4 Carbon Dioxide Reduction Catalysts
A number of products including CO, formate, methanol, and methane can be
formed by carbon dioxide reduction. Furthermore, since the potential for reduction
of protons to hydrogen is typically 100 mV more favorable, hydrogen evolution
usually competes with CO 2 reduction at transition metal sites. Thus, development
of selective, fast, and efficient carbon dioxide reduction catalysts has proven a
challenging research area. Enzymes carefully couple proton and electron movement
to generate multielectron-reduced states and catalysts specific for one product.
Control of proton movement has also proven an important theme in preparation
of synthetic catalysts. In this section, we start by considering some of the better
precious metal-based CO 2 reduction catalysts and then describe how these have
been modified to generate catalysts based on more abundant metals by incorporating bio-inspired features.
4.1 Structural Models of CODHs
Structural models of both aerobic (MoCu) and anaerobic Ni–CODHs have recently
been reviewed by Majumdar (see Fig. 5 for active site structures) [147]. The groups
of Holm, Tatsumi, and Young [148–150] have developed several new synthetic
approaches to create sulfide-bridged binuclear MoCu complexes as models for the
aerobic enzymes, and, as a result of this work, two significant challenges have come
to light. First, the formation of a Mo(μ 2 -S) 2 Cu rhomb is highly favored so that
forming singly bridged complexes is difficult. Second, Mo(VI) is unstable to
autoreduction, especially in the presence of anionic sulfur ligands. The latter
problem can be overcome by working with W complexes, but these seldom have
the same reactivity as the Mo analogues. With respect to the anaerobic CODHs,
attempts to mimic the spectroscopic properties of the NiFeS active site with
synthetic complexes were underway even before the determination of the active
site structure. There have been two major approaches: construction of [NiFe 3 S 4 ]
clusters, spearheaded largely by the Holm group [151–155], and efforts to make
heterobimetallic [NiFe] complexes, research closely related to the modeling of
[NiFe]-hydrogenases (see Sect. 3). Unfortunately, none of the complexes produced
in these efforts are reported to either reduce CO 2 or oxidize CO.
4.2 Functional Models of CODH: Molecular Electrocatalysts
for Reduction of CO 2 to CO
Functional models of CODH, i.e., molecular catalysts for CO 2 reduction to CO,
have been constructed using the precious metals Re and Ru as well as the first row
transition metals Co, Fe, Mn, and Ni.
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L. Gan et al.
A number of products including CO, formate, methanol, and methane can be
formed by carbon dioxide reduction. Furthermore, since the potential for reduction
of protons to hydrogen is typically 100 mV more favorable, hydrogen evolution
usually competes with CO 2 reduction at transition metal sites. Thus, development
of selective, fast, and efficient carbon dioxide reduction catalysts has proven a
challenging research area. Enzymes carefully couple proton and electron movement
to generate multielectron-reduced states and catalysts specific for one product.
Control of proton movement has also proven an important theme in preparation
of synthetic catalysts. In this section, we start by considering some of the better
precious metal-based CO 2 reduction catalysts and then describe how these have
been modified to generate catalysts based on more abundant metals by incorporating bio-inspired features.
4.1 Structural Models of CODHs
Structural models of both aerobic (MoCu) and anaerobic Ni–CODHs have recently
been reviewed by Majumdar (see Fig. 5 for active site structures) [147]. The groups
of Holm, Tatsumi, and Young [148–150] have developed several new synthetic
approaches to create sulfide-bridged binuclear MoCu complexes as models for the
aerobic enzymes, and, as a result of this work, two significant challenges have come
to light. First, the formation of a Mo(μ 2 -S) 2 Cu rhomb is highly favored so that
forming singly bridged complexes is difficult. Second, Mo(VI) is unstable to
autoreduction, especially in the presence of anionic sulfur ligands. The latter
problem can be overcome by working with W complexes, but these seldom have
the same reactivity as the Mo analogues. With respect to the anaerobic CODHs,
attempts to mimic the spectroscopic properties of the NiFeS active site with
synthetic complexes were underway even before the determination of the active
site structure. There have been two major approaches: construction of [NiFe 3 S 4 ]
clusters, spearheaded largely by the Holm group [151–155], and efforts to make
heterobimetallic [NiFe] complexes, research closely related to the modeling of
[NiFe]-hydrogenases (see Sect. 3). Unfortunately, none of the complexes produced
in these efforts are reported to either reduce CO 2 or oxidize CO.
4.2 Functional Models of CODH: Molecular Electrocatalysts
for Reduction of CO 2 to CO
Functional models of CODH, i.e., molecular catalysts for CO 2 reduction to CO,
have been constructed using the precious metals Re and Ru as well as the first row
transition metals Co, Fe, Mn, and Ni.
256
L. Gan et al.