Binuclear ZrOCo Metal-to-Metal Charge-Transfer Unit in Mesoporous Silica for Light-Driven CO2 Reduction to CO and Formate

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• 作者：Marisa L. Macnaughtan ; Han Sen Soo ; Heinz Frei
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：April 17, 2014
• 年：2014
• 卷：118
• 期：15
• 页码：7874-7885
• 全文大小：476K
• 年卷期：v.118,no.15(April 17, 2014)
• ISSN：1932-7455

文摘

Oxo-bridged heterobinuclear units of the type Zr(IV)OCo(II) covalently anchored on the pore surface of mesoporous silica SBA-15 have been synthesized with high selectivity. The unit exhibits a visible light absorbing metal-to-metal charge-transfer absorption (MMCT) extending to about 550 nm. The oxo-bridged structure of the binuclear moiety is manifested by spectral blue-shifts of the optical Co(II) spin鈥搊rbit bands due to reduced 蟺-electron donating ability of the bridging oxygen caused by the electron-withdrawing Zr center. EXAFS measurements of the Zr and Co K-edges and curve fitting analysis revealed a Zr to Co distance of 3.4 脜. The coordination geometry of the Zr and Co metal centers in monometallic Zr and Co-SBA-15 samples is closely preserved in the ZrOCo unit. Illumination of the MMCT absorption at 420 nm and shorter wavelengths resulted in the reduction of CO₂ to gas phase CO and HCO₂^{鈥?/sup>, the latter adsorbed on the silica pore surface. The branching between carbon monoxide and formate was found to be determined by the fate of the sacrificial donor (triethyl- or diethylamine), namely proton transfer versus H atom transfer to CO₂ interacting with the transient Zr(III) center. The ZrOCo(II) unit on a silica surface constitutes the first example of an all-inorganic heterobinuclear unit for the photoinduced splitting of CO₂ to free CO. Moreover, transient Co(III) formed upon MMCT excitation should possess sufficient oxidation potential for driving a catalyst for water oxidation, thereby opening up opportunities for replacing the sacrificial donor by water as electron source.}