Paper AS-TuP21
In-situ XPS Analysis of Co and Co²⁺ during Steam Reforming of Ethanol on Supported Cobalt Catalysts

Tuesday, October 19, 2010, 6:00 pm, Room Southwest Exhibit Hall

In catalysis, the oxidation state of metal nanoparticles on the surface is often unknown, especially under oxidizing conditions. The redox properties of the catalyst are typically investigated by temperature programmed reduction and oxidation or x-ray absorption spectroscopy. However, these are not the surface sensitive techniques and provide limited surface details especially under H₂/H₂O environments. We report the use of in-situ x-ray photoelectron spectroscopy (XPS) to determine the oxidation state of Co following exposure to O₂, H₂, and H₂/H₂O. We found that the type of support and catalyst pretreatment (calcinations and/or reduction temperature) have a strong effect on the Co⁰/Co²⁺ ratio. Our results indicate that Zn helps stabilize Co against oxidation by O₂ or H₂O. The in-situ XPS measurements allowed us to study the effect of Co⁰/Co²⁺ ratio on the catalytic activity and understand the role of Co²⁺ in the ethanol reforming reaction pathways. The catalytic tests show that both Co⁰ and Co²⁺ were active in the C-C bond cleavage and water gas shift reactions. However, Co⁰ is shown to be much more active than Co²⁺. Also, the reaction pathways for CO₂ and CH₄ formation appear to be different on Co⁰ and Co²⁺. Catalysts with higher Co⁰/Co²⁺ surface ratio exhibited lower selectivity to CH₄. Our results show that ethanol decomposition and CO methanation are more favored on Co²⁺ relative to Co⁰. In addition, we show that on both Co⁰ and Co²⁺, CO₂ is a secondary product forming by the water gas shift reaction.