FULL PAPER DOI: 10.1002/chem.201303185
Outstanding Catalytic Activity of Ultra-Pure Platinum Nanoparticles Aneta Januszewska,[a] Grzegorz Dercz,[b] Justyna Piwowar,[a] Rafal Jurczakowski,*[a] and Adam Lewera*[a] Abstract: Small (4 nm) nanoparticles with a narrow size distribution, exceptional surface purity, and increased surface order, which exhibits itself as an increased presence of basal crystallographic planes, can be obtained without the use of any surfactant. These nanoparticles can be used in many applications in an as-received state and are threefold more active towards a model
catalytic reaction (oxidation of ethylene glycol). Furthermore, the superior properties of this material are interesting not only due to the increase in their intrinsic catalytic activity, but also Keywords: heterogeneous catalysis · nanoparticles · oxidation · platinum · surface chemistry
Introduction In light of the high demand for new functional materials, significant effort has been devoted to developing new routes for the preparation of nanomaterials. Such materials are used as heterogeneous catalysts, for energy conversion, harvesting, and storage, as drug carriers, in high-density information storage devices, and in other applications.[1] The most common method of obtaining unsupported platinum nanoparticles (NPs) with a narrow size distribution is through the chemical reduction of platinum salts or complexes in a solution that consists of a reducing agent and size-stabilizing molecules. PtII or PtIV salts or complexes are reduced by such compounds as alcohols, especially ethylene glycol (EG),[2] hydrazine,[3] and sodium borohydride.[4] Size stabilization is obtained by the addition of organic molecules (surfactants), such as polyvinylpyrrolidone (PVP) or other strongly adsorbing polymers.[2–5] Although the synthesis of Pt NPs and other metals through the polyol route is well known, all of the reported methods were performed by using size-controlling agents. Additionally, capping agents, such as PVP, can facilitate the control of the surface, that is, through the formation of preferential domains.[6] However, the use of the surfactant (capping agent) during synthesis results in surfaces covered by strongly adsorbed molecules (either the surfactant used or products of its oxidation), [a] A. Januszewska, J. Piwowar, Dr. R. Jurczakowski, Dr. A. Lewera Department of Chemistry, University of Warsaw ul. Pasteura 1, 02-093 Warsaw (Poland) Fax: (+ 48) 22-822-0211 ext. 524 E-mail: [email protected] [email protected] [b] Dr. G. Dercz Institute of Materials Science, University of Silesia ul. Bankowa 12, 40-007 Katowice (Poland)
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due to the exceptional surface purity itself. The nanoparticles can be used directly (i.e., as-received, without any cleaning steps) in biomedical applications (i.e., as more efficient drug carriers due to an increased number of adsorption sites) and in energy-harvesting/data-storage devices.
which block the active surface sites and are usually difficult to remove entirely.[7] As a result, significant effort has been dedicated to developing a reliable decontamination procedure, which is typically based on electrochemical or chemical oxidation.[3, 5a, 8] Electrochemical cleaning usually consists of potential cycling to reach the platinum oxide formation (or even oxygen evolution) region until a stable voltammetric response is obtained. Chemical cleaning involves strong oxidizers, which can oxidize adsorbed organic molecules. Such pretreatments, in some cases, can facilitate the removal (oxidation) of contaminations adsorbed on the surface;[5a, 8] however, extensive research is required to determine whether decontamination is complete. It was reported that hard-to-detect carbonaceous products of the oxidative decomposition of capping agents were still present at the surface of NPs and blocked active sites.[9] Notably, electrochemical cleaning can be used only for very small amounts of NPs deposited on an electrode; thus, it is impractical for larger batches of synthesized NPs. Regardless of the nature of the cleaning procedure (either chemical or electrochemical oxidation), a significant drawback of oxidative decontamination is that it can succeed only for materials that are stable in highly oxidative environment, such as platinum. Less-noble materials will be destroyed in the oxidative environment. Another important issue is that oxidative cleaning requires large quantities of oxidizing agents, which are usually highly toxic to the environment.[10] There are methods that allow pure NPs to be obtained, such as cathodic corrosion or sputtering; however, the yields of such methods are too low to be practical. Recently, pure silver and gold colloids were obtained through the laser ablation of metal immersed in pure water.[11] This method is suitable for low metal concentrations because of the agglomeration of NPs and expensive infrastructure. On
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the contrary, chemical methods do not require sophisticated apparatus, and therefore, lead to the facile NP preparation of concentrated colloids, but the main drawback is the requirement for subsequent cleaning of the NPs obtained. Thus, there is the need for an environmentally friendly and simple synthetic method, which would be able to produce NPs with a relatively narrow size distribution, high catalytic activity, and which did not require oxidative cleaning of the resulting material.[12] Herein, we show, for the first time, a method for the direct synthesis of exceptionally pure NPs. The obtained NPs, even without the use of surfactants during synthesis, are small and characterized by a narrow size distribution and increased surface order in terms of the increased presence of basal crystallographic planes, such as (111) and (100). We also comment on the common assumption of surface purity for NPs prepared in the presence of surfactant and show that really pure NPs have a drastic enhancement in their catalytic activity. The role of surface purity has usually been overlooked due to the normalization method used (based on hydrogen adsorption/desorption charge), which is sensitive to the decontaminated surface sites only. The exceptional surface purity of the material obtained resulted in a threefold enhancement in catalytic activity, compared with literature data and the results obtained for the control sample, towards a model catalytic reaction: oxidation of EG. EG is considered to be one of the fuels that can be used in low-temperature fuel cells due to its numerous advantages from practical and mechanistic point of views.[13] Under some conditions, comparable[14] or even higher currents can be obtained for EG than that for methanol.[15] EG oxidation on platinum has been studied by many researchers,[16] and it has been shown that in this reaction EG adsorbs on the platinum surface and decomposes to adsorbed carbon monoxide adlayer (COad) in the potential range between 0.1 and 0.7 V versus a reversible hydrogen electrode (RHE), and that COad can be subsequently oxidized to CO2 at potentials above 0.5 V versus RHE.[16a, b, f] Overall, oxidation to carbon dioxide is not favored due to the high barrier of C C bond breaking, and current efficiencies of CO2 are usually in the range of a few percent,[16b, e, f, 17] a common problem for the oxidation of alcohols containing two or more carbon atoms.[13b, 18] This reaction is sensitive to the structure of the surface and from the basal Pt planes: PtACHTUNGRE(111) is the least active, but at this plane the reaction takes place at the lowest potential values.[19] Overall, our results show that the properties of functional nanomaterials can be significantly improved in the case of bare NPs, which in turn stresses the importance of the surface purity of nanomaterials regardless of their applications.
Results and Discussion The NPs studied were prepared through the so-called polyol method, with (for the sake of comparison) and without the presence of a capping agent (see the Experimental Section).
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The resulting samples are referred to as nanoPtACHTUNGRE(PVP) and nanoPt, respectively. Morphology analysis shows that the NPs obtained are characterized by very similar average sizes (Figure 1) of 3.8 and 4.5 nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. It
Figure 1. TEM micrographs and size distribution histograms based on measurements of at least 100 nanoPt (top) and nanoPtACHTUNGRE(PVP) (bottom) nanoparticles.
also confirms that ultra-pure Pt NPs in the sub-5 nm range and with a narrow size distribution can be obtained without the use of any size-controlling agent. For the nanoPt sample, the size is smaller and the size distribution is narrower than that in case of the nanoPtACHTUNGRE(PVP) NPs, which were also obtained by using the polyol method. Figure 2 shows the XRD patterns of samples of nanoPt and nanoPtACHTUNGRE(PVP) registered in classical Bragg–Brentano (B-B) geometry. Phase identification revealed that all lines
Figure 2. XRD patterns of nanoPt (top dashed line) and nanoPtACHTUNGRE(PVP) (lower solid line).
belonged to the platinum phase. To determine the lattice parameter, the XRD pattern was fitted by using the Rietveld method (the accuracy in determination found by using standard alumina plate SRM 1976 was 0.015 %).[20] The calculated values of the lattice parameters were 0.39196(2) and 0.39185(2) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively, whereas from the PDF-2 card (ICDD PDF 00-004-0802) it was 0.39231 nm.[21] The Williamson–Hall method was applied for the determination of the size of the crystallites as well as lattice deformation. Estimated average sizes of the
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platinum crystallites are 5(1) and 6(1) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. The lattice distortion revealed negligible values: 0.11(1) % and 0.08(1) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. If the lattice distortion is considered to be a measure of internal stresses, one can assume that the nanopowders are free from internal stresses. NPs were electrochemically characterized in the as-synthesized state and also after prolonged electrochemical oxidative cleaning, which consisted of cycling in the potential range between 50 and 1300 mV versus RHE with a scan rate of 50 mV s 1 until stable voltammograms were obtained (approx. 100 cycles). The very first electrochemical cycle for nanoPt does not exhibit any features related to EG (or EG fragments) or CO oxidation, which could be present as a product of EG decomposition on Pt in an aerated environment.[19] For as-received nanoPtACHTUNGRE(PVP), the surface is blocked by the adsorbed surfactant, which results in ill-defined hydrogen under-potential deposition (HUPD) peaks (Figure 3 C). In the case of as-synthesized NPs, the active surface area, and evolution of thereof, was determined. By assuming that the double-layer charging current was independent of the potential and taking QHUPD = 210 mC cm 2 for a monolayer of adsorbed hydrogen, the active surface area was calculated for stable voltammograms from the HUPD charge (Fig-
Figure 3. Comparison of cyclic voltammogram scans for A) as-received ultrapure nanoPt; B) electrochemically cleaned ultrapure nanoPt; C) asreceived nanoPtACHTUNGRE(PVP); and D) electrochemically cleaned nanoPtACHTUNGRE(PVP), registered at 5 mV s 1 in 0.1 mol dm 3 H2SO4.
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ure 3 B and D). For the nanoPtACHTUNGRE(PVP) sample, the active surface was 3.66 m2 g 1, whereas for the nanoPt sample the value was much higher value (24.3 m2 g 1). A comparison of voltammograms obtained for as-synthesized and electrochemically cleaned nanoPt NPs (Figure 3 A and B) confirms their high purity. Both regions, hydrogen adsorption/desorption (below 400 mV) and platinum oxide formation (anodic scan above 750 mV) and reduction (cathodic peak at 800 mV), are well defined, with sharp, separated peaks, which are typical for a clean platinum surface,[8] from the very first voltammetric cycle. In the case of as-received nanoPt, the surface is most likely to be partially covered with adsorbed OH species because an additional signal at about 750 mV is observed exclusively in the first cathodic scan (not shown). The charge, QPtOH, related to this peak is about two times smaller than that of QHUPD ; this implies that the surface coverage is close to 0.5. Because the cleaning step is not required in case of the NPs, we can also comment on the initial state of the platinum surface and on the influence of usual electrochemical cleaning on the surface morphology. A comparison of voltammograms for the as-received nanoPt sample in a relatively narrow potential range with voltammograms obtained after potential cycling in a broader potential range, including the region where platinum oxides are formed (above 800 mV vs. RHE; Figure 3 A and B, respectively), leads to the observation of particular differences. The voltammogram obtained after cycling in a broader potential range (Figure 3 B) is typical for polycrystalline platinum, which could be expected because it is well known that potential cycling of platinum electrodes in the far anodic region results in a loss of preferential single-faced domains.[22] However, the as-received NPs, when cycled in narrower potential range (Figure 3 A), show additional features typical for PtACHTUNGRE(100) and PtACHTUNGRE(111) facets, namely, an increase in intensity of the current associated with HUPD, which occurs on PtACHTUNGRE(100) at about 350 mV, and also from the adsorption of bisulfates.[22–23] The presence of these signals suggests that the surface of the NPs is more ordered after synthesis because features typical for (100) and (111) facets are more pronounced. This surface order is lost when those NPs are subjected to potential cycling in the range of platinum oxide formation (above 800 mV, as in the cleaning procedure). The difference in electrochemical response between the samples is most pronounced in the potential range from 350 to 450 mV, in which a voltammetric feature typical for (100) facets is located.[22] The charge associated with that feature is 14 mC cm 2, which is 3.5 times larger in case of as-received nanoPt sample (Figure 3 A) than in case of the nanoPt sample subjected to potential cycling (4 mC cm 2 ; Figure 3 B). Additionally, a hydrogen adsorption/desorption peak at 280 mV is higher for the as-received sample (Figure 3 A) than in case of NPs subjected to potential cycling (Figure 3 B). In particular, the background-corrected charge associated with the hydrogen desorption process that occurred at 280 mV decreased from 60 to 47 mC cm 2, for as-
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received and electrochemically cleaned samples, whereas the charge associated with the hydrogen desorption process that occurred at 145 mV remained virtually identical (37 vs. 34 mC cm 2, respectively). NPs with preferential (100) facets exhibited an increased height of the signal associated with the HUPD process occurring at 280 mV compared with NPs with preferential (111) facets.[6] These results suggest that the as-received NPs are characterized by a relatively high surface order, which is later lost due to potential cycling (electrochemical cleaning). The ratio between the (111) and (100) facets for as-received NPs shifted towards the latter. In a subsequent voltammetric experiment, the anodic potential limit was increased to 1.3 V and voltammograms were registered in the HUPD and oxide formation zones. The I–E curve for the nanoPt sample was already stable after the first run, whereas for nanoPtACHTUNGRE(PVP) NPs gradual surface purification was observed and manifested itself as a sluggish development of the HUPD peaks (not shown). After prolonged potential cycling (ca. 200 potential cycles) a stable response was finally attained. A comparison of voltammetric profiles obtained in both potential ranges and for both samples is shown in Figure 3. For nanoPt, the voltammograms are slightly different (see the surface order discussion above) before and after the same potential cycling as that used for oxidative cleaning of nanoPtACHTUNGRE(PVP), but the overall charge is constant. This implies a constant number of active sites and no surface cleaning. If the surface were cleaned, then additional surface sites would be opened and the charge would differ. This is not observed for the nanoPt sample. The voltammetric characteristic of nanoPt sample after potential cycling (after oxidation/reduction cycles) is typical for that of a polycrystalline platinum electrode[8] with some contribution from preferential simple facets, as discussed above. The HUPD charge remains practically unaltered. For nanoPtACHTUNGRE(PVP), the HUPD region is partially recovered and the active surface increases up to 9.57 m2 g 1. It is interesting to compare this value with that of the theoretical value, namely, the 3.75 nm spherical NPs should have a surface area of 76 m2 g 1, whereas 4.5 nm spherical particles should have a surface area of 63 m2 g 1. In the case of nanoPt, the difference between the measured and calculated surface areas is probably caused by agglomeration of the NPs. Using the measured active and theoretical surface areas determined for nanoPtACHTUNGRE(PVP) above, and assuming the same degree of agglomeration as that for nanoPt, we can estimate that oxidative cleaning is able to unblock, at most, 47 % of the real surface of nanoPtACHTUNGRE(PVP) NPs. Shao et al. recently showed that for platinum-containing NPs synthesized in presence of surfactant, the surface area determined from HUPD charge could be underestimated by nearly 50 %, but this effect was attributed to the presence of other metals (Ni for Pt Ni NPs) rather than surface purity [24] They used CO stripping and copper under-potential deposition (Cu UPD) as benchmark experiments for numerous NPs and, in general, even for commercially available NPs (see Table 1 in ref. [24]), the surface areas determined from HUPD were
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always lower; this is an example of the importance of surface purity. Notably, the Cu UPD is not necessarily the ultimate tool to determine the surface area because, in general, the surface area determined depends upon the adsorption strength of the probe atoms and, in general, the real surface area is still unknown. The difference in surface area values obtained for nanoPt and nanoPtACHTUNGRE(PVP) cannot be explained by the size difference of the NPs. In particular, based on geometric considerations, slightly smaller NPs (nanoPt) should have an area greater by a factor of 1.2. The relative surface value obtained based on HUPD experiment equals about 2.5. The explanation of this phenomenon, based on the different degrees of agglomeration, can also be excluded based on TEM images (Figure 1) in which nanoPt NPs are clearly more aggregated than that of nanoPtACHTUNGRE(PVP), so the effect should be the opposite. This suggests that other factors, probably surface purity, influence the active surface in the case of nanoPtACHTUNGRE(PVP). It should be reiterated here that because we used prolonged voltammetric cycling (ca. 100 cycles) in the potential range allowed by the sample (in which the Pt NPs are stable) and electrode material, and especially because the obtained voltammograms were stable, no further cleaning of nanoPtACHTUNGRE(PVP) NPs could be achieved, regardless of the method, because more aggressive conditions would lead to sample destruction. A comparison of charges related to surface oxidation and PtO reduction for both samples can be used as a support for the thesis of intrinsic surface contamination of the nanoPtACHTUNGRE(PVP) sample. For the nanoPt sample, both charges are practically the same and the balance is less than 0.1 % for all samples synthesized without the addition of PVP. For nanoPtACHTUNGRE(PVP) samples, the charge related to surface oxidation was always at least 10 % greater than that of oxide reduction, which indicated that the products of PVP oxidation might have been partially reduced at far cathodic potentials. Further irreversible oxidation of adsorbed PVP can be excluded because the voltammetric profile was stable during further potential cycling. To confirm the role of high surface purity, the catalytic activity of Pt NPs towards EG was studied in 0.5 m sulfuric acid for nanoPt and cleaned nanoPtACHTUNGRE(PVP). Figure 4 shows a comparison of voltammograms registered in a solution of 0.5 m sulfuric acid and 1.5 m EG for both samples. The data obtained were normalized by using mass and active surface area. The catalytic activity per surface area (determined in the HUPD experiment; the uncontaminated part of the total surface of the NPs) is virtually identical. When calculated per mass, the catalytic activity of the nanoPt sample reached 300 A g 1, compared with about 100 A g 1 for the reference nanoPtACHTUNGRE(PVP) sample. From the small difference in size between nanoPt and nanoPtACHTUNGRE(PVP), the expected increase in catalytic activity should not exceed about 20 %. Although our NPs in the as-received state has more (111) and (100) facets, and it has been reported that basal platinum planes are more active toward EG electro-oxidation,[19] increased surface order cannot be held responsible for the
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Figure 4. Electro-oxidation of EG in a voltammetric experiment with nanoPt (grey) and a reference sample, nanoPtACHTUNGRE(PVP) (black), which was synthesized in PVP and further electrochemically oxidative cleaned in 0.5 m sulfuric acid and 1.5 m EG. Inset: the same data recalculated per unit surface area (see text for details).
It is known that certain synthetic conditions and/or surfactants could lead to NPs with preferential crystallographic planes.[2b, f, 6, 22, 27] Herein, for the first time, we have shown that small NPs with increased surface order, which exhibited as an increased presence of basal crystallographic planes, could be obtained without the use of any surfactant. These NPs can be used in the as-received state without any decontamination procedures. Furthermore, the superior properties of our material can be profitable not only because of increased intrinsic catalytic activity, but also because of the exceptional surface purity itself. Our NPs can be used directly (i.e., as-received, without any cleaning steps) in biomedical applications (i.e., as more efficient drug carriers due to an increased number of adsorption sites) and as in energy-harvesting/data-storage devices.
Experimental Section observed enhancement of catalytic activity because EG oxidation occurs at a relatively far anodic potential, which introduces surface disorder, similar to that during electrochemical cleaning. The results obtained for the nanoPtACHTUNGRE(PVP) sample are in agreement with catalytic currents obtained in a voltammetric experiment under comparable conditions, as reported in the literature, namely, the EG voltammetric oxidation currents in acidic media are reported in the range of 0.25– 3.5 mA cm 2,[16d, 25] depending on the EG concentration and scan rate. Chetty and Scott,[16c] using catalyzed titanium mesh, reported EG oxidation currents in a voltammetric experiment (1 m EG in 0.5 m H2SO4, 20 mV s 1) in the range from about 55 for platinum to 120 A g 1 (see Figure 6 a in ref. [16c]) for platinum/ruthenium/tungsten; this is also in good agreement with our data for nanoPtACHTUNGRE(PVP), although the synthetic routes are different, they involve the thermal decomposition of an organic solution of metal salts, which may result in similar contamination as in the case of oxidative cleaning. Additionally Teran et al. reported a value of about 40 A g 1 for electro-oxidation of EG on ternary platinum catalysts containing platinum, rhodium, and tin;[26] this is lower than our results for nanoPtACHTUNGRE(PVP), probably due to different sample morphology. These results again suggest that the surface of nanoPtACHTUNGRE(PVP) NPs cannot be completely purified and may be covered, for instance, by some form of carbonaceous remains of PVP[9] and stress the importance of surface purity.
Conclusion We reported herein a direct method for the synthesis of exceptionally pure platinum NPs with a small size (ca. 4 nm) and narrow size distribution. We also stressed the importance of surface purity, and the method of determination thereof, on the catalytic activity of nanomaterials.
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The NPs studied were prepared through the polyol method, in which PtCl2 or K2PtCl4 were dissolved in and reduced by EG. To obtain ultrapure Pt NPs, nanoPt, K2PtCl4 was reduced in EG at 110 8C for 5 min. No other components were added. The reference sample, nanoPtACHTUNGRE(PVP), was prepared from PtCl2, which was reduced in boiling EG for 3 h in the presence of 150 mg of PVP (K30) per 267 mg (1 mmol) of PtCl2. Electrochemical measurements were performed for NPs supported on disposable Au (99.99 %) substrates (Au foil). To assure the highest possible cleanliness, the following procedure was utilized: first, the Au substrate was cleaned in piranha solution and subsequently subjected to aqua regia for 60 s to increase surface roughness, which helped to prevent detachment of the NPs. The cleanliness of gold substrate was confirmed by using cyclic voltammetry. Subsequently, a small aliquot of NPs suspended in water was deposited on the gold substrate and the sample was left to dry in an ultra-high pressure (UHP) Ar atmosphere at room temperature. The usual weight of deposited material was about 30 mg. To prevent aggregation, the suspension was subjected to ultrasonication in an ultrasound bath for 5 min prior to deposition. To normalize observed catalytic currents per catalyst mass, the gold substrate was preweighed by using an ultra-micro balance from Sartortius (SE2, resolution 0.1 mg, repeatability 0.25 mg), and the subsequently prepared electrode (substrate plus NPs deposit) was weighed prior to any electrochemical experiments. As a rule, every weighing was repeated ten times and an average value was used. To confirm that the mass did not change during experiments, the sample weight was confirmed after electrochemical measurements. No significant weight loss was detected (usually less than 5 %).
Acknowledgements This work was financially supported by the Ministry of Science and Higher Education (Poland) under project no. N N204 527739. We wish to thank Barbara Gralec for sample preparation and Marianna Gniadek for performing TEM imaging. The TEM images were obtained by using equipment purchased within CePT project no. POIG.02.02.00-14-024/0800. We also wish to thank the reviewer for helpful comments, which allowed us to improve our manuscript.
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Behm, J. Physical Chemistry C 2013, 117, 12689 – 12701; c) R. Chetty, K. Scott, J. Appl. Electrochem. 2007, 37, 1077 – 1084; d) D. M. Zeng, M. Schell, Electrochim. Acta 2011, 56, 4703 – 4710; e) H. Wang, Z. Jusys, R. J. Behm, J. Electroanal. Chem. 2006, 595, 23 – 36; f) H. Wang, Y. Zhao, Z. Jusys, R. J. Behm, J. Power Sources 2006, 155, 33 – 46; g) A. Dailey, J. Shin, C. Korzeniewski, Electrochim. Acta 1998, 44, 1147 – 1152; h) C. Lamy, Electrochim. Acta 1984, 29, 1581 – 1588. a) J. M. Lger, S. Rousseau, C. Coutanceau, F. Hahn, C. Lamy, Electrochim. Acta 2005, 50, 5118 – 5125; b) H. Hitmi, E. M. Belgsir, J. M. Leger, C. Lamy, R. O. Lezna, Electrochim. Acta 1994, 39, 407 – 415. a) J. Seweryn, A. Lewera, Appl. Catal. B 2014, 144, 129 – 134; b) J. T. Wang, S. Wasmus, R. F. Savinell, J. Electrochem. Soc. 1995, 142, 4218 – 4224. J. M. Orts, A. Fernandez-Vega, J. M. Feliu, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 1990, 290, 119 – 133. a) G. Dercz, B. Formanek, K. Prusik, L. Paja˛k, J. Mater. Proc. Tech. 2005, 162 – 163, 15 – 19; b) G. Dercz, L. Paja˛k, B. Formanek, J. Mater. Proc. Tech. 2006, 175, 334 – 337; c) G. Dercz, K. Prusik, L. Paja˛k, T. Goryczka, B. Formanek, J. Alloys Compd. 2006, 423, 112 – 115. Powder Diffraction Files PDF-2, International Center for Diffraction Data. J. Solla-Gulln, P. Rodriguez, E. Herrero, A. Aldaz, J. M. Feliu, Phys. Chem. Chem. Phys. 2008, 10, 1359 – 1373. M. E. Gamboaaldeco, E. Herrero, P. S. Zelenay, A. Wieckowski, J. Electroanal. Chem. 1993, 348, 451 – 457. M. H. Shao, J. H. Odell, S. I. Choi, Y. N. Xia, Electrochem. Commun. 2013, 31, 46 – 48. a) A. M. A. Ouf, A. A. Ibrahim, A. A. El-Shafei, Electroanalysis 2011, 23, 1998 – 2006; b) V. Selvaraj, M. Vinoba, M. Alagar, J. Colloid Interface Sci. 2008, 322, 537 – 544. F. E. Teran, D. M. Santos, J. Ribeiro, K. B. Kokoh, Thin Solid Films 2012, 520, 5846 – 5850. a) Q.-S. Chen, J. Solla-Gulln, S.-G. Sun, J. M. Feliu, Electrochim. Acta 2010, 55, 7982 – 7994; b) V. Grozovski, J. Solla-Gullon, V. Climent, E. Herrero, J. M. Feliu, J. Physical Chemistry C 2010, 114, 13802 – 13812; c) J. Solla-Gulln, F. J. Vidal-Iglesias, A. LopezCudero, E. Garnier, J. M. Feliu, A. Aldaza, Phys. Chem. Chem. Phys. 2008, 10, 3689 – 3698; d) F. J. Vidal-Iglesias, J. Solla-Gullon, E. Herrero, V. Montiel, A. Aldaz, J. M. Feliu, Electrochem. Commun. 2011, 13, 502 – 505.
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Received: April 26, 2013 Published online: November 12, 2013
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Outstanding Catalytic Activity of Ultra-Pure Platinum Nanoparticles Aneta Januszewska,[a] Grzegorz Dercz,[b] Justyna Piwowar,[a] Rafal Jurczakowski,*[a] and Adam Lewera*[a] Abstract: Small (4 nm) nanoparticles with a narrow size distribution, exceptional surface purity, and increased surface order, which exhibits itself as an increased presence of basal crystallographic planes, can be obtained without the use of any surfactant. These nanoparticles can be used in many applications in an as-received state and are threefold more active towards a model
catalytic reaction (oxidation of ethylene glycol). Furthermore, the superior properties of this material are interesting not only due to the increase in their intrinsic catalytic activity, but also Keywords: heterogeneous catalysis · nanoparticles · oxidation · platinum · surface chemistry
Introduction In light of the high demand for new functional materials, significant effort has been devoted to developing new routes for the preparation of nanomaterials. Such materials are used as heterogeneous catalysts, for energy conversion, harvesting, and storage, as drug carriers, in high-density information storage devices, and in other applications.[1] The most common method of obtaining unsupported platinum nanoparticles (NPs) with a narrow size distribution is through the chemical reduction of platinum salts or complexes in a solution that consists of a reducing agent and size-stabilizing molecules. PtII or PtIV salts or complexes are reduced by such compounds as alcohols, especially ethylene glycol (EG),[2] hydrazine,[3] and sodium borohydride.[4] Size stabilization is obtained by the addition of organic molecules (surfactants), such as polyvinylpyrrolidone (PVP) or other strongly adsorbing polymers.[2–5] Although the synthesis of Pt NPs and other metals through the polyol route is well known, all of the reported methods were performed by using size-controlling agents. Additionally, capping agents, such as PVP, can facilitate the control of the surface, that is, through the formation of preferential domains.[6] However, the use of the surfactant (capping agent) during synthesis results in surfaces covered by strongly adsorbed molecules (either the surfactant used or products of its oxidation), [a] A. Januszewska, J. Piwowar, Dr. R. Jurczakowski, Dr. A. Lewera Department of Chemistry, University of Warsaw ul. Pasteura 1, 02-093 Warsaw (Poland) Fax: (+ 48) 22-822-0211 ext. 524 E-mail: [email protected] [email protected] [b] Dr. G. Dercz Institute of Materials Science, University of Silesia ul. Bankowa 12, 40-007 Katowice (Poland)
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due to the exceptional surface purity itself. The nanoparticles can be used directly (i.e., as-received, without any cleaning steps) in biomedical applications (i.e., as more efficient drug carriers due to an increased number of adsorption sites) and in energy-harvesting/data-storage devices.
which block the active surface sites and are usually difficult to remove entirely.[7] As a result, significant effort has been dedicated to developing a reliable decontamination procedure, which is typically based on electrochemical or chemical oxidation.[3, 5a, 8] Electrochemical cleaning usually consists of potential cycling to reach the platinum oxide formation (or even oxygen evolution) region until a stable voltammetric response is obtained. Chemical cleaning involves strong oxidizers, which can oxidize adsorbed organic molecules. Such pretreatments, in some cases, can facilitate the removal (oxidation) of contaminations adsorbed on the surface;[5a, 8] however, extensive research is required to determine whether decontamination is complete. It was reported that hard-to-detect carbonaceous products of the oxidative decomposition of capping agents were still present at the surface of NPs and blocked active sites.[9] Notably, electrochemical cleaning can be used only for very small amounts of NPs deposited on an electrode; thus, it is impractical for larger batches of synthesized NPs. Regardless of the nature of the cleaning procedure (either chemical or electrochemical oxidation), a significant drawback of oxidative decontamination is that it can succeed only for materials that are stable in highly oxidative environment, such as platinum. Less-noble materials will be destroyed in the oxidative environment. Another important issue is that oxidative cleaning requires large quantities of oxidizing agents, which are usually highly toxic to the environment.[10] There are methods that allow pure NPs to be obtained, such as cathodic corrosion or sputtering; however, the yields of such methods are too low to be practical. Recently, pure silver and gold colloids were obtained through the laser ablation of metal immersed in pure water.[11] This method is suitable for low metal concentrations because of the agglomeration of NPs and expensive infrastructure. On
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the contrary, chemical methods do not require sophisticated apparatus, and therefore, lead to the facile NP preparation of concentrated colloids, but the main drawback is the requirement for subsequent cleaning of the NPs obtained. Thus, there is the need for an environmentally friendly and simple synthetic method, which would be able to produce NPs with a relatively narrow size distribution, high catalytic activity, and which did not require oxidative cleaning of the resulting material.[12] Herein, we show, for the first time, a method for the direct synthesis of exceptionally pure NPs. The obtained NPs, even without the use of surfactants during synthesis, are small and characterized by a narrow size distribution and increased surface order in terms of the increased presence of basal crystallographic planes, such as (111) and (100). We also comment on the common assumption of surface purity for NPs prepared in the presence of surfactant and show that really pure NPs have a drastic enhancement in their catalytic activity. The role of surface purity has usually been overlooked due to the normalization method used (based on hydrogen adsorption/desorption charge), which is sensitive to the decontaminated surface sites only. The exceptional surface purity of the material obtained resulted in a threefold enhancement in catalytic activity, compared with literature data and the results obtained for the control sample, towards a model catalytic reaction: oxidation of EG. EG is considered to be one of the fuels that can be used in low-temperature fuel cells due to its numerous advantages from practical and mechanistic point of views.[13] Under some conditions, comparable[14] or even higher currents can be obtained for EG than that for methanol.[15] EG oxidation on platinum has been studied by many researchers,[16] and it has been shown that in this reaction EG adsorbs on the platinum surface and decomposes to adsorbed carbon monoxide adlayer (COad) in the potential range between 0.1 and 0.7 V versus a reversible hydrogen electrode (RHE), and that COad can be subsequently oxidized to CO2 at potentials above 0.5 V versus RHE.[16a, b, f] Overall, oxidation to carbon dioxide is not favored due to the high barrier of C C bond breaking, and current efficiencies of CO2 are usually in the range of a few percent,[16b, e, f, 17] a common problem for the oxidation of alcohols containing two or more carbon atoms.[13b, 18] This reaction is sensitive to the structure of the surface and from the basal Pt planes: PtACHTUNGRE(111) is the least active, but at this plane the reaction takes place at the lowest potential values.[19] Overall, our results show that the properties of functional nanomaterials can be significantly improved in the case of bare NPs, which in turn stresses the importance of the surface purity of nanomaterials regardless of their applications.
Results and Discussion The NPs studied were prepared through the so-called polyol method, with (for the sake of comparison) and without the presence of a capping agent (see the Experimental Section).
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The resulting samples are referred to as nanoPtACHTUNGRE(PVP) and nanoPt, respectively. Morphology analysis shows that the NPs obtained are characterized by very similar average sizes (Figure 1) of 3.8 and 4.5 nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. It
Figure 1. TEM micrographs and size distribution histograms based on measurements of at least 100 nanoPt (top) and nanoPtACHTUNGRE(PVP) (bottom) nanoparticles.
also confirms that ultra-pure Pt NPs in the sub-5 nm range and with a narrow size distribution can be obtained without the use of any size-controlling agent. For the nanoPt sample, the size is smaller and the size distribution is narrower than that in case of the nanoPtACHTUNGRE(PVP) NPs, which were also obtained by using the polyol method. Figure 2 shows the XRD patterns of samples of nanoPt and nanoPtACHTUNGRE(PVP) registered in classical Bragg–Brentano (B-B) geometry. Phase identification revealed that all lines
Figure 2. XRD patterns of nanoPt (top dashed line) and nanoPtACHTUNGRE(PVP) (lower solid line).
belonged to the platinum phase. To determine the lattice parameter, the XRD pattern was fitted by using the Rietveld method (the accuracy in determination found by using standard alumina plate SRM 1976 was 0.015 %).[20] The calculated values of the lattice parameters were 0.39196(2) and 0.39185(2) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively, whereas from the PDF-2 card (ICDD PDF 00-004-0802) it was 0.39231 nm.[21] The Williamson–Hall method was applied for the determination of the size of the crystallites as well as lattice deformation. Estimated average sizes of the
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platinum crystallites are 5(1) and 6(1) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. The lattice distortion revealed negligible values: 0.11(1) % and 0.08(1) nm for nanoPt and nanoPtACHTUNGRE(PVP), respectively. If the lattice distortion is considered to be a measure of internal stresses, one can assume that the nanopowders are free from internal stresses. NPs were electrochemically characterized in the as-synthesized state and also after prolonged electrochemical oxidative cleaning, which consisted of cycling in the potential range between 50 and 1300 mV versus RHE with a scan rate of 50 mV s 1 until stable voltammograms were obtained (approx. 100 cycles). The very first electrochemical cycle for nanoPt does not exhibit any features related to EG (or EG fragments) or CO oxidation, which could be present as a product of EG decomposition on Pt in an aerated environment.[19] For as-received nanoPtACHTUNGRE(PVP), the surface is blocked by the adsorbed surfactant, which results in ill-defined hydrogen under-potential deposition (HUPD) peaks (Figure 3 C). In the case of as-synthesized NPs, the active surface area, and evolution of thereof, was determined. By assuming that the double-layer charging current was independent of the potential and taking QHUPD = 210 mC cm 2 for a monolayer of adsorbed hydrogen, the active surface area was calculated for stable voltammograms from the HUPD charge (Fig-
Figure 3. Comparison of cyclic voltammogram scans for A) as-received ultrapure nanoPt; B) electrochemically cleaned ultrapure nanoPt; C) asreceived nanoPtACHTUNGRE(PVP); and D) electrochemically cleaned nanoPtACHTUNGRE(PVP), registered at 5 mV s 1 in 0.1 mol dm 3 H2SO4.
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ure 3 B and D). For the nanoPtACHTUNGRE(PVP) sample, the active surface was 3.66 m2 g 1, whereas for the nanoPt sample the value was much higher value (24.3 m2 g 1). A comparison of voltammograms obtained for as-synthesized and electrochemically cleaned nanoPt NPs (Figure 3 A and B) confirms their high purity. Both regions, hydrogen adsorption/desorption (below 400 mV) and platinum oxide formation (anodic scan above 750 mV) and reduction (cathodic peak at 800 mV), are well defined, with sharp, separated peaks, which are typical for a clean platinum surface,[8] from the very first voltammetric cycle. In the case of as-received nanoPt, the surface is most likely to be partially covered with adsorbed OH species because an additional signal at about 750 mV is observed exclusively in the first cathodic scan (not shown). The charge, QPtOH, related to this peak is about two times smaller than that of QHUPD ; this implies that the surface coverage is close to 0.5. Because the cleaning step is not required in case of the NPs, we can also comment on the initial state of the platinum surface and on the influence of usual electrochemical cleaning on the surface morphology. A comparison of voltammograms for the as-received nanoPt sample in a relatively narrow potential range with voltammograms obtained after potential cycling in a broader potential range, including the region where platinum oxides are formed (above 800 mV vs. RHE; Figure 3 A and B, respectively), leads to the observation of particular differences. The voltammogram obtained after cycling in a broader potential range (Figure 3 B) is typical for polycrystalline platinum, which could be expected because it is well known that potential cycling of platinum electrodes in the far anodic region results in a loss of preferential single-faced domains.[22] However, the as-received NPs, when cycled in narrower potential range (Figure 3 A), show additional features typical for PtACHTUNGRE(100) and PtACHTUNGRE(111) facets, namely, an increase in intensity of the current associated with HUPD, which occurs on PtACHTUNGRE(100) at about 350 mV, and also from the adsorption of bisulfates.[22–23] The presence of these signals suggests that the surface of the NPs is more ordered after synthesis because features typical for (100) and (111) facets are more pronounced. This surface order is lost when those NPs are subjected to potential cycling in the range of platinum oxide formation (above 800 mV, as in the cleaning procedure). The difference in electrochemical response between the samples is most pronounced in the potential range from 350 to 450 mV, in which a voltammetric feature typical for (100) facets is located.[22] The charge associated with that feature is 14 mC cm 2, which is 3.5 times larger in case of as-received nanoPt sample (Figure 3 A) than in case of the nanoPt sample subjected to potential cycling (4 mC cm 2 ; Figure 3 B). Additionally, a hydrogen adsorption/desorption peak at 280 mV is higher for the as-received sample (Figure 3 A) than in case of NPs subjected to potential cycling (Figure 3 B). In particular, the background-corrected charge associated with the hydrogen desorption process that occurred at 280 mV decreased from 60 to 47 mC cm 2, for as-
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received and electrochemically cleaned samples, whereas the charge associated with the hydrogen desorption process that occurred at 145 mV remained virtually identical (37 vs. 34 mC cm 2, respectively). NPs with preferential (100) facets exhibited an increased height of the signal associated with the HUPD process occurring at 280 mV compared with NPs with preferential (111) facets.[6] These results suggest that the as-received NPs are characterized by a relatively high surface order, which is later lost due to potential cycling (electrochemical cleaning). The ratio between the (111) and (100) facets for as-received NPs shifted towards the latter. In a subsequent voltammetric experiment, the anodic potential limit was increased to 1.3 V and voltammograms were registered in the HUPD and oxide formation zones. The I–E curve for the nanoPt sample was already stable after the first run, whereas for nanoPtACHTUNGRE(PVP) NPs gradual surface purification was observed and manifested itself as a sluggish development of the HUPD peaks (not shown). After prolonged potential cycling (ca. 200 potential cycles) a stable response was finally attained. A comparison of voltammetric profiles obtained in both potential ranges and for both samples is shown in Figure 3. For nanoPt, the voltammograms are slightly different (see the surface order discussion above) before and after the same potential cycling as that used for oxidative cleaning of nanoPtACHTUNGRE(PVP), but the overall charge is constant. This implies a constant number of active sites and no surface cleaning. If the surface were cleaned, then additional surface sites would be opened and the charge would differ. This is not observed for the nanoPt sample. The voltammetric characteristic of nanoPt sample after potential cycling (after oxidation/reduction cycles) is typical for that of a polycrystalline platinum electrode[8] with some contribution from preferential simple facets, as discussed above. The HUPD charge remains practically unaltered. For nanoPtACHTUNGRE(PVP), the HUPD region is partially recovered and the active surface increases up to 9.57 m2 g 1. It is interesting to compare this value with that of the theoretical value, namely, the 3.75 nm spherical NPs should have a surface area of 76 m2 g 1, whereas 4.5 nm spherical particles should have a surface area of 63 m2 g 1. In the case of nanoPt, the difference between the measured and calculated surface areas is probably caused by agglomeration of the NPs. Using the measured active and theoretical surface areas determined for nanoPtACHTUNGRE(PVP) above, and assuming the same degree of agglomeration as that for nanoPt, we can estimate that oxidative cleaning is able to unblock, at most, 47 % of the real surface of nanoPtACHTUNGRE(PVP) NPs. Shao et al. recently showed that for platinum-containing NPs synthesized in presence of surfactant, the surface area determined from HUPD charge could be underestimated by nearly 50 %, but this effect was attributed to the presence of other metals (Ni for Pt Ni NPs) rather than surface purity [24] They used CO stripping and copper under-potential deposition (Cu UPD) as benchmark experiments for numerous NPs and, in general, even for commercially available NPs (see Table 1 in ref. [24]), the surface areas determined from HUPD were
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always lower; this is an example of the importance of surface purity. Notably, the Cu UPD is not necessarily the ultimate tool to determine the surface area because, in general, the surface area determined depends upon the adsorption strength of the probe atoms and, in general, the real surface area is still unknown. The difference in surface area values obtained for nanoPt and nanoPtACHTUNGRE(PVP) cannot be explained by the size difference of the NPs. In particular, based on geometric considerations, slightly smaller NPs (nanoPt) should have an area greater by a factor of 1.2. The relative surface value obtained based on HUPD experiment equals about 2.5. The explanation of this phenomenon, based on the different degrees of agglomeration, can also be excluded based on TEM images (Figure 1) in which nanoPt NPs are clearly more aggregated than that of nanoPtACHTUNGRE(PVP), so the effect should be the opposite. This suggests that other factors, probably surface purity, influence the active surface in the case of nanoPtACHTUNGRE(PVP). It should be reiterated here that because we used prolonged voltammetric cycling (ca. 100 cycles) in the potential range allowed by the sample (in which the Pt NPs are stable) and electrode material, and especially because the obtained voltammograms were stable, no further cleaning of nanoPtACHTUNGRE(PVP) NPs could be achieved, regardless of the method, because more aggressive conditions would lead to sample destruction. A comparison of charges related to surface oxidation and PtO reduction for both samples can be used as a support for the thesis of intrinsic surface contamination of the nanoPtACHTUNGRE(PVP) sample. For the nanoPt sample, both charges are practically the same and the balance is less than 0.1 % for all samples synthesized without the addition of PVP. For nanoPtACHTUNGRE(PVP) samples, the charge related to surface oxidation was always at least 10 % greater than that of oxide reduction, which indicated that the products of PVP oxidation might have been partially reduced at far cathodic potentials. Further irreversible oxidation of adsorbed PVP can be excluded because the voltammetric profile was stable during further potential cycling. To confirm the role of high surface purity, the catalytic activity of Pt NPs towards EG was studied in 0.5 m sulfuric acid for nanoPt and cleaned nanoPtACHTUNGRE(PVP). Figure 4 shows a comparison of voltammograms registered in a solution of 0.5 m sulfuric acid and 1.5 m EG for both samples. The data obtained were normalized by using mass and active surface area. The catalytic activity per surface area (determined in the HUPD experiment; the uncontaminated part of the total surface of the NPs) is virtually identical. When calculated per mass, the catalytic activity of the nanoPt sample reached 300 A g 1, compared with about 100 A g 1 for the reference nanoPtACHTUNGRE(PVP) sample. From the small difference in size between nanoPt and nanoPtACHTUNGRE(PVP), the expected increase in catalytic activity should not exceed about 20 %. Although our NPs in the as-received state has more (111) and (100) facets, and it has been reported that basal platinum planes are more active toward EG electro-oxidation,[19] increased surface order cannot be held responsible for the
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Figure 4. Electro-oxidation of EG in a voltammetric experiment with nanoPt (grey) and a reference sample, nanoPtACHTUNGRE(PVP) (black), which was synthesized in PVP and further electrochemically oxidative cleaned in 0.5 m sulfuric acid and 1.5 m EG. Inset: the same data recalculated per unit surface area (see text for details).
It is known that certain synthetic conditions and/or surfactants could lead to NPs with preferential crystallographic planes.[2b, f, 6, 22, 27] Herein, for the first time, we have shown that small NPs with increased surface order, which exhibited as an increased presence of basal crystallographic planes, could be obtained without the use of any surfactant. These NPs can be used in the as-received state without any decontamination procedures. Furthermore, the superior properties of our material can be profitable not only because of increased intrinsic catalytic activity, but also because of the exceptional surface purity itself. Our NPs can be used directly (i.e., as-received, without any cleaning steps) in biomedical applications (i.e., as more efficient drug carriers due to an increased number of adsorption sites) and as in energy-harvesting/data-storage devices.
Experimental Section observed enhancement of catalytic activity because EG oxidation occurs at a relatively far anodic potential, which introduces surface disorder, similar to that during electrochemical cleaning. The results obtained for the nanoPtACHTUNGRE(PVP) sample are in agreement with catalytic currents obtained in a voltammetric experiment under comparable conditions, as reported in the literature, namely, the EG voltammetric oxidation currents in acidic media are reported in the range of 0.25– 3.5 mA cm 2,[16d, 25] depending on the EG concentration and scan rate. Chetty and Scott,[16c] using catalyzed titanium mesh, reported EG oxidation currents in a voltammetric experiment (1 m EG in 0.5 m H2SO4, 20 mV s 1) in the range from about 55 for platinum to 120 A g 1 (see Figure 6 a in ref. [16c]) for platinum/ruthenium/tungsten; this is also in good agreement with our data for nanoPtACHTUNGRE(PVP), although the synthetic routes are different, they involve the thermal decomposition of an organic solution of metal salts, which may result in similar contamination as in the case of oxidative cleaning. Additionally Teran et al. reported a value of about 40 A g 1 for electro-oxidation of EG on ternary platinum catalysts containing platinum, rhodium, and tin;[26] this is lower than our results for nanoPtACHTUNGRE(PVP), probably due to different sample morphology. These results again suggest that the surface of nanoPtACHTUNGRE(PVP) NPs cannot be completely purified and may be covered, for instance, by some form of carbonaceous remains of PVP[9] and stress the importance of surface purity.
Conclusion We reported herein a direct method for the synthesis of exceptionally pure platinum NPs with a small size (ca. 4 nm) and narrow size distribution. We also stressed the importance of surface purity, and the method of determination thereof, on the catalytic activity of nanomaterials.
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The NPs studied were prepared through the polyol method, in which PtCl2 or K2PtCl4 were dissolved in and reduced by EG. To obtain ultrapure Pt NPs, nanoPt, K2PtCl4 was reduced in EG at 110 8C for 5 min. No other components were added. The reference sample, nanoPtACHTUNGRE(PVP), was prepared from PtCl2, which was reduced in boiling EG for 3 h in the presence of 150 mg of PVP (K30) per 267 mg (1 mmol) of PtCl2. Electrochemical measurements were performed for NPs supported on disposable Au (99.99 %) substrates (Au foil). To assure the highest possible cleanliness, the following procedure was utilized: first, the Au substrate was cleaned in piranha solution and subsequently subjected to aqua regia for 60 s to increase surface roughness, which helped to prevent detachment of the NPs. The cleanliness of gold substrate was confirmed by using cyclic voltammetry. Subsequently, a small aliquot of NPs suspended in water was deposited on the gold substrate and the sample was left to dry in an ultra-high pressure (UHP) Ar atmosphere at room temperature. The usual weight of deposited material was about 30 mg. To prevent aggregation, the suspension was subjected to ultrasonication in an ultrasound bath for 5 min prior to deposition. To normalize observed catalytic currents per catalyst mass, the gold substrate was preweighed by using an ultra-micro balance from Sartortius (SE2, resolution 0.1 mg, repeatability 0.25 mg), and the subsequently prepared electrode (substrate plus NPs deposit) was weighed prior to any electrochemical experiments. As a rule, every weighing was repeated ten times and an average value was used. To confirm that the mass did not change during experiments, the sample weight was confirmed after electrochemical measurements. No significant weight loss was detected (usually less than 5 %).
Acknowledgements This work was financially supported by the Ministry of Science and Higher Education (Poland) under project no. N N204 527739. We wish to thank Barbara Gralec for sample preparation and Marianna Gniadek for performing TEM imaging. The TEM images were obtained by using equipment purchased within CePT project no. POIG.02.02.00-14-024/0800. We also wish to thank the reviewer for helpful comments, which allowed us to improve our manuscript.
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Received: April 26, 2013 Published online: November 12, 2013
Chem. Eur. J. 2013, 19, 17159 – 17164
