Catalisadores baseados em óxidos de cério (CeOx, 3/2≤ x ≤ 2) como suporte são utilizados em várias reações de alto interesse econômico, por exemplo as reações de catalizadores de três-vias. Sabe-se que é possível melhorar as propriedade catalíticas da céria, através da mistura com óxido de zircônio e com a adição de partículas metálicas na superfície do material. Entretanto, a deposição de átomos de Zr sobre CeO2(111) é pouco explorada apesar de já ter sido utilizada para a síntese de nanopartículas de Ag de tamanho controlado. Uma das particularidades dos sistemas Zr/CeO2(111) é formar ilhas altura entre 1,5 e 3,0 Å sobre a superfície da céria que são sugeridas na literatura com camadas de Zr-O e O-Zr-O. Entretanto, a natureza e magnitudes das interações entre Zr e CeO2(111) ainda não são totalmente conhecidas, bem como as modificações causadas pelo Zr na superfície de CeO2 e os mecanismos que controlam a oxidação do Zr e a formação de ZrO2 na superfície. Visando entender as interações Zr/CeO2(111) e a formação de ZrO2 sobre CeO2(111), realizou-se um estudo teórico da adsorção de n (1 ≤n ≤ 4) adatomos de Zr sobre CeO2(111), e da formação de ZrO2 sobre CeO2(111). As análises de carga indicam transferências de carga do adatomos de Zr para a superfície e mudança no estado de oxidação das espécies. Os Zr se oxidam á Zr4+ e interagem com O2- da superfície, onde quatro cátions Ce4+ se reduzem á Ce3+. Analises energéticas indicam que o processo é muito estabilizante, mais de 10 eV por Zr. Com o aumento da quantidade n de adatomos de Zr na superfície, observa-se 4 × n reduções de Ce4+ e migrações de O2- de dentro da superfície para próximo dos Zr4+, formando agregados de ZrO2 sobre a superfície. A migração de O se deve a dois fatores, a interação dos O2- com Zr4+ no agregado é mais estável do que a interação dos O2- com Ce3+ dentro da superfície, e a migração de O diminuir a tensão causada pelo maior raio do Ce3+ em relação ao Ce4+. Em adição, foi encontrado uma tendência de estabilidade para os Zr4+ migrarem para sítios Ce dentro da superfície, devido a maior quantidade de coordenações Zr-O e a redução da tensão criada pelos Ce3+. / Cerium oxides (CeOx, 3/2≤ x ≤ 2) based catalysts are employed in several reactions with high economic interest, such as the reaction in three-way-catalysts. It is well know that is possible to improve the ceria catalytic properties, by mixing with zirconium oxide and adding metallic particles over the material surface. Meanwhile, the deposition of Zr atoms over CeO2(111) is little explored although it has already been used for synthesis Ag nanoparticles of controlled size. One of the particularities of the Zr/CeO2(111) systems is to form islands of height between 1,5 and 3,0 Å on the surface of the ceria, which are suggested in the literature to be Zr-O and O-Zr-O layers. However, the nature and magnitudes of interactions between Zr and CeO2 surface are little know, as well as the CeO2 modifications induced by Zr and the mechanisms for Zr oxidation and ZrO2 formation over the surface. Aiming to understand the Zr-CeO2(111) interactions and the ZrO2 formation over the CeO2(111), this mastering project perform a theoretical study of n (1 ≤ n ≤ 4) Zr adatoms absorption over CeO2(111), and the ZrO2 formation over CeO2(111). The charge analysis indicated charge transfer from Zr adatons to the surface together with change in species oxidation state. The Zr oxidize to Zr4+ and interact with surface O2- , where four Ce4+ cations reduce to Ce3+. Energetic analysis pointed out that the process is very stabilizing, more than 10 eV per Zr adatom. With the increase of quantity n of Zr adatoms over the surface, it is observer 4 × n Ce4+ reductions and O2- migrations from inside surface to close the Zr4+, forming ZrO2 aggregates over the surface. The O migration occurs because of two reasons, the O2- interaction with Zr4+ in the agregate is more stabilizer than the interaction of O2- with Ce3+ inside the surface, and the O migration decrease the strain produced bue to the radius of Ce3+ being greater than the Ce4+ radius. In addition, was found a stability trend for Zr4+ to migrate to inside surface Ce sites, due of the more Zr-O coordinations and release of the strain induced by Ce3+.

Catalisadores baseados em óxidos de cério (CeOx, 3/2≤ x ≤ 2) como suporte são utilizados em várias reações de alto interesse econômico, por exemplo as reações de catalizadores de três-vias. Sabe-se que é possível melhorar as propriedade catalíticas da céria, através da mistura com óxido de zircônio e com a adição de partículas metálicas na superfície do material. Entretanto, a deposição de átomos de Zr sobre CeO2(111) é pouco explorada apesar de já ter sido utilizada para a síntese de nanopartículas de Ag de tamanho controlado. Uma das particularidades dos sistemas Zr/CeO2(111) é formar ilhas altura entre 1,5 e 3,0 Å sobre a superfície da céria que são sugeridas na literatura com camadas de Zr-O e O-Zr-O. Entretanto, a natureza e magnitudes das interações entre Zr e CeO2(111) ainda não são totalmente conhecidas, bem como as modificações causadas pelo Zr na superfície de CeO2 e os mecanismos que controlam a oxidação do Zr e a formação de ZrO2 na superfície. Visando entender as interações Zr/CeO2(111) e a formação de ZrO2 sobre CeO2(111), realizou-se um estudo teórico da adsorção de n (1 ≤n ≤ 4) adatomos de Zr sobre CeO2(111), e da formação de ZrO2 sobre CeO2(111). As análises de carga indicam transferências de carga do adatomos de Zr para a superfície e mudança no estado de oxidação das espécies. Os Zr se oxidam á Zr4+ e interagem com O2- da superfície, onde quatro cátions Ce4+ se reduzem á Ce3+. Analises energéticas indicam que o processo é muito estabilizante, mais de 10 eV por Zr. Com o aumento da quantidade n de adatomos de Zr na superfície, observa-se 4 × n reduções de Ce4+ e migrações de O2- de dentro da superfície para próximo dos Zr4+, formando agregados de ZrO2 sobre a superfície. A migração de O se deve a dois fatores, a interação dos O2- com Zr4+ no agregado é mais estável do que a interação dos O2- com Ce3+ dentro da superfície, e a migração de O diminuir a tensão causada pelo maior raio do Ce3+ em relação ao Ce4+. Em adição, foi encontrado uma tendência de estabilidade para os Zr4+ migrarem para sítios Ce dentro da superfície, devido a maior quantidade de coordenações Zr-O e a redução da tensão criada pelos Ce3+. / Cerium oxides (CeOx, 3/2≤ x ≤ 2) based catalysts are employed in several reactions with high economic interest, such as the reaction in three-way-catalysts. It is well know that is possible to improve the ceria catalytic properties, by mixing with zirconium oxide and adding metallic particles over the material surface. Meanwhile, the deposition of Zr atoms over CeO2(111) is little explored although it has already been used for synthesis Ag nanoparticles of controlled size. One of the particularities of the Zr/CeO2(111) systems is to form islands of height between 1,5 and 3,0 Å on the surface of the ceria, which are suggested in the literature to be Zr-O and O-Zr-O layers. However, the nature and magnitudes of interactions between Zr and CeO2 surface are little know, as well as the CeO2 modifications induced by Zr and the mechanisms for Zr oxidation and ZrO2 formation over the surface. Aiming to understand the Zr-CeO2(111) interactions and the ZrO2 formation over the CeO2(111), this mastering project perform a theoretical study of n (1 ≤ n ≤ 4) Zr adatoms absorption over CeO2(111), and the ZrO2 formation over CeO2(111). The charge analysis indicated charge transfer from Zr adatons to the surface together with change in species oxidation state. The Zr oxidize to Zr4+ and interact with surface O2- , where four Ce4+ cations reduce to Ce3+. Energetic analysis pointed out that the process is very stabilizing, more than 10 eV per Zr adatom. With the increase of quantity n of Zr adatoms over the surface, it is observer 4 × n Ce4+ reductions and O2- migrations from inside surface to close the Zr4+, forming ZrO2 aggregates over the surface. The O migration occurs because of two reasons, the O2- interaction with Zr4+ in the agregate is more stabilizer than the interaction of O2- with Ce3+ inside the surface, and the O migration decrease the strain produced bue to the radius of Ce3+ being greater than the Ce4+ radius. In addition, was found a stability trend for Zr4+ to migrate to inside surface Ce sites, due of the more Zr-O coordinations and release of the strain induced by Ce3+.

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12680号 / 論工博第4082号 / 新制||工||1548(附属図書館) / 29813 / (主査)教授 田中 庸裕, 教授 江口 浩一, 教授 安部 武志 / 学位規則第4条第2項該当

Three-way catalysis (TWC) involves simultaneous removal of the three pollutants (i.e., CO, NOx, and HCs) which led to the branch of auto-exhaust catalysis. CeO2 has become the main component of TWC catalyst because of its oxygen storage storage (OSC) property to supply oxygen under excess fuel condition and store oxygen under lean condition. Substitution of smaller isovalent cations like Ti4+, Zr4+ and Hf4+ ions in CeO2 forming Ce1-xMxO2 (M = Ti, Zr &Hf) solid solution enhance the OSC property. XRD along with EXAFS study showed that cations arrange in FCC lattice but oxygen coordination around metal ions is split into 4 + 4 coordination in Ce1-xMxO2 instead of ideal 8 coordination in CeO2. The longer Ce/Ti/Zr – O bonds are weakly bound and can be easily removed by H2 giving high OSC value than pure CeO2. Among the three OSC systems studied here, Ce0.5Zr0.5O2 showed exceptionally high OSC which lead to formation of a new a pyrochlore, Ce2Zr2O6.3. This compound is nearly metallic. Ce0.85-xTi0.15PtxO2- (x = 0.01 & 0.02) crystallizes in fluorite structure and Pt is ionically substituted with 2+ and 4+ oxidation states. H/Pt atomic ratio at 30 oC over Ce0.84Ti0.15Pt0.01O2- is 5 and over Ce0.99Pt0.01O2-δ is 4 against just 0.078 for 8 nm Pt metal particles. Carbon monoxide and hydrocarbon oxidation activity are much higher over Ce1-x-yTixPtyO2 (x= 0.15, y= 0.01, 0.02) compared to Ce1-xPtxO2 (x= 0.01, 0.02). Synergistic involvement of Pt2+/Pt0 and Ti4+/Ti3+ redox couples in addition to Ce4+/Ce3+ due to the overlap of Pt(5d), Ti(3d), and Ce(4f) bands near EF is shown to be responsible for enhanced redox property and higher catalytic activity. On substitution of Pd ion in Ce1-xTixO2, more lattice oxygen is found to be more labile than Pd in CeO2. The easy removal of oxygen from the more reducible Ti4+ containing support plays a major role in showing higher catalytic activity of this material for CO oxidation, N2O and NO reduction by CO. The catalyst shows 100% N2 selectivity  240 oC in NO+CO reaction. It has been shown that oxide ion vacancy creation created by removal of lattice oxygen by CO is responsible for dissociation of NO or N2O at a lower temperature. Ionicity of Pd2+ ion in different support could be varied by varying the ionicity of the oxide support itself. Rates of CO oxidation increases or activation energy decreases over Ce1-xPdxO2-δ, Ti1-xPdxO2-δ and Ce1-x-yMxPdyO2-δ (M = Ti, Zr, Hf ; x = 0.25, 0.4 ; y = 0.02) is increased with ionicity of Pd2+ ion. The substitution of Sn in CeO2 forming Ce1-xSnxO2 (x = 0.1-0.5) solid solution was prepared using tin oxalate precursor by solution combustion method. These oxides can be promising support for noble metals because of the Sn4+  Sn2+ redox couple in addition to Ce3+/Ce4+. The two electron process involved in the redox reaction of Sn as well as easy reducibility of Sn4+ to Sn2+ offers a far better redox catalytic system hitherto not reported. Ce1-xSnxO2 solid solutions as well as Pd ion substituted Ce1-xSnxO2 was prepared for the first time.

The impacts of fluorine doping of ceria are studied by means of surface science experimental methods. Fluorine-doped and fluorine-free ceria layers are epitaxially grown on rhodium single crystals and their properties are compared in regular and inverse catalyst configurations. A procedure for epitaxial growth of CeO2(110) and CeOxFy(110) layers on Rh(110) single crystal is developed and described in detail. Shape alterations of Ce 3d spectrum brought about by fluorine doping are explained and a suitable deconvolution method is proposed. Special attention is focused towards stability of fluorine in ceria. Presented data show that fluorine incorporation in ceria lattice causes stable reduction of ceria, which withstands up to 200řC in near-ambient pressure conditions. Morphological changes are observed due to elongation of surface lattice constant of reduced ceria. Oxygen storage capacities and hydrogen oxidation reaction rates of four different studied systems are compared and discussed. The twofold nature of oxygen exposure of fluorinated ceria is discovered and explained. Oxygen repels fluorine from the surface, while the remaining part of fluorine is expelled to adsorbate positions, where its electronic state is altered. Moreover, such fluorine is prone to interact with atomic hydrogen. This reaction is...

Abstract The aim of the thesis was to find new information on the effect of BaO and oxygen storage capacity material on NOx reduction. A total of nine different kinds of Pd/Al2O3-based metallic monoliths were studied. Promoters such as oxygen storage capacity material (OSC material in our case Cex-1ZrxO2 mixed oxides) and stabilisers such as barium (BaO) were added into the catalyst alone or together to improve catalyst properties such as catalytic activity, selectivity, and thermal stability. The key aspects in this thesis can be divided into four parts. First, the behaviour of NO reduction and CO as well as C2H4 oxidation over Pd/Al2O3-based catalysts in rich, stoichiometric and lean conditions were studied. Secondly, the effect of BaO and the OSC material in NO reduction was considered. Thirdly, the preparation procedure, i.e. the addition order of BaO, OSC material, and Pd on the catalyst was considered. Finally, the effect of ageing on the behaviour of catalysts was investigated. Several characterization methods (in situ DRIFT, catalytic activity measurements, N2 physisorption, CO chemisorption, dynamic oxygen storage capacity measurements, and X-ray diffraction (XRD)) were utilised to find answers to the behaviour of these catalysts in the studied model reactions. The Rapid Ageing Hot in Laboratory (RAHLAB) method was used to age the catalyst. First, it was demonstrated that the CO and hydrocarbon oxidation and NO reduction reactions over the Pd/Al2O3-based catalyst behave differently. NO reduction and CO and hydrocarbon oxidation reactions were dependent on the reaction conditions and temperatures. Secondly, the effect of OSC and BaO on NO reduction was studied separately. In the case of OSC material, it was observed that the OSC material has an effect on NO reduction as well as on CO and C2H4 oxidation. In the case of BaO, it was observed that BaO has a positive effect on NO reduction as well as CO and C2H4 oxidation especially in rich conditions. Thirdly, the effects of the addition order of OSC and BaO on the catalyst was studied separately. It was shown that these have an effect on NO reduction, especially after ageing. First, on the fresh Pd/Al2O3 catalyst it was observed that the addition order of OSC on the catalyst has not an influence on catalyst performance. The effect of the BaO addition order on the Pd/OSC/Al2O3-based catalyst is found to be insignificant, especially, after the ageing process. Furthermore, it is observed that the effect of RAHLAB ageing was that the catalyst lost its catalytic activity, stability, and selectivity.

No description available.

Depuis la découverte de la piézoélectricité dans le quartz, les besoins en matériaux efficaces pour des applications technologiques piézoélectriques (PZ) et d’Optiques Non-Linéaires (ONL) ont considérablement augmenté. Les deux propriétés doivent présenter une stabilité thermique élevée dans une large gamme de température. Dans le cadre de ma thèse, les matériaux de type quartz BaXO2 (X=Zn ou Co) ont donc été étudiés car ils cristallisent dans le même groupe d’espace P3121 que le quartz-a l'un des matériaux les plus utilisés, principalement pour ses propriétés de PZ et d’ONL.Nous avons commencé ce travail par le composé BaZnO2 car il présenterait un coefficient de couplage électromécanique K11=35,7% supérieur à celui du quartz. Une étude combinée expériences/théorie a permis de i) caractériser la stabilité chimique et thermique jusqu’à 1273 K, et de ii) déterminer les propriétés diélectriques et vibrationnelles (IR-Raman) du matériau. Les propriétés ONL, calculées par DFT, seraient en outre supérieures à celles du quartz.Dans BaCoO2, le degré d’oxydation +II du Co en symétrie tétraédrique implique un ordre antiferromagnétique à température ambiante (TN =330 K). Associé à la structure type quartz, un couplage piézo-magnétique peut être envisagé. BaCoO2 a été caractérisé structuralement/vibrationnellement (résultats confrontés à la théorie) jusqu’à 1273 K et une capacité de stockage d’oxygène réversible exceptionnelle a pu être mise en évidence dans BaCoO2+x (0 ≤ x ≤ 1) par ATG, spectroscopie Raman et par diffraction de rayons X grâce à une séquence de transition topotactique. / Since the discovery of the piezoelectricity in quartz, the need of efficient materials for piezoelectric and non-linear optic technological applications has increased dramatically. Both properties have to exhibit a high thermal stability over a wide temperature range. In my thesis work, the BaXO2 quartz type structure materials (X = Zn or Co) have been studied since they crystallize in the same P3121/P3221 space group than quartz, which is still one of the most widely used minerals mainly due to its piezoelectric properties.We have started this work with the BaZnO2 compound since first-principles-based calculations predicted an electromechanical coupling, k_11=35.7%. Our combined experimental and theoretical study allowed i) to characterize the chemical and thermal stability of BaZnO2 up to 1273 K, and ii) to determine the dielectric and vibrational (IR-Raman) properties of the material. Additionally the non-linear optic responses are predicted to be one order of magnitude higher than those reported for quartz.In BaCoO2, the oxidation state of Co (+II) in a tetrahedral symmetry implies an antiferromagnetic order at room temperature ( T_N=330 K). Associated with the -quartz type structure, a piezo-magnetic coupling can be considered. BaCoO2 was structurally and vibrationally characterized up to 1273 K (as obtained results were confronted to theory) and an exceptional reversible oxygen storage capacity was pointed out in BaCoO_(2+δ )(0 < δ ≤ 1) based on thermogravimetric measurements, Raman spectroscopy and X-ray diffraction characterizations. The oxygen insertion/disinsertion mechanism is governed by a topotactic transition sequence.

Ceria (CeO2) containing materials are the subject of numerous investigations recently owing to their broad range of applications in various fields. Ceria is one of the most important components of three-way catalysts (TWC). Two unique features are responsible for making CeO2 a promising material for use either as a support or as an active catalyst: (a) the Ce3+/Ce4+ redox couple, and (b) its ability to shift between CeO2 and CeO2–δ under oxidizing and reducing conditions retaining fluorite structure. Despite widespread applications, pure CeO2 has a serious problem of degradation in performance with time at elevated temperatures. CeO2 undergoes rapid sintering under high operating temperatures, which leads to loss of oxygen buffer capacity and deactivation of the catalyst. In addition, the amount of lattice oxygen taking part in the redox reactions is small (δ ~ 0.05), and therefore unsatisfactory for practical applications. Therefore further improvement of OSC of CeO2 has led to development of new CeO2-based oxygen storage materials. Modifications of CeO2 with isovalent or aliovalent ion (noble metal, rare-earth or transition metal) confer new properties to the catalysts, such as better resistance to sintering and high catalytic activity. The demand for ceria-based oxygen storage materials were accelerated in the 1970s with the introduction of strict automotives exhaust treatment worldwide to combat the obnoxious gases released in the atmosphere causing deterioration of air quality. Significant developments have occurred in this field leading to better understanding of the catalysts synthesis, structure and improved catalytic activity. The introductory chapter 1 is a compendium to provide an overview of the topic, examine the critical lacunae in the field and the proposal for future developments. In chapter 2 we present the studies on synthesis and catalytic properties of Ce1– xSnxO2 (x= 0.1–0.5) solid solution and its Pd substituted analogue. A brief description of the single step solution combustion synthesis, catalysts characterization techniques such as powder X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) are given. Design and fabrication of temperature programmed reduction by hydrogen (H2-TPR) system in this laboratory is given in details. The home-made temperature programmed catalytic reaction system with a quadrupole mass spectrometer and an on-line gas-chromatograph for gas analysis is described. For the synthesis of Ce1–xSnxO2 solid solution by a single-step solution combustion method we have used tin oxalate as precursor for Sn. The compounds were characterized by XRD, XPS and TEM. Oxygen storage capacity of the Ce1–xSnxO2 solid solution was measured by H2-TPR. The cubic fluorite structure remained intact up to 50% of Sn substitution in CeO2, and the compounds were stable up to 700 °C. Oxygen storage capacity of Ce1–xSnxO2 was found to be much higher than that of Ce1–xZrxO2 due to accessible Ce4+/Ce3+ and Sn4+/Sn2+ redox couples at temperatures between 200 to 400 °C. Pd2+ ions in Ce0.78Sn0.2Pd0.02O2-δare highly ionic, and the lattice oxygen of this catalyst is highly labile, leading to low temperature CO to CO2 conversion. The rate of CO oxidation was 2 μmolg–1s–1 at 50 °C. NO reduction by CO with 70% N2 selectivity was observed at ~200 °C and 100% N2 selectivity below 260 °C with 1000-5000 ppm NO. Pd2+ ion substituted Ce1–xSnxO2 catalyst can be used for low temperature exhaust applications due to the involvement of the Sn2+/Sn4+ redox couple along with Pd2+/Pd0 and Ce4+/Ce3+ couples. With the goal to understand the improved OSC for Ce1–xSnxO2 solid solution, we have investigated the structure and its relative stability based on first-principles density functional calculations. In chapter 3, we present our studies on the relative stability of Ce1–xSnxO2 solid solution in fluorite in comparison to rutile structure of the other end-member SnO2. Analysis of relative energies of fluorite and rutile phases of CeO2, SnO2, and Ce1–xSnxO2 indicates that fluorite structure is most stable for Ce1–xSnxO2 solid solution. An analysis of local structural distortions reflected in phonon dispersion show that SnO2 in fluorite structure is highly unstable while CeO2 in rutile structure is only weakly unstable. Thus, Sn in Ce1–xSnxO2-fluorite structure is associated with high local structural distortion whereas Ce in Ce1–xSnxO2-rutile structure, if formed, will show only marginal local distortion. Determination of M–O (M = Ce or Sn) bond lengths and analysis of Born effective charges for the optimized structure of Ce1–xSnxO2 show that local coordination of these cations changes from ideal eight-fold coordination expected of Ce4+ ion in fluorite lattice, leading to generation of long and short Ce–O and Sn–O bonds in the doped structure. Bond valence analyses for all ions show the presence of oxygen with bond valence ~1.84. These weakly bonded oxygen ions are relevant for enhanced oxygen storage/release properties observed in Ce1–xSnxO2 solid solution. In chapter 4, we present detailed structural analysis of Ce1–xSnxO2 and Ce1–x– ySnxPdyO2–δsolid solutions based on our DFT calculations supported with EXAFS studies. Both EXAFS analysis and DFT calculation reveal that in the solid solution Ce exhibits 4 + 4 coordination, Sn exhibits 4 + 2 + 2 coordination and Pd has 4 + 3 coordination. While the oxygen in the first four coordination with short M—O bonds are strongly held in the lattice, the oxygens in the second and higher coordinations with long M—O bonds are weakly bound, and they are the activated oxygen in the lattice. Bond valence analysis shows that oxygen with valencies as low as 1.65 are created by the Sn and Pd ion substitution. Another interesting observation is that H2-TPR experiment of Ce1–xSnxO2 shows a broad peak starting from 200 to 500 oC, while the same reduction is achieved in a single step at ~110 oC in presence Pd2+ ion. Substitution of Pd2+ ion thus facilitates synergistic reduction of the catalyst at lower temperature. We have shown that simultaneous reduction of the Ce4+ and Sn4+ ions by Pd0 is the synergistic interaction leading to high oxygen storage capacity at low temperature. In chapter 5, we present the effect of substituting aliovalent Fe3+ ion on OSC and catalytic activity of ceria. Ce0.9Fe0.1O2–δ and Ce0.89Fe0.1Pd0.01O2–δ solid solutions have been synthesized by solution combustion method, which show higher oxygen storage/release property compared to CeO2 and Ce0.8Zr0.2O2. Temperature programmed reduction and XPS study reveal that the presence of Pd ion in Ce0.9Fe0.1O2–δ facilitates complete reduction of Fe3+ to Fe2+ state and partial reduction of Ce4+ to Ce3+ state at temperatures as low as 105 oC compared to 400 oC for monometal-ionic Ce0.9Fe0.1O2–δ. Fe3+ ion is reduced to Fe2 and not to Fe0 due to favorable redox potential for Ce4 + Fe2൅ → Ce3 + Fe3 reaction. Using first-principles density functional theory calculation we determine M—O (M = Pd, Fe, Ce) bond lengths, and find that bond lengths vary from shorter (2.16 Å) to longer (2.9 Å) bond distances compared to mean Ce—O bond distance of 2.34 Åfor CeO2. Using these results in bond valence analysis, we show that oxygen with bond valences as low as –1.55 are created, leading to activation of lattice oxygen in the bimetal ionic catalyst. Temperatures of CO oxidation and NO reduction by CO/H2 are lower with the bimetal ionic Ce0.89Fe0.1Pd0.01O2–δ catalyst compared to monometal-ionic Ce0.9Fe0.1O2–δ and Ce0.99Pd0.01O2–δ catalysts. From XPS studies of Pd impregnated on CeO2 and Fe2O3 oxides, we show that the synergism leading to low temperature activation of lattice oxygen in bimetal-ionic catalyst Ce0.89Fe0.1Pd0.01O2–δ is due to low-temperature reduction of Pd2 to Pd0, followed by Pd0 + 2Fe3൅ → Pd2 +2Fe2, Pd0 + 2Ce4൅ → Pd2 + 2Ce3redox reaction. In chapter 6, we simulate the structure of Ce1–xMxO2–δ (M = transition metal, noble metal and rare–earth ions) for theoretical understanding of origin of OSC in these oxides and to draw a general criteria required to increase the OSC in ceria. The relationship between the OSC and structural changes induced by the dopant ion was investigated by H2-TPR and first-principles based density functional calculations. Transition metal and noble metal ions substitution in ceria greatly enhances the reducibility of Ce1–xMxO2–δ (M = Mn, Fe, Co, Ni, Cu, Pd, Pt, Ru), whereas rare–earth ions substituted Ce1–xAxO2–δ (A = La, Y) have very little effect in improving the OSC. Our simulated optimized structure shows deviation in cation–oxygen bond length from ideal bond length of 2.34 Å (for CeO2). For example, our calculation for Ce28Mn4O62 structure shows that Mn—O bonds are in 4+2 coordination with average bond lengths of 2.0 and 3.06 Å respectively. While the four short Mn–O bond lengths for the calculated structure spans the bond distance region of Mn2O3, and the other two Mn–O bonds are moved to longer distances. The dopant transition and noble metal ions also affects Ce coordination shell and results in the formation of longer Ce—O bonds as well. Thus longer cation-oxygen bond lengths for both dopant and host ions results in enhanced synergistic reduction of the solid solution. With Pd ion substitution in Ce1–xMxO2–δ (M = Mn Fe, Co, Ni, Cu) further enhancement in OSC is observed in H2–TPR. This effect is reflected in our calculations by the presence of still longer bonds compared to the model without Pd ion doping. Synergistic effect is, therefore, due to enhanced reducibility of both dopant and host ion induced due to structural distortion of fluorite lattice in presence of dopant ion. For RE ions (RE = Y, La) our calculations show very little deviation of bonds lengths from ideal fluorite structure. The absence of longer Y— O/La—O and Ce–O bonds make the structure very less susceptible to reduction [8]. Since Pd substituted Ce1–xSnxO2 showed high OSC and catalytic activity towards CO oxidation and NO reduction, we tested this catalyst for water-gas shift (WGS) reaction and the results are presented in chapter 7. Over 99.5 % CO conversion to H2 is observed at 300 ± 25 oC. Based on different characterization techniques we found that the present catalyst is resistant to deactivation due to carbonate formation and sintering of Pt on the surface when subjected to longer duration of reaction conditions. The catalyst does not require any pre-treatment or activation between start-up/shut-down reaction operations. Formation of side products such as methane, methanol, formaldehyde, coke etc. was not observed under the WGS reaction conditions indicating the high selectivity of the catalyst for H2. Temperature programmed reduction of the catalyst in hydrogen (H2–TPR) shows reversible reduction of Ce4+ to Ce3+, Sn4+ to Sn2+ and Pt4+ to Pt0 oxidation state with oxygen storage capacity (OSC) of 3500 μmol g–1 at 80 oC. Such high value of OSC indicates the presence of highly activated lattice oxygen. CO oxidation in presence of stoichiometric O2 shows 100 % conversion to CO2 at room temperature. The catalyst also exhibits 100% selectivity for CO2 at room temperature towards preferential oxidation (PROX) of residual CO in presence of excess hydrogen in the feed. To further validate our DFT results presented in the thesis, DFT calculations on Ce2Zr2O8–Ce2Zr2O7 system were performed and the results are given in the last chapter 8. Ce2Zr2O7 does not show any oxygen storage/release property unlike Ce2Zr2O8 (=Ce0.5Zr0.5O2). Bond lengths obtained from DFT simulation on Ce2Zr2O7 structure showed well-defined Ce—O and Zr—O bonds expected of the pyrochlore structure, unlike distribution of bond lengths as has been observed for Ce1–xMxO2–δ case. Absence of bonds distribution indicates that the oxygen sublattice is not distorted in Ce2Zr2O7 in agreement with its closed packed structure. Filling of the 1/8 of the tetrahedral oxide ion vacancies will result in Ce2Zr2O8 structure, and DFT calculation for this structure show wide distribution of bond lengths. Long Ce—O and Zr—O bonds appear in the bond-distribution plot, suggesting substantial distortion of the oxygen sublattice. Thus absence of longer cation-oxygen bond in pyrochlore structure validates the structural calculations presented in this thesis. Based on the results derived in all the chapters, a critical review of the work is presented and major conclusions are given in the last chapter