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 (PW12O403- and (CuPW11O397-)-P-II), 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 PW12O403- 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 CO2, whereby W-V and Ce-IV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O403- cluster, it was also confirmed that the present methods call be extended to metal-substituted polyoxotungstates. Grafting of trivalent cerium ions to the copper-monosubstituted cluster ((CuPW11O397-)-P-II) 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.