Insight into the Role of Electropositive Promoters in Emission Control Catalysis: An In Situ DRIFTS Study of NO Reduction by C₃H₆ Over Na-Promoted Pt/Al₂O₃ Catalysts

The nature and the relative population of adsorbed species formed on the surface of unpromoted and Na-promoted Pt/γ-Al₂O₃ catalysts, during the NO reduction by propene, was investigated by means of in situ diffuse reflectance infrared Fourier transform spectroscopy, in an effort to elucidate the pronounced effect of electropositive promoters in emission control catalysis. It was found that under steady-state reaction conditions the surface of the unpromoted catalyst is mainly covered by carbon containing species, such as hydrocarbon fragments, carboxylates and cyanides. On the opposite, the surface of Na-promoted catalysts is predominantly covered by NOx ad-species, carbonyls and isocyanates, implying the significant effect of Na promoter on surface species formation. To further gain insight into the effect of Na-promoter on the nature and the reactivity of ad-species, the transient respond of IR spectra under the cycle: NO → He → C₃H₆ → NO, is also investigated. The results imply that over Na-free catalysts the interaction of propene with NOx pre-adsorbed species (mainly nitrates) leads to the formation of strongly bonded carboxylates and cyanides, which are inactive towards NO. In contrast, NO interaction with the surface of Na-promoted catalysts results in the formation of nitro/nitrite species as well as to nitrosyls, which are highly active towards propene, leading to active intermediates such as isocyanates (NCO). The present results demonstrate that the excellent catalytic performance of Na-promoted catalysts can be well interpreted on the basis of these significant modifications induced by electropositive promoters on catalysts surface chemistry.