Hierarchical Porous Structures with Aligned Carbon Nanotubes as Efficient Adsorbents and Metal ...

226 Pages·2016·7.9 MB·English

by  Hema Vijwani

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Hema Vijwani
Wright State University
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Hierarchical Porous Structures with Aligned Carbon Nanotubes
as Efficient Adsorbents and Metal-Catalyst Supports

A dissertation submitted in partial fulfillment
of the requirements for the degree of
Doctor of Philosophy

By

Hema Vijwani
M.S., Wright State University, 2011

2015
Wright State University
WRIGHT STATE UNIVERSITY
THE GRADUATE SCHOOL
May 21, 2015
I HEREBY RECOMMEND THAT THE DISSERTATION PREPARED UNDER MY
SUPERVISION BY Hema Vijwani ENTITLED Hierarchical Porous Structures with
Aligned Carbon Nanotubes as Efficient Adsorbents and Metal-Catalyst Supports
BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF Doctor of Philosophy.

_________________________________
Sharmila M. Mukhopadhyay, Ph.D.
Dissertation Director
___________________________________
Ramana V. Grandhi, Ph.D.
Director, Ph.D. in Engineering Program
_________________________________
Robert E. W. Fyffe, Ph.D.
Vice President for Research and Dean of the
Graduate School

Committee on Final Examination
________________________________
Sharmila M. Mukhopadhyay, Ph.D.
________________________________
Mallikarjuna N. Nadagouda, Ph.D.
________________________________
Amir A. Farajian, Ph.D.
________________________________
Hong Huang, Ph.D.
________________________________
Mark N. Goltz, Ph.D.
ABSTRACT

Vijwani, Hema. Ph.D., Department of Mechanical and Materials Science Engineering,
Engineering Ph.D. Program, Wright State University, 2015. Title: Hierarchical Porous
Structures with Aligned Carbon Nanotubes as Efficient Adsorbents and Metal-Catalyst
Supports.

The overall goal of this study is two-fold: synthesis of multiscale nanostructures
by growing aligned carbon nanotubes on porous foam substrates and investigation of
their applicability as adsorbents and catalyst supports for environmental remediation
applications. High purity, vertically-aligned arrays of carbon nanotubes (CNT) are grown
on open-cell interconnected porous carbon foams by pre-activating them with an oxide
buffer layer followed by chemical vapor deposition (CVD). This type of hierarchical
morphology provides the capability of increasing surface area by several orders of
magnitude, while tuning its morphology for targeted applications. Analytical models are
also proposed in this study for specific surface area calculations, those agree well with
the experimental measurements. These hierarchical carbon materials are seen to be
powerful adsorbents of aqueous pollutants such as methylene blue dye. Their monolayer
adsorption capacities correlate very well with the total CNT surface area determined from
analytical models and with BET measurements, indicating full utilization of the nanotube
surfaces.
The hierarchical structures can also serve as base supports for attachment of metal
nanoparticle catalysts. The catalysts investigated in this study are metallic palladium
(Pd), oxidized palladium (PdO), and silver-palladium (Ag-Pd) nanoparticles combination.
These are suitable for a variety of industrial applications such as hydrocarbon conversion,
hydrogen storage, fuel cell electrodes and pollutant degradation. The current architecture
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allows synthesis of highly active catalyst structures utilizing very small quantities of
precious metal that make the catalyst component significantly lighter and more compact
than conventional systems. Detailed characterization of structure and surface chemical
states of these nano-catalysts have been performed and their catalytic activities are tested
by measuring the degradation kinetics of organic contaminants via bench-scale
experiments. Catalytic degradation of atrazine, an emerging problematic contaminant,
was quantified using high-performance liquid chromatography. Among Pd, PdO, and Ag-
Pd nanoparticles, PdO in the presence of hydrogen was seen to provide the most rapid
reaction rate. These nanocatalysts also enable rapid degradation of chlorinated
hydrocarbons such as trichloroethylene and trichloroethane quantified using head-space
gas chromatography, with PdO providing the fastest kinetic route.
Durability tests indicated that the nano-particles and nanotubes are robust, and
remain attached to the base support after long periods of rapid rotation in water. These
results imply that such materials can provide compact and powerful surface active
materials in future applications such as adsorbents, catalysts, porous electrodes, and
energy storage devices.

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TABLE OF CONTENTS

1. Chapter 1: Introduction and Background .................................................................... 1
1.1 Importance of Surface Area ................................................................................. 1
1.2 Porous Materials ................................................................................................... 2
1.3 Hierarchical (Multi-Scale) Materials ................................................................... 3
1.3.1 Nature Inspired Hierarchical Materials ......................................................... 3
1.3.2 Emerging Hierarchical Materials .................................................................. 3
1.4 Current Trends in Water Treatment Applications ................................................ 4
1.4.1 Water Contamination and Purification Issues............................................... 4
1.4.2 Nanomaterials used in Water Purification .................................................... 5
1.5 Research Objectives ............................................................................................. 7
1.6 Dissertation Outline.............................................................................................. 7
2 Chapter 2: Materials and Methods ............................................................................ 10
2.1 Chemicals ........................................................................................................... 10
2.2 Porous Materials used in this Study ................................................................... 10
2.2.1 Micro-Cellular Carbon Foam (Ce-Foam) ................................................... 10
2.2.2 Reticulated Vitreous Carbon Foam (RVC-Foam) ...................................... 11
2.3 Experimental Methods ....................................................................................... 13
2.3.1 Fabrication of Carbon Nanotubes on Porous Structures ............................. 13
2.3.2 Synthesis of Supported Palladium Nano-Particles ..................................... 17
2.4 Characterization Techniques .............................................................................. 18
2.4.1 Scanning Electron Microscopy (SEM) ....................................................... 18
2.4.2 Energy Dispersive X-Ray Spectroscopy (EDS) ......................................... 19
2.4.3 X-Ray Photoelectron Spectroscopy (XPS) ................................................. 19
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2.4.4 X-Ray Diffraction (XRD) ........................................................................... 20
2.4.5 UV-Vis Spectrophotometry (UV-Vis) ........................................................ 20
2.4.6 Brunauer-Emmett-Teller Surface Area Analysis (BET) ............................. 20
2.4.7 Gas Chromatography – Mass Spectrometry (GC-MS) ............................... 20
2.4.8 Liquid Chromatography – Mass Spectrometry (LC-MS) ........................... 21
3 Chapter 3: Fabrication of Vertically Aligned Carbon Nanotubes through the Porous
Materials ........................................................................................................................... 22
3.1 Introduction ........................................................................................................ 22
3.1.1 Chemical Vapor Deposition ........................................................................ 25
3.2 Experimental ...................................................................................................... 26
3.2.1 Materials ..................................................................................................... 26
3.2.2 Support Details............................................................................................ 26
3.2.3 Synthesis of Vertically Aligned CNT on 3D Porous Structures ................. 26
3.2.4 Controlling CNT Fabrication Process Parameters ...................................... 28
3.3 Results and Discussion ....................................................................................... 30
3.3.1 Nanotubes Arrays on Porous Carbon Structures ........................................ 30
3.3.2 Structural and Chemical Characterization .................................................. 36
3.4 Factors Influencing CNT Growth Quality ......................................................... 37
3.4.1 Run-to-Run Variation – System Factors ..................................................... 37
3.4.2 Study of Key Process Parameters ............................................................... 40
3.5 Conclusion .......................................................................................................... 45
4 Chapter 4: Analysis of Specific Surface Area of CNT-Foam Structures ................. 46
4.1 Introduction ........................................................................................................ 46
4.2 Experimental ...................................................................................................... 47
4.2.1 Support Details............................................................................................ 47
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4.2.2 Varying Amount of CNT Coating .............................................................. 49
4.2.3 Estimating the Surface Area of CNT Arrays .............................................. 49
4.2.4 BET Characterization Method .................................................................... 50
4.3 Results and Discussion ....................................................................................... 50
4.3.1 Controlling CNT Carpet Length/Height ..................................................... 50
4.3.2 Estimating the Surface Area of CNT Arrays .............................................. 53
4.3.3 BET Measurements ..................................................................................... 60
4.4 Conclusion .......................................................................................................... 65
5 Chapter 5: Adsorption of Methylene Blue Dye Compound ..................................... 66
5.1 Introduction ........................................................................................................ 66
5.2 Experimental ...................................................................................................... 69
5.2.1 Materials and Chemicals ............................................................................. 69
5.2.2 Supports Used ............................................................................................. 69
5.2.3 Batch Studies – Adsorption, Desorption, and Repeatability Studies .......... 70
5.2.4 Data Treatment – Rate Kinetics and Adsorption Isotherms ....................... 71
5.3 Results and Discussion ....................................................................................... 75
5.3.1 Adsorption of Methylene Blue.................................................................... 75
5.3.2 Kinetics Analysis ........................................................................................ 79
5.3.3 Adsorption Isotherms .................................................................................. 80
5.3.4 Desorption of MB and Regeneration of the Adsorbent .............................. 86
5.3.5 Repeatability Test ....................................................................................... 86
5.4 Conclusion .......................................................................................................... 90
6 Chapter 6: Palladium and Palladium-Oxide Nanoparticles on Hierarchical Carbon
Nanostructures .................................................................................................................. 91
6.1 Introduction ........................................................................................................ 91
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6.2 Experimental ...................................................................................................... 92
6.2.1 Materials ..................................................................................................... 92
6.2.2 Support Preparation .................................................................................... 93
6.2.3 Synthesis of Supported Palladium Nano-Particles ..................................... 93
6.2.4 Synthesis of Supported Palladium-Oxide Nanoparticles ............................ 93
6.2.5 Materials Characterization .......................................................................... 93
6.3 Results and Discussion ....................................................................................... 94
6.3.1 Crystal Structure Characterization – XRD ................................................. 94
6.3.2 Microstructure Characterization – Pd on Various Porous Structures ......... 96
6.3.3 Microstructure of Pd-O Nanoparticles ........................................................ 97
6.3.4 Chemical State of Pd and Pd-O Nanoparticles ........................................... 97
6.4 Conclusion ........................................................................................................ 112
7 Chapter 7: Bimetallic Nanoparticles: Palladium – Silver Bimetallic Nanoparticles on
Hierarchical Carbon Nanostructures ............................................................................... 113
7.1 Introduction ...................................................................................................... 113
7.2 Experimental .................................................................................................... 114
7.2.1 Materials ................................................................................................... 114
7.2.2 Synthesis of Silver-Palladium Bimetallic Nanoparticles .......................... 114
7.3 Results and Discussion ..................................................................................... 118
7.4 Conclusion ........................................................................................................ 125
8 Chapter 8: Catalytic Dechlorination of Trichloroethylene and Trichloroethane using
Palladium Based Catalysts .............................................................................................. 126
8.1 Introduction ...................................................................................................... 126
8.2 Experimental .................................................................................................... 129
8.2.1 Chemicals .................................................................................................. 129
8.2.2 Stock Solution and Standards ................................................................... 129
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8.2.3 Batch Experiments .................................................................................... 130
8.3 Results and Discussion ..................................................................................... 131
8.3.1 Trichloroethylene (TCE) Experiments ..................................................... 131
8.3.2 Trichloroethane (TCA) Experiments ........................................................ 144
8.3.3 Dechlorination Mechanisms with the Pd/H System ................................ 154
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8.4 Conclusion ........................................................................................................ 155
9 Chapter 9: Catalytic Degradation of Emerging Contaminants – Atrazine using
Palladium Based Catalysts .............................................................................................. 156
9.1 Introduction ...................................................................................................... 156
9.2 Experimental .................................................................................................... 163
9.2.1 Materials and Chemicals ........................................................................... 163
9.2.2 Substrates used .......................................................................................... 163
9.2.3 Batch Degradation Studies ........................................................................ 163
9.2.4 Standard Concentration Analysis .............................................................. 165
9.2.5 Chemical Analysis – HPLC ...................................................................... 165
9.3 Results and Discussion ..................................................................................... 166
9.3.1 Atrazine Degradation ................................................................................ 166
9.3.2 Catalyst Chemical Analysis ...................................................................... 170
9.3.3 Daughter Products Formation ................................................................... 175
9.3.4 Atrazine Degradation Mechanism with Pd/H System ............................. 175
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9.4 Conclusion ........................................................................................................ 176
10 Summary ................................................................................................................. 178
11 Future Work ............................................................................................................ 180
12 Bibliography ........................................................................................................... 182
Appendix A: Methylene Blue Removal - using supported Pd-NPs ............................... 199
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