Platinum and palladium addition to supported rhodium catalysts for automotive emission control

Abstract Simultaneous catalytic control of the three major pollutants in automobile exhaust requires both reduction of nitric oxide and oxidation of carbon monoxide and hydrocarbons. A supported rhodium catalyst with low loading (0.002 wt% Rh) was found to have good activity for converting nitric oxide to nitrogen in a laboratory feedstream, but its oxidation activity was inadequate. Addition of platinum or palladium improved the oxidation activity. However, it was found that, compared to rhodium alone, these two-metal combinations (Pt-Rh or PdRh) form more ammonia under reducing conditions and decrease nitric oxide conversion under oxidizing conditions. Both of these undesired effects were circumvented by depositing the added metals on separate support beads. The rhodium catalyst was located in the front of the catalyst bed and was followed by a second layer of pellets containing the platinum or palladium. Evaluations in engine exhaust remain to be done, since laboratory results are not conclusive indicators of actual emission-control performance.