Reactivity of oxygen species formed upon N2O dissociation over Fe–ZSM-5 zeolite: CO oxidation as a model
The oxidative power toward CO of α-oxygen formed upon N2O 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 constant while a clear change from IV to II is calculated for the binuclear iron. According to DFT calculations CO addition on the α-oxygen from the 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 of α-oxygen and the rapid CO2 formation at low temperatures. Based on the calculated enthalpy values, the adsorption of CO is slightly more 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.
Journal: Catalysis Communications - Volume 11, Issue 12, 1 July 2010, Pages 1026–1031