Photochemical CO2 splitting by metal-to-metal charge-transfer excitation in mesoporous ZrCu(I)-MCM-41 silicate sieve.

Binuclear redox sites consisting of a Zr oxo-bridged to a Cu(I) center have been obtained on the pore surface of MCM-41 silicate sieve by a stepwise grafting procedure, along with isolated metal centers. The bimetallic site features a Zr(IV)-O-Cu(I) to Zr(III)-O-Cu(II) metal-to-metal charge-transfer (MMCT) absorption extending from the UV region to about 500 nm. The Zr-O-Cu linkage is revealed by a Cu(I)-O infrared stretch mode at 643 cm-1. Irradiation of the MMCT chromophore of ZrCu(I)-MCM-41 loaded with 1 atm of CO2 gas at room temperature resulted in growth of CO (2150 cm-1) and H2O (1600 cm-1). Photolysis experiments using 13CO2 and C18O2 demonstrate that carbon monoxide and the oxygen atom of the water product originate from CO2. This indicates splitting of the CO2 by the excited MMCT moiety to CO and a surface OH radical, followed by trapping of the products at Cu(I) centers (OH is reduced to H2O). This is the first observation of CO2 photoreduction at a binuclear MMCT site at the gas-solid interface.