Although the oxidation of water is efficiently catalysed by the oxygen-evolving complex in photosystem II (refs 1 and 2), it remains one of the main bottlenecks when aiming for synthetic chemical fuel production powered by sunlight or electricity. Consequently, the development of active and stable water oxidation catalysts is crucial, with heterogeneous systems considered more suitable for practical use and their homogeneous counterparts more suitable for targeted, molecular-level design guided by mechanistic understanding. Research into the mechanism of water oxidation has resulted in a range of synthetic molecular catalysts, yet there remains much interest in systems that use abundant, inexpensive and environmentally benign metals such as iron (the most abundant transition metal in the Earth's crust and found in natural and synthetic oxidation catalysts). Water oxidation catalysts based on mononuclear iron complexes have been explored, but they often deactivate rapidly and exhibit relatively low activities. Here we report a pentanuclear iron complex that efficiently and robustly catalyses water oxidation with a turnover frequency of 1,900 per second, which is about three orders of magnitude larger than that of other iron-based catalysts. Electrochemical analysis confirms the redox flexibility of the system, characterized by six different oxidation states between Fe(II)5 and Fe(III)5; the Fe(III)5 state is active for oxidizing water. Quantum chemistry calculations indicate that the presence of adjacent active sites facilitates O-O bond formation with a reaction barrier of less than ten kilocalories per mole. Although the need for a high overpotential and the inability to operate in water-rich solutions limit the practicality of the present system, our findings clearly indicate that efficient water oxidation catalysts based on iron complexes can be created by ensuring that the system has redox flexibility and contains adjacent water-activation sites.
The mechanism of O(2) evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, [Ru(terpy)(bpy)(OH(2))](2+), [Ru(tmtacn)(R(2)bpy)(OH(2))](2+) (R=H, Me, and OMe; R(2)bpy=4,4'-disubstituted-2,2'-bipyridines), and [Ru(tpzm)(R(2)bpy)(OH(2))](2+) (R=H, Me, and OMe), is investigated, where terpy=2,2':6',2''-terpyridine, bpy=2,2'-bipyridine, tmtacn=1,4,7-trimethyl-1,4,7-triazacyclononane, and tpzm=tris(1-pyrazolyl)methane. The kinetics of O(2) evolution is investigated as a function of either the catalyst concentration or the oxidant concentration by employing Ce(NH(4))(2)(NO(3))(6) as an oxidant; these catalysts can be classified into two groups that have different rate laws for O(2) evolution. In one class, the rate of O(2) evolution is linear to both the catalyst and Ce(4+) concentrations, as briefly reported for [Ru(terpy)(bpy)(OH(2))](2+) (S. Masaoka, K. Sakai, Chem. Lett. 2009, 38, 182). For the other class, [Ru(tmtacn)(R(2)bpy)(OH(2))](2+), the rate of O(2) evolution is quadratic to the catalyst concentration and independent of the Ce(4+) concentration. Moreover, the singlet biradical character of the hydroxocerium(IV) ion was realized by experimental and DFT investigations. These results indicate that the radical coupling between the oxygen atoms of a Ru(V)=O species and a hydroxocerium(IV) ion is the key step for the catalysis of [Ru(terpy)(bpy)(OH(2))](2+) and [Ru(tpzm)(R(2)bpy)(OH(2))](2+), while the well-known oxo-oxo radical coupling among two Ru(V)=O species proceeds in the catalysis of [Ru(tmtacn)(R(2)bpy)(OH(2))](2+). This is the first report demonstrating that the radical character provided by the hydroxocerium(IV) ion plays a crucial role in the catalysis of such ruthenium complexes in the evolution of O(2) from water.
The Ru(V)==O species and other intermediates in O(2) evolution from water catalyzed by [Ru(terpy)(bpy)(OH(2))](2+) were spectrophotometrically characterized, and the spectral components observed were identified based on the TD-DFT calculations. Moreover, important insights into the rapid paths after the RDS were given by the DFT studies.
The growing awareness of environmental issues and the requirements to establish solutions diminishing the impact on working environment as well as external environment has initiated ever increasing efforts to develop new, environmentally benign tribological systems for metal forming. The present paper gives an overview of these efforts substituting environmentally hazardous lubricants in cold, warm and hot forging as well as sheet forming and punching/blanking by new, less harmful lubricants and furthermore describes other more generic measures directed towards the same goal such as development of anti-seizure tool materials and coatings and application of structured workpiece and tool surfaces.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.