Hydrocarbon steam reforming on Ni alloys at solid oxide fuel cell operating conditions Eranda Nikolla, Johannes W. Schwank, Suljo Linic * Department of Chemical Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, MI 48109, United States 1. Introduction Steam reforming is a catalytic process for commercial hydrogen production. It is an endothermic reaction that involves the conversion of hydrocarbons and water into hydrogen and CO (1). In a catalytic reactor, this reaction is accompanied by the slightly exothermic water gas shift (WGS) reaction (2), which converts CO and steam into CO 2 and hydrogen. C n H m þ nH 2 O ) nCO þ n þ m 2   H 2 ðDH 298 > 0Þ (1) CO þ H 2 O , CO 2 þ H 2 ðDH 298 ¼41:2 kJ=molÞ (2) Steam reforming is also important in direct utilization of hydrocarbons in solid oxide fuel cells (SOFCs) [1–6]. In SOFCs, H 2 is electrochemically oxidized at the three-phase boundary (TPB) – TPB is the region where anode and electrolyte are in direct contact with each other – resulting in the formation of steam, which then internally reacts with the incoming fuel via steam reforming to form hydrogen and CO. Most steam reforming catalysts and SOFC anode electro-catalysts contain Ni metal supported on an oxide [7– 9]. Ni is the preferred catalytic material due to its excellent thermal and electronic characteristics, low cost, and high chemical activity. The ceramic oxide supports offer superior mechanical and thermal stability. One of the main issues with hydrocarbon steam reforming over supported Ni catalysts is the deactivation of Ni due to the formation of carbon deposits. The carbon-induced deactivation of Ni has been studied extensively [7,10–26]. For example, Rostrup- Nielsen and coworkers have reported that steam reforming of various liquid fuels on Ni leads to rapid catalyst deactivation due to the formation of encapsulating, whisker-like, or pyrolytic carbon on the catalyst [7,8,13]. Multiple studies have been performed to elucidate the mechanism of the formation and growth of carbon deposits on Ni [11,17,20]. For example, in situ transmission electron microscopy (TEM) studies have shown that during methane decomposition on Ni/MgAl 2 O 4 carbon nucleates at the under-coordinated Ni surface sites [17]. It was also shown that the extended carbon structures are formed in the process of the diffusion of C atoms and fragments on the surface of Ni and their attachment to the carbon nucleation centers on the low- coordinated Ni sites. It is possible to circumvent the carbon-induced deactivation of Ni by increasing the steam to carbon ratio (S/C) in the feed. While the presence of additional steam enhances the oxidation and removal of carbon in the form of CO and CO 2 , it also increases the mass flow rate through the reformer thus escalating the size of the reactor and capital cost. In addition, a substantial energy input is required to vaporize water and increase its temperature to the operating conditions. Another disadvantage of the high inlet steam Catalysis Today 136 (2008) 243–248 ARTICLE INFO Keywords: SOFC DFT Steam reforming Carbon poisoning Alloys ABSTRACT We demonstrate that supported Sn/Ni alloy catalyst is more resistant to deactivation via carbon deposition than supported monometallic Ni catalyst in steam reforming of isooctane at moderate steam to carbon ratios, irrespective of the average size of metal particles and the metal loading. The experiments were performed for average diameters of catalytic particles ranging from 30 to 500 nm and for the loading of active material ranging from 15 to 44 wt% with respect to the total mass of catalyst. The steam reforming reactions were performed at conditions that are consistent with typical solid oxide fuel cell (SOFC) operating conditions. DFT calculations show that the reasons for the enhanced carbon-tolerance of Sn/Ni compared to monometallic Ni are high propensity of Sn/Ni to oxidize carbon and lower driving force to form carbon deposits on low-coordinated metal sites. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 734 647 7984; fax: +1 734 764 7453. E-mail address: linic@umich.edu (S. Linic). Abbreviations: DFT, density functional theory; TEM, transmission electron microscopy; STEM, scanning transmission electron microscopy; SOFC, solid oxide fuel cell; YSZ, 8 mol% yttria stabilized zirconia; EELS, electron energy loss spectro- scopy; SEM, scanning electron microscopy; EDS, energy dispersive X-ray spectro- scopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction. Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2008.03.028