Oxygen Storage Capacity Of Ceria Research Articles

Recent advances and perspectives of CeO2-based catalysts: Electronic properties and applications for energy storage and conversion.

Cerium dioxide (CeO2, ceria) has long been regarded as one of the key materials in modern catalysis, both as a support and as a catalyst itself. Apart from its well-established use (three-way catalysts and diesel engines), CeO2 has been widely used as a cocatalyst/catalyst in energy conversion and storage applications. The importance stems from the oxygen storage capacity of ceria, which allows it to release oxygen under reducing conditions and to store oxygen by filling oxygen vacancies under oxidizing conditions. However, the nature of the Ce active site remains not well understood because the degree of participation of f electrons in catalytic reactions is not clear in the case of the heavy dependence of catalysis theory on localized d orbitals at the Fermi energy E F . This review focuses on the catalytic applications in energy conversion and storage of CeO2-based nanostructures and discusses the mechanisms for several typical catalytic reactions from the perspectives of electronic properties of CeO2-based nanostructures. Defect engineering is also summarized to better understand the relationship between catalytic performance and electronic properties. Finally, the challenges and prospects of designing high efficiency CeO2-based catalysts in energy storage and conversion have been emphasized.

Low temperature adsorption of nitric oxide on cerium impregnated biomass-derived biochar

This study investigates the catalytic oxidation of NO to NO2 over biomass-derived biochar at ambient temperature. Rubber seed shell (RSS) was used as lignocellulosic waste to develop biochar for NO capture. The NO adsorption capacity of pristine biochar was low, about 17.61 mg/g at 30 oC. To enhance the NO uptake capacity of biochar, cerium (Ce) was introduced into the biochar surface through simple impregnation method. Upon this, the NO adsorption capacity of 3 wt% Ce-loaded biochar profoundly increased to 75.59 mg/g at the same adsorption condition. This was confidently due to the excellent oxygen storage capacity of ceria which could react with free active sites on the biochar surface to form oxidized cites C(O). Characterization results indicated that the adsorbed species was in the form of -O-N=O, suggesting that the adsorption of NO was followed by reaction with surface oxidized sites to form NO2. Studying the kinetics of the NO adsorption using pseudo-second order, Avrami and Elovich models showed that chemisorption was the chief mechanism that governed the adsorption process and the activation energy for NO adsorption was estimated to be around −45 kJ/mol.

Ce promoted V2O5 catalyst in oxidation of SO2 reaction

Harmful environmental effects of SO2 on the start-up of the sulfuric acid plant process motivated studying ceria as a promoter for V2O5 catalyst that is employed in SO2 oxidation. Effects of SO2(%), ceria percentage, temperature and O2/SO2 were investigated to optimize ceria loading on V2O5 catalyst. To determine the main and interaction effects, a full factorial design of experiments including 315 experiments was used. The results showed that the catalyst with 7wt% ceria (VaCe7) possessed the highest catalytic performance. To study the effect of more parameters on this catalyst, other 135 post-experiments with full factorial designs were performed after the optimization of Ce contents. The results revealed that mean SO2 conversion increased with increasing SO2 (%) and O2/SO2 ratio, and decreasing GHSV. Also, the maximum mean conversion was found at the reaction temperature of 450°C by the trade-off of kinetic and thermodynamic factors of the reversible exothermic reaction. VaCe7 was found to have more catalyst activities than that the without-ceria (VaCe0) case. The synthesized catalysts were characterized by XPS, BET, FTIR, SEM, XRD and ICP. The SEM mapping revealed that the cerium oxide had better distribution on VaCe7 than VaCe9&11. FTIR, XRD and XPS showed the presence of CeVO4 in the Ce-promoted catalyst and confirmed that ceria promotes V2O5 by the formation of CeVO4 network, reduces the activation energy of reduction–oxidation and increases the rate of SO2 oxidation reaction. The stability tests demonstrated good catalytic performance stability of the synthesized catalyst up to 50h in the reaction. Results at the initial times of the stability tests showed that the catalyst with ceria promoter (VaCe7) had higher activity than VaCe0 at the start-up time. Also, VaCe7 had higher activity at the start-up than its normal conditions, which could be related to the good oxygen storage capacity of ceria because the catalyst at the start-up is faced to the oxygen of air. This high startup activity caused to use it in starting up of sulfuric acid plants that decreases SO2 emission to the environment.

ChemInform Abstract: Modeling Ceria‐Based Nanomaterials for Catalysis and Related Applications

AbstractReview: 169 refs.

Development of Fuel-Flexible SOFC

Efficient utilization of low-grade biomass resources (bio-wastes) such as sewage sludge, agricultural residues, livestock manures and garbage in a sustainable way must bring about a promising solution against global environmental issues. Biogas, mixture of CH4 and CO2, produced by anaerobic fermentation of bio-wastes is a useful fuel for power generation in a community abundant in biomass resources. For the biomass power generation, generally gas engine (GE) systems are applied, but then their low electrical efficiency of 20-30 % attributed to Carnot limitation and energy losses caused by multiple energy conversions can become critical problems for small-sized distributed power generation. Solid oxide fuel cell (SOFC), highly efficient energy conversion device, is one of the most promising candidates as a distributed power generator in the next generation, because fuel processing can be proceeded within the cell to produce syngas which will be directly converted to electricity without combustion process, in other words not through thermal energy. This operation mode is called direct internal reforming (DIR) operation. Hydrocarbon fuels including biogas can be converted into electrical energy with 2-3 times higher efficiency in comparison to GEs. In this study, for the global dissemination of SOFC, simplification of fuel processing was studied for further enhancement of energy conversion efficiency and creation of compact system, focusing on biogas as a fuel. With the above motivation, flexible paper-structured catalyst (PSC) has been developed. Especially, hydrotalcite (HT)-dispersed PSC (HT-PSC) exhibits high tolerance for sulfur poisoning in the methane dry reforming, and therefore it was applied to biogas-fuelled SOFC as a reaction field of the internal reforming or pre-reformer. In this study, planar-shaped compact reformer in which HT-PSC was embedded (PSC-reformer) was developed. The PSC reformer was applied as a pre-reformer to a stack testing fuelled by simulated biogas (equimolar mixture of CH4 and CO2). The PSC-reformer exhibited stable CH4 conversion leading to stable stack operation with power density nearly equal to that for pure H2 under the same load current during which residual CO2 can contribute to the reduction of anodic activation loss in the low current density region. In the high current density region, H2 amount is boosted up through water gas shift (WGS) reaction between CO and H2O produced by reforming and electrochemical oxidation of H2, respectively, which has positive impact on the stack operation. For further enhancement of the catalytic activity of PSC to suppress carbon formation during dry reforming, core-shell structure catalyst (Ni/CeO2) was prepared and dispersed in the fiber network. The reformate composition with CO/H2 > 1 suggests that Ni particles with the CeO2shell can promote reverse Boudouard reaction, resulting in the considerable suppression of carbon deposition. Moreover, in comparison with a conventional PSC, Ni loading amount can be suppressed to get a certain methane conversion probably due to the oxygen storage capacity of ceria. Technology of the fuel-flexible SOFC is now studied in the Vietnam-Japan International joint research project, “Project for Sustainable Development of Rural Area by Effective Utilization of Bio-wastes with Highly Efficient Fuel Cell Technology”, under the SATREPS program supported by Japan Science and Technology Agency (JST) and Japan International Cooperation Agency (JICA). Purpose of this joint research is to develop and demonstrate an SOFC-integrated energy circulation system for the environmental conservation and the stable energy supply in Mekong Delta region. Sludge accumulated at the bottom of a shrimp pond will be pumped out and fed to methane fermentation reactor to produce biogas. This biogas is supplied to SOFC system to generate electricity which will be used for the shrimp culture. In this system, PSC technology will be applied to increase energy conversion efficiency.

Modeling Ceria-Based Nanomaterials for Catalysis and Related Applications

Ceria (CeO2) is used as support material in heterogeneous catalysts since decades and, recently, ceria itself was also demonstrated to be catalytically active for some reactions. Atomistic and electronic structure details of the functioning of ceria in catalysis can nowadays be successfully uncovered with the help of computational modeling based on the density functional theory (DFT). Yet, the majority of such computational studies undertaken so far relied on extended models of surfaces, which are adequate for the description of surface science processes and phenomena, but neglect the nanostructured nature of ceria in many catalysts. This Perspective focuses on discussing DFT calculations of various nanostructured models of ceria and its composites relevant for catalysis. Pivotal consequences of ceria nanostructuring for its role in catalysts derived from the computational studies are documented and supported by experimental results. The presented case studies shed light on several actively debating issues of ceria usage in catalysis and other applications. For instance: What makes ceria nanoparticles in a certain size range dramatically more reactive in oxidative processes? Is the oxygen storage capacity of ceria solely due to its ability to easily form and heal oxygen vacancies or do alternative mechanisms also operate at the nanoscale? How prone are metal particles deposited on ceria to sintering or dispersion and how is this interplay controlled by the nanostructuring of the support? Under what conditions will the transfer of lattice oxygen atoms from ceria support to the metal particles deposited thereon become energetically favorable? How can the electron transfer across the metal-ceria interface be measured and its peculiarities rationalized? The discussed examples show that accounting for ceria nanostructuring in catalysts is essential for performing trustworthy computational modeling. Such realistic description is possible thanks to a variety of the recently developed dedicated models representative of ceria at the nanoscale.

Creating single-atom Pt-ceria catalysts by surface step decoration.

Single-atom catalysts maximize the utilization of supported precious metals by exposing every single metal atom to reactants. To avoid sintering and deactivation at realistic reaction conditions, single metal atoms are stabilized by specific adsorption sites on catalyst substrates. Here we show by combining photoelectron spectroscopy, scanning tunnelling microscopy and density functional theory calculations that Pt single atoms on ceria are stabilized by the most ubiquitous defects on solid surfaces—monoatomic step edges. Pt segregation at steps leads to stable dispersions of single Pt2+ ions in planar PtO4 moieties incorporating excess O atoms and contributing to oxygen storage capacity of ceria. We experimentally control the step density on our samples, to maximize the coverage of monodispersed Pt2+ and demonstrate that step engineering and step decoration represent effective strategies for understanding and design of new single-atom catalysts.

Au/CuO–CeO2 catalyst for preferential oxidation of CO in hydrogen-rich stream: Effect of CuO content

A series of Au/CuO–CeO2 with various amounts of CuO was prepared. CuO–CeO2 was prepared by co-precipitation and Au was loaded by deposition–precipitation. The aim of this study was to develop a catalyst which is highly active and selective in preferential oxidation of CO in hydrogen stream (PROX). These catalysts were characterized by N2-sorption, XRD, TEM, HR-TEM, and XPS. The PROX reaction was carried out in a fixed bed continuous flow reactor with a feed of CO: O2: H2: He = 1.33: 1.33: 65.33: 32.01 in volume ratios. The incorporation of copper ion into ceria lattice increased the oxygen storage capacity of ceria and enhanced the activity of the catalyst. The excess CuO would present on the surface of CeO2. Au particles were well dispersed well on the support and the particle size of gold was in the range of 2 and 4 nm. Au was the active species and CuO played the promoter role. Those CuO presented on the surface of CeO had negative effect for CO oxidation. The selectivity of oxygen reacting with CO increased with increasing CuO content. Adding suitable amount of CuO in Au/CeO2 is beneficial for CO conversion and suppressing hydrogen oxidation in fuel cell operation condition.

Effects of Thermal Fluctuations on the Hydroxylation and Reduction of Ceria Surfaces by Molecular H2

The hydroxylation of oxide surfaces driven by molecular H2 dissociation plays a central role in a wide range of catalytic redox reactions. The high reducibility and oxygen storage capacity of ceria (CeO2) surfaces account for its extensive use as active catalyst support in these redox reactions. By means of ab initio molecular dynamics simulations, we investigate the hydroxylation and reduction of ceria surfaces and demonstrate the so-far unrecognized effects of atomic thermal fluctuations into the mechanism and kinetics of H2 dissociation. The reaction free-energy hypersurface is sampled and mapped at finite temperature by combining Hubbard-U density functional theory (DFT+U), ab initio molecular dynamics, metadynamics, and umbrella sampling methods. Our molecular dynamics simulations show that the explicit inclusion of thermal fluctuations into the reaction thermodynamics alters the mechanism of H2 dissociation, changes the nature of the rate-limiting transition state, and decreases the activation tempe...

Spatiotemporal behavior of Pt/Rh/CeO2/BaO catalyst during lean–rich cycling

The experimental system that utilized an optical fiber and a spatially resolved capillary inlet mass spectrometer to study the dynamic oxygen storage capacity of ceria containing LNT catalyst during periodic lean (oxygen)–rich (propylene) operation. • Combined measurement of intrachannel temperature and concentration. • Detailed insight about the dynamics of a transient exothermic catalytic reaction. • Oxidation and reduction chemistry with ceria plays important role during cycling. • Ceria enables a more efficient oxidation to total oxidation products. • Cycling leads to much higher production of partial oxidation products H 2 and CO. Experiments were conducted to determine the impact of key operating variables (ceria loading, space velocity, cycle time, and rich pulse intensity) on the oxygen storage and release process of a Pt/Rh/CeO 2 /BaO monolithic catalyst during periodic lean (oxygen)–rich (propylene) operation. The concentrations were measured by spatially-resolved capillary inlet mass spectrometry (SpaciMS), and the temperature profile was measured by coherent optical frequency domain reflectometry (c-OFDR), providing detailed insight into the spatio-temporal features of the reaction system. The experiments revealed that the addition of ceria increased the breakthrough time of the propylene during a lean-to-rich transition due to an increased oxidation rate. Hydrogen was formed in the upstream and was consumed in the downstream section of the catalyst by reaction with ceria. Increasing the space velocity increased the upstream hydrogen formation rate and decreased its downstream oxidation rate. Increasing the lean and rich durations (while keeping their duration ratio constant) increased the oxygen uptake but resulted in propylene breakthrough and decreased the solid temperature. Increasing the rich period (at a fixed rich feed and total cycle time) allowed the catalyst to release more oxygen. A comparison of the ceria-containing catalyst to one without ceria revealed a more efficient complete oxidation for the former, while a comparison of the periodic to stationary operation revealed that less than half the propylene was converted to CO 2 by the former.

A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorption of CO2and the formation of H2 when a second water molecule adsorbs at a site where water had been previously dissociated (catalyzed by platinum), or the formation of a formate species from the interaction of the adsorbed CO2 with adjacent water molecules. Energy pathways are mapped for both processes, demonstrating the preference toward the initial direct redox pathway. However, the resulting formation of adsorbed hydroxide species in the oxygen vacancies adjacent to the platinum hinders this pathway. A bifunctional mechanism is then presented as a means to remove these species and thereby form formate. The activation of these adsorbed hydroxides is also studied.

Electronic and Geometric Structure of Ce3+Forming Under Reducing Conditions in Shaped Ceria Nanoparticles Promoted by Platinum

The structure of Ce3+, which is responsible for the low-temperature oxygen storage capacity of ceria, was determined by high-energy-resolution fluorescence detected X-ray absorption spectroscopy at the Ce L3 and L1 edges. Well-defined ceria nanoparticles (rods, truncated octahedra, and cubes) were synthesized hydrothermally and promoted by platinum nanoparticles. The electronic structure of Ce3+ does not depend significantly on the nature of the exposed crystallographic planes of CeO2 particles; it does, however, differ from the electronic structure of known stable compounds containing Ce3+ ions, such as CeAlO3, Ce(NO3)3·6H2O, and Ce2Zr2O7. Theoretical simulation of Ce L1 and L3 X-ray absorption spectra, quantitative analysis of the oxygen storage capacity, and X-ray diffraction data suggest that Ce3+ ions form both at the surface and in the near-surface layer. Surface and bulk Ce3+ ions are characterized by elongated Ce–O distances in the first coordination shell and almost the same Ce–Ce distances in the second coordination shell with respect to Ce4+ in stoichiometric CeO2. Ce3+ ions on the surface of the nanoparticles surface may have a smaller number of oxygen neighbors (as low as six), while in the near-surface layer they tend to have an 8-fold coordination, thus producing oxygen deficit structures similar to Ce11O20.

Copper and nickel catalysts supported on praseodymium-doped ceria (PDC) for the water-gas shift reaction

The aim of this work is to study the behavior of Cu and Ni catalysts supported on praseodymium-doped ceria (PDC) for the WGS reaction. Samples were prepared by impregnation of several PDC supports (praseodymium content in the range 0–50wt%) synthesized by the urea thermal decomposition method. Bare support and catalysts were characterized by BET, XRD, oxygen storage capacity and TPR. The activity tests were carried out in an atmospheric fixed bed reactor, using a CO/H2O/H2/N2 mixture as the feed stream. Doping of ceria with small loading of Pr increases the surface reducibility and the oxygen storage capacity of ceria. Nickel catalysts are more active than copper ones. In both cases, catalysts supported on PDC with 5 Pr wt% are more active than samples supported on pure ceria. Methanation, which is likely to occur on Ni catalysts, is moderated in samples supported on praseodymium-doped ceria.

Supercharged Low-Temperature Oxygen Storage Capacity of Ceria at the Nanoscale.

We provide an explanation for the experimental finding of a dramatically enhanced low-temperature oxygen storage capacity for small ceria nanoparticles. At low temperature, small octahedral ceria nanoparticles will be understoichiometric at both oxidizing and reducing conditions without showing explicit oxygen vacancies. Instead, rather than becoming stoichiometric at oxidizing conditions, such particles are stabilized through oxygen adsorption forming superoxo (O2(-)) ions and become in this way supercharged with oxygen. The supercharging effect is size-dependent and largest for small nanoparticles where it gives a direct increase in the oxygen storage capacity and simultaneously provides a source of active oxygen species at low temperatures.

Dynamic Response of a Three-Way Catalytic Converter

In an engine transient condition, effects of different exhaust mass flows, inlet signal amplitudes and periods on dynamic responses of the three-way catalytic converter were investigated experimentally. Experimental results show that exhaust mass flows, inlet signal amplitudes and periods have important influence on engine emissions and oxygen storage capacity of Ceria for the three-way catalytic converter with CeO2 catalyst in the washcoat. With the increase of the exhaust mass flow, the lambda outlet signal increases nonlinearly; emissions are better at the beginning and then turns deteriorated in the case of a high mass flow after the lean-rich transition. Outlet lambda increases almost linearly with the increase of exhaust mass flows and lambda outlet signal amplitudes after the richlean transition.

Stability improvement of ZrO2-doped MnCeOx catalyst in ethanol oxidation

Abstract ZrO 2 -doped manganese–cerium oxide catalyzed ethanol oxidation effectively and ethanol was fully oxidized to CO 2 at 453 K. The catalyst also showed quite promising stability for 120 h on-stream without obvious loss in ethanol conversion. Structural analyses have revealed that ZrO 2 -doping enhanced the oxygen storage capacity of ceria by generating more oxygen vacancies, and at the same time promoted the thermal stability through the formation of solid solution.

Soot combustion over silver-supported catalysts

In this study the characterization and soot oxidation activity of Ag-based catalysts deposited on alumina, ceria and zirconia have been investigated. The combustion of soot was shown to be promoted by the presence of silver, especially in a zero valent state, over all the supports investigated. Presence of silver in a positive oxidation state is favoured with CeO2, while zero valent silver dominates over alumina and zirconia. This is likely attributed to the oxygen storage capacity of ceria which stabilizes silver in an oxidized form. Soot oxidation starts at low temperature (around 500K for Ag on ZrO2) with a T50 in the range of 600–640K. Deactivation under strong ageing conditions (1023K for 12h) is negligible with alumina and zirconia, while in the presence of ceria, which slows down formation of metallic silver from Ag2O, loss of activity following thermal treatments at high temperatures becomes more intense.

A Density-Functional Theory Study of the Water−Gas Shift Mechanism on Pt/Ceria(111)

Density-functional theory has been used to model the interactions between ceria(111) and CO adsorbed on platinum in order to provide insights into the mechanism behind the water−gas shift reaction on this material. Morphological studies on ceria show the presence of various defect sites, both cerium and oxygen vacancies, which are believed to be the reason for the catalytic activity of the substrate. Of these defect sites, the reported calculations clearly show the preference for platinum to bind in cerium vacancies, as opposed to a defect-free surface or an oxygen deficiency site. Currently there are two main pathways proposed for this mechanism: a direct redox pathway and a formate-intermediate pathway. In both scenarios the initial step is the same: the oxygen storage capacity of ceria provides the oxygen necessary to oxidize CO into CO2; water then adsorbs and is dissociated in these oxygen vacancies helping reform the ceria surface. The key difference between the two mechanisms is either the desorpti...

Oxygen vacancy formation energy at the Pd/CeO2(111) interface

The atomic and electronic structure of Pd adsorption on the unreduced and reduced ceria (111) surfaces is studied using first principles projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (GGA + U). It is found that there exist different adsorption features for Pd on different surfaces (unreduced and reduced): (1) on the unreduced CeO2(111) surface, Pd prefers to be adsorbed on the O-bridge site (O2-site); (2) on the reduced CeO2(111) surface, the adsorption is much stronger than that on the unreduced CeO2(111) surface; (3) the formation of an oxygen vacancy is easier at the Pd/CeO2(111) interface than that on the clean CeO2(111) surface, indicating that the Pd adatom enhances the oxygen storage capacity (OSC) of ceria, and the interaction between the palladium catalyst and the ceria support enhances the lower temperature activity of the catalyst. The mechanisms for this promotion in the OSC of ceria at the Pd/ceria interface are discussed.

Effects of ceria in CO2 reforming of methane over Ni/calcium hydroxyapatite

In CO2 reforming of methane over a calcium hydroxyapatite-supported nickel catalyst, the carbon deposition occurred more severely with increase of the methane partial pressure and at temperatures below about 1,000 K. The effects of ceria that was added as a promoter to the nickel catalyst were investigated. It was observed that the ceria not only enhanced the catalyst stability but also increased the activity, and this is considered owing to the oxygen storage capacity of ceria. The TGA analysis demonstrated that the ceria promoted the removal of the deposited carbon. The optimum Ce/Ni mole ratio was ca. 0.3/2.5. The deposited carbon could easily be removed by oxygen treatment at 1,023 K and the catalytic activity could be restored.