Voltammetry of vanadyl sulfate hydrates in the absence of a deliberately added liquid phase

163

J. Electroanal. Chem., 323 (1992) 163-178

Elsevier Sequoia S.A., Lausanne

JEC 01813

Voltammetry of vanadyl sulfate hydrates in the absence of a deliberately added liquid phase Waldemar Gorski * and James A. Cox * l

Department of Chemistry, Miami University, Oxford, OH 45056 (USA)

(Received 1 July 1991; in revised form 2 September 1991)

Abstract Charge-transfer processes in solid samples of VOSO,.x H,O (x = 3 or 5) were studied using an ultramicroelectrode as the indicator and a 3 mm diameter glassy carbon disk as a quasi-reference electrode. The use of the solid matrix facilitated the electron transfer process in the V(IV)/V(V) couple. In contrast to solution-phase systems, reversible cyclic voltammograms were obtained, and the electrode surface did not undergo rapid poisoning. Voltammetty indicated that the electrode process was diffusion controlled for x = 5, but a surface-confined pathway predominated when x = 3. From constant potential chronoamperometry with solid samples of VOSO,.5 H,O, the apparent diffusion coefficient of charge propagation was 3 x lo-’ cm’ s-t, and the population of trapping sites was 1 mol dmd3. The redox conductivity of the solids was related to their crystallographic structures. The possibility of the existence of a liquid microphase in these solids is discussed.

INTRODUCTION

The sensitivity of charge-transfer processes to the local environment of trapping sites is an important consideration in both fundamental and applied studies of such topics as electrode passivation and electrocatalysis. One common approach to this problem is to investigate matrix effects on a chosen redox reaction. In contemporary electrochemical studies a variety of reaction media other than aqueous solutions are employed, including aqueous + organic mixtures, non-aqueous organic solvents, solid and concentrated-fluid electrolytes, fused salts, organic and inorganic polymer hosts, microemulsions, macromolecular assemblies and frozen solvents. The utility of certain of these media has been augmented by the theoretical and practical development of ultramicroelectrode techniques D-31.

l l

On leave from the Department of Chemistry, Warsaw University, Pasteura 1, Warsaw 02093, Poland. * To whom correspondence should be addressed.

0022-0728/92/$05.00

0 1992 - Elsevier Sequoia S.A. All rights reserved

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The present paper shows that charge-transfer processes in solid samples of simple inorganic salts, VOSO, .x H,O (x = 3 or 5), can be monitored by familiar voltammetric methods with ultramicroelectrodes as indicators. Demonstrated herein is that the elaborate pretreatment of solid electrode surfaces, which is essential for the appearance of reliable voltammetric signals of the V(IV)/V(V) system in aqueous solutions [4], is not necessary for obtaining reversible cyclic voltammetry of that couple in the solid state. This study also suggests a possible means of direct determination of the oxidation-reduction characteristics of solids. This is of particular interest for the preparation and characterization of solid catalysts. For example, high-valent vanadium compounds are known to be selective catalysts in solid/gas systems, including large-scale commercial processes [5,6]. These catalytic reactions are believed to proceed through the V(IV)/V(V) redox couple. Moreover, vanadyl sulfate hydrates, at least those with x G 3, exist as layered complexes-intercalates [7]; hence, they may serve as host matrices for solid state voltammetry of species incorporated therein. EXPERIMENTAL

Chemicals Samples of VOSO, * 3 H,O (99.99%, Aldrich) and VOSO, - 5 H,O (96%, Fluka) were used without further purification. Comparison of the voltammograms in both solid and solution-phase systems showed that the impurities in the latter salt are not electroactive in the range studied. As these compounds are hygroscopic, they were stored in tightly closed vessels in a desiccator. Immediately before an experiment they were ground with a mortar and pestle for 2 min in ambient atmosphere. Vanadyl sulfate hydrates consist of microcrystallites which stick together forming much larger domains, on the average, than the size of the ultramicroelectrode used. The cerium(IV) sulfate titrant (0.010 mol dme3) for the determination of the charges of the electrolysis products was prepared according to a standard analytical procedure from reagent grade salt and distilled water purified with a Sybron/Barnstead NANOpure II cartridge system. Electrodes and electrochemical cell Solid-state electrochemical experiments were carried out in the two-electrode mode. A 10 pm carbon disk, purchased from Bioanalytical Systems, Inc. (BAS, MF-20071, was the working electrode (WE) and a 3 mm diameter glassy carbon disk (BAS, MF-20212) served as the counter/quasi-reference electrode (QRE). Such an electrode configuration favors practical non-polarizability of the reference interface because the very small (within the nanoampere range) currents flowing through the working ultramicroelectrode result in a small current density at the QRE. The ratio of the area of the QRE to that of the WE is 9 X 104. We have found that potentials measured vs. QRE were stable and reproducible during experiments lasting several hours.

165

b)

Gc -

10

M

dia.

IT0 concktive glass

solid samDIe

tube

Fig. 1. Electrochemical trolysis experiments.

-3

GC m dia.

cmcktive

glass

on IT0 swface

soltd sample

cells for the study of solids. (a) Ultramicroelectrode

studies, and (b) bulk-elec-

The cell was constructed with both electrodes mounted in Teflon rings which slide inside a glass tube in a very precise manner (Fig. 1). The cell was loaded by placing a weighed sample of solid material (ca. 15 mg) on the QRE which was positioned in the holder. A glass tube was pressed tightly onto the Teflon ring around the electrode, and the WE was then gently lowered against the solid sample. Finally, the upper part of the cell was closed tightly with 100 g of mercury. Consequently, solid samples with a thickness of ca. 0.3 mm were held between the electrodes under a pressure of 1.6 kg cm -2. Voltammetric experiments were carried out under a dry argon atmosphere with the gas drains closed tightly (see Fig. la>. It should be pointed out that cyclic voltammograms generally were not stable during the first minutes after setting up the cell. Presumably, contact between the ultramicroelectrode surface and solid microcrystalline phase is not well-established initially, thereby affecting the current flowing through the system; desorption of water picked up during the grinding process may also be a factor. However, once the system stabilizes, cyclic voltammograms are repeatable for at least several hours. For example, with the trihydrate as the sample, voltammograms were constant over an 8 h experiment. Sample-to-sample variation in grain size and variation of the cell loading, which are problems discussed below, limited the reproducibility of the current to about 20-50%. The half-wave and the peak potentials were repeatable to within +5 mV. The glassy carbon (GC) electrodes were polished prior to the experiments on an Alpha A polishing cloth (Mark V Lab) with successively smaller particles (1.0, 0.3 and 0.05 pm diameter) of alumina suspended in 17.6 MR cm water. After each polishing step the slurry accumulated on the electrode surface was removed by a 2 min ultrasonication in a closed beaker of pure water, followed by washing with water. In some experiments the GC surface was oxidized at 1.8 V vs. an Ag/AgC1/3 M NaCl reference electrode (Bioanalytical Systems, Inc.) for 5 min in 0.05 mol dmp3 H,SO, prior to use. Indium-tin oxide (ITO) conductive glasses were washed consecutively for 5 min with a detergent solution, water, and methanol in an ultrasonic bath. After drying, the IT0 was immediately used in bulk electrolysis experiments on thin layers of

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vanadyl sulfate hydrates. A narrow scratch was made through the IT0 to provide a two-electrode (biamperometry) cell (Fig. lb). Potentiometric titrations of both fresh and electrolyzed samples of vanadium sulfate with an acidic cerium(IV) sulfate solution were carried out using a platinum indicator electrode and an Ag/AgC1/3 M NaCl reference electrode. All experiments were carried out at ambient temperature (23 f 2°C). Instrumentation

In the ultramicroelectrode experiments, the voltammograms were recorded using a BAS CV-37 Voltammograph and a Hewlett Packard 7015B x-y recorder. Current data in the single potential step chronoamperometric technique were acquired at 200 Hz using the BAS CV-37 Voltammograph in conjunction with a DASH 16 A/D interface board (Metrabyte Corp.) installed on an IBM PC/XT computer and controlled by an ASYSTANT+ software package (MacMillan Software). A BAS 100 Electrochemical Analyzer was used to perform bulk electrolysis experiments with 1 x 0.5 cm IT0 electrodes or two 3 mm diameter GC electrodes. The output was sent to either a Houston DMP-40 or a Hewlett Packard 7470A digital plotter. Ultramicroelectrode experiments were performed in a grounded Faraday cage. RESULTS AND DISCUSSION

Ultramicroelectrode studies of solid samples of VOSO, *5 H,O and VOSO, - 3 H,O

In the present study, ultramicroelectrodes were used primarily for two reasons. The low current flow inherent with ultramicroelectrodes causes essentially no change of the redox state of the bulk sample. Second, as the solids that were studied have greater resistance than typical electrolytes, the minimization of the ohmic distortion under the low current that is observed with these electrodes simplifies the interpretation of the data. Cyclic voltammograms (CVs> under certain conditions had the sigmoidal shape that is characteristic of the behavior under radial diffusion control in aqueous solutions [l-3]. For example, Fig. 2 shows CVs for the oxidation of solid VOSO, - 5 H,O samples on a carbon ultramicroelectrode recorded with the use of the cell shown in Fig. la. The CVs are sigmoidal with a well-defined plateau. In the scan rate range of OS-50 mV s-l the CVs are essentially free of background currents. The nature of the solid-state voltammograms was examined by a conventional semilogarithmic analysis, as we found that the currents on the positive-going potential scan followed the expression [g-lo]: E=E0,+2.303(nf)-’

log[Z/(I,-I)]

(1)

where f = F/RT. Plots of log[l/(l, - 111vs. E for the rising portions of the voltammograms are a parallel set of straight lines (correlation coefficients, 0.99950.9998) with inverse slopes equal to 58 f 1 mV. This behavior suggests that the

______-.----/’

/.

7

-f

/

I 0.0

25 nA

c~--._._.___/ E /V

Fig. 2. Ultramicroelectrode ) 0.5 mV s-l. (-

vs.

1 .o

GC

cyclic voltammetric curves of solid VOSO,.5

H,O. c-.-.-J

50; (- - -) 2;

overall electrode process is: V(N)

- e-z= V(V)

(2)

In addition to demonstrating that reaction (2) is reversible within the experimental time domain, the results indicate that ohmic drop effects in the system are negligible. Another characteristic feature of the CVs shown in Fig. 2 is that at slow sweep rates (u < 2 mV s-l) the current during the reverse scan almost retraces that observed during the forward scan. Further, the limiting current is almost independent of scan rate. Such behavior is indicative of virtual steady-state conditions. A true steady-state limiting current ZUSSj(eqn. 3) is independent of scan rate [2,3]: ZLCSSj = 4nFDcr

(3)

As the scan rate is increased above 2 mV s-l, the limiting currents of the voltammograms increase. In these cases, the currents of the reverse wave do not retrace the positive-going scan, and the difference between the anodic and cathodic half-wave potentials increases with scan rate. These observations demonstrate that the system does not reach a true steady state. The deviation of the voltammograms from true steady-state behavior can be estimated [9] with the theory elaborated by Aoki et al. [ll] using the equation: Zrn/ZI_os,= 0.34 exp( -0.66~)

+ 0.66 - 0.13 exp( -11/p)

+ 0.351~

(4)

where Z, is the maximum current (I, in our case), and the dimensionless parameter, p = [(r2u/D)nf]1/2, is a function of the microdisk electrode radius, r, the potential scan rate, o, and the diffusion coefficient, D. Generally, these parameters determine the mode of diffusion to the microdisk electrode, thereby controlling the shape of the voltammograms [ll]. The steady-state character of the voltammograms is favored by small values of p; when p + 0, the right hand side of

168

0.0

’ 0

1

2

3

4

5

6

t/s Fig. 3. Ultramicroelectrode chronoamperometric oxidation of solid VOSO,.5 H,O: (0) experimental ) chronoamperometric curve obtained by fitting of eqn. 5 (see text) to the experimental points; (points. Potential step, 0.2 V to 0.8 V vs. QRE (glassy carbon).

eqn. (4) approaches 1.0. Consequently, from the value of p, the percentage of deviation from the steady state can be estimated provided that D is known. Formally, D could be determined with the use of eqn. (3) and the voltammetric limiting current at u < 2 mV s-l. However, in the present case, a rather ambiguous a priori assumption about the reactant concentration, c, in a solid material would be needed. Instead, a chronoamperometric method [12,13] was used which allows the simultaneous determination of both D and c. The potential step was set from 0.20 V to 0.80 V, which correspond to the foot and the plateau, respectively, of the oxidative CVs in Fig. 2. A typical chronoamperometric Z-t transient is shown in Fig. 3. Each experimental point represents the average of several trials on a sample; the relative standard deviations are smaller than the plotted point diameters. It should be pointed out that the chronoamperometric and voltammetric data sets are consistent as chronoamperometric currents extrapolated to long times reach a value almost equal to the limiting current in voltammetry with u < 2 mV S--l.

The values of D and c were calculated by non-linear curve fitting of go-point experimental sets of current-time data to the equation proposed by Shoup and Szabo [13] Z/4nFDcr

= 0.7854 + 0.8862~-‘/~

+ 0.2146 exp( -0.7823r-‘/2)

(5)

where r is 4Dt/r2. Equation (5) is accurate to 0.6% for all times of chronoamperometric electrolysis [13]. During the fitting procedure the allowed difference in the sum of squares in the consecutive iterations was smaller than 0.01%. Figure 3 shows that the Z-t curve predicted by eqn. (5) agrees well with the experimental points. The final results of four independent experiments have shown that the calculated values of

169

the current agreed with the experimental ones within f 1% when D = 3.0 + 0.7 X lop7 cm* s-l and c = 1.0 f 0.1 mol dmm3. Now, taking D = 3 x 10e7 cm* s-l and using eqn. (4), an estimate of the deviation of the CVs in Fig. 2 from true steady-state can be made. Such calculations suggest that for u = 0.5 (p = 0.131, 2 (p = 0.26), and 50 (p = 1.28) mV s-r, the CVs deviate from true steady-state waves by 1.7%, 3.7% and 25.5%, respectively. These values agree quite well with the percent increase in the limiting currents with scan rate. A result of this data treatment is evidence that migrational effects on these voltammograms are not large 1141. As discussed previously, the semilogarithmic plots of the voltammetric curves shown in Fig. 2 are linear and parallel to each other even though they represent cases of increasing deviation from true steady state. This is rationalized by the recent report [15] that all points on a reversible wave should be reached equally fast, and, consequently, the near steady-state voltammograms can lie parallel to their true steady-state counterparts [16]. Such behavior of our system is another indication that reaction (2) occurs rapidly under the present conditions. Further, assuming that our system is “effectively reversible” according to Oldham’s nomenclature [17] and using an analogy to diffusion-controlled systems in solution, the lower limit for the standard rate constant of reaction (2) is 0.03 cm s-l from eqns. (4) and (6) in ref. 17. Quite different voltammetric behavior was found with VOSO, .3 H,O as the sample when studied over the same range of potential scan rates as that used on the pentahydrate. Cyclic voltammograms of solid samples of VOSO, * 3 H,O,

E /V

vs. GC

E IV

vs. GC

Fig. 4. Cyclic voltammetry of solid VOSO,.3 H,O at an ultramicroelectrode. mV s-l. Sensitivity S/nA: (1) 0.24; (2) 0.11; (3) 0.011; (4) 0.003.

(1) 50; (2) 16; (3) 2; (4) 0.5

170

which are shown in Fig. 4, are not sigmoidal; instead, they are peak-shaped. Moreover, the currents are much lower than those in the case of pentahydrate samples. Linear least-squares analysis of the data in Fig. 4 shows that the peak current is directly proportional to the scan rate (correlation coefficient, 0.9998). The total width at half-height of both the cathodic and anodic peaks is in the range 90-110 mV, with the latter peaks about 5 mV broader than the former. The integrated charges under the anodic and cathodic peaks (1.5 nC) are equal to within f5%, and the total charge under the i-E curves recorded at u 2 2 mV s-l is independent of u. These findings are generally characteristic of electrode processes that do not involve diffusion in the bulk sample [18]. Using the value of the charge obtained by integrating the voltammetric peaks and assuming that the concentration of the electroactive species in the solid sample is in the mol dmv3 range, the thickness of the electrolyzed material layer at the ultramicroelectrode surface is on the order of lop2 ,um. The electrode process related to the CVs in Fig. 4 approaches reversible behavior. The cathodic and anodic peak potentials are separated by only 10 mV at 2 mV s-l, and the peak shapes in the range 2 mV s-l Q u G 50 mV s-l are characteristic of the CVs for a reversible oxidation of a surface-confined component. An important feature of the trihydrate system is that as the potential scan rate is decreased below 2 mV s- 1 the CVs change from symmetrical peaks to currentpotential curves with diffusional tails (Fig. 4). This transition from thin layer to bulk transport behavior as the scan rate decreases is consistent with other ultramicroelectrode studies [19]. A difference in charge transport rates in VOSO, * 3 H,O and VOSO, * 5 H,O matrices is evident from a comparison of Figs. 2 and 4. The radial diffusion limit of the current with the pentahydrate is in marked contrast to the surface-confined limit with the trihydrate at scan rates u & 2 mV s-l. Electrochemical behavior and crystallographic structure The foregoing discussion shows that ultramicroelectrode theory, though elaborated for liquid systems, can explain the voltammetric behavior of solid hydrates of vanadyl sulfates. However, several important questions remain. These include identifying charge carriers in the system, elucidating the mechanism of the charge transport within the solid material, determining why reaction (2) is reversible in solid matrix voltammetry when it is usually irreversible in aqueous solution, and establishing the nature of the electrode process at the interface of the quasi-reference electrode in the ultramicroelectrode experiments. The structures of the solid materials studied are important in addressing these questions. The crystallographic structure of VOSO, .5 H,O can be considered to be a monoclinic structure composed of molecular units comprising a [VO,] distorted octahedron with a [SO,] tetrahedron sharing one apex. These units are held together in the crystal structure by an extensive net of hydrogen bonds. Two orthorhombic forms of the pentahydrate are known, but they are unstable, trans-

171

forming into the monoclinic structure [20,21]. The orthorhombic trihydrate is also unstable in air under ambient conditions in that it rapidly deliquesces 1221, a phenomenon that was not apparent in our experiments. Detailed analysis [20,23] of monoclinic VOSO, - 4 Hz0 has shown the following: each vanadium ion is coordinated by oxygens of four water molecules and one oxygen of the sulfate group, the latter being in a cis position to the vanadyl W=O) oxygen atom; the fifth water molecule of each asymmetric unit is not bound to the vanadium ion but instead exists as free water of crystallization, although it is also involved in hydrogen bonds; and the V=O bonds of the molecules are nearly parallel. In the monoclinic form of VOSO, * 3 H,O, all water molecules are bound directly to vanadium ions, and the basic units of the crystallographic pattern are composed of two [SO,] tetrahedra and two [VO,] octahedra linked together [24]. As in the case of the pentahydrate, the whole structure is maintained by an extensive network of hydrogen bonds. The questions concerning the charge carriers in the solid-state voltammetry of these salts relate to the problem of maintaining macroscopic electroneutrality inside the sample during the faradaic steps. In the absence of an external supporting electrolyte, the system has to use an internal supply; that is, “the ion budget” [25,26] involves only charged species from the vanadyl sulfate hydrates. In the simplest view, the passage of the current through the sample during the oxidation of V(W) to V(V) would be accomplished by the independent movement of the sulfate anions towards the anode and of the V(V) cations towards the cathode. However, this mechanism is unlikely as the vanadium and sulfate ions are bound in a manner analogous to a contact ion-pair in liquid media of low dielectric permittivity. Further, such a rearrangement of the bulk sample would induce gradual degradation of its structure, at least at the interface with the electrode, thereby affecting the voltammetric response of the system. In fact, continuous cyclic voltammetry over a period of several hours did not show any change in behavior. Apparently, the structure-determining cores of V02+ and SOi- are immobilized in the solid network. An alternative explanation is that the electrochemical charge is transported by electron hopping [271 between neighboring WV) and V(V) centers with the latter produced at the electrode surface. Electron hopping in solids is a widely recognized charge transport mechanism in other solid systems [28,29]. It assumes that the electron self-exchange occurs in conjunction with a charge-compensating motion of a counterion. Considering the acidic properties of high-valent transition metal cations [30], the counterion in our system is probably a proton. Indeed, IR spectroscopic studies [31] show that the O-H stretching band of the waters of hydration shifts towards smaller energy as the cation oxidation state and the electrolyte concentration of the solution are increased. Considering the solid hydrates of vanadyl sulfate as a limiting case of a very concentrated, high-valence electrolyte allows the hypothesis that the O-H bonds are polarized to the point where the proton is sufficiently acidic to transfer through the hydrogen bond network. This is especially feasible in the case of pentavalent vanadium ions.

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Relevant to this issue is the recent study on the role of lanthanum011) cations in developing strong acidity in solid zeolitic matrices [32]. The model of conduction by electron hopping between fixed V(W)-V(V) sites allows two possible interpretations of the meaning of the diffusion parameter (3 x IO-’ cm* s-l in the case of VOSO, * 5 H,O). The interpretations are related to whether the rate-determining step (rds) is the movement of counterion (proton) to preserve macroscopic neutrality or is the intrinsic rate of electron exchange between redox sites within the sample. Regarding the latter, in concentrated aqueous solutions (6.5 mol dme3 hydronium ion) the rate of electron exchange between V(N) and V(V) ions is fast, k = 1.5 X lo6 dm6 mol-* s-l [33]. However, in the present study, electron exchange is probably not the rds. The shorter minimum distance between vanadium ions in the crystallographic structure of VOSO, - 3 H,O (0.473 nm) than that in VOSO, - 5 H,O (0.561 nm> 1231 should favor electron hopping in the trihydrate relative to the pentahydrate. Consequently, a higher rate of charge transport within the structure of the trihydrate would be expected with electron exchange as the rds. In fact, the opposite is observed in voltammetry at ultramicroelectrodes. Thus, movement of the counterion (proton) is apparently the rds. A higher charge transfer rate in the pentahydrate than in the trihydrate is consistent with the difference in the water content of these structures. If the free water of crystallization in the network of the pentahydrate plays a crucial role in promoting proton mobility, hindered proton movement would occur with a decrease in water content. Consistent with that prediction, voltammetry performed with the vanadyl sulfate sample in a chamber that was evacuated to 6 x 10m3 Torr did not yield a measurable faradaic current. Consequently, the transport parameter, D, can be considered as the apparent diffusion coefficient of protons in the pentahydrate network. The chronoamperometric studies of VOSO, * 5 H,O samples at an ultramicroelectrode also yielded the concentration parameter, c. The meaning of the value, 1 mol dmp3, is somewhat ambiguous. Formally, this parameter should relate to the concentration of diffusing ions (protons); however, in this case 1 mol dme3 seems too high. In the study of a solid, c may also reflect the population of redox sites that are made electroactive in response to the movement of protons. This subset of redox sites is much lower than the total concentration of vanadium ions in solid VOSO, .5 H,O, 8.13 mol dmp3, which is calculated from crystallographic data in ref. 23. The low ratio of electroactive-to-total vanadium in the sample that is suggested can be rationalized by assuming that only the surface layer of the material grains is electroactive. It is interesting to note that such an interpretation agrees with the conclusion of Pepera et al. [6], which was drawn from pulse redox studies performed under quite different experimental conditions (WO),P,O, samples at 4OO”C),namely that the catalytic activity of the solid vanadyl salt in the solid/gas system is determined by the redox process V(N) - e- + VW) occurring in the near-surface layers of the material grains. The above interpretations involve the assumption that in the dry argon atmo-

173

sphere of the voltammetric experiments a liquid phase was not present. It is important to consider the possibility that a liquid microphase may be present, thereby accounting for the solution-like voltammetry of nominally solid vanadyl sulfate hydrates. Such a liquid microphase was postulated recently in the case of cryo-electrochemistry of multicomponent systems [34-361. The phase diagram of the VOSO, + Hz0 system (see Fig. 40 in ref. 37) does not preclude the presence of an Hz0 phase in the case of the pentahydrate at room temperature (23°C). The excess H,O may come from adsorption. However, the experiments were performed more than 40” below the appropriate peritectic point (665°C) for the trihydrate/ pentahydrate in the phase diagram. Hence, isothermic (23°C) phase transformation of the trihydrate exposed to the ambient atmosphere (here, only during the few minutes of sample grinding) should proceed macroscopically solely to the higher solid hydrate. The question is whether the pentahydrate domains (if any) formed in the trihydrate sample can be transformed further to yield a liquid microphase. Assuming that this transformation is feasible, the difference in voltammetric behavior between the penta- and trihydrate could be explained by a difference in the liquid microphase concentrations in these matrices. The formation of a liquid microphase cannot be discounted on the basis of the phase diagram, especially since slow diffusion in the solid state can promote heterogeneity. However, we consider this as an unlikely explanation for the voltammetric behavior because of our observation of facile electrochemical kinetics with the solid samples. A liquid microphase would be a concentrated (in the range of few mol dmP3) aqueous solution of vanadyl sulfate. Under such conditions, hydrolytic, polymerization, and ion association processes are generally very extensive. All these phenomena are known to affect the redox chemistry of vanadium by making the kinetics of reactions of the higher oxidation states complex and sluggish [4,38]. In accord with this behavior, we observed that prolonged bathing of solid vanadyl sulfate samples with water-saturated argon resulted in ill-defined and drawn out voltammograms of the pentahydrate and an increase in both the separation of voltammetric peaks and the widths of the peaks at half-height in the case of the trihydrate. These reports are in marked contrast to the simple and reversible redox behavior we observed for VOSO, .5 H,O and VOSO, +3 H,O in a static dry argon atmosphere. Bulk phase chemistry alone is probably not responsible for the marked difference between the voltammetric behavior observed in the present study and in aqueous solution. The activity of the indicator electrode surface is perhaps also an important factor, especially in comparison to studies in strongly acidic media. It is well known that the irreversibility of the oxidation of VW) to V(V) in acidic aqueous media is contributed to by the sensitivity of that couple to the state of surface of the solid electrode [4]. In fact, the V(IV)/V(V) couple in acidic aqueous solution behaves reversibly when studied with a platinized platinum electrode in an experimental design that includes nearly-continuous renewal of the surface layer 1391. In the solid-state experiments, reversible voltammograms are obtained with-

174

out any special treatment of the indicator electrodes. Moreover, the change in behavior with time that is often seen with solid electrodes in studies of solutions [40] is not observed with the present systems. An alternative explanation for the voltammetric reversibility is based on the possibility that the coordination spheres of the vanadium ions are frozen in the solid state. For example, in non-aqueous media, vanadyl ions with the first coordination sphere frozen by complexation with a multidentate ligand are oxidized in a reversible manner on a platinum electrode [41]. In the present case, the V=O entity may remain intact in the crystallographic structure during the [V02’] to [V03’] transition. Indeed, it is well known that the V=O possesses exceptional stability; V03+ ions are found in solid matrices such as VOPO, [42]. Although either the electrode surface factor or the stability of the coordination spheres of vanadium ions during electrolysis may account for the reversibility of reaction (2) in this study, the latter is considered to explain our observations primarily. An important question regarding the overall voltammetric behavior concerns the redox reaction occurring at the GC counter electrode in our system. Because V(W) is an intermediate oxidation state, it is possible that oxidation to V(V) at the ultramicroelectrode indicator and reduction to WIII) or WI) at the counter electrode occur. However, the electrochemical reduction of V(W) proceeds at a large overvoltage [4]. In this regard, we have found that electrolysis of solid vanadyl sulfate hydrates sandwiched between two large (0.07 cm21 glassy carbon electrodes occurs only when a large potential difference (1.8 V) is imposed on the system. This potential difference is about 1 V greater than that required to drive the oxidation on the ultramicroelectrode (see Figs. 2 and 4). Hence, a process other than the reduction of VW) is suggested to occur at the counter electrode during the oxidation of VW) at the ultramicroelectrode. This hypothesis is based on the acidic properties of the solids studied and the acid-base properties of the glassy carbon surface. Among the carbon-oxygen functional groups present on the carbon surface are carboxyls, phenols, quinones, and hydroquinones [43]. The acid-base properties of these groups account for the observed linear plots of the potential of carbon electrodes vs. the logarithm of acid concentration [44]. In the present case an interaction of these surface groups with the acidic sites of the solid vanadyl sulfate hydrates can result in establishment of a potential at the interface of the VOSO, *X H,O sample and the glassy carbon counter electrode. Moreover, this potential can be quite stable in the ultramicroelectrode experiment due to very low currents flowing through the system and the relatively large area of the GC counter electrode. Hence, under such conditions this junction may be largely non-polarizable, thereby serving as a quasi-reference electrode. To check this proposed model, measurements of potential vs. pH were performed at a GC electrode in solution. A comparison of the behavior of a freshly-polished GC surface with that of one which was oxidized in 0.05 mol dme3 H,SO, at 1.8 V for 5 min prior to potentiometric measurements is shown in Fig. 5. In all of the buffers used, the potential of the pre-anodized electrode is higher

175

?

0.80

t cn ui >

0.40

3 W

0.00

0

2

4

6

8

PH Fig. 5. Influence of pH on the potential of a freshly polished glassy carbon electrode ( -) and a glassy carbon electrode that was preanodized for 5 min at 1.8 V vs. SCE in 0.05 mol dme3 H,SO, (- - -1. The pH values were varied with commercial (Fisher) buffers. The potentials were read after a 30 min equilibration where less than 1 mV/min drift was observed.

than that of the nominally-clean GC surface. In accord with this observation, the ultramicroelectrode voltammograms of solid vanadyl sulfate hydrates, when recorded vs. the pre-anodized surface, are shifted towards less positive potentials relative to those recorded vs. a freshly polished GC (Fig. 6). In either case, a process related to the carbon-oxygen functionalities is apparently the counter reaction to the voltammetry at the ultramicroelectrode, but the greater density of these functionalities at the pre-anodized surface makes it superior as a quasi-reference electrode for this study. Finally, the most important conclusions based on the ultramicroelectrode experiments were supported by the results of bulk electrolysis (AE = 1.8 VI of solid vanadyl sulfate hydrates performed on conductive glasses (ITO, see Fig. lb). The

Fig. 6. Cyclic voltammetry of solid VOSO,.3 H,O at an ultramicroelectrode with a polished glassy carbon ( -----I and a preanodized glassy carbon (- - -1 counter electrode. Scan rate 50 mV s-l.

observed electrochromism during controlled potential electrolysis of thin layers of vanadyl sulfate, a blue to yellow-orange color change, confirmed a V(N)-to-V(V) transition [45,461. With electrolysis of thick layers of material (ca. 0.3 mm) the following points were noted: the color became green under oxidation; prolonged bulk electrolysis caused a transition from initially peak-shaped voltammetric curves to linear current-potential plots; and oxidation of V(N) occurred with low (ca. 10%) current efficiency. These findings demonstrate formation of the mixed-valence V(IV)/V(V) system [47,48] in the surface layers of the material grains. The slopes of the linear current-potential plots suggested a conductivity on the order of 10-3-10-4 0-l cm-‘, which is typical for semiconducting materials classified as mixed-valence class II systems by Robin and Day [49]. That the product was electronically conducting precluded a continued faradaic process and, hence, limited the current efficiency. SUMMARY AND CONCLUSIONS

The present work shows that with ultramicroelectrodes the redox characteristics of solid samples of a simple inorganic salt can be investigated by familiar electrochemical techniques such as chronoamperometry and cyclic voltammetry. Well-defined solid-state voltammograms of the VOSO, *x H,O (x= 3 or 5) samples were obtained in the absence of deliberately added liquids even when the bulk solid was not mixed-valent, a condition that has been suggested to be important for the study of solids with electrodes of conventional areas [SO].The redox process, V(N) - e+ V(V), is reversible in the solid state. The process is largely limited to the near-surface layers of the material grains. The electrochemical behavior of these solid samples is sensitive to water content; the electrode process of the V(IV)/V(V) couple is diffusion-limited in the case of the pentahydrate but surface-confined in the trihydrate. Further, vanadyl sulfate samples dehydrated under vacuum are not electroactive. The question of whether an aqueous phase was present even in the absence of deliberately added solution was addressed. Although the presence of a liquid microphase may explain the solution-like voltammetry observed with VOSO, .5 H,O, the redox kinetics suggested by the cyclic voltammetry experiments are not consistent with aqueous solution behavior. Bulk-scale electrolysis studies showed that the conductivity mode of the solid vanadyl sulfate hydrates changes from ionic to electronic as the concentration ratio, [V(V)]/[V(IV)l, in the material increases. The present study has potential practical applications. One that we are exploring is the preparation and characterization of catalysts for gas/solid systems. ACKNOWLEDGEMENTS

The work was supported in part by a Research Challenge grant from the Ohio Board of Regents to J.A. Cox. Helpful discussions with R.K. Jaworski and with I. Fabian on the computer-assisted data acquisition and analysis systems are acknowledged.

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