Real structure and catalytic activity of La1-xCaxMnO3+delta perovskites

Calcium-substituted lanthanum manganites La1-xCaxMnO3+delta (0 less than or equal to x less than or equal to 1) were synthesized from the solid oxide precursors using two routes: a traditional ceramic method and the mechanica l activation of the mixture of oxides in high power planetary ball mills, f ollowed by the product annealing at different temperatures. The samples str ucture was studied by XPD and TEM methods, while their surface composition was characterized by the differential dissolution method. A pronounced effe ct of the preparation method on samples bulk and surface properties, reflec ted in their reactivity characterized by the rare of CO catalytic oxidation , was observed. For samples prepared via the ceramic route by sintering at 1100 degreesC, continuous solid solutions are formed. In the case of mechan ical activation route, by annealing of activated mixture of oxides at 700-1 00 degreesC, solid solutions are formed only at the Ca content within 0 les s than or equal to x less than or equal to 0.4 range. At higher Ca content, microheterogeneous composites are formed, in which the nucleus of particle s is formed by the perovskite phase with x less than or equal to 0.4, while the surface layer is enriched by Ca. The catalytic activity in CO oxidatio n appears to correlate with the density of extended defects, being decrease d when Ca is segregated in the surface layer, (C) 2001 Elsevier Science B.V . All rights reserved.