Developing the science of self-healing catalysts

Abstract

It is the dream of every catalyst designer to design catalysts that continue to operate without loss of performance. However, heterogeneous catalysts are susceptible to loss of performance due to sintering, the loss of physical surface area and the growth of nanoparticles, especially catalysts that operate at elevated temperatures. To restore catalyst activity, a regeneration step is needed. This is possible in petroleum refining or alkane dehydrogenation where the reactor or the catalyst must be taken offline for regeneration. But this is not possible for emission-control catalysts, which must operate in the honeycomb monolith for the full useful life of the vehicle. Here, we elucidate the design features that allow emission-control catalysts to mitigate the loss of activity under normal operating conditions. We seek to determine why certain metal/support combinations perform better than others. In this study, we subjected catalysts to accelerated aging protocols, which are used in industry to screen catalyst formulations. For diesel oxidation catalysts, this involves heating under oxidizing conditions with steam at 800 °C. For three-way catalysts, the aging is performed in the presence of steam at 980 °C or higher, under cyclic reducing and oxidizing conditions. At these aging temperatures, the primary mechanism of sintering involves Ostwald ripening, where mobile species, typically single atoms or oxides or hydroxides, diffuse over the surface or are transported through the vapor phase and condense to form larger particles, leading to loss of activity. To allow these catalysts to continue to operate indefinitely, without requiring regeneration, the catalyst must provide opportunities to trap the atoms and then to reconstitute the active sites. Three-way catalysts operate under oscillatory conditions, providing opportunities for self-healing that are not available in diesel oxidation catalysts, which operate under lean conditions (excess oxygen). Unravelling the principles that lead to self-healing behaviour in both these application areas yields insights that can guide design of robust heterogeneous catalysts.