How to Efficiently Promote Transition Metal Oxides by Alkali Towards Catalytic Soot Oxidation

Abstract

The effect of surface and bulk potassium promotion on the transition metal oxides (Mn, Fe, Co) catalytic activity in catalytic soot oxidation was investigated. The surface promotion was obtained via incipient wetness impregnation, whereas the bulk promotion-nanostructuration-was obtained via wet chemical synthesis or solid state reaction leading to mixed oxide materials. The introduction of potassium into the transition metal oxide matrix results in the formation of tunneled or layered structures, which enable high potassium mobility. The structure of the obtained materials was verified by powder X-ray diffraction and Raman spectroscopy and the catalytic activity in soot oxidation was determined by TPO-QMS and TGA. It was found that the surface promotion does not alter the metal oxide structure and leads to either enhancement (Mn, Fe) or deterioration (Co) of the catalytic activity. Simultaneously, the catalytic activity in soot combustion of the potassium structured oxides is strongly enhanced. The most active, cobalt oxide based catalyst significantly lowers the temperature of 50 % of soot conversion by *350°C comparing to the non-catalyzed process. The study allows for the establishment of rational guidelines for designing robust materials for DPF applications based on ternary metal oxides.

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

“…3B) the set of characteristic bands was observed in the Raman spectra at ca. 185 cm −1 and 385 cm −1 (attributed to Mn-O-Mn bending vibrations within the MnO 2 octahedral lattice), 575 cm −1 (displacement of oxygen anions neighboring to manganese cations along the direction of octahedral chains) and 635 cm −1 (Mn-O stretching modes within tetrahedral sites), which remains in agreement with previously published literature data [12,15,33]. Based on the analysis of Raman active vibrational modes, it can be confirmed, that neither cobalt presence nor its concentration did not significantly influence the structure of both cryptomelane and birnessite.…”

“…270, 385 and 510 cm −1 can also be visible (Fig. 3A), as previously reported in literature [12,15]. Moreover, for birnessite impregnated with the cobalt solution ( Fig.…”

“…One of the possible solutions to avoid uncontrollable loss of alkalies is the stabilization of a given alkali metal within a transition metal oxide structure [7,10]. A good example of compounds which meet such requirements can be iron oxides doped with potassium stabilized inside of tunnels and/or between oxometallic layers-monoferrite KFeO 2 and β-ferrite K 2 Fe 22 O 34 [11,12], layered cobaltates K x CoO 2 [12,13] [12,[14][15][16]. They were investigated in the case of soot combustion, exhibiting high activity in this reaction with the last group deserving particular attention [7,9,10,[17][18][19].…”

“…Despite several reports on higher activity of cobalt-doped manganese systems in catalytic soot abatement, compared to that observed for the corresponding undoped manganese oxides, the promotion of cryptomelane and birnessite with cobalt has not been studied in detail in the context of total carbon oxidation [6,12,15]. The main goal of our work was thus to investigate in more detail how cobalt-doping can influence the catalytic activity of such Mn-based systems in soot combustion and to understand the behavior of the doped catalysts.…”

The Effect of Fe, Co, and Ni Structural Promotion of Cryptomelane (KMn8O16) on the Catalytic Activity in Oxygen Evolution Reaction

“…2. Upon doping with Co and Fe, a partial change of the structure into the birnessite type takes place, as indicated by an intensity increase of the high wavenumber peak and disappearance of the lowest wavenumber peak [38]. Unfortunately, the quantification of the cryptomelane and birnessite phases present in the samples is not feasible basing on the available XRD and Raman spectroscopy data.…”