Application of a Monel-Platinum Dual-Catalyst System to Automotive Emission Control 720209
A dual-catalyst-equipped test vehicle, containing Monel catalysts for the reduction of oxides of nitrogen followed by platinum oxidation catalysts, has controlled emissions of carbon monoxide, hydrocarbons, and oxides of nitrogen to levels of 8.0, 0.4, and 0.8 g/mile, respectively, using the single-bag 1972 federal test procedure. The Monel catalyst was fabricated in a low-mass, high-surface-area, open-mesh structure. The low mass contributed to rapid warmup of both the reduction and oxidation catalysts. The high geometric surface area permitted efficient conversion of all three pollutants simultaneously when the vehicle was operated only slightly richer than stoichiometric. As a result the amount of oxidation required of the platinum catalyst was greatly reduced.
The durability of Monel is a major problem. The mechanism of Monel strength loss, currently the limiting factor in catalyst life, was shown to be grain-boundary widening due to alternate oxidation-reduction reactions undergone by the metal. Reinforcement of Monel by an oxidation-resistant metal, particularly when accompanied by fabrication into a unitary structure, significantly improves strength and maintenance of low pressure drop. However, durability equivalent to 50,000 miles of vehicle operation has not yet been demonstrated.
A rapid method for Monel surface activation has been found to replace inconvenient vehicle activation. Heating in moist air for several hours develops the surface roughness necessary for high activity. Finally, laboratory tests of a technique for enhancing the warmup rate of the oxidation catalyst by means of an electrically heated “trigger” element are described. Expenditure of a relatively small amount of power through this catalytically active element can release sufficient heat of reaction to light-off the main oxidation catalyst.