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

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Chemisorption of water onto anhydrous nanophase manganese oxide surfaces promotes rapidly reversible redox phase changes as confirmed by calorimetry, X-ray diffraction, and titration for manganese average oxidation state. Surface reduction of bixbyite (Mn 2 O 3 ) to hausmannite (Mn 3 O 4 ) occurs in nanoparticles under conditions where no such reactions are seen or expected on grounds of bulk thermodynamics in coarse-grained materials. Additionally, transformation does not occur on nanosurfaces passivated by at least 2% coverage of what is likely an amorphous manganese oxide layer. The transformation is due to thermodynamic control arising from differences in surface energies of the two phases (Mn 2 O 3 and Mn 3 O 4 ) under wet and dry conditions. Such reversible and rapid transformation near room temperature may affect the behavior of manganese oxides in technological applications and in geologic and environmental settings.phase transformation | oxidation/reduction | water adsorption/desorption M anganese oxides are ubiquitous in the natural environment, often occurring as very fine grained (nanopahase) precipitates and coatings, and also find numerous technological applications, especially in catalysis. Easy variation of manganese oxidation state (Mn 2+ , Mn 3+ , Mn 4+ ) lends complexity to phase behavior and physical and chemical properties of manganese oxides. The thermodynamic stability of manganese oxide nanoparticles helps determine the robustness of such oxides and their behavior in soils and larger-scale geochemical cycles, as well as in applications in catalysis, renewable energy, and environmental remediation. The structure and energetics of nanoparticle surfaces are influenced by the phase present and its oxidation state and by the extent of surface hydration; thus dry surfaces are structurally and thermodynamically distinct from hydrated surfaces (1, 2). Using Mn 3 O 4 (hausmannite), Mn 2 O 3 (bixbyite), and MnO 2 (pyrolusite), previously, we showed that the position in temperature−oxygen fugacity space, of oxidation−reduction (redox) phase equilibria, is shifted at the nanoscale due to differences in nanophase surface energy as a function of particle size and surface hydration and that this thermodynamic shift is in favor of the lower surface energy phase (3). In the manganese oxides, the surface energy increases in the order hausmannite, bixbyite, pyrolusite, so small particle size favors the more reduced oxide phase.This paper summarizes observations in the nanoscale Mn 2 O 3 − Mn 3 O 4 system, which is found to undergo rapidly reversible redox phase transformations at room temperature induced by the adsorption/desorption of surface water. It is expected that the degree of reduction is a function of both average particle size and particle size distribution, but this work focuses on only one representative material to provide proof of concept. The rapid response of activated surfaces to changing hydration condition implies thermodynamic control and has implications for geochemistry, planetary scie...

Section: Discussion: Geochemical and Technological Implicationsmentioning

confidence: 86%

Section: Methodsmentioning

confidence: 99%

Rapidly reversible redox transformation in nanophase manganese oxides at room temperature triggered by changes in hydration

Birkner

Navrotsky

2014

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“…Currently, one of the biggest challenges is to develop a cheap and efficient catalyst for the former reaction with a low overpotential, which is still needed to overcome the kinetic barriers (already lowered by the catalyst) along the path from reactants to products. Birnessite, similar to the oxygen-evolving complex in Photosystem II for photosynthesis in regard to the coexistence of Mn(III) (nominal Mn 3+ ) and Mn(IV) (nominal Mn 4+ ) within its structure (1)(2)(3), has already shown a moderate catalytic performance with the overpotentials reported from 0.6 V to 0.8 V (4)(5)(6)(7)(8).…”

mentioning

confidence: 99%

Redox properties of birnessite from a defect perspective

Peng

McKendry

Ding

et al. 2017

Birnessite, a layered-structure MnO 2 , is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO 2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.birnessite MnO 2 | small polaron | catalysis | water oxidation P hotoelectrochemical (PEC) splitting of water into H2 and O2 (artificial photosynthesis) provides an attractive way to harvest solar energy. The process consists of the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), both of which are facilitated with catalysts in practice. Currently, one of the biggest challenges is to develop a cheap and efficient catalyst for the former reaction with a low overpotential, which is still needed to overcome the kinetic barriers (already lowered by the catalyst) along the path from reactants to products. Birnessite, similar to the oxygen-evolving complex in Photosystem II for photosynthesis in regard to the coexistence of Mn(III) (nominal Mn 3+ ) and Mn(IV) (nominal Mn 4+ ) within its structure (1-3), has already shown a moderate catalytic performance with the overpotentials reported from 0.6 V to 0.8 V (4-8).Birnessite has the general formula Mx MnO2·1.5(H2O), composed of layers of edge-sharing MnO6 octahedra as shown in Fig. 1 for the hexagonal phase, and an interlayer of randomly distributed water molecules and metal (M) cations (like K + , Na + , and Ca 2+ ). With the charge balanced by the interlayer cations, some of the Mn cations within the MnO2 layer are reduced from Mn(IV) to Mn(III). The coexistence of Mn(IV) and Mn(III), and further the "balanced equilibrium" or low energy barrier between them, has been suspected to be a critical factor for its catalytic activity (1). The importance of the high-spin Mn(III), through its e 1 g electronic configuration, has been realized in other manganese oxides for OER catalysis (2, 9) and also follows the design principle of Suntivich et al. (10) that a near-unity eg occupancy may imply good OER catalytic activity. This easy switching of the oxidation state is also a typical behavior for the transition metal cation undergoing back and forth changes between several oxidation states in OER catalysts during the water oxidation cycle (11-13). OER catalysts like Co-and Ni-doped hematite (12) and cobalt oxides (13), as well as ...

“…Mn-based cluster compounds (15, 16) and disordered birnessite nanoparticles (2) can exhibit analogous reactivity to the water-oxidizing center of photosystem II. Metal reduction is a key step in water oxidation using Mn oxide catalysts (2,15,17,18) with evidence that Mn(III) plays an important role in O 2 generation (19). However, no information on the intrinsic kinetics or efficiency of Mn(IV) reduction has been reported to date.…”

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confidence: 99%

“…However, no information on the intrinsic kinetics or efficiency of Mn(IV) reduction has been reported to date. The structural and chemical constraints on the mechanism of Mn photoreduction are not known for any Mn phase (17,18), although a recent study of MnO 2 -based water oxidation showed that the substitution of Na with Ca in the interlayer of MnO 2 greatly enhances reactivity (15). The mineralogy literature suggests that the interlayer cations, which balance the excess charge in the MnO 2 sheet, may influence its photoreactivity because the interlayer cations are known to bind water molecules to the neighboring MnO 2 octahedral sheets via hydrogen bonding, with the strength of the interactions dependent on the cation valence (20)(21)(22).…”

mentioning

confidence: 99%

Rate and mechanism of the photoreduction of birnessite (MnO ₂ ) nanosheets

Marafatto

Strader

Gonzalez-Holguera

et al. 2015

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO 2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO 2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca 2+ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.manganese oxide | photoreduction | band-gap excitation | pump-probe spectroscopy | water oxidation M anganese is a key element in environmental processes, catalytic materials, and biological systems due to its rich redox chemistry and ability to form species with a high oxidizing potential. Photochemical processes can enhance significantly the cycling of Mn between the +4, +3, and +2 valence states (1-3). Photoreduction of Mn(IV) is the first step in the reductive dissolution of birnessite minerals in the euphotic zone of marine and lacustrine environments (4-6). This process couples the biogeochemical cycle of Mn to the redox cycling of carbon and trace metals associated with Mn oxide phases. In addition, the greater role of Mn(IV) photoreduction relative to microbial Mn(II) oxidation leads to the predominance of dissolved over particulate Mn in the photic zone of natural waters (1). Thermodynamic calculations predict that direct photoexcitation of Mn oxides in water by visible light will lead to net metal reduction over a wide range of environmentally relevant pH values (7). However, experimental evidence of direct photoexcitation of MnO 2 and subsequent photoreduction of Mn(IV) in the absence of organic electron donors is currently lacking. Experimental studies on the photochemical cycling of Mn have incorporated natural organic ligands that can enhance metal reduction via multiple pathways (5,8,9). These studies have identified aqueous Mn(II) as a reaction end product but have not inve...