Catalytic oxidation of methane over hexaaluminates and hexaaluminate-supported Pd catalysts

An aqueous (NH₄)₂CO₃ coprecipitation method, based on that of Groppi et al. [Appl. Catal. A 104 (1993) 101–108] was used to synthesize Sr_1−xLa_xMnAl₁₁O₁₉₋ hexaaluminates. These materials were first synthesized by alkoxide hydrolysis. This synthesis route requires special handling of the starting materials and is not likely to be commercially practical. The materials prepared by (NH₄)₂CO₃ coprecipitation have similar surface areas as those prepared by the alkoxide hydrolysis method. Their CH₄ oxidation activity, measured as the temperature needed for 10% conversion of methane, is higher than those prepared by alkoxide hydrolysis. The La-substantiated material, LaMnAl₁₁O₁₉₋, shows high surface area with 19.3 m²/g after calcination at 1400°C for 2 h. It is active for CH₄ oxidation with T_10% at 450°C using 1% CH₄ in air and 70 000 cm³/h g space velocity. The stability and activity of LaMnAl₁₁O₁₉₋ prepared by (NH₄)₂CO₃ coprecipitation method is a simple and important step forward for the application of CH₄ catalytic combustion for gas turbines.     

Ethanol combustion over strontium- and cerium-doped LaCoO3 catalysts

Ce- or Sr-doped LaCoO₃ bulk perovskites were prepared by citric acid method as well as 10 wt.% of LaCoO₃ was deposited on alumina carrier stabilized with lanthanum. Properties of prepared materials were characterized by determination of surface area, acid-basic properties and XRD, XPS, TPDO₂, H₂-TPR measurements as well as catalytic activity and selectivity for ethanol combustion was tested. It was found that substitution of La in LaCoO₃ with either Sr or Ce has only small effect on its activity in ethanol combustion. Strontium inserted into LaCoO₃ structure increases basic character of the perovskite surface as well as selectivity to acetaldehyde (ACA). Substitution of La with cerium has no effect on the concentration of basic sites and does not affect the selectivity to ACA. Activity of LaCoO₃-based catalysts in ethanol combustion and their selectivity to ACA formation can be explained on the basis of the presence of both -oxygen species and sites with basic character on the material surface.

Acid-basic properties of supported LaCoO₃ are dominated by acidic character of the carrier. Results of XPS and H₂-TPR measurements of LaCoO₃ supported on La–Al₂O₃ suggest that perovskite remains in strong interaction with carrier and probably is partially decomposed. Deposition of perovskite on stabilized carrier significantly increases the rate of ethanol combustion.     

Catalytic combustion of gasified biomass: poisoning by sulphur in the feed

The influence of sulphur on the catalytic combustion of gasified biomass for gas turbine applications has been studied over precious metal and metal oxide based catalysts, namely Pd/LaAl₁₁O₁₈, Pt/LaAl₁₁O₁₈, Pt/La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉, La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉ and LaMnAl₁₁O₁₉. The samples were washcoated on cordierite monoliths and tested in a bench-scale reactor with a synthetic low-heating value fuel mixed with air. The fuel gas, that resembles the gas from air-blown fluidised bed gasification of wood, was composed of hydrogen, carbon monoxide, methane, ethene, carbon dioxide, water and nitrogen. Different concentrations of hydrogen sulphide as well as sulphur dioxide were added to the fuel gas. The results show that all samples were deactivated to some extent by addition of sulphur, although poisoning of the catalytic combustion for each fuel component varied depending both on the active phase and on the support and generally was reversible. The palladium catalyst was severely deactivated for combustion of methane, although activity for carbon monoxide and hydrogen was almost maintained. Platinum catalysts were more severely poisoned for carbon monoxide and hydrogen, but not for methane. Metal oxide catalysts were severely deactivated for all fuel components, especially for carbon monoxide, and the La_0.5Ba_0.5Mn_0.5Fe_0.5Al₁₁O₁₉-sample was irreversibly poisoned. The samples were also characterised by BET, XRD, ICP, SEM–EDX, XPS and SIMS.     

Effect of Ru on LaCoO3 perovskite-derived catalyst properties tested in oxidative reforming of diesel

The oxidative reforming of diesel over Co/La₂O₃ and Ru–Co/La₂O₃ catalysts derived from LaCoO₃ perovskite precursors was studied. Physicochemical characterization by XPS, TPR and XRD revealed that the incorporation of Ru to LaCoO₃ produces changes in LaCoO₃ evidenced by a smaller size of the LaCoO₃ particles and cobalt segregation on the LaCoO₃ surface. The modifications in the structure of LaCoO₃ induced by the addition of Ru directly affect the dispersion and morphology of Co particles developed under the reaction. The active phases derived from pretreatment of perovskites evolve differently with time on stream, being observed that the presence of a greater proportion of perovskite phase in the Ru/LaCoO₃ sample produces an increase in catalyst stability. TPD-MS analysis also indicates that bulk oxygen release from the Ru–Co/La₂O₃ sample could improve its catalytic behaviour. The characterization of used samples reveals that improvements in the cobalt surface concentration and Co–La₂O₃ interactions contribute to the better catalytic stability of the Ru–Co/La₂O₃-derived catalyst.     

Catalytic oxidation of 1,2-dichlorobenzene over ABO3-type perovskites

Several ABO₃-type perovskite oxides (A = La, Y, Nd or Gd; B = Fe, Mn, Cr or Co) have been investigated as catalysts for the oxidation of 1,2-dichlorobenzene (o-DCB), a model compound for the highly toxic polychlorinated dibenzodioxins. Initial transient and steady state activity measurements were conducted with all catalysts in the absence and presence of water. Perovskites containing Cr in the B-site were more active than perovskites containing other transition metals, with YCrO₃ being the most active catalyst among the different systems studied. Furthermore, YCrO₃ did not show any loss of its initial activity after several hours on stream. Other perovskites lost 10–20% of their initial activity within the first 5–10 h on stream. This loss was associated with a corresponding loss in BET surface area. With the exception of LaCoO₃, all perovskites retained their crystalline structure upon exposure to o-DCB under reaction conditions. LaCoO₃ was converted to LaOCl and Co₃O₄. The presence of water appeared to enhance the catalytic activity of some perovskites. This effect can be attributed to a faster removal of Cl⁻ ions from the catalyst surface via their reaction with water.     

Catalytic combustion of methane over bimetallic Pd–Pt catalysts: The influence of support materials

The effect of support material on the catalytic performance for methane combustion has been studied for bimetallic palladium–platinum catalysts and compared with a monometallic palladium catalyst on alumina. The catalytic activities of the various catalysts were measured in a tubular reactor, in which both the activity and stability of methane conversion were monitored. In addition, all catalysts were analysed by temperature-programmed oxidation and in situ XRD operating at high temperatures in order to study the oxidation/reduction properties.

The activity of the monometallic palladium catalyst decreases under steady-state conditions, even at a temperature as low as 470 °C. In situ XRD results showed that no decomposition of bulk PdO into metallic palladium occurred at temperatures below 800 °C. Hence, the reason for the drop in activity is probably not connected to the bulk PdO decomposition.

All Pd–Pt catalysts, independently of the support, have considerably more stable methane conversion than the monometallic palladium catalyst. However, dissimilarities in activity and ability to reoxidise PdO were observed for the various support materials. Pd–Pt supported on Al₂O₃ was the most active catalyst in the low-temperature region, Pd–Pt supported on ceria-stabilised ZrO₂ was the most active between 620 and 800 °C, whereas Pd–Pt supported on LaMnAl₁₁O₁₉ was superior for temperatures above 800 °C. The ability to reoxidise metallic Pd into PdO was observed to vary between the supports. The alumina sample showed a very slow reoxidation, whereas ceria-stabilised ZrO₂ was clearly faster.     

Au/LaCo_(1-x)Ce_xO_3催化剂的制备及其对CO催化氧化性能的研究

为进一步提高催化剂活性,用Ce对LaCoO3载体进行改性,并采用溶胶-凝胶法制备系列LaCo1-xCexO3(x=0~0.5)载体。其中,x=0.1和0.2时,载体为钙钛矿结构。采用沉积-沉淀法制备Au/LaCo1-xCexO3(x=0.1、0.2)催化剂,通过XRD、BET和H2-TPR等方法对催化剂进行催化活性评价及稳定性表征测试。结果表明,Au/LaCo0.9Ce0.1O3和Au/LaCo0.8Ce0.2O3催化剂能够在90℃将CO完全转化,在此温度进行的连续20h和30h的寿命实验中,CO转化率保持100%,催化活性和稳定性均优于Au/LaCoO3催化剂。表明掺杂Ce改性载体,能够提高催化剂活性和稳定性。     

Supported Co-based perovskites as catalysts for total oxidation of methane

Supported LaCoO₃ perovskites with 2, 5, 10, 15, 20 and 30 wt.% loading were prepared by impregnation of a Ce_0.8Zr_0.2O₂ support (40 m² g⁻¹) with: (i) a solution of La and Co nitrates and (ii) a “citrate” solution, namely containing La and Co nitrates, and citric acid. All precursors were decomposed and calcined at 700 °C for 5 h. XRD investigations indicated the formation of a pure perovskite phase only if citrates were used. These materials were tested as catalysts for methane combustion in the temperature range 300–700 °C. All catalysts showed a lower T₅₀ (the temperature at which the conversion level of methane is 50%) than the Ce_0.8Zr_0.2O₂ support or non-supported LaCoO₃. The activity increased continuously with the perovskite loading. The samples prepared from citrates were slightly more active than from nitrates. This is due to a more homogeneous surface, as indicated by XPS measurements. The presence of a well-characterized perovskite phase (as opposed to highly dispersed elements) seems necessary for good activity. A higher reaction rate per perovskite weight is observed for low loadings when compared to bulk LaCoO₃, but the variation with perovskite loading presents a breakpoint, suggesting complex interactions in the catalysts or in the oxidation mechanism.

In spite of the experimental impossibility to evaluate the area developed by the supported perovskite, an approximative approach strongly suggests a synergy between the support and supported species.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

Superior performance of Ir-substituted hexaaluminate catalysts for N2O decomposition

Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N₂O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N₂-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H₂-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al³⁺ to form BaIr_xFe_1−xAl₁₁O₁₉ after being calcined at 1200 °C, while the other part of iridium existed as IrO₂ phase. The activity tests for high concentration (30%, v/v) of N₂O decomposition demonstrated that the BaIr_xFe_1−xAl₁₁O₁₉ hexaaluminates exhibited much higher activities and stabilities than the Ir/Al₂O₃-1200, and the pre-reduction with H₂ was essential for activating the catalysts. By comparing BaIr_xFe_1−xAl₁₁O₁₉ with BaIr_xAl_12−xO₁₉ (x = 0–0.8), it was found that iridium was the active component in the N₂O decomposition and the framework iridium was more active than the large IrO₂ particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework.     

Catalytic combustion of CO---H2 on Manganese-substituted hexaaluminates

This work is devoted to a preliminary study on the feasibility of catalytic combustion for syngas fuelled gas turbines equipped with Mn-substituted hexaaluminate catalysts. Results of catalytic activity tests on combustion of CO---H₂ over BaMn₁Al₁₁O₁₉ and La_0.8Sr_0.2Mn₁Al₁₁O₁₉ are reported and analyzed by means of a mathematical model of the catalyst section of a hybrid combustor. Experimental activity levels are shown to be close to the operating requirements of the combustor.     

Oxygen storage capacity of La1−xBO3 perovskites (with A′ = Sr, Ce; B = Co, Mn)—relation with catalytic activity in the CH4 oxidation reaction

The aim of this work was to study the effect of cation-substitution on the reducibility of the perovskite, as well as the effect on the catalytic activity for the CH₄ oxidation reaction. Six perovskites (LaCoO₃, LaMnO₃, La_1−xSr_xMnO₃ (x = 0.2, 0.4), and La_1−xCe_xMnO₃ (x = 0.05, 0.1)) were synthesized by reactive grinding. The reducibility of the perovskite was studied by means of the oxygen storage capacity (OSC) measurement. OSC was performed at different temperatures on LaCoO₃ and LaMnO₃, in order to elucidate the different mechanisms of reduction involved at each temperature. The substituted samples showed that reduction profile is modified at high-substitution degrees; however, no differences were observed on the OSC values (amount of most active oxygen, calculated after one pulse of CO) between the pure lanthanum sample and the substituted ones.

Tested in the CH₄ oxidation reaction, the LaCoO₃ sample was found to present a little higher activity than LaMnO₃, even if the cobalt-based sample presented a smaller specific surface area. Moreover, all the substituted samples presented very slightly higher activities than the pure LaMnO₃ solid. Because of the supposed redox oxidation mechanism (Mars-Van-Krevelen), this agrees well with the OSC results obtained for the reducibility of the manganese on these samples, by which it was observed that substitution does not clearly affect the immediate reduction of the manganese.     

Cordierite monolith supported perovskite-type oxides — catalysts for the total oxidation of chlorinated hydrocarbons

The catalytic behaviour of LaMnO₃, LaCoO₃ and (La_0.84Sr_0.16)(Mn_0.67Co_0.33)O₃ perovskites supported on cordierite monoliths was studied in the total oxidation of chlorinated hydrocarbons (CHC). By-products (higher chlorinated hydrocarbons, lower molecular coupling and cracking products) were formed in the low temperature range depending on the kind and concentration of CHC in the feed and the reaction conditions. The preparation method (impregnation or coating) influences catalytic activity and by-product formation. Addition of water, methane or propane to the feed diminishes the by-product formation significantly. In the case of monolith supported LaCoO₃ perovskite, an irreversible catalyst deactivation was observed.     

High temperature catalytic combustion of methane and propane over hexaaluminate catalysts: NOx emission characteristics

Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH₄ and C₃H₈, in order to investigate the effect of the concentration of the fuels, O₂ and H₂O on NO_x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr_0.8La_0.2MnAl₁₁O₁₉₋ prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO_x emission capability. NO_x emission at 1500 °C was increased linearly with O₂ concentration, whereas water vapor addition decreased NO_x emission in CH₄ combustion over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst. In the catalytic combustion of C₃H₈ over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst, the amount of NO_x emitted was raised in the temperature range between 1000 and 1500 °C when the C₃H₈ concentration increased from 1 to 2 vol.%. It was found that NO_x emission in this temperature range was reduced effectively by adding water vapor.     

Effects of ZrO2/Al2O3 properties on the catalytic activity of Pd catalysts for methane combustion and CO oxidation

Zirconia supported on alumina was prepared and characterized by BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), temperature programmed desorption (TPD), and pulse reaction. 0.2% Pd/ZrO₂/Al₂O₃ catalyst were prepared by incipient wetness impregnation of supports with aqueous solution of Pd(NO₃)₂. The effects of support properties on catalytic activity for methane combustion and CO oxidation were investigated. The results show that ZrO₂ is highly dispersed on the surface of Al₂O₃ up to 10 wt.% ZrO₂, beyond this value tetragonal ZrO₂ is formed. The presence of a small amount of ZrO₂ can increase the surface area, pore volume and acidity of support. CO–TPD results show that the increase of CO adsorption capacity and the activation of CO bond after the presence of ZrO₂ lead to the increase of catalytic activity of Pd catalyst for CO oxidation. CO pulse reaction results indicate that the lattice oxygen of support can be activated at lower temperature following the presence of ZrO₂, but it does not accelerate the activity of 0.2% Pd/ZrO₂/Al₂O₃ for methane combustion. 0.2% Pd/ZrO₂/Al₂O₃ dried at 120 °C shows highest activity for CH₄ combustion, and the activity can be further enhanced following the repeat run. The increase of treatment temperature and pre-reduction can decrease the activity of catalyst for CH₄ combustion.     

The catalytic activity and surface characterization of Ln2B2O7 (Ln=Sm, Eu, Gd and Tb; B=Ti or Zr) with pyrochlore structure as novel CH4 combustion catalyst

Ln₂B₂O₇ (Ln=Sm, Eu, Gd and Tb; B=Zr or Ti) with pyrochlore structure was prepared by sol–gel method for the high-temperature catalytic combustion. The crystal structure of Ln₂B₂O₇ was identified by XRD and their surface area was about 4 m²/g after calcinations at 1200 °C. Catalytic activity of methane combustion was observed for Ln₂Zr₂O₇ series and the best catalyst was Sm₂Zr₂O₇. Its relative reaction rate per unit surface area at 600 °C was 2 cm³/m² min, which was twice higher than that of Mn-substituted Sr hexaaluminate. From surface analysis by XPS, the low binding energy of each Ln element of Ln₂Zr₂O₇ compared to that of Ln₂Ti₂O₇, gave the catalytic activity of methane combustion.     

Catalytic properties and surface modification of hexaaluminate microcrystals for combustion catalyst

Spinel oxide (Mn₃0₄) surface layers were produced on hexaaluminate microcrystals by means of the air-oxidation process. The surface layer completely covered the basal plane of hexagonal facets as evident from HREM observation and CO₂ chemisorption measurement. The catalytic activity of as prepared Mn₃0₄/hexaaluminate composites for methane combustion was evaluated as functions of the Mn₃0₄loading and the metal composition of the spinel surface layer. It was revealed that the air oxidationderived composites showed the excellent specific activities superior to those of the corresponding Mn₃0₄/hexaaluminates prepared from the conventional evaporation-to-dryness process. Partial substitution of Fe for Mn of the surface layer was effective in enhancing combustion activity in a whole temperature range.     

过渡金属Ni对Pd/Al_2O_3催化剂甲烷催化燃烧性能的影响

考察过渡金属Ni对Pd/Al_2O_3催化剂甲烷催化燃烧活性的影响以及过渡金属负载量及循环条件对甲烷降解性能的影响,采用扫描电镜、N_2吸附-脱附以及H_2程序升温还原技术对催化剂进行表征。结果表明,过渡金属Ni的添加对催化剂在(375~475)℃下的甲烷催化燃烧活性产生影响。催化剂经多次重复使用后,催化燃烧活性提高。分析原因为经过渡金属Ni改性后,可与载体形成NiAl_2O_4尖晶石,促进活性组分形成较小晶粒,并改善活性组分分散度,提高催化剂催化活性。     

四效催化剂La-K-Co-Mn同时去除CO、C_xH_y、PM和NOx的研究

四效催化剂的活性组分La0.8K0.2Co0.7Mn0.3O3的合成温度对柴油烟气的净化效果有较大影响,通过X射线衍射、电子扫描电镜、程序升温、比表面积检测和在线效率检测发现,750℃合成的四效催化剂净化柴油烟气的效果最好。通过K和Mn同时部分取代La Co O3中La和Co,探讨K和Mn对钙钛矿结构和烟气净化效果的影响。比表面积检测发现,涂覆改性γ-Al2O3后,整体式催化剂比表面积增加,催化性能得到有效改善。     

Catalytic combustion of gasified biomasses over Mn-substituted hexaaluminates for gas turbine applications

The catalytic combustion of gasified biomasses over Mn-substituted hexaaluminates with high thermal stability is addressed. Combustion activity tests of the main fuel components, i.e. CO, H₂, C₂H₄ and CH₄, have been performed, and the effects of H₂O and CO₂ on the fuel combustion have been investigated. The reactivity of NH₃ in the catalytic combustion has also been studied in view of its potential source of undesired fuel-NO_x. Lab-scale data have been preliminarily scaled up through mathematical modeling.