Cooperativity of Copper and Molybdenum Centers in Polyoxometalate

Nargis Anwar , Mikhail Vagin , Fathima Laffir , Gordon Armstrong , Calum ... Narpinder Kaur , Johan van Tol , Naresh S. Dalal , Bineta Keita , Louis N...
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Langmuir 2006, 22, 10416-10425

Cooperativity of Copper and Molybdenum Centers in Polyoxometalate-Based Electrocatalysts: Cyclic Voltammetry, EQCM, and AFM Characterization† Bineta Keita, Essadik Abdeljalil, Louis Nadjo,* Roland Contant, and Robila Belghiche Laboratoire de Chimie Physique, Groupe d’Electrochimie et de Photoe´ lectrochime, UMR 8000, CNRS, UniVersite´ Paris-Sud, Baˆ timent 350, 91405 Orsay Cedex, France ReceiVed April 28, 2006. In Final Form: July 8, 2006 Electrochemical behaviors of selected Dawson-type polytungstates including R2-K10[P2W15Mo2O610] where the symbol 0 designates a vacant site, R2-K7[Fe(OH2)P2W15Mo2O61], R2-K8[Cu(OH2)P2W15Mo2O61], R1- and R2K8[Cu(OH2)P2W17O61], R2-K8[Cu(OH2)P2W13Mo4O61], and R2-K8[Cu(OH2)P2W12Mo5O61] were investigated by cyclic voltammetry (CV) coupled with the electrochemical quartz microbalance (EQCM), and the results were completed by atomic force microscopy (AFM) observations of the electrodeposited films. The electrocatalytic abilities of these polyoxometalates (POMs) in the reduction of dioxygen, hydrogen peroxide, and NOx were also assessed by CV and EQCM. It turns out that the remarkable electrocatalysis obtained at the reduction potential of Mo centers within R2-K8[Cu(OH2)P2W15Mo2O61], but in a domain where Cu2+ is not deposited, benefits from the assistance of the copper center because such catalysis could not be observed in the absence of Cu2+. EQCM confirms that no copper deposition occurs under the experimental conditions used. Analogous behaviors are encountered in the electrocatalytic reduction of nitrite where assistance by the presence of the Cu2+ center induced the observation of catalysis at the potential location of Mo centers. Finally, the reduction of nitrate is triggered by electrodeposited copper but was remarkably favored by the presence of molybdenum atoms within these polyoxometalates (POMs). All of the results converge to indicate a cooperative effect between the Mo and Cu centers within these POMs. The various results suggest that copper deposition from these POMs should give morphologically different surfaces. AFM studies confirm this expectation, and the observed morphologies and sizes of particles were rationalized by taking into account the role of the POM skeleton and its atomic composition in the electrodeposition process.

1. Introduction The activation of small inorganic ions and molecules has received attention in recent decades. In electrochemistry, the main processes include the reduction of protons or the oxidation of hydrogen, the reduction of dioxygen, and the electrochemical processes of nitrogen oxides NOx and carbon oxides COx, complemented by the electrocatalysis of chlorate, bromate, and so forth. All of these reactions have important implications in environmental problems and/or are potentially considered to be abundant, inexpensive sources for the production of useful chemicals.1 The remarkable reliability of enzymes in performing such transformations has prompted ideas for designing and testing biomimetic systems. Mostly, attention is focused here on the role of several metal centers and their eventual interactions in realizing the transformations of interest. For instance, in the nitrogen cycle, dinitrogenase reductase contains a single 4Fe4S redox center, and dinitrogenase contains both molybdenum and iron.2 In nitrate reductases, which reduce [NO3]- to [NO2]-, molybdenum is considered to be an active site.3 Along these line, Tanaka et al.4,5 realized the catalytic reduction of NO3to NH3 on a glassy carbon electrode modified with (nBu4N)3[Mo2Fe6S8(SPh)9]. In enzymes that perform chemical energy †

Part of the Electrochemistry special issue. * Corresponding author. E-mail: [email protected]. Tel: 33 1 69 15 77 51. Fax: 33 1 69 15 43 28. (1) Epperly, W. R. CHEMTECH 1991, 429. (2) Nelson, D. L.; Cox; M. M. Lehninger Principles of Biochemistry, 4th ed.; W. H. Freeman and Company: New York, 2004; p 835. (3) Dervartanian, D. V.; Forget, P. Biochim. Biophys. Acta 1975, 379, 74. (4) Kuwabata, S.; Uezumi, S.; Tanaka, K.; Tanaka, T. J. Chem. Soc., Chem. Commun. 1986, 135. (5) Kuwabata, S.; Uezumi, S.; Tanaka, K.; Tanaka, T. Inorg. Chem. 1986, 25, 3018.

harnessing from molecular oxygen by its reductive activation to reactive oxygen species, copper and iron serve as cofactors in cytochrome oxidase.6-8 Evidence was published recently for Cu-O2 intermediates in superoxide oxidations by biomimetic copper(II) complexes.9 It is tempting to determine whether enzyme-inspired compositions could be useful in designing and synthesising polyoxometalates (POMs) that are efficient in the activation of some of the small inorganic ions and molecules cited previously. POMs are early transition metal-oxygen anionic clusters with remarkably rich redox- and photochemistry. These properties have made them attractive for applications in catalysis, electrocatalysis, materials science, photochemistry, photo- and electrochromism, analytical chemistry, magnetochemistry, and medicine.10-14 The ability of unsubstituted POMs to generate lacunary species and then metal cation-substituted derivatives opens the way for a virtually enormous family of compounds. At present, rational (6) Nelson, D. L.; Cox; M. M. Lehninger Principles of Biochemistry, 4th ed.; W. H. Freeman and Company: New York, 2004; p 191. (7) Nam, W. In ComprehensiVe Coordination Chemistry II; Que, L., Jr., Tolman, W. B., Eds.; Elsevier Pergamon: Amsterdam, 2004; Vol. 8, pp 281-307. (8) Halcrow, M. A. In ComprehensiVe Coordination Chemistry II; Que, L., Jr., Tolman, W. B., Eds.; Elsevier Pergamon: Amsterdam, 2004; Vol. 8, pp 395-436. (9) Smirnov, V. V.; Roth, J. P. J. Am. Chem. Soc. 2006, 128, 3683. (10) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: New York, 1983. (11) Hill, C. L., Guest Ed. Chem. ReV. 1998, 98, 1-389 (polyoxometalates). (12) Pope, M. T. In ComprehensiVe Coordination Chemistry II: Transition Metal Groups 3-6; Wedd, A. G., Ed.; Elsevier Science: New York, 2004; pp 635-678 (13) Hill, C. L. In ComprehensiVe Coordination Chemistry II: Transition Metals Groups 3-6; Wedd, A. G., Ed.; Elsevier Science, New York, 2004; pp 679-759. (14) Keita, B.; Nadjo, L. Electrochemistry of Isopoly and Heteropoly Oxometalates. In Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Scholz, F., Pickett, C. J., Eds.; Wiley: New York, 2006; Vol. 7, pp 607-700.

10.1021/la061159d CCC: $33.50 © 2006 American Chemical Society Published on Web 09/08/2006

Cu and Mo Center CooperatiVity in Electrocatalysts

and reproducible synthetic methods were devised and are available for the replacement of one or more of the skeletal d0 early transition metal cations in POMs with d- or p-block ions.12-18 The impetus for their study was triggered by the seminal remark of Baker19 that monosubstituted POMs can be considered to be analogues of metalated porphyrins and can be used in catalytic processes with the advantage, relative to their organic counterparts, of thermal stability, robustness, and inertness toward oxidizing environments. In addition, numerous properties were described.11,20-23 Recently, the siderophoric behavior of sandwichtype multi-iron POMs was stressed.24 Considered altogether, these ideas are used in the selection and/or synthesis of POMs that are expected to be active in the transformations of dioxygen, hydrogen peroxide, and nitrogen oxides. Alternatively, these small inorganic ions and molecules might serve as probes of electronic interactions within these POMs. In this article, cyclic voltammetry (CV), the electrochemical quartz crystal microbalance (EQCM), and atomic force microscopy (AFM) are used both for the identification of active sites within the catalysts and for eventual synergistic relationships between metallic sites, with the expectation of gaining more insight into the involved realistic chemical reaction pathways.

Key Prior Knowledge In the course of several studies of POM-based electrocatalyses, it appeared that cyclic voltammetry results might be explained by cooperativity effects between some mixed-metal addenda centers. The first series of examples concerns the electrocatalytic reduction of dioxygen, triggered by selected POMs. These include R2-K10[P2W15Mo2O610] (abbreviated as R2-[P2W15Mo20]), where the symbol 0 designates a vacant site, K6[P2W15Mo3O62] ([P2W15Mo3]), R2-K7[Fe(OH2)P2W15Mo2O61] (R2-[P2W15Mo2Fe]), R2-K8[Cu(OH2)P2W15Mo2O61] (R2-[P2W15Mo2Cu]), and R2-K8[Cu(OH2)P2W17O61] (R2-[P2W17Cu]), all of which were characterized first by electrochemistry in the absence of dioxygen.14 Except for the somewhat intricate example of R2-[P2W15Mo2Fe],25 the electroactive elements are reduced successively in the order Mo6+, Cu2+, and W6+ when the potential, referenced to SCE, is scanned from the positive to the negative domain. As concerns the electrocatalytic reduction of dioxygen, the following conclusions emerge from the data. With uncomplexed Cu2+ in solution or with R2-[P2W17Cu], dioxygen electrocatalytic reduction is triggered by electrodeposited Cu0 on the electrode surface. The lacunary species R2-[P2W15Mo20] and the plenary compound [P2W15Mo3] do not catalyze this reaction in the reduction potential domain of Mo moieties, whereas R2-[P2W15Mo2Cu] triggers this process. The paradox is that the electrocatalytic wave develops at the potential location of the molybdenum wave in the latter (15) Sadakane, A.; Steckhan, E. Chem. ReV. 1998, 98, 219. (16) Contant, R.; Herve´, G. ReV. Inorg. Chem. 2002, 22, 63. (17) Hill, C. L., Guest Ed. J. Mol. Catal. 1996, 114, 1-371 (chemical, polyoxometalates). (18) Briand, L. E.; Baronetti, G. T.; Thomas, H. J. Appl. Catal., A 2003, 256, 37. (19) Baker, L. C. Plenary Lecture Proceedings of the XV International Conference on Coordation Chemistry , Moscow, 1973. (20) Keita, B.; Mbomekalle, I. M.; de Oliveira, P.; Ranjbari, A.; Justum, Y.; Nadjo, L.; Pompon, D. J. Cluster Sci. 2006, 17, 221 (special issue on polyoxometalates). (21) Keita, B.; Contant, R.; Mialane, P.; Se´cheresse, F.; de Oliveira, P.; Nadjo, L. Electrochem. Commun. 2006, 8, 767. (22) Mbomekalle, I. M.; Keita, B.; Nadjo, L.; Berthet, P.; Hardcastle, K. I.; Hill, C. L.; Anderson, T. M. Inorg. Chem. 2003, 42, 1163. (23) Keita, B.; Mbomekalle, I. M.; Lu, Y. W.; Nadjo, L.; Berthet, P.; Anderson, T. M.; Hill, C. L. Eur. J. Inorg. Chem. 2004, 3462. (24) Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Anderson, T. M.; C. L. Hill, C. L. Inorg. Chem. 2004, 43, 3257. (25) Keita, B.; Girard, F.; Nadjo, L.; Contant, R.; Belghiche, R.; Abbessi, M. J. Electroanal. Chem. 2001, 508, 70.

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complex in a domain where the results of cyclic voltammetry and controlled potential electrolysis, coupled with UV-visible spectroscopy, converge to suggest strongly that no Cu0 deposition occurs at the electrode. In short, Mo5+ centers alone do not activate dioxygen, and Cu0 deposition can be discarded in the electrocatalytic experiments with R2-[P2W15Mo2Cu]. Then, because both Mo and Cu centers are necessary, some interaction between the Mo and Cu2+ centers must be admitted. With R2-[P2W15Mo2Fe] at pH 3, much the same electrocatalytic dioxygen reduction was observed at the potential location of the combined (Mo, Fe) wave, but the efficiency of this complex is considerably smaller than that of R2-[P2W15Mo2Cu]. Finally, in the electrocatalytic reduction of dioxygen, all of the results together support the conclusion that R2-[P2W15Mo2Cu] is a better catalyst than all of the other studied complexes, including Cu0 deposition from uncomplexed Cu2+.26 Also, in POM-triggered electrocatalytic reduction of NOx, the simultaneous presence of Mo and Cu centers proves beneficial. Nitrite is kept in the following text as a general term to represent the starting chemical dissolved in fairly acidic solutions, but nitrous acid and NO are the reactive species in several of the discussed examples. POM-based electrocatalytic reduction of HNO2 and NO was demonstrated in the late 1980s independently by Anson et al.27 and by Keita et al.28,29 In particular, Keita et al. were the first to demonstrate that even nonsubstituted POMs can perform this process.28,29 Since then, this reaction has been selected and is being used worldwide as a classical test of the electrocatalytic properties of POMs. The presence of Mo atoms within the framework of POMs turns out to be beneficial in the electrocatalytic reduction of nitrite. For example, purely molybdenum and molybdenum-tungsten POMs are better catalysts than the corresponding fully tungsten ones in the electroreduction of NO.28,30 Selected representatives of these three POM families reduce NO to N2O at pH 1 with a quantitative yield of 100%. The presence of even a single Mo atom substantially improves the overall catalytic process that is observed at the reduction potential of molybdenum. In pH >4 media where several of these catalysts are not stable, Fe- and Cu-substituted molybdenum-tungsten POMs are more efficient than fully tungsten ones for the reduction of nitrite.25,31 The addition of two more POMs to the series cited for dioxygen reduction, R2-K8[Cu(OH2)P2W13Mo4O61] (R2-[P2W13Mo4Cu]) and R2-K8[Cu(OH2)P2W12Mo5O61] (R2-[P2W12Mo5Cu]), has permitted us to show that the number and location of Mo atoms within the POM framework appear to constitute important parameters. For Fesubstituted Mo-rich POMs, more and more efficient catalysis is obtained at pH 5 when the number of Mo atoms increases. Finally, in the wealth of accumulated data, R2-[P2W15Mo2Cu] was found to be the best catalyst for NO electroreduction in the selected Cu-substituted POMs. Specifically, the electrocatalytic process commences readily with the Mo reduction wave of this complex in a potential regime where the interference of copper is not obvious.25 However, an analysis of the results highlights the beneficial influence of copper. As a consequence, complementary (26) Keita, B.; Benaı¨ssa, M.; Nadjo, L.; Contant, R. Electrochem. Commun. 2002, 4, 663. (27) Toth, J. E.; Anson, F. C. J. Am. Chem. Soc. 1989, 111, 2444. (28) Keita, B.; Nadjo, L.; Contant, R.; Fournier, M.; Herve´, G. (CNRS) French Patent 89/1,728, 1989. (29) Keita, B.; Nadjo, L.; Contant, R.; Fournier, M.; Herve´, G. (CNRS) Eur. Pat. Appl. EP 382,644, 1990. Keita, B.; Nadjo, L.; Contant, R.; Fournier, M.; Herve´, G. Chem Abstr. 1991, 114, 191882u. (30) Belhouari, A.; Keita, B.; Nadjo, L.; Contant, R. New J. Chem. 1998, 83. (31) Keita, B.; Mbomekalle, I. M.; Nadjo, L.; de Oliveira, P.; Ranjbari, A.; Contant, R. C. R. Chim. 2005, 8, 1057 (polyoxometalates issue).

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investigations are necessary for understanding the superiority of R2-[P2W15Mo2Cu] over analogous Cu-substituted POMs. Analogous behaviors were observed in the electrocatalytic reduction of nitrate. The electrocatalytic reduction of nitrate by reduced POMs was successfully achieved only recently.32,33 The presence of Cu2+ or Ni2+ substituents in the POM framework seems necessary except for an up to now unique example of an Fe-rich dimeric POM that is active for this reduction.14 The beneficial effect of the presence of Mo atoms is again demonstrated for the electrocatalytic reduction of nitrate.32 In contrast with the case of nitrite, the number of Mo atoms does not appear as a relevant parameter in the electrocatalytic reduction of nitrate. Altogether, the results on electrocatalytic reductions of dioxygen or NOx by R2-[P2W15Mo2Cu] raises the following interesting question: should the observed cooperativity between copper and molybdenum in the electrocatalytic reduction of dioxygen by R2-[P2W15Mo2Cu] be attributed to some deposition of the copper center that cyclic voltammetry fails to detect, or should some other mechanism be invoked? In the present communication, CV, EQCM, and AFM experiments are performed with the aim of providing additional data that are expected to shed further light on the reactions pathways both in the electrocatalytic reduction of dioxygen and in that of the NOx. 2. Experimental Section 2.1. Chemicals. Pure water was used throughout. It was obtained by passing through a RiOs 8 unit followed by a Millipore-Q Academic purification set. All of the chemicals were of high-purity grade and were used as received without further purification. The following POMs were selected for studying the electrocatalytic reduction of dioxygen: R2-K10[P2W15Mo2O610] (abbreviated as R2-[P2W15Mo20]), where the symbol 0 designates a vacant site, R2-K7[Fe(OH2)P2W15Mo2O61], (R2-[P2W15Mo2Fe]), R2-K8[Cu(OH2)P2W15Mo2O61] (R2-[P2W15Mo2Cu]), R2-K8[Cu(OH2)P2W17O61] (R2[P2W17Cu]), R1-K8[Cu(OH2)P2W17O61] (R1-[P2W17Cu]), R2-K8[Cu(OH2)P2W13Mo4O61] (R2-[P2W13Mo4Cu]), and R2-K8[Cu(OH2)P2W12Mo5O61] (R2-[P2W12Mo5Cu]). All of these POMs were prepared and characterized by published methods.14 In the present study, the pH 3 medium was made up with 0.2 M Na2SO4 + H2SO4, and the pH 4 and 5 media, with 0.4 M CH3COONa + CH3COOH. 2.2. Equipment and Apparatus. UV-visible spectra were recorded on a Perkin-Elmer Lambda 19 spectrophotometer. The solutions were deaerated thoroughly for at least 30 min with pure argon and kept under positive pressure of this gas during the experiments. The source, mounting, and polishing of the glassy carbon (GC, Le Carbone Lorraine, France) electrodes have been described.26 The glassy carbon samples had a diameter of 3 mm. The electrochemical setup was an EG & G 273 A driven by a PC with the 270 software. Potentials are quoted against a saturated calomel electrode (SCE). The counter electrode was a platinum gauze of large surface area. All experiments were performed at laboratory temperature. 2.3. Electrochemical Quartz Crystal Microbalance (EQCM). The EQCM setup used in this work was the system QCA 917 (Seiko/ EG&G) with 9 MHz AT-cut crystals. New crystals equipped with carbon electrodes were provided by Seiko and allow us to work in media where classical gold or platinum electrodes are inappropriate. The data from the QCA 917 setup were first recorded in the computer and then printed.34 Frequency variations are recorded and are the only data necessary for discussion in this work. However, the (32) Keita, B.; Abdeljalil, E.; Nadjo, L.; Contant, R.; Belghiche, R. Electrochem. Commun. 2001, 3, 56. (33) Jabbour, D.; Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Kortz, U. Eur. J. Inorg. Chem. 2004, 2036. (34) Keita, B.; Contant, R.; Abdeljalil, E.; Girard, F.; Nadjo, L. Electrochem. Commun. 2000, 2, 295.

Keita et al. expressions mass increase and mass decrease will also be used routinely as equivalent to frequency decrease and frequency increase, respectively. 2.4. Atomic Force Microscopy (AFM). The working electrode for electrochemical film depositions was a freshly cleaved HOPG surface (Le Carbone Lorraine, France). Ex-situ AFM imaging was carried out with a Nanoscope IIIa equipped with an extender electronic module. The images were acquired exclusively in tapping mode using a silicon cantilever. This most popular oscillatory AFM technique is particularly suited for the imaging of delicate samples. Two different types of data can be collected. Deflection data directly describe the change in the amplitude. Height data correspond to the change in piezo height needed to keep the vibrational amplitude of the cantilever constant. The extender electronic module offers new possibilities for tapping-mode AFM. It improves the results by providing phase and frequency detection capabilities. In addition, the phase and frequency signals are claimed to be generally immune to interference and artifacts. In the present work, we have been interested in phase imaging. Previously, we have used this technique to discriminate between metal clusters and the surfactant polymer used for the preparation35 and in other series of experiments.36 Size measurements were performed on images recorded in height and phase modes. Even though the difference in the imaging modes might restrict the comparability of the measured sizes, this way of obtaining data turns out to be useful for setting lower and upper limits on size measurements. Very good quality raw data were usually obtained, and no filtering was applied to the images shown in this article. Controlled potential copper deposition experiments were performed in a pH 3 medium (0.2 M Na2SO4 + H2SO4) containing 5 × 10-4 M CuSO4 or the appropriate POM. The amounts of charge consumed in the various experiments will be designated by q1, q2, q3, and q4, with the number indicating the appropriate multiple of q1 (q1 ) 0.8 mC). After copper deposition, all electrodes were rinsed with Millipore water before AFM imaging. We ensured that the HOPG working electrode was perfectly clean before each experiment by successive cleaving, rinsing with Millipore water, and imaging in air.

3. Results and Discussion 3.1. Cyclic Voltammetry Evidence of Element Cooperativity in POM-Based Electrocatalysts. The influence of the number and location of Mo atoms within the POM framework on the electroacatlytic reduction of dioxygen might be a relevant parameter, but it was not studied previously. The Mo centers are reduced at more positive potentials in R2-[P2W13Mo4Cu] and R2-[P2W12Mo5Cu] than in R2-[P2W15Mo2Cu]. At pH 3, the reduction peak potentials are -0.066 V for R2-[P2W15Mo2Cu], +0.144 V for R2-[P2W13Mo4Cu], and +0.116 V for R2-[P2W12Mo5Cu]. In Figure 1A, the cyclic voltammograms representing the electrocatalyic reduction of dioxygen by R2-[P2W15Mo2Cu], R2-[P2W12Mo5Cu], and R2[P2W17Cu] are compared. First, the electocatalysis is less efficient with the POM substituted only by Cu2+. For the R2-[P2W12Mo5Cu] complex, the electrocatalytic process occurs far negative of the Mo wave and is much less efficient than observed with R2-[P2W15Mo2Cu]. Figure 1B gives further insight and shows that the electrocatalytic reduction of dioxygen with R2-[P2W12Mo5Cu] appears in a potential domain where the reduction of Cu2+ has already begun. The same observations are valid for R2-[P2W13Mo4Cu]. At least two conclusions emerge from these and previously available observations. First, the presence of Mo is important in the electrocatalytic reduction of dioxygen by this family of POMs, but the electrocatalytic wave develops at the potential location of the Mo wave only in the case of R2-[P2W15(35) Keita, B.; Nadjo, L.; Gachard, E.; Remita, H.; Khatouri, J.; Belloni, J. New J. Chem. 1997, 21, 851. (36) Remita, H.; Keita, B.; Torogoe, K.; Belloni, J.; Nadjo, L. Surf. Sci. 2004, 572, 301.

Cu and Mo Center CooperatiVity in Electrocatalysts

Figure 1. Cyclic voltammograms showing the electrocatalytic activities of selected POMs toward dioxygen reduction in a pH 3 medium. The POM concentration is 10-4 M, the scan rate is 2 mV s-1, and the working electrode is glassy carbon. The excess parameter γ is defined as γ ) C°O2/C°POM. (A) Comparison of R2-[P2W15Mo2Cu], R2-[P2W12Mo5Cu], and R2-[P2W17Cu]. (B) Superposition of cyclic voltammogram of R2-[P2W12Mo5Cu] in the absence and presence of dioxygen.

Mo2Cu]. Provisionally, it is noted that the smaller separation between the Mo and Cu waves is observed with the latter complex (0.114 V compared to 0.318 V for R2-[P2W13Mo4Cu] and 0.314 V for R2-[P2W12Mo5Cu]). Second, the Mo waves of R2-[P2W13Mo4Cu] and R2-[P2W12Mo5Cu] behave exactly like the corresponding Mo waves in R2-[P2W14Mo4] and R2-[P2W13Mo5], respectively, which are insensitive to the presence of dioxygen. It must be concluded that the number of Mo atoms is not the only important criterion in this issue. This conclusion completes the study of possibly relevant parameters of electrocatalytic reduction of dioxygen by the selected POMs. Altogether, the results on electrocatalytic reduction of dioxygen by R2-[P2W15Mo2Cu] raises the following question: should the observed cooperativity between copper and molybdenum in the electrocatalytic reduction of dioxygen by R2-[P2W15Mo2Cu] be attributed to some deposition of the copper center that cyclic voltammetry fails to detect or should some other mechanism be invoked? The forthcoming study of this problem by EQCM is expected to shed more light on it. 3.2. EQCM Studies: Confirmation of CV Conclusions and New Insights. The study of the redox behaviors of Cu centers within POM frameworks is complicated by deposition possibilities that are not encountered with most other substituted POMs. Fortunately, the EQCM technique is particularly suited to such work. The main question addressed in the following text can be formulated as follows: does a detectable Cu0 deposition occur in potential domains that are useful to the electrocatalytic processes

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Figure 2. (A) Cyclic voltammograms showing the electrochemical behavior of R2-[P2W15Mo2Cu] in a pH 3 medium. The POM concentration is 5 × 10-4 M, the scan rate is 10 mV s-1, and the working electrode is carbon deposited on the quartz crystal of the EQCM. (B) EQCM frequency response associated with the solidline voltammogram in Figure 4A.

triggered by the selected POMs? An answer to this question might help in formulating realistic reaction pathways for further study. Second, what can we learn from the deposition conditions? 3.2.1. R2-[P2W15Mo2Cu]. Figure 2A shows the cyclic voltammetry pattern of R2-[P2W15Mo2Cu] restricted to the first several waves of interest in the study of the selected electrocatlytic processes. It was run in a pH 3 medium using the carbon film deposited on the quartz crystal as the working electrode. It must be emphasized that this pattern is the same as that obtained on a classical glassy carbon electrode, thus demonstrating the very good quality of the carbon film. In the considered potential domain, the cyclic voltammogram consists of three waves featuring three two-electron chemically reversible processes associated with the reductions of Mo6+, Cu2+, and W6+, respectively, in that order, roughly at the correct potential locations determined previously with a bulky glassy carbon electrode. The reduction pattern for Cu2+ is a composite. The large desorptive oxidation wave observed around 0.0 V during the reoxidation scan is characteristic of surface-adsorbed species and features an overlap of the oxidation of the deposited Cu0 with the reoxidation wave of Mo centers.26 Provided the potential domain is extended in the negative direction, it must be remembered, provisionally, that the cyclic voltammetry patterns of all of the Cu-substituted POMs in this work show a large current intensity pluri-electron wave featuring the further reduction of W and/or Mo centers and the proton reduction on the Cu0 film.14 Figure 2B shows the EQCM frequency response recorded simultaneously with the cyclic voltammetry pattern of Figure 2A. Several domains can be distinguished in Figure 2B. The mass increase (alternatively,

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the frequency decrease) is seen to starts at -0.2 V, a potential at which the reduction of Cu2+ to Cu0 begins. During copper deposition, an approximately linear mass increase is observed up to -0.51 V, where the W wave can be considered to commence developing. Then, a much slower mass change is observed until the reverse potential is fixed at -0.59 V. During the reverse potential scan, a weak deposition continues during the Wreoxidation wave (roughly from -0.59 V to -0.40 V) followed by a faster mass increase until approximately -0.18 V, the minimum potential to reduce Cu2+ to Cu0. Between -0.18 and -0.13 V, the electrode mass remains nearly constant, a feature that must be associated with the oxidation of Mo5+ to Mo6+. This observation is also in agreement with oxidation wave peak potentials of Mo5+ and Cu0 determined from cyclic voltammograms (-0.016 V and +0.004 V) and indicates a slightly easier oxidation for Mo5+. Finally, a mass decrease starts only at -0.13 V, where Cu0 oxidation begins. This potential, which cannot be determined from the cyclic voltammetry pattern, indicates that the onsets of the oxidation waves of Mo5+ and Cu0 are close to each other and confirm the composite character of the large current intensity wave following the combined oxidation of (Mo5+ + Cu0). The slope of mass decrease (Cu0 to Cu2+ oxidation) is larger than that of mass increase (Cu2+ to Cu0 reduction), reflecting the association of the reduction process with the sum of Cu2+ and W6+ and the oxidation with the sum of Cu0 and only a fraction of Mo5+. Upon complete reoxidation of Cu0, it is remarkable that the electrode recovers the initial mass it had before copper deposition. Finally, simultaneous recording of the cyclic voltammogram of R2-[P2W15Mo2Cu] and the associated frequency variations has allowed us to answer the following question raised by CV studies alone: does Cu0 form, eventually through an underpotential deposition process, during the reduction of Mo6+ to Mo5+ centers within R2-[P2W15Mo2Cu]? The response is clearly negative. We searched for a confirmation of this result by performing the experiments outlined in Figure 3. Figure 3A shows in superposition the CV associated with the supporting electrolyte and the CV of R2-[P2W15Mo2Cu] restricted to the Mo wave. The corresponding mass variations recorded simultaneously with the CVs appear in Figure 3B. They indicate comparable mass variations in both situations. Provisionally, it is pointed out that this observation remains valid for all scan rates tested (from 2 to 100 mV s-1). To favor an eventual copper deposition, the working electrode potential was held for 30 min at the reduction peak potential of the Mo centers, with simultaneous monitoring of mass variations. No deposition could be detected. Finally, frequency monitoring during dioxygen catalysis shows that there is no copper deposition during this process. Indeed, these observations do not rule out the possibility that Cu(I) is stabilized within the complexes by the presence of Mo, a possibility that will indicate an electronic cooperative effect. 3.2.2. R2-[P2W17Cu]. Figure 4A shows the cyclic voltammetry pattern of R2-[P2W17Cu] restricted to its first two waves, and Figure 4B shows the simultaneously recorded frequency variations detected by the quartz crystal microbalance. The first wave features an overall four-electron process and represents both the reduction of the Cu2+ center and the first W wave.14 The second wave is associated with a two-electron reduction of the W centers. Analogies and differences appear between Figure 2B, associated with R2-[P2W15Mo2Cu], and Figure 4B. For R2-[P2W17Cu], Cu0 deposition starts from the very beginning of the first wave, in agreement with the cyclic voltammogram. During the negative-going potential scan, three slopes can be distinguished instead of two in the case of the Mo-containing

Keita et al.

Figure 3. (A) Cyclic voltammogram of R2-[P2W15Mo2Cu] restricted to the Mo wave superimposed with that of the pure pH 3 medium. The POM concentration is 5 × 10-4 M, the scan rate is 10 mV s-1, and the working electrode is carbon deposited on the quartz crystal of the EQCM. (B) EQCM frequency response associated with both cyclic voltammograms.

complex: a slope change is observed after the first wave peak, and the third slope begins after the second W wave. These slopes indicate a gradual decrease of the deposition kinetics. Such slowing down of deposition kinetics was also observed in the potential domain corresponding to W reduction in R2-[P2W15Mo2Cu]. In further analogy between the two complexes, the existence of different slopes confirms the composite character of the first wave of R2-[P2W17Cu], with the competition between copper deposition and W reduction being in favor of the copper. At variance with R2-[P2W15Mo2Cu], very little deposition occurs during the reverse scan that is essentially dominated, between -0.73 and -0.1 V, by the two oxidation processes of W centers. It is worth mentioning that a single slope is associated with the Cu0 oxidation in the case of R2-[P2W15Mo2Cu] because only this process occurs in the corresponding potential domain. A difference that is worthy of note between the two complexes concerns the reoxidation of Cu0: after “complete” reoxidation, a sizable amount of copper remains in the case of R2-[P2W17Cu], in agreement with our previously stressed remark that electrochemical reoxidation, after electrolysis, is slower for Cu0 deposited from R2-[P2W17Cu] than from R2-[P2W15Mo2Cu] or the isomer R1-[P2W17Cu].14,37 Even for two successive runs, the electrode does not recover its initial mass, which results in significant baseline shifts (Figure SI1 A in Supporting Information). Incidentally, a close inspection of the cyclic voltammograms superposed in Figure 4A reveals slight differences from one run (37) Keita, B.; Abdeljalil, E.; Nadjo, L.; Avisse, B.; Contant, R.; Canny, J.; Richet, M. Electrochem. Commun. 2000, 2, 145.

Cu and Mo Center CooperatiVity in Electrocatalysts

Figure 4. (A) Cyclic voltammogram of R2-[P2W17Cu] restricted at most to its first two waves in a pH 3 medium. The POM concentration is 5 × 10-4 M, the scan rate is 10 mV s-1, and the working electrode is carbon deposited on the quartz crystal of the EQCM. (B) EQCM frequency response associated with the solid-line cyclic voltammogram in part A.

to the next, differences that disappear after several runs (Figure SI2 A). However, in this last situation, the frequency variation curves continue to indicate that the electrode has not returned to its initial mass, even though the residual deposit is less abundant than it was during the first runs (Figure SI2 B). In contrast, for R2-[P2W15Mo2Cu], the electrode reverts to its initial mass whatever the final potential selected for scan reversal (Figure SI1 B). All of these results together underscore the ability of the EQCM technique to reveal details that CV alone does not allow us to detect. Finally, an apparent paradox for copper deposition from R2-[P2W17Cu] must be pointed out (Figures SI1 A and SI2 B): the amount of deposited Cu0 (as measured by the surface area of the desorptive reoxidation wave) is larger when the potential scan is reversed just past the peak potential of the combined (W + Cu) wave than upon potential reversal after the wave attributed to the second W reduction. This paradox can probably be understood by observing (Figures SI1 A and SI2 B) that an important Cu0 deposition continues during the reverse scan in the case of the combined (W + Cu) wave, an explanation straightforwardly derived from EQCM abilities. Provisionally, it is pointed out that the same behavior is observed with a classical bulky glassy carbon electrode. The following complementary experiments aim to confirm the influence of the redox behaviors of Mo and W centers on the deposition of Cu0 or its reoxidation: (i) Cu0 deposition from a CuSO4 solution, a process in which only the redox behaviors of copper intervene and (ii) monitoring the frequency (or mass) variation during the electrocatalytic reduction of nitrate, a process

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that occurs on the Cu0 film.37 Because this catalysis is triggered by reduced W centers, it is expected that, in the corresponding potential domain, reoxidation of these centers by nitrate will impede a slowing down of copper deposition. The following results with CuSO4 (Figures SI3 A for the CV and SI3 B for the frequency variations) support our previous explanations relative to slope changes during Cu0 deposition from Cu-substituted POMs: (i) the deposition onset corresponds to that of Cu2+ reduction; (ii) a short domain between -0.120 and -0.210 V corresponding to the first copper wave is followed by a domain of linear mass increase; (iii) the mass increase continues during the reverse scan where Cu2+ undergoes a two-electron reduction, directly up to -0.210 V or through a disproportionation process (2Cu+ f Cu0 + Cu2+) in a higher-potential domain; and (iii) the mass decrease during film oxidation is linear. These straightforward conclusions contrast with those obtained with Cu-substituted POMs for which the redox behaviors of Mo and W centers must be taken into account to explain Cu0 deposition and oxidation processes. Also, the mass variations recorded during the electrocatalytic reduction of nitrate by POMs support these hypotheses. For example, mass variations were monitored during Cu0 deposition from R2-[P2W17Cu] in the absence and presence of nitrate (Figure SI4): in the presence of nitrate, the observed shape is the same as that obtained with CuSO4, a result interpreted to be a consequence of the suppression of slowing copper deposition through suppression of the competition between the Cu2+ and W6+ reductions by fast reoxidation of reduced W centers by nitrate. Furthermore, these results constitute a proof of participation of highly reduced W centers to the achievement of higher degrees of nitrate reduction not achievable by CuSO4 alone. Finally, EQCM shows a clear delineation between two kinds of situations encountered with the selected POMs, depending on the potential domains studied: the absence or presence of Cu0 film deposition. For the last situation, EQCM experiments highlight the influence of POM backbones, considered to be ligands for Cu2+, in Cu0 film deposition and reoxidation processes. 3.3. AFM Characterization of Electrocatalytic Films: Correlation between the Nature of the Ligand and the Catalyst Nanoparticle Size. One might wonder whether the differences suggested by EQCM between Cu0 films could correspond to different morphologies for deposited films, thus explaining differences between observed catalytic efficiencies. For a better characterization, AFM studies were performed to observe the morphologies of the electrodeposited catalyst films and to compare them with films deposited from CuSO4. Films were deposited potentiostatically from several Cusubstituted POMs. The film thickness increases quickly, at least during the consumption of the first several millicoulombs. To gain some insight into the nature of the deposit, a 3 h potentiostatic deposition was carried out from a sulfuric acid solution of R1[P2W17Cu] in the absence of dioxygen. XPS analysis of the relatively thick film indicates the presence of sulfur on the surface with the large majority of the deposit being metallic copper in the absence of dioxygen. Following this observation, it was admitted that the crystallites that were increasingly numerous or of larger size during the electrolysis of the solutions were essentially copper deposits. Further discussion of the actual nature of the deposit is beyond the scope of this work because the present films are considered only in relation to their electrocatalytic efficiencies for processes described in the preceding sections. The results will be described and compared mainly for q1 and q4, the minimum and the maximum charges consumed for the

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Figure 5. Ex-situ tapping-mode AFM images of the copper film deposited on an HOPG surface from a 5 × 10-4 M R2-[P2W15Mo2Cu] solution in a pH 3 medium. (A) Height-mode image after the consumption of the minimum charge designated by q1. (B) Phase-mode image after the consumption of the minimum charge designated by q1. (C) Phase-mode image after the consumption of the charge designated by q4 ) 4q1. (D) Enlargement of the phase-mode image represented in part C.

electrolyses, because these two charge quantities allow us to observe both incomplete and complete coverage of the electrode surface by the metal deposit. Raw data are shown. Figure 5 allows a comparison of the main images corresponding to the deposition results for R2-[P2W15Mo2Cu] with q1 and q4 amounts of charge at a potential of -0.380 V versus SCE. Qualitatively, these images present different features. Parts A and B of Figure 5 show the height-mode image and the corresponding phase image, respectively, after q1 charge consumption. They indicate an incomplete coverage of the electrode surface on which naked places were checked to show effectively HOPG. The deposit is constituted by a series of interconnected small islands. Parts C and D of Figure 5 correspond to q4 charge and are presented only in the phase-imaging mode to highlight the sharp details on the surface. Figure 5D shows an enlarged domain of Figure 5C. Now, the complete coverage of the surface is observed. Visually, images obtained with q1 and q4 are strikingly different. In Figure

5C, the uppermost layer of the deposit looks like a carpet of dead leaves, constituted of clearly visible small, densely packed motifs present all over the Figure and shown in more detail in Figure 5D. Even some of these motifs show partial veinlike alignment inside the leaves. Images obtained after q3 charge consumption show complete continuity between those recorded at q1 and q4. Quantitatively, complete or partial overlapping and also coalescence of the various motifs observed on the electrode surface somewhat complicate size determinations, but the complementary use of the height-mode and phase-mode images in this issue was beneficial. Strikingly, it was found that both on q1 and q4 images in Figure 5A a motif measuring 10 ( 2 nm in diameter is the main unit assembled into larger entities spanning roughly the range from 20 to 40 nm in diameter. Actually, the diameter of the majority of motifs in parts A and B of Figure 5 measures around 40 nm, and those in parts C and D of Figure 5, around 20 nm. This statistical observation reveals a difference between

Cu and Mo Center CooperatiVity in Electrocatalysts

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Figure 6. Ex-situ tapping-mode AFM images of a copper film deposited on an HOPG surface from a 5 × 10-4 M solution of the relevant POM or a CuSO4 solution in a pH 3 medium. Both height-mode and phase-mode images are shown for each compound. The charge consumed is q4. (A) CuSO4. (Β) R2-[P2W17Cu]. (C) R1-[P2W17Cu].

the extent of motif coalescence of the very first or the first several deposited layers and the subsequent ones. In short, even though several parameters are expected to govern deposition processes,

a stricking observation is worth stressing in the present case: the size of the basic motif that finally agglomerates into larger particles hardly depends on the charge consumed during the deposition

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Table 1. Main Sizes of Motifs Observed on AFM Images Deposited from r2-[P2W15Mo2Cu], r1-[P2W17Cu], r2-[P2W17Cu], and CuSO4 polyoxometalate

minimum size of constitutive motifs/nm

size of the majority of observed motifs/nm

R2-[P2W15Mo2Cu] R1-[P2W17Cu] R2-[P2W17Cu] CuSO4

10 ( 2 35 ( 5 45 ( 5 100 ( 10

20-25 200-250 300-350 450-600

process. Finally, the POM backbone might play the role of a stabilizing additive. This conclusion is also supported by the following result: increasing the charge consumed for copper deposition more than 8 times (q8) does not notably modify the size of the basic motif. Analogous behavior was observed for the other POMs in this work, as illustrated in the following section where they are also compared with CuSO4. Figure 6 gathers the height-mode and phase-mode images of CuSO4 (Figure 6A), R2-[P2W17Cu] (Figure 6B), and R1-[P2W17Cu] (Figure 6C). Depositions were carried out at -0.500, -0.450, and -0.450 V, respectively, for the three compounds, with a consumption of the same q4 charges in each experiment. Qualitatively, comparison with parts C and D of Figure 5 shows strikingly different features. Obviously, the randomly distributed motifs in Figure 6 are much larger than those previously observed in parts C and D of Figure 5, the largest being those of CuSO4, followed by those of R2-[P2W17Cu] and finally R1-[P2W17Cu]. The larger the size of the crystallites, the smaller the number and the more random the distribution. The large size of the crystallites deposited from CuSO4 makes them show very visible facets on the images. The important roughness of this deposit is a classical observation38 justifying the use of various efficient additives in studies devoted to copper electrocrystallization. Even though the deposition efficiency differs from one substrate to the next, the comparisons remain qualitatively significant, as suggested by the following experiment: for example, with R2-[P2W15Mo2Cu], it turns out that the consumption of a charge more than 2 times higher than that used for the electrolysis of CuSO4 leaves the observed crystallites markedly smaller than those obtained with the latter substrate. This observation underscores that particle size is not governed only by the charge consumed for deposition. Table 1 gathers the sizes of the constitutive motifs and those of the most frequently observed crystallites in the images. These values confirm the qualitative evaluation. Even though it must be borne in mind that the agglomeration of very fine particles might be ultimately deleterious to the desired catalytic efficiency,39,40 finely divided catalysts or electrocatalysts are usually expected to offer the best catalytic efficiencies, a feature that is followed in the experiments described in the preceding sections. The differences in film morphologies are important and might be explained by the following considerations. In comparing images and sizes in Table 1, the film deposited from R2-[P2W17Cu] is the closest to that obtained directly from CuSO4. This observation is in agreement with spectroelectrochemistry and EQCM results indicating the reduction of Cu2+ in this POM to be almost completed whereas that of W centers is still modest. Thus, the consumed charge during the electrolysis of R2-[P2W17Cu] is almost exclusively used for copper deposition. Because the presence of the POM backbone is the only difference in the (38) Welch, C. M.; Hyde, M. E.; Banks, C. E.; Compton, R. G. Anal. Sci. 2005, 21, 1421. (39) Hernandez-Creus, A.; Gimeno, Y.; Diaz, P.; Vasquez, L.; Gonzalez, S.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 2004, 108, 10785. (40) Chen, M.; Cai, Y.; Yan, Z.; D.W Goodman, D. W. J. Am. Chem. Soc. 2006, 128, 6341.

electrolysis bath of CuSO4, it can be assumed that the POM backbone acts as an additive that limits the crystallite size. In short, the role of POM as an additive might constitute a key explanation of the observed film morphologies. As expected, this role might vary from one POM to the next depending on various parameters, including the atomic composition. The particular role of Mo atoms was also confirmed and highlighted by depositing copper from R2-[P2W13Mo4Cu] (Figure SI5). Large domains are observed to be densely covered with uniformly sized particles. However, at variance with Figure 5C, no patches were obtained. The minimum size of the constitutive motifs measures 30 ( 5 nm. The mean diameter of the majority of motifs on the surface is around 60-75 nm. These complementary results underscore the importance of Mo centers in the present issues. The present films are believed to reproduce notably the properties of those actually active in electrocatalytic processes, triggered by electrodeposited copper films. In agreement with the expectation from film morphologies, R2-[P2W15Mo2Cu] is more efficient than R2-[P2W17Cu] in the electrocatalytic reduction of NOx.25 Also, the observation of smaller crystallites with R1[P2W17Cu] is in qualitative agreement with the better efficiency of this POM over that of its R2 isomer in the electrocatalytic reduction of nitrate.32 Finally, the important outcome from AFM experiments is to show the striking behavior of copper deposition from the POMs in this work: whatever the Cu-substituted POM, the sizes of crystallites remain limited compared to the sizes obtained in the deposition of copper from CuSO4. It is remarkable that these sizes of the constitutive motifs stop increasing even for fairly large charge consumptions, with the increase in the sizes of crystallites resulting only from the agglomeration of these motifs. This observation is sharply at variance with the behavior of CuSO4, which gives increasingly large crystals even for small charge consumption.

Concluding Remarks Cyclic voltammetry (CV), the electrochemical quartz crystal microbalance (EQCM), and atomic force microscopy (AFM) were used in conjunction or separately to obtain a deeper understanding of the behaviors of polyoxometalates (POMs) in the electrocatalytic reduction of dioxygen and nitrogen oxides. Several Dawson-type polytungstates were selected for this purpose. They include R2-[P2W17Cu], R1-[P2W17Cu], R2-[P2W15Mo2Fe], R2-[P2W15Mo2Cu], R2-[P2W17Cu], R2-[P2W13Mo4Cu], and R2-[P2W12Mo5Cu]. Following the idea that transition metal ion-substituted POMs might be considered to be analogues of metalated porphyrins, it is assumed that biomimetic behaviors might appear in appropriately substituted polytungstates. With this line of reasoning, particular attention was devoted to Mocontaining Cu-substituted polytungstates. It turns out that the remarkable electrocatalysis obtained at the reduction potential of Mo centers within R2-[P2W15Mo2Cu] but in a domain where Cu2+ is not deposited benefits from the assistance of the copper center because such catalysis could not be observed in the absence of copper. EQCM confirms that no copper deposition occurs under the experimental conditions used. Analogous behaviors are encountered in the electrocatalytic reduction of nitrite where assistance by the presence of the copper center induced the observation of the catalysis at the potential location of Mo centers. Finally, the reduction of nitrate is triggered by electrodeposited copper but was remarkably favored by the presence of molybdenum atoms within the POMs. Thus, the cooperative behavior of these two substituents could be shown. Detailed electrochemical

Cu and Mo Center CooperatiVity in Electrocatalysts

behaviors of the POMs were investigated by CV, and interpretations were confirmed by EQCM studies. It was expected from the various results that copper deposition from these POMs should give morphologically different surfaces. AFM studies confirm this expectation, and the observed morphologies and sizes of particles could be rationalized by taking into account the role of the POM skeleton and its atomic composition in the electrodeposition process. The confirmation that Cu0 is not deposited during several of the electrocatalytic processes studied in this work clarifies the issues to be discussed to learn more about the chemistry at the molecular level. Specifically, attention is focused here on the electrocatalytic reduction of dioxygen. Dioxygen is the favorite chemical for the reoxidation of reduced POMs after their participation in catalytic processes. The mechanism was generally considered to go through an intermediate adduct formation accompanied by inner-sphere electron transfers41 until further insight was recently gained with respect to this process and related processes. Duncan and Hill42 clearly established that simple outersphere electron transfer might be the actual pathway in the case of unsubstituted POMs; in the case of vanadium-substituted derivatives, the conclusion was less clear cut. The work of Duncan and Hill with isotopic labels does establish that POM oxygens are lost during POM reduction and subsequent reduction of O2 (reduced POM reoxidation). In addition, Neumann et al. showed that some molybdates do lose oxygens upon reduction and these are replenished by other oxygen sources upon reoxidation, thus following a Mars van Krevelen-type mechanism.43 Tentatively, we consider our results in the mechanistic framework of metal ion-substituted complexes. In short, the reasoning should parallel that used in dioxygen electroreduction by copper-containing organometallic complexes. In these examples, Cu(I)-O2 constitutes the favorite intermediate.9,44 Here, both the spatial (41) Hiskia, A.; Papaconstantinou, E. Inorg. Chem. 1992, 31, 163. (42) Duncan, D. C.; Hill, C. L. J. Am. Chem. Soc. 1997, 119, 243. (43) Khenkin, A. M.; Weiner, L.; Wang, Y.; Neumann, R. J. Am. Chem. Soc. 2001, 123, 8531. (44) Zhang, J.; Anson, F. C. J. Electroanal. Chem. 1992, 341, 323.

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proximity of Mo and Cu2+ centers within R2-P2W15Mo2Cu and the closeness of their reduction potentials, at appropriately selected pH values, strongly suggest that such a mechanism could be operative. For R2-[P2W13Mo4Cu] and R2-[P2W12Mo5Cu] for which the separations of the reduction potentials of Mo6+ and Cu2+ centers are larger compared to that for R2-P2W15Mo2Cu, no cooperativity is obtained. Tentatively, mixed acceptor orbitals containing both Mo and Cu contributions can be suggested to exist. Work in progress will consider several of these issues. Finally, a specificity of the Mo/Cu association might exist that favors the observed cooperativity. We propose to parallel the functioning obtained in homogeneous solution for the present molecule with that of catalytic solid electrodes prepared for the reduction of dioxygen in water: specifically, (Ru1 - xMox)ySeOz layers containing very small amounts of Mo realize the fourelectron reduction of dioxygen in acidic media.45 The presence of Mo was demonstrated to be required to favor the catalytic process. The mechanistic assumption that guided us to the synthesis and study of such layers is that oxygen is adsorbed by Mo as a result of its higher interaction energy among the surface atoms but electrocatalysis must occur at Ru. Such cooperativity can be considered to be a successful operational hypothesis. Further work is underway in several directions on such systems. Acknowledgment. This work was supported by the CNRS (UMR 8000) and the Universite´ Paris-Sud 11. Supporting Information Available: Frequency variations associated with the cyclic voltammograms of R2-[P2W17Cu] and R2[P2W15Mo2Cu]. Cyclic voltammograms of R2-[P2W17Cu] with different potential reversals and the associated frequency responses. Cyclic voltammograms of CuSO4 and the associated frequency response. Frequency variations associated with the cyclic voltammograms of R2[P2W17Cu] in the absence or presence of nitrate. Ex-situ tapping-mode AFM images of copper film deposited on an HOPG surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA061159D (45) Alonso-Vante, N.; Tributsch, H.; Solorza-Feria, O. Electrochim. Acta 1995, 40, 567.