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

The reduction of NO by CO over Rb-promoted Pt/gamma -Al2O3 catalysts has been investigated over a wide range of temperature (ca. 200-500 degreesC), partial pressures of reactants and promoter loadings. For purposes of comparison, K- and Cs-promoted Pt/gamma -Al2O3 catalysts were tested under the same conditions. Rubidium strongly enhanced both catalytic activity and N(2-)selectivity. Rate increases by factors as high as 110 and 45 for the production of N-2 and CO2, respectively, relative to unpromoted Pt were obtained, accompanied by substantial increase in N-2-selectivity (e.g. from 24 to 82% at 350 degreesC and [CO] = 0.5%, [NO] = 1%). Under stoichiometric, conditions, Rb-promoted catalysts gave 100% conversion of both reactants with 100% selectivity towards N-2 at T similar to 350 degreesC and at an effective reactant contact time of only similar to0.5 s. In contrast, under the same conditions unpromoted Pt delivered <30% conversion and poor N-2-selectivity (approximately <40%); even at 480 degreesC the conversion was only similar to 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 N-2-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. (C) 2001 Elsevier Science B.V. All rights reserved.