The role of carboxylic acid in cobalt Fischer-Tropsch synthesis catalyst deactivation

Oxygenated compounds have previously been detected on spent Co/Al₂O₃FTS catalyst and have also been proposed to be precursors for carbon formation. Build-up of polymeric carbon on the catalyst during Fischer-Tropsch synthesis (FTS) can negatively influence activity over an extended reaction time. Adsorbed oxygenates detected on spent Co/γ-Al₂O₃FTS catalyst are deduced to be located on the γ-Al₂O₃support using attenuated total reflectance infrared spectroscopy (ATR-IR). The formation of a metal-carboxylate compound is not detected (ATR-IR) and deduced to be unlikely since acetic acid decomposes at low temperature on a Co metal surface (single crystal Co(0 0 0 1) experiments under ultra-high vacuum conditions). Acetic acid undergoes dissociative adsorption on the γ-Al₂O₃(1 1 0) and (1 0 0) surfaces (DFT), forming an acetate species. Acetic acid vapor, contacted with reduced Co/Pt/Al₂O₃catalyst at model FTS conditions (i.e. 1 bar_(a)H₂/CO:2/1 at 230 °C), results in predominantly atomic carbon deposition on the catalyst. Co-feeding of excess acetic acid during FTS does not enhance Co/Pt/Al₂O₃catalyst deactivation nor does it significantly impact methane selectivity. Therefore, carboxylic acids can cause atomic carbon formation on Co/γ-Al₂O₃catalyst during FTS and result in strongly adsorbed carboxylates on γ-Al₂O₃support, but these factors do not significantly impact catalyst deactivation.