Thermodynamically, proton coupled multiple-electron reduction of CO is easier than single electron reduction. But to manage multiple proton coupled multiple-electron processes is a huge challenge kinetically. This leads to a high overpotential for electrochemical heterogeneous reduction of CO to hydrocarbons and alcohols. Even further heterogeneous reduction of singly reduced CO radical anion is difficult because of repulsive interaction between negatively biased electrode and negatively charged anion.
Figure 2 shows that in case of a p-type semiconductor/liquid junction photo generated electrons are available at the semiconductor/liquid interface under illumination. The reduction of redox species happens at less negative potential on illuminated p-type semiconductor compared to metal electrode due to the band bending at semiconductor/liquid interface. Figure 3 shows that thermodynamically, some of the proton-coupled multi-electron CO reductions are within semiconductors band gap. This makes it feasible to photo-reduce CO on p-type semiconductors. Various p-type semiconductors have been successfully employed for CO photo reduction including p-GaP, p-CdTe, p-Si, p-GaAs, p-InP, and p-SiC. Kinetically, however, these reactions are extremely slow on given semiconductor surfaces; this leads to significant overpotential for CO reduction on these semiconductor surfaces. Apart from high overpotential; these systems have a few advantages including sustainability (nothing is consumed in this system apart from light energy), direct conversion of solar energy to chemical energy, utilization of renewable energy resource for energy intensive process, stability of the process (semiconductors are really stable under illumination) etc. A different approach for photo-reduction of CO involves molecular catalysts, photosensitizers and sacrificial electron donors. In this process sacrificial electron donors are consumed during the process and photosensitizers degrade under long exposure to illumination.