Activation of surface lattice oxygen in the oxidation of carbon monoxide on silica

Abstract

Silica prepared by the sol–gel method from ethyl orthosilicate catalyses carbon monoxide oxidation at 850 K or above while no such activity is found with a commercial silica prepared from inorganic reagents. The activity of the silica prepared by the sol–gel method is increased by repetition of the carbon monoxide oxidation reaction. Labelled reactions with ¹⁸O₂ provide evidence to suggest that most of the lattice oxygen species on the surface of the silica participate in the reaction. After evacuation at 950 K or above, radical species identified as Si—O—O^–—Si are generated on the silica. Si—O^–—Si radical species are apparently formed during the reaction with the radical electron originating from the Si—O^–—Si species in the initial stage of the reaction. Adsorption of carbon monoxide and oxygen on the Si—O^–—Si species is proposed to cause formation of carbon dioxide and O^– which will react with carbon monoxide and provide the radical electron to the lattice oxygen species, that is, Si—O^–—Si and Si—O^–—O—Si. The present work shows that the lattice oxygen of silica can be involved in oxidation if the oxygen is activated by radical species such as O^–, which have been proposed as active species in catalytic oxidation processes.