Oxidant or Catalyst for Oxidation? A Study of How Structure and

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Oxidant or Catalyst for Oxidation? A Study of How Structure and Disorder Changes Selectivity for Direct vs. Catalytic Oxidation Mediated by Manganese (III,IV) Oxides Mayada Sabri, Hannah J. King, Rosalind J. Gummow, Xunyu Lu, Chuan Zhao, Michael Oelgemöller, Shery L. Y. Chang, and Rosalie K. Hocking Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03661 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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Chemistry of Materials

Oxidant or Catalyst for Oxidation? A Study of How Structure and Disorder Changes Selectivity for Direct vs. Catalytic Oxidation Mediated by Manganese (III,IV) Oxides

Mayada Sabri1,2,3, Hannah J. King1,2, Rosalind J. Gummow2, Xunyu Lu4, Chuan Zhao4, Michael Oelgemöller2, Shery L. Y. Chang5 and Rosalie K. Hocking1,2*

1

Department of Chemistry and Biotechnology, Faculty of Science, Engineering and

Technology, Swinburne University of Technology, Hawthorn, Melbourne, Victoria, 3122, Australia

2

College of Science and Engineering,

James Cook University, Townsville, Queensland, 4811, Australia

3

College of Education for Pure Science –Ibn Al-Haitham, University of Baghdad, 10071, Iraq

4

5

School of Chemistry, University of New South Wales, Sydney, NSW, 2052, Australia

LeRoy Eyring Centre for Microscopy, Arizona State University, 85287, United States of America

*Author to whom correspondence should be addressed: [email protected]

1 ACS Paragon Plus Environment


Abstract Structural type and disorder have become important questions in catalyst design with the most active catalysts often noted to be “disordered” or “amorphous”. To quantify the effects of disorder and structural type systematically, a test set of manganese (III,IV) oxides was developed and their reactivity as oxidants and catalysts tested against three substrates; methylene blue, hydrogen peroxide and water. We find that disorder destabilises the materials thermodynamically making them stronger chemical oxidants but not necessarily better catalysts. For the disproportionation of H2O2 and the oxidative decomposition of methylene blue- MnOx mediated direct oxidation competes with catalytically-mediated oxidation, making the most disordered materials the worst catalysts. Whereas, for water oxidation the most disordered materials and the strongest chemical oxidants are also the best catalysts. Even though the manganese (III,IV) oxide materials were able to oxidize both methylene blue and peroxides directly, the same materials were able to act as catalysts for the oxidation of methylene blue in the presence of peroxides. This implies that effects of electron transfer timescales are important and strongly affected by structural type and disorder. This is discussed in the context of catalyst design. TOC

Reactions of a Mn(III,IV) oxide test set and 3 substrates tells us about: disorder

(a) Thermodynamics

(b) Electron Transfer

vs.

MB e-

e-

disorder order

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MnOx Oxidant

MB

TBHP e-

MnOx as an oxidant and how they are important … for catalysis and oxidation

order e-

TBHP

MnOx Catalyst

d- only:

MnOx as a Catalyst


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Introduction Manganese (III,IV) oxides have long been known to catalyze a broad range of chemical reactions. Perhaps the best known of these is the classical chemical demonstration of the disproportionation of hydrogen peroxide (H2O2 → H2O + ½O2), which is catalyzed by a range of manganese oxide (and other metal oxide) types.1-4 Manganese (III,IV) oxides (MnOx) have also been implicated in catalyzing a range of organic chemical reactions including decarboxylation5-6, the oxidation of amines to imines 7, the de-alkylation of ketones to aldehydes8, the Maillard reaction9, the oxidative decomposition of dyes 3,

10

and water

oxidation11-17. In addition to being catalysts, manganese oxides are also very good chemical oxidants, with phases like birnessite being noted as amongst nature’s strongest known natural chemical oxidants.18-20

One of the challenges in the development of new catalysts is understanding how “structure” correlates with “function”.13, 21-23 In the water oxidation community it has been noted by many groups that the materials that are most efficient for catalysis of water oxidation are also often amorphous and highly disordered.11-13, 17, 21 The environmental science and geochemical communities have also noted that the most disordered manganese oxides are the strongest chemical oxidants and therefore most susceptible to chemical reduction.24-25 However, the relationship between the ability of a manganese oxide to act as an oxidant and it ability to act as a catalyst is not understood but is mechanistically important, as for any practical material the two reactions compete with each other.

To study the relationship between “redox chemistry” and “catalytic chemistry”, we examined five structurally well-defined manganese (III,IV) oxides, referred to in this study as the “test set” (Figure 1), and tested them for catalytic and oxidative activity using the three substrates; hydrogen peroxide (H2O2), methylene blue (MB) and water (H2O). The manganese oxides in the test set are structurally distinct, but are related by similar redox states with average Mn oxidation states between 3.5 and 4. Across the manganese oxide test set, the least stable materials were the strongest chemical oxidants and best catalysts for water oxidation – but the worst catalysts for the disproportionation of H2O2 and the oxidative decomposition of 3 ACS Paragon Plus Environment

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methylene blue. Based on this finding, the redox chemistry of manganese (III,IV) oxides must be tuned to optimize catalytic efficiency across all reaction types. The implications to the design of electro-catalysts and the general mechanism of oxidative chemical reactions are discussed.

1. 2D-MnOx (Room Temp.) 2. 2D-MnOx (110°C)

3. Hollandite

4. K+ birnessite

5. Pyrolusite

Figure 1. The “test set” of manganese oxides examined in this study. 1. 2D-MnOx (RT), 2. 2DMnOx (110°C), 3. hollandite, 4. K+ birnessite, and 5. pyrolusite.

2. Experimental Methods Reagent or analytical grade chemicals were sourced from commercial suppliers and used as received unless stated otherwise. Distilled water was used throughout. Synthesis of 2D-MnOx (RT), 2D-MnOx (110°C) and Hollandite. The 2D-MnOx, Room Temperature (RT) sample was synthesised using a one-pot reaction procedure described below and was used as precipitated without further chemical or heat treatment. The 2DMnOx (110 oC) and hollandite samples were synthesised by heat treating samples of 2D-MnOx (RT) to 110 oC and 500oC, respectively.

2D-MnOx (RT) was directly precipitated from the combination (1:1 volume ratio) of a Mn(II) solution (manganese(II) acetate tetrahydrate, Mn(OCOCH3)2(OH2)4, 200mM stock solution) and hypochlorite (4% ClO-). The product was separated by centrifugation from the ClOsolution and washed with water (10x) following each separation cycle then with acetone (2x). A sample of 2D-MnOx (RT) was then heated for 24 hours at 110°C to make 2D-MnOx (110oC). Materials prepared at room temperature and 110 °C were analysed using a combination of X4 ACS Paragon Plus Environment

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ray Diffraction (XRD), X-ray Absorption Spectroscopy (XAS), and Transmission Electron Microscopy (TEM). The results indicated that they were structurally similar to birnessite, but 2D-like or very highly c-disordered described further below.26 A sample of 2D-MnOx (RT) was then heated for 24 hours at 500oC and found to be hollandite by XRD, XAS, and TEM. This sample was used as a separate member of the test-set.

Synthesis of ordered K+ birnessite. Potassium (K+) birnessite was made according to a published method.22,23 In this procedure, potassium permanganate (KMnO4) was heated for 12 hours at 800°C in a muffle furnace. The K + birnessite product was separated by filtration and air dried.

Pyrolusite (MnO2). Was obtained via the procedure given in reference

27

. Briefly 0.45g of

KMnO4 was dissolved in deionised water (40 mL), to this mixture 1mL 10 M HCl was added, the sample was transferred to an autoclave and heated at 140 ° C for 12 hours after addition of a Titanium foil. The phase and chemical composition was confirmed by XRD and elemental analysis. A commercial sample of pyrolusite purchased from Sigma Aldrich (529664) was tested alongside the synthetic sample and found to produce results within error across analytical techniques of the synthetic version.

Formulation of MnOx materials. The chemical formula of the newly synthesised materials was characterised by a combination of ion chromatography, Atomic Emission Spectroscopy (AES), titration, weight changes upon heating to 110°C and analysis of the XRD data, see reference 26. Heating 2D-MnOx (RT) past 300°C transitions the material to hollandite with sodium in the tunnel, and further heating to 800°C causes a transition to Mn2O3. Both Hollandite and Mn2O3 are both crystalline phases with an unambiguous description.

2D-MnOx (RT) has substantial additional water in comparison to 2D-MnOx heated above the boiling point of water (110oC), referred to as 2D-MnOx (110 oC). The unheated material has a composition of Mn2+0.16Na0.12(H2O)4.5 [Mn4+0.95,(MnV)0.05O2], and the material freshly heated



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to 110°C were found to have a composition of Mn2+0.16Na0.12(H2O)0.9 [Mn4+0.90,(MnV)0.10O2], where MnV= manganese ion vacancy. More details of these analysis are given in reference 26.

Powder X-ray Diffraction. A SIEMENS D5000 X-Ray Diffractometer with a Cu anode X-ray tube (=0.154 nm) was used for powder X-ray diffraction analysis. Samples were ground into a fine powder for XRD analysis. The fine powder was pressed to form a flat surface flush with the rim of standard XRD loading trays before being scanned between 10-70° at 0.02° increments at a rate of 5 seconds per increment. XRD data was background corrected and the intensity was normalised using WINPLOTR28, part of the FULLPROF29 software package. XRD data was analysed further with Microsoft Excel 2010. Transmission Electron Microscopy. Transmission Electron Microscopy was performed using an aberration corrected Titan (FEI Company) microscope operated at 80kV. Images were analysed using the software “Image J”.30-31

Inductively Coupled Plasma–Atomic Emission Spectroscopy (ICP-MS). The manganese content of the MnOx materials was measured using ICP-MS. In preparation for analysis, a solid MnOx sample (~0.1 g) was dissolved in nitric acid (HNO3, 70%) and hydrogen peroxide (H2O2, 30%) and heated (90±5°C, 2 hrs) in accordance with the procedure described in ref

32

. The

ICP-AES analyses were carried out at the Advanced Analytical Centre (Townsville, Australia) using a Varian Liberty Series II instrument. X-ray Absorption Spectroscopy. Mn K-edge XAS spectra were recorded on the multipole wiggler XAS beam-line 12 ID at the Australian Synchrotron, in operational mode 1. The beam energy was 3.0 GeV and the maximum beam current was 200 mA. Data on the oxide powders was collected in transmission mode in He-filled ionisation chambers. Samples for the transmission X-ray experiment were prepared by grinding the sample together with boron nitride and loading into a 1 mm thin Al spacer and sealed with 63.5 M Kapton tape.33 XAS data was analysed using a combination of PySpline 34 and Microsoft Excel35 for background subtractions, and Artemis36-37 for EXAFS fitting. XAS was also used to examine the chemical changes in the MnOx materials after reactions with methylene blue and a cerium(IV) oxidant. Samples consisting of MnO x (5 mL, 0.01 M)


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and MB (5 mL 0.032 M) were reacted for 9 hours**, quenched by freezing (-20°C) and freeze dried (‘CRYODOS –50’ model freeze dryer), then analysed using a 100 element fluorescence detector. Samples consisting of MnOx (ca. 10 mg 2D-MnOx (RT), or 5 mg K+ birnessite), Ce4+ ((NH4)2Ce(NO3)6, 0.67 g and 0.2 g) and water (5 mL) were reacted and quenched at 1 hour and 48 hours (by freezing at -20°C) before being freeze dried. Due to spectroscopic interference between the Mn K and Ce L1 edges, Ce4+ was reacted at the concentration used in the gas chromatography experiments (0.67 g cerium oxidant) and at a reduced concentration (0.2 g cerium oxidant). All samples were ground prior to analysis by XAS.

** Note that 9 hours was

chosen as a comparison point for the reactions with methylene blue to facilitate a comparison between catalytic reactions and oxidation and reduction reactions. Identical trends across the series were observed at all-time points of the reaction so another time-point could have equally been chosen.

Ion chromatography (IC). Samples were dissolved using the procedure described above for AES and analysed b with a Metrohm 930 flex using a standard cation exchange column (METROSEP C6-250/4.0)

Titration. The redox state of manganese oxide was determined from XAS and titration analyses. In the titration process, an excess of sodium oxalate was first used to reduce all the manganese oxide sample to Mn2+.

A titration against a standardized solution of

permanganate was used then to quantitatively determine the oxalate remaining after the procedure was completed. The method is described in detail in reference 38.

Calorimetry. The specific heat of reductive dissolution was measured using a homemade calorimetry setup employing a modified open-mouthed Thermos brand food storage container (100 mL) and a polystyrene plug. In each experiment manganese oxide (ca. 0.1g measured to 4 decimal places) was reacted with EDTA (0.3 M, 10.00 mL) in acetate buffer (0.1 M, pH 3.5). Temperature changes upon mixing were measured to 0.1 °C. The maximum temperature change was observed after 5 minutes for all reactions.



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Gas evolution upon addition of hydrogen peroxide (H 2O2). The propensity of a manganese oxide phase to directly oxidize H2O2 vs. catalyse H2O2 disproportionation was tested by measuring the gas evolved from the reaction between a known amount manganese oxide and hydrogen peroxide as described previously.39 In this procedure, the manganese oxide (5.057x10-5 M, as a calibrated suspension), was reacted with hydrogen peroxide (H2O2, 2 mL 30%) in either acetate buffer (CH3COONa, 0.1 M, 5mL, pH = 5.0) or phosphate buffer (NaH2PO4, 0.1M, 5mL, pH = 8.0). The gas evolution was quantified using a gas burette (Figure S1). Four repeat measurements were made on each data set, and errors were estimated at the 95% certainty on the average measurement.

Tests for water oxidation. The Frey, Wiechen and Kurtz12, 40-41 gas chromatography (GC) analytical method was used to determine the moles of oxygen evolved over 1 hour of the manganese oxides. Each reaction solution consisted of the MnOx (ca. 10 mg for 2D-MnOx (RT) and 5 mg for all other MnOx materials) and cerium oxidant ((NH4)2Ce(NO3)6, 675 mg) mixed with distilled water (5 mL) in a Labco Exetainer© vial (12 mL, flat bottomed, non-evacuated, double wadded cap). At time steps of 10 min, 20 min and 60 min; 0.5 mL (injection volume) of the sample vial headspace was hand drawn for analysis. Molecular oxygen and nitrogen was measured using a Shimadzu GC-2014 equipped with a Restek Molesieve 5Å 80/100 column (10 ft × 2.1 mm) and thermal conductivity detector (TCD). The column temperature was 70oC, the injector temperature was 150°C and the TCD was 380°C. Ultra high purity He was used as a carrier gas at a flow rate of 30 mL/min.

Tests of the decomposition of methylene blue. The oxidative decomposition of methylene blue was monitored by following the absorption band at 660 nm using a Shimadzu UV-1800 spectrometer. Different reaction stoichiometries of manganese (III,IV) oxides, methylene blue and hydrogen peroxide / tertiary butyl peroxide solution (TBHP, luperox®) were tested to evaluate efficiency for the oxidative and catalytic chemistry of the test set in the presence of methylene blue. Data were collected at range of time points as the % MB decomposed calculated by comparison with the starting absorbance as it was related to concentration.


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3. Results and Discussion 3.1 Characterization The five manganese (III,IV) oxide samples were chosen for the test set because they have similar redox states (average Mn oxidation states between 3.5 and 4) and collectively span a range of properties common for manganese (III,IV) oxides. Within this series pyrolusite (MnO2) was made using a hydrothermal synthesis, K+birnessite was made by the thermal decomposition of potassium permanganate, 2D-MnOx (RT) was directly precipitated from the oxidation of Mn(II), and the remaining two samples (2D-MnOx (110°C) and hollandite) were prepared by heat treating 2D-MnOx (RT)42-43 at 110°C and 500°C, respectively. The effects of mild heating were included in this study because they were shown to have a measurable difference on the oxidative and catalytic efficiencies. The X-ray Absorption spectroscopy (XAS), X-ray Diffraction (XRD), with simulations from known crystal types, and Transmission Electron Microscopy (TEM) on the five manganese oxide samples are given in Figures 2, 3 and 4 respectively.

Of the five materials included in the test set, three of them have well defined crystal structure types: hollandite, K+birnessite and pyrolusite. For these materials the data are consistent with previous XAS, XRD and TEM reports of the materials, and the simulation of the XRD based on the known crystal structures is consistent with the materials as made (Figure 3).42-45 Hollandite has a 2x2 tunnel structure with sodium in the tunnel, K +birnessite has stacked layers of materials and pyrolusite is the manganese oxide phase with a rutile structure. (Figure 1). In addition to providing structural information, X-ray Absorption Spectroscopy can give us a good indication of redox state. Of the materials studied, Pyrolusite (MnO2) has an oxidation state of 4, K+Birnessite 3.8, 2D-MnOx 3.7 and Hollandite 3.6 based on a combination of XANES data and titration against oxalate. The two non-crystalline members of the test set have been previously described in detail by this group26. The 2D nature of these materials was confirmed from experimental and simulated XRD matches showing

Oxidant or Catalyst for Oxidation? A Study of How Structure and

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