Basic Research Needs for Solar Energy Utilization

low toxicity, and processibility. To meet these challenges, the tools of theory and experiment
must be brought together to understand and control catalytic processes.

Blueprint for Fuel-forming Catalyst Design

The essential requirements for electron-transfer driven, catalyzed production of solar fuels (such
as H 2 or a carbon based liquid fuel) are captured in Figure 43. The left-hand system features the
oxidation of water, and the right-hand side features (for purposes of illustration) reduction of
CO 2 to formic acid. Such systems share common features and illustrate a generic, systems
approach to successful catalyst design. The essential elements are as follows.

(1) An electron transfer interface. Catalyst systems for oxidation or reduction

are driven by electron transfer to or from an electron transfer interface. The

source of electrons is generic, potentially from molecular excitation-electron

transfer, a photovoltaic source, or even the excitation-electron transfer

apparatus of the natural photosynthetic apparatus.

(2) Proton-coupled electron transfer (PCET) for redox leveling and proton

addition or removal. The gain or loss of protons, which prevents the

accumulation of charge, is required for multi-electron transfer in order to

avoid high-energy proton intermediates, thereby reducing reaction barriers.

(3) Catalysis via atom, ion transfer, bond formation and breaking. The key

elements at this site are the utilization of atom (e.g., O or H) or ion (e.g., H-)

transfer reactions that carry out the complex chemical transformations

required with reaction barriers sufficiently low to ensure facile reactions on

the sub-millisecond time scale. All of these elements must be spatially

arranged to couple efficiently to ultimately generate fuel.

Blueprint for Fuel-Forming Catalyst Design

Oxidations Reductions

2H 2 O

O 2

4H+

4e–

CO 2

HCO 2 H

2e–

2H+

1 3 1 3

stepping beyond Marcus (1e–)

atom transfer

(bond-breaking/making)

(^1) multi-electron 3
transfer
(^2) proton-coupled
electron transfer
2
2
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Blueprint for Fuel-Forming Catalyst Design
Oxidations Reductions
2H 2 O
O 2
4H+
4e–
2H 2 O
O 2
4H+
2H 2 O
O 2
4H+
4e–
CO 2
HCO 2 H
2e–
2H+
CO 2
HCO 2 H
2e–
2H+
11 33 11 33
stepping beyond Marcus (1estepping beyond Marcus (1e––))
atom transfer
(bond-breaking/making)
(^1) multi-electron 3
transfer
(^2) proton-coupled
electron transfer
atom transfer
(bond-breaking/making)
(^1) multi-electron 33
transfer
(^11) multi-electron
transfer
(^22) proton-coupled
electron transfer
222
222
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Photoinduced
Charge Separation
Assembly
Figure 43 A blueprint for catalyst design detailed for both oxidation
and reduction catalysts (Cat = catalyst)