A dopant-mediated recombination mechanism in Fe-doped TiO2 nanoparticles for the photocatalytic decomposition of nitric oxide

• Presence of Fe dopant in TiO2 strongly suppresses photo-oxidation of NO to NO2.
• Fe3+ is photo-reduced to Fe2+, providing a favorable adsorption site for NO.
• Photo-generated hydroxyl radicals re-oxidize Fe and trigger release of NO.
• Dopant-mediated recombination can enhance selectivity of photocatalysts.
• A new IR band at 1805 cm−1 is observed for Fe:TiO2 and attributed to Fe2+(NO)2.

The photon-assisted adsorption and catalytic decomposition of nitric oxide (NO) over undoped and Fe-doped TiO2 nanoparticles have been investigated by in situ diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy, in situ X-ray photoelectron spectroscopy (XPS) and on-line NOx analysis. The DRIFT spectra and on-line NOx analysis reveal that the usual photo-oxidation of NO to NO2 is strongly suppressed by the Fe dopant. This is found to be caused by the photo-reduction of Fe3+ to Fe2+, which is an effective adsorption site for nitric oxide species. The DRIFT spectra indeed reveal a new band at 1805 cm−1, which is assigned to the NO stretch vibration in a Fe2+(NO)2 complex. Instead of producing NO2, photo-generated hydroxyl radicals oxidize the Fe2+ back to Fe3+. This causes the NO to desorb again, effectively closing an NO-mediated recombination loop. These results support the recently proposed reaction mechanisms for the photocatalytic decomposition of NO over undoped and Fe-doped TiO2, and provide new insights for the development of highly selective photocatalysts based on doped metal oxides.

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