By using a three-way catalyst containing a metal oxide, NO can be selectively converted to N//2//and N//2O without the formation of NH//3 even at the ″rich″ side of the air/fuel ratio. In order to elucidate the mechanism of this action of the added metal oxide, the NO-H//2 reaction on Pt-V//2O//5/Al//2O//3 catalyst was investigated using the pulse reaction technique. In the NO-H//2 reaction on Pt/Al//2O//3 catalyst, a significant amount of NH//3 was formed in the ″//rich″ region of the reactants (H//2/NO greater than 1), although NO was completely removed. In the NO-H//2 reaction on the Pt-V//2O//5/Al//2O//3 catalyst, on the other hand, no NH//3 was formed even in the ″rich″ region of the reactants; in addition, NO was completely removed. During the NO-H//2 reaction on the Pt-V//2O//5/Al//2O//3 catalyst, the V//2O//5 in the catalyst was found to be reduced from V**5** plus to V**4** plus . The reaction of NO with NH//3 and the reaction of NH//3 or H//2 with Pt-V//2O//5 mixed catalyts for various states of mixing were also investigated. This three-way catalyst is used in the removal of hydrocarbons from automatic exhaust emission.
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