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Catalytic oxidation of cinnamyl alcohol using Au–Ag nanotubes investigated by surface-enhanced Raman spectroscopy† Jean Claudio Santos Costa, Paola Corio and Liane Marcia Rossi* Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive detection of adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities on the same material. Here we report the application of well-defined Au–Ag nanotubes as both SERS substrates and catalysts for the oxidation of cinnamyl alcohol. The species adsorbed on the catalyst surfaces at different reaction times were analyzed by SERS. The bimetallic nanotubes prepared via a simple galvanic replacement reaction are highly active in the oxidation of cinnamyl alcohol, but do not avoid a radical-chain reaction and the cleavage of the carbon–carbon double bond. A comparison between changes in bulk
Received 13th February 2015, Accepted 3rd April 2015
composition and the nature of adsorbed species at the surface of the catalyst over time suggests that cin-
DOI: 10.1039/c5nr01064k
namaldehyde is formed on the catalyst surface (metal-catalyzed oxidation) and benzaldehyde is probably formed in the bulk solution via a radical-chain pathway. In the presence of 2,6-di-tert-butyl-4-methyl-
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phenol, the radical-chain reaction is suppressed and the oxidation reaction produces cinnamaldehyde.
Introduction Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive and single molecule detection with molecular fingerprint specificity, opening room for a large variety of chemical sensing applications.1,2 SERS is strongly related to plasmonics, and occurs through the coupling of the molecular excitations to localized surface plasmon resonances (LSPR) in metal nanostructures (especially silver or gold), leading to strong enhancement of the Raman cross section of the adsorbed species.3 SERS combines the advantages of high chemical specificity (vibrational Raman scattering), high sensitivity, and surface selectivity.4 These characteristics are extremely favourable for investigating molecular transformations on adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities in the same type of nanomaterial. For Au nanoparticles, while catalytic activity typically occurs for small nanoparticles with a diameter of 10 nm or less, sufficient plasmonic activity to provide significant SERS enhancement occurs for larger particles with an optimum size in the range 30–100 nm. Thus, the design of nanoparticles
Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, CP 26.077, 05513-970 São Paulo-SP, Brazil. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional analysis of the gas and liquid phase. SERS and normal Raman analysis of benzaldehyde, cinnamaldehyde and cinnamyl alcohol. See DOI: 10.1039/c5nr01064k
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capable of integrating catalytic and SERS activities remains challenging. Nanoparticles with core–shell and bimetallic hierarchical structures are able to expose both plasmonic and catalytic sites.5 Catalytic and SERS-responsive functionalities were achieved with architectures such as raspberry-like Au/Pt/Au particles,6 a bifunctional 3D superstructure comprising small gold satellites self-assembled onto a large shell-isolated gold core,4 Au–Pd alloy horns grown on the ends of single-crystal Au nanorods,5 multifunctional Fe3O4/C/Au nanoparticles,7 and Pt-coated Au nanowires.8 These nanomaterials were all employed for studying the reduction of 4-nitrothiophenol, a model molecule with high affinity for gold surfaces. The thiol moiety adsorbs on the metallic surface to form a stable monolayer, and the conversion of the nitro-aromatic compound to the corresponding aniline derivate 4-aminothiophenol was probed via SERS. A different approach towards the synthesis of catalytic and SERS active nanoparticles has been proposed by Jing et al.9 who created active high-index facets on plasmonic Ag structures. These monometallic particles were also investigated in the context of the chemical reduction of surfaceadsorbed 4-nitrothiophenol. SERS was used, to a lesser extent, for the study of reactions involving species without surface-anchoring groups. Gold nanoshells were used for investigating the role of the alkalinity in water-phase glycerol oxidation10,11 and Pd–Au nanoshells for studying hydrodechlorination of 1,1-dichloroethene.12 The development of nanostructures that combine both catalytic
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and SERS activities, especially for the spectroscopic study of adsorbed species without surface-anchoring groups, will expand the application of this technique in the field of catalysis. Porous bimetallic nanostructures with hollow interior and ultrathin walls have received considerable interest as catalysts, mainly because of their high specific surface area, and can be prepared by a simple galvanic replacement reaction.13 The shape and composition of the material can be controlled by employing sacrificial templates with different morphologies or by limiting the extent of the reaction. Nanocrystals with increased surface areas as compared to their solid counterparts are obtained in a simple one-pot reaction. The bimetallic nanoparticles display different optical and surface chemical properties with respect to the monometallic systems, being particularly attractive for applications in spectroscopy, photonics and catalysis.14–17 In particular, such bimetallic nanostructures can be used as both catalysts and highly sensitive substrates for SERS, allowing the investigation of catalytic reactions by assessing the intermediate species adsorbed on the catalyst surface. Here we synthesized porous Au–Ag nanotubes (Au–Ag NTs) that combine both catalytic and SERS activities and allowed the spectroscopic study of adsorbed species during the catalytic oxidation of alcohols that do not contain surface-anchoring groups. Cinnamyl alcohol was selected as a challenging substrate for the oxidation studies, because its oxidation can result in a complex reaction network.18,19 Gold20 and gold–palladium21 catalysts have attracted great interest for their high selectivity in aerobic oxidation of alcohols to aldehydes, and their unique chemoselectivity in the oxidation of α,β-unsaturated alcohols.22 The ideal catalyst should be able to distinguish the reactive R–H2C–OH and CvC moieties and perform the chemoselective oxidation of hydroxyl groups avoiding epoxidation, oxidative cleavage, isomerization or polymerization of the olefinic group.23 Competitive oxidation reactions via a radical-chain mechanism can affect reaction substrates and products, specially the aldehydes.24
Experimental Materials and instrumentation Analytical grade chemicals HAuCl4·3H2O (hydrogen tetrachloroaurate trihydrate), AgNO3 (silver nitrate, 99%), PVP ( polyvinylpyrrolidone, M.W. 55 000 g mol−1), BHT (2,6-di-tert-butyl-4methylphenol), EG (ethylene glycol), C9H10O (cinnamyl alcohol), C7H6O (benzaldehyde), C9H8O (cinnamaldehyde) and C8H10 ( p-xylene) were purchased from Aldrich or Merck, and were used as received. All solutions were prepared using deionized water (18.2 MΩ). Glassware was cleaned with a piranha solution (3 : 1 concentrated H2SO4 to 30% H2O2) before use. The scanning electron microscopy (FEG-SEM) images were obtained using a JEOL FEG-SEM JSM 6330F microscope operated at 5 kV. The samples for SEM were prepared by dropcasting an aqueous suspension of the nanostructures over a Si wafer, followed by drying under ambient conditions. Raman
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spectra were acquired on a Renishaw Raman InVia equipped with a CCD detector and coupled to a Leica microscope that allows rapid accumulation of Raman spectra with a spatial resolution of about 1 μm (micro-Raman technique). The laser beam was focused on the sample by a ×50 lens. Laser power was always maintained below 0.7 mW at the sample. The experiments were performed under ambient conditions using back-scattering geometry. The samples were irradiated with the 632.8 nm line of a He–Ne laser (Spectra Physics). Gas chromatography analyses were performed with a Shimadzu GC-2010 Gas Chromatograph–Flame Ionization Detector (FID) equipped with a capillary column Rtx-Wax 30 meter. The analyses were performed under the following conditions: initial temperature 50 °C, rate 20 °C min−1, final temperature 230 °C and gas flow of 3.58 mL min−1. Conversion and selectivity were calculated based on peak areas of GC calibrated with an internal standard ( p-xylene). Infrared measurements were performed on an IRPrestige-21 Shimadzu FTIR spectrometer with gas cells with a 5 cm light path. The spectra were recorded at 60 °C in the 500–4000 cm−1 range. Synthesis of Ag nanowires The synthesis of Ag nanowires followed a modified polyol process.25 In a typical synthesis, 5 mL of ethylene glycol was transferred to a round bottom flask and heated to 160 °C under stirring for 1 h. Then, 3 mL of a 0.1 mol L−1 AgNO3 solution in ethylene glycol and 3 mL of a 0.6 mol L−1 PVP solution in ethylene glycol were added simultaneously, using a peristaltic pump at a rate of 0.4 mL min−1, to pre-heated ethylene glycol. The reaction was allowed to proceed at this temperature for 1 h before the product was collected by centrifugation, washed three times with water and re-suspended in water. Synthesis of Au–Ag nanotubes The synthesis of Au–Ag NTs followed a modified galvanic reaction process.26,27 A suspension containing the Ag nanowires (100 μL) was added to 5 mL of an aqueous PVP solution (1 mg mL−1) under magnetic stirring. This system was heated at 100 °C for 10 min. Then, 2 mL of a 0.1 mmol L−1 HAuCl4 aqueous solution was added dropwise, causing a change in color from light brown to purple. The solution was maintained at 100 °C for another 10 min and the product was collected by centrifugation, washed with a saturated NaCl aqueous solution and re-suspended in water. Catalytic experiments The oxidation reactions were performed using a Fischer–Porter glass reactor. In a typical solventless reaction, the glass reactor was loaded with the porous bimetallic nanotubes (5 μmol) and cinnamyl alcohol (10 mmol). The reactor was loaded with O2 to the desired pressure. The temperature was maintained by using an oil bath placed on a hot stirring plate connected to a digital temperature controller. The reactions were maintained under magnetic stirring for the desired time. The collected products were centrifuged to separate the catalyst and analyzed by gas chromatography (GC) using p-xylene as the standard.
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The samples used for SERS were collected from the reaction mixture at different time intervals and the catalyst (separated by centrifugation) was placed over a glass slide for analysis. The starting materials, main intermediates and products of analytical grade obtained from commercial sources were analysed for comparison.
Results and discussion Silver nanowires of longitudinal dimension >5 µm and transversal dimension of ca. 100 nm were synthesized via the polyol approach.28 The silver nanowires were used as precursors for the synthesis of Au–Ag NTs by galvanic replacement reactions. The silver atoms of the template nanoparticle diffuse into the lattice of Au and form a particle-like porous bimetallic structure. Representative scanning electron microscopy (SEM-FEG) images of Ag nanowires and Au–Ag NTs with empty interior and uniform size and morphology are shown in Fig. 1a and b, respectively. The bimetallic nanotube width is ca. 140 nm, which corresponds to an increase of ca. 40 nm with respect to the average diameter of the template nanowires. The porous surface is comprised of islands of ca. 10–20 nm. The energy dispersive X-ray spectrum (EDS) mapping of a representative Au–Ag NT (Fig. 1c) confirmed the presence of both gold and silver. The bulk composition of the Au–Ag NTs was also obtained by flame atomic absorption spectrometry (FAAS) and corresponds to 53.4 wt% Ag and 46.6 wt% Au. The Au/Ag molar ratio obtained for this particular sample was 0.48, but it can be tailored by controlling the conditions of the galvanic reactions.26 The crystalline structure of the Ag NWs and Au–Ag NTs was studied by X-ray diffraction (XRD). The XRD pattern of the Ag NWs, Fig. 2a, shows only the metallic phase of Ag (JPCDS file 65-2871) with a cubic structure. The XRD pattern of the Au–Ag NTs (Fig. 2b) shows peaks at 2θ degrees of 38.1°, 44.3°, 64.5°, and 77.4° that can be assigned to the reflections from the (111), (200), (220), and (311) planes of Au (JPCDS 4-784) or Ag (JPCDS 4-783) face centered cubic structures. The Ag and Au have very close lattice parameters, which make it difficult to distinguish Au, Ag or even Au–Ag alloys using the XRD patterns only.29 The formation of an Au–Ag alloy can be better characterized by the presence of a single surface plasmon resonance (SPR) band in the optical absorption spectra of the Au–Ag NTs, while a physical mixture of Ag and Au monometallic NPs is expected to have two SPR bands at around 400 and 520 nm.29 The extinction spectra of the sacrificial Ag nanowires show a welldefined SPR band centered at 396 nm, while the extinction spectra of the Au–Ag NTs show a SPR band centered at 568 nm (Fig. 3). The presence of one red-shifted plasmonic absorption band has been attributed to the formation of the nanoalloys.30,31 The solventless oxidation of cinnamyl alcohol by Au–Ag NTs was carried out at 100 °C using a catalyst/substrate molar
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Fig. 1 (a) SEM images of Ag NWs, (b) SEM image of Au–Ag NTs and (c) EDS analysis of Au–Ag NTs.
ratio of 1/2000. The influence of oxygen reaction pressure on the reaction conversion and selectivity was investigated (Fig. 4). Selectivity refers to the formation of cinnamaldehyde and benzaldehyde, the only reaction products obtained by gas chromatography analysis of the liquid phase, and the rest up to 100% corresponds to unidentified by-products (see gas
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Fig. 2
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XRD pattern of Ag NWs and Au–Ag NTs.
Fig. 3 Extinction spectra recorded from aqueous suspensions containing (a) Ag nanowires and (b) Au–Ag NTs.
Fig. 4 Influence of O2 reaction pressure on cinnamyl alcohol conversion (catalysts/alcohol (1/2000), 100 °C, 2.5 h) and on oxidation product selectivity for the Au–Ag catalyst.
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phase analysis below). Increasing the oxygen reaction pressure from 1 to 6 bar increased reaction conversion, but decreased the selectivity to cinnamaldehyde to less than 20%, while benzaldehyde becomes the main reaction product. The formation of benzaldehyde with increasing oxygen pressure during oxidation of cinnamyl alcohol was reported before.32 The presence of this by-product suggests the oxidative cleavage of the carbon–carbon double bond. This reaction can occur by different mechanisms, either involving a cis-diol intermediate, an epoxidation step or a radical-chain oxidation pathway.33 Oxidative cleavage adds functionality to the substrates and has received special attention for the conversion of renewable substrates such as terpenes, unsaturated fatty acids and their esters into valuable products.32 The oxidation of cinnamyl alcohol was also studied under an inert atmosphere and under air (in atmospheric pressure) and in the absence of a catalyst. For comparison, the oxidation of cinnamyl alcohol catalyzed by the Ag NW (catalyst template) and Au–Ag nanoparticles (NPs), prepared by a similar galvanic replacement reaction of Ag NPs with HAuCl4, was studied under similar conditions as the Au–Ag NT catalyst. These and other selected results under O2 pressure are summarized in Table 1. The Au–Ag NTs were very active catalysts for the oxidation of cinnamyl alcohol in the absence of a base, which can in principle suggest a way to minimize side-reactions; however, the selectivity for cinnamaldehyde is low under the reaction conditions studied. After 2.5 h, the reaction was complete (>99% conversion), however benzaldehyde was the main product (Table 1, entry 1). The reaction was stopped at lower conversion (28%, after 1 h), but the formation of benzaldehyde was still quite high (Table 1, entry 2). The catalyst recovered after the oxidation reaction preserved its structure (Fig. S1†), but was not investigated in successive reactions, as this was not the scope of this study. The Ag NWs were not active (2% conversion) in the oxidation reaction studied under similar conditions as the bimetallic catalyst (Table 1, entry 6). Similarly, Au–Ag NPs were less active than the Au–Ag NTs when used as catalysts in the oxidation reaction under similar conditions (Table 1, entry 7). The reaction with the Au–Ag NT catalyst in air reached about 20% conversion and 65% selectivity to cinnamaldehyde (Table 1, entry 5) and the reaction conversion dropped to 99%), but the selectivity for cinnamaldehyde was only ca. 21% (Table 1, entry 1). The reaction performed with the catalyst in the presence of the radical trap BHT resulted in ca. 70% conversion and a pronounced increase of reaction selectivity to >96% of cinnamaldehyde (Table 1, entry 3). The high selectivity to cinnamaldehyde observed in the presence of the radical trap BHT suggests that the formation of benzaldehyde occurs via a radical-chain oxidation process. The main product observed in the absence of radical traps is benzaldehyde, which is known to be stable against radical oxidation.35 The formation of other small molecules (70%), and finally reached the catalyst surface. The results obtained by SERS did not provide any clear evidence for epoxide formation on the catalyst surface, which would suggest a different chain propagation mechanism. The characteristic Raman bands corresponding to epoxide vibration (breathing mode of the epoxide ring), which are expected in the 1220 cm−1–1280 cm−1 range,42 are coincident with a band at 1252 cm−1 present in the Raman spectrum of cinnamaldehyde (Fig. 5). There is also no evidence for peroxide adsorption on the catalyst surface. The characteristic Raman bands corresponding to organic peroxide stretching vibrations, which are expected in the 845–875 cm−1 range,43 are coincident with a band at 820–850 cm−1 present in the Raman spectrum of both cinnamyl alcohol and cinnamaldehyde (Fig. 5).
Conclusions Au–Ag NTs were used as both catalysts and SERS substrates to investigate the oxidation of cinnamyl alcohol. As catalysts, they exhibited high conversion of cinnamyl alcohol in the presence of relatively low oxygen pressure; however, benzaldehyde was obtained as the main product. In the presence of the radical trap BHT, the oxidation reaction exhibited >99% selectivity to cinnamaldehyde. These results suggest that the catalyst activity can be tuned to give either the product of oxidative cleavage of the CvC bond or the chemoselective oxidation of the hydroxyl group. Changes of the adsorbate species were observed by SERS at different reaction time intervals, providing a newfound ability to detect and identify reaction intermediates under ambient conditions. The SERS spectra suggest that cinnamaldehyde is adsorbed on the catalyst surface ( product of metalcatalyzed oxidation), but benzaldehyde is not adsorbed and is probably formed in the bulk solution (via a radical-chain pathway). The oxidative cleavage of CvC double bonds to give aldehydes (and then carboxylic acids) is a very important reac-
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tion in the context of conversion of renewable substrates such as terpenes, unsaturated fatty acids and their esters into valuable products, but is still limited to a few substrates. Further studies on the application of catalytic oxidative cleavage of relevant molecules, as an alternative to ozonolysis, are under study in our group.
Acknowledgements The authors gratefully acknowledge support from FAPESP, CNPq and CAPES.
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18 L. J. Durndell, C. M. A. Parlett, N. S. Hondow, K. Wilson and A. F. Lee, Nanoscale, 2013, 5, 5412–5419. 19 C. M. A. Parlett, L. J. Durndell, K. Wilson, D. W. Bruce and N. S. Hondow, Catal. Commun., 2014, 44, 40–45. 20 A. Abad, P. Concepción, A. Corma and H. García, Angew. Chem., Int. Ed., 2005, 44, 4066–4069. 21 D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362– 365. 22 A. Abad, C. Almela, A. Corma and H. Garcia, Chem. Commun., 2006, 3178–3180. 23 A. Abad, A. Corma and H. Garcia, Pure Appl. Chem., 2007, 79, 1847–1854. 24 L. Nie, K. K. Li, W. S. Xin and X. P. Zhou, Catal. Commun., 2007, 8, 488–492. 25 J. Chen, B. J. Wiley and Y. Xia, Langmuir, 2007, 23, 4120– 4129. 26 Y. Sun and Y. Xia, J. Am. Chem. Soc., 2004, 126, 3892– 3901. 27 Y. Sun and Y. Xia, Adv. Mater., 2004, 16, 264–268. 28 Y. Sun, B. Gates, B. Mayers and Y. Xia, Nano Lett., 2002, 2, 165–168. 29 K. S. Shin, J. H. Kim, I. H. Kim and K. Kim, J. Nanopart. Res., 2012, 14, 735–744. 30 Y. Sun, Nanoscale, 2010, 2, 1626–1642. 31 J. Yuanyuan, L. Yizhong, H. Dongxue, Z. Qixian and N. Li, Nanotechnology, 2012, 23, 105609.
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32 M. Caravati, D. M. Meier, J.-D. Grunwaldt and A. Baiker, J. Catal., 2006, 240, 126–136. 33 P. Spannring, P. C. A. Bruijnincx, B. M. Weckhuysen and R. J. M. Klein Gebbink, Catal. Sci. Technol., 2014, 4, 2182– 2209. 34 I. B. Niklasson, T. Delaine, M. N. Islam, R. Karlsson, K. Luthman and A.-T. Karlberg, Contact Dermatitis, 2013, 68, 129–138. 35 V. A. Aver’yanov, D. V. Vlasov, A. A. Svechnikova and A. I. Ermakov, Kinet. Catal., 2000, 41, 152–158. 36 P. C. Vasconcellos, L. R. F. Carvalho and C. S. Pool, J. Braz. Chem. Soc., 2005, 16, 1210–1216. 37 T. Shiraishi, Y. Soma, O. Ishitani and K. Sakamoto, J. Environ. Monit., 2001, 3, 654–660. 38 R. A. Sheldon and J. K. Kochi, Metal-catalyzed oxidations of organic compounds, Academic Press, New York, 1981. 39 M. Conte, H. Miyamura, S. Kobayashi and V. Chechik, Chem. Commun., 2010, 46, 145–147. 40 M. Abdelsalam, P. N. Bartlett, A. E. Russell, J. J. Baumberg, E. J. Calvo, N. G. Tognalli and A. Fainstein, Langmuir, 2008, 24, 7018–7023. 41 R. A. Alvarez-Puebla, J. P. Bravo-Vasquez, P. Cheben, D.-X. Xu, P. Waldron and H. Fenniri, J. Colloid Interface Sci., 2009, 333, 237–241. 42 M. P. Kapoor, W. Fujii, Y. Kasama, M. Yanagi, H. Nanbu and L. R. Juneja, J. Mater. Chem., 2008, 18, 4683–4691. 43 V. Vacque, B. Sombret, J. P. Huvenne, P. Legrand and S. Such, Spectrochim. Acta, Part A, 1997, 53, 55–66.
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Catalytic oxidation of cinnamyl alcohol using Au–Ag nanotubes investigated by surface-enhanced Raman spectroscopy† Jean Claudio Santos Costa, Paola Corio and Liane Marcia Rossi* Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive detection of adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities on the same material. Here we report the application of well-defined Au–Ag nanotubes as both SERS substrates and catalysts for the oxidation of cinnamyl alcohol. The species adsorbed on the catalyst surfaces at different reaction times were analyzed by SERS. The bimetallic nanotubes prepared via a simple galvanic replacement reaction are highly active in the oxidation of cinnamyl alcohol, but do not avoid a radical-chain reaction and the cleavage of the carbon–carbon double bond. A comparison between changes in bulk
Received 13th February 2015, Accepted 3rd April 2015
composition and the nature of adsorbed species at the surface of the catalyst over time suggests that cin-
DOI: 10.1039/c5nr01064k
namaldehyde is formed on the catalyst surface (metal-catalyzed oxidation) and benzaldehyde is probably formed in the bulk solution via a radical-chain pathway. In the presence of 2,6-di-tert-butyl-4-methyl-
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phenol, the radical-chain reaction is suppressed and the oxidation reaction produces cinnamaldehyde.
Introduction Surface-enhanced Raman spectroscopy (SERS) enables ultrasensitive and single molecule detection with molecular fingerprint specificity, opening room for a large variety of chemical sensing applications.1,2 SERS is strongly related to plasmonics, and occurs through the coupling of the molecular excitations to localized surface plasmon resonances (LSPR) in metal nanostructures (especially silver or gold), leading to strong enhancement of the Raman cross section of the adsorbed species.3 SERS combines the advantages of high chemical specificity (vibrational Raman scattering), high sensitivity, and surface selectivity.4 These characteristics are extremely favourable for investigating molecular transformations on adsorbed species at the catalyst surface. However, it is quite difficult to combine catalytic and SERS activities in the same type of nanomaterial. For Au nanoparticles, while catalytic activity typically occurs for small nanoparticles with a diameter of 10 nm or less, sufficient plasmonic activity to provide significant SERS enhancement occurs for larger particles with an optimum size in the range 30–100 nm. Thus, the design of nanoparticles
Departamento de Química Fundamental, Instituto de Química, Universidade de São Paulo, CP 26.077, 05513-970 São Paulo-SP, Brazil. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional analysis of the gas and liquid phase. SERS and normal Raman analysis of benzaldehyde, cinnamaldehyde and cinnamyl alcohol. See DOI: 10.1039/c5nr01064k
This journal is © The Royal Society of Chemistry 2015
capable of integrating catalytic and SERS activities remains challenging. Nanoparticles with core–shell and bimetallic hierarchical structures are able to expose both plasmonic and catalytic sites.5 Catalytic and SERS-responsive functionalities were achieved with architectures such as raspberry-like Au/Pt/Au particles,6 a bifunctional 3D superstructure comprising small gold satellites self-assembled onto a large shell-isolated gold core,4 Au–Pd alloy horns grown on the ends of single-crystal Au nanorods,5 multifunctional Fe3O4/C/Au nanoparticles,7 and Pt-coated Au nanowires.8 These nanomaterials were all employed for studying the reduction of 4-nitrothiophenol, a model molecule with high affinity for gold surfaces. The thiol moiety adsorbs on the metallic surface to form a stable monolayer, and the conversion of the nitro-aromatic compound to the corresponding aniline derivate 4-aminothiophenol was probed via SERS. A different approach towards the synthesis of catalytic and SERS active nanoparticles has been proposed by Jing et al.9 who created active high-index facets on plasmonic Ag structures. These monometallic particles were also investigated in the context of the chemical reduction of surfaceadsorbed 4-nitrothiophenol. SERS was used, to a lesser extent, for the study of reactions involving species without surface-anchoring groups. Gold nanoshells were used for investigating the role of the alkalinity in water-phase glycerol oxidation10,11 and Pd–Au nanoshells for studying hydrodechlorination of 1,1-dichloroethene.12 The development of nanostructures that combine both catalytic
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and SERS activities, especially for the spectroscopic study of adsorbed species without surface-anchoring groups, will expand the application of this technique in the field of catalysis. Porous bimetallic nanostructures with hollow interior and ultrathin walls have received considerable interest as catalysts, mainly because of their high specific surface area, and can be prepared by a simple galvanic replacement reaction.13 The shape and composition of the material can be controlled by employing sacrificial templates with different morphologies or by limiting the extent of the reaction. Nanocrystals with increased surface areas as compared to their solid counterparts are obtained in a simple one-pot reaction. The bimetallic nanoparticles display different optical and surface chemical properties with respect to the monometallic systems, being particularly attractive for applications in spectroscopy, photonics and catalysis.14–17 In particular, such bimetallic nanostructures can be used as both catalysts and highly sensitive substrates for SERS, allowing the investigation of catalytic reactions by assessing the intermediate species adsorbed on the catalyst surface. Here we synthesized porous Au–Ag nanotubes (Au–Ag NTs) that combine both catalytic and SERS activities and allowed the spectroscopic study of adsorbed species during the catalytic oxidation of alcohols that do not contain surface-anchoring groups. Cinnamyl alcohol was selected as a challenging substrate for the oxidation studies, because its oxidation can result in a complex reaction network.18,19 Gold20 and gold–palladium21 catalysts have attracted great interest for their high selectivity in aerobic oxidation of alcohols to aldehydes, and their unique chemoselectivity in the oxidation of α,β-unsaturated alcohols.22 The ideal catalyst should be able to distinguish the reactive R–H2C–OH and CvC moieties and perform the chemoselective oxidation of hydroxyl groups avoiding epoxidation, oxidative cleavage, isomerization or polymerization of the olefinic group.23 Competitive oxidation reactions via a radical-chain mechanism can affect reaction substrates and products, specially the aldehydes.24
Experimental Materials and instrumentation Analytical grade chemicals HAuCl4·3H2O (hydrogen tetrachloroaurate trihydrate), AgNO3 (silver nitrate, 99%), PVP ( polyvinylpyrrolidone, M.W. 55 000 g mol−1), BHT (2,6-di-tert-butyl-4methylphenol), EG (ethylene glycol), C9H10O (cinnamyl alcohol), C7H6O (benzaldehyde), C9H8O (cinnamaldehyde) and C8H10 ( p-xylene) were purchased from Aldrich or Merck, and were used as received. All solutions were prepared using deionized water (18.2 MΩ). Glassware was cleaned with a piranha solution (3 : 1 concentrated H2SO4 to 30% H2O2) before use. The scanning electron microscopy (FEG-SEM) images were obtained using a JEOL FEG-SEM JSM 6330F microscope operated at 5 kV. The samples for SEM were prepared by dropcasting an aqueous suspension of the nanostructures over a Si wafer, followed by drying under ambient conditions. Raman
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spectra were acquired on a Renishaw Raman InVia equipped with a CCD detector and coupled to a Leica microscope that allows rapid accumulation of Raman spectra with a spatial resolution of about 1 μm (micro-Raman technique). The laser beam was focused on the sample by a ×50 lens. Laser power was always maintained below 0.7 mW at the sample. The experiments were performed under ambient conditions using back-scattering geometry. The samples were irradiated with the 632.8 nm line of a He–Ne laser (Spectra Physics). Gas chromatography analyses were performed with a Shimadzu GC-2010 Gas Chromatograph–Flame Ionization Detector (FID) equipped with a capillary column Rtx-Wax 30 meter. The analyses were performed under the following conditions: initial temperature 50 °C, rate 20 °C min−1, final temperature 230 °C and gas flow of 3.58 mL min−1. Conversion and selectivity were calculated based on peak areas of GC calibrated with an internal standard ( p-xylene). Infrared measurements were performed on an IRPrestige-21 Shimadzu FTIR spectrometer with gas cells with a 5 cm light path. The spectra were recorded at 60 °C in the 500–4000 cm−1 range. Synthesis of Ag nanowires The synthesis of Ag nanowires followed a modified polyol process.25 In a typical synthesis, 5 mL of ethylene glycol was transferred to a round bottom flask and heated to 160 °C under stirring for 1 h. Then, 3 mL of a 0.1 mol L−1 AgNO3 solution in ethylene glycol and 3 mL of a 0.6 mol L−1 PVP solution in ethylene glycol were added simultaneously, using a peristaltic pump at a rate of 0.4 mL min−1, to pre-heated ethylene glycol. The reaction was allowed to proceed at this temperature for 1 h before the product was collected by centrifugation, washed three times with water and re-suspended in water. Synthesis of Au–Ag nanotubes The synthesis of Au–Ag NTs followed a modified galvanic reaction process.26,27 A suspension containing the Ag nanowires (100 μL) was added to 5 mL of an aqueous PVP solution (1 mg mL−1) under magnetic stirring. This system was heated at 100 °C for 10 min. Then, 2 mL of a 0.1 mmol L−1 HAuCl4 aqueous solution was added dropwise, causing a change in color from light brown to purple. The solution was maintained at 100 °C for another 10 min and the product was collected by centrifugation, washed with a saturated NaCl aqueous solution and re-suspended in water. Catalytic experiments The oxidation reactions were performed using a Fischer–Porter glass reactor. In a typical solventless reaction, the glass reactor was loaded with the porous bimetallic nanotubes (5 μmol) and cinnamyl alcohol (10 mmol). The reactor was loaded with O2 to the desired pressure. The temperature was maintained by using an oil bath placed on a hot stirring plate connected to a digital temperature controller. The reactions were maintained under magnetic stirring for the desired time. The collected products were centrifuged to separate the catalyst and analyzed by gas chromatography (GC) using p-xylene as the standard.
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The samples used for SERS were collected from the reaction mixture at different time intervals and the catalyst (separated by centrifugation) was placed over a glass slide for analysis. The starting materials, main intermediates and products of analytical grade obtained from commercial sources were analysed for comparison.
Results and discussion Silver nanowires of longitudinal dimension >5 µm and transversal dimension of ca. 100 nm were synthesized via the polyol approach.28 The silver nanowires were used as precursors for the synthesis of Au–Ag NTs by galvanic replacement reactions. The silver atoms of the template nanoparticle diffuse into the lattice of Au and form a particle-like porous bimetallic structure. Representative scanning electron microscopy (SEM-FEG) images of Ag nanowires and Au–Ag NTs with empty interior and uniform size and morphology are shown in Fig. 1a and b, respectively. The bimetallic nanotube width is ca. 140 nm, which corresponds to an increase of ca. 40 nm with respect to the average diameter of the template nanowires. The porous surface is comprised of islands of ca. 10–20 nm. The energy dispersive X-ray spectrum (EDS) mapping of a representative Au–Ag NT (Fig. 1c) confirmed the presence of both gold and silver. The bulk composition of the Au–Ag NTs was also obtained by flame atomic absorption spectrometry (FAAS) and corresponds to 53.4 wt% Ag and 46.6 wt% Au. The Au/Ag molar ratio obtained for this particular sample was 0.48, but it can be tailored by controlling the conditions of the galvanic reactions.26 The crystalline structure of the Ag NWs and Au–Ag NTs was studied by X-ray diffraction (XRD). The XRD pattern of the Ag NWs, Fig. 2a, shows only the metallic phase of Ag (JPCDS file 65-2871) with a cubic structure. The XRD pattern of the Au–Ag NTs (Fig. 2b) shows peaks at 2θ degrees of 38.1°, 44.3°, 64.5°, and 77.4° that can be assigned to the reflections from the (111), (200), (220), and (311) planes of Au (JPCDS 4-784) or Ag (JPCDS 4-783) face centered cubic structures. The Ag and Au have very close lattice parameters, which make it difficult to distinguish Au, Ag or even Au–Ag alloys using the XRD patterns only.29 The formation of an Au–Ag alloy can be better characterized by the presence of a single surface plasmon resonance (SPR) band in the optical absorption spectra of the Au–Ag NTs, while a physical mixture of Ag and Au monometallic NPs is expected to have two SPR bands at around 400 and 520 nm.29 The extinction spectra of the sacrificial Ag nanowires show a welldefined SPR band centered at 396 nm, while the extinction spectra of the Au–Ag NTs show a SPR band centered at 568 nm (Fig. 3). The presence of one red-shifted plasmonic absorption band has been attributed to the formation of the nanoalloys.30,31 The solventless oxidation of cinnamyl alcohol by Au–Ag NTs was carried out at 100 °C using a catalyst/substrate molar
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Fig. 1 (a) SEM images of Ag NWs, (b) SEM image of Au–Ag NTs and (c) EDS analysis of Au–Ag NTs.
ratio of 1/2000. The influence of oxygen reaction pressure on the reaction conversion and selectivity was investigated (Fig. 4). Selectivity refers to the formation of cinnamaldehyde and benzaldehyde, the only reaction products obtained by gas chromatography analysis of the liquid phase, and the rest up to 100% corresponds to unidentified by-products (see gas
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Fig. 2
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XRD pattern of Ag NWs and Au–Ag NTs.
Fig. 3 Extinction spectra recorded from aqueous suspensions containing (a) Ag nanowires and (b) Au–Ag NTs.
Fig. 4 Influence of O2 reaction pressure on cinnamyl alcohol conversion (catalysts/alcohol (1/2000), 100 °C, 2.5 h) and on oxidation product selectivity for the Au–Ag catalyst.
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phase analysis below). Increasing the oxygen reaction pressure from 1 to 6 bar increased reaction conversion, but decreased the selectivity to cinnamaldehyde to less than 20%, while benzaldehyde becomes the main reaction product. The formation of benzaldehyde with increasing oxygen pressure during oxidation of cinnamyl alcohol was reported before.32 The presence of this by-product suggests the oxidative cleavage of the carbon–carbon double bond. This reaction can occur by different mechanisms, either involving a cis-diol intermediate, an epoxidation step or a radical-chain oxidation pathway.33 Oxidative cleavage adds functionality to the substrates and has received special attention for the conversion of renewable substrates such as terpenes, unsaturated fatty acids and their esters into valuable products.32 The oxidation of cinnamyl alcohol was also studied under an inert atmosphere and under air (in atmospheric pressure) and in the absence of a catalyst. For comparison, the oxidation of cinnamyl alcohol catalyzed by the Ag NW (catalyst template) and Au–Ag nanoparticles (NPs), prepared by a similar galvanic replacement reaction of Ag NPs with HAuCl4, was studied under similar conditions as the Au–Ag NT catalyst. These and other selected results under O2 pressure are summarized in Table 1. The Au–Ag NTs were very active catalysts for the oxidation of cinnamyl alcohol in the absence of a base, which can in principle suggest a way to minimize side-reactions; however, the selectivity for cinnamaldehyde is low under the reaction conditions studied. After 2.5 h, the reaction was complete (>99% conversion), however benzaldehyde was the main product (Table 1, entry 1). The reaction was stopped at lower conversion (28%, after 1 h), but the formation of benzaldehyde was still quite high (Table 1, entry 2). The catalyst recovered after the oxidation reaction preserved its structure (Fig. S1†), but was not investigated in successive reactions, as this was not the scope of this study. The Ag NWs were not active (2% conversion) in the oxidation reaction studied under similar conditions as the bimetallic catalyst (Table 1, entry 6). Similarly, Au–Ag NPs were less active than the Au–Ag NTs when used as catalysts in the oxidation reaction under similar conditions (Table 1, entry 7). The reaction with the Au–Ag NT catalyst in air reached about 20% conversion and 65% selectivity to cinnamaldehyde (Table 1, entry 5) and the reaction conversion dropped to 99%), but the selectivity for cinnamaldehyde was only ca. 21% (Table 1, entry 1). The reaction performed with the catalyst in the presence of the radical trap BHT resulted in ca. 70% conversion and a pronounced increase of reaction selectivity to >96% of cinnamaldehyde (Table 1, entry 3). The high selectivity to cinnamaldehyde observed in the presence of the radical trap BHT suggests that the formation of benzaldehyde occurs via a radical-chain oxidation process. The main product observed in the absence of radical traps is benzaldehyde, which is known to be stable against radical oxidation.35 The formation of other small molecules (70%), and finally reached the catalyst surface. The results obtained by SERS did not provide any clear evidence for epoxide formation on the catalyst surface, which would suggest a different chain propagation mechanism. The characteristic Raman bands corresponding to epoxide vibration (breathing mode of the epoxide ring), which are expected in the 1220 cm−1–1280 cm−1 range,42 are coincident with a band at 1252 cm−1 present in the Raman spectrum of cinnamaldehyde (Fig. 5). There is also no evidence for peroxide adsorption on the catalyst surface. The characteristic Raman bands corresponding to organic peroxide stretching vibrations, which are expected in the 845–875 cm−1 range,43 are coincident with a band at 820–850 cm−1 present in the Raman spectrum of both cinnamyl alcohol and cinnamaldehyde (Fig. 5).
Conclusions Au–Ag NTs were used as both catalysts and SERS substrates to investigate the oxidation of cinnamyl alcohol. As catalysts, they exhibited high conversion of cinnamyl alcohol in the presence of relatively low oxygen pressure; however, benzaldehyde was obtained as the main product. In the presence of the radical trap BHT, the oxidation reaction exhibited >99% selectivity to cinnamaldehyde. These results suggest that the catalyst activity can be tuned to give either the product of oxidative cleavage of the CvC bond or the chemoselective oxidation of the hydroxyl group. Changes of the adsorbate species were observed by SERS at different reaction time intervals, providing a newfound ability to detect and identify reaction intermediates under ambient conditions. The SERS spectra suggest that cinnamaldehyde is adsorbed on the catalyst surface ( product of metalcatalyzed oxidation), but benzaldehyde is not adsorbed and is probably formed in the bulk solution (via a radical-chain pathway). The oxidative cleavage of CvC double bonds to give aldehydes (and then carboxylic acids) is a very important reac-
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tion in the context of conversion of renewable substrates such as terpenes, unsaturated fatty acids and their esters into valuable products, but is still limited to a few substrates. Further studies on the application of catalytic oxidative cleavage of relevant molecules, as an alternative to ozonolysis, are under study in our group.
Acknowledgements The authors gratefully acknowledge support from FAPESP, CNPq and CAPES.
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