Effects of La₂O₃ on the mixed higher alcohols synthesis from syngas over co catalysts: A combined theoretical and experimental study

The promoting role of lanthanum oxide (La₂O₃) in the catalytic synthesis of mixed higher (C₂-C₆) alcohols from syngas on Co-based catalysts was investigated using a combination of in situ and ex situ X-ray diffraction (XRD), photoelectron spectroscopy (XPS), catalyst reactivity performance studies, and ab initio molecular dynamics (AIMD) simulations. XRD measurements show that doping with La (0.5 wt %) onto activated carbon (AC) supported Co catalyst enhances the Co₂C phase formation, whereas the Co₂C phase formation is largely suppressed on alumina supported Co catalyst. A strong correlation of the selectivity toward alcohols with the ratio of Co₂C/Co particles present in the catalysts was observed. AIMD simulations suggest that on AC supports La exists as an oxide phase in the form of small clusters in the vicinity of Co particles. It was found that Co₂C formation is energetically favorable, especially for smaller Co particles because of the formation of surface carbide. Theoretical mechanistic studies indicate that oxygenated hydrocarbons can be formed on these catalysts by multiple routes involving the formation of CH_xO and CH_xCO species at the interface between the La₂O ₃ phase and Co/Co₂C. A detailed comparison with previous findings in the literature as well as discussion of the implications of these results upon the improvement of the selectivity of these catalysts toward higher alcohols is presented.