ISIS Report 01/07/09
Making Fuel from Water
An efficient and robust catalyst for oxidizing water brings us closer
to converting sunlight into fuel Dr.
Mae-Wan Ho
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The holy grail of artificial photosynthesis is to mimic and improve on the
green plant’s ability to turn sunlight directly into electrochemical energy
that can be used as fuel [1] (Harvesting
Energy from Sun with Artificial Photosynthesis, SiS 43). Research
and development in this area within the OECD (Organisation for Economic Co-operation
and Development) countries date back to the 1970s; and major efforts have been
renewed by the United States Department of Energy (DoE) since 2007 [2].
These efforts are paying off. Important
progress has been made by researchers Heinz Frei and Feng Jiao at DoE’s
Lawrence Berkeley National Laboratory recently, bringing the dream of making
fuel from water a closer to market. They’ve found that nano-sized crystals of cobalt oxide improves the status
of the art by 1 550-fold
Effective photo-oxidation requires
a catalyst that is both efficient in using solar photons and fast enough to
keep up with the solar flux to avoid wasting those photons. Clusters of cobalt
oxide nanocrystals are sufficiently efficient and fast, and also robust and
abundant,” said Frei [3]. “They perfectly fit the bill.”
Efficient and robust catalysts required
The direct conversion of carbon dioxide and water to fuel depends on the availability
of efficient and robust catalysts for the photochemical transformations [4]
(see Splitting Water with
Ease, SiS 43). Catalysts need to have high turnover frequency (TOF)
and density to keep up with the solar flux at ground level (1 000 Wm-2)
to avoid wasting incident solar photons. For example, a catalyst with a TOF
of 100 s-1 requires a density of one catalytic site per square nanometre.
Catalysts with lower rates or taking up a larger space will require
a high surface area nanostructure support that provides tens to hundreds of
catalytic sites per square nanometre. Furthermore, catalysts need to work
close to the thermodynamic potential of the redox reaction [1] so that a maximum
fraction of the solar photon energy is converted to chemical energy. Stability
considerations favour all-inorganic materials, as does the ability to withstand
harsh reaction conditions of pH or temperature.
For the water oxidation half reaction, Jiang and Frei had found
that iridium oxide fulfils these requirements in robustness, and has a reported
TOF of 40 s-1 for IrO2 colloidal particles suspended
in water. The catalyst was driven by a [Ru3+ (bpy)3]
unit (bpy, 2,2-bipyridine), generated photochemically with visible light using
the established [Ru2+(bpy)3]/persulphate (electron donor/acceptor)
system and a modest overpotential of 0.37V. (The overpotential is the potential
in excess of the theoretical electrochemical potential of 1.23V required [1]
due to inefficiencies in the system.)
The researchers have previously demonstrated that the all-inorganic
IrO2 nanoclusters (~ 2nm) directly coupled to a single centre chromium(VI)
or a binuclear TiCrIII charge-transfer chromophore (a chemical
group that gives colour to the molecule) [4] gave oxygen evolution under visible
light with good quantum yield. While iridium oxide closely approaches the
efficiency and stability required as catalyst for water oxidation, iridium
is the least abundant metal on earth and is therefore not suitable for use
on a very large scale. So Jiao and Frei explored more abundant metals, inspired
by nature’s MnCa cluster of photosystem II; nature tends to use the most abundant
materials [5] (Living with Oxygen, SiS
43). So they focussed on Co3O4 nanoclusters, and struck
gold [6].
Nano-structured cobalt oxide the answer
To form the Co3O4 nanoclusters, they used mesoporous
silica (SBA-15) as the scaffold. The mesocopic structure of the silica consists
of hollow channels connected by micropores. The CO3O4
clusters are formed exclusively inside the channels as parallel bundles of
nanorods linked by short bridges, formed by CO3O4 growth
in the micropores interconnecting the mesoscale channels. They loaded the
silica at 4.2 and 8.6 percent by weight of CO3O4 in
wet impregnation.
Transmission electron microscope images showed that the average
spheroid-shaped bundle of CO3O4 has a short diameter
of 35 nm and a long diameter of 65 nm for the sample prepared with 4.2 percent
CO3O4; for the 8.6 percent sample, the short and long
diameters were 65 and 170nm respectively. X-ray crystallographic analysis
showed that the 4.2 percent samples were poorly crystallized, while the 8.6
percent sample corresponds to a 7.6 nm diameter rod structure. The 4.2 percent
sample gave the highest rate of oxygen evolution when tested at pH 5.8 and
22 C (with an overpotential of 0.35V), about 40 percent higher than the 8.6
percent sample. The rate was linear for the first 30 minutes before gradually
levelling off. When fresh Na2S2O8 electron
acceptor was added and the pH value readjusted, oxygen evolution resumed at
the initial rate. This finding confirmed that the slowdown was principally
due to the consumption of the persulfate acceptor, and demonstrated that the
activity of the CO3O4 nanoclusters did not degrade during
photocatalysis in the several hours investigated.
In comparison, NiO nanocrystals in silica or micron sized CO3O4
particles were not effective. An estimated TOF of 1 140 s-1 per CO3O4
cluster was obtained in the 4.2 percent sample. The calculation is based on
the geometry of the bundles of CO3O4 nanorods, bundle
diameter 35 nm, rod diameter 7.6 nm, typically 14 rods per bundle, average
rod length 50 nm. For the larger CO3O4 clusters (8.6
percent) the estimated TOF is 3450 s-1. The calculation assumed CO3O4
nanorod spheroid bundles of 48 per bundle, rod diameter 7.6 nm, average rod
length 130 nm. The oxygen yield was 65 times smaller for the aqueous suspension
of 200 mg of bare CO3O4 particles compared with the
4.2 percent nanocrystals impregnated in silica. When normalised to the same
amount of CO3O4 the O2 yield for the silica
impregnated nanocrystals at 4.2 percent exceeds that of the bare micron-sized
particles by a factor of 1 550.
This was the first observation of efficient water oxidation,
which is only half the artificial photosynthesis reaction. Nevertheless, the
abundance of the metal oxide, the stability of the nanoclusters under use,
the modest overpotential required, and the mild pH and temperature conditions
make it a promising catalytic component for developing a viable integrated
system for converting sunlight to fuel.
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