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.