Abstract
Different low-cost cobalt precursors (acetate, chloride) and thermal treatments (air calcination/H2 reduction versus direct H2-activation) were investigated to alter the interaction between cobalt and silica. H2-activated catalysts prepared from cobalt chloride had large Co0 particles (XRD, chemisorption) formed by weak interactions between cobalt chloride and silica (temperature programmed reduction (TPR), TPR with mass spectrometry (TPR-MS), TPR with extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES) techniques) and retained Cl-blocked active sites, resulting in poor activity. In contrast, unpromoted Co/SiO2 catalysts derived from cobalt acetate had strong interactions between Co species and silica (TPR/TPR-MS, TPR-EXAFS/XANES); adding Pt increased the extent of the Co reduction. For these Pt-promoted catalysts, the reduction of uncalcined catalysts was faster, resulting in larger Co0 clusters (19.5 nm) in comparison with the air-calcined/H2-activated catalyst (7.8 nm). Both catalysts had CO conversions 25% higher than that of the Pt-promoted catalyst prepared in the traditional manner (air calcination/H2 reduction using cobalt nitrate) and three times higher than that of the traditional unpromoted Co/silica catalyst. The retention of residual cobalt carbide (observed in XANES) from cobalt acetate decomposition impacted performance, resulting in a higher C1–C4 selectivity (32.2% for air-calcined and 38.7% for uncalcined) than that of traditional catalysts (17.5–18.6%). The residual carbide also lowered the α-value and olefin/paraffin ratio. Future work will focus on improving selectivity through oxidation–reduction cycles.
Highlights
Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed reaction used to produce high-value synthetic hydrocarbons, which are further upgraded to produce transportation fuels and chemicals from fossil and renewable sources [1]
While minor pore blocking occurred for all cobalt chloride-derived catalysts, the extent of pore blocking was slightly higher for cobalt acetate-derived catalysts, especially with air calcination
Cobalt chloride is an even worse compound than the traditional one, Co3O4, derived from the air calcination of cobalt nitrate, the latter of which results in large Co0 particles (e.g., 24–52 nm) on silica following activation in H2
Summary
Fischer-Tropsch synthesis (FTS) is a heterogeneously catalyzed reaction used to produce high-value synthetic hydrocarbons, which are further upgraded to produce transportation fuels and chemicals from fossil and renewable sources [1]. Improving CO conversion on a per gram of catalyst basis requires the development of catalysts with higher surface Co0 active site densities [3,4]. In conventional Co-based FTS catalysts, the Co0 active site density is typically improved by decreasing the Co0 particle size and increasing the extent of the reduction of cobalt oxides prepared from the calcination of cobalt nitrate particles. The resulting average size of Co0 clusters following the activation of the catalyst in H2 depends on the interaction between cobalt oxide species and the support. Stronger interactions between the support and cobalt oxides, such as those observed with 39 catalysts [4,5], result in a smaller average Co0 nanoparticle size following H2 activation but a lower extent of reduction. If the Co particles are too small (e.g.,
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