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In the light of a recent study which identiÐed the beneÐcial inÑuence of poorly crystallised ferrihydrite on the activity of CO conversion catalysts comprising gold nanoparticles on oxidic iron, we Fe 5 HO 8 É 4H 2 O have investigated three series of ferrihydrite-rich samples prepared by coprecipitation. The samples were structurally and chemically characterised using X-ray di †raction and both 57Fe and 197Au Mo ssbauer spectroscopy, and tested for CO oxidation at room temperature using a microreactor with on-line GC. The highest activity, 100% conversion after 20 min on line, was observed in a dried sample that contained ferrihydrite and a non-crystalline and possibly hydrated gold oxyhydroxide phase, The AuOOH É xH 2 O. activity of the same materials after calcination, where the gold was transformed to 3È5 nm Au metal particles and the ferrihydrite to hematite, was less than ca. 7%. This is the Ðrst report of a synergistic interaction between and ferrihydrite resulting in an active catalyst for room temperature CO oxidation, AuOOH É xH 2 O and contrasts with previous work which has been interpreted in terms of the requirement for metallic Au nanoparticles.

57Fe and 197Au Mo ssbauer data have been reported and discussed for a variety of catalysts of gold supported on iron oxide. The 57Fe Mo ssbauer spectra show that iron is present as ferrihydrite, haematite and goethite in amounts that depend on the conditions of preparation. The highest amount of ferrihydrite is observed in the samples prepared by coprecipitation. The 197Au Mo ssbauer spectra reveal the formation of two main gold species, i.e., metallic gold and oxidic trivalent gold. The presence of gold favours the formation of ferrihydrite over that of haematite. No direct correlation was found between catalytic activity towards CO oxidation and the Au species identiÐed by Mo ssbauer spectroscopy, but catalyst activity was found to increase with the relative amount of ferrihydrite. It is suggested that the ferrihydrite is important for activation of molecular oxygen.

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