Catalytic activity of PdO/ZrO2 catalyst for methane combustion

Catalytic activity of ZrO₂ supported PdO catalysts for methane combustion has been investigated in comparison with Al₂O₃ supported PdO catalysts. It was found that the drop of catalytic activity owing to decomposition of PdO at a high temperature region (600–900°C) was suppressed by using ZrO₂ support. Temperature-programmed reduction (TPR) measurements of the catalyst with hydrogen revealed that the PdO of PdO/Al₂O₃ catalyst was reduced at the temperature less than 100°C, whereas in PdO/ZrO₂ catalyst the consumption of hydrogen was also observed at 200–300°C. This result indicates that the stable PdO species were present in the PdO/ZrO₂ catalyst. In order to confirm the formation of the solid solution of PdO and ZrO₂, X-ray diffraction (XRD) analyses of the mixtures of ZrO₂ and PdO calcined at 700–900°C in air were carried out. The lattice volume of ZrO₂ in the mixture was larger than that of ZrO₂. Furthermore, the Pd thin film on ZrO₂ substrate was prepared as a model catalyst and the depth profile of the elements in the Pd thin film was measured by Auger electron spectroscopy (AES). It was confirmed that Zr and O as well as Pd were present in the Pd thin film heated at 900°C in air. It was considered that the PdO on ZrO₂ support might be stabilized by the formation of the solid solution of PdO and ZrO₂.     

Kinetics of PdO combustion catalysis

The kinetics of the catalytic combustion of methane by supported palladium oxide catalysts (2 wt.-% Pd/La₂O₃·11A1₂O₃ and 5 wt.-%Pd/ γ-A1₂0₃ were examined for several oxygen partial pressure levels over the temperature range from 40–900°C using temperature-programmed reaction and slow ramp and hold temperature-time transient techniques. Combustion rates were measured by differential reaction in a fixed bed of powdered catalyst at lower temperatures (200–500°C). Also, by preparing the catalysts as thin (ca. 10 μm) coatings on an alumina tube and conducting the experiments with very high flows of dilute methane and oxygen in helium, the rate measurements were extended up to 900°C without significant contribution from gas phase reactions. The specific combustion activity of supported PdO shows a persistent hysteresis between 450 and 750°C, i.e., the rate of combustion between these temperature limits depends strongly on whether the catalyst is cooling from above 750°C or heating from below 450°C. This region is also notable for negative apparent activation energy in the rate of methane oxidation, i.e., the rate increases with decreasing temperature during reoxidation of the Pd metal and decreases with increasing temperature (especially with low oxygen partial pressure) prior to decomposition of the bulk oxide. Detailed time-temperature transient kinetic analyses were performed for supported PdO catalysts within the 450–750°C temperature range. The hysteresis in methane combustion rate is caused by a higher activation energy for reduction of oxygen chemisorbed on metallic Pd and by suppressed reoxidation of Pd metal relative to PdO decomposition.     

Carbon formation from methane in combustion products

The rates of deposition of carbon on alumina surfaces and on soot particles, have been measured in a pilot scale tubular reactor in which cold methane was mixed with combustion products at 1920°K. A hard grey metallic film of carbon, quite free of soot, was deposited on alumina surfaces for initial methane concentrations between 12 and 24 per cent. An induction period of slow growth rate, before a film covered the surface completely, was followed by a constant growth rate. Measured growth rates were from 0·06 × 10^?6 to 1·43 × 10^?6 g/cm² sec of carbon on alumina at 1270°K to 1450°K, and from 0·1 × 10^?4 to 1·14 × 10^?4 g/cm² sec on soot particles at 1370°K to 1700°K. Methane decomposition rates were much higher than predicted by the unimolecular mechanism indicating a predominance of radical reactions. Carbon deposition rates were related to the mole fraction, χ, of hydrocarbons in the gas which bear more than three carbon atoms per molecule, by, $\text{m}\text{?}{}_{\mathrm{f}}\text{= 1·0 × 10}{}^2\text{n.χ}$. exp (?42,300/RT_f), g/cm²sec for carbon film, $\text{m}\text{?}{}_{\mathrm{s}}\text{= 4·6 × 10}{}^3\text{nχ}$ exp (? 46,100/RT_g), g/cm² sec for soot. A precoat of soot increased the growth rate of film carbon by 1·8 to 7·8 times yielding a hard adherent dull brown film     

Effect of hydrothermal ageing on the performance of Ce-promoted PdO/ZrO2 for methane combustion

The performance of different Ce-modified PdO/ZrO₂ catalysts for methane oxidation in lean mixtures (5000 ppm of CH₄) in presence of external water has been studied in this work. Deactivation experiments carried out in presence of 20,000 ppm of external water showed that water reversibly inhibits the reaction. However, it was observed that these catalysts can increase their activity in presence of water at low temperature (350 °C).In order to explain this behaviour, different samples of this catalyst were treated with wet air (20,000 ppm of H₂O for 30 h). After this pre-treatment, their activity and stability for methane combustion were studied by recording light-off curves for the fresh catalysts and the catalyst after 50 h on stream for the oxidation of methane at 500 °C. As general trend, the hydro-ageing at the lowest temperature (300 °C) leaded to a very active catalyst (similar activity than the parent one), but it was more markedly deactivated. Hydro-ageing of the catalyst at higher temperatures enhanced its thermal stability.     

Low-temperature combustion of methane using PdO/Al2O3 catalyst: Influence of crystalline phase of Al2O3 support

Palladium oxide (PdO) catalysts supported on various alumina supports (θ-, δ-, κ-, η-, and γ-Al₂O₃) were prepared and the effect of the crystalline phases of the supports on the catalytic performance in methane combustion was investigated. Among the catalysts examined, the PdO/θ-Al₂O₃ catalyst, which showed the lowest reduction temperature in the temperature-programmed reduction of methane and the highest oxygen capacity in methane pulse experiments, exhibited the highest CH₄ conversion and stability for methane combustion. However, all the catalysts were deactivated because of the growth of PdO particles and strong adsorption of water vapor.     

Catalytic activity of powder and monolith perovskites in methane combustion

Honeycomb monolith perovskite catalysts were prepared from ultradispersed powders of mixed oxides of rare-earth metals (La–Ce or Dy–Y) and transition metals (Ni, Fe, Mn) by mechanochemical methods. A plasmochemical method was used to obtain La–Ni containing monoliths. The catalytic activity of powders and monoliths was compared in the catalytic combustion of methane. The intrinsic catalytic properties of the active components (apparent kinetic constant and energy of activation) were not significantly affected by the manufacturing procedure of monoliths in a large range of temperatures. Best performance was exhibited by La–Ni oxides containing monoliths which possess the highest pore volume and fraction of macropores.     

水蒸气对oxy-coal燃烧中NO生成的影响

为了研究H₂O对燃料N向NO转化各个阶段的影响,采用可沿程取样的沉降炉反应器,研究了一种烟煤在1273 K温度下,O₂/N₂、O₂/CO₂以及O₂/CO₂/H₂O气氛下燃烧的燃尽与NO生成的情况.并通过在停留时间为0.2、0.3、0.5、1.1 s与O₂浓度为5%、21%及30%情况下的实验来研究H₂O对NO生成的影响.实验结果表明,相对于O₂/CO₂气氛,H₂O的添加抑制了NO的生成,且该影响主要集中在燃烧初期挥发分N的氧化过程.随着O₂浓度的增加,H₂O添加对oxy-coal燃烧方式下NO生成影响加大.     

Methane oxidation over PdO x : on the mechanism for the hysteresis in activity and oxygen content

A conceptual picture is developed to explain the peculiar kinetic features of methane oxidation over supported Pd catalysts (observed by several investigators), notably the hysteresis in activity accompanying temperature cycles. Experiments were performed with supported Pdcatalysts to illustrate these features. The activity hysteresis is closely coupled with a hysteresis in oxygen content. The latter is in turn attributed to the properties of the PCT-diagram of the involved three-phase system; gas phase O₂ and the two solid phases, Pd and PdO_x. The two main ingredients in the mechanism are: (i) the so-called absorption and decomposition plateau pressures for the O₂-Pd-PdO _x system are different, i.e., show a hysteresis, (ii) these pressures are not independent ofx, but increase with increasing oxygen content. Both features are deviations from the ideal three-phase system and are frequently observed for H₂-metalmetal hydride systems.     

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.     

Remarkable activity enhancement in the catalytic combustion of methane on supported palladium catalysts

The activity of supported palladium catalysts for the combustion of methane has been determined. It has been observed that the activity increases with time on stream irrespective of whether the catalysts are pre-calcined or pre-reduced. Careful re-reduction experiments have shown that the enhanced activity is maintained. It is concluded that the increase in activity with time on stream is not attributable to slow variations in the chemical state of the palladium particles, but to changes in the morphology of the palladium crystallites under reaction conditions. The possible role of dissolved carbon in stabilising the reconstructed crystallites against reduction is discussed.     

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.     

Effects of hydrogen addition on methane combustion

In this study, the effect of hydrogen on methane combustion characteristic was tested. The ignition temperature (T₁₀) and burn off temperature (T₉₀) was carried out in a quartz reactor at atmospheric pressure with the mixture flow rate of 800 mL/min. The compositions of outlet gas were measured online by Gasmet DX4000 FTIR gas analyzer. The results showed that hydrogen enhanced the activity of methane. For all methane concentration range, the T₁₀ of methane could decrease 50 °C–70 °C with the H₂/CH₄ mole ratio at 0.1. For 1 vol.% methane combustion, when the H₂/CH₄ was equal to 0.05, the T₁₀ and T₉₀ could decrease 45 °C and 42 °C, respectively. When the H₂/CH₄ was 2.5, the T₁₀ and T₉₀ could decrease about 170 °C and 180 °C, respectively. Further more, CO generated in a wider temperature range when the hydrogen was added.     

Effect of support on the apparent activity of palladium oxide in catalytic methane combustion

The support effect on the low temperature catalytic oxidation of methane over palladium catalysts was studied by comparing a series of metal oxides as the support. Samples of 0.010 g/g Pd catalysts supported on different grades and/or phases of TiO₂, Al₂O₃, and ZrO₂ were prepared via incipient impregnation and their catalytic activity was evaluated using a laboratory plug-flow reactor. The specific surface area of the supports determined by nitrogen adsorption varied from about 13-220 m²/g. Initial experiments conducted with titania (anatase) as a support showed a low apparent activity and a poor thermal stability. Focusing on anatase, we have successfully improved its thermal stability by additions of Al₂O₃ or by doping with CeO₂, or La₂O₃. However, contrary to expectations based on some information in the literature, we have found that the activity decreased in the sequence of Al₂O₃ > ZrO₂ > TiO₂, and was not a direct function of specific surface area. This was especially evident in the case of titania. The surface structure of the support and the nature of its interaction with the active component PdO seem to play a far more important role in activity than the apparent specific surface area. Moreover, anatase-supported catalysts present a very rapid deactivation, whereas rutile-supported catalysts are relatively stable. The observed phenomena could potentially be related to the interaction between support and the active phase of palladium. Several models have been proposed to describe the strong metal-support interaction, but either charge transfer or encapsulation seems to be the most probable.     

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.     

LaMnFexAl11-xO19-δ催化剂制备及其甲烷燃烧活性

以甲烷催化燃烧为目标反应,采用了碳酸铵为沉淀剂的共沉淀法制备了LaMnFexAl11-xO19-δ(x=1,2,4,6,8)六铝酸盐催化剂.在不同的焙烧温度下通过SEM、XRD进行表征分析,确定了最佳焙烧温度.对催化剂进行了XRD表征,研究了不同Fe加入量下所制备催化剂的化学结构及其对甲烷催化燃烧活性的影响.结果表明焙烧温度在1200℃左右焙烧4h可以形成完整的六铝酸盐晶型.当Fe的取代量为2时,所得催化剂的活性最佳.