Structure and catalytic activity of La1-xFeO3 system (x = 0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.35) for the NO+CO reaction

Solids of the nominal formula La1-xFeO3 where x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.35, and containing only the perovskite (small x) and the perovskite plus Fe2O3 crystal phases (large X) were examined in the catalytic reaction of (NO+CO). The solids were prepared by heating at T=1000 degrees C under similar to 10(-5) Torr vacuum and showed appreciable catalytic activity at temperatures from 280 degrees C to 480 degrees C when SV is similar to 300 h(-1). XRD analysis and Rietveld refinement showed that for x=0.00 and 0.05 only the perovskite phase is apparent. For x greater than or equal to 0.10 an additional crystal phase of Fe2O3 appears which increases with x up to a maximum of 4.1% at x=0.35, Moessbauer examination indicated that iron exists in the perovskite structure at x=0.00 and 0.05 but for x greater than or equal to 0.10 the extra-perovskite Fe3+ increases proportional to the parameter x of the solids La1-xFeO3 and reaches 43% at x=0.35. These results are explained assuming that in the catalyst particles with x greater than or equal to 0.10 a Fe2O3 core of increasing size is covered by a LaFeO3 shell. The sample LaFeO3 showed lower catalytic activity for the NO+CO reaction than the rest of La1-xFeO3 solids. The Arrhenius plots showed two distinct regions of activity one at low temperature with high apparent activation energies and another at high temperature with lower apparent activation barriers. At low temperatures the low activity at x=0.00 is related to high apparent activation energies while for x greater than or equal to 0.05 the opposite is true. A detailed scrutinization of the apparent activation energies resulted in an estimation of the heats of adsorption of NO on LaFeO3 and La1-xFeO3. The reaction of NO+CO also resulted in small amounts of N2O which showed a maximum at similar to 320 degrees C. The dependence of N2O production and elimination on the temperature made it possible to determine the activation energies of its formation as well as the heat of desorption of nitrogen from the catalyst surface. The catalytic activity of the solids is destroyed if they are heated under atmospheric conditions. (C) 1999 Elsevier Science B.V. All rights reserved.