II. Structure of Perovskites 1982 A. Crystal Structure 1982 B. Nonstoichiometry in Perovskites 1983 1. Oxygen Nonstoichiometry 1983 2. Cation Nonstoichiometry 1984 C. Physical Properties 1985 D. Adsorption Properties 1986 1. CO and NO Adsorption 1986 2. Oxygen Adsorption 1987 E. Specific Surface and Porosity 1987 F. Thermal Stability in a Reducing Atmosphere 1989 III. Acid−Base and Redox Properties 1990 A. Acidity and Basicity 1990 B. Redox Processes 1991 1. Kinetics and Mechanisms 1992 2. Reduction−Oxidation Cycles 1993 C. Ion Mobility 1993 1. Oxygen Transport 1993 2. Cation Transport 1994 IV. Heterogeneous Catalysis 1995 A. Oxidation Reactions 1995 1. CO Oxidation 1995 2. Oxidation of Hydrocarbons 1996 B. Pollution Abatement 2001 1. NOx Decomposition 2001 2. Exhaust Treatment 2002 3. Stability 2004 C. Hydrogenation and Hydrogenolysis Reactions 2004 1. Hydrogenation of Carbon Oxides 2004 2. Hydrogenation and Hydrogenolysis Reactions 2006

Chemical structures and performance of perovskite oxides.

Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

Perovskite oxides with formula ABO3 or A2BO4 are a very important class of functional materials that exhibit a range of stoichiometries and crystal structures. Because of the structural features, they could accommodate around 90% of the metallic natural elements of the Periodic Table that stand solely or partially at the A and/or B positions without destroying the matrix structure, offering a way of correlating solid state chemistry to catalytic properties. Moreover, their high thermal and hydrothermal stability enable them suitable catalytic materials either for gas or solid reactions carried out at high temperatures, or liquid reactions carried out at low temperatures. In this review, we addressed the preparation, characterization, and application of perovskite oxides in heterogeneous catalysis. Preparation is an important issue in catalysis by which materials with desired textural structure and physicochemical property could be achieved; characterization is the way to explore and understand the textura...

Perovskite Oxides: Preparation, Characterizations, and Applications in Heterogeneous Catalysis

Perovskites as substitutes of noble metals for heterogeneous catalysis: dream or reality.

Increasingly stringent legislation controls emissions from internal combustion engines to the point where alternative power sources for vehicles are necessary. The hydrogen fuel cell is one promising option, but the nature of the gas is such that the conversion of other fuels to hydrogen on board the vehicle is necessary. The conversion of methanol, methane, propane, and octane to hydrogen is reviewed. A combination of oxidation and steam reforming (indirect partial oxidation) or direct partial oxidation are the most promising processes. Indirect partial oxidation involves combustion of part of the fuel to produce sufficient heat to drive the endothermic steam reforming reaction. Direct partial oxidation is favored only at high temperatures and short residence times but is highly selective. However, indirect partial oxidation is shown to be the preferred process for all fuels. The product gases can be taken through a water–gas shift reactor, but still retain ∼2% carbon monoxide, which poisons fuel-cell ca...

Onboard fuel conversion for hydrogen-fuel-cell-driven vehicles

Oxygen-deficient SrFeO3-δ (0.02 < δ < 0.26) perovskites were prepared by soft-chemistry procedures, followed by annealing under different conditions, including high oxygen pressure. These materials contain Fe cations in the mixed Fe3+−Fe4+ valence state. The samples were characterized by X-ray and neutron powder diffraction, thermal analysis under reducing conditions, temperature-programmed reduction and desorption, specific surface area measurements, and XPS. The most deficient oxide, SrFeO2.74, shows a complex neutron diffraction diagram corresponding to a superstructure of perovskite in which half of the Fe cations are pentacoordinated to oxide anions in a pyramidal configuration. By contrast, the other half are octahedrally coordinated. The materials were tested as catalysts for methane oxidation. At moderate temperatures, a significantly higher catalytic activity was observed for the most oxygen-deficient sample, which is consistent with the oxygen desorption ability exhibited by this material, conta...

SrFeO3-δ Perovskite Oxides: Chemical Features and Performance for Methane Combustion

La 1− x Sr x NiO 3 ( x =0.00–0.20) mixed oxides have been prepared and tested in the combustion of methane. X-ray diffraction (XRD) profiles revealed the presence of a single perovskite structure for substitutions 0≤ x ≤0.05, whereas a mixture of the major perovskite phase and minor NiO, SrO and SrCO 3 phases were observed for substitutions x ≥0.10. Temperature-programmed reduction (TPR) profiles showed: (i) a first reduction step at 520–650, due to reduction of Ni 3+ to Ni 2+ , in which the perovskite structure is preserved although distorted by the oxygen vacancies; (ii) an intermediate reduction step at 589–650 K associated to reduction of the segregated NiO phase; (iii) a final step at 720–800 K assigned to the complete reduction of the Ni 2+ -containing phases into La 2 O 3 and Ni o . The partial substitution of Sr by La alters the oxide stoichiometry, yielding a mixture of Ni II /Ni III oxidation states and oxygen vacancies, together with surface enrichment of Sr-containing phases. A close relationship has been found between substitution degree ( x ) and the oxygen non-stoichiometry. The highest intrinsic activity for the CH 4 combustion in sample x =0.10 is associated not only with the largest proportion of oxygen non-stoichiometry but also with the lowest Sr segregation on the surface.

Role of bulk and surface structures of La1−xSrxNiO3 perovskite-type oxides in methane combustion

Polycrystalline LaCuO3−δ (δ=0.45, 0.35, 0.17 and 0.05) powders have been prepared by a citrate technique followed by annealing under 200 bar O2 pressure at temperatures 1073–1273 K. The crystal structure of three selected samples was refined by the Rietveld method from room temperature neutron powder diffraction data. Thermogravimetric analysis of the samples in a H2/N2 flow allowed the determination of the oxygen nonstoichiometry. These TG profiles showed two clear reduction processes: the first one corresponds to a partial reduction to give intermediate perovskites and in the second step the sample is completely reduced to Cu0 and La2O3. However, surface analysis of a representative sample (LaCuO2.55) revealed that at intermediate reduction temperatures, a fraction of copper is already reduced to metallic state while the other fraction still contains CuIII stabilized in the lattice. These samples were tested in the catalytic oxidation of CO under flow condition, and the intrinsic activity was found to depend strongly on the oxygen nonstoichiometry. The increasing structural instability observed in LaCuO3−δ phases as δ increases appears to be responsible for the enhancement of the catalytic properties of this perovskite based system.

Horacio Falcon

Papers

SrFeO3-δ Perovskite Oxides: Chemical Features and Performance for Methane Combustion

Role of bulk and surface structures of La1−xSrxNiO3 perovskite-type oxides in methane combustion

Defect LaCuO3−δ (δ=0.05−0.45) perovskites

An Oxygen‐Deficient Perovskite as Selective Catalyst in the Oxidation of Alkyl Benzenes

Vanadium and titanium oxide supported on mesoporous CMK-3 as new catalysts for oxidative desulfurization