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

In the light of a recent study which identified the beneficial influence of poorly crystallised ferrihydrite Fe5HO8. 4H(2)O on the activity of CO conv ersion catalysts comprising gold nanoparticles on oxidic iron, we have inve stigated three series of ferrihydrite-rich samples prepared by coprecipitat ion. The samples were structurally and chemically characterised using X-ray diffraction and both Fe-57 and Au-197 Mossbauer spectroscopy, and tested f or CO oxidation at room temperature using a microreactor with on-line GC. T he highest activity, 100% conversion after 20 min on line, was observed in a dried sample that contained ferrihydrite and a non-crystalline and possib ly hydrated gold oxyhydroxide phase, AuOOH . xH(2)O. The activity of the sa me 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%. Th is is the first report of a synergistic interaction between AuOOH . xH(2)O and ferrihydrite resulting in an active catalyst for room temperature CO ox idation, and contrasts with previous work which has been interpreted in ter ms of the requirement for metallic Au nanoparticles.