Electrochemical promotion in emission control catalysis: The role of Na for the Pt-catalysed reduction of NO by propene

Abstract

Conventional heterogeneous metal catalysts are commonly enhanced by the addition of so-called promoter species that are used to modify intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical promotion (EP) provides an in situ, reversible and efficacious means of catalyst promotion and it allows for a systematic study of the role of promoters in heterogeneous catalysis. EP studies relevant to the three-way' catalytic chemistry i.e., control of automotive CO, NO and hydrocarbons emissions, demonstrate that major enhancements in activity of Pt catalyst supported on â-Al 2 O 3 (a sodium ion conductor) are possible when Na is electrochemically pumped to the catalyst surface. In the case of the important reactions involving NO reduction by CO or by hydrocarbons, major enhancements in selectivity towards N 2 (from 15 to 70%) have been also achieved. The promotional effect of Na is due to enhanced NO chemisorpion and pronounced NO dissociation on the Pt surface. The Pt-catalysed reduction of NO by propene, which is of particular importance with respect to emission control catalysis, exhibits strong electrochemical promotion by spillover of Na from the β-Al 2 O 3 support. In the promoted regime rate increase by an order of magnitude are achievable. At sufficiently high loading of Na, the system exhibits poisoning, and excursion between the promoter and poisoned regimes are fully reversible. XPS was used in order to shed light on several important questions emerging for the role and the form of Na promoter on the catalyst surface. We have also demonstrated that EP of Pt catalyst very substantially enhances the ability of NO to oxidise propene even in the presence of oxygen.