Identification of active phases in Au–Fe catalysts for low-temperature CO oxidation

Abstract

In the light of a recent study which identified the beneficial influence of poorly crystallised ferrihydrite Fe₅HO₈·4H₂O on the activity of CO conversion catalysts comprising gold nanoparticles on oxidic iron, we have investigated three series of ferrihydrite-rich samples prepared by coprecipitation. The samples were structurally and chemically characterised using X-ray diffraction and both ⁵⁷Fe and ¹⁹⁷Au Mössbauer 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, AuOOH·xH₂O. The activity of the same materials after calcination, where the gold was transformed to 3–5 nm Au metal particles and the ferrihydrite to hematite, was less than ca. 7%. This is the first report of a synergistic interaction between AuOOH·xH₂O and ferrihydrite resulting in an active catalyst for room temperature CO oxidation, and contrasts with previous work which has been interpreted in terms of the requirement for metallic Au nanoparticles.