Catalytic carbon monoxide oxidation over strontium, cerium and copper-substituted lanthanum manganates and cobaltates

Abstract

The influence of either A or B-site substitution in perovskite-type mixed oxides on the catalytic oxidation of carbon monoxide has been studied. The following systems were investigated: (La,Sr) MnO 3, La(Mn,Cu)O 3, (La,Sr)CoO 3 and (La,Ce)CoO 3. Cobaltates are generally more active than the manganates. Substitution in the A or B-site improved the catalytic activity with oxidation starting from 75 °C. A volcano plot of activity versus composition was obtained for each series with up to a 10-fold increase in catalytic activity for the substituted compounds. Lattice oxygen participates in the reaction even under stoichiometric conditions. The catalysts show a positive rate dependence on the carbon monoxide partial pressure so that under reducing conditions, the reaction is not inhibited. A bistability in the rate of catalytic oxidation at high carbon monoxide concentration was observed over La 1− x Sr x MnO 3 and LaMn 1− x Cu x O 3 (0⩽x⩽0.2). This bistability has been attributed to a carbon monoxide-driven reconstruction of the reduced surface, leading to pairs of Mn 2 ions with a Mn-Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for carbon monoxide oxidation and oxygen chemisorption. Such metal-metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4-layered packing which is the normal crystal structure of SrMnO 3. The change back to the less active state is due to reoxidation of the surface. It was confirmed that a low mobility of lattice oxygen is a necessary condition for hysteresis in these oxides.