Oxygen storage in three-way-catalysts is an equilibrium controlled process: Experimental investigation of the redox thermodynamics

Abstract

A titration method is presented that measures the redox thermodynamics of oxygen storage in three way catalysts under typical operating conditions. The titration scheme starts with an oxygen pulse to completely oxidize the catalyst. The catalyst is then brought into equilibrium with an H2/H2O mixture and the resulting oxidation state is determined from the amount of H2 consumed to establish the equilibrium. Measurements were performed in the temperature range of 300°C–500°C for two commercial three-way catalysts containing a doped ceria-zirconia solid solution of the composition Ce0.404Zr0.552Nd0.044O1.978. The results confirm that under typical automotive operating conditions the oxygen available from the oxygen storage is limited by the equilibrium thermodynamics. Even under very rich conditions (8000ppm H2/10% H2O) at 500°C only about 20% of the theoretically available oxygen can be used. At the typical steady state operating point of a three-way catalyst (lambda sensor voltage of 650mV, 70ppm H2), at 500°C only about 6% of the ceria is reduced. Virtually identical oxygen storage performance is obtained for two catalysts with identical oxide composition but different precious metal loadings of 100g/ft3 and 10g/ft3.In a second part of the study, the dynamical oxygen breakthrough is studied experimentally in the H2/H2O/O2 system. First, the catalyst is brought into the typical steady state oxidation state and then it is exposed to an oxygen pulse of varying length.If the thermodynamic data is implemented in a kinetic model, the H2 and O2 signals during the dynamic oxygen breakthrough experiments are very well described. A sensitivity study shows that the H2 response to small O2 pulses, where no O2 breakthrough is observed yet, is completely controlled by the thermodynamics of the oxygen storage. As the O2 pulses get longer and O2 breakthrough is observed, besides thermodynamics also kinetics and internal as well as external mass transfer become important.