Co3O4 nanoparticles as oxygen carriers for chemical looping combustion: A materials characterization approach to understanding oxygen carrier performance

• Published in: Chemical engineering journal (Lausanne, Switzerland : 1996) Switzerland : 1996), 2017-07, Vol.319, p.279-287
• Main Authors: Alalwan, Hayder A., Cwiertny, David M., Grassian, Vicki H.
• Format: Article
• Language: English
• Subjects: Chemical looping combustion; Cobalt oxides; Methane oxidation; Nanotechnology; Nucleation and nuclei growth model; Surface spectroscopy
• ISSN: 1385-8947; 1873-3212
• DOI: 10.1016/j.cej.2017.02.134

[Display omitted] •Co3O4 reduction proceeded through CoO then to metallic Co during CLC.•Superior performance of CoO/Co mixed phases.•CoO reduction to Co follows a nucleation and nuclei growth mechanism.•Mixed metal-metal oxide phases may help to improve reactivity.•Continued performance of Co3O4 through several cycles, illustrates its potential. Currently, there is relatively limited information about the use of Co3O4 as an oxygen carrier for chemical looping combustion, with the promise afforded by its high oxygen capacity often overlooked because of its moderate temperature of decomposition. Here, we employ a materials characterization approach to investigate the coupled surface chemical and bulk material processes influencing the oxidation of methane by Co3O4 (CO2 yield and reactive longevity). Methane (CH4) oxidation by Co3O4 was studied in a continuous flow reactor across a range of temperatures (500–700°C) and gas hourly space velocities (125–375h−1). At the highest temperatures considered (700°C), Co3O4 reduction proceeded through CoO to an ultimate end product of metallic Co, with the rate and extent of CH4 oxidation to CO2 decreasing monotonically with increasing CoO content because of its more highly coordinated lattice oxygen. In contrast, at lower temperatures (e.g., 500°C) the initial decrease in CH4 oxidation coinciding with Co3O4 conversion to CoO was followed by a period of increasing CO2 yield as some CoO was further converted to Co, unexpected behavior not observed at higher temperatures. Complementary bulk and surface analyses indicate that CoO reduction to Co follows a nucleation and nuclei growth mechanism within the particle bulk. We thereby attribute the greater reactivity of lattice oxygen in mixed CoO/Co phases to a lower cohesive energy for oxygen atoms at the interface of these phases. Our results suggest that for CLC applications in lower temperature regimes, the use of mixed metal-metal oxide phases may help to improve reactivity. More practically, our results demonstrate the performance of Co3O4 through several cycles, illustrating its potential for use in CLC.