Investigations of the Efficient Electrocatalytic Interconversions of Carbon Dioxide and Carbon Monoxide by Nickel-Containing Carbon Monoxide Dehydrogenases - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 91

Figure 12 UpperPanel: Cartoon showing a hybrid system for light-driven CO 2 reduction using
CODH I Ch on CdS nanoparticles. Numbers refer to potentials in V versus SHE. D represents an
electron donor. LowerPanel: Comparisons of the performance of CODH I Ch adsorbed on various
CdS nanoparticles: NR ¼ nanorods, QD ¼ quantum dots, CdS calc ¼ calcined sample. Adapted with
permission from [ 62 ]; copyright 2012 Royal Society of Chemistry.
We have already discussed how CODH I Ch and CODH II Ch attached to graphite
electrodes show high activities of CO oxidation and CO 2 reduction with a minimal
overpotential [ 17 ]. The catalytic bias is largely determined by the fact that the reduction
potential of the D-cluster lies in the region of the potential for the CO 2 /CO redox couple
[ 47 ]. When TiO 2 or CdS are used as electrode materials, the electrocatalytic activity is
biased instead strongly in favor of CO 2 reduction, an observation that is explained by
the potential dependence of electron availability in the material. Semiconductors have a
low carrier density and, in simplistic physics terms, the availability of electrons for
transfer into a catalyst is governed by the position of the flatband potential E FB .Both
TiO 2 and CdS are n -type semiconductors and a potential more negative than E FB is
required for electrons to become concentrated close to the surface and available for
transfer. At potentials well above E FB no current flows. Results comparing CODH I Ch