La-Ce-Ca perovskites as catalysts for exhaust gas depollution

A series of La1-xCexCoO3+delta perovskite-type catalysts, with x ranging 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 temperatures ranging from 373 to 723 K. Analysis by X-ray diffraction, electron probe microanalysis, temperature-programmed desorption-temperature-programmed reaction and electron paramagnetic resonance, coupled with catalytic activity data, showed that the active sites on these catalysts are very likely localised onto Co ions, which coordinate O-2(-) ions as intermediates for reaction 2, the latter taking place essentially through a suprafacial mechanism. For reaction 1, taking place through both a suprafacial and an intrafacial mechanism, the Co-based sites play also an additional role, favouring the electron transfer from site to site and so enhancing the transfer of oxygen species from surface 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.