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J . Phys. Chem. 1985, 89, 5178-5179
substituted ana10gues.l~ In Table I the frequencies and assignments of the RRS spectrum of Ag(I1)TPP are also included for comparison purpose. The table indicates that all the samples give the band mainly due to the C,-C, stretching vibration near 1540 cm-], the band mainly due to the C,-N stretching vibration near 1340 cm-I, and the band ascribable to the porphine ring deformation vibration around 400 cm-I. The similarity in frequency between the corresponding bands clearly demonstrates that Ag(II)TPP, Ag(II)TPyP(2), Ag(II)TMPyP(4), and Ag(I1)TMPyP(2) in the layered structure take a virtually identical backbone structure. By comparing the SERRS spectra of the porphines in str I, it is known that the extent of the conversion to the corresponding Ag(I1) complex is roughly in the order TMPyP(4) = TMPyP(2) > TPyP(2) > TPyP(4) > TPP, reflecting the order of the reactivity of each porphine to the Ag atoms. According to Hengline,14 atomic silver and silver clusters possess unusual redox properties; e.g., the standard potential of a single Ag atom in equilibrium with Ag ions is -1.8 V vs. the NHE, which has to be compared with the standard potential of bulk silver of 0.799 V vs. the NHE. The result suggests that the Ag atom has a strong reducing activity, which may be one of the driving forces of the Ag(I1) complex formation in the layered structure (str I). The surface spectra of the meso-substituted porphines in str I1 prove that the porphines show a tendency to form the Ag(I1) complex on deposition to a vacuum-deposited Ag film, the tendency of TMPyP(4) and TPP being much larger than that of the other porphines. Although it is expected that the preparation condition of the Ag layer severely affects this kind of reaction, the results clearly demonstrate the existence of the active sites, which cause the formation of the argentic complexes, on the surface of the Ag film. Presumably the surface shows roughness on a variety of scales such as an atomic scale roughness (e.g., steps, (13) Kozuka, M.; Iwaizumi, M. Bull. Chem. SOC.Jpn. 1983, 56, 3165. (14) Hengline, A. Ber. Bunsenges. Phys. Chem. 1977,8Z, 556.
kinks, adatoms, and clusters), a submicroscopic roughness ( 5100
A), and a rather coarse r0~ghness.l~It is reasonable to consider
that the surface portion with an atomic scale roughness has a chemical reactivity similar to that of atomic Ag and Ag clusters, which means that the portion has a high reducing activity. As in the case of the Ag(I1) complex formation in str I, the high reducing activity may be one of the factors which cause the Ag(I1) complex formation on the vacuum-deposited Ag surface. By monitoring the extent of the conversion (porphine Ag(I1)porphine) taking place on Ag films prepared under various conditions, we can obtain insight into the relationship between the surface morphology and the reactivity of the Ag films. Table I indicates that the bands observed at 1595, 1034, and 898 cm-’ for Ag(II)TPyP(2) can be ascribed to the pyridyl group and the bands near 1640,1220,1190, and 790 cm-l observed for Ag(II)TMPyP(4) and Ag(II)TMPyP(2) to the N-methyl group. It is a well-known fact that pyridine and N-methylpyridine adsorbed on Ag surfaces exhibit strong SERS bands and that the adsorbates ascribable to the SERS bands interact with the Ag surface through the nitrogen atom of the samples.’3l6 As Figures 5-8 show, the SERRS spectra of the porphines do not show any selective enhancement of the bands due to the peripheral groups compared with the other porphine ring modes, suggesting the absence of a specific interaction between the peripheral groups and the Ag surface.
-
Acknowledgment. This work was partly supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture in Japan. Registry No. TPP, 917-23-7; TPyP(4), 16834-13-2; TPyP(2), 40904-90-3; TMPyP(4), 38673-65-3; TMPyP(2), 59728-89-1; CaF,, 7789-75-5; Ag, 7440-22-4. (15) Otto, A. In “Light Scattering in Solids”; Cardona, M., Guntherodt, G., Eds.; Springer-Verlag: West Berlin, 1984; Vol. IV, p 289. (16) Bunding, K. A.; Bell, M. I.; Durst, R. A. Chem. Phys. Let?. 1982,89, 1.
The Oscillating Belousov-Zhabotlnsky Type Reaction with Saccharides Peter bvEik* and h b i c a AdamZkovii Department of Physical Chemistry, Comenius University, 842 15 Bratislava, Czechoslovakia (Received: April 12, 1985; In Final Form: June 19, 1985)
Nine saccharides, arabinose, xylose, glucose, fructose, mannose, galactose, lactose, maltose, and sucrose, give rise to sustained oscillations of the Belousov-Zhabotinsky type when a nitrogen flow is used. Mn(II1) ions induce the reaction between HOBr and/or Br, and saccharides.
Introduction The classical Belousov-Zhabotinsky (BZ) reaction is the metal ion catalyzed oxidation of malonic acid by an acidic bromate solution. In a closed system the substrates known so far to give rise to oscillation are easily brominated.’ Noszticzius and Bodiss2 discovered an oscillator in which the organic substrate is not subject to bromination but in which the product bromine is removed from solution by a constant stream of gas. A completely inorganic oscillating system of the BZ type containing sodium hypophosphite in place of the organic substrate and with nitrogen flow was described too.3 OrbSn et aL4 discovered the “minimal bromate
oscillator” involving bromate, bromide, and a metal ion in a stirred tank reactor (CSTR). More recently it has been shownSthat the bromide flow can be replaced by a flow of a series of inorganic reductants. In this paper we report that a group of saccharides gives rise to sustained oscillation using a Mn(I1) catalyst when a nitrogen flow is used. These systems are intermediate in character between the classical closed BZ configuration and the open (CSTR) system. Oscillations have been obtained with the following substrates including pentoses, hexoses, and disaccharides: arabinose, xylose, g l u m , fructose, mannose, galactose, lactose, maltose, and sucrose.
(1) Gurel, 0.; Gurel, D. In “Topics in Current Chemistry”; SpringerVerlag: West Berlin, 1983; Vol. 118. (2) Noszticzius, Z.; BAiss, J. J . Am. Chem. SOC.1979, IOZ, 3177.
(3) Adamcikovi, L.; Sevcik, P. Inr. J . Chem. Kine?.1982, 14, 735. (4) Orbin, M.; De Kepper, P.; Epstein, I. R. J . Am. Chem. Soc. 1982,104, 2657. (5) Alamgir, M.; Orbin, M.; Epstein, I. R. J . Phys. Chem. 1983, 87, 3725.
0022-3654/85/2089-5 178%01.50/0 0 1985 American Chemical Society
Belousov-Zhabotinsky Type Reaction with Saccharides
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Figure 1. Oscillations in the potential of a platinum electrode with [Mn(II)] = 0.001 M, [glucose] = 0.05 M, [H2S04]= 1 M, N, flow = 600 mL/min, and T = 25 T:(a) [BrOC] = 0.05 M; (b) [BrOC] = 0.01 M; (c) [BrOC] = 0.005 M.
Figure 2. Oscillations in the potential of a bromide selective electrode: (a) [BrOr] = 0.01 M, [arabinose] = 0.05 M; (b) [BrOC] = 0.01 M, [fructose] = 0.05 M; (c) [BrOr] = 0.02 M, [galactose] = 0.06 M; Other constraints as in Figure 1.
Experimental Section All reagents used were the highest grade commercially available, and were used without further purification (the D-saccharides). Experiments were performed at a temperature 25.0 f 0.1 OC in a thermally regulated reaction vessel, from which gaseous products were transferred by nitrogen into a polarographic Kalousek cell. The experimental details have been described elsewhereS6 The potential of a platinum redox electrode and a bromide selective electrode, placed in the reaction vessel, as well as a polarographic current of bromine transferred against a mercurous sulfate reference electrode were continuously recorded.
the regeneration of Br- is now attributed to the oxidation of substrates by HOBr. Therefore, the rate of reactions of the saccharides with Br2 and/or HOBr was followed in a series of batch experiments under pseudo-first-order conditions (at 400 nm, [saccharide] = 0.05 M, [BrJ = 0.0007 M, and [HzSO,] = 1 M). The rates of bromine consumption (corresponding to the rates of bromide ion production) were in the order: arabinose > galactose > xylose N glucose > sucrose N lactose N maltose > fructose = mannose. These reactions are not rapid. The pseudo-first-order rate constant of the fastest reaction of them was found to be kBT2= 4.18 X s-]. However, addition of [Mn(III)] = 0.001 M to the reaction mixtures of the same composition has a dramatic effect. The rates of bromide ion production were increased by 2-3 orders of magnitude, and were in the order: arabinose > galactose > xylose > mannose = fructose = glucose > maltose > lactose > sucrose. Under the above conditions the rate constants were 2.66 X lo-*, 5.51 X and 1.65 X s-l for arabinose, glucose, and sucrose, respectively. This new order corresponds (except for fructose) to the order of the rate of reactions of saccharides with Mn(II1) ions (monitored at 480 nm). Therefore, Mn(II1) ions induce the reaction between HOBr and/or Br2 and saccharides. This process can provide Br- ions, as in the Noyes model: and is required, and expected. Different results were found when Ce(1V) ions were used in place of Mn(II1). Lower rates in all batch Ce(1V)-saccharides reactions were observed, and the rate of Br- production was appreciably slower in the batch Br2-saccharides reactions. For example, with the same concentration as in Mn(II1) experiments, the pseudo-first-order rate constant of Ce(1V) reaction with glucose is 7.93 X s-I, and the ratio of the rate constants for the Ce(IV) and Mn(II1) reactions is 2.6 X lo-,. The rate of Br- production in the Brz-glucose reaction in the presence of 0.001 M Ce(1V) ions is only slightly, 2.7 times, increased. Tysong has shown that the revised Oregonator predicts oscillations only over a restricted range of the appropriate dimensionless ratio of rate constants, and only for a restricted range of bromate concentrations. This prediction is in accord with our experimental results. We believe that the appropriate ratio of the rate constants for steps five and six of Noyes' revised Oregonator has been achieved with Mn(II), but not with Ce(II1) catalyst.
Results and Discussion Typical oscillatory responses of platinum and bromide selective electrodes obtained with glucose and with arabinose, fructose, and galactose as the reductants are shown in Figures 1 and 2. The general shape of oscillation cycle is similar under the same conditions with any of the saccharides. However, the induction period, frequency, amplitude, and total number of oscillations widely depend on conditions (see Figure 1). At fixed flow rate of N 2 (600 mL/min), [Mn(II)] = 0.001 M , [glucose] = 0.05 M, [H2S04] = 1 M (the standard conditions), oscillations appeared if 0.1 < [BrO,-]/ [glucose] < 1.4. The oscillation range of monosaccharides resembles each other. The optimum range (the highest number and amplitudes of the oscillations) is about 0.2-0.4 for monosaccharides, while a slightly higher value was found for disaccharides. For the standard conditions, the shortest induction period with arabinose (7.5 min), and the longest with disaccharides, was obtained. However, the induction period can be suppressed entirely, e.g., increasing the Mn(I1) concentration from 0.001 to 0.005 M under the standard conditions with arabinose. Oscillations of the polarographic current of bromine transferred into a Kalousek cell were observed as well. In the studied systems a substantially smaller concentration of bromine is maintained, and the amplitude of Br2 oscillations is about an order smaller than in the system with oxalic acid substrate.6 N o oscillations with Ce(II1) catalyst or without nitrogen bubbling were found. The calculations suggest5 that a species which reacts too slowly with HOBr and/or Br2 will be incapable of generating oscillations. In response to recent objections raised by Noszticzius, Farkas, and Schelly,' Noyes8 has revised the Oregonator model so that (6) Sevcik, P.;Adamcikovl, L. Collect. Czech. Chem. Commun. 1982, 47, 891. (7) Noszticzius, 2.;Farkas, H.; Schelly, 2.A. J . Chem. Phys. 1984, 80, 6062. ( 8 ) Noyes, R.M. J . Chem. Phys. 1984,80, 6071.
Registry No. Mn(II), 16397-91-4; arabinose, 10323-20-3; xylose, 58-86-6; glucose, 50-99-7; fructose, 57-48-7; mannose, 3458-28-4; galactose, 59-23-4; lactose, 63-42-3; maltose, 69-79-4; sucrose, 57-50-1. (9) Tyson, J. J. J . Chem. Phys. 1984, 80, 6079.
