Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates

Anchored visible light-absorbing metal oxide nanoclusters, consisting of trivalent cerium ions and Keggin-type polyoxotungstates (PW 12 O 40 3− and Cu II PW 11 O 39 7− ), have been synthesized on the pore surface of mesoporous silica (MCM-41) as a new class of visible light sensitive photocatalysts. The diffuse reflectance UV−vis, FT-Raman, and Ce L III -edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW 12 O 40 3− cluster on the silica surface provided a broad absorption tail extending from the UV region to 530 nm assigned to a metal-to-metal charge transfer (MMCT) transition of an oxo-bridged binuclear W VI −O−Ce III unit. Photoexcitation of W VI −O−Ce III → W V −O−Ce IV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO 2, whereby W V and Ce IV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW 12 O 40 3− cluster, it was also confirmed that the present methods can be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster (Cu II PW 11 O 39 7− ) produced an oxo-bridged trinuclear Cu II /W VI /Ce III unit and exhibited photoactivity 4 times higher than that for the binuclear W VI /Ce III cluster. Cu II -substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of Cu II to Cu 0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from Cu II to Cu 0 is accomplished by sequential photoinduced MMCT through the confined W VI −O−Ce III linkages, which results in acceleration of the photocatalytic two-electron O 2 reduction process.