Optimal promotion by rubidium of the CO + NO reaction over Pt/γ-Al2O3 catalysts

The reduction of NO by CO over Rb-promoted Pt/γ-Al2O3 catalysts has been investigated over a wide range of temperature (ca. 200–500°C), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/γ-Al2O3 catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N2-selectivity. Rate increases by factors as high as 110 and 45 for the production of N2 and CO2, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N2-selectivity (e.g. from 24 to 82% at 350°C and [CO]=0.5%, [NO]=1%). Under stoichiometric conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N2 at T∼350°C and at an effective reactant contact time of only ∼0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N2-selectivity (approximately <40%); even at 480°C the conversion was only ∼60%. The observed promotional effects are ascribed to alkali-induced changes in the chemisorption bond strengths of CO, NO and NO dissociation products which lead to the observed activity enhancement and dependence of N2-selectivity on promoter loading. The effects of K-promotion mirror those of Rb-promotion, but are significantly less pronounced. Rb is the best alkali promoter.