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Freshly H2-reduced catalyst samples and FTS catalyst samples (i.e., freshly reduced and immediately exposed to the onset of FTS conditions corresponding to 50 % CO conversion) were prepared. Each sample was coated in situ using molten polywax and solidified so that an air-protected sample was obtained, which was stored in inert gas. XAS was utilize...
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... the aim of this work being to investigate the susceptibility of small cobalt crystallites to reoxidation at the onset of FT reaction at 50 % CO con- version, these activation temperatures are deemed appropriate. Figure 2 shows normalized XANES spectra of reference compounds (e.g., Co 3 O 4 , CoO, CoAl 2 O 4 , and Co 0 foil). A major characteristic of Co 0 is the edge peak at 7,709 eV. ...
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Citations
... To increase the extent of reduction, promoters such as Pt, Re, and Ru are used [3][4][5][6][7][8][9][10][11][12][13][14][15]. If the Co particles are too small (e.g., <2-4 nm), however, they are susceptible to deactivation phenomena, including oxidation and cobalt-support compound formation [16][17][18][19][20][21]. ...
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.
... For the reduction of CoO to Co 0 , the reduction temperatures of Co 3 O 4 /SiC samples are increased in the order of PCT15 > SG15 > IWI15, which is in accordance with the order of Co 3 O 4 grain size reduction (see Table 1). It has been reported that the particle size affects significantly the second reduction peak of CoO to metallic Co 0 [25], and the reduction of larger particles is easier than that of smaller particles, so for IWI15 and SG15 lower reduction temperatures are obtained. Interestingly, although the crystallite sizes of Co 3 O 4 in IWI15 is nearly 1.6 times that in SG15, the second reduction peak temperature for IWI15 is only 2°C lower than that for SG15. ...
... These complications highlight a complex mechanism that may be related to chemical-assisted sintering of Co FTS catalysts through a combination of the effect of CoO reduction during the initial activation of the catalysts and water exposure during operation. First, CoO, present either due to incomplete reduction of the catalysts [368] or oxidation of the small (<2 nm) crystallites as suggested by Davis' group [369,370] can apparently increase the sintering rate due to mobility that allows them to aggregate into larger CoO clusters that are subsequently reduced to metallic Co, as inferred from evidence presented in a number of studies [79,362,[368][369][370][371]. Primarily, X-ray absorption near edge (XANES) analysis shows simultaneous increasing extent of reduction and increasing Co-Co coordination, due both to removal of oxygen and increases in particle size. ...
... These complications highlight a complex mechanism that may be related to chemical-assisted sintering of Co FTS catalysts through a combination of the effect of CoO reduction during the initial activation of the catalysts and water exposure during operation. First, CoO, present either due to incomplete reduction of the catalysts [368] or oxidation of the small (<2 nm) crystallites as suggested by Davis' group [369,370] can apparently increase the sintering rate due to mobility that allows them to aggregate into larger CoO clusters that are subsequently reduced to metallic Co, as inferred from evidence presented in a number of studies [79,362,[368][369][370][371]. Primarily, X-ray absorption near edge (XANES) analysis shows simultaneous increasing extent of reduction and increasing Co-Co coordination, due both to removal of oxygen and increases in particle size. ...
Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the practice of industrial catalytic processes. Costs to industry for catalyst replacement and process shutdown total tens of billions of dollars per year. [...]
... However, surface oxidation was only clearly evident by STEM-EELS after an excursion to 340 °C and 100% CO conversion. B.H. Davis and coworkers exposed a freshly reduced catalyst directly to a water vapor pressure equivalent to 50% conversion [56]. They observed oxidation of a fraction of the smaller cobalt crystallites when supported on alumina or activated carbon, and recommended that careful crystallite size management is required for a commercial catalyst. ...
Deactivation of commercially relevant cobalt catalysts for Low Temperature Fischer-Tropsch (LTFT) synthesis is discussed with a focus on the two main long-term deactivation mechanisms proposed: Carbon deposits covering the catalytic surface and re-oxidation of the cobalt metal. There is a great variety in commercial, demonstration or pilot LTFT operations in terms of reactor systems employed, catalyst formulations and process conditions. Lack of sufficient data makes it difficult to correlate the deactivation mechanism with the actual process and catalyst design. It is well known that long term catalyst deactivation is sensitive to the conditions the actual catalyst experiences in the reactor. Therefore, great care should be taken during start-up, shutdown and upsets to monitor and control process variables such as reactant concentrations, pressure and temperature which greatly affect deactivation mechanism and rate. Nevertheless, evidence so far shows that carbon deposition is the main long-term deactivation mechanism for most LTFT operations. It is intriguing that some reports indicate a low deactivation rate for multi-channel micro-reactors. In situ rejuvenation and regeneration of Co catalysts are economically necessary for extending their life to several years. The review covers information from open sources, but with a particular focus on patent literature.
... These complications highlight a complex mechanism that may be related to chemical-assisted sintering of Co FTS catalysts through a combination of the effect of CoO reduction during the initial activation of the catalysts and water exposure during operation. First, CoO, present either due to incomplete reduction of the catalysts [368] or oxidation of the small (<2 nm) crystallites as suggested by Davis' group [369,370] can apparently increase the sintering rate due to mobility that allows them to aggregate into larger CoO clusters that are subsequently reduced to metallic Co, as inferred from evidence presented in a number of studies [79,362,[368][369][370][371]. Primarily, X-ray absorption near edge (XANES) analysis shows simultaneous increasing extent of reduction and increasing Co-Co coordination, due both to removal of oxygen and increases in particle size. ...
... These complications highlight a complex mechanism that may be related to chemical-assisted sintering of Co FTS catalysts through a combination of the effect of CoO reduction during the initial activation of the catalysts and water exposure during operation. First, CoO, present either due to incomplete reduction of the catalysts [368] or oxidation of the small (<2 nm) crystallites as suggested by Davis' group [369,370] can apparently increase the sintering rate due to mobility that allows them to aggregate into larger CoO clusters that are subsequently reduced to metallic Co, as inferred from evidence presented in a number of studies [79,362,[368][369][370][371]. Primarily, X-ray absorption near edge (XANES) analysis shows simultaneous increasing extent of reduction and increasing Co-Co coordination, due both to removal of oxygen and increases in particle size. ...
Deactivation of heterogeneous catalysts is a ubiquitous problem that causes loss of catalytic rate with time. This review on deactivation and regeneration of heterogeneous catalysts classifies deactivation by type (chemical, thermal, and mechanical) and by mechanism (poisoning, fouling, thermal degradation, vapor formation, vapor-solid and solid-solid reactions, and attrition/crushing). The key features and considerations for each of these deactivation types is reviewed in detail with reference to the latest literature reports in these areas. Two case studies on the deactivation mechanisms of catalysts used for cobalt Fischer-Tropsch and selective catalytic reduction are considered to provide additional depth in the topics of sintering, coking, poisoning, and fouling. Regeneration considerations and options are also briefly discussed for each deactivation mechanism.
... To probe the role of oxidation, a recent XANES study was made whereby freshly reduced unpromoted and Pt-promoted cobalt/alumina catalysts were subjected to H 2 :CO:H 2 O mixtures typical of the 50% conversion condition of a slurry phase reactor [36] for one hour. A lower-than-commercial loading of 10% cobalt was utilized in order to favor the formation of small cobalt crystallites after activation that fall in the range of being susceptible to reoxidation. ...
... A lower-than-commercial loading of 10% cobalt was utilized in order to favor the formation of small cobalt crystallites after activation that fall in the range of being susceptible to reoxidation. The average cobalt cluster size (i.e., cluster of crystallites) was ~5 nm [36]. Even though the 10%Co/Al 2 O 3 catalyst was reduced at 550 °C as opposed to 400 °C for the 0.5%Pt-10%Co/Al 2 O 3 catalyst, the extent of reduction from XANES indicated that the Pt-promoted catalyst had a higher extent of reduction, as defined by the intensity of the white line. ...
... Even though the 10%Co/Al 2 O 3 catalyst was reduced at 550 °C as opposed to 400 °C for the 0.5%Pt-10%Co/Al 2 O 3 catalyst, the extent of reduction from XANES indicated that the Pt-promoted catalyst had a higher extent of reduction, as defined by the intensity of the white line. When switching to conditions to mimic 50% conversion, the white line intensity in the XANES spectra of both catalysts increased significantly (Figure 4), but the change was more severe for the Pt-promoted catalyst [36]. This reoxidation occurred rapidly, is confined to a fraction of cobalt, and is not associated with the initial decay period commonly observed in FTS reaction tests, which may take on the order of days to establish. ...
This focused review article underscores how metal reduction promoters can impact deactivation phenomena associated with cobalt Fischer-Tropsch synthesis catalysts. Promoters can exacerbate sintering if the additional cobalt metal clusters, formed as a result of the promoting effect, are in close proximity at the nanoscale to other cobalt particles on the surface. Recent efforts have shown that when promoters are used to facilitate the reduction of small crystallites with the aim of increasing surface Co-0 site densities (e. g., in research catalysts), ultra-small crystallites (e. g., < 2-4.4 nm) formed are more susceptible to oxidation at high conversion relative to larger ones. The choice of promoter is important, as certain metals (e. g., Au) that promote cobalt oxide reduction can separate from cobalt during oxidation-reduction (regeneration) cycles. Finally, some elements have been identified to promote reduction but either poison the surface of Co0 (e. g., Cu), or produce excessive light gas selectivity (e. g., Cu and Pd, or Au at high loading). Computational studies indicate that certain promoters may inhibit polymeric C formation by hindering C-C coupling.
Supported nano-sized metal crystallites as catalysts in the Fischer-Tropsch synthesis have become a major research focus due to their high mass specific surface area and resulting lower cost. Such small supported cobalt crystallites have been reported to show a very different resistance with regard to deactivation compared to larger cobalt particles. The Fischer-Tropsch product water is reported to have a severe effect on the deactivation of cobalt-based Fischer-Tropsch catalysts. Compared to other water-induced deactivation mechanisms, hydrothermal sintering of cobalt nanoparticles is fairly well established in literature. A previously hypothesised interconnection between oxidation of cobalt nanoparticles and hydrothermal sintering has – for the first time – been captured in situ in the presented study. High concentrations of water induce oxidation of the cobalt nanoparticles increasing their mobility and resulting in crystallite growth via particle migration and coalescence whilst in the oxidised state. A well-defined model catalyst comprising highly dispersed cobalt nanoparticles on a relatively inert exfoliated graphite support in combination with an in situ magnetometer allowed for these observations, which resulted in irreversible deactivation of the catalyst.
