. Around the Ce/O core of 1-3 is an incomplete monolayer of Mnn ions disposed as four Mn3, two Mn7, and four Mn5 units, respectively. This represents a clear structural similarity with composite (phase-separated) CeO2/MnOx mixtures where at high Ce:Mn ratios the Mn atoms segregate on the surface of CeO2 phases. Variable-temperature dc and ac magnetic susceptibility studies have revealed S = 2, S = 1/2, and S = 3/2 ground states for 1-3, respectively. Fitting of the 5.0-300 K dc data for 1 to a two-J model for an asymmetrical V-shaped Mn3 unit with no interaction between the end MnIII ions gave an excellent fit with the following values: J1 = 5.2(3) cm-1, J2 = -7.4(3) cm-1, and g = 1.96(2).

Research of Structural, Strength and Thermal Properties of ZrO2—CeO2 Ceramics Doped with Yttrium

In this work, using a mechanochemical solid-phase synthesis method, ZrO2—CeO2 ceramics doped with yttrium were obtained, which have great prospects for use as a basis for dispersed nuclear fuel materials or inert nuclear fuel matrices. The purpose of this work was to study the formation of the ZrO2—CeO2 phase composition, depending on the concentration of yttrium dopant, as well as to study their structural and strength properties. The relevance of this study is in obtaining new data on the properties of composite ceramics based on oxides having a cermet structure, as well as the effect of doping with yttrium on increasing the resistance of ceramics to deformation and thermal properties. During the studies, the dynamics of the phase transformations depending on the concentration of the dopant, as well as changes in the structural characteristics and dislocation density, were established. It was found that at a dopant concentration of 0.25 mol, the main phase in the structure was Ce3ZrO8–triclinic P1 (1), the formation of which led to an increase in the mechanical and strength properties of the ceramics as well as a 1.5-fold increase in the thermal conductivity coefficient.

Effect of Substrate Temperature on Structural and Optical Properties of Radio‐Frequency Magnetron‐Sputtered Ce0.97Co0.03O2 Thin Films

Single‐phase Ce0.97Co0.03O2 films are successfully deposited on Si(100) by radio‐frequency magnetron sputtering. The effects of substrate temperature on the structural and optical properties of Ce0.97Co0.03O2 films are investigated. X‐ray diffractometer (XRD) and Raman spectra confirm the formation of the pure CeO2 phase with highly (111) preferred orientation for all films. The crystallite size increases with increasing substrate temperature. Atomic force microscope (AFM) images of Ce0.97Co0.03O2 films exhibit dense and uniform microstructures with grain sizes of ≈18, 28, and 35 nm for substrate temperatures of 30, 500, and 700 °C, respectively. The surface roughness is increased from 1.49 to 6.72 nm with increase in substrate temperature. Emission at centers of 335, 350, 375, and 392 nm is observed in photoluminescence (PL) spectra. Emission peaks of all films with different substrate temperatures have similar shapes and different intensities. Optical characteristics are studied by spectroscopic ellipsometry (SE) in the UV−NIR region. The optical constants (n and k) are determined by analyzing the SE spectra. The results show that with increase in substrate temperature, the values of n and k increase, while the value of E g decreases. The obtained results can serve as a database for the optical properties of Ce0.97Co0.03O2 films.

Green synthesis of cerium oxide, Co-doped cerium oxide nanoparticles and its dielectric properties

In this article, we have synthesized CeO2 and Co-doped CeO2 nanoparticles (NPs) using the sol–gel method in a greener route by using Acacia Concinna fruit extract as a natural surfactant. Different doping concentrations of Ce1-xCoxO2 (where x = 0.02, 0.04, and 0.06 w/w) were prepared by adding varying concentrations of cobalt nitrate to ceric ammonium nitrate in a stoichiometric ratio. The characterizations were done by different techniques like XRD, SEM, Raman spectroscopy, and FT-IR. XRD pattern shows a single-phase cubic fluorite type structure for the CeO2 and Co-doped CeO2 NPs. The crystallite sizes calculated from the XRD pattern were 22.7 nm for CeO2 NPs and for Co-doped CeO2 NPs, it increased from 38.15 nm to 46.04 nm, when the concentration of cobalt doping was increased from 2 % to 6 %. The formation of the cubic phase of CeO2 in both pure and Co-doped CeO2 NPs was further confirmed through Raman spectroscopy, which signified the vibrational mode of cubic fluorite type structure of CeO2 NPs at 462 cm−1. SEM images illustrated a porous structure for both the doped and un-doped NPs, the porosity being more for Co-doped CeO2 NPs. FT-IR analysis showed characteristic absorption bands for Co-O-Ce and CO at 1118 cm−1 and 2924 cm−1, respectively. The effects of the composition of the dopant on the dielectric properties of Co-doped CeO2 NPs were studied. The dielectric properties of the as-synthesized NPs vary with the percentage of cobalt doping.

Tunning Ce valence and optimizing intergranular phase to prepare high abundance (La, Ce, Pr)-Fe-B permanent alloys with excellent properties

The regulation of Ce valence and the optimization of intergranular phase can effectively improve magnetic properties, which is beneficial in promoting the application of high abundance rare earth in permanent magnets. In this work, [(La0.3Ce0.7)xPr1-x]17Fe78B6 (x = 0.1–0.7) melt-spun alloys are prepared. The results illustrate that, Ce shows the mixed valency consisting of Ce3+ and Ce4+ configuration, and when x = 0.3, the alloy possesses the highest Ce3+ ratio. Meanwhile, the highest Ce4+ ratio can be determined in x = 0.1 alloy. TEM characterization demonstrates that, the intergranular rare earth oxides including CeO2 phase, Pr2O3 phase, (Pr, Ce)2O3 as well as La2O3 phase can be determined, among which CeO2 phase, existed in x = 0.3 alloy, is more beneficial in elevating the coercivity. Thus, the optimum comprehensive magnetic properties with the remanence of 6.5 kG and the coercivity of 14.9 kOe can be obtained in x = 0.3 alloy. In addition, due to more uniform microstructure of melt-spun alloy with x = 0.3, higher thermal stability can be obtained. Elemental metallurgical behavior reveals that La, Ce and Pr elements are enriched in intergranular phase, and main phase region is rich for Fe element.

In English

X-ray phase and thermogravimetric analysis, scanning electron microscopy and energy-dispersion spectroscopy were used to study the products of phase formation during the precipitation of lanthanum and cerium salts in the presence of silver nitrate and recipients of precipitators, nucleating agents and hydrolysis regulators. Thermogravimetric analysis shows the completion of the La(OH)3 lattice dehydroxylation process at a temperature of ~ 300 °С and probable destruction of sulfates at a temperature of ~ 340 °С. The phase interaction of lanthanum oxide(III) with silver ends at T ~ 400 °C. The DTG curve shows a two-stage weight loss, which characterizes the destruction of lanthanum and silver hydroxides (250 °C) and the removal of sulfates (~ 340 °C), respectively. According to the TG, the total weight loss is 21.6 %. For the cerium-containing system the only endothermic effect of dehydroxylation of cerium hydroxide at T = 250 °C with its conversion into cerium dioxide is observed. The destruction of nitrates (anionic component of solutions) takes place at the temperature of 400 °C. Weight loss takes place at T = 150 °C and is 53.9 %. Thus, on the basis of TG-DTA data, it can be assumed that the formation of composites particles based on lanthanum and cerium oxides, modified with silver, ends at the temperature of 400 °C. The X-ray diffraction data shows that at the initial stage the system undergoes the formation of cerium and lanthanum hydroxides, and during lyophilization of the precipitate (T = 160 °C) the crystal lattice of hydroxides partial dehydroxylation takes place with the formation of trigonal oxides La2O3 and Ce2O3. It has been found that the presence of silver cations in the solution can affect the phase composition of lyophilized structures and the formation of the CeO2 phase. It is shown that the hydroxylamine chloride injection into the system can initiate the silver restoration on the lanthanum oxide surface and also partially restore it to the LaO phase. Temperature treatment of the samples (T = 400 °С) promotes homogenization of the precipitate composition: formation of 30 nm cerium dioxide particles with silver clusters evenly distributed on its surface, and hexagonal lanthanum oxide plates with individual silver particles as the second phase. In three-component systems, two modifications of lanthanum oxides (trigonal and cubic), cerium dioxide and metallic silver are formed. It is found that the chemical composition of the precipitates contains the main elements – La, Ce, O, Ag and impurity – S or Cl, as the anionic component of the initial solutions, N and K in the composition of the initial suspension. It is shown that the morphology of the samples is represented by hexagonal structures of lanthanum hydroxide and oxide, spherical and pseudocubic particles of cerium dioxide and lanthanum oxide, spherical clusters of silver.

Release of Nanoparticles in the Environment and Catalytic Converters Ageing.

A Three-Way Catalyst (TWC) contains a cordierite ceramic monolith coated with a layer of Al2O3, CexZr1−xO2 and platinoids mixture. Under standard operation, the platinoid concentration decreases, exposing the remaining washcoat structure. After that particle release stage, the sintering process follows where the crystalline CexZr1−xO2 solution is broken and begins to separate into ZrO2 and CeO2 phases. ZrO2 is released to the environment as micro and nanoparticles, while a small amount of CeO2 generates a new AlxCe1−xO2 composite. The main effect of Ce capture is the growth in the size of the polycrystal structure from 86.13 ± 16.58 nm to 225.35 ± 69.51 nm. Moreover, a transformation of cordierite to mullite was identified by XRD analysis. Raman spectra showed that the oxygen vacancies (Vö) concentration decreased as CexZr1−xO2 phases separation occurred. The SEM-EDS revealed the incorporation of new spurious elements and microfractures favouring the detachment of the TWC support structure. The release of ultrafine particles is a consequence of catalytic devices overusing. The emission of refractory micro to nanocrystals to the atmosphere may represent an emerging public health issue underlining the importance of implementing strict worldwide regulations on regular TWCs replacement.

Elucidating the Role of the Metal Catalyst and Oxide Support in the Ru/CeO2-Catalyzed CO2 Methanation Mechanism.

This study addresses the yet unresolved CO2 methanation mechanism on a Ru/CeO2 catalyst by means of near-ambient-pressure X-ray photoelectron spectroscopy (NAP–XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) complemented with periodic density functional theory (DFT) calculations. NAP–XPS results show that the switch from H2 to CO2 + H2 mixture oxidizes both the Ru and CeO2 phases at low temperatures, which is explained by the CO2 adsorption modes assessed by means of DFT on each representative surface. CO2 adsorption on Ru is dissociative and moderately endergonic, leading to polybonded Ru-carbonyl groups whose hydrogenation is the rate-determining step in the overall process. Unlike on Ru metal, CO2 can be strongly adsorbed as carbonates on ceria surface oxygen sites or on the reduced ceria at oxygen vacancies as carboxylates (CO2–δ), resulting in the reoxidation of ceria. Carboxylates can then evolve as CO, which is released either via direct splitting at relatively low temperatures or through stable formate species at higher temperatures. DRIFTS confirm the great stability of formates, whose depletion relates with CO2 conversion in the reaction cell, while carbonates remain on the surface up to higher temperatures. CO generation on ceria serves as an additional reservoir of Ru-carbonyls, cooperating to the overall CO2 methanation process. Altogether, this study highlights the noninnocent role of the ceria support in the performance of Ru/CeO2 toward CO2 methanation.

Impact of CeO2 Nanoparticle Morphology: Radical Scavenging within the Polymer Electrolyte Membrane Fuel Cell

This paper reports a systematic investigation of the radical scavenging behaviour of ceria with different morphological shapes inside Nafion membrane. All the ceria nanostructure is synthesized using a template-free hydrothermal route. Distinct crystal planes of CeO2 have different numbers of broken bonds and reaction sites having different surface energies. So, the preparation of CeO2 with various uncovered planes may enhance its scavenging activity. The crystal structure, morphology, and lattice structure are investigated using transmission electron microscopy (TEM), and x-ray diffraction (XRD). The results show that the radical scavenging efficiency of ceria strongly depends on the active surface plane, and decreases in the following order: nanorods > nanocube > nanosphere. Furthermore, the concentrations of surface oxygen vacancies and lattice cerium (III) are correlated with the morphology of the nanoparticles. This relationship clarifies the vital scavenging mechanism of CeO2 that mitigates degradation inside the polymer electrolyte membrane. This is because the inherent lattice strain on the active planes of nanosized ceria with different shapes affects their surficial reactions. The existence of prominent concentration of oxygen vacancy in the nanometric dimension of ceria leading to greater Ce3+ generation by exposed active phase CeO2 nanoparticles is key to achieve a durable hybrid Nafion membrane.

Mechanochemically Prepared Co3O4-CeO2 Catalysts for Complete Benzene Oxidation

Considerable efforts to reduce the harmful emissions of volatile organic compounds (VOCs) have been directed towards the development of highly active and economically viable catalytic materials for complete hydrocarbon oxidation. The present study is focused on the complete benzene oxidation as a probe reaction for VOCs abatement over Co3O4-CeO2 mixed oxides (20, 30, and 40 wt.% of ceria) synthesized by the more sustainable, in terms of less waste, less energy and less hazard, mechanochemical mixing of cerium hydroxide and cobalt hydroxycarbonate precursors. The catalysts were characterized by BET, powder XRD, H2-TPR, UV resonance Raman spectroscopy, and XPS techniques. The mixed oxides exhibited superior catalytic activity in comparison with Co3O4, thus, confirming the promotional role of ceria. The close interaction between Co3O4 and CeO2 phases, induced by mechanochemical treatment, led to strained Co3O4 and CeO2 surface structures. The most significant surface defectiveness was attained for 70 wt.% Co3O4-30 wt.% CeO2. A trend of the highest surface amount of Co3+, Ce3+ and adsorbed oxygen species was evidenced for the sample with this optimal composition. The catalyst exhibited the best performance and 100% benzene conversion was reached at 200 °C (relatively low temperature for noble metal-free oxide catalysts). The catalytic activity at 200 °C was stable without any products of incomplete benzene oxidation. The results showed promising catalytic properties for effective VOCs elimination over low-cost Co3O4-CeO2 mixed oxides synthesized by simple and eco-friendly mechanochemical mixing.

Photocatalytic degradation of persistent brilliant green dye in water using CeO2/ZnO nanospheres

In the present work, ceria/zinc oxide (CeO2/ZnO) nanocomposite has been synthesized by controlled annealing of cerium and zinc precursors. The obtained CeO2/ZnO nanocomposite was characterized using common analytical methods such as Fourier transform infrared spectroscopy (FT-IR), UV-Visible diffuse reflectance spectroscopy (UV-DRS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive spectrum (SEM with EDS) and elemental mapping measurements. The band gap energy of CeO2/ZnO nanocomposite was about ∼2.9 eV calculated from the Tauc plot. The XRD pattern of CeO2/ZnO nanocomposite consisted mixed phases of CeO2 and ZnO with crystalline nature. The XPS spectrum of CeO2/ZnO revealed the presence of C, Zn, N, O and Ce elements, where Zn and Ce existed in 2+ and 4+ oxidation states, respectively. The particle size and shape of synthesized CeO2/ZnO nanocomposite was approximately 0.32 µm with a distorted spherical shape identified from the SEM images. The synthesized CeO2/ZnO nanocomposite effectively degraded BG in water and the calculated kinetic rate constant was 0.0471 min−1. The results suggested that CeO2/ZnO nanocomposite behave as a good photocatalyst towards the degradation of BG, identified from the quick degradation as compared with pure ZnO and CeO2. The enhanced catalytic performance of CeO2/ZnO nanocomposite was attributed to the synergistic effect of both CeO2 and ZnO semiconductors, which decrease the bandgap energy and increase the degradation kinetics.

Chemical Composition, In Vitro and In Situ Antimicrobial and Antibiofilm Activities of Syzygium aromaticum (Clove) Essential Oil.

The essential oil of Syzygium (S.) aromaticum (CEO) is known for its good biological activity. The aim of the research was to evaluate in vitro and in situ antimicrobial and antibiofilm activity of the essential oil produced in Slovakia. The main components of CEO were eugenol 82.4% and (E)-caryophyllene 14.0%. The antimicrobial activity was either weak or very strong with inhibition zones ranging from 4.67 to 15.78 mm in gram-positive and gram-negative bacteria and from 8.22 to 18.56 mm in yeasts and fungi. Among the tested bacteria and fungi, the lowest values of MIC were determined for Staphylococcus (S.) aureus and Penicillium (P.) expansum, respectively. The vapor phase of CEO inhibited the growth of the microscopic filamentous fungi of the genus Penicillium when tested in situ on bread. The strongest effect of mycelia inhibition in a bread model was observed against P. expansum at concentrations of 250 and 500 μL/mL. The best antimicrobial activity of CEO in the carrot model was found against P. chrysosenum. Differences between the mass spectra of Bacillus (B.) subtilis biofilms on the tested surfaces (wood, glass) and the control sample were noted from the seventh day of culture. There were some changes in mass spectra of Stenotrophomonas (S.) maltophilia, which were observed in both experimental groups from the fifth day of culture. These findings confirmed the impact of CEO on the protein structure of older biofilms. The findings indicate that, besides being safe and sensorially attractive, S. aromaticum has antimicrobial activity, which makes it a potential substitute for chemical food preservatives.

Dual-Site Cooperation for High Benzyl Alcohol Oxidation Activity of MnO2 in Biphasic MnOx–CeO2 Catalyst Using Aerial O2 in the Vapor Phase

Selective oxidation of benzyl alcohol to benzaldehyde in the vapor phase has drawn growing interest recently. In this work, MnOx–CeO2 mixed oxide compositions have been prepared by a coprecipitation method and tested for their oxidation activities of benzyl alcohol to benzaldehyde in the vapor phase. Detailed structural analyses have indicated that MnOx–CeO2 catalysts contain two phases, namely, α-MnO2 and fluorite CeO2 phases. The benzyl alcohol oxidation activity of pure MnO2 is more than 7 times higher compared to that of CeO2, indicating a much higher intrinsic oxidation ability of the MnO2 phase. Further, enhancement of the benzyl alcohol oxidation rate over MnO2 in 10%MnOx–CeO2 catalyst by 13 times is observed in relation to pure MnO2. The role of CeO2 in the MnOx–CeO2 catalyst has also been investigated, which indicates that the oxidation activity is almost independent of CeO2. However, stronger adsorption of benzyl alcohol over the MnOx–CeO2 catalysts compared to that of MnO2 points to the role of CeO2 in adsorption. Thus, both the CeO2 and the MnO2 components have different roles in the catalytic process—adsorption of benzyl alcohol on the CeO2 surface and its oxidation on MnO2 at the interface. The cooperation between the two sites toward oxidation could happen due to jumping of adsorbed benzyl alcohol from the surface of the CeO2 phase to the closest Mn4+ site in the MnO2 phase at the contact surface with MnO2 by thermal motion.

Methylene blue and metformin photocatalytic activity of CeO2-Nb2O5 coatings is dependent on the treatment time of plasma electrolytic oxidation on titanium

Despite UV photolysis is common industrial water treatment, existent approaches are still not fully effective nor practical due to the growing amount of discharged effluents. Titanium dioxide (TiO2) has been used as a semiconductor for photocatalytic degradation of pollutants, but its efficiency is limited due to its rapid reverse reaction. Thus, novel approaches are needed to improve the quality and cost benefits of photocatalysts. In this study, the processing time effect of CeO2-Nb2O5 coatings on titanium synthesized via a one-step method of Plasma Electrolytic Oxidation (PEO) in an electrolytic solution containing ammonium niobium oxalate salt [NH4NbO(C2O4)2] and cerium oxide (CeO2) powder at 300, 450, and 600 s. The structural and optical properties, morphology, wettability, coating thickness, and elemental chemical composition were investigated. The performance of CeO2-Nb2O5 coatings regarding the photocatalytic activity was evaluated under UV irradiation, using methylene blue (MB) and metformin as model pollutants. Energy Dispersive Spectroscopy confirmed the incorporation of Ce and Nb into the oxide film. The Scanning Electron Microscopy demonstrated that the PEO process produced CeO2-Nb2O5 coatings with a thickness from 9.6 to 55.8 µm and irregular and porous structures that changed in proportion and size according to treatment time. X-ray diffraction (XRD) analysis revealed that the coatings demonstrated crystallization corresponding to CeO2 and Nb2O5 phases. All the experimental groups presented an estimated band gap of 3.3 eV, suggesting that the obtained coatings could be active in the UV light range. Also, such coatings showed an increased surface area that varied from 3.93 to 5.62 cm2 as high it was the treatment time, which could facilitate the degradation of pollutants. These results were consistent with the photocatalytic activity assay, where samples treated for 600 s showed the most significant degradation of MB under UV irradiation, especially at 15 and 30 min of irradiation when compared to untreated titanium and photolysis process (p < 0.05). Regarding metformin, after 120 min of UV exposure, 27% was degraded due to photolysis. Whereas after 210 min, 67% of the pharmaceutical degradation was observed on the 600 s-treated CeO2-Nb2O5 coating. A crystalline CeO2-Nb2O5 coating with improved surface characteristics and photocatalytic activity was synthesized by tailoring the treatment time process. This study makes advantageous for degrading pollutants and medicines, since sewage treatment systems are sometimes slow or not fully effective.

Green synthesis and characterisation of nanocrystalline NiO-GDC powders with low activation energy for solid oxide fuel cells

This work reports the preparation of nanocrystalline Ni-Gd0.1Ce0.9O1.95 (NiO-GDC) anode powders using a novel single-step co-precipitation synthesis method (carboxylate route) based on ammonium tartrate as a low-cost green precipitant. The thermogravimetric analysis (TGA) of the synthesised powder showed the complete calcination/crystallisation of the resultant precipitates to take place at 500 °C. The prepared NiO-GDC powder was coated on a GDC electrolyte disc and co-sintered at 1300 °C. A mixture of La0.6Sr0.4Co0.2Fe0.8O3−δ and GDC was used as the cathode material and subsequently coated onto the anode-electrolyte bilayer, resulting in the fabrication of a NiO-GDC|GDC|La0.6Sr0.4Co0.2Fe0.8O3−δ-GDC cell. The crystallite size of both NiO and CeO2 phases were estimated using the X-ray powder diffraction (XRD) profiles and were calculated to be ~14 nm. Applied H2 temperature-programmed reduction (H2-TPR) analysis indicated a synergetic effect among different anode composites' constituents, where an intense interaction between the dispersed NiO nanocrystalline particles and the GDC crystallite phase had weakened the metal-oxygen bonds in the synthesised anode composites, resulting in a strikingly high catalytic activity at temperatures as low as 300 °C. The electrochemical impedance spectroscopy (EIS) and the electrochemical performance of the fabricated cells were measured over a broad range of operating temperatures (500–750 °C) and H2/Ar-ratios of the anode fuel (e.g. 100%–15%). Quantitative analysis from the EIS data and the application of the distribution of relaxation times (DRT) method allowed for the estimation of the activation energies of the anodic high and intermediate frequency processes that were 0.45 eV and 0.76 eV, respectively. This is the first report of a NiO-GDC synthesis, where a considerable improvement in activation energy is observed at the low-temperature region. Such low activation energies were later associated with the adsorption/desorption process of water molecules at the surface of NiO-GDC composite, indicating a high activity towards hydrogen oxidation.