DOC Deactivation

W. Addy Majewski

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• Introduction
• Thermal Deactivation
• Poisoning
• PGM Oxidation

Introduction

In early DOC applications, catalyst deactivation was not expected to be a major issue. Most catalytic materials used in early diesel catalysts had been developed for gasoline applications, where the maximum exhaust gas temperature and the temperature raise (exotherm) were much higher than those in diesel engines. With time, however, new materials have been introduced to DOC formulations (e.g., zeolites) that may require thermal stabilization, and diesel aftertreatment configurations have been developed where catalysts can be exposed to very high temperatures. An example is the heat-up catalyst used to support the regeneration of diesel particulate filters. Heat-up catalysts are exposed to both very high temperatures and to fouling by unburned hydrocarbons and coke—it is a very demanding catalyst application that requires excellent durability.

Most diesel oxidation catalysts utilize platinum group metals (PGM) such as Pt and Pd. Important modes of deactivation of PGM-based catalysts include [5628][5629]:

• Thermal degradation (sintering)—a loss of oxidation performance due to changes in particle size and morphology of PGM and washcoat particles.
• Phosphorus poisoning—catalyst deactivation by phosphorus and other metal and non-metal elements that originate primarily from lubricating oil additives.
• SOx poisoning—catalyst deactivation by sulfur compounds originating from diesel fuel and engine lubricating oil.
• PGM oxidation—oxidation of the Pt/Pd catalyst to a higher and less active valence state.

The first two deactivation modes are permanent, while sulfur poisoning and PGM oxidation are temporary or reversible, either partially or fully.

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