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Spontaneous adsorption of heteropolytungstates and heteropolymolybdates on the surfaces of solid electrodes and the electrocatalytic activity of the adsorbed anions
ELSEVIER
Inorganica Chimica Acta 242 (1996) 1l-16
Spontaneous adsorption of heteropolytungstates and heteropolymolybdates on the surfaces of solid electrodes and the electrocatalytic activity of the adsorbed anionslY Chaoying Rang, Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena CA 91125, USA
Abstract Heteropolytungstate and heteropolymolybdate anions spontaneously adsorb from aqueous solutions onto mercury, gold, pyrolytic graphite and glassy carbon electrodes. The adsorption persists when the electrodes are transferred to pure supporting electrolyte
solutions where the electrochemical responses from the adsorbed anions can be observed. The adsorption is much less extensive from non-aqueous solvents. The adsorbed anions can serve as catalysts for various electrode reactions. Examples are given of the electrocatalytic reductions of hydrogen peroxide to water and of nitrite to ammonia. Keywords: Electrocatalysis; Adsorption; Heteropolytungstate anion; Heteropolymolybdate anion
1. Introduction Recently we described the strong spontaneous adsorption of several heteropolytungstate anions on the surface of mercury electrodes [ 11. In some cases the adsorption
was so strong that monolayers of the adsorbed heteropolytungstates were formed from solutions containing only a few nanomoles per liter of the anion. Such extensive adsorption from such dilute solutions allows the electrochemistry of the adsorbed species to be examined in the absence of significant contributions from the reactant present in solution. In addition, we have found that the spontaneous adsorption of heteropolyanions on mercury also occurs on a variety of solid electrodes (gold, glassy carbon, pyrolytic graphite) with considerable variation in the intensity of the adsorption with the electrode material. At both mercury and solid electrodes some of the adsorbed heteropolyanions behave as electrocatalysts for the reductions of otherwise unreactive substrates. Characteristics of the adsorption of representative hetero-
* Corresponding author. ’ Contribution no. 9072. 2 This paper is dedicated to Professor Harry B. Gray.
0020-1693/96/$15.00 Q 1996 Elsevier Science S.A. All rights reserved SSDI 0020-1693(95)04843-5
polyanions on solid electrodes and their application as electrocatalysts for the reduction of hydrogen peroxide and nitrite at mercury or solid electrodes are described in this report. 2. Experimental 2.1. Materials
The heteropolymetallates employed were prepared by procedures given in the cited references: H~[PMo~~O~]. 23320,
(NH4)6[~2~~1&3~621~~
lJ320
and
&dp2w180621-
11H20 [2a], &[PW, r0srJZe(OH2)].14H20 and KS [SiW1103sPe(OH2)]*14H20[2b]. Other chemicals were reagent grade and were used as received. Solutions were prepared from laboratory distilled water that was further purified by passage through a purification train (Millipore). Hanging mercury drop electrodes (Brinkman Model 410) were prepared from triply distilled mercury (Bethlehem Apparatus Co.). Commercially available (Bioanalytical Systems, Inc.) glassy carbon and gold electrodes were employed. Pyrolytic graphite electrodes were prepared by sealing commercially available cylinders (Union Carbide) to metal shafts with heat-shrinkable tubing.
12
C. Rang, F. C. Anson / Inorganica Chimica Acta 242 (1996) 1I-l 6
2.2. Apparatus and procedures
Conventional electrochemical cells and instrumentation were employed. The solid electrodes were pretreated before each experiment by the following procedures. Glassy carbon: polished with 0.05pm alumina; sonicated for 20 s in purified water. Edge plane pyrolytic graphite: polished with 0.3 ,um alumina; sonicated for 2 min in purified water. Gold: cycled between +1.70 and -0.55 V in 0.5 M NaHS04 adjusted to pH 1 until the voltammetric response matched that expected for clean gold electrodes [3]. The electrode areas for mercury, glassy carbon, edge plane pyrolytic graphite and gold were 0.014, 0.07, 0.33 and 0.02 cm*, respectively. Potentials were measured and are reported with respect to a Ag/AgCl (3 M NaCl) reference electrode with a potential of 0.21 V versus the standard hydrogen electrode. Experiments were conducted at the ambient laboratory temperature, 22 + 2°C.
EPG
I;:_:; A
IS
&
C
dt
3. Results and discussion 3.1. Adsorption on solid electrodes
0.2
0
-0.2 E/V
In our recent report the unusually strong adsorption of heteropolytungstate anions on mercury was emphasized [I]. The adsorption of the same anions on glassy carbon (GC) or edge plane pyrolytic graphite (EPG) is weaker but readily detectable from dilute solutions of the anions. The adsorption occurs spontaneously upon exposure of the electrodes to solutions of the anions. For example, in Fig. 1 cyclic voltammograms recorded at GC and EPG electrodes in a 0.01 mM solution of [P2W1sO& are shown (curves A and B). The response from the dissolved reactant is essentially negligible at the low concentration employed so the peaks apparent in curves A and B arise from the adsorbed reactant. The peak potentials match those obtained from the dissolved reactant when more concentrated solutions are employed (curves C and D in Fig. 1) and the three waves correspond to two oneelectron transfers followed by a two-electron transfer to delocalized orbitals of the polyoxotungstate anion [4]. The adsorption persists when the solid electrodes are transferred to solutions containing none of the heteropolyanions as shown in curves E and F of Fig. 1. Much stronger adsorption at both mercury and solid electrodes is exhibited by the structurally similar heteropolymolybdate anion, [P2Mo,s062]6-. Shown in Fig. 2 are cyclic voltammograms for this anion recorded with GC, EPG, Au and Hg electrodes. With 0.01 mM solutions of the anion the prominent, symmetrically shaped peaks are clear indicators of the substantial adsorption of the anion on all four electrodes (Fig. 2, curves A-D). The range of potentials encompassed by the voltammograms is different for the solid and mercury electrodes because of the oxidation of Hg at potentials more positive than ca. 0.3 V and the commencement of hydrogen ion reduction at the
-0.4
0.2
0
F
-0.2
-0.4
E/V
Fig. 1. Cyclic voltammograms for adsorbed and dissolved [p2Wtt30& at glassy carbon (GC) and edge plane pyrolytic graphite (EPG) electrodes. Concentration of the heteropoly anion: (A,B) 0.01 mM, (CD) 0.5 mM. Voltammograms E and F were recorded after the electrodes used to obtain voltammograms C and D were washed and transferred to the pure supporting electrolyte solution. Supporting electrolyte: 0.5 M NaHS04 adjusted to pH 1. Scan rate: 50 mV s-t. Current scale: (A,E) S = 1 PA, (B,C,F) S = SpA, (D) S = 20/A.
solid electrodes at potentials more negative than ca. -0.1 V. When the concentration of the heteropolymolybdate anion was increased to 0.5 mM the voltammetric responses took on the shapes expected for reactants dissolved in solution (Fig. 2, curves E-H) but transfer of the solid electrodes used to record curves E-G to the pure supporting electrolyte yielded voltammograms that showed the adsorbed anion was strongly retained on the surfaces of the GC and EPG electrodes (curves I, J) and more weakly on the Au electrode (curve K). Comparison of Figs. 1 and 2 makes it clear that the [P2M018062]6anion is more strongly adsorbed than the [P2W18062]6 anion despite their identical charges and similar sizes and structures. Some of the various factors that might control the extent of adsorption of heteropolymetallates have been suggested [l] but the interplay between the electronic charge on the electrode surface and the structurebreaking properties and negative charge of the adsorbing anion has yet to be fully understood. Each of the three reduction waves exhibited by the [P2M018062]6-anion in solution at potentials between 0.5 and 0 V in Fig. 2 corresponds to a two-electron reaction [5]. The smaller peak current and broader shape of the wave near 0.1 V in curves E-G may be the result of a
13
C. Rong, F.C. Anson / Inorganica C&mica Acta 242 (I 996) 11-I 6 EPG
GC
AU
JT+ G
I
0.6
I
I
0.4 E/V
0.2
I
I
0
0.6
t
I
0.4
0.2
I
I
0
0.6
WV
I
I
0.4 E/V
0.2
I
0
0.2
0
-0.2
E/V
at glassy carbon, edge plane pyrolytic graphite, gold and mercury elecFig. 2. Cyclic voltammograms for adsorbed and dissolved [pzMot80&trodes. Concentration of the heteropolyanion: (A-D) 0.01 mh4, (E-H) 0.5 mM. Voltammograms (I-K) were recorded after the electrodes used to obtain voltammograms (E&) were transferred to the pure supporting electrolyte. Other conditions as in Fig. 1. Current scale: (A,G,I) S = 1 PA, (B,E,J) S=5flA.(C,K)S=0.2~A.(D)S=O.l~A,(F,H)S=20~A.
decrease in the rate constant for the electron transfer as the charge on the anion increases. The response obtained at the Hg electrode (curve H in Fig. 2) deviates from those at the solid electrodes. The behavior resembles that observed previously with solutions of several heteropolytungstate anions at Hg electrodes [l]. The inequivalent current peaks were shown to be the result of electrostatic repulsion between the anions adsorbed on the electrode surface and those in solution diffusing to the electrode to be reduced [l]. The magnitude of such repulsion depends upon the electronic charge on the electrode at the potential where the anions are reduced, the ionic charge of the anion and the homogeneity of the electrode surface. The inhomogeneities resulting from irregularities in the surfaces of the solid electrodes may be the reason that these electrodes produce voltammetric patterns that differ from that obtained at Hg electrodes. The shapes of the voltammetric waves arising from the adsorbed anion are also somewhat dependent on the electrode material (compare curves A-D in Fig. 2). There is little reason to suspect that the actual electrode halfreactions could depend on the kind of electrode on which
the anion is adsorbed. Instead, we believe the variations in the wave shapes are the result of differences in the interactions of the adsorbed anions with each other and with the underlying electrode on the different electrode surfaces. The interactions of the adsorbing heteropolymolybdate anions with the electrode surface can be stabilizing. For example, the [PMoi20~]~- anion is known to dissociate extensively as its concentration is decreased [6] and attempts to adsorb it onto GC or EPG electrodes from 0.01 mM solutions of the anion produced only weak responses in the cyclic voltammograms (Fig. 3, curves A and B). Voltammograms recorded in much more concentrated solutions of the anion are shown in curves C and D of Fig. 3. Three overlapping waves are present with magnitudes that are dependent on the electrode material. When the electrodes used to record curves C and D were transferred to pure supporting electrolyte the voltammograms shown in curves E and F were obtained. Extensive adsorption of the complex had clearly occurred from the more concentrated solutions. The adsorbed complex exhibited three waves that were more clearly defined than
C. Rong, F.C. Anson / Inorganica Chimica Acta 242 (1994) II-16
14
gen-bonded structure of aqueous solutions by the anions and by the weak solvation that is the result of their large size and correspondingly low charge densities [7]. Both factors could favor their exclusion from aqueous media and foster their adsorption on surfaces. To test this proposal the [P2M018062]6-anion was adsorbed on an EPG electrode and the voltammetry of the adsorbed complex was recorded in pure aqueous supporting electrolyte (Fig. 4A). The coated electrode was then removed from the aqueous electrolyte, rinsed repeatedly with pure acetonitrile and returned to the aqueous electrolyte where the voltammogram was recorded again (Fig. 4B). It is clear that rinsing the surface with acetonitrile removed much of the adsorbed anion from the electrode. Rinsing the electrode with water instead of acetonitrile produced very little loss of the adsorbed anion from the electrode surface. When the electrode used to record Fig. 4A was transferred to a non-aqueous supporting electrolyte to record the voltammogram there was no detectable response (Fig. 4C). These results show that the adsorption of the heteropolyanions from a non-aqueous solvent is greatly diminished which supports the idea that the very strong adsorption from aqueous solution may be driven, at least in part, by a tendency to maintain the hydrogen bonded aqueous structure by excluding ions that disrupt it.
GC
44
0.6
0.4
0.2
0
0.6
0.4
E/V
F
t
0.2
0
-0.2
EPG
EIV
Fig. 3. Cl_clic voltammograms for adsorbed and dissolved [PMot2040] at GC (A,C,E) and EPG (B,D,F) electrodes. Concentration of [PM0t20~]~-: (A,B) 0.01 mM, (C, D) 0.5 mM. The voltammograms in (E) and (F) were recorded after the electrodes used to obtain voltammograms (C) and (D) were transferred to the pure supporting electrolyte. Other conditions as in Fig. 1. Current scale: (A,E) S = 0.2pA, (B,C) S = 2pA, (D) S = 20/4A, (F) S = ~/LA).
those obtained from the 0.5 mM solution of the anion in curves C and D. In addition, the current peaks were much larger in curves E and F than in curves A and B and they were stable to repeated cycling of the electrode potential. Apparently once the anion is adsorbed on the electrode surface its decomposition is retarded even when it is transferred to solutions containing none of the dissolved heteropolyanion.
I
0.6
3.2. Effect of solvent on the adsorption The tendency of a variety of heteropolytungstate anions to adsorb spontaneously on electrodes clearly extends beyond the mercury surfaces that were documented in our previous report [ 11. The generality of the adsorption indicates that it is driven by factors other than specific interactions between the anions and the surfaces on which they adsorb. We speculated previously [l] that the adsorption might be driven by the disruption of the hydro-
I
0.4
I
I
0.2
0
EIV Fig. 4. (A) Cyclic voltammogmm for [P2MotsOe& adsorbed on a EPG electrode from a 0.5 mM solution of the anion. The electrode was immersed in the anion solution for ca. 10 s, rinsed with water and transferred to the pure supporting electrolyte solution (0.5 M NaHSO4 adjusted to pH 1). (B) After the electrode used in (A) was rinsed extensively with acetonitrile and returned to the aqueous supporting electrolyte. (C) The electrode from (A) was transferred to a supporting electrolyte composed of 0.1 M tetrabutylammonium fluoroborate in acetonitrile. Other conditions as in Fig. 1.
C. Rang, F.C. Anson / Inorganica Chimica Acta 242 (1996) 11-16
B
15
tive in the range of electrode potentials in Fig. 5. The catalytic wave appears near the potential where the Fern center of the heteropolyanions is reduced to Fen which implicates the reduced, Fe” form of the heteropolyanions as the active form of the catalyst. The same conclusion was reached in a previous report [lo] in which the homogeneous reduction of H202 by the [SiW,1039Fe(OH2)]5- anion was examined. Digital simulation was employed to obtain an estimate of the rate constants for the reduction of H202 by the two adsorbed heteropolyanions. A comparison between the experimental and simulated voltammograms is shown in Fig. 5C,D. In calculating the simulated voltammograms the needed rate constant and formal potential for reaction 1 were estimated from the dashed curves in Fig. 5A,B without inclusion of possible interactions among the adsorbed catalyst molecules. Better agreement between the experimental and simulated voltammograms could be obtained if interaction parameters were included in the simulation [15] but such refinements in the simulation procedure were beyond the intended scope of the present study. The reaction mechanism assumed in carrying out the simulation was similar to that given in our previous study of the homogeneous reaction between [SiW11039Fe(OH2)]6-and H202, namely
--J__... __ ...___, I-1
I
0.2
0 E/V
-0.2
0
-0.2 E/V
-0.4
0.2
I
0
-0.2
I
-0.4
E/Y
Fig. 5. Catalysis of electroreductions of H202 by adsorbed heteropolytungstates. (A) Dashed curve: Cyclic voltammogram of 9 X lo-” mol cme2 of [PWl1030Fe(OH2)]4- adsorbed on a mercury electrode. The solid curve is the response obtained after the solution was made 0.5 mM in H202. Supporting electrolyte: 0.5 M acetate buffer adjusted to pH 4.7 and also containing 0.005 mM [PW11020Fe(OH2)]4-. Scan rate: 50 mV s-‘; S = 0.5 PA. (B) Repeat of A except 5 X lo-*’ mol cme2 of [SiWt,020Fe(OH2)]5- was adsorbed from a 0.005 mM solution of the anion. S = 0.3,~A. (C) Voltammetric response calculated for the mechanism given in reactions (l)-(4) using the experimental conditions s cified in (A) plus: Diffusion coefficient of H202 = 1.6 x 10v5 cm F s-‘, electrode area = 0.014 cm2, Elf = -0.04 V, ks” = 45 s-l. The solid line is the experimental voltammogram and the points were calculated. The best agreement was obtained with kf= 4 X lo4 M-’ s-l. S = 0.5 ,uA. (D) Repeat of (C) for the experimental conditions in (B) and El’= -0.20 V, ks” = 45 s-l. The best agreement was obtained with kf= 3 X IO4 M-’ s-l. S = 0.3 PA. (E) Repeat of (A) except that the electrode was EPG (0.33 cm2) instead of Hg (0.014 cm’) and 9.5 X lo-l1 mol cme2 of [PW11030Fe(OH2)]4- was adsorbed on the electrode.
3.3. Catalysis of electroreductions by adsorbed heteropolyanions
[MWl10,9Fe(OH2)]$;
+e<
E”k’ +
[MWl,0,,Fe(OH2)]!$~‘)-
(M = Si, P; m = 5,4) (1)
[MW,,O,,Fe(OH,)]!$~+‘)-
+H,O,
6
[~,,O,,Fe(O,H,)l~~‘)The strong and irreversible adsorption of the heteropolyanions on electrode surfaces makes them attractive candidates for application in electrocatalysis. Such applications have been pursued by a number of groups using both solutions of the heteropolytungstates or polymer coatings in which the anions were incorporated [8-141. We were interested in examining the electrocatalysis with the adsorbed anions alone under conditions where there was little or no contribution to the catalytic currents from the catalysts themselves. In Figs. 5A,B are shown the electroreduction of H,O, as catalyzed by [PW11039Fe(OH2)]4-‘5- or [SiW11039Fe(OH2)]5-/6-adsorbed on a hanging mercury drop electrode. The concentration of the heteropolyanions in solution was too low (0.005 mM) to contribute to the current in the absence of H202 (dashed curves in Fig. 5A,B) and the catalytic currents obtained after the addition of H20z (solid curves in Fig. 5A,B) were not affected if the concentration of the catalyst in solution was doubled, showing that the adsorbed anion was the seat of the electrocatalysis. In the absence of the heteropolytungstate the H202 was unreac-
[MWllo3,Fe(02H2
+H,O
(m+l)- k,
)]a,%
[MW,,O,FeOH]!$“‘-
NW,
(2)
(m+t)- + HO. + H+
10 39Fe(OH 2 Ilads
[~,1039WOH2)1$;
+HO,
(3)
fast
+H20
(4)
The second order rate constants obtained by fitting the simulated to the experimental voltammograms M-* were kobs= Kkr[H2023-’= 4 x lo4 s-* for [PWl103~e(OH2)]5- and kobs= 3 X 104 M-* s-l for [SiW1,03$e(OH2)]“. The corresponding rate constant measured previously for the homogeneous reaction between [SiWl10,9Fe(OH2)]6- and H202 was 9 X lo2 M-i s-i [lo]. These results indicate a substantially greater reactivity for the adsorbed compared with the dissolved [SiW11039Fe(OH2)]” anion, perhaps because of a
16
C. Rang, F. C. Anson / Inorganica Chimica Acta 242 (I 996) I1 -I 6
B was obtained in the presence of 1 mM NO;. The voltammograms obtained indicate that the catalysis proceeds in two steps just as it does when the same heteropolytungstate is used as a homogeneous catalyst [lo]. At the first peak, near -0.1 V, the Fe”-nitrosyl complex is formed. At the second peak, near -0.6 V, the coordinated nitrosyl is reduced to NH3 [lo]. 4. Conclusions
I
0
I
-0.2
I
-0.4
I
-0.6
E/V Fig. 6. Catalysis of the electroreduction of NO1 by [SiWtJO3oFe(OH& adsotbed on mercury electrodes. Experimental conditions were as in Fig. 5B except that the supporting electrolyte was 0.1 M NaC104 + 0.01 M HC104. (A) Cyclic voltammogram for the adsorbed complex in the absence of NO2-. (B) After the solution was made 1 mM in N01.
The results presented in this study have demonstrated that heteropolymetallate complexes adsorb strongly on a variety of electrode surfaces where their electrochemistry in the adsorbed state can be monitored in solutions containing little or none of the dissolved reactant. The adsorbed anions retain the catalytic activities that they exhibit in homogeneous solution and their strong, spontaneous adsorption facilitates their potential usefulness as electrocatalysts. Acknowledgements
larger value of the equilibrium constant of reaction (2). The formal potential of the Fern/Fen couple in the anion is almost 200 mV less [PWt AWOW4-/5negative than that of the [SiW,,03&e(OH2)]5~~ anion so the larger rate constant observed for the reduction of H202 by [PWII0,sFe(OH2)]5-,d, by the mechanism given above is somewhat surprising and suggests that alternative mechanisms may need to be considered in continuing studies of this reaction. The importance of the nature of electrode surface in influencing the kinetics of the electroreduction of H202 as catalyzed by adsorbed heteropolytungstates is demonstrated in Fig. 5E. The catalytic peak current density obtained when the [PW,,0s9Fe(OH2)]” anion is adsorbed on tbe surface of an EPG electrode is only 25% as large as that obtained when it is adsorbed on the Hg electrode (Fig. 5A). The underlying reasons for this difference in catalytic efficiency are unclear but one factor could be partitioning of the OH radicals formed in reaction (3) between reaction (4), which contributes to the measured current, and irreversible oxidation of the graphite surface, which might not contribute to the current. Another possibility is that a portion of the H202 undergoes disproportionation instead of reduction at the catalyst-coated electrode surface by means of the mechanism proposed previously to account for the consumption of fewer than two electrons per H202 molecule during its electroreduction in solutions containing the [SiW1rOs~e(OHz)]5- catalyst 1111. The catalysis of the reduction of NO; by [SiW110s9Fe(OH2)]5- adsorbed on Hg electrodes is shown in Fig. 6. Curve A is the voltammogram recorded for the adsorbed catalyst in the absence of NO; and curve
This work was supported by the National Science Foundation. We are grateful to Dr. Yuanwu Xie for assistance with the simulation of the voltammograms in Fig. 5. Discussions with Dr. Beat Steiger and Dr. Chunnian Shi were highly helpful. It is a pleasure to dedicate this paper to our friend and colleague, Harry Gray, and to join his many admirers in offering felicitations on his sixtieth birthday. References
111C. Rong and F.C. Anson, Anal. Chem., 66 (1994) 3124. 121(a) H. Wu, J. Biol. Chem., 43 (1920) 189. (b) F. Zonnevijlle, CM. Toume and G.F. Toume, Inorg. Chem., 21 (1980) 2751. r31 S.E. Creager, L.A. Hackett and G.K. Rowe, Langmuir, 8 (1992) 854. [41 (a) M.T. Pope and E. Papaconstantinou, Inorg. Chem., 6 (1967) 1147. (b) M. Kozik, CF. Hammer and L.C.W. Baker, J. Am. Chem. Sot., 108 (1986) 2748. PI P. Souchay, R. Contant and J.-M. Fruchart, C. R. Acad. AC. Paris, Series C, 264 (1967) 976. bl F. Chauveau, P. Souchay and R. Schaal, Bull. Sot. Chim. Fr., (1959) 1190. r71 L.C.W. Baker, in S. Kirschner (ed.), Advances in the Chemistry of Coordination Compounds, Macmillan, New York, 1961, p. 604. PI B. Keita and L. Nadjo, .I Electroanal. Chem., 243 (1988) 87. r91 G. Bidan, E.M. Genies and M. Lapkowski, J. Chem. Sot. Chem. Commun., (1988) 533; Synth. Metals, 31(1989) 334. 1101 I.E. Toth and F.C. Anson, J. Am. Chem. Sot., 111 (1989) 2444. [Ill J.E. Toth, J.D. Melton, D. Cabelli, B.H.J. Bielski and F.C. Anson, Inorg. Chem., 29 (1990) 1952. u21 C. Rong and M.T. Pope, J. Am. Chem. Sot., I14 (1992) 2932. u31 S. Dong and M. Liu, J. Electroanal. Chem., 372 (1994) 95. u41 B. Keith, A. Belhouari, L. Nadjo and R. Contant, J Electroanal. Chem., 381(1995) 243. WI Y. Xie and F.C. Anson, J. Electroanal. Chem., 384 (1995) 145.
Inorganica Chimica Acta 242 (1996) 1l-16
Spontaneous adsorption of heteropolytungstates and heteropolymolybdates on the surfaces of solid electrodes and the electrocatalytic activity of the adsorbed anionslY Chaoying Rang, Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena CA 91125, USA
Abstract Heteropolytungstate and heteropolymolybdate anions spontaneously adsorb from aqueous solutions onto mercury, gold, pyrolytic graphite and glassy carbon electrodes. The adsorption persists when the electrodes are transferred to pure supporting electrolyte
solutions where the electrochemical responses from the adsorbed anions can be observed. The adsorption is much less extensive from non-aqueous solvents. The adsorbed anions can serve as catalysts for various electrode reactions. Examples are given of the electrocatalytic reductions of hydrogen peroxide to water and of nitrite to ammonia. Keywords: Electrocatalysis; Adsorption; Heteropolytungstate anion; Heteropolymolybdate anion
1. Introduction Recently we described the strong spontaneous adsorption of several heteropolytungstate anions on the surface of mercury electrodes [ 11. In some cases the adsorption
was so strong that monolayers of the adsorbed heteropolytungstates were formed from solutions containing only a few nanomoles per liter of the anion. Such extensive adsorption from such dilute solutions allows the electrochemistry of the adsorbed species to be examined in the absence of significant contributions from the reactant present in solution. In addition, we have found that the spontaneous adsorption of heteropolyanions on mercury also occurs on a variety of solid electrodes (gold, glassy carbon, pyrolytic graphite) with considerable variation in the intensity of the adsorption with the electrode material. At both mercury and solid electrodes some of the adsorbed heteropolyanions behave as electrocatalysts for the reductions of otherwise unreactive substrates. Characteristics of the adsorption of representative hetero-
* Corresponding author. ’ Contribution no. 9072. 2 This paper is dedicated to Professor Harry B. Gray.
0020-1693/96/$15.00 Q 1996 Elsevier Science S.A. All rights reserved SSDI 0020-1693(95)04843-5
polyanions on solid electrodes and their application as electrocatalysts for the reduction of hydrogen peroxide and nitrite at mercury or solid electrodes are described in this report. 2. Experimental 2.1. Materials
The heteropolymetallates employed were prepared by procedures given in the cited references: H~[PMo~~O~]. 23320,
(NH4)6[~2~~1&3~621~~
lJ320
and
&dp2w180621-
11H20 [2a], &[PW, r0srJZe(OH2)].14H20 and KS [SiW1103sPe(OH2)]*14H20[2b]. Other chemicals were reagent grade and were used as received. Solutions were prepared from laboratory distilled water that was further purified by passage through a purification train (Millipore). Hanging mercury drop electrodes (Brinkman Model 410) were prepared from triply distilled mercury (Bethlehem Apparatus Co.). Commercially available (Bioanalytical Systems, Inc.) glassy carbon and gold electrodes were employed. Pyrolytic graphite electrodes were prepared by sealing commercially available cylinders (Union Carbide) to metal shafts with heat-shrinkable tubing.
12
C. Rang, F. C. Anson / Inorganica Chimica Acta 242 (1996) 1I-l 6
2.2. Apparatus and procedures
Conventional electrochemical cells and instrumentation were employed. The solid electrodes were pretreated before each experiment by the following procedures. Glassy carbon: polished with 0.05pm alumina; sonicated for 20 s in purified water. Edge plane pyrolytic graphite: polished with 0.3 ,um alumina; sonicated for 2 min in purified water. Gold: cycled between +1.70 and -0.55 V in 0.5 M NaHS04 adjusted to pH 1 until the voltammetric response matched that expected for clean gold electrodes [3]. The electrode areas for mercury, glassy carbon, edge plane pyrolytic graphite and gold were 0.014, 0.07, 0.33 and 0.02 cm*, respectively. Potentials were measured and are reported with respect to a Ag/AgCl (3 M NaCl) reference electrode with a potential of 0.21 V versus the standard hydrogen electrode. Experiments were conducted at the ambient laboratory temperature, 22 + 2°C.
EPG
I;:_:; A
IS
&
C
dt
3. Results and discussion 3.1. Adsorption on solid electrodes
0.2
0
-0.2 E/V
In our recent report the unusually strong adsorption of heteropolytungstate anions on mercury was emphasized [I]. The adsorption of the same anions on glassy carbon (GC) or edge plane pyrolytic graphite (EPG) is weaker but readily detectable from dilute solutions of the anions. The adsorption occurs spontaneously upon exposure of the electrodes to solutions of the anions. For example, in Fig. 1 cyclic voltammograms recorded at GC and EPG electrodes in a 0.01 mM solution of [P2W1sO& are shown (curves A and B). The response from the dissolved reactant is essentially negligible at the low concentration employed so the peaks apparent in curves A and B arise from the adsorbed reactant. The peak potentials match those obtained from the dissolved reactant when more concentrated solutions are employed (curves C and D in Fig. 1) and the three waves correspond to two oneelectron transfers followed by a two-electron transfer to delocalized orbitals of the polyoxotungstate anion [4]. The adsorption persists when the solid electrodes are transferred to solutions containing none of the heteropolyanions as shown in curves E and F of Fig. 1. Much stronger adsorption at both mercury and solid electrodes is exhibited by the structurally similar heteropolymolybdate anion, [P2Mo,s062]6-. Shown in Fig. 2 are cyclic voltammograms for this anion recorded with GC, EPG, Au and Hg electrodes. With 0.01 mM solutions of the anion the prominent, symmetrically shaped peaks are clear indicators of the substantial adsorption of the anion on all four electrodes (Fig. 2, curves A-D). The range of potentials encompassed by the voltammograms is different for the solid and mercury electrodes because of the oxidation of Hg at potentials more positive than ca. 0.3 V and the commencement of hydrogen ion reduction at the
-0.4
0.2
0
F
-0.2
-0.4
E/V
Fig. 1. Cyclic voltammograms for adsorbed and dissolved [p2Wtt30& at glassy carbon (GC) and edge plane pyrolytic graphite (EPG) electrodes. Concentration of the heteropoly anion: (A,B) 0.01 mM, (CD) 0.5 mM. Voltammograms E and F were recorded after the electrodes used to obtain voltammograms C and D were washed and transferred to the pure supporting electrolyte solution. Supporting electrolyte: 0.5 M NaHS04 adjusted to pH 1. Scan rate: 50 mV s-t. Current scale: (A,E) S = 1 PA, (B,C,F) S = SpA, (D) S = 20/A.
solid electrodes at potentials more negative than ca. -0.1 V. When the concentration of the heteropolymolybdate anion was increased to 0.5 mM the voltammetric responses took on the shapes expected for reactants dissolved in solution (Fig. 2, curves E-H) but transfer of the solid electrodes used to record curves E-G to the pure supporting electrolyte yielded voltammograms that showed the adsorbed anion was strongly retained on the surfaces of the GC and EPG electrodes (curves I, J) and more weakly on the Au electrode (curve K). Comparison of Figs. 1 and 2 makes it clear that the [P2M018062]6anion is more strongly adsorbed than the [P2W18062]6 anion despite their identical charges and similar sizes and structures. Some of the various factors that might control the extent of adsorption of heteropolymetallates have been suggested [l] but the interplay between the electronic charge on the electrode surface and the structurebreaking properties and negative charge of the adsorbing anion has yet to be fully understood. Each of the three reduction waves exhibited by the [P2M018062]6-anion in solution at potentials between 0.5 and 0 V in Fig. 2 corresponds to a two-electron reaction [5]. The smaller peak current and broader shape of the wave near 0.1 V in curves E-G may be the result of a
13
C. Rong, F.C. Anson / Inorganica C&mica Acta 242 (I 996) 11-I 6 EPG
GC
AU
JT+ G
I
0.6
I
I
0.4 E/V
0.2
I
I
0
0.6
t
I
0.4
0.2
I
I
0
0.6
WV
I
I
0.4 E/V
0.2
I
0
0.2
0
-0.2
E/V
at glassy carbon, edge plane pyrolytic graphite, gold and mercury elecFig. 2. Cyclic voltammograms for adsorbed and dissolved [pzMot80&trodes. Concentration of the heteropolyanion: (A-D) 0.01 mh4, (E-H) 0.5 mM. Voltammograms (I-K) were recorded after the electrodes used to obtain voltammograms (E&) were transferred to the pure supporting electrolyte. Other conditions as in Fig. 1. Current scale: (A,G,I) S = 1 PA, (B,E,J) S=5flA.(C,K)S=0.2~A.(D)S=O.l~A,(F,H)S=20~A.
decrease in the rate constant for the electron transfer as the charge on the anion increases. The response obtained at the Hg electrode (curve H in Fig. 2) deviates from those at the solid electrodes. The behavior resembles that observed previously with solutions of several heteropolytungstate anions at Hg electrodes [l]. The inequivalent current peaks were shown to be the result of electrostatic repulsion between the anions adsorbed on the electrode surface and those in solution diffusing to the electrode to be reduced [l]. The magnitude of such repulsion depends upon the electronic charge on the electrode at the potential where the anions are reduced, the ionic charge of the anion and the homogeneity of the electrode surface. The inhomogeneities resulting from irregularities in the surfaces of the solid electrodes may be the reason that these electrodes produce voltammetric patterns that differ from that obtained at Hg electrodes. The shapes of the voltammetric waves arising from the adsorbed anion are also somewhat dependent on the electrode material (compare curves A-D in Fig. 2). There is little reason to suspect that the actual electrode halfreactions could depend on the kind of electrode on which
the anion is adsorbed. Instead, we believe the variations in the wave shapes are the result of differences in the interactions of the adsorbed anions with each other and with the underlying electrode on the different electrode surfaces. The interactions of the adsorbing heteropolymolybdate anions with the electrode surface can be stabilizing. For example, the [PMoi20~]~- anion is known to dissociate extensively as its concentration is decreased [6] and attempts to adsorb it onto GC or EPG electrodes from 0.01 mM solutions of the anion produced only weak responses in the cyclic voltammograms (Fig. 3, curves A and B). Voltammograms recorded in much more concentrated solutions of the anion are shown in curves C and D of Fig. 3. Three overlapping waves are present with magnitudes that are dependent on the electrode material. When the electrodes used to record curves C and D were transferred to pure supporting electrolyte the voltammograms shown in curves E and F were obtained. Extensive adsorption of the complex had clearly occurred from the more concentrated solutions. The adsorbed complex exhibited three waves that were more clearly defined than
C. Rong, F.C. Anson / Inorganica Chimica Acta 242 (1994) II-16
14
gen-bonded structure of aqueous solutions by the anions and by the weak solvation that is the result of their large size and correspondingly low charge densities [7]. Both factors could favor their exclusion from aqueous media and foster their adsorption on surfaces. To test this proposal the [P2M018062]6-anion was adsorbed on an EPG electrode and the voltammetry of the adsorbed complex was recorded in pure aqueous supporting electrolyte (Fig. 4A). The coated electrode was then removed from the aqueous electrolyte, rinsed repeatedly with pure acetonitrile and returned to the aqueous electrolyte where the voltammogram was recorded again (Fig. 4B). It is clear that rinsing the surface with acetonitrile removed much of the adsorbed anion from the electrode. Rinsing the electrode with water instead of acetonitrile produced very little loss of the adsorbed anion from the electrode surface. When the electrode used to record Fig. 4A was transferred to a non-aqueous supporting electrolyte to record the voltammogram there was no detectable response (Fig. 4C). These results show that the adsorption of the heteropolyanions from a non-aqueous solvent is greatly diminished which supports the idea that the very strong adsorption from aqueous solution may be driven, at least in part, by a tendency to maintain the hydrogen bonded aqueous structure by excluding ions that disrupt it.
GC
44
0.6
0.4
0.2
0
0.6
0.4
E/V
F
t
0.2
0
-0.2
EPG
EIV
Fig. 3. Cl_clic voltammograms for adsorbed and dissolved [PMot2040] at GC (A,C,E) and EPG (B,D,F) electrodes. Concentration of [PM0t20~]~-: (A,B) 0.01 mM, (C, D) 0.5 mM. The voltammograms in (E) and (F) were recorded after the electrodes used to obtain voltammograms (C) and (D) were transferred to the pure supporting electrolyte. Other conditions as in Fig. 1. Current scale: (A,E) S = 0.2pA, (B,C) S = 2pA, (D) S = 20/4A, (F) S = ~/LA).
those obtained from the 0.5 mM solution of the anion in curves C and D. In addition, the current peaks were much larger in curves E and F than in curves A and B and they were stable to repeated cycling of the electrode potential. Apparently once the anion is adsorbed on the electrode surface its decomposition is retarded even when it is transferred to solutions containing none of the dissolved heteropolyanion.
I
0.6
3.2. Effect of solvent on the adsorption The tendency of a variety of heteropolytungstate anions to adsorb spontaneously on electrodes clearly extends beyond the mercury surfaces that were documented in our previous report [ 11. The generality of the adsorption indicates that it is driven by factors other than specific interactions between the anions and the surfaces on which they adsorb. We speculated previously [l] that the adsorption might be driven by the disruption of the hydro-
I
0.4
I
I
0.2
0
EIV Fig. 4. (A) Cyclic voltammogmm for [P2MotsOe& adsorbed on a EPG electrode from a 0.5 mM solution of the anion. The electrode was immersed in the anion solution for ca. 10 s, rinsed with water and transferred to the pure supporting electrolyte solution (0.5 M NaHSO4 adjusted to pH 1). (B) After the electrode used in (A) was rinsed extensively with acetonitrile and returned to the aqueous supporting electrolyte. (C) The electrode from (A) was transferred to a supporting electrolyte composed of 0.1 M tetrabutylammonium fluoroborate in acetonitrile. Other conditions as in Fig. 1.
C. Rang, F.C. Anson / Inorganica Chimica Acta 242 (1996) 11-16
B
15
tive in the range of electrode potentials in Fig. 5. The catalytic wave appears near the potential where the Fern center of the heteropolyanions is reduced to Fen which implicates the reduced, Fe” form of the heteropolyanions as the active form of the catalyst. The same conclusion was reached in a previous report [lo] in which the homogeneous reduction of H202 by the [SiW,1039Fe(OH2)]5- anion was examined. Digital simulation was employed to obtain an estimate of the rate constants for the reduction of H202 by the two adsorbed heteropolyanions. A comparison between the experimental and simulated voltammograms is shown in Fig. 5C,D. In calculating the simulated voltammograms the needed rate constant and formal potential for reaction 1 were estimated from the dashed curves in Fig. 5A,B without inclusion of possible interactions among the adsorbed catalyst molecules. Better agreement between the experimental and simulated voltammograms could be obtained if interaction parameters were included in the simulation [15] but such refinements in the simulation procedure were beyond the intended scope of the present study. The reaction mechanism assumed in carrying out the simulation was similar to that given in our previous study of the homogeneous reaction between [SiW11039Fe(OH2)]6-and H202, namely
--J__... __ ...___, I-1
I
0.2
0 E/V
-0.2
0
-0.2 E/V
-0.4
0.2
I
0
-0.2
I
-0.4
E/Y
Fig. 5. Catalysis of electroreductions of H202 by adsorbed heteropolytungstates. (A) Dashed curve: Cyclic voltammogram of 9 X lo-” mol cme2 of [PWl1030Fe(OH2)]4- adsorbed on a mercury electrode. The solid curve is the response obtained after the solution was made 0.5 mM in H202. Supporting electrolyte: 0.5 M acetate buffer adjusted to pH 4.7 and also containing 0.005 mM [PW11020Fe(OH2)]4-. Scan rate: 50 mV s-‘; S = 0.5 PA. (B) Repeat of A except 5 X lo-*’ mol cme2 of [SiWt,020Fe(OH2)]5- was adsorbed from a 0.005 mM solution of the anion. S = 0.3,~A. (C) Voltammetric response calculated for the mechanism given in reactions (l)-(4) using the experimental conditions s cified in (A) plus: Diffusion coefficient of H202 = 1.6 x 10v5 cm F s-‘, electrode area = 0.014 cm2, Elf = -0.04 V, ks” = 45 s-l. The solid line is the experimental voltammogram and the points were calculated. The best agreement was obtained with kf= 4 X lo4 M-’ s-l. S = 0.5 ,uA. (D) Repeat of (C) for the experimental conditions in (B) and El’= -0.20 V, ks” = 45 s-l. The best agreement was obtained with kf= 3 X IO4 M-’ s-l. S = 0.3 PA. (E) Repeat of (A) except that the electrode was EPG (0.33 cm2) instead of Hg (0.014 cm’) and 9.5 X lo-l1 mol cme2 of [PW11030Fe(OH2)]4- was adsorbed on the electrode.
3.3. Catalysis of electroreductions by adsorbed heteropolyanions
[MWl10,9Fe(OH2)]$;
+e<
E”k’ +
[MWl,0,,Fe(OH2)]!$~‘)-
(M = Si, P; m = 5,4) (1)
[MW,,O,,Fe(OH,)]!$~+‘)-
+H,O,
6
[~,,O,,Fe(O,H,)l~~‘)The strong and irreversible adsorption of the heteropolyanions on electrode surfaces makes them attractive candidates for application in electrocatalysis. Such applications have been pursued by a number of groups using both solutions of the heteropolytungstates or polymer coatings in which the anions were incorporated [8-141. We were interested in examining the electrocatalysis with the adsorbed anions alone under conditions where there was little or no contribution to the catalytic currents from the catalysts themselves. In Figs. 5A,B are shown the electroreduction of H,O, as catalyzed by [PW11039Fe(OH2)]4-‘5- or [SiW11039Fe(OH2)]5-/6-adsorbed on a hanging mercury drop electrode. The concentration of the heteropolyanions in solution was too low (0.005 mM) to contribute to the current in the absence of H202 (dashed curves in Fig. 5A,B) and the catalytic currents obtained after the addition of H20z (solid curves in Fig. 5A,B) were not affected if the concentration of the catalyst in solution was doubled, showing that the adsorbed anion was the seat of the electrocatalysis. In the absence of the heteropolytungstate the H202 was unreac-
[MWllo3,Fe(02H2
+H,O
(m+l)- k,
)]a,%
[MW,,O,FeOH]!$“‘-
NW,
(2)
(m+t)- + HO. + H+
10 39Fe(OH 2 Ilads
[~,1039WOH2)1$;
+HO,
(3)
fast
+H20
(4)
The second order rate constants obtained by fitting the simulated to the experimental voltammograms M-* were kobs= Kkr[H2023-’= 4 x lo4 s-* for [PWl103~e(OH2)]5- and kobs= 3 X 104 M-* s-l for [SiW1,03$e(OH2)]“. The corresponding rate constant measured previously for the homogeneous reaction between [SiWl10,9Fe(OH2)]6- and H202 was 9 X lo2 M-i s-i [lo]. These results indicate a substantially greater reactivity for the adsorbed compared with the dissolved [SiW11039Fe(OH2)]” anion, perhaps because of a
16
C. Rang, F. C. Anson / Inorganica Chimica Acta 242 (I 996) I1 -I 6
B was obtained in the presence of 1 mM NO;. The voltammograms obtained indicate that the catalysis proceeds in two steps just as it does when the same heteropolytungstate is used as a homogeneous catalyst [lo]. At the first peak, near -0.1 V, the Fe”-nitrosyl complex is formed. At the second peak, near -0.6 V, the coordinated nitrosyl is reduced to NH3 [lo]. 4. Conclusions
I
0
I
-0.2
I
-0.4
I
-0.6
E/V Fig. 6. Catalysis of the electroreduction of NO1 by [SiWtJO3oFe(OH& adsotbed on mercury electrodes. Experimental conditions were as in Fig. 5B except that the supporting electrolyte was 0.1 M NaC104 + 0.01 M HC104. (A) Cyclic voltammogram for the adsorbed complex in the absence of NO2-. (B) After the solution was made 1 mM in N01.
The results presented in this study have demonstrated that heteropolymetallate complexes adsorb strongly on a variety of electrode surfaces where their electrochemistry in the adsorbed state can be monitored in solutions containing little or none of the dissolved reactant. The adsorbed anions retain the catalytic activities that they exhibit in homogeneous solution and their strong, spontaneous adsorption facilitates their potential usefulness as electrocatalysts. Acknowledgements
larger value of the equilibrium constant of reaction (2). The formal potential of the Fern/Fen couple in the anion is almost 200 mV less [PWt AWOW4-/5negative than that of the [SiW,,03&e(OH2)]5~~ anion so the larger rate constant observed for the reduction of H202 by [PWII0,sFe(OH2)]5-,d, by the mechanism given above is somewhat surprising and suggests that alternative mechanisms may need to be considered in continuing studies of this reaction. The importance of the nature of electrode surface in influencing the kinetics of the electroreduction of H202 as catalyzed by adsorbed heteropolytungstates is demonstrated in Fig. 5E. The catalytic peak current density obtained when the [PW,,0s9Fe(OH2)]” anion is adsorbed on tbe surface of an EPG electrode is only 25% as large as that obtained when it is adsorbed on the Hg electrode (Fig. 5A). The underlying reasons for this difference in catalytic efficiency are unclear but one factor could be partitioning of the OH radicals formed in reaction (3) between reaction (4), which contributes to the measured current, and irreversible oxidation of the graphite surface, which might not contribute to the current. Another possibility is that a portion of the H202 undergoes disproportionation instead of reduction at the catalyst-coated electrode surface by means of the mechanism proposed previously to account for the consumption of fewer than two electrons per H202 molecule during its electroreduction in solutions containing the [SiW1rOs~e(OHz)]5- catalyst 1111. The catalysis of the reduction of NO; by [SiW110s9Fe(OH2)]5- adsorbed on Hg electrodes is shown in Fig. 6. Curve A is the voltammogram recorded for the adsorbed catalyst in the absence of NO; and curve
This work was supported by the National Science Foundation. We are grateful to Dr. Yuanwu Xie for assistance with the simulation of the voltammograms in Fig. 5. Discussions with Dr. Beat Steiger and Dr. Chunnian Shi were highly helpful. It is a pleasure to dedicate this paper to our friend and colleague, Harry Gray, and to join his many admirers in offering felicitations on his sixtieth birthday. References
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