Electrical properties of ceria Co-doped with Sm and Nd

Ceria co-doped with Sm³⁺ and Nd³⁺ powders are successfully synthesized by citric acid–nitrate low-temperature combustion process. In order to optimize the electrical properties of the series of ceria co-doped with Sm³⁺ and Nd³⁺, the effects of co-doping, doping content and sintering conditions on grain and grain boundary conductivity are investigated in detail. For the series of Ce_0.9(Sm_xNd_1−x)_0.1O_1.95 (x = 0, 0.5, 1) and Ce_1−x(Sm_0.5Nd_0.5)_xO_δ (x = 0.05, 0.10, 0.15, 0.20) sintered under the same condition, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 exhibits both higher grain and grain boundary conductivity. Compared with Ce_0.9Gd_0.1O_1.95 and Ce_0.8Sm_0.2O_1.9, Ce_0.9(Sm_0.5Nd_0.5)_0.1O_1.95 sintered at 1350–1400 °C shows higher total conductivity with the value of 1.0 × 10⁻² S cm⁻¹ at 550 °C. In addition, it can be found the trends of grain and grain boundary activation energies of Ce_1−x(Sm_0.5Nd_0.5)_xO_δ are both consistent with those of Ce_1−xNd_xO_δ, but different from those of Ce_1−xSm_xO_δ, which can be explained as: the local ordering of oxygen vacancies maybe occurs more easily in Nd-doped ceria than in Sm-doped ceria; the segregation amount of Sm³⁺ is more than that of Nd³⁺ to the grain boundaries in ceria co-doped with Sm³⁺ and Nd³⁺, which is confirmed by X-ray photoelectron spectroscopy (XPS).     

A comparative study of differently prepared rare earths-modified ceria-supported gold catalysts for preferential oxidation of CO

The preferential oxidation of CO in H₂-rich gas was studied over gold catalysts supported on ceria modified by rare earths (RE = La, Sm, Gd and Y). The ceria supports were prepared by mechanochemical activation or co-precipitation. The amount of RE₂O₃ was 10 wt%. Gold (2 wt%) was added by the deposition-precipitation method. The samples were characterized using XRD, HRTEM, HAADF, TPR, and Raman spectroscopy. It was established that catalysts prepared by co-precipitation were more active than samples made by mechanochemical activation. A gold catalyst on yttrium-modified ceria, prepared by co-precipitation, exhibited the highest catalytic activity and selectivity, and high stability. No substantial differences in the size distribution and average size of the nanogold particles in the studied catalysts were observed. The main reason for the differences in PROX activity of these gold catalysts was searched into the role of the ceria supports, depending on the preparation method, and the nature of the modifier.     

Intermediate temperature single-chamber methane fed SOFC based on Gd doped ceria electrolyte and La0.5Sr0.5CoO3−δ as cathode

Single-chamber fuel cells with electrodes supported on an electrolyte of gadolinium doped ceria Ce_1−xGd_xO_2−y with x = 0.2 (CGO) 200 μm thickness has been successfully prepared and characterized. The cells were fed directly with a mixture of methane and air. Doped ceria electrolyte supports were prepared from powders obtained by the acetyl-acetonate sol–gel related method. Inks prepared from mixtures of precursor powders of NiO and CGO with different particle sizes and compositions were prepared, analysed and used to obtain optimal porous anodes thick films. Cathodes based on La_0.5Sr_0.5CoO₃ perovskites (LSCO) were also prepared and deposited on the other side of the electrolyte by inks prepared with a mixture of powders of LSCO, CGO and AgO obtained also by sol–gel related techniques. Both electrodes were deposited by dip coating at different thicknesses (20–30 μm) using a commercial resin where the electrode powders were dispersed. Finally, electrical properties were determined in a single-chamber reactor where methane, as fuel, was mixed with synthetic air below the direct combustion limit. Stable density currents were obtained in these experimental conditions. Temperature, composition and flux rate values of the carrier gas were determinants for the optimization of the electrical properties of the fuel cells.     

Transport and deposition of CeO2 nanoparticles in water-saturated porous media

Ceria nanoparticles are used for fuel cell, metal polishing and automobile exhaust catalyst; however, little is known about the impact of their release to the environment. The stability, transport and deposition of engineered CeO₂ nanoparticles through water-saturated column packed with sand were studied by monitoring effluent CeO₂ concentration. The influence of solution chemistry such as ionic strength (1-10 mM) and pH (3-9) on the mobility and deposition of CeO₂ nanoparticles was investigated by using a three-phase (deposition-rinse-reentrainment) procedure in packed bed columns. The results show that water chemistry governs the transport and deposition of CeO₂ nanoparticles. Transport is significantly hindered at acidic conditions (pH 3) and high ionic strengths (10 mM and above), and the deposited CeO₂ particles may not be re-entrained by increasing the pH or lowering the ionic strength of water. At neutral and alkaline conditions (pH6 and 9), and lower ionic strengths (below 10 mM), partial breakthrough of CeO₂ nanoparticles was observed and particles can be partially detached and re-entrained from porous media by changing the solution chemistry. A mathematical model was developed based on advection-dispersion-adsorption equations and it successfully predicts the transport, deposition and re-entrainment of CeO₂ nanoparticles through a packed bed. There is strong agreement between the deposition rate coefficients calculated from experimental data and predicted by the model. The successful prediction for attachment and detachment of nanoparticles during the deposition and re-entrainment phases is unique addition in this study. This work can be applied to access the risk of CeO₂ nanoparticles transport in contaminated ground water.     

Harnessing the Beneficial Attributes of Ceria and Titania in a Mixed-Oxide Support for Nickel-Catalyzed Photothermal CO2 Methanation

Solar-powered carbon dioxide (CO₂)-to-fuel conversion presents itself as an ideal solution for both CO₂ mitigation and the rapidly growing world energy demand. In this work, the heating effect of light irradiation onto a bed of supported nickel (Ni) catalyst was utilized to facilitate CO₂ conversion. Ceria (CeO₂)-titania (TiO₂) oxide supports of different compositions were employed and their effects on photothermal CO₂ conversion examined. Two factors are shown to be crucial for instigating photothermal CO₂ methanation activity: ① Fine nickel deposits are required for both higher active catalyst area and greater light absorption capacity for the initial heating of the catalyst bed; and ② the presence of defect sites on the support are necessary to promote adsorption of CO₂ for its subsequent activation. Titania inclusion within the support plays a crucial role in maintaining the oxygen vacancy defect sites on the (titanium-doped) cerium oxide. The combination of elevated light absorption and stabilized reduced states for CO₂ adsorption subsequently invokes effective photothermal CO₂ methanation when the ceria and titania are blended in the ideal ratio(s).     

Catalytic performance of ceria fibers with phosphatase-like activity and their application as protein carriers

Ceria (CeO₂) synthesized by cerium nitrate hexahydrate in alkaline solution under hydrothermal treatment produces a fiber structure that allows high O-P bond cleavage activity. Brunauer–Emmett–Teller (BET) analysis revealed that fiber-morphological CeO₂ with high surface area (73.9 m²/g) and pore volume (0.42 cm³/g) showed better hydrolytic activity than nanopolyhedral and cubic morphologies. The CeO₂ fiber displayed hydrolytic activity in a tris(hydroxymethyl)aminomethane (Tris) buffer; however, no reaction occurred in a phosphate buffer. From analysis by based on scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX), the phosphate group in the buffer was seen to be immediately adsorbed on the surface of CeO₂ particles; therefore, the CeO₂ catalysts could not attack the phosphoric esters as a substrate. In addition, the CeO₂ fiber showed hydrolytic activity for deoxyribonucleic acid (DNA). Moreover, enzymes loaded on the CeO₂ fiber particles retained activity levels equivalent to free-solution enzymes. It is thought that the findings of the present study regarding the properties of CeO₂ fiber will have a significant impact in the fields of not only antibacterial and antimicrobial reagents, but also biosensors and biocatalysts.     

A lamellar ceria structure with encapsulated platinum nanoparticles

A novel lamellar feather-like CeO₂ structure has been fabricated by using a triblock copolymer as the structure-directing agent. This material was characterized in detail by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and BET surface area measurements. Compared with conventional spherical shaped ceria prepared by ammonia gelation, the ceria feathers have superior ability to support nanosized platinum particles due to their special structure. The “skeletons” of ceria feathers can serve as an ideal host matrix to anchor the platinum particles. Furthermore, the inter-crossing pattern of the “skeletons” also acts as a partition to separate platinum particles, allowing the Pt nanoparticles (average diameter ∼6 nm) to be highly dispersed in the structure. The Pt/feather-like CeO₂ catalyst exhibits high activity in the water gas shift reaction.      

Heat transfer and solar absorption analysis of multiscale CeO2 reduction for rapid H2 production prediction

The characters of transient heat transfer and solar absorption are analyzed based on the multiscale CeO₂ reduction to predict the H₂ production, and the results show that the heat absorption of CeO₂ is affected by the diameter and the value of concentration ratio. The proportion of energy absorbed by CeO₂ increases from 0.55% to 42.73% as the diameter of CeO₂ increases from 0.1 to 100 μm. However, the time required for 0.1 and 100 μm CeO₂ to reach 1775 K is 0.07 and 0.83 s, respectively. The peak of the solar-to-fuel energy conversion efficiency (η_solar-fuel) for 1 μm CeO₂ is 16.43%, which is the maximum for all selected diameters. After the analysis of the radiation intensity of blackbody and the absorptance of 1 μm CeO₂ as a function of wavelength, we find that comparing with no cutoff wavelength coating, the peak of η_solar-fuel increases from 16.43% to 18.91% as the cutoff wavelength is 600 nm. To reach the same temperature of 1773 K, it takes 0.15 and 0.05 s as C is 50 and 200, and η_solar-fuel is improved from 14.43% to 17.93%.     

Zr promotion effect in CO2 methanation over ceria supported nickel catalysts

Carbon dioxide methanation is an interesting way to reduce greenhouse effect gases emission and, simultaneously, provide a renewable energy source of methane. Ceria and 15 at.% Zr-doped ceria supported nickel catalysts were characterized by means of various techniques (BET, XRD, Raman, H₂-TPR, CO₂-TPD, O₂-TPO, OSC and H₂-chemisorption) and evaluated in carbon dioxide methanation. Zr incorporation into catalyst formulation reduced catalyst's basicity but favored its reducibility, nickel availability and oxygen storage capacity. These characteristics gave rise to an improved catalytic performance both in terms of activity and stability: temperature required to achieve 50% conversion was reduced in 20 °C and low temperature (250 °C) stability was improved in around 8%. Initial rates approach was employed to determine reaction rates and apparent activation energies for CO₂ methanation, which resulted in 113 and 121 kJ mol⁻¹for Ni/CeO₂ and Ni/Ce_0.85Zr_0.15O₂, respectively.     

Improvement of splitting performance of Ce0.75Zr0.25O2 material: Tuning bulk and surface properties by hydrothermal synthesis

Thermochemical cycles received renewed interest as CO₂ and H₂O energy-upgrading processes using solar energy as source. The two-step cycles, based on self-reduction in a solar reactor at high temperature (above 1300–1400 °C) and re-oxidation by CO₂ and/or H₂O flow, are the most interesting due to their simplicity and high theoretical solar-to-fuel efficiency. In the two-step cycle, ceria has been recognized as the benchmark material but it suffers from high reduction temperature, low re-oxidation kinetics as well as low stability, thus hindering practical application. In this work, the redox properties of two Ce_0.75Zr_0.25O₂ materials prepared by hydrothermal synthesis were compared with those of a co-precipitated sample with the same nominal composition used as reference. Samples were characterized by X-Ray Diffraction (XRD), N₂ physisorption, Scanning Elecron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and Electron Paramagnetic Resonance (EPR); their self-reducibility and CO₂ splitting activity were tested in a Thermogravimetric (TG) balance, while H₂O splitting properties were studied in an ad-hoc fixed bed reactor on H₂ pre-reduced samples. Characterization results and activity tests agreed that the Ce³⁺ fraction both on the surface and in the bulk of ceria-zirconia can be increased by hydrothermal synthesis, thus providing improved redox properties and higher splitting activity with respect to the co-precipitated sample. So, hydrothermal synthesis, providing a controlled nucleation and growth of crystallites, appears as a promising route for the preparation of ceria-based materials with tuned oxygen vacancies.