Catalytic Decomposition of Nitric Oxide and Carbon Monoxide Gases Using

Home , Carbon monoxide, Nitrogen oxide

CATALYTIC DECOMPOSITION OF NITRIC OXIDE AND CARBON MONOXIDE GASES USING

NANOFIBER BASED FILTER MEDIA OF VARYING DIAMETERS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Master of Science

Renee L. Petty

August 2010

CATALYTIC DECOMPOSITION OF NITRIC OXIDE AND CARBON MONOXIDE GASES USING

NANOFIBER BASED FILTER MEDIA OF VARYING DIAMETERS

Renee L. Petty

Thesis

Approved: Accepted:

______Advisor Department Chair Dr. George G. Chase Dr. Lu-Kwang Ju

______Committee Member Dean of the College Dr. Edward A. Evans Dr. George K. Haritos

______Committee Member Dean of the Graduate School Dr. Bi-Min Zhang Newby Dr. George R. Newkome

______Date ii

ABSTRACT

Nitrogen Oxide (NO) and carbon monoxide (CO) are major pollutants in the exhaust streams of automobiles, power plants, and other combustion processes. The growing concerns for the environment have resulted in increasingly restrictive emission standards. The removal of NO and CO from exhaust gases is a challenging task.

One method for harmful gas removal is using a catalyst for dissociation. This work explored an alternative method for catalytic reduction of NO. Polymer solutions with palladium catalyst and ceramic precursors were electrospun to form polymer nanofibers. These nanofibers were heated to form ceramic nanofibers with catalyst nanoparticles and were mixed with microfibers to form a nonwoven fibrous catalyst support structure. The concentration of the polymer was varied to create nanofibers with diameters ranging from 100 to 700 nm with a constant mass of catalyst particles per mass of fiber. The effect of the fiber diameter on the corresponding catalyst structure performance was tested. A surface area comparison test was completed to determine whether the reactions occur strictly on the surface of the catalyst or if diffusion occurs. An aging comparison was also completed which tested 1 week old catalytic filters compared to 6 months old. A conventional catalytic converter was

iii tested to verify the performance was similar to the catalytic fibrous filter media containing only palladium.

Experiments were carried out using a lab reactor to expose the media to a mixture of gases simulating an exhaust stream at room temperature to a maximum of

450oC. The reactor exhaust concentrations are measured using gas chromatography

(GC) to determine the catalyst performance. Results indicated that the catalytic reaction performance was about the same for fiber sizes ranging from 100 to 700 nm on a mass basis with a reduction temperature of 325 – 350oC. The surface area comparison filter reduced at 275oC which showed that both surface catalyst particles and particles within the fibers are available for reaction. Furthermore, a conventional catalytic converter reduced at approximately 325oC which exhibits comparable catalytic performance with the catalytic filters. Model theory and equations were also developed for decomposition reactions of NO and CO using elementary reactions.

ACKNOWLEDGEMENTS

I would like to thank Dr. George Chase for all of his support during this research.

I learned so much both personally and academically from him and I will use those skills in my future. I would also like to thank Mempro Ceramics Corporation for providing the materials and equipment to conduct this research. I would also like to sincerely thank

Sneha Swaminathan for continuous support and knowledge about the project as well as project set-up and organization. I would like to thank my fiancé, Michael Coe, for his help with this Thesis and for his unconditional love and support while I was finishing my education. I would also like to thank my good friend Linda Kuhajda for her help with this

Thesis. I am also thankful for the whole multiphase group for making my education a wonderful experience. I would also like to give a special thanks to Mr. Frank Pelc and

Gabriel Manzo for their help throughout my research. I would like to thank Dr. Edward

Evans and Dr. Bi-Min Newby for being on my committee and giving me valuable input and helping with my Thesis.

TABLE OF CONTENTS

Page

LIST OF TABLES…………………………………………………………….……………………………………………....x

LIST OF FIGURES…………………………………………………………….……..……………………………………..xi

CHAPTER

I. INTRODUCTION……………………………………………………………………………………………………..…..1

1.1 Background and overview of work………………………………………...……………………….….1

1.2 Problem statement………………………………………………………….….……………………………..4

1.3 Objectives…………………………………………………………………...…...... 5

1.4 Hypothesis……………………………………………………………………………………………………….…5

1.5 Thesis outline…………………………………………………………...……………………………………..…6

II. LITERATURE REVIEW……………………………………………………………...... 8

2.1 Introduction……………………………………...…………………………………………………………..…..8

2.2 Formation of nitrogen oxide in combustion processes…….……………………………….10

2.3 Current available technology and mechanisms for NOx reduction……….…………..11

2.4 Conventional catalytic converter……………………………………….……………………………..16

2.5 Deactivation of catalysts…………………………………………………………………………………..20

2.6 Benefit and technological importance of the catalytic filter with nanofibers…….21

2.7 Introduction to electrospinning…………………………………….…………...... 23

2.8 Technology for the high production of nanofibers and applications……...…………25

III. DESIGN OF EXPERIMENT…………………………………………...…………...... 32

3.1 Experimental set-up for electrospinning……………………………………………………………32

3.2 Calcining Polymer nanofibers……………………………………………………………………………35

3.3 Vacuum mold set-up for making filter medium…………………………………………………36

3.4 Flowing hydrogen reduction apparatus……………………………………………...…………….38

3.5 Catalytic reaction experimental set-up….………….………………………………………………40

3.6 Instruments…………………………..…………………………………….…...... 43

IV. SYNTHESIS OF PALLADIUM SUPPORTED ON CERAMIC NANOFIBERS AND MAKING FILTER MEDIA...……………………………………………...………………………………………………………45

4.1 Palladium nanoparticles supported by alumina nanofibers synthesized by electrospinning……………….…………………………………………..……………………………………45

4.2 Ceramic fibrous filter media incorporated with electrospun nanofibers……………51

V. REACTION OF NITRIC OXIDE AND CARBON MONOXIDE USING FILTER MEDIA..……….56

5.1 Introduction…………………………………………………………………………………………………….56

5.2 Reactions on 100 nm nanofibers with palladium catalyst……………………….…………63

5.3 Reactions on 250 nm nanofibers with palladium catalyst……………………….…………69

5.4 Reactions on 300 nm nanofibers with palladium catalyst……………………….…………73

5.5 Reactions on 350 nm nanofibers with palladium catalyst……………………….…………77 vii

5.6 Reactions on 700 nm nanofibers with palladium catalyst……………………….…………81

5.7 Reactions on 700 nm nanofibers with same surface area as 100 nm fibers………85

5.8 Reactions on 100 nm nanofibers for aging analysis…………………………………………..91

5.9 Reactions with catalytic converter…………………………………………………………………….98

5.10 Discussion and Conclusions…………………………………………………………………………..101

VI. CONCLUSIONS AND FUTURE WORK…………………..…………………………………………………107

6.1 Conclusions……………..……………………………………………………………………………………..107

6.2 Recommended future work………………………………………..………………………………….110

REFERENCES…………………………………………………….…………...……………………………………………112

APPENDICIES……………………………………………………………………………………………………………...115

APPENDIX A. ADDITIONAL INFORMATION ON THE SYNTHESIS AND CHARACTERIZATION OF CERAMIC NANOFIBERS….…...…………………………………………………………….116

APPENDIX B. FLOWING HYDROGEN REDUCTION APPARATUS OPERATING PROCEDURE…...... 120

APPENDIX C. GC OPERATING PROCEDURE……………………………………………………………….122

APPENDIX D. GC CALIBRATION…………………………………………………………………………………124

APPENDIX E. CATALYTIC FILTER RAW DATA OBTAINED FROM GC…………………………….133

APPENDIX F. ERROR ANALYSIS………………………………………………...... 158

APPENDIX G. EQUIPMENT TROUBLESHOOTING………………………………………………………161

APPENDIX H. CATALYTIC FILTER IDENTIFICATION TABLE………………………………………….164

APPENDIX I. ELEMENTARY REACTION MODEL FOR NITRIC OXIDE AND CARBON MONOXIDE REACTIONS……………………………………………………………………....166

viii

I.1 Introduction………………………………………………………………….………………………………..166

I.2 Basis for Assumptions...... 169

I.3 Development of model equations………………..…………………….…....…………………….182

APPENDIX J. CHARACTERIZATION OF CATALYTIC CONVERTER...... 204

LIST OF TABLES

Table Page

2.1 Summary of EPA current vehicle emission standards (PM is particulate matter)…………………………………………………………………………………………………………….9

2.2 Typical emissions from gasoline and diesel engines………………………………………..11

2.3 Proposed mechanisms for NO and CO reactions on palladium catalyst...... 13

4.1 Slurry ingredients to make catalytic filter...... 52

5.1 Proposed elementary reaction mechanism with detailed kinetic data on Pt (1 1 1)...... 58

5.2 Test conditions for the 100 nm nanofibers with palladium catalyst...... 63

5.3 Test conditions for the 250 nm nanofibers with palladium catalyst...... 70

5.4 Test conditions for the 300 nm nanofibers with palladium catalyst...... 74

5.5 Test conditions for the 350 nm nanofibers with palladium catalyst...... 78

5.6 Test conditions for the 700 nm nanofibers with palladium catalyst...... 82

5.7 Test conditions for the 700 nm nanofibers with palladium catalyst with the same surface area as 100 nm nanofibers...... 88

5.8 Test conditions for the 100 nm nanofibers with palladium catalyst for aging analysis...... 92

LIST OF FIGURES

Figure Page

2.1 Mechanisms for the direct decomposition of NO.………………………………………....16

2.2 Commercially available catalytic converter (with permission) (a) Catalytic converter with steel cover, (b) packing mat made of ceramic fibers, (c) catalytic converter showing dimensions, (d) honeycomb structure catalytic converter with 1mm x 1mm square holes (Chevy, 3L engine)...... 19

2.3 The length of a catalytic filter puck compared to a conventional catalytic converter with equivalent surface areas...... 22

2.4 Multijet set-up utilized in the lab for increased production of nanofibers...... 27

2.5 Electrodes available for use in the Nanospider™...... 29

2.6 Optimum electrode used for producing nanofibers with the Nanospider™...... 29

2.7 Nanospider™ machine (Model NS LAB 200S) utilized at the University of Akron...... 30

3.1 Rotating cylinder electrospinning apparatus used for obtaining nanofibers.....34

3.2 Furnace used for calcination of polymer nanofibers after electrospinning...... 35

3.3 Vacuum mold apparatus to prepare filter from slurry of electrospun nanofibers and alumina microfibers...... 37

3.4 Flowing hydrogen reduction apparatus set-up to reduce metal oxide to metal...... 39

3.5 The schematic diagram of the reduction apparatus...... 40

3.6 Flow Diagram for measuring catalytic performance using a gas chromatograph...... 41

3.7 Set-up for measuring catalytic performance using gas chromatography...... 42

3.8 Lab reactor to hold catalytic filter (with permission) (a) actual reactor and (b) detail diagram of reactor...... 43 xi

4.1 SEM images for palladium oxide supported on alumina nanofibers after heating at 600oC for 4 hours (a) 8 wt.% (b) 10wt.% (c) 12.5wt.% (d) 15 wt.% (e) 17.5wt.%...... 47

4.2 Summary of Weight % PVP vs. Fiber Diameter...... 48

4.3 Length weighted frequency distribution of fiber diameter of varying concentrations of PVP in ethanol...... 49

4.4 TEM image of the 17.5 wt.% solution nanofibers with an average particle diameter of 12.3 ± 7.5 nm...... 50

4.5 Filter media with 2.3 cm diameter (a) 8 wt % filter after calcining, hydrogenation, and running in the GC, (b) Surface area comparison filter after 600oC calcination, and (c) commercial catalytic converter cut with the same dimension as the catalytic filters...... 53

4.6 Pycnometer used for estimating the porosity of filter media...... 54

5.1 The results of blank test (no filter) for NO and CO gases at different temperatures...... 59

5.2 The results of blank test (no filter) with NO and He gases at different temperatures...... 60

5.3 The results of blank test (no filter) with CO and He gases at different temperatures...... 61

5.4 The results of blank test (no filter) with He gas only at different temperatures...... 62

5.5 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #1)...... 67

5.6 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter # 2)...... 68

5.7 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #3)...... 69

5.8 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #4)...... 71

5.9 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #5)...... 72

xii

5.10 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #6)...... 73

5.11 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #7)...... 75

5.12 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #8)...... 76

5.13 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #9)...... 77

5.14 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #10)...... 79

5.15 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #11)...... 80

5.16 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #12)...... 81

5.17 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #13)...... 83

5.18 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #14)...... 84

5.19 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #15)...... 85

5.20 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #16)...... 89

5.21 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #17)...... 90

5.22 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #18)...... 91

5.23 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #19)...... 94

5.24 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #20)...... 95 xiii

5.25 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #21)...... 96

5.26 Filter media that fell out of the filter puck under microscope after sonication for 3 hours...... 97

5.27 Concentration versus temperature for the reaction of NO and CO on the Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #19 - Rerun)...... 98

5.28 Concentration versus temperature for the NO-CO reaction over catalytic converter after reducing with total flowrate of 31.4 cc/min...... 100

5.29 Concentration versus temperature for the NO-CO reaction over catalytic converter without reducing with total flowrate of 31.4 cc/min...... 101

5.30 The results for fiber diameter effect on NO and CO gases using palladium supported by alumina nanofibers...... 103

5.31 The results for effect of age of filter media on NO and CO gases using palladium supported by alumina nanofibers...... 104

5.32 The results for effect of surface area on NO and CO gases using palladium supported by alumina nanofibers. The surface area was constant hence 7 times more nanofibers and 7 times more catalyst by mass were used in each filter...... 104

5.33 The effect of concentration (ppm) of NO and CO gases on the corresponding decomposition temperature using palladium supported by alumina nanofibers...... 105

xiv

CHAPTER I

INTRODUCTION

1.1 Background and overview of work

Meeting rising energy requirements and protecting the environment are among the most important applications of catalyst technology. Nitrogen oxide (NO) and carbon monoxide (CO) are major pollutants in the exhaust of various combustion processes.

The growing concerns for the environment have resulted in increasingly restrictive emission standards. Removal of NO and CO from the exhaust streams of automobiles, power plants, and other combustion processes is a challenging task. The catalyst market for energy and environmental applications is a multi billion dollar industry and the demand will only increase with restrictive emission requirements.

Conventional catalytic converters are expensive, relatively heavy, and use a large amount of catalyst. They contain multiple precious metals, including platinum palladium, and rhodium. As of May 2010, the prices of platinum, palladium, and rhodium were $1,651, $504, and $ 2,760 per ounce, respectively [1]. Catalytic converters are estimated to contain 3 - 7 grams of platinum and 1 - 2 grams of rhodium

[2]. The ability to decrease the amount of catalyst needed for catalytic converters can save a large amount in material costs.

Catalysts used in converters are relatively large particles and since the reactions with NO and CO only occur on the surface of these particles, approximately 90% of the catalyst is not utilized [3]. Well distributed nano sized noble metal particles show better potential for catalyst than bulk materials typically used in catalytic converters. Catalyst nanoparticles are a promising alternative as they decrease the amount of catalyst needed because of the increased surface area available for reaction. The rate limiting step for a conventional catalytic converter is the surface reaction. No diffusion is required because all of the catalyst is on the surface of the honeycomb structure.

A promising alternative catalytic support structure for heterogeneous catalytic reactions are metal oxide nanofibers [4]. These nanofibers can be fabricated by the electrospinning method with polymer solutions and ceramic precursors. Heating the fibers produces ceramic fibers with diameters ranging from a few nanometers to micrometers [5-7]. Nanofibers can be used to construct a catalyst support structure that has a large surface area, is lightweight, as well as chemically and thermally stable.

The catalytic nanofibers differ from a conventional catalytic converter in that diffusion is required into the nanofibers to react with the internal catalyst. However, since the nanofiber diameters are relatively small, diffusion time is negligible and the process is surface reaction limited. This is discussed in detail in Appendix I.

Noble metal catalysts can be supported on alumina fibers for use in NO and CO reactions [7, 8]. Typically, these fibers for catalyst support are prepared by wet impregnation [9], but methods such as depositing noble metal nanoparticles directly on carbon nanofibers by physical deposition from solutions [10] or adding metal nanoparticles directly to the polymer solution prior to electrospinning have also been done [11]. The use of nanofibers for catalyst structure support is relatively new. Park

[7] studied catalyst performance of varying amounts of rhodium, platinum, and palladium and compared the reaction kinetics of NO and CO. Varying the NO concentration and determining an appropriate binder for catalytic filters was also completed [7]. Nanofiber diameter effects on reaction kinetics and aging of catalytic filters are areas lacking in research.

In this work, catalyst particles in the range of about 10 nm supported by alumina nanofibers were synthesized via the electrospinning method. This method eliminates the need for physical deposition or wet impregnation. The catalytic support structure consists of varying diameter nanofibers with catalyst, alumina microfibers, and binding agents. The effect of nanofiber diameter on the reaction kinetics was explored using a gas chromatography set-up with flowing exhaust simulation gases (NO & CO). These results were compared to using a conventional catalytic converter at the same conditions. An analysis on storage of catalytic filters was also explored to determine the shelf life. A series of surface area comparison experiments are also completed to determine if catalyst is reacting strictly on the surface of the nanofibers, or if diffusion

3 into the fiber allows reaction on the internal pores of the catalytic support. An elementary reaction mechanism is also discussed. This mechanism may form the basis for future models of the reaction kinetics for the catalytic filters.

1.2 Problem statement

Noble catalyst metals have been successfully incorporated in ceramic nanofibers.

Nanofiber diameters may significantly impact the catalytic performance. Smaller diameter fibers are typically more challenging to make whereas large diameter fibers require more polymer to fabricate which result in lower yields of ceramic nanofibers per mass of polymer. The optimum fiber diameter needs to be determined without compromising the catalytic reactivity while taking into account these other factors.

Furthermore, it is not known whether the chemical reactions of reducing both NO and

CO occur strictly on the surface of the catalyst particles or if the gases diffuse into the nanofibers and react with catalyst present in the internal pores. Aging of fabricated filters is also an area that is lacking in research. Determining the shelf life and proper storage of these catalytic filters will facilitate economic comparison with conventional catalytic converters.

1.3 Objectives

The specific objectives are:

(i) Fabrication of ceramic nanofibers of varying diameters with catalyst.

(ii) Develop filter media with microfibers and varying diameter nanofibers.

(iii) Utilize a flowing hydrogen apparatus to reduce catalyst from metal oxide to pure metal.

(iv) Operate an experimental reactor with NO and CO to test the catalytic filters.

(v) Compare the catalytic filter results with a conventional catalytic converter.

(vi) Conduct a surface area comparison experiment using higher diameter fibers with the same surface area as the smallest fibers.

(vii) Compare 6 month old catalytic filters to 1 week old to assess potential aging issues.

(viii) Determine appropriate model equations for decomposition reactions of NO and CO using elementary reactions.

1.4 Hypothesis

The hypotheses of this work are the following:

 Ceramic nanofibers of varying diameters can be fabricated by the

electrospinning method by varying the concentration of the polymer in solution. 5

 A decrease in fiber diameter will result in more catalyst on the surface so there is

a higher catalytic reactivity which in turn reduces the size needed for the catalyst

support structure.

 Reactions reducing NO and CO take place strictly on the surface of the

nanofibers so diffusion of gases and reaction on internal pores is negligible.

 Catalytic filters have enough surface area for reaction to be competitive with

conventional catalytic converters.

 Aging of catalytic filters are not a significant factor.

 A model using elementary reactions can be set up, that if solved, could predict

catalytic performance.

1.5 Thesis Outline

Chapter II of this thesis provides a literature review of current methods to reduce nitric oxide and carbon monoxide, including catalytic converters. The benefit and technological importance of the catalytic filter with nanofibers and the electrospinning method is also discussed. Chapter III consists of the experimental design and instrumentation. Chapter IV discusses ceramic fibrous filter media of alumina nanofibers supporting palladium nanoparticles synthesized by electrospinning. Chapter

V presents catalytic reaction data of nitric oxide and carbon monoxide gases using filter media fabricated using varying diameter nanofibers and a commercial catalytic converter. Chapter VI includes the elementary reaction model theory and equations for 6 nitric oxide and carbon monoxide reactions. Chapter VII summarizes the conclusions drawn from this research and recommended future work for the continuation of this study.

CHAPTER II

LITERATURE REVIEW

2.1 Introduction

When fuel is combusted at high temperatures, harmful components such as nitrogen oxides (NOx) and carbon monoxide (CO) are formed. Nitrogen oxides are precursors to the formation of ozone and to smog formation. They also contribute to the formation of acid rain and cause some respiratory system illnesses in humans [12].

Carbon monoxide (CO) is formed as a product of incomplete combustion. This is caused by partial oxidation of carbon in the fuel rather than full oxidation to carbon dioxide

(CO2). Carbon monoxide is very harmful to humans as it reduces oxygen flow in the bloodstream which is mainly dangerous to persons with heart disease [13]. The primary sources of nitrogen oxides are fuel burning processes such as power generation, transportation, and off-road equipment [12]. Both the automotive and the power industries are required to follow regulations introduced by the Environmental

Protection Agency (EPA). Regulations have become increasingly severe for automobile exhaust emissions. A summary of EPA current vehicle emission standards can be found

8 in Table 2.1. They separate the latest standards (Tier 2 Program) into different ‘Bins’ which determine the emission requirements and corresponding air pollution score [14].

Table 2.1 Summary of EPA current vehicle emission standards (PM is particulate matter)

[14].

2.2 Formation of nitrogen oxide in combustion processes

There are three primary sources of NOx in combustion processes which consist of thermal, fuel, and prompt NOx [15]. There are many factors that determine which mechanism will occur such as the type of fuel being burned, the temperature, and the air to fuel ratio. Thermal NOx is extremely temperature dependent and proceeds by the extended Zeldovich mechanism which is governed by the following equations [16].

O + N2 N + NO (2.1)

N + O2 O + NO (2.2)

N + OH H + NO (2.3)

As shown in the equations, NO is formed by oxygen and nitrogen that are in the combustion air. This mechanism is initiated at high temperatures.

Fuel NOx is formed by the reaction of nitrogen bound in the fuel with oxygen in the combustion air. The nitrogen compounds are decomposed to NH, NH2, NH3, HCN, and CN, which react to form NO. Fuel NOx is not typically a problem in gaseous fuels, but in oils that contain a significant amount of fuel-bound nitrogen, fuel NOx can account for 50% to 70% of the total NOx emissions [17].

A third source of NOx formation is prompt NOx which is generally minor compared to the overall amount of NOx generated from combustion. Prompt NOx forms from the rapid reaction of atmospheric nitrogen with hydrocarbon radicals that

10 are produced from combustion processes [17]. This reaction results in the formation of

- nitrogen species such as NH, HCN, H2CN, and CN which can oxidize to NO.

Diesel engines typically have high nitrogen oxide emissions. Table 2.2 summarizes emissions from both gasoline and diesel engines [18]. Diesel engine information from this source are the results of averaging 7.0 L engines including 9 diesel,

15 gasoline, and 3 heavy duty gasoline engines with three way catalysts.

Table 2.2 Typical emissions from gasoline and diesel engines [18].

2.3 Current available technology and mechanisms for NOx reduction

Current methods for NOx reduction include (1) catalytic converter based catalysis reacting NO and CO over Pt, Pd, and Rh noble metals (2) selective catalytic reduction (SCR) of NO using ammonia gas, (3) three way catalysis using Pt, Pd, and Rh catalysts for the removal of NO, CO, and hydrocarbons from automobile exhausts, and

(4) using Cu-based catalysts for the direct decomposition of NO. This chapter gives more detail about these technologies for NO removal.

2.3.1 NO and CO reaction over noble metals

CO is available in automobile exhaust to use as a reducing agent for NO. Noble metals that are typically used to eliminate NOx include Pt, Pd, and Rh. The possible reactions on a supported noble metal catalyst are shown in equations 2.4 – 2.6. These show that CO initially reacts with NO to form N2O and CO2. N2O reacts further with CO to produce additional CO2 and N2 [19].

2NO + CO → N2O + CO2 (2.4)

N2O + CO → CO2 + N2 (2.5)

2NO + 2CO → N2 + 2CO2 (2.6)

Chuang et al. [20] used palladium to react NO and CO on the surface. The same

o products of CO2 and N2 were produced and the proposed mechanisms at 200 C are shown in Table 2.3.

Table 2.3 Proposed mechanisms for NO and CO reactions on palladium catalyst [20].

2.3.2 Selective catalytic reduction with ammonia

The most widely used method of NOx control is selective catalytic reduction

(SCR) that is typically used for stationary diesel engines or for industrial sources such as downstream of boilers for coal fired power plants. An SCR utilizes a reactant such as ammonia or urea that is injected and properly mixed with the exhaust gases before entering the catalyst chamber. The catalyst promotes a reaction between ammonia

(NH3) and the nitrogen oxides and oxygen in the exhaust stream. This reaction forms the products nitrogen and water and is shown in Equations 2.7 and 2.8.

4NO + 4NH3 + O2  4N2 + 6H2O (2.7)

2NO2 + 4NH3 + O2  3N2 + 6H2O (2.8)

The disadvantage to an SCR is the special storage, handling, and metering required for the use of ammonia to prevent the harmful release of unreacted ammonia

(ammonia slip) [21].

Major factors which determine the extent of NOx reduction include the type of

SCR catalyst used, the operating temperature, and potential contaminants such as particulate and catalyst poisons. Typical catalysts used are manufactured from various ceramic materials using a support structure such as titanium oxide. Active catalytic components are usually oxides of base metals, zeolites, and some noble metals.

Vanadium and tungsten are base metal catalysts that operate well at typical industrial and utility boiler temperatures, but they lack high thermal durability [22]. Zeolite catalysts have the ability to function at much higher temperatures and can withstand prolonged operation at approximately 630oC and transient conditions of up to around

850oC [22].

2.3.3 Three way catalysts

A three way catalytic converter has three simultaneous tasks to remove pollutants including reduction of nitrogen oxides to nitrogen and oxygen, oxidation of carbon monoxide to carbon dioxide, and oxidation of unburnt hydrocarbons which are shown in Equations 2.9, 2.10, and 2.11.

2NOx → xO2 + N2 (2.9) 14

2CO + O2 → 2CO2 (2.10)

2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O (2.11)

The extent of NOx, CO, and hydrocarbon removal depends on the air-to-fuel ratio (A/F ratio). Conversion of all three pollutants is nearly complete when the engine is running slightly above the stoichiometric point with an A/F between 14.6 and 14.8 by weight [23]. Outside of that range, the conversion efficiency decreases rapidly. The system is running lean when there is more oxygen than required. This is considered in oxidizing condition and the oxidation of CO and hydrocarbons are favored. On the other hand, when there is more fuel than needed, the engine is running rich and the reduction of NOx is favored at the expense of CO and HC oxidation reactions. The catalytic converter that is commercially used for the three way catalysts is described in more detail section 2.4.

2.3.4 Direct decomposition of NO

Another method of NOx removal is by the direct decomposition of nitric oxide to nitrogen and oxygen. This method is environmentally friendly and is thermodynamically favored in diesel exhaust engines. Cu-ZSM-5 zeolites have been used for direct NO decomposition, however, they are inhibited by water and are sensitive to poisoning caused by sulfur dioxide. Chuang et al. studied the mechanism of direct NO decomposition over copper-ion-exchanged-ZSM-5 which are shown in Figure 2.1 [24]. 15

Cu+ initiates the NO decomposition process. Adsorbed oxygen from dissociated NO

+ 2+ - changes the oxidation state of Cu ion, causing the formation of Cu (NO3 ). Oxygen is

2+ - formed from the decomposition of Cu (NO3 ) and from the desorption of adsorbed oxygen on Cu−ZSM-5.

Figure 2.1 Mechanisms for the direct decomposition of NO [24].

2.4 Conventional catalytic converter

A catalytic converter is used to reduce the toxicity of emissions from internal combustion engines. It provides an environment for a chemical reaction where toxic combustion by-products such as NOx and CO are converted to less toxic substances such as nitrogen, oxygen, and carbon dioxide. Catalysts lower the activation energy required for reactions, thus speed up the reaction rates between exhaust gas components. Good contact between the catalyst and the exhaust gases is important to achieve desired

16 conversion. A large catalytic surface area is used to provide this higher contact which is typically achieved by utilizing a ceramic honeycomb monolithic support and finely dispersing the catalyst on it. The honeycomb structure has a large number of small parallel channels (1mm x 1mm) running in the axial direction which is shown in Figure

2.2 (d).

The noble metals typically incorporated in catalytic converters include platinum, palladium, and rhodium. Palladium is prone to interact with poisonous species such as sulphur (S) and lead (Pb) [25]. Rhodium is the best noble metal to promote the reduction of NO to N2 in three way catalytic converters. Platinum and rhodium are used as a reduction catalyst, while platinum and palladium are used as an oxidization catalyst. The amount of noble metals contained in catalytic converters is usually kept confidential by manufacturers. This is a very important parameter for both cost and performance of the catalytic converter. According to Park [7], the conventional catalytic converter used in Chapter V consists of approximately 20:1:1.5 wt% ratio of Pt: Pd: Rh as the main noble metals.

The catalyst coating is typically applied to the support material using both wash coating and wet impregnation of noble metals. The catalyst support is typically a porous inorganic material such as activated aluminum oxide (Al2O3), titanium dioxide (TiO2), or silicon oxide (SiO2). Wash coating is applied to this support using a water based slurry and the catalyst support is dried and calcined at high temperature. The noble metals

17 are then attached by impregnation where the wash coated monolith is exposed to catalytic precursors within a water based solution and is dried and calcinated.

Catalytic converters have catalyzed substrates that are packaged into steel shells as shown in Figure 2.2 (a). Ceramic fiber packing mats are used to protect the ceramic substrates and evenly distribute external forces acting on it as shown in Figure 2.2 (b).

To compare reaction efficiency of a catalytic converter with the catalytic filters used in this research, the catalyzed substrate that is shown in Figure 2.2 (c) was used and results are discussed in Chapter V.

b) a)

c) d)

Figure 2.2 Commercially available catalytic converter (with permission) (a) Catalytic converter with steel cover, (b) packing mat made of ceramic fibers, (c) catalytic converter showing dimensions, (d) honeycomb structure catalytic converter with 1mm x

1mm square holes (Chevy, 3L engine) [7].

The light-off temperature is the temperature at which the catalytic converter begins to function. A conventional catalytic converter light-off temperature ranges from approximately 200oC to 315oC [26].

2.5 Deactivation of catalysts

Knowing catalyst deactivation mechanisms is important to maximize the life of catalysts used. Mechanisms of catalyst deactivation include aging or sintering, coking or fouling, washcoat losses, and poisoning [27].

Sintering is caused by catalysts being exposed to high temperature gases for long periods of time. The activity decreases because of crystal agglomeration, the growth of metals that are deposited on the support structure, or by the pore sizes within the catalyst decreasing.

Coking or fouling is common in hydrocarbon reactions. A carbonaceous (coke) material is deposited on the surface of the catalyst which causes decay. This decay can be slowed or eliminated by adding a rare earth metal such as Lanthanum (La).

Washcoat losses are when the catalyst is physically separated from its ceramic support structure. This can be caused by abrasions due to high velocities, particle collisions, temperature changes, and differences in thermal expansion between the catalyst and support structure.

Poisoning of catalyst can occur when a contaminant such as lead in gasoline becomes irreversibly chemisorbed to active sites. This decreases the surface area of catalyst available for reaction so more unreacted pollutants such as NOx, CO, and hydrocarbons are released into the atmosphere. Other catalyst poisons include manganese, silicone, phosphorus, and zinc [27]. 20

2.6 Benefit and technological importance of the catalytic filter with nanofibers

Using ceramic nanofibers as catalyst support structures are expected to decrease the cost and improve the performance of catalytic materials. Nanofibers provide a large surface area per unit volume which results in more catalyst available for reaction. Noble metal particles are well distributed and are in the 10 nm range. These show better potential for catalytic reactions than bulk materials typically used in catalytic converters.

Small catalyst particles decrease the amount of wasted catalyst and they increase the surface area available for reaction compared to conventional catalytic converters.

A comparison of the total surface area of a catalytic filter puck to a conventional catalytic converter was completed to determine the length of a catalytic converter required to obtain the same surface area as a filter puck. The ratio of the catalytic converter surface area to the filter puck surface area was determined to be 5.13. A catalytic converter length that is 7.7 cm is required in order to give the same surface area as the filter pucks as shown in Figure 2.3. The surface area of the catalytic filter puck includes both the microfibers (3 micrometer diameter) and nanofibers (100 nm diameter assuming the 8 wt. % solution as described in Chapter IV).

Figure 2.3 The length of a catalytic filter puck compared to a conventional catalytic converter with equivalent surface areas.

Ceramic nanofibers also significantly reduce the pore size with only a small increase in pressure drop. These materials provide a light weight, efficient, and thermally and chemically stable catalyst support structure for a variety of applications.

The ceramic nanofibers can also remove sub-micron particulates in combustion exhaust gases, serving as filter media. Black smoke soot is emitted as a result of heterogeneous combustion processes or diffusion type combustion from diesel engines.

Soot or particulates ranging from 100 to 500 nm are not effectively captured by electrostatic mechanisms or diffusion because they are too large. The particulates in this range are also too small to be captured by straining methods, impaction, or interception using microfiber filters. The use of nanofibers can eliminate this problem and can efficiently capture these particulates.

2.7 Introduction to electrospinning

Fabrication of nanofibers ranging from a few nanometers to a few microns can be completed using the electrospinning technique [28]. A variety of solutions consisting of polymers, composites, and ceramic precursors can be electrospun forming nonwoven fibers. This method has successfully produced hundreds of varieties of polymer and ceramics nanofibers. Catalysts can be added directly to polymer solutions as shown in Chapter IV and secondary processing has also been completed to give the nanofibers catalytic function.

Electrospinning uses an electrical charge to form a mat of fine fibers. The standard set-up for electrospinning consists of a spinneret with a metallic needle, a syringe pump, a high-voltage power supply, and a grounded collector. Figure 3.1 shows a laboratory electrospinning set-up. A polymer solution is loaded into the syringe and this liquid is driven to the needle tip by a syringe pump, forming a droplet at the tip.

The solution is typically charged with a voltage ranging from 5-30 kV. Because of its charge, the solution is drawn toward a grounded collector (usually a metal screen, plate, or rotating drum), that is usually between 5 and 30 cm away, as a jet. When a voltage is applied to the needle, the droplet first stretches into a shape called a Taylor cone [29].

The Taylor cone forms due to the competing forces of the static electric field and the liquid’s surface tension. When the electric field is strong enough, a jet launches from the drop and the jet stretches and elongates due to the repulsions of the electric charges. As the jet stretches solvent evaporates. As long as the solvent concentration is 23 high enough that the viscous and surface tension forces are weaker than the electric field, the jet continues to stretch even as the jet diameter reaches the nanometer range.

When the solvent concentration decreases enough, the electrical forces will not be strong enough to continue the stretching and the jet (now a fiber) diameter becomes fixed. The charged polymer fiber accumulates on the grounded target and the charge on the fibers eventually dissipates into the surrounding environment. The resulting product is a non-woven fiber mat that is composed of tiny fibers with diameters between 50 nm and 10 μm [30].

There are many parameters that can affect this process. System parameters include molecular weight, molecular-weight distribution, architecture (branched, linear etc.) of the polymer, and solution properties (viscosity, conductivity, and surface tension). Process parameters include electric potential, flow rate, and concentration of the polymer solution. Other parameters include the distance between the capillary and collecting screen, ambient parameters such as temperature, humidity, and air velocity in the chamber, and the motion of the target screen [30].

A challenge with fabricating nanofibers using electrospinning is to make small diameter fibers with a uniform surface. Another challenge is obtaining nanofibers with catalyst particles evenly distributed on the surface. Parameters that have a direct effect on fiber diameter include the gap distance, applied voltage, polymer flow rate, as well as concentration of ceramic precursor and polymer. Literature shows that increased concentration of ceramic precursor in polymer solution increases the diameter of the 24 nanofiber [30]. Fiber diameter decreases with increased electric potential and as a result of increasing the distance between the metallic needle and the grounded collector. Fiber diameter also increases with an increased flow rate of the solution to be electrospun [30].

Fiber diameter is most effectively controlled by altering the concentration of polymer in the electrospinning solution. Demir et al. found that the fiber diameter was proportional to the cube of the polymer concentration [31]. Varying the concentration of the polymer in electrospinning solutions was used to obtain fibers of varying diameters as shown in Chapter IV.

2.8 Technology for the high production of nanofibers and applications

The nanofiber market is growing rapidly due to the large amount of potential applications. In order to keep up with the demand, higher production rates are needed for electrospinning fibers which are achieved by utilizing multiple jets simultaneously or by using a porous tube. The typical metal needle method is used in laboratory applications, whereas these high production methods are used for commercial applications. Three methods for increased production of nanofibers include a multijet set-up, a multijet scale-up process, and the Nanospider™.

Chase et al. utilized an electrospinning device with multiple jets from a porous tube. Their mass production rate showed 250 times greater than over the single needle

25 jet [32]. The multijet utilizes a reservoir where the electrospinning solution is loaded and is driven by air pressure (1.0 inH2O) into a plastic tube with 10, 0.2 mm holes where the solution is fed through. Within the plastic tube is a wire mesh used as an electrode that is charged with high voltage up to 60 kV. The solution is charged and collects on a rotating drum covered with aluminum foil that is located 20 cm away from the plastic tube. The multijet yield is in the range of 0.425 – 0.6765 g/hr using 10 jets (55 kV, 20 cm distance, PVP/Aluminum Acetate solutions and with catalyst) [33]. Figure 2.4 shows a picture of the multijet set-up utilized in the lab to produce 17.5% fibers as described in

Chapter IV.

Figure 2.4 Multijet set-up utilized in the lab for increased production of nanofibers.

MemPro Ceramics Corporation is working on scaling-up the multijet process.

This plant will be located in New Mexico and will have the capability to produce large amounts of ceramic nanofibers. The plastic tubing will be 1 meter long with 100 – 200 holes in it for the solution to be driven though. There will be a number of tanks used to aid in the solution making process as well as recirculation through the porous tubing.

The collector will be similar to that of the multijet, however, it will be above the plastic tubing. 27

Another machine available for production of nanofibers on an industrial scale is the Nanospider™ which is manufactured by Elmarco Ltd [34] using their patented technique. The Nanospider™ uses a cylinder (electrode) that is partially immersed in a polymer solution to form the fibers. The cylinder is charged with a maximum of 82 kV.

The cylinder rotates and part of the polymer solution is carried to the top of the cylinder where multiple Taylor cones are created throughout the entire length of the cylinder which initiates jet formation. These multiple Taylor cones give a high production capacity for nanofibers that gives an average yield of 2.24 g/hr for the Nanospider™

(Model: NS LAB 200S, Electrospinning conditions: 55 kV, 19 cm distance, and solution

4.5% PVP in Aluminum Acetate) [33]. The nanofibers are collected 19 cm above the electrode on a rotating cloth that is covered with aluminum foil. There are multiple electrodes available (Figure 2.5) and their rotating speed can be varied. It was found that increasing the speed of rotation decreases the standard deviation of nanofiber diameters; the optimum speed is between 5 and 10 rpm. The electrode that gives the greatest yield and smallest standard deviation of nanofibers is shown in Figure 2.6.

Figure 2.7 shows a picture of a Nanospider™ machine utilized at the University of Akron

[33].

Figure 2.5 Electrodes available for use in the Nanospider™ [35].

Figure 2.6 Optimum electrode used for producing nanofibers with the Nanospider™

[33].

Figure 2.7 Nanospider™ machine (Model NS LAB 200S) utilized at the University of Akron

[33].

An important characteristic of electrospinning is the ability to make fibers with diameters in the range of nanometers to a few microns. Consequently, these fibers have a large surface area per unit mass so that nanowoven fabrics of these nanofibers collected on a screen can be used for example, in filtration of small particles in separation industries, biomedical applications, such as wound dressing in medical industry, tissue engineering scaffolds, and artificial blood vessels [30]. The use of

30 electrospun fibers at critical places in advanced composites to improve crack resistance is also promising.

Currently, ceramic precursors are used for catalysis and other applications such as membranes, hydrogen storage batteries, fuel cells, solar cells, structural applications requiring high mechanical strength, as well as in biology for biosensors, biomolecular machines, and tissue engineering [36, 37]. Adding ceramic precursor to nanofibers is a relatively novel application [30]. Electrospinning a ceramic precursor with catalyst can make nanofibers that can be used in the production of filters for use in baghouses to remove Elemental Mercury (Hg+) and for catalytic converter applications to remove NO and CO in combustion exhaust gases.

CHAPTER III

DESIGN OF EXPERIMENT

This chapter introduces the experimental design and equipment used to electrospin the nanofibers, calcine polymer nanofibers, create the catalytic filter using a vacuum mold set-up, a flowing hydrogen reduction apparatus to reduce the catalyst from metal oxide to pure metal, and the gas chromatography (GC) set-up to determine the reactant and product concentrations as a function of temperature for NO and CO simulation exhaust gas reactions.

3.1 Experimental set-up for electrospinning

The set-up for electrospinning consists of a spinneret with a metallic needle, a syringe pump, a high-voltage power supply, and a grounded collector as shown in Figure

3.1. A polymer solution consisting of polymer, solvent, and organic metal precursor is filled into two syringes. More detail of this polymer solution can be found in Chapter 4.

Teflon tubing is used to connect these syringes to steel needles. These two syringes are loaded onto a syringe pump (World Precision Instruments, SP101i) to control the flow rate of the polymer solution at 2 µL/min. The syringe pump can hold one or two

32 syringes, depending on the amount of fibers desired. High voltage power (Gamma High

Voltage, ES 30P-5) is applied to the metallic needles. When the voltage is applied, the polymer solution droplet first stretches into a shape called a Taylor cone [29]. This forms due to the competing forces of the static electric field and the liquid’s surface tension.

When the electric field is strong enough, a jet launches from the drop and the jet stretches and elongates due to the repulsions of the electric charges. This elongation causes the jet diameter to decrease which produces nanometer to submicron size fibers. The fibers are drawn to a grounded collector wrapped in aluminum foil. This collector is a cylindrical rotating drum that is made of copper wires through notches machined on the outer diameter of Pyrex glass disks. The drum is rotated at controlled rates using a motor. More detail of the electrospinning process can be found in Chapter

Syringes Needles Teflon Tubing

High Voltage Syringe Pump Terminal Grounding

Rotating Cylindrical Motor Collector

High Voltage Supply

Figure 3.1 Rotating cylinder electrospinning apparatus used for obtaining nanofibers.

3.2 Calcining Polymer nanofibers

After electrospinning, the polymer fibers are calcined by heating [38] in a furnace (BI Barnstead Thermolyne, 1400 Furnace) in air for 4 hours at 600:C with a ramp rate of 10 :C per minute. The furnace is shown in Figure 3.2 below. This burns away the polymer away while converting the metal salts to metal oxide giving ceramic nanofibers with catalyst particles. This occurs by the following reaction:

PdCl2 + O2  PdO + Cl2 (3.1)

Figure 3.2 Furnace used for calcination of polymer nanofibers after electrospinning.

3.3 Vacuum mold set-up for making filter medium

These ceramic nanofibers are mixed with microfibers, corn starch, and alumina binder in water to form a slurry of fibers. Acid is added to maintain the slurry at a pH ~

6.0 to evenly disperse the fibers and starch is added to enhance the binding capability of binder and fibers. After mixing this slurry with an agitator overnight, a nonwoven fibrous catalyst support structure is formed using a vacuum molding apparatus. The vacuum molding apparatus consists of a mixing tank, collecting tank and vacuum pump.

The slurry is poured into the mixing tank where it is agitated using air jets. A small vacuum pressure is applied to a Plexiglas hollow mold (internal diameter = 2.3 cm) at the bottom of the mixing tank to draw the slurry through a steel mesh on which the fibers are collected in a disk filter shape. The steel mesh is lined with Whatman® 114

Wet Strengthened filter paper to prevent fibers from passing through the steel mesh.

The acidic waste solution enters a collecting tank with a maximum capacity of 7 liters.

The filter is heated in a furnace for 4 hours at 600oC to remove any liquid remaining after vacuum molding to thermally set the fibers. Figure 3.3 shows this vacuum mold apparatus.

Vacuum Pump

Mixing Tank

Collecting Tank

Plexiglas Mold

Hose Connecting Plexiglas Mold to Collecting Tank

Figure 3.3 Vacuum mold apparatus to prepare filter from slurry of electrospun nanofibers and alumina microfibers.

3.4 Flowing hydrogen reduction apparatus

The dried filter is reduced by hydrogen at a high temperature to convert the catalyst metal oxides to metal. This apparatus consists of a nitrogen gas cylinder, 20% hydrogen / 80% nitrogen gas cylinder, mass flow meter, tubular furnace, vacuum pump, and a blower. The filter is placed in a ceramic tube with an inner diameter of 2.5 cm.

This tube runs through a Thermolyne tube furnace (SYBRON, Maximum temperature:

1200 oC). The reduction apparatus is flushed with nitrogen to remove oxygen so that the hydrogen will be effective. The nitrogen gas flow is controlled by a mass flow meter at 95 cc/min. Nitrogen gas flow is followed by the introduction of nitrogen and hydrogen (80:20) at a flow rate of 95 cc/min. The furnace temperature is increased to

400oC and held at this temperature for 4 hours. The actual temperature inside the furnace is also controlled by an external thermocouple (Omega HH82 Digital

Thermometer). The exhaust gas line is connected to a vacuum pump and is vented through plastic piping to a fume hood using a blower. A portable hydrogen sensor

(VWR, Dräger safety, Pac Ш) is used as a safety precaution to detect any hydrogen gas leakage from the apparatus. The sensor was programmed to alarm when the hydrogen concentration exceeds 200 ppm and detects a maximum of 2,000 ppm. This reduction process is completed one day before running the filter in the GC and the filter is kept in the oxygen free environment until just before placing the filter into the lab reactor. This reduces the chance of the pure metal reacting with oxygen which could convert it back to metal oxide. Please reference Appendix B for a detailed operating procedure for the

38 reduction apparatus. The set-up and a schematic of the system is shown in Figure 3.4 and Figure 3.5, respectively.

Ceramic 20% H2 Tube 80% N2 Nitrogen Gas Blower Cylinder Cylinder

Mass Flow Meter

Thermocouple

Hydrogen Sensor Vacuum Pump

Figure 3.4 Flowing hydrogen reduction apparatus set-up to reduce metal oxide to metal.

Figure 3.5 The schematic diagram of the reduction apparatus.

3.5 Catalytic reaction experimental set-up

The prepared filters were placed inside a catalytic filter cell reactor. Three gases

(He, CO, NO) flowed through the reactor with a continuous controlled flow rate using three flow meters (Omega, FMA 5400/FMA 5500). Helium was used as the carrier gas and to increase the velocity of the NO and CO gases. The lab reactor was wrapped with heating tape connected to a temperature controller to control the reaction from room temperature up to a maximum of 450oC. The reactor exhaust concentrations were measured using a gas chromatography (GC, SRI 8610C) to determine the catalyst performance. The outlet of the lab reactor was directly connected to the GC and computer where Peak Simple software (Version 3.29) was used to calculate and display the peaks of the components and concentrations. A detailed operating procedure for the GC can be found in Appendix C. All of the catalytic reaction experimental set-up was 40 constructed of stainless steel hardware and tubing to prevent corrosion from the gases.

A flow diagram of the experimental set-up is shown in Figure 3.6 and a photograph in

Figure 3.7.

Figure 3.6 Flow Diagram for measuring catalytic performance using a gas chromatograph.

Computer with Peak Simple Software Gas Cylinders Flow Meters

Lab Reactor

Gas Chromatograph

Gas Inlet (CO, NO, He) Temperature Controller

Figure 3.7 Set-up for measuring catalytic performance using gas chromatography.

The lab reactor consisted of two cylindrical stainless steel blocks held together by 3 metal supports and screwed together using stainless steel knobs. The reactor was oversized due to the pressure and high temperatures of the reaction. The reactor had a hole with a diameter of 2.3 cm where the catalytic filter was inserted. Gasket material was used as a sealant for the reactor and was made of ceramic fibers (later it was determined that the gasket material was not necessary to seal the reactor). The bottom and top parts of the reactor were connected with 1/8” stainless steel tubing.

The inlet to the reactor and the reactor was wrapped with heating tape connected to

42 the temperature controller. As shown in Figure 3.7, insulation was used to minimize heat loss while heating the reactor for experimentation. The details of the filter holder are shown in Figure 3.8.

a) b)

Figure 3.8 Lab reactor to hold catalytic filter (with permission) (a) actual reactor and (b) detail diagram of reactor [7].

3.6 Instruments

A gas chromatograph (GC, SRI 8610C) was used for experimentation. The GC was equipped with a thermal conductivity detector (TCD) to detect reactants and product gases from reacting nitric oxide and carbon monoxide (O2, N2, CO, NO, N2O and

CO2). Each of these compounds was thermally conductive and their concentrations

43 could be measured with the TCD detector. Helium was used as a carrier gas due to its high thermal conductivity and safety, which is often used with TCD detectors. Columns

(6’ silica gel and 6’ Molecular Sieve) inside the GC oven were connected to the detector.

The inlet to the GC was set for automatic injection from the reactor exhaust line. The automatic injection was triggered by pressing the space bar on the computer or pressing of the start run button located on the GC. The inlet valve opened at this time and was set to close after 10 minutes. Because the reactor was run at high temperatures up to

450oC, an excess of stainless steel tubing was used on the exhaust of the reactor to pass the gas through an ice bath to cool down the gas temperature before entering the GC.

High temperatures may damage the GC and TCD detector, so the GC was programmed to operate at a constant temperature of 26oC. To determine the concentrations of each gas, the peak areas obtained from the Peak Simple Software were calibrated to known gas concentrations. The calibration curves for each gas can be found in Appendix D.

CHAPTER IV

SYNTHESIS OF PALLADIUM SUPPORTED ON CERAMIC NANOFIBERS AND MAKING FILTER

MEDIA

4.1. Palladium nanoparticles supported by alumina nanofibers synthesized by electrospinning

Palladium was chosen for these experiments because of the relatively low cost compared to rhodium and platinum. This metal also has a large catalytic activity for oxidation [39], hydrogenation [40], and hydrogenolysis [41] reactions.

Specifically, palladium catalyst supported on alumina has been used for simultaneous CO oxidation and NO reduction [42]. Park [7] incorporated palladium into alumina nanofibers by dissolving the catalyst in solution prior to electrospinning which eliminates the need for physical deposition or wet impregnation steps. This method will be duplicated in this study.

4.1.1 Experimental

Solutions ranging from 8 wt% - 17.5 wt% of polyvinylpyrrolidone (PVP, Aldrich,

MW 1,300,000) in ethanol (AAPER alcohol) were prepared. Each varying concentration solution was mixed with an aluminum acetate solution consisting of aluminum acetate

(Alfa Aesar, Basic hydrate), water, and formic acid in the weight ratio of 1:2.5:1, respectively. Palladium chloride (Sigma-Aldrich, 60 % Pd) was dissolved in each solution in the ratio of 7.87 wt% PdCl2 with respect to aluminum acetate. These solutions were sonicated to aid in dissolving the catalyst and were placed in a shaker held at 40oC until dissolved.

These solutions were loaded into plastic syringes with steel needles and were electrospun by applying 20 kV (Gamma High Voltage Research Inc.). A syringe pump

(World Precision Instruments SP1011) was used to control the flow rate of solution at 2

μl/min. The grounded collector with aluminum foil was placed approximately 20 cm away from the needle tip. After electrospinning, the nanofibers were calcined at 600oC for 4 hrs.

4.1.2 Results and Discussion

The solution of polymer and alumina precursor was electrospun to make nanoparticles supported on alumina nanofibers. The fiber diameters were determined

46 using an environmental scanning electron microscope (FEI Quanta 200 FSEM) after calcining at 600oC for 4 hours. Such SEM images can be found in Figure 4.1 below.

a) b)

c) d)

Figure 4.1 SEM images for palladium oxide supported on alumina nanofibers after heating at 600oC for 4 hours (a) 8 wt.% (b) 10wt.% (c) 12.5wt.% (d) 15 wt.% (e)

17.5wt.% 47

A fiber size distribution was determined by measuring the diameter and length of fibers in SEM images using ImageJ 1.43 software. Average fiber diameters and the length weighted frequency fiber diameter distributions are summarized in Figure 4.2 and Figure 4.3, respectively.

Figure 4.2 Summary of Weight % PVP vs. Fiber Diameter.

Figure 4.3 Length weighted frequency distribution of fiber diameter of varying concentrations of PVP in ethanol.

The diameter of 8 wt% PVP in ethanol alumina nanofibers is around 100 nm with a narrow size distribution. As fiber diameter increases, the size distribution also increases. The highest concentration of PVP in ethanol (17.5 wt%) gave approximately

700 nm fibers. A description of error analysis for determining fiber diameters can be found in Appendix F. Solutions including 20%, 22.5%, and 25% were also electrospun and observed using an SEM, but these high concentrations of polymer resulted in fibers with inconsistent and large diameters so were not used for this research.

A transmission electron microscope (TEM - FEI Tecnai 12) was used to obtain images of the catalyst particles within and on the nanofibers. Only the 17.5 wt.%

49 solution (700 nm diameter fibers) was viewed using the TEM which is shown in Figure

4.4. The average diameter of the palladium particles are 12.3 ± 7.5 nm. Images of the other diameter nanofibers were not obtained due to the TEM being out of order.

Figure 4.4 TEM image of the 17.5 wt.% solution nanofibers with an average particle diameter of 12.3 ± 7.5 nm.

In depth characterization of nanofibers before and after hydrogenation were completed by Park [6,7]. Park [6,7] determined that alumina nanofibers are in amorphous form after heating to 600oC rather than crystalline. It was also determined by X-ray photoelectron spectroscopy that palladium is no longer in its chloride form after calcining at 600 °C, but is rather palladium oxide. Palladium oxide is then converted to its metallic form after hydrogenation [6,7]. This proves that nanofibers of 50

Pd/Al2O3 were fabricated using electrospinning followed by calcination, and hydrogen reduction techniques.

4.2 Ceramic fibrous filter media incorporated with electrospun nanofibers

Combustion gases consist of particulates that are harmful to both the environment and the human body. The benefit to using catalytic filters is to not only provide a catalyst to react with combustion gases, but to also provide a filtration medium for particulates. Conventional catalytic converters are themochemically and mechanically stable. Catalytic filters have both of these qualities and possess the advantages of cost reduction and space savings. This work utilizes palladium catalyst supported on alumina nanofibers mixed with alumina microfibers. Catalytic filters fabricated using nanofibers have a high surface area per unit volume and are expected to improve catalytic performance compared to conventional catalytic converters [5, 43].

These nanofibers can also serve as a filter media to remove sub-micron particles from combustion gases.

4.2.1 Construction of filter media

Park [7] compared alumina from different sources and two different binders and found alumina (SAFFIL HA) micro fibers (approximate diameter is 3.5 μm), alumina

51 rigidizer (Zircar Ceramics, 99% alumina), and corn starch was the best combination for binding the filter media. Based on those findings, the same binding materials were used in this study. The filter media consists of the ingredients in Table 4.1 mixed in 4 L of water.

Table 4.1 Slurry ingredients to make catalytic filter.

Acid is added to maintain the slurry at a pH ~ 6.0 to evenly disperse the fibers and starch is added in order to enhance the binding capability of binder and fibers. This slurry is mixed with an agitator for 24 hours, and additional mixing is completed using a blender. The mixed slurry is vacuum molded under a vacuum of less than 5 psi to form a nonwoven fibrous catalyst support structure with a diameter of approximately 2.3 cm.

This filter was heated up to 600oC for 4 hours to remove moisture and to calcine and

o again heated to 400 C with hydrogen gas (20 % H2, 80 % N2) for 4 hours.

A top view of an 8 wt % filter after calcining, hydrogenation, and running in the

GC is shown in Figure 4.5 A. Figure 4.5 B shows a surface area comparison filter after calcining which is fabricated with 17.5 wt% fibers with the same surface area as 8 wt%

52 fibers. More detail of the surface area comparison filter can be found in Chapter 5.

Figure 4.5 C shows a commercial catalytic converter (Advance Auto Parts, USA,

Maremont Catalytic Converter part no. 38402) cut to approximately the same dimensions as the fabricated catalytic filters.

b) c)

Figure 4.5 Filter media with 2.3 cm diameter (a) 8 wt % filter after calcining, hydrogenation, and running in the GC, (b) Surface area comparison filter after 600oC calcination, and (c) commercial catalytic converter cut with the same dimension as the catalytic filters.

4.2.2 Characterization

Porosity of the catalytic filter was measured using the Pycnometer shown in

Figure 4.6 using the gas expansion method [44].

Figure 4.6 Pycnometer used for estimating the porosity of filter media.

Filters were fabricated using palladium supported on alumina nanofibers, alumina microfibers, and binders. The porosity of the catalytic filters ranged from 0.88 to 0.99. The porosity of filters is tabulated and can be found in Appendix H.

Future work is to calculate the permeability of the catalytic filter using the

Frazier Air Permeability tester (Frazier Precision Instrument Co.). Permeability can then

54 be used to calculate the pressure drop of the filter media using Darcy’s Law for fibrous filters as shown in the following equation [45].

(4.1)

Where Q = Volumetric flow rate, A = Cross sectional area of the filter medium, k =

Permeability, and μ = Viscosity.

CHAPTER V

REACTION OF NITRIC OXIDE AND CARBON MONOXIDE USING FILTER MEDIA

This chapter presents the results of the reactions of nitric oxide and carbon monoxide using catalytic filters which are introduced in Chapter IV. The reaction temperature is varied from room temperature to a maximum of 450oC. Results from filters of varying diameter (100 – 700 nm), surface area comparison filters, and aging filters are presented. Three of each type of filter was fabricated and run in the GC to obtain statistically significant results. A commercial automobile monolithic honeycomb catalytic converter was cut to the same size as the catalytic filters and was tested under the same conditions.

5.1 Introduction

The most common catalyst used for catalytic reactions with NO and CO are palladium, platinum, and rhodium. Palladium was used in this research due to its low cost relative to Pt and Rh. This lower cost is because it is more widely distributed under the earth’s crust and thus is more readily available than the other metals.

The elementary reactions for NO and CO taking place in the lab reactor are proposed by D. Mantri and P. Aghalayam [46]. They also provide the pre-exponential factors and activation energies on Pt and Ir catalysts which are needed to predict the reaction kinetics associated with the reactions. The proposed elementary reactions and parameters are shown in Table 5.1. In this table the pre-exponential factor units will vary depending on the order of the reaction. The reactions that are first order have units of s−1, while 2nd order reactions units are liters/mole/sec for k. The proposed reactions that are reversible are bound by the principle of detailed balance which is discussed in more detail in Appendix I. The NO and CO react on the catalyst surface and forms mostly nitrogen (N2) and carbon dioxide (CO2) gaseous products. The products oxygen (O2) and nitrous oxide (N2O) are also formed. Nitrous oxide can undergo further reaction with CO to produce the desired products nitrogen and carbon dioxide.

Appendix I discusses the elementary reaction model for the decomposition of nitric oxide and carbon monoxide gases and derivation of a transport model for predicting outlet concentrations from the catalytic filters.

Table 5.1 Proposed elementary reaction mechanism with detailed kinetic data on Pt (1 1

1) [46].

A test was conducted without any sample (filter) in the reactor at a flow rate of 1 cc/min for CO and 0.4 cc/min for NO gases to study the effect of temperature on the experiment. The concentrations flowing into the GC of NO and CO are 12,739 ppm and

31,847 ppm, respectively. Throughout the experiments, the initial GC output concentrations fluctuate and can be corrected by multiplying by a correction factor to obtain the expected initial concentrations of NO and CO. The correction factors are reported for each filter if applicable. The concentration of NO and CO did not change from room temperature up to 350oC, however, when the temperature was above 350oC,

o there is a slight decrease in NO and CO. At 200 C there is an unexpected increase in N2

58 and O2 which can be attributed to a leak forming at elevated temperatures due to the thermal expansion of the stainless steel reactor. The additional air in the GC and lines result in N2O and CO2 formation. These results show at elevated temperatures, a homogeneous reaction (or a heterogeneous reaction with the reactor walls) may occur.

The blank test results are shown in Figure 5.1 where the concentration (ppm) is volume to volume ratio (v/v). The correction factors for the NO and CO are 1.35 and 1.04, respectively. The change in concentrations due to this homogeneous reaction is relatively small compared to the reactions with the catalyst.

Figure 5.1 The results of blank test (no filter) for NO and CO gases at different temperatures.

In order to confirm the increase in N2 and O2 is due to a leak of air forming at approximately 200oC, the GC was run with NO and He, CO and He, and He only. These results are shown in Figures 5.2, 5.3, and 5.4, respectively. These experimental runs are to verify there are not any reactions occurring with the stainless steel reactor.

Figure 5.2 The results of blank test (no filter) with NO and He gases at different temperatures.

Figure 5.3 The results of blank test (no filter) with CO and He gases at different temperatures.

Figure 5.4 The results of blank test (no filter) with He gas only at different temperatures.

Based on the results from these blank runs, there is a leak that forms at elevated temperatures of approximately 200oC. Leak tests are completed at room temperature, but due to the design of the reactor there is no way to check for leaks and prevent them at elevated temperatures because of the heating tape and insulation wrapped around the reactor. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions will specifically be used to determine the decomposition temperatures of

NO and CO for each catalytic filter and are not used to determine the product amounts.

There was also an unexpected increase in reactants (i.e. NO and CO) when running at elevated temperatures. This can also be caused by the reactor leaking or 62 mass flow meter errors if the controller did not hold the setpoint properly. However, this increase indicates that there is no reaction occurring with the stainless steel reactor.

5.2 Reactions on 100 nm nanofibers with palladium catalyst

In this section, catalytic filters constructed with 100 nm nanofibers with palladium catalyst and 3 µm alumina microfibers were tested for NO and CO reactions.

The electrospun solution is as described in Chapter IV consisted of 8 wt.% of PVP in ethanol and overall PVP in solution was 5.18 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect to Al2O3) and 0.5 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table

5.2 shows the detail test conditions for the 100 nm nanofibers with palladium catalyst.

Table 5.2 Test conditions for the 100 nm nanofibers with palladium catalyst.

The detailed calculation and assumptions for the surface area of Pd catalyst particles are shown below.

Assumptions: Catalyst particles are spherical in shape.

The average diameter of catalyst particle is 10 nm.

Density of Pd is 12,024 kg/m3=12.024×106 g/m3

Mass ratio of catalyst to nanofibers = WCF = 0.07875

The volume of one palladium particle is:

The mass of one palladium particle is calculated by the following:

The surface area of one palladium particle is calculated below:

The total mass of nanofibers in the filter is calculated by the following:

The total mass of palladium in the filter fibers are

The number of catalyst particles in the filter is given by

The total surface area of palladium particles in each filter is

The total surface area of palladium per gram of palladium is below.

The surface area of nanofibers in the filter disk was calculated based on the following assumptions.

Assumptions: Mass of ceramic nanofibers added into Filter Puck = 0.05 g

The average diameter of the fibers is 100 nm

Mass of Pd within ceramic nanofibers = 7.87%

6 3 Density of Al2O3 = 3.85×10 g/m

Density of Pd = 12.024×106 g/m3

The density of nanofibers is calculated by the following

The mass of nanofibers is given by

Since the mass of nanofibers is known, rearrange equation to solve for L (the length of the nanofibers)

The surface area of the nanofibers is calculated below.

Figure 5.5 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #1).

The correction factors for the NO and CO are 0.91 and 1.07, respectively. A filter identification table including measured properties and fabrication info can be found in

Appendix H. Results will reference the specific filter identification number from this table. Park [7] calculated the pressure drop for all catalytic filter reactions to be around

0.779 kPa which is similar for a catalytic converter. Filter #1 showed the concentrations of NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 200oC, which is consistent with results of Park [7].

As temperature increased to 350oC, NO and CO completely converted, forming the products O2, N2, N2O, and CO2. Each filter made with varying fiber diameters were made in triplicates to obtain statistically significant results. Figures 5.6 and 5.7 are filter

#2 (Correction factors: NO = 1.10 and CO = 1.07) and #3 (Correction factors: NO = 1.21 and CO = 1.17), respectively with the same fiber diameter of 100 nm. These show consistent results with filter #1. However, filter #3 showed a reduction of NO at a slightly lower temperature of 325oC, which can be due to the slightly lower initial concentration of NO compared to filters #1 and #2. A description of error analysis for these results can be found in Appendix F.

Figure 5.5 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #1). 67

Figure 5.6 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter # 2).

Figure 5.7 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #3).

5.3 Reactions on 250 nm nanofibers with palladium catalyst

In this section, catalytic filters constructed with 250 nm nanofibers with palladium catalyst and 3 µm alumina microfibers were tested for NO and CO reactions.

The electrospun solution consisted of 10 wt.% of PVP in ethanol and overall PVP in solution was 6.48 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect

69 to Al2O3) and 0.5 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table 5.3 shows the detail test conditions for the 250 nm nanofibers with palladium catalyst.

Table 5.3 Test conditions for the 250 nm nanofibers with palladium catalyst.

Figure 5.8 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #4).

The correction factors for the NO and CO are 1.03 and 1.16, respectively. Filter #4 showed the concentrations of NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 200oC, which is consistent with results of the 100 nm diameter fibers. As temperature increased to above 350oC, the NO concentration completely reduces while CO completely converts at approximately 400oC. Results from duplicated filters #5 (Correction factors: NO = 1.19 and CO = 1.19) and #6 are shown in Figures 5.9 and 5.10, respectively. These show

70 consistent results with filter #4, however, filter #6 showed a reduction of NO at a slightly lower temperature of 325oC. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of

NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.8 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #4).

Figure 5.9 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #5).

Figure 5.10 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 250 nm (Filter #6).

5.4 Reactions on 300 nm nanofibers with palladium catalyst

Catalytic filters constructed with 300 nm nanofibers with palladium catalyst and

3 µm alumina microfibers were tested for NO and CO reactions. The electrospun solution consisted of 12 wt.% of PVP in ethanol and overall PVP in solution was 7.77 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect to Al2O3) and 0.5

73 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table 5.4 shows the detail test conditions for the 300 nm nanofibers with palladium catalyst.

Table 5.4 Test conditions for the 300 nm nanofibers with palladium catalyst.

Figure 5.11 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #7).

The correction factors for the NO and CO are 1.63 and 1.15, respectively. Filter #7 showed the concentrations of NO and CO were approximately the same as that at room temperature until reaction temperature reached approximately 200oC, which is consistent with results of Filter #1 - #6. The decomposition temperatures of NO and CO are 300oC and 400oC, respectively. Results from duplicated filters #8 (Correction factors:

NO = 1.04 and CO = 0.99) and #9 (Correction factors: NO = 0.99 and CO = 1.00) are shown in Figures 5.12 and 5.13, respectively. These show slightly different results

74 compared to filter #7 with an NO decomposition temperature of 350oC for filter #8.

Filter #9 completely reduced NO and CO at 350oC. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.11 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #7).

Figure 5.12 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #8).

Figure 5.13 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 300 nm (Filter #9).

5.5 Reactions on 350 nm nanofibers with palladium catalyst

In this section, catalytic filters constructed with 350 nm nanofibers with palladium catalyst and 3 µm alumina microfibers were tested for NO and CO reactions.

The electrospun solution consisted of 15 wt.% of PVP in ethanol and overall PVP in solution was 9.72 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect

77 to Al2O3) and 0.5 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table 5.5 shows the detail test conditions for the 350 nm nanofibers with palladium catalyst.

Table 5.5 Test conditions for the 350 nm nanofibers with palladium catalyst.

Figure 5.14 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #10).

The correction factors for the NO and CO are 0.54 and 0.72, respectively. Filter #10 showed the concentrations of NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 300oC. This is not consistent with previous diameter results and could be to the unusually large concentration of both NO and CO for this run. Duplicate filters #11 (Correction factors:

NO = 1.02 and CO = 0.93) and #12 (Correction factors: NO = 1.30 and CO = 1.08) results are shown in Figure 5.15 and 5.16, respectively. Filter #11 gave a decomposition

78 temperature of slightly over 350oC for CO and 325oC for NO while Filter #12 gave 350 for both NO and CO. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of NO and

CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.14 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #10).

Figure 5.15 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #11).

Figure 5.16 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 350 nm (Filter #12).

5.6 Reactions on 700 nm nanofibers with palladium catalyst

In this section, catalytic filters constructed with 700 nm nanofibers with palladium catalyst and 3 µm alumina microfibers were tested for NO and CO reactions.

The electrospun solution consisted of 17.5 wt.% of PVP in ethanol and overall PVP in solution was 11.34 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect

81 to Al2O3) and 0.5 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table 5.6 shows the detail test conditions for the 700 nm nanofibers with palladium catalyst.

Table 5.6 Test conditions for the 700 nm nanofibers with palladium catalyst.

Figure 5.17 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #13).

The correction factors for the NO and CO are 0.89 and 0.98, respectively. Filter #13 showed the concentrations of NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 350oC, which is higher than other nanofiber diameter results. This filter had significantly higher concentrations for NO and CO which may be the cause of the decomposition temperature around

450oC (the heating tape was malfunctioning and would not reach 450oC in order to obtain the exact compositions). This filter was also approximately 3 months old before

82 tested in the GC. The filter duplicates were Filter #14 (Correction factors: NO = 1.29 and CO = 1.00) and #15 and the results are shown in Figures 5.18 and 5.19, respectively.

Both of these filters were approximately 3 weeks old and had consistent results with a decomposition temperature of 300oC for NO and 325oC for CO. The inconsistency with

Filter #13 may be due a combination of the larger initial concentrations and the age of the filter media. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of NO and

CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.17 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #13).

Figure 5.18 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #14).

Figure 5.19 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm (Filter #15).

5.7 Reactions on 700 nm nanofibers with same surface area as 100 nm fibers

In addition to the 5 different size fiber filters, a surface area comparison was also completed using 17.5% fibers, with the same surface area as the lowest diameter fiber

(8%). The amount of fibers needed for this comparison was determined using the following equations:

Surface area in filter media: (hold surface area constant instead of mass)

Ad d 2 L m  f A  d L m  f 4 external f 4

m1  A1,d1 (8% fibers)

m2  A1,d2 (17.5% fibers - keep A1 constant)

A1d2

A1d1 A1d2 m2 4 d2 687nm m1  m2      6.5175 4 4 m1 A1d1 d1 105nm 4

To determine whether only surface particles are active, 3 catalytic filters were constructed each consisting of 6.52 times the amount of 700 nm nanofibers in the original 17.5% filter. These filters contained 0.5 g of alumina microfibers and 0.326 g of alumina nanofibers with palladium catalyst and were tested for NO and CO reactions.

These tests will determine whether catalyst particles within the nanofiber are active or if the surface particles are only available for reaction. If these catalytic filters are more reactive than the 100 nm fiber filters with the same surface area, then it is an indication that a constant amount of catalyst particles give the same performance, regardless of where they are located. On the other hand, if only surface particles are active, then we will expect to get the same catalyst performance when the surface areas are equal.

Since the 17.5% solutions consist of a larger amount of PVP, it takes a lot more electrospinning to obtain the same yield of nanofibers compared to the lower percentage of PVP in solution. This is due to the polymer degrading during calcining at

600oC. The multijet was used to electrospin the fibers used for the 3 surface area comparison filters since such a large amount is needed. The multijet fiber diameters were measured using SEM photos to verify they have the same diameter as fibers obtained from electrospinning using syringe pumps. The diameters were confirmed to be equal with an average diameter of 685 nm for the multijet compared to 687 nm using the syringe. Details on the multijet process can be found in Chapter 2.

For the testing of the surface area comparison filters, the flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table

5.7 shows the detail test conditions for the 700 nm nanofibers with palladium catalyst with the same surface area as 100 nm nanofibers.

Table 5.7 Test conditions for the 700 nm nanofibers with palladium catalyst with the same surface area as 100 nm nanofibers

Figure 5.20 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with 700 nm nanofibers with the same surface area as 100 nm nanofibers (Filter #16). Filter #16 showed the concentrations of

NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 200oC, which is consistent with other nanofiber diameter results. The decomposition temperature is lower than that of filters fabricated with 100 nm fibers with both NO and CO at 300oC. Duplicated filters #17 (Correction factors: NO = 0.92 and CO = 0.91) and #18 (Correction factors: NO = 1.17 and CO =

1.19) results are shown on Figures 5.21 and 5.22, respectively. These showed similar results with decomposition temperatures for NO and CO of 275oC for Filter #17. Filter

#18 showed temperatures of 325oC for CO and 300oC for NO. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the

88 decomposition temperatures of NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.20 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #16).

Figure 5.21 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #17).

Figure 5.22 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 700 nm with the same surface area as 100 nm fibers (Filter #18).

5.8 Reactions on 100 nm nanofibers for aging analysis

The previous 100 nm nanofibers were run in the GC 6 months after fabrication.

The aging analysis compared the results from 6 month old filters to 1 week old filters.

This was considered due to the inconsistent results of filters made with 700 nm (17.5%) nanofibers that were aged compared to filters run in the GC within 3 weeks of making.

The old 17.5% catalytic filter had a decomposition temperature of 450oC compared to the 3 week old filters with a decomposition temperature of 325oC for CO and 300oC for

NO. In this section, catalytic filters constructed with 100 nm nanofibers with palladium catalyst and 3 µm alumina microfibers were tested for NO and CO reactions within one week of fabricating to determine if aging of filter media compromises catalytic performance. The electrospun solution consisted of 8 wt.% of PVP in ethanol and overall PVP in solution is 5.18 wt.%. The catalytic filter media contained a constant amount of catalyst of 0.05 g palladium supported on alumina nanofibers (7.87 wt% Pd with respect to Al2O3) and 0.5 g of alumina microfibers. The flow rates of nitric oxide and carbon monoxide were held constant at 0.4 and 1 cc/min, respectively. Table 5.8 shows the detail test conditions for the 100 nm nanofibers with palladium catalyst for aging analysis.

Table 5.8 Test conditions for the 100 nm nanofibers with palladium catalyst for aging analysis.

Figure 5.23 shows the concentration profile versus temperature plot for the reaction of NO and CO on Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #19) for aging analysis. The correction factors for the NO and CO are 2.47 and 1.50, respectively. Filter #19 showed the concentrations of NO and CO were almost the same as that at room temperature until reaction temperature reached approximately 200oC, which is consistent the aged 100 nm filters. The decomposition temperatures for NO and CO are 325oC and 350oC, respectively. Duplicated filters #20 (Correction factors:

NO = 3.05 and CO = 1.59) and #21 (Correction factors: NO = 2.46 and CO = 1.34) results are shown in Figures 5.24 and 5.25, respectively. These filters are consistent with a decomposition temperature of 300oC for both NO and CO. These results suggest there is an aging effect between 1 week and 6 month old catalytic filters. The aging could be caused by catalyst falling out of the filter media as it sits or possible reactions of Pd and aluminum foil may be taking place while stored (filters are wrapped in aluminum foil and placed in a Ziploc bag for storage). Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.23 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #19).

Figure 5.24 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #20).

Figure 5.25 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #21).

To determine whether catalyst falling out of the filter media is an issue, filter #19 was sonicated for 3 hours and rerun in the GC (additional reduction under flowing hydrogen was not completed). After sonication, the matter that fell out of the filter media looked like mostly microfibers and a small amount of nanofibers containing catalyst from what could be seen under a microscope. The image is shown in Figure

5.26.

Figure 5.26 Filter media that fell out of the filter puck under microscope after sonication for 3 hours.

Results from rerunning this filter in the GC are shown in Figure 5.27. The correction factors for the NO and CO are 3.75 and 1.74, respectively. A decomposition temperature of 350oC was noted for CO which is consistent with the first run of Filter

#19. However, the decomposition temperature of NO decreased to 200oC, which would be due to the lower initial concentration of NO. There was not a significant change in performance between Filter #19 initial run and after sonication, so catalyst falling out of fibers may not be the cause of aging. Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance.

Results from reactions are specifically used to determine the decomposition

97 temperatures of NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.27 Concentration versus temperature for the reaction of NO and CO on the

Pd/Al2O3 catalyst with fiber diameter of 100 nm (Filter #19 - Rerun).

5.9 Reactions with catalytic converter

A conventional catalytic converter with a honeycomb structure was cut to the same dimensions as the catalytic filters to test in the GC for NO and CO reactions. Due to limitations with the testing apparatus, only low filter media face velocities can be 98 tested, approximately 0.14 cm/sec (and three times higher). Face velocities of a 3 liter engine operating at 3,000 rpm are around 2,300 cm/sec, which is significantly higher than face velocities obtained with the GC testing apparatus. However, both face velocities give Reynolds numbers that suggest laminar flow through the catalytic filters

(0.002 for GC apparatus and 299 for an engine). For tube flow, laminar flow is considered having a Reynolds number less than 2300, so although the face velocity is lower for the GC testing apparatus, it is virtually identical for both the catalytic filter and the conventional catalytic converter and can results can be directly compared.

Results are shown in Figure 5.28 for a catalytic converter that was reduced with flowing hydrogen before running in the GC as was done with catalytic filter media. The correction factors for the NO and CO are 1.81 and 1.37, respectively. Results from a conventional catalytic converter without reducing are shown in Figure 5.29. The correction factors for the NO and CO are 2.35 and 1.48, respectively. These results show that the catalyzed ceramic nanofiber filter media achieved similar results to that obtained by a conventional catalytic converter. Characterization of this catalytic converter can be found in Appendix J.

Due to the calibration or a leak the concentrations of the product species are much higher than they should be for the mass balance. Results from reactions are specifically used to determine the decomposition temperatures of NO and CO for the catalytic filters and are not used to determine the product concentrations.

Figure 5.28 Concentration versus temperature for the NO-CO reaction over catalytic converter after reducing with total flowrate of 31.4 cc/min.

100

Figure 5.29 Concentration versus temperature for the NO-CO reaction over catalytic converter without reducing with total flowrate of 31.4 cc/min.

5.10 Discussion and Conclusions

The results of fiber diameter effect on NO and CO decomposition temperature are shown in Figure 5.30. The decomposition temperatures range from 300oC - 400oC and there is no apparent trend for fiber diameter verses decomposition temperature. A t-test assuming unequal variances was completed on the data which gave a p-value of

101

0.051 which is greater than 0.05 so it can be concluded that there is no difference between the decomposition temperatures of different fiber diameters.

Aging results are summarized in Figure 5.31. Results show an increase in decomposition temperature for both NO and CO of approximately 33oC between 1 week old and 6 month old catalytic filters. Therefore, 6 month old filters have decreased catalytic performance compared to 1 week old catalytic filters. The p-value of this comparison is 0.030 which is less than 0.05 so it can be concluded that there is a statistically significant difference between the means. However, taking into account all of the data points and standard deviations, the p-value for the CO decomposition temperatures is 0.18 which is greater than 0.05 so we cannot conclude whether or not there is a significant difference between the 1 week old and 6 month old catalytic filters.

Figure 5.32 shows the results for the surface area comparison test. The 100 nm fiber results are compared to the 700 nm fibers containing 7 times the amount of fibers, hence 7 times the amount of catalyst on a mass basis which gives equivalent surface areas. The 700 nm fibers with the same surface area as 100 nm fibers showed a 50oC decrease in decomposition temperature for both NO and CO compared to the 100 nm fiber filters. A t-test assuming unequal variances gave a p-value of 0.014 which is less than 0.05 so it can be concluded that there is a significant difference between the catalytic filters.

102

The concentration in ppm of CO and NO varied due to fluctuation in the GC. This was later determined to be caused by buildup of residual gases in the GC or in the lines leading to the GC which can be eliminated by running the GC for an extended amount of time with only helium to flush any remaining gases. Park [7] noted with a decrease in nitric oxide flowrate, the temperature that NO reduces decreased. It was also observed when the nitric oxide concentration was higher, there was higher consumption of carbon monoxide. A similar trend is noted in this study as shown in Figure 5.33. There is a slight trend that increased concentration also increases the decomposition temperature.

Figure 5.30 The results for fiber diameter effect on NO and CO gases using palladium supported by alumina nanofibers.

103

Figure 5.31 The results for effect of age of filter media on NO and CO gases using palladium supported by alumina nanofibers.

Figure 5.32 The results for effect of surface area on NO and CO gases using palladium supported by alumina nanofibers. The surface area was constant hence 7 times more nanofibers and 7 times more catalyst by mass were used in each filter. 104

Figure 5.33 The effect of concentration (ppm) of NO and CO gases on the corresponding decomposition temperature using palladium supported by alumina nanofibers.

Some catalytic filter reaction graphs violated the mass balance which is a concern. This could be due to air leaks at elevated temperatures of reaction which would increase both the N2 and O2 concentrations. Error could also be due to possible inconsistency in manual integration to determine the gas peaks using PeakSimple

Software. Specifically, CO2 is the last gas to appear in the GC data and depending on the length of the tail of the CO2 peak, the concentrations could fluctuate. This could also be due to the GC not being properly zeroed before starting the runs at each temperature.

This could result in uneven peaks which could be misinterpreted. Error can also be due to the controller not holding the setpoint properly.

105

With this said, values in the given plots are the best we can determine with the given experimental equipment. In principle, the N2 and N2O concentrations should be half that of the initial NO concentration in order to satisfy the mass balance. Similarly, the CO2 concentration should be no more than that of the initial CO concentration because that is the only source of carbon in the reaction. Future work is to figure out exactly where excess gases are coming from. Although the mass balance is incorrect, we are still able to interpret when the reaction goes to completion. However, we cannot rely on the quantified values of the concentrations for the products.

This research has successfully produced catalytic filter media containing palladium supported on ceramic nanofibers with diameters ranging from 100 to 700 nm.

The concentrations of NO and CO reduced and the products N2, O2, CO2, and N2O form as the reaction went to completion. The catalytic performance was tested with these catalytic filters containing only palladium and was compared to results obtained from a conventional catalytic converter which has a variety of catalysts including Pd, Pt, and Rh as well as other metals such as Mo, Ba, La, Ce, Si, Mg, Al, and Ni [7]. Results show that catalytic filters have competitive advantages over conventional catalytic converters such as using less catalyst.

All of the filters tested completely decomposed the NO gas with carbon monoxide gas at temperatures in the range of 200 - 400oC. Catalytic filters have the potential for use in car exhaust streams which range from 200 to 600oC.

106

CHAPTER VI

CONCLUSIONS AND FUTURE WORK

6.1 Conclusions

An alternative method for catalytic reaction of NO and CO using palladium supported by varying diameter alumina nanofibers was introduced. These alumina nanofibers with palladium were synthesized by the electrospinning technique and the nanofibers were calcined to obtain the ceramic nanofibers. The diameter of the alumina nanofibers were controlled by varying the concentration of polymer in the solutions which yielded diameters ranging from 100-700 nm. Filter media with high temperature durability were constructed using a vacuum mold apparatus with a constant amount of palladium supported on alumina nanofibers and alumina microfibers. The palladium oxide nanoparticles were reduced to pure palladium by flowing hydrogen at 400oC.

The catalytic filters with varying diameters were made in triplicates to obtain statistically significant results. The reaction temperature varied from room temperature up to a maximum of 450oC. NO and CO were flowed into the reactor at a constant flow

107 and are converted to the products N2, O2, CO2, and N2O. Filters with nanofibers ranging from 100 to 350 nm showed relatively consistent results with NO and CO peaks reducing around 350oC. Filters made with 700 nm fibers reduced at 325oC. These results indicate that the catalytic reaction performance is about the same for fiber sizes ranging from

100 to 700 nm on a mass basis. The lower reduction temperature of the 700 nm fibers may be due to the age of the filter media that can be caused by catalyst falling out of the filter media or reactions with aluminum foil when stored for extended periods. The

700 nm filters were fabricated within 3 weeks of running them in the GC, while the fibers ranging from 100 - 350 nm were made approximately 6 months before experiments were completed. An aging analysis was completed using filters made with

100 nm fibers. These filters were tested approximately 1 week after making compared to the initial filters that were 6 months old. Results show an increase in decomposition temperature for both NO and CO of approximately 33oC between 1 week old and 6 month old catalytic filters. Therefore, 6 month old filters have decreased catalytic performance compared to 1 week old catalytic filters.

Surface area comparison tests were completed and compared to the 100 nm fiber results. The surface area comparison filters used 700 nm fibers containing 7 times the amount of fibers on a mass basis which gives an equivalent surface area compared to the 100 nm fibers. The surface area comparison filter results show reduction at

275oC - 300oC which shows that both surface catalyst particles and particles within the fibers are available for reaction. The 700 nm fibers with the same surface area as 100

108 nm fibers showed a 50oC decrease in decomposition temperature for both NO and CO compared to the 100 nm fiber filters.

Error in results may be attributed to the fluctuation in the concentration of CO and NO throughout the filter tests, which is caused by buildup of residual gases in the

GC or in the lines leading to the GC. A slight trend was observed that increased concentration also increases the decomposition temperature.

The results of running a conventional catalytic converter show reduction of NO and CO at approximately 300oC - 325oC. Catalytic filter media used in these experiments only contain palladium while catalytic converters consist of additional noble catalytic metals such as Platinum (Pt) and Rhodium (Rh). These results show that the catalytic filters tested are consistent with a catalytic converter and exhibit comparable catalytic performance.

Elementary reaction model equations were used to derive equations that can be used to predict and compare the performance of catalytic filters as well as calculate reaction kinetic parameters in future work.

The results from the surface area comparison tests, coupled with the relatively consistent results of fiber sizes ranging from 100 to 700 nm is significant because it tells us fiber size does not significantly affect the reaction rate. However, fiber size does affect pressure drop and the amount of PVP required to make the fibers.

109

Results from this research will determine what size fibers should be produced on a large scale after taking into account the catalyst reactivity and pressure drop across the filter media. Results from this research will also be used to make small engines more environmental friendly. Conventional catalytic converters may be too large for practical use with small engines. With nanofibers the catalyst support structures may be smaller.

6.2 Recommended future work

This research can be extended in a number of ways and the following are some recommendations for future work:

1. Continue aging analysis to determine how long catalytic filters can be stored

without compromising the catalytic performance (i.e. 1 year or more). Compare

storage in aluminum foil to other methods of storage (plastic or glass).

2. Determine a more effective binding method and test the number of runs it takes

for decreased catalytic performance. Conventional catalytic converters last for

around 10 years, and catalytic filters should be comparable.

3. Find a better method to remove catalytic filters from reactor to allow reuse.

4. Determine the maximum activity for varying catalyst loading.

5. Run tests on a small lawnmower engine to test actual engine exhausts compared

to strictly NO and CO.

110

6. Use catalyst supported on titania nanofibers and titania microfibers and

compare the catalytic performance with alumina fibers used in this research.

Other supporting ceramic materials such as silica and zeolites can be tested

because of their ability to withstand high temperatures.

7. Study catalyst poisoning of Pd from compounds commonly found in fuels (Pb, P,

and S) and self-poisoning by hydrocarbons. Consider adding Lanthanum (La) to

reduce the poisoning of palladium catalyst.

8. Explore methods of dissolving catalyst in solutions to decrease the catalyst

particle size which could improve catalytic performance. Catalyst size should be

examined using a TEM.

9. Determine whether heating the fabricated filters in an oven at 80oC for 2 hours

before placing in the furnace at 600oC has a significant effect on binding.

10. Study the filtration of particulate matter on catalytic performance and the

pressure drop as particulate matter is captured by filter media.

11. Study the results at higher temperatures (above 450oC).

12. Increase the face velocity of the experimental set-up to closer simulate that of

actual engine exhaust.

13. Solve elementary model equations to predict reaction kinetics using a genetic

algorithm (GA) programmed in FORTRAN.

111

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Mussmann. Investigation of the selective catalytic reduction of NO by NH3 on Fe-ZSM5 monolith catalysts. Applied Catalysis B. 2006. 66. 208. [22] C. Lambert, G. Cavataio, Y. Cheng, D. Dobson, J. Girard, P. Laing, J. Patterson, S. Williams. Urea SCR and DPF System for Tier 2 Diesel Light-Duty Trucks. DOE Presentation. August 24, 2006. [23] J. Kašpar, P. Fornasiero, N. Hickey. Catalysis Today. 2003. 77. 419. [24] M. Konduru, S. Chuang. Investigation of Adsorbate Reactivity during NO Decomposition over Different Levels of Copper Ion-Exchanged ZSM-5 Using in Situ IR Technique. J. Phys. Chem. B. 1999. 103 (28). 5802–5813. [25] P. Albers, J. Pietschb, S. Parker. Poisoning and deactivation of palladium catalysts. Journal of Molecular Catalysis A: Chemical. 2001. 173. [26] Catalytic Converter. Retrieved 5/19/2010. . [27] H.S. Fogler. Elements of Chemical Reaction Engineering. 3rd Edition. Prentice Hall. 2002. 636-637. [28] H. Fong, D. Reneker. Journal of Polymer Science B: Polymer Physics 37. 1999. 3488. [29] A. Yarin, S. Koombhongse, D. Reneker. Taylor cone and jetting from liquid droplets in electrospinning of nanofibers. Journal of Applied Physics. 2001. 90(9). 4836-4846. [30] Y. Mencelog lu, M. Papila, O. Yordem. Effects of electrospinning parameters on polyacrylonitrile nanofiber diameter: An investigation by response surface methodology. Materials & Design. 2008. Vol 29. Issue 1. [31] M. Demir, I. Yilgor, E. Yilgor, B. Erman. Polymer. 2002. 43. 3303. [32] O. Dosunmu, G. Chase, W. Kataphinan, D. Reneker. Nanotechnology. 2006. 17(4). 1123. [33] Kitchaporn Nartetamrongsutt. Multiphase Group Member. The University of Akron. Personal Interview. 5/20/2010. [34] Elmarco Ltd. Retrieved 5/25/2010. . [35] Nanospider™ NS LAB 200S Owner’s/Operating Manual. Elmarco Ltd. [36] R. Ramaseshan, S. Sundarrajan, R. Jose. Nanostructured Ceramics by Electrospinning. Journal of Applied Physics. 2007. 102, 111101.

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[37] W. Son, J. Youk, T. Lee, W. Park. The effects of solution properties and polyelectrolyte on electrospinning of ultrafine poly(ethylene oxide) fibers. Science Direct. 2004. Polymer. 45. 2959–2966. [38] E. Bender, P. Katta, G. Chase, R. Ramsier. Spectroscopic Investigation of the Composition of Electrospun Titania Nanofibers. Surface and Interface Analysis. 2006. 38(9). 1252 – 1256. [39] G. Hoflund, H. Hagelin, J. Weaver, G. Salaita. App. Surf. Sci. 2003. 205. 102. [40] M. Demir, M. Gulgun, Y. Menceloglu, B. Erman, S. Abramchuk, E. Makhaeva, A. Khokhlov, V. Matveeva, M. Sulman. Macromolecules. 2004. 37. 1787. [41] F. Urbano, J. Marinas. J Mol Catal A. Chemical. 2001. 173. 329. [42] K. Thirunavukkarasu, K. Thirumoorthy, J. Libuda, C. Gopinath, J Phys Chem B. 2005. 109. 13272. [43] R. Teye-Mensah, V. Tomer, W. Kataphinan, J. Tokash, N. Stojilovic, G. Chase, E. Evans, R. Ramsier, D. Smith, D. Reneker. J. Phys. Condens. Matter. 2004. 16. 7557. [44] F. Dullien. Porous media fluid transport and pore structure. Academic Press, New York. 1992. 2nd edition. 8. [45] G. Vasudevan, S. Hariharan, G. Chase. Journal of porous media. 2005. 8. 299. [46] D. Mantri, P. Aghalayam. Detailed surface reaction mechanism for reduction of NO by CO. Catalysis Today. 2007. 119. 88-93. [47] M. Davis. Numerical methods and modeling for chemical engineers. John Wiley & Sons, New York, NY. 1984. ISBN 0-471-88761-7. 24-28. [48] ImageJ 1.43 Software. Download at . [49] J. Varabhas, G. Chase, D. Reneker. Electrospun nanofibers from a porous hollow tube. Polymer 49. 2008. 4226–4229. [50] R. Alberty. Principle of Detailed Balance in Kinetics. Journal of Chemical Education. 2004. 81. 8. [51] R. Bird, W. Stewart, E. Lightfoot. Transport Phenomena. Revised Second Edition. 2007. 563-566. [52] J. Maxwell. A Treatise on Electricity and Magnetism. 3rd edition. Clarendon Press. New York. 1891.

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APPENDICIES

115

APPENDIX A

ADDITIONAL INFORMATION ON THE SYNTHESIS AND CHARACTERIZATION OF CERAMIC NANOFIBERS

Table A.1 Measured fiber diameters for varying solution concentrations electrospun at 2 µL/min, 20kV, and 20 cm gap distance obtained from SEM images taken on 1-22-10.

116

117

118

Table A.2 Average fiber diameters for varying solution concentrations electrospun at 2 µL/min, 20kV, and 20 cm gap distance obtained from SEM images taken on 1-22-10.

119

APPENDIX B

FLOWING HYDROGEN REDUCTION APPARATUS OPERATING PROCEDURE

Procedure for Hydrogenation

While running the set-up, one person should always be at the set-up.

1. Clean system with sample at high temperature 2. Run sample with slowly increasing the temperature and checking the pressure

Start up Procedure:

1. Insert the sample inside the furnace using the ceramic boat 2. Close the vent valve (clockwise) 3. Turn on the blower 4. Turn on the vacuum pump and wait until it shows stable pressure (around 50 mtorr) 5. Open the in-line valve on the top of the pump station and pump until a base pressure is reached (~200 mtorr) 6. Purge with nitrogen gas at 95 cc/min for 15 minutes 7. Pump out the lines again until pressure is (~200 mtorr) 8. Flow the gas (20% H2: 80% N2) at 95 cc/min 9. Check every joint with hydrogen sensor before turning on the furnace for heating 10. Turn on the furnace and slowly increase up to 400:C (set at approximately 2.7 on Thermolyne Tube Furnace – Sybron). Furnace takes approximately 1 hour to heat to 400:C, then decrease setting to 2.1. 11. Monitor the temperature of the furnace with a thermocouple every 20 minutes while operating 12. Check every joint with the hydrogen sensor and exhaust gas inside the vacuum hood every 20 minutes 13. Check the temperature of fixtures 120

Shut down procedure:

1. Turn off the furnace 2. After 30 minutes, stop the gas flow 3. Pump out all of the lines 4. Purge with nitrogen gas for 15 minutes 5. Close the in-line valve 6. Stop the nitrogen gas flow (inert gas) 7. Turn off the vacuum pump 8. Turn off the blower 9. Open the vent valve 10. Wait for the furnace to cool down 11. Remove the sample

121

APPENDIX C

GC OPERATING PROCEDURE

GC Operating Procedure for Catalytic Filters

1. Open the reactor and place the filter inside 2. Close the reactor and make sure fittings are tight to prevent leaks 3. Wrap heating tape around the reactor 4. Wrap temperature controller wire into heating tape. 5. Wrap reactor with insulation (can tie it on with copper wires) 6. TURN ON THE HELIUM FLOW TO THE GC 7. CHECK TO MAKE SURE THERE IS FLOW (Rotameter at 30 cc/min) 8. Turn on the GC. The switch is on the outside of the GC on the left side near the back 9. Turn on the Peaksimple software 10. Wait for “TCD Cell” and “detector 1” lights to start blinking. This means they are at temperature. 11. Turn on the TCD. Flip the switch on the front of the GC below the ion current light under detector parameters. 12. Turn on the TCD Amplifier under the GC hood to LOW. 13. Turn on the HID Amplifier under the GC hood to MEDIUM. 14. Make sure the valves going to the other GC are closed. 15. Open all 4 valves to GC. 16. Make sure GREEN Purge valve is open. 17. SAVE ALL CHROMATOGRAMS 18. Run 2 normal runs at room temperature with only helium (25 minutes each) 19. BAKE AFTER EACH RUN 20. Turn on NO and CO gases and set flow to 1 cc/min. ALWAYS LEAVE HELIUM ON. 21. We only want to analyze the outlet, so close the INLET valve to the GC (top valve near the hood) after the NO and CO gases are running. The outlet valve is the bottom valve near the hood & should be open. 22. Run NO and CO gases at room temperature for 2 runs.

122

23. Plug in Temperature Controller and heating tape. 24. Set the Temperature Controller to 100C (MENU to set and ENTER to run) and let it reach this temperature. 25. MAKE SURE TO GET ICE FROM THE POLYMER BUILDING AND PUT IN PLASTIC TUB TO LOWER THE TEMPERATURE OF THE REACTOR OUTLET BEFORE GOING INTO THE GC. HIGH TEMPERATURES CAN DESTROY THE GC. 26. Run GC with NO and CO for 2 runs at 100C. 27. Increase temperature to 200C, 300C, 350C, 400C, and 450C (optional). Run 2 at each temperature. 28. When NO and CO peaks become less than 100 then stop running the GC. 29. SHUTTING DOWN: Turn off NO and CO flow. 30. Flow ONLY helium through the GC to purge any remaining harmful gases. 31. Turn off the TCD Amplifier (under GC hood) – flip to the center 32. Turn off the HID Amplifier (under GC hood) – flip to the center 33. Turn off the TCD (the switch on the front of the GC below the ion current light) The yellow light above it should turn off. 34. Turn off the GC (Back left switch) 35. Turn off the Helium Gas 36. Turn off Peaksimple software.

Notes:

After 10 minutes the valve closes to the GC so you can increase the temperature.

Max temp is 450:C

Peak Order on GC:

O2 N2 NO CO N2O CO2

CO2 and N2 will become bigger as temperature increases

N2 increases when NO decreases

When NO & CO become less than 100 then stop running GC

123

APPENDIX D

GC CALIBRATION

Old Calibration (12/2009)

Figure D.1 Calibration curve of carbon dioxide to convert peak area to concentration.

Figure D.2 Calibration curve of carbon monoxide to convert peak area to concentration. 124

Figure D.3 Calibration curve of nitric oxide to convert peak area to concentration.

Figure D.4 Calibration curve of nitrogen to convert peak area to concentration.

125

Figure D.5 Calibration curve of N2O to convert peak area to concentration.

Figure D.6 Calibration curve of oxygen to convert peak area to concentration.

126

Old Calibration (2/2010)

Figure D.7 Calibration curve of carbon dioxide to convert peak area to concentration.

Figure D.8 Calibration curve of carbon monoxide to convert peak area to concentration.

127

Figure D.9 Calibration curve of nitric oxide to convert peak area to concentration.

Figure D.10 Calibration curve of nitrogen to convert peak area to concentration.

128

Figure D.11 Calibration curve of N2O to convert peak area to concentration.

Figure D.12 Calibration curve of oxygen to convert peak area to concentration.

129

New Calibration (6/2010)

Figure D.13 Calibration curve of carbon dioxide to convert peak area to concentration.

Figure D.14 Calibration curve of carbon monoxide to convert peak area to concentration. 130

Figure D.15 Calibration curve of nitric oxide to convert peak area to concentration.

Figure D.16 Calibration curve of nitrogen to convert peak area to concentration.

131

Figure D.17 Calibration curve of N2O to convert peak area to concentration.

Figure D.18 Calibration curve of oxygen to convert peak area to concentration. 132

APPENDIX E

CATALYTIC FILTER RAW DATA OBTAINED FROM GC

Table E.1 Blank run raw data obtained from GC

133

Table E.2 Filter #1 raw data obtained from GC

134

Table E.3 Filter #2 raw data obtained from GC

135

Table E.4 Filter #3 raw data obtained from GC

136

Table E.5 Filter #4 raw data obtained from GC

137

Table E.6 Filter #5 raw data obtained from GC

138

Table E.7 Filter #6 raw data obtained from GC

139

Table E.8 Filter #7 raw data obtained from GC

140

Table E.9 Filter #8 raw data obtained from GC

141

Table E.10 Filter #9 raw data obtained from GC

142

Table E.11 Filter #10 raw data obtained from GC

143

Table E.12 Filter #11 raw data obtained from GC

144

Table E.13 Filter #12 raw data obtained from GC

145

Table E.14 Filter #13 raw data obtained from GC

146

Table E.15 Filter #14 raw data obtained from GC

147

Table E.16 Filter #15 raw data obtained from GC

148

Table E.17 Filter #16 raw data obtained from GC

149

Table E.18 Filter #17 raw data obtained from GC

150

Table E.19 Filter #18 raw data obtained from GC

151

Table E.20 Filter #19 raw data obtained from GC

152

Table E.21 Filter #20 raw data obtained from GC

153

Table E.22 Filter #21 raw data obtained from GC

154

Table E.23 8% filter rerun after sonication raw data obtained from GC

155

Table E.24 Catalytic converter with reducing first raw data obtained from GC

156

Table E.25 Catalytic converter without reducing first raw data obtained from GC

157

APPENDIX F

ERROR ANALYSIS

The error associated with the measurement and equipment is listed in Table D.1.

Table D.1 Error associated with measurements.

The fiber diameters and lengths for each solution concentration were measured using ImageJ 1.43 software [48]. For each solution, not fewer than 100 fibers from not fewer than 4 SEM images were measured as shown in Appendix A. The length weighted diameters, and standard deviations, , of the distributions were calculated using the following equations obtained from Varabhas et al. [49]( not used in the log form).

158 where and is the length of the fiber segments of diameter . The frequency distributions are determined from:

Length weighted frequency fiber diameter distributions are shown in Figure D.1.

These results show that the narrowest size distribution and smallest fibers are obtained with the 8% solution. The size distribution increases as the solution concentration increases.

Figure D.1 Length weighted frequency distribution of fiber diameter of varying concentrations of PVP in ethanol.

159

For the concentration measurements of the inlet or outlet gases using the GC were measured only two times for each temperature due to the long amount of time each filter takes to run. Error bars from the concentration profiles in Chapter V was calculated by taking the average of the concentrations of the two runs and taking the difference between the average and the data points using the following equation.

Where is the average of the concentrations and is the actual concentration for 1 run. For example, in Figure 5.2 the CO gas concentration error at 25oC is 463 and the error bars do not show. In this same figure, the concentration error at 300oC is 1838 which can be seen in the figure. The CO gas calibration curve showed a good enough fit

(R2 = 0.9994) that the calibration curve does not introduce significant additional error into the calculated quantities.

160

APPENDIX G

EQUIPMENT TROUBLESHOOTING

Gas Chromatograph

 Issue: Reactor leaking

o Perform a leak test using soapy water

o Make sure there is no debris on reactor surfaces that would cause leaks

o Polish the reactor using a lathe machine every 2 months because pitting

of the metal occurs due to the high and fluctuations in temperature.

 Issue: Low or incorrect NO peak

o Check to make sure the flow meters are functioning properly (switch flow

meters to make sure the peaks are consistent)

o Check the regulator on the gas cylinder to see if it is functioning properly

o Make sure there is no debris in the lines to the GC (such as parts of

filters) – use a vacuum cleaner to remove debris from reactor area or

take out lines and run water through them to make sure there is no

blockage

161

o Increase the pressure on the gas cylinder regulator to 44 psi to match CO

pressure.

o Turn off the NO gas

o Open the inlet to the NO mass flow meter to release possible buildup of

gasses and pressure

o Reduce the pressure on the NO regulator to 0

o Close the inlet to the NO mass flow meter

o Start the NO gas flow

o Slowly increase the pressure until up to 44 psi

 Issue: Inconsistent peaks

o Bake the GC at 200oC for 4 hours

 Issue: Thermocouple not working

o Make sure wires are connected properly – if needed, correct wire

attachment.

 Issue: Heat tape not working

o Replace if wires are exposed – the high temperatures cause the heating

tape to disintegrate

o To prevent heating tape breaking prematurely, wrap copper wire around

heating tape end ties and use to tie around reactor (the heating tape ties

disintegrate and will become brittle – using the copper wire will extend

life)

162

 Issue: NO and CO gas increasing with subsequent runs

o Make sure to run the GC to get rid of remaining NO and CO gases

o The increase was caused by buildup of gases in lines and the GC since the

GC was not run with only helium until all the NO and CO was gone

Reduction Apparatus

 Issue: Hydrogen is not coming out of hood exhaust

o Make sure blower is working

o Make sure hydrogen sensor is functioning properly (replace batteries)

o Make sure valve in reduction apparatus is open

Vacuum Mold Set-up

 Issue: Slurry is no longer being vacuumed properly

o Check the vacuum pump

o Make sure the hose is connected properly

o Make sure the collecting tank has not reached the maximum capacity of 7

liters

o Make sure to clean the wire mesh thoroughly after using because the

binder can get stuck and clog the mesh which causes the slurry to not

vacuum properly

o Replace the wire mesh

163

APPENDIX H

CATALYTIC FILTER IDENTIFICATION TABLE

Table H.1 Catalytic filter identification table.

164

165

APPENDIX I

ELEMENTARY REACTION MODEL FOR NITRIC OXIDE AND CARBON MONOXIDE REACTIONS

I.1 Introduction

This appendix discusses the elementary reaction model for the decomposition of nitric oxide and carbon monoxide gases and derivation of a transport model for predicting outlet concentrations from the catalytic filters. Solution to the transport model is left for future work. The plug flow reactor (PFR) used in these experiments is assumed to be isothermal. The reactor outlet concentrations depend on temperature at which the reaction is run. This appendix introduces the elementary reaction model equations that can be used to calculate reaction kinetic parameters in future work.

These reactions assume a single site mechanism. The model equations include terms to 166 account for the gas velocity, the amount of catalyst in the filter, and the nanofiber diameter within the filter media. The proposed reactions that are reversible (i.e. 1 & 2,

7 & 8, and 10 & 11) are bound by the principle of detailed balance. The principle of detailed balance relates forward and reverse reactions by an equilibrium relationship that was explored by Alberty [50]. Alberty states that at equilibrium, the forward rate of each reaction step is equal to the reverse rate of that step which is the principle of detailed balance. This principle infers that we cannot independently solve for rate constants for forward and reverse reactions because they are related by a thermodynamic quantity (k) that sets the concentrations and conversion of reactants for a given temperature.

The detailed balance at equilibrium requires that the forward reaction exactly balance the reverse reaction:

Where is the thermodynamic quantity for the forward reaction and is the thermodynamic quantity for the reverse reaction. Hence

The two reactions are balanced at equilibrium hence

167

And

The ratio is the equilibrium constant .

Equation C-8 in Fogler [27] relates (concentration equilibrium constant) and

(pressure equilibrium constant) by

Where is the change in the total number of moles per mole of A reacted.

Equation C-9 in Fogler [27] shows that if you have the value of at one temperature you can calculate at another temperature. This equation can be combined with

Equation C-8 and is the following:

Where is the heat of reaction.

The value of K is related to the Gibbs free energy by Equation C-10 in Fogler [27].

Where is the change in Gibbs free energy.

168

In practice, you should let the reaction reach equilibrium in a batch reactor and measure the component concentrations. K is product of concentrations of reactant products divided by the product of the concentrations of reactants at equilibrium.

1.2 Basis for Assumptions

Park [7] determined the appropriate reactor type for experimentation and that a plug flow model could be used. The Reynolds number was calculated for slow reaction through the porous media in Park’s study and was determined to be 2 x 10-4 according to the following equation.

Park [7] also estimated the dispersion coefficient for the case of gas-solid

catalytic reactions. It was determined the quantity has an approximate value of

100,000. The axial dispersion coefficient (D) was determined to be 1.5 x 10-7 m2/s while

the inversed vessel dispersion number was 0.0076. Park [7] concluded the reactor

for experimentation is convection controlled a plug flow reactor (PFR) with small amount of dispersion.

A concern is whether or not diffusion into the interior and through the boundary layer of the nanofibers is important for catalytic filter performance. The boundary layer

169 thickness depends on the velocity of the gases flowing through the experimental reactor. Park [7] varied the face velocity of the reaction gases running at 0.14 cm/sec,

0.28 cm/sec, 0.42 cm/sec with the same flow ratio of NO and CO gases. It was determined that the face velocity does not have a significant effect on the catalytic filter performance.

Bird, Stewart, & Lightfoot derives an effectiveness factor for diffusion and chemical reaction inside a spherical porous catalyst [51]. This same method is applied to derive the effectiveness factor in cylindrical coordinates for diffusion into nanofibers which is occurring within the catalytic filters. This is to prove that diffusion is not the rate limiting step for the series of reactions with catalytic filters; the surface reaction itself is the rate limiting step. The calculations for the catalyst effectiveness factor is for a 1st order reaction, however the other reaction orders give similar results. The shell mass balance method and Fick’s first law are first applied. A cylindrical porous catalyst nanofiber of radius R is used. These nanofibers are in a catalytic reactor and reactant A diffuses through and reacts to form the product B. A mass balance for species A on a cylindrical shell of thickness Δr within a single nanofiber is the following:

Where = number of moles of A passing in the r direction and the source term

is the molar rate of production of A by chemical reaction in the shell of thickness .

170

Dividing by and letting gives

Using the definition of the first derivative,

Effective diffusivity for species A in the porous medium is defined as,

Where is the concentration of the gas A contained within the pores. The effective diffusivity must be measured experimentally and depends generally on pressure and temperature.

Inserting into the derivative term gives,

Letting a be the available catalytic surface per unit volume and ,

This equation is to be solved using the following boundary conditions:

171

Rearranging gives,

The product rule gives,

Simplifying gives

Multiplying each side by to obtain the Bessel function form gives

Which gives the Bessel function in the form of

Where

172

Where the solution is in the form:

Applying the boundary condition

Therefore,

173

Applying the second Boundary Condition:

Solving for A gives

And

Giving

174

Where is the available surface per unit volume of solids and voids, is the reaction rate, and is the effective diffusivity. The constant represents if a fiber material has pores. This analysis estimates as the surface area of the nanofibers within a catalytic filter. In future work, a BET analysis should be run to determine the actual surface area.

Where is the reaction rate.

175

Set and

Which is the effectiveness factor for the nanofibers in cylindrical coordinates.

is the effective diffusivity of molecules going through pores which can be estimated using Maxwell’s equation [52].

: the effective diffusivity

: Molecular diffusivity (species in helium)

176

: Void fraction of the intrinsic fiber itself. We can assume the porosity is less than

10% from TEM images ( )

To estimate the molecular diffusivity we can use correlations given in Ch 17 of

Bird, Stewart, and Lightfoot [51].

The molecular diffusivity of both NO—He and CO—He are calculated using

Equation 17.3-12 which is the following:

Where , , T K, and p atm

Where is the “collision integral” for diffusion and is a function of the dimensionless temperature .

The Lennard-Jones potential parameters are found in Appendix E on page 864 of Bird,

Stewart, and Lightfoot and are the following:

Bird, Stewart, and Lightfoot defines the following equations in Equation 17.3-14,15.

177

Table E.2 in Bird, Stewart, and Lightfoot gives given

Using the values tabulated above:

The reaction is run in the GC at temperatures ranging from 25oC to 450oC. Each temperature is used for calculating

The reaction rate is estimated using the pre-exponential factors given by Mantri

[46]. A large Thiele modulus is expected for fast reactions which will push towards 0.

This is the case for reactions with a preexponential factor of 1 x 1013 1/sec which is the fastest reactons. However, calculating the overall effectiveness factor for the rate limiting step (0.03 1/sec) gives overall effectiveness factor values close to 1 as shown in the following figure:

178

Figure I.1 Temperature effect of effectiveness factor for each fiber diameter.

It can be noted from this figure that at as the fiber diameter decreases, the effectiveness factor, approaches 1. Also, as the temperature of the reaction increases, the effectiveness factor also approaches 1.

The worst case for effectiveness factor occurs at room temperature as shown in the following figure:

179

Figure I.2 Effectiveness factor for varying fiber diameters at a reaction temperature of

25oC.

However, as the reaction temperature increases, the effectiveness factor approaches 1. The light-off temperature of typical catalytic converters and catalytic filters is above 200oC which is when the reaction starts taking place. The effectiveness factor for each fiber diameter is shown in the following figure for reactions taking place at 200oC. It can be noted that all fiber diameter effectiveness factors are very close to 1 at this temperature.

180

Figure I.3 Effectiveness factor for varying fiber diameters at a reaction temperature of

200oC.

For a reaction temperature of 300oC, the lowest effectiveness factor is 0.9884.

We can conclude from these calculations that all diameters give values close to 1 which indicates that nearly the whole surface area of the catalyst is available for reaction. The catalyst effectiveness factor will be important for fibers under room temperature conditions. However, since the reactions only take place at elevated temperatures, we can assume an effectiveness factor close to 1.

The advantage of using nanofibers as catalytic support structure is there is a very short distance for diffusion. There is diffusion into the alumina and the catalyst within pores is potentially all utilized in the reaction.

The process of reaction includes convection, diffusion, attachment, reaction, detachment, diffusion, and convection. From this analysis, we can conclude that fiber 181 size is not a significant factor. Under these reaction conditions, diffusion into the boundary and internal is not the rate limiting step. The rate limiting step is the reaction itself, therefore the process is surface reaction limited.

I.3 Development of model equations

The general multiphase ith chemical species balance is used to set up the isothermal reaction for the plug flow reactor.

(I.1)

From left to right, the terms in Equation (I.1) account for the rate of accumulation in the

α phase, rate of convection, diffusion of species A, homogeneous reaction, and the heterogeneous terms which are defined on page 201.

The following assumptions are used to simplify the species balances for the model.

1) Steady state

2) Isothermal

3) Dilute gas flow

4) One dimension (z-directional) flow

5) No phase change and molecular diffusion at the interface is dominated

by the heterogeneous reaction term

6) Heterogeneous reaction and no homogeneous reaction

7) Gas phase volume fraction, = constant (i.e., porosity is constant)

182

8) over medium is small and has negligible effect on (constant gas phase density)

9) Convection dominates diffusion (Convection >> Diffusion)

10) Only two phases (gas and solid fiber) hence the slip generation term is zero

From the assumptions, Equation (I.1) is simplified to

(I.2)

The mass continuity equation is used to obtain the gas velocity.

(I.3)

(I.4)

( is set by the flow meters on the GC set-up) (I.5)

(I.6)

The ideal gas law gives

(I.7)

Substituting Equation (I.7) into (I.6),

(I.8)

Molar concentration is given by

183

(I.9)

Substituting into the simplified i species balance, Equation (I.2)

(I.10)

Substituting Equation (I.8) in for

(I.11)

Canceling porosity terms and bringing out constant terms from the differential yields

(I.12)

Dividing Equation (I.12) by Mi gives

(I.13)

Defining the term of Equation (I.13)

(I.14)

184

Where (I.15)

Substituting in Equation (I.13) and rearranging gives

(I.16)

dimensionless posn, L=filter thickness (I.17)

(I.18)

where (I.19)

Assuming pressure does not change much over range of flows for j streams,

(I.20)

where Pm and Tm are the pressure and temperature in the mass flow meter, respectively.

(I.21)

th Where is the molar fraction in the gas stream of species k in the j stream.

(I.22)

then Tm = same for all streams

(I.23)

185

Where the helium flow dominates and

Substituting Equation (I.23) into Equation (I.16) gives

(I.24)

Canceling P and R and rearranging gives

(I.25)

Let (I.26)

the species balances for the i species in the gas phase is given by the following equation

(I.27)

(I.28)

(I.29)

Where N is the number of catalyst particles per volume of nanofiber

From Equation (I.19), ac is defined as

(I.30)

186

Where fCAT is the fractional area of catalyst particles exposed to reaction.

Substituting Equation (I.29) into Equation (I.30) gives

(I.31)

Simplifying gives

(I.32)

The reaction model equations assume elementary reactions and the forward reaction only and are the following:

(I.33)

Where is the stoichiometric coefficient for species i in the reaction equation j

and (I.34)

Where kj is given by the Arrhenius equation in terms of frequency factor (Aj) and activation energy (Ej).

(I.35)

187

The i species participating in the series of reactions are the following: i Species

1. NO 2. CO 3. O2 4. N2 5. N2O 6. CO2 7. * 8. NO* 9. CO* 10. O* 11. N* 12. N2O*

The reactions proposed by D. Mantri and P. Aghalayam [46] and the elementary reactions are the following assuming a single site mechanism:

Table I.1 Reactions proposed by D. Mantri and P. Aghalayam [46] assuming a single site mechanism and the corresponding elementary reactions.

Reaction No. Reaction Elementary Reaction 1. NO + *  NO* S1 =k1C1C7 (I.36)

2. NO*  NO + * S2 =k2C8 (I.37) 3. NO* + *  N* + O* S3 = k3C8C7 (I.38) 4. 2N*  N2 + 2* S4 =k4C11C11 (I.39) 5. NO* + N*  N2O* + * S5 =k5C8C11 (I.40) 6. N2O*  N2O + * S6 =k6C12 (I.41) 7. CO + *  CO* S7 =k7C2C7 (I.42) 8. CO*  CO + * S8 =k8C9 (I.43) 9. CO* + O*  CO2 + 2* S9 =k9C9C10 (I.44) 10. O2 + 2*  2O* S10 =k10C3C7C7 (I.45) 11. 2O*  O2 + 2* S11 =k11C10C10 (I.46)

188

th The rate of generation of the i species, , in each species balance is given by

R1 = -S1 + S2 (I.47)

R2 = -S7 + S8 (I.48)

R3 = -S10 + S11 (I.49)

R4 = S4 (I.50)

R5 = S6 (I.51)

R6 = S9 (I.52)

R7 = - S1 + S2 – S3 + 2S4 + S6 – S7 + S8 + 2S9 – 2S10 + 2S11 (I.53)

R8 = S1 – S2 – S3 – S5 (I.54)

R9 = S7 – S8 – S9 (I.55)

R10 = S3 – S9 + 2S10 – 2S11 (I.56)

R11 = S3 – 2S4 – S5 (I.57)

R12 = S5 – S6 (I.58)

Information that must be input includes the following:

Measured inlet concentration

Measured outlet concentration

Temperature of gas in flow meters

189

Temperature of the reactor

Total flow rate at temperature of flow meters

Porosity of filters

Diameter of nanofibers

Diameter of catalyst particles

Mass of catalyst / mass of nanofibers

Mass of microfibers

Mass of nanofibers

The unknowns to find include the following:

Ai (i =1 to 11)

Ei (i =1 to 11)

The constants within the equations include

A=filter area

ρMF =density of alumina

ρCAT =density of the catalyst material

There are 6 differential equations to solve for i species 1 - 6 using Equation (I.27)

(I.27)

Where is the molecular weight of species i 190

Using Equations (I.17) and (I.26) gives the following

(I.59)

Where is the filter cross sectional area, is the total volume rate of flow at , and is the reactor temperature.

Defining the variables in :

(I.60)

Where is the fraction of catalyst particles surface that is available for reaction which is suspected to be dependent upon the NF diameter ( ), the catalyst material, and the catalyst particle diameter.

Note: Experiments should include data only for same , and catalyst material for

to be a constant.

= mass cat/mass NF

includes the density of the catalyst as shown in the following

(I.61)

Where and are the densities of the microfibers and nanofibers, respectively.

The volume of the nanofibers, , is given by the following

191

(I.62)

(I.63)

Equation (I.19) gives and is shown below

(I.19)

D. Mantri and P. Aghalayam [46] assume for = 8 to 12. Hence, it assumes that and are constants.

Setting R8 through R12 = 0 gives

R8 = 0 = S1 - S2 - S3 - S5 = k1C1C7 - k2C8 - k3C8C7 - k5C8C11 (I.64)

R9 = 0 = S7 - S8 - S9 = k7C2C7 - k8C9 - k9C9C10 (I.65)

R10 = 0 = S3 - S9 + 2S10 - 2S11 = k3C8C7 - k9C9C10 + 2k10C3C7C7 - 2k11C10C10 (I.66)

R11 = 0 = S3 - 2S4 - S5 = k3C8C7 - 2k4C11C11 - k5C8C11 (I.67)

R12 = 0 = S5 - S6 = k5C8C11 - k6C12 (I.68)

Where (I.69)

192

From R12  C12 = f(C11,C8) (I.70)

From R9  C10 = f(C7,C9) (I.71)

From R8  C7 = f(C8,C11) (I.72)

Combine R8 +R11

(I.73)

(I.74)

Rearranging to obtain the quadratic formula form gives

(I.75)

(I.76)

(I.77)

where (I.78)

Requires b2-4ac≥0 (I.79)

Simplifying the coefficients for the quadratic equation for gives the following 193

(I.80)

(I.81)

(I.82)

1 (I.83)

(I.84)

(I.85)

Combining Equations (I.84) and (I.85) to determine if gives

(I.86)

From Equation (I.86) we find that is still needed to determine if

. This means a computer program used to solve these equations will have to accommodate for situations when the guessed values for and do not give a realistic solution.

Combine R9 + R10 to obtain C9

194

(I.87)

(I.88)

(I.89)

Equation (I.89) gives a cubic equation in the form

(I.90)

The solution to the cubic Equation (I.83) gives = f(C8, C7)

Therefore, we have one unknown, and can use Equation (I.91) to obtain

(I.91)

To simplify equations further, Equation (I.71) can be plugged into Equation (I.56) and

simplified into a cubic equation as shown below:

(I.71)

S3  2S10  S9 2S11  0 (I.56)

195  S3  2S10 k9C9C10 2k11C10C10  0 (I.92)

S  2S S  k C 2k C C  0  3 10 7 8 9 11 10 10 (I.93)

2  1   S  2S  S  k C  2k S  k C  0  3 10 7  8 9 11  7 8 9 (I.94) k9C9 

2 3 2 3 2 2 2 S3  2S10  S7k9C9  k8k9C9  2k11S7  2S7k8C9  k8C9  0  (I.95)

k k 2C3  S  2S  S k 2  2k k 2 C2  4k k S C  2k S2  0  8 9 9  3 10 7 9 11 8  9 11 8 7 9 11 7 (I.96)

 S  2S  S k 2 2k k   4k S  2k S 2 3 3 10 7 9 11 8 2  11 7  11 7 C9   2  2 C9  2 C9  2  0 k k k  k  k k  11 9 9   9  8 9 (I.97)

3 2 C9  aC9  bC9  c  0 (I.98)

The method to solve these equations using a computer program requires an iterative

simultaneous solution and the steps follow:

Guess C8  calculate C7, C10, C11, C12 from Equations (I.72), (I.71), (I.78), and

(I.70), respectively.

Calculate C9 from CTOT Equation (I.91)

Calculate new C8 from cubic Equation (I.90)

196

Davis presents a Multistep (predictor) Method in Numerical Methods and Modeling for

Chemical Engineers [47] which can be applied and is shown below

(I.99)

(I.100)

The Trapezoidal Rule is

(I.101)

If is non-linear, cannot be solved directly.

We can attempt to obtain by iteration.

Predict a 1st approximation by:

(I.102)

Then calculate correct value using:

(I.103)

For most problems convergence usually occurs in a few iterations.

Therefore, from Equation (I.27)

(I.104)

197

Where is species and is the position step

The computer model described above could be used to fit the unknown parameters to the experimental data using a method such as a Genetic Algorithm.

Such a calculation scheme is as follows:

Inlet k=0

x=0

C1, C2…C6 = known BC

C7, C8…C12 = unknown

Sets values for

C8, fCAT, kj, (Aj, Ej)

Where from Equation (I.69)

(I.69)

Calculate C11  C12  C7  C9  C10  CTOT (hold constant)

Step in k

198

k=k+1

(I.105)

Iteration s=0

(I.106)

(I.107)

(I.108)

Yes  Ci for i=7-12

No  next k (iterate)

Using parameter values given in literature

Use CTOTѲi=Ci (I.109) where Ѳi =fractional concentration (Hence a computer program would search for numbers between 0 and 1)

Because CTOT=constant, then CTOT becomes part of the reaction coefficient

199

Therefore let C7, C8,…C12 = Ѳk where 0≤Ѳk≤1 and k=7-12

Ѳk=1 (I.110)

This modification impacts solving equations by the following:

Rk=0 (1) 1= Ѳk

(2) C12

(3) =

(4)

(5)

a’,b’,c’,=f(C8,C7)

(6)

6 equations and 6 unknowns, therefore there should be an exact solution.

Guess C8 and iterate using equation 1 to calculate C8

Let F = C8guess - C8calc . Then using a bisection method the value of F can be driven to zero

Let x2 = 0, x1 = 1

200

(I.111)

Check

Revise

Repeat to find value of x that drives F towards zero.

NOMENCLATURE

: Mass transfer across the interface between the phases due to phase change

: Molecular diffusion of species A across the interface (without reaction)

: The ‘slip’ transfer across the interface (when 3 or more phases are present) due to

non-parallel interfacial boundaries on the flux vector

: The generation of species A at the interface due to heterogeneous reaction

: Species I diffusive flux vector in phase

: Volume fraction of the phase

: Molecular weight of bulk gas (mostly He)

201

: Face area of filter

: Absolute pressure

: Pressure at mass flow meter

: Absolute temperature of the reactor

: Temperature at mass flow meter

: Mass flow rate of gases (determined from sum of flow meters at Tm)

: Fraction area of catalyst particles exposed to reaction (also accounts for number

of active sites catalyst area)

: Area ratio of catalyst to nanofibers

: Mass ratio of catalyst to nanofibers

: Fiber diameter

: Volume of nanofibers

: Density of nanofibers

: Density of microfibers

: Mass of nanofibers

: Mass of microfibers

202

: Number of catalyst particles per volume of nanofibers

: Molar fraction in the gas stream

203

APPENDIX J

CHARACTERIZATION OF CATALYTIC CONVERTER

Figure J.1 SEM images of catalytic converter.

204

205

Figure J.2 EDX Spectrum of Catalytic Converter

Element Weight %

O 34.37

Al 29.83

Na 1.42

Rh 0.31

Pd 0.20

Ba 11.84

Mg 2.94

La 6.71

Pt 1.5

Ce 10.84

206