(100b) Fischer-Tropsch Synthesis: Metal-Support Interfacial Contact Governs Oxygenates Selectivity Over CeO2 Supported Pt-Co Catalysts

             The reaction of carbon monoxide and hydrogen to form hydrocarbons using either an iron or cobalt catalyst usually produces a broad range of products which consists of mainly paraffins and olefins with a carbon number of up to and greater than 100, and low levels of oxygenates including alcohols, aldehydes, ketones and acids. Iron catalysts usually produce more olefins and oxygenates compared to supported cobalt catalysts at elevated temperatures and pressures [1,2]. For Fischer-tropsch (FT) synthesis, cobalt has been reported to produce mainly n-alkanes over a wide range of molecular weights.  The cobalt active phase is generally deposited over an oxide support (e.g., Al₂O₃, SiO₂, or TiO₂), which provides good mechanical strength (i.e., attrition resistance) and thermal stability under reaction conditions. Factors such as particle size and shape, metal loading and dispersion, nature of the support and strength of metal-support interactions have been widely studied over supported cobalt-based catalysts [3,4]. Many interesting studies have been devoted to ascertain the influence of oxide support on the FT activity, selectivity, and stability of cobalt catalysts.

              Information regarding the influence of CeO₂ during FT synthesis is scarce and only limited data are available in the open literature considering the use of ceria as either a promoter or a support. Recently, Spadaro et al. [5] obtained a higher selectivity to oxygenates using CeO₂ supported cobalt catalysts at very low CO conversions (~1.6); however, the authors did not provide an explanation regarding their mechanism of formation. In general, the amount of oxygenates produced under typical cobalt FT synthesis conditions is very small, and they are therefore often neglected. The objective of the present study is to examine the FT activity and selectivity of Pt promoted cobalt catalysts supported over high surface area ceria at moderate temperatures and pressures and compare oxygenates selectivity to a standard Pt promoted Co/γ-Al₂O₃ catalyst.  In addition, different cobalt loadings were used to achieve different cobalt particle sizes, and thus, different degrees of interaction with the support.  In this way, the aim was to assess the plausibility of a metal-support bifunctional pathway in the formation of higher alcohols.  From this analysis, a plausible mechanism (Scheme 1) is proposed for the formation of oxygenates over Pt promoted Co-Ceria catalysts.

Scheme 1 A plausible pathway of alcohol formation on Pt promoted Co-CeO₂ catalysts.

Acknowledgements

The work carried out at the CAER was supported in part by funding from the Commonwealth of Kentucky. 

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