Ethan Lawrence1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Internal reforming of hydrocarbons in solid oxide fuel cells (SOFCs) can enhance electrochemical conversion efficiencies by eliminating the need for external fuel reforming [1]. Long-term stability of SOFCs may be limited by carbon deposition from the fuels onto the anode catalyst, causing deactivation of the ceramic-metal composite structure. Ni/CeO2-based catalysts may inhibit carbon deposition through a process involving lattice oxygen exchange. Natural gas consists primarily of methane (CH4) but also contains several percent of higher hydrocarbons such as ethane (C2H6) and propane (C3H8), which have been shown to deposit carbon more easily than CH4 [2]. We are interested in understanding how CeO2 can affect carbon deposition at the gas-solid interface by observing structural and chemical changes at the nanoscale. In situ environmental transmission electron microscopy (ETEM) provides the ability to observe atomic-level structural changes under simulated reaction conditions [3]. In situ electron energy-loss spectroscopy (EELS) has allowed dynamic changes in the local oxidation state of CeO2 to be determined during reduction [4]. A fundamental study of the local structural and chemical changes occurring at Ni/CeO2 interfaces under reaction conditions will elucidate the mechanisms that enable Ni/CeO2-based catalysts to inhibit carbon deposition from light hydrocarbons.

In situ ETEM techniques were employed to investigate the atomic-level three-phase interactions occurring at the metal-support interface during carbon deposition from C2H6 and C2H4 over a model Ni/CeO2 catalyst. Structural and chemical interfacial changes occurring during species-dependent carbon deposition were determined using atomic-level imaging and EELS. During exposure to C2H6, no carbon deposition occurred, and localized reduction zones formed at the Ni/CeO2 interface through a Mars van Krevelen carbon oxidation mechanism. In contrast, less pronounced reduction zones formed during C2H4 exposure, and carbon deposition occurred on Ni surfaces. Rapid dehydrogenation and subsequent graphite formation occurred on Ni surfaces during C2H4 exposure, whereas the metal-support interface catalyzed the oxidative dehydrogenation of C2H6 and oxidized the resulting carbonaceous species during C2H6 exposure. These experiments demonstrate that the ability of the interfacial sites on Ni/CeO2 to inhibit carbon deposition during reforming is strongly influenced by thermodynamic and kinetic considerations which may show significant variation among different hydrocarbon species [5].
[1] Gür, T.M., et al, Progress in Energy and Combustion Science 54 (2016), p. 1-64.
[2] Bierschenk, D.M., et al, Fuel Cells 10 (2010), p. 1129-1134.
[3] Tao F. and Crozier P.A., Chemical Reviews 116 (2016) p. 3487-3539.
[4] Sharma R., et al, Philosophical Magazine 84 (2004) p. 2731-2747.
[5] We gratefully acknowledge support of NSF grant DMR-1308085 and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.