Effect of metal-doping (Me = Fe, Ce, Sn) on phase composition, structural peculiarities, and CO oxidation catalytic activity of cryptomelane-type MnO2

Abstract

Manganese oxides attract considerable attention as redox catalysts for abatement of environmental pollutants due to their relatively high activity and low costs contrary to noble metal catalysts. The present work is focused on the effect of cryptomelane-type MnO 2 catalyst modification with Fe 3+ , Ce 3+ /Ce 4+ , Sn 4+ on their phase composition, structural peculiarities, and catalytic activity in CO oxidation. The samples are investigated by XRD, Raman spectroscopy, XPS, XRF, and comprehensive HRTEM, with their catalytic activity being studied under the model dry conditions and simulated real exhaust conditions comprising CO, O 2 , and water balanced by inert. The Mn 4+ substitution in the cryptomelane structure by Me n+ is revealed to result in its monoclinic distortion, with the Sn- and Fe-doped cryptomelane-type MnO 2 formation being accompanied by the formation of SnO 2 and Fe 1−x Mn x O 3 /Fe 1+x Mn 2−x O 4 , respectively. For Ce-doped samples, the K + presence in the cryptomelane tunnels is essential to stabilize the cryptomelane structure and nanorod morphology. The cryptomelane structure stabilized by only Ce n+ cations is collapsed during the sample calcination to form Mn 2−x Ce x O 3 and Mn 3−x Ce x O 4 , while high Ce content does not favor the formation of the cryptomelane-type MnO 2 . The composites based on the Me-doped cryptomelane-type α-MnO 2 , especially those doped with Ce, are shown to be promising candidates to develop effective oxide-based catalysts for CO oxidation. • Mn 4+ substitution by Me n+ results in monoclinic distortion of cryptomelane structure. • K + ions are essential to stabilize cryptomelane structure and nanorod morphology. • Doped cryptomelane-type MnO 2 is promising for designing of new CO oxidation catalysts.