Au/FeOx catalysts of different degree of iron oxide reduction

Au/FeOx catalysts were prepared by deposition of Au on Fe3O4 (commercial Cerac, a mixture of magnetite and maghemite) with the deposition-precipitation method, and heating of the precursors in air or in Ar. The catalysts were characterized by XPS, Mössbauer spectroscopies, H2TPR, TEM, decomposition of isopropanol (probe reaction for acid–base properties), and tested in CO oxidation.The heating atmosphere influences composition, physicochemical and catalytic properties. The catalysts prepared in Ar contain magnetite, MT and maghemite, MG as the main iron oxide phases, for those calcined in air MG is the only phase observed. Higher amounts of weakly bound oxygen are observed in H2TPR measurements for the catalysts prepared in air. They exhibit smaller acidity and higher basicity compared to the samples prepared in Ar. The mean particle size of nano Au is not affected markedly by the preparation conditions and amounts to ∼4.5 nm. Calcination in Ar appears, however, to stabilize smaller particles (<4 nm) against the sintering. Catalytic activity of the Au/FeOx catalysts in CO oxidation is higher for catalysts prepared in air, containing MG, as compared to those containing MT. This can be related to the higher amounts of weakly bound oxygen for the former catalysts. In the reaction mixture CO + O2, MT phase is oxidized to MG. For all the catalysts the activity is considerably enhanced by pretreatment in hydrogen. This can be associated with the increase in the content of the surface OH groups.

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► Catalytic activity of Au/FeOx catalysts in CO oxidation is higher for catalysts containing maghemite (γ-Fe2O3) as compared to those containing magnetite (Fe3O4).
► In the reaction mixture CO + O2 magnetite phase is oxidized to maghemite.
► Catalysts containing maghemite show higher amounts of weakly bound oxygen and lower acidity.
► The activity is considerably enhanced by pretreatment in hydrogen. This can be associated with the increase in the content of the surface OH groups.