Role of Surface Oxygen Vacancy Concentration on the Dissociation of Methane over Nonstoichiometric Ceria

Chemical looping reforming of methane over ceria-based materials is a promising route for the production of synthetic liquid fuel precursors, H-2 and CO. In this work, a comprehensive kinetic model was established, based on thermogravimetric experiments, to describe the previously unresolved, surface-mediated mechanism of methane dissociation via ceria oxygen removal, the first heterogeneous, noncatalytic reaction within the aforementioned redox cycle. Prior studies have suggested that either a surface oxygen anion or vacancy is responsible for the activation of methane. However, here, these two theories are combined to unambiguously show that the prominence of each pathway is dependent on the availability of surface oxygen. Thus, at sufficiently high oxygen nonstoichiometries or for low-surface-area samples, as examined in this work, the vacancy-mediated dissociation of methane is predominant. This assertion was elucidated by mathematically describing a series of rate-determining steps based on surface interactions of known reactive intermediates and fitting the postulated reaction mechanism to temperature-dependent measurements obtained with multistage isothermal thermogravimetry. Corresponding Arrhenius parameters were extracted with excellent agreement between the model predictions and experimentally measured rates. Further validation of the hypothesized reaction mechanism is supported by (1) close similarity between activation energies obtained through model fits and separate isoconversional techniques and (2) expected trends observed with acceptor-doped ceria that has a higher concentration of extrinsic oxygen vacancies at the onset of the reaction. The quantitative kinetic insight obtained from the model presented herein allows for the evaluation and optimization of ceria-based materials in larger-scale, chemical looping processes.