***hhνν++++ --++++ ----PhotonCapture***Charge SeparationEnergy Storage
H 2 O H 2
CO 2 CH 3 OH
Regeneration
H 2 O O 2Energy Storage
H 2 O H 2
CO 2 CH 3 OH
Regeneration
H 2 O O 2Figure 11 Solar production of fuels by the
following sequence: (1) light absorption, (2) charge
separation, and (3) use of the separated charges to
produce fuel (e.g., H 2 , CH 3 OH) and co-product
(e.g., O 2 from water oxidation). The reactions
shown in the black box require efficient catalysts.reactions must produce the desired fuel (e.g., H 2 ,
CH 3 OH) and a desirable co-product (e.g. O 2 from
water oxidation). In so-called heterogeneous
(insoluble) catalysts, requirements (1) and (2) are
usually implemented by using semiconductor
assemblies, while in homogeneous (soluble)
catalysts these functions are performed by using
molecular assemblies in solution. The critical issue
for requirement (3) is the coupling of the one-
photon light absorption events in (1) and (2) to the
proton-coupled, multi-electron processes required
for catalysis. Most previous efforts have used
catalysts driven by one-electron reactions to avoid
high-energy intermediates. However, fuel
production requires multi-electron oxidation and
reduction reactions, so new catalysts that couple
single-photon events to the accumulation of
multiple redox equivalents are essential.
For solar fuel production to be economically and environmentally attractive, the fuels must be
formed from abundant, inexpensive raw materials such as water or CO 2. The thermodynamics
for generation of fuels such as H 2 , CH 3 OH, or CH 4 by photodecomposition of water or CO 2 from
aqueous solutions are known (Arakawa et al. 2001; Sutin et al. 1997). Assuming a 100% charge
separation efficiency and a catalyst coverage density of 1/nm^2 , the catalyst turnover rate must be
about 100/s in order for the fuel generation/regeneration reactions to keep up with the solar
production of electrons and/or holes. Currently available catalysts for CO 2 reduction or water
oxidation have turnover frequencies that are far below those required for a viable catalyst.
Homogeneous Photocatalytic CO 2 Reduction. Most systems studied to date generally use
transition metal complexes (e.g., containing Ru and Ir) as catalysts for photoreduction of CO 2
because they absorb a significant part of the solar spectrum, have long-lived excited states (about
1 μs), and can transfer electrons to or from small molecules. Typical systems also include a
secondary metal complex (e.g., containing Co) as a co-catalyst to carry out the reduction of CO 2.
Metal hydride complexes are also important because bimolecular reactions of hydrides or their
reactions with H 2 O/H 3 O+ are responsible for the formation of H 2. While both metal hydride and
metal-CO 2 complexes pertinent to some of the systems are known, in no case has the reduction
mechanism been completely determined. For example, the best systems reported thus far show
quantum efficiencies as high as 38% using a Re sensitizer, but have disappointing turnover
frequencies of CO formation of <10/h.
Homogeneous Photocatalytic Water Oxidation. Photochemical splitting of water into
molecular H 2 and O 2 has yet to be realized on a large, cost-effective, efficient scale. Yet recent
advances related to this field have been made, including the reported use of a compound
containing two interacting Rh atoms to photocatalyze the reduction of HBr to H 2 (Heyduk and