Hydrothermal synthesis of LA-MN-Hexaaluminates for the catalytic combustion of methane

Hydrothermal synthesis by using urea hydrolysis at 1.0-3.0 MPa and 120-130 ‡C was employed to prepare Mn-substituted hexaaluminate catalysts for methane combustion. The results from DTA-MS demonstrated that CO_3- and Off anions co-exist in the hydrothermal reaction. XRD reveals that the components of carbonates and hydroxides in the hydrothermal reaction are more favorable than those in the (NH₄)₂CO₃ co-precipitation for the formation of the Mn-substituted hexaaluminate phase. After calcination at 1,200 ‡C for 2 h, LaMnAl₁₁O₁₉ is the major phase of the catalyst prepared by the hydrothermal synthesis method while LaAlO₃ is the major one of the catalysts prepared by NH₄OH and (NH₄)₂CO₃ co-precipitation. The catalyst prepared by hydrothermal synthesis has higher activity than that prepared by NH₄OH and (NH₄)₂CO₃ co-precipitation. The major reason is that more Mn²⁺ ions have incorporated into the hexaaluminate lattice. The effect of drying methods on the formation of hexaaluminate phase was also discussed.     

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.     

Stability of palladium-based catalysts during catalytic combustion of methane: The influence of water

The stability of methane conversion was studied over a Pd/Al₂O₃ catalyst and bimetallic Pd–Pt/Al₂O₃ catalysts. The activity of methane combustion over Pd/Al₂O₃ gradually decreased with time, whereas the methane conversion over bimetallic Pd–Pt catalysts was significantly more stable. The differences in combustion behavior were further investigated by activity tests where additional water vapor was periodically added to the feed stream. From these tests it was concluded that water speeds up the degradation process of the Pd/Al₂O₃ catalyst, whereas the catalyst containing Pt was not affected to the same extent. DRIFTS studies in a mixture of oxygen and methane revealed that both catalysts produce surface hydroxyls during combustion, although the steady state concentration on the pure Pd catalyst is higher for a fixed temperature and water partial pressure. The structure of the bimetallic catalyst grains with a PdO domain and a Pd–Pt alloy domain may be the reason for the higher stability, as the PdO domain appears to be more affected by the water generated in the combustion reaction than the alloy. Not all fuels that produce water during combustion will have stability issues. It appears that less strong binding in the fuel molecule will compensate for the degradation.     

Supported palladium-platinum catalyst for methane combustion at high pressure

Catalytic combustion of methane over a supported bimetallic Pd-Pt catalyst and a monometallic Pd catalyst has been investigated experimentally. Two different reactor configurations were used in the study, i.e. a tubular lab-scale reactor working at atmospheric pressure and a high-pressure reactor working at up to 15 bar. The results showed that the bimetallic catalyst has a clearly more stable activity during steady-state operation compare to the palladium only catalyst. The activity of the bimetallic catalyst was slightly higher than for the palladium catalyst. These results were established in both test facilities. Further, the impact of pressure on the combustion activity has been studied experimentally. The tests showed that the methane conversion decreases with increasing pressure. However, the impact of pressure is more prominent at lower pressures and levels out for pressures above 10 bar.     

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.     

Bimetallic Pt/Pd diesel oxidation catalysts: Structural characterisation and catalytic behaviour

Extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) studies on supported bimetallic Pt/Pd diesel oxidation catalyst (Pt:Pd weight ratio 2:1) after various treatments were compared with those of monometallic Pd and Pt catalysts prepared under similar conditions. After calcination and thermal ageing, the coexistence of alloyed bimetallic Pt/Pd particles and of tetragonal PdO has been found in the bimetallic Pt/Pd catalyst. PdO is present in form of crystals at the surface of the Pt/Pd particles or as isolated PdO crystals on the support oxide. Bimetallic Pt/Pd nanoparticles were already formed in the Pt/Pd catalyst after calcination. Hydrogen treatment causes the formation of randomly alloyed Pt/Pd nanoparticles. In the thermally aged catalyst, a strong indication for an enrichment of Pt in the interior of the particle and of Pd at its outer shell was found. In the monometallic catalyst, the Pd is found to be completely oxidised already after calcination and to consist of metallic Pd in zero-valent state exclusively after reductive treatment. Ageing under hydrothermal oxidative atmosphere leads to complete oxidation of the Pd species. After calcinations, the catalytic activity of the Pt/Pd catalyst studied is comparable to those of monometallic Pt catalysts. In contrast to monometallic Pt catalysts, the alloyed system show significant stabilisation against sintering and a much higher activity after the thermal ageing step. This stabilisation of dispersion and the presence of Pt atoms on the surface of the Pt/Pd particles are considered to cause the higher catalytic activity of metallic particles for the oxidation of carbon monoxide and propene after ageing.     

The application of “superacidic” metal oxides and their platinum doped counterparts to methane combustion

A range of metal oxides have been compared as methane combustion catalysts. The effect of modification to generate “superacidic” behaviour on the activities of ZrO₂ and Fe₂O₃ systems has been studied. It has been shown that whilst sulphation lowers the activity of Fe₂O₃, sulphation and, particularly, molybdation enhance the performance of ZrO₂. Despite enhancing the activity of the unmodified base oxides, the addition of low levels of Pt has been demonstrated to poison the activity of “superacidic” zirconias.     

Molybdenum ZSM-5 zeolite catalysts for the conversion of methane to benzene

This paper describes characterization studies of Mo-HZSM-5 zeolite catalysts active for the non-oxidative conversion of methane to benzene. FTIR, and NMR evidence is presented for migration of molybdenum into the zeolite pores during catalyst calcination at high temperatures. Mo K-edge EXAFS confirms that calcination produces highly dispersed oxomolybdenum or molybdate species which are converted to a molybdenum carbide phase under reaction conditions. Factors determining catalyst performance are discussed.     

Study of catalysts for catalytic burners for fuel cell power plant reformers

Catalytic burners for fuel cell power plant reformers are alternatives to conventional flame burners. Their application is expected to provide uniform temperatures in the reformer, efficient use of low-calorific gaseous by products and reduction of pollutant emissions. For testing in the burners, a series of spherical Pd/CeO₂/Al₂O₃ catalysts were prepared. An optimum concentration of ceria providing the highest thermal stability of catalysts was determined. An effect of catalyst activation in the reaction mixture-1% methane in air was observed. A series of Mn containing oxide catalysts on spherical γ-Al₂O₃ or (γ+Χ)-Al₂O₃, both pure and doped with La, Ce and Mg oxides were prepared. The catalysts were characterized by chemical analysis, X-ray phase analysis, BET surface area and activity measurements in methane oxidation. A batch of Mn-Mg-La-Al-O catalyst was prepared for further long-term testing in a model reformer with a catalytic burner. A model reformer with a catalytic burner was designed and fabricated for testing in the composition of the bench-scale Fuel Cell Power Plant. Preliminary testing of this catalyst showed that it provided complete methane combustion at the specified operational temperatures over 900 °C.     

Flow reversal reactor for the catalytic combustion of lean methane mixtures

This paper describes an experimental investigation of a pilot scale reverse flow reactor for the catalytic destruction of lean mixtures of methane in air. It was found that using reverse flow it was possible maintain elevated reactor temperatures which were capable of achieving high methane conversion of methane in air streams at methane concentrations as low as 0.19% by volume. The space velocity, cycle time and feed concentration are all important parameters that govern the operation of the reactor. Control of these parameters is important to prevent the trapping of the thermal energy within the catalyst bed, which can limit the amount of energy that can be usefully extracted from the reactor.     

Perovskite-like catalysts for the catalytic flameless combustion of methane

Modified LaCoO₃ and LaMnO₃ were investigated as catalysts for low tvemperature flameless combustion of methane. Modifications were carried out by the substitution part of La for Sr²⁺ and Ce⁴⁺, by the addition of 0.5% of Pt or Pd and by the substitution with Ag, which have limited solubility in the perovskite structure and may exist as intraframework Ag⁺ and extraframework metallic silver. Catalysts were synthesized by flame pyrolysis, which lead to a significant increase of both surface area and thermal resistance in comparison with the catalysts prepared by traditional sol-gel method. Samples were mainly characterized by XRD, BET and TPR techniques. Catalytic activity for the flameless combustion of methane was investigated by means of bench scale continuous apparatus, equipped with a quadrupolar mass spectrometer. In addition the resistance of every catalysts against sulphur poisoning was tested by using tetrahydrothiophene (THT) as poisoning agent. In most cases modification of perovskites led to an activity improvement, which was much more evident in the case of silver substitution. All the FP-prepared catalysts showed full methane conversion below 600°C, with CO₂ and H₂O as the sole detected products. Sr-substitution and addition of noble metals increased resistance to sulphur poisoning, while silver was not effective from this point of view, its main advantage being a substantial increase of the initial activity, which lead to satisfactory performance even after poisoning.     

Experimental and modelling studies of the catalytic combustion of methane

Ceramic honeycomb monoliths with a noble metal-alumina based washcoat were used as burners for the combustion of very lean methane-air mixtures below the conventional lower flammability limit without the emission of CO, NO_x, or unburned fuel gas. Measurements and modelling in the steady state proved that the near zero emissions could have been equally due to gas phase combustion than to catalytic combustion for the long monoliths. However, only catalytic oxidation reactions could account for the complete and clean combustion observed for the shortest burners, indicating that even in the longest monoliths, the combustion had been catalytic. Thus the onset of gas phase combustion was inhibited by catalytic combustion. This phenomenon was investigated using numerical modelling and experimental studies on a catalytic stagnation point flow reactor, with a polycrystalline Pt foil as the catalyst. These studies showed the extent of the phenomenon of inhibition of gas phase ignition and how catalytic combustion is an extremely stable and clean process.     

Catalytic combustion of natural gas as the role of on-site heat supply in rapid catalytic CO2---H2O reforming of methane

Development in highly active catalysts for the reforming of methane with H₂O, CO₂, and H₂O+CO₂, and partial oxidation of methane was conducted to produce hydrogen with high reaction rates. A Ni-based three-component catalyst such as Ni---La₂O₃---Ru or Ni---Ce₂O₃---Pt supported on alumina wash-coated ceramic fiber in a plate shape was very suitable for both reactions. The catalyst composition was set at 10 wt.-% Ni, 5.6 wt.-% La₂0₃, and 0.57 wt.-% Ru for example, or molar ratios of these components were 1:0.2:0.03. Even with such a low concentration, the precious metal enhanced the reaction rate markedly, and this synergistic effect was ascribed to the hydrogen spillover effect through the part of precious metal and it resulted in a more reduced surface of the main catalyst component. In particular, a marked enhancement in the reaction rate of CO₂-reforming of methane was observed by the modification of a low concentration Rh to the Ni---Ce₂0₃---Pt catalyst. Very high space-time yields of H₂ (i.e., 8300 mol/1 h in partial oxidation of methane at 600°C with a methane conversion of 37.5%, and 3585 mol/1 h in CO₂reforming of methane at 600°C with a methane conversion of 58%) were realized in those reactions. By combining the catalytic combustion reaction, methane conversion to syngas was markedly enhanced, and even with a very short contact time (10 ms) the conversion of methane increased more than that at 50 ms. The space-time yield of hydrogen amounted to 2,780 mol/1 h with a methane conversion of 90% at 700°C. Furthermore, in a reaction of CH₄---CO₂---H₂O---O₂ on the four components catalyst, an extraordinarily high space-time yield of hydrogen, 12 190 mol/1 h, could be realized under the conditions of very high space velocity (5 ms).     

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.     

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.     

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.     

Materials for catalytic gas combustion

Catalytic combustion, which permits to burn lean fuel/air mixtures is the key to environmentally preferable utilization of natural gas as an energy source and to removal of organic combustible gases from industrial effluents. The range of potential applications of catalytic combustion is large and can vary in temperatures of operation. Successful wide implementation of existing and of new catalytic combustion technologies will largely depend on the availability of suitable low cost catalytic materials. Since no single material can meet all demands, development of new catalysts needs to be orchestrated with the specific requirements of a given technology. The challenge is to combine existing knowledge and expertise in the area of combustion catalysts with innovations in their synthesis, improved formulations and applications in new specific composite forms. This paper outlines the current state of art and then focuses on perovskites for applications below 1,000 K. Examples of highly active formulations and of further enhancement of their activity through controlled synthesis and suitable support combinations are given. Criteria for the design of highly performing materials for high temperature catalytic combustion are also presented.     

Optimization of a flow reversal reactor for the catalytic combustion of lean methane mixtures

This paper describes a parametric study of a catalytic flow reversal reactor used for the combustion of lean methane in air mixtures. The effects of cycle time, velocity, reactor diameter, insulation thickness, thermal mass and thermal conductivity of the inert sections are studied using a computer model of the system. The effects on the transient behaviour of the reactor are shown. Emphasis is placed on the effects of geometry from a scale-up perspective. The most stable system is obtained when the thermal mass of the inert sections is highest, while thermal conductivity has only a minor effect on reactor temperature. For a given operation, the stationary state depends on the combination of velocity and switch time. Provided that complete conversion is achieved, highest reactor temperature is achieved with the highest switch time. The role of the insulation is not only to prevent heat loss to the environment, but also to provide additional thermal mass. During operation heat is transfer to and from the insulation. The insulation effect leads to higher reactor temperature up to a maximum thickness. The insulation effect diminishes as the reactor diameter increases, and results in higher temperatures at the centreline.     

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.     

The effect of dopants on the activity of MoO3/ZSM-5 catalysts for the dehydroaromatisation of methane

The effect of a number of dopants – Co²⁺, Fe³⁺, Al³⁺, Ga³⁺ and P^(V) – on the catalytic activity of MoO₃/ZSM-5 dehydroaromatisation catalysts has been investigated. Promotional effects were observed with Fe³⁺, Al³⁺ and Ga³⁺, whereas P^(V) added in the form of phosphomolybdic acid decreased the benzene formation rate. Doping with Co²⁺ was not observed to produce any promotional effects, aside from an initial high conversion and burst of hydrogen which was short-lived. In the case of iron, although the benzene formation rate was close to that for the non-doped parent catalyst in the early stages, benzene formation was enhanced at longer times on stream and the addition of iron was observed to enhance methane conversion, principally through enhanced coke formation. The addition of gallium was found to produce an enhancement of benzene production in the early stages of reaction, but its principal effect was the reduction of hydrogen producing side reactions associated with the formation of coke. In comparing the effects of metal ion addition, it is notable that the impregnation solutions of Fe³⁺, Al³⁺ and Ga³⁺ are acidic. Possible effects of these dopants such as zeolite dealumination, modification of catalyst acidity and the formation of mixed molybdenum containing phases or re-dispersion of the MoO₃ phase can be advanced as potential explanations for the observed effects.