High Temperature Substrate and Catalyst System 850553
Catalytic converters in heavy duty truck application are exposed to significantly higher temperatures than the temperatures considered safe for the conventional oxidation catalysts. A research program, which was conducted to evaluate high temperature substrates and catalyst systems in laboratory and engine tests, revealed that silicon nitride and stabilized aluminum titanate substrates provide melt resistance up to 3100°F. In contrast, the conventional cordierite substrates melt at approximately 2600°f. However, the questionable thermal stability of the aluminum titanate and the production feasibility silicon nitride substrates are open issues and the cordierite still remains as the best production feasible substrate. The results of the program also showed that several catalysts, including 50 gm/ft3 Pt on γ-almina washcoat and 25-40 gm/ft3 Pt-Pd in 2:1 to 1:4 ratios on BaO stabilized alumina washcoats, on stabilized aluminum titanate substrates retain nearly 90% of the initial activity after 2200°F - 17 hrs. aging. These catalysts also retained at least 80% of the initial activity after ten over temperature cycles extending into 2600°-3000°F temperature range simulating ignition malfunction condition, α-silicon nitride whiskers showed excellent stability up to 2400°F as compared to 2200°F limit for γ-alumina washcoat. Catalysts with 25-50 gm/ft3 Pt on substrates with whiskers retained over 95% of initial activity after 2200°F aging.