Abstract
The kinetic behavior of Rh supported on SiO2, Al2O3, and TiO2 in the CO oxidation reaction has been investigated. It was found that the maximum turnover frequency (TOF) is approximately the same over the three catalysts, although the maximum is shifted to lower CO pressures for Rh/Al2O3 and Rh/TiO2 than for Rh/SiO2 catalyst. At high CO pressures, a change from −1. to −0.4 in the reaction order with respect to CO was observed over Rh/Al2O3, while in the case of Rh/TiO2 a similar change from −1 to −0.2 was observed. These changes are attributed to the contribution of the CO dissociation step to the total oxidation rate. Oscillatory behavior and hysteresis were observed on all catalysts at 393 K. However, the window of oscillation with respect to the CO pressure was much wider on Rh/SiO2 than on the other catalysts. On-line mass spectrometry was used to follow the precise oscillatory patterns as a function of reaction temperature over the Rh/SiO2 catalyst. The oxidation/reduction of the rhodium surface is proposed to be the dominant mechanism responsible for the observed oscillatory behavior.
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