Why would some photocatalyst reduce CO2 to CH4 only even though its conduction band is more negative than the reduction potential of CO2/CH4, CO2/CO or CO2/CH3OH? What is the reason for this? Why does it not reduce CO2 to CO or CH3OH even the band position is feasible?
The band structure is not the only factor responsible for the reactivity. In this case the adsorption strength of particular intermediates on the catalyst surface play the crucial role. Even if you have perfect band gap, and CO2 does not adsorb on the surface - you don't have the reaction. If CO desorbs easily, the reaction goes only to CO, and not CH3OH or CH4. Check the papers of the group of R. van Santen on the Fischer-Tropsch process for details.
Hi There,
Its entirely the mechanism. It depends on the type of catalyst and its reducing power, and how it goes about the reduction. Common products are CO and formate, with methane being less common and methanol and formaldehyde the least common, in general.
See: http://www.annualreviews.org/doi/abs/10.1146/annurev-physchem-032511-143759
The review has some good references with mechanistic information.
Kyle Grice