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 Mossbauer 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.