Catalytic combustion of methane on palladium single crystals

Catalytic methane combustion was studied over the palladium single crystals Pd(1 1 1), Pd(1 0 0) and Pd(1 1 0). Under lean reaction conditions at 600 K (O₂:CH₄ = 10:1), stoichiometric palladium oxide was formed with an increase in surface area by a factor of approximately two. The oxide phase formed a “cauliflower-like” surface structure composed of approximately 4 nm sized semispherical oxide agglomerates. This oxide structure was independent of the original metal single crystal orientation. The turnover rates over the oxide structure starting with metal single crystals were 0.7 s⁻¹ on Pd(1 1 1), 0.9 s⁻¹ on Pd(1 0 0) and 0.9 s⁻¹ on Pd(1 1 0) at 600 K, 160 Torr O₂, 16 Torr CH₄, 1 Torr H₂O and N₂ balance to 800 Torr, suggesting that the methane combustion reaction is independent of the initial structure of the catalyst. Methane combustion on palladium single crystals experienced an activation period in which the initial turnover rates based on the initial Pd surface area were about 1/8–1/4 of the steady-state rates determined based on the oxide surface area. This activation period was caused by the slow oxidation of palladium single crystals and concomitant surface area increase during reaction. The increase in surface area happened mostly in the first 10 min of reaction. Carbon dissolution into the crystal was not found during methane combustion under reaction conditions in excess oxygen.     

C–H bond activation in hydrocarbon oxidation on heterogeneous catalysts

The activation of C–H bonds in different hydrocarbons on the surfaces of metal oxide and metal catalysts is considered. On oxides, it appears that the initial activation may occur through either homolytic or heterolytic scission of the C–H bond, but the reaction is surface-catalysed. The activation of methane requires highly basic sites which are susceptible to severe poisoning by carbon dioxide. With metal surfaces, the extent of oxidation of the surface can strongly affect the oxidation activity. For rhodium catalysts, it is shown that the intrinsic activity for methane combustion is high. However, rhodium is strongly deactivated under oxidising conditions when alumina is used as the support: deactivation is not observed when the support is zirconia. Transient effects on the activity of an alumina-supported palladium catalyst are reported and these show that the steady state is not easily established. Water severely inhibits the methane combustion reaction on palladium, and chlorine and sulphur dioxide are strong poisons. In contrast, for the combustion of propane on alumina-supported platinum catalysts, sulphur dioxide is a significant promoter of the reaction.     

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.     

Deactivation of palladium catalyst in catalytic combustion of methane

Catalytic combustion of natural gas, for applications such as gas turbines, can reduce NO_x emissions. Palladium-on-stabilised alumina has been found to be the most efficient catalyst for the complete oxidation of methane to carbon dioxide and water. However, its poor durability is considered to be an obstruction for the development of catalytic combustion. This work was aimed at identifying the origin of this deactivation: metal sintering, support sintering, transformation or coking.

Catalytic combustion of methane was studied in a 15 mm i.d. and 50 mm length lab reactor and in a 25 mm i.d. pilot test rig on monolithic honeycomb substrates. Experiments were performed at GHSV of 50 000 h⁻¹ in lab test and 500 000 h⁻¹ in pilot test. The catalysts used were palladium on different supports on cordierite substrate. The catalysts were characterised by XRD, STEM, ATG and XPS.

In steady-state conditions, deactivation has been found to be dependent on the air/methane ratio, the palladium content on the washcoat and the amount of washcoat on the substrate. An oscillating behaviour of the methane conversion was even observed under specific conditions, due to the reducibility of palladium oxide PdO to Pd. The influence of the nature of the support on the catalyst deactivation was also investigated. It has been shown that some supports can surprisingly eliminate this oscillating behaviour. However, in pilot test, deactivation was found to be very rapid, even with stabilised alumina supports. Furthermore, successive tests performed on the same catalyst revealed that the activity (light-off temperature, conversion) falls strongly from one test to another.

Then, the stabilised alumina support was calcined at 1230°C for 16 h prior to its impregnation by palladium, in order to rule out its sintering. Experiments carried out on precalcined catalysts point out that deactivation is mostly correlated to the metal transformation under reaction conditions: activity decreases gradually as PdO sinters, but it dropped much more steeply in relation to appearance of metallic palladium.     

Polymer-supported catalysts—selective hydrogenation of acid chlorides over palladium/polyamide

The catalytic behaviour of palladium supported on aromatic polyamides was studied in the liquid phase hydrogenation of benzoyl chloride at 1 atm total pressure and between 348 and 408 K. The specific activity of the catalysts as a function of palladium concentration was found to increase with metal loading. It is suggested that palladium in a metallic state is the active site for the acid chloride hydrogenation. A decrease in the reaction rate has been observed at the highest temperatures indicating that under these conditions the availability of hydrogen becomes the rate determining step. Results on the liquid phase hydrogenation of acyl- and aroyl-chlorides are reported. No strong influence of the nature of the substituents was observed on reaction parameters.     

Methane combustion over supported palladium catalysts I. Reactivity and active phase

The performance of Al₂O₃, ZrO₂ and ZrO₂ stabilized with SiO₂ (ZrO₂-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd (1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdO_x (0<x≤1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd⁰/PdO_x, is the most active phase for methane combustion, and that a redox mechanism may occur on this phase.     

Combustion of methane over palladium/zirconia: effect of Pd-particle size and role of lattice oxygen

The catalytic combustion of methane over palladium/zirconia catalyst prepared by oxidation and subsequent reduction of Pd/Zr glassy metals has been investigated with special emphasis on the influence of the catalyst structure (particle size and specific surface area) on the catalytic performance and the impact of a redox mechanism on the product formation. The reaction of 1% methane and 4% oxygen (balance of He) in a fixed-bed microreactor was found to be strongly dependent on the particle size of palladium, which was controlled by appropriate reduction before catalytic testing. Pre-reduction of PdO resulted in an enhanced activity compared to the unreduced catalysts. The structural changes induced upon reduction were accompanied by altered physico-chemical properties, evidenced by different behaviors in the decomposition of PdO and the reduction of PdO by methane. The correlation established between catalytic performance and the rate of reduction of PdO by methane led to the postulation of a redox mechanism, involving the reaction of methane with the active palladium oxide phase and subsequent reoxidation of metallic Pd by oxygen, which was independently confirmed by the use of ¹⁸O labeled catalysts and pulse methods.     

Chaos and synchronisation in heterogeneous catalytic systems: CO oxidation over Pd zeolite catalysts

The influence of experimental parameters on the structure of global reaction rate oscillations and the coupling of local oscillators on a catalyst bed in a continuous stirred tank reactor is studied for the oxidation of CO on zeolite supported palladium catalysts. Global coupling can be achieved via mass transfer through the gas phase or via heat transfer in the case of a support of high heat conductivity. Characteristic differences in the activity of catalysts as well as in the period and the amplitude of the oscillations are related to the size of the palladium clusters and can be simulated by adding the state of the oxidation of the metal surface as a parameter to a common kinetic model. The analysis of observed chaotic behaviour leads to the conclusion that diffusional chaos characteristic of a distributed system is observed on the level of the zeolite crystallite that supports the palladium clusters.     

The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane

Supported palladium catalysts are very active in the combustion of methane, but still little is known about the kinetic parameters. In this paper a rate expression is presented for an alumina-supported palladium oxide catalyst in the temperature range 180–515°C. Special care was taken to ensure differential conditions during the experiments. In this way, an apparent activation energy of 151±15 kJ/mol was found. The orders in methane, oxygen and water were 1.0±0.1, 0.1±0.1 and −0.8±0.2, respectively. For carbon dioxide a zero order was observed under all conditions. Inhibition by water produced during the reaction was demonstrated to cause non-differential conditions, when a dry feed was used. The rate constant that corrects for this effect could be derived.     

Easily separable molecular catalysis

Homogeneous metal catalysts offer greater selectivity and controllability because their molecular nature ensures that only one type of active site is present. However, the majority of large-scale, industrial chemical processes employs heterogeneous catalysts because of ease of their separation from products, thermal stability, and amenability to continuous processing. Heterogenization of homogeneous catalysts explores the cross-fertilization of two systems to combine most of their advantages. Some examples are discussed of heterogenized palladium complex catalysts applied for low-temperature CO oxidation, carbonylation for the synthesis of ibuprofen or naproxen, oxidative carbonylation for the synthesis of diphenyl carbonate, and functionalization of methane. There appears no significant change in reaction mechanisms when active centers are transferred from solution to the solid surface. In addition to the easy recovery, heterogenized homogeneous catalysts can have advantages in the productivity over homogeneous catalysts when a suitable support material is selected considering its hydrophilicity and surface reactivity.     

Colloidal submicron-palladium particles stabilized with acetate

Palladium spherical particles 0.23 μm in diameter were synthesized by reducing palladium acetate with hydrazine in the presence of surfactant, with an aim of exhibiting both easy separation by filtration and easy dispersion for a catalyst. The particles in the suspension were sedimented slowly but not aggregated. The suspension showed voltammetric redox waves. The anodic wave was ascribed to the oxidation of Pd to Pd²⁺, whereas the cathodic one was to the reduction of the palladium acetate moiety to Pd. The current ratio of the anodic peak to the cathodic one 4:1, was close to the ratios by the partial chemical oxidation with permanganate and by the thermogravimetry, suggesting the composition of 80% palladium metal and 20% palladium acetate in the molar ratio. Heating the palladium particles at 300 °C yielded palladium metal. The decomposition proceeded to the first-order reaction with the activation energy of 40 kJ mol⁻¹. The particle catalyzed the reduction of methylene blue with hydrazine. The reaction rate was of the first-order with respect to methylene blue. The rate constant was proportional to the geometrical surface area of the palladium particle, suggesting a surface catalysis.     

Synthetic gas bench study of a natural gas vehicle commercial catalyst in monolithic form: On the effect of gas composition

With growing concerns about global environmental and increased focus on population health is renewed interest in transportation sector. The challenge is to find and develop cost effective ways to improve urban air quality without scarifying economy. The natural gas, as fuel, constitutes an alternative to the traditional fuels because its impact on the reduction of the global warming impact is consequent compared to the traditional fuels such as gasoline. This study deals with a study of NGV’s catalysts. Natural gas vehicles’ catalysts were characterized by XPS, STEM in order to determine the various active phases. The results of characterization showed that the palladium oxidized Pd²⁺ was the principal active site. The experimental runs were carried out close to the real operating conditions using a sample of monolith. Numerous reactions were identified, such as NO reduction by hydrogen, carbon monoxide at low temperature and methane at high temperature. Moreover, methane reforming and water-gas shift were clearly found on commercial system. The influence of each reaction on the kinetics behaviour of the whole mixture makes difficult to uncouple the different reactions of oxidations and reductions, because the rate of these reactions was highly dependant on the reactant present in the automobile exhaust gases as well as their relative concentration. To identify each involved reaction in NGV TWC process, we remove alternatively reducing agents or oxidizing agents.     

Complete methane oxidation over Pd catalyst supported on α-alumina. Influence of temperature and oxygen pressure on the catalyst activity

The influence of the reaction parameters including temperature, oxygen concentration, and of in situ hydrogen reduction on the Pd catalyst activity towards complete methane oxidation is studied experimentally.Zero porosity α-alumina plates are used as a support for Pd catalyst. This lowers the influence of metal–support interaction on the catalyst state as confirmed by UV–visible spectroscopy. A plug flow reactor with a high linear gas velocity is used to measure the reaction rate. Overall conversion is kept low for most of the experiments so that the reaction is in the kinetically limited regime. The oxidation state of the catalyst before and after the reaction is determined using UV–visible reflectance spectroscopy of the plate surface. Changes in the catalyst activity with time are monitored after stepwise changes in the reaction parameters.Activity was found to decrease with time at low temperatures and high oxygen concentrations (condition when PdO phase is stable) and to increase with time at high temperatures and low oxygen concentrations (conditions when Pd is stable). A sharp increase in conversion was observed after the in situ hydrogen reduction of the sample.The experimental data is consistent with the reduced Pd form of the catalyst being more active towards methane oxidation than the oxidized PdO form at high temperatures. Possible particle size and morphology effects are discussed.     

Chemical modelling and measurements of the catalytic combustion of CH4/air mixtures on platinum and palladium catalysts

This work reports experimental measurements and a modelling study carried out on palladium and platinum based catalytic monoliths used as methane combustors for heating purposes. It concentrates on the effects of operating conditions on combustion, heat transfer efficiency and pollutant formation. The development of a detailed homogeneous/heterogeneous chemical kinetics model for methane–air combustion over palladium using literature data was undertaken to model the behaviour of one of the experimental catalytic heaters. In addition, a published detailed chemical mechanism for methane combustion over platinum was used in the platinum catalyst model. The fuel–air equivalence ratios ranged from 0.3 to 0.6 and the space velocities used were between 24 000 and 72 000 h⁻¹. Although the model assumed perfectly stirred reactor (PSR) conditions and was applied to localised regions of the monoliths where little radial gradients of temperature and concentrations were measured, it predicted the surface temperature, methane slippage, CO and NO_x at the downstream face of the monolith with reasonable accuracy in some cases, but also highlighted the shortcomings of the PSR assumption in other cases.     

Importance of palladium dispersion in Pd/Al2O3 catalysts for complete oxidation of humid low-methane–air mixtures

The catalytic oxidation is considered as an environmental benign method for utilization of various methane-poor gas mixtures, including humid post-ventilation air of coal mines. The small crystallites of palladium phase in the Pd/Al₂O₃ catalyst decrease temperatures necessary to ignite the methane oxidation reaction and to achieve complete conversion of methane. The isotopic exchange of oxygen between the catalyst and the gas phase, the temperature-programmed reduction (TPR) with methane and the X-ray photoelectron spectroscopy studies suggest that it can result from a higher number of the Pd–PdO sites present on the catalysts with small palladium crystallites. The inhibiting effect of water vapour present in the reaction mixture increases with lower dispersion of palladium phase as well as with the water concentration in the feed. The larger palladium crystallites are more significantly affected by the presence of water. It is suggested that water vapour blocks the Pd–PdO active sites. The catalysts with small crystallites (<6.6 nm) of palladium can be successfully used for mitigation of the emission of methane from coal mine post-ventilation air and, after increasing of the methane concentration to 1–2 vol.%, for its utilization for the energy production. In the case of such catalysts even a high concentration of water vapour has the least negative influence on the catalyst activity and it will not interfere with obtaining of the 100% conversion of methane below 650 °C.     

Pure hydrogen production by methane steam reforming with hydrogen-permeable membrane reactor

Low temperature steam reforming of methane mainly to hydrogen and carbon dioxide (CH₄ + 2H₂O → 4H₂ + CO₂) has been performed at 773 and 823 K over a commercial nickel catalyst in an equilibrium-shift reactor with an 11-μm thick palladium membrane (Mem-L) on a stainless steel porous metal filter. The methane conversion with the reactor is significantly higher than its equilibrium value without membrane due to the equilibrium-shift combined with separation of pure hydrogen through the membrane. The methane conversion in a reactor with an 8-μm membrane (Mem-H) is similar to that with Mem-L, although the hydrogen permeance through Mem-H is almost double of that through Mem-L. The amount of hydrogen separated in the reaction with Mem-H is significantly large, showing that the hydrogen separation overwhelms the hydrogen production because of the insufficient catalytic activity.     

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.     

The effect of mass transfer on the catalytic combustion of benzene and methane over palladium catalysts supported on porous materials

Catalytic combustion of benzene and methane over palladium catalysts supported on FAU and MOR zeolites and MCM-41 and KIT-1 mesoporous materials were studied to illustrate the effect of pore size and shape of supports on their catalytic activities. The palladium catalysts supported on mesoporous materials showed high activity and a steep increase in the conversion of benzene with rising temperature. The low activity of palladium catalysts supported on FAU zeolite was ascribed to mass transfer limitation. However, conversion profiles of methane on palladium catalysts were similar, although their supports were different as zeolites and mesoporous materials. The catalytic behavior of palladium catalysts in the combustion of benzene and methane was explained by the diffusion properties of fuels in the pores of zeolites and mesoporous materials.     

Kinetics of methane combustion over Pt and Pt–Pd catalysts

A kinetic study was performed over thermally aged and steam-aged Pt and Pt–Pd catalysts to investigate the effect of temperature, and methane and water concentrations on the performance of catalysts in the range of interest for environmental applications. It was found that both catalysts permanently lose a large portion of their initial activity as result of exposure to 5 vol.% water in the reactor feed. Empirical power-law and LHHW type of rate equations were proposed for methane combustion over Pt and Pt–Pd catalysts respectively. Optimization was used to determine the parameters of the proposed rate equations using the experimental results. The overall reaction orders of one and zero in methane and water concentration was found for stabilized steam-aged Pt catalyst in the presence and absence of water. The apparent self-inhibition effect caused by methane over Pt–Pd catalyst in the absence of water was associated with the inhibiting effect of water produced during the combustion of methane. A significant reversible inhibition effect was also observed over steam-aged Pt–Pd catalyst when 5 vol.% water vapor was added to the reactor feed stream. A significant reduction in both activity and activation energy was observed above temperatures of approximately 550 °C for steam-aged Pt–Pd catalyst in the presence of water (the activation energy dropped from a value of 72.6 kJ/mol to 35.7 kJ/mol when temperature exceeded 550 °C).     

DFT法研究钯原子催化活化甲烷碳氢键的反应机理

首先采用M06L泛函研究了钯原子催化活化甲烷C-H的反应势能面(PES),并与已有的实验结果和理论基准进行了比较。随后,对反应复合物(RC)、过渡态(TS)和产物(P)分子进行了电子密度拓扑分析。势能面结合拓扑分析结果更清晰明确地表述了反应机理。