Mn oxide/nanodiamond composite: a new water-oxidizing catalyst for water oxidation

Abstract

Herein, we report nanosized Mn oxide/nanodiamond composites as water-oxidizing compounds. The composites were synthesized by easy and simple procedures by the reaction of Mn(II) and MnO₄⁻ in the presence of ND (1), the reaction of MnO₄⁻ and ND at different temperatures (2 and 4) and by the simple mixing of Mn–Ca oxide and ND (3). The compounds were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction spectrometry, Fourier transform infrared spectroscopy and atomic absorption spectroscopy. The water-oxidizing activities of these compounds were also considered in the presence of cerium(IV) ammonium nitrate. A Mn oxide/nanodiamond resulting from the reaction of MnO₄⁻ and ND shows a turnover frequency of ∼1 (mmol O₂/mol Mn.s). However, other Mn oxide/nanodiamond composites show lower activity toward water oxidation.

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Introduction

Water oxidation is known as a source of “cheap electrons”.¹ In future, the reaction may provide electrons not only for proton reduction but also for other reduction reactions, which are equally important in artificial photosynthesis. Thus, water oxidation and a super anode for the reaction is an important subject in science.²

The water-oxidizing complex of photosystem II (PSII)³ is very efficient, and the only system to catalyze water oxidation in Nature. The water-oxidizing complex (WOC) is a Mn₄O₅Ca cluster housed in a protein environment in PSII that controls the reaction coordinates, proton movement, and water access.⁴ In 2011, Shen and co-workers reported the crystal structure of the Mn–Ca cluster at an atomic resolution.⁴ In this structure, metal ions, namely one Ca and four Mn ions, are bridged by five oxygen atoms. Four water molecules were also found in this structure, with two of them suggested to be the substrates for water oxidation.⁴

Many compounds have been reported as water-oxidizing catalysts, but the use of them for global scale water oxidation is problematic owing to concerns about low earth abundance, toxicity, and high cost. Mn is considered suitable for water oxidation, because it is not only earth abundant, environmentally friendly, and low cost, but also it is efficiently used by Nature as a water-oxidizing catalyst for biological water oxidation.⁵ However, few Mn complexes have been discovered so far that are able to act as a homogeneous or heterogeneous catalyst⁶ for the oxidation of water, although Mn oxides have been reported as heterogeneous catalysts for water oxidation by Glikman and Shcheglova,⁷ Shilov,⁸ Morita,⁹ Harriman,¹⁰ and other groups.¹¹ Interestingly, Mn oxides are also proposed as true catalysts for water oxidation in many Mn-based water-oxidation reactions.^12,13 Recently, layered Mn oxides have been studied as one of the most efficient Mn-based catalysts for water oxidation.¹⁴ However, the efficiency of these oxides is much lower compared with the water-oxidizing complex in Nature.

Layered Mn oxides, Mn oxide clusters supported on mesoporous silica in the presence of Ru(bpy)₃³⁺,¹⁵ gold particles on layered Mn oxide,¹⁶ MnO_x/glassy carbon,¹⁷ and very pure β-MnO₂, R-MnO₂, α-MnO₂, δ-MnO₂, λ-MnO₂, LiMn₂O₄, Mn₂O₃, and Mn₃O₄ compounds have been reported by different groups.¹⁸ Many Mn oxides in the presence of cerium(IV) ammonium nitrate (Ce(IV)) or in electrochemical water oxidation are converted to layered Mn oxides after a few hours.^19–21 Self-healing for Mn oxides has also been reported.²² In the self-healing reaction, decomposition products from Mn oxide in the water oxidation reaction can react or combine to remake Mn oxide.²² Recently, Dau's group, by a specific electrodepostion protocol, synthesized a Mn oxide that oxidizes water at neutral pH.²³

Carbon is an interesting element that can form various original structures, such as C₆₀, carbon nanotubes (CNT), graphene (G), graphene oxide (GO), and nanodiamond (ND). The compounds show exceptional structural and chemical properties.²⁴

Mn oxide/CNTs were shown as promising composites for water oxidation.²⁵ Many factors such as the surface, the oxidation state of Mn oxide, dispersion, calcination temperature, and crystallinity are important in the water-oxidizing activity.¹¹ It is known that, compared to other allotropic forms of carbon such as C₆₀ and CNT, ND constituted by sp³ carbons are remarkably inert.^26a Recently, optical and biological applications of ND were considered.^26b ND was also used as an electrode material in electrochemical applications.^26a On the other hand, ND-supported metal oxides or other metal compounds exhibit excellent catalytic activity for different reactions. In these reactions, ND acts as an excellent support material for catalysts.^26b

Regarding these interesting properties of ND, we considered Mn oxide/ND composite as an effective water-oxidizing catalyst.

Experimental

Material and methods

All reagents and solvents were purchased from the commercial sources and were used without further purification. Nanopowder diamond (ND), <10 nm particle size (TEM) and ≥97% trace metals basis, was purchased from Sigma-Aldrich company. TEM and SEM were carried out with a Philips CM120 and LEO 1430VP, respectively. The X-ray powder patterns were recorded with a Bruker D8 ADVANCE diffractometer (CuK_α radiation). Mn atomic absorption spectroscopy (AAS) was performed on an Atomic Absorption Spectrometer Varian Spectr AA 110. Prior to the analysis, the compounds were added to 1 mL of concentrated nitric acid and H₂O₂, and left at room temperature for at least 1 h to ensure that the oxides were completely dissolved. The solutions were then diluted to 25.0 mL and analyzed by AAS.

Synthesis

We used four methods to synthesize the composites:

1: Solution 1: ND (200 mg) in water (5 mL) was sonicated and added to 2 mL water containing Mn(OAc)₂·4H₂O (122 mg), and stirred for 5 minutes. Solution 2: KOH (40 mg) and KMnO₄ (40 mg) was added in 8 mL water. Solution 2 was added to solution 1, and the mixture was stirred for one hour. Then, it was dried at 90 °C and the solid was washed with water.

2: ND (200 mg) in water (10 mL) was sonicated and added to 10 mL water containing 20 mg KMnO₄. The mixture was dried at 90 °C and then the solid was washed with water.

3: ND (100 mg) in water (5 mL) was sonicated and added to 5 mL sonicated water containing 100 mg Mn–Ca oxide.¹⁶ The mixture was stirred at 60 °C until dry to obtain a powder.

4: ND (250 mg) in water (10 mL) was sonicated and added to 10 mL water containing 50 mg KMnO₄. The mixture was stirred for one day at 25 °C, and the solid was then separated and washed to remove KMnO₄. The solid was dried at 60 °C.

Water oxidation

Oxygen evolution from the aqueous solutions in the presence of Ce(IV) was investigated using an HQ40d portable dissolved oxygen-meter connected to an oxygen monitor with a digital readout. The reactor was maintained at 25.0 °C in a water bath. In a typical experiment, the instrument readout was calibrated against air-saturated distilled water stirred continuously with a magnetic stirrer in the air-tight reactor. After ensuring a constant baseline reading, water in the reactor was replaced with Ce(IV) solution. Without the catalyst, Ce(IV) was stable under these conditions and oxygen evolution was not observed. After deaeration of the Ce(IV) solution with argon, Mn oxides as several small particles were added, and oxygen evolution was recorded with the oxygen meter under stirring (Scheme S1†). The formation of oxygen followed and the oxygen formation rates per Mn site were obtained from linear fits of the data by the initial rate.

Results and discussion

We used different methods to synthesize the Mn oxide/ND composites. In these procedures, we tried to synthesize nanolayered Mn oxides that are known to be efficient water-oxidizing catalyst.

1–4 were synthesized by simple methods. The reaction of Mn(II) and MnO₄⁻ ions in the presence of ND forms 1. 2 and 4 were synthesized by the reactions of MnO₄⁻ and ND:^26c

4MnO₄⁻ + 3C + H₂O → 4MnO₂ + CO₃²⁻ + 2HCO₃⁻

However, we used different temperatures for the synthesis of 2 and 4, respectively.

As discussed by Eder from the University of Cambridge,²⁷ in some cases simple van der Waals interactions are sufficient to provide a strong enough adhesion for the inorganics/nanocarbons.²⁷ For 3, we simply mixed layered Mn–Ca oxide with ND.

In the IR spectrum of ND, a broad band at ∼3200–3500 cm⁻¹ related to antisymmetric and symmetric O–H stretching and at ∼1630 cm⁻¹ related to H–O–H bending are observed (Fig. S1†). Bands at 1110–1600 cm⁻¹ are related to the C–H, C=O, C=C, and C–O stretching and bending deformation modes of the hydroxyl, carboxylic acid, and the anhydride, carbonyl, CH, and C=C surface functional groups.²⁸

In 1–4, the absorption bands characteristic for a MnO₆ core in the region ∼500–600 cm⁻¹ assigned to the stretching vibrations of Mn–O bonds in Mn oxide was also observed in the FTIR spectra of 1–4. To characterize the morphology of the prepared oxides, they were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM and TEM images are shown in Fig. 1, S2 and S3.† SEM images for 1 show aggregated particles with diameters of 30–40 nm. The images indicate that a mixing occurs for ND and Mn oxide. TEM and HRTEM for 1 also show an appreciated good mixing for ND and Mn oxides. These images show that the diameters of the ND particles are ∼10 nm.

Fig. 1 TEM and HRTEM from 1 (a,b), 2 (c,d), 3 (e) and 4 (f,g). Blue, yellow and red arrows show layered Mn oxide, ND, and covered ND by amorphous Mn oxide. SEM images of 1 (h), 2 (i), 3 (j), and 4 (k).

SEM images for 2 show aggregated ND. TEM and HRTEM images show covered ND particles by Mn oxide as black materials.

SEM images for 3, in addition to ND particles, show bigger aggregated particles (∼50–70 nm) related to Mn–Ca oxides. TEM and HRTEM images, in addition to the ND particles (∼10 nm), show layered Mn oxide.

SEM images for 4, similar to 2, show aggregated ND with diameters of around 30–40 nm. TEM and HRTEM images show covered ND particles by Mn oxide as black materials. These images show that the diameter of each ND particle is ∼10 nm.

TEM images show some ND particles are covered by Mn oxide in 2 and 4. Such ND particles are darker than usual ND. TEM images for 1 and 3 show both Mn oxide and ND in the structures.

In XRD of 1 and 3 (Fig. 2b and S5–S7†), patterns related to ND and the crystalline phase of Mn oxides such as Mn₂O₃ or MnO(OH) are observed. In 1 and 3, patterns for ND are not as sharp as related patterns for 2 and 4, most probably because of the oxidation of ND. The peaks for Mn oxides in 2 or 4 are not sharp, but may be related to layered Mn oxide without long-range order (Fig. S4 and S6†). Such a structure can be known as amorphous Mn oxide.

Fig. 2 XRD patterns from 1 (blue), 2 (black), 3 (red), and 4 (green) (a). XRD for 1 with related peaks for MnO(OH), Gama-MnO₂, and Mn₂O₃.

In the next step, we considered the water-oxidation reaction (Fig. 3).

Fig. 3 Oxygen evolution by ND (red) and 2 (5 mg) in the presence of Ce(IV) (0.11 M) (a). The rates of oxygen evolution in the presence of different amounts of 1 and 2 ([Ce(IV)]: 0.11 M) (b). Oxygen evolution of an aqueous solution of Ce(IV) (40 mL, 0.11–0.44 M) at 25.0 °C in the presence of 1–4 (c).

Ce(IV) is a non-oxo transfer, soluble in water, and a stable oxidant, and thus is a usual oxidant in water-oxidation reactions. It oxidizes Mn oxides, and in the next step the oxide can oxidize water.⁴⁷ Water oxidation by ND, without Mn oxide, is not observed (Fig. 3a). The turnover frequency (mmol O₂/mol Mn.s) (TOF)'s are the same for different amounts of each catalyst. The effect of the concentration of Ce(IV) on water oxidation is shown in Fig. 3c. Both water oxidation and catalyst decomposition (such as MnO₄⁻ formation, for detail see ref. 33) occurs in a high concentration of Ce(IV) and, thus, the effect of concentration of Ce(IV) on water oxidation is complicated. Among these compounds, 2 shows promising water oxidation at 0.22 M of Ce(IV) (TOF ∼ 1). The TOF is among efficient Mn-based catalysts toward water oxidation (Table 1). Regarding the XRD results, the reaction of Mn oxide and ND causes the decomposition of both Mn oxide and ND. In 1 and 3, Mn₂O₃ and MnO(OH) are observed, suggesting that neither of them are good catalysts for water oxidation. However, in 2, such phases are not observed and amorphous Mn oxide is the major phase. On the other hand, the TEM and HRTEM images show that in 2, a higher dispersion of Mn oxide on ND is observed. These factors may cause the higher activity for 2 toward water oxidation. The cause for the relatively low activity of 4 toward water oxidation is unknown, but AAS shows that a small amount of Mn oxide is placed on ND in 4.

Table 1 The rate of water oxidation by the various Mn-based catalysts for water oxidation in the presence of a non-oxygen transfer oxidant chemical oxidant

Compound	Oxidant	TOF mmol O2/mol Mn	References
Optimistic Ca–Mn oxide	Ce(IV)	3.0	29
Nanoscale Mn oxide within a NaY zeolite	Ce(IV)	2.62	30
Layered Mn-calcium oxide	Ce(IV)	2.2	31
Layered Mn–Al, Zn, K, Cd and Mg oxide	Ce(IV)	0.8–2.2	32 and 33
Layered Mn/ND	Ce(IV)	∼1	This work
CaMn2O4·H2O	Ce(IV)	0.54	34
Amorphous Mn oxides	Ru(bpy)3^3+	0.06	35
	Ce(IV)	0.52
CaMn2O4·4H2O	Ce(IV)	0.32	34
Mn oxide nanoclusters	Ru(bpy)3^3+	0.28	36
Mn oxide-coated montmorillonite	Ce(IV)	0.22	37
Nano-sized α-Mn2O3	Ce(IV)	0.15	38
Octahedral molecular sieves	Ru(bpy)3^3+	0.11	35
	Ce(IV)	0.05
MnO2 (colloid)	Ce(IV)	0.09	39
α-MnO2 nanowires	Ru(bpy)3^3+	0.059	40
CaMn3O6	Ce(IV)	0.046	41
CaMn4O8	Ce(IV)	0.035	41
α-MnO2 nanotubes	Ru(bpy)3^3+	0.035	40
Mn2O3	Ce(IV)	0.027	34
β-MnO2 nanowires	Ru(bpy)3^3+	0.02	40
Ca2Mn3O8	Ce(IV)	0.016	42
CaMnO3	Ce(IV)	0.012	42
Nanosized λ-MnO2	Ru(bpy)3^3+	0.03	43
Bulk α-MnO2	Ru(bpy)3^3+	0.01	40
Mn complexes	Ce(IV)	0.01–0.6	44
PSII	Sunlight	100–400 × 10^3	45 and 46

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In this condition, in addition to the efficiency of catalysts, ND may improve other properties, such as conductivity, of the Mn oxides.

Conclusions

We have concluded that the synthesis of amorphous nanosized Mn oxide/ND composites as efficient water-oxidizing catalysts by the reaction of MnO₄⁻ and ND is possible. However, other methods, such as mixing Ca–Mn oxide and ND or the reaction of MnO₄⁻ and Mn(II) in the presence of ND, cause the reduction of Mn oxide to Mn₂O₃ and MnO(OH). The simple van der Waals interactions between ND and these Mn oxides are sufficient to provide a strong enough adhesion for the inorganics/nanocarbon. Thus, although ND has a fragile structure, using new strategies may allow its application as a support for heterogeneous catalysts. The results clearly show that where amorphous Mn oxide remain intact, good water oxidation is observed by the amorphous Mn oxide/ND composite (TOF = 1 mmol O₂ per mol Mn per second). Similar results were reported by CNT, GO, G, and C60.^48,49

Acknowledgements

MMN and MA are grateful to the Institute for Advanced Studies in Basic Sciences and the National Elite Foundation for financial support. This work was supported by Grant-in-Aids for Scientific Research from the Ministry of Education of Japan (22370017 and 24370025), and a grant from JST PRESTO to TT. SIA was supported by grants from the Russian Foundation for Basic Research (no: 13-04-91372, 14-04-01549, 14-04-92102, 14-04-92690), and by Molecular and Cell Biology Programs of the Russian Academy of Sciences. The authors thank Fahime Rahimi for preparation of Mn–Ca oxide.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06181k

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