The effects of the use of weak organic acids on the improvement of oxygen storage and release properties of aged commercial three-way catalysts

Abstract

The efficiency of weak organic acid solutions for improving the oxygen storage and release properties of aged commercial three-way catalysts (TWC) has been investigated. For the first time, three candidate leaching solutions, namely acetic acid (CH3COOH), oxalic acid ((COOH)2) and citric acid (C6H8O7) were tested for their efficiency in removing various contaminants, such as P, Fe, Zn, Cr, Pb, Ni and Cu that potentially reduce the oxygen storage capacity (OSC) of three-way catalysts. It was shown that over a severely aged (242,000 km) commercial TWC, oxalic acid appears to be the most efficient in removing P and Cu, while citric acid in removing Zn, Fe, Cr, Pb and Ni. For all three weak organic acid solutions investigated, higher quantities of contaminants were removed with increasing aging of TWC. All regeneration procedures applied led to the partial recovery of the OSC of aged TWCs likely due to the removal of at least some of the contaminants mostly responsible for their negative effect on the processes of oxygen storage and release in CeO2 and CexZr 1 - xO2 "oxygen storage" materials present. It was found that oxalic acid leads to the largest improvement of OSC quantities, very likely because of its ability to remove P, the latter being known to cause severe reduction of OSC. Based on transient redox experiments in measuring the dynamic OSC of TWC (O2/He → He → CO/He), it was found that the peak maximum position of the transient response curve of CO2 obtained over the regenerated TWC samples shifts to higher reaction times compared to that of aged TWC. This behavior is very likely related to the increase of the rate constant (k) of the oxygen back-spillover step, based on mathematical simulations of the same transient experiment performed on a model Pd/CeO2 catalyst. © 2005 Elsevier B.V. All rights reserved.