Reduction Of Ceria Research Articles

Role of Cluster Morphology in the Dynamics and Reactivity of Subnanometer Pt Clusters Supported on Ceria Surfaces

We study subnanometer (sub-nm) Pt clusters supported by highly reducible oxide surfaces and establish the role of cluster morphology in the thermodynamics and kinetics of surface processes relevant for reactivity, namely cluster mobility, reverse oxygen spillover, and oxygen vacancy formation. The relationships between cluster morphology and reactivity are rarely considered in computational studies because of the large domain and complexity of the potential energy surface, particularly in the presence of strong metal–support interaction. Global optimization algorithms together with Hubbard-corrected density functional theory calculations (DFT+U) are used to identify the stable and metastable morphologies of Pt3–Pt6 clusters supported on pristine and defective CeO2(111) surfaces. Our systematic exploration for these sub-nm Pt particles shows that the charge of the supported cluster, its bonding to the substrate, and the degree of ceria reduction depend on the metal/oxide interface area and on the cluster morphology. Concerning reaction thermodynamics and kinetics, the use of global optimization methods leads to very different results as compared to usual minimization procedures. By allowing for morphology changes during reaction, the energetics of reverse O spillover changes from highly endothermic to exothermic and leads to new minimum-energy reaction and diffusion mechanisms. The diffusion kinetics predicts clusters as small as Pt6 to be resistant to sintering on ceria surfaces. The relevance of these findings for larger metal clusters and for supporting oxide nanoparticles is discussed as well as their connection with the recent literature.

Electrochemically enhanced metal-support interaction of highly dispersed Ru nanoparticles with a CeO2 support

Small particle size (1.1nm) Ru nanoparticles were supported on a mixed ionic-electronic conductor, CeO2, with a low metal loading (1wt%), interfaced with a YSZ electrolyte. A pronounced enhancement (up to about 2.5 times) of the catalytic rate for the complete oxidation of ethylene was observed for negative polarization. The opposite effect was observed for positive polarization. Apparent Faradaic efficiencies up to 96 were determined, indicating a non-Faradaic effect. The modification of the cerium oxidation state (i.e., reduction from Ce4+ to Ce3+) is proposed to enhance the catalytic performance of the Ru nanoparticles. XPS analysis was performed to confirm this reduction of ceria. The enhancement of catalytic activity is attributed to the presence of more oxygen vacancies in the ceria interlayer causing a stronger metal-support interaction. Results demonstrate the feasibility of in-situ modification of the metal support-interaction between Ru nanoparticles and CeO2 catalytic support by a small current (−2μA) application.

Cerium Reduction at the Interface between Ceria and Yttria-stabilised Zirconia and Implications for Interfacial Oxygen Non-stoichiometry

Cerium Reduction at the Interface between Ceria and Yttria-stabilised Zirconia and Implications for Interfacial Oxygen Non-stoichiometry

Nanostructured manganese doped ceria solid solutions for CO oxidation at lower temperatures

Noble metal-free nanostructured Ce0.7Mn0.3O2−δ (CM) solid solutions were prepared by a simple, facile, and high yielding coprecipitation method from aqueous solutions of cerium and manganese nitrates. The influence of calcination temperature from 773 to 1073K on the structural features and catalytic behaviour of the CM solid solutions has been thoroughly investigated. The synthesized materials were characterized using XRD, ICP-OES, BET surface area, TEM and HRTEM, UV–vis DRS, Raman, XPS, EPR, and H2-TPR techniques. The XRD results indicated that the samples calcined at various temperatures from 773 to 973K did not show any diffraction peaks corresponding to manganese oxides and thereby ensured the formation of Ce0.7Mn0.3O2−δ solid solution. However, the diffraction peak corresponding to Mn2O3 was clearly observed at 1073K due to the occurrence of phase segregation between Mn–oxide and ceria. The TEM and HRTEM observations showed the nanocrystalline nature of the solid solutions with a particle size in the range of ∼7–20nm. The doping of Mn into the ceria lattice increased the concentration of structural oxygen vacancies, which are beneficial for oxidation reaction as revealed by UV–vis DRS and Raman results. XPS analysis confirmed that Ce and Mn exist in +4 or +3 and +3 or +2 oxidation states, respectively on the surface of the samples. The EPR studies showed the presence of isolated Mn2+ and Mn3+ species in different structural locations such as ceria-lattice defect sites, framework of Ce4+ locations, interstitial locations, and at the surface of the ceria. H2-TPR measurements showed a pronounced enhancement in the reduction of ceria by manganese doping. Among the various catalysts examined, the CM catalyst calcined at 773K exhibited superior CO oxidation activity at much lower temperature with T50=390K and 100% CO conversion at ∼470K in comparison to pure ceria and MnOx. The observed remarkable enhancement in the reactivity for CO oxidation at lower temperatures has been attributed to a highly dispersed state of Mn2+/Mn3+ in the ceria matrix, facile redox behaviour, and a synergistic Mn–Ce interaction.

Study of compaction and sintering of nanosized oxide powders by in situ electrical measurements and dilatometry: Nano CeO2—case study

Densification and sintering of CeO2 nanoparticles and their electrical properties were simultaneously studied as a function of temperature in controlled atmosphere using a modified dilatometer. CeO2 nanoparticles simultaneously shrink and become more resistive upon initial heating, associated with desorption of water. The electrical conductance G at 300–550 °C revealed a pO2 dependence described by log (G) = A + n × log (pO2) with n ~ −1/6, consistent with n-type conduction. The results were analyzed with a defect equilibrium model based on the reduction of ceria and formation of doubly ionized oxygen vacancies and electrons. The activation energy was found equal to (1.3 ± 0.1) eV, which results in an enthalpy of reduction of (2.7 ± 0.4) eV, considerably lower than that for bulk ceria (~4.5 eV). The coarsening of particles during heat treatment at 800 °C were analysed assuming grain boundary diffusion-limited sintering. Although the coarsened powder shows a similar pO2 dependence, the activation energy was considerably higher (1.9 ± 0.1) eV, leading to a reduction enthalpy of (4.5 ± 0.4) eV. The decrease in the enthalpy of reduction with decreasing particle size is consistent with the increasing fraction of oxide ions residing at the surface. Alternate interpretations based on space charge effects and surface adsorption/desorption were considered and found to be less consistent with the experimental results.

Thermodynamic Analysis of Syngas Production via the Solar Thermochemical Cerium Oxide Redox Cycle with Methane-Driven Reduction

The present work considers the convergence of two approaches for syngas production: solar fuels via the cerium oxide (ceria) redox cycle and the partial oxidation of methane. The chemical thermodynamics of the ceria–methane system reveal that coupling the reduction of ceria to the partial oxidation of methane enables isothermal cycling at temperatures as low as 1223 K with the additional production of high-quality syngas during the reduction step. The equilibrium non-stoichiometry of the oxidation step has a substantial impact on the conversion of the oxidizer to fuel, with important implications for cycle efficiency. A model of the process thermodynamics is used to evaluate the efficiency of the cycle and its sensitivity to oxidation non-stoichiometry, temperature, and concentration ratio. Reduction with methane enables significant gains in efficiency over other proposed approaches, with plausible solar-to-fuel efficiencies reaching 40% without any heat recovery.

Geochemistry of the Apulian karst bauxites (southern Italy): Chemical fractionation and parental affinities

This study focuses on a late Cretaceous karst bauxite deposit in the Murge area of the Apulia district, southern Italy. The first analysis of the vertical distributions of a wide range of elements (including REEs and selected trace elements) throughout the deposit was shown in order to identify the processes determining element fractionation during the evolution of the bauxite.The studied karst bauxite deposit exhibits an ooidal texture, is mineralogically homogeneous, and contains higher abundances of boehmite than of hematite, kaolinite, and anatase. The major element composition of the bauxite is dominated by elevated concentrations of SiO2, TiO2, Al2O3, and Fe2O3, and analyses of element mobility within the bauxite indicate that all elements except for Nb and Ni, and to a lesser extent Cr, are depleted relative to the immobile element Ti. R-mode factor analysis revealed a number of key findings: (i) some low-solubility elements (e.g., Zr, Th, Ti, V, Ga) were concentrated in detrital zircon and monazite (Zr, Th), in anatase (Ti, V), and possibly in boehmite and hematite (Ga) during the later stages of bauxitisation; (ii) Fe and Cr were concentrated during wet conditions, whereas Al and Co accumulated during dry conditions; (iii) distributions of the light REEs (LREEs) and the heavy REEs (HREEs) are controlled by the same processes, suggesting that little LREE/HREE fractionation occurred during bauxite formation; and (iv) the behaviour of cerium is different from that of the other REEs, and highly variable cerium anomalies are observed across the deposit, with three characteristic Ce/Ce* maxima with values of >2. Parisite was the only authigenic cerium mineral detected during this study. Thus, we propose a three-step model to explain the distribution of Ce: cerianite is dissolved by cerium reduction, is transported by downward-moving meteoric water (per descensum), and finally parisite is precipitated. This cycle was repeated several times in the Apulian karst bauxite in response to Eh decreases under alkaline conditions, promoted by fluctuations in the groundwater level. Finally, we used the value of the Eu anomaly to discuss the parental affinity or protolith of the bauxite. The value of the Eu anomaly (min. Eu/Eu*=0.76, max. Eu/Eu*=0.89) indicates that the bauxite was not derived from carbonates, but rather, that the majority of the bauxite was influenced by intermediate to mafic magmatic sources. Eu/Eu* vs Sm/Nd diagram suggests that the parental material for the bauxite was derived from a combination of a distant magmatic source and clastic material derived from a continental margin (northern Africa) to the south.

Efficient Splitting of CO2 in an Isothermal Redox Cycle Based on Ceria

An isothermal thermochemical cycle to split CO2 based on nonstoichiometric reduction and oxidation of ceria is demonstrated. Carbon monoxide is produced via an oxygen partial pressure swing by alternating inert sweep gas and CO2 flows over the ceria. The rates of reduction and oxidation at 1500 °C in a porous ceria particle bed are measured for sweep gas and CO2 flow rates from 50 to 600 mL min–1 g–1 and analyzed to identify cycle operating conditions (gas flow rates and reduction and oxidation durations) that maximize process efficiency. For a solar reactor assumed to operate at 3000 suns concentration and with 90% of the sensible heat of the gases recovered, the optimal cycle uses 150 mL min–1 g–1 sweep gas and 50 mL min–1 g–1 CO2 at reduction and oxidation periods of 100 and 155 s, respectively. This cycle is demonstrated in an IR imaging furnace over 102 cycles, yielding a stable average rate of CO production of 0.079 μmol s–1 g–1 and a projected reactor efficiency of 4%. The optimal conditions apply at large scale if the flow rates are scaled in proportion to the ceria mass.

Delamination-resistant bi-layer electrolyte for anode-supported solid oxide fuel cells

One of the critical fuel cell degradation phenomena is ‘cell imbalance’ in a series-connected stack, which can cause abnormal operation under a negative cell voltage and consequently rapid degradation by anode interface delamination. In a previous study, the effect of electrolyte composition on the electrochemical degradation of solid oxide fuel cell (SOFC) was investigated, and it was observed that a small amount of ceria (an electronic conducting material) prevents anode delamination under abnormal (negative voltage) operation. However, the open circuit voltage (OCV) was lowered as a result of reduction of ceria. In the present study, bi-layer, YSZ (8mol % yttria doped zirconia, a predominantly ionic conductor) at the cathode side and 8CYSZ (8mol % ceria doped YSZ, a mixed ionic-electronic conductor) at the anode side were fabricated for anode-supported cells with a Pt probe embedded in each layer to estimate the internal oxygen chemical potential and tested under a negative voltage. The results indicated that the OCV was close to the theoretical value (similar to that of a YSZ single layer cell) and no delamination was observed under negative voltage operation (similar to the case of an 8CYSZ single-layer cell). Therefore, the bi-layer-structured electrolyte (with locally increased electronic conduction at the anode side) is effective in preventing anode/electrolyte delamination as well as maintaining open circuit voltage.

Effects of Support and Rh Additive on Co-Based Catalysts in the Ethanol Steam Reforming Reaction

The effect of the nature of the support and the promotion achieved by a Rh additive on Co-based catalysts in the ethanol steam reforming reaction were studied. The catalysts with 2% Co loading were characterized by temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). In situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS) identified the surface intermediates formed during the reaction, whereas gas phase products were detected by gas chromatography (GC). Upon heating in hydrogen to 773 K, cobalt could not be reduced to Co0 on alumina, but on silica the reduction was almost complete. On ceria, half of the Co could be reduced to the metallic state. By the presence of a small amount (0.1%) of Rh promoter, the reduction of both cobalt and ceria was greatly enhanced. For Co on the acidic Al2O3 support, the dehydration mechanism was dominant, although on the basic CeO2 support, a significant amount of hydrogen was also formed. Addition of a small amount of Rh as...

Cerium reduction at the interface between ceria and yttria-stabilised zirconia and implications for interfacial oxygen non-stoichiometry

Epitaxial CeO2 films with different thickness were grown on Y2O3 stabilised Zirconia substrates. Reduction of cerium ions at the interface between CeO2 films and yttria stabilised zirconia substrates is demonstrated using aberration-corrected scanning transmission electron microscopy combined with electron energy-loss spectroscopy. It is revealed that most of the Ce ions were reduced from Ce4+ to Ce3+ at the interface region with a decay of several nanometers. Several possibilities of charge compensations are discussed. Irrespective of the details, such local non-stoichiometries are crucial not only for understanding charge transport in such hetero-structures but also for understanding ceria catalytic properties.

Facile synthesis of catalytically active CeO 2 –Gd 2 O 3 solid solutions for soot oxidation

CeO2–Gd2O3 oxides were synthesized by a modified coprecipitation method and subjected to thermal treatments at different temperatures to understand their thermal behaviour. The obtained samples were characterized by XRD, BET, TEM, Raman and TPR techniques. Catalytic efficiencies for oxygen storage/release capacity (OSC) and soot oxidation were evaluated by a thermogravimetric (TG) method. XRD and Raman results indicated the formation of Ce0.8Gd0.2 O 2−δ (CG) solid solutions at lower calcination temperatures, and TEM studies confirmed nanosized nature of the particles. Raman studies further confirmed the presence of oxygen vacancies and lattice defects in the CG sample. TPR measurements indicated a facile reduction of ceria after Gd3+ addition. Activity studies revealed that incorporation of Gd3+ into the ceria matrix favoured the creation of more structural defects, which accelerates the oxidation rate of soot compared to pure ceria.

Thermal Reduction of Ceria within an Aerosol Reactor for H2O and CO2 Splitting

An aerosol reactor was tested for the thermal reduction of ceria as part of a solar thermochemical redox cycle for producing H2 and CO from H2O and CO2. The design is based on the downward aerosol flow of ceria particles, counter to an argon sweep gas, which are rapidly heated and thermally reduced within residence times of less than 1 s. When operating in the temperature range of 1723–1873 K and at oxygen partial pressures between 5 × 10–5 and 1.2 × 10–4 atm, reduction extents of small particles (Dv50 = 12 μm) approached those predicted by thermodynamics. However, heat- and mass-transfer effects were found to limit their conversion when the ceria mass flow rate was increased above 100 mg s–1. This reactor concept inherently results in separation of the reduced ceria and evolved O2(g), operates isothermally throughout the day, and decouples the reduction and oxidation steps in both space and time for potential 24-h syngas generation.

TPR, XRD and XPS characterisation of ceria-based materials synthesized by freeze-drying precursor method

A freeze-drying precursor method was used to obtain submicrometric powders of ceria-based materials such as Ce1−xGdxO2−δ (x=0, 0.01, 0.05, 0.10 and 0.20), 80%CeO2–20%ZrO2, 80%CeO2–20%Al2O3 and (1−y)Ce0.99Gd0.01O2−δ–(y)Al2O3 (y=0.01, 0.02, 0.05, 0.10 and 0.30) at temperatures as low as 400°C. The phase formation and evolution with the temperature was studied by X-ray diffraction (XRD). Also, temperature programmed reduction (TPR) was performed to investigate the reducibility of the ceramic powders. It was observed that after reduction of the ceria-based materials the fluorite structure of the samples was retained. The TPR profiles showed two peaks which are associated to the surface and bulk ceria reduction processes. Likewise, after the TPR measurements the resulting powders have showed high phase stability and reproducibility. XPS results confirmed the reduction of Ce4+ to (Ce3++Ce4+) ratio with alumina doping.

Effect of support on the deep oxidation of propane and propylene on Pt-based catalysts

The complete oxidations of propane and propylene were studied on Pt supported on CeO2, TiO2 and Al2O3. The catalyst activities were evaluated through conversion versus temperature (light-off curves) and conversion versus time tests. Propane oxidation turnover rates (TOF) followed the order: Pt/TiO2>Pt/CeO2>Pt/Al2O3. The higher activity on Pt/CeO2 than on Pt/Al2O3 was interpreted by considering that the combustion of C3H8 on Pt/CeO2 occurs not only on Pt0 sites but also on perimeter Pt0–Ce3+ sites providing an additional oxidation pathway. Propane uptake on Pt/TiO2 was 5.5times higher than on Pt/CeO2. This drastic increase of the density of adsorbed C3H8 molecules around the metallic Pt active sites would explain the high TOF values observed on Pt/TiO2 because the reaction is positive order with respect to propane. The propylene oxidation turnover rate trend was Pt/CeO2>Pt/Al2O3≅Pt/TiO2. Kinetic studies showed that on the three catalysts the apparent activation energy of propylene oxidation was about the same while the reaction orders were positive in oxygen and negative or zero in propylene. The higher activity of Pt/CeO2 catalyst was explained by considering that the Pt-catalyzed reduction of ceria forms oxygen vacancies that would improve the mobility of lattice oxygen of the support and its transfer to the propylene species adsorbed on the metal.

Near ambient pressure XPS investigation of the interaction of ethanol with Co/CeO2(1 1 1)

Near ambient pressure X-ray photoelectron spectroscopy was applied to study the interaction of ethanol (CH3CH2OH) with a well-ordered CeO2(111) film on Cu(111) and with a Co/CeO2(111)/Cu(111) model catalyst. The oxidation state of the surface and the chemical nature of reaction intermediates were analyzed. At 300K, the oxidation state of ceria decreased gradually with increasing ethanol pressure. At a constant pressure of 0.1mbar, the reduction of Ce4+ to Ce3+ increased significantly between 320 and 600K due to a higher mobility of oxygen or Ce3+ centers at elevated temperatures. The main intermediate, ethoxide, was formed by dissociative adsorption of ethanol at room temperature. No coke formation was observed up to 600K on CeO2. Upon deposition of metallic cobalt, partial reduction of ceria was observed, leading to the formation of Co2+ sites but still leaving metallic Co in the metal particles. During the reaction of ethanol with the Co/CeO2(111) model catalyst, the amount of Co2+ decreased drastically with increasing temperature, and at 600K, the majority of Co was metallic. This process was accompanied by the severe reduction of ceria and the formation of significant carbon deposits.

Influence of ceria on the NOx reduction performance of NOx storage reduction catalysts

Influence of ceria on the NOx storage and reduction behavior of NSR catalysts was investigated in a systematic manner over γ-Al2O3, Ba/Al, Ce/Al, Ba/Ce/Al, Pt/Al, Pt/Ce/Al and Ba/Pt/Ce/Al systems using BET, XRD, Raman spectroscopy and in situ FTIR. Although ceria promotion does not seem to have a substantial influence on the overall NOx storage capacity, it does have a clearly positive effect on the NOx reduction via H2(g) during catalytic regeneration under rich conditions which is associated with the enhancement in the total amount of activated hydrogen on the catalyst surface and lowering of the thermal threshold for hydrogen activation. A strong metal support interaction (SMSI) between Pt sites and the BaOx/CeOx domains leads to a complex redox interplay including oxidation of the precious metal sites, reduction of ceria, formation of BaO2 species as well as the formation of Pt–O–Ce interfacial sites on the Ba/Pt/Ce/Al surface. Ceria domains also act as anchoring sites for Pt species, limit their surface diffusion, enhance dispersion and hinder sintering at elevated temperatures. On the Ba/Pt/Ce/Al catalyst surface, reduction of the stored nitrates under relatively mild conditions via H2(g) initially leads to the formation of surface –OH and –NHx species and gas phase N2O, as well as the destruction of surface nitrate species, leaving bulk nitrates mostly intact. Reduction proceeds with the conversion of N2O(g) into N2(g) along with the partial loss of surface –OH and –NHx groups, dehydration and the loss of bulk nitrates.

Total oxidation of dichloromethane and ethanol over ceria–zirconia mixed oxide supported platinum and gold catalysts

Ce0.5Zr0.5O2 prepared by sol–gel method was used as a support for platinum and gold catalysts that were tested in total oxidation of dichloromethane and ethanol. It was shown that the deposition of platinum and gold on Ce0.5Zr0.5O2 support enhanced the reducibility of surface ceria. This phenomenon was more pronounced for platinum catalysts. Introduction of both noble metals led to the decrease of catalysts’ acidity. In total oxidation of dichloromethane, the noble metal catalysts showed lower catalytic performance compared to parent Ce0.5Zr0.5O2, due to lower amount of acid sites that act as chemisorption sites for chlorinated compounds. On the other hand, platinum catalysts exhibited significantly enhanced selectivity to CO2 in comparison with the Ce0.5Zr0.5O2 support. In total oxidation of ethanol, the deposition of platinum on Ce0.5Zr0.5O2 resulted in a significant increase in catalytic performance, while the introduction of gold had only a minor effect. Moreover, the positive effect of higher noble metal loading on catalytic performance was more pronounced for Pt catalysts. For all investigated catalysts, the temperature of the H2-TPR peak corresponding to the reduction of surface ceria correlated with their catalytic performance. The influence of Pt loading on the mechanism of ethanol oxidation was revealed. Based on the concentration profiles of individual by-products/products detected by on-line FTIR analysis, the possible pathways of their formation were suggested. Both noble metal catalysts exhibited higher selectivity to CO2 than the pristine Ce0.5Zr0.5O2.

Surface Reduction Mechanism of Cerium–Gallium Mixed Oxides with Enhanced Redox Properties

The doping of CeO2 with different types of cations has been recognized as a significant factor in controlling the oxygen vacancies and improving the oxygen mobility. Thus, the catalytic properties of these materials might be determined by modifying the redox properties of ceria. A combined experimental and theoretical study of the redox properties of gallium-doped cerium dioxide is presented. Infrared spectroscopy and time-resolved X-ray diffraction were used for temperature programmed reduction (H2) and oxidation (with O2 and H2O) studies. Additionally, X-ray absorption near edge spectroscopy shows that only Ce4+ is reduced to Ce3+ in the ceria–gallia mixed oxides when annealed up to 623 K. The oxygen storage capacity (OSC) measurements show a pronounced enhancement on the reduction of ceria by gallium doping. Theoretical calculations by density functional theory (DFT) confirm the higher reducibility of gallium-doped ceria oxides and give a molecular description of the stabilization of the doped material...

Steam Reforming of Ethanol on Ni/CeO2: Reaction Pathway and Interaction between Ni and the CeO2 Support

The steam reforming of ethanol on a Ni-based CeO2-supported catalyst was studied using in situ X-ray diffraction (XRD), operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and mass spectroscopy (MS) with a focus on the structural characterization of the catalysts, chemical identification of the reaction pathway, and understanding of the interaction between Ni and the CeO2 support. Ethoxy, acetate, carbonate, and hydroxyl species are identified by DRIFTS as surface intermediates that appear during the reaction process. The oxidation of ethoxy to acetate and the decomposition of acetate are two key steps in the steam reforming process. The CeO2 support facilitates the oxidation of ethoxy to acetate below 350 °C. Above 350 °C, the Ni metal catalyzes dissociation of the C–C bond in acetate to form carbonate and methyl, something that the CeO2 support is not able to do. The Ce(III) sites produced by the reduction of ceria in ethanol help to dissociate water forming the surface hydroxyl groups, which react with the methyl groups to produce CO2 and inhibited the methyl groups’ progress to CH4. Post-reaction transmission electron microscopy (TEM) images of the Ni/CeO2 catalyst reveal two types of carbon configurations: encapsulating carbon and filamentous carbon. A water-rich atmosphere favors formation of carbon filaments, which do not deactivate the catalyst.