Tailoring the product selectivity of Co/SiO₂ Fischer-Tropsch synthesis catalysts by lanthanide doping

The effect of the nature of the lanthanide (Ln = La, Ce, Pr, Sm, Gd) on the structure and reactivity of Co/SiO₂ catalysts for CO hydrogenation (i.e., Fischer Trospch synthesis) was investigated. In-situ temperature programmed reduction with extended x-ray absorption fine structure and x-ray absorption near edge spectroscopy (TPR-EXAFS/XANES) of the structure of both Co and Ln containing phases under activation and CO hydrogenation conditions were performed. Concerning catalyst selectivity (made at comparable conversion levels), while methane selectivity was higher for the Gd-doped catalyst (∼100%, relative) compared to the unpromoted catalyst, the selectivity to olefins plus alcohols and C₂-C₄ products was higher (∼35%, relative), compared to the unpromoted catalyst. The Ce-promoted Co/SiO₂ catalyst presented the highest oxygenate/olefin selectivity (∼40%), among the promoted catalysts tested at a similar conversion level of ∼20%. Under reactive conditions (both following H₂ activation and during CO+H₂ flow), a mixture containing small LnO_X/CoO/Co° nanocrystallites likely constitute the active sites during reaction. These results imply that the presence of the lanthanide likely introduces surface defects in the oxides (LnO_X and/or CoO) located at the metallic cobalt nanoparticle rim which may serve as active sites for active O-containing species (e.g., mobile Type II OH groups) that may either serve as chain termination species, or generate chain terminating species such as formates (i.e., essentially molecularly adsorbed CO) upon CO adsorption.