Elucidation of Unexpectedly Weak Catalytic Effect of Doping with Cobalt of the Cryptomelane and Birnessite Systems Active in Soot Combustion

Abstract

In this work the influence of cobalt doping on both properties and catalytic activity of cryptomelane and birnessite in soot combustion was investigated in detail. The investigated samples have been synthesized by a microwave-assisted hydrothermal method and cobalt was introduced in two ways-by impregnation or by addition to the precursor mixture before hydrothermal treatment. The obtained catalysts have been characterized by XRF, BET, XRD, RS, XPS and ATR-IR. Surprisingly, no distinct effect of cobalt presence on catalytic activity has been found. Most probably cobalt was localized in the position of manganese within the cryptomelane/birnessite structure or within segregated manganese-cobalt complex oxides, where its oxidation level was fixed. A series of reference manganese-cobalt mixed oxides (100-0% Mn) have been synthesized to elucidate this effect. These samples have been structurally characterized and thermal evolution of their structures was profoundly investigated. A high tendency of cobalt incorporation into the manganese oxide matrix and its subsequent stabilization in the spinel forms was proposed as an explanation of the observed lack of a promotional effect of this additive in soot combustion.

Contrasting Effects of Potassium Addition on M3O4 (M = Co, Fe, and Mn) Oxides during Direct NO Decomposition Catalysis

“…Based on previous literature reports of similar oxides, these peaks are tentatively indexed to cryptomelane KMn 8 O 16 (PDF# 00-044-1386). Typically, pure-phase cryptomelane is synthesized at elevated pressures via hydrothermal methods, however, its formation has also been observed over CoAlMn-based oxides co-precipitated by KOH [25,30]. Cryptomelane also has tendency to oxidize to bixbyite above 600 • C in air, but this has not been observed under the current reaction conditions (~1% NO/He) as the XRD patterns of the spent K-containing samples displayed no notable increase in relative intensity of bixbyite reflections.…”

Catalytic Soot Combustion─General Concepts and Alkali Promotion

“…Detailed data used for the compilation of Figure are reported in the Table , Table , and Table S1 in the Supporting Information. Depending on their chemical composition and morphology, the catalysts were divided into six groups: platinum metal group, ,− ceria–zirconia-based, − ,,,,,,,,,,− alkali-nanostructured, ,,,,− ,,,,,,,,− ,− spinel structure, ,,,,− ,,,− perovskite structure, ,,,,,,,,,…”

Fungal transformation of natural and synthetic cobalt‐bearing manganese oxides and implications for cobalt biogeochemistry

“…K-birnessite, Na-birnessite and acid birnessite can be easily prepared to accommodate geochemical and biogeochemical investigations (Jokic, et al, 2001;Tebo et al, 2004;Toner and Sposito, 2005;Händel et al, 2013;Wang et al, 2013). A range of studies have used metal-doped birnessites, including those containing Co, Cu, Al and Ni (Lucht and Medoza-Cortez, 2015;Yadav et al, 2017;Arias et al, 2019;Legutko et al, 2019). In this work, a synthetic Co-doped birnessite was used to provide a general example for the fate of Co during Mn oxide transformations, to complement examination of the Nkamouna laterite.…”