Catalytic Activity For Methane Combustion Research Articles

Synthesis, structure and catalytic properties of Fe-substituted barium hexaaluminates

A sol–gel method using Ba and Al isopropylates and iron nitrate has been used to synthesise barium hexaaluminate partially substituted with iron. After calcination under oxygen at 1200°C the β-alumina structure was obtained. Formation of the mixed BaFexAl12-xO19 phase occurred for x=1–4. XRD measurements showed a good crystallinity of the structure and expansion of unit cell parameters due to the presence of larger Fe3+ ions substituting Al3+ ones in octahedral sites only. Mossbauer spectroscopy revealed that Fe3+ ions are present in four different octahedral sites slightly distorted. Catalytic activity in methane combustion showed that an optimum was obtained for solid containing 2 Fe ions per unit cell: the increase of the amount of introduced iron was counterbalanced by the decrease of specific surface area. Intrinsic activities have been calculated for the four solids in both the fresh and aged states. It is observed that increasing iron content increases relative activities in the same ratio as the populations of iron located in two sites as deduced from Mossbauer spectroscopy. It is then tentatively assumed that activity is attributed to octahedrally coordinated Fe3+ ions in some specific sites.

Methane combustion over B-site partially substituted perovskite-type LaFeO 3 prepared by sol-gel method

Ultrafine perovskite-type complex oxides LaFe 1−yA yO 3−λ (A = Mn, Al, Co) were prepared by sol-gel method using citric acid as complex agent, and characterized by XRD, TPR, TG and XPS techniques. The catalytic activities for methane combustion were tested and compared with that of the large particle oxides having the same composition. It was shown that the catalytic activities of ultrafine LaFe 1−yA yO 3−λ complex oxides were much higher than that of LaFeO 3 sample due to the increase of high valence B-site cations and lattice oxygen content by the substitution of B-site elements. Much higher catalytic activities were also observed for the ultrafine perovskite-type complex oxides compared to their large particle counterparts.

Catalytic combustion of methane over barium hexaferrites

Barium hexaferrite BaFe12O19 and iridium-containing barium hexaferrites have been prepared by the citrates gel method. Their catalytic activity in methane combustion has been evaluated. BaFe12O19 is an efficient catalyst for this reaction, and the introduction of iridium in the hexaferrite structure does not improve this activity. Mossbauer spectroscopy suggests that a part of the iridium ions are incorporated in the hexaferrite structure, however in crystallographic sites where they cannot interact with the gas phase. Infrared study of CO adsorption reveals the presence of two types of iridium particles in the surface: small Ir particles, in strong interaction with the hexaferrite structure, and some larger Ir particles which were not incorporated into the lattice.

Changes of Crystalline Phase and Catalytic Properties by Cation Substitution in Mirror Plane of Hexaaluminate Compounds

Mn-substituted hexaaluminate compounds have been investigated as catalysts for high temperature combustion above 1000°C. In this study, changes of crystalline phase and catalytic properties of Sr1−xLnxMnAl11O19−α(Ln= Pr, Nd, Sm, Gd) were investigated. Two kinds of hexaaluminate phases existed as equilibrium phases in the unsubstituted sample, theLnions were preferentially soluble in one of these phases. Accompanied with the dissolution, unequilibrium LnAlO3phase was sometimes formed for highxsamples. The amount of this phase decreases with an increase in temperature and/or ionic radius ofLn. The surface area of the samples generally decreased with increasing extent ofLnsubstitution. Catalytic activity for methane combustion at low conversion level was improved by the substitution.

Structure and Catalytic Property of Coherent Spinel Surface Layers on Hexaaluminate Microcrystals

Formation of coherent surface layers of Mn3O4 on hexaaluminate microcrystals has been studied as a structural modification to enhance the catalytic activity for methane combustion. The spinel oxide surface layer was successfully produced on planar microcrystals of hexaaluminate by employing the air oxidation of an aqueous Mn(II) solution. As evident from TEM, XPS, and CO2 chemisorption measurements, as-prepared surface layers completely covered the basal plane of planar microcrystals. This characteristic structure appears to be a result of liquid phase epitaxy at the structurally coherent interface between hexaaluminate and spinel. A completely different structure was observed for the samples from the conventional impregnation method, which contain insular Mn3O4 particles dispersed on hexaaluminate. The catalytic activity for methane combustion was evaluated as a function of the Mn3O4 loading and the metal composition of the spinel surface layer. It was revealed that the higher specific activity was observed over air-oxidation-derived samples than over the impregnated samples. Partial substitution of Fe for Mn on the surface layer was effective in enhancing the combustion activity as a result of promoting the reduction/oxidation property.

Catalytic and Thermal Behavior of Cerium Oxide Supported onSiO2 and Al2O3 for Methane Combustion

Abstract CeO2/SiO2 and CeO2/Al2O3 catalysts oxidized at 1173 K in O2 stream were employed to investigate the catalytic activity for methane combustion and the behavior of CeO2 on supports by the oxidation at 1173 K. CeO2/SiO2 catalyst gave a poor catalytic activity by comparison with that of CeO2/Al2O3 catalyst. The crystallite size of CeO2 of the silica catalyst was found to be 1.5 times larger. In addition, Auger Electron Spectroscopy (AES) analysis after the oxidation at 1173 K proved that cerium species existed on and within the silica support. We conclude that the low surface density of CeO2 might contribute to the poor activity of CeO2/SiO2 catalyst for methane combustion.

ChemInform Abstract: Influence of the A‐Site Cation in ACoO3 (A: La, Pr, Nd, and Gd) Perovskite‐Type Oxides on Catalytic Activity for Methane Combustion.

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Influence of the A-site cation in ACoO 3 ( A = La, Pr, Nd, andGd) perovskite-type oxides on catalytic activity for methane combustion

The effect of rare-earth ions (La, Pr, Nd, and Gd) in ACoO 3 perovskites on the thermal behavior and on the catalytic activity for methane oxidation has been studied. The thermal reduction, as investigated by oxygen desorption analysis, revealed for all perovskites one evolution step occurring above 1100 K. Transmission electron micrographs at the beginning of the desorption step revealed the presence of superstructures, which are attributed to the formation of oxygen-deficient domains ( ACoO 2.5 ≤ ACoO 3- x ≤ ACoO 3) adopting perovskite-related brownmillerite-type structures. A model for the sequence of the structural changes of the perovskites during the reduction is proposed. The amount of oxygen which evolved above 1100 K increased with decreasing size of the lanthanide ion. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 830 K, expressed as reaction rates referred to the BET surface area, increased in the order PrCoO 3 ⪡ LaCoO 3 < NdCoO 3 < GdCoO 3. However, no significant influence of the A-site cations on the catalytic activity was found, with exception of praseodymium. PrCoO 3 showed a 30 times lower activity than the other samples. The catalytic activity of ACoO 3 perovskites can be markedly affected by the presence of Co 3O 4 impurity in the perovskites.

Influence of the A-site cation in AMnO3+x and AFeO3+x (A = La, Pr, Nd and Gd) perovskite-type oxides on the catalytic activity for methane combustion

The effect of rare-earth ions (La, Pr, Nd and Gd) in AMnO3+x and AFeO3+x perovskites on the thermal behavior and on the catalytic activity for the deep oxidation of methane has been studied. AMnO3+x perovskites showed after preparation an oxidative non-stoichiometry. Oxygen desorption analysis revealed for the four manganites different desorption steps occurring between 930 and 1370 K. Stoichiometry was reached after the first desorption step. Heating the samples at temperatures above 1300 K resulted in phase segregation to the simple oxides. AFeO3+x perovskites were more stable towards thermal decomposition than the Mn-perovskites, showing no oxygen evolution up to 1400 K. The reducibility of these perovskites in hydrogen correlated inversely with the relative effective ionic radii of the trivalent rare-earth cations. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 770 K, expressed as reaction rates referred to the BET surface area, varied between 1.4 × 10−7 and 2.9 × 10−7 mol s−1 m−7 for the AMnO3+x, and between 1.1 × 10−7 and 1.6 × 10−7 mols−1m−2for the AFeO3+x perovskites.

β- and β″-Gallate, Gallium Analogues of β- and β″-Alumina, as Catalysts for High Temperature Combustion of Methane

Abstract β- and β″-Gallate, gallium analogues of β- and β″-alumina, were tested for their catalytic activity for methane combustion. In spite of small surface areas, K- and Rb-β″-gallate gave promising results.

ChemInform Abstract: Catalytic Properties of BaMAl11O19‐α (M: Cr, Mn, Fe, Co, and Ni) for High‐Temperature Catalytic Combustion.

AbstractThe cation‐substituted barium hexaaluminates of title exhibit high sintering resistance and retain a surface area of more than 10 m2/g even after annealing at 1300 °C. The Mn‐substituted system BaMnxAl12‐xO19‐α shows the highest catalytic activity for methane combustion.