X-ray Absorption Spectroscopic Study of a Highly Thermally Stable

Sep 22, 2014 - ... (ORR) in aqueous alkaline solution that outperformed the activity of .... Curtis Guild , Wenqiao Song , Rampi Ramprasad , James F. ...
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X‑ray Absorption Spectroscopic Study of a Highly Thermally Stable Manganese Oxide Octahedral Molecular Sieve (OMS-2) with High Oxygen Reduction Reaction Activity Abdelhamid M. El-Sawy,†,‡ Cecil K. King’ondu,†,§ Chung-Hao Kuo,† David A. Kriz,† Curtis J. Guild,† Yongtao Meng,† Samuel J. Frueh,† Saminda Dharmarathna,† Steven N. Ehrlich,∥ and Steven L. Suib*,†,⊥ †

Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt § Department of Physical Science, School of Pure and Applied Science, South Eastern Kenya University, Post Office Box 170, Kitui 90200, Kenya ∥ National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York 11973, United States ⊥ Institute of Materials Science and Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269-3222, United States ‡

S Supporting Information *

ABSTRACT: The development of catalysts with high thermal stability is receiving considerable attention. Here, we report manganese oxide octahedral molecular sieve (OMS-2) materials with remarkably high thermal stability, synthesized by a simple one-pot synthesis in a neutral medium. The high thermal stability was confirmed by the retention of the cryptomelane phase at 750 °C in air. Mechanistic studies were performed by X-ray absorption nearedge structure (XANES) spectroscopy and ex situ X-ray diffraction (XRD) to monitor the change in oxidation state and the phase evolution during the thermal transformation. These two techniques revealed the intermediate phases formed during the nucleation and growth of highly crystalline cryptomelane manganese oxide. Thermogravimetric analysis, Fourier transform infrared spectroscopy (FTIR), time-dependent studies of field emission scanning electron microscopy (FE-SEM), and high-resolution transmission electron microscopy (HR-TEM) techniques confirm the formation of these intermediates. The amorphous phase of manganese oxide with random nanocrystalline orientation undergoes destructive reformation to form a mixture of birnessite and hausmannite during its thermal transformation to pure crystalline OMS-2. The material still has a relatively high surface area (80 m2/g) even after calcination to 750 °C. The surfactant was used as a capping agent to confine the growth of OMS-2 to form short nanorods. In the absence of the surfactant, the OMS-2 extends its growth in the c direction to form nanofibers. The particle sizes of OMS-2 can be controlled by the temperatures of calcination. The OMS-2 calcined at elevated temperatures (400−750 °C) shows high remarkable catalytic activity for oxygen reduction reaction (ORR) in aqueous alkaline solution that outperformed the activity of synthesized solvent-free OMS-2. The activity follows this order: OMS-2500 °C > OMS-2750 °C > OMS-2400 °C. The developed method reported here can be easily scaled up for synthesis of OMS-2 for use in high-temperature (400−750 °C) industrial applications, e.g., oxidative dehydrogenation of hydrocarbons and CO oxidation.

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INTRODUCTION

heterogeneous catalysts. They have superbly tailorable functionalities that have been attained routinely via adapting the crystallinity, particle size, pore size, oxidation state, morphology, thermal stability, and surface area, and as such, a very strong size/structure property functionality relationship has been established for these materials. OMS-2 materials have shown remarkable performance in a plethora of applications. In

Current environmental and energy concerns are increasingly motivating researchers to develop new nanomaterials for heterogeneous catalysis that are capable of retaining their structure reactivity and selectivity during high-temperature (500−1000 °C) reactions.1−3 The need for thermally stable catalysts is pivotal for a number of high-temperature catalytic processes, such as those in solid oxide fuel cells (SOFCs) and oxidative dehydrogenation of hydrocarbons.4 The manganese oxide octahedral molecular sieve (OMS) family of materials has been extensively investigated as © 2014 American Chemical Society

Received: August 5, 2014 Revised: September 22, 2014 Published: September 22, 2014 5752

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the environmental field, for instance, OMS-2 has been used in CO oxidation,5 decomposition of pollutants, such as methylene blue,6 and photo-oxidation of 2-propanol.7 In the energy field, OMS-2 has shown remarkable performance in water splitting.8,9 In general, this material is highly efficient in the aerobic oxidation of alcohols to aldehydes10 and thiols to disulfides11 and many more important organic transformations.12,13 The high catalytic activity of OMS-2 materials is attributed to the variable oxidation state of Mn and the tunnel structure. Under ideal conditions, the average oxidation state of OMS-2 is 3.8, but this can vary depending upon the synthesis method. There are many methods to prepare OMS-2. One of the conventional methods is reflux.14 Drawbacks of this method are long reaction times (24 h) and the use of high concentrations of HNO3, which is a strong oxidizing acid. Reflux is therefore not suitable for large-scale production, which ideally requires mild conditions and a very fast reaction for high-volume production. Microwave15 and sonication16 methods have shorter reaction times compared to the reflux method. However, they both suffer from difficulties in scale up and use of high concentrations of HNO3. In addition, OMS-2 can be formed by the hydrothermal method at a high temperature (∼200 °C) and pressure to obtain a pure OMS-2 phase. Manganese oxides have also been synthesized at room temperature using simple primary alcohols, such as methanol, ethanol, and pentanol, to give MnO2, while glycerol, isopropanol, and ethylene glycol give Mn2O3.17 Identifying the intermediate structures between amorphous manganese oxide (AMO) and OMS-2 may help to achieve better control of crystallinity and purity. This will also help to understand and explain the mechanism of high thermal stability of this material. Previously, in situ synchrotron X-ray diffraction (XRD) was used to study the formation of OMS-2 by hydrothermal treatment of amorphous MnOOH. However, there was no detailed information about the change of the oxidation state of Mn.18 On the other hand, XRD cannot exactly follow the nucleation, transformation, and rearrangement from amorphous materials to crystalline materials. X-ray absorption spectroscopy (XAS) provides the ability to monitor changes in the oxidation state online. In situ XAS has been used to monitor the change of the Mn K-edge during charging and discharging of batteries19 and supercapacitors,20 for oxygen reduction reactions (ORRs), for oxygen evolution reactions (OERs),21 and in water oxidation.22 Thermal transformation of layered double hydroxide (LDH) of metalsubstituted (Cu and Fe) Zn2Al(OH)6·nH2O was followed with XAS to investigate the structures of intermediates during the formation of final products.23 The ORR is very important in energy conversion applications, such as fuel cells and lithium−air batteries. In these applications, platinum is the catalyst of choice; however, because of its scarcity and high cost, there is huge research interest in developing alternative oxygen reduction catalysts using inexpensive and abundant elements, such as manganese. Manganese oxide can be used as a bifunctional catalyst for the ORR and the OER.9,24 Doping precious metals (In25 and Ag26) in OMS-2 enhances catalytic activity. Oxygen deficiencies into β-MnO2 have been found to enhance the ORR activity of βMnO2 by calcination at 450 °C; above that temperature, βMnO2 undergoes a phase change, which will affect the activity.27 Herein, we report a very simple approach to prepare highly thermally stable OMS-2 within 8 h without using strong acids

or bases or sophisticated techniques. The method is amenable to scale up. First, an AMO is precipitated by the reaction between KMnO4 and ethanol in water slightly above room temperature with or without the presence of cetyltrimethylammonium bromide (CTAB). The presence of CTAB modifies the final microstructure. In addition, the present study gives a new understanding of the changes in the oxidation state of Mn in OMS-2 materials achieved by XAS. Such a comprehensive XAS study has not been reported for OMS-2 materials.

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EXPERIMENTAL SECTION

Materials. CTAB, potassium permanganate (KMnO4), hexagonal boron nitride (h-BN), manganese sulfate monohydrate (MnSO4· H2O), nitric acid (HNO3), manganese dioxide (MnO2), and 60% polytetrafluoroethylene (PTFE) were obtained from Sigma-Aldrich, and absolute ethanol was obtained from Pharmco-Aaper. Different manganese oxide standards (MnO, Mn3O4, and Mn2O3) were purchased from Alfa Aesar. These materials were used as received without any further purification. Synthesis. To obtain short nanorods of OMS-2, 3 g of CTAB (3.2 mmol) and 1.58 g of KMnO4 (10 mmol) were dissolved in 100 mL of distilled deionized water (DDW) and the mixture was stirred until all KMnO4 dissolved. Then, 50 mL of pure ethanol (99%) was added and heated at 50 °C for 30 min. The ethanol acts as a reducing agent to manganese in KMnO4. The brown precipitate, which formed after the addition of ethanol, was then separated by centrifuging, washed several times with DDW, and then dried at 90 °C before calcining at different temperatures for 8 h to provide material for ex situ mechanistic studies, as depicted in Scheme S1 of the Supporting Information. The product obtained after calcination at 500 °C is OMS-2. Because of high thermal stability of the 500 °C calcined material, we have called this HT-OMS2. To synthesize long nanofibers of OMS-2, the same procedures were followed but in the absence of CTAB. Reflux OMS-2 was prepared as previously reported.14 Characterization. The OMS-2 materials were characterized by XAS using the X18A beamline of the National Synchrotron Light Source (NSLS), XRD, Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), thermogravimetric analysis (TGA), temperature-programmed desorption− mass spectroscopy (TPD−MS), and Brunauer−Emmett−Teller (BET). Detailed characterization procedures are presented in the Supporting Information. ORR. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were conducted using a computer-controlled CHI660A electrochemical workstation in a three-electrode electrochemical cell. Pt wire was used as the counter electrode with a saturated calomel electrode as the reference. The working electrode, loaded with 20 μL of ink, was dried at room temperature overnight before measurements. The ink was made by sonication of 10 mg of OMS-2, 10 mg of activated carbon, and 5 μL of PTFE in 5 mL of deionized water for 30 min. The measurements were conducted by purging O2 for 30 min. CV and LSV were then recorded at a scan rate of 5 mV/s. Rotating disk electrode (RDE) measurements were carried out the same way as LSV, but the working electrode was rotated from 400 to 2400 rpm. Koutecky−Levich (K−L) plots (J−1 versus ω−1/2) were used to determine the number of electrons transferred at different potentials from their slopes of the best linear fit on the basis of the K−L equation (eq 1)

1 1 1 1 1 = + = + J JL JK JK Bω1/2

(1)

B = 0.62nFCoDo2/3ν−1/6 where J is the measured current density, JL and JK are the limiting and kinetic diffusion-limiting current densities, respectively, B is the reciprocal of the slope, ω is the angular velocity of the electrode (rad 5753

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Figure 1. (A) Ex situ XRD patterns showing transformation of AMO materials into OMS-2 with calcination time at 500 °C. XRD peaks corresponding to OMS-2, birnessite, hausmannite, and an unknown phase are labeled as o, #, ∗, and x, respectively. (B) Enlarged Ex situ XRD patterns from 10° to 20° 2θ. (C) XRD patterns of OMS-2 calcined at different temperatures. (D) Effect of the calcination temperature on the particle size of OMS-2. s−1), n is the number of electrons transferred, F is the Faraday constant (96500 C mol−1), Co is the saturated concentration of oxygen in 0.1 M KOH (1.14 × 10−6 mol cm−3), Do is the oxygen diffusion coefficient (1.73 × 10−5 cm2 s−1), and ν is the kinetic viscosity of the electrolyte (0.01 cm2 s−1).

further increase in the calcination time led to an increase and a decrease of peak intensities and widths, respectively. Figure 1B shows that the AMO layered structure transforms into the OMS-2 tunnel structure during the calcination process. As shown in Figure 1B, the (002) peak of birnessite progressively shifted to higher 2θ with an increase in the calcination time and formation of the (110) XRD peak of OMS-2. Figure 1C shows the calcination of AMO at different temperatures (400−800 °C) for 8 h. As shown in this figure, the crystallinity (the peak intensity) increased and the full width at half-maximum (fwhm) decreased with increasing the calcination temperature. In terms of crystal phase stability, the OMS-2 materials retained the tetragonal cryptomelane phase up to 750 °C, above which the structure started to collapse to form a mixed phase of Mn3O4 and Mn2O3. On the other hand, the particle size increased with the calcination temperature from 17.14 nm at 400 °C to 33.59 nm at 600 °C and then decreased to 30.88 nm at 750 °C, as shown in Figure 1D. FTIR. FTIR is very sensitive to functional groups of materials. The absorption peaks at 2922 and 2834 cm−1, as shown in Figure 2, for as-prepared material correspond to C−H stretches for CH3 and CH2 groups of CTAB, respectively. These peaks disappeared when the sample was heated as the CTAB was thermally decomposed, leaving remaining peaks at 477, 533, and 581 cm−1, which are characteristic peaks for Mn− O vibrations. Morphology. The morphologies of OMS-2 synthesized in the presence and absence of CTAB are shown in Figure 3. In the presence of CTAB (panels A and B of Figure 3), the OMS2 shows very short nanorods with average lengths and widths of 150 and 20 nm, respectively. In the absence of capping agent (CTAB) (panels C and D of Figure 3), relatively long rods with an average length and diameter of 2 μm and 40 nm, correspondingly, were formed. To understand the growth mechanism of short OMS-2 nanorods, a time-dependent morphology study was carried out during the calcination. Asprepared manganese oxide materials were made up of

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RESULTS Upon the addition of ethanol to the permanganate solution, the deep violet color of MnO4− gradually disappeared and a brown color of manganese oxide precipitate was formed. The brown precipitate of manganese oxide was obtained as depicted in Scheme S1 of the Supporting Information. XRD is a suitable technique to study the phases of metal oxides. Figure S1A of the Supporting Information shows thermal treatment of as-prepared material in different atmospheres (air and N2). Calcination, under air, of the asmade amorphous materials synthesized in both the presence and absence of CTAB leads to growth of highly crystalline OMS-2 materials, with all diffraction peaks indexed to the pure tetragonal cryptomelane phase (card number 29-1020, space group I4/m, with unit cell parameters a = b = 9.815 Å and c = 2.847 Å; see Figure S1A of the Supporting Information). The OMS-2 materials obtained by this method show high crystallinity compared to their reflux-synthesized OMS-2 counterparts (see Figure S1B of the Supporting Information). On the other hand, the calcination of as-made material under an inert atmosphere (N2) leads to the formation of Mn2O3 and Mn3O4 mixed phases (see Figure S1B of the Supporting Information). Ex situ XRD was used to track the evolution of the OMS-2 phase with the calcination time. Figure 1A shows XRD patterns of as-made materials calcined at 500 °C for 0.5−8 h. The XRD pattern of as-prepared material shows weak diffraction peaks corresponding to the XRD pattern of AMO. After 4 h of calcination, the XRD begins to show some peaks corresponding to the OMS-2 phase that starts to appear, and after 6 h of continuous heating, all peaks of pure OMS-2 were observed. A 5754

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(Figure 4E). The HR-TEM micrograph in Figure 4F shows long-range order of crystallinity of the HT-OMS-2 materials evident from the well-formed lattice fringes with interplanar spacings of 0.48 nm, corresponding to the (200) OMS-2 planes shown in Figure 4G. As shown by the low- and highmagnification TEM images in panels H and I of Figure 4, OMS2 materials prepared without CTAB are composed of relatively long and thin nanorods. These nanorods show lattice fringes of 0.69 nm spacings, corresponding to the (110) OMS-2 planes. The selected area electron diffraction (SAED) for short nanorods and long nanofibers of OMS-2 (insets of panels E and H of Figure 4) showed that the OMS-2 is highly crystalline and the diffraction peaks can be indexed to (110), (200), (220), (310), (211), and (301) planes. The reciprocal distance of these diffraction patterns are consistent with obtained XRD planes. Thermal Stability. To confirm the high thermal stability exhibited by OMS-2 materials prepared by our new method, TGAs were performed. Figure 5 shows the TGA profiles of asprepared materials and HT-OMS-2 in air and nitrogen atmospheres. As-prepared materials can be divided into three stages, with the first stage representing about 8% weight loss at 350 °C. This is corroborated by TPD−MS results, which show CO2 and H2O desorption peaks (see Figure S3A of the Supporting Information). The weight loss stabilizes between 350 and 500 °C. The second stage, which starts at around 500 °C and ends at ∼850 °C, represents a weight gain of about 3%, arising from the uptake of oxygen because of the oxidation of the materials. This O2 uptake was confirmed by TPD−MS, which shows an O2 uptake at 450−480 °C, as shown in Figure S3A of the Supporting Information. This uptake peak was also more clearly observed in the absence of CTAB but at a lower temperature of 380−400 °C, which means that the presence of CTAB causes a phase transition (see Figure S3B of the Supporting Information). The third stage between 850 and 1000 °C shows another weight loss (10%). On the other hand, the TGA profile of as-prepared material under an inert atmosphere (N2) shows a different profile. The first major weight loss was observed at 25−200 °C, and the second weight loss occurs between 400 and 600 °C, with a final stage at 700− 800 °C. Figure 5 shows TGA profiles of OMS-2 materials calcined at 600 °C. These materials showed high thermal stability up to 750 and 650 °C under air and N2, respectively. Above these temperatures, the OMS-2 lost lattice oxygen, giving a total weight loss from the initial weight of about 8%, which was also confirmed by TPD−MS (see Figure S3C of the Supporting Information) of OMS-2 calcined at different temperatures (500, 600, and 750 °C). The oxygen evolved above 900 °C is due to the strong Mn−O bond in this HTOMS-2. In Situ X-ray Absorption Near-Edge Structure (XANES). XAS is a powerful technique used to monitor the AOS of the atom under investigation. To obtain the oxidation state of Mn, a series of manganese oxides (MnO, Mn3O4, Mn2O3, and MnO2) with different oxidation states was measured (see Figure S4 of the Supporting Information). The standard calibration curve (see Figure S5 of the Supporting Information) was used to calculate the AOS of Mn. The in situ XANES K-edge for Mn in manganese oxide calcined at 450 °C was measured to monitor the change in the oxidation state of Mn as AMO transformed into OMS-2, as shown in Figure 6. Upon heating the as-prepared material, the rising edge of Mn−K is decreased to lower energy. When the

Figure 2. FTIR of as-prepared material in the presence of CTAB and OMS-2 after calcination at 500 °C for 10 h.

Figure 3. FE-SEM images of OMS-2: (A and B) short nanorods prepared in the presence of CTAB and (C and D) long nanorods prepared in the absence of CTAB.

nanoparticles with an average particle size of 10 nm (see Figure S2 of the Supporting Information). Upon heating, these particles aggregate to form large particles. The large particles then grow in the c-axis direction as they continually lose surfactant (CTAB), which acts as a capping agent, to form very short nanorods of OMS-2. TEM. The low-magnification TEM micrograph, shown in Figure 4A, for as-prepared material shows aggregated nanoparticles of about 10 nm in size. The HR-TEM shows the randomly oriented short-order crystallinity of the AMO particles (Figure 4B). Upon calcination at 500 °C, the AMO transformed into mixed phase materials consisting of birnessite and hausmannite. Figure 4C shows the formation of pseudospherical aggregated particles of mixed phases. Figure 4D shows the relatively long-order crystallinity of birnessite, where the lattice fringes of AMO became more oriented, giving an interplanar distance of 0.71 nm. OMS-2 materials synthesized with CTAB show very short nanorods with the same lengths as those obtained by FE-SEM 5755

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Figure 4. TEM micrographs: (A and B) low- and high-magnification images of as-made AMO materials, (C and D) low- and high-magnification images of mixed phases of birnessite and hausmannite obtained by calcination of as-made AMO at 500 °C for 0.5 h, (D) lattice fringes for (002) planes of birnessite, (E) low-magnification of OMS-2 prepared in the presence of CTAB calcined at 500 °C (inset of panel E is SAED along with the index of short nanorod OMS-2 planes), (F and G) high-magnification image of a single nanorod showing lattice fringes of 0.48 nm corresponding to (200) OMS-2 planes, and (H and I) low- and high-magnification images of long OMS-2 nanorods prepared in the absence of CTAB calcined at 500 °C [inset of panel H is SAED along with the index of long nanofiber OMS-2 planes, and inset of panel I shows the lattice fringes of 0.69 nm corresponding to the (110) OMS-2 planes].

Figure 5. TGA profiles for as-made material and HT-OMS-2 under N2 and air atmospheres.

time of calcination is increased, the rising edge of Mn−K starts to shift again to a higher energy. In situ XANES was used to detect the intermediate oxidation states during the thermal transformation of AMO. The major change in the rising edge of Mn−K happened in the first scans (Figure 6). The most stable intermediates were formed during the first stage of thermal transformation. To validate this finding, ex situ XANES was used. Ex situ XANES results (Figure 7) are consistent with the in situ XANES findings. The change in the oxidation state of Mn with the calcination time is

Figure 6. In situ XANES spectra for thermal transformation of AMO to OMS-2 at 450 °C.

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are shown in Figure 9A. The cathodic currents of OMS-2 calcined at 500 and 750 °C were shifted more positive than the cathodic current of OMS-2 calcined at 400 °C, indicating that the activity of OMS-2 increases with the calcination temperature. The performance parameters for ORR activity of OMS-2 calcined at difference temperatures are summarized in Table 1 and compared to the activity of solvent-free OMS-2.9 The onset potential (Eo) of OMS-2 calcined at 400, 500, and 750 °C is −0.12, −0.10, and −0.11 V, respectively. This material showed higher activity than solvent-free OMS-2, which has an Eo of −0.13 V. The OMS-2 calcined at 500 and 750 °C also has outstanding activity at medium potentials (E1/2) and also at potentials of 3 mA cm−2 (EJ). To obtain further information about the activity of OMS-2, RDE measurements were performed at various rotation speeds (400−2400 rpm) (Figure 9B). The slope of the K−L plot (Figure 9C), shows that the average number of electrons transferred for the OMS-2 calcined at 400 °C is 3.94 in the potential range from −0.2 to −0.4 V. This is similar to the state-of-the-art Pt catalyst, with the number of electrons transferred being 4. The chronoamperometeric measurement at a potential of −0.5 V for OMS-2 calcined at 500 °C (Figure 9D) exhibited excellent stability and did not degrade over 3600 s. On the contrary, the Pt/C catalyst has time-dependent deflection during long times of reaction.28

Figure 7. Ex situ XANES spectra for thermal transformation of AMO to OMS-2 at 450 °C.

measured during the ex situ XANES study. Figure S6 of the Supporting Information shows the change in the oxidation state of Mn during the ex situ XANES study. BET. N2 adsorption and desorption of OMS-2 material calcined at 500 °C show a typical type II adsorption isotherm (Figure 8). At relatively low P/Po, a monolayer of N2 was

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DISCUSSION

Thermal stability is one critical issue in the development of catalysts. In this work, we developed a very simple route to synthesize and control the mesostructure of OMS-2 with high thermal stability. A detailed mechanistic study of thermal transformation of AMO to highly thermally stable and crystalline OMS-2 has not been performed before. XANES, XRD, TEM, and other techniques give unique information about the intermediates formed during the thermal transformation. Crypotmelane-type manganese oxides were formed via a thermal transformation of AMO obtained by a redox reaction between ethanol and potassium permanganate in neutral medium according to the following equation: KMnO4(aq) + C2H5OH(l) → K xMnO(3 − x)(s) + CH3CHO(l) + H 2O(l)

Figure 8. N2 adsorption/desorption isotherms for OMS-2 calcined at 500 °C.

(2)

Upon heating in air at 500 °C, AMO transformed into a mixture of birnnesite and hausmannite, after which the mixture was oxidized to form the OMS-2 phase via K+ cation templates. In addition to acting as templates, K+ cations play a role of charge balance. In the absence of O2, manganese in the AMO could not be oxidized to the right proportions of Mn2+, Mn3+, and Mn4+, critical to the formation of the OMS-2 phase, and this led to a mixture of Mn2O3 and Mn3O4 phases. This is supported by the TGA and TPD data (Figure 5 and Figure S3 of the Supporting Information), both of which show uptake of O2 by AMO. The uptake of O2 leads to the transformation of birnessite and hausmannite mixed phases into the pure crystalline phase of OMS-2. Time and temperature play an important role in nucleation and growth of crystalline OMS-2 phases. The effect of time was evident in the transformation of AMO into OMS-2, as shown in panels A and B of Figure 1. The XRD patterns of as-made AMO show broad peaks with very low intensity similar to previously reported AMO.16,29 The broadness of the peaks and

formed, and at high P/Po, capillary condensation became predominant, leading to a rapid increase in the amount of N2 adsorbed. The BET surface areas for OMS-2 calcined at 500 and 750 °C were 85 and 80 m2/g, respectively. ORR. The catalytic activity of the prepared highly thermally stable OMS-2 materials was evaluated with ORR experiments. CV of HT-OMS-2 materials in 0.1 M KOH electrolyte saturated with O2 and Ar (see Figure S7 of the Supporting Information) show a distinct peak at −0.2 V corresponding to the ORR activity of the OMS-2 materials in O2-rich 0.1 M KOH electrolyte. This peak was not observed when the Ar atmosphere was used in the same potential range, confirming the catalytic activity of the HT- OMS-2 in ORR. Rotating disk electrode (RDE) measurements were used to measure the activity of OMS-2 prepared at different temperatures of calcination. RDE linear sweep voltammograms in saturated O2 and 0.1 M KOH at rotation speeds of 1500 rpm 5757

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Figure 9. (A) RDE voltammograms for OMS-2 calcined at different temperatures (400, 500, and 750 °C) in O2-saturated 0.1 M KOH solution at room temperature with a rotation speed of 1500 rpm at a scan rate of 5 mV s−1. (B) RDE voltammograms for OMS-2 calcined at 400 °C in O2saturated 0.1 M KOH solution at room temperature at different rotation speeds (400−2400 rpm) at a scan rate of 5 mV s−1. (C) K−L plots of OMS2 calcined at 400 °C derived from RDE voltammograms in panel B at different electrode potentials. (D) Chronoamperometric response of OMS-2 calcined at 500 °C at a potential of −0.5 V.

the energy barrier required for phase transformation. When the temperature is increased from 400 to 500 °C, the rate of phase transformation increased, thereby increasing the rate of OMS-2 crystal growth. As a result, the particle size increased from 17.14 to 18.77 nm (Figure 1D). The gradual increase in particle size observed between 400 and 500 °C as opposed to the dramatic increase in particle size (from 17.14 to 33.59 nm) observed between 500 and 600 °C is suggested to be due to the capping effect of the surfactant, which hinders the particle growth, thereby decreasing the particle size. When the calcination temperature is increased from 500 to 600 °C, the amount of surfactant in the material decreased greatly, making the particle size increase by sintering. This explains the drastic increase in the particle size from 17.14 to 33.59 nm (Figure 1D). Above 600 °C, the crystal growth rate was very high. Therefore, the particle size decreased from 33.59 to 30.88 nm at 750 °C. In the realm of crystal growth processes, high growth rates lead to a small particle size and vice versa. The high stability of OMS-2 can be correlated to the crystallinity, as previously reported for sonochemically prepared OMS-2, which showed low crystallinity relative to reflux OMS2 and, hence, low thermal stability.16 Morphology. The OMS-2 synthesis by thermal transformation of AMO in the presence of surfactant CTAB shows short nanorods, as shown in panels A and B of Figure 3. The surfactant acts as a capping agent, which prevents the crystal nucleation growth of nanorods in the c direction, leading to the

Table 1. ORR Activity of Different OMS-2 Calcined at Different Temperatures HT-OMS-2500 °C HT-OMS-2750 °C HT-OMS-2400 °C solvent-free OMS-2

Eo (V)

E1/2 (V)

EJ (V)

reference

−0.10 −0.11 −0.12 −0.13

−0.20 −0.22 −0.27 −0.22

−0.19 −0.21 −0.27 −0.25

this study this study this study 9

low intensity are due to small crystallite sizes and poor crystallinity, respectively. Upon increasing the calcination time to 0.5, 1, 2, 4, 6 8, and 10 h, more peaks appeared because of the evolution of birnessite, hausmannite, and eventually OMS2. AMO has been demonstrated to have the hexagonal layered structure of birnessite.8 With an increasing time of calcination, AMO is suggested to have lost its layered structure, transforming into mixed phases of birnessite and hausmannite. The progressive shift of the (002) peak of birnessite to higher 2θ with calcination time (Figure 1B) was attributed to the decrease in the interlayer spacing of birnessite to that of the (110) planes of the OMS-2 phase. Figure 1D shows that the particle size is greatly affected by the calcination temperature. The crystal growth of OMS-2 depends upon two factors: first, the energy required for the thermal transformation of AMO to OMS-2 (crystal growth rate) and, second, the rate of removal of the capping agent (CTAB). Thermal transformation requires energy to overcome 5758

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of 4.13 (see Figure S6 of the Supporting Information). This average oxidation state (4.18) decreased drastically to 2.87 upon heating the materials at 450 °C for 15 min. After 15 min, the AOS increased again and stabilized at 3.81, which is the average oxidation state of OMS-2. The decrease of the A1/A2 ratio of the pre-edge peak (A) and the red shift of the rising edge peak (B) during the initial stage (15 min) of calcination and increase of the A1/A2 ratio of the pre-edge peak (A) and blue shift of the rising edge peak (B) upon further heating indicate that the material is first reduced before being oxidized. ORR. The highly thermally stable OMS-2 materials calcined at different temperatures (400, 500, and 750 °C) showed excellent catalytic actvity in the ORR. The HT-OMS-2 calcined at 500 and 750 °C showed higher catalytic activty for ORR than that calcined at 400 °C (Figure 9A). Moreover, the ORR activity of HT-OMS-2 materials is higher than the solvent-free OMS-2, which was recently reported to be the highest active MnO2. This outstanding ORR activity observed for 500 and 750 °C calcined materials is suggested to be due to oxygen deficiencies in these materials. Cheng et al.27 observed that ORR activity of β-MnO2 materials increased with the calcination temperature and attributed this to structural oxygen deficiencies. OMS-2 may lose some oxygen, especially the surface oxygen, without its overall oxidation state changing appreciably, as confirmed by the TPD−MS results (see Figure S3C of the Supporting Information) and XANES measurements of calcined OMS-2 at different temperatures (see Figure S8 of the Supporting Information). This explains the retention of the catalytic activity of these calcined OMS-2 materials. The OMS-2 calcined at 750 °C shows lower activity than the OMS2 calcined at 500 °C because of the removal of some surface oxygen of OMS-2 at this elevated temperature. Our developed highly thermally stable OMS-2 is still highly active for ORR up to 750 °C, which is a very high temperature for this metal oxide to maintain activity.

formation of short nanorods with lengths and widths of 150 and 20 nm, respectively. The absence of CTAB leads to the formation of longer nanofibers with lengths of 500 nm and diameters of 40 nm, as shown in panels C and D of Figure 3. The time-dependent study by SEM (see Figure S2 of the Supporting Information) shows that the crystal growth of OMS-2 follows a dissolution recrystallization mechanism.30,31 First, the as-prepared material has small nanoparticles with poor crystallinity, as shown from XRD data (Figure 2A), and also a random orientation of nanocrystals (HR-TEM in Figure 4B). These nanoparticles aggregate to form larger particles to decrease surface energy. The XRD data also show that the calcination of AMO with a lamellar structure starts to decompose and then undergoes self-assembly to form mixed phases of hausmannite and birnessite on its route to the formation of OMS-2. Thermal Stability. The TGA profiles in Figure 5 show that HT-OMS-2 materials have very high thermal stability up to 800 and 700 °C in air and N2 atmospheres, respectively, and the total weight loss is 1%. Above that temperature, the OMS-2 starts to lose its lattice oxygen, which means that the Mn−O bond strength is very strong. The TPD data (see Figure S3 of the Supporting Information) for as-prepared materials with and without CTAB show an uptake of O2 from the atmosphere at temperatures of 350−460 °C, which is consistent with TGA of the as-prepared material in air, which starts to gain weight at this range of temperatures. This weight gain corresponds to a phase transition. The nitrogen adsorption/desorption isotherm plot shows that HT-OMS-2 follows a type II hysteresis (Figure 7). The surface areas of OMS-2 calcined at 500 and 750 °C are 85 and 80 m2/g, respectively, and are comparable to that of the conventional reflux OMS-2 (about 75 m2/g). The surface areas of previously reported calcined OMS-2 syntheses using different cross-linking agents, such as poly(vinyl alcohol), glycerol, and glucose, at 800 °C are 11, 18, and 14 m2/g. This low surface area was attributed to the sintering effect of OMS-2 at this high temperature.32 The newly developed method does not have this sintering effect and, thus, a relatively high surface area. Figures 6 and 7 of in situ and ex situ XANES show two distinct peaks. The first weak peak (A) is due to electric dipole forbidden transitions from 1s to 3d levels. This peak splits into two peaks A1 (from 1s to 3d t2g) and A2 (from 1s to 3d eg) because of the degeneracy of 3d orbitals. The 3d−4p orbital mixing arises from the non-symmetric environment of the slightly distorted MnO6 octahedra and electric quadrupole coupling, which make the 1s to 3d forbidden transitions become partially allowed. The Mn4+ cations have a t2g3eg0 configuration, which correspond to A1, and Mn3+ cations have a t2g3eg1 configuration, corresponding to A2.20,33 The as-prepared materials (inset of Figure 7) show a high ratio of A1/A2, which means that the amount of Mn4+ is higher than Mn3+. When the calcination time is increased, this ratio decreased probably because of an increase in the amount of Mn3+; however, upon further increase in the calcination time, the ratio remarkably increased. This is indicative of Mn3+ oxidation to Mn4+. The main peak (B) at 6551 eV in the XANES spectrum corresponds to 1s to 4p transitions. This peak was red-shifted to lower energy upon heating the materials for 15 min. When the calcination time was increased beyond 15 min, the peak started to blue shift to a higher energy. The asprepared manganese oxide material was found to have an AOS

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CONCLUSION Short OMS-2 nanorods with remarkably high thermal stability were successfully synthesized by a simple one-pot method via reduction of Mn7+ by ethanol. These materials retained the pure cryptomelane crystal structure up to 750 °C and showed a high surface area of 80 m2/g compared to the previously reported calcined counterpart (11 m2/g) at 800 °C. High crystallinity enhanced the thermal stability of OMS-2. XANES, ex situ XRD, and HR-TEM techniques confirm the formation of birnessite and hausmannite as intermediate phases during the thermal transformation of AMO to OMS-2. The OMS-2 calcined at an elevated temperature of 750 °C showed much greater activity than the solvent-free OMS-2 in the ORR reactivity, and this makes this material a better candidate for use in high-temperature fuel cells and also in other catalytic reactions that require highly thermally stable catalysts.

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ASSOCIATED CONTENT

* Supporting Information S

Details of characterization, XRD for AMO in different atomspheres, time-dependent studies for the formation of OMS-2, TPD, XAS, and standard calibration curves of different manganese oxide standards, changes in the oxidation state with time, CV data for OMS-2, and XANES of OMS-2 calcined at different temperatures (Figures S1−S8), and synthesis and in situ XAS studies (Schemes S1 and S2). This material is available free of charge via the Internet at http://pubs.acs.org. 5759

dx.doi.org/10.1021/cm5028783 | Chem. Mater. 2014, 26, 5752−5760

Chemistry of Materials

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Article

(24) Gorlin, Y.; Jaramillo, T. F. J. Am. Chem. Soc. 2010, 132, 13612− 13614. (25) Liu, Z. X.; Xing, Y.; Chen, C. H.; Zhao, L. L.; Suib, S. L. Chem. Mater. 2008, 20, 2069−2071. (26) Huang, H.; Meng, Y. T.; Labonte, A.; Dobley, A.; Suib, S. L. J. Phys. Chem. C 2013, 117, 25352−25359. (27) Cheng, F.; Zhang, T.; Zhang, Y.; Du, J.; Han, X.; Chen, J. Angew. Chem., Int. Ed. Engl. 2013, 52, 2474−2477. (28) Liang, Y. Y.; Li, Y. G.; Wang, H. L.; Zhou, J. G.; Wang, J.; Regier, T.; Dai, H. J. Nat. Mater. 2011, 10, 780−786. (29) Villegas, J. C.; Garces, L. J.; Gomez, S.; Durand, J. P.; Suib, S. L. Chem. Mater. 2005, 17, 1910−1918. (30) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414−3439. (31) King’ondu, C. K.; Iyer, A.; Njagi, E. C.; Opembe, N.; Genuino, H.; Huang, H.; Ristau, R. A.; Suib, S. L. J. Am. Chem. Soc. 2011, 133, 4186−4189. (32) Liu, J.; Son, Y. C.; Cai, J.; Shen, X. F.; Suib, S. L.; Aindow, M. Chem. Mater. 2004, 16, 276−285. (33) Farges, F. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the Division of Chemical, Geological, and Biological Sciences, Office of Basic Energy Sciences, United States Department of Energy, under Grant DE-FGO2-86ER13622.A000. Abdelhamid M. El-Sawy thanks the Ministry of Higher Education in Egypt for the financial support.

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