Modes of Deactivation of Exhaust Purification Catalysts 741079
Crushed samples of Engelhard PTX (II-B) catalyst were calcined at temperatures between 900-2700°F. When the temperature exceeds about 1100°F in air, the crystallites of precious metals (platinum-palladium) grow larger, exposing less surface for catalysis. When the catalyst is exposed to temperatures on the order of 1500-1800°F for extended periods of time (16 h or more), the wash-coat tends to lose a substantial percentage of its surface area. Finally, above about 2300°F, the cordierite of the ceramic becomes converted to mullite and amorphous material.
The intrinsic rate constant for the oxidation of pure propylene is inversely related to Pt-Pd crystallite size. Thus, in the absence of poisoning, changes in intrinsic oxidation rate constant can be accounted for by changes in degree of dispersion of precious metal.
Catalysts examined after 8000-20,000 equivalent miles of steady-state (70 mph) on a stationary engine with catalyst temperatures at 1350-1500°F show results consistent with those obtained from our calcination studies. After service in normally operated 1971 vehicles, severe deactivation sometimes occurred, which could be accounted for only by attainment of temperatures in excess of 2300°F.
Calcinations in the presence of lead compounds result in the conversion of most of the precious metal from the active metallic state to a state in which it is dissolved in the ceramic.
Pore volume distribution data after artificial or real deactivation conditions were used to obtain diffusion coefficients for use in a published model. The changes had little effect on the results.
As lead deposited during steady-state engine testing increased in the 0.2-5% range, the intrinsic propylene oxidation rate decreased. Examination of the catalysts confirmed that lead caused conversion of the Pt-Pd from an active metallic state to one in which they became dissolved in the ceramic, just as in the calcination experiments.
We thus estimate the intrinsic propylene oxidation activity of deactivated catalyst from Pt-Pd crystallite size if no lead is present, or from percent lead. Division by the activity for fresh catalyst gives a relative activity.
Computations of CVS test results using a published computer model, which takes heat and mass transfer effects into account as well as intrinsic activity, show that large changes in intrinsic activity have much less effect on the CVS absolute conversion. Thus, a moderate increase in the percentage of precious metal or a moderate degree of precious metal deactivation or of lead poisoning will have little effect.