Article pubs.acs.org/IC
Influence of Polyoxometalate Protecting Ligands on Catalytic Aerobic Oxidation at the Surfaces of Gold Nanoparticles in Water Mingfu Zhang,† Jingcheng Hao,† Alevtina Neyman,‡ Yifeng Wang,*,† and Ira A. Weinstock*,‡ †
Key Laboratory of Colloid and Interface Science of the Education Ministry, Department of Chemistry and Chemical Engineering, Shandong University, Ji’Nan 250100, P. R. China ‡ Department of Chemistry and the Ilse Katz Institute for Nanoscale Science & Technology, Ben-Gurion University of the Negev, Beer Sheva, 84105, Israel S Supporting Information *
ABSTRACT: Metal oxide cluster-anion (polyoxometalate, or POM) protecting ligands, [α-PW11O39]7− (1), modify the rates at which 14 nm gold nanoparticles (Au NPs) catalyze an important model reaction, the aerobic (O2) oxidation of CO to CO2 in water. At 20 °C and pH 6.2, the following stoichiometry was observed: CO + O2 + H2O = CO2 + H2O2. After control experiments verified that the H2O2 product was sufficiently stable and did not react with 1 under turnover conditions, quantitative analysis of H2O2 was used to monitor the rates of CO oxidation, which increased linearly with the percent coverage of the Au NPs by 1 (0−64% coverage, with the latter value corresponding to 211 ± 19 surface-bound molecules of 1). X-ray photoelectron spectroscopy of Au NPs protected by a series of POM ligands (K+ salts): 1, the Wells−Dawson ion [α-P2W18O62]6− (2) and the monodefect Keggin anion [α-SiW11O39]8− (3) revealed that binding energies of electrons in the Au 4f7/2 and 4f5/2 atomic orbitals decreased as a linear function of the POM charge and percent coverage of Au NPs, providing a direct correlation between the electronic effects of the POMs bound to the surfaces of the Au NPs and the rates of CO oxidation by O2. Additional data show that this effect is not limited to POMs but occurs, albeit to a lesser extent, when common anions capable of binding to Au-NP surfaces, such as citrate or phosphate, are present.
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analogues” of heterogeneous metal oxide supports.7 Since that seminal discovery, numerous POMs have been used as inorganic stabilizing ligands for metal(0) NPs prepared from a variety of transition metals.8−14 Many of these, in turn, feature high catalytic activities and selectivities in reductive and oxidative transformations of organic substrates.13,15−23 At the same time, these unique reactivities are generally attributed either to the activities of the metal(0) cores, with the POMs simply providing a stabilizing function, or to the reversible redox properties of the POM ligands, capable, for example, of quenching organic-radical intermediates.23 A direct role of the POM ligands in modifying the catalytic activities of the metal(0) cores themselves has never been documented.24 We now demonstrate this for the first time using the monodefect Keggin cluster anion [α-PW11O39]7− (1; Figure 1) as a protecting ligand for 14-nm Au NPs in water. Taking advantage of the relatively large electron density of the W atoms (Z = 74) in heteropolytungsate cluster anions, cryogenic sample preparation for transmission electron microscopy (cryoTEM) has provided unprecedented insight into the structures of POM-monolayer shells on Au NPs in water and, when combined with traditional solution-state spectroscopic methods, has made it possible to elucidate the electrostatic forces that control their formation and stability.25−29 Insights and
INTRODUCTION In heterogeneous catalysis, metal oxide supports play important roles in the formation and stabilization of dispersed metal(0) nanoparticles (NPs). Moreover, interactions between the support and metal(0) NPs can significantly modify their catalytic activities. In this context, the term “Strong Metal− Support Interaction” is used to describe the enhanced catalytic activity observed when reducible metal oxide supports partially cover the edges of supported metal(0) NPs.1,2 Although this phenomenon is well established for solid-state catalysts, much less is known about the possible effects of stabilizing ligands on the catalytic properties of colloidal metal(0) NPs. In this regard, the organic protecting ligand3 poly(N-vinyl-2pyrrolidone) was recently shown4 to modify the electronic structure of 1.5 nm core gold nanoparticles (Au NPs) via electron donation from the O atoms of the pyrrolidone moieties of the polymeric stabilizer. This increased the electron density at the Au cores, resulting in enhanced catalytic activity in the aerobic (O2) oxidation of alcohols.4−6 The negatively charged Au cores were more effective at transferring electrons from the alcoholic substrates to the π* lowest unoccupied molecular orbitals (LUMOs) of O2, generating superoxo- or peroxo-like species at the surfaces of the Au clusters.4−6 Turning to inorganic ligands, Finke showed in 1990 that metal oxide cluster anions (polyoxometalates, or POMs) can bind directly to catalytically active colloidal metal(0) NP surfaces and described the POM components as “soluble © XXXX American Chemical Society
Received: September 7, 2016
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DOI: 10.1021/acs.inorgchem.6b02167 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
specific volumes of water and 2 (4 mM, pH 6.2). The final volumes were 3.0 mL, and the concentrations of 2 ranged from 0 to 2.0 mM. After aging in the dark for 24 h, the solution in each vial was analyzed by UV−vis spectroscopy. The binding isotherm was then derived using a previously published method.29 Briefly, the absorbance of each solution at 521.5 nm was measured and plotted against the added POM concentration to give a Langmuir type I isotherm. The following equation and the OriginLab 9.0 software were used for the nonlinear curve fitting:
A = (A 0 + A1K[POM])/(1 + K[POM])
in which A0 is the experimental absorbance of citrate-stabilized Au NP colloids, A1 is the fitted value of 100% coverage POM-stabilized Au NP colloids, and K is the Langmuir constant. Equation 2 is used to calculate the fractional coverage (Θ) of a certain species when the Langmuir constant and concentration (C) are provided.
Figure 1. Polyhedral structure of 1. The red tetrahedron at the center represents PO4, and the green polyhedra are WO6 units, with O atoms at their vertices.
methods developed in those studies are now used to document the role of POMs on colloidal Au-NP catalysis of an important model reaction, the aerobic (O2) oxidation of CO to CO2.30−32 Using this model reaction, monolayer domains of 1 on the surfaces of the Au NPs were shown to play an important role in modulating the catalytic activities of the Au cores.
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(1)
Θ = KC /(1 + KC)
(2)
Adsorption Isotherm of K7PW11O39 during CO Oxidation (under Turnover Conditions). Different from the above, specific amounts of solid K7PW11O39 were directly dissolved in 3.0 mL of Au NPs (6.2 nM). After aging in the dark for 24 h, a CO/O2 mixture (the composition is provided immediately below) was bubbled into the solution until no further UV−vis absorbance change was observed (ca. 3 min). UV−vis spectra obtained before and after introduction of the CO/O2 mixture to each solution were used to determine the binding isotherm of 1 under turnover conditions (the method used was identical with that described above). CO Oxidation Reactions. Typically, Au NP-catalyzed CO oxidation reactions were performed by gently bubbling a CO/O2 mixture into the colloidal solutions after the systems were thoroughly purged with N2. The concentrations of CO and O2 in solution were controlled and calculated by their partial pressures (pi, 27.3 and 72.7% for CO and O2, respectively; the saturated concentrations of CO and O2, Ci0, are 9.30 × 10−4 and 1.23 × 10−3 M, respectively; C = Ci0pi; note that the steady-state concentration near the Au surface is smaller than that calculated), which are tuned by their individual flow rates. The flow rates were controlled using float-flow meters calibrated carefully before each CO-oxidation experiment. Formation of peroxide was used to quantify the rates of CO oxidation (by O2). To measure the total concentration of peroxide formed during the experiments, aliquots of the solutions were withdrawn every 2 min and quantitatively diluted in 3.0 M CsCl solutions, after which Au precipitate was removed by filtration and the DPD method was adopted (ε = 21000 M−1 cm−1 at λ = 551 nm) to measure the total peroxide concentration.35 Control experiments show that 0.5 mM K7PW11O39 had no significant effect on the accuracy of the measured [H2O2] values. Because five points provide enough accuracy for linear fitting, the peroxide formation rates (which increased linearly with time and correspond in a 1:1 fashion to the CO-oxidation rates) were measured over the course of 8 min reactions. Determination of the Stoichiometry of the Catalytic Reaction. For this, a N2 flow was used to purge the dissolved CO2 in (either the as-prepared citrate-stabilized or the K71-stabilized) Au colloidal solution for 5 min. After that, a CO/O2 mixture was bubbled into this solution, and the outlet gas (containing CO2) was absorbed by a Ba(OH)2 solution (0.01 M). After a certain time, the CO/O2 mixture was switched to N2 to make sure all of the CO2 produced was purged from the Au colloid to the Ba(OH)2 solution. Then the unreacted Ba(OH)2 was quantified by back-titration using a 0.01 M oxalic acid solution. A pH meter was used to monitor the pH jump and to determine the endpoint of the titration. The amount of CO2 produced during the catalytic reaction was then calculated. The amount of peroxide formed was also measured using the DPD method. Direct determination of the CO2 formation rates in the kinetic study of CO oxidation was not adopted because of the technical difficulties involved in the required operations and the lack of adequate precision in the corresponding times of reaction. Hence, rapid and quantitative
EXPERIMENTAL SECTION
Materials. The POM salts α-K7PW11O39·13H2O (K71),33 αK6P2W18O62·24H2O (K62),34 and α-K8SiW11O39·13H2O (K83)34 were synthesized according to published methods and analyzed by 31 P NMR and Fourier transform infrared spectroscopy. Trisodium citrate (Na3Ct) was obtained from Sigma-Aldrich. HAuCl4·3H2O (>99.9%) and N,N-dimethyl-p-phenylenediamine (DPD) were purchased from J&K Chemicals. Horseradish peroxidase was purchased from Aladdin (China). Deionized water (18.25 MΩ) was used for all experiments, and glassware for handling the Au colloid was pretreated with aqua regia. Instrumentation. A centrifuge (maximum centrifugal force 20000 g) equipped with a temperature controller was used for purification of the Au NPs. UV−vis spectral and kinetic data were obtained using an Agilent Cary 60 spectrophotometer. NMR spectra were acquired using a Bruker 400 MHz instrument. IR spectra were obtained on a PerkinElmer Spectrum Two IR spectrometer. ζ-potential measurements were obtained using a ZetaPlus instrument from the Malvern Co. X-ray photoelectron spectroscopy (XPS) was performed on an Omicron (ESCA+) spectrometer, using an Al Kα X-ray source (1486.6 eV) equipped with a flood gun. Binding energies were calibrated using a C 1s band (BE = 284.7 eV). Preparation of 14-nm-Diameter Au NPs. A citrate-stabilized Au colloid was prepared by minor modification of the Turkevich method, described elsewhere.29 The Au NPs were close to spherical and relatively monodisperse in size, with an average diameter of 13.8 ± 0.9 nm, and thus the concentration of Au NP is ca. 6.2 nM (0.5 mM Au). The citrate concentration was 2.0 mM before formation of the Au colloid. If necessary, the Au NPs were purified to remove excess citrate: the colloid was centrifuged (10000 g for 5 min at 20 °C) and redispersed in deionized water. This process was repeated, and the final concentration of the redispersed Au NPs was measured through the spectrometric method. The final citrate concentration is estimated to be 1 > 3. Because the percent coverages of the Au NPs are nearly identical for the Keggin ions 1 (94%) and 3 (96%), the distinct differences between the BE results (i.e., 83.0 vs 82.8 eV for Au 4f7/2) are clearly due to the POM charges of 7− and 8−, respectively. For the 2-protected Au NPs, the smaller degree of coverage (77%) could also contribute to the larger BE value (83.5 eV). To account for differences in the POM surface coverage, the charge densities of the POM monolayers were “normalized” by dividing the charge density12 of each cluster anion by the corresponding fractional-coverage values (see Table S1). This revealed a linear correlation between the BE values and the normalized charge densities (Figure 7D). Notably, this also shows that an increase in the POM surface coverage results in a linear increase in electron donation to the Au NP surface. Importantly, this finding is consistent with the linear response of the catalytic activity of 1-protected Au NPs to incremental increases in the surface coverage by 1 (Figure 6). Mechanistic Considerations. The data in Figures 6 and 7 establish a linear relationship between the degree to which POM ligands donate charge density to the Au NP surface and the rate of formation of H2O2 via the aerobic (O2) oxidation of CO. In addition, equal numbers of equivalents of CO2 and H2O2 products are observed (Figure 2). Because CO is the only reducing agent present and control experiments rule out any significant decomposition of H2O2 (or other likely side reactions), the reaction is reasonably described as written in eq 4. Moreover, the rate at which this occurs increases as more charge density is donated to the Au surface by the POM ligands. Precisely how this occurs, however, cannot be reliably established without detailed knowledge about what is occurring at the Au surface. In this regard, the remarkable ability of small (2−3) Au NPs on oxide supports to rapidly catalyze the aerobic oxidation of CO at even very low temperatures (i.e., below 300 K)52 has led to numerous mechanistic studies (experimental and computational) that, even for this relatively simple model reaction, point to an impressive variety of elementary steps occurring at the Au surface.53,54 While these generally involve the adsorption of both O2 and CO at the Au surface,55 different rate-limiting steps appear operative as functions of Au NP (or cluster) size, surface structure [e.g., exposed (101) facets as well as edge versus terrace structures], the roles of gold(0), alone or in combination with gold(I), donation of negative charge density to the Au surface by ligands, the presence or absence of water,56 and the nature and properties of the interfaces between the Au NP surface and oxide supports.54 This information has largely been obtained by experimental investigations of heterogeneous systems involving supported Au NPs or via computational studies.53,54 The even more difficult task of acquiring definitive information about elementary steps occurring on the surfaces of colloidal Au NPs in solution is beyond the scope of the present work. Nevertheless, orders of the reaction with respect to concentrations of the POM (1), CO, and O2 were examined. For [1], a highlight of the present study is our observation of a first-order dependence of the rate not on the bulk concentration of 1 but rather on the percent coverage by 1 of the Au NP surface, [1surface] (at 0.5 mM 1 and partial pressures of 27.3 and 72.7% for CO and O2, respectively).
rate = kapp[1surface ]1 [CO]1 [O2 ]x
(5)
Notably, these findings highlight the direct (linear) effect of the POM surface coverage on the rate, and the rapid, energetically favorable binding of CO to the catalytically active Au surface. The less than first-order dependence on [O2] is consistent with both a weaker binding to the Au surface (as compared to CO)54 and some complexity regarding the nature and reactivity of adsorbed O2. This is not surprising given the ongoing uncertainty regarding this complex issue.53,54,57 In addition, the robust linear correlations between the rates, surface coverage, and electronic structure of Au atoms at the NP surface indicate that the negative charge density, rather than the specific nature of the interface between POM-monolayer domains and POM-free regions on the Au NP surface, is responsible for the rate acceleration observed to accompany increases in the POM surface coverage. As such, the findings from detailed mechanistic studies of CO oxidation by O2 at the surfaces of Au NPs are relevant to the present discussion. Those studies highlight two related phenomena: the donation of electron density to LUMOs (π* orbitals) of bound O2 and CO. If O2 activation is rate-determining, the observed rate enhancement could reflect more extensive charge transfer to surface-bound O2.4,5 This follows Tsukuda’s proposal4based on data by Kim5that more rapid aerobic oxidations of alcohols by Au NPs with electron-donating organic ligands reflect partial ET to the LUMOs (π* orbitals) of O2. For metallic Au in particular, which has a small metal− oxygen bond energy (relative to metals that form stable oxides, such as ruthenium), the reaction of CO with O2 often follows the Langmuir−Hinshelwood model,54 in which the ratelimiting step is the activation of molecular oxygen at the metal surface.4,5,54,57−59 This is one way in which increased coverage by 1 in the present case would result in more rapid catalytic turnover,60 despite the ongoing debate concerning the precise nature of the active oxygen intermediate.53,57 In addition, increased negative charge density weakens the C−O bond of Au-bound CO (again via the population of π* orbitals), which could also kinetically play a role in the conversion of CO to CO2.61 An alternative, and more general, way to interpret the observed rate enhancement is to view it as an anion-adsorptioninduced electrochemical potential effect. Viewed from this perspective, the Au NPs behave as individual nanoelectrodes, and the adsorbed anions shift the electrode potential in the negative direction. This would increase the thermodynamic driving force (and, hence, the rate) of O2 reduction. If the reaction rate is strongly dependent on O2 reduction, as is the case in many aerobic oxidation processes, including the present F
DOI: 10.1021/acs.inorgchem.6b02167 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry *E-mail:
[email protected].
one, increased anion coverage would accelerate the overall reaction. Effects of Common Oxoanions on the Au NP Reactivity. To assess whether the rate enhancement provided by 1 might be a general phenomenon, occurring when common oxoanions are adsorbed onto metal(0) NPs in water, we investigated the rate effects of partially protonated PO43− and citrate anions (pH 6−7) and the fully deprotonated NO3− and SO42− anions (Figures S6−S8). Rate enhancement was indeed observed as a function of the PO43− coverage (Figure S6, inset). The rate increased from ca. 0.49 μM s−1, in the absence of added PO43−, to nearly 1.2 μM s−1 at 2.0 mM [PO43−]. A similar, ca. 100% rate enhancement was observed when citrate concentrations were increased from ca. 0.1 to 2.0 mM (Figure S7). By contrast, no rate enhancement was observed for NO3− or SO42− (Figure S8; 0.1 mM citrate), both of which are strong electrolytes and much less effective than citrate anions at binding to and stabilizing Au NPs.21,62,63 These findings show that millimolar concentrations of simple oxoanions such as phosphate and citrate can enhance CO oxidation rates by >100%. Because ions are spontaneously adsorbed onto the surface of NPs in water, this indicates that common anions ubiquitous to aqueous solutions of Au NPs can have significant effects on the rates of aerobic catalysis.
ORCID
Ira A. Weinstock: 0000-0002-6701-2001 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Y.W. thanks the National Natural Science Foundation of China (Grant 21473104) and Chinese Postdoctoral Science Foundation (Grant 2015M572042), and I.A.W. thanks the Israel Science Foundation (Grant ISF 190/13) and the I-CORE Program of the Planning and Budgeting Committee and the ISF (Grant 152/11).
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CONCLUSIONS In conclusion, quantitatively well-defined coverage of Au NPs by metal oxide cluster-anion (POM) protecting ligands, in combination with a model reaction, the aerobic oxidation of CO, has made it possible to obtain detailed information regarding the effects of POM anions on catalysis at the Au surface. Combined cryo-TEM imaging, UV−vis spectroscopic analysis, and XPS data reveal a linear correlation between the electronic effects of negative charge density of precisely defined degrees of surface coverage by POM monolayer domains and the rates of Au-catalyzed CO oxidation by O2. Hence, not only in a structural sense may POM ligands be viewed as “soluble analogues” of heterogeneous metal oxide supports.7 Indeed, just as catalysis by metal(0) NPs is influenced by metal oxide supports in heterogeneous catalysis, POM ligands, through donation of negative charge density, modify the electronic properties and reactivities of colloidal Au NPs in solution. Moreover, simple oxoanion protecting ligandsphosphate and citratehave a rate-enhancing effect, while nitrate and sulfate, which do not bind significantly to Au NPs, provide no observable rate enhancement. Taken together, these findings suggest that modulation of the electronic structure of Au NP surfaces by anionic (as well as electronrich) 4 protecting ligands may prove to be a general phenomenon, perhaps broadly applicable to catalysis by metal(0) NPs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02167. Supplementary data and discussion (PDF)
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REFERENCES
(1) Baker, R. T. K., Tauster, S. J., Dumesic, J. A. Strong Metal− Support Interactions; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. (2) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Strong Interactions in Supported-Metal Catalysts. Science 1981, 211, 1121− 1125. (3) Saha, K.; Agasti, S. S.; Kim, C.; Li, X.; Rotello, V. M. Gold Nanoparticles in Chemical and Biological Sensing. Chem. Rev. 2012, 112, 2739−2779. (4) Tsunoyama, H.; Ichikuni, N.; Sakurai, H.; Tsukuda, T. Effect of Electronic Structures of Au Clusters Stabilized by Poly(N-vinyl-2pyrrolidone) on Aerobic Oxidation Catalysis. J. Am. Chem. Soc. 2009, 131, 7086−7093. (5) Kim, Y. D.; Fischer, M.; Ganteför, G. Origin of Unusual Catalytic Activities of Au-based Catalysts. Chem. Phys. Lett. 2003, 377, 170−176. (6) More generally, using organic ligands, which associate with a nanomaterial surface via electrostatic interaction and/or other types of weak secondary bonds, to modify the catalytic activities of a nanomaterial is not limited to metal(0) NPs. For example, a polyelectrolyte can modify the catalytic activity of carbon nanotubes in the oxygen reduction reaction. See: Wang, S.; Yu, D.; Dai, L. J. Am. Chem. Soc. 2011, 133, 5182−5185. (7) Lyon, D. K.; Finke, R. G. Polyoxoanions as Soluble Metal Oxide Analogs. 6. Catalytic Activity and Initial Kinetic and Mechanistic Studies of Polyoxoanion-Supported, Atomically Dispersed Iridium(I), (1,5-COD)Ir·P2W15Nb3O628‑. Inorg. Chem. 1990, 29, 1787−1789. (8) Villanneau, R.; Roucoux, A.; Beaunier, P.; Brouri, D.; Proust, A. Simple Procedure for Vacant POM-stabilized Palladium(0) Nanoparticles in Water: Structural and Dispersive Effects of Lacunary Polyoxometalates. RSC Adv. 2014, 4, 26491−26498. (9) Hsu-Yao, T.; Browne, K. P.; Honesty, N.; Tong, Y. J. Polyoxometalate-Stabilized Pt Nanoparticles and Their Electrocatalytic Activities. Phys. Chem. Chem. Phys. 2011, 13, 7433−7438. (10) Graham, C. R.; Ott, L. S.; Finke, R. G. Ranking the Lacunary (Bu4N)9{H[α2-P2W17O61]} Polyoxometalate’s Stabilizing Ability for Ir(0)n Nanocluster Formation and Stabilization Using the Five-Criteria Method Plus Necessary Control Experiments. Langmuir 2009, 25, 1327−1336. (11) Troupis, A.; Hiskia, A.; Papaconstantinou, E. Synthesis of Metal Nanoparticles by Using Polyoxometalates as Photocatalysts and Stabilizers. Angew. Chem., Int. Ed. 2002, 41, 1911−1914. (12) Wang, Y.; Weinstock, I. A. Polyoxometalate-Decorated Nanoparticles. Chem. Soc. Rev. 2012, 41, 7479−7496. (13) Keita, B.; Liu, T.; Nadjo, L. Synthesis of Remarkably Stabilized Metal Nanostructures Using Polyoxometalates. J. Mater. Chem. 2009, 19, 19−33. (14) For more references on the synthesis of POM-stabilized metal(0) NPs, see citations in refs 12 and 13. (15) Mitchell, S. G.; de la Fuente, J. M. The Synergistic Behavior of Polyoxometalates and Metal Nanoparticles: from Synthetic Approaches to Functional Nanohybrid Materials. J. Mater. Chem. 2012, 22, 18091−18100.
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DOI: 10.1021/acs.inorgchem.6b02167 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry (16) Biboum, R. N.; Keita, B.; Franger, S.; Nanseu Njiki, C. P.; Zhang, G.; Zhang, J.; Liu, T.; Mbomekalle, I.-M.; Nadjo, L. Pd0@ Polyoxometalate Nanostructures as Green Electrocatalysts: Illustrative Example of Hydrogen Production. Materials 2010, 3, 741−754. (17) Maayan, G.; Neumann, R. Direct Aerobic Oxidation of Secondary Alcohols Catalysed by Pt(0) Nanoparticles Stabilized by PV2Mo10O405‑ Polyoxmetalate. Catal. Lett. 2008, 123, 41−45. (18) Vasylyev, M. V.; Maayan, G.; Hovav, Y.; Haimov, A.; Neumann, R. Palladium Nanoparticles Stabilized by Alkylated Polyethyleneimine as Aqueous Biphasic Catalysts for the Chemoselective Stereocontrolled Hydrogenation of Alkenes. Org. Lett. 2006, 8, 5445−5448. (19) Maayan, G.; Neumann, R. Direct Aerobic Epoxidation of Alkenes Catalyzed by Metal Nanoparticles Stabilized by the H5PV2Mo10O40 Polyoxometalate. Chem. Commun. 2005, 4595−4597. (20) Lin, Y.; Finke, R. G. Novel Polyoxoanion- and Bu4N+-Stabilized, Isolable, and Redissolvable, 20−30 Å Ir300−900 Nanoclusters: The Kinetically Controlled Synthesis, Characterization, and Mechanism of Formation of Organic Solvent-Soluble, Reproducible Size, and Reproducible Catalytic Activity Metal Nanoclusters. J. Am. Chem. Soc. 1994, 116, 8335−8353. (21) Ö zkar, S.; Finke, R. G. Nanocluster Formation and Stabilization Fundamental Studies: Ranking Commonly Employed Anionic Stabilizers via the Development, then Application, of Five Comparative Criteria. J. Am. Chem. Soc. 2002, 124, 5796−5810. (22) Aiken, J. D.; Finke, R. G. Polyoxoanion- and Tetrabutylammonium-Stabilized Rh(0)n Nanoclusters: Unprecedented Nanocluster Catalytic Lifetime in Solution. J. Am. Chem. Soc. 1999, 121, 8803− 8810. (23) For more information on the catalytic activities of POMstabilized metal(0) NPs, see the book chapter (Wang, Y.; Weinstock, I. A. In Polyoxometalate Chemistry, Some Recent Trends; Secheresse, F., Ed.; World Scientific Publishing Co.: Singapore, 2013; pp 1−48) and the references cited therein. (24) The effects of the types and concentrations of POMs in bulk solution on the growth and reactivity of Ir(0) NPs are described in the following articles: (a) Watzky, M. A.; Finke, R. G. Transition Metal Nanocluster Formation Kinetic and Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow, Continuous Nucleation and Fast Autocatalytic Surface Growth. J. Am. Chem. Soc. 1997, 119, 10382−10400. (b) Lin, Y.; Finke, R. G. A More General Approach to Distinguishing ″Homogeneous″ from ″Heterogeneous″ Catalysis: Discovery of Polyoxoanion- and Bu4N+-Stabilized, Isolable and Redissolvable, High-Reactivity Ir∼190−450 Nanocluster Catalysts. Inorg. Chem. 1994, 33, 4891−4910. (25) Wang, Y.; Zeiri, O.; Sharet, S.; Weinstock, I. A. Role of the Alkali-Metal Cation Size in the Self-Assembly of PolyoxometalateMonolayer Shells on Gold Nanoparticles. Inorg. Chem. 2012, 51, 7436−7438. (26) Wang, Y.; Zeiri, O.; Meshi, L.; Stellacci, F.; Weinstock, I. A. Regioselective Placement of Alkanethiolate Domains on Tetrahedral and Octahedral Gold Nanocrystals. Chem. Commun. 2012, 48, 9765− 9767. (27) Sharet, S.; Sandars, E.; Wang, Y.; Zeiri, O.; Neyman, A.; Meshi, L.; Weinstock, I. A. Orientations of Polyoxometalate Anions on Gold Nanoparticles. Dalton Trans. 2012, 41, 9849−9851. (28) Wang, Y.; Weinstock, I. A. Cation Mediated Self-Assembly of Inorganic Cluster Anion Building Blocks. Dalton Trans. 2010, 39, 6143−6152. (29) Wang, Y.; Neyman, A.; Arkhangelsky, E.; Gitis, V.; Meshi, L.; Weinstock, I. A. Self-Assembly and Structure of Directly Imaged Inorganic-Anion Monolayers on a Gold Nanoparticle. J. Am. Chem. Soc. 2009, 131, 17412−17422. (30) Della Pina, C.; Falletta, E.; Prati, L.; Rossi, M. Selective Oxidation Using Gold. Chem. Soc. Rev. 2008, 37, 2077−2095. (31) Rodriguez-Rivera, G. J.; Kim, W. B.; Evans, S. T.; Voitl, T.; Dumesic, J. A. Hydrogenation of Benzene Using Aqueous Solution of Polyoxometalates Reduced by CO over Gold Catalysts. J. Am. Chem. Soc. 2005, 127, 10790−10791.
(32) Kim, W. B.; Voitl, T.; Rodriguez-Rivera, G. J.; Dumesic, J. A. Powering Fuel Cells with CO via Aqueous Polyoxometalates and Gold Catalysts. Science 2004, 305, 1280−1283. (33) Haraguchi, N.; Okaue, Y.; Isobe, T.; Matsuda, Y. Stabilization of Tetravalent Cerium upon Coordination of Unsaturated Heteropolytungstate Anions. Inorg. Chem. 1994, 33, 1015−1020. (34) Klemperer, W. G. Early Transition Metal Polyoxoanions. In Inorganic Synthesis; Ginsberg, A. P., Ed.; John Wiley & Sons: New York, 1990; Vol. 27, pp 71−132. (35) Wu, T.; Liu, G.; Zhao, J.; Hidaka, H.; Serpone, N. Evidence for H2O2 Generation during the TiO2-Assisted Photodegradation of Dyes in Aqueous Dispersions under Visible Light Illumination. J. Phys. Chem. B 1999, 103, 4862−4867. (36) Contant, R. Relations entre les tungstophosphates apparentés à l’anion PW12O403−. Synthèse et propriétés d’un nouveau polyoxotungstophosphate lacunaire K10P2W20O70·24H2O. Can. J. Chem. 1987, 65, 568−573. (37) Couto, F. A. R. S.; Cavaleiro, A. M. V.; Pedrosa de Jesus, J. D.; Simão, J. E. J. Study of Polyoxotungstates with the Keggin Structure by Cyclic Voltammetry. Inorg. Chim. Acta 1998, 281, 225−228. (38) Weinstock, I. A.; Barbuzzi, E. M. G.; Wemple, M. W.; Cowan, J. J.; Reiner, R. S.; Sonnen, D. M.; Heintz, R. A.; Bond, J. S.; Hill, C. L. Equilibrating Metal-Oxide Cluster Ensembles for Oxidation Reactions Using Oxygen in Water. Nature 2001, 414, 191−195. (39) Zhang, M.; Weinstock, I. A.; Wang, Y. Electrocatalysis by Polyoxometalate-Protected Gold Nanoparticles. J. Cluster Sci. 2014, 25, 771−779. (40) Geletii, Y. V.; Hill, C. L.; Atalla, R. H.; Weinstock, I. A. Reduction of O2 to Superoxide Anion (O2•‑) in Water by Heteropolytungstate Cluster-Anions. J. Am. Chem. Soc. 2006, 128, 17033−17042. (41) Weinstock, I. A. Homogeneous-Phase Electron-Transfer Reactions of Polyoxometalates. Chem. Rev. 1998, 98, 113−170. (42) Sadakane, M.; Steckhan, E. Electrochemical Properties of Polyoxometalates as Electrocatalysts. Chem. Rev. 1998, 98, 219−238. (43) Teranishi, M.; Naya, S.-i.; Tada, H. In Situ Liquid Phase Synthesis of Hydrogen Peroxide from Molecular Oxygen Using Gold Nanoparticle-Loaded Titanium(IV) Dioxide Photocatalyst. J. Am. Chem. Soc. 2010, 132, 7850−7851. (44) Li, G.; Edwards, J.; Carley, A. F.; Hutchings, G. J. Direct Synthesis of Hydrogen Peroxide from H2 and O2 and in situ Oxidation using Zeolite-Supported Catalysts. Catal. Commun. 2007, 8, 247−250. (45) Shao, M. H.; Adzic, R. R. Spectroscopic Identification of the Reaction Intermediates in Oxygen Reduction on Gold in Alkaline Solutions. J. Phys. Chem. B 2005, 109, 16563−16566. (46) Park, Y.; Lee, B.; Kim, C.; Kim, J.; Nam, S.; Oh, Y.; Park, B. Modification of Gold Catalysis with Aluminum Phosphate for OxygenReduction Reaction. J. Phys. Chem. C 2010, 114, 3688−3692. (47) Duncan, D. C.; Chambers, R. C.; Hecht, E.; Hill, C. L. Mechanism and Dynamics in the H3[PW12O40]-Catalyzed Selective Epoxidation of Terminal Olefins by H2O2. Formation, Reactivity, and Stability of {PO4[WO(O2)2]4}3‑. J. Am. Chem. Soc. 1995, 117, 681− 691. (48) Derrouiche, S.; Gravejat, P.; Bianchi, D. Heats of Adsorption of Linear CO Species Adsorbed on the Au0 and Ti+δ Sites of a 1% Au/ TiO2 Catalyst Using in Situ FTIR Spectroscopy under Adsorption Equilibrium. J. Am. Chem. Soc. 2004, 126, 13010−13015. (49) Zheng, Z.; Huang, B.; Qin, X.; Zhang, X.; Dai, Y.; Whangbo, M.H. Facile in situ Synthesis of Visible-Light Plasmonic Photocatalysts M@TiO2 (M = Au, Pt, Ag) and Evaluation of their Photocatalytic Oxidation of Benzene to Phenol. J. Mater. Chem. 2011, 21, 9079− 9087. (50) Leppelt, R.; Schumacher, B.; Plzak, V.; Kinne, M.; Behm, R. J. Kinetics and Mechanism of the Low-Temperature Water−Gas Shift Reaction on Au/CeO2 Catalysts in an Idealized Reaction Atmosphere. J. Catal. 2006, 244, 137−152. (51) Kitagawa, H.; Kojima, N.; Nakajima, T. Studies of MixedValence States in Three-Dimensional Halogen-Bridged Gold ComH
DOI: 10.1021/acs.inorgchem.6b02167 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry pounds, Cs2AuIAuIIIX6, (X = Cl, Br or I). Part 2. X-Ray Photoelectron Spectroscopic Study. J. Chem. Soc., Dalton Trans. 1991, 3121−3125. (52) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. Low-Temperature Oxidation of CO over Gold Supported on TiO2, α-Fe2O3, and Co3O4. J. Catal. 1993, 144, 175− 192. (53) Takei, T.; Akita, T.; Nakamura, I.; Fujitani, T.; Okumura, M.; Okazaki, K.; Huang, J.; Ishida, T.; Haruta, M. Heterogeneous Catalysis by Gold. In Advanced Catalysis; Bruce, C. G., Friederike, C. J., Eds.; Academic Press: New York, 2012; Chapter 1, Vol. 55, pp 1−126. (54) Min, B. K.; Friend, C. M. Heterogeneous Gold-Based Catalysis for Green Chemistry: Low-Temperature CO Oxidation and Propene Oxidation. Chem. Rev. 2007, 107, 2709−2724. (55) Weiher, N.; Bus, E.; Delannoy, L.; Louis, C.; Ramaker, D. E.; Miller, J. T.; van Bokhoven, J. A. Structure and Oxidation State of Gold on Different Supports Under Various CO Oxidation Conditions. J. Catal. 2006, 240, 100−107. (56) Mullen, G.; Gong, J.; Yan, T.; Pan, M.; Mullins, C. The Effects of Adsorbed Water on Gold Catalysis and Surface Chemistry. Top. Catal. 2013, 56, 1499−1511. (57) Gong, J.; Mullins, C. B. Surface Science Investigations of Oxidative Chemistry on Gold. Acc. Chem. Res. 2009, 42, 1063−1073. (58) Gong, J. Structure and Surface Chemistry of Gold-Based Model Catalysts. Chem. Rev. 2012, 112, 2987−3054. (59) Outka, D. A.; Madix, R. J. The Oxidation of Carbon Monoxide on the Au(110) Surface. Surf. Sci. 1987, 179, 351−360. (60) At the same time, the observed rate enhancement indicates that the electronic effect of the POM anions overcomes the decrease in the concentration of the active sites on the Au surface that accompanies the formation of POM-monolayer domains. A similar situation was observed by Tsukuda in a detailed study of the rate-enhancing effects of poly(N-vinyl-2-pyrolidone) ligands on the Au NP-catalyzed aerobic (O2) oxidation of CO in CH2Cl2. See ref 4. (61) Yoon, B.; Häkkinen, H.; Landman, U.; Wörz, A. S.; Antonietti, J.-M.; Abbet, S.; Judai, K.; Heiz, U. Charging Effects on Bonding and Catalyzed Oxidation of CO on Au8 Clusters on MgO. Science 2005, 307, 403−407. (62) Silva, F.; Martins, A. Surface Structural Effects on Specific Adsorption of Oxoanions on Gold Single Crystal Electrodes. J. Electroanal. Chem. 1999, 467, 335−341. (63) Melendres, C. A.; Hahn, F.; Bowmaker, G. A. Oxyanion Adsorption and Competition at a Gold Electrode. Electrochim. Acta 2000, 46, 9−13.
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DOI: 10.1021/acs.inorgchem.6b02167 Inorg. Chem. XXXX, XXX, XXX−XXX
