Nitric oxide reduction performance of automotive palladium catalysts

Reduction of nitric oxide over palladium catalysts under reducing conditions was examined by using cycled feeds and steady non-cycled feeds. The effects of lanthanum(III) oxide on the catalytic properties of α-alumina-supported palladium for the reduction of nitric oxide were studied by X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and chemisorption of propene. The reduction of nitric oxide on palladium catalysts is significantly inhibited by hydrocarbon. However, the reduction activity is improved by both the periodic operation effect and the addition lanthana to the catalyst. The mechanism of the periodic operation effect is interpreted in terms of the strong adsorption of hydrocarbons. The addition of lanthana to the catalyst substantially moderated this hydrocarbon poisoning. The conversion of nitric oxide on the Pd/La2O3 catalyst with cycled feeds was similar to that on a rhodium catalyst. The XPS, TPR and hydrocarbon chemisorption studies showed that the presence of lanthana provided palladium oxide in a hardly-reduced state and suppressed the chemisorption of hydrocarbons on the palladium oxide.

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