Exceptional Low-Temperature CO Oxidation over Noble-Metal-Free Iron-Doped Hollandites: An In-Depth Analysis of the Influence of the Defect Structure on Catalytic Performance

A family of iron-doped manganese-related hollandites, K(x)Mn(1-y)FeyO(2-delta) (0 <= y <= 0.15), with high performance in CO oxidation have been prepared. Among them, the most active catalyst, K0.11Mn0.876Fe0.123O1.80(OH)(0.09), is able to oxidize more than 50% of CO at room temperature. Detailed compositional and structural characterization studies, using a wide battery of thermogravimetric, spectroscopic, and diffractometric techniques, both at macroscopic and microscopic levels, have provided essential information about this never-reported behavior, which relates to the oxidation state of manganese. Neutron diffraction studies evidence that the above compound stabilizes hydroxyl groups at the midpoints of the tunnel edges as in isostructural beta-FeOOH. The presence of oxygen and hydroxyl species at the anion sublattice and Mn3+, confirmed by electron energy loss spectroscopy, appears to play a key role in the catalytic activity of this doped hollandite oxide. The analysis of these detailed structural features has allowed us to point out the key role of both OH groups and Mn3+ content in these materials, which are able to effectively transform CO without involving any critical, noble metal in the catalyst formulation.