MASTER THESIS
Ahmet EREM
DEMS STUDY OF ETHANOL AND CYCLOHEXANE
ELECTROOXIDATION AT BORON DOPED DIAMOND ELECTRODE
DEPARTMENT OF CHEMISTRY
ADANA, 2009
DEMS STUDY OF ETHANOL AND CYCLOHEXANE
ELECTROOXIDATION AT BORON DOPED DIAMOND ELECTRODE
By Ahmet EREM
A THESIS OF MASTER of CHEMISTRY DEPARTMENT
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DEMS STUDY OF ETHANOL AND CYCLOHEXANE
ELECTROOXIDATION AT BORON DOPED DIAMOND ELECTRODE Ahmet EREM
DEPARTMENT OF CHEMISTRY
INSTITUTE OF NATURAL AND APPLIED SCIENCES UNIVERSITY of ÇUKUROVA
Supervisor : Prof. Dr. Birgül YAZICI Year: 2009 , Pages: 70
Jury : Prof.Dr. Birgül YAZICI Assoc.Prof.Gülfeza KARDAŞ Asst.Prof.Muzaffer ÖZCAN
The electrochemical oxidation of 1.10-3M ethanol and 1.10-4M cyclohexane in 0.5M H2SO4 was studied on boron-doped diamond electrode (BDD) using differential electrochemical mass spectrometry (DEMS) where the dual thin-layer flow-through cell connected directly to the mass spectrometer. We studied the electro-oxidation of ethanol due to its importance as fuel in fuel cell because it produce 12 e- in case of it is complete oxidation CO2 but until now there is a lack in the catalyst which active the cleavage of C-C. It is considered the major renewable biofuel, less toxic than methanol and it has a high energy density. We should break C-C bond and convert to CO2. And cyclohexane is very interesting molecule and we wanted to study the electro-oxidation of cyclohexane. Cyclohexane which is flammable molecule, volatile, is not reactive but very stable. we used BDD electrode as a working electrode because Boron-doped diamond thin films (BDD) have recently attracted as a new electrode material. The major advantage of BDD electrodes is their wide electrochemical window which allows oxidation of organic compounds in aqueous electrolytes. At high electrode potentials hydroxyl radicals (OH•) are produced at the surface. The other properties of BDD are Tough, Resistant to degradation and thermal shock, electrical conductivity, suitable for wastewater treatment and water disinfection, sterilisation and chemically stable. In this work we obtained acetaldehyde as a main intermediate and we obtained 12-24% CO2 and 30- 40% acetaldehyde (depends on flow rate and potential ) as main product during ethanol oxidation. For cyclohexane oxidation, we obtained 10-15% CO2 (depend on flow rate ) as a main product.
Key Words: BDD electrode, DEMS, Electro-oxidation, Ethanol, Cyclohexane.
ETANOL VE SİKLOHEKSANIN BDD ELEKTROTTA
ELEKTROOKSİDASYONUNUN DEMS CİHAZINDA ÇALIŞILMASI
Ahmet EREM
ÇUKUROVA ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
KİMYA ANABİLİM DALI Danışman: Prof. Dr. Birgül Yazıcı
Yılı: 2009, Sayfa: 70
Jüri: Prof.Dr. Birgül YAZICI Doç. Dr. Gülfeza KARDAŞ Yrd.Doç.Dr. Muzaffer ÖZCAN
Bu çalışmada; BDD elektrot üzerinde 1.10-3M ethanol ve 1.10-4M sikloheksanın 0.5M H2SO4 içerisindeki çözeltilerinin, direk kütle spektroskopisine bağlanan bir hücre ile DEMS cihazında elektro-oxidasyonları çalışılmıştır. Yakıt hücresi için çok önemli olmasından dolayı etanolün elektrooksidasyonunu incelemek istedik çünkü etanol CO2 e kadar oksidasyona uğradığı zaman 12e- üretebilmektedir. Etanol, biyokimyasal yakıtlarda kullanılabilirliği, metanolden daha az toksik olması ve enerji bakımından tercih edilmektedir. Etanol yapısındaki C-C bağları karşımıza çıkan en büyük sorundur. Biz bu C-C bağlarını kırıp CO2’e dönüştürmemiz gerekmektedir.
Sikloheksan çok merak uyandıran bir moleküldür bu yüzden sikloheksanın elektrooksidasyonunu çalışmak istedik. Sikloheksan; yanıcı, uçucu , çok fazla reaktif olmayan fakat çok kararlı bir moleküldür. Bu çalışmada BDD elektrotu çalışma elektrotu olarak kullandık çünkü BDD elektrot son zamanlarda üstün özelliklerinden dolayı araştırmacıların ilgisini çekmektedir. BDD elektrotun en büyük avantajı organik bileşiklerin sulu çözeltilerinin elektro-oksidasyonunu çok geniş bir potansiyel aralığında gerçekleştirmesidir. Çok yüksek potansiyellerde OH•
radikalleri elektrot yüzeyinde oluşmaktadır. BDD elektrotun diğer önemli özellikleri;
çok sağlam olması, ayrışmaya ve termal olarak bozunmaya dayanıklı olması, elektriksel iletkenliği iyi olması, atık su arıtımı, suyun dezenfektan edilmesi ve sterilizasyonuna uygun olaması, ve kimyasal olarak kararlı olması. Bu çalışmada etanol oksidasyonu sırasında asetaldehit’i ara basamak olarak elde ettik ve ayrıca
%12-24 CO2 ve % 30-40 asetaldehiti ana ürün olarak elde ettik ( çözelti akış hızına ve potansiyele bağlı olarak değişiklik gösteriyor). Sikloheksan elektroksidasyonu için
% 10-15 oranında CO ’i ( çözelti akış hızına göre değişiklik gösteriyor) ana ürün
University. And I want to thank all of members in our physicochemistry department.
All of the my work performed in electrochemistry department of Bonn University. I went this university for all of my work via Erasmus program. I want to thank a lot Doc. Dr. G. Kardaş and our international office for their interest about Erasmus program.
Thanks to all the members of the jury for reading and evaluating my thesis as well as assisting in my exam.
Special Thanks to Prof. Helmut Baltruschat for accepting me for one year in his research group in Bonn. I was very pleased from meet in the airport to end all of my study. When I went there, I met wonderful people there. Thank you very much all of Prof.H.Baltruschat’s group for your warm welcome, great help for all of my work. I do not forget all of you.
I must also thank Dr. S.Ernst for his interest about my discussions and helped me to make interpretation about new subject for me. Thank you very much Abd el Aziz Ab del Salehin for your great help in DEMS measurements also during the my writing. You are very good friend and very good teacher.
ÖZ………...………..…II ACKNOWLEDGEMENTS……….………...III CONTENTS ……….……….…IV TABLE CAPTIONS………...VII FİGURE CAPTIONS………..………...VIII
SYMBOLS AND ABBREVIATIONS………XIII
1.INTRODUCTION……….………...………..…...…...1
2. THEORETICAL ………..…...……3
2.1.Electrochemistry Of Boron Doped Diamond Electodes…..………...3
2.1.1. Synthesis of Boron-Doped Diamond Electrodes……...…...……...….3
2.1.2. Chemical Vapour Deposition ………...4
2.1.3. CVD Polycrystalline Diamond …………...……...…...5
2.2. Properties And Applications of BDD Electrodes…..… ……....………...…6
2.2.1. Properties of BDD...………....…...6
2.2.2. Applications of BDD.. ……….…...7
2.3. Organic compounds oxidation on BDD.……….….……..7
2.4. Mechanism of C1 organic oxidation on BDD……...8
2.5. Differential electrochemical mass spectrometry (DEMS).… ………..10
2.5.1. DEMS Set up .………...11
2.5.2. DEMS CELL.………..…………...…12
2.5.2.1 Dual Thin Layer Flow Through Cell .………..…………...12
2.6. Effect Of Hydroxyl Radicals (HO•) at Boron Doped Diamond.……... 13
2.7. Ethanol ………..15
2.7.1. Properties of Ethanol ...15
2.7.2 Physical Properties of Ethanol .…...………...16
2.8. Cyclohexane………..………...….17
2.8.1 Properties of Cyclohexane .………...…………..……….….17
2.10. Water Treatment With Boron Doped Diamond Electrodes.…...…...……..20
3. MOTIVATION AND AIM OF THIS WORK……….…………..22
4. PREVIOUS EXPERIMENTS .………...…..………...23
4.1 DEMS study electrooxidation of Methanol on BDD …………..……...23
4.2 DEMS study electrooxidation of Formic acid on BDD ……..…………..….25
4.3. DEMS study electrooxidation of Acetic acid on BDD ………..…...26
4.4. DEMS Study Electro-oxidation of bulk ethanol on Pt(Pc)…………...…….29
5.EXPERIMENTAL ...………..………...……33
5.1. Chemicals .…………..………..……….….…..33
5.2. Solutions ………..………...……….…...33
5.3. Cleanliness . ………..……...33
5.4. DEMS Study .………..……….…..…...34
5.5. Calibration of DEMS .………..………..……….…...….35
5.5.1. Acetaldehyde Calibration . ………..…………...…....35
5.5.2 CO2 Calibration ...………...………….... 36
5.5.2.1 Adsorption Test .……….…….….….…….…38
5.6.Data Record .………..…………..……….………….….…...39
5.7 Calculations ..…………..……….………...……....40
5.7.1. Number Of Electron ( Z ) ………..…….……….…40
5.7.2 Current efficiency for acetaldehyde ..………...…41
5.7.3 Current efficiency for CO2..………...………41
6.1.1 Flow rate 5µL/s………..…….………...………..……...…...46
6.1.2 Flow rate 1.6 µL/s ……..………...………….….…..52
6.1.3 Flow rate 10 µL/s ………..………54
6.2 Electrooxidation Of CycloHexane .………..……….….………55
6.2.1 Flow rate 5µL/s ………..………..……….…55
6.2.2 Flow rate 1.6µL/s……..………...…………...………59
6.2.3 Flow rate 15µL/s ….…….………...…………...…..………61
7.CONCLUSION.………..…………63
8.SUMMARY.……….….……..……….…..65
REFERENCES………..………67
CURRICULUM VITAE……….70
Table 2.2. Physical Properties of Cyclohexane………..17
Table 4.1. The current efficiencies for CO2 and d4-acetaldehyde during the potential step experiment of bulk d6-ethanol on smooth polycrystalline platinum in 0.01M d6-ethanol + 0.1M H2SO4 + 0.5 M HClO4 solution at different flow rate and potential………..32
Table 5.1. Chemicals which is used in all experimental part………...33
Table 5.2. Calibration Constant of Acetaldehyde depend on flow rate………35
Table 5.3. Calculation of CO oxidation………38
Table 7.1. Number of electron (Z), the current efficiencies for CO2 and acetaldehyde in 1.10-3 M ethanol + 0.5 M H2SO4 and number of electron and current efficiencies for CO2 in 1.10-4 M Cyclohexane + 0.5M H2SO4 at 2.5 V and different flow rate………..64
Figure 2.1. Differences between Diamond and Graphite lattice………..…3 Figure 2.2. solid BDD electrode...4 Figure 2.3. schemes of Chemical Vapour Deposition ………....5 Figure 2.4. A microscopic view shows the surface morphology and
the polycrystalline nature of the diamond structure………6 Figure 2.5. Scheme of the electrochemical oxidation of organics compounds on active anodes (reactions (1), (2), (3), and (4)), and non-active anodes (reactions (1), (5), and (6)); M is an active site at the electrode
Surface………8 Figure 2.6. shows Schematic representation of a typical experimental DEMS
setup; (1) electrochemical cell, (2) MS connection to
the electrochemical cell, (3) connection to the calibration leak, (4) turbo molecular pump, (5) rotary pump, (6) ion source, (7) quadrupole rods, (8) turbo molecular pump, (9) rotary pump,
(10) secondary electron multiplier……….….11 Figure 2.7. Dual Thin Layer Flow Through Cell………..12 Figure 2.8. Side and top view of the dual thin layer cell (1) BDD electrode,
(2–3) Teflon spacer, (4) porous Teflon membrane, (5) stainless steel frit, (6) connection to the vacuum system and the mass spectrometer, (7) capillaries for flushing with gas
(8)inlet-outlet capillaries, (9) connecting capillaries…………...……13 Figure 2.9. Shows electro-oxidation at BDD……….14 Figure 2.10. Ethanol molecule………...…15 Figure 2.11. Scheme of ethanol oxidation……….…....16 Figure 2.12. A cyclohexane molecule in chair conformation. Hydrogen atoms in axial positions are shown in red, while those in equatorial
on BDD in 1 M HClO4; scan rate 10 mV/s, flow rate 5µL/s;T = 25 ˚C..23 Figure 4.2. Simultaneously recorded CV (a) and MSCV for (b) m/z = 31 (CH3OH), (c) m/z = 32 (O2) and (d) m/z = 44 (CO2) on BDD in 1 mM CH3OH;
scan rate 10 mV/ s, flow rate 5µL/s , electrolyte 1 M HClO4;
T = 25 ˚C………..24 Figure 4.3. Simultaneously recorded CV (a) and MSCV for (b) m/z = 44 (CO2) and (c) m/z = 32 (O2) on BDD in 1 mM HCOOH; scan rate
10 mV/s, flow rate 5µL/s, electrolyte 1 M HClO4;T = 25 ˚C…………25 Figure 4.4. Simultaneously recorded cyclic voltammogram (CV) (a) and mass spectrometric CV (MSCV) for (b) m/z = 44 (CO2) and
(c) m/z = 32 (O2) on BDD in 1 mM CH3COOH; scan rate
10 mV/s, flow rate 5µL/s, electrolyte 1 M HClO4;T = 25 ˚C...26 Figure 4.5. Simultaneously recorded cyclic voltammogram (CV) (a) and mass spectrometric CV (MSCV) for (b) m/z = 44 (CO2) and
(c) m/z = 32 (O2) on BDD in (1) 0 mM, (2) 1 mM, (3) 10 mM, (4) 50 mM, (5) 500 mM CH3COOH; experimental conditions as
in Fig. 8.3...27 Figure 4.6. Simultaneously recorded CV and MSCV for m/z = 44 and
m/z = 29 during the electrooxidation of bulk ethanol on
smooth polycrystalline platinum in 0.01M ethanol + 0.1M H2SO4 + 0.5 MHClO4 solution. Scan rate: 10 mV/s. Electrolyte
flow rate: 5 µL/s. 3 cycles are shown………..……….…..29 Figure 4.7. Simultaneously recorded CV and MSCV for m/z = 44 and
m/z = 30 during the electrooxidation of bulk d6-ethanol on
smooth polycrystalline platinum in 0.01M d6-ethanol + 0.1M H2SO4 + 0.5M HClO4 solution. Scan rate: 10 mV/s. Electrolyte flow
rate: 5 µL/s…...………...31 Figure 5.1. Calibration current of Acetaldehyde……….. 36
of adsorbed CO on polycrystalline Pt-electrode; electrolyte
0.5M H2SO4; scan rate 10 mV/s, flow rate 5 µL/s……..………..37 Figure 5.3. scheme of adsorption test………38 Figure 6.1. The cyclic voltammetry of BDD in 0.5 M H2SO4 at 5 µL/s and
room temperature , solid curve at 50mV/s , and dotted curve at
10mV/s………..……….…….43 Figure 6.2. Simultaneously recorded CV and MSCV for m/z = 31
m/z = 32 (O2),m/z= 44(CO2) and m/z = 29 on BDD in 0.5 M H2SO4; scan rate 10 mV/s , flow rate 5µL/s T = 25 ˚C………...…44 Figure 6.3. Simultaneously recorded CV and MSCV for
m/z = 31 (CH3CH2OH), m/z = 32 (O2), m/z = 44 (CO2) and
m/z = 29 (acetaldehyde + ethanol) on BDD in 1.10-3 M CH3CH2OH + 0.5 M ethanol; scan rate 10 mV/s , flow rate 5 µL/s, electrolyte
0.5 M H2SO4 T = 25 ˚C………..………….46 Figure 6.4 Simultaneously recorded CV and MSCV for
m/z = 43 (acetic acid), m/z = 29 (acetaldehyde+ethanol) and
m/z = 15(CO2) on BDD in 1.10-3 M CH3CH2OH; scan rate 10 mV/s , flow rate 5 µL/s electrolyte 0.5 M H2SO4 T = 25 ˚C………….…….48 Figure 6.5 Simultaneously recorded CV and MSCV for
m/z = 31 (CH3CH2OH), m/z = 32 (O2), m/z = 44 (CO2) and
m/z = 29 (acetaldehyde + ethanol) on BDD in 1.10-3 M CH3CH2OH;
scan rate 10 mV/s , flow rate 5 µL/s, electrolyte 0.5 M H2SO4
T = 25 ˚C………..…….49
m/z = 44 (CO2) and m/z = 90(Diethyl peroxide) on
BDD in 1.10-3 M CH3CH2OH; scan rate 10 mV/s , flow rate
5 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C………..…50 Figure 6.7 Simultaneously recorded CV and MSCV for
m/z =45 (CH3CH2O Ethoxy radical ), m/z = 32 (O2),
m/z = 31 (ethanol) and m/z = 62(main peak of Diethyl peroxide) on BDD in 1.10-3 M CH3CH2OH; scan rate 10 mV/s , flow rate
5 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C……….…51 Figure 6.8 Simultaneously recorded CV and MSCV for
m/z = 31 (CH3CH2OH), m/z = 32 (O2), m/z = 44 (CO2) and
m/z = 29 (acetaldehyde + ethanol) on BDD in 1.10-3 M CH3CH2OH;
scan rate 10 mV/s , flow rate 1.6 µL/s, electrolyte 0.5 M H2SO4
T = 25 ˚C………...……..52 Figure 6.9 Simultaneously recorded CV and MSCV for
m/z = 31 (CH3CH2OH), m/z = 32 (O2), m/z = 44 (CO2) and
m/z = 29 (acetaldehyde + ethanol) on BDD in 1.10-3 M CH3CH2OH;
scan rate 10 mV/s , flow rate 10 µL/s, electrolyte 0.5 M H2SO4
T = 25 ˚C……….…..54 Figure 6.10 Simultaneously recorded CV and MSCV for m/z = 56 (C6H12),
m/z = 32 (O2), m/z = 44 (CO2) and m/z = 78 (Benzene) on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate
5 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C………....55
on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate 5 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C………..57
Figure 6.12 Simultaneously recorded CV and MSCV for m/z = 56 (C6H12), m/z = 32 (O2), m/z = 44 (CO2) and m/z = 55 ( cyclohexanon )
on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate 5 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C……….….…58 Figure 6.13 Simultaneously recorded CV and MSCV for m/z = 56 (C6H12),
m/z = 32 (O2), m/z = 44 (CO2) and m/z = 78 (Benzene) on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate
1.6 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C………..…..59 Figure 6.14 Simultaneously recorded CV and MSCV for m/z = 56 (C6H12),
m/z = 32 (O2), m/z = 44 (CO2) and m/z = 55 ( cyclohexanon ) on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate 1.6 µL/s, electrolyte 0.5 M H2SO4 T = 25 ˚C………….………...……60 Figure 6.15 Simultaneously recorded CV and MSCV for m/z = 56 (C6H12),
m/z = 32 (O2), m/z = 44 (CO2) and m/z = 57 ( Cyclohexanol ) on BDD in 1.10-4 M Cyclohexane ; scan rate 10 mV/s , flow rate
10 µL/s,electrolyte 0.5 M H2SO4 T = 25 ˚C………...……..61 Figure 8.1. Yearly research publications on diamond electrochemistry………65
I29 = Ionic current of acetaldehyde
*K29 = Calibration constant of acetaldehyde
Cacetaldehyde = Concentration of acetaldehyde
QMS =Mass spectrometric charge QF = The faradic charge
ΔC = Consumed concentration of organic molecule
IF = Faradaic current K* = Calibration constant
˚n = Number of molecule measure to mass spectrometer
u = Flow rate (µl/s) I ionic = İonic current (A)
F = Faraday constant (mol-1) CE = Current efficiency
ald I 29 = ionic current of m/z 29 for acetaldehyde z = Number of electron
I*f (CO2) = The partial faradaic current corresponding to the formation of CO2
Ii (44) = The ionic current of CO2
BDD = Boron doped Diamond
DEMS = differential electrochemical mass spectrometry
1.INTRODUCTION
In this work boron doped diamond electrode was used as a working electrode because boron doped diamond electrodes produce a wide range of oxidising species, and particularly hydroxyl radicals. Boron-doped diamond (BDD) films have been recognized as a material highly suitable for electrochemical applications such as wastewater treatment, oxidation of organic compounds, electroanalysis, and electrosynthesis because of its chemical inertness, mechanical hardness, low fouling (Pleskov, 2002), (Panizza, 2005).
Diamond exhibits very high overpotentials for the chemical species following the inner-sphere mechanism because of the inertness of the surface for adsorption.
For this reason it also exhibits high overpotentials for the oxygen and hydrogen evolution resulting in a wide electrochemical potential window. While this helps in the electroanalysis of a wide variety of chemical species, the inactiveness of diamond electrode for certain species (Yang, 1994), (Higson, 1992), (Zhang, 1994).
The boron-doped diamond film electrode represents an attractive anode material for the degradation of refractory or priority pollutants such as ammonia, cyanide, phenol, chlorophenols, aniline, various dyes, surfactants, alcohols and many other compounds. Unlike PbO2, SnO2 and TiO2, the BDD thin films deposited on Si, Ta, Nb and W by Chemical vapour deposition (CVD) have shown excellent electrochemical stability. However, the application of BDD electrodes for wastewater treatment has been mostly studied with Si-supported devices, in spite of the difficulties related to their industrial application, due to the fragility and the relatively low conductivity of the Si substrate (Marselli, 2003), (Martinez-Huitle, 2004), (Beck, 2000), (Hattori, 2003), (Fryda, 1999).
We wanted to work with ethanol and cyclohexane electrooxidation in 0.5 M H2SO4 solutions, because some authors studied the electrooxidation of methanol and acetic acid. This is the first study for the electrooxidation of ethanol and cyclohexane at BDD surface. we want the determine the mechanism of oxidation, the current
The products of ethanol and cyclohexane were studied by differential electrochemical mass spectrometry (DEMS) which allows the immediate detection of reaction products after their formation during potential sweep. As a result, in parallel to faradaic current, the ionic current of m/z = 44 (CO2), m/z = 32 (O2), m/z = 31 (CH3CH2OH), m/z = 29 (acetaldehyde), m/z = 43 (acetic acid), m/z =45 (CH3CH2O Ethoxy radical ), m/z = 90(Diethyl peroxide) , m/z = 62(main peak of Diethyl peroxide) , m/z = 56 (C6H12), m/z = 78 (Benzene), m/z = 57 (Cyclohexanol ) , m/z = 55 ( cyclohexanon ) , were recorded during oxidation of 1.10-3M ethanol and 1.10-4M acetaldehyde in 0.5 M H2SO4. We applied a continuous flow to avoid the further oxidation of intermediates.The percentage of volatile products species which formed at BDD surface was also determined.
2.THEORETICAL
2.1.Electrochemistry Of Boron Doped Diamond Electodes 2.1.1. Synthesis of Boron-Doped Diamond Electrodes
BDD electrodes are made of polycrystalline diamond formed by Chemical Vapour Deposition (CVD) in a high temperature microwave process. In the tetrahedral diamond lattice, each carbon atom is covalently bonded (sp3) to its neighbours. Some carbon atoms in the lattice are substituted with boron to provide electrical conductivity. Boron acts as an electron acceptor due to its electron deficiency in its outer shell.
Figure 2.1 Differences between Diamond and Graphite lattice (www.advoxi.com).
Boron doping levels are in the range 1.1019 to 1.1021 atoms/cm3 (up to 8000 ppm) for sufficient conductivity.
The BDD electrodes are available as solid wafers or as coatings on a range of suitable substrates which can include silicon, tungsten, niobium and tantalum.
Figure 2.2 Solid BDD electrode 2.1.2. Chemical Vapour Deposition
Chemical Vapour Deposition (CVD) is a technique for growing materials from the gas phase on to a substrate. It is most commonly used in the semiconductor industry as part of device fabrication. By heating the gases to more than 2000ºC, usually a mixture of hydrogen and methane, and carefully controlling the conditions of the substrate it is possible to grow layers of pure diamond, in the absence of stabilising conditions of high pressure and high temperature (HPHT). Freed from the constraints of HPHT apparatus, it is possible to isolate the kinetically stable product, diamond, and hence grow large area polycrystalline diamond electrodes.
Figure 2.3 Schemes of Chemical Vapour Deposition 2.1.3. CVD Polycrystalline Diamond
During the initial stages of the CVD process diamond crystallites nucleate on the substrate. As synthesis progresses the randomly orientated grains gradually align
together. Typically, polycrystalline diamond has large crystallites, where the grains at the surface are approximately 10% of the total film thickness.
Figure 2.4. A microscopic view shows the surface morphology and the polycrystalline nature of the diamond structure (www.advoxi.com).
2.2. Properties And Applications of BDD Electrodes 2.2.1. Properties Of BDD
Boron doped diamond (BDD) electrodes which include a wide electrochemical potential window in aqueous and non-aqueous media are very attractive for electrolysis and electroanalytical applications due to their outstanding properties which are for the oxidation of organic and inorganic compounds. The electrodes are tough and resistant to degradation. They are chemically stable, resistant to thermal shock and exhibit extreme electrochemical stability. The boron doping provides the necessary electrical conductivity. These are the ultimate electrodes for anodic oxidation and are suitable for wastewater treatment and water disinfection and sterilization. Diamond exhibits very high over potentials for the chemical species following the inner-sphere mechanism because of the inertness of the surface for adsorption. This is the reason it exhibits high overpotentials for the oxygen and hydrogen evolution resulting in a wide electrochemical potential window.
2.2.2. Applications of BDD
All properties have led to application of Boron doped diamond electrodes in electrosynthesis, electrolysis, electro-mineralization of organic pollutants, preparation of strong oxidants , recovery of heavy metals , and electroanalysis.
BDD electrodes are passive in nature and don`t interact or bind to organic pollutants nor do they catalyse the oxidation of pollutants. Direct anodic oxidation can take place by the subtraction of electrons from covalent bonds of polluting substances. However, the best feature of BDD electrodes is that, when the current is switched on, they produce a wide range of oxidising species, and particularly hydroxyl radicals.
2.3. Organic Compounds Oxidation On BDD
It is well established that oxidation of organics at BDD anodes takes place in the potential region of oxygen evolution, through reaction steps in which OH•
radicals are involved: these radicals can react with organic compounds to give more oxidised substances, or with water to give oxygen. Because of the high reactivity of OH• radicals these reactions are confined to a thin film adjacent to the electrode surface. A mathematical model was implemented, which accounts for chemical and electrochemical reactions, as well as for the transport phenomena involved in the process: the parameters of the model were derived from experimental data. The model allowed calculation of the trend with time of reactant concentration, reaction intermediates and oxidisable agents: their space profiles in the reactor were also obtained.
Figure 2.5. Scheme of the electrochemical oxidation of organics compounds on active anodes (reactions (1), (2), (3), and (4)), and non-active anodes (reactions (1), (5), and (6)); M is an active site at the electrode surface.
2.4. Mechanism of C1 organic oxidation on BDD
C1 organic oxidation on boron-doped diamond electrodes ;
(1)
(2)
(3)
ke and ke´ are the electrochemical rate constants of water discharge (Eq. (1)) and anodic discharge of hydroxyl radicals (Eq. (3)), respectively, kr is the
electrochemical rate constant of organics, R, oxidation (Eq. (2)). In general, the electrochemical rate constants k can be expressed as the following equation:
( 4)
kφ is linked to the standard rate constant k◦ and standard electrode potential E◦ and α= 0.5 by the following equation:
( 5)
Assuming that neither the organic nor its oxidation intermediates are adsorbed on BDD and that hydroxyl radicals are loosely adsorbed (physisorbed), the corresponding rates ν (molm−2 s−1) of reactions (1)–(3) can be expressed as
(6)
(7)
(8)
where Γ is the density of surface sites available for hydroxyl radicals, HO*, adsorption, ΘHO* is the fractional surface coverage by HO*, [R] is the organic concentration and (1 − ΘHO* ) is the fraction of unoccupied surface.At steady-state (ν(1) = ν(2) + ν(3)) the fractional coverage of hydroxyl radicals can be given by the following equation:
( 9 )
Inserting this relation into Eq. (6) , the reaction rate in terms of current density is expressed as the following equation:
( 10 )
From this equation it is clear that the higher is the reaction rate of organics oxidation (Eq. (2)), the lower is the coverage of hydroxyl radicals (higher unoccupied surface) and consequently the higher is the current density at a given potential. Moreover, if reaction (2) is fast enough to influence notably the HO*
coverage, the higher is the concentration of the organic compound the higher current is expected. Investigations of electrochemical oxygen transfer reaction on boron- doped diamond electrodes ( A.Kapalka 2007)
2.5. Differential Electrochemical Mass Spectrometry (DEMS)
For this technique, an electrochemical cell is placed at the inlet to a vacuum chamber connected to a quadrupole mass spectrometer. A teflon membrane serves as the interface between the electrochemical cell, at surrounding pressure, and the vacuum chamber.Volatile or gaseous intermediates and products formed during the electrochemical reaction at the electrode go through the Teflon membrane and into the mass spectrometer, where the flux of the species can be monitored in parallel to the faradaic current during a potential sweep (cyclic voltammetry), resulting in the so-called mass spectrometric cyclic voltammogram (MSCV) and a cyclic voltammogram (CV), respectively. Differential pumping usually is employed.(Baltruschat, 2004 ), (Baltruschat, 1999 ), (B. Bittins-Cattaneo, 1990)
2.5.1. DEMS Set up
It includes of an electrochemical cell (1) attached to the inlet of a mass spectrometer (2), which is depleted differentially by two turbomolecular pumps: at position (4) with a working pressure of about 10-2bar, and (8), placed after the
ionization chamber, with a working pressure of less than 10-5 mbar. The electrodes of the electrochemical cell are connected to the potentiostat driven by a function generator. Volatile electrolysis products are ionized in the ionization chamber by electron bombardment, then the produced ions are accelerated and analyzed with a quadrupole mass filter (quadrupole rodes (7) m/e ratio), and finally detected by a secondary electron multiplier (10). The control of the whole instrument and data acquisition is carried out by a computer.
Figure 2.6. Shows Schematic representation of a typical experimental DEMS setup; (1) electrochemical cell, (2) MS connection to the electrochemical cell, (3) connection to the calibration leak, (4) turbo molecular pump, (5) rotary pump, (6) ion source, (7) quadrupole rods, (8) turbo molecular pump, (9) rotary pump, (10) secondary electron multiplier.
2.5.2. DEMS CELL
2.5.2.1 Dual Thin Layer Flow Through Cell
The electrochemical cell used in the DEMS experiments consisted of a Teflon block into which three holes were drilled to hold the three electrodes and the solution. The electrochemical cell was press fitted onto the membrane-flange assembly, with the membrane serving as the interface between the mass spectrometer main chamber at high vacuum and the electrochemical cell at atmospheric pressure.
Figure 2.7. Dual Thin Layer Flow Through Cell
Figure 2.7 shows, Cross section through the dual thin layer cell. The diameter of the electrode is 1 cm. The upper and lower compartment are connected via six capillaries, two of which are visible.
Figure 2.8. Side and top view of the dual thin layer cell (1) BDD electrode, (2–
3) Teflon spacer, (4) porous Teflon membrane, (5) stainless steel frit, (6) connection to the vacuum system and the mass spectrometer, (7) capillaries for flushing with gas, (8) inlet-outlet capillaries, (9) connecting capillaries
2.6. Effect Of Hydroxyl Radicals (HO•) at Boron Doped Diamond:
The major advantage of BDD electrodes is their wide electrochemical window which allows the efficient oxidation of organic compounds in aqueous electrolytes.
At high electrode potentials hydroxyl radicals (OH•) are produced in abundance.
These are the strongest oxidising species known and formed at the anode.
2H2O → 2OH• + 2H+ + 2e- (anode)
The rates of reactions involving hydroxyl radicals are very fast since they are strong oxidising agents towards almost all organic compounds (www.advoxi.com).
peroxodicarbonates respectively. These are all powerful oxidants generated in the waste stream as it passes through the reactor. Even hydrogen peroxide and ozone are produced during electrolysis.
Figure 2.9. Shows electro-oxidation at BDD (www.advoxi.com).
2.7. Ethanol
Ethanol is a volatile, flammable, colorless liquid. its molecular formula is C2H5OH. Its empirical formula is C2H6O. An alternative notation is CH3-CH2-OH, which indicates that the carbon of a methyl group (CH3-) is attached to the carbon of a methylene group (-CH2-), which is attached to the oxygen of a hydroxyl group (- OH). It is a constitutional isomer of dimethyl ether.
Figure 2.10. Ethanol molecule 2.7.1. Properties of Ethanol :
Ethanol is a monohydric alcohol which can be considered as derivatives of alkanes obtained by replacing one hydrogen atom by a hydroxyl group. Similarly, they can be considered as derivatives of water obtained by replacing one hydrogen atom by an alkyl group. For this reason ethanol exhibits the properties of both ethane and water.
Ethanol will oxidise to an aldehyde and given further oxidation it will become a carboxylic acid.
C
2H
5OH + 3H
2O 2CO
2+ 12H
++12e
-CH
3CHO +3H
2O 2CO
2+ 10H
++10e
-CH
3COOH
-2H+-2e-
-2H+ +H2O
G. A. Camara, and T. Iwasita; J. Electroanal. Chem. 578(2004)315.
Schematic representation of the parallel pathways for ethanol oxidation
The parallel pathways for ethanol oxidation
Figure 2.11. Scheme of ethanol oxidation (Camara, 2005)
2.7.2 Physical Properties of Ethanol :
Table 2.1. Physical Properties of Ethanol (http://en.wikipedia.org ) Molecular formula : C2H6O
Boiling Point: 78.4 °C, 352 K, 173 °F Melting Point: −114.3 °C, 159 K, -174 °F
Flash Point: 13 °C (55.4 °F)
Density: 0.789 g/cm3
Appearance colorless liquid
Acidity (pKa) 15.9
Molar mass 46.07 g mol−1
Viscosity 1.200 cP (20 °C)
2.8. Cyclohexane
Cyclohexane is a cycloalkane with the molecular formula C6H12 which is used as a nonpolar solvent for the chemical industry. Cyclohexane is produced by reacting benzene with hydrogen for the industry Due to its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard.
2.8.1 Properties of Cyclohexane :
Cyclohexane is a colorless, flammable, mobile liquid with a pungent odor. It is insoluble in water and soluble in alcohol, acetone, benzene, ethanol, ethyl ether, olive oil, and carbon tetrachloride. Chyclohexane is a non-corrosive liquid and wil volatilize quickly. It sublimes between -5 to 5 oC (HSDB, 1993).
2.8.2 Physical Properties of Cyclohexane
Table 2.2.Physical Properties of Cyclohexane (Howard, 1990; HSDB, 1993; Merck, 1989; Sax, 1987)
Molecular Weight: 84.18
Boiling Point: 807 oC
Melting Point: 6.47 oC
Flash Point: -18 oC (closed cup)
Vapor Density: 2.98 (air = 1)
Vapor Pressure: 97.6 mm Hg at 25 oC
Density/Specific Gravity: 0.779 at 20/4 oC (water = 1) Log Octanol/Water Partition Coefficient: 3.18 (est)
Conversion Factor: 1 ppm = 3.44 mg/m3
2.8.3 Chemical Conformation of Cyclohexane
In the chair form, 12 extracyclic bonds fall in two classes. Six of the bonds lie parallel to the main axis of symmetry and are designated as axial, while the other six extend radially outward at +109.5 degree angles to the axis and are designated as equatorial (Merck, 1989).
Cyclohexane has the lowest angle and torsional strain of all the cycloalkanes, as a result cyclohexane has been deemed a 0 in total ring strain, a combination of angle and torsional strain. This also makes cyclohexane the most stable of the cycloalkanes and therefore will produce the least amount of heat (per CH2 unit) when burned compared to the other cycloalkanes (E.W. Warnhoff, 1996 ).
Figure 2.12. A cyclohexane molecule in chair conformation. Hydrogen atoms in axial positions are shown in red, while those in equatorial positions are in blue.
(M.Price, 1995)
2.9. Cyclic Voltammetry :
Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement. In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time like linear sweep voltammetry. Cyclic voltammetry
takes the experiment a step farther than linear sweep voltammetry which ends when it reaches a set potential. When cyclic voltammetry reaches a set potential, the working electrode's potential ramp is inverted. This Inversion can happen multiple times during a single experiment. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace. Cyclic voltammetry is generally used to study the electrochemical properties of an analyte in solution (Bard, 2000).
In some experiments an electroactive species is fixed to the surface of the electrode, for instance in microparticle voltammetry. Potentiodynamic techniques also exist that add low-amplitude ac perturbation to a potential ramp and measure variable response in a single frequency (ac voltammetry) or in many frequencies simultaneously (potentiodynamic electrochemical impedance spectroscopy). The response in alternating current is two-dimensional – it is characterised by amplitude and phase. The amplitude and phase depend differently on frequency for constituents of ac response attributed to different processes (charge transfer, diffusion, double layer charging, etc.). Frequency response analysis enables simultaneous monitoring of the various processes that contribute to the potentiodynamic ac response of electrochemical system (Nicholson, 1964).
Cyclic voltammetry is not a hydrodynamic technique. In a hydrodynamic technique flow is achieved at the electrode surface by stirring the solution, pumping the solution, or rotating the electrode as is the case with rotating disk electrodes and rotating ring-disk electrodes. These techniques target steady state conditions which appear the same scanned from the positive or the negative, thus limiting them to linear sweep voltammetry (Heinze, 1984).
Figure 2.13. Typical cyclic voltammogram
2.10. Water Treatment With Boron Doped Diamond Electrodes:
Water treatment with BDD (Boron Doped Diamond) electrodes can be divided into the areas of disinfecting and waste water treatment (both areas have, in common actually). With the assistance of an electrochemical procedure impurities such as bacteria, viruses, algae, oils, emulsions, chemical and pharmaceutical residues can be removed. A remarkable fact about using BDD electrodes is the extremely high over- voltage exhibited for water electrolysis. Instead of the production of oxygen very effective OH• radicals are formed directly from the water without any additional chemical input. Through the efficient use of electrochemical procedures, using BDD electrodes, it is possible to replace existing procedures and to be prepared for further future requirements. It is possible to save both on space and maintenance costs and to be ready for the future through a simple scale up procedure.
Figure 2.14. Complete Water Treatment systems with Boron doped diamond.
3. MOTIVATION AND AIM OF THIS WORK :
The motivation of this work is ethanol and cyclohexane are model compounds fundamental reactions of organic species with OH radicals.
The late of years Fuel cell and waste water treatment were studied frequently.
Methanol was tested before and somebody wants to see effect of ethanol for fuel cell because ethanol is much less toxic than methanol, ethanol’s energy density higher than methanol (8.1 kWh/kg) (Methanol ≈6.1), ethanol available from ren ewable resources very easily and also ethanol has got another good properties like Simple handling, storage and transportation on the other hand disadvantages of Ethanol that oxidize to CO2 is associated with the cleavage of the C-C bond, which requires a higher activation energy than C-H bond breaking. Because of all of these properties, we wanted to study with ethanol and also we wanted to study boron doped diamond electrode because boron doped diamond electrode has got very good properties which we explained in introduction part and BDD electrode which was used very much recently, is a new surface.
Electro-oxidation of organic compounds on BDD proceeds via electrochemical generation of hydroxyl radicals (OH ) by water discharge and their subsequent reactions. Hydroxyl radicals are extremely reactive and cause the oxidative degradation of organics (Marselli, 2003), (Foti, 2004).
Before nobody worked with ethanol or cyclohexane on BDD electrodes.
Therefore we want to examine the effect of the organic molecules.We should break the C-C bond producing CO2. This should be possible at boron dopped diamond electrode because it generates free OH radicals active for electro-oxidation to produce CO2 or acetic acid.
The other molecule is cyclohexane which is very interesting and not very reactive, is very stable. And we wanted to see, is it possible to electrochemically oxidize the complete cyclohexane molecule to CO2.
4. PREVIOUS EXPERIMENTS
4.1 DEMS study electrooxidation of Methanol on BDD
Figure 4.1. Simultaneously recorded (a) cyclic voltammogram (CV) and mass spectrometric CV (MSCV) for (b) O2 (m/z = 32), (c) CO2 (m/z = 44) on BDD in 1 M HClO4; scan rate 10 mV/s, flow rate 5µL/s; T = 25 ˚C ( A.Kapalka, 2008).
In this experiment, Agnieszka Kapalka works with supporting electrolyte as 1M HClO4 on boron doped diamond electrode. It can be seen that during anodic scan of potential, in parallel to oxygen evolution (b), some traces of CO2 were detected.
Using the current efficiency of CO2 formation at 2.85 V vs. RHE.
Figure 4.2. Simultaneously recorded CV (a) and MSCV for (b) m/z = 31 (CH3OH), (c) m/z = 32 (O2) and (d) m/z = 44 (CO2) on BDD in 1 mM CH3OH; scan rate 10 mV/ s, flow rate 5µL/s , electrolyte 1 M HClO4; T = 25 ˚C ( A.Kapalka, 2008).
In this Figure 4.2 (a) shows that faradic current If starts to increase at ~2.3 V vs. RHE simultaneously to the decrease of the ionic current for methanol Ii(31) (b).
Ii(31) starts decrease ~2.3 V vs. RHE. the effect of Ii(31) limitation on If can be observed in the form of an oxidation wave in the vicinity of oxygen evolution (a). (c) shows that oxygen evolution starts to proceed from ~2.6 V vs. RHE, i.e., once the
Ii(31) limitation is attained. The ionic current for CO2, Ii(44), increases from ~2.5 V vs. RHE reaching a limitation in parallel to Ii(31) ( A.Kapalka, 2008).
4.2 DEMS study electro-oxidation of Formic acid on BDD
Figure 4.3. Simultaneously recorded CV (a) and MSCV for (b) m/z = 44 (CO2) and (c) m/z = 32 (O2) on BDD in 1 mM HCOOH; scan rate 10 mV/s, flow rate 5µL/s, electrolyte 1 M HClO4; T = 25 ˚C ( A.Kapalka, 2008).
Agnieszka Kapalka shows in her own study that Formic acid starts to be oxidized from ~2.4 V vs. RHE, resulting in increase of both the faradic current If and the ionic current, Ii(44), for CO2. During its oxidation, no intermediates were found.
transfer limitation) at higher potentials. The oxygen evolution, a secondary reaction, starts to proceed from ~2.6 V ( A.Kapalka, 2008).
4.3. DEMS study electrooxidation of Acetic acid on BDD
Figure 4.4. Simultaneously recorded cyclic voltammogram (CV) (a) and mass spectrometric CV (MSCV) for (b) m/z = 44 (CO2) and (c) m/z = 32 (O2) on BDD in 1 mM CH3COOH; scan rate 10 mV/s, flow rate 5µL/s, electrolyte 1 M HClO4; T = 25 ˚C ( A.Kapalka, 2008).
Agnieszka Kapalka shows in her own study that It can be seen that the ionic current of CO2 continuously increases from around 2.45 V vs. RHE reaching a limitation at higher potentials (b). The limiting current may indicate that the
oxidation of acetic acid is mass transfer limited. O2 starts to evolve at around 2.55 V vs. RHE (c), simultaneously to CO2 production. As a consequence, the effect of Ii(44) limitation on the faradaic current (a), which sums up the current for both CO2 and O2 formation, is not visible, i.e., neither a peak nor an oxidation wave is observed( A.Kapalka, 2008).
Figure 4.5. Simultaneously recorded cyclic voltammogram (CV) (a) and mass spectrometric CV (MSCV) for (b) m/z = 44 (CO2) and (c) m/z = 32 (O2) on BDD in (1) 0 mM, (2) 1 mM, (3) 10 mM, (4) 50 mM, (5) 500 mM CH3COOH; ( A.Kapalka, 2008).
Agnieszka Kapalka explained Effect of acetic acid concentration in her own study like that , In the oxygen evolution region (> 2.5 V vs. RHE), the faradaic current depends strongly on the acetic acid concentration (a). At 1 and 10 mM concentration of acetic acid, the faradaic current increases with respect to that of the supporting electrolyte (curve 1 (a) ), whereas starting from 50 mM a decrease of the current is observed. This phenomenon can be explained by the physisorption of acetic acid on the electrode surface resulting in the auto-inhibition of its oxidation.
As a consequence, the oxidation of acetic acid is shifted to higher potentials.
Figure 4.5 b shows that the ionic current of CO2 for 50mM acetic acid, however, is much higher than that for 10 mM, indicating an unimpeded oxidation of acetic acid. To explain the decrease of faradaic current for 50 mM acetic acid, one has to take into account the oxygen evolution reaction. In fact, in comparison with 10 mM acetic acid, oxygen evolution is almost negligible in 50 mM acetic acid. Because it hardly contributes to the faradaic current, the latter, decreases with increasing acetic acid concentration. The further decline of faradaic current in 500 mM acetic acid is due to the decrease of the rate of its oxidation. It can be seen that Ii(44) for 500 mM acetic acid is three times lower than that for 50 mM acetic acid ( A.Kapalka, 2008).
4.4. DEMS Study Electro-oxidation of bulk ethanol on Pt(Pc)
-0.02 0.00 0.02 0.04 0.06 0.08 0.10
0 10 20 30 40 50 60
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0 20 40 60 80 100 -0.02 0.00 0.02 0.04 0.06 0.08 0.10
-0.01 0.00 0.01
0.0 0.2 0.4
0.00
s0505807PA-1 in Ethanol oxidation1 PtPc
s05058a
E / V vs. RHE
2
I / mA 1
1
2
I 44 / pA
44
I 29 / pA
29
E / V vs. RHE
I / mA
s0505807
2 1
I / mA
1 2
E / V vs. RHE
I / mA
Figure 4.6. Simultaneously recorded CV and MSCV for m/z = 44 and m/z = 29 during the electrooxidation of bulk ethanol on smooth polycrystalline platinum in 0.01M ethanol + 0.1M H2SO4 + 0.5 MHClO4 solution. Scan rate: 10 mV/s.
Electrolyte flow rate: 5 µL/s. 3 cycles are shown. (A. A. Abd-El-Latif, 2009)
corresponding mass spectrometric cyclic voltammograms (MSCV) for m/z = 44 (CH3CHO + CO2) and m/z = 29 (acetaldehyde) at flow rate 5 µL/s. In the cyclic voltammogram (Fig.4.6.a): the hydrogen desorption peak is still present in the first anodic sweep because the adsorption of ethanol does not take place at the potential of 50 mV at which ethanol containing solution is replacing the supporting electrolyte.
The onset of the oxidation at about 300 mV is followed by two peaks at 0.65 and 0.8 V and a third anodic peak in the oxygen region at 1.3 V (Wang, 2004; Camara, 2005). During the cathodic sweep, an oxidation peak is present at 0.6 V after the complete desorption of oxygen from the Pt-surface.
In the second sweep the hydrogen desorption peak is suppressed due to the blocking of Pt-surface by the adsorbed intermediates formed in the preceding cathodic sweep. The first anodic peak is also decreased because of these adsorbates, which are only oxidized at higher potentials.
Fig. 4.6.(b and c) show the mass spectrometric cyclic voltammetry of ionic signals of m/z = 44 and m/z = 29. The shape of the ionic signal for mass 29 is similar to the faradic current for both the first and subsequent sweeps. This means that the current efficiencies for the production of acetaldehyde during ethanol oxidation at polycrystalline Pt is independent of the applied potential and it is produced over the whole potential range. Also, the ionic signal of mass 44 follows the ionic signal of mass 29 in the third anodic peak. However, at the first faradaic peak at 0.6 V, both the faradaic current and the ionic peak of mass 29 are lower than that in the second peak at 0.8V, whereas, the ionic peak of mass 44 is higher. At 0.8 V the ion current for m/z=44 is that expected for the molecular peak of acetaldehyde (≈55% of I 29).
However, at 0.6 V, the ion current of m/z = 44obviously is higher, and a part of the ion current is due to the formation of CO2. Thus, the main product at 0.8 V is acetaldehyde, but at 0.6 V there is some amount of CO2 produced from the oxidation of adsorbed species like COad resulting from the breaking of C-C bond at low potential (E<0.5 V).
In order to better distinguish between CO2 and fragments of acetaldehyde, we used ethanol-d6. The most abundant fragment of acetaldehyde-d4 is then CDO at m/z
= 30. On m/z = 44 there is a contribution of C2D2O which should have an intensity of
7% of that of the m/z = 30 signal. (C2H2O has an abundance of 7% of the m/z = 29 signal in the case of acetaldehyde-h4.) (A. A. Abd-El-Latif, 2009).
-0.04 -0.02 0.00 0.02 0.04 0.06
0.0 2.0x10-12 4.0x10-12
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.0 2.0x10-11 4.0x10-11
a1007822
in Ethanol oxidation1 PtPc
Copy of a10078-1
E / V vs. RHE
I f / mA
a1007822-1
I 44 / A
44
10 point AA Smoothing of A1007822_44
I 30 / A
30 basey
10 point AA Smoothing of A1007822_30
Figure 4.7. Simultaneously recorded CV and MSCV for m/z = 44 and m/z = 30 during the electrooxidation of bulk d6-ethanol on smooth polycrystalline platinum in 0.01M d6-ethanol + 0.1M H2SO4 + 0.5M HClO4 solution. Scan rate: 10 mV/s.
Electrolyte flow rate: 5 µL/s(A. A. Abd-El-Latif, 2009).
A. A. Abd-El-Latif shows in his own study that the CV and MSCV of mass 44
fig.4.6.(a), but all currents are nearly two times lower than that of normal ethanol due to the kinetic of isotope effect.
The shape of ionic signals for m/z 30 is similar to that for m/z = 29 in fig.1. In the anodic sweep, the ion current for m/z =44, however, is drastically decreased at 0.6 V and only shows a peak at 0.8V.
In the oxidation peak during the cathodic sweep, the ion current for m/z = 44 is 7% of that for m/z = 30 and thus results from acetaldehyde. In the first anodic sweep, the ion current for m/z = 44 is 10% of that for m/z = 30, in the subsequent sweeps even 20% at 0.8 V.
The higher ionic current of m/z = 44 in the second and third sweep as compared to that in the first sweep above 0.7 V this due to the oxidation of adsorbed intermediate which were formed in the previous sweeps (A. A. Abd-El-Latif, 2009).
Table 4.1. The current efficiencies for CO2 and d4-acetaldehyde during the potential step experiment of bulk d6-ethanol on smooth polycrystalline platinum in 0.01M d6- ethanol + 0.1M H2SO4 + 0.5 M HClO4 solution at different flow rate and potential (A. A. Abd-El-Latif, 2009).
Flow rate µL/s Ead
CD3CD2OD CH3CH2OH A44% A30% A44% A29%
1.6 0.6 0 120 0 113
0.7 0.2 70 0 104
0.8 0.5 116 0 103
5 0.6 0 245 0 260
0.7 0 250 0 236
0.8 0 177 0 250
5.EXPERIMENTAL 5.1. Chemicals
Table 5.1. Chemicals which is used in all experimental part.
Name Formula Purity Company
Sulfuric Asid H2SO4 95-97 % MERCK
Ethanol CH3CH2OH 99.9 % MERCK
Argon Ar 5.0 Air products
Cyclohexane C6H12 99.9 % MERCK
Carbon monoxide CO 4.7 Messer griesheim
Water H2O 100 % – 3ppb Millipore system
5.2. Solutions
All solutions which are used in my experimental work are prepared with my supporting electrolyte and own high purity solutions. My supporting electrolyte is 0.5 M sulfuric acid which is prepared with ultra pure water (Millipore 18.2 MΩ cm, < 3 ppb TOC ) and sulfuric acid ( purity ; 95-97 % ).
5.3. Cleanliness
Before each experiment, all of my glassware was cleaned and reference electrode was prepared. After my experimental work we put all of my glassware into the chromic acid over night and the day after we put them out and washed carefully with millipore water, because they may include some organic compounds and some anions. Reference electrode was prepared with Millipore water and supporting electrolyte before every experimental work because may be some bubbles were inside the connections; therein we avoided some disconnecting for electrode.
order to remove any impurity of organic molecules or volatile oils because sometimes it contains impurity from organic molecule or oils. And after one day we washed with Millipore water very carefully.
5.4. DEMS Study
We used a BDD electrode in a dual thin layer electrochemical cell with DEMS. This cell contains two compartments; one of them the electrochemical compartment with the electrolyte inlet, and the mass spectrometric compartment with the electrolyte outlet.
In the electrochemical compartment the electrolyte containing the products are transported through six capillaries to the mass spectrometric compartment, in this part the volatile species can evaporate through the Teflon membrane to the quadruple mass spectrometer (Pfeiffer Vacuum Balzer QMG 422). Because of this in my measurements, the ionic current corresponding to a given species can be recorded in parallel to the faradaic current. For all of my measurements, a hydrogen electrode (RHE) was used as a reference electrode and two Pt wires were used as counter electrodes with different applied resistances in the outlet and inlet to optimize the current distribution. The electrolyte volume and the geometric surface area (0.283 cm2) of the working electrode (BDD) are defined by a thin (5-7mm) Teflon ring (spacer) placed on the disc shaped electrode. All my measurements were performed at room temperature, in 0.5 M H2SO4 as a supporting electrolyte. Both faradaic and ionic current were measured as a function of potential to obtain the cyclic voltammograms (CV) and the mass spectrometric cyclic voltammograms (MSCV), respectively. The solutions were deaerated with argon during measurements.
Nearly all of experiments are done between -0.7 V to 2.8 V potential. I applied sweep rate 10mV/s during the measurement with MS. All of my experiments are done at different flow rate as 1.6, 5, 10, 15 µL/s using a prelistic pump at outlet (SPETEC 50-60Hz-70 W).
5.5. Calibration Of DEMS
Not only the qualitative detection of the volatile electrochemical products, but also the determination of the amount of these products or its formation rate, is possible with DEMS. To convert the ion currents determined by DEMS to the amount of species, the mass spectrometer needs to be calibrated under the same experimental conditions.
5.5.1. Acetaldehyde Calibration
Acetaldehyde calibration was done by 1.10-3M Acetaldehyde solution (supporting electrolyte is 0.5 M H2SO4 ) and watched the ionic signal of masses 15, 29 and 44. Until the mass signal become stable at for different flow rates. The ion currents were plotted vs. the acetaldehyde concentration. The slope of the linear relationship is the calibration constant (*K29), that includes all the DEMS parameters.
I used Kel-f as a working electrode during the calibration of acetaldehyde.
I29 = *K29. Cacetaldehyde
I29 = Ionic current of acetaldehyde
*K29 = Calibration constant of acetaldehyde
Cacetaldehyde = Concentration of acetaldehyde
Table 5.2. Calibration Constant of Acetaldehyde depend on flow rate
Flow Rate *K29
1.6 µL/s 9.4 E-8
5 µL/s 1.125 E-7
10 µL/s 1.43 E-7
0,2 0,4 0,6 0,8 1,0 1,2
-10,05 -10,00 -9,95 -9,90 -9,85
log I29=logb + a * log u
Parameter value logb -10,0693 a 0,1873 I29 = b.ua
logI29 = logb +a logu
log I
29log u
logI29
Lineares Fitten von Data1_logI29
Figure 5.1. Calibration curve of Acetaldehyde.
5.5.2 CO2 Calibration
We cleaned polycrystalline platinum electrode by the potential between 0.05 and 1.5 V in 0.5M H2SO4 at 50 mV/s scan rate then stop the potential at between 50 and 70 mV and the supporting electrolyte was changed for saturated CO solution after adsorption of CO on the electrode surface, then we started the potential again at a scan rate 10 mV/s and anodic scan was continued in order to oxidize adsorbed CO to CO2, and the calibration constant (K*) was calculated.
K* = z QMS/QF
