Selective Reduction of Nitrogen Monoxide with Hydrogen on Pt/Al2O3 Containing a Metal Oxide

Abstract

Abstract By using a three-way catalyst containing a metal oxide, NO can be selectively converted to N2 and N2O without the formation of NH3 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–H2 reaction on Pt–V2O5/Al2O3 catalyst was investigated using the pulse reaction technique. In the NO–H2 reaction on Pt/Al2O3 catalyst, a significant amount of NH3 was formed in the “rich” region of the reactants (H2/NO>1), although NO was completely removed. In the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, on the other hand, no NH3 was formed even in the “rich” region of the reactants; in addition, NO was completely removed. During the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, the V2O5 in the catalyst was found to be reduced from V5+ to V4+. The reaction of NO with NH3 and the reaction of NH3 or H2 with Pt–V2O5 mixed catalysts for various states of mixing were also investigated. From these results, the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst has been shown to be composed of the following three main steps: Step I, the formation of NH3 by the reduction of NO with H2 on Pt; Step II, the NO–NH3 reaction on Pt to form N2 or N2O; and Step III, the reduction of V2O5 in the catalyst by H2 which is accelerated by the “hydrogen spil-lover.” In the NO–H2 reaction on Pt/Al2O3 catalyst, NO is first reduced to NH3 in Step I and the NO–NH3 reaction in Step II follows. In the “rich” region, the formation of NH3 takes place more readily than the subsequent NO–NH3 reaction. In the NO–H2 reaction on the Pt–V2O5/Al2O3 catalyst, the formation of excess NH3 is suppressed by the removal of hydrogen from Pt in Step III, and the NH3 thus produced reacts with NO to form N2 or N2O selectively in Step II. The rates of Steps I, II, and III were also discussed in terms of the structure of the catalysts and composition of the reactants.