Binuclear TiOMn charge-transfer chromophore in mesoporous silica

Abstract

An all-inorganic heterobinuclear chromophore consisting of Ti(IV) oxo-bridged to a Mn(II) center has been assembled on the surface of silica pores of MCM-41 material. The key step of covalent attachment on the pore surface is the reaction of a Mn(II) precursor featuring weakly held CH3CN ligands with the OH group of a previously anchored titanol site. The optical diffuse reflectance spectrum reveals a Ti(IV)OMn(II) --> Ti(III)OMn(III) metal-to-metal charge-transfer (MMCT) absorption extending from the UV throughout to visible into the red spectral region. FT-IR, FT-Raman and optical spectroscopy confirm that the material is free of Mn oxide clusters, while EPR and Mn K-edge X-ray absorption spectra indicate that the donor center is predominantly in oxidation state +2. In situ FT-IR spectroscopy allowed detection of visible light-induced redox chemistry of the MMCT unit using O2 (18O2) and methanol as acceptor and donor probe molecules, respectively. Formate and water were observed as primary products, with methyl formate emerging as a secondary condensation product. The observed photochemistry demonstrates that excitation of the Ti(IV)OMn(II) --> Ti(III)OMn(III) results in complete transfer of an electron from donor to acceptor center, with the charge separation sufficiently long lived for initiation of redox chemistry to occur. With donor and acceptor redox potentials appropriate for driving multi-electron catalysts for water oxidation, proton or CO2 reduction, the TiOMn(II) unit is an attractive candidate as a charge-transfer chromophore in a solar fuel generating system.

Photocatalytic reduction of CO2 in methanol to methyl formate over CuO–TiO2 composite catalysts

“…The potential of conduction band of CuO (E cb = À0.78 V) is more negative than those of formic acid and formaldehyde yield, and the potential of valence band of TiO 2 was more positive than those of methanol oxidation [21][22][23]. The potential of methanol oxidation by a hole to ÁCH 2 OH is E 0 CH 3 OH=CH 2 OH = 0.927 V [24,25]. Methyl formate can be produced through the esterification of formic acid and methanol and dimerization of formaldehyde via Tishchenko reaction [26].…”

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

“…Units with appropriate redox potentials are capable of driving a water oxidation catalyst, others reduce CO 2 to CO or formate. Todate, a dozen different systems featuring Ti or Zr as acceptor and a first or second row transition metal as donor center have been developed [86][87][88][89][90][91][92][93][94][95][96][97]. The detailed structure of several units on the surface of mesoporous silica supports, including the ZrOCo II system shown in Fig.…”

Coupling carbon dioxide reduction with water oxidation in nanoscale photocatalytic assemblies

“…Donor centers represent most of the upper row transition metals as well as some group B metals, while acceptors are the d 0 elements, Ti or Zr. [3][4][5][6][7][8][9][10][11][12][13][14][15][16] The flexibility in the choice of the donor metal provides a means of matching the redox potential of the light absorber with that of the multi-electron catalyst. In our most recent example, ZrOCo II units were assembled on the nanopore surface of mesoporous silica SBA-15 (1-dimensional system of 8 nm channels separated by 2 nm walls 5 ) by anchoring tripodal Zr-OH groups using an established method 6 followed by the exposure of the resulting powder of Zr-SBA-15 particles to Co II (NCCH 3 ) 2 Cl 2 in acetonitrile at room temperature.…”