Nickel-copper alloys, marketed under the name Monel, have been found to be extremely active NOx reduction catalysts. At temperatures above 1300 F, and under net reducing conditions, Monel will catalyze the removal of 90% or more of the NO in automotive exhaust at space velocities of up to 100,000 v/v/hr. On unleaded fuel, Monel catalysts have shown good activity maintenance in mileage accumulation runs as long as 31,000 miles. Catalyst life is limited by physical deterioration of the catalyst which causes increases in exhaust back pressure. On unleaded fuel, Monel in its present form will last approximately 10,000 miles at 1700 F (∼60 mph) before back pressure begins to rise rapidly. The presence of lead in the fuel substantially increases the rate of Monel deterioration.
When Monel is used as part of a dual-bed catalyst system two problems, which appear to be generic to dual-bed catalyst systems, arise. First, under normal operating conditions, roughly 10% of the NO in the untreated exhaust reacts with H2 to form NH3 in the Monel bed. NH3 thus formed can be oxidized to NO in the oxidation bed. Ammonia formation can be decreased by careful control of exhaust composition. Alternatively, the concentration of nitric oxide in the untreated exhaust can be lowered by exhaust gas recycle so that even with ammonia formation the nitric oxide concentration in the treated exhaust will be low enough to meet low emissions standards. Second, placing Monel or any other NOx reduction catalyst ahead of the oxidation catalyst slows the warm-up of the oxidation catalyst. This problem has been attacked by placing the catalyst system close to the exhaust ports and modifying the exhaust system to conserve exhaust gas sensible heat.