(245e) A First-Principles Study on the Catalytic Roles of Co0 and Co2+ in Ethanol Steam Reforming

The ethanol steam reforming (ESR) reaction is a promising way of producing hydrogen from bio-renewable materials. However the practical application of ESR requires a low-cost catalyst that shows fast kinetics, high selectivity to CO₂ and H₂ as well as long-term stability. As a candidate catalyst, cobalt based materials, such as ceria supported Co, show good performance as judged by the aforementioned criteria. Previous studies have suggested that two oxidation states of Co, namely Co⁰ and Co²⁺, coexist under ESR reaction conditions and they both contribute to the ESR catalytic activity, however their separate roles are still an open question. We used first-principles based methods, such as density functional theory (DFT) and ab initio atomistic thermodynamics, to comparatively study the ESR reaction pathways on Co metal and CoO surfaces. Specifically, we examined the initial steps of ethanol decomposition through either dehydrogenation into acetaldehyde or dehydration into ethylene on both the Co metal and the CoO surfaces. We also investigated water gas shift (WGS) reactions on both surfaces since WGS may be important in determining the ESR reaction selectivity to CO₂ or CO. Our results suggest that neither Co⁰ nor Co²⁺ is active for all ESR reaction steps, and a collaboration of both oxidation states, for example the existence of a Co⁰/Co²⁺ interface, is very likely required for achieving high ESR catalytic activity. We also found that different facets of Co²⁺ surfaces, for example CoO(111) and CoO(100), have distinct activities towards ESR reactions. Controlling the dominant Co facets, for example through tailoring the morphology of the ceria support, may enhance the performance of Co-based catalysts.