Kenji Baba

Deactivation by poisoning of three-way catalyst for natural gas-fuelled engines

Abstract A Pt Rh three-way catalyst used in natural gas-fuelled engine systems for 21 000 h was investigated employing chemical analyses, EPMA, surface area measurements, catalytic activity tests, as well as tests using model poisoned catalysts. Specific deactivation characteristics, such as the decrease in the selectivity of nitric oxide reduction, which cannot be reproduced by heat treatment nor explained by physical poisoning such as blockage of micropores, were observed on the used catalyst. P, Ca, Zn, Fe and trace amounts of Pb were detected as elements which increased in the used catalyst, and Zn, Fe and Pb were observed in the wash coat layer of the catalyst, while P and Ca existed on the surface of the wash coat. Of the model Zn-, Fe-, and Pb-poisoned catalysts prepared by dipping heat-treated catalysts into an aqueous solution of metal salt, only model Pb-poisoned catalyst showed a considerable decrease in the selectivity of nitric oxide reduction. Further, all the characteristics of the deactivation on the used catalyst were qualitatively reproduced by the measurement of the activity of the model Pb-poisoned catalyst at high space velocity. From these results, it is concluded that the selectivity of nitric oxide reduction is decreased by chemical poisoning due to trace amounts of Pb on the used catalyst, and that the absolute rates of nitric oxide reduction as well as the other reactions are additionally reduced by an decrease in the effective surface area of the catalyst due to the covering the surface of the wash coat layer by accumulated compounds consisting of P, Ca, Zn and Fe. Therefore, trace amounts of Pb as well as ash derived from oil additives become an issue even in natural gasfuelled engine systems when a Pt Rh three-way catalyst is used for more than several ten thousand hours.

Deactivation by poisoning and the improvement of three way catalysts for natural gas-fueled engines

A Pt-Rh three way catalyst used in natural gas-fueled engine systems for 21,000 h was investigated to analyze its deactivation mechanism. Specific deactivation characteristics, such as the decrease in the selectivity of NO reduction, which cannot be reproduced by heat treatment, were observed on the used catalyst. P and Ca were detected on the surface of the wash coat and Zn, Fe and trace amounts of Pb were observed in the wash coat layer. Of the model-poisoned catalysts a decrease in the selectivity of NO removal was observed only for the Pb-poisoned catalyst. All the characteristics of the deactivation on the used catalyst were reproduced on the model Pb-poisoned catalyst at high SV. Thus, it is concluded that the selectivity of NO reduction is decreased by chemical poisoning due to Pb on the used catalyst; moreover the absolute rates of NO reduction as well as for other reactions are reduced by a decrease in the effective surface area due to physical clogging in addition to a thermal effect. Furthermore, it was found that an improved catalyst, which can maintain a high Rh surface area even after thermal deactivation by separating high Pt/Rh ratio particles from low Pt/Rh ratio particles at a high Rh concentration, possesses high poison-resistance in the activity tests using model Pb-poisoned catalysts as well as field tested catalyst. Finally, its durability has been proven for more than 30,000 h.