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, 0.2 0, 0.25, 0.35, and containing only the perovskite (small x) and the perovsk ite plus Fe2O3 crystal phases (large X) were examined in the catalytic reac tion of (NO+CO). The solids were prepared by heating at T=1000 degrees C un der 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 th an 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 resul ts are explained assuming that in the catalyst particles with x greater tha n or equal to 0.10 a Fe2O3 core of increasing size is covered by a LaFeO3 s hell. The sample LaFeO3 showed lower catalytic activity for the NO+CO react ion than the rest of La1-xFeO3 solids. The Arrhenius plots showed two disti nct regions of activity one at low temperature with high apparent activatio n energies and another at high temperature with lower apparent activation b arriers. 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 ener gies resulted in an estimation of the heats of adsorption of NO on LaFeO3 a nd La1-xFeO3. The reaction of NO+CO also resulted in small amounts of N2O w hich showed a maximum at similar to 320 degrees C. The dependence of N2O pr oduction and elimination on the temperature made it possible to determine t he activation energies of its formation as well as the heat of desorption o f nitrogen from the catalyst surface. The catalytic activity of the solids is destroyed if they are heated under atmospheric conditions. (C) 1999 Else vier Science B.V. All rights reserved.