Using Ni, Co–ZrO2 photocatalyst, the dissociation of CO2 into CO proceeded on the ZrO2 surface, while the splitting of H2 and/or H2O followed by the consecutive hydrogenation starting from CO to CH4 proceeded on the metal nanoparticles. However, the detailed dynamic reaction mechanism of O–C dissociation are unknown. To clarify these problems, the energy change along the reaction coordinate and the density of states in each reaction step were investigated by DFT calculations. The vacuum layer was set to 1.0 nm thick slab model was used for monoclinic ZrO2(0 0 1) and (1 1 1) facets in the presence/absence of metallic Ni and Co. On the ZrO2(1 1 1) facets, O–C bond of O–COH dissociated under the UV light irradiation. Then, dissociated O filled O vacancy (Ov) site and COH species hopped onto the nearest O atom, followed by the dissociation to CO and hydroxy species (Fig. 1A). In contrast, on the (0 0 1) facets, the hopping could not occur due to the greater interval between the O sites. Alternative associative mechanism occurred both (1 1 1) and (0 0 1) facets and the activation energy (Eact) was 1.3 eV (Fig. 1B). However, Eact for hopping mechanism was by far lower: 0.9 eV (Fig. 1A). The LUMO of CO2-adsorbed ZrO2 system populated on OCOH species, in which the antibonding node at O–C bond resulted in O–C dissociation owing to charge separation in ZrO2 by light. The Eact value of 0.9 eV was further reduced to 0.4 eV by the presence of OH species on Zr atoms neighboring to the surface Ov site. Furthermore, H atom transfer to O resulting in CO formation occurred only by the Eact value of 0.3 eV. The O vacancy (Ov) on the ZrO2surface was energetically favorable for CO2 dissociation, while the Ni or Co surface was favorable for consecutive hydrogenation to CH4. Based on these results, dual catalytic mechanism, charge separation to reduce CO2 into CO over ZrO2 (0.4 eV) followed by consecutive hydrogen addition into CH4 (0.6 eV) on metal due to heat transformed from light energy was quantitatively confirmed.

Electron transfer process and reaction mechanism of CO2 photoreduction in metal nanoparticle supported on ZrO2

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