Strong promotional effects of Li, K, Rb and Cs on the Pt-catalysed reduction of NO by propene

The catalytic activity and selectivity of Pt dispersed on γ-Al2O3 for the reduction of NO by propene is promoted extremely strongly by Li, K, Rb and Cs alkalis in a wide temperature range ca. 450-800K. Remarkable and unprecedented effects on both activity and selectivity are found. The best promotion effects are achieved by Rb-promotion, for which rate increases as high as 420-, 280- and 25-fold are obtained for the formation rates of N2, CO2 and N2O, respectively, in comparison with the performance of un-promoted (alkali-free) Pt/γ-Al2O3. From the other hand, the selectivity towards N2 is improved from 20% over the alkali-free unpromoted Pt catalyst, to >90% over the optimally alkali-promoted catalyst. These effects are understandable in terms of the effect of alkali promoter on the relative adsorption strengths of reactant species. Alkali promotes the adsorption, and consequently the dissociation, of electronegative adsorbates (NO) and inhibits the adsorption of electropositive adsorbates (propene), on a catalyst surface predominantly covered by propene and its fragments