Scanning electron micrograph of the water-splitting cobalt oxide catalyst.
Hydrogen is a clean fuel, producing only water vapor when it burns. But
generating hydrogen in large quantities and in a “green” fashion is not
Biological photosynthesis includes an efficient reaction step that splits
water into hydrogen and oxygen with the help of catalysts that have been used
as models for synthetic catalysts.
Working at the U.S. Department of Energy’s Advanced Photon Source (APS)
at Argonne National Laboratory, a team of Argonne scientists has determined the
structure of one such catalyst, a complex cobalt oxide. The material is
composed of crystalline domains containing just 13 or 14 cobalt atoms, with
distortions at their boundaries that may be important for its catalytic
The water-splitting cobalt oxide catalyst was discovered serendipitously
about three years ago, in experiments where an electric current ran through a
solution containing dissolved cobalt salts. Because of the amorphous nature of
the catalyst film, the details of its structure have remained unclear.
Previous X-ray and other spectroscopy studies showed that cobalt in the
catalyst exists in an octahedral coordination environment, CoO6,
with Co at the center and O at the vertices. This conformation is analogous to
the lattice sheet domains of the non-catalytic crystalline cobalt oxides, but
is present here in domains of undetermined molecular-scale dimensions.
However, some earlier publications suggested that the X-ray spectroscopy
data could also be interpreted in terms of cobalt oxide cubane structures, Co4O4,
that are analogous to the structure and spectroscopy of the water-splitting
catalyst that occurs in biological photosynthesis systems.
The Argonne researchers prepared the cobalt oxide material by
electrodeposition from a solution buffered with potassium phosphate.
Working at X-ray Science Division X-ray beamline 11-ID-B at the APS, they
conducted X-ray diffraction studies on samples in the form either of dry powder
or aqueous slurry.
The amorphous nature of the catalyst’s composition makes it impossible to
infer definitive crystallographic structure from standard diffraction
measurements. Instead, the researchers used a straightforward mathematical
technique—essentially a Fourier transform—to turn the observed X-ray scattering
data into the electron pair density distribution function, which appears as a
series of peaks corresponding to a set of distances between the atoms in the
Interpreting the results was then a matter of modeling the expected pair
distribution function (PDF) from a supposed structure and seeing how well it
fit the experimental data.
The researchers found that the best fit came from structures with domains
consisting of CoO6 octahedra with shared edges, and careful analysis
allowed them to deduce the most likely number of cobalt atoms in the domains.
Domains with 12 cobalt atoms or fewer didn’t fit the PDF for interatomic
distances greater than about 10 Å, while domains with 15 or more cobalt atoms
failed to match the PDF at smaller distances.
The team thus concluded that each domain in the amorphous material must
have 13 or 14 cobalt oxide octahedra.
The model-fitting exercise also revealed some characteristic differences
between the pair distribution functions observed experimentally and those
calculated using crystalline lattice models for the domains, demonstrating that
the structure for the lattice domains in the catalyst differs from those of
non-catalytic crystalline cobalt oxides.
The analysis showed that slight alterations to the coordination
geometries for the oxygen atoms bound to the cobalt atoms at the edges of the
domains (for example a 4 degree shift from their normal positions)
could account for most of the discrepancy between the experimental and model
Cubane-type inclusions were also found to be possible additional sparse
defects that could bring experiment and model into better alignment. Also
significant was the finding that phosphate anions, which are required for
catalytic activity, were detected as disordered constituents of the film.
Ongoing work is investigating the correlation between structure and
catalytic function of the cobalt oxide catalysts, and developing more highly
The researchers suggest that the distorted cobalt coordination geometries
in the catalyst may be a distinguishing feature that confers the ability to
catalyze the water-splitting reaction compared to the non-catalytic crystalline
cobalt oxides, although Chupas acknowledges that this suggestion is somewhat speculative.
“It will take a while to figure out the [catalytic] mechanism,” he says,
but it’s crucial to know the structure of the material and “this is the best
structure that’s out there right now.”
Source: Argonne National Laboratory