J . Phys. Chem. 1990,94, 6458-6464
6458
Transport Phenomena Accompanying Redox Switching in Polythionlne Films Immersed in Aqueous Acetic Acid Solutions Stanley Bruckenstein,* C. Paul Wilde,+ Department of Chemistry, University at Buffalo, State University of New York, Buffalo, New York 14214
and A. Robert Hillman School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 I TS, Great Britain (Received: January 19, 1990)
The transport of neutral molecules in electroactive polymer films is a problem of considerable importance and is addressed , solvent and undissociated weak here for thin (ca. 10 nm thick) polythionine films. In weak acid media, pH < P K H ~both acid are present in the polymer. Raising the pH above pKHAprogressively removes HA. Using the quartz crystal microbalance, we determined the film weight changes accompanying the redox switching process. At all pH's studied, a mass decrease accompanies reduction. The magnitude of this change is consistent with the expulsion of one water molecule per redox site where, at low pH, HA within the film acts as the sole source of counterion, A-. At higher pH, counterions must increasingly be supplied by the bathing solution, with the result that the mass change becomes less negative. These results are interpreted from a purely thermodynamic viewpoint, and then we discuss a coordination model as a relevant, special case.
Introduction
Transfer of mobile species into/out of surface-immobilized electroactive polymer films is an issue presently attracting considerable attention. We recently outlined a general framework describing the changes in equilibrium populations of the mobile species upon redox switching in terms of their activities.' An important general result emerged-there is no a priori restriction to integer values of stoichiometric coefficients in the overall redox process. This contrasts with the practice of expressing the film redox process in terms of integer stoichiometric coefficients. In this paper we examine the special circumstances that must prevail for this simple, integer stoichiometric case to apply and illustrate these conclusions through a study of the polythionine system in aqueous weak acid media. First we introduce the simple, integer case. The compositional changes accompanying redox switching of a polymer film can be inferred from the electrode half-reaction provided two conditions are met. First, the oxidation states must each be a single phase (compound) of invariant chemical composition. Second, transfer of mobile species into/out of the film must not alter their activities in either the film or the bathing solution. (An ideally permselective redox membrane electrode is one example. A silver/silver chloride electrode covered with a film and in contact with a saturated KCI solution is another.) Such a situation leads to a description of the population changes of mobile species in terms of the halfreaction's integer stoichiometric coefficients. However, the general, noninteger case applies if either phase is not of invariant composition. (A nonpermselective redox membrane electrode is such an instance as is an imperfect ion-selective electrode.) Then continuously variable activities for both redox polymer forms are required to define the system, leading to nonintegral stoichiometric coefficients appearing in the overall redox reaction. The redox switching process occurring in polythionine films at pH < 5 in aqueous media has been represented2 by the half-reaction TH+ + 2e
+ 3H+ = TH4*+
(1)
Electroneutrality requires that there be one (two) counteranions per redox site in the oxidized (reduced) film. This formulation requires a source of anion on film reduction. For pH >> 5 , the half reaction has been represented2 as TH+
+ 2e + 2H+ = TH,+
(2) In this case no counteranion source is required by electroneutrality. 'Now at Department of Chemistry, University of Ottawa, Ottawa, Ontario K I N 6NS. Canada.
0022-3654/90/2094-6458$02.50/0
In experiments using the electrochemical quartz crystal microbalance (EQCM) to test ion-transport predictions in strong acid solutions based on eq 1, we obtained the unexpected result that the direction of mass change, A M , depended on the strong acid concentration in the bathing ~ o l u t i o n .Simple ~ electroneutrality considerations based on eq 1 could not adequately represent the results obtained by using the in situ EQCM t e c h n i q ~ e .By ~ utilizing a particular case of a general model,' which considered both thermodynamic and electroneutrality restraints, we were able to interpret this behavior in terms of an ion aggregation model for species present in the polymer film phase. The change in sign of AM with electrolyte concentration implied that solvent transport played a significant role. As an extension of the strong acid experiments, here we study the behavior of polythionine films in aqueous acetic acid bathing solution as a function of pH at different constant total acetate concentrations. Again we find that simple electroneutrality considerations do not describe the results. Indeed, for all the experiments reported here, mass decreases accompanied film reduction-the exact opposite of predictions from eq 1 based solely on electroneutrality. We utilize our general theory to consider the specific case of an electroactive polymer film phase in contact with a bathing solution containing a single, uncharged weak acid (acetic acid here) and its conjugate base. As before3 we allow ion aggregation in the polymer film phase but now consider the situation where the solution source of counter and co-ions is a weak electrolyte that partitions into the polymer film phase. Analysis of the experimental data for the acetic acid system indicates that it is a special case in which the activities of net neutral polymer phase solutes are constant and their activity coefficients hardly change on redox switching. Integer differences in the stoichiometric coefficients are found. The equilibrium considerations are consistent with the EQCM mass changes if we assume that both water and weak acid may be bound, to some extent, to both oxidation states of polythionine. Hence we also discuss a model based on coordination of water and acetic acid with polymer that predicts such a result as being a special case within our thermodynamic framework. Theory A complete description of charge transfer in redox polymers requires consideration of concomitant changes in the populations ( 1 ) Bruckenstein, S.; Hillman, A. R. J. Phys. Chem. 1988, 92, 4837 (2) Albery, W. J.; Foulds, A. W.; Hall, K. J.; Hillman, A. R. J. Elecrrochem. Soc. 1980, 127, 654.
(3) Bruckenstein, S . ; Wilde, C. P.; Shay, M.; Hillman, A. R. J. Phys. Chem. 1990, 94, 787
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6459
Redox Switching in Polythionine Films of all species in the polymer phase, The relevant polymer phase species are ions, ion aggregates, molecular acid, redox sites, and solvent. The bathing solution contains only an uncharged weak acid and its conjugate base, hydronium ion, and sufficient indifferent cation to satisfy electroneutrality. The aqueous phase activities of all forms of the acid and hydronium ion are needed to describe the system. In this work we particularize and extend our general thermodynamic description' to the polythionine system in acetic acid solutions of varying pH. Our original model recognized the requirement that the activities of all species (charged or neutral) in both phases be determined by thermodynamic restraints in addition to the electroneutrality restraint. These are not separate equilibrium restraints but are coupled and arise naturally by formulation of the problem using the electrochemical potential. A feature of the model, which proved to be central to the interpretation of our strong acid studies,' is that solvent transfer accompanies the redox-switching process. Here we consider the pH dependence of the mass response of a polythionine film in a bathing solution containing a single uncharged Brernsted acid. We include the formation of ion aggregates in the polymer phase, since they will be a significant reservoir of their component species (proton, anion, immobilized charge sites, and solvent). The various species present are subject to the following equilibria, described by the designated equilibrium constants (nomenclature as in ref 3): H 2 0 p = H20, TH+, TH4'+,
+ A-, + A-,
K1
(3)
= TH'A-,
K4
= THd2+A-,
KS
(5) (6)
The activity of water in the polymer film phase, designated by subscript p, is maintained constant by the activity of water in the vast excess of bathing electrolyte, designated by subscript s. Different values of the ion association constants include all situations from complete to negligible ion association; for the polythionine system we need consider only net-neutral, fixed redox ~ the mobile sites because of extensive ion aggregate f ~ r m a t i o n .Of species in the film, only net neutral ion aggregates may be important in bringing about weight changes. Vapor-phase absorption studies performed at dry polythionine films4 show that significant amounts of both H 2 0 and H A can exist in the polymer film phase. This is consistent with in situ ellipsometric s t u d i e ~which , ~ show that the film comprises ca.equal quantities (by volume) of polymer film and all other species. Therefore, we allow for neutral Brernsted acid (and conjugate base) partition, proton exchange with water in both phases, and adducts with water in the oxidized/reduced polymer film phases. The species HA.H20, represents all undissociated, mobile forms of HA containing water in the film that are not bound (coordinated) to the polymer: HzO,
+ HA, = H30+, + A-, H A , = HA,
K7 (=KHA)
Kg
+ H20, = HAsH20, H20.HA, = H30+, + A-, HA,
TH+A-,
+ 2e + 3HA, = TH42+(A-)2,p+ 2A-,
(7)
(8) KS
(9)
(1 1)
are useful representations of the redox process from the viewpoint of calculating mass changes at low pH. Other representations of the redox process such as TH'A-,
or TH+A-,
+ 2e + 3HA,
+ 2e + 3H20.HA,
+
= TH4Z+(A-)2,p 2A-,
(12)
= TH42+(A-)z,p+ 3H20,
+ 2A-, (13)
or TH+A-,
+ 2e + H20.HA, + 2H30+, = THd2+(A-)2,,
(4)
KS
TH42+p+ 2A-, = THd2+(A-)2,,
the bathing solution can affect whether the redox process is described by analogy with eq l or 2 and whether the undissociated weak acid HA, or its anion A-, is the principal bathing solution species. We consider some likely cases. Reduced Form Protonation State = TH42+.pH < pKHA. In general, interpretation of mass changes requires a knowledge of all species whose film concentrations change as a result of the redox process. In this pH range, the dissociation of HA, according to eq 7 is negligible, so that at fixed total concentration of HA, species and low ionic strength, the activity of HA, is fixed. Thus, according to eqs 3, 8, and 9, the activities of all mobile HAcontaining species in the polymer film phase are fixed by the constant activities of HA, and HzO,. Consequently, only reactions that involve sources in the aqueous phase, such as
+ 3H20,
while thermodynamically valid representations of the redox process, are not useful descriptions of the process for purposes of calculating the mass change. Useful formulations of the reaction for purposes of calculating mass changes must be expressed only in terms of major polymer phase species whose activities may change. This is not the case for H20,! HA,, and HA-HTO,. The detailed analysis of the model described by eqs 1-10 is given in Appendix A. We start by noting that the EQCM technique measures the total mass change, AM. AM is obtained by summing the individual mass change components, Ag.mj (where mi are the relative molar masses), associated with the participating film species: AM = XAgjmj = E(gjmj)finai- kjmjlinit
(10)
We define all equilibrium constants to be independent of redox composition6 Consequently, the activity coefficients of solutes in the polymer film phase will change in a manner that reflects medium effects accompanying redox switching. Also the pH of
+ mHzO + yHA = TH+A-,(H20),(HA), THd2+(A-)2,, + nHzO + (Z - I)HA = TH42+(A-)2,,(H20)n(HA),_,
(16) ( l 7,
Then another general way of describing the redox switching process is TH+A-,(HzO),(HA),
(4) Wilde, C. P.; Bruckenstein, S.; Hillman, A. R. Unpublished quartz crystal microbalance studies. (5) Hamnett, A,; Hillman, A. R . J . Electrounal. Chem. 1987, 233, 125. (6) Marinsky, J . A. In Aquutic Surfuce Chemistry: Chemical Processes ut the Purticle- Wuter Interface; Stumm, W., Ed.; Wiley: New York, 1987; Chapter 3
(15)
Agj is the change in film population of species j , gj,final- g,,init, associated with the redox process. Individual Ag values are obtained by subtracting stoichiometric coefficients for film species appearing in the balanced redox equation. Solution species, even if they appear in the balanced equation, are not sensed by the EQCM and thus do not appear in the expression for AM. We find in Appendix A, by making use of the constraints imposed by eqs 1-10, that this model always leads to an increase in film weight on reduction, contrary to our experimental data obtained with acetic acid solutions. Hence we are forced to propose the existence of other source(s)/sink(s) for HzO and HA in the polymer film. We now allow for specific binding of water and weak acid by the polymer, according to TH+A-,
Klo
(14)
+ 2H30+, + 2e =
TH42'(A-)~.p(H~O)n(HA)(n-,) + ( m - n + ~ ) H z O+, 0, - z)HA, (18)
The values of m, n, y , and z determine whether a loss or gain in weight occurs. The predicted mass change is ( z - y)mHA+
Bruckenstein et al.
6460 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 ( n - m)m,,.@+ 2mH+. In the case where y = z (no anion expulsion from the film) and n - m = -1 (one water molecule is expelled), AM equals -I 6 g mol-’. This is the experimental result we found for acetic acid. Reduced Form Protonation State = TH42+.pH > pK,. We assume in this case that the increase in pH deprotonates only H&, and both of the polymer charged sites retain their protons. This limiting situation is not a well-defined experimentally accessible state for the polythionine/acetic acid system as the switch from eq 1 to eq 2 occurs in the vicinity of PKHA. The description of the electrode process would be
TH+A-p(H20),,,(HA), + (2 + z - y)H30+, + 2e = T H ~ 2 + ( A - ) 2 , ~ ( H 2 0 ) n ( H A+) ~(m ~ --, ~n + 2 + I -y)H20, + Lv - z)A-, (19) Thus eq 19 differs from eq 18 in that the A-, has replaced HA, as the principal weak acid form in the aqueous bathing solution phase. Consequently m, n,y, and z are the same in both equations, and the predicted mass change for this case will not differ from that for eq 18. Reduced Form Protonation State = TH,’. pH < PKHA. This situation corresponds to a bathing solution containing weak acid whose pK is > - 7 . No anion source is required. It would be described by TH+A-p(H20),,,,(HA),, + 2H30+, + 2e = TH3+A-,(H20)nt(HA),J+ (m’- n’+ 2)H20, + (y’- z’)HA, (20) (n’ Thus a substantial weight change ( A M = (z’-y’)mHA - m3mHI0 + 2mH+)is allowed, provided the coefficients of mHA and/or mHIOare nonzero even though eq 2 does not suggest this as being possible. The pKHA of acetic acid is very close to the pH at which the reduced form of the polymer changes to TH3+, so this situation will not be realized. Reduced Form Protonation State = TH3+. pH > pK,,. The decrease of activity of HA in the aqueous phase could remove some or all forms of HA from the film as compared to the previous situation. This leads to the following formulation:
+
+ +
TH+A-,,(H,O),,,,(HA),, ( 2 z ’ - y?H30+, + 2e = TH3+A-p(H20)n~(HA)z, + ( w I ’ - n’+ 2 + z’-y’)H2OS @’- z ~ A - ,(21)
+
The values of m’, n’, y’, and z’need not be the same as those in the previous case. Again, no anion source is required by the redox process, but a weight change resulting from bound HA and/or water is not ruled out. In the limit that all bound HA is removed from the film, any mass effects must arise primarily from bound water motion, A M = ( n ’ - m’)mHZo+ 2mH+:
+
TH+A-p(H20),,,,+ 2H3O+, 2e = TH3+A-,(H20), + (2
+ m’-
n 9 H 2 0 , (22)
Recapitulation. As a consequence of the redox process, the polymer film phase concentrations of H 3 0 + and A- must adjust to maintain electroneutrality. Simultaneously, the activities of all species must satisfy the equilibria (3)-(10). Because of extensive ion association in the film, the major form of the mobile species are net neutral ion aggregates. The two redox forms of the polythionine are net neutral and are associated with some HA and water. The changes in the concentrations of net neutral species in the film determine the weight change. However, the activities of all net neutral mobile polymer phase species do not change on redox switching. Thus, sources and sinks of H 2 0 and HA associated with the polymer phase were postulated to rationalize the experimental data. Thermodynamics does not a priori require that any of the coefficients be integral in the half-reactions. However, in the special situation for which both the oxidized and reduced forms of polythionine correspond to single phases of unique composition, activities assume fixed values and integer coefficients in the electrode half-reaction describe the mass change. This special
situation arises when the activities of solvent (water) and HA in the oxidized and reduced film are constant for all external phase compositions. Two possible circumstances leading to this situation are solvent content limitations imposed by saturation of the polymer film and stoichiometric coordination effects. These circumstances are considered in the Results and Discussion. We stress that the above model is merely an aid to visualizing the reality. This model is based upon a microscopic view of the situation that agrees with the reality that thermodynamic relationships between the activities of the various species and phases control. The rates of attainment of the various equilibria may differ markedly as discussed e l ~ e w h e r e . ~We stress the necessity of ruling out kinetic phenomena which may mimic some of these effects.
Experimental Section The apparatus and technique for simultaneously recording mass vs potential and current vs potential are described elsewhere.8 Deposition of polythionine films on the evaporated gold electrode of the quartz crystal was performed according to the standard procedure,2 substituting perchloric acid for sulfuric acid as the deposition medium. Films used in acetate medium were rinsed with acetate supporting electrolyte several times and then potential cycled in the supporting electrolyte until a constant response was achieved. The procedure was designed to ensure that (a) the dominant anion species in the film was acetate ion and (b) the film was equilibrated with respect to all other species in the supporting electrolyte. Two series of acetate solutions, 0.1 and 1 .O mol dm-3 total acetate, were prepared from pure acetic acid by addition of suitable quantities of concentrated sodium hydroxide. At the end of a series of experiments, the measurement using the first solution of the series was repeated. Solution pH values were measured directly in the electrochemical cell before and after all experiments. Reagents, water, and nitrogen were of quality similar to those used in refs 2 and 3. Experiments were performed at room temperature, 20 f 2 O C , over a several-day period. The duration of a single experiment was never more than 200 s; hence any temperature effects are negligible. All potentials were measured and are quoted with respect to the SCE. Cyclic voltammetric experiments used a scan rate of 5 mV s-l. Results and Discussion Figure 1 shows the current/potential, charge/potential, and mass changefpotential response obtained in 0.1 mol dm-) acetic acid at pH = 2.9. The slow scan rate for which we present data (5 mV s-I) was employed to minimize kinetic effect^.^ Figures 2 and 3 summarize the observed mass changes (nanograms) per redox center (nanomoles) associated with total film reduction in 0.1 and 1 .O mol d m 3 total acetate solutions of varying pH. There are four noteworthy points. First, the mass changes are always negative, with a value of -16 g (mol thionine)-’ when pH < - 5 . This contrasts with the prediction, based on electroneutrality alone, that counterions must enter the film. Second, for pH < ca. PKHA, the mass change is independent of pH. Third, PKHA, the mass change becomes progressively less at pH negative. Fourth, the mass change is insensitive to the total acetate concentration in the aqueous bathing phase. pH < 4 . The change in mass is ca. -16 g mol-’ for both 0.1 and 1.O mol dm-3 total acetate over this pH range in the bathing solution. In the aqueous solution, the predominant acetate species at equilibrium is undissociated acetic acid, whose bathing solution concentration
-
is equal to the total acetate content, C, Over the pH range studied the change in ionic strength is relatively small; hence the activity of HA, remains constant and proportional to the total acetate (7) Bruckenstein, S.; Wilde, C. P.; Shay, M.; Hillman, A. R.; Loveday, D.
C.J . Electroanal. Chew. 1989, 258, 457.
(8) Bruckenstein, S.; Shay, M. Elecrrochim. Acra 1985, 30, 1295.
The Journal of Physical Chemistry, Vol. 94, No. 16. 1990 6461
Redox Switching in Polythionine Films 0 I+A
5 0
5 00
a -
10
mC
01
02
03
04
Figure 1. Current-(i), charge-(q), and mass change- ( A M ) potential curves obtained during the course of a cyclic voltammetric experiment using a polythionine coated electrode. The total concentration of acetate species was 0.1 mol dm-’, and the pH was 2.9. The voltage scan rate was 5 m V s-I.
o1n CH,CWH
” * . ” ’ . . Figure 2. Plot of normalized mass change on total reduction to polyleucothionine of a polythionine film as a function of pH for a series of acetic acid solutions containing 0.1 mol dm-’ total acetate species. The mass data are normalized with respect to the total charge passed to account for any slight variation in coverage. -20
Figure 3. Plot of normalized mass change on total reduction to polyleucothionine of a polythionine film as a function of pH for a series of total acetate species. The acetic acid solutions containing 1 .O mol mass data are normalized as in Figure 2.
concentration. Acetate ion and hydrogen ion activities change reciprocally by -3 orders of magnitude. Consequently, in view of the experimental observation of constant mass change over this pH range in the film, charged acetate or protonated species do not contribute to the mass change. Since the polythionine sites are net neutral, there are hardly any free anions or cations in the film.
We initially assume that there is no coordination of water and HA with either the oxidized or reduced polythionine. Thus only H 2 0 , HA, HA.H20p, and H30+, remain to be considered as possible sources of water and HA. However, eqs 3, 8, and 9 show that the activities of the neutral species are constant on redox switching and are determined by the activity of water and acetic acid in the bathing solution. The insensitivity of the mass change to total aqueous solution acetate concentration can be explained in one of two ways, both of which lead to rejection of the assumption. First, assume that the film is not saturated with either HA or HA.H20,. Then both HAP and HA.H20, should increase 18 times with a 10 times change in aqueous phase concentration. As shown in Appendix A, the mass change depends on two factors (see eqs A4 and A8). One is proportional to the aqueous phase activities of the mobile species, and the other factor contains volume and activity effects associated with redox switching. The experimental lack of sensitivity to aqueous-phase concentrations contradicts the initial assumption, and so it must be rejected. Second, assume the film is saturated with either HA or HA.H20, at both 0.1 and 1.0 mol dm-3 total aqueous-pgase acetate concentration. Then the activity of both these species does not change with HA, or oxidation state. Since the activity of water does not change with total acetate concentration in the bathing solution, this assumption leads to the conclusion that the weight changes must be associated with activity coefficient effects and/or film volume changes on redox switching. It would be a remarkable coincidence for such effects to produce an integral change in mass. This seems unlikely since \k in eq A8 would have to be one for mobile HA, species. Again, we infer that mobile HA-containing species in the polymer film phase do not contribute to the weight change and conclude that a model analogous to our strong-acid model3 does not describe this acetic acid case. We considered various models and propose that described by eq 18 as being consistent with the thermodynamic restraints, weight change data, and ellipsometric data (see below). This is a coordination model, which the experimental results force upon us. It may not describe other weak acid systems. The limiting value of AM at low pH was -16 and -12 g mol-’ (*lo%) for the total acetate concentrations employed, 0.1 and 1 mol d ~ n - respectively. ~, This result is consistent with the description given by eq 18. As the pH is increased, the bathing solution activity of HA decreases and the description of the redox process shifts toward a description encompassed by either eq 19 or eq 21. pH 5. This situation represents one intermediate to eqs 18 and 19 or eq 21. As the description of the process shifts from that given by eq 18, the mass change moves in a positive direction, i.e., becomes less negative. This is seen in Figures 2 and 3 also. Increasing the higher total acetate concentration in the bathing solution would have no effect on the relative contributions of eqs 18 and 19 to the mass change. However, since the stoichiometric coefficients m,n, y , and z in eq 18 and their primed counterparts in eq 21 may differ, the shift in the mass change break point can be rationalized by choosing appropriate values of m’, n’, z’, and Y’. p H > 5. At higher pH, the polythionine redox process converts to a 2H+/2e process (see eq 2), and the requirement for a change in the film’s total anion population vanishes. All that is now required by electroneutrality is a source of (two) protons. This must be H 3 0 + in solution (see above) or a neutral H A species in the film. Experimentally, we observe an upward trend in the mass change to less negative values of Ah4 (see Figures 2 and 3). In this pH range, the dominant form of the reduced sites is shifting from TH42+(A-)2to TH3+A- and suggests that either eq 21 or eq 22 may be used to describe the reduction process. Within the range of pH consistent with use of an acetate buffer, our data (Figures 2 and 3) show the mass change is becoming less negative. Values for m’, n’, z’, and y’exist that would yield this experimental result. A Coordination Model. We emphasize that thermodynamic relations are general and may be interpreted in a number of ways.
-
6462 The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
In the previous discussion we considered an ion aggregate model because it formally described the behavior of the switching process in strong acid s o l ~ t i o n s .This ~ description did not suffice for an aqueous acetic acid bathing solution. The thermodynamic analysis of our data led to a description of the polymer film in terms of reduced and oxidized phases containing bound water and/or HA. We now discuss a coordination model that allows a particular molecular explanation consistent with the thermodynamic theory. The special feature of this coordination model is that only two chemical species need be invoked. The two polythionine oxidation states represent these different species, each with a unique standard state, Le., each oxidation state has constant activity and unit activity coefficient. Thus, the electrode half-reaction, with its integer stoichiometric coefficients, is sufficient to describe the mass change (see Introduction). In the two redox forms of polythionine there are four potential coordination sites (two terminal amines, and heterocyclic nitrogen and sulfur atoms). I n the oxidized form, the charge is conventionally shown as residing principally on the sulfur atom2 In the reduced form, the two terminal amines carry the positive charges2 Here we explore the consequences of assuming that water and acetic acid in the film compete for these sites, to which the winners are strongly bound. Two independent pieces of evidence are consistent with this view. First, vapor-phase sorption studies performed at dry polythionine films using the EQCM4 confirm that both water and acetic acid enter the film in amounts not unduly different from the quantity of thionine sites. In particular, acetic acid is rather difficult to remove. Second, ellipsometric studies5 indicate that the volume fraction of polymer in the film is ca. 50%. Molar volume arguments imply that the remaining 50% of the volume could accommodate approximately three small molecules such as water or acetic acid. There is no evidence for aqueous regions of the type found in polymers such as Nafion. If such regions existed or if the mole ratio of ligands to coordination sites becomes very large, this coordination model would be inappropriate. Given the intimate level of mixing of the various species, it appears reasonable to expect them to interact, i.e., we assume that water and HA coordinate with the polymer. We interpret eq 18, the redox reaction from the previous section, in terms of this coordination model for pH < pKHA or eq 21 for pH > PKHA, where the coefficient for hydronium is chosen to satisfy electroneutrality. Note that these representations do not specify whether the principal interaction of the water is with HA or polymer film sites. The straightforward deduction from the mass data of Figure 2 at low pH, where the principal acetate species is HA, is that H 2 0 is the only "heavy" species transferred. At higher pH, this water is retained in the film. This explanation requires that sufficient HA is already in the film to satisfy the polymer site requirement for anion and one of the protons. Note that our definition of the stoichiometric coefficients for HA permits the electroneutrality condition to be partially satisfied by internal proton transfer within the film from HA to TH+. This model leads to a set of simultaneous equations and inequalities in the unknown stoichiometric coefficients m, n, y, and z . The solution to this problem is given in Appendix B, where it is shown that for pH < pKHA TH+A-p(H20)(HA)X 2e 2H30+, =
+ +
where X may be either acetic acid or water, and the number of X's is determined by the maximum coordination number. This chemically interesting result follows from the experimental requirement that one water molecule be expelled from the film on reduction. A priori, it was not necessary that the analysis of the coordination model in Appendix B yield reasonable integer coordination numbers for water and HA. At higher pH values, as seen in Figure 2, AM becomes less negative. This change begins in the vicinity of the pK of acetic acid. The extent of binding of HA with the film is directly related
Bruckenstein et al. to the activity of its molecular form in the solution. Thus converting acetic acid to its conjugate base in the solution will ultimately remove it from the film, and as we traverse the pH axis in Figure 2, water progressively replaces coordinated acetic acid. In the intermediate range of pH values water will compete more successfully within the film for the available coordination sites, and a pH value will eventually be reached at which the HA content of the film is no longer able to supply the required proton and anion, Le., y < 1. In this switchover region, the coordination numbers for water and HA become nonintegral because the chemical identity of the oxidized and reduced forms of polythionine is changing. Eventually, new integral values for water and HA characteristic of polythionine sites at high pH are achieved. The situation is complicated by the fact that leucothionine has a pK not far from the acetic acid, so that we do not have experimental access to the high-pH limit, reaction 21. Experiments shown in Figure 3 using a total acetate concentration of 1.O mol dm-3 are consistent with this model. The limiting value of AM at low pH was the same as that in Figure 2 within experimental error (*lo%). However, the break point in the AM-pH plot is shifted to higher pH values by about 0.75 f 0.25 unit. The direction of this shift indicates that (z - y ) increases with the equilibrium concentration of acetic acid in the aqueous phase. This is additional strong evidence for the coordination of HA by the thionine centers. Conclusion
The change in mass associated with the reduction of polythionine in weak acid media depends on the pH of the bathing solution. Analysis of mass change data at low pH according to a thermodynamic model shows that activities of mobile species in the film are unaltered by the redox switching process. Under these special circumstances, a coordination model involving water and HA competition for a minimum of three sites fits the data. At low pH, (coordinated) HA in the polymer film is able to satisfy the counterion requirement associated with reduction. There is also (coordinated) water present in the oxidized polymer, and for each redox center one molecule is expelled on reduction. As the pH is raised, HA in the film is progressively replaced by water, and any counterion required by reduction must increasingly be supplied from the bathing solution. Ultimately, the acid/base behavior of the redox sites obviates the requirement for counteranion transfer. Acknowledgment. This work was supported by the Air Force Office of Scientific Research (AFOSR Grant 87-0037) and the NATO Scientific Affairs Division (Grant 86/0830). Appendix A
At all pH values the oxidized sites may be represented by TH'A-. The nature of the reduced site depends on the pH, as implied by eqs 1 and 2. For pH > 5 it is TH3+A-. This has an important consequence for the particular weak acid used here, acetic acid, whose pK is 4.8. At low pH, the dominant aqueous acetate species is HA, Le., Ais not available. For pH >> pK, when A- is available, it is not required by eq 2. The conclusion is that we need not consider A- at any pH. Uncoordinated Polymer Oxidation States. Half-reaction 1 represents the redox reaction for the monomeric analogue in solution if we assume that the two oxidation states of polythionine are not coordinated to HA- or H20-containing species. The general representation of the polymer phase redox process explicitly including hydronium ion, A-, ion aggregates, water, weak acid, and weak acid associated with water in the polymer film phase (subscript p) and the solution phase (subscript s) and with the two redox forms of the polymer is
+ ( 1 - u)TH+A-, + xH~O', + 2H30+, + (x + u)A-, + hHA, + h*HA, + (w - x)HA.H20, + 2e = rTH42+p+ u T H ~ ~ + A+ - , (1 - r -
vTH+,
u ) T H , ~ + ( A - ) ~+, ~(2r
+ u)A-, + (2 + w)H20, ( A l )
Redox Switching in Polythionine Films
The Journal of Physical Chemistry, Vol. 94, No. 16. 1990 6463
where h+h*+w=l
(A2)
We specify the phase for each species. Their stoichiometric coefficients are chosen to satisfy electroneutrality within each phase and the overall mass balance. Equation A1 is formulated so as to be completely general with respect to values of the equilibrium constants and the activity coefficients. Under any given set of experimental conditions, certain species will predominate. The unknown quantities in eq A1 are related through the thermodynamic relationships eqs 3-10. However, as was shown previously in the case of a strong acid bathing ele~trolyte,~ net neutral ion aggregates are the principal species in the polymer film phase. We do not repeat the detailed arguments based upon the strong-acid data justifying this conclusion but merely state that the arguments apply analogously in the weak acid situation. The polymer film phase behaves as a low dielectric solvent in which extensive ion aggregation yields net neutral species. Thus the polymeric half reaction can be expressed as TH+A-,
+ 2H30+, + hHA, + h*HA, + wHA-H20, + 2e = THd2+(A-)2,, + (2
+ w)H20,
(A3)
second is small if the product of activity coefficient and volumes is near unity. In either case, the experimental results rule out a significant contribution from the change in concentration of the mobile polymer species. Thus we conclude that this model does not describe the behavior of polythionine in contact with an acetic acid bathing solution even though an analogous one was successful in strong-acid bathing electrolyte^.^ The distinguishing characteristic of this system and the previous one is that in this weak-acid case the polymer phase activity of A--containing species is fixed and independent of bathing solution pH, while this is not the situation in the strong acid case. Next we consider a model that allows for specific binding by (i) one or both oxidation states of the polymer by (ii) H 2 0 and/or HA. Coordinated Polymer Oxidation States. We expand upon the case just treated, in which all mobile polymer phase species either do not change concentrations on redox switching or are present in such low concentrations that they cannot contribute significantly to mass changes. We assume that the reduced form of the polymer has the formula TH+A- (H20),(HA),, while the oxidized form is TH42+(A-)2,p(H20)n~A~,I). Then the redox reaction can be written TH+A-,(H20),(HA),
+ 2H30+, + qHA, + (r - 2)H20, +
The coefficients {gjlin eqs A1 and A3 are related to activities by the activity coefficients rj = uj/Cj,where Cj are the concentrations, the subscript j refers to the species, and the standard state is infinite dilution in the bathing solvent phase. These gj values are required to calculate the mass change by using eq 15. Since gjmj represents the number of moles in a film of volume, V, participating in the redox process
where m r = n and y + q = z . This formulation allows for both proton and weak acid sources from the aqueous bathing solution. It assumes the existence of well-defined complexes in which m, n, y, and z are integer (or rational fractions). The change in film mass on reduction is
Ag, = A{C,V = I ViC,,i}{VfCj,f/ VjC,,i - I ) =
AM = CAgjmj = rmHZO+ ( q - 1)mHA + 2mH+
I Viaj,i/rj,ill(aj,~j,i/aj,irj,f)( Vf/ V,) - 11 (A4) where the subscripts i and f refer to the initial (oxidized) and final (reduced) polymer states. The term in the first set of braces corresponds to the number of moles of the j t h species that is present in the oxidized form of the polymer. We refer to it as the concentration term, while the second set of braces contains the “redox term”, the part of gj associated with the switching process. All the terms are thermodynamically defined. We confine our discussion to aqueous-phase pH values much less than the pK of the weak acid and low ionic strengths in water. Under these conditions, the activities of HA and water in the aqueous phase do not change with pH. Furthermore the volume of the aqueous bathing phase is enormously larger than that of the polymer phase, so that no change of aqueous concentrations occurs because of the redox switching processes. Thus according to eq 3, the activity of water remains constant in the polymer on redox switching. The activity of water is independent of the total acetate concentration in the aqueous bathing solution. Furthermore, from eqs 8 and 9 we see that the activities of HA, and HA-H20pare both proportional to the total aqueous phase concentration of HA. In this situation it is convenient to define the quantities CY
= (rPHA.i/YPHA,f)(Vf/Vj)
(‘45)
2e = TH42+(A-)~,p(H20)nHA(r-l) (A9)
+
(‘410)
The experimental decrease in mass, -16 g mol-I, is found if, for example, r = -1 (one water molecule is expelled from the film) and q = 0 (no H A enters or leaves the film). This leads to a formulation of the redox reaction as TH’A-,(H20),(HA),
+ 2H30+, + 2e =
TH42+(A-)2,pH20(m-,)HA~l)
+ 3H20s
(A1
Thermodynamic arguments do not allow anything more to be said about the absolute values of m and n. This point is discussed in more detail in Appendix B.
Appendix B The low-pH limit of the data shown in Figure 2 indicates that AM for the reduction process is about -16 g mo1-I of redox sites. The equations to be solved in the framework of the coordination model are as follows. Mass change: AM = 18(n - m)
+ 60(z - y) + 2
(B1)
Site occupation: (i) oxidized form
m+y=3
(B2)
(ii) reduced form
n+z-1
(B3)
where the individual coordination numbers cannot be negative. For reaction 18: since the change in the number of moles for these species is then given by ASj =
(viaj,i/rj,ill*
- 11
(‘48)
where 9 = a,fl, or 6 and j = HA, H 2 0 , or HA.H20. This result shows that all net neutral, mobile species in the polymer phase are likely to change in a similar manner. As the experimental mass change is insensitive to the concentration of HA in the aqueous bathing phase, either or both of the terms in the braces in eq A8 must be small. The first term is small if the species does not exist in appreciable concentration in the film. The
OIn+z-113 Only the values -3 I y - z + 1 I3 are allowed.
(B4a)
For reaction 2 1 : 0 I n’+ z’I3
Only the values -3 I (y’- z’) I +3 are allowed.
(B4b) (B5b)
We do not treat the high pH case, reaction 2 1, because we do not have experimental access to AM for this limiting case. The procedure would be analogous to that given below for the low pH limit. In the case of acetic acid at low pH (see main text)
J. Phys. Chem. 1990, 94, 6464-6467
6464
Eliminating m and n between eqs B1, B2, and B4a yields
- 18 I4 2 b - Z )
I36
037)
Since y and z represent integer coordination numbers, they must be equal to satisfy eq B7. The significance of this in reaction 18 is that one initially coordinated HA transfers its proton and provides the charge compensating anion on reduction of TH+. Substituting y = z into eq B1 yields n-m=-l
(B8)
This relation tells us that one water molecule is expelled from the film on reduction. Furthermore, by combining eqs 82, B6, and B8 and recalling that n cannot be negative
or m = 2 and y = l We can now write eq 20 as TH+A-&H,O)(HA)X
(B9b)
+ 2H3O+, + 2e = TH42'(A-)2,d(
+ 3Hz0, (B10)
where X may be either acetic acid (eq B9a) or water (eq B9b). Note that had we assumed a coordination number larger than 3, say 3 + K, then "X" would simply be replaced by UXK+ln.In other words, the model is blind to species not participating in the overall redox process. Registry No. Polythionine, 87257-37-2; acetic acid, 64-1 9-7.
Enhanced Oxygen Storage Capacity of Cerium Oxides in CeO2/La2O3/AI2O, Containing Precious Metals Takeshi Miki, Takao Ogawa, Masaaki Haneda, Noriyoshi Kakuta, Akifumi Ueno,* Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan
Syuji Tateishi, Shinji Matsuura, and Masayasu Sat0 Cataler Industrial Co., Chihama. Daitocho, Ogasagun, Sizuoka 437- 14, Japan (Received: January 22, 1990)
Effects of the addition of precious metals (PM; Pt, Rh) on Ce02/A1203and CeO2/LaZO3/Al2O3were confirmed to enhance the oxygen storage capacities (OSC). Increments in the OSC of the added CeO2/LazO3/AI2O3catalysts were much greater than those in the OSC of the PM added CeO2/Al2O3.The enhanced OSC is ascribed to the interaction between PM and a Ce02-La203solid solution formed during the catalyst preparation. No enhancements in the OSC were observed on physical mixing of CeO2/La2O3/AI2O3and Pt-Rh/A1203, although the composition ratio of PM:Ce02:La203is the same as that in the PM added Ce02/La203/A1203.This indicates that the intimate contacts between the precious metals and Ce02particles dispersed on A1203are essential for the enhanced OSC of cerium oxides.
Introduction
Cerium oxide has often been dispersed in an automotive exhaust catalyst to prevent an active alumina, the catalyst carrier, from the thermal sintering.I The role of cerium oxide is not only the thermal stabilization of the active alumina but the extension of the "window" of the air/fuel ratio where the catalysts can work to reduce NO, and to oxidize C O and C,H, simultaneously.z Because of the low redox potential between Ce3+ and Ce4+ (1.7 V),3 CeOz dominates in the oxidative atmosphere, while in the reducing circumstances CezO3 becomes predominant: 2Ce0, F? Ce2O3 '/202. Thus, according to a cyclic rich-lean composition fluctuation in the automotive exhaust gas, the cerium oxides can either provide oxygen for the oxidation of C O and C,H, or remove oxygen from the gas phase for the reduction of NO,.4 The amounts of oxygen reversibly provided in and removed from the gas phase are called "OSC" of ceria.s An enhanced OSC of ceria in CeO2/AI2O3has been reported when precious metals such as Pt, Pd, and Rh are highly dispersed on the catalyst.6 The enhanced OSC has been attributed to an
increase in the dispersion of ceria on alumina, caused by the PM addition. Recently, the interaction between precious metals and ceria in PM/CeO2/AI2O3catalysts has been studied by XPS to demonstrate that the precious metals reversibly facilitate the reaction CeA103 F? Ce02 A1203.6,7Jin et aI.* are of the opinion that a lattice oxygen and, conversely, an oxygen vacancy in a ceria crystallite play a substantial role in the oxidation of C O and the reduction of C 0 2 over Pt/Ce02 catalyst. Dissolution of La3+ ions into CeOz lattice to form a La202-Ce02 solid solution has been reported by Zintl and C r ~ a t t o . Miyoshi ~ et a1.I0 have also found the dissolution of La3+ ions into C e 0 2 lattice to be responsible for the enhanced activity of a PM/CeOz/La2O3/AlZO3catalyst for the simultaneous abatements of NO, CO, and C,H, at low temperatures.Il They have concluded that the enhanced activities are attributed to an accelerated diffusion of oxygen ions in the ceria-lanthana solid solution. In the present work, PM/CeO2/(La2O3)/AI2O3catalysts were prepared by two different methods. One is a subsequent impregnation of alumina with an aqueous solution containing Ce4+
( I ) Yu-Yao, Y. F.; Kummer, J. T. J . Caral. 1987, 106, 307. Su,E. C.; Rothchild, W. G. J . Catal. 1986, 99, 506. Su, E. C.; Montreuil, C. N.; Rothchild, W . G. Appl. Caral. 1985, 17, 75. (2) Herz, R. K. Ind. Eng. Chem. Prod. Res. Dew. 1981, 20, 451; 1983, 22, 387. Gandhi, H. S.: Piken, A. G.;Stepien, H . K.: Shelef, M.; Delosh, R. G.; Heyde, M. E. S A E Preprint 770196, Detroit, MI, 1977. Kim, G. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 267. ( 3 ) Kagaku-Binran, Kisohen II; Japanese Chemical Society, Ed.; Maruzen: Tokyo, 1984; p 11-476 (in Japanese). (4) Loof, P.; Kasemo, B.; Keck, K.-E. J . Catal. 1989, 118, 339. (5) Yao, H . C.: Yu-Yao, Y. F. J . Catal. 1984, 86, 254.
(6) Shyu, J . Z.; Otto, K. J . Catal. 1989, / I S , 16. Shyu, J. Z.; Otto, K.; Watkins, W. L. H.; Graham, G.W.; Belitz, R. K. J . Caral. 1988, 114, 23. (7) Mizuno, M.; Berjoan, R.; Coutures, J. P.: Ferox, M.; Yogyo-KyokaiShi 1975, 83, SO. (8) Jin, T.: Okuhara, T.; Mains, G.J.; White, J. M. J . Phys. Chem. 1987, 91, 3310. (9) Zintl, E.; Croatto, U . Z . Anorg. Allg. Chem. 1939, 242, 79. (IO) Miyoshi, N.; Matsumoto, S.; Kimura, M.; Muraki, H. Preprint in Meeting (A) of Catalysis Society of Japan, 4E07: 1987 (in Japanese). ( 1 !) Ozawa, M.; Kimura, M.; Miyoshi, N.; Matsumoto, S. Preprint in Meeting (A) of Catalysis Society of Japan, 4B222; 1988 (in Japanese).
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0022-3654/90/2094-6464$02.50/0 0 1990 American Chemical Society
