CrO_x/ZrO_2催化剂上甲烷催化燃烧性能

采用沉淀法制备了CrO_x/ZrO_2催化剂,考察800℃高温焙烧的CrO_x/ZrO_2催化剂对CH_4燃烧的催化性能。采用X射线衍射和拉曼光谱等技术对催化剂进行物相结构表征。结果表明,Cr物种以Cr_2O_3形式存在,随着焙烧温度升高,催化剂中ZrO_2和Cr_2O_3的晶粒明显增大。CrO_x/ZrO_2催化剂的比表面积大于相应的纯Cr_2O_3和ZrO_2。催化剂的CH_4燃烧活性随着Cr含量的增加而提高,Cr质量分数20%时活性最高,CH_4完全燃烧温度为450℃。     

负载型钯催化剂上甲烷催化燃烧的研究进展

概述了负载型钯催化剂对甲烷完全氧化的催化机理,以及钯催化剂在甲烷催化燃烧中的性能特点。     

Metallic monolith supported LaMnO3 perovskite-based catalysts in methane combustion

Metallic monolith supported LaMnO₃ perovskite-based catalysts are characterized by a high activity in methane combustion (95.5% conversion at 745 °C) and by a high thermal resistance. The activity of the catalysts depends on the duration and temperature of LaMnO₃ calcination. The same relation holds for the chemical composition of the catalyst surfaces when they are determined by the XPS method. The shortening of the time of LaMnO₃ perovskite calcination from 12.5 h to 8 h (700 °C) reduces the conversion of methane over a fresh catalyst. This is attributable to the lower amount of manganese (Mn:La = 0.48) on the surface of this catalyst compared to the catalyst whose perovskite was calcined for 12.5 h (Mn:La = 1.8). The extension of calcination time from 8 h to 12.5 h (at 700 °C) brings about a decrease in the specific surface area (SSA) from about 13.7 m²/g to 9.4 m²/g. After approximately 6 h on stream, the activities of the two catalysts become comparable. Aging of the catalyst with an LaMnO₃ active layer at 920 °C for 24 h reduces methane combustion to 82.5% (at 745 °C). The aging process changes the catalyst surface, where Al and C content increases and the Mn:La ratio decreases. The activity of the monolithic LaMnO₃ catalyst rises with the increase in the amount of the active layer from 11.5% to 17.8%. Methane conversion is greater over catalysts with an LaMnO₃ than with an LaCoO₃ active layer, but the LaMnO₃ catalysts show a lower resistance to thermal shocks.     

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 °C and completely convert methane to CO₂ below 550 °C where the conversion rate reaches 35 μmol (CH₄)/g_cat s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.     

Catalytic combustion of methane on novel catalysts derived from Cu-Mg/Al-hydrotalcites

Novel Cu-Mg/Al mixed oxides (designated as i-CMAO-800) were prepared by calcinations of Cu-Mg/Al hydrotalcites [(Cu²⁺ +Mg²⁺)/Al³⁺= 3] at 800 °C. Their performance for the catalytic combustion of methane was investigated. The oxides and their precursors were characterized by XRD, TG-DSC, TPR and N₂ adsorption/desorption techniques. The results showed that BET surface areas and the stability of the resultant oxides were greatly influenced by the copper contents in hydrotalcite precursors, bringing about difference in their activities for methane catalytic combustion. XRD results indicated that Cu was highly dispersed in hydrotalcite precursors in case of low copper contents, (Cu 40 wt%). For higher Cu contents, Cu(OH)₂ was formed, and, consequently, a separate phase of CuO was detected in the oxide catalysts after calcination. As indicated by the TG-DSC results, different decomposition behaviors were observed for various hydrotalcites. Thermal calcination promoted the formation of copper aluminates and segregation of CuO from the bulk phases. TPR results showed 15CMAO-800 has the highest reduction rate, and the catalytic activities of iCMAO-800 mixed oxides depend on both the reduction rates and the amounts of copper ions in mixed oxides. The catalyst 15-CMAO-800 showed the best performance.     

MnOx/Mg-Al2O3催化甲烷燃烧反应的研究

在Al2O3上浸渍Mg（NO3）2溶液,焙烧后制得Mg改性Al2O3（Mg-Al2O3）。以Mg-Al2O3为载体,制备了MnOx/Mg-Al2O3系列催化剂,测试了这些催化剂对甲烷燃烧反应的催化活性。结果表明：Mg的加入有效抑制了Al2O3在高温焙烧时发生γ相变,提高了Al2O3的热稳定性,MnOx/10%Mg-Al2O3对甲烷燃烧的催化活性较高,Mg的适宜加入量为10%。     

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.     

Low-temperature total oxidation of methane by pore- and vacancy-engineered NiO catalysts

Designing methane combustion catalysts operated under low temperature (<400°C) remains a huge challenge, especially for noble-metal–free catalytic systems. With NaCl as a crystalline scaffold, NiO catalyst with abundant oxygen vacancies and an ultra-high–specific surface area of 181 m² g⁻¹ is obtained. The mesoporous NiO exhibits outstanding CH₄ combustion performance (T₉₀ = 370°C at the weight hourly space velocity (WHSV) = 20,000 mL g⁻¹ h⁻¹). X-ray photoelectron spectroscopy (XPS), H₂-temperature-programmed reduction (TPR), kinetic measurements, and O¹⁸ isotope-labeling experiments together disclose the key role of surface lattice oxygen and reaction mechanism by NiO catalysts. More importantly, the excellent stability of NiO by doping La was obtained (low-temperature thermal stability: 385°C, 400 h, 4 vol% H₂O).     

The nature of active sites for the oxidation of methane on La-based perovskites

Highly crystalline, monophasic LaFeO₃ and LaCoO₃ perovskites, prepared by the explosion method, are shown to be heterogeneous at surface level. The outmost atomic layers of these perovskites contain high concentrations of carbonate-type species. Their specific activities for methane combustion are in fact identical to La₂O₂CO₃ and air-exposed La₂O₃. These results compared with pertinent data from the literature hint that surface heterogeneity may be often present in mixed oxides catalysts.     

Synthesis and properties of PdO/CeO2-Al2O3 catalysts for methane combustion

This study focuses on the loading of catalytic materials, e.g., palladium on the surface of supporting materials, with the aim to obtain catalysts with high activity for methane combustion. The catalyst PdO/CeO₂-Al₂O₃ was prepared by impregnation under ultrasonic condition. The effect of different activation methods on the activity of catalysts for methane catalytic combustion was tested. The properties of reaction and adsorption of oxygen species on catalyst surface were characterized by H₂-temperature programmed reduction (H₂-TPR), and O₂-temperature programmed desorption (O₂-TPD). Furthermore, the sulfur tolerance and sulfur poisoning mode were investigated. The results indicate that the catalyst PdO/CeO₂-Al₂O₃ activated with rapid activation shows higher activity for methane combustion and better sulfur tolerance. The result of sulfur content analysis shows that there is a large number of sulfur species on the catalyst’s surface after reactivation at high temperature. It proves that the activity of catalysts cannot be fully restored by high-temperature reactivation.     

Monte Carlo simulation studies of the catalytic combustion of methane

A Monte Carlo (MC) simulation is carried out of the catalytic oxidation of methane based on the pseudo-Langmuir-Hinshelwood (LH) mechanism proposed by Iglesia et al., that attempts to interpret the behavior of this reaction, which from experiments is assumed to be of the Mars-van Krevelen type. The results interpret reasonably some experimental findings (reaction order with respect to H₂O, CH₄, activity and structural sensitivity) if certain criteria based on the experiment and some laboratory data for the kinetic and thermodynamic parameters from the literature are considered.     

The effect of SO2 on the activity of Pd-based catalysts in methane combustion

The effect of SO₂ on Pd-based catalysts for the combustion of methane has been investigated. It is shown that while SO₂ poisons Al₂O₃- and SiO₂-supported catalysts, pre-treatment of Pd/ZrO₂ by SO₂ enhances the activity substantially.     

Catalytic combustion of methane over barium hexaferrites

Barium hexaferrite BaFe₁₂O₁₉ and iridium-containing barium hexaferrites have been prepared by the citrates gel method. Their catalytic activity in methane combustion has been evaluated. BaFe₁₂O₁₉ is an efficient catalyst for this reaction, and the introduction of iridium in the hexaferrite structure does not improve this activity. Mössbauer spectroscopy suggests that a part of the iridium ions are incorporated in the hexaferrite structure, however in crystallographic sites where they cannot interact with the gas phase. Infrared study of CO adsorption reveals the presence of two types of iridium particles in the surface: small Ir particles, in strong interaction with the hexaferrite structure, and some larger Ir particles which were not incorporated into the lattice.     

Mn、Fe取代六铝酸盐的结构和甲烷催化燃烧性能

采用化学可控共沉淀法制备了系列取代六铝酸盐LaMe_xAl_12-xO_19-δ（Me＝Fe、Mn）催化剂,研究了焙烧温度和Fe、Mn的离子取代量对催化剂比表面、结构及甲烷催化燃烧活性的影响.结果表明,催化剂前驱物经1000℃焙烧,催化剂中开始有六铝酸盐晶相生成; 当焙烧温度提高到1200℃时,样品主要以六铝酸盐晶相存在.增加Fe、Mn离子取代量可以提高六铝酸盐晶相的结晶度,但同时导致晶粒增大,引起比表面下降.由XPS和TPR分析表明,Mn在六铝酸盐结构中以+2价和+3价混合价态存在,而Fe以+3价形式存在.用Fe和Mn离子取代晶格中的Al³⁺大大提高了六铝酸盐对甲烷催化燃烧活性,当Mn离子的取代数为1,Fe离子的取代数为2时催化剂的活性最高.     

硫化处理对负载钯催化剂甲烷低温催化燃烧性能的影响

采用浸渍法制备氧化硅负载钯催化剂,考察硫化气氛和硫化时间对催化剂性能的影响,并采用XRD对催化剂进行表征。结果表明,硫化后形成的Pd4S物种可以提高催化剂抗硫中毒性能,同时催化剂表面的SPd4S/SPd对催化活性有重要影响,改变硫化气氛和硫化时间可以调节Pd4S物种的形成和SPd4S/SPd。     

微型燃烧器内甲烷预混催化燃烧的数值研究

微尺度燃烧存在热量损失大、易熄火、燃烧不完全、转化效率不高等问题,因此对微型燃烧器内甲烷的燃烧采取预混催化燃烧方式来提高燃烧的稳定性和转化效率,为微型发动机碳氢燃料燃烧技术奠定基础。采用连续介质层流有限速率模型和二阶离散方法对微型燃烧器微流道内的催化燃烧、流动和传热进行了三维数值模拟。结果表明,甲烷质量流量和过量空气系数对催化转化效率有一定影响,壁面温度是影响催化转化效率的主要因素。甲烷质量流量、壁面温度与最佳过量空气系数之间具有一定的变化关系。可根据催化温度选择富燃料或富氧燃烧方式来提高微尺度催化转化效率。恒壁温边界条件下,催化燃烧主要发生在燃烧腔的下壁面。     

煤层气非催化燃烧除氧实验研究

煤层气由于热值及浓度较低,达不到工业利用要求,工业应用中需进行浓缩。由于煤层气中含有氧,给浓缩过程带来了不安全因素,因此在浓缩前必须将氧除去。简要介绍了煤层气除氧的几种方法,并在除氧反应器装置上对煤层气非催化燃烧除氧进行了实验研究。结果发现,通过非催化燃烧法除氧,O2浓度能够降至1%以下;选用的除氧燃料须具有反应活性好、热值低的特点;除氧温度接近600℃时,CH4损失率小于5%,CH4的裂解是造成CH4损失的主要原因。     

Pd catalysts supported on silicon nitride for the combustion of methane: Influence of the crystalline and amorphous phases of the support and of the preparation method on the catalytic performances

Non-oxide refractory materials, such as silicon nitride having high thermal stability and thermal conductivity can be used as catalytic supports. The influence of the Si₃N₄ support nature and of the chemical compounds used for preparations on the physical-chemistry and catalytic properties of the palladium systems in the total oxidation of methane was investigated. A strong influence of the phase composition and the crystalline state of supports on the catalytic properties in the total oxidation of methane of the Pd catalysts was found. The activity of Pd catalysts increases with the -Si₃N₄ content and crystallization state of the support. The catalytic activity of Pd/-Si₃N₄ is also strongly affected by the preparation procedure. The Pd/-Si₃N₄ catalyst obtained by aqueous impregnation is less active and less stable. It was proposed that if water is used as an impregnation solvent, the surface acid-based properties of Si₃N₄ support and/or of the Pd active phase are irreversibly damaged. Pd supported on -Si₃N₄, prepared by impregnation of the Pd precursors in toluene solutions are found to be the most active and stable under reaction conditions.     

硫化处理对钯锆纤维催化剂甲烷低温催化燃烧性能的影响

采用胶体喷吹成型法制备钯锆-γ-氧化铝纤维催化剂。将催化剂在不同硫化气氛进行硫化处理,置于扩散式催化燃烧取暖器。考察硫化气氛、热负荷和耐久试验时间对燃烧取暖器的烟气浓度、起燃温度和燃烧板面平均温度的影响。使用烟气分析仪和红外热成像仪对取暖器的烟气浓度和板面平均温度进行分析。结果表明,纤维催化剂经10%H₂S-H₂气氛硫化120 min,可以制得初始活性良好、稳定性较好且完全符合燃气取暖器CE标准指标的催化剂。     

Catalytic removal of methane over thermal-proof nanostructured catalysts for CNG engines

Five spinel-type catalysts AB₂O₄ (Co_0.8Cr₂O₄, CoCr₂O₄, MnCr₂O₄, MgFe₂O₄ and CoFe₂O₄) were prepared and characterized by XRD, BET and FESEM techniques. The activity of these catalysts towards the combustion of methane was evaluated in a Temperature Programmed Combustion (TPC) apparatus. The half conversion temperature of methane over the Co_0.8Cr₂O₄ catalyst was 369 °C with a W/F = 0.12 g s/cm³. On the basis of Temperature Programmed Desorption (TPD) of oxygen as well as of catalytic combustion runs, the prevalent activity of the Co_0.8Cr₂O₄ catalyst could be explained by its higher capability to deliver suprafacial chemisorbed oxygen species. This catalyst, promoted by the presence of 1 wt% of Pd, deposited by wet impregnation, was lined on cordierite monoliths and then tested in a lab-scale test rig. The combination of Pd and Co_0.8Cr₂O₄ catalysts enables half methane conversion at 340 °C (GHSV = 10,000 h⁻¹), a performance similar to that of conventional 4 wt% Pd-γ Al₂O₃ catalysts but guaranteed with just a four-fold lower amount of noble metal. Both the catalysts in powder and the monolith hosting the Co_0.8Cr₂O₄ + 1 wt% Pd catalyst, submitted to a thermal ageing treatment in air at 700 °C for 12 h, displayed a negligible deactivation.