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 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.     

Al-MCM-41 supported palladium catalyst for methane combustion: Effect of the preparation methodologies

This work aimed at elucidating the beneficial effect of plasma treatment on the catalytic performance of palladium (Pd) catalysts in methane combustion with the ordered mesoporous molecular sieve Al-MCM-41 as the model support. The plasma treated Pd/Al-MCM-41 catalyst exhibited a higher initial activity and a better stability in comparison with the untreated counterpart catalyst. To clarify the plasma effect, the catalysts were characterized by N₂ sorption analysis, X-ray diffraction (XRD), temperature-programmed desorption of ammonia (NH₃-TPD), pyridine adsorption-infrared spectroscopy (Py-IR), high resolution-transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (CH₄-TPR) experiments. The results obtained confirmed that palladium oxide (PdO) was the active phase. Plasma treatment enhanced the acidity of catalyst and improved the dispersion of PdO particles, which lead to a higher initial activity. The better stability for plasma treated Pd-based catalyst was proved to be closely related to the stronger interaction between palladium oxide and the molecular sieve support. In addition, the sintering of PdO particles over the plasma treated catalyst was not significant during the stability test. These findings may provide useful guidelines for further catalyst design for methane combustion.     

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。     

Low temperature complete combustion of methane over titania-modified alumina-supported palladium

Effects of titania on the catalytic property of Pd/Al₂O₃ towards methane combustion were examined. The results revealed that the catalytic activity of the Pd/Al₂O₃ catalyst was considerably improved by pre-coating the alumina support with titania at low temperature (below 700 °C). Hydrogen chemisorption and BET measurements revealed that the titania-modified alumina supports could modify the support characterization to achieve a high dispersion of palladium. Temperature-programmed reduction and temperature-programmed desorption study further demonstrated that the coating of Pd/Al₂O₃ catalysts with titania can weaken the bond strength of Pd-O and enhance their catalytic activity towards methane combustion at lower temperature.     

Catalytic combustion of methane over supported bimetallic Pd catalysts: Effects of Ru or Rh addition

A series of Pd/γ-Al₂O₃ catalysts with various amounts of Ru or Rh with, and/or without, BaO were prepared by successive incipient wetness impregnation. The catalysts were investigated for the catalytic methane combustion before, and after, H₂S poisoning in an oxygen-rich atmosphere. The addition of ruthenium enhanced the catalytic activity for methane oxidation even after H₂S poisoning while maintaining the initial catalytic activity of the fresh catalyst. These results are explained in terms of dispersion of palladium by ruthenium and poisoning resistance of ruthenium. The addition of rhodium did not improve the overall activity in methane oxidation.     

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.     

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.     

Structured reactors for kinetic measurements under severe conditions in catalytic combustion over palladium supported systems

The collection of chemical kinetics data in catalytic combustion over very active palladium catalysts under conditions relevant to practical applications (e.g. gas turbine combustors) is extremely difficult, mainly due to strong exothermicity and very fast rate of combustion reactions. Within this purpose in this paper two types of laboratory structured reactors, which closely resemble industrial monolith catalysts, are investigated: (a) the annular reactor, consisting of a catalyst coated ceramic tube, co-axially placed in a quartz tube; (b) the metallic plate-type reactor, consisting of an assembled packet of metallic slabs coated with a ceramic catalytic layer.

The design of the annular reactor configurations for kinetic investigations is first addressed by mathematical modeling. The resulting advantages, including: (i) negligible pressure drops; (ii) minimal impact of diffusional limitations in high temperature–high GHSV experiments; (iii) effective dissipation of reaction heat are then experimentally demonstrated for the case of CH₄ combustion over a PdO/γ-Al₂O₃ catalyst with high noble metal loading (10% (w/w) of Pd).

The feasibility of a near-isothermal operation with the metallic plate-type reactor by an extremely effective dissipation of reaction heat through proper selection of highly conductive support material and of the geometry of the metallic slabs is finally discussed and experimentally demonstrated for the case of combustion of CO at high concentrations over a PdO/γ-Al₂O₃ (3% (w/w) of Pd) catalyst.     

Catalytic combustion of methane over bimetallic catalysts a comparison between a novel annular reactor and a high-pressure reactor

The effects of adding a co-metal, Pt or Rh, to Pd/γ-Al₂O₃ catalysts were studied with respect to the catalytic activity for methane combustion and compared to a Pd/γ-Al₂O₃ catalyst, using both a pressurized pilot-scale and a lab-scale annular reactor. Temperature programmed oxidation (TPO) experiments were also carried out to investigate the oxygen release/uptake of the catalyst materials. Palladium showed an unstable behavior both in the pilot and lab-scale experiments at temperatures well below the PdO to Pd transformation. An addition of Pt to Pd stabilized, and in some cases increased, the catalytic activity for methane combustion.

The TPO experiments showed that the oxygen release peak was shifted to lower temperatures even for low additions of Pt, i.e. Pd:Pt=2:1. For additions of rhodium only small beneficial effects were seen. The steady-state behavior of the lab-scale annular reactor correspond well to the pressurized pilot-scale tests.     

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.     

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.     

Al18B4O33 aluminium borate: A new efficient support for palladium in the high temperature catalytic combustion of methane

Well crystallised aluminium borate Al₁₈B₄O₃₃ has been synthesised from alumina and boric acid with a BET area of 18 m²/g after calcination at 1100 °C. Afterwards, 2 wt.% Pd/Al₁₈B₄O₃₃ was prepared by conventional impregnation of Pd(NO₃)₂ aqueous solution and calcination in air at 500 °C. The catalytic activity of Pd/Al₁₈B₄O₃₃ in the complete oxidation of methane was measured between 300 and 900 °C and compared with that of Pd/Al₂O₃. Pd/Al₁₈B₄O₃₃ exhibited a much lower activity than Pd/Al₂O₃ when treated in hydrogen at 500 °C or aged in O₂/H₂O (90:10) at 800 °C prior to catalytic testing. Surprisingly, a catalytic reaction run up to 900 °C in the reaction mixture induced a steep increase of the catalytic activity of Pd/Al₁₈B₄O₃₃ which became as active as Pd/Al₂O₃. Moreover, the decrease of the catalytic activity observed around 750 °C for Pd/Al₂O₃ and attributed to PdO decomposition into metallic Pd was significantly shifted to higher temperatures (820 °C) in the case of Pd/Al₁₈B₄O₃₃. The existence of two distinct types of PdO species formed on Al₁₈B₄O₃₃ and being, respectively, responsible for the improvement of the activity at low and high temperature was proposed on the basis of diffuse reflectance spectroscopy and temperature-programmed desorption of O₂.     

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.     

Catalytic combustion of benzene over supported metal oxides catalysts

Catalytic combustion of benzene over supported metal oxides has been investigated. The catalysts have been prepared by incipient wetness method and characterized by XRD, FT-Raman, ESR and TPR. Among supported metal oxides, CuO_x, supported on TiO₂ is found to have the highest activity for benzene oxidation. In addition, among the catalysts of copper oxide supported on TiO₂, A1₂O₃ and SiO₂, titania-supported catalyst (CuO_x/TiO₂) gives the highest catalytic activity. CuO_x/TiO₂ (Cu loading 5.5 wt%) shows the total oxidation of benzene at about 250 °C. From the ESR and FT-Raman results, the CuO dispersed on the TiO₂ surface acts as an active site of CuO_x/TiO₂ catalysts on the oxidative decomposition of benzene. The catalytic activity gradually increases with an increase of Cu loading on TiO₂. When Cu loading reaches 5.5 wt%, the total conversion temperature is lowered to 300 °C. However, the catalytic activity considerably decreases at 7 wt% Cu loading. The catalytic activity increased with an increase of oxygen concentration but the concentration of benzene showed no difference in the benzene conversion. This result suggests that the rate determining step is the adsorption of oxygen.     

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.     

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.     

Oscillatory behaviour during the oxidation of methane over palladium metal catalysts

Oscillatory reactions over palladium foil and wire catalysts during the oxidation of methane have been investigated over a wide range of reaction temperatures and argon/methane/oxygen feed gas compositions. Characterisation of the catalyst has also been carried out using scanning electron microscopy (SEM) techniques, which revealed the presence of a porous surface. This suggested that the metal surface has undergone a change since the reaction commenced, and using X-ray powder diffraction (XRD) techniques the palladium phase was shown to be the dominant phase present. Hysteresis phenomena were observed in the activity of the reaction as the temperature was cycled up and down, showing that the metal surface was continually changing throughout the reaction. The activation energies of the reaction during the high reactivity mode, PdO, and low reactivity mode, Pd, were also calculated. Oscillation rates were observed to depend on the dominant surface. Oscillations were frequent when the high reactivity mode was dominant while the activation energy of this mode was found to be low. When the low reactivity mode was dominant, the oscillations were slower and the activation energy was three times larger. The results obtained imply that the behaviour of the palladium surface, switching back and forth from the reduced state to the oxidised state, is responsible for the oscillatory behaviour seen in this system.     

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.     

Total combustion of m-xylene over palladium catalysts supported by stainless steel flakes

Total combustion of m-xylene over Pd/stainless steel flakes has been investigated. Ignition temperature of the catalytic combustion of m-xylene mixtures over palladium catalysts is lowered by about 330°C compared to that of thermal oxidation. Hysteresis in the conversion-temperature curves recorded in temperature programmed modes is controlled not only by the operating conditions (the temperature ramp, concentration of the reactants, contact time) but also by catalyst pretreatments. High and low activity states have been observed with typical ignition temperatures of 200–250°C and about 400°C, respectively. In high activity state the combustion is likely initiated by the catalyst followed by a chain reaction which propagates into the gas phase. This is strongly supported by the abrupt increase in the conversion profile. During the induction period preceding the combustion, active species are accumulated on the surface which are responsible for this process.