Catalytic reduction of NO x in gasoline engine exhaust over copper- and nickel-exchanged X–zeolite catalysts

Abstract

Catalytic removal of NO x in engine exhaust gases can be accomplished by non-selective reduction, selective reduction and decomposition. Noble metals are extensively used for non-selective reduction of NO x and up to 90% of engine NO x emissions can be reduced in a stoichiometric exhaust. This requirement of having the stoichiometric fuel–air ratio acts against efficiency improvement of engines. Selective NO x reduction in the presence of different reductants such as, NH 3, urea or hydrocarbons, requires close control of the amount of reductant being injected which otherwise may be emitted as a pollutant. Catalytic decomposition is the best option for NO x removal. Nevertheless, catalysts which are durable, economic and active for NO x reduction at normal engine exhaust temperature ranges are still being investigated. Three catalysts based on X–zeolite have been developed by exchanging the Na + ion with copper, nickel and copper–nickel metal ions and applied to the exhaust of a stationary gasoline engine to explore their potential for catalytic reduction of NO x under a wide range of engine and exhaust conditions. Some encouraging results have been obtained. The catalyst Cu–X exhibits much better NO x reduction performance at any temperature in comparison to Cu–Ni–X and Ni–X; while Cu–Ni–X catalyst exhibits slightly better performance than Ni–X catalyst. Maximum NO x conversion efficiency achieved with Cu–X catalyst is 59.2% at a space velocity (sv) of 31 000 h −1; while for Cu–Ni–X and Ni–X catalysts the equivalent numbers are 60.4% and 56% respectively at a sv of 22 000 h −1. Unlike noble metals, the doped X–zeolite catalysts exhibit significant NO x reduction capability for a wide range of air/fuel ratio and with a slower rate of decline as well with increase in air/fuel ratio.