Reactivity of oxygen species formed upon N2O dissociation over Fe-ZSM-5 zeolite: CO oxidation as a model

The oxidative power toward COofα-oxygen formed uponN2O dissociation over isolated and binuclear Fe/ZSM-5 zeolite is investigated by means of DFT calculations. The two α-sites [Fe-O]+ and [Fe-(μO)-(μOH)-Fe]+ exchanged in ZSM-5 were considered since their activity in the N2O decomposition was recently shown. Computed electronic properties, charge transfers and frequency analysis of α-oxygen and iron in [O-Fe-O]+ and [OFe-(μO)-(μOH)-FeO]+ suggest a FeII character for the isolated and FeIV for the binuclear Fe-ZSM-5 sites. Addition of CO on oxygen atoms reveals that along the oxidation reaction the valence state for the isolated iron is II and remains relatively constantwhile a clear change fromIV to II is calculated for the binuclear iron. According to DFT calculations CO addition on the α-oxygen fromthe iron active sites induces a significant length increase of the Fe-α-oxygen bond. Whatever the α-sites, the addition of CO is strongly exothermic and leads to stable minima resembling an adsorbed CO2 on iron active site. This reactivity is in line with the well known high reactivity ofα-oxygen and the rapid CO 2 formation at lowtemperatures. Based on the calculated enthalpy values, the adsorption of CO is slightlymore favourable on binuclear [OFe-(μO)-(μOH)-FeO]+ than over isolated [O-Fe- O]+ iron site. A comparison of the entropic parameters suggests the opposite with a stronger oxidative power of α-oxygen from isolated over those from binuclear iron site. © 2010 Elsevier B.V. All rights reserved.