steric hindrance for an electrophilic attack of CO 2 to the μ 3 -S ligand
({[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ](CO 2 )}
+
). In addition, a two-electron reduction and subsequent reaction with CO 2 provided anionic species {[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ]
(CO 2 ) 2 }
À . The coupling of two CO 2 molecules bonded on adjacent μ 3 -S and Ir
centers in {[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ](CO 2 ) 2 }
À produced C 2 O 4
2À . It should also be
noted that the electrochemical conversion of CO 2 to C 2 O 4
2À has been achieved using
Fe [38], Ni [39, 40], Cu [41], and Ru complexes [42], in addition to [(Cp*Ir) 3 (μ 3 -
S) 2 ]
2+ .
5 Conclusion
The development of iridium-based catalysts for the electrolytic reduction of CO 2 has
been conducted for more than 30 years. Due to the increasing importance of CO 2
utilization technologies in recent years, selective CO 2 conversion has become
possible, with dramatically improved catalyst performances being obtained through
the development of new ligands. In addition, the overpotential and current density
for the electroreduction of CO 2 have also received attention in terms of the energy
consumption and reaction rate. In terms of the overpotential, it is important to
understand the function of metal-hydride species, as discussed in this chapter.
Furthermore, it has been found that the reaction system, and more specifically the
electrode materials and mass diffusion, has a significant effect on the reaction
efficiency. Hence, to achieve efficient reactions with high current densities and
low overpotentials, the design of an appropriate catalyst and overall electrochemical
system must be optimized.
In the context of future catalyst developments, compounds that require C-O bond
cleavage and multiple C-H bond formation, such as CH 3 OH, will be targeted.
Indeed, the catalytic functions of single C-H bond formation and C-O bond cleavage,
such as in the cases of HCO 2
À formation and CO formation, have been studied. In
terms of the synthesis of CH 3 OH, the integration of individual catalyst design
principals will be necessary.
References
1. Rogelj J, den Elzen M, Höhne N, Fransen T, Fekete H, Winkler H, Schaeffer R, Sha F, Riahi K,
Meinshausen M (2016) Nature 534:631–639
2. Ilas A, Ralon P, Rodriguez A, Taylor M (2018) International Renewable Energy Agency
(IRENA). Abu Dhabi, UAE, pp 1–160
3. Gotz M, Lefebvre J, Mors F, Koch AM, Graf F, Bajohr S, Reimert R, Kolb T (2016) Renew
Energy 85:1371–1390
4. Schemme S, Samsun RC, Peters R, Stolten D (2017) Fuel 205:198–221
5. Whipple DT, Kenis PJA (2010) J Phys Chem Lett 1:3451–3458
6. Benson EE, Kubiak CP, Sathrum AJ, Smieja JM (2009) Chem Soc Rev 38:89–99
338
R. Kanega
({[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ](CO 2 )}
+
). In addition, a two-electron reduction and subsequent reaction with CO 2 provided anionic species {[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ]
(CO 2 ) 2 }
À . The coupling of two CO 2 molecules bonded on adjacent μ 3 -S and Ir
centers in {[(Cp*Ir) 3 CH 2 CN(μ 3 -S) 2 ](CO 2 ) 2 }
À produced C 2 O 4
2À . It should also be
noted that the electrochemical conversion of CO 2 to C 2 O 4
2À has been achieved using
Fe [38], Ni [39, 40], Cu [41], and Ru complexes [42], in addition to [(Cp*Ir) 3 (μ 3 -
S) 2 ]
2+ .
5 Conclusion
The development of iridium-based catalysts for the electrolytic reduction of CO 2 has
been conducted for more than 30 years. Due to the increasing importance of CO 2
utilization technologies in recent years, selective CO 2 conversion has become
possible, with dramatically improved catalyst performances being obtained through
the development of new ligands. In addition, the overpotential and current density
for the electroreduction of CO 2 have also received attention in terms of the energy
consumption and reaction rate. In terms of the overpotential, it is important to
understand the function of metal-hydride species, as discussed in this chapter.
Furthermore, it has been found that the reaction system, and more specifically the
electrode materials and mass diffusion, has a significant effect on the reaction
efficiency. Hence, to achieve efficient reactions with high current densities and
low overpotentials, the design of an appropriate catalyst and overall electrochemical
system must be optimized.
In the context of future catalyst developments, compounds that require C-O bond
cleavage and multiple C-H bond formation, such as CH 3 OH, will be targeted.
Indeed, the catalytic functions of single C-H bond formation and C-O bond cleavage,
such as in the cases of HCO 2
À formation and CO formation, have been studied. In
terms of the synthesis of CH 3 OH, the integration of individual catalyst design
principals will be necessary.
References
1. Rogelj J, den Elzen M, Höhne N, Fransen T, Fekete H, Winkler H, Schaeffer R, Sha F, Riahi K,
Meinshausen M (2016) Nature 534:631–639
2. Ilas A, Ralon P, Rodriguez A, Taylor M (2018) International Renewable Energy Agency
(IRENA). Abu Dhabi, UAE, pp 1–160
3. Gotz M, Lefebvre J, Mors F, Koch AM, Graf F, Bajohr S, Reimert R, Kolb T (2016) Renew
Energy 85:1371–1390
4. Schemme S, Samsun RC, Peters R, Stolten D (2017) Fuel 205:198–221
5. Whipple DT, Kenis PJA (2010) J Phys Chem Lett 1:3451–3458
6. Benson EE, Kubiak CP, Sathrum AJ, Smieja JM (2009) Chem Soc Rev 38:89–99
338
R. Kanega