Abstract
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- (O2c) and tri-coordinated (O3c) oxygen anions, located on the (100) and (110) planes of the cryptomelane nanorods, in the ignition process.
Highlights
Diesel engine devices and vehicles are one of the major sources of soot particle emission, contributing to proliferation of particulate matter, and polluting air in urban areas.[1,2] Micrometric soot grains can be described as a mixture of crystalline and amorphous forms of carbon, which are intermingled with polycyclic and long-chain hydrocarbons.[3]Constant strengthening of the diesel PM emission standards (EURO 6 or China 6) provides a steady incentive for development of effective technologies for soot emission abatement.[4,5] The temperature of autogenic soot combustion is higher than that of the exhaust gases produced by diesel engines (200−400 °C)
The imaging results of the nanometric scale agglomeration and intermingling between the cryptomelane nanorods and the soot nanoparticles are presented in Figure 1b1−b4, c1−c4, for the loose and TC modes, respectively. Due to their elongated morphology, the K-OMS-2 nanorods exhibit a clear tendency for agglomeration into rounded bundles (Figure 1b1,c1)
In the temperature range of 450−550 °C, the values approach a nearly stoichiometric level of 0.47−0.49. These results reveal that the redox properties of the K-OMS-2 catalyst in the temperature window of soot oxidation interest are fairly well equilibrated, when the momentary deviations from the stoichiometry, controlled by the size of the CO/O2 pulses, are reasonably small
Summary
Diesel engine devices and vehicles are one of the major sources of soot particle emission, contributing to proliferation of particulate matter (mainly PM < 2.5 μm), and polluting air in urban areas.[1,2] Micrometric soot grains can be described as a mixture of crystalline and amorphous forms of carbon, which are intermingled with polycyclic and long-chain hydrocarbons.[3]. A similar effect was observed in the case of Ag doping.[39] the observed high soot oxidation activity even at low temperature could, again, be related to an increased contribution of such species in this reaction.[35] Based on sophisticated TAP studies of soot combustion over bare and Pt- or La-modified ceria, a more substantiated mechanism has been proposed, where soot oxidation was carried out by the Osur2− species, and not due to a direct reaction between O2 and soot particles.[40] despite the progress made in this field, a systematic evaluation of the role of various oxygen species in the soot combustion processes over oxide catalysts, including dynamic competition between the involvement of the adsorbed/gas phase oxygen (suprafacial mechanism) and the surface/lattice oxygen (intrafacial mechanism) at various stages of the reaction, is still not definitely resolved Both types of the redox mechanisms are widely recognized, their experimental verification is quite difficult, and often based on isotopically labeled reactants of single enrichment only. Mechanistic studies resolving the involvement of suprafacial (adsorbed/gas phase) and intrafacial (surface/lattice) oxygen, and providing direct insights into the TC and LC mechanisms of total soot combustion over K-OMS-2 and other oxide catalysts have not been carried out so far
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