J. Phys. Chem. 1991,95, 701-705 N,(X,u’? to both uI and u3 of SiH4 is resonant to within kT up to u” = 15. The most likely pathway for silane decomposition would involve having N2(X,v’? supply sufficient vibrational energy to the SiH4 to allow H atom stripping by N(4S) to become exoergic. Only two N2(X,u’? quanta need to be supplied for this process to become possible, although several more quanta may be required before the stripping process becomes efficient. The other possible pathway would be SiH4 dissociation by N2(X.u”). Formation of either SiH3and a hydrogen atom or SiH2 and a hydrogen molecule could occur in one step for collisions with N2 in vibrational levels 15 or 9, respectively. More probable, of course, would be a stepwise excitation of silane by a number of successive collisions with molecular nitrogen containing lower amounts of vibrational energy. With an N2(X,v) number density on the order of a milliTorr, each silane molecule will undergo roughly 15 collisions within a millisecond. Thus, even singlequantum transfers would be sufficiently frequent to effect the decomposition of the silane on the time scale of the experiments, 10-20 ms, were not intramode relaxation within the silane likely to redistribute the absorbed energy. A number of successive singleand multiple-quantum transfers would be necessary to dissociate the silane. After the first decomposition step, atomic nitrogen can react exoergically with the SiH, fragments to generate smaller fragments. If these reactions are moderately fast, this further decomposition can occur rapidly even at the modest N(4S) number densities of the present experiments (- 1013atoms ~ m - ~ )The . final decomposition step, that between SiH and N(’S), is endoergic to produce Si atoms and N H but is exoergic by 1.5 eV for generating SiN and H atoms. The channel producing Si and N H is exoergic if the nitrogen atoms are electronically excited. Atomic silicon also can be produced in an exoergic reaction between H atoms and SiH. Once the silane is fragmented, the observed emissions can be excited in energy-transfer reactions with the nitrogen metastables which are generated by N atom recombination. Even at number
701
densities as low as 107-108 molecules ~ m - metastable ~, Nz(A) can excite strong emissions from fragments such as N H , SiH, and Si given number densities of these latter species on the order of 109-10’0 molecules ~ m - ~ . The much larger intensity in the spectrum excited under condition 4 compared to those excited under conditions 2 and 3 can be rationalized by the fact that the Nz(X,v) and Nz(A) number densities are much larger for condition 4. These larger number densities will greatly increase the rates of both silane decomposition and fragment excitation. The slight increase in the intensities in changing from condition 2 to condition 3 most likely results from a somewhat more rapid fragmentation that is possible with the metastable nitrogen atoms that result when N(4S) quenches N2(A). In addition, although the N2(A) has been quenched by the atomic nitrogen to levels below our detection limit, lo7 molecules ~ m - the ~ , N,(A) number density might still be somewhat in excess of the steady-state number density generated in N atom recombination. Clearly these effects are small because the intensity enhancement is only about a factor of 2. Summary
Atomic and molecular metastable nitrogen, N(2D), N2(A), and N2(a’), react rapidly with silane, leading to SiH4 fragmentation. In addition, silane appears to be decomposed efficiently by vibrationally excited, ground-electronic-state molecular nitrogen. This latter fragmentation most probably occurs when N ( 5 ) strips H atoms from silane molecules that have absorbed two or more N2(X,u’? quanta. Also possible would be silane dissociation by N2(X,u’? either in one step with many quanta (on the order of 10) transferred to the SiH4or by a series of successive transfers a few quanta at a time.
Acknowledgment. We appreciate support for this work from PSI under internal research funds. We also acknowledge helpful discussions and correspondence with Peter Haaland on silane thermochemistry.
New Features of Stirring Sensitivities of the Belousov-Zhabotinskii Reaction L.Uipez-Tomiis and F. Sagu6s* Departament de Quimica Fisica, Uniuersitat de Barcelona, Diagonal 647, Barcelona 08028, Spain (Received: March 5, 1990; In Final Form: July 6, 1990)
The sensitivity to stirring shown by the oscillatory Belousov-Zhabotinskii (BZ) reaction when carried out in batch mode has been experimentally examined. Different series of experiments were designed in order to test the influence of both the flow rate of a deaerating agent and the reactant initial concentrations on the observed stirring susceptibility. In addition, distinctive stirring effects associated with the different phases of the oscillatory reaction were analyzed. The experimental results indicate that stirring effects strongly depend on the initial concentrations of the BZ mixture and on the detailed chemical dynamics inherent in each phase of the reaction.
I. Introduction Chemical instabilities, as paradigms of nonlinear effects corresponding to dynamics operating in far from equilibrium regimes, have been mostly described in past years in terms of homogeneous models, avoiding any reference to heterogeneities. Yet it is presently well-documented that effects originated in nonideal mixing conditions not only exist under the usual operating modes of real experiments but may also exert noticeable influences enhancing, suppressing, or modifying normal patterns of behavior such as temporal oscillations or multistability.’ Even recently, ( I ) Nicolis, G.; Baras, F. Physico 1985, flD,345.
0022-365419 112095-0701$02.50/0
features of irreproducibility likely mediated by stirring, which have been observed in certain chemical systems, have motivated preliminary experimental2 and theoretical studiesa3 Commonly, stirring effects within the context of oscillatory or multistable chemical dynamics have attracted the attention of both theoreticianseI0 and experimentalists.6*’1-22The existing literature refers (2) NagypB1, I.; Epstein, I. R. J . Phys. Chem. 1986, 90,6285; J . Chem. Phys. 1988,89, 6925. (3) SaguC, F.; Sancho, J. M. J . Chem. Phys. 1988,89,3793. SaguCs, F.; Ramirez-Piscina, L.; Sancho, J. M. J . Chem. Phys., in press. (4) Horsthemke, W.; Hannon, L. J . Chem. Phys. 1984,81,4363. Hannon, L.; Horsthemke, W. J . Chem. Phys. 1987,86, 140.
0 1991 American Chemical Society
702 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 either to batch reactorsll-17 or to reactors operating under open flow conditions (CSTR).c9**&22In what follows we will essentially concentrate on stirring effects corresponding to the batch oscillatory mode of the cerium-catalyzed Belousov-Zhabotinskii (BZ) reaction. In order to put our work here into a better perspective, let us first briefly recall what is known about stirring effects on batch oscillatory reactions. From the experiments reported by Farage et al.,” it is well-accepted that, for oscillating reactions under aerobic conditions, stirring sensitivity results in modifications of their oscillation parameters, typically periods and amplitudes, the periods being particularly sensitive to the stirring rate.23 A series of experiments performed by Menzinger et al.,” originally aimed at evidence of concentration fluctuations in a stirred batch reactor with and without a free surface for atmospheric contact, confirmed the major role of interfacial oxygen exchange in explaining the dependence of both the concentration fluctuations and oscillation parameters on the stirring conditions. This was further corroborated by the experimental work of Li et aI.,l4 who proved the nonexistence of a higher limit for stirring rates compatible with oscillatory behavior when operating the BZ reaction under an inert nitrogen or argon atmosphere, in contrast with what had been previously reported” for an oxygen atmosphere. However, the experiments previously quotedI3J4 also showed the persistence of stirring effects under anaerobic conditions. In addition, stirring sensitivities were also observed to be reaction dependent when results from the cerium- and ferroin-catalyzed reactions were compared. Further results were also reported by Sevcik et al.,15 who tested, under strictly oxygen-free-atmosphere conditions, the influence of having or not having a free surface allowing the exchange of dissolved gases. In this regard, Menzinger et al.I7 have recently suggested an explanation for stirring effects under anaerobic conditions for closed batch reactors in terms of two different surface processes: wall adsorption (e.g. of Br$, particularly in Plexiglas reactors and catalytic phenomena originating on the surfaces of Pt electrodes. From this account of experimental results, it is clear that different mechanisms have to be invoked to interpret realistically any observed indication of stirring sensitivities for reactions oscillating in batch modes. Moreover, given a particular system displaying stirring effects, the question of the actual relative weight that should be assigned to any of these complex phenomena in explaining the observed behavior remains unelucidated. Further theoretical and experimental work is certainly necessary to better understand this intricate question. This has motivated the work here reported on stirring sensitivities observed from experiments on the batch-operated BZ reaction. Actually, we have examined three different aspects of the problem that, to our knowledge, had been hardly discussed previously in the literature. The first question is related to the role of the deaerating agent that provides the anaerobic conditions used in our experiments. (5) Dewel, G.; Borckmans, P.; Walgraef, D. Phys. Rev. 1985, A31, 1983. (6) Kumpinski, E.; Eptein, I. R. J. Chem. Phys. 1985, 82, 53. (7) Bar-Eli, K.; Noyea, R. M. J . Chem. Phys. 1986,85, 3251. (8) Boiaconade, J.; De Kepper, P. J . Chem. Phys. 1987,87, 210. (9) F’uhl, A.; Nicolis, G. J . Chem. Phys. 1987,87, 1070. Puhl, A,; Altares, V.; Nicolis. G. Phys. Reu. 1988, A37,3039. (IO) Nicolis, G.; Altares, V. J . Phys. Chem. 1989, 93, 2861. ( 1 1 ) Farage, U. J.; Jancic, D. Chimiu 1980. 34, 342; 1981, 35, 289. (12) Ruoff, P. Chem. Phys. krr. 1982, 90, 76. (13) Menzinger, M.; Jankowski, P. J . Phys. Chem. 1986, 90,1217. (14) Li, R. S.;Li, J. Chem. Phys. k r r . 1988, 96. 144. (IS) Sevcik, P.; Adamcikova, I. Chem. Phys. Lrrr. 1988, 146, 419. (16) Sevcik, P.; Adamcikova, I. J . Chem. Phys. 1989, 91, 1012. (17) Menzinger, M.; Jankowski. P. To be published. (18) Roux, J. C.; De Kepper, P.; B o i n a d e . J. Phys. mi.1983,97A, 169. (19) Menzinger, M.; Boukalouch, M.; De Kepper, P.; Boimnadc. J.; Roux, J. C.; Saadaoui, H. J . Phys. Chem. 1986, 90,313. (20) Menzinger. M.; Giraudi, A. J . Phys. Chem. 1987, 91, 4391. (21) Luo, Y.;Eptein, 1. R. J . Chem. Phys. 1986,85, 5733. (22) Menzinger, M.; Dutt. A. K. J. Phys. Chem. 1990, 94, 4510. Dutt, A. K.; Menzinger, M. J . Phys. Chem. 1990, 94, 4867. (23) Actually, there are also indications of observed stirring sensitivities in oscillatory reactions which date back to the early 19708: Kasperec, G.J.; Bruice, T. C. Inorg. Chem. 1971, 10, 382. Bowers P. G.;Caldwell, K. E.; Prendergast, D. F. J . Phys. Chem. 1972, 76, 2185.
Ldpez-Tomls and SaguEs
TABLE I: Global Periods for Different Gas Flows and Stirring Rates” period, s stirring rate, rpm 1000 210
nitrogen 3 L/min 205 136
argon 3 L/min
1 L/min
201
198 129
I44
“Initial concentrations are the same as for Figure I . The periods actually correspond to the second oscillation cycle.
To this end we operated our reaction with a free surface, and a series of experiments, with reactive mixtures identically prepared, were repeated with various flow rates of an inert gas passing above the solution. Even without bubbling the inert gas through the solution during the course of the reaction, which would render the appropriate control of the stirring conditions difficult, one might expect that gas exchange between the solution and the inert atmosphere above it would be favored when increasing the flow rate of the deaerating inert gas, and this, in turn, would affect the stirring sensitivity of the reaction. The second examined question refers to the dependence of stirring sensitivities on the concentrations of the BZ reactants. Since it is well-known that the oscillation attributes, e.g. induction periods, periods, and amplitudes, depend largely on the initial concentrations of reactants, it seemed worth analyzing this dependence relative to stirring effects. We think this simple study should help us to better characterize the relevance of stirring, since in changing the initial concentrations of the BZ reactants, we modify, in the easiest way, the time scales of the oscillatory reaction. The last point here addressed concerns the relative importance of stirring effects when measured in different phases of the oscillating reaction. The pioneering work of Field, Koros, and no ye^^^ clearly revealed that a global oscillation in the concentration of the intermediate species of the reaction may be appropriately resolved into normally well-differentiated phases or partial periods, commonly associated with different rates of production or consumption of those intermediates. Although mechanistic discrepancies still preclude complete knowledge of the BZ medium, it is apparent that those phases correspond to different steps of the whole mechanism whose stirring sensitivities are likely to be largely different. One might even suspect that a precise knowledge of how stirring affects those different phases of the reaction would help to clarify remaining BZ mechanistic controversies. 11. Experimental Section
The studied solutions were prepared with KBr03, Ce(SO& (Merck, pea.), H2S04(Merck, suprapurum), and CH2(COOH)2 (Carlo Erba, p.a). Laboratory water was supplied by a Millipore-MilliQ system. The reactants were used without further purification. The experiments were carried out in a batch reactor (volume 50 mL) with a thermostating jacket that maintained a temperature of 25 f 0.1 OC. The reaction was monitored with a platinum-wire electrode and a bromide-selective electrode (Orion 94-35). The reference electrodes were saturated mercury/mercurous sulfate (Metrohm 6.0703.100). Potential measurements were done on millivoltmeter (Metrohm 632). The reaction progress was recorded on Cornig and J. J. Instruments x-t recorders. The stirring rate was adjusted with a Heidolph M R 2002 magnetic stirrer. The reaction solution was kept under a blanket of N2 or Ar by feeding these gases continuously into the volume above the solution. In each experimental run, H2S04, KBr03, and CH2(COOH)2 were added first to the reactor. Then the solution was purged with N2 or Ar at a rate of 3 L/min, with the stirring rate fixed at a value of 700 rpm. Twenty minutes later the N2or Ar flow was routed to pass above the solution, the stirring rate was adjusted to the working value, and the Ce(S04)2 solution was added. (24) Field, R. J.; K8r8s. E.; Noyes, R. M. J . Am. Chem. Soc. 1972. 94, 8649.
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 103
New Features of the Belousov-Zhabotinskii Reaction I
-8
-1
I-
w
0 c
X
2 0
10
30
20
LO
time lminl
Figure 1. Potentiometric trace of the bromide-selective electrode for different flow rates of deaerating agent. Initial concentrations: [H$O,] = 0.8 M;[KBrOJ = 0.063 M;[CH,(COOH),] = 0.032 M;[Ce(SO,),]
0
=O.OOI M.
TABLE 11: Global Periods for Different Initial Compositions and Stirring Ratesa
stirring rate. rDm lo00 210
20
time ( m i d
period, s a
68 60
b 205
C
d
194
136
198
68 56
'See section I1 in text for compositions a-d. The periods are averaged over five cycles. Up to eight different sets of initial concentrations were tested during the experiments here reported, corresponding to different concentration ratios for the four main ingredients of the BZ system, i.e. CH2(COOH)2, KBr03, Ce(S04)2, and H2S04. However, only four initial conditions displayed a stirring sensitivity significative and reproducible enough to be analyzed here. These four initial compositions, to which the experiments in section 111.2 refer, are as follows: (a) [CH2(COOH)2]= 0.130 M, [H2S04] = 0.8 M, [KBr03] = 0.063 M, [Ce(SO,),] = 0.001 M; (b) [CH2(COOH)2 0.032 M, [H2S04] = 0.8 M, [ K B Q ] = 0.063 M, [Ce(SO,),] = 0.001 M; (c) [CH2(COOH)2] = 0.130 M, [HSO,] = 0.8 M, [KBr03] = 0.016 M, [Ce(SO,),] = 0.001 M; (d) [CH,(COOH),] = 0.130 M, [H2S04]= 0.8 M, [KBr03] = 0.063 M, [Ce(SO,),] = 0.005 M. 111. Results III.1. Influence of the Flow Rate of Inert Gas. Experiments were performed for the b set of concentrations above by using different inert gases and different flow rates to test their influence, if any, on the stirring sensitivity of the BZ reaction. In Table I we summarize the results for the oscillation periods corresponding to use of two gases, N 2 and Ar, two flow rates of Ar, and two stirring rates for each running condition. Figure 1 shows the bromide-selective electrode response for an experiment conducted with a fixed stirring rate and varying flow rates of Ar during the course of the reaction. Figure 1 clearly shows that no appreciable change in the oscillating behavior is found when we modify the flow rate of the blanket of Ar. This demonstrates the efficiency of the inert gas flowing at rates greater than 0.5 L/min in assuring anaerobic conditions for the BZ reaction mixture and agrees with what was observed in the experiments of Li et al.I4 On the other hand, the results quoted in Table I indicate that stirring effects, at least when manifested in global magnitudes, there exemplified by the oscillation periods, do not appreciably depend on the nature and flow rates of the deaerating agent, provided that anaerobic conditions are strictly maintained. III.2. Influence of Initial Reactant Concentrations. A second set of experiments were performed with N2 passing at a constant flow rate above the solutions prepared with different reagent concentrations. Two stirring conditions were used for each set of initial concentrations. In Table I1 we summarize the results for the calculated global periods. Oscillation amplitudes are, according to our experiments, far less significative in analyzing stirring effects in oscillatory reactions, and we will make little use of them in what follows. The first general conclusion from these results is that the oscillation periods tend to increase in going to higher stirring rates. This is in complete agreement with what has been reported in the literature."J4Js However, what is important to observe is that
Figure 2. Phases of a potentiometric trace of the bromide-selective electrode, labeled according to ref 24. TABLE 111: Partial Periods for Different Phases of Oscillrtion, Initial Compositions, a d Stirring Rates" stirring period, s init compn rate, rpm EF FG GH HE 44 5 10 9 a 1000 31 5 9 9 210 115 5 13 12 b lo00 63 6 54 13 210 154 8 12 20 C 1000 153 13 7 25 210 41 9 10 8 d lo00 34 5 1 10 210
'Phases are defined in text and shown in Figure 2. Initial compositions are the same as for Table 11. The periods are averaged over five cycles. this variation largely depends on the initial concentrations in the reactant mixture. This is especially noticeable in comparing the results for composition b, with a very small amount of CH*(COOH)2, to those for composition c, with a deficiency of KBrO3. In the first case, the period increases by nearly 40% whereas it does not change, within our instrumental precision, in the second case. 111.3. Resolved Stirring Effects in Different Phases of the BZ Reaction. Figure 2 shows a typical record of the BZ reaction monitored by the bromideselective electrode. This potentiometric trace is resolved into different phases labeled according to the nomenclature introduced by Field, Koros, and Noyesa2' Essentially, four different phases can be distinguished in the oscillatory regime that follows the induction period CD. A rapid bromide production period, DE or HE, increases the bromide concentration to its maximum during the course of the oscillations. This stage is followed by a period of slow bromide consumption, EF, which is suddenly interrupted when the bromide concentration falls to a certain critical value. At that point, a period of rapid bromide consumption, FG, is initiated which decreases the bromide concentration to its minimum. Finally, the limit cycle closes with a slow bromide production stage, GH. Similar considerations, this time referred to the ratio [Ce(IV)]/[Ce(III)], would apply to the trace monitored by the platinum electrode. In order to examine the stirring sensitivities to the different phases of the oscillatory regime, two kinds of experiments were conducted: runs with stirring rate kept constant during the whole course of the reaction were supplemented with others where the stirring rate was changed during the oscillations at different phases of the reaction. In the first set of experiments, two stirring conditions were used for the whole series of initial concentrations listed in section 111.2. In the second kind of experiments, we restricted our measurements to composition b, which displays the maximum stirring sensitivity as reported above. In Table 111 we summarize the results limited to the oscillation periods, corresponding to the experiments conducted with constant stirring conditions. The important conclusion from these results is that, for most of the experimental conditions, the lengthening of the global periods in going to higher stirring rates is concentrated in the stage of slow bromide consumption, except for composition b, with a low concentration of CH2(COOH)2,for which the phase
704 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
I I 0
I 10
20
30
LO
50
tine (ain I
Figure 3. Simultaneous potentiometric traces of the bromide-selective
and platinum electrodes. The arrows show when the stirring rate was modified: arrows pointing up, from 210 to 1000 rpm; arrows pointing down, from 1000 to 210 rpm. Initial concentrations were the same as for Figure I . of slow bromide production also lengthens significantively. On the other hand, the phases of rapid bromide variations are less well defined, since it is normally very difficult to assign exact times to points G and H in Figure 2. In this sense, we think that the minor variations observed in Table I11 for phases of rapid bromide variation, apart from being practically irrelevant to their contribution to the modification of the global periods by stirring, are far less significative of the stirring sensitivity shown by the BZ system. This suggests that heterogeneities, whose existence is mediated by stirring, definitively couple with the local chemical dynamics and indeed probe it along the different phases of the oscillatory reaction. Similar conclusions referred to the system’s noise susceptibility may also be found in ref 13. For the experiments conducted while the stirring rate was varied, records from both electrodes are displayed in Figure 3. Preliminary runs indicated that no effect was detectable when the stirring rate was changed during the phases of rapid bromide variation, in accordance with our conclusions above. Therefore, we concentrated on changes made during the phases of slow consumption and production of bromide. Actually, these two phases display slightly different stirring sensitivities. The phase of slow bromide consumption, EF, generally shows a smaller instantaneous susceptibility to stirring modifications than the phase of slow bromide production, GH. However, phase E F considerably lengthens (shortens) with increasing (decreasing) stirring rate, whereas this effect is far less pronounced during phase GH, in agreement with the results in Table 11.
IV. Discussion and Conclusions We have formed three main conclusions from the above results. The first refers to the influence of the flow rate of deaerating agents. According to the results in section 111.1, stirring effects do not appear to appreciably depend on the blanket gas flow. Notice that this conclusion does not rule out any eventual role of gas exchange in generating heterogeneities and consequent stirring sensitivities of oscillatory reactions in batch reactors with initially free surfaces, but this effect must be, in any case, intrinsically interfacial and not related, at least under the anaerobic conditions here studied, to the actual removal rate of the gas phase inside the reactor. The second point to be considered concerns the influence of initial reactant concentrations on global stirring sensitivities. Although the set of experiments reported in section 111.2 should be completed in order to enlarge the set of investigated concentrations, stirring effects definitely depend on the initial concentrations of the BZ reagents. The most important conclusion in this regard is that the maximum global stirring sensitivity is observed when the BZ reaction is conducted with a low concentration of CH2(COOH)2,Le. composition b in Table 11. On the other hand, notice that with changes in the initial concentration of the other major reactant of the BZ system, KBr03, no apparent
Lbpez-TomPs and Sagues global sensitivity to stirring is found, in spite of the fact that the global period is greatly changed. Compare, for example, global periods either at loo0 or 210 rpm for compositions a and c in Table 11, and the two global periods for composition c, 194 s for 1000 rpm and 198 s for 210 rpm, which are equal within our experimental error. Actually, the values corresponding to the ratio of initial CH2(COOH)2 and K B r 0 3 concentrations ( r [CH2(COOH),]/ [KBr03]) for the set of investigated compositions are respectively (a) = 2, (b) = (c) = 8, and (d) = 2. It is then tempting to try to correlate the observed global stirring sensitivity with the value of r. Maximum and minimum effects would correspond respectively to low and high values of r. In this respect, notice the similarity of the global stirring sensitivities observed under conditions a and d, having the same value of r but different Ce(S0.J2 initial concentrations. Although this conclusion is at this point merely speculative, we believe it would be worth examining further for other compositions of the BZ system. The last point we want to stress here is the largely different stirring sensitivities found in the different phases of the global oscillation. According to Figure 3, it seems that, at least for composition b, for which global stirring effects are more prominent, the kinetics of slow bromide consumption during phase EF is not apparently affected by stirring, since no modification on the trace of the bromide-selective electrode is observed on changing the stirring rate. The critical concentration corresponding to point F, however, seems to decrease on going to higher stirring rate, the final result being the noticeable lengthening of period EF and of the global oscillation itself. On the other hand, we observe a much more pronounced instantaneous response to stirring modifications during phase G H of slow bromide production, for which the trace of the bromide-selective electrode is definitely altered on changing the stirring rate. Apparently, this very different behavior could be associated with a different degree of coupling between the specific kinetic laws governing both phases of the reaction and the heterogeneity-creating mechanisms mediated by stirring. However, we must be cautious on this particular conclusion since it is also true that because an electrode potential is, assuming an ideal Nernstian behavior, logarithmically related to the actual bromide concentration, equally small absolute variations in the bromide concentration, occurring during the phases of relatively high concentration (EF) and low concentration (GH), will result in quite different modifications of the corresponding electrode signal. Analogously, distinctive stirring sensitivities are also found on the trace recorded by the platinum electrode. However, from the traces shown in Figure 3, we conclude that maximum instantaneous stirring sensitivities are indeed asynchronous. In any case, as is also apparent from Figure 3, the highest instantaneous stirring sensibility on the trace recorded by the platinum electrode occurs, as in the case of the trace recorded by the bromide-selective electrode, when the [Ce(IV)]/[Ce(III)] ratio reaches its phase of minimum potential. Now let us finish by interpreting our results in terms of what have been previously proposed in the literature as possible mechanisms responsible for stirring effects. As stated in the Introduction, according to the experiments by Menzinger et al.,13J7 Li et aI.,l4 and Sevcic et al.,I5 two phenomena seem to play an essential role in explaining stirring effects in batch reactors: gas exchange (0, absorption and/or Br2 loss) at the liquid-gas interface, if existent, and other heterogeneous processes, invoked by Menzinger et aI.,l7 related to adsorption processes of Br2 on the walls of the reactor or to surface catalytic effects at the Pt electrode. In this case, we think that our results should be interpreted as arising from gas-exchange effects at the liquid-gas interface inside the reactor involving molecules of elemental bromine. No reference is made to O2exchange, since we operated under an 02-free atmosphere. We can even try to correlate this assumption with an elementary description of the BZ mechanism in order to better understand the distinctive stirring effects observed in our experiments. To this end, we refer to the classic papers in the 1 9 7 0 ~ wherein ~ ~ 3 ~ three ~ overall processes are proposed to explain
J . Phys. Chem. 1991,95, 705-709 the oscillatory behavior of the BZ system. We are aware of the considerable progress made during the past 15 years in gaining a more complete understanding of the BZ reaction, but we believe that a minimal description, like the one invoked here, retains the essential ingredients that could explain our observations on stirring effects. The three main points of the BZ mechanism mentioned above correspond to (A) the reaction of Br- and Br03- to give Brl, through the intermediates HOBr and HBr02, which further brominates (CH2(COOH), (B) an autocatalytic loop in the oxybromine chemistry during which HBr02 is formed simultaneously with the rapid oxidation of Ce(II1) to Ce(IV), and (C) the regeneration of Br- from CHBr(COOH), via the reduction of Ce(IV) to Ce(II1). Our general interpretation is that Br2 losses favored by stirring should generally affect the rate of bromination of CH2(COOH)z, slowing down this process and finally resulting in a lengthening of the oscillation period. The important fact that this effect is especially noticeable under low concentrations of CH2(COOH)z may be then simply understood, since under these conditions CH2(COOH)2brominates slowly, and thus Br2 may escape most easily through the surface of the solution. In order to understand why this effect may be better observed during phase EF, one should keep in mind that, during this slow phase, process A is leading the reaction until, at point F, Br- concentration has decreased enough to fail to inhibit the HBr02 autocatalytic cycle ( p r y B). If Br2 is kept in solution under poor mixing conditions,
705
then the concentration levels of intermediates HOBr and HBrOz are kept higher than expected, and the control of the reaction is shifted to process B prematurely, at higher Br- concentration, resulting finally in shorter periods of slow bromide consumption. On the other hand, under sufficiently low concentrations of CH2(COOH),, the existence itself of a slow recovering of Brconcentration during period G H can be interpreted in the sense that the rate of consumption of CH2(COOH)2 is low enough so that it is unable to trigger process C, preventing an inmediate increase in Br- production. Under poor mixing conditions, this is also accompanied by higher values of the ratio [Ce(IV)]/ [Ce(III)] that may prematurely inhibit process B, causing control of the oxybromine chemistry to be rapidly regained by process A and finally shortening the period of slow bromide production. We hope that our work reporting new features of the problem of the stirring effects on complex nonlinear chemical reactions, such as the BZ system investigated here, may help to keep alive the interest of theoreticians and experimentalists in examining the intricate coupling of chemical kinetic laws and transport processes mediated by stirring.
Acknowledgment. We thank Dr. Theo Plesser for encouraging us to initiate this work, Dr. R. Albalat for her valuable advice during the course of the experiments, and Dr. S. Nagy-Ungvarai for fruitful discussions. We also thank the reviewers of a previous version of this paper for their comments, which have permitted us to improve its final presentation. This work was supported in part by grants from the ComissiB Interdepartamental de Recerca i lnnovacid Tecnoldgica (CIRIT), DGICYT Project PB87.0014, and Stiftung Volkswagenwerk.
(25) Edelson, 0.; Field, R. J.; Noyes, R. M. Int. J . Chem. Kinet. 1975,7, 417.
Acetylene Polymerization in a H-ZSM-5 Zeolitet C. Pereira, G. T. Kokotailo, and R. J. Gorte* Department of Chemical Engineering and Laboratory for the Research in the Structure of Matter. University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received: March 16, 1990; In Final Form: October 1. 1990)
We have examined the acid-catalyzed reaction of acetylene in H-ZSM-5 zeolites in a batch reactor between 298 and 550 K, with acetylene pressures between 10 and lo00 Torr, in an attempt to form zeolite-encapsulated polyacetylene. The samples were then characterized by temperature-programmed desorption (TPD), thermogravimetric analysis (TGA), and transmission IR, reflectance UV-visible, and 13CNMR spectroscopies. The reaction rate for acetylene was proportional to the zeolite AI concentration, indicating that reaction occurs at the Bransted sites associated with the AI atoms in the zeolite framework. The reaction is activated and substantial rates were observed only above 425 K. Zeolite pore volumes decreased linearly with acetylene conversion, indicating that the products remain in the pore structure. TPD-TGA results showed that the products decomposed above -550 K, a temperature similar to that observed for polyacetylene, with 70% of the hydrocarbons in the zeolite desorbing as simple aromatics and olefins. The IR spectra provided further evidence that reaction occurs at Bransted sites in the zeolite but were inconclusive for identifying the hydrocarbon species. I3C NMR showed the presence of substantial quantities of sp3-hybridized C, implying that substantial defects probably exist in the oligomer chains. The hydrocarbon products in the zeolite also interact strongly with adsorbed bases, as demonstrated by the UV-visible spectra, which showed a broad band between 350 and 700 nm that decreased reversibly following adsorption of ammonia.
Introduction In recent years, several group have reported the polymerization of acetylene in zeolites in an attempt to form well-oriented, isolated chains.'-3 Polyacetylene, which is of interest due to its high conductivity in the doped state,e6 is unstable in air and has properties that depend on the synthesis conditions and postsynthesis treatment.' Therefore, the encapsulated polymers may be very useful as models for understanding the physical properties of conducting polymers7**and may possess unique properties due to the regular, crystalline array in which the polymer is formed. Most
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'This work was suuwrted bv the NSF. MRL Program. under Grant DMR .. 88-19885. Author to whom correspondence should be addressed.
0022-3654/91/2095-0705$02.50/0
of the previous studies of acetylene polymerization in zeolites have involved using alkali-metal cations or transition metals to carry (1) Dutta, P. K.; Puri, M. J . Catal. 1988, I l l , 453. (2) Tsai, P.; Cooney, R. P.; Heaviside, J.; Hendra, P. J. Chem. Phys. Lett. 1978, 59, 510. (3) Pichat, P.; Vedrine, J. C.; Gallezot, P.; Imelik, B. J . Catal. 1974, 32, 190. (4) Chien, J. C. W. Polyacetylene: Chemistry, Physics, and Materials Science; Academic Pres: Orlando, FL, 1984; pp 325ff.
(5) Shirakawa, H.;Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J . Chem. Soc., Chem. Commun. 1977, 518. (6).Chiang, C. K.; Drury, M. A.; Gau, S. C.; Heeger, A. J.; Louis, E. J.; MacDiarmid, A. 0.;Park, Y. W.; Shirakawa, H. J . Am. Chem. Soc. 1978, 100. 1013. (7) Bein, T.; Enzel, P. Synrh. Met. 1989, 29, E163. ~
0 1991 American Chemical Society
