Effect of different promoters on Ni/CeZrO2 catalyst for autothermal reforming and partial oxidation of methane

Ni/CeZrO₂ catalysts promoted by Ag, Fe, Pt and Pd were investigated for methane autothermal reforming and partial oxidation of methane. The catalysts properties were determined by BET surface area, X-ray diffraction (XRD), H₂ temperature-programmed reduction (TPR), temperature-programmed desorption of CO₂ (CO₂-TPD) and UV–vis diffuse reflectance spectroscopy (DRS). Nickel dispersions were evaluated using a model reaction, the dehydrogenation of cyclohexane. BET surface area results showed that the catalysts prepared by successive impregnation presented lower surface area which favored the smaller nickel dispersion. XRD analysis showed the formation of a ceria–zirconia solid solution. TPR experiments revealed that the addition of Pt and Pd as promoters increased the reducibility of nickel. CO₂-TPD results indicated that the AgNiCZ catalysts presented the best redox properties among all catalysts. The autothermal reforming of methane showed that, among different promoters, the sample modified with silver, AgNiCZ, presented higher methane conversion and better stability during the reaction. These results are related to the good reducibility and to the higher redox capacity observed in TPR and CO₂-TPD analysis. Samples prepared by successive impregnation technique resulted in a smaller catalytic activity. For partial oxidation of methane, just as happened in autothermal reforming, AgNiCZ also presented the best performance during the 24 h of reaction and the addition of silver by successive impregnation resulted in a lower methane conversion, probably, due to the smaller metal dispersion.     

Synthesis of flower-like La or Pr-doped mesoporous ceria microspheres and their catalytic activities for methane combustion

La or Pr-doped flower-like mesoporous ceria microspheres were prepared by a unique calcination of La or Pr-doped Ce(OH)CO₃ microspheres hydrothermally synthesized with the aid of glucose and acrylic acid. Prepared flower-like ceria-based materials have a novel hierarchical architecture with mesoporous structure and high surface area, which can obviously facilitate the catalytic combustion of methane, compared with general La or Pr-doped ceria. The presence of La or Pr can promote the production of oxygen vacancies and improve oxygen mobility, which result in enhancing the oxygen-storage capacity of the flower-like ceria and its catalytic performance for the methane combustion.     

Autothermal Reforming of Methane Over CeO2–ZrO2–La2O3 Supported Rh Catalyst

Autothermal reforming of methane was studied over La-doped ceria–zirconia-supported Rh catalysts. The CH₄ conversion increased from 49 to 60% on increasing the content of La³⁺ from 5 to 15%, while further increase in the La³⁺ content led to a slight decrease on both CH₄ conversion and H₂ selectivity. H₂-TPR and UV–vis DRS spectrum showed that the interaction between Rh and the support was enhanced by increasing the content of La. We speculated that a so-called “Rh–La interfacial species” was formed on the surface of the support, which played an important role in catalytic activity. The balance between exposed Rh and the “Rh–La interfacial species” was necessary to improve the catalytic activity. Upon increasing the Rh loading on 15% La-doped ceria–zirconia support, the balance was built, i.e., CH₄ conversion increased from ~60 to 69% by increasing Rh loading from 0.1 to 0.5 wt% and only 2% conversion was elevated by doubling the Rh loading from 0.5 to 1.0 wt%.     

Methane conversion over Nb-doped ceria

Methane steam reforming and dry methane conversion over ceria, and ceria doped with 1.4 and 5% Nb cation has been investigated at 900 °C (a typical solid oxide fuel cell temperature). The influence of the calcination atmosphere on the Nb-doped ceria has also been studied. Use of reducing conditions leads to significantly lower crystallite size, higher specific surface area and greater Nb solubility. Nb-doping lowers activity mainly as a consequence of strong segregation of Nb to the ceria surface. Kinetics of steam reforming are interpreted in terms of a redox mechanism.     

Complete oxidation of methane on Pd/YSZ and Pd/CeO2/YSZ by electrochemical promotion

In this work, methane combustion over Pd/YSZ and Pd/CeO₂/YSZ catalyst was investigated at a temperature range of 470–600 °C. For the first time, the feasibility of electrochemical promotion on palladium films prepared by wet impregnation was reported. The catalytic activity of palladium was found to increase over 160% via transference of oxygen ions from the solid electrolyte to the catalyst film. In addition, palladium supported over ceria and yttria-stabilized zirconia showed the highest activity. As expected, the presence of ceria allowed improving the oxygen storage capacity of the catalyst system.     

Conversion of Methane Facilitated by Solid Oxide Electrolysis Cells

A novel Fe-Al/SiO₂ catalyst for direct non-oxidative conversion of methane to ethylene, benzene, and naphthalene was synthesized. An Fe-Al/SiO₂ catalyst integrated solid oxide electrolysis cell (SOEC) membrane reactor design is further developed. It could be successfully demonstrated that the SOEC enabled the in situ removal of H₂, which helps to shift the chemical equilibrium of the methane conversion reaction to enhance the conversion efficiency.     

Effects of ceria in CO2 reforming of methane over Ni/calcium hydroxyapatite

In CO₂ reforming of methane over a calcium hydroxyapatite-supported nickel catalyst, the carbon deposition occurred more severely with increase of the methane partial pressure and at temperatures below about 1,000 K. The effects of ceria that was added as a promoter to the nickel catalyst were investigated. It was observed that the ceria not only enhanced the catalyst stability but also increased the activity, and this is considered owing to the oxygen storage capacity of ceria. The TGA analysis demonstrated that the ceria promoted the removal of the deposited carbon. The optimum Ce/Ni mole ratio was ca. 0.3/2.5. The deposited carbon could easily be removed by oxygen treatment at 1,023 K and the catalytic activity could be restored.     

Stability improvement of ZrO2-doped MnCeOx catalyst in ethanol oxidation

ZrO₂-doped manganese–cerium oxide catalyzed ethanol oxidation effectively and ethanol was fully oxidized to CO₂ at 453 K. The catalyst also showed quite promising stability for 120 h on-stream without obvious loss in ethanol conversion. Structural analyses have revealed that ZrO₂-doping enhanced the oxygen storage capacity of ceria by generating more oxygen vacancies, and at the same time promoted the thermal stability through the formation of solid solution.     

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.     

Thermal stability of palladium on ceria-doped alumina

Several palladium on alumina and ceria/alumina catalysts were prepared and oxidized in air between 400 and 1000°C. The metal dispersion was determined by hydrogen titration of adsorbed oxygen. Dispersions above 50% were maintained on 0.2% Pd/Al₂O₃ up to 900°C. Adding 5.0% ceria, or increasing the metal loading to 2.5%, greatly reduces the thermal stability of the palladium, such that the dispersion falls rapidly at 600°C. The rates of methane oxidation (moles of CO₂/g Pd h) at 250°C and 5% excess oxygen are nearly equal on 0.22–2.50% Pd/3.5–5.2% CeO₂/Al₂O₃, dispersion 14–42%, and 0.20–0.46% Pd/Al₂O₃, dispersion 59–86%, but are 10 to 20 times lower than the rate on 2.3% Pd/Al₂O₃, dispersion 11%. The lower rate of methane oxidation on ceria-promoted and highly dispersed palladium on alumina might be due to the conversion of the palladium into less active palladium oxide during reaction.     

Stability Improvement of Rh/γ-Al2O3 Catalyst Layer by Ceria Doping for Steam Reforming in an Integrated Catalytic Membrane Reactor System

Significant improvement over the equilibrium methane conversion level was achieved by performing the reforming of methane in a catalytic membrane reactor, which was prepared by integrating a microporous silica membrane with a sandwiched-type Rh/γ-Al₂O₃ catalyst layer. However, the methane conversion activity decreased progressively owing to the deactivation of the intermediate catalyst layer under the reaction environments. On the other hand, addition of CeO₂ as a promoter for the Rh/γ-Al₂O₃ catalyst significantly improved the catalyst stability. The improvement was achieved probably by the kinetic and oxidative stabilization of the catalyst matrix with CeO₂. However, compared to the nonpromoted system, the ceria-promoted systems displayed lower catalytic activities based on the Rh/Ce ratios. The results led to the conclusion that a controlled interplay of the catalytic potentiality of Rh and the stabilization effect of Ce are essential to obtain an acceptable system. The membrane quality and its performance decreased especially with high Ce incorporation in the catalyst layer, possibly as a result of the observed microstructural variations in the catalyst layer with the Ce addition. Therefore, a consensus between the activity and stability of the material as a catalyst and the textural characteristics of the catalyst layer as a support layer for the silica membrane is considered to be an important factor that decides the success of the approach. A possible mechanism has been suggested to explain the role of ceria as a promoter in the Rh/γ-Al₂O₃ system.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Low-temperature catalytic combustion of methane over MnO x –CeO2 mixed oxide catalysts: Effect of preparation method

The effect of preparation method on MnO _x –CeO₂ mixed oxide catalysts for methane combustion at low temperature was investigated by means of BET, XRD, XPS, H₂-TPR techniques and methane oxidation reaction. The catalysts were prepared by the conventional coprecipitation, plasma and modified coprecipitation methods, respectively. It was found that the catalyst prepared by modified coprecipitation was the most active, over which methane conversion reached 90% at a temperature as low as 390 °C. The XRD results showed the preparation methods had no effect on the solid solution structure of MnO _x –CeO₂ catalysts. More Mn⁴⁺ and richer lattice oxygen were found on the surface of the modified coprecipitation prepared catalyst with the help of XPS analysis, and its reduction and BET surface area were remarkably promoted. These factors could be responsible for its higher activity for methane combustion at low temperature.     

Conversion of natural gas to liquids via acetylene as an intermediate

This paper describes an experimental investigation of the conversion of natural gas to liquid transportation fuels through acetylene as an intermediate. The first step is the direct thermal conversion of methane to acetylene utilizing a thermal plasma heat source to dissociate the methane. The dissociation products react to form a mixture of acetylene and hydrogen. Significant improvements over the prior art were observed; these improvements may be attributed to an improved methane injection configuration and minimization of radial temperature gradients. Conversion efficiencies (percent methane converted) approached 100% and acetylene yields in the 90-95% range with 2-4% solid carbon production were obtained. A variety of methods were examined for the second step, the conversion of acetylene to liquid products. The most promising technology was the reaction of acetylene with hydrogen over a shape-selective zeolite to form C₃-C₅+ aliphatics.     

Steam reforming of ethanol over Pt/ceria with co-fed hydrogen

Metal/ceria catalysts are receiving great interest for reactions involving steam conversion, including CO for low-temperature water–gas shift, and the conversion of chemical carriers of hydrogen, among them methanol, and ethanol. The mechanism by which ROH model reagents are activated on the surface of the Pt/partially reduced ceria catalyst was explored using a combination of reaction testing and infrared spectroscopy. In this particular investigation, the activation and turnover of ethanol were explored and compared with previous investigations of methanol steam reforming and low-temperature water–gas shift under H₂-rich conditions, where the surface of ceria is in a partially reduced state. Under these conditions, activation of ethanol was found to proceed by dissociative adsorption at reduced defect sites on ceria (i.e., Ce surface atoms in the Ce³⁺ oxidation state), yielding an adsorbed type II ethoxy species and an adsorbed H species, the latter identified to be a type II bridging OH group. In the presence of steam, the ethoxy species rapidly undergoes molecular transformation to an adsorbed acetate intermediate by oxidative dehydrogenation. This is analogous to the conversion of type II methoxy species to formate observed in previous investigations of methanol steam reforming. In addition, although formate then decomposes in steam to CO₂ and H₂ during methanol steam reforming, in an analogous pathway for ethanol steam reforming, the acetate intermediate decomposes in steam to CO₂ and CH₄. Therefore, further H₂ production requires energy-intensive activation of CH₄, which is not required for methanol conversion over Pt/ceria.     

Synthesis,densification and characterization of Ag doped ceria nanopowders

Nanosized Ag-doped ceria (Ce_1-xAl_xO_2-δ)powders (0.1 ≤ x ≤ 0.04) were obtained by self-propagating room temperature reaction. The solid solubility of Ag into ceria lattice was the highest reported so far. X-ray diffraction analysis and field emission scanning microscopy results showed that the doped samples are single phase solid solutions with fluorite-type structure and all prepared powders were nanometric in size. The average size of Ce1-xAgxO2-▯ particles lies at about 4 nm. Raman spectra revealed an increase in the amount of oxygen vacancies with the increase of Ag concentration, such as is foreseen. The thermal stability of solid solution was followed by XRD. Microstructure development was studied by scanning electron microscopy. By controlling the processing variables, it was possible to obtain high density samples with homogeneous microstructure at low sintering temperature.     

BINARY MgO-BASED SOLID SOLUTION CATALYSTS FOR METHANE CONVERSION TO SYNGAS

The excellent catalytic performance and high stability of MgO–NiO solid solution catalysts in CH₄ conversion to syngas generated the recent outburst of interest for the binary MgO-based solid solutions. This review will focus on the relationship between the catalytic performance of the binary MgO-based solid solution and its properties in the CO₂ reforming, the partial oxidation and the steam reforming of methane. First, the development of methane conversion to syngas will be summarized. Second, the role of the basicity and of the solid solution in the design of a catalyst that can inhibit carbon deposition and active metal sintering will be examined. Third, the main results regarding the catalytic performance of the MgO-based solid solutions will be presented. Fourth, detailed information regarding the effects of the NiO/MgO composition, surface area, pore distribution, crystal lattice parameter, precursors, and preparation condition on its catalytic behavior will be provided.     

Oxygen ion conductivity of the ceria-samarium oxide system with fluorite structure

Ionic conduction of oxygen in the ceria-samarium oxide system was investigated as a function of temperature, partial pressure of oxygen and the oxide composition, together with its crystal structure, density and defect structure. The ionic conductivity of (CeO₂)_1–x(SmO_1.5)_x was the highest in ZrO₂-, ThO₂- and CeO₂-based oxide systems. The system CeO₂-SmO_1.5 consisted of the solid solution with a fluorite structure atx<50 at.%. The ionic transference number was nearly unity between 600 and 900°C. With an increase in Sm₂O₃ content, the ionic conductivity gradually decreased due to a decrease in mobility of oxygen ions. The samarium oxide-doped ceria was less reducible than pure and alkaline earth oxide-doped ceria.     

An experimental study of oxidative coupling of methane in a solid oxide fuel cell with 1 wt%Sr/La2O3-Bi2O3-Ag-YSZ membrane

A solid oxide fuel cell with 1 wt%Sr/La₂O₃-Bi₂O₃-Ag-YSZ membrane was applied to oxidative coupling of methane. Membrane composition had a great effect on the reaction and current generated. An increase in the current generated was accompanied by a decrease in C₂ selectivity and an increase in CH₄ conversion. There is an optimal temperature for C₂-selectivity. CH₄ conversion decreased, C₂-selectivity increased and current generated decreased slightly with a rise in total flow rates. CH₄ conversion and the current generated increased with a rise in oxygen concentration. If only C₂-selectivity and current were concerned, the higher the methane concentration, the more favourable for the cogeneration of electrical energy and ethane and ethylene. Stability of the membrane was also tested.     

An investigation of metal ion diffusion in ceria‐based solid oxide fuel cell with barium‐containing anode

Metal ion diffusion is an effective strategy to suppress the internal electronic short circuit in ceria‐based solid oxide fuel cells (SOFCs). This could be achieved by fabricating an electron‐blocking layer between the barium‐containing anode and ceria‐based electrolyte. In this paper, a 0.6NiO‐0.4BaZr_0.1Ce_0.7Y_0.2O_3‐δ (NiO‐BZCY) anode‐supported cell based on Gd_0.1Ce_0.9O_2‐δ (GDC) electrolyte was employed to evaluate the internal metal ion diffusion behavior. The high open circuit voltages of about 1 V obtained at 550‐700°C can be attributed to in situ formation of an electron‐blocking interlayer between NiO‐BZCY and GDC. Microstructural analyses of the interlayer grains obtained by traditional solid‐state reaction were carried out. Phase identification demonstrated that the electron‐blocking interlayer had a perovskite structure. SEM and TEM analyses indicated formation of a new compound in the interlayer, of which the composition was determined as Zr, Y, and Ni co‐doped BaCe_0.9Gd_0.1O₃ with orthorhombic structure.