Deactivation of diesel oxidation catalysts

Licentiate thesis, 2006

To meet the demand for heavy- and light-duty diesel vehicles on expansive markets while complying with current and future emission standards, it is necessary to reinsure the long-livety of the applied exhaust aftertreatment technology. Loss of catalytic performance may be associated with environmental as well as economical consequences. To aid the development of improved exhaust control components, such as the diesel oxidation catalyst (DOC), and optimise the use of raw materials, methods for accelerating the deactivation process are used. Catalysts can be aged in an engine-bench setup to simulate conditions in the field and thereby shorten the test procedure. The most resource efficient approach relies on using synthetic aging procedures to achieve such effect, which is the main focus of this thesis. To confirm a relevant effect of thermal and chemical deactivation, model DOCs were prepared and exposed to a variety of conditions, including atmospheres containing SO2 and H2O, prior to characterisation of activity for CO oxidation, Pt dispersion, BET surface area, C3H6 adsorption capacity (TPD) and surface composition (XPS). Hydrothermal treatment affected the HC trap function, critical for cold-start performance, and catalytic activity. Sulfur, adsorbed during low-temperature exposure, was the main cause for loss of Pt dispersion and activity for CO oxidation despite removal of the major part of SO2 in succeeding heat treatment. A fresh commercial DOC (0 km) was subjected to different aging procedures and compared to samples dismantled from vehicles with different driving distance and sulfur content in the fuel. The deactivation of the vehicle-aged catalysts was confirmed by measuring emission levels in dynamometer tests and in lab reactor measurements. A similar degree of deactivation was observed, from flow reactor experiments and TEM analysis of Pt sintering, by using a 15-30 h rapid aging procedure combining poisoning at low temperature with (hydro)thermal treatment. The different sources of catalyst poisoning were studied with XPS. While S and Zn could be removed in reducing atmosphere at 700ºC, results confirmed that P and Ca cause irreversible chemical deactivation of the vehicle-aged catalysts. The strong interaction of the sulfates with the washcoat, shown for both aging methods, highlights the importance of using low-sulfur diesel fuel. The described methods for rapid aging can be used for simulating catalyst poisoning in general terms, where accumulation of poisons during long-term exposure to diesel exhaust of low temperature is of concern. Hydrothermal treatment mimic periods of higher engine load and exotherms from regeneration processes.