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Direct DRIFTS evidences for theactive surface intermediates responsible for the improved catalytic performanceof Na-promoted Pt/γ-Al2O3 catalysts during NO reduction by hydrocarbons

Matsouka Vasiliki, Konsolakis Michail, Yentekakis Ioannis.V.

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Summary

The reactivity of species formed on the surface of un-promoted and sodiumpromoted Pt catalysts supported on γ-Al2O3 was investigated by the transient response of IRspectra under the cycle: NO→He→C3H6→NO. It was found that NO adsorption on the unpromotedPt/Al2O3 catalyst results mainly in the formation of small amounts of nitrate species,which are relatively inactive towards C3H6, leading to the formation of limited amounts offormates, acetates and cyanide compounds. The latter species are very stable towards NO,indicating that carboxylate and cyanide species should be considered as spectator species; theinactivity of such species is consistent to the low activity of un-promoted Pt/γ-Al2O3 catalystduring the NO reduction by propene. In complete contrast, NO interaction with the surface ofNa-promoted catalysts results in the formation of nitro/nitrite as well as nitrosyl species, whichare highly active towards propene, leading to the formation of enolic species (R-CH=C-O),organic compounds (CxHyNOz), isocyanates (NCO) and carbonyl (CO) species. Among thesespecies isocyanates are very active towards NO, leading to the formation of reaction products.The present results strongly indicate that the excellent catalytic behaviour of sodium modifiedPt/Al2O3 catalysts during the NO reduction by propene, can be attributed to the pronouncedeffect of alkalis on the NO chemisorption bond, which in turn favours the formation of activeintermediates such as isocyanates.

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