Single CeO2 Research Articles

The effect of La3+, Ti4+ and Zr4+ dopants on the mechanism of WGS on ceria-doped supported Pt catalysts

Platinum nanoparticles supported on single CeO2 and TiO2 metal oxides and Ce0.8Ti0.2O2−δ solid solution were prepared to investigate the effect of Ti4+-doping of ceria on important mechanistic and kinetic aspects of the Water–Gas Shift (WGS) reaction in the 200–300°C range, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS, and (ii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. The relationship between the chemical nature of dopant (Zr4+, Ti4+ and La3+) and the concentration of active C-pool and H-pool of reaction intermediates as well as that of specific rate per gram basis (rCO, μmolg−1s−1) for the ceria-doped supported Pt is illustrated for the first time based on relevant results previously reported (Zr4+ and La3+-doped ceria). The 0.5wt% Pt supported on Ce0.8Ti0.2O2−δ (Ti4+-doped CeO2) exhibits significantly higher WGS activity in terms of CO conversion (%) and specific kinetic rate (μmolCOg−1s−1 or μmolCOcm−1s−1) compared to Pt/CeO2, Pt/TiO2, Pt/Ce0.8La0.2O2−δ and Pt/Ce0.5Zr0.5O2−δ catalysts. This was explained mainly by: (i) the larger concentration of active C-pool of reaction intermediates formed around each Pt nanoparticle, and (ii) the higher reactivity of sites (k, s−1) along the Pt-support interface responsible for CO2 and H2 formation. A very good correlation between the concentration of active C-pool and the specific reaction rate, rCO (μmolg−1s−1) as a function of the dopant (Zr4+, La3+ and Ti4+) was found. The concentration of labile surface oxygen and its mobility in Ce0.8Ti0.2O2−δ compared to CeO2 (undoped), La3+ or Zr4+-doped ceria are also important factors. It is proposed that on Pt/CeO2-doped catalysts the WGS reaction follows both the “redox” and “associative formate” mechanisms, where the extent of participation of each mechanism depends on the chemical nature of the dopant (Zr4+, La3+ and Ti4+).

Anomalous Lattice Shrink of a Single CeO2 Submicrometer Particle in an Optical Trap

Anomalous lattice shrink is observed in a single submicrometer-sized CeO2 particle with X-ray diffraction using an optical-trap sample holder. A CeO2 particle 380 nm in size is successfully levitated with radiation pressure force exerted by counterpropagating laser beams and irradiated by a high-flux focused synchrotron radiation beam. The diffraction pattern of a single particle is consistent with that of CeO2 but the lattice shrink approaches 1.22% of the lattice constant of a particle assemblage.

On the use of bastnasite ore as a phosphor material

Bastnasite ore obtained from Indian Rare Earth (IRE) was investigated for its possible use as a phosphor material. The material was characterized by X-ray diffraction (XRD), energy dispersive X-ray fluorescence (EDXRF), photoacoustic spectroscopy (PAS), photoluminescence (PL) and electron paramagnetic resonance spectroscopy (EPR) techniques. XRD studies revealed the semi processed ore to be consisting of single phase CeO2 with no other impurities. EDXRF studies revealed the presence of ‘Th’ and traces of ‘Sm’ along with ‘Ce’ in the sample. PAS studies revealed the presence of strong charge transfer from oxygen to cerium in the system. PL studies confirmed the presence of at least four trivalent rare earths, viz. Sm, Eu, Dy and Tb in the system in trace quantities. The emission spectrum and decay time data were evaluated. It was observed that the rare earth ions are situated at distorted sites in the system surrounded by defect centers. EPR studies confirmed the presence of Ce3+in the system along with electron trapped in oxygen ion vacancies. CIE indices for the ore sample were evaluated and it was seen that the overall emission from the system was in the ‘magenta’ region of the visible spectrum. The emission intensities were also compared with that of commercial samples.

The effect of La3+-doping of CeO2 support on the water-gas shift reaction mechanism and kinetics over Pt/Ce1−xLaxO2−δ

Platinum nanoparticles (dPt=1.0–1.2nm) supported on single CeO2 and La2O3 metal oxides and Ce0.8La0.2O2−δ solid solution were prepared to investigate for the first time the effect of La3+-doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300°C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D2O) and C-path (use of 13CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce1−xLaxO2−δ (x=0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol COg−1s−1) increased by a factor of 2.0 and 2.8 at 300°C on 0.5wt% Pt supported on Ce0.8La0.2O2−δ compared to CeO2 and La2O3, respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites (k, s−1) responsible for CO2 and H2 formation on Pt/Ce0.8La0.2O2−δ compared to Pt/CeO2 and Pt/La2O3. Active OH groups is suggested to be formed on defect sites (Ce3+□s) of Ce0.8La0.2O2−δ as a consequence of the introduction of La3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

One-step synthesis of ternary nanostructured CuO/Ti0.8Ce0.2O2 catalysts for low temperature CO oxidation

Ternary nanostructured CuO/Ti0.8Ce0.2O2 catalysts were prepared by a one-step surfactant-assisted method of nanoparticle assembly. The textural and structural properties of the CuO/Ti0.8Ce0.2O2 catalysts were characterized by XRD, TGA, BET, XPS and H2-TPR. Their catalytic performance for low-temperature CO oxidation was studied by using a catlab system. CuO supported on binary Ti0.8Ce0.2O2 support showed higher catalytic activity than CuO supported on single CeO2 or TiO2 support. The calcination temperature had a remarkable influence on the catalytic activity of the CuO/Ti0.8Ce0.2O2 catalysts. The CuO/Ti0.8Ce0.2O2 catalyst calcined at 500 °C exhibited the highest catalytic activity with T50% and T100% at 82 and 123 °C, respectively. According to the XRD, BET and H2-TPR analyses, the higher surface areas and more highly dispersed small particle size CuO should be responsible for the high catalytic activity of catalysts.

Development of Solution Buffer Layers for RABiTS Based YBCO Coated Conductors

The main objective of this research is to find a suitable alternate solution based seed layer for the standard RABiTS three-layer architecture of physical vapor deposited CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cap/YSZ barrier/Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> seed on Ni-5%W metal tape. In the present work, we have identified CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffer layer as a potential replacement for Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> seeds. Using a metal-organic deposition (MOD) process, we have grown smooth, crack-free, epitaxial thin films of CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (pure and Zr, Cu and Gd-doped) directly on biaxially textured Ni-5W substrates in short lengths. Detailed XRD studies indicate that a single epitaxial CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase with slightly improved out-of-plane texture compared to the texture of the underlying Ni-W substrates can be achieved in pure, undoped CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples. We have also demonstrated the growth of YSZ barrier layers on pure CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> seeds using sputtering. Both sputtered CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cap layers and MOD-YBCO films were grown epitaxially on these YSZ-buffered MOD-CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ni-5W substrates. High critical currents per unit width, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 264 A/cm (critical current density, J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of 3.3 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) at 77 K and 0.01 T was achieved for 0.8 μm thick MOD-YBCO films grown on MOD-CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> seeds. These results indicate that CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> films can be grown directly on Ni-5W substrates and still support high performance YBCO coated conductors. This work holds promise for a route for producing low-cost buffer architecture for RABiTS based YBCO coated conductors.

Crystalline evolution of various CeO2 morphology via a solvothermal method

Preparation of cerium compounds with different structures and morphology is still a challenge in powder synthesis. CeOHCO3 and CeO2 have received widespread attention because of their potential applications. In this study, a solvothermal method with one-step precipitation was used to synthesize CeO2 with different structures and morphology without use of surfactant. The effects of different experimental conditions on the morphology of CeO2 were investigated. Both X-ray diffraction and SAED patterns revealed that products changed from a single CeOHCO3 phase to a mixture of CeOHCO3 and CeO2. A single CeO2 phase could always be obtained from CeOHCO3 by calcination in air at 673 K for 1 h. Moreover, the morphology of CeOHCO3 was retained. Thus CeO2 particles with different morphology, for example rod-like, prism-like, polygon-like, sphere-like, and cube-like, could be synthesized. Possible mechanisms of phase and morphology evolution are discussed.

Solar thermochemical process for hydrogen production via two-step water splitting cycle based on Ce1−xPrxO2−δ redox reaction

Ce1−xPrxO2−δ (x=0, 0.05, 0.1, 0.2 and 0.3), as reactive ceramics synthesized by combustion method for H2 production via two-step water splitting reaction with concentrated solar energy, was investigated in present work. XRD analysis of the samples shows that Pr6O11 underwent a phase transition into Pr2O3and Pr2O3 cannot be oxidized by steam to yield H2 in two-step water splitting reaction. The solid solutions of the samples containing Ce were formed with a single cubic CeO2 fluorite structure after synthesis and even after two-step water splitting reaction. Although Pr cannot be oxidized by steam to generate H2 in two-step water splitting reaction, the promotion effect is present with the introduction of 5mol% and 10mol% Pr into CeO2 on the amounts of O2 and H2, but the volume of H2 evolved is enhanced or not, strongly dependent on the H2-generation temperature. The optimum H2-generation temperature is 1023K.

Pt nanoparticles supported over Ce–Ti–O: the solvothermal and photochemical approaches for the preparation of catalytic materials

Ce–Ti–O supports with different Ce/Ti molar ratios were synthesized by the solvothermal method using hexadecyltrimethylammonium bromide. Pt nanoparticles were then supported by photochemical deposition. The shape, size, and structure of these materials were analyzed by high-resolution transmission electron microscopy. The single CeO2 support was also prepared, consisting of agglomerated cubic particles ranging from ~3 to 8 nm. When titania was combined with ceria, a nanostructured architecture was produced, evidencing the strong influence of Ti in the support structure. Photodeposition of Pt nanoparticles is more efficient on Ce–Ti–O supports than in pristine CeO2. Crystalline Pt nanoparticles (mainly of ~2 to 4 nm) were detected. The catalytic properties of the materials were tested in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol. It was observed that Pt supported on Ce–Ti–O is more active and selective than Pt on CeO2 or TiO2 separately. The catalyst with 40 mol% Ce leads to total conversion of cinnamaldehyde in a few minutes; however, higher selectivity toward the desired product (cinnamyl alcohol) was obtained with higher amounts of Ce (50 mol%).

Effect of the Preparation Conditions of Ru/CeO2 Catalysts for the Liquid Phase Oxidation of Benzyl Alcohol

Ceria colloidal particles with a mean crystallite size of 2 nm were synthesized by a solvothermal reaction. The Ru/CeO2 catalyst prepared from the CeO2 colloids exhibited higher activity than the catalyst prepared from Ce(NO3)3. Temperature-programmed reduction analysis indicated that the reduction of surface Ce4+ was accelerated by highly dispersed Ru species on the CeO2 particles and occurred at low temperatures. The single component CeO2 sample prepared by the coagulation of the CeO2 colloid was more easily reduced and re-oxidized than the CeO2 sample prepared by the precipitation method from Ce(NO3)3. The higher activity of Ru/CeO2 prepared from the CeO2 colloids came from the inherent nature of the CeO2 support itself.

Single CeO2 Nanowire Gas Sensor Supported with Pt Nanocrystals: Gas Sensitivity, Surface Bond States, and Chemical Mechanism

We present a new type of CeO2 nanodevice in which a single CeO2 nanowire was used as the sensing unit. It was found that incorporation of Pt nanocrystals on CeO2 nanowire could significantly increase the sensor response. A possible mechanism was discussed based on the study of morphology and surface bond states of the nanowire using local electron energy loss spectroscopy and X-ray photoemission spectroscopy. The results provide a pathway to improve the performance of gas sensors with a good understanding of the nanowire’s surface physics and chemistry.

Characteristics of sol–gel dip coated Ceria films

Cerium oxide(CeO2) thin films were deposited by the sol–gel dip coating technique using cerium chloride, acrylamide and N,N bis methylene acrylamide. The as deposited films were heat-treated at different temperatures in air. X-ray diffraction studies indicated the films to be of single phase CeO2. Optical bandgap in the range of 3.53–3.60 eV was obtained from optical studies. Laser Raman studies exhibited Raman bands around 457 cm−1.

Co-evaporated YBCO/doped-CeO2/Ni–W coated conductors oxygen improved using a supersonic nozzle

A novel process for the coated conductors (CC) deposition, characterized by a single CeO2 buffer layer architecture, and a new oxygenation device for the YBCO layer has been developed.In CC technology, usually the ceria layer thickness must be less than 100nm to avoid the formation of cracks; in order to ensure an efficient barrier effect, complex and costly multi-buffer layers architectures must be grown. In this work, we describe the way to increase the thickness of crack-free single buffer layer.Single CeO2 buffer layer is grown on biaxially textured Ni–5at.%W substrates (RABiTS) by e-beam evaporation technique, introducing different doping elements at various percentages in pure Ceria targets. Since the crack formation in cerium oxide layers mainly depends on the stresses introduced by the lattice mismatch with Ni alloy substrate and on the arrangement of the oxygen vacancies in the lattice, it is possible to reduce these stresses by optimizing the doping in the Ceria layer, thus obtaining very thick and crack-free CeO2 buffer layers.The texture quality of our samples is tested by XRD, giving a strong in-plane and out-of-plane orientation (FWHM values of about 5°). Optical and electron microscopy shows dense and crack-free thick layers (up to 450nm) and confirms excellent insulating properties.A novel technique has been developed to enhance the oxygen pressure on the YBCO film during the deposition as the oxygen content is known to be critical for the growth of YBCO films with high Tc. A supersonic oxygen nozzle (SNEO) incorporated into the vacuum chamber enables a ×103 increasing of the local oxygen pressure close to the YBCO layer keeping a 10−4mbar background pressure.The work demonstrates that the single thick doped-Ceria buffer layer could be a viable way to reduce CC complexity and production costs and that SNEO apparatus can be used to make the oxygenation process of the YBCO films easier and more reliable.

Preparation and Characterization of Ce0.3Ti0.7O2and Supported CuO Catalysts for NO+CO Reaction

The Ce x Ti1−x O2 mixed oxides at different mole ratios (x=0.1–1.0) were prepared by co-precipitation of TiCl4 and Ce(NO3)3. The structural and reductive properties of the Ce x Ti1− x O2 were affected by calcination temperature. At x=0.1–0.3, CeTi2O6 phase was formed and mainly as amorphous after calcination at 650°C. At x=0.3, only CeTi2O6 was formed after calcination at 750°C and CeTi2O6 crystallized completely after calcination at 800°C. TPR analyses showed that the amount of H2 consumption by Ce x Ti1−xO2 (650°C) (except x=0.1) was greater than that by single CeO2, and the valence of CeO2was the lowest (+3.18) at x=0.3. CuO/Ce0.3Ti0.7O2 was prepared by the impregnation method and catalytic properties were examined by means of a GC micro-reactor NO+CO reaction system, BET, TPR, XRD, XPS and NO-TPD. It was found that CuO/Ce0.3Ti0.7O2 calcined at 650°C had the highest activity in NO+CO reaction with 100% NO conversion at reaction temperature of 300°C, and at 650°C Ce0.3Ti0.7O2just began to crystallize. The catalytic activities were largely affected by the pre-treatment conditions. At low reduction temperature (100°C), CuO species was difficult to reduce. When high degree of reductions took place, both CuO species and Ce0.3Ti0.7O2 reduced and thus a part of CuO species on the support surface would be covered. The XPS and NO-TPD analyses showed that CuO/Ce0.3Ti0.7O2 had four NO absorption centers (Cu+, Cu2+(I), Cu2+(II) and Ce3+). The CuO species involving in NO+CO reaction included Cu2+(I) and Cu+, and CeO2 species (Ce3+ and Ce4+).

Study of the double layer CeO2/Nb2O5 thin film

A double layer CeO2/Nb2O5 thin film by magnetron sputtering was prepared. The property and microstructure of the thin film were carefully characterized by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy, and high resolution transmission electron microscopy. XPS measurement clearly demonstrated that the Nb2O5 layer underlying CeO2 as a electrical-conductive layer could greatly improve the reduction ability of CeO2 thin film compared with that of a single layer CeO2 thin film. Fitting to the XPS experimental data and deconvoluting the spectra showed the increase of Ce3+ concentration in the double layer CeO2/Nb2O5 thin film. The microstructure of the double layer thin film was composed of three distinguishable layers, which are a polycrystalline CeO2 layer, an amorphous Nb2O5 layer, and an intermediate amorphous layer with regard to the silicon substrate. The interface between CeO2 and Nb2O5 was rough and indistinct. A detailed analysis was given from the viewpoint of interdiffusion at the CeO2/Nb2O5 interface.

Hydrothermal Synthesis and Sintering of Fine Powders in CeO&lt;sub&gt;2&lt;/sub&gt;-ZrO&lt;sub&gt;2&lt;/sub&gt; System

Fine powders in CeO2-ZrO2 system have been prepared from the precipitate mixtures of cerium (III) nitrate solution, zirconium oxychloride solution and an excess amount of ammonia under hydrothermal conditions at 180°C, and their changes in crystal phase and surface area by calcination, sinterability, the crystal phase and the microstructure of the sintered bodies have been investigated. Twelve mol% CeO2-doped fine zirconia powders hydrothermally synthesized showed metastable cubic zirconia phase without monoclinic phase and they gradually transformed into tetragonal phase by calcining at above 600°C. The samples except for 12mol% CeO2-ZrO2 consisted of two phase mixtures of zirconia and ceria. The crystallite size of 12mol% CeO2-doped zirconia particles and CeO2 particles in the 70mol% CeO2-ZrO2 powder was about 6nm and 20nm, respectively. Dense CeO2-ZrO2 ceramics were obtained by sintering these powders at above 1400°C for 1h. The sinterability of the powders increased with increasing CeO2 concentration. The calcined 80mol% CeO2-ZrO2 powders pressed at 98MPa reached a maximum density with single cubic CeO2 solid solution at 1300°C for 1h. The average grain size of 12mol% CeO2-doped tetragonal zirconia ceramics sintered at 1500°C was 1.5μm. Although the grain size of the sintered body in the two-phase region (tetragonal zirconia and cubic CeO2 solid solution) was below 1.0μm, it increased with increasing CeO2 concentration within the single phase region of cubic CeO2 solid solution.