Perovskite-type Nd 1-x Sr x CoO 3-y catalysts with various Sr mole fraction were prepared and investigated for the effect of Sr substitution on their catalytic activities in the oxidation of carbon monoxide. Utilizing the static and flow methods, kinetic studies have been carried out between 373 and 523 K. The initial reaction was investigated by the static reactor system using a differential photoacoustic cell, and for the study of reaction stage showing a constant catalytic activity after an initial stage characterized by high reaction rates, the flow reactor system using on-line gas chromatography was employed. The catalytic activity increased with increasing amounts of Sr substitution for Nd in NdCoO 3 compounds, and it also increased with higher reaction temperature within the range of 373-523 K. Kinetic data obtained in an initial reaction stage by CO 2 photoacoustic spectroscopy showed that the reaction partial orders to CO and O 2 were 0.8-0.9 and 0, respectively. In the reaction stage showing a constant catalytic activity after an initial stage, the oxidation was found to be first order with respect to CO and 0.5 order with respect to O 2 . The concentration of oxygen vacancy in the solid catalyst was shown to be the controlling factor for the oxidation of carbon monoxide. According to the experimental results, the mechanisms of the CO oxidation processes are discussed, and it is believed that O 2 adsorbs on the oxygen vacancies (V o x ) formed by Sr substitution while CO adsorbs on the lattice oxygens during the reaction process.
CO2 photoacoustic spectroscopy was employed to analyze kinetically the CO2/CH4 reaction catalyzed by 14 wt% Ni/Al2O3 and 14 wt% Ni/TiO2. The catalytic reaction was carried out in the temperature range of 673-923 K at various partial pressures of CO2 and CH4 (40 Torr total pressure) in a closed-circulating reactor system. The CO2 photoacoustic signal, measured by using a differential photoacoustic cell, was recorded as a function of reaction time. Under these conditions, Al203 and TiO2 used as supports do not promote the reaction as noted by the lack of changes in the CO2 photoacoustic signal. Reactions run in the presence of H2-reduced supported Ni catalysts are associated with significant time dependent changes in the CO2 photoacoustic signal, while processes carried out in the presence of unreduced catalysts do not. Changes in the CO2 photoacoustic signal at early reaction times provide precise data for the rate of CO2 disappearance. The rate of CO2 disappearance is observed to increase with increasing temperature in the range of 673-923 K. Apparent activation energies for CO2 consumption were calculated to be 15.4 kcal mol(-1) for the Ni/Al2O3- and 14.3 kcal mol(-1) for the Ni/TiO2-catalyzed reactions. Reaction orders, determined from initial rates of CO2 disappearance at 873 K, were found to be 0.48 in CH4 and 0.45 in CO2 for the Ni/Al2O3-promoted process, and 0.38 in CH4 and 0.32 in CO2 for the Ni/TiO2-catalyzed reaction. The results of this effort were compared with those reported previously and were used to construct a mechanism for the low pressure CO2/CH4 reaction.
Ni/La 2 O 3 with a high dispersion was prepared by reduction of LaNiO 3 perovskite oxide to examine the catalytic activity for the CO 2 -CH 4 reaction. The Ni/La 2 O 3 catalyst was found to be highly active for the reaction. The ratios of H 2 /CO were measured in a flow of the reaction mixture containing CO 2 /CH 4 /Ar using an on-line gas chromatography system operated at 1 atm and found to be varied with temperature between 0.66 and 1 in the temperature range of 500-800 o C. A kinetic study of the catalytic reaction was performed in a static reactor at 40 Torr total pressure of CO 2 /CH 4 /N 2 by using a photoacoustic spectroscopy technique. The CO 2 photoacoustic signal varying with the concentration of CO 2 during the catalytic reaction was recorded as a function of time. o C under various partial pressures of CO 2 and CH 4 , the reaction orders were determined to be 0.43 with respect to CO 2 and 0.33 with respect to CH 4 . The kinetic results were compared with those reported previously and used to infer a reaction mechanism for the Ni/La 2 O 3 -catalyzed CO 2 -CH 4 reaction.
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