Total oxidation of lean methane over cobalt spinel nanocubes—Mechanistic vistas gained from DFT modeling and catalytic isotopic investigations

Abstract Mechanistic investigations based on combination of catalytic isotopic experiments with DFT modeling were used to probe the pathways by which CH 4 in transformed into H 2 O and CO 2 over cobalt spinel nanocubes exposing the (100) surface exclusively. The catalyst synthetized by the hydrothermal method was thoroughly characterized by means of XRD, XRF, Raman and SEM techniques. Combustion of methane was performed in a TPSR mode using sole 16 O 2 or a 1:1 mixture of 16 O 2 and 18 O 2 . Two distinct mechanistic pathways of the C H bond activation, and subsequent oxidation of the consecutive intermediates involve a suprafacial mechanism operating in the oxygen covered surface and an interfacial mechanism operating on the oxygen depleted/bare surface. It was found that depending on the conditions they may act separately in the early and late stages of the reaction, and simultaneously in the middle phase, in response to differences in the nature and accessibility of the surface and lattice oxygen species, however the activation of the first C– bond remains the kinetically relevant step. The cobalt-oxo Co T 2c –O species exhibit the highest activity in the suprafacial C H bond breaking, and the peroxo O 2 2– species are engaged in the intermediate reaction steps leading to formation of final H 2 O and CO 2 products. On the oxygen exhausted surface, where the interfacial mechanism operates, dual Co–O sites constituted by Co O 5c –O 3O and Co T 2c –O 2O,1T pairs may activate the C H bond in a concerted way, with the former being more active. For all the mechanistic steps the transition and stable states of the involved intermediates and products were identified and quantified in terms of their structure and the associated free enthalpy profiles. The three dimensional (log( k intra / k supra ) versus (Δ G a supra -Δ G a supra ) and Θ O ) diagrams were constructed to discriminate between the suprafacial (Eley-Rideal/Langmuir-Hinshelwood) and interfacial (Mars-van Krevelen) mechanism of CH 4 combustion. It is believed that the main features of the established mechanism are also applicable in the case of the methane oxidation over other electron rich transition metal oxides.