4. PREVIOUS EXPERIMENTS
4.4. DEMS Study Electro-oxidation of bulk ethanol on Pt(Pc)
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
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
QMS and QF are mass spectrometric and the faradic charges, respectively, and z
= 2 is the number of electrons for CO oxidation to CO2 CO + H2O → CO2 + 2H+ + 2 e¯ spectrometric CV (MSCV) for CO2 (m/z = 44) during oxidation of adsorbed CO on polycrystalline Pt-electrode; electrolyte 0.5M H2SO4; scan rate 10 mV.s-1, flow rate 5 µL/s.
Table 5.3. Calculation of CO oxidation.
Flow Rate (µL/s) QF (mc) QMS (mc) K*
1.6 5.8x10-2 1.24x10-7 4.25x10-6
5 7.5x10-2 1.67x10-7 4.4x10-6
5.5.2.1 Adsorption Test
This is for cleaning the polycrystalline platinum electrode.
0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
-0,06 -0,05 -0,04 -0,03 -0,02 -0,01 0,00 0,01 0,02
E
D
C B
A
I / m A
E / V vs. RHE
Adsp. test Pt(Pc) in 0.5 M H2SO4 5 µL/s
Figure 5.3. Scheme of adsorption test
At section A is desorption of hydrogen from the platinum surface. The hydrogen adlayer is desorbed in two steps, the reactions being the reverse of those presented above.
H (ads) → H+(aq) + e -
At section B there is no current flows in the system, this point is double layer.
At section C; at nearly 0.8V the adsorption of an oxygen species to form a Pt surface oxide is under way. Oxygen-containing species become strongly chemisorbed and eventually may enter into the bulk.
Pt + H-O-H → Pt-O-H + H+ +e –
Pt-O-H + H-O-H → Pt-(OH)2 + H+ + e –
Pt- (O-H)2 → Pt-O + H-O-H
At section D is oxygen desorption in cathodic direction at nearly 1.2 V.
Pt-O + 2H + + 2e - → Pt + H2O
At section E a hydrogen layer is adsorbed onto the surface in two distinguishable stages.
Pt + H+ + e - → Pt-H (ads)
This experimental part for the cleaning. In this part working electrode , supporting electrolyte and DEMS system is controlled. If there is a unwanted molecule, we can do another cleaning methods or adsorption test again.
5.6. Data Record
computer(486 DX2-66) with DASH 1602 measuring board together with software for the data collection developed in our department(potmadash) were used. Parallel to the faradaic current, datafrom the mass spectrometer were registered with special software Quadstar (Pfeiffer Vacuum) which makes it possible to detect the ion current for each m/z separately. The data were evaluated with Origin (Originlabs).
5.7 Calculations
EtOH I 29 = 0.34 x EtOH I 31
acet I 44 = 0.8 x acet I 29
5.7.1. Number Of Electron ( Z )
To calculate the number of e- exchanged during the electrooxidation of ethanol and cyclohexane. I should determine first the concentration of consumed species using calibration factor similar to acetaldehyde calibration ( I31 = *K31. Cethanol , I56=
*K56. Ccyclohexane ).
ΔC= ΔI ionic / K*ionic = ˚n / u
If = z x ˚n x F
z = (˚n x F) / If
ΔC = consumed concentration of organic molecule
If = Faraday current (mA)
K* = calibration constant
˚n = number of molecule measure to mass spectrometer
u = flow rate (µl/s)
F = Faraday constant = 96.500 C mol-1 5.7.2 Current efficiency for acetaldehyde
First of all we should correct ionic signal of m/z 29 because m/z 29 include both ethanol and acetaldehyde. We know that how many percent of ethanol include in m/z 29 and we should arrange ionic current of 29 for only acetaldehyde.
K*29 = ald I 29 / C
˚n = Cx u
29 If = ˚n x z x 96500
29CE = ald If / If
CE = current efficiency
ald I 29 = ionic current of m/z 29 for acetaldehyde
K29* = calibration constant for acetaldehyde
z = 2 (number of electron transfer during the electro-oxidation of ethanol to acetaldehyde)
5.7.3 Current efficiency for CO2
The current efficiency CE of CO2 formation was determined from the faradic current If and ionic current Ii (44) of CO2, according to Eqs.
I*f (CO2) = Z .Ii (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, Ii (44) is the ionic current of CO2, respectively, If 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 If 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 iethanol29 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 iethanol29 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 If starts to increase at ~2.3 V vs. RHE. At the same time to the decrease of the ionic current for ethanol Ii(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
The top of this figure, there is a CV of 1.10-4 M cyclohexane in 0.5 M