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.     

Exhaust-gas reforming using precious metal catalysts

Rh-only and Rh bimetallic catalysts have been screened for exhaust-gas reforming, under conditions that mimic the output of an autoignition gasoline engine. Propane has been used as a model fuel, with simulated exhaust-gas providing the co-reactants (O₂ and H₂O) needed to generate hydrogen. Based on oxygen-conversion as a measure of light-off, Pt–Rh on ceria–zirconia shows the highest activity. In the presence of SO₂, adsorbed sulphur species do not inhibit the oxidation reactions that induce light-off, but suppress the major pathway to hydrogen (steam reforming). By excluding platinum and using silica-enriched alumina as the underlying support, light-off is delayed, but the steam reforming reaction becomes much more insensitive to the presence of sulphur. The Pt–Rh catalyst is most suited to exhaust-gas reforming systems in which the engine runs on a sulphur-free fuel, whereas the Rh-only catalyst is the better choice when the fuel is conventional gasoline.     

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.     

Sulphur poisoning of transition metal oxides used as catalysts for methane combustion

Different bulk metal oxides (NiO, CuO, Mn₂O₃, Cr₂O₃ and Co₃O₄) were prepared and tested for the combustion of methane–air lean mixtures (5000 ppmV of CH₄) in presence of SO₂ (40 ppmV). Methane combustion experiments were carried out at ambient pressure, 425 and 625 °C and a space time of 93.3 g h mol_CH4⁻¹. Catalysts aged (60 h on stream) both in absence and in presence of SO₂, were characterised by nitrogen physisorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature-programmed desorption (TPD-MS) and infrared spectroscopy (DRIFTS). It was observed that Cr₂O₃ is not deactivated at the studied conditions, whereas all the other materials present fast deactivation in presence of SO₂. Aged catalysts characterisation reveals that the outstanding behaviour of the Cr₂O₃ catalyst is caused by the absence of formation of surface sulphates. By contrast, Mn₂O₃ and Co₃O₄ are more active than Cr₂O₃ for methane oxidation in absence of sulphur species, but they are strongly deactivated in presence of SO₂.

Finally, the performance of the Cr₂O₃ catalysts was compared to the corresponding to Pd/Al₂O₃ catalyst and to a highly sulphur-tolerant perovskite (La_0.9Ce_0.1CoO₃) for the oxidation of methane in a real industrial emission from a coke oven, containing different inorganic gases (NH₃, N₂, H₂, H₂O, CO, CO₂, SO₂ and H₂S). Cr₂O₃ catalyst shows to be also the most stable catalyst for the treatment of these emissions.     

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.     

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.     

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.     

Deactivation and regeneration of Pt/γ-alumina and Pt/ceria–alumina catalysts for methane combustion in the presence of H2S

Ceria has been widely explored as an additive in alumina-supported precious metal catalysts due to a number of unique properties. The success of ceria and ceria-based materials is mainly attributed to the unique combination of an elevated oxygen transport capacity coupled with the ability to shift easily between reduced and oxidised sates. In this study the influence of CeO₂ addition to a Pt/Al₂O₃ catalyst for low temperature (<540 °C) methane oxidation in an oxidising environment has been investigated. The resistance to H₂S-poisoning and influence on catalyst regeneration by oxidation or reductive treatments has been studied. The addition of CeO₂ to the support creates an increase in the level of activity based primarily on the oxygen storage capacity offered by the cerium oxide, causing an increase in oxygen activation. The ceria–alumina-supported catalyst showed a greater shift to poorer activity upon exposure to H₂S. It appears sulphur compounds react with the oxygen storage component causing a decrease in oxygen transfer, removing any benefit offered by the ceria. However, the level of Pt-agglomeration and support changes were reduced with the incorporation of ceria, emphasising the stabilising effect and promotion of metal particle dispersion associated with ceria. In order to obtain the maximum benefit of ceria addition to the support structure in terms of activity a reductive pretreatment is required. Upon exposure to a reducing atmosphere, it appears a Pt–CeO₂ interaction generates greater levels of activity.     

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.     

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 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.     

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.     

Catalytic combustion concept for gas turbines

Catalytic combustion for gas turbines was investigated, based on a partial catalytic combustion section followed by a homogeneous combustion zone. A pressurized test rig (<25 bar) was built to test the influence of various parameters on this concept using Pd and Pt catalysts.

The pressure influence on the apparent catalytic reaction rate was of the order 0.4, assuming that the reaction kinetics could be described by a power rate function which was of first order with respect to methane. Pd catalysts showed a pressure-dependent temperature for the transition of the active PdO to the much less active Pd. Combining Pd and Pt within one catalyst resulted in a considerably lower transition temperature.

Homogeneous combustion reactions set on from 650°C, depending on the methane concentration, pressure and flow. With inlet temperatures above 800°C the homogeneous combustion always started. At outlet temperatures below 1050°C high CO concentrations could be measured. At higher temperatures the CO, CH₄ and NO_x concentrations were lower than 5 ppm. During several experiments total conversion of CH₄ and CO was observed.     

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.     

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.     

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.     

Effect of washcoat modification with metal oxides on the activity of a monolithic Pd-based catalyst for methane combustion

The deposition of Ni, Co, Ce or Fe oxides onto the washcoat surface in the 0.5%Pd/Al₂O₃ catalyst enhances conversion of CH₄. Catalytic activity of the Pd-catalysts containing cobalt oxide depends on the incorporated amount of cobalt oxide and the method of incorporation. The highest activities were those of the 0.5%Pd/0.3%Co/Al₂O₃ and 1%Pd/0.3%Co/Al₂O₃ catalysts (cobalt oxide deposited onto the surface of Al₂O₃) and the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst (mixed washcoat). Total SSA, Pd dispersion and Pd crystallite size in the x%Pd/y%Co/Al₂O₃ catalysts depend on the incorporated amount of PdO and cobalt oxide. Pd dispersion in the 1%Pd/Al₂O₃ catalyst increases from 4% to 20% upon deposition of 14 wt.% Co₃O₄ (by mass Al₂O₃) onto the Al₂O₃ surface (1%Pd/0.3%Co/Al₂O₃). This increase in Pd dispersion influence the increase in the activity of the 1%Pd/Al₂O₃ catalyst. On the surface of the 0.5%Pd/5%Co₃O₄–Al₂O₃ catalyst Pd occurs mainly in the form of PdO and displays considerable mobility under conditions of temperature variations—cyclically undergoing reduction and oxidation. At 500 °C, in vacuo, the reduction was irreversible and parallelled by the agglomeration of metallic Pd crystallites. At room temperature, cobalt occurred on the catalyst surface in the form of Co⁺² ions (CoAl₂O₄) and was reduced to Co⁰ at 500 °C (in vacuo). Up to 500 °C, the reduction of Co was reversible.     

Total oxidation catalysts based on manganese or copper oxides and platinum or palladium II: Activity, hydrothermal stability and sulphur resistance

Deactivation of catalysts based on either manganese oxides, copper oxides, platinum, palladium or combinations of these metal oxides and noble metals supported on γ-alumina was studied. The activity of the catalysts for the oxidation of carbon monoxide, naphthalene and methane, in a mixture resembling the flue gases from wood combustion, was measured before and after exposure of the catalysts either to a temperature of 900°C in the presence of steam or to sulphur dioxide. Most of the mixed catalysts were more resistant to hydrothermal and sulphur treatments than the catalysts with a single active component. After the hydrothermal treatment the activity of the MnO_x catalyst was enhanced. When Pt is combined with MnO_x or CuO_x, the loss of activity of Pt was decreased during the hydrothermal treatment. Also, the hydrotreated mixed MnO_x–Pd and CuO_x–Pd catalysts were more active than the treated Pd catalyst for the oxidation of methane. After sulphur treatment, the activities of the mixed MnO_x–Pt (Pt: 0.05 mol%), MnO_x–Pd and CuO_x–Pd catalysts were improved for the oxidation of carbon monoxide and naphthalene. Among the catalysts studied, the MnO_x–Pt, CuO_x–Pt and CuO_x–Pd catalysts, with a metal oxide and a noble metal loading of 10 and 0.1 mol%/γ-alumina, respectively, had the best combination of activity, thermal stability and resistance to sulphur treatment.     

Complete oxidation of methane at low temperature over Pt and Pd catalysts for the abatement of lean-burn natural gas fuelled vehicles emissions: influence of water and sulphur containing compounds

The catalytic activity of fresh Pd and Pt catalysts supported on γ-alumina in the complete oxidation of CH₄ traces under lean-burn conditions was studied in the presence or the absence of water or H₂S. Steam-aged catalysts were also studied in order to simulate long-term ageing in real lean-burn natural gas fuelled vehicles (NGVs) exhaust conditions. Without water or H₂S added to the feed, Pd catalysts exhibit a superior catalytic activity in methane oxidation compared to Pt ones, whatever the catalysts were fresh or aged. The addition of 10 vol.% water vapour to the feed strongly affects the activity of the fresh Pd catalyst, thus being only slightly more efficient than the fresh Pt one. H₂S has a strong poisoning effect on the catalytic activity of Pd catalysts, while Pt catalysts are more resistant. The fresh H₂S-poisoned Pd/Al₂O₃ catalyst was studied by TPD in O₂/He. Poisoning species decompose above 873 K as SO₂ and O₂ in relative concentrations consistent with the decomposition of surface sulphate species. However, a treatment in O₂/He at temperatures as high as 923 K does not allow the complete regeneration of the catalytic activity of H₂S-poisoned Pd/Al₂O₃. A mechanism involving the poisoning of PdO by sulphate species is proposed. Different diffusion processes by which these sulphate species can migrate back and forth between PdO and the support, depending on the experimental conditions, are suggested.     

Adsorption and regeneration dynamic characteristics of methane and hydrogen binary system

In order to optimize the performance of an adsorption column, the adsorption and regeneration dynamic characteristics were studied for 20% methane and 80% hydrogen binary system on nonisothermal and nonadiabatic conditions. The adsorption dynamic characteristics were studied at various flow rates, 7.2 LPM to15.8 LPM, and at various adsorption pressures, 6 to 12 atm. Also, regeneration dynamic characteristics were studied at various purge rates, 1.5 to 3.5 LPM, and constant pressure, 1.2 atm. Nonisothermal and nonadiabatic models, considering linear driving force model and Langmuir-Freundlich adsorption isotherm model, were considered to compare between prediction and experimental data.