Authors: Ivelina Zaharieva, M. Mahdi Najafpour, Mathias Wiechen, Michael Haumann, Philipp Kurz and Holger Dau
Journal: Energy and Environmental Science, 2011, 4, 2400-2408
Published: March 3, 2011
Main Affiliations: Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195, Berlin, Germany; Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, 24118, Kiel, Germany.
What did the researchers do?
The researchers synthesized two types of manganese oxides that have the ability to oxidize water. Through spectroscopy, they determined that their Mn-Ca oxides and the Mn4Ca complex of photosystem II (PSII) (core shown above, labeled A) were structurally similar. Therefore, these materials are the closest synthetic analogs to the core of native PSII discovered so far.
Why is this work important?
The research contributes to the field of artificial photosynthesis, the conversion of solar energy into high-energy fuels. A major goal of that field is to find cheaper and more efficient water oxidation and reduction catalysts.
Although the researcher’s Mn-Ca oxides did not perform better (ie. low turnover frequencies) than nature’s Mn4Ca complex in PSII, this work is important in the biomimicry of the Mn4Ca complex in PSII (see figure top left). The mechanism of how PSII splits water is still unclear. Therefore, the researcher’s Mn-Ca oxides provide a good structural and functional model to understanding how PSII may work.
How did the researchers do it?
The Mn-Ca oxides were synthesized by comproportionating Mn2+ and MnO4– in the presence of Ca2+ (essentially they just mix them all together). Then the MnO2 product was heated under pressure in aqueous solution (called hydrothermal treatment) at 210 °C to yield the first Mn-Ca compound. The second Mn-Ca compound was made by further heating the first compound to 400 °C. Hydrothermal treatment is important in the synthesis because it inserts oxygen atoms (obtained from water) into the material and also makes the material more crystalline.
A big part of this paper was characterizing the structure of these Mn-Ca oxides with X-ray spectroscopy and then comparing them to two model compounds (marokite and β-MnO2) and the active site of PSII.
Mn oxidation states were determined with X-ray absorption near-edge structure (XANES). The XANES spectra of the Mn-CA oxides were compared to the spectra for marokite (CaMn2O4 where Mn = +3) and β-MnO2 (Mn = +4) (see figure bottom left). Because the XANES spectra lies closer to the β-MnO2 spectra, the researchers determined that the oxidation state of Mn in the Mn-Ca oxides was +3.8.
Structure comparisons were completed with extended X-ray absorption fine structure (EXAFS). The EXAFS spectra of the Mn-Ca oxides were compared with spectra from various model complexes including the Mn4Ca site in PSII (see figure right). The results indicated that the synthesized Mn-Ca oxides were structurally similar to the Mn4Ca site in PSII
What did they learn?
From the structural characterization, the researchers proposed three common features that may be necessary for water oxidation by Mn oxides: (i) a mean oxidation state between +3 and +4, (ii) a layered-oxide structure with oxo-bridges and hydroxyl bridges between Mn ions, and (iii) redox-inert cations (such as Ca+) that are connected to Mn ions by oxygen bridges. These features are explained in greater detail in the actual paper.