Dynamics and Selectivity of N2O Formation/Reduction During Regeneration Phase of Pt-Based Catalysts
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The formation of N2O has been studied by means of isothermal lean-rich experiments at 150, 180 and 250 °C over Pt–Ba/Al2O3 and Pt/Al2O3 catalysts with H2 and/or C3H6 as reductants. This allows to provide further insights on the mechanistic aspects of N2O formation and on the influence of the storage component. Both gas phase analysis and surface species studies by operando FT-IR spectroscopy were performed. N2O evolution is observed at both lean-to-rich (primary N2O) and rich-to-lean (secondary N2O) transitions. The production of both primary and secondary N2O decreases by increasing the temperature. The presence of Ba markedly decreases secondary N2O formation. FT-IR analysis shows the presence of adsorbed ammonia at the end of the rich phase only for Pt/Al2O3 catalyst. These results suggest that: (i) primary N2O is formed when undissociated NO in the gas phase and partially reduced metal sites are present; (ii) secondary N2O originates from reaction between adsorbed NH3 and residual NOx at the beginning of the lean phase. Moreover, N2O reduction was studied performing temperature programming temperature experiments with H2, NH3 and C3H6 as reducing agents. The reduction is completely selective to nitrogen and occurs at temperature higher than 250 °C in the case of Pt–Ba/Al2O3 catalyst, while lower temperatures are detected for Pt/Al2O3 catalyst. The reactivity order of the reductants is the same for the two catalysts, being hydrogen the more efficient and propylene the less one. Having H2 a high reactivity in the reduction of N2O, it could react with N2O when the regeneration front is developing. Moreover, also ammonia present downstream to the H2 front could react with N2O, even if the reaction with stored NOx seems more efficient.
KeywordsN2O reduction N2O formation Lean NOx Trap NOx storage reduction Pt–Ba/Al2O3 Pt