Current efficiency for CO 2

The current efficiency CE of CO₂ formation was determined from the faradic current I_f and ionic current I_i (44) of CO₂, according to Eqs.

I*_f (CO₂) = Z .I_i (44) / K*(44) CE (CO2 ) = I*f (CO2) / If

where I*_f (CO₂) is the partial faradaic current corresponding to the formation of CO₂ respectively, I_i (44) is the ionic current of CO₂, respectively, I_f is the total faradaic current, K*(44) is the calibration constant for CO₂ (K*(44)= 4,4 x 10 ^-6 ) determined from the calibration experiment on Pt electrode, and Z is the number of electrons Z= 6 involved in the oxidation of ethanol. (Wang, 2001)

In case of complete oxidation of ethanol to CO₂, it will produce 12 e- according to the following equation;

CH3CH2OH + 3H2O → 2CO2 + 12 e^- + 12 H⁺ Z = Number of electron exchanges per one CO2 molecule.

6.RESULTS AND DISCUSSION Characteristic CV of BDD

-1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0

-0,10 -0,08 -0,06 -0,04 -0,02 0,00 0,02 0,04 0,06 0,08 0,10 0,12

I f, mA

E, V vs. RHE

(50mV/s) basey (10mV/s)

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

Figure 6.1 which is specific for I_f of H₂S0₄ shows that faradaic current of BDD at scan rate is 50 mV/s starts to increase at ~2.1 V vs. RHE. And at potential higher than 2.5 V there is abrabt increase in the faradic current due to the oxygen evolution besides the burning of diamond to produce CO2 ( as shown in MSCV of fig 5.2 ) and at potential 2.8 V we sweeped the potential cathodicaly to -700 mV and the faradaic

oxidation with potential window rage between 50 mV and 1.5 V ( see fig 4.5) the BDD has alarge potential range. The other scan rate is 10 mV/s that starts to increase at ~2.2V vs. RHE.

Figure 6.2 shows the simultaneously measured of CV and MSCV for m/z = 31, m/z = 32 (O₂) , m/z = 44 (CO₂) and m/z = 29. Only supporting electrolyte is used this figure, there is no organic molecule inside the solution therefore there is no ethanol or acetaldehyde peak in m/z = 29 or m/z = 31. At the figure 6.2.a shows there is a high faradaic current which due to the oxygen evolution and burning of diamond (from carbon atoms) starts to increase ~2.6 V. In figure Figure 6.2.b shows that oxygen evolution starts to proceed from ~2.6 V vs. RHE. Figure 6.2.c shows as the background of the main peak of ethanol. Figure 6.2.e shows as the background of the main peak of acetaldehyde on 0.5 M H2SO4 at 10 mV/s. Figure 6.2.d starts to increase ~2.6 V vs. RHE, all of effect due to the burning of diamond and we should take in my during apply the supporting electrolyte (blank) from the charge of O2 and CO2 produced during ethanol and cyclohexane electrooxidation.

6.1 Electrooxidation of ethanol :

The top of figure 6.3 shows that the CV of 1.10^-3 M CH₃CH₂OH in 0.5 M H₂SO₄ is different from that in the supporting electrolyte at boron doped diamond electrode. The main difference is that ethanol is oxidized earlier than the supporting electrolyte.

This Figure also shows the MSCV for m/z = 31, m/z = 32 (O₂), m/z = 44 (CO₂) and m/z = 29 at the same time recorded on BDD during oxidation of 1.10^-3 M CH3CH2OH in 0.5 M H2SO4 . In the Figure, top of graph shows that faradic current If

starts to increase at ~2.3 V vs. RHE. At the same time the ionic current for ethanol Ii

(31) decreases. Ii (31) reaches a limitation at higher potentials which can be attributed, as a first approximation, to the mass transfer limitation. As a consequence, the effect of Ii (31) limitation on If can be observed in the form of an oxidation wave in the vicinity of oxygen evolution (at the top). The highest potential is 2.79V which is limited current for consumption of ethanol (Ii (31)). Two molecules give a contribution to mass 29: one of them is ethanol and the other one is acetaldehyde (main peak).The ionic current for m/z 29 increases at ~2,2V vs. RHE due to the effect of acetaldehyde. When the current reaches ~2.4V, it starts decrease due to the effect of ethanol until all of ethanol is oxidized. Oxygen evolution (m/z 32) starts to proceed from ~2.7 V vs. RHE, once the Ii(31) diffusion limitation is attained. The ionic current for CO2, Ii (44), first increases from ~2.3 due to the acetaldehyde, in this point acetaldehyde oxidize to CO₂ . At the main increases from ~2.5 V vs. RHE reaching a limitation in parallel to Ii (31) ( ethanol oxidize to CO2 ). The difference between the onset potential of CH₃CH₂OH oxidation and CO₂ formation is about 0.2 V, what indicates the formation of intermediates.

-0,10 m/z = 29 (acetaldehyde+ethanol) and m/z = 15 (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.

In this figure 6.4.b there is no peak of m/z = 43(Acetic acid) present because acetic acid is not volatile enough to diffuse through the teflone membrane directly to the mass spectrometry. In figure 6.4.c shows the peak of m/z 29 which is the overlapping between the main peak of acetaldehyde and the peak of ethanol. The higher I29 at 2.3 V is due to the formation of acetaldehyde and at higher potential I29

in lowered to the minimum value than the background (ethanol) due to the consumption of ethanol. Figure 6.4.d follows the same behavior of fig 6.4.c because I₁₅ is acetaldehyde + ethanol.

-0,2

corrected for i^ethanol₂₉ and shifted 29

of acetic acid inside the solution. After this result we may say there are another products show effect but we do not know actually which product show own effect.

The other situation is there is a experimental error because number of electron (Z) is low and may be balance is not ok and something missing.

-0,2 radical formation. Figure m/z = 32 (O2) shows that oxygen evolution starts to proceed from ~2.7 V vs. RHE. . Figure m/z = 44(CO2) shows that; mass 44 starts increases from ~2.25 due to the acetaldehyde. At the main increases from ~2.5 V vs.

RHE due to the ethanol effect. Figure m/z = 45(Ethoxy radical) starts to decrease from ~2.25 V vs. RHE. All of effect same with the ethanol until ~2.6 V. At this point graph is starts a little bit increase and quickly decrease and continue the same effect with ethanol. This point a little bit strange and my suggestion this is due to the ¹³C isotope and ¹³CO₂ is occurred. And the last mass is 90 but there is not shown any effect for m/z = 90 peak and there is not peroxide into the solution.

-0,2

This experiment is done because in the previous experiment for Diethyl peroxide is watched but in molecular peak of it (m/z 90), there is no shown any peak and we wanted to watch this experiment, are there any peak in m/z 62 (main peak of Diethyl peroxide) but we did not see any peak also m/z 62 therefore we may say correctly there is no effect of peroxide. And also in this graph is shown that m/z= 45 and m/z= 31 show nearly same effect without effect of ¹³C isotope which is explained previous experiment.

corrected for i^ethanol₂₉ and shifted 29

Figure 6.8 shows the CV and the MSCV for m/z = 31 (CH₃CH₂OH), m/z = 32 (O₂) , m/z = 44 (CO₂) and m/z = 29 (acid aldehyde) recorded on BDD during oxidation of 1.10^-3 M CH₃CH₂OH in 0.5 M H₂SO₄ . In the Figure, the top graph shows that faradaic current I_f starts to increase at ~2.3 V vs. RHE. At the same time to the decrease of the ionic current for ethanol I_i(31). The other masses are the same as the figure 6.3

For this figure some calculations were done at 2.5 V potential, First of it Z of ethanol was calculated, it equals 3.2, normaly this means, my solutions include acetaldehyde and acetic acid. After that current efficiency of acetaldehyde and CO2

was calculated, CECO2 = 20% and CEAcetaldehyde = 30 %. Normally total 50% (two) product inside the solution and we do not know the rest of product because acetic acid is not volatile enough to be detected by DEMS. May be % 50 acetic acid inside the solution but my suggestion there are not acetic acid like this rate because we calculated 3.2 e- and after that we knew electro-oxidation of ethanol to acetaldehyde produce 2 e-, 2CO2 needs 12 e- and acetic acid needs 4 electrons. From all of this data I thought that there is much acetic acid inside the solution but we could not say anything about percentage of acetic acid because it is not volatile. And there were another different product inside the solution but actually we are not able to detect.

Only my suggestion is may be there are formic acid inside the solution may be a little bit effect of methanol.

The advantage of low flow rate used in this experiment is; in the cell mixing is good and homogenous distribution of the product in the solution is obtained.

Disadvantage is that delay time which is a difference between a faraday peak and corresponding ionic peak in case of 1.6 µL/s is about 5 sec.(volume of electrolyte inside the cell is 7 µL, therefore 7 µl/ 1.6µL/s = 5s ).

6.1.3 Flow rate: 10 µL/s

In this experiment of fig 6.9 we used high flow rate; advantage of high flow rate is fast response of ionic current due to incomplete mixing a quantitative calculation of product formation and current efficiencies is not possible.

6.2 Electrooxidation of cyclohexane :

The top of this figure, there is a CV of 1.10^-4 M cyclohexane in 0.5 M H₂SO₄.The current starts to increase at ~2.2V vs. RHE. This value is a little bit lower than in the case of ethanol. Figure m/z = 32 (O₂) shows that oxygen evolution starts to proceed from ~2.25V vs. RHE, similarly to the faraday current. This may be due to the starting of cyclohexane oxidation because cyclohexane starts oxidation earlier.

Figure m/z= 44 shows that it starts to increase at ~2.2V vs. RHE but after 2.25 V, curve continue with sagging. My suggestion in this situation, is due to the H₂O₂ (peroxide) because in this potential OH• radicals can constitute H₂O₂. Figure m/z= 56 shows that it starts to decrease ~1.8 V it shows that cyclohexane oxidation starts earlier and longer than ethanol, methanol and acetic acid. This graph shows 3 different section first between 1.7 V to 2 V intermediates are formed: OHad which is formed via H2O ( H2O = OHad + H⁺ + e- ), reacts with Cyclohexane to form the intermediate (OHad + C6H12 = I (Intermediate) ).In this section there are not enough OH• radicals formed to oxidize cyclohexane to CO2 completely. Second section is between 2V and 2.25V. In this section the rate of OH• formation increases.The formation of CO2 starts with the higher number of OH• radicals, it is possible to attack the intermediate, further and oxidize part of them to CO( first step : H2O = OH• + H⁺ +e^- , second step ; OH• + C6H12 = I + CO2 + e- ). Third curve between 2.25V and 2.5. Above 2.25 V oxygen evolution starts to increase until in the consumption of C6H12 becomes smaller. The number of OH radicals increases further to a level, where the chance of OH• radical to meet a cyclohexane molecule or an intermediate (resulting from the reaction of another OH• radical with C₆H₁₂) has to complete with the chance to meet another OH radical leading to formation H₂O₂ and eventually O₂. ( 2 OH• = H₂O₂ = O₂ + 2^- + 2H⁺ ). Figure m/z= 78 shows that benzene is formed.

-0,5 which can be possible to see any effect in cyclohexane solution in 0.5 M H2SO4. First

calculated current efficiency of CO₂ it`s equal 11%. For cyclohexane oxidation per CO₂ molucule needs 6 e- ( C₆H₁₂ + 12 H₂O = 6 CO₂+ 36 H⁺ + 36^- ) but we did not find this result. And we found 2.7 number of electron may be it means effect of cyclohexane.The other masses nearly same to figure 5.10.

-0,5

In figure 6.12 the possible formation of cyclohexanon was elucidated no effect of cyclohexanon which approximately same the effect of cyclohexane. The other masses nearly same to figure 6.10.

6.2.2 Flow rate : 1.6 µL/s

In this experiment, we wanted to see formation of cyclohexane at 1.6µL/s.There is a difference only m/z 56(main peak of cyclohexane) due to the delay time of low flow rate and due to the good complete mixing into the DEMS cell.

Also we calculated Z and current efficiency of CO₂ for this flow rate. At 2.5 V z of acetaldehyde equals 2.4 per C₆H₁₂, current efficiency of CO₂ equals %13.

Cyclohexane produce to CO₂ with 6 electron but in this result 2.4 number of electron are consumed and nearly 0.8 electron used for CO₂ ( 0.13 x 6 ). May be there were some other product but for this result we can not say any name of product because we checked cyclohexanol , cyclohexanon and benzene but there were not any effect of this molecules.

We did this experiment because we did not see any effect of cyclohexanon in figure 5.12 and we tried it in this flow rate but we did not see any effect of it too. The other masses and CV approximately same the other result.

6.2.3 Flow rate : 10 µl/s

The shoulder in I₅₆ (1.6V-1.9V) correspond to a small shoulder in the faradic current, which becomes visible at a higher current resolution.

7. CONCLUSION

DEMS measurements performed on a BDD electrode indicate that the direct oxidation of ethanol and Cyclohexane. DEMS was used to identify products and intermediates of the reaction, monitored online during the potential sweep. The measurements showed that the main product of ethanol oxidation is CO₂, acetaldehyde and cyclohexane oxidation is CO₂. The indirect mechanism of electrooxidation of ethanol and cyclohexane, mediated by quasi free hydroxyl radicals, is assumed. It has been shown that oxidation of both ethanol and cyclohexane is a fast reaction, whereas the rate determining step is the water discharge to hydroxyl radicals. In this work potential range is used between -0.7 V to 2.8 V because 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.

Highly boron-doped conductive diamond electrodes are shown here to exhibit excellent performance for the electrochemical oxidation of ethanol and cyclohexane.

Well-defined sweep rate- dependent cyclic voltammograms and mass spectroscopies were obtained. Consider a simplified mechanism of oxygen evolution on BDD electrodes, in which the first step is the discharge of water leading to formation of quasi free hydroxyl radicals.

H₂O → OH• + H⁺ + e

-These hydroxyl radicals are further discharged to oxygen, probably via formation of hydrogen peroxide, according to the global reaction.

2OH• → O2 + 2H⁺ + 2e

The indirect oxidation of organic compounds R mediated by quasi free hydroxyl radicals on BDD surface can be expressed as:

R + OH• → RO + H⁺ + e

During oxidation of ethanol and cyclohexane on BDD electrodes, in addition to CO₂ and O₂ evolution, acetaldehyde (m/z=29) and acetic acid were detected as intermediates.

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.

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.

Ethanol Cyclohexane

FlowRate (µL/s)

Z CO2 % Acetaldehyde % Z CO2 %

1.6 2.4 24 40 2.7 11

5 3.2 20 30 2.4 13

8. SUMMARY

The first extensive study of use of diamond electrodes in electrochemistry was done by Pleskov in 1987. Since than, the synthetic BDD thin films have attracted attention of researchers resulting in a rapid increase in the number of publications (Fig. 8.1) and patents. Until now, the main electrochemical applications of diamond electrodes are in the domains of: electrosynthesis, electrochemical treatment of organic pollutants electroanalysis, preparation of strong oxidants and recovery of heavy metals (Pleskov,1987), (A. Morao, 2004).

Figure 8.1. Yearly research publications on diamond electrochemistry (Agnieszka Kapalka 2008).

Conductive boron-doped chemical vapor-deposited diamond thin films have emerged as unique electrode materials in electrochemistry due to their attractive properties, including very low background current, a wide electrochemical potential window in aqueous media, high resistance to corrosion, mechanical stability and

conventional carbon-based electrodes such as glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG) in some electroanalytical applications. First, the wide electrochemical potential window of the diamond electrode allows the sensitive electroanalytical detection of chemical species that react at relatively high potentials.

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İlköğretim Okulu’ and I continued intermediate school in ‘ DSİ Baraj İlköğretim Okulu’ After that I finished to high school in ATO Anatolian high school in 2002 and continued my study in chemistry in University of Çukurova a BSc in chemistry in 2003 and I started to master in Institute of Naturel and Applied Sciences University of Çukurova in 2007. Last year I studied in Bonn üniversity for second year of my master via erasmus program.