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Study of perovskites catalytic activity for CO oxidation

Koilia Evangelia

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Summary

The catalytic oxidation of CO is a widespread reaction of converting CO to CO2 which has been the subject of scientific research. In present thesis, the activity of perovskite materials with chemical formula La1-xSrxMnO3 (x=0, 0.3, 0.5 and 0.7) for the oxidation of CO in conditions of excess O2, was experimentally studied. Perovskites with structure ABO3 show very good catalytic properties, are stable at high temperatures and due to their utility in various catalytic redox reactions are able to replace noble metals in catalytic three-way converters. To perform the kinetic experiments, LSMx perovskites were studied in a temperature range of 100°C - 450°C, with a feed gas composition of 1% CO + 5% O2 in equilibrium with He and FT=160 cc/min. Their catalytic activity was studied after various pre-treatment steps, i.e., after pre-reduction and pre-oxidation, while their thermal stability was studied after applying a specific thermal aging protocol that included successive oxidation cycles at 600°C and 750°C. In addition, the physicochemical and structural properties of the perovskites were evaluated by various characterization techniques such as BET, XRD and H2- TPR. Finally, the usage of LSMx as active supports for the deposition of noble metals, such as Ru and Ir, was studied. According to the results of characterization techniques, it was found that the specific surface of the perovskites varies in low values in the range of 6.8 to 12 m2/gr. Furthermore, XRD measurements revealed the formation of the perovskite structure, as well as the existence of other Mn and La oxides. From the H2-TPR experiments, the oxygen storage capacity (OSC) values were calculated, which range from 670 to 1220 μmol O2/gr, while a gradual increase is observed with the simultaneous increase in the substitution of La by Sr. From the kinetic experiments carried out, it was found that LSMx perovskites are active in the oxidation of CO (in excess O2 conditions) in the temperature range of 100°C to 450°C, however their catalytic activity depends on the pre-treatment stage, since when the reduced LSMx give better results than the pre-oxidized. Furthermore, in the pre-reduced state of LSMx, the reverse hysteresis effect is observed, which gradually decreases as the substitution of La by Sr increases.The catalytic activity of LSMx remains stable even after successive aging cycles at high temperatures and under oxidizing conditions. Finally, LSMx can be used as supports for the deposition of noble metals (e.g., Ir, Ru) giving good results. However, this area of research needs further investigation. These results are quite optimistic and with further study they can be shown to be promising in more complex reactions.

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