A principle mode of TWC deactivation is the agglomeration of Rh under high temperature lean conditions. This is particularly pronounced with aging cycles that incorporate a fuel shut-off, since the latter creates a severely oxidizing environment.
We have attempted to stabilize Rh based on the fact that Rh3+, which is the dominant species under the above-mentioned conditions, is highly interactive, forming ternary oxides with a large number of base metal oxides (BMOs). Such compound formation can be expected to decrease the tendency to agglomerate. A potential pitfall, however, is the need for the BMO-Rh3+ complex to undergo reductive decomposition, releasing active Rh0, under stoichiometric/reducing conditions. Thus, the BMO-Rh3+ interaction must be intermediate in strength.
Consistent with these expectations, an exploratory program showed that weakly basic BMOs were superior to strongly basic BMOs. In some cases, significant promotional effects were seen after a simulated fuel-cut cycle. Unfortunately, these benefits were realized after intermediate-term aging, but were not seen after longer aging.