Langmuir 1996,11, 3603-3604
3603
Reversible Vanadium(M Reduction by Voltammetry of Vanadyl Sulfate Trihydrate in the Absence of a Solution Phase Waldemar Gorski and James A. Cox* Department of Chemistry, Miami University, Oxford, Ohio 45056
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Received April 3, 1995. In Final Form: June 27, 1995
Introduction Recent developments in methodology for electrochemical investigations allow the use of common techniques such as cyclic voltammetry in studies of electroactive solids in the absence of an added solution phase. The present study is part of our program to apply such methodology to the electrochemistry of gas-solid systems at room temperature. Our choice ofVOS04.3H20 for this study was based on the fact that vanadium compounds frequently have been employed as catalysts in the chemical redox of gassolid systems. For example, high-valence vanadium phosphates are selective catalysts for hydrocarbon oxidation in gas-solid systems, including large-scale commercial processes.1,2 The V(IV,V) couple is believed to be involved in these catalytic reactions. Vanadium pentoxides at high temperature catalyze the conversion of CO and of SO2 to their higher oxide^.^^^ Several solid-state electrochemical studies on ionicallyconducting, inorganic compounds have now been reported. Among the examples are the following. Feldman and Murray measured the electron self-exchangerate constant for fedferrocyanide lattice sites of Prussian blue by voltammetry on films over interdigitated array electrode^.^ The development of Faradaic processes in solid Prussian blue was facilitated by the mixed-valence nature of this material;6 transport of Kf was identified as the currentlimiting step.7 We demonstrated by the reduction of hydrated Moo3 to a bronze that a mixed-valence system was not required to sustain electrolysis of a solid.8 Using a cell consisting of a carbon quasi-reference electrode and a carbon fiber indicator electrode, we elucidated the anodic behavior of VOS04 hydrates in the absence of a liquid electrolyte using theory developed for solution-phase studies.g In the present study, the reversible electrochemical reduction ofV(IV) to V(II1) in the solid state is described. The behavior is contrasted to that observed for the electrochemistry of V(IV) in solution.
Experimental Section Most experimentswere performed with VOS04.3HzO (Aldrich,
99.99%purity)as the solid electrolyte. Certainexperimentsused dimethyl sulfoxide (DMSO)rather than water as the solvate on vanadyl sulfate. The VOS04.3DMSO was prepared by a pro-
* Corresponding author:
MUOHIO.EDU.
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(1)Schiot, B.; Jorgensen, K. A.; Hoffman, R. J.Phys. Chem. 1991, 95,2297. (2)Pepera, M. A,; Callahan, J. L.; Desmond, M. J.; Milberger, E. C.; Blum, P.R.;Bremer, N. J. J.Am. Chem. SOC.1986,107,4883. (3)Baiker, A.; Dollenmeier, P.; He, R.; Wokaun, A. J. Catal. 1986, 100,345. (4)Clark, F.T.; Spring”, F. C.; Willcox, D.; Wachs, I. E. J. Catal. 1993,139,1. (5)Feldman, B. J.;Murray, R. W. Inorg. Chem. 1987,26,1702. (6)Kulesza, P. J . Znorg. Chem. 1990,29,2395. (7)Kulesza, P. J.; Galus, Z. J. Electroanal. Chem. 1992,323, 261. (8)Jaworski, R. K.; Cox, J. A. Electrochim. Acta 1992,37,5. (9)Gorski, W.; Cox, J . A. J. Electroanul. Chem. 1992,323,163.
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Figure 1. Cyclic voltammetry of VOS04.3Hz0 powder at a carbon ultramicroelectrode in an argon atmosphere. The displayed voltammogram is the current-voltage behavior at steady-state (current not a function of cycle number) which is achieved after a few cycles. Scan rate, 50 mV3-l; AE,voltage applied us the carbon quasi-reference electrode. Cathodic currents are above the origin, the position of which is at the initial potential for the voltammetry rather than at 0 V.
cedure in the literature.1° Argon ((99,9%)was used without further purification. Cyclic voltammetricand chronoamperometric measurements were made with an EG&G PAR Model 273A potentiostau galvanostat. All electrochemical experiments were performed at ambient room temperature (20 3~ 2 “C)in a grounded Faraday cage. The solid-stateelectrochemical measurementswere made in a gas-tight,two-electrode cell we previously de~cribed.~ After the powdered sample was loaded (about 20 mg), the volume of the cell, 1.2 cm3, was flushed by flowing argon through the cell for at least 2 min after which the inlet and drain ports were closed. The indicator electrode was a 10 pm diameter carbon disk from BioanalyticalSystems, Inc. (BAS). A 3 mm diameter glassy carbon disk (BAS) served as the counterlquasi-reference electrode. Prior to the experimentsthe electrodeswere polished with alumina. All potentials are reported relative to the carbon quasi-reference electrode.
Results and Discussion Because of their potential application to electrocatalysis, generating reduced forms of VOS04.3H20 was the initial goal of the present study. Figure 1 contains a cyclic voltammogram of this solid obtained over the range 0.9 to -1.3 V. The peaks a t 0.55 V are due to the V(V,IV) couple; those at -0.97 Vrelate to a previously-unreported process. The small change in the background that appears near 0.0 V apparently is from oxidation of a surface species on the carbon electrode. Consistent with solution-phase voltammetry at carbon, its amplitude depends upon the time for which the electrode is anodized, and its development requires a subsequent negative potential excursion. The behavior of the peaks a t -0.97 V was studied by varying the scan rate, v , over the range 20-150 mV*s-’ (sixpoints). The cathodic and anodic peak currents were directly proportional to u . Linear least squares analysis ofthe relationship between the cathodic peak current and u yielded the following parameters: slope, 35 f 5 pAwmV-’; r , 0.998; and peak current ratio, ipJipa,1.00 f 0.03. The half-widths of the peaks were both 110 f5 mV. The average charges under these anodic and cathodic peaks were both 5.6 f 0.2 nC. These data are consistent with the assignment of a reversible reduction of thin layer (about 140 monolayers) of V(IV) at -0.97 V. The same behavior previously was demonstrated for the oxidation of VOSO4.3HzO.’ Voltammetric data suggest that the product of the above reduction is V(II1). The primary evidence is obtained from a comparison of the behavior a t -0.97 V to that for the one-electron process a t 0.55 V. The charge under the cathodic peak at -0.97 V, 5.6 f 0.2 nC, is statistically identical to that under the anodic peak at 0.55 V, 5.4 f (10)Selbin, J.; Holmes, L. H. J. Znorg. Nucl. Chem. 1962,24,1111.
0743-746319512411-3603$09.00/0 0 1995 American Chemical Society
Notes
3604 Langmuir, Vol. 11, No. 9, 1995 0.2 nC. Analysis of the peak at -0.97 V provides direct evidence for a one-electron reduction of V(IV). A plot of log i us E over the potential range where i, is between 5% and 35% of ,i is linear (r,0.999) with a slope (67 f 2 mV) which approaches that for a reversible, one-electron process. Finally, the difference between the peak and half-peak potentials, 60 f2 mV, supports the assignment of a one-electron reduction of V(IV). The solid state voltammetry for the reduction of V(IV) is markedly different from that in solution. In contrast to the above-demonstrated reversible behavior in the solid state, the reduction of V(IV) in solution is irreversible. The relative behavior can be explained by a difference between the chemical forms of V(111)generated by solution and solid phase electrolysis. In the former case, the V+ aquo-ion is the product, so the reaction involves cleavage of V=O bonds. However, the behavior of the solid phase electrochemistry is consistent with the reduction of VOz+ to VO+,a process that does not require the energy to break the V=O bonds. A second differencebetween the solid and solution phase electrochemistry of the reduction of V(IV) is that V(I1) is not produced in the former. Either the formal potential for the V(II1,II) is more negative than the potential at which the water that is coordinated to VOS04is reduced (about -1.5 V) or the electron transfer kinetics for the reduction of V(II1) are unfavorable for observing its reduction in the potential window available in the system. Because the formation of V(I1) by solid state voltammetry is a goal of our program, we attempted to circumvent any problems related to the water of crystallization by eliminating it from the system while maintaining the salt as a n ionic conductor. As described in the Experimental Section, DMSO was substituted for water in the solvation
of voso4. This solvent was selected after attempts to use acetonitrile, dimethylformamide, and 1,3-dimethyl3,4,5,6-tetrahydro-2-(lH)-pyrimidinone failed because of their oxidation by the dissolved vanadium salt a t the elevated temperatures that are required in the synthesis. When the experiment described in Figure 1 was repeated with VOSOc3DMSO as the solid, current was not observed, even when the potential limits were changed to 1.5 and -2.0 V. This solid apparently is not a n ionic conductor. The lack of a Faradaic process in this case demonstrates the importance of the coordinated solvent in imparting ionic conductivity to this salt. Moreover, the result confirms that the data in Figure 1 are not controlled by water impurities that can be introduced during the sample preparation steps. Specifically precluded is that adsorbed water forms a microphase in the cell which causes the solid state voltammetry to resemble solution phase results. The reversibility of the V(IV,III) couple along with the large potential range over which the voltammogram is featureless suggests that vanadyl sulfate will be useful as a solid-state electrolyte for electrochemical study of gaseous species. The former characteristic may permit mediation of redox by V(III), and the latter will allow the study of redox of gaseous species at the carbon electrode. For example, we have observed the oxidation of carbon monoxide at 0.0 V in the solid-state cell described herein.
Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research through Grant 26538-AC5. L4950269+
