Optimized Three-Way Catalysts for Emission Control on a Heavy-Duty Stoichiometric Natural Gas Engine

Natural gas (NG) engines have attracted increasing attention in the heavy duty (HD) vehicle market as an alternative to conventional diesel fuel often due to the abundance and low price of NG. However, it is challenging to meet the increasingly stringent China VI legislation, particularly for hydrocarbons (mainly CH₄), carbon monoxide (CO), nitrogen oxides (NO_x) and ammonia (NH₃). In this work, approaches were explored in which a gasoline three-way catalyst (TWC) was modified through optimization of promoters, OSC materials, and catalyst structure. The optimized TWC was evaluated in a laboratory reactor and with a HD stoichiometric NG engine. Lab reactor results showed that promoters can improve CH₄ light off performance with T₅₀ decreases of 20 ^oC and 13 ^oC for the fresh and aged catalyst respectively. Modelling results indicated that promoters could reduce the CH₄ activation barrier energy over both the PdO(100) and PdO(101) crystal faces, which consistently correlates with light off performance. The TWC modified with an advanced oxygen storage material can reduce CH₄ and NOx emission simultaneously as shown on NG bench testing results. NH₃ is a by-product during the operation of TWC on a stoichiometric NG engine. NH₃ slip catalyst modifications are discussed which were demonstrated to reduce NH₃ emissions due to the relatively low NH₃ limit of 10 ppm for China VI legislation. Data are reported demonstrating the optimized TWC passed NG engine durability testing and PEMS emissions of HD vehicle.