Abstract
The influence of ceria on the kinetic behavior of noble metals in the reduction of NO by CO has been investigated at 120 and 300°C on a freshly prepared Pt–Rh/Al2O3–CeO2 catalyst, and on an aged catalyst after reaction for 16 h at 500°C, within pressure ranges of 2.3–9×10−3 atm for CO and 1.5–8×10−3 atm for NO, using a differential fixed-bed flow reactor. These two temperatures have been selected from temperature-programmed experiments because they correspond to two very different regimes of activity of noble metals with ceria. It has been found that a rate equation derived from a bifunctional mechanism involving reaction paths either on metal or on ceria can correctly fit rate measurements performed at 120°C. In contrast a conventional mechanism earlier proposed for modeling the CO+NO reaction on Pt–Rh/Al2O3 at 300°C, where only noble metals are involved, enables modeling of rate measurements at 300°C on Pt–Rh/Al2O3–CeO2, which suggests that the interaction between ceria and noble metals is suppressed at that temperature. The temperature dependency of the rate constants and equilibrium constants for NO and CO adsorption has been quantified in order to explain such changes in the kinetic behavior of Pt–Rh/Al2O3–CeO2.
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