Abstract Catalytic Performance And

ABSTRACT

CATALYTIC PERFORMANCE AND CHARACTERIZATION OF ZN-DOPED CRYPTOMELANE-TYPE MANGANESE DIOXIDE FOR ETHANOL OXIDATION

by Lulu Jiang

Cryptomelane (KMn8O16) was prepared by two methods: 1) a sol-gel method, and 2) a proprietary method that uses alkaline battery waste as a starting material. These cryptomelane materials were investigated as volatile organic carbon (VOC) oxidation catalysts and compared. To better understand the roles of processing steps on the physical characteristics and catalytic activity, the effects of zinc dopant and calcination temperature were studied with the sol-gel cryptomelane materials. Zinc dopant was included as a test variable because it is present in alkaline battery waste, and cyrptomelane prepared from battery waste also contains zinc (<5 wt%). The oxidation of ethanol was used to assess catalytic activity of the cryptomelane materials. It was found that cryptomelane-based manganese dioxide prepared by both the sol-gel method and from battery waste can be used as catalysts in the catalytic oxidation of ethanol in air. The zinc dopant (<5 wt%) has very little impact on catalytic activity, although it has some impact on the morphology and crystallinity of the materials. Calcination temperature, however, which impacts the surface area of the material, has a much more significant role, where the higher the calcination temperature, the lower the surface area, and hence, the lower the catalytic activity.

CATALYTIC PERFORMANCE AND CHARACTERIZATION OF ZN-DOPED CRYPTOMELANE-TYPE MANGANESE DIOXIDE FOR ETHANOL OXIDATION

A Thesis Submitted to the Faculty of Miami University In partial fulfillment of the requirements for the degree of Master of Science Department of Paper and Chemical Engineering by Lulu Jiang Miami University Oxford, Ohio 2012

Advisor: Dr. C. Almquist Dr. M. Krekeler

Reader: Dr. S. Lalvani

Reader: Dr. Lei Kerr

Contents

List of Tables ...... iv List of Figures ...... v ACKNOWLEDGEMENTS ...... vii I. Introduction ...... 1 II. Literature Review ...... 3 2.1 Preparing methods ...... 3 2.1.1 Overview ...... 3 2.1.2 Reflux Method ...... 3 2.1.3 Sol-gel Method ...... 3 2.1.4 Solid-State/Milling Method ...... 4 2.1.5 Dopant Methods ...... 4 2.2 Catalytic property of cryptomelane ...... 4 2.2.1 Catalytic Mechanism ...... 5 2.2.3 Effect of calcination temperature ...... 6 2.2.4 Transition metal doped cryptomelane as a catalyst ...... 6 2.2.4 Catalytic deactivation ...... 7 III Hypothesis ...... 8 3.1 Synthesis ...... 8 3.2 Characterization ...... 9 3.3 Catalysis ...... 10 IV. Experiment...... 12 4.1 Catalyst Synthesis ...... 12 4.2 Characterization ...... 13 4.2.1 BET ...... 13 4.2.2 XRD ...... 14 4.2.3 SEM EDS ...... 14 4.2.4 TEM EDS ...... 14 4.3 Catalysis ...... 14 4.3.1 Preparation ...... 14 4.3.2 Performance experiment ...... 15 V. Results and Analysis ...... 17 5.1 Characterization ...... 17

ii 5.1.1 BET surface area ...... 17 5.1.2 X-ray Diffraction ...... 18 5.1.3 SEM ...... 21 5.1.4 TEM ...... 28 5.2 Catalytic Performance ...... 32 5.2.1 The effect of calcination temperature on catalytic activity for ethanol oxidation...... 32 5.2.3 Battery-derived cryptomelane performance ...... 33 5.2.4 Activation Energy...... 34 5.2.3 Mass balance in catalytic reaction ...... 37 5.2.4 Temperature at which ethanol half decomposed ...... 38 References ...... 41

iii List of Tables

Table 4.1 Summary of chemicals used in this study...... 12 Table 5.1 Summary of BET surface areas of cryptomelane samples ...... 17 Table 5.2 Crystal length and width estimated from SEM pictures...... 24 Table 5.3 Summary of carbon balances conducted at each reaction temperature for a selected experimental trial...... 37

iv List of Figures

Figure 3.1 Schematic of test system used for the catalytic oxidation of ethanol. 11 Figure 4.1 Procedure for synthesizing cryptomelane using the sol-gel method . 13 Figure 4.2 Schematic diagram of the reaction system ...... 15 Figure 5.1 Effect of zinc dopant and calcination temperature on the BET surface area of cryptomelane...... 17 Figure 5.2 Comparison of X-ray diffraction patterns for four Zn-doped cryptomelane samples calcined at 350℃ , 400℃ , 500℃, and 600℃...... 19 Figure 5.3 XRD crystal structures at different calcination temperatures with pure cryptomelane (upper) and 2.5 weight % Zn/cryptomelane (lower). Cryptomelane peaks have been marked above. # Refers to birnessite crystal peaks...... 20 Figure 5.4 SEM image of pure cryptomelane under (a) 350℃ (b) 400℃ (c) 500℃ (d) 600℃calcination temperatures with the same magnification of 100,000x (e) 400℃ (f) 500℃ under 50,000x ...... 22 Figure 5.5 An overview of the crystal quantity that grows from the sample rock following calcination at 400℃ (left) and 600℃ (right)...... 23 Figure 5.6 Calcination temperature vs. crystal length (a, upper) and crystal width (b, lower) ...... 25 Figure 5.7 SEM image of cryptomelane calcined at 500℃ with 0%(left), 2.5%(middle) and 5%(right) zinc/cryptomelane under a magnification of 100,000x ...... 26 Figure 5.8 SEM image of the 500℃ 2.5% cryptomelane before (upper) and after (lower) catalytic reaction under the same magnification of 50,000x ...... 27 Figure 5.9 SEM image of the battery-derived cryptomelane under magnification of 50,000 x (left) and 10,000 x (right) ...... 28 Figure 5.10 TEM images and diffraction patterns of 400℃-calcined samples (a) 0% (b) 1% of zinc/cryptomelane under different magnifications, the scale is on the picture...... 29 Figure 5.11 TEM images of in pure cryptomelane crystal under 400℃ (left) and 600 ℃(right) calcination temperatures in the same scale ...... 30

v Figure 5.12 TEM image of 400℃-calcined pure cryptomelane under the same magnification; the tunnel and lattice structure can be clearly seen near the white arrow...... 31 Figure 5.13 Ethanol conversion as a function of reaction temperature and cryptomelane calcination temperature...... 32 Figure 5.14 Reaction temperature vs. average ethanol conversion in different zinc/cryptomelane weight loadings ...... 33 Figure 5.15 Reaction temperature vs. average ethanol conversion in battery- derived sample...... 33 Figure 5.16 Representative plots to calculate activation energies...... 35 Figure 5.17 Activation energy vs. calcination temperature (upper) and zinc dopant (lower) ...... 36 Figure 5.18 Reactor effluent composition vs. reaction temperature with the catalyst calcined at 350℃ and with 2.5% Zn...... 37 Figure 5.19 Reactor effluent composition vs. reaction temperature with the catalyst calcined at 500℃ and with 2.5% Zn...... 38 Figure 5.20 Zinc percentage vs. reaction temperature when the ethanol outlet percentage reaches 50% ...... 39

ACKNOWLEDGEMENTS

I would like to thank my advisors, Dr. Almquist and Dr. Krekeler, for their help during both my study and my research in the two years of study in graduate school.

I would also give my gratitude for John Morton, for tutoring the X-ray Diffraction and helping me with identify metals; Mattew Duley, for the help of guiding me Scanning Electron Microscopy; Richard Edelmann, for the help with the characterization with my sample.

I would also like to thank my committee members, Dr. Lalvani and Dr. Kerr, for their suggestions in my thesis.

Finally, I would thank with all my gratitude, to all my friends in the Chemical and Paper Engineering Department at Miami University, for their inspirational help with both my study and research.

vii I. Introduction

Cryptomelane is a manganese oxide octahedral molecular sieve (OMS-2) that has been investigated as a volatile organic compound (VOC) oxidation catalyst because of its unique structure and properties (1). Cryptomelane is a material with an ideal stoichiometric formula of KMn8O16. The K+ cations are contained within the 2x2 tunnel structures. There exists a mixture of Mn4+, Mn3+ and Mn2+ ions in the cryptomelane structure, with an average oxidation state of Mn near 3.8 (1). The pore size of this material is about 0.45 to 0.7 nm.

Recently, battery waste has been investigated as a raw material to synthesize cryptomelane (2). Alkaline batteries rely on the reaction of MnO2 and Zn and contain a potassium hydroxide electrolyte. Nearly 3 billion alkaline batteries are disposed each year in the United States (2; 3). Therefore, this waste stream presents a unique opportunity for waste reduction if a value-added product can be made from alkaline battery waste.

In this study, cryptomelane was prepared by two methods, a sol-gel method and a proprietary method that uses alkaline battery waste as a starting material. Both preparations were investigated as an oxidation catalyst. Ethanol was used as the representative volatile organic compound (VOC) for these experiments.

Ethanol was selected for this study because it is a renewable fuel that already has widespread use in the US as an oxygenate in transportation fuels and as a fuel supplement. In addition, the amended Renewable Fuels Standard, which resulted from the 2007 Energy Independence and Security Act (4), will significantly increase the production, storage, and distribution of ethanol for blending with transportation fuels. Therefore, control of ethanol vapor emissions is crucial for the renewable and transportation fuels sectors. Furthermore ethanol is commonly used as an analog for VOC oxidation studies.

1 Common VOC catalysts utilize precious metals, such as Pt and Pd. However, due to their short life and expensive cost (5), transition metal oxides, such as Ba (6) , Co (7), Cd, Cu (5) , Mn (8; 9) , and Cr (10) along with supports, have been studied as VOC oxidation catalysts in recent publications. Cryptomelane has also been reported to be an efficient, environmentally friendly, and inexpensive catalyst for VOC oxidation (11; 12).

Dopants or mixed metal oxides often result in significant improvement in catalytic activity compared to single metal oxides, increasing active sites in the support materials. Therefore, a series of transition metal-doped cryptomelane have been synthesized to enhance the catalytic activity. Dopants have included CuO (13) , V (14), Fe, Cr (15) , and Ag (16) , among others. Some of the dopant materials, such as V and Ag, improved the observed catalytic activity of cryptomelane (14; 16).

This study focuses on the preparation, characterization and evaluation of Zn- doped cryptomelane for the gas-phase catalytic oxidation of ethanol. In this study, two preparation methods were compared: 1) a sol-gel method to synthesize cryptomelane with and without zinc dopant and calcined in air at temperatures ranging from 350℃ to 600℃, and 2) a method that starts with alkaline battery waste. The physical characteristics and catalytic activity of these cryptomelane samples were investigated and compared to assess the roles of zinc dopant and calcination temperature on cryptomelane structure and catalytic activity.

2 II. Literature Review

2.1 Preparing methods

2.1.1 Overview

Various methods have been used in the laboratory to synthesize cryptomelane. The synthesis method has an impact on the ultimate morphology, surface area, and crystallinity, hence the synthesis method will also impact the catalytic activity of cryptomelane. The major methods of synthesis include reflux, sol-gel, and milling methods.

2.1.2 Reflux Method

The reflux method is the most common preparation method to make cryptomelane. It requires redox reactions between permanganate salts and organic acids (11). Material precipitates from a solution of potassium permanganate and manganese salts, such as manganese sulfate monohydrate or manganese acetate. Typically the solution is refluxed for up to 24 hours before the resulting solids are filtered, washed, dried, and calcined to produce cryptomelane (11).

2.1.3 Sol-gel Method

In the sol-gel method, a highly-concentrated potassium permanganate (KMnO4) solution reacts with fumaric acid (C4H4O4) in a molar ratio of 3:1 to form a brown gel (17) . The gel is then filtered, washed, dried, and finally calcined in air at 450℃ for 2h. Finally the black cryptomelane is washed again with both deionized water and acid to remove the surface potassium. The acidity of the solution is also important for sol-gel synthesis of cyrptomelane. If the pH increases to 9, the formation of Mn2O3 will occur. The sol-gel method requires less time for cryptomelane synthesis than other methods of synthesis. Moreover, the doping process is achieved by mixing the dopant into the initial synthesis solution. The BET surface area of sol-gel cyrptomelane is often higher than that for cryptomelane prepared by other methods (18).

A modification of the sol-gel synthesis method is to add cross-linking reagents (PVA, glycerol, or glucose) into the sol. The cross-linking agents can change the morphology of the cryptomelane from nanorods to nanoneedles to nanowires. The cross-linking agent is added to the sol in a ratio of 1:2 manganese salt/cross-linking reagent. The solution is evaporated and then calcined in air or burned in a flame. Cross-linking could largely increase the length of the cryptomelane crystal.

2.1.4 Solid-State/Milling Method

The solid-state or milling method is the reaction between Mn7+ and Mn2+ in a milling environment, mostly by adding KMnO4 and Mn(CH3COO)2*4H2O in 2:3 stoichiometric ratio. After 1 hour the product is washed, dried under 80℃ and calcined to obtain cryptomelane.

2.1.5 Dopant Methods

Dopants can be added to cryptomelane by adding the dopant salt to the cryptomelane precursors during synthesis. Under an acidic environment, transition metals can be doped into the tunnel or framework by an ion-exchange reaction. Using the ion exchange, the amount of dopant that can be loaded onto cryptomelane is limited. Liu et. al. found that the dopant metal contents in cryptomelane are limited due to their morphology (18).

2.2 Catalytic property of cryptomelane

Cryptomelane has been investigated as a useful and active catalyst in oxidation reactions with VOC compounds such as ethyl acetate, 2-propanol, benzene (19) (20), toluene (21), benzyl alcohol (22), and ethanol (23; 24). In the catalytic performance test of ethyl acetate performed by Santos, the pure cryptomelane show a 100% of CO2 selectivity at a reaction temperature of 200℃ (11). The same result was also obtained with benzyl alcohol and ethanol at reaction temperatures lower than 300 C (11).

4 2.2.1 Catalytic Mechanism

The presence of mixed ionic states of Mn (Mn3+/Mn4+ redox couples) and the lattice oxygen in cryptomelane enables VOC catalytic oxidation reactions (25). The Mars Van Krevelen catalytic mechanism has been investigted for VOC oxidation on cryptomelane. This mechanism is represented by the expressions provided in Equations 2.1 and 2.2. In these equations, M=O represents the manganese oxide, S represents an organic compound, and M is the reduced catalyst (25).

M=O+S M+S=O (2.1) 2M+O22M=O (2.2)

In this mechanism, lattice oxygen of cryptomelane participates the oxidation reaction (Equation 2.1) and subsequently reacts with oxygen in air to return to M=O (Equation 2.2). Two electrons are donated by the oxygen atom at the same time in the second step (25).

2.2.2 Effect of Synthesis Method The effect of synthesis method on the catalytic activity of cryptomelane was previously studied (26; 27). The synthesis method can have a significant impact on the resulting structure and morphology of cryptomelane, which, in turn, will impact the catalytic activity for VOC oxidation reactions. Cryptomelane prepared by the reflux method showed higher catalytic activity for the oxidation of toluene compared to cryptomelane prepared by the milling method (26). The milling method disfavored the [0 0 1] growth (27), which may have impeded the activity for the catalyst for VOC catalytic oxidation reactions. One explanation for the observed poor performance is that there were fewer active lattice oxygen sites in the layers near the surface, since they usually are present in the [0 0 1] plane (28). Different crystal orientation or discontinuous growth of the tunnel may affect the availability of lattice oxygen in the surface for the reaction. High performance cryptomelane for alcohol oxidation may relate to different planes or crystal defects (24).

5 2.2.2 Effect of calcination temperature

Jing et al. investigated the influence of calcination temperature on the catalytic activity of cryptomelane for the oxidation of benzyl alcohol (29). It was observed that amorphous MnOx had a better mass specific activity and a lower onset decomposition temperature compared to highly crystallized cryptomelane that had been calcined under a higher temperature (29).

Liu et al investigated the effect of calcination temperature on crystal growth (18). They found that a cryptomelane phase first appears at 300℃, but the majority is

γ-Mn2O3. The γ-Mn2O3 peaks disappear following calcination at 500℃. In the 500℃ to 800℃ region, the intensities of cryptomelane peaks increase. The cryptomelane peaks become weaker when the calcination temperature reaches 900℃, and a hausmannite phase is formed.

Calcination temperature impacts surface area, where, in general, the higher the calcination temperature, the lower the surface area (28) (24). Some researchers have emphasized that cryptomelane surface area is not connected with catalytic activity (30). However, others believe larger cryptomelane surface area will result in greater conversion in VOC degradation (30).

2.2.3 Transition metal doped cryptomelane as a catalyst

Liu et al. doped cryptomelane with transition metals and calcined the resulting materials at 800℃. The results showed that only Fe and Co-doped cryptomelane contained a crystalline phase. Co-doped cryptomelane was found to be especially thermally stable. Birnessite and cryptomelane were both found to have a high absorbency for cobalt. Tang et al. synthesized 2% V/cryptomelane by the reflux method and calcined the resulting materials at 500℃. The V-doped cryptomelane had improved catalytic activity over pure cryptomelane. The improvement was likely due to the increased concentration of oxygen vacancy sites on the surface of the catalyst (14).

Although transition metals can be doped into cryptomelane, their detection can be challenging. Hernandez et al. synthesized 10% Cu-, Ni-, Co- Zn-modified

6 cryptomelane manganese dioxide using a milling method (27). The absence of CuO, NiO, CoO or ZnO on X-ray diffraction patterns indicates three possibilities: 1) the content of transition metals is under the detection limit; 2) transition metal cations occupy the Mn4+ or Mn3+ cations within the cryptomelane structure; and 3) transition metals are incorporated into the channel of the nanomaterial. (27)

Liu et al. have designed experiments on metal-doped cryptomelane calcined at 800℃. The results show that only Fe and Co-substituted materials contain crystalline phases while others just form amorphous MnOx mixtures. Co-doped cryptomelane was found to be especially stable, likely due to the cobalt ion’s greater stability in the tunnel structures compared to the K+ ion.

2.2.4 Catalytic deactivation

Deactivation of cryptomelane is typically due to coke deposition on the porous structure (11). Catalytic activity of cryptomelane is associated with its crystal structure and morphology and its ability to transfer oxygen to the surface. Therefore, stabilizing the cryptomelane for prolonged service life in VOC oxidation reactions is still a challenge that must be overcome.

2.3 Battery-derived cryptomelane Recently a method of recycle cryptomelane from waste battery has been invented (2). The resulting material shows cryptomelane crystal morphology in both SEM and TEM pictures as well as in X-ray diffraction patterns. Zn is also found in the material at concentrations of 1~5 wt% in the battery-derived cryptomelane (2).

7 III Hypothesis

It is hypothesized that zinc-doped cryptomelane will be a better catalyst in ethanol oxidation reactions than pure cryptomelane. A second hypothesis is that cryptomelane made with battery waste will have comparable activity to those zinc-doped catalysts made in the lab. In order to validate the hypothesis the following objectives will be met:  Investigate the effects of calcination temperature on the physical properties and catalytic activity of cryptomelane prepared by the sol-gel method.  Assess the effects of zinc dopant on the catalyst activity of Zn-doped cryptomelane, as measured by the vapor phase oxidation of ethanol.  Compare the catalytic activities of the lab-made catalysts with those made from battery wastes. The objectives can be spilt into three tasks: synthesis, characterization, and catalysis.

3.1 Synthesis

The sol-gel method was used to synthesize cryptomelane in this study. Table 3.1 summarizes the catalysts prepared for this this study.

Table 3.1 Synthesis matrix contain zinc concentration

Calcination Zinc-Doped Cryptomelane Temperature℃ 0 wt % 1 wt % 2.5 wt % 5 wt % 350 350, 0% 350, 1% 350, 2.5% 350, 5% 400 400, 0% 400, 1% 400, 2.5% 400, 5% 500 500, 0% 500, 1% 500, 2.5% 500, 5% 600 600, 0% 600, 1% 600, 2.5% 600, 5%

The catalysts made from battery waste were prepared by Dr. Mark Krekeler (Department of Geology & Environmental Earth Science, Miami University, Oxford, Ohio).

8 3.2 Characterization

Each sample made in the Table 3.1 was characterized by methods listed in Table 3.2. The BET surface area, XRD, SEM, and TEM provide data for the surface area, crystallinity and crystal structure, and morphology of the cryptomelane samples respectively.

Table 3.2 Summary of characterization methods used for cryptomelane samples Characterization Equipment Property Brunauer, Emmett and Teller (BET) Surface area X-ray Diffraction (XRD) Crystal structure Scanning Electron Microscopy (SEM) Surface information Crystal image and Transmission Electron Microscopy (TEM) structure

9 3.3 Catalysis

All of the samples in Table 3.1 and a selected catalyst made from battery waste were used as catalysts for the oxidation of ethanol under constant conditions. The test system used for the catalytic oxidation tests is shown in Figure 3.1. Ethanol vapor was generated at a concentration of approximately 2000 ppm in air using a diffusion cell. The ethanol vapor in air was directed through a ¼” OD stainless steel tube reactor containing 100 mg catalyst in each run. The reactor effluent was collected in impingers filled with water containing an internal standard (isopropanol). The resulting water solutions were analyzed by gas chromatography with flame ionization detection (GC/FID). The GC/FID was calibrated for ethanol in water and for acetaldehyde in water, using isopropanol as an internal standard.

Ethanol oxidation results in two predominant products: acetaldehyde and carbon dioxide. The ethanol oxidation reaction is shown in Equation 3.1.

C2H5OH +1/2 O2  CH3CHO + H2O (3.1)

CH3CHO +5/2 O2  2 CO2 +2 H2O

Carbon dioxide was collected by a precipitation method in KOH/ethanol solution. The carbon dioxide reacts with KOH to form potassium bicarbonate, which is insoluble in ethanol. The reaction is shown in Equation 3.2.

CO2 + KOH  KHCO3(s) (3.2)

The potassium bicarbonate was subsequently filtered, dried, and weighed to gravimetrically determine the amount of CO2 captured. After each experimental trial, a carbon balance was conducted to assess losses of carbon. The carbon balances confirm that the predominant carbon-containing compounds in the

10 inlet and effluent streams are ethanol, acetaldehyde, and carbon dioxide.

Figure 3.1 Schematic of test system used for the catalytic oxidation of ethanol.

The goals of the catalyst performance testing were to determine the following:  The effect of Zn loading on cryptomelane catalytic activity.  The effect of calcination temperatures on catalytic activity.

11 IV. Experiment

The total experiment is divided into three parts: synthesis, characterization, and catalysis. All the chemicals used during those three processes are listed below in Table 4.1.

Table 4.1 Summary of chemicals used in this study.

Name Chemical Formula CAS Purity/ Manufacturer Number Conc. Potassium KMnO4 7722-64-7 98% Alfa Aesar permanganate Zinc acetate (C2H3O2)2Zn 557-34-6 99.99% Aldrich Fumaric acid COOHCH=CHCOOH 110-17-8 99% Alfa Aesar Hydrogen HCl/H2O 7647-01-0 1 mol Fisher Scientific chloride soln Potassium KOH 1310-58-3 98% Fisher Scientific hydroxide Ethanol C2H5OH 64-17-5 100% Fisher Scientific

4.1 Catalyst Synthesis

To synthesize cryptomelane, a solution was prepared first by dissolving 6.58 g (4.2 mmol) of potassium permanganate, a desired percentage (0%, 1%, 2.5%, 5% ) of zinc acetate salt, and 1.625 g (1.4mmol) of fumaric acid in 200 ml water. The molar ratio of salts was accurately measured to be a 3:1 ratio. The mixture was magnetically stirred for 30 minutes at room temperature to form a brownish solution. The mixture was then left to settle until a gel formed, typically within an hour. After a brownish gel had been formed, the gel was filtered and washed with 100 mL of deionized water four times. The gel was dried in an oven at 110 C overnight, and aliquots of the sample were calcined at different temperatures (350℃, 400℃, 500℃, 600℃ ) in air for two hours and slowly cooled down.

The crude products form black luster powders. The powders were then washed with both DI water and 0.1mol hydrogen chloride solution three times each. The

12 final products were crushed and ground into fine black powders. A pictoral diagram of the catalyst synthesis process is shown in Figure. 4.1.

Figure 4.1 Procedure for synthesizing cryptomelane using the sol-gel method

4.2 Characterization

4.2.1 BET

A Beckman Coulter SA3100 surface area analyzer was used to measure the BET surface areas of the cryptomelane samples by nitrogen adsorption at 77K. To conduct the BET surface area analyses, each sample cell was loaded with a measured amount of catalyst (approximately 0.1~0.2 g). The catalyst was outgassed in the SA3100 BET surface area analyzer under vacuum at 125℃ for 15 minutes. Following outgassing, the sample’s surface area was measured by

13 nitrogen adsorption in a liquid nitrogen environment (-196℃). The SA3100 collects adsorption isotherm data, from which the BET surface area is calculated.

4.2.2 XRD

X-ray diffraction (XRD) analyses were performed on a Scintag X1 X-Ray Diffractometer, and Scintag software was used to compare the XRD patterns to reference patterns to identify crystal structure. A filament current of 35 mA and voltage of 40kV was maintained. A diffractometer scan rate of 0.2 degree per 4 seconds was used, which takes 3.5 hours to finish one sample. Phase identification analysis was performed using powder X-ray diffraction (XRD) methods and the computer program JADE.

4.2.3 SEM EDS

A Zeiss Supra35 Scanning Electron Microscope (SEM) was used to visualize the morphology and surface structures on the cryptomelane samples. Energy dispersive X-ray spectrometry with the SEM was used to measure the atomic compositions of the cryptomelane samples.

4.2.4 TEM EDS

A JEM2100 Transmission Electron Microscope (TEM) was used to investigate the crystal morphology, compositions and crystallinity of the cryptomelane samples. The samples were prepared by placing proper amounts of fine powder cryptomelane into 5 mL of pure ethanol. After shaking for several minutes, one drop of the highly dispersed slurry was placed onto a silicon grid and dried.

4.3 Catalysis

4.3.1 Preparation

An Agilent 6890 gas chromatograph with a flame ionization detector (GC/FID) was calibrated for ethanol and acetaldehyde in water. To reduce error due to the small injection volumes (1/2 μL) used for GC/FID analyses, isopropanol in the water was used as an internal standard, and the ethanol and acetaldehyde peak areas were compared to the isopropanol peak area for quantitative

14 analysis. The isopropanol internal standard was in a concentration of approximately 400 mg/L.

Carbon dioxide was generated at concentrations too low for detection by the Thermal Conductivity Detector (TCD) that was available on the GC. Therefore, it was collected in a solution of KOH in ethanol, and the resulting KHCO3 was collected as a precipitate via filtration, and subsequently analyzed gravimetrically. The KOH concentration in ethanol was 2.5 g/L. A control catalysis test was conducted with only glass wool in the reactor. Under the conditions of the catalytic experiments, less than 20 % of the ethanol had degraded at a reactor temperature of 200℃.

4.3.2 Performance experiment

The catalytic oxidation of ethanol experiments were performed at 1 atmosphere pressure. An air flow through the reactor was held constant at 90 ml/min using an MKS Instruments mass flow controller. The ethanol vapor in the air was generated by a temperature-controlled diffusion cell. For each test, 100 mg of catalyst was used. The reactor was a ¼ inch OD stainless steel tube with glass wool on both sides of the catalyst positioned in the center of the tube. The tube was set into a Lindburg Blue tube furnace.

Figure 4.2 Schematic diagram of the reaction system

15 After 1 hour at each reactor temperature, the reactor effluent was collected for 10 minutes with a midget impinger (ACE Glass) containing 15ml of water with the isopropanol internal standard. The water solution was subsequently analyzed for ethanol and acetaldehyde concentrations using the GC/FID. A separate impinger was used to collect the reactor effluent in 15 mL of the

KOH/ethanol solution to analyze the CO2 generated in the reactor. Reactor temperatures of 25℃, 100℃, 150℃, 175℃, 200℃, 225℃ were used in this study.

Figure 4.3 Procedure by which catalytic oxidation data were collected.

16 V. Results and Analysis

5.1 Characterization

5.1.1 BET surface area

All of the cryptomelane samples (summarized in Table 3.1) have been analyzed for BET surface area. The results are summarized in Table 5.1 and in Figure 5.1.

Table 5.1 Summary of BET surface areas of cryptomelane samples

Calcination Temperature Surface Area (m2/g) ℃ 0% Zn 1% Zn 2.5% Zn 5% Zn 350 74 67 100 65 400 45 48 61 69 500 32 71 39 40 600 17 26 26 27

100 0 0.025

80 0.01 0.05

BET Surface Area (m2/g) Area Surface BET 0 350 400 450 500 550 600 Calcination temperature (℃)

Figure 5.1 Effect of zinc dopant and calcination temperature on the BET surface area of cryptomelane.

17 From Table 5.1 and Figure 5.1, the BET surface area decreases with calcination temperature, likely due to sintering of small crystal structures and crystal growth (31). However, the trend with zinc dopant is less apparent. The zinc dopant appears to enhance the surface area slightly, which is likely due to defects imparted to the microstructure of the cryptomelane. This was also observed by Jia Liu, who found a 15-20 m2/g increase in surface area when cryptomelane was doped with Li or Pb (32).

5.1.2 X-ray Diffraction

The X-ray diffraction patterns for the cryptomelane samples prepared in this study by the sol-gel method are shown in Figure 5.2. Noted is that the crystallinity of the samples increase with calcination temperature but decrease with zinc dopant. This is shown by the sharpening of the peaks as calcination temperature increases and the broadening of the peaks as zinc dopant increases. Figure 5.4 compares the XRD patterns of pure cryptomelane and with those of cryptomelane doped with 2.5% zinc. Noted is that after calcination at 600℃, the crystal phase, birnessite, is observed. In Figure 5.4, this structure is marked with a ‘#’ symbol.

350 C 500 C

400 C 600 C

Figure 5.2 Comparison of X-ray diffraction patterns for four Zn-doped cryptomelane samples calcined at 350℃ , 400℃ , 500℃, and 600℃.

Figure 5.3 XRD crystal structures at different calcination temperatures with pure cryptomelane (upper) and 2.5 weight % Zn/cryptomelane (lower). Cryptomelane peaks have been marked above. # Refers to birnessite crystal peaks.

20 5.1.3 SEM

Figure 5.4 shows SEM images of cryptomelane calcined at different temperatures. Crystal lengths significantly increase with calcination temperature, which supports the observed increase in sharpness of the XRD patterns in Figure 5.3. After calcination at 350℃, aggregate spherical cryptomelane can be found in Chen et al.’s work (33). He used a higher magnification image to explain that the observed ball shaped material consists of small cryptomelane crystals. After calcination at 600℃, a plate morphology is observed. This plate morphology is consistent with the presence of birnessite (34), which also supports the presence of birnessite that was observed in XRD patterns in Figure 5.3..

Figure 5.4 SEM image of pure cryptomelane under (a) 350℃ (b) 400℃ (c) 500℃

(d) 600℃calcination temperatures with the same magnification of 100,000x (e)

400℃ (f) 500℃ under 50,000x

Figure 5.5 An overview of the crystal quantity that grows from the sample rock following calcination at 400℃ (left) and 600℃ (right).

The needle-like crystal structures are present in much higher concentrations after calcination at 600℃ compared to a calcination temperature of 400℃, as shown in Figure 5.5. Based upon the SEM images, an estimation of both crystal length and width as functions of calcination temperature were made. The results are listed in Table 5.2 and are shown graphically in Figures 5.6 a and b. Noted is that the crystal length increases significantly between calcination temperatures of 350℃ and 400˚ C, and then the length increases only modestly between calcination temperatures of 400℃ and 600℃. The crystal width trend is less obvious; however, there appears to be an increasing trend in crystal width with calcination temperature.

23 Table 5.2 Crystal length and width estimated from SEM pictures.

Calcination Upper Lower Average Length Temperature℃ Limit (nm) Limit (nm) (nm) 350 554 200 377 400 1429 714 1071 500 1700 750 1225 600 1846 831 1338 Calcination Upper Lower Average Width Temperature℃ Limit (nm) Limit (nm) (nm) 350 31 62 46 400 143 57 100

500 75 33 54 600 77 38 58

Figure 5.6a and 5.6b show a trend line between calcination temperature and crystal length and width, respectively. The trend indicates a significant initial increase in length between calcination temperatures of 350℃ and 400℃. However, the trend in crystal width is not as apparent with calcination temperature.

Figure 5.6 Calcination temperature vs. crystal length (a, upper) and crystal width (b, lower)

Figure 5.7 SEM image of cryptomelane calcined at 500℃ with 0%(left), 2.5%(middle) and 5%(right) zinc/cryptomelane under a magnification of 100,000x

Figure 5.7 shows a decrease in crystallinity with zinc dopant after calcination at 500℃. This is noted by the shortening of the crystal length and thinning of the crystal diameter.

Although deactivation of cryptomelane was not investigated in this study, Figure 5.8, which shows SEM images of a sample of cryptomelane calcined at 500℃ before and after being used in the ethanol oxidation reaction, suggests that the crystals become coated, likely with carbon deposition. Further investigation is needed to identify the particle composition.

Figure 5.8 SEM image of the 500℃ 2.5% cryptomelane before (upper) and after (lower) catalytic reaction under the same magnification of 50,000x

27 The SEM image of battery-derived cryptomelane is shown in Figure 5.9. The surface of this metal oxide is likely to be a mixture of crystal and bulk morphology. EDS shows the smooth surface area in the 10,000x image

(indicated with a black arrow) is likely K2S. The sulfur likely originates with the manganese oxide used to manufacture the alkaline batteries.

Figure 5.9 SEM image of the battery-derived cryptomelane under magnification of 50,000 x (left) and 10,000 x (right)

5.1.4 TEM

TEM images and diffraction patterns of cryptomelane calcined at 500℃ with 0% and 1% zinc dopant are shown in Figure 5.10. The ring and dot pattern in Figure 5.10a show the crystal structure of the material. However, in Figure 5.10b, the ring and dot pattern are indicative of a more amorphous structure. This supports the observations made with XRD and SEM analyses, where the presence of zinc decreases the crystallinity of the cryptomelane samples.

Figure 5.10 TEM images and diffraction patterns of 400℃-calcined samples (a) 0% (b) 1% of zinc/cryptomelane under different magnifications, the scale is on the picture.

Figure 5.11 TEM images of in pure cryptomelane crystal under 400℃ (left) and 600 ℃ (right) calcination temperatures in the same scale

The crystal shape changes with the calcination temperature. In Figure 5.11, the crystal structure is shorter and wider after calcination at 400℃ and much longer and thinner after calcination at 600℃.

The cryptomelane tunnel structure can be seen clearly in Figure 5.12, where the tunnel structure is pointed to with a white arrow. The image is very similar to the TEM images in Santos et al.’s work (35).

Figure 5.12 TEM image of 400℃-calcined pure cryptomelane under the same magnification; the tunnel and lattice structure can be clearly seen near the white arrow.

31 5.2 Catalytic Performance

5.2.1 The effect of calcination temperature on catalytic activity for ethanol oxidation.

Figure 5.13 shows the effect of calcination temperature on the catalytic activity of cryptomelane for the catalytic oxidation of ethanol. The data represent average ethanol conversions observed as a function of reaction temperature for cryptomelane calcined at 350℃, 400℃, 500℃, and 600℃. Noted is that the apparent catalytic activity decreases with increasing calcination temperature. This is likely due to the reduction in surface area of the cryptomelane with calcination temperature. The same trend could be found in other catalyst systems. For example, Susumu Tsubota et al. discovered that the effective catalytic activity decreases for CO oxidation with increasing calcination temperature for Au/TiO2 (35).

Figure 5.13 Ethanol conversion as a function of reaction temperature and cryptomelane calcination temperature.

5.2.2 The effect of Zn dopant on catalytic activity for ethanol oxidation

Figure 5.14 graphically shows that zinc dopant on the sol-gel cryptomelane samples used in this study have no observable effect on catalytic activity.

32 100% 0%

80% 1% 2.50% 60% 5%

40%

20% Ethanol Conversion Ethanol 0% 25 50 75 100 125 150 175 200 225 ℃ Reaction Temperature ( ) Figure 5.14 Reaction temperature vs. average ethanol conversion in different zinc/cryptomelane weight loadings

5.2.2 Battery-derived cryptomelane performance

Figure 5.15 shows the performance of battery-derived cryptomelane. This material was calcined at 550℃, and its surface area was approximately 5 m2/g. Its performance as a catalyst for ethanol oxidation is comparable to the sol-gel cryptomelane calcined at 500℃. This implies that if battery waste can be used to prepare cryptomelane with higher surface area, the apparent catalytic activity will also increase.

100%

80% 60% 40%

Conversion 20% Relative EtOH EtOH Relative 0% 25 50 75 100 125 150 175 200 225 250 Reaction Temperature (℃ )

Figure 5.15 Reaction temperature vs. average ethanol conversion in battery-derived sample

33 5.2.3 Activation Energy

By assuming the catalytic oxidation of ethanol in air is a first-order reaction, a relationship between the concentration and reaction rate can be written as:

(5.1)

where A represents the available surface area on the catalyst; k (s-1 cm-2) is the reaction rate constant, and C is the concentration of ethanol (mg/cm3). Upon integration:

(5.2)

where τ is the residence time (s). The activation energies were calculated using Arrhenius’s equation:

⁄ (5.3) where Ao is a constant; E is activation energy (kJ/mole); R is the gas constant (kJ/mole K); and T is absolute temperature (K). Plotting ln k vs 1/T should result in a linear correlation with a slope that equals -E/R.

⁄ (5.4)

Figure 5.16 shows representative plots of by which the activation energies were calculated.

2.5

2 400 C 600 C 1.5 1 0.5 y = -6038.7x + 14.88 R² = 0.9988 0 y = -10359x + 23.895 -0.5 R² = 0.9997

ln (apparent rate constant, k) constant, rate (apparent ln -1 -1.5 0.002 0.0022 0.0024 0.0026 0.0028 Inverse Temperature (1 / K)

Figure 5.16 Representative plots to calculate activation energies. Figure 5.17a and b show the activation energies calculated as functions of calcination temperature and zinc dopant, averaged for all samples investigated in this study. Noted is that the activation energies averaged approximately 43 kJ/mole for cryptomelane calcined at temperatures at or below 500℃. The activation energy increases for cryptomelane calcined at 600℃ to approximately 68 kJ/mole. Since the rate constants were normalized with the surface area of the catalysts, this increase in activation energy is likely a function of crystal structure and morphology. Similar results could be found in Susumu et al.’s work in the CO oxidation, where the activation energy increased with calcination temperature in the Au/TiO2 catalyst system (35). There is little to no observed trend regarding the effect of zinc dopant on the activation energy.

100

20 Activation Energy (kJ/mole) Energy Activation

0 350 C 400 C 500 C 600 C Calcination Temperature ( C )

100

20 ActivationEnergy (kJ/mole)

0 0% 1% 2.50% 5% Zinc Dopant (wt%)

Figure 5.17 Activation energy vs. calcination temperature (upper) and zinc dopant (lower)

36 5.2.4 Mass balance in catalytic reaction

Figures 5.18 and 5.19 show the composition of the reactor effluent for selected trials as a function of reaction temperature. Noted is that ethanol partially oxidizes to acetaldehyde and then ultimately to carbon dioxide. A carbon balance was obtained for each reaction temperature, as shown in Table 5.3. The carbon balance error shows supports that acetaldehyde and carbon dioxide are predominant oxidation products in our experiments.

Table 5.3 Summary of carbon balances conducted at each reaction temperature for a selected experimental trial.

Reaction Ethanol Ethanol Acetald. CO2 Carbon Carbon % Temp IN OUT OUT OUT IN OUT Error (℃) umol/min μmol/min μmol/min μmol/min umol/min umol/min 25 6.1 6.1 0.0 0.0 12.2 12.2 0 100 6.1 4.3 0.0 2.0 12.2 10.6 13.1% 150 6.1 1.8 0.6 5.2 12.2 10 18.0% 175 6.1 1.1 0.8 9.8 12.2 13.6 -11.4% 200 6.1 0.0 0.8 12.2 12.2 13.8 -13.1% 225 6.1 0.0 0.7 15.2 12.2 16.6 -36.1%

0.008 Calcuated 0.5*CO2 0.007 EtOH inlet 0.006 Acealdehyde 0.005 0.004 0.003 0.002

0.001 Flowrate (mmol/min) Flowrate 0 25 75 125 175 225 Reaction Temperature (℃ )

Figure 5.18 Reactor effluent composition vs. reaction temperature with the catalyst calcined at 350℃ and with 2.5% Zn.

37 0.014

0.012 0.01 Calcuated 0.5*CO2 EtOH inlet 0.008 Acealdehyde 0.006 0.5*CO2 0.004

0.002 Flowrate (mmol/min) Flowrate 0 25 75 125 175 225 ℃ Reaction Temperature ( ) Figure 5.19 Reactor effluent composition vs. reaction temperature with the catalyst calcined at 500℃ and with 2.5% Zn.

5.2.5 Temperature at which ethanol half decomposed

Figure 5.20 shows the temperature at which 50% conversion of the ethanol was observed as functions of calcination temperature and zinc dopant. Noted is that a clear trend exists for calcination temperature, where the temperature at which 50% ethanol is degraded increases with calcination temperature. However, the effect of zinc dopant is not clear, as indicated by the relatively flat lines with zinc dopant concentration.

)

200 ℃

150

100 350C 400C 50

Reaction Temperature ( Temperature Reaction 500C 600C

0 0% 1% 2% 3% 4% 5% Zn Percentage Figure 5.20 Zinc percentage vs. reaction temperature when the ethanol outlet percentage reaches 50%

39 VI. Conclusion

The conclusions of this study are:

1) The calcination temperature, hence the surface area, plays a key role in the apparent activity of cryptomelane for the oxidation of ethanol. 2) There is very little effect of zinc dopant on the catalytic activity. 3) Zinc dopant may impact the crystallinity and crystal structure of sol-gel synthesized cryptomelane.

These conclusions have implications with respect to the battery-waste derived cryptomelane:

1) Can cryptomelane be prepared from battery waste with higher surface area? 2) The zinc impurity in the battery waste should have little effect on the catalytic activity of battery waste-derived cryptomelane.

Future work should include the following:  Long-term catalysis experiments with cryptomelane to investigate catalyst deactivation and its mechanisms.  Catalysis with other representative VOCs to assess its performance for other oxygenates, aromatics, and chlorinated hydrocarbons.  The role of zinc as a dopant on the surface of cryptomelane as opposed to within its crystal structure.  Reasons for zinc dopant being limited in concentration on/within sol-gel- derived cryptomelane.

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