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Detection of hydrocarbons in air by adsorption on Pt-electrodes using continuous impedance measurements
Sensors and Actuators B 42 (1997) 31 – 37
Detection of hydrocarbons in air by adsorption on Pt-electrodes using continuous impedance measurements R. Oelgeklaus, H. Baltruschat * Institute of Physical Chemistry, Uni6ersity of Bonn, Ro¨merstr. 164, D-53117 Bonn, Germany Received 2 July 1996; accepted 16 May 1997
Abstract In this paper, it is shown that the detection of many organic compounds such as benzene, its derivatives and simple halogenated hydrocarbons is possible by means of their adsorption at a Pt-sensor electrode. The change of electrode capacity due to adsorbate formation is continuously monitored by ac measurements. This is possible despite the high oxygen reduction currents at the adsorption potentials. A special potential program serves to repeatedly strip the adsorbate from the surface, thus allowing the continuous detection of such compounds with sensitivities in the low ppm range. © 1997 Elsevier Science S.A. Keywords: Impedance; Adsorption; Organic gases; Electrode; Platinum
1. Introduction In previous publications [1 – 3] we have shown that many volatile hydrocarbons in air can be detected by an electrochemical sensor using a potentiodynamic method. In brief, the species are allowed to adsorb at the metal/electrolyte interface of a porous Pt-electrode and are then oxidized during an anodic potential cycle. A special potential programme is continuously applied and the oxidation current, after correction for pseudocapacitive effects like oxygen adsorption, is a relative measure for the amount of species adsorbed. This, in turn, is proportional to the concentration of the species in air since the adsorption time is constant. (Adsorption kinetics can safely be assumed to be of first order in most cases. In particular, this has been shown for CH2Cl2 [4].) Sensitivities in the lower ppm range have thus been obtained for species like benzene, tetrachloroethene, vinylacetate and others [2]. Here, we will show that also the impedance change during adsorption of the organic species can be used for their detection. We will demonstrate that a combination of both detection principles will lead to a sensor which automatically optimizes its sensitivity and response time to the concentration. * Corresponding author. Tel.: + 49 228 550234; fax: + 49 228 686292; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 8 2 - 2
The adsorption of tetrachloroethene and other halogenated and unsaturated species has recently been studied using differential electrochemical mass spectrometry [5–14] and other techniques. In this study, we use benzene as a model compound because its adsorption has been well studied and its adsorption isotherm on Pt in sulfuric acid (with a maximum coverage between 400 and 500 mV versus NHE) is well known [15]. Comparative results will be presented for tetrachloroethene because of its similar ecological and human toxicologic relevance. Impedance measurements are often used for the characterization of sensor cells; in addition, their possible use for a continuous performance control is discussed [16–19]. Preidel and co-workers [20,21] oxidize glucose at a noble metal electrode covered with a membrane without using biomolecules. The concentration is determined by applying a step-profile-potential to the electrode and superimposing an ac-voltage. On the basis of the real and imaginary component of the impedance at specific potentials, the charge transfer resistance and the capacity which serve for the determination of the glucose concentration are calculated. Biomolecules could also be detected by impedance spectroscopy, since the antibody–antigene binding possibly causes a change of the dielectrical constant or an increasing thickness of the dielectrical layer [22].
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R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
2. Experimental For the experiments with smooth Pt, a standard electrochemical glass cell using the hanging electrolyte method was used. The laboratory design of our sensor cell was described previously [1,2]. All solutions were made of Milli-Q water and, in the case of the glass cell, purged by pure nitrogen; the sulphuric acid and perchloric acid were obtained from Merck (Darmstadt), benzene and tetrachloroethene from Roth (Karlsruhe). We used an impedance measurement system from EG&G (potentiostat 273A, lock-in type 5210) and a simulation software from Boukamp [23]. Special care was necessary to avoid the contamination of the smooth Pt surface in the supporting electrolyte during the relatively long impedance measurements. Therefore, the potential was stepped for 1 min before each measurement at a given frequency to a more positive potential, at which possible contaminants are oxidized. After recording the complete impedance spectrum, the cleanliness was checked again by CV (see [24] for details). The test gases were generated by diluting the corresponding pure gas by the carrier gas (nitrogen or synthetic air). Volume flow rates corresponding to the respective concentrations were adjusted using flow meters (UCAR). For investigations of readily volatile liquid species the carrier gas was bubbled through the respective liquids. The gas saturated with the liquid vapor formed this way was further diluted by mixing with the carrier gas.
3. Results The basic idea of using impedance measurements for the detection of organics using their adsorption on Pt can be explained by the CV in Fig. 1. It compares the (well known) cyclic voltammograms of a polycrystalline
Fig. 1. CV at the smooth Pt electrode in 0.5 M H2SO4 saturated with nitrogen containing 360 ppm benzene and in absence of benzene (n = 100 mV s − 1; reference electrode: NHE).
Pt-electrode in sulfuric acid without and with benzene. It can be clearly seen that hydrogen adsorption is suppressed by adsorbed benzene. It was our aim to use this decrease, determined by electrode capacity measurements, to not only determine the amount of adsorbed species, but also the rate of adsorption and thus its concentration. According to Eq. (1) the variation of the surface coverage U in time is proportional to the concentration c of the adsorbing species in the vicinity of the electrode surface. Furthermore, the relation (Eq. (2)) between the coverage U and the capacity C is valid in the case of the adsorption of organic species [25]: dU/dt= 6ad = kc(1− U)$ kc
for
U 0 (or t0) (1)
U=(C0 − C)/(C0 − Cm)
(2)
dU dC = − (C0 − Cm) k · c = −(C0 − Cm) · dt dt
(3)
where k is the adsorption rate constant, C0 and Cm are the capacities of a free and a completely covered electrode surface. Therefore, at short times, the variation of the capacity in time (dC/dt) is proportional to the concentration c of the substance to detect. For a continuous monitoring of concentrations, the surface has to be cleaned periodically from the adsorbed substance by potential cycles. Various obstacles have to be considered before this can be used in a sensor cell: 1. The rate of adsorption must be high at the potential where the capacity change is highest (or vice versa). Therefore, the potential dependence of the capacity change should be known. 2. The sensor cell (esp. the sensing gas diffusion electrode) should allow meaningful impedance measurements, from which a reliable determination of the electrode capacity is possible. This is not possible with usual amperometric gas sensors, due to their extremely large roughness factor. 3. The determination of the capacity should be possible at a single frequency and should not be disturbed by the large currents due to oxygen reduction. We first studied the potential dependence of the capacity change at smooth, polycrystalline Pt-electrodes because these serve as a reference for the impedance measurements with the sensor electrodes. Therefore, impedance spectra of a smooth Pt-electrode were recorded in a frequency range between n= 0.1 Hz–30 kHz for potentials between Ead = 100 mV and Ead = 500 mV, containing no or 3 · 10 − 5 M benzene. The hydrogen adsorption is so fast at Pt in H2SO4 (in contrast to KOH, see [24]) that it is impossible to separate the hydrogen adsorption capacity from the pure double-layer capacity. Impedance spectra were
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
Fig. 2. The potential dependence of the electrode capacity at the smooth Pt electrode in 0.5 M H2SO4 in benzene (360 ppm in N2) containing solution and in absence of benzene.
simply fitted by an equivalent circuit consisting of a solution resistance Re and a capacity C. The same equivalent circuit could be used to fit the impedance data in the case of benzene-containing solutions, as no charge transfer inhibition or diffusion inhibition were observed. As expected, the potential dependence of the electrode capacity reflects the shape of the cyclic voltammograms, since the current in the CV is proportional to the electrode (pseudo) capacity (Fig. 2). The relative change of the electrode capacity due to benzene adsorption is shown in Fig. 3. It shows a maximum in the potential range between 250 and 300 mV, i.e. in the range of the hydrogen adsorption, where it amounts to about 80%. Obviously due to the displacement of adsorbed hydrogen, the hydrogen adsorption capacity decreased more strongly because of adsorbed benzene than the pure double-layer capacity. This can be understood by taking into consideration that theoretically the hydrogen adsorption should decrease by 100% in the case of a full monolayer (Cm = 0 in Eq. (2)), whereas the double layer capacity only
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decreases due to the larger distance of ionic charge carriers from the electrode surface. At more cathodic potentials the change of the capacity gets smaller again, as here benzene is desorbed anew to some extent [11]. Fig. 3 also shows the relative change in capacity of a porous Pt electrode in the sensor cell (¥ = 8 mm, 5 M H2SO4). The sensor was exposed to air or 360 ppm benzene in air, respectively. In order to fit the impedance spectra recorded in the sensor cell, the equivalent circuit (series circuit of Re and C) has to be supplemented with a resistance Rp in parallel to the capacity. The resistance Rp symbolizes the charge transfer resistance due to the oxygen reduction (EB0.9 V). It is important to note that the determination of the electrode capacitance is not hindered by the large oxygen reduction current (typically 10–20 mA cm − 2). Therefore, even in presence of synthetic air a relative change of the capacity of 80% can be obtained. This result reproduces the value at the smooth electrode. The potential of the maximum sensitivity is found somewhat more cathodically than at the smooth Pt electrode because of the IR-drop. The IR-drop results from the oxygen reduction current and the series resistance Re ($ 5 V), which is only in part due to the solution resistance, but mainly due to the resistance of the thin Pt film of the sensing electrode. For a working sensor, recording of a complete impedance spectrum would be too time-consuming. Instead, a signal obtained at a single frequency has to be used. This can be done as follows: Eqs. (4) and (5) give the imaginary and real parts of the electrode admittance (which is the reciprocal of the impedance). vC (Re/Rp + 1)2 + (vReC)2
(4)
(1/Rp)(Re/Rp + 1)+ (1/Re)(vReC)2 (Re/Rp + 1)2 + (vReC)2
(5)
Yim = Yre =
For Rp Re and Re 1/vC these equations reduce to Yim $ vC
(6)
and Yre $ 1/Rp
Fig. 3. The potential dependence of the relative change of the capacity due to the adsorption of benzene at the smooth Pt electrode and at the sensor electrode (¥ =8 mm, 360 ppm in nitrogen or synthetic air, respectively).
(7)
i.e. at low frequencies, when 1/vC becomes large, the capacity is given by the imaginary part of the admittance. In Fig. 4, the so-called ‘real part of the capacity’, which is defined as Yim/v, is plotted versus frequency on a logarithmic scale. It can be seen that the above relation holds up to about 20 Hz, both with and without benzene. Therefore, it is possible to determine the electrode capacity by superimposing an ac voltage of, e.g. 10 Hz on the electrode potential and measuring the imaginary part of the ac current, which is proportional to the admittance. It is crucial, however, that Re
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R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
tween the first and the last sweep at a given potential (e.g. 1.44 V) serve as a sensor signal for the potentiodynamic sensor. Using a special electronic circuitry, the ac signal is only superimposed on the dc potential during the adsorption period. Otherwise, the detection of the oxidation current transients is disturbed. Fig. 6 demonstrates the shape of several successive imaginary current transients for long adsorption times without benzene in air and during a sudden exposure to a high benzene concentration. A slight decrease of the imaginary current in air is due to impurities. When benzene exposure starts, it suddenly decreases to the value of corresponding to saturation, i.e. by nearly 80%. When the benzene exposure is stopped, the deFig. 4. A section of the admittance spectra in benzene-containing solution (360 ppm benzene in synthetic air) and in benzene-free solution recorded at the sensor electrode (¥ = 8 mm, Pt catalyst, 5 M H2SO4) at the potential of the maximum sensitivity (Ead = 200 mV).
is low enough. An increase by a factor of 10 due to low electrolyte conductivity or bad electrode conductance would lead to a decrease of the upper frequency limit to about 1 Hz, making the determination of the imaginary part of the current by a look-in amplifier time-consuming and thus decreasing the response time. It is not sufficient to simply detect the overall ac current since this is largely influenced by the faradaic current due to oxygen reduction (Yre $1/Rp). As described above, the change in electrode capacity per unit of time is proportional to the concentration of the species to be detected. For practical applications, the sensor electrode has to be cleaned periodically by potential cycling, and the change in capacity in between the potential cycles has to be recorded. Because of this, we combined this procedure with our previously described potentiodynamic detection principle [1,2] (Fig. 5). During an adsorption period tad, the substance (tetrachloroethene in this case) is allowed to adsorb at a constant adsorption potential Ead. A small ac-voltage is superimposed during this period. The imaginary part of the ac current is simultaneously detected and decreases due to adsorption of the organic species as described above (Eq. (3)). Afterwards, the adsorbate is oxidatively desorbed in several potential sweeps from 0.8 – 1.45 V. Several sweeps are applied to ensure that desorption is complete. Short potential pulses to 0.1 or 0.0 V are applied for 0.5 s after each anodic sweep to reduce the adsorbed oxygen. After six sweeps, a new adsorption period begins. The dc current transients during the anodic sweeps (Fig. 5c) are due to oxygen adsorption. During the first of these (and to a smaller extent also during the second) they are also due to the oxidation of the organic adsorbate. As described before [1,2], the difference be-
Fig. 5. (a) The specific potential program exemplary for the detection of C2Cl4 in synthetic air (c =180 ppm) at the sensor electrode (Pt catalyst, ¥= 6 mm, 1 M HClO4) using the following parameters: dE/dt =300 mV s − 1, tad =20 s, tred =0.5 s, Ead =100 mV, Ered = 0 V, Eac =10 mV, n= 10 Hz; (b) the response of the imaginary component of the ac-current; (c) the response of the oxidation current according to the potentiodynamic procedure.
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
Fig. 6. Change of sensor electrode capacity during a sudden exposure to 180 ppm benzene (¥= 8 mm, 5 M H2SO4, dE/dt= 300 mV s − 1, tad = 40 s, tred =0.5 s, Ead = Ered = 0.2 V, Eac = 10 mV, n =8 Hz).
crease becomes less noticeable. (Due to the high benzene concentration used in this experiment, it takes some time before the concentration of benzene in air drops to zero again.). The initial slope dIim/dt of the imaginary current transient should be proportional to the concentration of the adsorbing species. Alternately, it should be possible and easier to use the difference of the currents at the beginning (i.e. at the maximum) and the end of the adsorption period. This should be a good approximation provided the adsorption period is not too long. The signals usable in such a sensor device therefore are: Signal 1:
dIim/dt
Signal 2:
DIim
Signal 3:
DIox/Iox5
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Fig. 7. The response characteristics of the sensor (Pt catalyst, ¥ = 6 mm, 1 M HClO4) for a benzene/synthetic air mixture for Signal 1, 2, and 3 (concentration specifications in ppm). dE/dt = 300 mV s − 1, tad =20 s, tred =0.5 s, Ead =200 mV, Ered =0 V, Eox 1.45 V, Eac = 10 mV, n =10 Hz.
those adsorption times and concentrations for which the surface coverage of the organic adsorbate becomes appreciable (see Eq. (1)). Signal 1, on the contrary, is directly proportional to the initial adsorption rate and therefore to the concentration, since it is determined at short adsorption times corresponding to low coverages. The saturation effect is even more pronounced for Signal 3. This may be due to the fact that it is not obtained from an evaluation of the total oxidation
Initial slope of the time dependence of the imaginary component of the ac-current The difference of the imaginary components of the ac-current at the beginning and the end of the adsorption phase The difference of the oxidation currents of the first and the fifth cycle standardized to that of the fifth cycle (Potentiodynamic procedure, cf. above and in [1])
Fig. 7 compares the response of these different signals to various concentrations of benzene in air. The response time (35 s) is determined by the adsorption time (20 s) and the potential sweep rate (300 mV s − 1) during the oxidation cycles. While the initial slope value of the ac-current signal increases linearly with the concentration, Signal 2 and Signal 3 show a saturation characteristic Fig. 8a. Since Signal 2 should be proportional to the coverage at the end of the adsorption phase, this is expected for
Fig. 8. The concentration dependence of Signal 1, 2, and 3 for a mixture of (a) benzene (data taken from Fig. 7); (b) tetrachloroethene in synthetic air (same parameters as in Fig. 7, except Ead =100 mV).
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R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
change, but only for the current, i.e. the oxidation rate at a given potential. The concentration dependence for tetrachloroethene is demonstrated in Fig. 8b, confirming the results for benzene.
cies like CO would be adsorbed, and their presence would decrease the adsorption time. However, since their adsorbate is oxidized below 1 V, CO could not simulate the presence of benzene or tetrachloroethene.
Acknowledgements 4. Discussion We have demonstrated that by using an appropriate sensor design meaningful measurements of the sensor electrode impedance are feasible. It is possible to determine the electrode capacity using an impedance measurement at a single frequency even in the presence of oxygen, i.e. during high faradaic reduction currents. The decrease of the double layer capacity due to the adsorption of organic species can be monitored during adsorption using the imaginary part of the ac current. Using the same dc potential program as used for a potentiodynamic sensor, it is thus possible to sense organic species. By itself, such a sensor is not very selective. Many other species which also adsorb at Pt would also lead to a decrease of electrode capacity. On the other hand, the potentiodynamic sensor which uses the oxidation current of the species as the sensor signal (Signal 3= DIox/ Iox5) is much more selective because only a limited class of volatile organic species are oxidized at the same potential as benzene or tetrachloroethene. In particular, ethanol and methanol (including their adsorbate) are oxidized at much lower potentials and are therefore recognized. However, this potentiodynamic sensor has one major disadvantage: its limited dynamic range, due to saturation of the surface with adsorbate at high concentrations and long adsorption times (which are used to obtain a high sensitivity). We, therefore, propose to combine both detection principles in one sensor: The adsorption time is not fixed, but determined by the decrease of the imaginary part of the ac current: that the adsorption period should be terminated as soon as Iim drops to, e.g. 10% of its initial value. The normalized oxidation current (sensor Signal 3), divided by the variable tad, would still serve as the sensor signal. Such a sensor has the following characteristics: In the absence of adsorbable species or at low concentrations, the adsorption time is high, leading to high sensitivities. The corresponding slow response is acceptable. At high concentrations, tad automatically becomes low, corresponding to a short response time. A sudden increase in concentration during the adsorption period (Fig. 6) is immediately detected and leads to a fast interruption of the adsorption. Saturation of the surface with adsorbate does not occur, leading to a wide dynamic range of the sensor. The selectivity, on the other hand, is as good as for the ‘simple’ potentiodynamic sensor. Spe-
This work was supported by BMFT (grant number 01-VQ 9012/7).
References [1] H. Baltruschat, R. Oelgeklaus, J. Rose, Nachweis flu¨chtiger organischer Lo¨semittel mit Hilfe von potentiodynamischen Gassensoren, GIT 39 (1995) 556 – 558. [2] H. Baltruschat, I. Kramphausen, R. Oelgeklaus, J. Rose, M. Wahlekamp, Detection of volatile organic solvents using potentiodynamic gas sensors, Anal. Chem. 69 (1997) 743 – 748. [3] H. Baltruschat, Verfahren und Vorrichtung zum fortlaufenden Nachweis wenigstens einer Substanz in einem gasfo¨rmigen oder flu¨ssigen Gemisch mittels einer Sensorelektrode, Patent DE 195 15 524 A1. [4] U. Mu¨ller, A. Du¨lberg, A. Stoyanova, H. Baltruschat, Displacement of adsorbed benzene by CU-UPD: A DEMS-study at polycrystalling Pt and Pt(110)-Electrodes, in: Proc. 6th Int. Symp. on Electrode Processes, The Electrochemical Society, Pennington, 1996, pp. 257 – 268. [5] G.L. Zucchini, V.H. Andreev, V.E. Kazarikov, Investigation of chloroform adsorption on a platinum electrode by the tracer method, J. Electroanal. Chem. 210 (1986) 161 – 166. [6] J.M. Zhu, T. Hartung, D. Tegtmeyer, H. Baltruschat, J. Heitbaum, The electrochemical reactivity of toluene on Pt-electrodes, J. Electroanal. Chem. 244 (1988) 273 – 286. [7] B. Ba¨nsch, T. Hartung, H. Baltruschat, J. Heitbaum, Reduction and oxidation of adsorbed acetone at Pt-electrodes studied by DEMS, J. Electroanal. Chem. 259 (1989) 207 – 215. [8] N. Batina, S.A. Chaffins, J.Y. Gui, F. Lu, J.W. McCargar, J.W. Rovang, D.A. Stern, A.T. Hubbard, Electrochemical oxidation of adsorbed terminal alkenes as a function of chain length at Pt(111) electrodes, J. Electroanal. Chem. 284 (1990) 81–96. [9] H. Baltruschat, A. Du¨lberg, U. Schmiemann, Zum elektrochemischen reaktionsverhalten umweltrelevanterorganischer Verbindungen: Untersuchhungen mit DEMS, Dechema Monographien 25 (1992) 489 – 500. [10] H. Baltruschat, U. Schmiemann, The adsorption of unsaturated organic species at single crystal electrodes studied by differential electrochemical mass spectrometry, Ber. Bunsenges. Phys. Chem. 97 (1993) 452 – 460. [11] U. Schmiemann, H. Baltruschat, The influence of the single crystal orientation on the electrocatalytic hydrogenation of benzene and the H/D-exchange, J. Electroanal. Chem. 347 (1993) 93 – 109. [12] H. Baltruschat, M. Beltowska-Brzezinska, A. Du¨lberg, Reactions of halogenated hydrocarbons at platinum group metals. I. A DEMS study of the adsorption of CH3CCl3, Electrochim. Acta 38 (1993) 281 – 284. [13] S. Wasmus, W. Vielstich, Electro-oxidation and electroreduction of acetonitrile in aqueous acid solution. A DEMS study, J. Electroanal. Chem. 345 (1993) 325 – 335. [14] U. Schmiemann, U. Mu¨ller, H. Baltruschat, The influence of the surface structure on the adsorption of ethene, ethanol and cyclohexene as studied by DEMS, Electrochim. Acta 40 (1995) 99 – 107.
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37 [15] W. Heiland, E. Gileadi, J.O’M. Bockris, Kinetic and thermodynamic aspects of the electrosorption of benzene on platinum electrodes, J. Phys. Chem. 70 (1966) 1207–1216. [16] J. Gutierrez, L. Area, M.C. Horillo, I. Sayago, J. Agapito, L. Lopez, Use of complex impedance spectroscopy in chemical sensor characterization, Sensors and Actuators B 4 (1991) 359 – 363. [17] J. Szeponik, W. Moritz, F. Sellam, Investigation of sensor characteristics of chemical semiconductor sensors with impedance spectroscopy, Sensors and Actuators B 15/16 (1993) 233 – 237. [18] J.M. Chovelon, N. Jaffrezic, Y. Cros, J.J. Fombon, D. Pedone, Monitoring of ISFET encapsulation aging by impedance measurements, Sensors and Actuators 3 (1991) 43–50. [19] D. Kitzelmann, C. Gottschalk, Elektrochemische GassensorenWirkungsweisen und Mo¨glichkeiten zur Funktionsu¨berwachung, Technisches Messen 4 (1995) 152–159. [20] W. Preidel, I. von Lucadou, W. Lager, L. Ruprecht, S. Saeger, Glucose measurements by electrocatalytic sensor in the extracorporeal blood circulation of a sheep, Sensors and Actuators B 2 (1990) 257 – 263. [21] W. Preidel, S. Saeger, Zyklisches Potentilasprungverfahren mit u¨berlagerter Wechselspannung Zur Glucosebestimmung im Blut, Dechema Monographien 117 (1989) 59–72. [22] E. Souteyrand, J.R. Martin, C. Martelet, Direct detection of biomolecules by electrochemical impedance measurements, Sensors and Actuators B 20 (1994) 63–69. [23] B.A. Boukamp, A nonlinear least squares fit procedure for analysis of immittance data of electrochemical systems, Solid State Ionics 20 (1986) 31–44. [24] R. Oelgeklaus, J. Rose, H. Baltruschat, On the rate of hydrogen and iodine adsorption on polycrystalline Pt and Pt(111), J. Electroanal. Chem. 376 (1994) 127–133. [25] A.N. Frumkin, B.B. Damaskin, in: J.O’M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, vol. 3, Butterworth, London, 1964, pp. 149–223.
Biographies Rainer Oelgeklaus studied physics at Mu¨ster University between 1984 and 1990. In 1990, he became a
.
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Master of Science with a thesis on the detection of the local density of states using energy loss spectra, with Professor L. Reimer, Institute of Physics, Mu¨nster University. In 1995 he got his PhD with a thesis on analysis of adsorption kinetics at poly- and monocrystalline Pt-electrodes analysed by impedance spectroscopy. Since December 1995, he has been teacher-training to qualify as a teacher for secondary school in physics and chemistry. Helmut Baltruschat studied chemistry at Marburg and Bonn University between 1972 and 1981. Between 1976 to 1977 he was at the University of Paris VII with a scholarship by the French Government and DAAD– German Academic Exchange Service. In 1981, he became a Master of Science with a thesis on the kinetics of silver dissolution in cyanide-containing solutions, with Professor W. Vielstich, Bonn University. In 1985 he got his PhD with a thesis on surface-enhanced Raman spectroscopy, with Professor J. Heitbaum, Bonn University. During 1985–1986 he had a postdoctoral fellowship in Professor Hubbard’s research group at the University of California/Santa Barbara (analysis of electrosorbates at Pt-single crystal electrodes using auger electron spectroscopy and LEED). From 1986– 1993, he was a research assistant at the Institute of Physical Chemistry at the University of Witten/ Herdecke. In November 1992, he received ‘Habilitation’ with a thesis on the characterization of electrochemical adsorbates by non-traditional methods. Since December 1993 he has been a Professor of Physical Chemistry (Electrochemistry) at the Institute of Physical and Theoretical Chemistry at Bonn University.
Detection of hydrocarbons in air by adsorption on Pt-electrodes using continuous impedance measurements R. Oelgeklaus, H. Baltruschat * Institute of Physical Chemistry, Uni6ersity of Bonn, Ro¨merstr. 164, D-53117 Bonn, Germany Received 2 July 1996; accepted 16 May 1997
Abstract In this paper, it is shown that the detection of many organic compounds such as benzene, its derivatives and simple halogenated hydrocarbons is possible by means of their adsorption at a Pt-sensor electrode. The change of electrode capacity due to adsorbate formation is continuously monitored by ac measurements. This is possible despite the high oxygen reduction currents at the adsorption potentials. A special potential program serves to repeatedly strip the adsorbate from the surface, thus allowing the continuous detection of such compounds with sensitivities in the low ppm range. © 1997 Elsevier Science S.A. Keywords: Impedance; Adsorption; Organic gases; Electrode; Platinum
1. Introduction In previous publications [1 – 3] we have shown that many volatile hydrocarbons in air can be detected by an electrochemical sensor using a potentiodynamic method. In brief, the species are allowed to adsorb at the metal/electrolyte interface of a porous Pt-electrode and are then oxidized during an anodic potential cycle. A special potential programme is continuously applied and the oxidation current, after correction for pseudocapacitive effects like oxygen adsorption, is a relative measure for the amount of species adsorbed. This, in turn, is proportional to the concentration of the species in air since the adsorption time is constant. (Adsorption kinetics can safely be assumed to be of first order in most cases. In particular, this has been shown for CH2Cl2 [4].) Sensitivities in the lower ppm range have thus been obtained for species like benzene, tetrachloroethene, vinylacetate and others [2]. Here, we will show that also the impedance change during adsorption of the organic species can be used for their detection. We will demonstrate that a combination of both detection principles will lead to a sensor which automatically optimizes its sensitivity and response time to the concentration. * Corresponding author. Tel.: + 49 228 550234; fax: + 49 228 686292; e-mail: [email protected] 0925-4005/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S 0 9 2 5 - 4 0 0 5 ( 9 7 ) 0 0 1 8 2 - 2
The adsorption of tetrachloroethene and other halogenated and unsaturated species has recently been studied using differential electrochemical mass spectrometry [5–14] and other techniques. In this study, we use benzene as a model compound because its adsorption has been well studied and its adsorption isotherm on Pt in sulfuric acid (with a maximum coverage between 400 and 500 mV versus NHE) is well known [15]. Comparative results will be presented for tetrachloroethene because of its similar ecological and human toxicologic relevance. Impedance measurements are often used for the characterization of sensor cells; in addition, their possible use for a continuous performance control is discussed [16–19]. Preidel and co-workers [20,21] oxidize glucose at a noble metal electrode covered with a membrane without using biomolecules. The concentration is determined by applying a step-profile-potential to the electrode and superimposing an ac-voltage. On the basis of the real and imaginary component of the impedance at specific potentials, the charge transfer resistance and the capacity which serve for the determination of the glucose concentration are calculated. Biomolecules could also be detected by impedance spectroscopy, since the antibody–antigene binding possibly causes a change of the dielectrical constant or an increasing thickness of the dielectrical layer [22].
32
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
2. Experimental For the experiments with smooth Pt, a standard electrochemical glass cell using the hanging electrolyte method was used. The laboratory design of our sensor cell was described previously [1,2]. All solutions were made of Milli-Q water and, in the case of the glass cell, purged by pure nitrogen; the sulphuric acid and perchloric acid were obtained from Merck (Darmstadt), benzene and tetrachloroethene from Roth (Karlsruhe). We used an impedance measurement system from EG&G (potentiostat 273A, lock-in type 5210) and a simulation software from Boukamp [23]. Special care was necessary to avoid the contamination of the smooth Pt surface in the supporting electrolyte during the relatively long impedance measurements. Therefore, the potential was stepped for 1 min before each measurement at a given frequency to a more positive potential, at which possible contaminants are oxidized. After recording the complete impedance spectrum, the cleanliness was checked again by CV (see [24] for details). The test gases were generated by diluting the corresponding pure gas by the carrier gas (nitrogen or synthetic air). Volume flow rates corresponding to the respective concentrations were adjusted using flow meters (UCAR). For investigations of readily volatile liquid species the carrier gas was bubbled through the respective liquids. The gas saturated with the liquid vapor formed this way was further diluted by mixing with the carrier gas.
3. Results The basic idea of using impedance measurements for the detection of organics using their adsorption on Pt can be explained by the CV in Fig. 1. It compares the (well known) cyclic voltammograms of a polycrystalline
Fig. 1. CV at the smooth Pt electrode in 0.5 M H2SO4 saturated with nitrogen containing 360 ppm benzene and in absence of benzene (n = 100 mV s − 1; reference electrode: NHE).
Pt-electrode in sulfuric acid without and with benzene. It can be clearly seen that hydrogen adsorption is suppressed by adsorbed benzene. It was our aim to use this decrease, determined by electrode capacity measurements, to not only determine the amount of adsorbed species, but also the rate of adsorption and thus its concentration. According to Eq. (1) the variation of the surface coverage U in time is proportional to the concentration c of the adsorbing species in the vicinity of the electrode surface. Furthermore, the relation (Eq. (2)) between the coverage U and the capacity C is valid in the case of the adsorption of organic species [25]: dU/dt= 6ad = kc(1− U)$ kc
for
U 0 (or t0) (1)
U=(C0 − C)/(C0 − Cm)
(2)
dU dC = − (C0 − Cm) k · c = −(C0 − Cm) · dt dt
(3)
where k is the adsorption rate constant, C0 and Cm are the capacities of a free and a completely covered electrode surface. Therefore, at short times, the variation of the capacity in time (dC/dt) is proportional to the concentration c of the substance to detect. For a continuous monitoring of concentrations, the surface has to be cleaned periodically from the adsorbed substance by potential cycles. Various obstacles have to be considered before this can be used in a sensor cell: 1. The rate of adsorption must be high at the potential where the capacity change is highest (or vice versa). Therefore, the potential dependence of the capacity change should be known. 2. The sensor cell (esp. the sensing gas diffusion electrode) should allow meaningful impedance measurements, from which a reliable determination of the electrode capacity is possible. This is not possible with usual amperometric gas sensors, due to their extremely large roughness factor. 3. The determination of the capacity should be possible at a single frequency and should not be disturbed by the large currents due to oxygen reduction. We first studied the potential dependence of the capacity change at smooth, polycrystalline Pt-electrodes because these serve as a reference for the impedance measurements with the sensor electrodes. Therefore, impedance spectra of a smooth Pt-electrode were recorded in a frequency range between n= 0.1 Hz–30 kHz for potentials between Ead = 100 mV and Ead = 500 mV, containing no or 3 · 10 − 5 M benzene. The hydrogen adsorption is so fast at Pt in H2SO4 (in contrast to KOH, see [24]) that it is impossible to separate the hydrogen adsorption capacity from the pure double-layer capacity. Impedance spectra were
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
Fig. 2. The potential dependence of the electrode capacity at the smooth Pt electrode in 0.5 M H2SO4 in benzene (360 ppm in N2) containing solution and in absence of benzene.
simply fitted by an equivalent circuit consisting of a solution resistance Re and a capacity C. The same equivalent circuit could be used to fit the impedance data in the case of benzene-containing solutions, as no charge transfer inhibition or diffusion inhibition were observed. As expected, the potential dependence of the electrode capacity reflects the shape of the cyclic voltammograms, since the current in the CV is proportional to the electrode (pseudo) capacity (Fig. 2). The relative change of the electrode capacity due to benzene adsorption is shown in Fig. 3. It shows a maximum in the potential range between 250 and 300 mV, i.e. in the range of the hydrogen adsorption, where it amounts to about 80%. Obviously due to the displacement of adsorbed hydrogen, the hydrogen adsorption capacity decreased more strongly because of adsorbed benzene than the pure double-layer capacity. This can be understood by taking into consideration that theoretically the hydrogen adsorption should decrease by 100% in the case of a full monolayer (Cm = 0 in Eq. (2)), whereas the double layer capacity only
33
decreases due to the larger distance of ionic charge carriers from the electrode surface. At more cathodic potentials the change of the capacity gets smaller again, as here benzene is desorbed anew to some extent [11]. Fig. 3 also shows the relative change in capacity of a porous Pt electrode in the sensor cell (¥ = 8 mm, 5 M H2SO4). The sensor was exposed to air or 360 ppm benzene in air, respectively. In order to fit the impedance spectra recorded in the sensor cell, the equivalent circuit (series circuit of Re and C) has to be supplemented with a resistance Rp in parallel to the capacity. The resistance Rp symbolizes the charge transfer resistance due to the oxygen reduction (EB0.9 V). It is important to note that the determination of the electrode capacitance is not hindered by the large oxygen reduction current (typically 10–20 mA cm − 2). Therefore, even in presence of synthetic air a relative change of the capacity of 80% can be obtained. This result reproduces the value at the smooth electrode. The potential of the maximum sensitivity is found somewhat more cathodically than at the smooth Pt electrode because of the IR-drop. The IR-drop results from the oxygen reduction current and the series resistance Re ($ 5 V), which is only in part due to the solution resistance, but mainly due to the resistance of the thin Pt film of the sensing electrode. For a working sensor, recording of a complete impedance spectrum would be too time-consuming. Instead, a signal obtained at a single frequency has to be used. This can be done as follows: Eqs. (4) and (5) give the imaginary and real parts of the electrode admittance (which is the reciprocal of the impedance). vC (Re/Rp + 1)2 + (vReC)2
(4)
(1/Rp)(Re/Rp + 1)+ (1/Re)(vReC)2 (Re/Rp + 1)2 + (vReC)2
(5)
Yim = Yre =
For Rp Re and Re 1/vC these equations reduce to Yim $ vC
(6)
and Yre $ 1/Rp
Fig. 3. The potential dependence of the relative change of the capacity due to the adsorption of benzene at the smooth Pt electrode and at the sensor electrode (¥ =8 mm, 360 ppm in nitrogen or synthetic air, respectively).
(7)
i.e. at low frequencies, when 1/vC becomes large, the capacity is given by the imaginary part of the admittance. In Fig. 4, the so-called ‘real part of the capacity’, which is defined as Yim/v, is plotted versus frequency on a logarithmic scale. It can be seen that the above relation holds up to about 20 Hz, both with and without benzene. Therefore, it is possible to determine the electrode capacity by superimposing an ac voltage of, e.g. 10 Hz on the electrode potential and measuring the imaginary part of the ac current, which is proportional to the admittance. It is crucial, however, that Re
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R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
tween the first and the last sweep at a given potential (e.g. 1.44 V) serve as a sensor signal for the potentiodynamic sensor. Using a special electronic circuitry, the ac signal is only superimposed on the dc potential during the adsorption period. Otherwise, the detection of the oxidation current transients is disturbed. Fig. 6 demonstrates the shape of several successive imaginary current transients for long adsorption times without benzene in air and during a sudden exposure to a high benzene concentration. A slight decrease of the imaginary current in air is due to impurities. When benzene exposure starts, it suddenly decreases to the value of corresponding to saturation, i.e. by nearly 80%. When the benzene exposure is stopped, the deFig. 4. A section of the admittance spectra in benzene-containing solution (360 ppm benzene in synthetic air) and in benzene-free solution recorded at the sensor electrode (¥ = 8 mm, Pt catalyst, 5 M H2SO4) at the potential of the maximum sensitivity (Ead = 200 mV).
is low enough. An increase by a factor of 10 due to low electrolyte conductivity or bad electrode conductance would lead to a decrease of the upper frequency limit to about 1 Hz, making the determination of the imaginary part of the current by a look-in amplifier time-consuming and thus decreasing the response time. It is not sufficient to simply detect the overall ac current since this is largely influenced by the faradaic current due to oxygen reduction (Yre $1/Rp). As described above, the change in electrode capacity per unit of time is proportional to the concentration of the species to be detected. For practical applications, the sensor electrode has to be cleaned periodically by potential cycling, and the change in capacity in between the potential cycles has to be recorded. Because of this, we combined this procedure with our previously described potentiodynamic detection principle [1,2] (Fig. 5). During an adsorption period tad, the substance (tetrachloroethene in this case) is allowed to adsorb at a constant adsorption potential Ead. A small ac-voltage is superimposed during this period. The imaginary part of the ac current is simultaneously detected and decreases due to adsorption of the organic species as described above (Eq. (3)). Afterwards, the adsorbate is oxidatively desorbed in several potential sweeps from 0.8 – 1.45 V. Several sweeps are applied to ensure that desorption is complete. Short potential pulses to 0.1 or 0.0 V are applied for 0.5 s after each anodic sweep to reduce the adsorbed oxygen. After six sweeps, a new adsorption period begins. The dc current transients during the anodic sweeps (Fig. 5c) are due to oxygen adsorption. During the first of these (and to a smaller extent also during the second) they are also due to the oxidation of the organic adsorbate. As described before [1,2], the difference be-
Fig. 5. (a) The specific potential program exemplary for the detection of C2Cl4 in synthetic air (c =180 ppm) at the sensor electrode (Pt catalyst, ¥= 6 mm, 1 M HClO4) using the following parameters: dE/dt =300 mV s − 1, tad =20 s, tred =0.5 s, Ead =100 mV, Ered = 0 V, Eac =10 mV, n= 10 Hz; (b) the response of the imaginary component of the ac-current; (c) the response of the oxidation current according to the potentiodynamic procedure.
R. Oelgeklaus, H. Baltruschat / Sensors and Actuators B 42 (1997) 31–37
Fig. 6. Change of sensor electrode capacity during a sudden exposure to 180 ppm benzene (¥= 8 mm, 5 M H2SO4, dE/dt= 300 mV s − 1, tad = 40 s, tred =0.5 s, Ead = Ered = 0.2 V, Eac = 10 mV, n =8 Hz).
crease becomes less noticeable. (Due to the high benzene concentration used in this experiment, it takes some time before the concentration of benzene in air drops to zero again.). The initial slope dIim/dt of the imaginary current transient should be proportional to the concentration of the adsorbing species. Alternately, it should be possible and easier to use the difference of the currents at the beginning (i.e. at the maximum) and the end of the adsorption period. This should be a good approximation provided the adsorption period is not too long. The signals usable in such a sensor device therefore are: Signal 1:
dIim/dt
Signal 2:
DIim
Signal 3:
DIox/Iox5
35
Fig. 7. The response characteristics of the sensor (Pt catalyst, ¥ = 6 mm, 1 M HClO4) for a benzene/synthetic air mixture for Signal 1, 2, and 3 (concentration specifications in ppm). dE/dt = 300 mV s − 1, tad =20 s, tred =0.5 s, Ead =200 mV, Ered =0 V, Eox 1.45 V, Eac = 10 mV, n =10 Hz.
those adsorption times and concentrations for which the surface coverage of the organic adsorbate becomes appreciable (see Eq. (1)). Signal 1, on the contrary, is directly proportional to the initial adsorption rate and therefore to the concentration, since it is determined at short adsorption times corresponding to low coverages. The saturation effect is even more pronounced for Signal 3. This may be due to the fact that it is not obtained from an evaluation of the total oxidation
Initial slope of the time dependence of the imaginary component of the ac-current The difference of the imaginary components of the ac-current at the beginning and the end of the adsorption phase The difference of the oxidation currents of the first and the fifth cycle standardized to that of the fifth cycle (Potentiodynamic procedure, cf. above and in [1])
Fig. 7 compares the response of these different signals to various concentrations of benzene in air. The response time (35 s) is determined by the adsorption time (20 s) and the potential sweep rate (300 mV s − 1) during the oxidation cycles. While the initial slope value of the ac-current signal increases linearly with the concentration, Signal 2 and Signal 3 show a saturation characteristic Fig. 8a. Since Signal 2 should be proportional to the coverage at the end of the adsorption phase, this is expected for
Fig. 8. The concentration dependence of Signal 1, 2, and 3 for a mixture of (a) benzene (data taken from Fig. 7); (b) tetrachloroethene in synthetic air (same parameters as in Fig. 7, except Ead =100 mV).
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change, but only for the current, i.e. the oxidation rate at a given potential. The concentration dependence for tetrachloroethene is demonstrated in Fig. 8b, confirming the results for benzene.
cies like CO would be adsorbed, and their presence would decrease the adsorption time. However, since their adsorbate is oxidized below 1 V, CO could not simulate the presence of benzene or tetrachloroethene.
Acknowledgements 4. Discussion We have demonstrated that by using an appropriate sensor design meaningful measurements of the sensor electrode impedance are feasible. It is possible to determine the electrode capacity using an impedance measurement at a single frequency even in the presence of oxygen, i.e. during high faradaic reduction currents. The decrease of the double layer capacity due to the adsorption of organic species can be monitored during adsorption using the imaginary part of the ac current. Using the same dc potential program as used for a potentiodynamic sensor, it is thus possible to sense organic species. By itself, such a sensor is not very selective. Many other species which also adsorb at Pt would also lead to a decrease of electrode capacity. On the other hand, the potentiodynamic sensor which uses the oxidation current of the species as the sensor signal (Signal 3= DIox/ Iox5) is much more selective because only a limited class of volatile organic species are oxidized at the same potential as benzene or tetrachloroethene. In particular, ethanol and methanol (including their adsorbate) are oxidized at much lower potentials and are therefore recognized. However, this potentiodynamic sensor has one major disadvantage: its limited dynamic range, due to saturation of the surface with adsorbate at high concentrations and long adsorption times (which are used to obtain a high sensitivity). We, therefore, propose to combine both detection principles in one sensor: The adsorption time is not fixed, but determined by the decrease of the imaginary part of the ac current: that the adsorption period should be terminated as soon as Iim drops to, e.g. 10% of its initial value. The normalized oxidation current (sensor Signal 3), divided by the variable tad, would still serve as the sensor signal. Such a sensor has the following characteristics: In the absence of adsorbable species or at low concentrations, the adsorption time is high, leading to high sensitivities. The corresponding slow response is acceptable. At high concentrations, tad automatically becomes low, corresponding to a short response time. A sudden increase in concentration during the adsorption period (Fig. 6) is immediately detected and leads to a fast interruption of the adsorption. Saturation of the surface with adsorbate does not occur, leading to a wide dynamic range of the sensor. The selectivity, on the other hand, is as good as for the ‘simple’ potentiodynamic sensor. Spe-
This work was supported by BMFT (grant number 01-VQ 9012/7).
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Biographies Rainer Oelgeklaus studied physics at Mu¨ster University between 1984 and 1990. In 1990, he became a
.
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Master of Science with a thesis on the detection of the local density of states using energy loss spectra, with Professor L. Reimer, Institute of Physics, Mu¨nster University. In 1995 he got his PhD with a thesis on analysis of adsorption kinetics at poly- and monocrystalline Pt-electrodes analysed by impedance spectroscopy. Since December 1995, he has been teacher-training to qualify as a teacher for secondary school in physics and chemistry. Helmut Baltruschat studied chemistry at Marburg and Bonn University between 1972 and 1981. Between 1976 to 1977 he was at the University of Paris VII with a scholarship by the French Government and DAAD– German Academic Exchange Service. In 1981, he became a Master of Science with a thesis on the kinetics of silver dissolution in cyanide-containing solutions, with Professor W. Vielstich, Bonn University. In 1985 he got his PhD with a thesis on surface-enhanced Raman spectroscopy, with Professor J. Heitbaum, Bonn University. During 1985–1986 he had a postdoctoral fellowship in Professor Hubbard’s research group at the University of California/Santa Barbara (analysis of electrosorbates at Pt-single crystal electrodes using auger electron spectroscopy and LEED). From 1986– 1993, he was a research assistant at the Institute of Physical Chemistry at the University of Witten/ Herdecke. In November 1992, he received ‘Habilitation’ with a thesis on the characterization of electrochemical adsorbates by non-traditional methods. Since December 1993 he has been a Professor of Physical Chemistry (Electrochemistry) at the Institute of Physical and Theoretical Chemistry at Bonn University.