Electron energy loss spectroscopy (EELS), X-ray photoelectron
spectroscopy (XPS), and transmission electron microscopy have been used to
study iron catalysts for Fischer–Tropsch synthesis. When
silica-containing iron oxide precursors are activated in flowing CO, the
iron phase segregates into iron carbide crystallites, leaving behind some
unreduced iron oxide in an amorphous state coexisting with the silica
binder. The iron carbide crystallites are found covered by characteristic
amorphous carbonaceous surface layers. These amorphous species are
difficult to analyze by traditional catalyst characterization techniques,
which lack spatial resolution. Even a surface-sensitive technique such as
XPS shows only broad carbon or iron peaks in these catalysts. As we show
in this work, EELS allows us to distinguish three different carbonaceous
species: reactive amorphous carbon, graphitic carbon, and carbidic carbon
in the bulk of the iron carbide particles. The carbidic carbon K edge
shows an intense “π*” peak with an edge shift of about 1
eV to higher energy loss compared to that of the π* of amorphous
carbon film or graphitic carbon. EELS analysis of the oxygen K edge allows
us to distinguish the amorphous unreduced iron phase from the silica
binder, indicating these are two separate phases. These results shed light
onto the complex phase transformations that accompany the activation of
iron catalysts for Fischer–Tropsch synthesis.