Belousov Zhabotinsky reaction

Transcription

1 22 September 2 Ž. Chemical Physics Letters Comparative study of chemical waves and temporal oscillations in the Ruž bpy/ 2q 3 -catalyzed photosensitive Belousov Zhabotinsky reaction Toshimasa Akagi a,), Noriaki Okazaki b, Tatsuo Yoshinobu c, Takeko Matsumura-Inoue a a Department of Chemistry, Nara UniÕersity of Education, Takabatake, Nara , Japan b National Institute for Research in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 35-44, Japan c The Institute of Scientific and Industrial Research, Osaka UniÕersity, 8-1, Mihogaoka, Ibaraki, Osaka , Japan Received 26 July 2; in final form 8 August 2 Abstract Properties of the chemical waves emerging from the mask edge were investigated for the RuŽ bpy. 3 2q -catalyzed Belousov Zhabotinsky reaction under steady illumination while varying the initial concentration of malonic acid ŽwMAx. systematically. The wave velocity, the wavelength and the thickness of the oxidized band were found to decrease monotonously with wma x. The obtained results were qualitatively interpreted based on the temporal oscillations measured in a batch reactor. A key effect caused by increasing wmax was identified to be the enhancement of the reduction rate of RuŽ bpy. 3q. q 2 Published by Elsevier Science B.V Introduction The photoresponse of the Belousov Zhabotinsky X Ž BZ. reaction catalyzed by trisž2,2 -bipyridine.- rutheniumž II. ion ŽRuŽ II.. wx 1 has been the subject of intensive studies in recent years. Photo-induced biw2,3x and photo- furcations such as photo-induction inhibition w2,4 6x of chemical oscillations in wellstirred homogeneous solutions were discovered. The primary light absorber in the visible region was identified to be RuŽ II. from the action spectrum studies w4,7 x. The effect of light on the spatiotemporal behavior in an unstirred thin-layer system has also been attracting much interest because of the ) Corresponding author. akagit@eva.hi-ho.ne.jp possible application to new photochemical imagew8,9x demonstrated processing devices. Kuhnert et al. that the photosensitive BZ system could be used as a photochemical memory device. The photo-induced pattern formation w1x and photo-induced retardation of wave propagation w11x were reported. Light illuw12x and con- mination was also used for initiating trolling w13,14x spiral waves. Some photochemical processes are proposed w4,5,7,8,14 18x and modeled w2,21x to account for the photo-illumination effects. Although the exact reaction mechanism is still indefinite, both Br y and HBrO2 are considered to be photo-produced more or less depending on the experimental conditions. Recently, we studied the non-uniform illumination effect on the thin-layer RuŽ II. -catalyzed BZ rr$ - see front matter q 2 Published by Elsevier Science B.V. Ž. PII: S

2 system and succeeded in controlling the chemical wave generation by partially blocking the light with a black mask w22 x. Under appropriate light intensity, the chemical waves emerge only from the mask edge and spread outward into the illuminated area. The phenomenon was termed the mask effect. We have also discovered that the characteristics of the chemical waves such as wave velocity, wavelength and oxidized-band width can be effectively controlled by the initial concentration of malonic acid Ž MA.. The controllability of wave initiation as well as wave properties by simply tuning the light intensity andror the chemical composition should be a great advantage in the practical design of image-processing devices with desired functionality. In this context, it is an important subject to clarify the role of MA in the properties of chemical waves under light illumination. In this Letter, we report on a comparative study carried out for the chemical waves generated by the mask effect and the temporal oscillations in a batch reactor while changing the initial concentration of MA systematically. Characteristic features of the chemical waves and their dependence on the initial concentration of MA are discussed based on the temporal oscillations measured in a batch reactor. T. Akagi et al.rchemical Physics Letters Experimental 2.1. Materials The RuŽ bpy. Ž ClO was prepared by the microwave method w23 x. Reagent grade NaBrO Ž Wako. 3 was recrystallized from ethanol. Other chemicals, CH Ž COOH., NaBr and H SO Ž Wako of reagent grade were used without further purification. All of the stock solutions were prepared with distilled water and were deoxygenated with N Chemical waões in a thin-layer system The experimental setup for the thin-layer BZ system is shown in Fig. 1. The BZ-reaction mixture with a layer thickness of 1 mm was placed in a Petri dish. The initial concentrations of the reactants are given in Table 1. The sample was illuminated from Fig. 1. Experimental setup for the thin-layer BZ system. the bottom by a 2-W halogen lamp through a blue filter centered at 48 nm Ž HOYA, B48.. The light intensity was fixed at 4.7 mwrcm 2. A circular light mask with a diameter of 5 mm was placed immediately beneath the Petri dish. The chemical waves were monitored by a CCD camera Ž Victor, TK-17. mounted on a binocular microscope Ž Nikon, SMZ-U., and the images were stored on a video recorder Ž Toshiba, A-79HFD.. The recorded images were digitized by a video capture board ŽMicrotechnica, MT981-FMM. and were processed on a personal computer Ž NEC, PC-981 FA. for the region of 17.5=15.8 mm 2. The temperature was regulated at 188C.

3 216 T. Akagi et al.rchemical Physics Letters Table 1 Chemical compositions employed in this study Reactant Chemical Temporal waves oscillations wruž bpy. Ž ClO. x Ž mm wnabro x Ž M wmax Ž M wnabrx Ž M..8.8 wh SO x Ž M Temporal oscillations in batch reactor A quartz optical cell with an optical path length of 1 mm was used as a reactor. The cell was placed in a thermostated water bath Ž Lauda, K4R. to keep the reaction temperature constant at 258C. A reaction mixture Ž 3 ml. containing H 2SO 4, NaBrO 3, MA, NaBr and RuŽ II. was stirred by a magnetic stirrer at 5 55 rpm. The initial concentrations are employed as listed in Table 1. A platinum electrode was used as an indicator electrode in combination with an AgrAgCl reference electrode connected through a Teflon-tube salt bridge filled with saturated KNO - 3 agar solution. The platinum-electrode potential, E Pt, was continuously measured by a digital voltmeter Ž Yokogawa, and was recorded on a personal computer Ž NEC, PC-981FA. for further analysis. The potential EPt is related to the concentration ratio wruž III.xrwRuŽ II.x through the Nernst equation E se q59 log RuŽ III. r RuŽ II. wmvx Ž 1. Pt f Ž. where Ef is the formal redox potential of the RuŽ II. rruž III. redox pair. The value of E was f estimated to be 126 mv by the titration of RuŽ II. solution with Ce 4q standard solution. Note that the conservation condition RuŽ II. q RuŽ III. s RuŽ II. Ž 2. should hold at all times, where suffix represents the initial concentration of the reactant species. The existence ratio of RuŽ II., g swruž II.xrwRuŽ II. ll, was calculated from the measured platinum potential using Eqs. Ž. 1 and Ž Results 3.1. Phenomenology of the mask effect in a thin-layer BZ system Fig. 2 shows typical examples of the mask effect in an illuminated thin-layer BZ system. When a part of the thin-layer solution is masked against illumination by a black film, the chemical waves emerge only from the mask edge at a constant interval and propagate toward the illuminated area to display a single target pattern. Photo-production of Br y is considered to take place predominantly in a thin-layer BZ solution w19 x; therefore the illumination makes the system bifurcate into the reduced steady state with moderate excitability, whereas the masked dark area remains in an oscillatory state. Thus, the mask can act as a sole pacemaker for triggering chemical waves. With stronger light illumination, the propagation of chemical waves is completely inhibited. Fig. 2. Photo images of the chemical waves emerging from the mask edge: wmax sž A..6 M, Ž B..18 M. Other initial concentrations are: wruž bpy. 2q x s.6 mm, wnabro x s.15 M, wh SO x s.8 M and wnabrx s.8 M

4 T. Akagi et al.rchemical Physics Letters The characteristics of the generated chemical waves such as wave velocity, wavelength and width of the oxidized band are found to be sensitive to the initial concentrations of the reactants and the light intensity. In particular, the nature of chemical waves under illumination is found to be extremely sensitive to the initial concentration of MA. At low wma x, the chemical waves exhibit a thick oxidized band with a sharp wavefront and a relatively broad waveback. With increasing wma x, the waveback becomes sharper and the thickness of the oxidized band becomes narrower, resulting in a sharply contrasted Fig. 3. Dependence of wave properties on wma x : Ž. a wave velocity, Ž. b wavelength and Ž. c period of wave generation at the mask edge. Initial concentrations other than wmax are the same as those in Fig. 1.

5 218 T. Akagi et al.rchemical Physics Letters photo image Ž Fig. 2B. compared to the case of low wmax Ž Fig. 2A.. Other characteristic parameters of the chemical waves are plotted as a function of wmax in Fig. 3. As can be seen from the figure, the wave velocity, wavelength and period of wave generation are all found to decrease monotonously with wma x. The critical light intensity for the inhibition of wave propagation has also been found to decrease with wma x, indicating that photosensitivity of the system is enhanced by the increase in wma x BehaÕior of the batch-mode reaction Experiments in a batch reactor have been carried out while changing wmax systematically. In Fig. 4, the oscillatory amplitude of g Ž ll the existence ratio of RuŽ II.. is plotted against wma x, where the oscillatory amplitude is measured 1 min after the reaction has started. Within the time range from 1 to 2 min after starting the reaction, no remarkable change in the oscillatory waveform has been observed irrespective of the wmax employed in this study. At wmaxs.3 M, the system does not oscillate and virtually all of the Ru complex remains in the form of RuŽ III., suggesting that the concentration of MA andror BrCHŽ COOH. Ž BrMA. 2 is not sufficient to reduce RuŽ III. to RuŽ II.. At wmax s.6 M, the g ll exhibits small-amplitude oscillations in the range of 1 18%. Under these low MA concentrations, the Fig. 4. Variation in the oscillatory amplitude of g Ž ll the existence ratio of RuŽ II.. with wma x. Experimental conditions are given in Table 1. oscillations take place with the predominance of RuŽ III.. With increasing wma x, the oscillatory maximum of g ll rapidly increases, leading to an increase in oscillatory amplitude. The oscillatory amplitude becomes maximum at around wmax s.18 M. A further increase in wmax causes a gradual increase in the oscillatory minimum of g ll, and the oscillatory amplitude again tends to shrink. At wmaxs.4 M, the g ll oscillates in the range of 9 1%. With high wma x, the system shows oscillations with the predominance of RuŽ II.. The oscillatory period has been found to decrease monotonously with wma x, mostly due to the enhancement of the reduction rate of RuŽ III. as discussed below. 4. Discussion The properties of the chemical waves triggered by the light mask and their dependence on wmax are discussed below on the basis of the results obtained for the batch-mode reaction WaÕe Õelocity It is well-known that the velocity of chemical waves in a dark BZ system is predominantly determined by the square root of the concentration prody uct of BrO and H q w24x 3 and is almost independent of wma x. In this context, the relatively strong depenwmax observed in this dence of wave velocity on study may seem rather peculiar. In an illuminated thin-layer BZ system, it has been reported that an increase in light intensity causes the retardation of wave propagation w11 x. This phenomenon has been ascribed to the inhibitory effect of photo-produced y Br from BrMA w11,19 x. In our system, the increase in wmax makes the average concentration of RuŽ II. higher as has been confirmed by the batch experiments. Because the light absorbing species in the RuŽ II. -catalyzed BZ reaction is RuŽ II., an increase in wruž II.x makes the system more photosensitive. Furthermore, the concentration of BrMA should increase with wma x, which promotes the photo-production of Br y from BrMA. We believe that the decrease in wave velocity with wmax can be reasonably explained by the enhanced photo-production of Br y due to the increase in wruž II.x and wbrma x.

6 T. Akagi et al.rchemical Physics Letters Table 2 Maximum oxidationrreduction rate in the temporal oscillations wmax Oxidation rate Reduction rate Ž M. Ž mvrs. Ž mvrs WaÕelength The wavelength is determined directly from the wave velocity and the triggering period of the pacemaker. Because the solution under the light mask resides in an oscillatory state, the wave-generation interval should correspond to the oscillatory period of a batch-mode reaction in darkness. Hence, the triggering interval is expected to decrease with wma x. In addition, the wave velocity tends to decrease with wmax as stated above. Accordingly, the wavelength becomes narrower with increasing wmax to give densely distributed wave trains Thickness of the oxidized band The width of the oxidized band is considered to vary depending on the rates of redox reactions of RuŽ II. rruž III. and the wave velocity. The oxidation of RuŽ II. to RuŽ III. occurs at the wavefront, and the reduction of RuŽ III. to RuŽ II. takes place at the waveback. Because the former process is an autocatalytic reaction that generally gives rise to a sharp wavefront, the latter reduction process is more important as a determining factor of the oxidized-band width. In Table 2, the maximum rates of the redox reactions of RuŽ II. rruž III. are compared for various wmax measured in a batch reactor. As can be seen from Table 2, the reduction rate rapidly increases with wma x; the reduction rate becomes about one order of magnitude faster as wmax is increased from.6 to.24 M. Besides this, the decrease in wave velocity with wmax may also contribute to the narrowing of the oxidized band. Hence, the increase in wmax makes the wave back sharper, leading to a fine oxidized-band structure. In the practical application of BZ reaction to the image-processing devices, the fine band structure with high wmax seems to be favorable for improv- ing spatial resolution. However, excess increase of wmax in turn leads to the shrinkage of oscillatory amplitude and thus the degradation of image contrast in the chemical waves. A good image with clear contrast and sufficiently high resolution can be obtained under the optimal condition where the oscillatory amplitude takes the maximum Že.g., wmax s.18 M in this study.. 5. Conclusions Properties of the chemical waves generated at the mask edge were investigated for the RuŽ II. -catalyzed BZ reaction under steady illumination while changwmax systematically. The wave velocity, the ing wavelength and the thickness of the oxidized band were found to decrease monotonously with wma x. Qualitative explanations were given for these tendencies based on the behavior of batch-mode temporal oscillations. A key effect caused by increasing wmax was identified as the enhancement of the reduction rate of RuŽ III.. Finally, we would like to emphasize the high controllability of the chemical waves in the RuŽ II. - catalyzed BZ system realized by the mask effect. The feasibility of generating stable chemical waves from a desired point at a desired frequency should be a great advantage in designing chemical circuits w25 x. The high sensitivity of the oxidized-band width to wmax may be utilized for optimizing some kind of image processing operations such as contour extraction and contrast enhancement. Thus, the mask effect is expected to increase its importance in practical applications of the photosensitive BZ reaction. References wx 1 J.N. Demas, D. Diemente, J. Chem. Educ. 5 Ž wx 2 V. Gaspar, G. Bazsa, M.T. Beck, Z. Physik. Chem. Ž Leipzig. 264 Ž wx 3 Y. Mori, Y. Nakamichi, T. Sekiguchi, N. Okazaki, T. Matsumura, I. Hanazaki, Chem. Phys. Lett. 211 Ž wx 4 P.K. Srivastava, Y. Mori, I. Hanazaki, Chem. Phys. Lett. 19 Ž wx 5 M. Jinguji, M. Ishihara, T. Nakazawa, J. Phys. Chem. 96 Ž

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