Strong-metal–support interaction by molecular design: Fe–silicate interactions in Fischer–Tropsch catalysts

Metal–support interactions in the form of iron–silicate were investigated by an inverse approach, that is, modification of nano-sized iron oxide with surface silicate groups. The presence of surface silicate groups in the calcined catalyst precursor was confirmed using diffuse reflectance infra-red Fourier transform analysis. The genesis of the various iron phases in the presence of surface silicate groups after H2-activation and the Fischer–Tropsch synthesis was followed. The surface silicate groups are preserved after a hydrogen treatment at 350 °C for 16 h, and these surface ligands are associated with the residual iron oxide phase, wüstite. During the Fischer–Tropsch synthesis, α-Fe is mostly converted into χ-Fe5C2, whereas FeO is the main source for ε-Fe2C. The activity per unit surface area of hexagonal carbide, ε-Fe2C, is ca. 25% higher than that of χ-Fe5C2. The presence of surface silicate ligands on ε-Fe2C results in a further enhancement of the rate per unit surface area of ε-Fe2C by a factor of ca. 3. This is being ascribed to the enhanced availability of hydrogen on the surface due to the presence of the surface silicate groups, which also results in an increase in the methane selectivity, a decrease in the olefin content and a decrease in formation of branched product compounds.

The modification of nano-sized iron oxide crystallites with surface silicate ligands can be used as a model to investigate metal-support interactions. The reduction in hydrogen of iron oxide nano crystallites modified with surface silicate ligands stabilizes the formation of wüstite, which is transformed during the Fischer-Tropsch synthesis into ε-Fe2C possibly via the formation of Fe3O4.Figure optionsDownload high-quality image (60 K)Download as PowerPoint slideHighlights
► Surface ligands on nano-crystallites: a model for strong-metal–support interactions.
► Surface silicates on Fe2O3 stabilizes the formation of wüstite upon H2-activation.
► Under FT-conditions ε-Fe2C is formed from FeO modified with surface silicate groups.
► The presence of surface silicate groups enhances the FT-activity of ε-Fe2C.