Synergistic effects of crystal phases and mixed valences in La-Sr-Ce-Fe-O mixed oxidic/perovskitic solids on their catalytic activity for the NO plusCO reaction

Mixed oxidic and perovskite-type materials based on the La, Sr, Ce and Fe e lements were prepared using a mixture of nitrates salts and heating at 1000 degrees C. Three groups of solids were synthesized: (i) La1-yCeyFeO3 (y = 0.2, 0.3, 0.5), (ii) La1-xSrxFeO3 (x = 0.2, 0.3, 0.5) and (iii) La1-x-ySrxC eyFeO3 (x/y = 0.05/0.15, 0.15/0.05,.1/0.2, 0.2/0.1, 0.2/0.3 and 0.3/0.2). T he structure of the solids was examined by XRD and the main crystal phases determined were LaFeO3, alpha-Fe2O3 and CeO2 in group (i), LaFeO3 and SrFeO 3-x, in group (ii), and LaFeO3, alpha-Fe2O3 SrFeO3-x, and CeO2 in group (ii i), while traces of La(OH)(3) and SrFe12O19 were also detected. The precise determination of the percentage amount of the iron-containing crystal phas es in each solid composition was-determined by Mossbauer spectroscopy at 20 K. The ceramic materials had low surface areas and were tested for their c atalytic activity for the NO + CO reaction in a flow reactor in the range o f 280-560 degrees C. Conversions as high as 90% were achieved at 550 degree s C at a GHSV = 54000h(-1). The reaction rate of NO conversion is favored b y the increased amount of CeO2 in groups (1) and (iii) of solids that conta in cerium. In the case of solids without CeO2 (group ii), the NO conversion is favored by the existence of SrFeO3-x, phase at low temperatures (280-44 0 degrees C), while it decreases at high temperatures (440-560 degrees C). The double substituted solids La1-x-ySrxCeyFeO3 with x+y>0.3 and y>x were f ound to be the best catalysts for the NO+CO reaction as compared to the sin gle substituted mixed oxides. Temperature programmed desorption (TPD) studi es of NO and CO2 support the view that a synergistic effect takes place bet ween the two phases of CeO2 and SrFeO3-x, whose co-existence results in the maximum enhancement of activity, via alternative oxidation-reduction cycle s in the two phases. (C) 2000 Elsevier Science B.V. All rights reserved.