Basic Research Needs for Solar Energy Utilization

CATALYSTS FOR CO^2 REDUCTION

Photo-driven catalysts for CO 2 reduction have made use of

the versatile photochemistry and redox properties of Ru

complexes. Photo-excitation of Ru(bpy) 3 2+ (below, top panel)

results in formation of an excited state that reacts readily with

the sacrificial donor triethanolamine. The reduced Ru(bpy) 3 +

can then act as a source of electrons to drive a catalytic cycle

of the type shown in the bottom panel below (Pugh et al.

1991). In this cycle, another Ru complex catalyzes the

reduction of CO 2 to formate ion. Input of electrons is required

at two points in the overall cycle. The overall mechanism for

the catalytic reduction of CO 2 is complex, so that a great deal

of work remains to find optimal catalysts that are well-

coupled to the photochemical agents that use solar energy to

provide a source of electrons to drive the catalysts.

N

N N

N

Ru

+

CO

H

N

N N

N

Ru

0

CO

H

N

N N

N

Ru

0

CO

NCCH 3

N N

N

N

Ru

0

CO

O

-.

-.

O

H

-.

-.

O-

O

CH 3 CN

CO 2

e-

e-

H 2 O, CO 2

CH 3 CN, HCO 3 -

N

N N

N

N N

Ru

2+

N

N N

N

N

N

Ru

+

N(CH 2 CH 2 OH) 3 +N(CH 2 CH 2 OH) 3

decompositionproducts

hν

CATALYSTS FOR WATER OXIDATION

Catalysts with sufficient oxidizing power to split water remain

relatively rare. Yet, several recent catalysts based on Mn and

Ru have demonstrated water oxidation following addition of

strong oxidants to access the higher oxidation states of these

metals. As an example, the top panel shows the structure of

a Mn 2 complex that generates O 2 obtained from water using

NaOCl as an added oxidant (Limberg et al. 1999). The

mechanism of catalysis is shown in the lower panel. Thus far,

the turnover frequencies and the stability of water oxidation

catalysts remain low. In addition, coupling of a photo-driven

oxidant to these catalysts has not been accomplished.