Abstract.

The catalytic oxidative coupling of methane over perovskite CaTiO, prepared by a modified ceramic method was carried out and gave a C, yield of 13% at 830°C and promoted with Na, P20, to improve considerably catalyst performance. From reactor test and the mechanism studies it is believed that charge deficient oxygen O- generated by transforming oxygen adsorbed on surfacial defects and dissolved into bulk vacancies is responsible for activating the methane (activating sites). The adsorbed oxygen generates oxidizing sites for suppressing activated methane. Strict conditions are required for generating and regenerating of the activating sites in the lattice.

INTRODUCTION

The heterogeneous oxidative coupling of methane has been extensively studied. The use of doped alkaline earth materials has been a rather fruitful area of work'. It is believed that the efficacy of these catalysts results from the presence of oxygen vacancies in their surface and bulk. Therefore being good ionic conductors, the use of defect rich materials as catalysts has been proposed. In fact perovskite oxides of formule ABO, and similar oxides have been shown to be catalytically active for CO and hydrocarbon oxidation as well as oxidative coupling of methane2.

A few members of the perovskite oxides are cited as good electronic conductors (P-type mode). In this work, we measured the catalytic properties of well defined bulk solid whose structure, stability and electrical properties are well known. The results will be discussed considering the mechanism base on the solid conductivity.

EXPERIMENTAL Perovskite catalysts were prepared by the ceramic (Sol-sol) method3 by adding the proper oxides of Ti-Ca to deionized water to form a slurry solution. A polymeric binder and mineral solids were added to the slurry solution while stirring fast. After drying and crushing the viscous blend, the powder was calcined at 850°C for 8–12 hours to form a perovskite phase of the nominal composition CaTiO, with the other oxides. Characterization of the catalysts was carried out.

The reactor experiments were carried out in a flow fixed bed quartz reactor (9 mm ID) at a temperature range of 800–850°C by using 3 ml catalyst with a feed composition: CH4/02/He = 32/8/60 and at total gas flowrate of 100 ccfmin. The apparatus has been described elsewhere4.

RESULTS AND DISCUSSION The solid-solid method employed for preparing the catalyst offers the advantage of simplicity. High temperatures are required for a complete reaction between the single phases to take place, thus leading to a drastic loss of surface area by sintering, because the surface area was measured very low at less than 1 m2/g. The catalysts have the well defined structure of

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