The
formation of mixed-metal cobalt oxides, representing potential metal-support
compounds for cobalt-based catalysts, has been observed at high conversion
levels in the Fischer-Tropsch synthesis over metal oxide-supported cobalt
catalysts. An often observed increase in the carbon dioxide selectivity at
Fischer Tropsch conversion levels above 80% has been suggested to be inter-linked to the formation of water-gas shift active oxidic cobalt species. Mixed-metal
cobalt oxides, namely cobalt aluminate and cobalt titanate, were therefore
synthesised and tested for potential catalytic activity towards the water-gas
shift reaction. We present a preparation route for amorphous mixed-metal oxides
<i>via</i> thermal treatment of metal
precursors in benzyl alcohol. Calcination of the as prepared nanoparticles
results in highly crystalline phases. The nano-particulate mixed-metal cobalt
oxides were thoroughly analysed by means of X-ray diffraction, Raman
spectroscopy, temperature-programmed reduction, X-ray absorption near edge
structure spectroscopy, and high-resolution scanning transmission electron
microscopy. This complementary characterisation of the synthesised materials
allows for a distinct identification of the phases and their properties. The cobalt
aluminate prepared has a cobalt-rich composition (Co<sub>1+<i>x</i></sub>Al<sub>2-<i>x</i></sub>O<sub>4</sub>)
with a homogeneous atomic distribution throughout the nano-particulate
structures, while the perovskite-type cobalt titanate (CoTiO<sub>3</sub>)
features cobalt-lean smaller particles being associated with larger ones with
an increased concentration of cobalt. The cobalt aluminate prepared showed no
water-gas shift activity in the medium-shift temperature range, while the
cobalt titanate sample was shown to catalyse the conversion of water and carbon
monoxide to hydrogen and carbon dioxide after an extended activation period. However,
this perovskite underwent vast restructuring forming metallic cobalt, a known
catalyst for the water-gas shift reaction at temperatures exceeding typical
conditions for the cobalt-based Fischer-Tropsch synthesis, and anatase-TiO<sub>2</sub>
<i>via</i> a partial reduction of the
mixed-metal cobalt oxide and segregation as identified by means of post-run
X-ray diffraction.
Hydrogen Production from the Water-Gas Shift Reaction on Iron Oxide Catalysts
