Reaction mechanism in the photoreduction of CO2 with CH4 over ZrO2

The surface species arising during the photoreduction of carbon dioxide with methane over zirconium oxide is observed by infrared spectroscopy. Two definite species have been expected to exist on the surface during the photoreaction. One has been supposed to be a reaction intermediate and decomposed to CO at around 623 K, and the other has not been decomposed even at 673 K which should be a carbonaceous residue. From the resemblance of the IR spectral features, the latter species is assigned to the surface acetate ion. Several properties of the former species are found to be quite similar to those of the surface formate ion, which is a reaction intermediate of the photoreduction of CO2 by H-2 over ZrO2. The former species is therefore assigned to the surface formate, which is also supposed to be the reaction intermediate of the photoreaction between CO2 and CH4. The existence of another carbonaceous residue than the surface acetate is suggested. As no IR bands assigned to the C-H vibration are observed in the spectrum of the carbonaceous residues, the other residue is supposed to be a highly carbonaceous species. The EPR spectrum indicates the photoexcitation of adsorbed CO2 to the CO2- anion radical, and the interaction of the CO2- radical with CH4 in the dark. On the basis of these results, a possible reaction mechanism in this reaction is proposed.