Low-temperature Methane Oxidation Research Articles

Low-temperature methane oxidation over oxide-supported Pd catalysts: inhibitory effect of water vapor

The influence of water vapor on the activity for low-temperature methane oxidation over oxide-supported catalysts such as Pd/Al 2O 3, Pd/SnO 2, and Pd/Al 2O 3-36NiO was studied. It was found that Pd/Al 2O 3 was deactivated most significantly due to water vapor, and that Pd/Al 2O 3-36NiO was most insensitive to water vapor. The catalytic activity of Pd/Al 2O 3 decreased monotonically as water vapor concentration increased, whereas Pd/SnO 2 and Pd/Al 2O 3-36NiO showed almost constant activity under higher water vapor concentrations. The catalytic activity at high steam concentration was in the following order: Pd/SnO 2>Pd/Al 2O 3-36NiO>Pd/Al 2O 3. Kinetic analysis with methane adsorption as the rate-limiting step was applied to evaluate the water inhibiting effect. Pd/Al 2O 3 displayed the most negative value of the enthalpy of water adsorption, while Pd/SnO 2 and Pd/Al 2O 3-36NiO exhibited similar water adsorption enthalpy. Deactivation and regeneration of Pd/SnO 2 and Pd/Al 2O 3 catalysts were investigated by cyclic feed of water vapor. Both the catalysts were deactivated rapidly upon switching on water feed, and then they regenerated gradually to the initial activity after the water feed was switched off.

Low temperature oxidation of methane over Pd-based catalysts—effect of support oxide on the combustion activity

Pd catalysts supported on metal oxides were investigated for low temperature catalytic combustion of methane. Of the catalysts examined, Pd/Al 2O 3-36NiO and Pd/SnO 2 catalysts demonstrated excellent activities, despite its low specific surface area as compared with the conventional Pd/γ-Al 2O 3 catalyst. It was observed by a transmission electron microscope and an X-ray line broadening analysis that the PdO phase was well-dispersed on the surface of the SnO 2 and Al 2O 3-36NiO supports, respectively, despite their low surface areas. Temperature-programmed desorption (TPD) of oxygen has revealed that the catalytic activity depends on the adsorption state of oxygen on palladium. It was suggested from TPD of O 2, in situ XPS, and in situ XRD that the dissociation of oxygen from the PdO bulk, as well as from its surface proceeded at higher temperatures for the Pd/Al 2O 3-36NiO and Pd/SnO 2 catalysts than for Pd/Al 2O 3 because of the strong palladium-support interaction. The catalytic activity on Pd/SnO 2 was zero-order with respect to the oxygen concentration, whereas an increase in water concentration significantly deteriorated the catalytic activity. This suggests that adsorption of both oxygen and water is strong on the Pd surface. The support oxides appear to modify relative adsorption strength oxygen and water, and then combustion activity of palladium.

Low-Temperature Selective Oxidation of Methane into Formic Acid with H2–O2 Gas Mixture Catalyzed by Bifunctional Catalyst of Palladium–Heteropoly Compound

The selective oxidation of methane catalyzed by heteropoly compounds having the formulas MxCs2.5H0.5−2x+yPVyMo12−yO40 (M=Pd2+, Rh2+, Ru2+, Pt2+, Mn2+, Hg2+, Fe3+, Co2+, Cu2+; x=0–3, y=0–3) was investigated. It was demonstrated that addition of Pd and incorporation of V had a strong influence on oxidation with a H2−O2 gas mixture and that Pd0.08Cs2.5H0.34PVMo11O40 showed the highest yield of formic acid. The reaction proceeded at temperatures as low as 423–593 K. Moreover, the reaction rate reached 1.2×10−4 mol h−1 g−1 at 573 K; this value is about 300 times higher than that with FePO4 catalyst. Addition of steam promoted the production of formic acid, and the yield reached a maximum at a partial pressure of steam of 9.1 kPa. The coexistence of H2 and O2 was indispensable for the selective oxidation of methane. It is suggested that an active oxygen species is formed by the reaction of H2 with O2 catalyzed by Pd and acidic sites of supports. Pressure dependencies were expressed by −dPCH4/dt=kPH21.0PO21.0PCH41.0, and is consistent with the idea that the reaction of an active species formed from H2 and O2 with CH4 is rate determining.

Low temperature oxidation of methane over Pd/SnO 2 catalyst

Catalytic activities of supported Pd were investigated for low temperature oxidation of methane. Pd/SnO 2 catalysts demonstrated excellent activity for methane oxidation in spite of their low surface area. The catalytic activity of Pd/SnO 2 was strongly affected by the preparation procedure. Impregnation of Pd on SnO 2 using aqueous solution of Pd(CH 3COO) 2 was most effective in enhancing the catalytic activity. The catalytic activity was also improved when well-crystallized SnO 2 was employed as a support material. TEM observations revealed that catalytic activity is strongly influenced by the dispersion state of Pd. For the active catalysts, strong interaction between Pd and SnO 2 support was observed in the adsorption of oxygen.

Low temperature oxidation of methane over Pd catalyst supported on metal oxides

Supported Pd catalysts were investigated for low temperature oxidation of methane for catalytic combustor. Both, Pd/ZrO 2 and Pd/SnO 2 demonstrated excellent activity in spite of its low surface area. The activity of Pd/ZrO 2 was strongly dependent on the crystal phase of the support materials. ZrO 2 with a monoclinic phase enhanced the activity than that with a tetragonal phase. The catalytic activity of Pd/SnO 2 was affected by the preparation procedure. Impregnation of Pd on SnO 2 using Pd(C 5H 7OO) 2 aqueous solution was most effective in enhancing the catalytic activity. It is considered that catalytic activity is strongly influenced by the existence of interaction between palladium and support materials.

Low temperature oxidation of methane: the influence of nitrogen oxides

An experimental investigation of methane oxidation in the presence of NO and NO2 has been made in an isothermal plug-flow reactor at 750-1250K. The temperature for on-set of oxidation was lowered by 250 K in the presence of NO or NO2 at residence times of 200 ms. At shorter residence times (140 ms) this enhancement effect is reduced for NO but maintained for NO2. Furthermore two temperature regimes of oxidation separated by an intermediate regime where only little oxidation lakes place exist at residence times of 140 ms, if NO is the only nitrogen oxide initially present. The results were explained by the competition between three reaction paths from CH3 to CH2O. A direct high temperature path (A), a two-step NO2 enhanced low temperature path (B) and a slow three step NO enhanced path (C), which may produce NO2 to activate path B. The negative temperature coefricient behaviour was explained by a competition between paths (A) and (C). A previously published reaction set was modified to take these reaction patterns into account. The processes discussed here presumably occur to some extent in the exhaust of natural gas engines, but conditions may be modified, either to ensure an enhanced activity to oxidize methane in the exhaust, or alternatively to decrease the activity 10 reduce the production of unwanted intermediates such as formaldehyde.

Low-Temperature Oxidation of Methane over Pd Supported on SnO2-Based Oxides

Abstract Pd catalysts supported on various metal oxides were investigated for the low-temperature catalytic combustion of methane. Of the catalysts examined, a Pd/SnO2 catalyst demonstrated excellent activity despite its low specific surface area. The activity of the Pd/SnO2 catalyst was lowered with the addition of M ( = Al, Ce, Fe, Mn, Ni, and Zr) oxide to the support, Pd/SnO2–MOx. It was observed by a transmission electron-microscope observation that the PdO phase was well-dispersed on the outer surface of SnO2support uniformly, observed as an intimately contacted layer on fine SnO2 particles. The temperature-programmed desorption of oxygen has revealed that the catalytic activity depends on the adsorption state of oxygen on palladium. An X-ray photoelectron spectroscopic analysis suggested that reduction of the palladium surface proceeded slowly under a strong palladium-support interaction for Pd/SnO2. In situ X-ray diffraction indicated that the PdO phase was stabilized at higher temperature on SnO2 than on the Al2O3 support. It is considered that the catalytic activity is strongly influenced by the interaction between palladium and SnO2.

A Highly Catalytic Bimetallic System for the Low-Temperature Selective Oxidation of Methane and Lower Alkanes with Dioxygen as the Oxidant

This report describes a highly catalytic bimetallic system for the low temperature selective oxidation of methane, ethane, and butane with dioxygen as the oxidant. The catalytic system consists of a mixture of copper chloride and metallic palladium and operates in a 3:1 mixture (v/v) of trifluoroacetic acid and water in the presence of dioxygen and carbon monoxide. Methane was selectively converted to methanol. The dependence of the reaction rate on the partial pressure of methane was measured, and saturation kinetics was observed. The dependence of the rate on the partial pressure of carbon monoxide was measured at two different pressures of methane, and a first-order dependence on the partial pressure of carbon monoxide was observed in both instances. The activation parameters for the overall reaction were obtained under the reaction conditions when the rate was first order in both methane and carbon monoxide. The values obtained were A = 2 × 104 s-1 and Ea = 15.3 kcal mol-1. Both C−H and C−C cleavage products were observed for ethane and n-butane, resulting in the formation of methanol, ethanol, and acetic acid. The rate of formation of methanol from methane with the bimetallic system was ca. 65 × 10-4 M/min at 145−150 °C. This rate may be compared to the rate of formation of acetic acid from methanol in the benchmark Monsanto process: 380 × 10-4 M/min at 180 °C.

Low-temperature total oxidation of methane over Ag-doped LaMO3 perovskite oxides

Partial substitution of La by Ag, instead of Sr, in LaMO3(M = Co, Mn, Ni) perovskite causes a large increase in the catalytic activity of the perovskite in the complete combustion of dilute methane for its emission control, at low temperature (< 700 °C) and high space velocity (51 000 cm3 g–1 h–1); Ag-doped LaCoO3 shows the highest methane combustion activity.

Low-temperature catalytic selective partial oxidation of methane to carbon monoxide and hydrogen over nickel/ytterbium sesquioxide

Ni/Yb{sub 2}O{sub 3} showed high activity/selectivity in catalytic oxidative conversion of methane to CO and H{sub 2} at low temperature (300-700 {degree}C) and extremely high space velocity (5 {times} 10{sup 5} cm{sup 3}{center_dot}g{sup {minus}1}{center_dot}h{sup {minus}1}) at atmospheric pressure. The low-temperature catalysts process operated far away from the thermodynamic equilibrium and also follows a reaction path totally different from those considered for the high-temperature noncatalytic and catalytic processes. The formation of surface carbon species in the initial reaction period seems to play an important role in the catalytic process. 8 refs., 1 fig., 1 tab.

Low-temperature selective oxidation of methane to carbon monoxide and hydrogen over cobalt-MgO catalysts

Reduced CoO-MgO catalysts (Co/Mg mole ratio = 1–10) show high activity and selectivity in oxidative conversion of methane to carbon monoxide and hydrogen with extremely high space-time-yield at low temperatures (⩽ 700 °C, down to 300 °C).

An Analytical Study of Methane Oxidation in a Steady-Flow Reactor

Abstract The high temperature reaction of methane in a flow system was investigated analytically using a reaction scheme consisting of 24 reaction steps and involving 15 species. Over the range of conditions considered the predicted role of H2CO compared favourably with the experimental evidence available in the literature. The results also indicated the importance of H2O2 in the low temperature oxidation of methane particularly within the pre-ignition stage. Calculated values of the induction period were generally in good agreement with experimental results reported in the literature.