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

• Published in: Catalysis today Vol. 343; pp. 80 - 90
• Main Authors: Ribeiro, Mauro C., Gnanamani, Muthu K., Garcia, Richard, Jacobs, Gary, Rabelo-Neto, Raimundo C., Noronha, Fabio B., Gomes, Igor F., Davis, Burtron H.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.03.2020
• Subjects: Ceria; Cobalt; Fischer-Tropsch synthesis; Higher oxygenate synthesis; Lanthanides; Higher oxygenate synthesis; Fischer-Tropsch synthesis; Lanthanides; Cobalt; Ceria
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2018.10.064

[Display omitted] •Lanthanide promotion of Co/SiO2 catalysts increases selectivity toward olefins/oxygenates.•Lanthanide promotion of Co/SiO2 catalysts increases selectivity toward light products and methane.•The Ln promoter limits CoO reduction and contribute to smaller sized Co0 nanoparticles.•Reduced defects in LnOX, measured by XANES, contribute to the observed shifts in the selectivity. 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 LnOX/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 (LnOX 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.