Published: February 25, 2011 r2011 American Chemical Society 279 dx.doi.org/10.1021/cs200014j | ACS Catal. 2011, 1, 279–286 RESEARCH ARTICLE pubs.acs.org/acscatalysis Catalytic Roles of Co 0 and Co 2þ during Steam Reforming of Ethanol on Co/MgO Catalysts Ayman M. Karim, † Yu Su, † Mark H. Engelhard, ‡ David L. King, † and Yong Wang* ,†,§ † Institute for Interfacial Catalysis and ‡ Environmental Molecular Science Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, Washington 99164-2710, United States 1. INTRODUCTION Hydrogen is a clean energy carrier and has been considered for use in fuel cells for pollution-free generation of electricity. H 2 production from ethanol steam reforming (ESR) has recently caught particular attention because of the high H 2 yield and the established technology of ethanol production from biomass. 1,2 Noble metal based catalysts, that is, Rh, Pt, Pd, and Ru, have been extensively investigated 3-8 and have been shown to be highly active for ESR. However, their application is to a large extent limited by their high cost. Transition metal catalysts, such as Ni and Cu, have also been studied, 9-16 but their performance suffers from either low activity and/or fast carbon deposition. Co-based catalysts, in comparison, are less expensive than the noble metals, and have been shown to exhibit high activity and selectivity (to H 2 and CO 2 ) during ESR. 1,17,18 The performance of Co-based catalysts is greatly dependent on the catalyst support, preparation method, 19 metal precursor, 17 calcination temperature, 20 and the Co crystalline structure. 21 Several research efforts investigating the reaction mechanism of ESR over Co catalysts have been reported. 1,22-27 Llorca et al. found the reaction pathways are strongly dependent on the support used. 1 With the aid of in situ DRIFTS experiments, they concluded that the first step of ESR over Co/ZnO catalyst was the ethanol dehydrogenation to acetaldehyde. 24 An in situ magnetic study from the same group indicated that both Co 0 and Co 2þ were observed under the reaction conditions, with an easy exchange between the two species. 22 However, because of the low Co loading and the activity contributed by the support, it was not possible to unambiguously determine whether the active site was Co 0 or Co 2þ . 22 O’ Shea et al. investigated the evolution of Co 3 O 4 under operando conditions with X-ray diffraction (XRD), and observed that Co 3 O 4 was not active, but the evolved material with CoO and Co 0 was active and selective in ESR. 25 On the other hand, Batista et al. believed Co 0 was the only site that was important in ESR. 23 To identify the different roles of Co 0 and Co 2þ , Tuti et al. 26 investigated bulk Co 3 O 4 (both oxidized and reduced), Co supported on MgO and CoO-MgO solid solution. They concluded that the Co 0 was mainly responsible for the reforming of ethanol, while ethanol dehydrogenation could occur on Co 2þ . 26 However, the specific roles and reactions proceeding on different Co sites have not been identified. In addition, the effect of the different Co 0 /Co 2þ ratios on the activity and product distribution has not been studied. The goal of this work was to investigate the specific roles of Co 0 and Co 2þ , as well as the effect of their relative ratio on ESR. MgO was selected as the support because of its ability to form an oxide solid solution with Co, making it possible to stabilize Co 2þ . 26,28 Catalysts with different Co 0 fraction, Co 0 /(Co 0 þ Co 2þ ), were obtained through different calcination and/or reduction pretreatments, and were characterized by N 2 adsorption, Received: January 11, 2011 Revised: February 2, 2011 ABSTRACT: The catalytic roles of Co 0 and Co 2þ during steam reforming of ethanol were investigated over Co/MgO catalysts. Catalysts with different Co 0 /(Co 0 þCo 2þ ) fraction were prepared through calcination and/or reduction at different temperatures, and the Co 0 fraction was quantified by temperature programmed reduction (TPR) and in situ X-ray photoelectron spectroscopy (XPS). Higher temperature calcination of Co/MgO allowed us to prepare catalysts with more nonreducible Co 2þ incorporated in the MgO lattice, while lower calcination temperatures allowed for the preparation of catalysts with higher Co 0 /(Co 0 þCo 2þ ) fractions. The catalytic tests on Co 0 , nonreducible Co 2þ , and reducible Co 2þ indicated that Co 0 is much more active than either reducible or nonreducible Co 2þ for C-C cleavage and water gas shift reaction. In addition, catalysts with a higher Co 0 surface fraction exhibited a lower selectivity to CH 4 . KEYWORDS: ethanol steam reforming, reaction pathway, Co 0 , Co 2þ , XPS, cobalt oxidation state