Deuterium Kinetic Isotope Effect in the Photocatalyzed Dissociation of Methanol on TiO2(110)

Deuterium kinetic isotope effect (KIE) in the photochemistry of methanol on TiO2(110) has been studied to find the rate determining step (RDS) and understand the reaction mechanism using two-photon photoemission spectroscopy. Deuterium substitution of the methyl hydrogen has little effect on the kinetics of this reaction, suggesting that neither the break of the C-H(D) bond nor the transfer of the H(D) atoms to the bridging sites is the RDS in the transformation of methanol into formaldehyde. In contrast, the reaction rate of MeOH is similar to 1.3 times that of MeOD, suggesting that the cleavage of O-H(D) is the RDS in the photocatalyzed dissociation of methanol on TiO2(110). The results contradict the common fact that C-H(D) is more difficult to break than O-H(D) based on ground-state energetics, implying the involvement of photogenerated charge carriers in the reaction of the C-H breakage, whereas the cleavage of O-H is likely a thermal reaction. Difference in the activation energy of O-H and O-D dissociation reaction in the methanol/TiO2(110) system has been calculated based on the KIE measurements. Our work is consistent with the fact that methoxy is photocatalytically more reactive than methanol and suggests that the conversion of methanol into methoxy is crucial in the photochemistry of methanol on TiO2(110) and probably other metal oxide semiconductor surfaces.