Oxidative coupling of methane at 600 °C on CaNiK oxide catalysts

We are herewith reporting new data which modifies, explains and extends our earlier work on this subject. Previous observations of low CH₄ conversion and high selectivity to C₂ hydrocarbons were erroneous due to the formation of bulk CaCO₃ in the catalyst. The carbonate was detected by powder X-ray diffraction and was shown to accumulate during the reaction and decompose during regeneration. Catalytic runs which incorporated an internal standard revealed a deficit in the C balance consistent with carbonate formation. Actual CH₄ conversions were 20% with 15% selectivity to hydrocarbons. The effect of steam in promoting coupling over combustion was affirmed.     

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.     

Oxidative dehydrogenation of n-butane: a comparative study of thermal and catalytic reaction using Fe–Zn mixed oxides

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier. This revised version was published online in July 2006 with corrections to the Cover Date.     

Temperature-programmed reduction and CO oxidation studies over Ce–Sn mixed oxides

The reduction behaviour of Ce–Sn mixed oxides has been studied by a temperature-programmed hydrogen reduction technique and compared with that of pure SnO₂ and CeO₂. The mixed oxides were found to reduce at lower temperature as compared to that of individual oxides. Carbon monoxide oxidation studies showed that mixed oxides have better activity for CO oxidation reaction than the constituent oxides, which is in conformity with their surface reduction behaviour. The improved oxidation activity is attributed to a synergetic effect existing in these mixed oxides.     

Electrochemical methane conversion over SrFeO3−δ perovskite on an yttrium stabilized zirconia membrane

Oxidative methane coupling and the related chemical reactions have been studied in an electrochemical membrane cell of the type: CH₄, (O₂), SrFeO_3– , Au¦8%Y₂O₃:ZrO₂¦Ag, air. The results are compared to a fixed bed study of SrFeO_3– . The C₂₊ selectivity and the alkene/alkane ratio may be higher in the cell reactor than in the fixed bed reactor, but the C₂₊ yield never exceeded fixed bed data. The maximum C₂₊ yield observed in the cell reactor was 3.1%. The electric fields in the cell when electrodes were connected influenced the selectivity to CO₂ in a manner which may be related to the NEMCA effect.     

Dehydrogenation of methylcyclohexane over Pt supported on Mg–Al mixed oxides catalyst: The effect of promoter Ir

To enhance the hydrogen release during hydrogen storage, several Pt–Ir supported on Mg–Al mixed oxide catalysts were prepared and then applied into the dehydrogenation of methylcyclohexane (MCH) in this study. The effects of iridium content, reduction temperature on the activity and stability of the catalysts were studied in detail. In the presence of Ir, metal particle size was decreased and electron transfer between Ir and Pt was observed. High reduction temperature increased the metallic Ir content but enlarged the particle size of active sites. During the dehydrogenation reaction on Pt–Ir bimetallic catalyst, MCH was efficiently converted into toluene and PtIr-5/Mg–Al-275 exhibited the highest activity. After prolonging the residence time and raising the reaction temperature to 350 °C, the conversion and hydrogen evolution rate were increased to 99.9% and 578.7 mmol·(g Pt)⁻¹·min⁻¹, respectively. Moreover, no carbon deposition was observed in the spent catalyst, presenting a high anti-coking ability and good potential for industrial application.     

Water gas shift reaction over Cu–Mn mixed oxides catalysts: Effects of the third metal

Highly efficient Cu–Mn spinel catalysts for water gas shift (WGS) reaction were achieved by a single step urea-nitrate combustion method. A series of doped Cu–Mn-M catalysts (M = Ce, Zr, Zn, Fe, Al) were prepared by the same method. Effects of dopants on WGS activity and stability of doped Cu–Mn catalysts were investigated. The doped catalysts were characterized by BET, XRD and TPR. XRD results showed that non-doped samples and Zr-doped samples are mainly composed of Cu_1.5Mn_1.5O₄ phase, while CuO, Cu₂O and Cu_1.5Mn_1.5O₄ for other doped samples. It was further found that WGS activities depend strongly on the natures of the dopant employed despite of their lower content, varying in the order of Zr > Fe > non-doped > Ce > Al > Zn. TPR profiles revealed that all dopants shift the reduction peaks to lower temperature region, indicating no direct correlation between WGS activity and the reducibility. In addition, Zr-doped Cu–Mn catalyst with 5 wt.% content showed the best catalytic performance and, optimal stability exposed to oxygen-stream and on-stream operation. It indicates that ZrO₂ is an effective promoter for Cu–Mn catalyst, and the catalytic performances are related to the existence of a Cu_1.5Mn_1.5O₄ phase and ease reducibility of the catalysts.     

Oxidative dehydrogenation of ethane over mixed metal oxide catalysts: Autothermal or cooled tubular reactor design?

A kinetic model consistent with experimentally measured reaction orders for the oxidative dehydrogenation of ethane (ODHE) over a MoVTeNbO catalyst is developed and applied to compare autothermal and multi-tubular reactor designs for the same. The results suggest that autothermal reactor configurations are more favorable compared to multi-tubular ones for this highly exothermic reaction, and become even more so with increasing active site density. A bifurcation analysis based on ignition and extinction behavior is presented as a function of adiabatic temperature rise (varied by altering the feed ethane to oxygen molar ratio) and catalyst active site density (varied by altering the pre-exponential factor), thereby providing additional insights into strategies for successful scale-up of ODHE reactors. These strategies for the design of viable reactor configurations may be more broadly applicable to high temperature catalytic partial oxidation reactions currently in practice or under consideration in the context of large-scale processes for chemical production.     

Oxidative dehydrogenation of ethane and propane over magnesium–cobalt phosphates CoxMg3−x(PO4)2

The magnesium–cobalt phosphates Co_xMg_3–x(PO₄)₂ belonging to the olivine-type structure were synthesized by coprecipitation and then investigated in the oxidative dehydrogenation (ODH) of ethane and propane. The best yields, with the exception of Co_0.5Mg_2.5(PO₄)₂, were achieved with the compositions ranging between 1x2.5. Magnesium phosphate Mg₃(PO₄)₂ displayed no activity and pure cobalt phosphate Co₃(PO₄)₂ was found to be the less active component of the solid solution. Comparison of the catalysts performances showed that they all have similar activity in ethane and propane ODH, albeit, they are more selective in propylene than in ethylene production. The Co_xMg_3–x(PO₄)₂ solid solution was also studied, for characterization purposes, in butan-2-ol conversion. The samples presented acid–base properties due essentially to the (PO–H) groups but they do not bear conventional redox centers. All the catalysts were active at low temperatures in the alcohol dehydration. The dehydrogenation activity versus the phosphates composition displayed two maxima around x=1 and 2, respectively. Similar striking behavior was also observed in ethane and propane ODH. UV-visible investigations of Co_xMg_3–x(PO₄)₂ showed, in agreement with the XRD data, that the Co^{2 +} ions are distributed in the phosphate framework between six- and five-coordinated sites. The cobalt atoms in the five-coordinated sites Co(5) and their Co(5)–Co(5) interatomic distances were assumed to play the main role in the C–H bond activation and the appearance of maxima in the activity. Magnesium cations presumably intervene in acid–base properties of the samples and O₂ activation. Characterization of the samples showed that they do not undergo any noticeable transformation after the catalytic tests.     

The catalytic conversion of CO2 to hydrocarbons over Fe–K supported on Al2O3–MgO mixed oxides

Al₂O₃–MgO mixed oxides prepared by a co-precipitation method have been used as supports for potassium-promoted iron catalysts for CO₂ hydrogenation to hydrocarbons. The catalysts have been characterized by XRD, BET surface area, CO₂ chemisorption, TPR and TPDC techniques. The CO₂ conversion, the total hydrocarbon selectivity, the selectivities of C₂–C₄ olefins and C₅₊ hydrocarbons are found to increase with increase in MgO content upto 20 wt% in Fe–K/Al₂O₃–MgO catalysts and to decrease above this MgO content. The TPR profiles of the catalysts containing pure Al₂O₃ and higher (above 20 wt%) MgO content are observed to contain only two peaks, corresponding to the reduction of Fe₂O₃ to Fe⁰ through Fe₃O₄. However, the TPR profile of 20 wt% MgO catalyst exhibits three peaks, which indicate the formation of iron phase through FeO phase. The TPDC profiles show the formation of three types of carbide species on the catalysts during the reaction. These profiles are shifted towards high temperatures with increasing MgO content in the catalyst. The activities of the catalysts are correlated with physico-chemical characteristics of the catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Highly selective synthesis of 2‐butoxy ethanol over Mg/Al/V mixed oxides catalysts derived from hydrotalcites

The catalytic activity of a series of mixed oxides obtained by the thermal decomposition of hydrotalcite‐like precursors was assessed for the alkoxylation of n-butanol with ethylene oxide. The calcination products of a decavanadate intercalated magnesium–aluminium layered double hydroxide were shown to possess extremely high activity for the alkoxylation reaction achieving up to 100% conversion in batch reaction. In all cases, the catalysts exhibit a much higher selectivity towards the monoglycol adduct than that obtained with the industrial catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of doping on flash sintering conditions in SrTi1−xFexO3−δ

Joule heating in flash sintering depends on the sample resistance, which is linked to the charged defects concentration. To study the contribution of defects on the flash sintering mechanism, this link should be untied. In SrTi_1−xFe_xO_3−δ (STFO) system, point defect concentration can be pinned while the resistance is influenced by the oxygen partial pressure (pO₂). SrTi_0.97Fe_0.03O_2.985 onset temperature at different pO₂ was examined at constant oxygen vacancy concentration. The furnace onset temperature decreases with increasing pO₂. The onset temperature of five STFO samples was predicted based on resistance-temperature dependency of the porous green ceramics in a dynamic heat balance simulation. High compatibility of model and experimental results showed reduction of onset temperature with increasing doping. Ex-situ impedance measurements of green samples reveal an overlapping Nyquist plots close to the sample onset temperature. This indicates that the onset is determined by the green body resistance regardless how it has been achieved.     

Solubility studies and thermophysical properties of uranium–neodymium mixed oxides system

Uranium–neodymium mixed oxides (U_1−yNd_y)O_2±x (y=0.2–0.85) were prepared by citrate gel-combustion and characterized by XRD. Single phase fluorite structure was observed up to y=0.80. For solid solutions with y>0.80 additional lines pertaining to hexagonal neodymium oxide were observed. Lattice thermal expansion of these samples was investigated by using high temperature X-ray diffraction (HTXRD). The coefficients of thermal expansion for (U_1−yNd_y)O_2±x for y=0.2, 0.4, 0.6, and 0.8 in the temperature range 298–1973 K were found to be 16.46, 16.64, 16.79, and 16.89×10⁻⁶ K⁻¹, respectively. Heat capacity and enthalpy increment measurements were carried out by using DSC and drop calorimetry in the temperature range 298–800 K and 800–1800 K respectively. The C_p,m values at 298 K for (U_1−yLa_y)O_2±x (y=0.2, 0.4, 0.6, and 0.8) are 63.4, 64.3, 61.8, and 58.9 J K⁻¹ mol⁻¹ respectively.     

Oxidative dehydrogenation of ethanol over AgLi–Al2O3 catalysts containing different phases of alumina

Oxidative dehydrogenation of ethanol over the AgLi–Al₂O₃ catalysts having different phase compositions of alumina was investigated. The pure gamma (CHI00), pure chi (CHI100) and equally mixed phases (CHI50) derived from the solvothermal synthesis can play important roles on the physicochemical properties of AgLi–Al₂O₃ catalysts. Especially, the amount of weak basic sites, the oxidation state of Ag, and the reduction behaviors of catalysts are crucial in determining the ethanol conversion and product selectivity. It was found that increased amounts of weak basic sites and Ag_n^δ + clusters enhanced the catalytic activity as seen for the AgLi–CHI50 catalyst.     

Selective catalytic reduction of nitrogen oxides with hydrocarbons over Zn–Al–Ga complex oxides

Selective reduction of NO with hydrocarbons was studied using metal oxide catalysts having a spinel structure. A Zn–Al–Ga complex oxide was found to be very active and selective for the catalytic reduction of NO with both C₃H₆ and CH₄. It was revealed that the role of oxygen at the initial stage of the reaction strongly depends on the reductants; oxygen is mainly used for NO oxidation to NO₂ in the reduction with CH₄, whereas it is used both for NO oxidation to NO₂ and oxidation of C₃H₆ to an active intermediate in the reduction with C₃H₆. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic dehydrogenation of isobutane over ordered mesoporous Cr2O3–Al2O3 composite oxides

A series of ordered mesoporous Cr₂O₃–Al₂O₃ composite oxides synthesized via improved one-pot evaporation induced self-assembly strategy were investigated as the catalysts for catalytic dehydrogenation of isobutane. These mesoporous catalysts with good structural properties and thermal stability performed excellent catalytic properties. Besides, the effect of the ordered mesopore structure on improving catalytic properties was also studied. Compared with non-mesoporous catalyst, the current mesoporous catalyst could accommodate the gaseous reactant with more “accessible” active sites. Therefore, the present materials were considered as promising catalyst candidates for catalytic dehydrogenation of isobutane.     

Ketonization of fatty methyl esters over Sn−Ce−Rh−O catalyst

A mixture of methyl esters of fatty acids obtained by transterification of nonerucic rape oil was ketonized. The starting material, diluted with methanol, was converted at atmospheric pressure over a catalyst that contained Sn, Ce, and Rh oxides in a molar ratio of 90:9:1. At a temperature of 385°C ketones were obtained with a total yield of 63% at the 96% conversion of starting material. The reported experiments prove that catalysts other than iron that are active in ketonization of primary alcohols can be successfully used in ketonization of esters of fatty acids. The kind of diluent used plays a crucial role in the conversion.     

Direct conversion of methane to synthesis gas using lattice oxygen of CeO2–Fe2O3 complex oxides

Three kinds of complex oxides oxygen carriers (CeO₂–Fe₂O₃, CeO₂–ZrO₂ and ZrO₂–Fe₂O₃) were prepared and tested for the gas–solid reaction with methane in the absence of gaseous oxidant. These oxides were prepared by co-precipitation method and characterized by means of XRD, H₂-TPR and Raman. The XRD measurement shows that Fe₂O₃ particles well disperse on ZrO₂ surface and Ce–Zr solid solution forms in CeO₂–ZrO₂ sample. For CeO₂–Fe₂O₃ sample, only a small part of Fe³⁺ has been incorporated into the ceria lattice to form solid solutions and the rest left on the surface of the oxides. Low reduction temperature and low lattice oxygen content are observed over ZrO₂–Fe₂O₃ and CeO₂–ZrO₂ samples, respectively by H₂-TPR experiments. On the other hand, CeO₂–Fe₂O₃ shows a rather high reduction peak ascribed to the consuming of H₂ by bulk CeO₂, indicating high lattice oxygen content in CeO₂–Fe₂O₃ complex oxides. The gas–solid reaction between methane and oxygen carriers are strongly affected by the reaction temperature and higher temperature is benefit to the methane oxidation. ZrO₂–Fe₂O₃ sample shows evident methane combustion during the reducing of Fe₂O₃, and then the methane conversion is strongly enhanced by the reduced Fe species through catalytic cracking of methane. CeO₂–ZrO₂ complex oxides present a high activity for methane oxidation due to the formation of Ce–Zr solid solution, however, the low synthesis gas selectivity due to the high density of surface defects on Ce–Zr–O surface could also be observed. The highly selective synthesis gas (with H₂/CO ratio of 2) can be obtained over CeO₂–Fe₂O₃ oxygen carrier through gas–solid reaction at 800 °C. It is proposed that the dispersed Fe₂O₃ and Ce–Fe solid solution interact to contribute to the generation of synthesis gas. The reduced oxygen carrier could be re-oxidized by air and restored its initial state. The CeO₂–Fe₂O₃ complex oxides maintained very high catalytic activity and structural stability in successive redox cycles. After a long period of successive redox cycles, there could be more solid solutions in the CeO₂–Fe₂O₃ oxygen carrier, and that may be responsible for its favorable successive redox cycles performance.