1. *Both fresh compounds have Fe3+ in spinel structure*
Catalytic properties of some cobalt ferrites in propylene oxidation P. Chesler1, M. Caldararu1*, C. Munteanu1, M. Scurtu1, V. Bratan1, M. Feder2, L. Diamandescu2, I. Bibicu2 1-“Ilie Murgulescu” Institute of Physical Chemistry, P.O. BOX 194, Bucharest, Romania, 2-National Institute of Material Physics, Bucharest-Magurele, P.O. BOX MG-7, Romania * - corresponding author,
Introduction
CoFe2O4 (cobalt ferrite) is an inverse spinel having a structure of the type (Fe)A[CoxFe2-x ]BO4, where A stands for tetrahedral sites and B for octahedral ones. The cation distribution strongly depends on the synthesis method and on reaction conditions. The catalytic activity of spinels seems to be generally controlled by the cations located in the octahedral sites. Cations located in tetrahedral sites are apparently inactive.
Experimental
Cobalt ferrite-based samples, with excess of iron (Co0.8Fe2.2O4) or containing manganese (CoMnxFe2-xO4 , with 0<x<0.6) have been prepared by classical ceramic method (solid phase reaction) at 900oC for 5 hours. The samples were tested in propylene oxidation in oxygen-rich gas mixture (C3H6:air = 1:22 and C3H6:air = 1:30), between oC, at a contact time of 1.3 seconds. The fresh and used samples were characterized by X-ray diffraction (XRD) and Mössbauer spectroscopy.
*Both fresh compounds have Fe3+ in spinel structure*
Fe3+(Oh) is regained in 2 other new magnetic sub-lattices, ≈16% as Fe2+(probably Td) and ≈21% as Fe0; ○ data,― fit, ▲ octahedral positions, tetrahedral positions,  new magnetic sub-lattice, ■ iron
Part of Fe3+ (Td) (≈20%) is reduced to Fe 2+(possibly Td) in a new sub-lattice; • data,― fit,  octahedral positions,  tetrahedral positions,  new magnetic sub-lattice,  wustite
Co0.8Fe2.2O4 powder XRD spectra: (a)-fresh sample: 77.1% spinel phase-22.9% hematite; (b)-used sample, after catalytic test with: 78.6% spinel, 4.5% FeO, and 16.9% Fe
CoFe1.8 Mn0.2O4 powder XRD spectra, which shows a 97.9% spinel phase-(a)-fresh sample, and 82.4% spinel-17.6% FeO-(b)-used sample, after catalytic test
CoMnxFe2−xO4 samples obtained from synthesis, as a function of x value
Discussion
As shown in the figure below, the cobalt ferrite with excess of iron is a good catalyst for deep oxidation. Propylene conversion jumps suddenly above 200oC, when also a very high oxygen consumption was recorded. Conversion up to approximately 100% to carbon oxides (CO in important amounts) was obtained on Co0.8Fe2.2O4 at 400oC. The use of C3H6: air 1:30 feed does not seem to change the conversion very much, but decreases the selectivity to CO.
CoFe1.8Mn0.2O4 is obviously less active. On the other side, the CO selectivity on manganese containing compound was much lower, an important fact while using this material as a depollution catalyst.
Conclusions
The present study reveals good catalytic properties for deep propylene oxidation of Co0.8Fe2.2O4 (propylene conversions of about 100% were obtained at 400oC). CoFe1.8Mn0.2O4 was less active. It is suggested that by appropriate optimization concerning composition and oxygen content in feed, cobalt ferrites can be proposed as possible depollution catalysts
The obvious difference of activity between the two samples points to the role of iron sites in this type of catalyst. The increase of selectivity for partial oxidation (CO formation) observed at high temperature, coupled with evolution of hydrogen among products (dehydrogenation) indicates that in this temperature range the surface is in a reduced state (surface reduction is faster than surface reoxidation)
Part of Fe3+ (Td) (≈20%) is reduced to Fe 2+(possibly Td) in a new sub-lattice , which indicates that the reaction may occur through a Mars van Krevelen mechanism.
Acknowledgements
Acknowledgement is made to the Romanian Ministry of Education and Research-NASR, for the support of this research under CEEX Project nr. C-73 S9/2006.