Nanomaterials as photocatalysts for the CO2 reduction to methanol in water

CO2 can be converted into methanol, through the intermediate steps of reduction to formic acid and formaldehyde using a triad of enzymes such as formate dehydrogenase (FatoDH), formaldehyde dehydrogenase (FaldDH) and alcohol dehydrogenase (ADH). In each reductive step one mole of NADH is oxidized to NAD+ that has to be converted back to NADH in order to make the process acceptable from an economic point of view. Such regeneration can be accomplished by chemical, electrochemical, photochemical or photoelectrochemical processes. We have recently shown[1] that the photosystems can be coupled with the three enzymes listed above for the reduction of CO2 to methanol, using glycerol as H-donor. New photocatalysts have been prepared, such as: transition metal sulphides and nonstoichiometric mixed sulphides, composites of metal oxides like Cu2O/TiO2. Here, we describe the behaviour of selected semiconductors and the working mechanism (electron injection in the conduction band or hole injection in the valence band) and show that they are interesting agents for the reduction of NAD+ and the regeneration of NADH