Manganese–lanthanum oxides modified with silver for the catalytic combustion of methane

Abstract

The characterization of manganese–lanthanum oxides modified with silver has been performed in order to identify factors responsible for the variation of their activity in the oxidation of methane. A significant increase in the activity per unit surface area in silver-containing catalysts occurred above 800 K, where a new source of surface oxygen appeared. It is probably oxygen released from filled oxygen vacancies, more weakly bound in the oxides structure in comparison with lattice oxide ions, more mobile, and therefore easily accessible to methane oxidation. Such oxygen is probably neighboring with silver ions. The remaining part of the catalyst may constitute a reservoir of oxygen ions with which the vacancies are filled and which is supplemented with the gaseous oxygen. A consequence of filling up oxygen vacancies is the appearance of a larger number of manganese ions in the unstable oxidation state Mn⁴⁺. The rate of methane oxidation is a function of the Mn⁴⁺/Mn³⁺ surface ratio which is a parameter characterizing the intrinsic properties of the manganese–lanthanum oxides, influencing their activity.

Introduction

Interest in the catalytic combustion of methane results mainly from the possibility of lowering the combustion temperature so that minor amounts of nitrogen oxides are produced, smaller than those which remain in the waste gases even after treatment with the most effective methods [1], [2], [3], [4], [5], [6]. A catalyst may also make it possible to purify waste gases of methane when they contain such small amounts of methane that they would not undergo ignition in ordinary burners.

Manganese oxides are among the various oxide catalytic materials showing a fairly good activity in flameless methane combustion [1], [7], [8], [9], [10]. Their heat resistance, and thus also the possibility of industrial utilization, improves when manganese oxide becomes bound in the perovskite structure of the general formula ABO₃, e.g., lanthanum-based perovskite LaMnO₃, which is one of the most active perovskite oxide catalysts for the combustion of methane [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The rare earth metals in the perovskite provide thermal stability of transition metal oxides [9], [11], [19]. Lanthanum oxide alone also shows activity in the reaction of the complete oxidation of methane, though much lower than manganese oxide [8]. In recent years it has been demonstrated that doping of perovskites LaNiO₃, LaMnO₃, LaCoO₃ [20], [21], LaFeO₃, and LaFe_0.5O₃ [22] with silver increases their activity in methane combustion. Moreover, Ag-doped LaCoO₃ showed no deactivation in methane combustion during 50 h at 600 °C [20]. Even greater activity enhancement in comparison with LaMnO₃ was observed in the case of composite Ag/Mn/perovskites catalysts in CO oxidation [19], [21]. In perovskites some part of basic cations may be replaced by other metal cations with similar ionic radii, with perovskite crystalline structure being almost unaltered. In the case of LaMnO₃ silver cations may only replace lanthanum cations (the ionic radii being Ag⁺ 1.40 Å, La³⁺ 1.22 Å, Mn³⁺ 0.66 Å, Mn⁴⁺ 0.56 Å [12]), forming perovskites of the type La_1−xAg_xMnO₃. Naturally, such cation exchange must entail the appearance of structural defects compensating the charge and preserving the electroneutrality of the perovskite, which affect its catalytic activity. An increase in the activity of perovskites in the reactions of the full oxidation of CO and hydrocarbons was also obtained following substitution of lanthanum ions by cations of other metals, e.g., Sr, Ca, Ce [9], [12], [13], [15], [18], [23].

The promoting action of silver is not confined only for perovskites. There have been reports of activity enhancement in CO and VOC oxidation [24], [25], [26], [27], [28], [29], [30], [31] after doping of manganese or cobalt oxides—unsupported or supported on Al₂O₃—with silver. It has been suggested that the increased activity of these silver-modified catalysts results from a synergistic interaction between silver oxide and manganese or cobalt oxides and from increase of the lability of lattice oxygen. Manganese–silver and cobalt–silver oxides had a higher activity than each individual catalyst [25], [27], [28], [29]. Differences in the activities of pure manganese and silver catalysts depended on the accepted basis of comparison. In unsupported catalysts the activity of silver oxide in CO oxidation in reference to its weight was much smaller than that of manganese oxide [25], [29] and of cobalt oxide [25], [28], but on a surface area basis silver oxide exhibited very high activity [28]. For Al₂O₃-supported catalysts the temperature required for 98% conversion of CO and VOCs on the silver catalyst depended on Ag loading but generally was lower than on the manganese catalyst [27], which is a reverse dependence than in the case of unsupported catalysts.

Silver alone is known to be a good partial oxidation catalyst; it is used in industry for the partial oxidation of methanol to formaldehyde and for the oxidation of ethylene to ethylene epoxide. The reports cited above indicate that silver may also be a catalyst of complete oxidation. Studies [32] also show that silver supported on zirconia may catalyze the complete oxidation of methane (the only oxidation products were carbon dioxide and water). Methane conversion strongly depended on silver state and dispersion: the catalyst activity was higher when metallic Ag crystallites were larger than 10 nm and partially oxidized.

The present paper reports experimental results concerning the preparation, characterization, and activity of silver-modified, manganese–lanthanum oxide composite catalysts in the complete oxidation of methane. The catalysts were prepared by a coprecipitation method with various precipitation agents and by various drying procedures. The characterization of the catalysts was performed by X-ray diffraction, infrared spectroscopy, photoelectron spectroscopy, temperature-programmed reduction, and temperature-programmed oxygen desorption, in order to identify factors responsible for the variation of the catalysts activity and to elucidate the modifying role of silver in these catalysts.