Ordered Mesoporous Cobalt Oxide as Highly Efficient Oxygen Evolution Catalyst Jonathan Rosen, Gregory S. Hutchings, and Feng Jiao* Center for Catalytic Science & Technology, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States * S Supporting Information ABSTRACT: Oxygen evolution from water by use of earth- abundant element-based catalysts is crucial for mass solar fuel production. In this report, a mesoporous cobalt oxide with an ultrahigh surface area (up to 250 m 2 ·g -1 ) has been fabricated through Mg substitution in the mesoporous Co 3 O 4 spinel, followed by a Mg-selective leaching process. Approximately a third of Mg cations were removed in the leaching process, resulting in a highly porous cobalt oxide with a significant amount of defects in the spinel structure. The activated mesoporous cobalt oxide exhibited high oxygen evolution activities in both the visible-light-driven [Ru(bpy) 3 ] 2+ -persulfate system and the Ce 4+ /Ce 3+ chemical water oxidation system. Under a strong acidic environment, a high turnover frequency (TOF) of ∼2.2 × 10 -3 s -1 per Co atom was achieved, which is more than twice the TOF of traditional hard-templated, mesoporous Co 3 O 4 . ■ INTRODUCTION Solar fuel production from abundant sources (e.g., water and CO 2 ) with sunlight as the energy source is a very attractive approach toward a sustainable and clean energy future. 1-6 A range of methods, including solar thermal, photoelectrochem- ical, and photochemical approaches, have been proposed to produce solar fuel efficiently and economically. 1,2,7-9 Catalytic oxygen evolution from water by use of solar energy is an important reaction, because it is one of the critical reactions that are able to provide a large-scale source of electrons and protons for solar fuel production through either proton reduction to hydrogen or proton-assisted CO 2 reduction to hydrocarbons. 3,10-17 Compared to the reduction half-reaction, the oxygen evolution reaction (OER) usually has slow kinetics and requires large overpotentials. Therefore, an efficient oxygen evolution catalyst is essential to enhance the reaction rate and/ or lower the overpotential. 3 In the past few years, many earth-abundant metal oxides have been investigated as potential OER catalysts to replace expensive Ir- and Ru-based compounds. 14,18-26 Among all the catalysts, cobalt-based materials exhibited high activities in water oxidation reaction through photocatalytic and electro- catalytic approaches. 10,14,16,22,26-29 For example, Kanan and Nocera and co-workers 10,30,31 reported a Co-P i -based electro- catalyst prepared from Co 2+ in phosphate-containing solution through an electrodeposition process. The as-prepared Co-P i electrocatalyst showed high activity for oxygen evolution in a neutral aqueous solution. In situ X-ray absorption study revealed that the Co-O cubane units were formed in the electrodeposition process, and these cubanes may be the active sites. 31 Another example is our recent report on cobalt oxide nanoclusters supported by mesoporous silica, which also showed high turnover frequencies (TOFs) for water oxidation driven by visible light. 32,33 The cobalt oxide nanoclusters have a typical spinel Co 3 O 4 structure, which consists of Co 2+ at tetrahedral sites and Co 3+ at octahedral sites. In the Co 3 O 4 spinel structure, octahedral cobalt and oxygen atoms form Co 4 O 4 cubanes, which might be the active sites for four- electron oxygen evolution in light of the discovery of Mn 4 O 4 Ca as the catalytic core in nature’s photosystem II. The unique property of spinel structure was also reported by Dismukes and co-workers. 34 Mesoporous transition metal oxides are of particular interest in heterogeneous catalysis, because they combine large internal surface area, nanosized walls, and d electrons in an open shell. 35,36 In the past few years, a hard templating method has been developed and a wide range of transition metal oxides with highly ordered mesostructures have been successfully synthesized. 37-41 Some of them exhibited unique electronic, magnetic, and catalytic properties compared with their bulk and nanoparticulate counterparts. 38,40 Here, we reported a cobalt oxide-based oxygen evolution catalyst that has an ordered mesoporous structure, very high surface area, and highly crystalline spinel walls. A Mg-substituted Co 3 O 4 (Mg-Co 3 O 4 ) with three-dimensional ordered mesoporous structure was first fabricated through a hard-templating method, followed by a Mg 2+ leaching/activation process, in which approximately a third of the Mg ions in Mg-Co 3 O 4 were removed. In previous studies, lithium has been incorporated into cobalt oxide to Received: January 17, 2013 Published: February 28, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 4516 dx.doi.org/10.1021/ja400555q | J. Am. Chem. Soc. 2013, 135, 4516-4521