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Cite this: Chem. Commun., 2014, 50, 10809 Received 24th June 2014, Accepted 23rd July 2014
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Facile synthesis of ultra-small rhenium nanoparticles† ab c d d Tug ˘ çe Ayvalı, Pierre Lecante, Pier-Francesco Fazzini, Ange ´lique Gillet, ab d Karine Philippot* and Bruno Chaudret*
DOI: 10.1039/c4cc04816d www.rsc.org/chemcomm
Ultra-small monodisperse rhenium nanoparticles (Re NPs; ca. 1.0–1.2 nm) were easily prepared by reducing the organometallic complex [Re2(C3H5)4] under a dihydrogen atmosphere under mild reaction conditions (3 bar H2; 120 8C). The particles can be stabilized by a ligand, hexadecylamine, or a polymer, polyvinylpyrrolidone and accommodate surface hydrides.
Nanomaterials have been experiencing significant development over the last two decades due to their physical and chemical properties at the boundary of those of molecular and bulk species.1 However, this development has remained limited to a few classes of elements. For example, the preparation and characterization of nanoparticles of group 7 transition metals have been reported in very few articles.2 Refractoriness, mechanical strength, high melting point and chemical resistance poisoned from N, S and P3 make rhenium nanoparticles (Re NPs) attractive for electronics and electrical engineering. It has also been observed that rhenium has a positive contribution in terms of conversion and selectivity in complex catalytic processes such as glycerol reforming, hydrocarbon transformations or hydrogenation of difficult functional groups.4 This shows the interest of rhenium for different catalytic reactions. Surprisingly, the synthesis of pure Re NPs has been overlooked for a long period of time. Several synthetic methods have been attempted to prepare Re NPs such as wet chemical reduction,5 impregnation followed by calcination under H2 gas,6 radiation,5b,7 thermal decomposition,8 solid state thermolytic demixing9 and lately alcohol assisted
reduction.10 Most of these reports present weak-points such as difficult synthetic protocols, polydispersity and/or lack of information on the oxidation state of the resulting particles, to cite only a few. Thus, the synthesis of uniformly dispersed and well-defined Re NPs still remains a challenge. In this communication, we report a facile one-pot synthesis of ultra-small Re NPs following an organometallic approach11 and their characterization by several techniques such as TEM, HAADF-STEM, WAXS, FT-IR spectroscopy and quantification of surface hydrides. Our long-term interest concerns with the use of rhenium-containing nanomaterials in catalysis. The Re NPs were synthesized (Experimental, ESI†) by decomposition of the dirhenium(II)tetraallyl complex [Re2(C3H5)4]12 at 120 1C under 3 bar H2 in anisole as a solvent and in the presence of either polyvinylpyrrolidone (PVP) or hexadecylamine (HDA) as a stabilizing agent (Scheme 1). The reactions led to stable dark brown colloidal solutions in both cases. After solvent evaporation and purification steps PVP-stabilized nanoparticles (Re/PVP NPs) were isolated as a dark brown fine powder. In contrast, HDA-stabilized nanoparticles (Re/HDA NPs) could only be isolated as a sticky dark brown solid owing to the presence of long alkyl chain amines linked to the metal surface, as previously observed with other metals like Ru, Pd and Pt.13 Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with high annular dark field detector (HAADF) analyses of purified samples revealed the presence of spherical and well-dispersed NPs having a mean size of
a
Laboratoire de Chimie de Coordination, CNRS, LCC, 205 Route de Narbonne, F-31077 Toulouse, France. E-mail: [email protected]; Tel: +33 (0)561333182 b Universite´ de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France c CNRS UPR 8011, CEMES (Centre d’Elaboration des Mate´riaux et d’Etudes Structurales), 29 Rue Jeanne Marvig, F-31055 Toulouse, France d LPCNO, Laboratoire de Physique et Chimie des Nano-Objets, UMR 5215 INSACNRS-UPS, Institut des Sciences Applique´es, 135 Avenue de Rangueil, F-31077 Toulouse, France. E-mail: [email protected]; Tel: +33 (0)561559655 † Electronic supplementary information (ESI) available: Experimental details: TEM, WAXS and FT-IR data. See DOI: 10.1039/c4cc04816d
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Scheme 1 Synthesis of Re NPs stabilized either by PVP or HDA from organometallic complex [Re2(C3H5)4].
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Fig. 1 (a), (c) TEM and (b), (d) HAADF-STEM images of Re/HDA and Re/ PVP NPs respectively. Insets show the corresponding size histograms.
1.2(0.3) and 1.0(0.3) nm for Re/PVP and Re/HDA samples, respectively (Fig. 1). Fourier transform infrared (FT-IR) spectra of purified Re/HDA and Re/PVP NPs (S1, ESI†) showed absorption bands in the same region as for free HDA and PVP, thus evidencing the presence of each stabilizer (Experimental, ESI†). The crystalline structure of the purified Re NPs was investigated by wide-angle X-ray scattering (WAXS). In the reciprocal space (S2, ESI†), only very broad peaks could be observed, as expected for very small particles. The intensity pattern was in good agreement with a hcp structure computed for bulk Re. From radial distribution functions (RDFs), coherence lengths of ca. 1.2 nm and 1.0 nm were derived for Re/PVP and Re/HDA NPs, respectively. These values agree with the mean sizes measured by TEM. In order to determine the NP structure, the experimental RDFs were compared (Fig. 2) with a theoretical function computed from a very small hcp model (spherical; 1.3 nm) and taking into account a single and large static disorder factor. All the distances from the experimental RDFs adequately match the ones derived from the 0.274 nm
Fig. 2 WAXS analysis of Re NPs (from top to bottom: Re/HDA NPs, hcp simulation for comparison, Re/PVP NPs).
10810 | Chem. Commun., 2014, 50, 10809--10811
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bond length in bulk rhenium. However the fast decay of the amplitude with distance indicates the presence of a short-range order indicative of low crystallinity. Given that changing experimental parameters may induce modulation of the NP morphology, we then studied the influence of temperature, [stabilizer]/[metal] ratio and reaction time. The temperature effect was investigated by performing the synthesis at 100 1C instead of 120 1C while keeping the other parameters the same. At 100 1C, the kinetics of the reaction drastically slowed down, and one additional day was necessary to achieve total reduction of the precursor (Experimental, ESI†). At the end of the reaction, mean sizes of 1.0(0.2) and 1.3(0.3) nm were measured for spherical Re/HDA and Re/PVP NPs, respectively, by using TEM and WAXS analysis with data in good agreement (S3 and S4, ESI†). These results show that temperature does not have a significant influence on the morphology of Re NPs but it affects the rate of the reaction. A change in the [stabilizer]/[metal] ratio clearly influenced the mean size of NPs as seen by TEM (S5, ESI†). When the [HDA]/ [Re] ratio was decreased from 1 to 0.5 molar equiv., the mean size of the Re/HDA NPs increased to 1.3(0.3) nm, as expected in the presence of lower amounts of the stabilizer. In addition, some Re NPs were shown to display a slightly elongated shape as previously observed for Ru, Pd and Pt NPs when using HDA.13 This shape modification may arise either from the organization of HDA in solution forming a ‘‘soft’’ template matrix or from a specific coordination of HDA on given faces of the growing nanocrystals.11,13 When Re/PVP NPs were prepared with less Re content (5 wt%), a smaller NP mean size (1.0(0.3) nm) was obtained. These results are again consistent with the theoretical expectations since higher amounts of polymers can interfere in the early stage of nucleation and provide smaller NPs. Altering the reaction time from 2 to 4 days at 120 1C resulted in a slight broadening of the size distribution without a significant change in size and shape of the NPs (S6, ESI†). Since dihydrogen was used as a reducing agent to synthesize Re NPs, the presence of hydrides at their surface could be expected as known with Ru NPs.14,15 The amount of surface hydrides was monitored by a home-made titration method which is based on the hydrogenation of a simple olefin like norbornene15 without addition of dihydrogen (Experimental, ESI†). No significant hydrogenation of norbornene was observed at room temperature (RT) but the reaction proceeded smoothly at 80 1C leading to norbornane. Assuming a mean size of 1.2 nm for Re/PVP NPs which thus corresponds to ca. 74% of surface Re atoms, we found ca. 1.1 hydride per surface Re atom. Similarly, for Re/HDA NPs, we calculated 0.7 hydride per surface Re atom by assuming a mean size of 1.0 nm displaying ca. 81% surface atoms. These results show that hydrides are coordinated at the surface of Re NPs and that heating is necessary to induce their reactivity, while Ru NP surface hydrides were reactive at RT. In addition, under a CO atmosphere (3 bar) at RT, the metallic surface was not very reactive towards CO adsorption. However, at 90 1C, a clear coordination of CO was visible with absorption bands at ca. 1890 and 2000 cm 1 for Re/PVP NPs and at ca. 1905 and 1985 cm 1 for Re/HDA NPs (S7, ESI†), corresponding in each case to the presence of bridging and terminal carbonyl groups16 as a result of hydride displacement. These observations are in agreement with the molecular chemistry of Re and Ru for which
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ReH7(PPh3)217 is a reactive species only under forcing conditions whereas the corresponding RuH6(PCy3)218 is highly reactive at RT. Given the interest for rhenium oxide catalysts,4,19 solid samples of particles were reacted with 3 bar dioxygen for 2 days. The resulting NPs were analyzed by FT-IR spectroscopy, TEM and WAXS. By FT-IR spectroscopy, a new and intense band was visible at 907 cm 1 which can be attributed to a rhenium oxygen bond20 for O2-treated Re NPs (S8, ESI†). TEM analysis did not show any variation in size or in size distribution for Re/HDA and Re/PVP NPs (S9a and c, ESI†), but some organization was visible on the TEM micrographs of Re/HDA NPs. WAXS analysis revealed a partial oxidation of Re NPs for both Re–HDA and Re–PVP systems. Both the amplitude and coherence length were strongly reduced; however, the remaining peaks were still consistent with those of metallic Re. Moreover, the reduction of the coherence length is consistent with smaller metallic domains and strongly suggests that oxidation leads to a smaller metallic core surrounded by an amorphous shell of Re oxide without a significant change in the overall size, as observed by TEM analysis. Partially oxidized Re NPs were previously observed by Abu-Omar et al.10a during the catalytic dehydrogenation of secondary aliphatic alcohols using NH4ReO4 as a precatalyst under air. Finally, in order to recover the rhenium zero oxidation state, Re NPs were reacted with 3 bar H2 at RT for 3 days and for 1 additional day at 80 1C. No significant change was noticed by FT-IR analysis showing that oxidation is irreversible under these conditions unlike the case of Ru NPs.14b To sum up the hydrogenolysis of [Re2(C3H5)4] provides ultrasmall Re NPs (ca. 1–1.2 nm) in the presence of a polymer (PVP) or a ligand (HDA). The resulting NPs display a spherical shape and adopt a highly disordered hcp structure. They accommodate surface hydrides which are strongly coordinated to rhenium in agreement with the corresponding chemistry of rhenium complexes. Under dioxygen, these Re NPs show a partial oxidation, suggesting a core–shell system with a metallic core and an amorphous oxide shell. For the first time, this work offers a simple route towards monodisperse rhenium nanoparticles as well as basic useful information on their surface state. Surface reactivity studies on these Re NPs and the design of more complex systems such as bimetallic nanostructures are presently on-going in our group. Moreover, these nanosystems will be investigated in the hydrogenation of difficult functional groups. The authors are grateful to Prof. K. Mertis for valuable `re discussions on the synthesis of the rhenium precursor. V. Collie and L. Datas at UPS-TEMSCAN, Y. Coppel and C. Bijani from CNRSLCC are acknowledged for electron microscopy and NMR facilities, respectively. This work was supported by CNRS, UPS and FP7NMP2-Large program grant (Synflow 2010-246461).
This journal is © The Royal Society of Chemistry 2014
Communication
Notes and references 1 (a) Nanoparticles. From Theory to Application, ed. G. Schmid, WileyVCH, Weinheim, 2004; (b) Nanoparticles and Catalysis, ed. D. Astruc, Wiley-Interscience, New York, 2008. 2 (a) A. A. Ensafi, H. Karrimi-Maleh, M. Ghiaci and M. Arshadi, J. Mater. Chem., 2011, 21, 15022; (b) D. H. Lee, J. A. Lee, W. J. Lee, D. S. Choi, W. J. Lee and S. O. Kim, J. Phys. Chem. C, 2010, 114, 21184; (c) H. A. Ngwa, A. Kanthasamy, Y. Gu, N. Fang, V. Anantharam and A. G. Kanthasamy, Toxicol. Appl. Pharmacol., 2011, 256, 227. 3 T. E. Tietz and J. W. Wilson, Behavior and Properties of Refractory Metals, Tokyo University International Edition, USA, 1965, pp. 206–222. 4 V. G. Kessler and G. A. Seisenbaeva, Minerals, 2012, 2, 244. 5 (a) M. C. Zenghelis and C. Stathis, Compt. Rend., 1939, 209, 797; (b) M. R. Mucalo and C. R. Bullen, J. Colloid Interface Sci., 2001, 239, 71; (c) K. M. Babu and M. R. Mucalo, J. Mater. Sci. Lett., 2003, 22, 1755; (d) A. A. Revina, M. A. Kuznetsov and A. M. Chekmarev, Dokl. Akad. Nauk, 2013, 450, 47. 6 (a) N. Yang, G. E. Mickelson, N. Greenlay, S. D. Kelly, F. D. Vila, J. Kas, J. J. Rehr and S. R. Bare, AIP Conf. Proc., 2007, 882, 591; (b) U. G. Hong, H. W. Park, J. Lee, S. Hwang, J. Yi and I. K. Song, Appl. Catal., A, 2012, 415, 141; (c) S. R. Bare, S. D. Kelly, F. D. Vila, E. Boldingh, E. Karapetrova, J. Kas, G. E. Mickelson, F. S. Modica, N. Yang and J. J. Rehr, J. Phys. Chem. C, 2011, 115, 5740. 7 (a) C. Vollmer, E. Redel, K. Abu-Shandi, R. Thomann, H. Manyar, C. Hardacre and C. Janiak, Chem. – Eur. J., 2010, 16, 3849; (b) Y. Y. Chaong, W. Y. Chow and W. Y. Fan, J. Colloid Interface Sci., 2012, 369, 164. 8 G. Y. Yurkov, A. V. Kozinkin, Y. A. Koksharov, A. S. Fionov, N. A. Taratanov, V. G. Vlasenko, I. V. Pirog, O. N. Shishilov and O. V. Popkov, Composites, Part B, 2012, 43, 3192. 9 C. D. Valenzuela, M. L. Valenzuela, S. Caceres and C. O’Dwyer, J. Mater. Chem. A, 2013, 1, 1566. 10 (a) J. Yi, J. T. Miller, D. Y. Zemlyanov, R. Zhang, P. J. Dietrich, F. H. Ribeiro, S. Suslov and M. M. Abu-Omar, Angew. Chem., Int. Ed., 2013, 52, 1; (b) N. I. Buryak, O. G. Yanko and S. V. Volkov, Ukr. Khim. Zh., 2013, 79, 79. ´re, 11 C. Amiens, B. Chaudret, D. Ciuculescu-Pradines, V. Collie K. Fajerwerg, P. Fau, M. Kahn, A. Maisonnat, K. Soulantica and K. Philippot, New J. Chem., 2013, 37, 3374. 12 A. F. Masters, K. Mertis, J. F. Gibson and G. Wilkinson, New J. Chem., 1977, 1, 389. 13 (a) C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P. Lecante and M.-J. Casanove, J. Am. Chem. Soc., 2001, 123, 7584; (b) E. Ramirez, S. Jansat, K. Philippot, P. Lecante, M. Gomez, A. M. Masdeu-Bulto and B. Chaudret, J. Organomet. Chem., 2004, ´s, K. Philippot, P. Lecante and 689, 4601; (c) E. Ramirez, L. Erade B. Chaudret, Adv. Funct. Mater., 2007, 17, 2219. 14 (a) F. Novio, K. Philippot and B. Chaudret, Catal. Lett., 2010, 140, 1; (b) F. Novio, D. Monahan, Y. Coppel, G. Antorrena, P. Lecante, K. Philippot and B. Chaudret, Chem. – Eur. J., 2014, 20, 1287. 15 J. Garcia-Anton, M. R. Axet, S. Jansat, K. Philippot, B. Chaudret, T. Pery, G. Buntkowsky and H.-H. Limbach, Angew. Chem., Int. Ed., 2008, 47, 2074. 16 F. Solymosi and T. Bansagi, J. Phys. Chem., 1992, 96, 1349 and the references therein. 17 D. Baudry and M. Ephritikhine, J. Chem. Soc., Chem. Commun., 1980, 6, 249. 18 S. Sabo-Etienne and B. Chaudret, Coord. Chem. Rev., 1998, 381, 178. 19 (a) J. C. Mo, Catal. Today, 1999, 51, 289; (b) R. Hamtil, N. Zilkova, H. Balcar and J. Cejka, Appl. Catal., A, 2006, 302, 193. 20 P. S. Kirlin, F. A. Dethomas, J. W. Bailey, H. S. Gold, C. Dybowski and B. C. Gates, J. Phys. Chem., 1986, 90, 4882.
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Cite this: Chem. Commun., 2014, 50, 10809 Received 24th June 2014, Accepted 23rd July 2014
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Facile synthesis of ultra-small rhenium nanoparticles† ab c d d Tug ˘ çe Ayvalı, Pierre Lecante, Pier-Francesco Fazzini, Ange ´lique Gillet, ab d Karine Philippot* and Bruno Chaudret*
DOI: 10.1039/c4cc04816d www.rsc.org/chemcomm
Ultra-small monodisperse rhenium nanoparticles (Re NPs; ca. 1.0–1.2 nm) were easily prepared by reducing the organometallic complex [Re2(C3H5)4] under a dihydrogen atmosphere under mild reaction conditions (3 bar H2; 120 8C). The particles can be stabilized by a ligand, hexadecylamine, or a polymer, polyvinylpyrrolidone and accommodate surface hydrides.
Nanomaterials have been experiencing significant development over the last two decades due to their physical and chemical properties at the boundary of those of molecular and bulk species.1 However, this development has remained limited to a few classes of elements. For example, the preparation and characterization of nanoparticles of group 7 transition metals have been reported in very few articles.2 Refractoriness, mechanical strength, high melting point and chemical resistance poisoned from N, S and P3 make rhenium nanoparticles (Re NPs) attractive for electronics and electrical engineering. It has also been observed that rhenium has a positive contribution in terms of conversion and selectivity in complex catalytic processes such as glycerol reforming, hydrocarbon transformations or hydrogenation of difficult functional groups.4 This shows the interest of rhenium for different catalytic reactions. Surprisingly, the synthesis of pure Re NPs has been overlooked for a long period of time. Several synthetic methods have been attempted to prepare Re NPs such as wet chemical reduction,5 impregnation followed by calcination under H2 gas,6 radiation,5b,7 thermal decomposition,8 solid state thermolytic demixing9 and lately alcohol assisted
reduction.10 Most of these reports present weak-points such as difficult synthetic protocols, polydispersity and/or lack of information on the oxidation state of the resulting particles, to cite only a few. Thus, the synthesis of uniformly dispersed and well-defined Re NPs still remains a challenge. In this communication, we report a facile one-pot synthesis of ultra-small Re NPs following an organometallic approach11 and their characterization by several techniques such as TEM, HAADF-STEM, WAXS, FT-IR spectroscopy and quantification of surface hydrides. Our long-term interest concerns with the use of rhenium-containing nanomaterials in catalysis. The Re NPs were synthesized (Experimental, ESI†) by decomposition of the dirhenium(II)tetraallyl complex [Re2(C3H5)4]12 at 120 1C under 3 bar H2 in anisole as a solvent and in the presence of either polyvinylpyrrolidone (PVP) or hexadecylamine (HDA) as a stabilizing agent (Scheme 1). The reactions led to stable dark brown colloidal solutions in both cases. After solvent evaporation and purification steps PVP-stabilized nanoparticles (Re/PVP NPs) were isolated as a dark brown fine powder. In contrast, HDA-stabilized nanoparticles (Re/HDA NPs) could only be isolated as a sticky dark brown solid owing to the presence of long alkyl chain amines linked to the metal surface, as previously observed with other metals like Ru, Pd and Pt.13 Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with high annular dark field detector (HAADF) analyses of purified samples revealed the presence of spherical and well-dispersed NPs having a mean size of
a
Laboratoire de Chimie de Coordination, CNRS, LCC, 205 Route de Narbonne, F-31077 Toulouse, France. E-mail: [email protected]; Tel: +33 (0)561333182 b Universite´ de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France c CNRS UPR 8011, CEMES (Centre d’Elaboration des Mate´riaux et d’Etudes Structurales), 29 Rue Jeanne Marvig, F-31055 Toulouse, France d LPCNO, Laboratoire de Physique et Chimie des Nano-Objets, UMR 5215 INSACNRS-UPS, Institut des Sciences Applique´es, 135 Avenue de Rangueil, F-31077 Toulouse, France. E-mail: [email protected]; Tel: +33 (0)561559655 † Electronic supplementary information (ESI) available: Experimental details: TEM, WAXS and FT-IR data. See DOI: 10.1039/c4cc04816d
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Scheme 1 Synthesis of Re NPs stabilized either by PVP or HDA from organometallic complex [Re2(C3H5)4].
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Fig. 1 (a), (c) TEM and (b), (d) HAADF-STEM images of Re/HDA and Re/ PVP NPs respectively. Insets show the corresponding size histograms.
1.2(0.3) and 1.0(0.3) nm for Re/PVP and Re/HDA samples, respectively (Fig. 1). Fourier transform infrared (FT-IR) spectra of purified Re/HDA and Re/PVP NPs (S1, ESI†) showed absorption bands in the same region as for free HDA and PVP, thus evidencing the presence of each stabilizer (Experimental, ESI†). The crystalline structure of the purified Re NPs was investigated by wide-angle X-ray scattering (WAXS). In the reciprocal space (S2, ESI†), only very broad peaks could be observed, as expected for very small particles. The intensity pattern was in good agreement with a hcp structure computed for bulk Re. From radial distribution functions (RDFs), coherence lengths of ca. 1.2 nm and 1.0 nm were derived for Re/PVP and Re/HDA NPs, respectively. These values agree with the mean sizes measured by TEM. In order to determine the NP structure, the experimental RDFs were compared (Fig. 2) with a theoretical function computed from a very small hcp model (spherical; 1.3 nm) and taking into account a single and large static disorder factor. All the distances from the experimental RDFs adequately match the ones derived from the 0.274 nm
Fig. 2 WAXS analysis of Re NPs (from top to bottom: Re/HDA NPs, hcp simulation for comparison, Re/PVP NPs).
10810 | Chem. Commun., 2014, 50, 10809--10811
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bond length in bulk rhenium. However the fast decay of the amplitude with distance indicates the presence of a short-range order indicative of low crystallinity. Given that changing experimental parameters may induce modulation of the NP morphology, we then studied the influence of temperature, [stabilizer]/[metal] ratio and reaction time. The temperature effect was investigated by performing the synthesis at 100 1C instead of 120 1C while keeping the other parameters the same. At 100 1C, the kinetics of the reaction drastically slowed down, and one additional day was necessary to achieve total reduction of the precursor (Experimental, ESI†). At the end of the reaction, mean sizes of 1.0(0.2) and 1.3(0.3) nm were measured for spherical Re/HDA and Re/PVP NPs, respectively, by using TEM and WAXS analysis with data in good agreement (S3 and S4, ESI†). These results show that temperature does not have a significant influence on the morphology of Re NPs but it affects the rate of the reaction. A change in the [stabilizer]/[metal] ratio clearly influenced the mean size of NPs as seen by TEM (S5, ESI†). When the [HDA]/ [Re] ratio was decreased from 1 to 0.5 molar equiv., the mean size of the Re/HDA NPs increased to 1.3(0.3) nm, as expected in the presence of lower amounts of the stabilizer. In addition, some Re NPs were shown to display a slightly elongated shape as previously observed for Ru, Pd and Pt NPs when using HDA.13 This shape modification may arise either from the organization of HDA in solution forming a ‘‘soft’’ template matrix or from a specific coordination of HDA on given faces of the growing nanocrystals.11,13 When Re/PVP NPs were prepared with less Re content (5 wt%), a smaller NP mean size (1.0(0.3) nm) was obtained. These results are again consistent with the theoretical expectations since higher amounts of polymers can interfere in the early stage of nucleation and provide smaller NPs. Altering the reaction time from 2 to 4 days at 120 1C resulted in a slight broadening of the size distribution without a significant change in size and shape of the NPs (S6, ESI†). Since dihydrogen was used as a reducing agent to synthesize Re NPs, the presence of hydrides at their surface could be expected as known with Ru NPs.14,15 The amount of surface hydrides was monitored by a home-made titration method which is based on the hydrogenation of a simple olefin like norbornene15 without addition of dihydrogen (Experimental, ESI†). No significant hydrogenation of norbornene was observed at room temperature (RT) but the reaction proceeded smoothly at 80 1C leading to norbornane. Assuming a mean size of 1.2 nm for Re/PVP NPs which thus corresponds to ca. 74% of surface Re atoms, we found ca. 1.1 hydride per surface Re atom. Similarly, for Re/HDA NPs, we calculated 0.7 hydride per surface Re atom by assuming a mean size of 1.0 nm displaying ca. 81% surface atoms. These results show that hydrides are coordinated at the surface of Re NPs and that heating is necessary to induce their reactivity, while Ru NP surface hydrides were reactive at RT. In addition, under a CO atmosphere (3 bar) at RT, the metallic surface was not very reactive towards CO adsorption. However, at 90 1C, a clear coordination of CO was visible with absorption bands at ca. 1890 and 2000 cm 1 for Re/PVP NPs and at ca. 1905 and 1985 cm 1 for Re/HDA NPs (S7, ESI†), corresponding in each case to the presence of bridging and terminal carbonyl groups16 as a result of hydride displacement. These observations are in agreement with the molecular chemistry of Re and Ru for which
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ReH7(PPh3)217 is a reactive species only under forcing conditions whereas the corresponding RuH6(PCy3)218 is highly reactive at RT. Given the interest for rhenium oxide catalysts,4,19 solid samples of particles were reacted with 3 bar dioxygen for 2 days. The resulting NPs were analyzed by FT-IR spectroscopy, TEM and WAXS. By FT-IR spectroscopy, a new and intense band was visible at 907 cm 1 which can be attributed to a rhenium oxygen bond20 for O2-treated Re NPs (S8, ESI†). TEM analysis did not show any variation in size or in size distribution for Re/HDA and Re/PVP NPs (S9a and c, ESI†), but some organization was visible on the TEM micrographs of Re/HDA NPs. WAXS analysis revealed a partial oxidation of Re NPs for both Re–HDA and Re–PVP systems. Both the amplitude and coherence length were strongly reduced; however, the remaining peaks were still consistent with those of metallic Re. Moreover, the reduction of the coherence length is consistent with smaller metallic domains and strongly suggests that oxidation leads to a smaller metallic core surrounded by an amorphous shell of Re oxide without a significant change in the overall size, as observed by TEM analysis. Partially oxidized Re NPs were previously observed by Abu-Omar et al.10a during the catalytic dehydrogenation of secondary aliphatic alcohols using NH4ReO4 as a precatalyst under air. Finally, in order to recover the rhenium zero oxidation state, Re NPs were reacted with 3 bar H2 at RT for 3 days and for 1 additional day at 80 1C. No significant change was noticed by FT-IR analysis showing that oxidation is irreversible under these conditions unlike the case of Ru NPs.14b To sum up the hydrogenolysis of [Re2(C3H5)4] provides ultrasmall Re NPs (ca. 1–1.2 nm) in the presence of a polymer (PVP) or a ligand (HDA). The resulting NPs display a spherical shape and adopt a highly disordered hcp structure. They accommodate surface hydrides which are strongly coordinated to rhenium in agreement with the corresponding chemistry of rhenium complexes. Under dioxygen, these Re NPs show a partial oxidation, suggesting a core–shell system with a metallic core and an amorphous oxide shell. For the first time, this work offers a simple route towards monodisperse rhenium nanoparticles as well as basic useful information on their surface state. Surface reactivity studies on these Re NPs and the design of more complex systems such as bimetallic nanostructures are presently on-going in our group. Moreover, these nanosystems will be investigated in the hydrogenation of difficult functional groups. The authors are grateful to Prof. K. Mertis for valuable `re discussions on the synthesis of the rhenium precursor. V. Collie and L. Datas at UPS-TEMSCAN, Y. Coppel and C. Bijani from CNRSLCC are acknowledged for electron microscopy and NMR facilities, respectively. This work was supported by CNRS, UPS and FP7NMP2-Large program grant (Synflow 2010-246461).
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