LA-CE-CA PEROVSKITES AS CATALYSTS FOR EXHAUST-GAS DEPOLLUTION

A series of La1-xCexCoO3+delta perovskite-type catalysts, with x rangi ng from 0 to 0.20, showed to be quite active for reduction of NO by CO (reaction 1) and for oxidation of CO by air oxygen (reaction 2) at te mperatures ranging from 373 to 723 K. Analysis by X-ray diffraction, e lectron probe microanalysis, temperature-programmed desorption-tempera ture-programmed reaction and electron paramagnetic resonance, coupled with catalytic activity data, showed that the active sites on these ca talysts are very likely localised onto Co ions, which coordinate O-2(- ) ions as intermediates for reaction 2, the latter taking place essent ially through a suprafacial mechanism. For reaction 1, taking place th rough both a suprafacial and an intrafacial mechanism, the Co-based si tes play also an additional role, favouring the electron transfer from site to site and so enhancing the transfer of oxygen species from sur face to bulk and vice versa. Ce ions seem to act only as a stabiliser of O-2(-) ions, helping in keeping them at the catalyst surface.