The Effect of Chelating Agent on the Catalytic and Structural Properties of Sm2Zr2O7 as a Methane Combustion Catalyst

The Sm₂Zr₂O₇ of pyrochlore structure was studied as a catalyst for high temperature combustion. It was prepared by the sol-gel method with and without acetylacetone as a chelating agent. The order of crystallinity and the surface area were improved when acetylacetone was used. The catalytic activity of CH₄ combustion was also enhanced and superior to that of Mn-substituted hexaaluminate above 550 °C.     

Low temperature complete combustion of methane over titania-modified alumina-supported palladium

Effects of titania on the catalytic property of Pd/Al₂O₃ towards methane combustion were examined. The results revealed that the catalytic activity of the Pd/Al₂O₃ catalyst was considerably improved by pre-coating the alumina support with titania at low temperature (below 700 °C). Hydrogen chemisorption and BET measurements revealed that the titania-modified alumina supports could modify the support characterization to achieve a high dispersion of palladium. Temperature-programmed reduction and temperature-programmed desorption study further demonstrated that the coating of Pd/Al₂O₃ catalysts with titania can weaken the bond strength of Pd-O and enhance their catalytic activity towards methane combustion at lower temperature.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Catalytic Combustion of Volatile Organic Compounds on Supported Precious Metal Catalysts

Catalytic combustion of volatile organic compounds (VOCs) was investigated on supported precious metal catalysts. The activities for the combustion of methane and acetaldehyde were closely related to the reducibility of the precious metal oxides of the catalysts. On the other hand, light-off temperatures for toluene combustion on PdO/Al₂O₃, PdO/SnO₂, and PdO/CeO₂ were around 200 °C, although PdO/ZrO₂ showed a higher temperature of 240 °C. This result indicated that light-off temperatures depend on not only the catalytic activities but also the catalyst structure because of low concentration of toluene and weak interaction between catalysts and toluene. In this experiment, the PdO/SnO₂ catalyst showed highest activity for the combustion of methane and VOCs.     

Synthesis and characterization of pyrochlore lanthanide (Pr,Sm) zirconate ceramics

Three different lanthanide zirconate powders: Pr₂Zr₂O₇, Sm₂Zr₂O₇ and PrSmZr₂O₇ were prepared by combustion synthesis. The synthesis initially yielded amorphous powders, which crystallized after subsequent thermal treatment. Well-crystallized compounds were formed after calcination at temperature as low as 950 °C. Effect of the thermal treatment on the phase evolution was studied by X-ray powder diffraction (XRD). The powders calcined at the highest temperature (1550 °C) showed that all compositions possess the pyrochlore structure with the space group No. 227. The obtained powders were compacted and pressureless sintered without additive at 1600 °C for 4 h in the air. Microstructure development was examined by field emission scanning electron microscopy, as well as by transmission electron microscopy. It was found that the lowest value for thermal conductivity, 1.2 W/m K, was obtained for mixed lanthanide composition with pyrochlore structure (PrSmZr₂O₇). The effect of chemical composition on micro-hardness and thermal conductivity of the obtained zirconates was studied.     

Facile one-pot synthesis of porous Ln2Ti2O7 (Ln = Nd,Gd, Er) with photocatalytic degradation performance for methyl orange

Porous nanocrystalline Ln₂Ti₂O₇ (Ln = Nd, Gd, Er) was prepared by a facile self-propagating combustion method using metal nitrates (Ln(NO₃)₃ (Ln = Nd, Gd, Er), TiO(NO₃)₂) and glycine. The photocatalytic activity of Ln₂Ti₂O₇ (Ln = Nd, Gd, Er) was evaluated by the photocatalytic degradation of methyl orange (MO). The results showed that the photocatalytic activity of Ln₂Ti₂O₇ (Ln = Gd, Er) with cubic pyrochlore structure was higher than that of Nd₂Ti₂O₇ with monoclinic perovskite structure and Gd₂Ti₂O₇ showed the best photocatalytic activity. The different photocatalytic activities observed for Ln₂Ti₂O₇ (Ln = Nd, Gd, Er) could be related to its different crystal structures and Ln 4f shells.     

Aqueous Chemical Synthesis of Ln2Sn2O7 Pyrochlore‐Structured Ceramics

Pyrochlore‐structured lanthanide stannate ceramic (Ln₂Sn₂O₇) has been synthesized via a new complex precipitation method. A suite of characterization techniques, including FTIR, Raman, X‐ray, and electron diffraction as well as nitrogen sorption were employed to investigate the structural evolution of the synthesized and calcined powder. Raman, XRD, and selected area electron diffraction results confirm the presence of the pyrochlore structure after calcination of the powder above 1200°C. TEM imaging shows fine crystallites gradually increased in size from approximately 100 nm to about 500 nm with higher calcination temperatures. Grain growth and powder densification upon increasing the calcination temperature was confirmed by nitrogen sorption results. This aqueous synthetic method provides a simple pathway for the preparation of homogeneous lanthanide stannate ceramics.     

Catalytic combustion of methane over mixed oxides derived from Co-Mg/Al ternary hydrotalcites

Co_xMg_3 − x /Al composite oxides (xCoMAO-800) were prepared by calcination of Co_xMg_3 − x/Al hydrotalcites (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively) at 800 °C. The materials were characterized using XRD, TG-DSC, N₂ adsorption-desorption and TPR. The methane catalytic combustion over the xCoMAO-800 was assessed in a fixed bed micro-reactor. The results revealed that cobalt can be homogenously dispersed into the matrices of the hydrotalcites and determines the structure, specific surface areas and porosity of the derived xCoMAO-800 oxide catalysts. The thermal stability and homogeneity of the hydrotalcites markedly depends on the cobalt concentration in the hydrotalcites. The Co-based hydrotalcite-derived oxides exhibit good activity in the catalytic combustion of methane. The catalytic activity over the xCoMAO-800 oxides enhances with increasing x up to 1.5, but subsequently decreases dramatically as cobalt loadings are further increased. The 1.5CoMAO-800 catalyst shows the best methane combustion activity, igniting methane at 450 °C and completing methane combustion around 600 °C. The catalytic combustion activity over the xCoMAO-800 oxides are closely related to the strong Co-Mg/Al interaction within the mixed oxides according to the TG-DSC, TPR and activity characteristics.     

Glycine–nitrate synthesis of Sr doped La2Zr2O7 pyrochlore powder

Abstract

Abstract

Materials with A₂B₂O₇ (pyrochlore) structure have received significant attention for their applications as new protonic conductors and materials used in electronic devices. One of the unique synthesis routes for La₂Zr₂O₇ (pyrochlore) powders is the glycine–nitrate combustion method, which shows superior properties of the synthesised powder using glycine as a complexing agent. The Sr doped La₂Zr₂O₇ powders in pure pyrochlore structure were produced using this approach. Selected characteristics of the synthesised powders, such as crystal structure, lattice parameters, crystallite size, the vibrational properties, the morphology of the particles, along with the specific surface area and particle size, have been investigated. The dependence of some properties on annealing temperatures of the powders has been studied.     

Low Thermal Conductivity of Rare‐Earth Zirconate‐Stannate Solid Solutions (Yb2Zr2O7)1−x(Ln2Sn2O7)x (Ln = Nd,Sm)

Dense monoliths of rare‐earth zirconate‐stannate solid solutions (Yb₂Zr₂O₇)_1?x(Ln₂Sn₂O₇)_x (Ln = Nd, Sm) were prepared by solid‐state reaction. Characterized by XRD, Raman, SEM, and TEM, a double‐phase structure of Yb₂Zr₂O₇‐rich fluorite and Nd₂Sn₂O₇‐rich pyrochlore was observed in the specimens of x = 0.4 and 0.5 of (Yb₂Zr₂O₇)_1?x(Nd₂Sn₂O₇)_x series while complete solid solutions were formed within the whole composition range of (Yb₂Zr₂O₇)_1?x(Sm₂Sn₂O₇)_x series. Except for the defect phonon scattering, lattice softening caused by order–disorder phase transformation between pyrochlore and fluorite structures also plays an important role in minimizing the thermal conductivity. Low thermal conductivity with positive temperature dependence is achieved in both the series. Considering the structure stability and low thermal conductivity, rare‐earth zirconate‐stannate solid solutions may be promising materials for thermal insulating applications, such as thermal barrier coatings.     

Effect of support on the conversion of methane to synthesis gas over supported iridium catalysts

A partial oxidation of methane was carried out using iridium catalysts supported on several metal oxides. The productivity of the synthesis gas from methane was strongly affected by the choice of support oxides for the catalysts. The synthesis gas production proceeded basically via a two-step reaction consisting of methane combustion to give H₂O and CO₂, followed by the reforming of methane from CO₂ and steam. Although the combustion and the reforming of methane from steam did not depend upon the catalyst support, a large variation in the catalytic activity for the reforming of methane from CO₂ was observed over Ir catalysts with different supports. The support activity order in the reforming of methane from CO₂ with iridium catalysts was as follows: TiO₂≧ZrO₂≧Y₂O₃>La₂O₃>MgO≧Al₂O₃>SiO₂. The same order was observed in the synthesis gas production from the partial oxidation of methane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phase equilibria and electrical properties of pyrochlore and zirconolite phases in the Bi2O3-ZnO-Ta2O5 system

The complete subsolidus phase diagram of the system Bi₂O₃-ZnO-Ta₂O₅, including cubic pyrochlore and monoclinic zirconolite phases, has been determined at 950-1050 °C. Through systematic heat treatment and X-ray diffraction of over 100 compositions, the layout of compatibility triangles (both 2-phase and 3-phase) and single phase solid solution areas has been determined. Pyrochlore and zirconolite phases have ideal nominal compositions Bi_1.5Zn_1.0Ta_1.5O₇ and Bi₂(Zn_1/3Ta_2/3)₂O₇ respectively, but both form solid solution areas. The sintering condition of pyrochlore pellets has been optimised to obtain high density ceramics with minimal weight loss: optimised condition is 1100 °C for pellets covered with sacrificial powder. Permittivity, ?′ dielectric loss and temperature coefficient of capacitance, TCC, of single phase materials were measured using impedance spectroscopy; ?′ and TCC show little variation with composition but the losses are higher for Zn-deficient compositions.     

Catalytic Oxidation of CO Over Synthesized Nickel Ferrite Nanoparticles from Fly Ash

Catalytic oxidation of carbon monoxide (CO) gas over nanosized nickel ferrites prepared from fly ash has been investigated. X-ray diffraction analyses showed that pure crystalline nickel ferrite, NiFe₂O₄, phase can be obtained by thermal treatment of the precursors at temperature >800 °C for 120 min in the studied pH range, from 7 (neutral) to 12 (highly alkaline). In the temperature range 500 ≤ T ≤ 800 °C, impure low crystalline NiFe₂O₄ phase formed. The main impurities are FeO (OH) and Fe₂O₃ · H₂O phases. Higher magnetization (32 emu/g) is obtained for a precursor precipitated at pH 10 and thermally treated at 1,200 °C for 120 min. The catalytic oxidation of CO over nanocrystalline NiFe₂O₄ powders was studied using quadrupole mass gas analyzer system. The main parameters as crystal size, surface area and firing temperature are used to clarify the efficiency of using NiFe₂O₄ powders in catalytic oxidation of CO. It was found that the efficiency of catalytic oxidation decreased by increasing firing temperature and crystallite size of the samples. The lower crystal size (2–8.5 nm), the higher surface area (25–55 m²/g) and the presence of impurities FeO(OH) phase enhanced CO adsorption and consequently its oxidation.     

Axial Changes of Catalyst Structure and Temperature in a Fixed-Bed Microreactor During Noble Metal Catalysed Partial Oxidation of Methane

The catalytic partial oxidation of methane (CPO) over flame-made 2.5%Rh–2.5%Pt/Al₂O₃ and 2.5%Rh/Al₂O₃ in 6%CH₄/3%O₂/He shows the potential of in situ studies using miniaturized fixed-bed reactors, the importance of spatially resolved studies and its combination with infrared thermography and on-line mass spectrometry. This experimental strategy allowed collecting data on the structure of the noble metal (oxidation state) and the temperature along the catalyst bed. The reaction was investigated in a fixed-bed quartz microreactor (1–1.5 mm diameter) following the catalytic performance by on-line gas mass spectrometry (MS). Above the ignition temperature of the catalytic partial oxidation of methane (310–330 °C), a zone with oxidized noble metals was observed in the inlet region of the catalyst bed, accompanied by a characteristic hot spot (over-temperature up to 150 °C), while reduced noble metal species became dominant towards the outlet of the bed. The position of both the gradient in oxidation state and the hot spot were strongly dependent on the furnace temperature and the gas flow (residence time). Heating as well as a higher flow rate caused a migration of the transition zone of the oxidation state/maximum in temperature towards the inlet. At the same time the hydrogen concentration in the reactor effluent increased. In contrast, at low temperatures a movement of the transition zone towards the outlet was observed at increasing flux, except if the self-heating by the exothermic methane oxidation was too strong. The results indicate that in the oxidized zone mainly combustion of methane occurs, whereas in the reduced part direct partial oxidation and reforming reactions prevail. The results demonstrate how spatially resolved spectroscopy can help in understanding catalytic reactions involving different reaction zones and gradients even in micro scale fixed-bed reactors. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of hydrogen addition on methane combustion

In this study, the effect of hydrogen on methane combustion characteristic was tested. The ignition temperature (T₁₀) and burn off temperature (T₉₀) was carried out in a quartz reactor at atmospheric pressure with the mixture flow rate of 800 mL/min. The compositions of outlet gas were measured online by Gasmet DX4000 FTIR gas analyzer. The results showed that hydrogen enhanced the activity of methane. For all methane concentration range, the T₁₀ of methane could decrease 50 °C–70 °C with the H₂/CH₄ mole ratio at 0.1. For 1 vol.% methane combustion, when the H₂/CH₄ was equal to 0.05, the T₁₀ and T₉₀ could decrease 45 °C and 42 °C, respectively. When the H₂/CH₄ was 2.5, the T₁₀ and T₉₀ could decrease about 170 °C and 180 °C, respectively. Further more, CO generated in a wider temperature range when the hydrogen was added.     

Direct Formation and Luminescence of Nanocrystals in the System Eu2Sn2O7–Gd2Sn2O7 Complete Solid Solutions

Nanocrystals with orange‐reddish luminescence based on the pyrochlore‐type complete solid solutions with cube‐like morphology in the Eu₂Sn₂O₇–Gd₂Sn₂O₇ system were directly formed from the precursor solutions of SnCl₄, GdCl₃, and EuCl₃ under weakly basic hydrothermal conditions at temperatures higher than 180°C for 5 h. The crystallite of Gd₂Sn₂O₇ pyrochlore gradually grew from 10 to 37 nm as the hydrothermal treatment temperature rose from 180°C to 240°C. The lattice parameter of cubic phase linearly increased with increased europium concentration according to the Vegard's law. The characteristic orange‐reddish photoluminescence spectra of Gd₂Sn₂O₇:Eu³⁺ cubelike nanocrystals with crystallite size from 34 to 37 nm that were formed at 240°C for 5 h were attributed to the most sharp orange (586 nm) luminescence with high intensity and quite broad red (610–630 nm) emission with weak intensity, according to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺, respectively. At a composition of (Eu_0.09Gd_0.91)₂Sn₂O₇, the intensity of orange emission reached the maximum. The Red‐to‐Orange (⁵D₀→⁷F₂/⁵D₀→⁷F₁) (R/O) emission intensity ratio was in the low range from 0.10 to 0.14, which was a characteristic of Gd₂Sn₂O₇:Eu³⁺.     

Catalytic test of supported Ni catalysts with core/shell structure for dry reforming of methane

Supported nickel catalysts with core/shell structures of Ni/Al₂O₃ and Ni/MgO-Al₂O₃ were synthesized under multi-bubble sonoluminescence (MBSL) conditions and tested for dry reforming of methane (DRM) to produce hydrogen and carbon monoxide. A supported Ni catalyst made of 10% Ni loading on Al₂O₃ and MgO-Al₂O₃, which performed best in the steam reforming of methane (97% methane conversion at 750 °C) and in the partial oxidation of methane (96% methane conversion at 800 °C), showed also good performance in DRM and excellent thermal stability for the first 150 h. The supported Ni catalysts Ni/Al₂O₃ and Ni/MgO-Al₂O₃ yielded methane conversions of 92% and 92.5%, respectively and CO₂ conversions of 95.0% and 91.8%, respectively, at a reaction temperature of 800 °C with a molar ratio of CH₄/CO₂ = 1. Those were near thermodynamic equilibrium values.     

Ultra-rapid synthesis of syngas by the catalytic reforming of methane enhanced byin-situ heat supply through combustion

Development in highly active catalysts for the reforming of methane with CO₂ and partial oxidation of methane was conducted to produce hydrogen and carbon monoxide with high reaction rates. An Ni-based four-components catalyst, Ni-Ce₂O₃-Pt-Rh, supported on an alumina wash-coated ceramic fiber in a plate shape was suitable for the objective reaction. By combining the catalytic combustion of ethane or propane, methane conversion was markedly enhanced, and a high space-time yield of syngas, 25,000 mol/l·h was obtained at a catalyst temperature of 700 ‡C or furnace temperature of 500 ‡C. The extraordinary high space-time yield of syngas was also confirmed even under the very rapid flow rate conditions as a contact time of 3 m-sec by using a monolithic shape of catalyst bed without back pressure.     

Conversions of Methane to Synthesis Gas over Co/γ-Al2O3 by CO2 and/or O2

The goal of the paper was to investigate the effect of the catalyst precursor on the catalytic activity. For this reason, the structure, the reducibility and the reaction behavior of -Al₂O₃-supported Co (24 wt%) catalysts as a function of calcination temperature (T _c) were investigated using X-ray diffraction, temperature-programmed reduction, CO chemisorption, pulse reaction with pure CH₄, and the catalytic reactions of methane conversion to synthesis gas. Depending on T _c, one, two, or three of the following Co-containing compounds, Co₃O₄, Co₂AlO₄, and CoAl₂O₄, were identified. Their reducibility decreased in the sequence: Co₃O₄>Co₂AlO₄>CoAl₂O₄. Co₃O₄ was generated as a major phase at a T _c of 500°C and Co₂AlO₄ and CoAl₂O₄ at a T _c of 1000°C. The reduced Co/-Al₂O₃ catalysts, obtained via the reduction of the 500 and 1000°C calcined catalysts, provided high and stable activities for the partial oxidation of methane and the combined partial oxidation and CO₂ reforming of methane. They deactivated, however, rapidly in the CO₂ reforming of methane. Possible explanations for the stability are provided.     

Promotional effect of Ca on the Pd/Ce–Zr/Al2O3 catalyst for low-temperature catalytic combustion of methane

Promotional effect of Ca on the catalytic property of Pd/Ce–Zr/Al₂O₃ catalyst towards methane combustion is examined. The surface properties and the oxidation/reduction behavior of these catalysts are investigated by BET, TEM, XPS, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that addition of Ca to Pd/Ce–Zr/Al₂O₃ can promote remarkably its low-temperature activity. The thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst to the exposure at high temperature is also enhanced by Ca loading. XPS and TEM results show that the addition of Ca to Pd/Ce–Zr/Al₂O₃ catalyst generates well-dispersed PdO particles on support. H₂–TPR, O₂–TPO and CH₄/O₂–TPSR experiments show that the addition of Ca improves the reduction/reoxidation properties and thermal stability of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.