Nanostructured manganese oxide clusters supported on mesoporous silica as efficient oxygen-evolving catalystsw

Generation of a solar fuel on a terawatt scale requires that water is used as electron source. Catalysts for the fourelectron oxidation of water to oxygen need not only to fulfill the rate and size requirements for keeping up with the solar flux, but must be robust and made of earth abundant elements. Benefiting from explorations of first row transition metal oxides as O2 evolving electrocatalysts 2,3 and, in a few cases, chemically driven catalysts over the past several decades, progress on Co oxide-based water oxidation catalysts has recently been reported on two fronts. Nocera’s group has employed electrodeposition of Co-containing films on an indium tin oxide anode from an aqueous Co phosphate solution and achieved O2 evolution at neutral pH and with modest overpotential (10 mA current, 100 percent Faradaic efficiency, 410 mV overpotential). In another direction, a nanostructured silica support has enabled the synthesis of nanometer-sized Co3O4 (spinel) clusters which possess a very high surface area (bundles of nanorods of 8 nm diameter). Using a standard visible light sensitizer method for driving the catalyst, we have achieved a turnover frequency (TOF) of 1140 s 1 per nanocluster catalyst (pH 5.8, RT, 350 mV overpotential). Projected on a plane, this corresponds to a TOF of 1 s 1 nm . Therefore, a hundred such nanocluster catalysts stacked in a nanoporous scaffold, which can readily be achieved, will result in a TOF of 100 s 1 nm , the rate required for keeping up with the solar flux (1000 W m , AM 1.5). The high TOF of the Co3O4 nanoclusters is mainly attributed to the very high surface area afforded by the nanorod structure, and the high activity of Co surface centers due to the sharply curved nanorod surface. A recent detailed study by Tilley and Bell et al. of the size-dependence of the catalytic water oxidation activity for cubic Co3O4 nanoparticles (range 5 to 50 nm) loaded on Ni foam anodes in alkaline aqueous solution showed a clear, linear dependence of surface area thus confirming the surface size effect. In light of the fact that Mn is not only earth abundant but also environmentally friendly, this metal is particularly attractive for use as a catalyst material. In fact, electrochemists have conducted extensive studies on the use of Mn oxides as anode coatings for catalytic O2 evolution from water. 2,3 While work was typically conducted under basic conditions around pH 14 to minimize the overvoltage, Tamura et al. studied water oxidation over a wide range of pH values including neutral aqueous solution for which they reported current densities around 100 mA cm 2 corresponding to a TOF of Z 0.013 s 1 at an overvoltage of 440 mV (the TOF is a lower limit because no BET surface area was reported; therefore, the estimate assumes that all deposited Mn centers are catalytically active). Also under mild conditions (pH 5), Harriman et al. demonstrated oxygen evolution in aqueous suspensions of mm-sized Mn2O3 particles using photochemically generated Ru(bpy)3 as oxidant for driving the catalyst. 4 Taken together, these literature reports suggest that Mn oxides hold promise for developing robust, efficient water oxidation catalysts that can be operated under mild pH and temperature conditions if nano-sized clusters with high surface area can be made. Here we report efficient O2 evolution at nanostructured Mn oxide clusters in mesoporous silica under very mild conditions for the first time. For driving the catalyst with visible light, the established Ru(bpy)3–persulfate sensitizer system was used. Transmission electron microscopic (TEM) images of Mn oxide nanoclusters synthesized in KIT-6 silica material at 8.0 0.3% loading (per ICP-MS) by wet impregnation followed by controlled calcination are shown in Fig. 1.w By examining many KIT-6 particles (3D network of 8 nm channels), we confirmed that Mn oxide clusters are exclusively formed inside the silica host and do not disrupt the cubic mesopore structure. The latter conclusion is further supported