Anal. Chem. 1997, 69, 2669-2672
Electrochemical Sensor for Acetylene Larry R. Jordan and Peter C. Hauser*
Department of Chemistry, The University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland
The fixed potential amperometric sensing of acetylene in the gas phase on a gold-Nafion electrode was found to suffer from catalytic poisoning effects with resultant loss of sensitivity. However, by applying a potential pulse program, the sensitivity of the electrode could be restored. With suitable choice of applied potentials, electrochemical adsorption of acetylene took place, allowing an increase in sensitivity. For an adsorption time of 60 s, a 20-fold increase in sensitivity and a detection limit of 20 ppb were obtained. Compounds which, in a fixed potential amperometric mode, would be considerable interferants were found not to preadsorb along with acetylene. An increase in selectivity was, therefore, also achieved.
The monitoring of acetylene relies principally on gas chromatography, which provides good sensitivity and selectivity. However, this method is not well suited for use as a simple, portable measuring device. An amperometric method, which utilized a gold electrode deposited on a Teflon gas diffusion membrane, has been described for the monitoring of aceylene in gas extracted from high-power transformer insulating oil, in which its detection is a useful diagnosis of a faulty transformer.1,2 Although a suitable detection limit of 1 ppm acetylene was achieved, this method was not ideal, since ethylene, which may also be present in transformer oil applications, was found to interfere.1 CH4 and C2H6 are also present in transformer insulating oils; however, they did not interfere. We recently described an amperometric ethylene sensor that utilized gold chemically deposited onto a Nafion membrane as the sensing electrode.3 Acetylene was also seen to react on this electrode, producing a signal ∼7 times greater than that seen for ethylene. Acetylene oxidizes on a gold electrode in acid solution in the same potential region as that seen for ethylene,4 commencing oxidation at +0.95 V (SHE), and, as was found to be the case for ethylene, is inhibited by a gold oxide layer which forms at ∼+1.35 V (SHE). Studies of both acetylene and ethylene reactions on gold electrodes in 0.5 M H2SO4 solution have shown differences in the catalytic reaction of the two gases.5 The oxidation of ethylene results in the formation of the partial oxidation product, acetaldehyde, whereas acetylene oxidizes completely to CO2 via the following overall equation:4 (1) Ishiji, T.; Takahashi, K. J. Appl. Electrochem. 1993, 23, 771-4. (2) Torkos, K.; Borossay, J. J. Chromatogr. 1984, 286, 317-21. (3) Jordan, L. R.; Hauser, P. C.; Dawson, G. A. Anal. Chem. 1997, 69, 558-62. (4) Schmidt, V. M.; Pastor, E. J. Electroanal. Chem. 1994, 369, 271-4. (5) Schmidt, V. M.; Pastor, E. J. Electroanal. Chem. 1994, 376, 65-72. S0003-2700(97)00058-9 CCC: $14.00
© 1997 American Chemical Society
C2H2 + 4H2O f 2CO2 + 10H+ + 10eThis has been attributed to different adsorption mechanisms of the two gases. Ethylene reversibly adsorbs onto the gold surface, whereas acetylene bonds irreversibly, forming a chemisorbate according to the following possible schemes:5
Au + C2H2 f Au(C2H)ad + H+ + eAu + C2H2 f Au(C2)ad + 2H+ + 2eThis adsorption process was investigated here for acetylene sensing with the aim of preadsorbing acetylene prior to its oxidation with an expected increase in sensitivity and selectivity. This detection method requires potential pulsing since the exclusive electrochemical adsorption of acetylene occurs in a potential region distinct to its oxidation. Pulsed amperometric detection in the gas phase is often hindered by the presence of oxygen since its concomitant reduction can interfere with the analytical signal. Schiavon et al.6 have described the gas phase detection of H2S on a silver electrode by the anodic formation of Ag2S, followed by its reduction at a lower potential. Unfortunately, the oxygen reduction process was found to occur at the same applied potential as that required for silver sulfide reduction, resulting in the need to flush the sensor with pure nitrogen gas during the measurement process. The detection of CO2 by the reoxidation of cathodically formed “reduced CO2” species on a platinum electrode also showed interference by oxygen.7 Ethanol detection in the gas phase on a gold-Nafion electrode in a NaOH solution using pulsed amperometric detection has also been described.8 The sole purpose of this procedure for detecting ethanol was not preadsorption but to retain high sensitivity by removing the buildup of poisoning reaction products. In this work, a gold electrode deposited on a Nafion membrane was used to detect acetylene since this configuration has been shown to give high sensitivities and low detection limits.3,6,8 Both direct amperometric detection and a regime in which the acetylene first forms a chemisorbate on the electrode surface before its subsequent oxidation at a higher potential are described. EXPERIMENTAL SECTION Electrode Assembly. The electrochemical cell has been described previously.3 The sensing electrode used was a gold(6) Schiavon, G.; Zotti, G.; Toniolo, R.; Bontempelli, G. Anal. Chem. 1995, 67, 318-23. (7) Kuver, A.; Vielstich, W. J. Electroanal. Chem. 1993, 353, 255-63. (8) Schiavon, G.; Comisso, N.; Toniolo, R.; Bontempelli, G. Electroanalysis 1996, 8, 544-8.
Analytical Chemistry, Vol. 69, No. 14, July 15, 1997 2669
Nafion electrode with a geometric area of 0.8 cm2 and a real surface area of ∼160 cm2, as determined by cyclic voltammetry.9 This gave a sensing electrode with a surface roughness factor of ∼200. The reverse side of the Nafion membrane was contacted with a 0.5 M H2SO4 electrolyte containing a gold wire (Advent Research Materials Ltd., Suffolk, England) as counter electrode and a mercury-mercurous sulfate reference electrode (MSE; REF-601, Radiometer Analytical, Lyon, France) with a 0.5 M H2SO4 filling solution. All potentials reported here have been converted to the standard hydrogen scale by adding 640 mV. Chemicals and Instrumentation. A BAS-100B/W potentiostat (Bioanalytical Systems, West Lafayette, IN) was used for applying potentials. The triple-pulse technique was mainly used. This pulsing methodology has been described in detail elsewhere.10 First, a high “cleaning” potential (E1) in the gold oxide formation region was applied, followed by a second, low potential (E2), at which the gold oxide layer was reduced and acetylene adsorbed. Third, the measuring potential (E3) was applied, at which the preadsorbed gas was oxidized and the analytical signal obtained. The BAS-100B/W software allowed a maximum pulse width of 65 s at each potential. The data were gathered at the end of each pulse for a sample time of 100 ms, over which it was averaged to give the analytical signal. During the final analysis, the potential values for the pulses were kept constant. However, in initial experiments, either the adsorption or measuring potential values were incremented, as explained by LaCourse and Johnson,11 in order to optimize the values. Mass flow controllers of type 1159B (MKS Instruments Inc., Munich, Germany) were used to provide gas mixtures of known concentration by blending of a calibration gas mixture of 10 ppm acetylene in nitrogen with pure nitrogen or air (Carbagas, Basel, Switzerland). The mass flow controllers were connected to the gas cylinders via 1/4 in. (o.d.) tubing using Swagelok fittings. To mix the gases from the mass flow controllers, a Swagelok T piece was used. An adapter from 1/4 in. Swagelok to 1/4 in. × 28 fittings (built in-house) was used for connection to the sensor via 1 mm (i.d.) Teflon tubing (Alltech, Auckland, New Zealand).
RESULTS AND DISCUSSION Reaction of Acetylene on a Gold-Nafion Electrode. Linear sweep voltammograms, both with a pure nitrogen gas flow and with 10 ppm acetylene in nitrogen, are shown in Figure 1. Before the linear sweep, the potential was held at 0 V (SHE) for 60 s to produce a clean gold surface free of oxides. The potential was then swept over the range from 0 to +1.8 V (SHE). Figure 1 shows the region over which acetylene was seen to react and the start of gold oxide formation. The peak seen in the voltammogram in the presence of acetylene is believed to be due to the oxidation of acetylene which has adsorbed during the sweep before the region where acetylene is oxidized. After the peak, the current is thought to be due to concomitant acetylene adsorption and oxidation. Also shown in Figure 1 is the inhibition of acetylene oxidation in the region where gold oxide forms. Fixed Potential Detection. The dynamic response of the gold-Nafion amperometric sensor to ppm levels of acetylene is shown in Figure 2. A potential of +1.25 V, which is close to the (9) Kita, H.; Nakajima, H. Electrochim. Acta 1986, 31, 193-200. (10) Johnson, D. C.; LaCourse, W. R. Anal. Chem. 1990, 62, 589A-97A. (11) LaCourse, W. R.; Johnson, D. C. Anal. Chem. 1993, 65, 50-5.
2670 Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
Figure 1. Linear sweep voltammogram on gold-Nafion electrode. (a) Acetylene (10 ppm), (b) nitogen. Sweep rate, 20 mV s-1. Gas flow, 100 cm3 min-1.
Figure 2. Dynamic response of sensor to acetylene (0-10 ppm). Gas flow, 100 cm3 min-1. Applied potential, +1.25 V vs SHE.
highest potential at which acetylene is oxidized, was applied to the sensing electrode. The gas side of the sensing electrode was exposed to a gas stream with a constant flow of 100 cm3 min-1, and on the reverse side a 0.5 M H2SO4 electrolyte was used to contact the membrane. A detection limit of below 100 ppb (signal/ noise ) 3) was obtained, which compares favorably with the 1 ppm reported for the Teflon membrane gas diffusion electrode.1 This is consistent with the reported high sensitivity of these noble metal-Nafion electrodes, which has been attributed to fast diffusion of gas to the electrode surface.3,12 As can be seen in Figure 2, at higher acetylene concentrations or longer reaction times, a decay in signal was observed. This decay is irreversible under fixed potential conditions, as no recovery of electrode reactivity was observed when the electrode was flushed with pure nitrogen. A decay in current output has also been noted in fuel cell and related research, where the reaction intermediate, linearly adsorbed CO, has been identified as the likely cause.13 It has been demonstrated, although not specifically for acetylene, that the reactivity of the electrode can be restored by cyclic voltammetry or potential pulsing in which a high potential is applied to (12) Schiavon, G.; Zotti, G.; Bontempelli, G.; Farnia, G.; Sandona, G. Anal. Chem. 1990, 62, 293-8. (13) Parsons, R.; Van der Noot, T. J. Electroanal. Chem. 1988, 257, 9-45.
Figure 3. Potential step to +1.25 V after adsorption at +0.6 V for 60 s. (SHE). (a) Acetylene (10 ppm), (b) nitrogen. Gas flow, 100 cm3 min-1.
oxidize the poisoning species.8,10 Potential pulsing may, therefore, also be useful in maintaining the sensitivity of the electrode in this application. Pulsed Detection. The electrode response to a potential step in the presence of nitrogen (bottom curve) and 10 ppm acetylene (top curve) is shown in Figure 3. The potential was held at +0.6 V, where acetylene adsorption is expected to occur, for 60 s and then stepped up to +1.25 V for 5 s to oxidize the adsorbed acetylene. As can be seen in Figure 3, the current for acetylene oxidation decays over the first few seconds, whereas the background current, which is thought to be mainly due to doublelayer charging, reaches a low level after only 250 ms. Therefore, to obtain the greatest sensitivity to acetylene, while at the same time discriminating against the background current, a pulse width of 350 ms, with a current sampling width of 100 ms, at the end of the pulse was used for the measuring step. The optimal potential (E2) for acetylene adsorption was determined by varying the adsorption potential from +0.25 to +1.05 V and determining the relative coverage of acetylene by measuring the current in its subsequent oxidation at +1.25 V. The resulting plot is shown in Figure 4. The acetylene oxidation current is seen to be relatively constant between +0.55 and +0.85 V but to decay sharply on either side of this range. This is probably due to concomitant acetylene reduction or nonadsorption at lower potentials and acetylene oxidation at higher potentials during the adsorption step. The background current, measured with pure nitrogen flow through the sensor, is seen to decay steadily as the potential is increased. The chosen potential for acetylene adsorption/gold oxide reduction was +0.75 V, which is suitably high to reduce the residual current but not so high as to oxidize the adsorbed acetylene. This value is also high enough to avoid oxygen reduction3,12 and its possible interference in the adsorption of acetylene. The choice of oxidation potential (E3) was determined in a similar experiment, in which the measuring potential was incremented from +1.05 to +1.25 V. It is shown in Figure 5 that, at higher oxidation potentials, larger currents are produced. At potentials higher than +1.25 V, interference by gold oxide formation occurred, making +1.25 V a suitable potential for acetylene detection.
Figure 4. Adsorption potential optimization. E1 ) +1.7 V; pulse width, 20 s. E2 ) +0.25 to +1.05 V, increment 20 mV; pulse width, 60 s. E3 ) +1.25 V; pulse width, 350 ms; sample width, 100 ms, (a) Acetylene (10 ppm), (b) nitrogen. Gas flow, 100 cm3 min-1.
Figure 5. Measurement potential optimization. E1 ) +1.7 V; pulse width, 20 s. E2 ) +0.75 V; pulse width, 60 s. E3 ) +1.05 to +1.25 V, increment 20 mV; pulse width, 350 ms, sample width, 100 ms. Acetylene (10 ppm). Gas flow, 100 cm3 min-1.
The cleaning potential (E1) of +1.7 V was chosen to be as high as possible to efficiently oxidize acetylene and reaction intermediates, but not so high as to oxidize the supporting electrolyte. It was found that the gas flow rate had a scarce influence on the sensor response at flow rates higher than 50 cm3 min-1 but caused a decrease in the response at lower flow rates. The gas flow rate dependence seen at lower flow rates must be due to the mass transport limitation of acetylene to the sensing electrode during the adsorption step, resulting in less acetylene getting adsorbed and subsequently oxidized. Flow insensitivity is favorable since accurate control of the gas flow rate is then not required. For this reason and to obtain high sensitivity, a gas flow rate of 100 cm3 min-1 was used in this work. Sensor Performance. In Figure 6, the dynamic response to acetylene using pulsed amperometric detection is shown. Acetylene was measured using an oxide reduction/acetylene adsorption value of +0.75 V (SHE), an acetylene oxidation value of +1.25 V with a pulse width of 350 ms, and a sample width of 100 ms. The high cleaning potential of +1.7 V was applied for 20 s. The adsorption time was 60 s, which was near the maximum time allowed by the software. In contrast to the fixed potential case, Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
2671
Figure 6. Dynamic response of sensor to acetylene (0-10 ppm). E1 ) +1.7 V; pulse width, 20 s. E2 ) +0.75 V; pulse width, 60 s. E3 ) +1.25 V; pulse width, 350 ms, sample width, 100 ms. Gas flow, 100 cm3 min-1.
the response to acetylene showed no decay at a fixed concentration. For an adsorption time of 60 s, a 20-fold increase in sensitivity and a detection limit of 20 ppb acetylene, for a signal-to-noise ratio of 3, were obtained. The response was linear up to nearly 5 ppm acetylene, beyond which the sensitivity steadily decreased. This is thought to be due to competition for adsorption sites when, at higher concentrations, the acetylene already adsorbed blocks the surface to further adsorption. When the adsorption time was decreased to 5 s, the calibration curve became linear up to the 10 ppm limit given by the acetylene standard. However, the sensitivity was reduced over 6-fold from 0.25 to 0.04 mA/ppm. In principle, the choice of adsorption time could be optimized, depending on what concentration is being measured, to give a linear response over the concentration range of interest. Selectivity. SO2 (10 ppm), NO2 (10 ppm), NO (10 ppm), ethylene (10 ppm), and oxygen (air) have been shown to be reactive on gold electrodes in acidic conditions.3 These compounds were, therefore, tested as interferants in both fixed potential and pulsed amperometric detection. At a fixed applied potential of +1.25 V and a gas flow rate of 100 cm3 min-1, the strongest interferant was found to be SO2, which, nevertheless, displayed a signal over 5 times less than that of acetylene. The relative sensitivities obtained were as follow: acetylene, 1.0; NO, 0.24; NO2, 0.04; SO2, 0.34; and ethylene, 0.14. When pulsed detection was employed, a 20-fold increase in sensitivity to acetylene was obtained, whereas no significant increase in response was noted for the other compounds. The relative sensitivities of acetylene (1.0), NO (0.03), NO2 (0.01), SO2 (14) Chang, S.-C.; Stetter, J. R. Electroanalysis 1990, 2, 359-65.
2672 Analytical Chemistry, Vol. 69, No. 14, July 15, 1997
(0.02), and ethylene (0.01) demonstrate that an increase in selectivity to acetylene of ∼20-fold was obtained with pulsed amperometric detection. No difference in sensitivity was seen when the acetylene standard was diluted with nitrogen or pure air, indicating that oxygen has no influence in the adsorption or detection of acetylene. This can be attributed to the oxygen reduction process occurring at more cathodic potentials on gold than that applied for acetylene adsorption. Since acetylene was found to preadsorb, whereas the interferants tested here did not, it was found possible to further improve the selectivity by stopping the flow before the measurement step so that no interferants were transported to the electrode and, ideally, only the preadsorbed acetylene was oxidized. This mode was tested on acetylene and two of the possible interferants, ethylene (500 ppm) and SO2 (200 ppm). When the gas flow was stopped during the measurement step, the sensitivity to ethylene was found to decrease by a factor of 1.8, and that to SO2 by a factor of 8. Acetylene (10 ppm) showed no decrease in response. The greater discrimination against SO2 compared with ethylene is thought to be due to SO2, but not ethylene, oxidizing at the adsorption potential, resulting in more ethylene than SO2 remaining dissolved in the Nafion membrane and producing a signal when the measuring potential is applied. Overall, with preadsorption of acetylene and modulated flow, the sensor response to acetylene was 180 times greater than that to ethylene and 400 times greater than that to SO2. CONCLUSION The detection limit for pulsed amperometric detection of acetylene was 20 ppb. However, the main benefit of pulsed amperometric detection of acetylene is the increase in sensitivity and the resulting selectivity. The selectivity to acetylene is high for a number of reasons. First, the electrocatalytic oxidation of organic compounds on gold, in acid electrolyte, is very selective to only unsaturated compounds.5 In the application of fault gas detection in transformer oils, this rules out cross sensitivity to CH4 and C2H6 which are also produced in faulty high-power transformers. The complete oxidation of acetylene on gold produces 10 electrons per acetylene molecule, compared with two electrons, for example, in the oxidation of NO2,14 with resulting current increase. With pulsed amperometric detection, the preadsorption of acetylene results in increased sensitivity only to acetylene, giving an increase in selectivity. Also, with pulsed detection, the selectivity to acetylene could be further increased by stopping the gas flow during the measurement step. Received for review January 16, 1997. Accepted May 8, 1997. AC9700585 X
Abstract published in Advance ACS Abstracts, June 15, 1997.
