Catalytic CO2 reforming of methane over Ir/Ce0.9Gd0.1O2−x

CO₂ reforming of methane over Ir loaded Ce_0.9Gd_0.1O_2−x (Ir/CGO) has been studied between 600 and 800 °C and for CH₄/CO₂ ratios between 2 and 0.66 in order to evaluate its potential use as an anode material for direct conversion of biogas at moderate temperatures in solid oxide fuel cells. The catalyst exhibited a superior catalytic activity compared to the support alone and other Ir based catalysts. High CH₄/CO₂ ratios and temperatures were required to obtain the maximum H₂/CO ratio, which could never exceed unity. Long-term experiments were carried out, showing the excellent stability of the catalyst with time on stream. Carbon formation was totally inhibited (in most experimental conditions) or very limited in the most severe conditions of the study (800 °C, CH₄/CO₂ = 2). This carbon was found to be highly reactive towards O₂ upon TPO experiments.     

An Investigation of Catalytic Activity for CO Oxidation of CuO/Ce x Zr1–x O2 Catalysts

A series of CuO/Ce _x Zr_1–x O₂ catalyst powders with different Ce/Zr ratio were prepared via an impregnation method and characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman), H₂-Temperature-programmed reduction (TPR) and X-ray photoelectron spectra techniques. The catalytic properties of the catalysts were evaluated by means of a microreactor-GC system. XRD results showed that the addition of CuO had no effect on the crystalline lattice of the support. The structures of the Ce _x Zr_1–x O₂ samples were confirmed by XRD analyses and FT-Raman results. The H₂-TPR profiles for these catalysts had three peaks, which could be attributed to the reduction of three kinds of CuO species, i.e., the highly dispersed CuO, the larger CuO species and the bulk CuO. The TPR analyses and catalytic property tests indicated that the Ce/Zr ratio of CuO/Ce _x Zr_1–x O₂ had an effect on the dispersion degree of CuO and the catalytic activity of the catalysts.     

Surfactant-Controlled and Microwave-Assisted Synthesis of Highly Active Ce x Zr1−x O2 Nano-Oxides for CO Oxidation

Uniform nanosized, mesoporous and high specific surface area ceria–zirconia (1:1 mole ratio) solid solutions were synthesized employing a poly-block copolymer surfactant combined with microwave or thermal treatment. For comparison purpose an identical composition Ce _x Zr_1?x O₂ mixed oxide was also prepared by a conventional coprecipitation method. The surface and bulk structure of the synthesized samples were investigated using X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area and BJH pore size distribution (PSD) methods. The catalytic activity was evaluated for CO oxidation at normal atmospheric pressure. The Ce _x Zr_1?x O₂ solid solutions obtained through surfactant use exhibited high specific surface area and mesoporosity. The XRD measurements revealed the presence of cubic Ce_0.75Zr_0.25O₂ and Ce_0.6Zr_0.4O₂ phases in the case of conventional coprecipitation and surfactant controlled synthesized samples, respectively. The Raman measurements revealed existence of more oxygen defects in the surfactant-controlled and microwave treated sample. The SEM images suggested that all samples consist of typical spherical agglomerates with almost uniform size within the nanometer size range. The TEM measurements revealed nanosized crystallites in a narrow range between 4 and 8 nm, and densely packed in the case of conventional precipitation and microwave treated samples. Interestingly, the Ce _x Zr_1?x O₂ solid solution obtained by surfactant-controlled method and treated with microwave radiation exhibited better CO oxidation activity than other samples. Enhanced activity of surfactant-controlled and microwave treated sample is correlated with its unique physicochemical characteristics.     

Carbon Monoxide Oxidation over Au/Ce1−x Zr x O2 Catalysts: Effects of Moisture Content in the Reactant Gas and Catalyst Pretreatment

The Au/Ce_1?x Zr _x O₂ (x = 0, 0.25, 1) catalysts were synthesized, characterized by BET, XRD, TPR-H₂, HRTEM, AAS and tested in CO oxidation. The effect of moisture in the reactant gas on CO conversion has been studied in a wide range of concentrations (~0.7–6000 ppm). Moisture generates a positive effect on catalytic activity and wet conditions gave higher CO conversions. The optimum concentration of moisture for CO oxidation over Au/CeO₂ and Au/Ce_0.75Zr_0.25O₂ is 200–1000 ppm, while further increase in the moisture content suppresses CO conversion. The activity of the studied Au catalysts depends on the amount of moisture adsorbed on the catalyst rather than on its content in the feed stream, which suggests that the reaction involves water-derived species on the catalysts surface. The effect of the catalysts pretreatment in air, dry He, H₂ stream as well as H₂ + H₂O gas mixture on their catalytic performance in CO oxidation has been also investigated. The model of the active sites for CO oxidation over the studied catalysts was proposed.     

Preparation and electrochemical characterization of Ti/Ru x Mn1−x O2 electrodes

DSA^® type electrodes of ruthenium–manganese mixed oxides (30 at % Ru < 100) supported on titanium were prepared by the spray-pyrolysis technique, using Ru(NO)(OH) _x (NO₃)_3–x and Mn(NO₃)₂ as precursors. Electrodes were characterized by SEM, XRD and cyclic voltammetry. Their behaviour as anode for the chlorine and oxygen evolution reactions was also evaluated by polarization curves. The stability of the mixed oxides was determined through accelerated tests of service life. It has been verified that the best performance on the apparent electrocatalytic activity of both reactions as well as on stability is achieved at a composition of about 70% Ru.     

NiMn x Fe2 − x O4 ferrites: Combustion synthesis and characterization

Ferrites NiMn _x Fe_{2 ? x} O₄ (x = 0.0, 0.2, 0.4, and 0.6) were synthesized by SHS and hot sintering and comparatively characterized by ESM, EDS, and XRD; and a tentative mechanism of SHS reaction was suggested.     

State of manganese atoms in the Nd1+x Sr2−x Mn2O7−δ solid solution

The oxidation state of manganese in the Nd_2?x Sr_1+x Mn₂O_7?δ solid solution was determined by X-ray photoelectron spectroscopy and by calculating the oxygen nonstoichiometry based on the gravimetric data. As a result of the heterovalent replacement of Nd³⁺ with Sr²⁺, the change in the oxidation state of manganese occurs in different ways, i.e., it increases at x > 0 and decreases at x < 0. In the latter case, some oxygen ions acquire the oxidation state of ?1. The samples slowly cooled under oxidative conditions possess a significant positive oxygen non-stoichiometry, which tends to decrease after Nd³⁺ is replaced with Sr²⁺. An excess of oxygen stabilizes the crystal structure of Nd_1+x Sr_2?x Mn₂O_7?δ.     

Magnetic and Optical Properties of Mn1−xZnxFe2O4 Nanoparticles

Mn_1?xZn_xFe₂O₄ (x = 0.0?1.0) NPs (MZF NPs) were synthesized by a citric acid assisted sol–gel process. MZF NPs show superparamagnetic characteristics at room temperature. Saturation magnetization (M_s) of MnFe₂O₄ NPs is 70.52 emu/g is very close to the bulk saturation magnetization value of 80 emu/g. The observed M_s = 35.90 emu/g value for ZnFe₂O₄ particles is much greater than the bulk M_s value of 5 emu/g. This case is attributed to cation distribution change from normal spinel to mixed structure. The small M_r/M_s ratios (the maximum 0.147) specify uniaxial anisotropy in the Mn_1-xZn_xFe₂O₄ NPs. The average crystallite diameter (D _mag) was evaluated from magnetic analyses. The obtained D _mag values are between 27.67 and 33.60 nm and this range is in great accordance with the results calculated from XRD measurements. Among the NPs, the samples with more zinc content show higher diffuse reflectance. The optical direct band gap of MZF NPs is found to decrease from 2.1 to 1.90 eV as the zinc content rises.     

Catalytic combustion of toluene over Fe–Mn mixed oxides supported on cordierite

The catalytic combustion of toluene over Fe–Mn mixed oxides supported on cordierite was investigated. The catalysts were synthesized by the impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and BET specific surface area measurement. The effects of the mole ratio of Fe to Mn, the loading of Fe–Mn mixed oxides on the catalyst support and the calcination temperature were all investigated. The results indicate that Fe–Mn/cordierite catalysts with a 4 mol ratio of Fe to Mn, used with 10 wt% loading, and calcined at 500 ^°C showed the highest catalytic activities as measured by the oxidation of toluene. Compared to unsupported powder catalysts of Fe–Mn mixed oxides, the Fe–Mn/cordierite catalyst showed higher activity for the catalytic combustion of toluene with less active component.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.     

Preparation of Y1−x Yb x Ba2Cu3O7−y superconducting films by chemical vapor deposition

High Tc Y_1−x Yb _x Ba₂Cu₃O_7−y films were prepared on SrTiO₃(100) substrates by chemical vapor deposition method. Yb₁Ba₂Cu₃O_7−y films were obtained at higher oxygen partial pressure compared with Y₁Ba₂Cu₃O_7−y films at the same deposition temperature. Tc,o (R=0) decreased about 1.5 K when Y was fully substituted with Yb. The caxis lattice parameter of Y_1−x Yb _x Ba₂Cu₃O_7−y films also decreased as the amount of Yb(x) increased.     

Varistor and electrical properties of MgO.(Fe2O3)1−x(Bi2O3)x ceramics

In this study, MgO.(Fe₂O₃)_1−x(Bi₂O₃)_x (x = 0.01, 0.02, 0.04, 0.08) samples were prepared by the conventional ceramic process. Microstructure studies revealed that the samples contain MgFe₂O₄ grains surrounded by insulating Bi₂O₃-rich phases. DC electrical resistivity of the samples was increased by Bi₂O₃ content up to 1.1 MΩ.cm for the sample with x = 0.08. Current density- electric field investigations suggested that the samples with x = 0.01, 0.02 and 0.04 exhibited varistor properties. The sample with x = 0.01 showed excellent varistor properties with a non-linear coefficient of 45 and a threshold electric field value of 160 V/cm. Variation of D.C electrical conductivity versus temperature indicated that the activation energy values for the conduction were increased by Bi₂O₃ content from 0.334(5) eV to 0.857(5) eV. A.C electrical conductivity spectra of the samples obey the universal power law and the charge transport mechanism is based on electron hopping via sudden translational motion between the ferric and ferrous ions.     

Production of hydrogen by steam reforming of methanol over Cu/CeO2 catalysts derived from Ce1−xCuxO2−x precursors

The Ce_1−xCu_xO_2−x mixed oxides were synthesized using the coprecipitation method, and were reduced to form the Cu/CeO₂ (cop) catalysts for the steam reforming of methanol. All the Cu-containing catalysts tested in this study showed high selectivities to CO₂ (over 97%) and H₂. A 3.9 wt% Cu/CeO₂ (cop) catalyst showed a conversion of 53.9% for the steam reforming of methanol at 513 K, which was higher than those over Cu/ZnO (37.9%), Cu/Zn(Al)O (32.3%) and Cu/Al₂O₃ (11.2%) with the same Cu loading under the same reaction conditions. It is likely that the high activity of the 3.9 wt% Cu/CeO₂ (cop) catalyst was due to the highly dispersed Cu metal particles and the Cu⁺ species stabilized by the CeO₂ support.     

Comparative Study on Catalytic Performances for Low-temperature CO Oxidation of Cu–Ce–O and Cu–Co–Ce–O Catalysts

Two series of Cu–Ce–O and Cu–Co–Ce–O catalysts were prepared by co-precipitation method. The prepared catalysts were characterized by XRD, IR, TPR, XPS, BET and ICP-AES. The catalytic activities of the catalysts for low-temperature CO oxidation were evaluated through a microreactor-GC system. TPR results indicate that the addition of cobalt to the Cu–Ce–O can increase the dispersion of copper oxide, and the interaction between cobalt and copper can enhance the reducibility of each other. XPS analysis show that Ce⁴⁺, Cu²⁺, along with Co₃O₄, are present on the surface of Cu_0.4Co_0.6Ce₄ catalyst. The Co/Cu atomic ratio and the calcination temperature have significant effect on the activities of the catalysts. Compared with Cu₁Ce₄ catalyst, the Cu_0.4Co_0.6Ce₄ catalyst has better activity and thermal stability.     

Thermal decomposition of K1 − x Cs x BSi2O6 borosilicates

Thermal decomposition of mixed K_{1 ? x} Cs _x BSi₂O₆ borosilicates by the methods of thermal analysis, annealing, and quenching, with the following refining of the structure by the Rietveld method on the example of solid solutions with x = 0.3 and 0.7 crystallizing in the space groups $I4\bar 3d$ and $Ia\bar 3d$ , respectively, is studied. It is shown that the solid-phase decomposition of borosilicates proceeds with the release of the gas- eous phase and formation of cristobalite and/or tridymite of SiO₂ at the final stage. In this case the solid solutions enriched by potassium decompose in one stage with the formation of SiO₂, while the solutions enriched by cesium decompose with the formation of the intermediate zeolite-like CsBSi₅O₁₂ borosilicate, which also decomposes during further thermal treatment. According to the data of the structure, the refining of K_{1 ? x} Cs _x BSi₂O₆ solid solutions obtained by thermal treatment at 1000°C for 20, 65, 80, and 100 h, it is detected that for the samples with x = 0.3 and 0.7 the parameter of the cubic cell of the leucite-like phase decreases and the vacancies in the position of alkaline cation appear. During the solid-phase decomposition of K_{1 ? x} Cs _x BSi₂O₆ solid solutions the structure degrades.     

Selective Catalytic Oxidation (SCO) of Ammonia to Nitrogen over Hydrotalcite Originated Mg–Cu–Fe Mixed Metal Oxides

Abstract  

Mg–Cu–Fe oxide systems, obtained from hydrotalcite-like precursors, were tested as catalysts for the selective catalytic oxidation (SCO) of ammonia. Copper containing catalysts were active in low-temperature SCO processes; however, their selectivity to nitrogen significantly decreased at higher temperatures. The optimum composition of the catalyst to guarantee high activity and selectivity to N₂ was proposed. Temperature-programmed experiments, SCO catalytic tests performed with various contact times and additional tests on the samples in the selective catalytic reduction of NO with ammonia showed that the SCO process over the studied calcined hydrotalcites proceeds according to the internal SCR mechanism and oxidation of ammonia to NO is a rate-determining step in the low-temperature range.     

The Effect of Copper Substitution on La2Ni1−x Cu x O4 Catalysts Activity for Simultaneous Removal of NOx and Diesel Soot

A series of the La₂Ni_1?x Cu _x O₄ (0 ≤ x ≤ 1.0) perovskite-like complex oxide catalysts, prepared and characterized by XRD, H₂-TPR, O₂-TPD and XPS, and catalytic activity tests, proved to be effective in the simultaneous removal of NOx and soot. The results indicated that the catalysts show the single orthorhombic K₂NiF₄-like phase. The doping of Cu led to the increase of orthorhombic distortion and the decrease of capability to accommodate non-stoichiometric oxygen, as well as the increase of the reducibility of lattice oxygen, resulting in improving catalytic activities. The La₂Ni_0.4Cu_0.6O₄ catalyst showed the highest activity. The maximum conversion of nitrogen oxide to molecular nitrogen was 15.6% and the ignition temperature decreased from 440 to 246 °C, as compared with the uncatalyzed soot combustion reaction.     

Carbon Deposition and Catalytic Deactivation during CO2 Reforming of CH4 over Co/γ-Al2O3 Catalysts

The reaction behavior and carbon deposition during the CO₂/CH₄ reforming reaction have been investigated over the γ-Al₂O₃-supported Co catalysts as a function of Co loading (between 2 and 20 wt%) and calcination temperature (T_c=500 or 1000°C). It was found that the stability of Co/γ-Al₂O₃ catalysts was strongly dependent on the Co loading and calcination temperature. For some loadings (6 wt% for T_c=500°C and 9 wt% for T_c=1000°C), stable activities have been achieved. However, over the catalysts with high Co loadings (>12 wt%), notable amounts of carbon were accumulated during reforming, and deactivation was observed. Moreover, severe deactivation was also noted over the 2 wt% catalysts, both when carbon deposition occurred (T_c=500°C) or was absent (T_c=1000°C). In the latter case, the oxidation of the metallic sites was responsible for the deactivation. Hence, there are two different deactivation mechanisms, namely, carbon deposition and oxidation of metallic sites. The activities were stable when a balance between carbon formation and its oxidation could be achieved.     

Microwave dielectric properties of (1−x)ZnTa2O6−xMgNb2O6 ceramics

Single phase MgNb₂O₆ and ZnTa₂O₆ powders were synthesized by solid-state method, and the high quality factor composite ceramics of (1?x)ZnTa₂O₆?xMgNb₂O₆ (x=0, 0.05, 0.10, 0.15, 0.20, 0.25 and 1.0) were prepared using the as-synthesized powders. The microwave dielectric properties, microstructure, phase transition and sintering behavior of the composite ceramics were investigated. The X-ray diffraction analysis revealed that solid solution between ZnTa₂O₆ and MgNb₂O₆ phases appeared in the composite ceramic. SEM results show that the grain sizes of the composite ceramics increased with increasing x values. The temperature coefficient of resonant frequency of (1?x)ZnTa₂O₆?xMgNb₂O₆ composite ceramics reaches near-zero of 1.02 ppm/°C with ε_r=35.58 and a high quality factor of 65500 GHz when x=0.20 and sintered at 1350 °C for 2 h.