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 (PW12O40 3− and CuIIPW11O39 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 LIII-edge X-ray absorption measurements showed that grafting of trivalent cerium ions to the preimmobilized PW12O40 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 WVI−O−CeIII unit. Photoexcitation of WVI−O−CeIII → WV−O−CeIV MMCT in the anchored clusters initiated the photooxidation reactions of gaseous isopropanol to CO2, whereby WV and CeIV work as a site-specific reducing and oxidizing centers, respectively. In addition to the PW12O40 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 (CuIIPW11O39 7−) produced an oxo-bridged trinuclear CuII/WVI/CeIII unit and exhibited photoactivity 4 times higher than that for the binuclear WVI/CeIII cluster. CuII-substituted polyoxotungstates are known to initiate a multielectron dioxygen reduction reaction and have a unique redox property in that the two-electron reduction of CuII to Cu0 occurs in a concerted manner. Therefore, the enhanced photoactivity for the trinuclear unit demonstrates that the activation of a single Cu center from CuII to Cu0 is accomplished by sequential photoinduced MMCT through the confined WVI−O−CeIII linkages, which results in acceleration of the photocatalytic two-electron O2 reduction process.