Total Oxidation Of Methane Research Articles

Catalytic properties of stoichiometric and non-stoichiometric LaFeO 3 perovskite for total oxidation of methane

Perovskite LaFeO 3 and derived materials La (1− ϵ) FeO (3−1.5 ϵ) with ϵ=0.1, 0.2, 0.3 have been prepared as polycrystalline powders by thermal low-temperature decomposition of La and Fe nitrates, a method which ensures a high surface area. They have been used in catalytic methane combustion in the range 300–450 °C, showing a 100% selectivity to carbon dioxide. The reaction is indeed characterised by two ranges with different dependence of the reaction rate on temperature, as shown by the Arrhenius plot, whose slope decreases markedly above about 375 °C. Temperature programmed desorption experiments of oxygen allowed us to ascertain the presence of two surface oxygen species, whose desorption occurs, respectively before and after the above indicated temperature on this basis, we propose here an hypothesis on the way of working of the catalysts. The characterisation of the catalysts has been then completed by means of temperature programmed reduction, whose results contribute to strengthen the hypothesis that the catalyst reduction operated by methane involves a smaller number of lattice planes than reduction of hydrogen.

Preparation of high surface area La 1− xA xMnO 3 (A=Ba, Sr or Ca) ultra-fine particles used for CH 4 oxidation

Ultra-fine particles of La 1− x A x MnO 3 (A=Ba 2+, Sr 2+ or Ca 2+, x⩽0.3) with high surface area and single perovskite structure were prepared by a Na 2CO 3–NaOH coprecipitation method. The so-prepared ultra-fine particles exhibited high catalytic activity for CH 4 total oxidation. The ultra-fine particles of La 1− x Ba x MnO 3 prepared with this method were thermally more stable than LaMnO 3 and La 1− x Sr x MnO 3, and its surface areas were close to 20 m 2/g after sintering at 1000 °C for 2 h. La 1− x Ba x MnO 3 exhibited higher activity for methane total oxidation than La 1− x Sr x MnO 3, and among the La 1− x Ba x MnO 3 series La 0.8Ba 0.2MnO 3 showed highest activity.

Kinetics of the photocatalytic total oxidation of different alkanes and alkenes on TiO2 powder

AbstractAlthough photocatalyzed total oxidation reaction of hydrocarbon species has been discussed in the literature, most of these studies were performed to obtain an appropriate reaction mechanism. Studies on the kinetics of this type of reaction are rare. Using titanium dioxide as the photo catalyst, the kinetics of the total oxidation of methane, ethane, ethene, as well as propene, have been investigated using a continuous‐stirred tank reactor. In the experiments, the hydrocarbon concentrations, the oxygen concentration, and the irradiation intensity were varied. The results obtained are evaluated on the basis of a kinetic model to derive rate equations which can be used for reactor design.

Influence of barium and cerium oxides on alumina supported Pd catalysts for hydrocarbon combustion

The effects of baria and ceria additions to alumina-supported palladium catalysts have been investigated. Two series of binary Al 2O 3–BaO and ternary oxides Al 2O 3–CeO 2–BaO were prepared using a sol–gel method and then characterized, after high-temperature treatments, by XRD, BET, TG, TEM and EDXS analyses. The influence of baria and ceria on the thermal stability and morphology of the supports was examined. A baria content of 12 wt.% appears the most effective for alumina stabilization. Palladium catalysts were prepared by impregnation method using Pd(NH 3) 4(NO 3) 2 and were characterized by XRD and TEM. The catalysts showed high activity in the total oxidation of methane, in the temperature range 200–600 °C. The methane oxidation activity of both PdAlBa and PdAlCeBa series increases with the specific surface area (SSA), ceria addition does not produce significant differences of activities. The ethene combustion was studied, also, on the series PdAlCeBa, as a model structure-sensitive reaction chosen to investigate possible palladium–ceria interactions. The reactivity was compared with that of PdAlBa catalyst with the same baria content.

Kinetics of the Total Oxidation of Methane over a La0.9Ce0.1CoO3 Perovskite Catalyst

The kinetic study of the total oxidation of methane over a La0.9Ce0.1CoO3 catalyst has been achieved. A discrimination among the five multistep rate-controlling models, preselected in a preceding study (Stud. Surf Sci. Cat. 2001, 133, 599-604), was carried out on the basis of experimental data from an integral fixed-bed reactor using a sequential experimental design. According to the selected model, the overall rate is controlled by the rate of three elementary steps, i.e., (a) active surface oxygen formation, (b) the reaction between surface oxygen species and gas-phase methane, and (c) the desorption of reaction products. The kinetic constants of the model were determined using weighted regression. The experimental conversions could be predicted with an average error of lower than 5%. Both reaction products inhibit the reaction, although H2O causes much stronger inhibition. When the partial pressure of water in the feed is significantly higher than the one produced in the reaction, the desorption becomes an equilibrium process and the overall rate is determined only by the two elementary steps a and b.

Evaluation of methane oxidation in rice plant-soil system

Mechanisms of methane oxidation in the plant-soil system of rice were studied in a pot experiment using two cultivars (PSBRc-30 and IR72) at two growth stages (flowering and heading). Methane emission was measured by chambers, while methane oxidation was determined through propylene amendment as an alternative substrate to be propylene oxide (PPO) and acetylene as an inhibitor for methane oxidizing (methanotrophic) bacteria. Cell numbers (methanotrophic and methanogenic bacteria) were determined by the most probable number method. The cultivar PSBRc-30 consistently showed higher methane emission rates than IR72. Methane flux clearly decreased from flowering to heading stages in both cultivars. This observation was largely reflected by trends in the mechanisms involved: either methanogenic cell numbers or activities decreased with plant age while methanotrophic cell numbers or activities generally showed an increasing trend. The methanogenic population was in the order of 105 g−1 dry soil, while the population of methanotrophs ranged from 104 to nearly 106 g−1 dry soil. Methanotrophic activity followed the order; root (1.7–2.8 nL PPO g−1 DM h−1) > shoot (0.7–2.0) > soil (0–0.4) when the consumption of alternative substrate was related to dry matter. Derived from the estimated amounts of soil and plant biomass in the pot experiment, however, the soil generally accounted for more than 90% of the total methane oxidation. Within the plant segments, methane oxidation activities in the root exceeded those of the shoot by factor of approximately 10.

Hexane total oxidation on LaMO3 (M = Mn, Co, Fe) perovskite-type oxides

LaMO3 (M=Mn, Fe, Co) perovskites were prepared by calcining the citrate precursors at 800°C for 5h. PdO 3% /Al2O3, a largely used combustion catalyst, was also studied for comparison. The catalytic behavior was examined in the hexane total oxidation. The following activity scale was found: LaFeO3>LaCoO3>LaMnO3. >3% PdO/Al2O3, the three perovskites activating the CH bond cleavage of hexane better than PdO/Al2O3. At temperature around 250–300°C mobile surface oxygen species are involved in the oxidation process, whereas at T>300°C oxygen mobile species are no more present and so, particularly for LaCoO3 and PdO/Al2O3, the activation mechanism becomes more energetic. The activity values were compared with those obtained from the total oxidation of methane.

Role of the Ceria–Zirconia Support in the Reactivity of Platinum and Palladium Catalysts for Methane Total Oxidation under Lean Conditions

Platinum or palladium catalysts deposited on a Ce0.67Zr0.33O2 solid solution were tested in methane total oxidation. The ceria–zirconia mixed oxide is itself an active catalyst for methane combustion, in the 673–1073 K temperature range. Deposition of platinum or palladium on this support results in a strong increase in activity, which now takes place at low temperature (473–773 K). The ceria–zirconia support is particularly beneficial for the activity of platinum, when compared to a Pt/Al2O3 catalyst. Methane oxidation takes place at the active ceria–zirconia/metal interface, according to a redox mechanism involving the reaction of dissociated methane with lattice oxygen from the support. However, a time-on-stream deactivation is observed at moderate temperatures (573–623 K), which is associated with an oxidized state of the catalysts, whereas a reduction at 573 K strongly activates the solids. In situ electrical conductivity measurements show that the reduction at 573 K of the Pt/Ce0.67Zr0.33O2 and Pd/Ce0.67Zr0.33O2 catalysts creates a large amount of oxygen vacancies. Although the reduced ceria–zirconia support is largely reoxidized by oxygen at 573 K, some vacancies still remain and require a higher oxidation temperature (773 K) to be filled. The presence of vacancies, even in small amounts, seems to favor activity because of an electron transfer to the noble metals and an improvement of the lattice oxygen mobility. The slow filling of these vacancies is probably an important factor contributing to the observed time-on-stream deactivation.

Influence of the Metallic Precursor and of the Catalytic Reaction on the Activity and Evolution of Pt(Cl)/δ-Al2O3 Catalysts in the Total Oxidation of Methane

In the light-off temperature range of methane total oxidation (250–600°C), two types of Pt/δ-Al2O3 catalysts prepared from H2PtCl6 or Pt(NH3)4(OH)2 show different catalytic behaviors. Initially very active, fresh chlorine-free catalysts deactivate in isothermal conditions of reaction, while fresh chlorine-containing catalysts, initially poorly active, undergo an activation with time on stream. Consequences of dechlorination of chlorine-containing catalysts and of impregnation by various amounts of NH4Cl on both types of catalysts are examined. Chlorine is responsible for the inhibition observed on ex-H2PtCl6 catalysts, which could be attributed, more specifically, to the chloride ions remaining at the platinum–support interface after reduction of platinum. After aging of the catalysts under reaction, sintering of the platinum particles is observed on all samples, regardless of their preparation mode. This phenomenon is linked to the presence of oxygen in excess in the gas phase and can account for the deactivation of chlorine-free catalysts. The water produced during the reaction removes the inhibiting chlorine on all chlorine-containing catalysts. The activity of the largest sintered particles is shown to vary with the nature of the fresh catalyst, the aged ex-H2PtCl6 catalysts giving the best results: the molecular nature of the fresh catalyst would also affect the activity of the stable aged catalyst. Both the initial presence of chloride ions at the metal–support interface and the composition of the reacting mixture are key factors to this positive reconstruction.

Methane catalytic combustion on La-based perovskite catalysts

Two series of La-based perovskites oxides (La 1– x Sr x MO 3– y with M=Fe or Co) have been used as methane total oxidation catalysts. The best catalytic performances, in isothermal, high temperature conditions (900 °C), in the presence of water and carbon dioxide, were obtained for both series when 20 % of lanthanum cations are replaced by strontium. The catalytic behavior of the two series of catalysts is quite similar, although the cobalt-containing samples are easily reducible by H 2 or CH 4 , whereas the iron-containing ones are not reduced in the same conditions. It is proposed, to explain this apparent inconsistency, that the active sites reoxidation process occurs directly from the molecular oxygen of the gas phase, without participation of the bulk oxygen species mobility. Version française abrégée – Combustion catalytique du méthane sur des oxydes à base de lanthane, de structure pérovskite. Deux séries d'oxydes de structure pérovskite à base de lanthane (La 1– x Sr x MO 3– y avec M=Fe ou Co) ont été utilisées comme catalyseurs de combustion catalytique du méthane. Une étude cinétique a pu être conduite en conditions quasi isothermes à haute température (900 °C), en présence de CO 2 et de vapeur d'eau, grâce à l'utilisation d'une pression partielle faible de méthane (≈1 Torr), d'une dilution du mélange réactionnel par de l'hélium et du catalyseur par de la pierre ponce, limitant ainsi l'augmentation de température due à l'exothermicité de la réaction. Dans ces conditions, on montre que les meilleures performances catalytiques sont obtenues pour les échantillons dont 20 % des atomes de lanthane sont substitués par du strontium, quelle que soit la nature de l'autre cation (Fe ou Co). La caractérisation de ces échantillons par diffraction des rayons X et par réduction en température programmée a montré que les échantillons contenant du cobalt étaient très réductibles, alors que ceux contenant du fer ne l'étaient pratiquement pas. Ces résultats, confirmés par l'envoi de pulses de CH 4 et/ou d'oxygène à 900 °C montrent que les échantillons contenant du cobalt possèdent une plus grande mobilité de l'oxygène que ceux qui contiennent du fer, bien que les résultats catalytiques soient similaires pour les deux séries d'échantillons. Toutefois, cette apparente contradiction pourrait trouver son explication dans le fait que la réoxydation du site actif puisse se produire directement par l'oxygène moléculaire de la phase gazeuse, sans qu'il y ait participation des espèces oxygène du réseau et donc sans que leur mobilité ait une influence importante sur l'activité catalytique.

Partial oxidation of methane: Effect of reaction parameters and catalyst composition on the thermal profile and heat distribution

The partial oxidation of methane was carried out using new catalysts obtained by calcination and reduction of Mg/Al hydrotalcite-type precursors, containing Rh, Ni or Rh/Ni. The preparation methodology ensured high dispersion of the metal and activity in the partial oxidation of methane. To study the activity, the reaction in autothermal conditions was investigated using IR thermography to monitor the surface temperature. This technique made it possible to monitor the thermal profile of the catalytic bed and the gas/solid heat distribution, following its changes with residence time and reagent concentration. Furthermore, it allowed the diffusion limits to be estimated as a function of the initial ratio between total and partial oxidation of methane. The surface temperature profile was found to be very dependent on the catalyst composition and controlled by the Ni oxidation. For the reforming reactions the outlet composition was compared with the data at equilibrium, calculated at the outlet surface temperature in all reaction conditions.

Thermal evolution and catalytic activity of Pd/Mg/Al mixed oxides obtained from a hydrotalcite-type precursor

The synthesis, thermal evolution and catalytic activity in the total oxidation of methane of a novel Pd/Mg/Al (5:66:29, as atomic ratio) catalyst, obtained from a hydrotalcite-type (HT) anionic clay [or layered double hydroxide (LDH)] was investigated. The HT phase was prepared by co-precipitation at pH 10.0 from an aqueous solution of the corresponding nitrates. The XRD powder patterns of the sample obtained by calcination at 923 and 1173 K showed, together with MgO-type and/or MgAl 2O 4-type phases, the presence of increasing amounts of Pd-containing (PdO or metallic Pd) segregated phases. Further information was collected with an in-situ temperature-programmed experiment, where the evolving phases were investigated by neutron diffraction. Moreover, the effect of different heating atmospheres (oxidizing, reducing or inert) on the phase segregation was also investigated using differential thermal analysis. The catalyst always showed high activity in the total oxidation of methane, regardless of the heating atmosphere. However, PdO enhanced the activity at low temperature and low methane conversion, while the complete oxidation of methane was achieved at lower temperatures in the presence of large amounts of metallic Pd.

Catalytic behaviour of Cl-free and Cl-containing Pd/Al 2O 3 catalysts in the total oxidation of methane at low temperature

Pd/Al 2O 3 catalysts prepared by the impregnation of alumina free of chlorine with H 2PdCl 4 and Pd(NO 3) 2 precursors were characterised and their catalytic activity in methane oxidation was measured under oxygen-rich conditions. The presence of residual chlorine ions either originating from the precursor or from impregnation with HCl was shown to strongly inhibit the conversion of methane. Residual chlorine ions could be slowly removed under reaction conditions in the form of HCl. This was observed by in situ measurement of HCl departure and catalytic activity. Experimental data were explained by a simple model in which residual chlorine would block PdO surface active sites during the reaction. The resulting catalyst, free of chlorine ions, was similar to the one prepared from the nitrate precursor in terms of dispersion and activity. Chlorine-free catalysts exhibited slow deactivation with time on stream, which was tentatively attributed to the slow conversion of the active PdO phase into a less active Pd(OH) 2 phase. This interpretation was supported by the poisoning effect of water vapour observed upon addition of water to the feed while CO 2 had no influence. Regeneration of the catalyst could be achieved by purging in dry carrier above 500°C, which could be ascribed to the decomposition of Pd(OH) 2 into PdO.

Total Oxidation of Methane Over La, Ce and Y Modified Manganese Oxide Catalysts

A series of catalysts based on high specific surface area MnO2 precursor, and modified by La, Ce or Y ions were prepared and applied for methane deep oxidation. XRD and BET results indicate that La, Ce and Y ions as additives can impede the crystallization of catalysts at high temperatures. Thus, some catalysts that can maintain much higher specific surface area than unmodified MnOx were obtained. The main species of all catalysts are proved to be crystalline or amorphous Mn2O3 by XRD and H2-TPR. The activities for methane total oxidation over these catalysts are compared with each other. It is found that all modified catalysts show higher activities than unmodified MnOx below 420°C, but show a little lower activity above this temperature.

Pd/Si 3N 4 catalysts: preparation, characterization and catalytic activity for the methane oxidation

Silicon nitride (Si 3N 4) is a refractory material showing high thermal conductivity at high temperature. We can therefore expect very good stability for both the catalytic phase and the support when using it as a support of active phases, even for very exothermic reactions occurring at high temperature. Pd supported on silicon nitride catalysts were prepared by impregnation of Si 3N 4 with Pd(II) acetylacetonate. The catalytic precursors were decomposed under oxygen at 350°C and reduced under hydrogen at 300°C; they were tested for the methane total oxidation reaction. Such catalysts were found to be highly stable, even after ageing at temperatures higher than 800°C under the reactive (oxidizing) mixture.

Catalytic combustion of hydrocarbons with Mn and Cu-doped ceria–zirconia solid solutions

The catalytic activity of a series of CeO 2–ZrO 2 mixed oxides in the total oxidation of methane and light hydrocarbons has been investigated. The influence of dopants like Mn and Cu has also been studied. It is shown that both MnO x and CuO at low loading dissolve within the ceria–zirconia lattice. This strongly influences the redox behaviour of the catalysts by promoting low-temperature reduction of Ce 4+. In addition, the ternary oxides show better stability to repeated redox cycles, which is attributed to the presence of ZrO 2. The catalytic activity of pure CeO 2 is also enhanced in the presence of ZrO 2, reaching a maximum with Ce 0.92Zr 0.08O 2; a further promotion of activity is observed with the introduction of MnO x and CuO dissolved into CeO 2–ZrO 2 lattice.

CO and CH4 total oxidation over manganese oxide supported on ZrO2, TiO2, TiO2–Al2O3 and SiO2–Al2O3 catalysts

Zirconia, titania, titania–alumina and silica–alumina supported and unsupported MnOx catalysts were prepared and characterized by X-ray diffractometry and photoelectron spectroscopy, infrared spectroscopy and nitrogen sorptometry. Their catalytic activities were tested towards total oxidation of carbon monoxide and methane. The results show the unsupported MnOx (exposed on an α-Mn2O3 bulk structure) to catalyze CO oxidation at ⩽250°C and CH4 oxidation at 250°C, and to remain chemically and structurally stable. The CO oxidation activity of MnOx is improved when dispersed on zirconia, whereas its CH4 oxidation activity is improved on silica–alumina. These results may help in concluding that (i) CO oxidation is not the rate determining step of the oxidation of CH4, (ii) availability of strong acid sites (as those exposed on silica–alumina) is important for CH4 oxidation and (iii) the difference in the catalytic activity towards the oxidation of CO and CH4 resides in the need for different catalytic functions for each reaction, which are, therefore, not related in terms of kinetic control.

Catalytic partial oxidation of methane over Ni-, Co- and Fe-based catalysts

Partial oxidation of methane to synthesis gas has been investigated over Fe-, Co- and Ni-based catalysts in fixed bed reactor in view of a possible application in a fluidized bed reactor. Catalytic testing of the oxidic catalysts show an increasing activity for total oxidation of methane in the following order: Fe 2O 3>CoO>NiO. After reduction of the metal, the Ni and Co catalysts give equilibrium conversion to synthesis gas. Co is much more easier oxidized compared to Ni at decreasing temperature under experimental conditions and equilibrium is not followed at temperatures below 550°C. Low reforming activity is obtained over the Fe catalyst. Investigations at 1000–1100°C are necessary to further explore Fe as a reforming catalyst.

Isotopic tracing experiments in syngas production from methane on Ru/Al 2O 3 and Ru/SiO 2

The catalytic production of syngas from methane was investigated over alumina- and silica-supported ruthenium catalysts. It is shown that the support is involved in reactions with reactants and products somewhat modifying the catalytic behaviour. From isotopic tracing experiments it is concluded that the syngas production reaction follows the same indirect pathway over the two catalysts, i.e. the sequence of total oxidation of methane, followed by reforming of the unconverted methane with carbon dioxide and steam to syngas. Recent literature is analysed in the light of our results.

5676766 Solar cell having a chalcopyrite absorber layer

In the light-off temperature range of methane total oxidation (250–600°C), two types of Pt/δ-Al2O3 catalysts prepared from H2PtCl6 or Pt(NH3)4(OH)2 show different catalytic behaviors. Initially very active, fresh chlorine-free catalysts deactivate in isothermal conditions of reaction, while fresh chlorine-containing catalysts, initially poorly active, undergo an activation with time on stream. Consequences of dechlorination of chlorine-containing catalysts and of impregnation by various amounts of NH4Cl on both types of catalysts are examined. Chlorine is responsible for the inhibition observed on ex-H2PtCl6 catalysts, which could be attributed, more specifically, to the chloride ions remaining at the platinum–support interface after reduction of platinum. After aging of the catalysts under reaction, sintering of the platinum particles is observed on all samples, regardless of their preparation mode. This phenomenon is linked to the presence of oxygen in excess in the gas phase and can account for the deactivation of chlorine-free catalysts. The water produced during the reaction removes the inhibiting chlorine on all chlorine-containing catalysts. The activity of the largest sintered particles is shown to vary with the nature of the fresh catalyst, the aged ex-H2PtCl6 catalysts giving the best results: the molecular nature of the fresh catalyst would also affect the activity of the stable aged catalyst. Both the initial presence of chloride ions at the metal–support interface and the composition of the reacting mixture are key factors to this positive reconstruction.