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The effect of alcohol concentration on the mass signal of CO2 detected by differential mass spectrometry
Electrochemistry Communications 48 (2014) 10–12
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
The effect of alcohol concentration on the mass signal of CO2 detected by differential mass spectrometry Wei Chen 1, Qian Tao 1, Jun Cai 1, Yan-Xia Chen ⁎ Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 9 July 2014 Received in revised form 1 August 2014 Accepted 4 August 2014 Available online 10 August 2014 Keywords: Differential electrochemical mass spectrometry (DEMS) Mass calibration constant Concentration effect Methanol oxidation Fuel cell
a b s t r a c t Differential electrochemical mass spectrometry (DEMS) is one of the powerful techniques for quantitative analysis of multiple products and their distribution from complex reactions occurred at the electrode/electrolyte interface, in which precise calibration of the mass signal is a prerequisite for accurate determination of product rate and current efficiencies of various products. In this work, we report the effect of alcohol concentration on the detected DEMS signal of CO2, a common final product from electrochemical oxidation of small organic molecules. Comparing to the solution with low alcohol concentration and the same CO2 concentration, we observe a significant reduction of mass signal of CO2 in a concentrated methanol or ethanol solution (N 0.1 M). Possible origins for this are: 1) alcohol molecules evaporate into the vacuum chamber and reduce the ionization probability for CO2 via competition for electrons; and 2) CO2 is trapped in the hydrophobic alcohol cages formed in the aqueous electrolyte. A proper way to eliminate alcohol concentration effect on the detected CO2 mass signal in DEMS studies on fuel cell related electrocatalysis is suggested. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Differential electrochemical mass spectrometry (DEMS) has been extensively used for studying fuel cell related electrocatalytic reactions. Quantitative data such as the production rate and the current efficiency for volatile products, e.g., CO2 and HCOOH from the oxidation of small alcohol molecules [1–4] and H2O2 from oxygen reduction reaction provided by DEMS, have been demonstrated to be crucial for deriving molecular level insights for such processes [5,6]. Since only a small fraction of the volatile species produced can be finally detected by the mass spectrometer, precise calibration of the mass signal is a prerequisite for the quantitative analysis. CO2 is the final product from the oxidation of organic fuels, whose production rate and current efficiency are among the most important parameters to judge catalysts’ performance. In most previous studies, the mass calibration constant for CO2 is derived from oxidative stripping of a saturated adlayer of CO adsorbed at the catalyst surface or from bulk oxidation of CO in CO saturated supporting electrolyte. The mass calibration constant (K) is expressed as following: K ¼ 2 Q m=e¼44 =Q CO
to CO2
ð1Þ
where Qm/e = 44 is the integrated mass signal for CO2 detected by DEMS setup, while Q CO to CO2 is the integrated Faradaic charge from CO oxidation. ⁎ Corresponding author. Tel./fax: +86 551 63600035. E-mail address: [email protected] (Y.-X. Chen). 1 Tel./fax: +86 551 63600035.
http://dx.doi.org/10.1016/j.elecom.2014.08.001 1388-2481/© 2014 Elsevier B.V. All rights reserved.
In order to eliminate the mass transport effect and to simulate the real working conditions for fuel cells, high alcohol concentration (≥0.1 M) is preferred for such DEMS studies [4,7,8]. It is expected that high alcohol concentration may affect the detected mass signal of CO2 dissolved in an aqueous solution by changing the Henry constant of CO2 or the ionization probability, which may lead to significant deviation of the mass calibration constant for CO2. However, such effects are not well recognized at all [4,7–11]. In this work, we will provide experimental evidences on how the alcohol concentration affects the detected DEMS signal of CO2. Possible origins for this effect and method to correctly derive the mass calibration constant for CO2 in solutions with high alcohol concentration are discussed.
2. Experimental 2.1. DEMS set-up The mass spectrometer used is the Hiden HPR-40 DSA Bench-Top Membrane Inlet Gas Analysis System with a dual thin-layer flow-cell (flow rate = 100 μL/s) [12,13]. Solutions with x M CH3OH or C2H5OH (AR, Sinopharm, x = 0.1, 1, and 2) are prepared by ultrapure water (18.2 MΩ cm). To simulate the solution containing various amounts of CO2 produced during electrocatalytic oxidation of alcohols, the solution is purged with a gas mixture of N2 + CO2 (99.999%, Nanjing Special gas) for at least 20 min until an equilibrium is reached. The flow rate of each gas (in the range of N2:CO2 = 500 sccm:0 to 5 sccm, sccm stands for standard-state cubic centimeter per minute) is regulated by a flow
W. Chen et al. / Electrochemistry Communications 48 (2014) 10–12
meter controller (Sevenstar, total gas pressure is kept at 1 atm). The mass signal of CO2 is recorded at a frequency of 20 Hz.
8 2.2. Titrimetric analysis
6
3. Results and discussion Fig. 1 gives the mass signal of CO2 recorded when switching from N2 saturated a) pure water, b) 0.1 M, c) 1 M and d) 2 M CH3OH solution to the same solution in equilibrium with N2:CO2 = 500 sccm:0 to 5 sccm at 0 s. Immediately after switching from a CO2 free solution to CO2 containing solution, the mass signal for CO2 increases and reaches a steady-state value at longer time. Similar phenomenon is also observed in an ethanol containing solution. The steady-state mass signal of CO2 as a function of CO2 flow rate and the concentration of methanol or ethanol are plotted in Fig. 2. From Figs. 1 and 2, for each solution, it is seen that the mass signal of CO2 increases linearly with the flow rate of CO2. At a fixed flow rate of CO2, the mass signal for CO2 is nearly the same in pure water and in a solution with 0.1 M CH3OH or C2H5OH. When the alcohol concentration is above 0.1 M, the mass signal for CO2 decreases with the increase in alcohol concentrations. Furthermore, the decay in the CO2 mass signal in a C2H5OH containing solution is much higher than that in a CH3OH containing solution under otherwise identical conditions, e.g., for the cases with 2 M CH3OH or C2H5OH, the CO2 mass signal is just ca. 0.58 and 0.25 times that in pure water. Since we bubble CO2 into the solutions to simulate its formation during alcohol oxidation, reasons for the decrease in CO2 mass signal in a solution with the increase in alcohol concentration may be: i) the equilibrium concentration of CO2 in a solution with high alcohol concentration may be lower than that in 0.1 M alcohol or in an alcohol
(a)
(b)
(c)
(d)
6
MS44 / x 10-8 torr
4 2 0 6
1 sccm 3 sccm 5 sccm
0 40
60
0
time / s
20
40
(a)
1. water 2. 0.1 M 3. 1.0 M 4. 2.0 M
C2H5OH
(b)
2 0
8 6 4 2 0 0
1
2
3
4
5
CO2 / sccm Fig. 2. The steady-state (symbols) and linear fitting (lines) of mass signal of CO2 in 1) pure water (triangle), 2) 0.1 M (diamond), 3) 1 M (square) and 4) 2 M (circle) (a) CH3OH or (b) C2H5OH as a function of CO2 flow rate in the N2 + CO2 gas mixture (N2:CO2 = 500 sccm:x sccm, x = 0 to 5). The above solutions and the gas mixture are in equilibrium.
free solution; and ii) highly concentrated alcohol may change the mass calibration constant for CO2. In order to figure out the major factor for such effect, two sets of parallel titration experiments have been carried out and the results are given in Table 1. At 25 °C when pure water is in equilibrium with the gas mixture with a ratio of N2:CO2 = 100:1, the concentration of CO2 in the solution is ca. 3.44 × 10− 4 mol·L−1 . This corresponds to χCO2 = 6.18 × 10−6 on the mole fraction scale, which agrees well with the literature reports [14]. This confirms that the accuracy of our titration measurement is good enough. From Table 1, when the partial pressure of CO2 is kept at 0.01 atm (N2:CO2 = 500 sccm:5 sccm), the equilibrium concentration of CO2 increases slightly with the increase in alcohol concentration, e.g. it is ca. 12 and 25% higher in 2 M CH3OH and C2H5OH, respectively. Thus, the decrease in the detected mass
Table 1 The concentration of CO2 in a) pure water, b) 1 M CH3OH, c) 2 M CH3OH, d) 1 M C2H5OH, and e) 2 M C2H5OH after being equilibrated with the N2 + CO2 gas mixture at a total pressure of 1 atm and N2:CO2 = 500 sccm:5 sccm, as determined from the titration measurement.
2
20
CH3OH
2 sccm 4 sccm
4
0
1. water 2. 0.1 M 3. 1.0 M 4. 2.0 M
4
Mass44 x10-8 /Torr
Actual CO2 concentration in solutions with x M CH3OH or C2H5OH (as prepared above, x = 1 and 2) equilibrated with the N2 + CO2 gas mixture is titrated with the NaOH standard solution (calibrated by potassium acid phthalate, using phenothalin as indicator, AR grade, Sinopharm). In order to avoid the interference of gas atmosphere (releasing of CO2 into air or dissolving of CO2 from the air into the solution) during the titration, the solution is sealed in a conical flask with a special cap where only the burette can go through and the solution is stirred with a magnetic stirrer during the titration.
11
60
Fig. 1. Mass signal of CO2 recorded when switching from N2 saturated a) pure water, b) 0.1 M, c) 1 M and d) 2 M CH3OH solution to the same solution in equilibrium with the N2 + CO2 gas mixture (N2:CO2 = 500 sccm:x sccm, x = 0 to 5) at 0 s. The mass signal for CO 2 in a N2 saturated solution recorded under otherwise identical condition, from − 10 to 0 s, is set to zero.
Equilibrium concentration cCO2/10−4 mol·L−1
Pure water
1M MeOH
2M MeOH
1M EtOH
2M EtOH
Parallel set c1 Parallel set c2 Average of c1 and c2 Mole fraction χCO2 Ratio of cCO2 in an alcoholic solution to that in pure water
3.49 3.40 3.44 6.18 × 10−6 /
3.65 3.54 3.59 / 104.36%
3.91 3.79 3.85 / 111.92%
3.85 3.79 3.82 / 111.05%
4.33 4.31 4.32 / 125.58%
12
W. Chen et al. / Electrochemistry Communications 48 (2014) 10–12
signal of CO2 with alcohol concentration must come from the decrease in the mass calibration constant of CO2. There are two possible origins for the decrease in the mass calibration constant of CO2. One is that the ionization probability for CO2 is reduced. With alcohol concentration increase, more alcohol molecules enter the vacuum chamber (supported by the fact that the total pressure of vacuum chamber increased from ca. 6.8 × 10−6 mTorr to 9.4 × 10−6 mTorr after the solution switch). Since we use electron impact [15] to ionize the volatile species, alcohol molecules will compete with CO2 for electrons to be ionized and some ionized CO2 may also lose its charge by colliding with the alcohol molecules. As a result, the higher the alcohol concentration, the lower the ionization probability for CO2 (and lower mass calibration constant for CO2) is. The other origin may be that less CO2 molecules can reach the vacuum chamber. It is proved by a neutron diffraction study that alcohol molecules dissolved in water may form cages with its hydrophobic group pointing to the center and OH group pointing to surrounding water [16]. Such cage with hydrophobic zone may trap the CO2 molecules so that free CO2 molecules dissolved in water become less. Those trapped CO2 molecules may not easily reach the vacuum chamber to be detected. Anyway, we consider this effect to be much smaller than first one. The present results reveal that in a solution with high alcohol concentration, alcohol molecules will significantly affect the mass calibration constant for CO2. When the alcohol concentration is above 0.1 M, it is inappropriate to use the mass calibration constant derived from CO oxidation in an alcohol free solution as usually done in the literature. In order to ensure the accuracy of DEMS results, calibration in an alcohol containing solution (the same concentration as in DEMS experiment) should be applied. Otherwise, a separate measurement with titration as carried out in the present study or other method such as gas chromatography and so on is necessary. 4. Conclusion The effect of alcohol concentration on the mass signal of CO2 detected by differential electrochemical mass spectrometry has been examined carefully. We found that when the alcohol concentration is above 0.1 M, the mass signal of CO2 decreases significantly with an increase in alcohol concentration; for the cases with 2 M CH3OH or C2H5OH, the CO2 mass signal is just ca. 0.58 and 0.25 times that in pure water. Titration experiments ascribe this to the decrease in the CO2 mass calibration constant rather than the decrease in CO2 concentration in the solution. Possible origins leading to such an effect are: i) more alcohol molecules enter the vacuum chamber and reduce the ionization probability of CO2 molecules and ii) alcohol molecules form a cage structure which traps CO2 molecules and prevents them from going into the vacuum. Our results demonstrated that when using DEMS to study electrocatalytic oxidation of organic fuels, the change of mass calibration of CO2 induced by high alcohol concentration should be carefully considered. It should be emphasized that besides alcohol, other volatile solutes can also cause a similar problem in the calibration of DEMS signal. Great
attention should be paid when using DEMS for quantitative analysis of volatile species in fuel cell related electrocatalysis. Conflict of interest We declare that we have no conflict of interest. Acknowledgment We appreciate invaluable discussion with S. Ye from Hokkaido Univ. and G. M. Huang from the Univ. Sci. Tech. of China. This work was supported by the National Natural Science Foundation of China (no. 21273215), National Instrumentation Program (no. 2011YQ03012416) and 973 program from the Ministry of Science and Technology of China (project's no. 2010CB923302). References [1] Z. Jusys, R.J. Behm, Methanol, formaldehyde, and formic acid adsorption/oxidation on a carbon-supported pt nanoparticle fuel cell catalyst: a comparative quantitative DEMS study, in: M. Koper (Ed.), Fuel Cell Catalysis: A Surface Science Approach, John Wiley & Sons Ltd., Hoboken, 2009, pp. 411–464. [2] H. Baltruschat, Differential electrochemical mass spectrometry, J. Am. Soc. Mass Spectrom. 15 (2004) 1693–1706. [3] Y.E. Seidel, A. Schneider, Z. Jusys, B. Wickman, B. Kasemo, R.J. Behm, Transport effects in the electrooxidation of methanol studied on nanostructured Pt/glassy carbon electrodes, Langmuir 26 (2010) 3569–3578. [4] Gabriel A. Planes, Gonzalo García, E. Pastor, High performance mesoporous Pt electrode for methanol electrooxidation. A DEMS study, Electrochem. Commun. 9 (2007) 839–844. [5] A.M. Gómez-Marín, K.J.P. Schouten, M.T.M. Koper, J.M. Feliu, Interaction of hydrogen peroxide with a Pt(111) electrode, Electrochem. Commun. 22 (2012) 153–156. [6] J.L. Rodríguez, E. Pastor, C.F. Zinola, V.M. Schmidt, Heterogeneously assisted oxidation of adsorbates from carbon monoxide, methanol and ethanol by hydrogen peroxide solutions on platinum electrodes in sulphuric acid, J. Appl. Electrochem. 36 (2006) 1271–1279. [7] L. Colmenares, H. Wang, Z. Jusys, L. Jiang, S. Yan, G.Q. Sun, R.J. Behm, Ethanol oxidation on novel, carbon supported Pt alloy catalysts—model studies under defined diffusion conditions, Electrochim. Acta 52 (2006) 221–233. [8] T. Seiler, E.R. Savinova, K.A. Friedrich, U. Stimming, Poisoning of PtRu/C catalysts in the anode of a direct methanol fuel cell: a DEMS study, Electrochim. Acta 49 (2004) 3927–3936. [9] H. Wang, H.D. Abruña, Electrocatalysis of direct alcohol fuel cells: quantitative DEMS studies, Struct. Bond. 141 (2011) 33–83. [10] D.A. Cantane, W.F. Ambrosio, M. Chatenet, F.H.B. Lima, Electro-oxidation of ethanol on Pt/C, Rh/C, and Pt/Rh/C-based electrocatalysts investigated by on-line DEMS, J. Electroanal. Chem. 681 (2012) 56–65. [11] Hongsen Wang, Christoph Wingenderr, Helmut Baltruschat, M. Lopez, M.T. Reetz, Methanol oxidation on Pt, PtRu, and colloidal Pt electrocatalysts: a DEMS study of product formation, J. Electroanal. Chem. 509 (2001) 163–169. [12] Y.X. Chen, M. Heinen, Z. Jusys, R.J. Behm, Kinetics and mechanism of the electrooxidation of formic acid—spectroelectrochemical studies in a flow cell, Angew. Chem. Int. Ed. 45 (2006) 981–985. [13] Y.X. Chen, M. Heinen, Z. Jusys, R.J. Behm, Kinetic Isotope effects in complex reaction networks: formic acid electro-oxidation, ChemPhysChem 8 (2007) 380–385. [14] John J. Carroll, J.D. Slupsky, A.E. Mather, The solubility of carbon dioxide in water at low pressure, J. Phys. Chem. Ref. Data 20 (1991) 1201–1208. [15] Achille Cappiello, G. Famiglini, P. Palma, Peer reviewed: electron ionization for LC/ MS, Anal. Chem. 75 (2003) 496 A–503 A. [16] S. Dixit, J. Crain, W.C.K. Poon, J.L. Finney, A.K. Soper, Molecular segregation observed in a concentrated alcohol–water solution, Nature 416 (2002) 829–832.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
The effect of alcohol concentration on the mass signal of CO2 detected by differential mass spectrometry Wei Chen 1, Qian Tao 1, Jun Cai 1, Yan-Xia Chen ⁎ Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China
a r t i c l e
i n f o
Article history: Received 9 July 2014 Received in revised form 1 August 2014 Accepted 4 August 2014 Available online 10 August 2014 Keywords: Differential electrochemical mass spectrometry (DEMS) Mass calibration constant Concentration effect Methanol oxidation Fuel cell
a b s t r a c t Differential electrochemical mass spectrometry (DEMS) is one of the powerful techniques for quantitative analysis of multiple products and their distribution from complex reactions occurred at the electrode/electrolyte interface, in which precise calibration of the mass signal is a prerequisite for accurate determination of product rate and current efficiencies of various products. In this work, we report the effect of alcohol concentration on the detected DEMS signal of CO2, a common final product from electrochemical oxidation of small organic molecules. Comparing to the solution with low alcohol concentration and the same CO2 concentration, we observe a significant reduction of mass signal of CO2 in a concentrated methanol or ethanol solution (N 0.1 M). Possible origins for this are: 1) alcohol molecules evaporate into the vacuum chamber and reduce the ionization probability for CO2 via competition for electrons; and 2) CO2 is trapped in the hydrophobic alcohol cages formed in the aqueous electrolyte. A proper way to eliminate alcohol concentration effect on the detected CO2 mass signal in DEMS studies on fuel cell related electrocatalysis is suggested. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Differential electrochemical mass spectrometry (DEMS) has been extensively used for studying fuel cell related electrocatalytic reactions. Quantitative data such as the production rate and the current efficiency for volatile products, e.g., CO2 and HCOOH from the oxidation of small alcohol molecules [1–4] and H2O2 from oxygen reduction reaction provided by DEMS, have been demonstrated to be crucial for deriving molecular level insights for such processes [5,6]. Since only a small fraction of the volatile species produced can be finally detected by the mass spectrometer, precise calibration of the mass signal is a prerequisite for the quantitative analysis. CO2 is the final product from the oxidation of organic fuels, whose production rate and current efficiency are among the most important parameters to judge catalysts’ performance. In most previous studies, the mass calibration constant for CO2 is derived from oxidative stripping of a saturated adlayer of CO adsorbed at the catalyst surface or from bulk oxidation of CO in CO saturated supporting electrolyte. The mass calibration constant (K) is expressed as following: K ¼ 2 Q m=e¼44 =Q CO
to CO2
ð1Þ
where Qm/e = 44 is the integrated mass signal for CO2 detected by DEMS setup, while Q CO to CO2 is the integrated Faradaic charge from CO oxidation. ⁎ Corresponding author. Tel./fax: +86 551 63600035. E-mail address: [email protected] (Y.-X. Chen). 1 Tel./fax: +86 551 63600035.
http://dx.doi.org/10.1016/j.elecom.2014.08.001 1388-2481/© 2014 Elsevier B.V. All rights reserved.
In order to eliminate the mass transport effect and to simulate the real working conditions for fuel cells, high alcohol concentration (≥0.1 M) is preferred for such DEMS studies [4,7,8]. It is expected that high alcohol concentration may affect the detected mass signal of CO2 dissolved in an aqueous solution by changing the Henry constant of CO2 or the ionization probability, which may lead to significant deviation of the mass calibration constant for CO2. However, such effects are not well recognized at all [4,7–11]. In this work, we will provide experimental evidences on how the alcohol concentration affects the detected DEMS signal of CO2. Possible origins for this effect and method to correctly derive the mass calibration constant for CO2 in solutions with high alcohol concentration are discussed.
2. Experimental 2.1. DEMS set-up The mass spectrometer used is the Hiden HPR-40 DSA Bench-Top Membrane Inlet Gas Analysis System with a dual thin-layer flow-cell (flow rate = 100 μL/s) [12,13]. Solutions with x M CH3OH or C2H5OH (AR, Sinopharm, x = 0.1, 1, and 2) are prepared by ultrapure water (18.2 MΩ cm). To simulate the solution containing various amounts of CO2 produced during electrocatalytic oxidation of alcohols, the solution is purged with a gas mixture of N2 + CO2 (99.999%, Nanjing Special gas) for at least 20 min until an equilibrium is reached. The flow rate of each gas (in the range of N2:CO2 = 500 sccm:0 to 5 sccm, sccm stands for standard-state cubic centimeter per minute) is regulated by a flow
W. Chen et al. / Electrochemistry Communications 48 (2014) 10–12
meter controller (Sevenstar, total gas pressure is kept at 1 atm). The mass signal of CO2 is recorded at a frequency of 20 Hz.
8 2.2. Titrimetric analysis
6
3. Results and discussion Fig. 1 gives the mass signal of CO2 recorded when switching from N2 saturated a) pure water, b) 0.1 M, c) 1 M and d) 2 M CH3OH solution to the same solution in equilibrium with N2:CO2 = 500 sccm:0 to 5 sccm at 0 s. Immediately after switching from a CO2 free solution to CO2 containing solution, the mass signal for CO2 increases and reaches a steady-state value at longer time. Similar phenomenon is also observed in an ethanol containing solution. The steady-state mass signal of CO2 as a function of CO2 flow rate and the concentration of methanol or ethanol are plotted in Fig. 2. From Figs. 1 and 2, for each solution, it is seen that the mass signal of CO2 increases linearly with the flow rate of CO2. At a fixed flow rate of CO2, the mass signal for CO2 is nearly the same in pure water and in a solution with 0.1 M CH3OH or C2H5OH. When the alcohol concentration is above 0.1 M, the mass signal for CO2 decreases with the increase in alcohol concentrations. Furthermore, the decay in the CO2 mass signal in a C2H5OH containing solution is much higher than that in a CH3OH containing solution under otherwise identical conditions, e.g., for the cases with 2 M CH3OH or C2H5OH, the CO2 mass signal is just ca. 0.58 and 0.25 times that in pure water. Since we bubble CO2 into the solutions to simulate its formation during alcohol oxidation, reasons for the decrease in CO2 mass signal in a solution with the increase in alcohol concentration may be: i) the equilibrium concentration of CO2 in a solution with high alcohol concentration may be lower than that in 0.1 M alcohol or in an alcohol
(a)
(b)
(c)
(d)
6
MS44 / x 10-8 torr
4 2 0 6
1 sccm 3 sccm 5 sccm
0 40
60
0
time / s
20
40
(a)
1. water 2. 0.1 M 3. 1.0 M 4. 2.0 M
C2H5OH
(b)
2 0
8 6 4 2 0 0
1
2
3
4
5
CO2 / sccm Fig. 2. The steady-state (symbols) and linear fitting (lines) of mass signal of CO2 in 1) pure water (triangle), 2) 0.1 M (diamond), 3) 1 M (square) and 4) 2 M (circle) (a) CH3OH or (b) C2H5OH as a function of CO2 flow rate in the N2 + CO2 gas mixture (N2:CO2 = 500 sccm:x sccm, x = 0 to 5). The above solutions and the gas mixture are in equilibrium.
free solution; and ii) highly concentrated alcohol may change the mass calibration constant for CO2. In order to figure out the major factor for such effect, two sets of parallel titration experiments have been carried out and the results are given in Table 1. At 25 °C when pure water is in equilibrium with the gas mixture with a ratio of N2:CO2 = 100:1, the concentration of CO2 in the solution is ca. 3.44 × 10− 4 mol·L−1 . This corresponds to χCO2 = 6.18 × 10−6 on the mole fraction scale, which agrees well with the literature reports [14]. This confirms that the accuracy of our titration measurement is good enough. From Table 1, when the partial pressure of CO2 is kept at 0.01 atm (N2:CO2 = 500 sccm:5 sccm), the equilibrium concentration of CO2 increases slightly with the increase in alcohol concentration, e.g. it is ca. 12 and 25% higher in 2 M CH3OH and C2H5OH, respectively. Thus, the decrease in the detected mass
Table 1 The concentration of CO2 in a) pure water, b) 1 M CH3OH, c) 2 M CH3OH, d) 1 M C2H5OH, and e) 2 M C2H5OH after being equilibrated with the N2 + CO2 gas mixture at a total pressure of 1 atm and N2:CO2 = 500 sccm:5 sccm, as determined from the titration measurement.
2
20
CH3OH
2 sccm 4 sccm
4
0
1. water 2. 0.1 M 3. 1.0 M 4. 2.0 M
4
Mass44 x10-8 /Torr
Actual CO2 concentration in solutions with x M CH3OH or C2H5OH (as prepared above, x = 1 and 2) equilibrated with the N2 + CO2 gas mixture is titrated with the NaOH standard solution (calibrated by potassium acid phthalate, using phenothalin as indicator, AR grade, Sinopharm). In order to avoid the interference of gas atmosphere (releasing of CO2 into air or dissolving of CO2 from the air into the solution) during the titration, the solution is sealed in a conical flask with a special cap where only the burette can go through and the solution is stirred with a magnetic stirrer during the titration.
11
60
Fig. 1. Mass signal of CO2 recorded when switching from N2 saturated a) pure water, b) 0.1 M, c) 1 M and d) 2 M CH3OH solution to the same solution in equilibrium with the N2 + CO2 gas mixture (N2:CO2 = 500 sccm:x sccm, x = 0 to 5) at 0 s. The mass signal for CO 2 in a N2 saturated solution recorded under otherwise identical condition, from − 10 to 0 s, is set to zero.
Equilibrium concentration cCO2/10−4 mol·L−1
Pure water
1M MeOH
2M MeOH
1M EtOH
2M EtOH
Parallel set c1 Parallel set c2 Average of c1 and c2 Mole fraction χCO2 Ratio of cCO2 in an alcoholic solution to that in pure water
3.49 3.40 3.44 6.18 × 10−6 /
3.65 3.54 3.59 / 104.36%
3.91 3.79 3.85 / 111.92%
3.85 3.79 3.82 / 111.05%
4.33 4.31 4.32 / 125.58%
12
W. Chen et al. / Electrochemistry Communications 48 (2014) 10–12
signal of CO2 with alcohol concentration must come from the decrease in the mass calibration constant of CO2. There are two possible origins for the decrease in the mass calibration constant of CO2. One is that the ionization probability for CO2 is reduced. With alcohol concentration increase, more alcohol molecules enter the vacuum chamber (supported by the fact that the total pressure of vacuum chamber increased from ca. 6.8 × 10−6 mTorr to 9.4 × 10−6 mTorr after the solution switch). Since we use electron impact [15] to ionize the volatile species, alcohol molecules will compete with CO2 for electrons to be ionized and some ionized CO2 may also lose its charge by colliding with the alcohol molecules. As a result, the higher the alcohol concentration, the lower the ionization probability for CO2 (and lower mass calibration constant for CO2) is. The other origin may be that less CO2 molecules can reach the vacuum chamber. It is proved by a neutron diffraction study that alcohol molecules dissolved in water may form cages with its hydrophobic group pointing to the center and OH group pointing to surrounding water [16]. Such cage with hydrophobic zone may trap the CO2 molecules so that free CO2 molecules dissolved in water become less. Those trapped CO2 molecules may not easily reach the vacuum chamber to be detected. Anyway, we consider this effect to be much smaller than first one. The present results reveal that in a solution with high alcohol concentration, alcohol molecules will significantly affect the mass calibration constant for CO2. When the alcohol concentration is above 0.1 M, it is inappropriate to use the mass calibration constant derived from CO oxidation in an alcohol free solution as usually done in the literature. In order to ensure the accuracy of DEMS results, calibration in an alcohol containing solution (the same concentration as in DEMS experiment) should be applied. Otherwise, a separate measurement with titration as carried out in the present study or other method such as gas chromatography and so on is necessary. 4. Conclusion The effect of alcohol concentration on the mass signal of CO2 detected by differential electrochemical mass spectrometry has been examined carefully. We found that when the alcohol concentration is above 0.1 M, the mass signal of CO2 decreases significantly with an increase in alcohol concentration; for the cases with 2 M CH3OH or C2H5OH, the CO2 mass signal is just ca. 0.58 and 0.25 times that in pure water. Titration experiments ascribe this to the decrease in the CO2 mass calibration constant rather than the decrease in CO2 concentration in the solution. Possible origins leading to such an effect are: i) more alcohol molecules enter the vacuum chamber and reduce the ionization probability of CO2 molecules and ii) alcohol molecules form a cage structure which traps CO2 molecules and prevents them from going into the vacuum. Our results demonstrated that when using DEMS to study electrocatalytic oxidation of organic fuels, the change of mass calibration of CO2 induced by high alcohol concentration should be carefully considered. It should be emphasized that besides alcohol, other volatile solutes can also cause a similar problem in the calibration of DEMS signal. Great
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