Shifting and Switching between Chemical Steady States through

J. Phys. Chem. 1994,98, 3927-3929

3927

Shifting and Switching between Chemical Steady States through Electrode Processes G. Dechert and F. W. Schneider; Institute of Physical Chemistry, University of Wurzburg, Marcusstrasse 911 1, D- 97070 Wiirzburg, Germany Received: December 28, 1993; In Final Form: February 24, 1994@

Transitions between two steady states in the bistability region of the Belousov-Zhabotinsky (BZ) reaction have been induced by the application of an electric current. The current causes redox processes which produce the perturbing species on Pt electrodes in two electrically coupled stirred flow reactors. One reactor is the anode while the other is the cathode, depending on the polarity of the applied current. On the thermodynamic branch outside the bistability region at low flow rates the steady state is shifted toward higher redox potentials in the anodic reactor proportional to the current flow. On the kinetic branch outside the bistability region at high flow rates the system is shifted by a threshold current to a lower steady state in the cathodic reactor. On both branches the system returns to its original steady states when the current is switched off, whereas in the bistability region the system remains on a given steady state.

Introduction

Bistability, the existence of two stable states for the same set of conditions,l is well-known in the Belousov-Zhabotinsky (BZ) reaction2 and other nonlinear systems.3-8 We demonstrate that an electrical current induces transitions between steady states in the bistable region (Figure 2) when the BZ reaction is conducted in a continuous flow stirred tank reactor (CSTR). This is caused by the application of a dc voltage to two platinum electrodes which are immersed in two identical electricallycoupled reactors. One BZ reactor represents the anode and the other BZ reactor represents the cathode compartment which are separated by a salt bridge. Redox processes occur at each electrode which produce or remove the perturbing species. In the specific case of the BZ reaction the electrode process in the cathode reactor involves mainly the reduction of Ce4+to Ce3+ whereas oxidation in the anode reactor produces Ce4+ from Ce3+.9 With this experimental setup the effect of concentration perturbations on the steady states in each of the two well-stirred reactors may be studied separately. During the electrochemicalperturbation the identical flow rates through the two reactors remain constant. Furthermore, mass is conserved; i.e., the sums of particular redox species, like Ca3+ and Ce4+, remain constant throughout the perturbing process.lG14

Experimental Section The experimental setup of the two electricallycoupled CSTRs (6.4-mLvolumeeach) is shown in Figure 1. The reactors contain Pt working electrodes (1) (3.4-cm2surface) which are connected to a potentiostat (2) supplying a constant potential at a given current. Mass exchange between the reactors is avoided by using a salt bridge (9,through which a solution of 1.5 M H2SO4 flows constantly. Permeability between both CSTRs and the salt bridge is guaranteed by Teflon membranes (4) (1-2-pm pore size). Potential changes are measured with a Pt/Ag/AgCl reference electrode (3) inserted in each reactor. The signals are registered with a two-channel x-t chart recorder and a computer where the data are collected at 1 Hz. Due to the variations in the sensitivity of the redox electrodes,we present a normalized output in arbitrary units. The latter output is obtained by an appropriate adjustment of the difference of signals by amplification. Each reactor is fed by three reactant feed streams,deliveredby a precisethree-channel syringe pump. The feed streams enter through the bottom of the CSTRs. They contain the following solutions: (a) 4.5 M H2-

* To whom correspondence should be addressed. 8

Abstract published in Aduance ACS Abstracts, April 1, 1994.

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Figure 1. Two electricallycoupled CSTRs: (1) working Pt electrodes, (2) potentiostat, (3) Pt/Ag/AgCl reference electrodes in each reactor, (4) Teflon membranes(1-2-pm pore size),and (5) salt bridge with sulfuric acid solution (1.5 M) which flows from the bottom to the top. Three inlets for the reactants are placed at the bottom of each CSTR. The solutions are thermostated at 25 i 0.2 OC and stirred with a Tefloncoated magnetic stirrer (6) at 600 rpm. The arrows indicate the outflow of solution.

SO4,(b) 6.0 X le3 M KBr03, and (e) 1.0 X lW3 M Ce2(S04)3 and 0.1 M malonic acid. To obtain the reactor concentrations, divide by 3. The malonic acid was recrystallized from acetone.15J6 The other reactants were used without further purification. The experiments were carried out at 25.0 f 0.2 OC at a stirring rate of 600 rpm using a magnetic stirrer (6).

In the present system, the bifurcation parameter is represented by the flow rate kf and the order parameter is proportional to the Ce4+ concentration. The bifurcation diagram (Figure 2) representing the steady states is determined by starting the measurements of the redox potentials either on the thermodynamicbranch A or on the kinetic branch C. The flow rate is then raised or lowered in steps after 3-4 residence times have been allowed to elapse at each kf value to ensure that a particular steady state has been reached. The transition from A to C occurs in the bistability region B in a narrow interval around kf = 0.0014 f 2.6 X 10-5s-1 for ascending flow rates. The down transition from C to A was observed at 0.0008 f 2.7 X 10-5s-l. Thus, a hysteresis loop is observed (Figure2). The bistabilityregion B is independent of the number of traversal times. 0 1994 American Chemical Society

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3928 The Journal of Physical Chemistry, Vol. 98, iuo. 15, 1994

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Effect of Electric Current. The main redox processesoccurring at the surface of the Pt working electrodes are the oxidation of Ce3+in the anodicreactor and the reduction of Ce4+in the cathodic reactor? The electrodereactions produce concentrationchanges of the above species as long as the electric current flows. The measured potentialsare proportionalto the logarithm of the Ce4+/ Ce3+ ratio in the homogeneous reaction medium. The absolute value of the Ce4+concentration may be determined by spectroscopic means (absorption of Ce4+ at 350 nm) in an individual reactor. The response of the reacting system to the electrical current may be classified into three regions. Thermodynamic Branch A. When the electric current is set to -61 pA (point b, Figure 3), the reference potential shows a rapid rise and overshoot, finally reaching a value of 170arbitrary

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units in the anodic reactor (Figure 3a) at the steady state ( k f = 5.74 X 10-4 s-l). Subsequent settings of the electric current at -31 pA (point c) and -121 pA (point d) lead to steady states represented by a voltage of 90 and 290 arbitrary units, respectively. Turning off the current (point a) restores the original chemical steady state at 30 arbitrary units. Thus, by application of an electrical current, the steady state may be shifted to higher Ce4+values. The cathodic reaction is hardly affected by the electrical current (Figure 3b, 75 arbitrary units), since the thermodynamic branch is characterized by negligible concentrations of Ce4+. Region of Bistability B. The application of a threshold value of the anodic current (>I240 PA))causes a transition (point bl, Figure 4a) from the lower steady state to the upper steady state where it remains even after the current has been turned off at point a (Figure 4). In order to reverse this switching process at the constant flow rate of kf = 1.14 X 10-3 s-1, the polarities of the two working electrodes are reversed such that the anode becomes the cathode that reduces Ce4+to Ce3+ (point b2). Here a transition is induced from the upper to the lower steady state. Turningoff the current (point a) does not affect the redox potential in the lower steady state (150 arbitrary units). Similarly, in the other reactor (Figure 4b) the application of a cathodic current (point bl) to the lower steady state does not affect the measured redox potential. However, a switch in polarity causes this reactor to be the anode, and a transition to the upper steady state occurs (point b2). The chemical reaction remains in the upper Ce4+ steady state after the current has been turned off (point a). The transitions from the lower (upper) to the upper (lower) state show threshold values of the applied voltage that decrease (increase) with increasing flow rate in the bistable region. Kinetic Branch C. Figure 5a showscurrent-induced transitions on the kinetic branch C to a steady state of lower Ce4+ concentration (state X)which is stable only during the duration of current flow in the cathodic reactor (kf= 1.52 X le3 s-l). A

The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 3929

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threshold value (- 240 PA) of the electric current was observed for the establishment of the lower steady state X. When the current is turned off (point a), the chemical reactions rapidly shifts back (with an overshoot) to its upper steady state without any sign of hysteresis. On the other hand, the anodic reactor (Figure 5b) does not show any changes during oxidative current flow as expected, since the upper steady state is “buffered” kinetically and the electrochemically produced Ce4+is readily used up in the chemical reaction. Discussion

In the absence of a chemical reaction, the overpotential of a solutioncontaining Ce4+and Ce3+ions will follow the well-known Butler-Volmer equation if a capillaryelectrodecloseto the surface of the working electrode is used instead of a reference electrode. A Tafel plot will lead to the exchange current density and the symmetryparameter.” In the present system,however, the redox potentialis measured during the chemical reaction in well-stirred reactors containing reference electrodes. In the presence of the BZ reaction large perturbationscause switchingprocesses between stable steady states in the bistability region when the polarity of the current applied to a Pt electrode is reversed. This switching

process takes approximately 2-3 residence times to reach a given steady state. On the kinetic branch outside the bistability region the steady state X is stable only when the cathodic current is on. This indicates that cathodic reduction produces Ce3+from Ce4+, which inhibits the autocatalytic production of HBr02. When the current is turned off, the reaction returns from steady state X to its previous steady state with a pronounced overshoot. Overshoots have been studied for systems without redox On the thermodynamic branch higher Ce4+ concentrations than steady state may be obtained by oxidation in the anodic reactor while the electric current is turned on. Here a rate increasein the autocatalytic process by the speciesproduced (by the anodic current) leads to an increase of Ce4+ without reaching any defined steady state since the system is below its bistability region. The switching and shifting process in the bistability region and on the kinetic branch are characterized by a threshold in the applied current (Figures 4 and 5 ) . It is noted that the levels of Ce4+ concentrations are reversed on the thermodynamic and kinetic branches of the minimal bromate ~ystem.l~ In*the ~ ~present BZ system the Ce4+concentration of the kinetic branch will also approach zero at very high flow rates since Ce3+ is in the inflow. Thus, electrode processes may induce transitions or shift chemical steady states. Such processes are of interest in the construction of simple chemical computers.23 Acknowledgment. We thank the Volkswagen Stiftung, the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie for partial support of this work. We thank M. Hauser, K.-P. W. Zeyer, and D. Lebender for useful discussions. References and Notes (1) De Kepper, P.; Boissonade, J. In Oscillations and Traveling Waves in ChemicalSystems;Field, R. J., Burger, M., Eds.; Wiley: New York, 1985; p 223. (2) De Kepper, P.; Rossi, A.; Pacault, A. C. R. Acad. Sci. Paris, Ser. C

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