Characterization and CO oxidation catalytic behavior of CuO/CeO2 catalysts

Summary Effects of the lattice oxygen mobility, Pt dispersion and the surface features of Pt/Ce_1-xPr_xO_2-y samples on their performance in the selective oxidation of methane into syngas, in the steam and dry reforming of methane at short contact times are considered.     

Tap study of the impact of the oxidation state of Pt/PrCeZrO and Pt/GdCeZrO catalysts on their reactivity in the partial/deep oxidation of methane

The temporal analysis of products (TAP) technique coupled with the oxygen TPD was used to elucidate the effects of the Pt-supported fluorite-like doped ceria–zirconia oxide chemical composition and the type of pretreatment on their oxygen bonding strength, mobility, and reactivity as related to catalytic properties in the partial oxidation of methane into synthesis gas. A rapid evolution of hydrogen under CH₄ pulse observed for oxidized catalysts agrees with the direct route of the methane selective oxidation into syngas. This route is favored by the Pt-support interaction and a moderate bonding strength of surface oxygen species along with a high lattice oxygen mobility.     

Reactivity of surface and bulk oxygen in La1-xCaxFeO3-y system with respect to methane oxidation

The reactivity and mobility of the surface and lattice oxygen in a La_1-xCa_xFeO_3-y system prepared via a ceramic route were studied by using CH₄-TPR. While the middle-temperature (400-700^oC) oxygen lattice mobility reaches maximum for samples with disordered anion vacancies, the high-temperature lattice oxygen flux appears to be controlled by a rapid transfer along disordered domain walls. In the high-temperature range, a high selectivity to syngas of methane oxidation by the lattice oxygen was achieved both for pure lanthanum ferrite and for samples with a microdomain structure. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nanostructured perovskite oxides as promising substitutes of noble metals catalysts for catalytic combustion of methane

In this review, we summarize the recent development of nanostructured perovskite oxide catalysts for methane combustion, and shed some light on the rational design of high efficient nanostructured perovskite catalysts via lattice oxygen activation, lattice oxygen mobility and materials morphology engineering.     

Reactivity of La1 ? xSrxFeO3 ? y (x = 0–1) Perovskites in Oxidation Reactions

Oxygen species and their reactivity in La_{1 − x} Sr _x FeO_{3 − y} perovskites prepared using mechanochemical activation were studied by temperature-programmed reduction (TPR) with hydrogen and methane. The experimental data were compared with data on the catalytic activity in oxidation reactions. It was found that the rates of CO and methane oxidation on the perovskites in the presence of gas-phase oxygen correlated (k = 0.8) with the amount of reactive surface oxygen species that were removed by TPR with hydrogen up to 250°C. Maximum amounts of this oxygen species were released from two-phase samples (x = 0.3, 0.4, and 0.8), which exhibited an enhanced activity in the reaction of CO oxidation. In the absence of oxygen in the gas phase, methane is oxidized by lattice oxygen. In this case, the process activity and selectivity depend on the mobility of lattice oxygen, which is determined by the temperature, the degree of substitution, the degree of reduction, and the microstructure of the oxide. Thus, the high mobility of oxygen, which is reached at high concentrations of point defects or interphase/domain boundaries, is of importance for the process of deep oxidation. However, the process of partial oxidation occurs in single-phase samples at low degrees of substitution (x = 0.1–0.2). __________ Translated from Kinetika i Kataliz, Vol. 46, No. 5, 2005, pp. 773–779. Original Russian Text Copyright ? 2005 by Isupova, Yakovleva, Alikina, Rogov, Sadykov.     

NiO and ZrO2-based catalysts in the reaction of complete methane oxidation

The reaction of complete methane oxidation over deposited catalysts (NiO/ZrO₂ and NiO/YSZ) and binary oxides NiO_ZrO₂ produced by co-precipitation, by the sol gel method, and using a bio-template (NiO content in the samples, 10.1 mol %) is investigated. It is shown that binary oxides cause methane oxidation at lower temperatures than their deposited analogue: the temperature of methane half-transformation is 470, 500, and 520°C for binary oxides, while T ₅₀ = 570°C for NiO/ZrO₂. Major factors affecting the activity of binary oxides in the methane oxidation reaction are determined: the dispersion of the active phase (NiO) and the availability of the second component with high mobility of the lattice oxygen.     

Low temperature conversion of methane to syngas using lattice oxygen over NiO-MgO

The conversion of methane to syngas (H₂ and CO) is an important route to produce high value-added products. Oxidize methane into syngas in the absence of gaseous oxidants is an economical route. In this work, NiO-MgO composite is successfully synthesized via an impregnation method. At 764 K, methane is directly converted to syngas on the NiO-MgO without gaseous oxidants. A synergistic effect of NiO and MgO was observed, in which NiO induced lattice oxygen of MgO mobility to oxidize methane and suppressed the formation of intermediates for side reaction. As a result, NiO-MgO exhibited enhancement of catalytic activity with the H₂ production rate of 1241.0 µmol g^?1 min^?1, which was 3.4 times higher than that of pure MgO. This work provides a direct guidance to understand of methane oxidation via lattice oxygen under low temperature (< 773 K).     

The influence of La2O3 and TiO2 on NiO/MgO/α-Al2O3

Summary The effect of La₂O₃ and TiO₂ on product selectivity, methane conversion and coke formation over NiO/MgO/ α -Al₂O₃ catalyst were studied in a simultaneous steam and CO₂ reforming of methane to syngas. La₂O₃ and TiO₂ were added to the catalyst via incipient wetness impregnation and bulk precipitation techniques and catalyst activity was tested in a fixed bed quartz reactor. Results reveal that although the addition of these oxides has no effect on the product selectivity and methane conversion, but can reduce coke formation on the surface of the catalysts as it can enhance the mobility of lattice oxygen anions. The results further show that the catalysts prepared by bulk precipitation technique decrease the coke formation more effectively.     

Ce-Fe-Zr-O/MgO整体型氧载体用于化学链部分氧化甲烷制合成气

以MgO为载体,采用球磨法制备了Ce-Fe-Zr-O/MgO粉末状氧载体,进而采用挤压成型法制备了整体型氧载体。研究了两种氧载体化学链部分氧化甲烷制合成气的性能,并通过XRD、H₂-TPR对氧载体进行表征。结果表明,粉末状氧载体中的储氧组分以Ce-Fe-Zr-O固溶体形式存在,而整体型氧载体的制备过程会导致Zr、Fe游离氧化物的形成。粉末状氧载体和整体型氧载体上均存在表面晶格氧和体相晶格氧,其中,体相晶格氧具有高选择性氧化甲烷的性能,可以将甲烷转化成CO和H₂。粉末状氧载体与甲烷反应活性较高,但其存在高含量的表面氧,易导致甲烷的完全氧化。整体型氧载体上体相晶格氧占据优势,可将甲烷选择性氧化为CO和H₂。氧化还原循环实验表明,粉末状氧载体在还原反应发生短时间内容易引起甲烷裂解导致产物气中的H₂/CO物质的量比显著大于2.0,同时产生大量积炭,制约了其循环性能。而整体型氧载体经10次循环实验后,全程反应过程中合成气H₂/CO物质的量比一直维持在2.0附近,显示了较高的循环稳定性能。     

Influence of the composition of composites based on Y-and Sc-stabilized zirconium dioxide on catalytic properties in the oxidative conversion of methane

The effect of the composition of composites based on Y-and Sc-stabilized zirconium dioxide doped with CeO₂ and transition metal (Cu, Co, Ni) oxides on catalytic properties in the oxidative conversion of methane was studied. The activity of the composites correlated with the quantity and mobility of oxygen in them. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 3, pp. 184–188, May–June, 2006.     

Methane catalytic combustion on La-based perovskite catalysts

Two series of La-based perovskites oxides (La_1–xSr_xMO_3–y with M=Fe or Co) have been used as methane total oxidation catalysts. The best catalytic performances, in isothermal, high temperature conditions (900 °C), in the presence of water and carbon dioxide, were obtained for both series when 20 % of lanthanum cations are replaced by strontium. The catalytic behavior of the two series of catalysts is quite similar, although the cobalt-containing samples are easily reducible by H₂ or CH₄, whereas the iron-containing ones are not reduced in the same conditions. It is proposed, to explain this apparent inconsistency, that the active sites reoxidation process occurs directly from the molecular oxygen of the gas phase, without participation of the bulk oxygen species mobility.     

Heterogeneous methane oxidative coupling using perovskites

Catalytic oxidative coupling of methane over perovskite CaTiO₃ prepared by modified ceramic method has been studied. The C₂ yield was 13% at 830 °C and promotion with Na₄P₂O₇ did not improve considerably the catalyst peformance. Regarding reactor test and mechanism studies it is believed that charge deficient, oxygen O^– generated by transforming oxygen adsorbed on surface defects and dissolved into bulk vacancies is responsible for activating methane (active site). Adsorbed oxygen generates the oxidizing sites for actived methane. Strict conditions are required for generation and regeneration of the activating sites in lattice.     

Influence of the mobility of oxygen in a complex oxide carrier on the mechanism of partial oxidation of methane

The catalytic partial oxidation of methane was studied over single channels of monolith catalysts Pt/PrCeZrO/α-Al₂O₃ and Pt/GdCeZrO/α-Al₂O₃ using the temporal analysis of products (TAP) and kinetic transients. Effects of catalyst composition, oxidation state, time offset between O₂ and CH₄ pulses on activity, selectivity and dynamics of product formation were elucidated. Realization of the direct pyrolysis-CH₄ partial oxidation route was reliably established. This route is favored by optimum lattice/surface oxygen mobility and reactivity controlled by the dopant type (Gd, Pr) and oxidation state of the complex cerium/zirconium oxide.     

Isotope effects in the oxidative functionalization of methane in the presence of rhodium-containing homogeneous catalytic systems

The combined oxidation of carbon monoxide, CH₄, and CD₄ by molecular oxygen (¹⁶O₂ and ¹⁸O₂) in aqueous solutions of trifluoroacetic acid labeled with ¹⁸O in the presence of rhodium and copper compounds and potassium iodide has been studied. The distribution of ¹⁸O in isotopically substituted products suggests that oxygen entered into the methane molecule from an active oxidizing agent. This oxidizing agent was produced from molecular oxygen under the action of reagents and catalytic system components. The kinetic isotope effect observed for methane (k _H/k _D=3.9?4.3) suggests a nonradical character of the step at which the oxygen atom passes from an active oxidizing agent to the methane molecule or its fragments—transition-state components of the corresponding step.     

Comparative Effectiveness of Oxygen and Nitrous Oxide in the Partial Oxidation of Methane on V2O5/SiO2

It is shown that at relatively low temperatures the rate of oxidation of methane by nitrous oxide over the catalyst V₂O₅/SiO₂ exceeds the rate of oxidation of methane by oxygen, whereas at higher temperatures the opposite relation is observed between the comparative rates of the reactions. This effect is explained on the basis of a heterogeneous chain mechanism for the oxidation of methane.     

Deep Oxidation of Methane over Nano-Sized Ferrites with Spinel Structures

Ferrites with spinel structures as catalysts for the deep oxidation of methane have been studied by X-ray phase analysis and temperature programmed reduction. The most active catalysts are the nano-sized ferrites of cobalt and nickel, prepared by the decomposition of polynuclear complexes, with Al₂O₃ as carrier and with addition of a surface active agent to increase the thermal stability of the catalysts. At low temperatures (up to 450 °C), the effect of the size factor appears with the increase in specific catalytic activity of cobalt and nickel ferrites with decreasing of their particles. A correlation of the catalytic activity with the quantity and mobility (reactivity) of oxygen in the ferrites has been established.     

Methane Conversion to C2 Hydrocarbons Using Dielectric-barrier Discharge Reactor: Effects of System Variables

The partial oxidation of methane to C₂ hydrocarbons was investigated experimentally in a dielectric-barrier discharge (DBD) reactor. The effects of reactor wall temperature, input gas flow rate and volumetric ratio of methane to oxygen over methane conversion and C₂ production were investigated. The highest C₂ selectivity of about 50% was achieved at 1.8% methane conversion. Finally the model equations were used to correlate methane conversion and ethylene selectivity with the system variable within the studied range of them. The correlation equation shows the sole effects and interaction effects of system variables on methane conversion and ethylene selectivity.     

三维有序大孔钙钛矿型氧化物LaFeO3的合成及甲烷化学链重整性能

采用无皂乳液聚合法制备了聚苯乙烯(PS)聚合物微球,并采用胶晶模板法制备了三维有序大孔3DOM LaFeO₃钙钛矿型氧化物.通过扫描电镜、X射线衍射和傅里叶变换红外光谱等手段对氧化物的性能进行了表征.利用程序升温还原和多次氧化还原循环反应评价了氧化物的反应性,并在固定床反应器上研究了其甲烷氧化性能.结果表明,与离心法和蒸发法相比,垂直沉积法获得的PS微球模板排列更均匀有序;前驱物溶剂及浓度对最终的三维有序大孔材料的结构有显著影响,利用乙醇为前驱物溶剂所制备的样品比利用乙烯为溶剂的样品具有更好的三维有序大孔结构,前驱物乙醇溶液浓度在1.0 mol/L为宜.甲烷氧化实验表明,3DOM-LaFeO₃钙钛矿型氧化物中存在两种氧物种:表面吸附氧和体相晶格氧.表面吸附氧主要在反应初期将甲烷完全氧化为CO₂和水蒸汽,而体相晶格氧主要将甲烷部分氧化为H₂和CO.在甲烷部分氧化反应中,三维有序大孔LaFeO₃钙钛矿型氧化物比相同质量的纳米LaFeO₃氧化物提供了更多的氧,并且可使甲烷在较宽的反应阶段生成H₂和CO摩尔比为2:1的合成气,从而更有利于后续的费托合成等工艺.     

Oxidative coupling of methane in porous Vycor membrane reactors

Porous Vycor membrane tubes were used in shell-and-tube type membrane reactors to study the effect on the oxidative coupling of methane of metering the oxygen into the catalyst bed. Experimental studies showed that under conditions of complete oxygen conversion, Vycor membrane reactors packed with Sm₂O₃ catalyst exhibited enhanced hydrocarbon (C₂) selectivity. C₂ yields were comparable to those of the conventional co-feed packed bed reactors operated under the same conditions. The higher C₂ selectivity in the membrane reactors indicated that, for methane coupling, regulating the supply of oxygen along the length of the packed bed may be beneficial to C₂ formation.     

Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion

A mixed-conducting perovskite-type Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO) ceramic membrane reactor with high oxygen permeability was applied for the activation of methane. The membrane reactor has intrinsic catalytic activities for methane conversion to ethane and ethylene. C₂ selectivity up to 40–70% was achieved, albeit that conversion rate were low, typically 0.5–3.5% at 800–900°C with a 50% helium diluted methane inlet stream at a flow rate of 34 ml/min. Large amount of unreacted molecular oxygen was detected in the eluted gas and the oxygen permeation flux improved only slightly compared with that under non-reactive air/He experiments. The partial oxidation of methane to syngas in a BSCFO membrane reactor was also performed by packing LiLaNiO/γ-Al₂O₃ with 10% Ni loading as the catalyst. At the initial stage, oxygen permeation flux, methane conversion and CO selectivity were closely related with the state of the catalyst. Less than 21 h was needed for the oxygen permeation flux to reach its steady state. 98.5% CH₄ conversion, 93.0% CO selectivity and 10.45 ml/cm² min oxygen permeation flux were achieved under steady state at 850°C. Methane conversion and oxygen permeation flux increased with increasing temperature. No fracture of the membrane reactor was observed during syngas production. However, H₂-TPR investigation demonstrated that the BSCFO was unstable under reducing atmosphere, yet the material was found to have excellent phase reversibility. A membrane reactor made from BSCFO was successfully operated for the POM reaction at 875°C for more than 500 h without failure, with a stable oxygen permeation flux of about 11.5 ml/cm² min.