Nano-size amorphous calcium–manganese oxide as an efficient and biomimetic water oxidizing catalyst for artificial photosynthesis: back to manganese

Najafpour, Mohammad Mahdi; Nayeri, Sara; Pashaei, Babak

doi:10.1039/c1dt11048a

Cited by 94 publications

(87 citation statements)

References 31 publications

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“…Inspired by nature's catalyst for water oxidation, functional Mn-based molecular mimics [10][11][12] and Mn oxides [13][14][15][16][17][18] have been synthesized. Aside from fully molecular systems for fuel production which assume the presence of an electron acceptor, the water oxidation catalysts need to be connected to an electrode, photoanodes or photoactive semiconductor particles.…”

Section: Introductionmentioning

confidence: 99%

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

Zaharieva

Chernev

Risch

et al. 2012

Energy Environ. Sci.

410

583

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In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn 4 Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-m-oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of Mn III sites and prevented formation of well-ordered and unreactive Mn IV O 2 . Structure-function relations in Mn-based wateroxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use. Broader contextThreatening global climate changes and unsecured supply of fossil fuels call for a global transition toward sustainable energyconversion systems. The storage of wind or solar energy by formation of energy-rich fuel molecules could play a central role in both transient storage of the intermittently provided energy and replacement of fossil fuels in the transportation sector. Whether hydrogen or a carbon-based fuel is the target, in any event the extraction of reducing equivalents and protons from water (that is, water oxidation) is pivotal. In search of water-oxidation catalysts that ultimately could play a role at a global scale, we and others are aiming at development of simple routes towards formation of Mn-based catalysts. Manganese excels by high availability and low toxicity; and in oxygenic photosynthesis, nature has demonstrated that a Mn-based catalyst can oxidize water efficiently. When aiming at an 'artificial leaf' with a Mn-based catalyst directly coupled to a solar-energy-converting material, the activity of the catalyst (per area) needs to cope with the incoming photon flux. Moreover the device design typically requires the use of benign synthesis and operation conditions, that is, temperatures close to room temperature and pH close to neutral. As an important first step, we report a simple protocol for e...

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Section: Introductionmentioning

confidence: 99%

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

Zaharieva

Chernev

Risch

et al. 2012

Energy Environ. Sci.

410

583

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“…Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).…”

mentioning

confidence: 99%

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Birkner

Nayeri²,

Pashaei³

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Previous measurements show that calcium manganese oxide nanoparticles are better water oxidation catalysts than binary manganese oxides (Mn 3 O 4 , Mn 2 O 3 , and MnO 2 ). The probable reasons for such enhancement involve a combination of factors: The calcium manganese oxide materials have a layered structure with considerable thermodynamic stability and a high surface area, their low surface energy suggests relatively loose binding of H 2 O on the internal and external surfaces, and they possess mixed-valent manganese with internal oxidation enthalpy independent of the Mn 3+ / Mn 4+ ratio and much smaller in magnitude than the Mn 2 O 3 -MnO 2 couple. These factors enhance catalytic ability by providing easy access for solutes and water to active sites and facile electron transfer between manganese in different oxidation states.nanomaterials | thermochemistry R esearch on calcium manganese oxides (CaMnOs) has been inspired by the water-oxidation centers in photosystem II (1-5), which is a manganese-calcium cluster Mn 4 CaO 5 (H 2 O) 4 supported by a protein environment. Because the capability for efficient water oxidation is unique to photosystem II among all biological photosystems (6, 7), these CaMnO materials that mimic the elemental composition, manganese oxidation state, and particle size of the photosynthetic water oxidation center are of special interest (8)(9)(10)(11)(12). Although a number of other metal compounds function as water-oxidizing catalysts (13), many contain rare and expensive metals like iridium and ruthenium; the advantage of manganese oxides (Mn-oxides) is that they are earth-abundant, inexpensive, and environmentally friendly (1)(2)(3)(4)(5)14).CaMnO phases have short-range order structure and lamellar morphology (12), which are hallmarks of phyllomanganates (classically known as hydrous Mn-oxides), and they possess a layered structure (15)(16)(17)(18)(19)(20). In this family of structures, manganese octahedra (and vacancies) form the layers, and charge-balancing cations (i.e., alkali and alkaline earth ions, protons) and water occupy the interlayer space.Our previous calorimetric studies of various oxide nanoparticles, including those of manganese, iron, and cobalt (21), show that different polymorphs and phases with different oxidation states have significantly different surface energies. For particle sizes below 100 nm, these differences affect the free energies of phase transitions and of oxidation-reduction reactions, shifting the latter by as much as several log(fO 2 ) units at a given temperature. This thermodynamic effect on redox equilibria is significant when one considers electrochemical potentials for water splitting or other catalytic reactions involving nanoparticles (14). In a chargecompensated layered structure, the Mn(III)/Mn(IV) equilibrium will be different from that of binary Mn-oxide phase assemblages, namely, Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite). Thus, particle size, morphology, crystal structure, and mixed valence in the more complex ...

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“…Nano-sized amorphous manganese-calcium oxide has been synthesized and its water oxidation activity in the presence of cerium (IV) ammonium nitrate was reported. The nano-sized amorphous manganese-calcium oxide is very closely related to the water-oxidizing cluster in PSII, not only because of similarity in the elemental composition, oxidation number of manganese ions, similarity of structure and function to CaMn 4 O 5 (H 2 O) 4 cluster in PSII, but also because of more similar catalyst particle size when compared with previously reported micro-size amorphous manganese-calcium oxides [40]. The oxide was amorphous, and XRD data for the compound were of very poor resolution.…”

Section: Nano-size-layered Manganese -Calcium Oxidesmentioning

confidence: 81%

“…These readily synthesized layers of manganese-calcium oxides are efficient catalysts of water oxidation and are the closest structural and functional analogues to the native PSII catalyst found so far [46]. However, the molecular modelling of the CaMn 4 O 5 (H 2 O) 4 cluster in PSII shows that it has a dimension of about approximately 0.5 nm [40]. Thus, nano-scale, and more interesting ångström-scale, manganese or manganese-calcium oxides will be better structural and functional models for the CaMn 4 O 5 (H 2 O) 4 in PSII.…”

Section: Nano-size-layered Manganese -Calcium Oxidesmentioning

confidence: 99%

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Nano-sized manganese oxides as biomimetic catalysts for water oxidation in artificial photosynthesis: a review

Najafpour

Rahimi

Aro

et al. 2012

J. R. Soc. Interface.

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127

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There has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating the water-oxidizing complex (WOC) of photosystem II (PSII). This is crucial because the water oxidation half reaction is overwhelmingly rate-limiting and needs high over-voltage (approx. 1 V), which results in low conversion efficiencies when working at current densities required for hydrogen production via water splitting. Particular attention has been given to the manganese compounds not only because manganese has been used by nature to oxidize water but also because manganese is cheap and environmentally friendly. The manganese-calcium cluster in PSII has a dimension of about approximately 0.5 nm. Thus, nano-sized manganese compounds might be good structural and functional models for the cluster. As in the nanometre-size of the synthetic models, most of the active sites are at the surface, these compounds could be more efficient catalysts than micrometre (or bigger) particles. In this paper, we focus on nano-sized manganese oxides as functional and structural models of the WOC of PSII for hydrogen production via water splitting and review nano-sized manganese oxides used in water oxidation by some research groups.

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Nano-size amorphous calcium–manganese oxide as an efficient and biomimetic water oxidizing catalyst for artificial photosynthesis: back to manganese

Abstract: A nano-size amorphous calcium-manganese oxide shows efficient water oxidation activity in the presence of cerium(IV) ammonium nitrate.

Cited by 94 publications

References 31 publications

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

Energetic basis of catalytic activity of layered nanophase calcium manganese oxides for water oxidation

Nano-sized manganese oxides as biomimetic catalysts for water oxidation in artificial photosynthesis: a review

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