Alternatives to Noble Metal Catalysts for Automotive Exhaust Purification

Abstract Our screening programme showed that supported Cu/Cr oxide catalysts are most effective for CO oxidation and NO reduction by CO, and this mixed oxide system was studied further. Aspects that were considered were the effect of temperature, loading, Cu/Cr ratio, particle size and the pretreatment in reducing or oxidizing environment on the activity and selectivity of the CO oxidation and the NO reduction with CO, both individually and simultaneously. A Cu/Cr ratio of two turned out to be optimum. Higher loadings yielded higher activities per amount of catalyst, but above 10 wt.% Cu+Cr the intrinsic activity per metal atom decreased. The presence of oxygen inhibits the NO reaction with CO, so in combined experiments the first part of the catalyst bed will perform the CO oxidation, untill all oxygen is consumed, and the rest will reduce the NO with CO in the absence of O 2 . A thorough kinetic study of the two reactions, including the phenomena of N 2 O formation and the reduction of N 2 O with CO, provided a kinetic model that accounts for the observed reaction behaviour of the catalyst, and which can serve to model a converter based on the Cu/Cr catalyst. Bench scale experiments were performed under stationary and oscillating conditions over the range λ = 0.95–1.05 in reducing/oxidizing gas ratios. The gas mixture contained NO, CO, O 2 , hydrocarbons and H 2 O and CO 2 . Under stationary conditions only between λ = 0.99 and 1.01 a good overall performance was observed, like the three-way catalysts. Under oscillating conditions the HC and NO conversions were improved, especially under lean conditions. Addition of SO 2 to the feed gas destroyed the catalytic activity at high concentrations ( 2 . Catalysts have been prepared that exhibit a high thermostability and that keep their activity even after extreme treatment conditions. The experimental results indicate that these oxidic Cu/Cr catalysts have a three way performance and that they may find application in the purification of exhaust gases of low sulphur contents.