Thermal Transformation of Birnessite (OL) Towards Highly Active Cryptomelane (OMS-2) Catalyst for Soot Oxidation

Abstract

A layered potassium–manganese oxide, birnessite (OL: KMn4O8) was subjected to long-term post-calcination thermal treatment aimed at stimulating its activity in catalytic soot oxidation. The 12\xa0h thermal treatment at several temperatures (425, 500, 550 and 650\xa0°C) in atmospheric conditions resulted in the swift transformation of the layered material into the tunnel-shaped octahedral molecular sieve cryptomelane (OMS-2: KMn8O16). The obtained materials were thoroughly characterized using powder X-ray diffraction, Raman spectroscopy, N2-BET specific surface area analysis, and transmission electron microscopy techniques. With increasing temperature of thermal treatment, the concentration of OMS-2 increased, which was confirmed by Rietveld analysis, and dominated the material properties, notably the lowering of work function of the catalysts (Δϕ\u2009=\u20090.2 and 0.4\xa0eV in air and vacuum, respectively). The obtained results reveal the high catalytic activity of the OMS-2 formed from the thermal transformation of the OL material compared to both the parent OL material and the uncatalyzed soot oxidation reaction. The achieved catalytic activity showed direct correlation with the temperature of pre-treatment, with the most active catalyst being calcined at 550\xa0°C/12\xa0h, and lowering the temperature of 50% soot conversion (T50%) by spectacular 150\xa0°C in loose contact. The catalytic activities were found to correlate well with the work function (low work function–high catalytic activity), confirming the electron transfer from the catalyst surface to oxygen molecule as the important step in the soot oxidation mechanism over mixed potassium–manganese oxides. The beneficial effect of thermal pre-treatment was found to last over multiple runs, maintaining a T50% lower by 100\xa0°C compared to the untreated parent material. The obtained results indicate the importance of the temperature treatment for the catalytic performance of potassium promoted transition metal oxides as catalytically active phases for efficient soot removal in the conditions present in combustion engine exhaust gases.Graphical Abstract