Strong Enhancement of deSoot Activity of Transition Metal Oxides by Alkali Doping: Additive Effects of Potassium and Nitric Oxide

Abstract

A series of potassium-promoted spinels (Mn, Fe, Co) were prepared with various K ? promoter locations: on the surface (surface promotion) or in the bulk (formation of new layered and tunneled nanostructures via solid state reaction). All prepared samples were characterized by means of X-ray diffraction, Raman spectroscopy, X-ray fluorescence and N 2 -BET specific surface area analysis. Catalytic activity in soot combustion in different reaction conditions was investigated (tight contact, loose contact, loose contact with NO addition). It was shown that in all cases the nanostructuration is more effective than the surface promotion, with the layered structures of KCo 4 O 8 , KMn 4 O 8 being the most catalytically active phases, lowering the soot combustion down to 250°C. The difference in activity between tight and loose contacts can be bridged in the presence of NO due to its transformation into NO 2 , which acts as the oxygen carrier from the catalyst surface into soot particles, eliminating the soot-catalyst contact difference.

Oxygen Evolution Reaction of Co-Mn-O Electrocatalyst Prepared by Solution Combustion Synthesis

“…Raman spectra for mixed materials in Figure 4 are also in agreement with the EDX and XRD results. In comparison to with the Raman spectra of Co 3 O 4 , different activated modes were observed for the mixed materials with Co = 1.2, which could be assigned to the 6 vibrational modes of tetragonal spinel CoMn 2 O 4 [17]. The peaks at 180, 320, 372, 495, 585, and 664 cm −1 in the spectra of Co = 1.2 are due to the tetragonal spinel structure of CoMn 2 O 4 [18].…”

“…Broadened Raman peaks at 180 cm −1 , between 320 and 370 cm −1 , and between 495 and 585 cm −1 can be attributed to the low crystallinity and nanosized grain of the mixed materials with Co = 1.2. The Raman peaks of the mixed materials shifted slightly to higher frequencies with increase in the Co content, owing to changes in the cation-anion bonds in the lattice constriction of Co 3 O 4 [17]. In particular, the most significant shift to a lower frequency was observed for the A 1g reflection, from 690 to 664 cm −1 , for the octahedral cation sites on the substitution of Co cations with Mn cations.…”

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

“…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.…”

Nanostructured Potassium-Manganese Oxides Decorated with Pd Nanoparticles as Efficient Catalysts for Low-Temperature Soot Oxidation

“…This is meant to increase the number of contact points between the catalyst surface and soot particles, which is essential for soot oxidation [17][18][19][20]. The class of compounds investigated in this context are mixed oxides built of d-metal cations (Fe, Mn, Co), which works as a redox center, and an alkali cation, most often potassium [21]. The combination of these elements can lead to the formation of nanostructured materials, where alkali cations are located in the tunnels or layers.…”