Mechanistic Insights into Oxygen Dynamics in Soot Combustion over Cryptomelane Catalysts in Tight and Loose Contact Modes via ¹⁸O₂/¹⁶O₂ Isotopic Variable Composition Measurements – A Hot Ring Model of the Catalyst Operation

A cryptomelane K-OMS-2 model catalyst of a well-defined nanorod morphology, dominated by the (100) and (110) planes, was synthesized, and thoroughly examined by means of XRD, Raman, XPS, XAS, TEM/EDX/SAED, and computed X-ray microtomography techniques. The catalyst oxidation performance was elucidated in the tight (TC) and loose (LC) contact modes using isotopic ¹⁸O₂/¹⁶O₂ mixtures of various compositions. A simple methodology disentangling the relative involvement of the suprafacial (adsorbed/gas phase) and intrafacial (surface/lattice) reactive oxygen species in the soot oxidation was developed, allowing for straightforwardly unraveling the mechanistic details of the LC and TC combustion of soot particles. A functional “hot ring model” was established to rationalize the catalyst operation. It was found that apart from the ROS generation function, the catalyst acts in the TC mode as an igniter, whereas in the LC mode as a booster that drives the combustion process until the afterburning stage is reached. The obtained results were accounted for by molecular DFT and atomistic thermodynamic modeling, justifying the particular role of the surface di- (O_2c) and tri-coordinated (O_3c) oxygen anions, located on the (100) and (110) planes of the cryptomelane nanorods, in the ignition process.