Investigations of element spatial correlation in Mn-promoted Co-based Fischer–Tropsch synthesis catalysts

Abstract

a b s t r a c tMaking connections between performance and structure in bimetallic catalysts requires knowledge of how the two elements are spatially associated. Elemental maps obtained by analytical TEM methods are an invaluable tool for identifying the location of different elements, but for many samples, visual inspection of elemental maps is insufficient for assessing the degree of element spatial correlation. This is particularly true for beam-sensitive materials where short mapping acquisition times lead to images with high noise and low color depth. In these situations, statistical analysis of elemental maps can be used to identify spatial correlations among the elements in a sample. In this work, the relationship between catalyst performance and bimetallic spatial association was explored using Mn-promoted Cobased Fischer-Tropsch synthesis catalysts prepared by different pretreatment methods. Mn was used as a catalyst additive to suppress methane formation. Catalysts that underwent calcination before reduction produced more methane and fewer long-chain hydrocarbons than catalysts that were directly reduced. The extent to which Co and Mn were spatially associated was assessed using correlation metrics, colocation plots, and histograms generated using data from STEM-EDS maps. Although both catalysts yielded visually similar elemental maps, the results of statistical analysis suggested that the calcined catalyst exhibited greater spatial segregation between the Co and Mn. These findings support the hypothesis that having Mn in close proximity to the Co is essential for the manifestation of Mn promotion effects in Co-based FTS catalysts.

Rate and selectivity hysteresis during the carbon monoxide hydrogenation over promoted Co/MnOx catalysts

“…The authors suggested that Mn facilitates the dissociation and disproportionation of CO and suppresses H 2 adsorption on the catalyst surface, thereby creating a relatively C-rich and H-lean surface chemical environment. Similar arguments were previously advanced by Johnson et al 30,31 to explain the high selectivity of C 5+ products and olefins, at the expense of methane. Overall, however, little is known about how the kinetics of the Co–Co 2 C phase transformation drive both reaction rate and selectivity.…”

“…In a previous study with Mn(II)-promoted Co/SiO 2 , Johnson et al 30,31 considered the high selectivity of C 5+ formation with their catalyst as being due to an increase of CO ad concentrations, i.e., a decrease of the H ad /CO ad ratios, following the strong Lewis acid–base interaction between Mn(II) and CO ad . While the present paper provides clear evidence for the partial pressure ratio P H2 /P CO to have a profound influence on the selectivity of the reaction, the amounts of Mn(II) (in MnO) or Mn(II)–Mn(IV) (in Mn 5 O 8 ) employed in our study are far beyond the low Mn(II) promoter concentrations in Co/SiO 2 .…”

Effects of Lewis acidity of metal oxide promoters on the activity and selectivity of Co-based Fischer–Tropsch synthesis catalysts

“…For example, work by de Jong et al (4) (5) and Weckhuysen et al (6) (7) (8) (9) (10) has reported that promotion with MnO enhances the rate of CO consumption and C 5+ selectivity. Studies by these authors and others have shown that these effects are a consequence of good contact between metallic Co and MnO (11) (12).…”

Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer–Tropsch Synthesis Catalysts

“…Ali et al speculated that Co active sites at the interface with ZrO2 may exhibit enhanced activity toward CO hydrogenation (13), a hypothesis that originates from studies of metal oxide-promoted Rh catalysts (20) (21). This hypothesis has also been used to rationalize the catalyst structure-performance relationships resulting from Mn promotion of Co-based FTS catalysts (22) (23). In this study, we demonstrate that Zr promotes the catalyst activity and selectivity in a manner analogous to promotion by Mn.…”