In photo reduction the promoted electron of the metal oxide is accepted by an acceptor species. In the case of CO reduction, shown in the table below, the absence of dissolved oxygen in the aqueous system favors reduction of protons to form hydrogen radicals which then go on to reduce CO to HCOOH. HCOOH can then be further reduced to HCOH and water. Further reduction leads to the production of CH that can combine in a number of ways to produce CH or CH, etc.