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

Abstract

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.

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

“…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).…”

“…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.…”

“…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).…”

Síntese de óxidos mistos SiO2 /Mn xOy para aplicação na reação de redução de O2

“…Em função de sua estabilidade química, de seu baixo custo e de ser ambientalmente benigna, a família dos óxidos de manganês tem recebido grande atenção atualmente [9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], e esses óxidos têm se destacado por suas potenciais aplicações na área de materiais para aplicação em energia, por exemplo, como catalisadores na oxidação da água, em sistemas de fotossíntese artificial [23][24][25][26][27], ou como catalisadores na reação de redução de oxigênio molecular (RRO) em pilhas a combustível alcalinas [27][28][29][30]. Recentemente, vários artigos vêm estudando a atividade catalítica dos óxidos de manganês na reação de redução de oxigênio molecular [11][12][13][14][15][16][17][27][28][29][30].…”

Titania Nanotubes by Electrochemical Anodization for Solar Energy Conversion

“…201 A variety of Mn oxides have been found to exhibit significant activity toward water oxidation, and particular oxidation states and structures seem to be favored over others. [202][203][204] What is often disregarded is that under OER testing these materials dynamically change their structure and oxidation state, converting often to hydroxides. In addition, the structure of the material is much less important than that of the surface, which has been characterized by Doyle as an octhedrally coordinated surface structure of metal complexes.…”