Abstract
Abstract By using a three-way catalyst containing a metal oxide, CO is removed as CO2 even in the “rich” region of the air/fuel ratio. In order to elucidate the mechanism of this action of the added metal oxide, the reaction of CO with O2 on a Pt–V2O5/Al2O3 catalyst was investigated using a rectangular pulse-reaction apparatus. In the CO–O2 reaction on a Pt/Al2O3 catalyst, a significant amount of unreacted CO was exhausted in the “rich” region of the reactants. Furthermore, the conversion of CO decreased markedly with an increase in the CO/O2 ratio in the reactants. In the CO–O2 reaction on the Pt–V2O5/Al2O3 catalyst, on the other hand, the conversion of CO was high and did not decrease markedly with an increase in the CO/O2 ratio in the “rich” region. As for the reaction on the Pt–V2O5/Al2O3 at 558 K and above, the CO was completely removed as CO2 at any of the CO/O2 ratios examined. These effects of the added V2O5 were found to be due to: (i) the reaction of CO with an oxygen of V2O5 in the Pt–2O5/Al2O3 catalyst and (ii) the enhanced oxidation of CO with a gaseous O2 on the Pt/Al2O3 catalyst upon the addition of V2O5. On the basis of the kinetics of the oxidation of CO with the oxygen of V2O5 in the Pt–V2O5/Al2O3 and Pt black–V2O5 mixtures, its mechanism was proposed to be as follows; the oxygen of V2O5 at the Pt–V2O5 interface is activated by contact with Pt (this activated oxygen is denoted by [Pt···O···V]). The oxidation of CO with the oxygen of the Pt–V2O5/Al2O3 and Pt black–V2O5 mixtures proceeds according to the following two steps. Step I is the reaction of a gaseous CO with the active oxygen at the Pt–V2O5 interface, [Pt···O···V], to form CO2 and a reduced site, [Pt V]. Step II is the oxidation of the reduced site with the bulk oxygen in V2O5 to reproduce [Pt···O···V]. The enhanced oxidation of CO with a gaseous O2 caused by the addition of V2O5 to Pt/Al2O3 was found also to be understandable in terms of the active site, [Pt···O···V], where the reduced site, [Pt V], is oxidized by the gaseous O2 to reproduce [Pt···O···V] instead of the bulk oxygen in V2O5.
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