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Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light irradiation conditions
Accepted Manuscript Title: Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light irradiation conditions Authors: Priyanka Verma, Yasutaka Kuwahara, Kohsuke Mori, Hiromi Yamashita PII: DOI: Reference:
S0920-5861(18)30116-0 https://doi.org/10.1016/j.cattod.2018.06.051 CATTOD 11540
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
23-2-2018 24-4-2018 30-6-2018
Please cite this article as: Verma P, Kuwahara Y, Mori K, Yamashita H, Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light irradiation conditions, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.06.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light
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irradiation conditions
Priyanka Vermaa, Yasutaka Kuwaharaa,b, Kohsuke Moria,b,c and Hiromi Yamashitaa,b*
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Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka
Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 606-
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b
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University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
c
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8501, Japan
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JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
*
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Email: [email protected]
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Graphical abstract
Highlights
Ag-based plasmonic nanocatalyst supported on ceria modified mesoporous silica is designed. The prepared catalysts showed excellent catalytic performances in the p-nitrostyrene
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reduction.
Significant enhancements in the product formation were observed under light irradiation conditions attributing to the Ag-plasmonic effect.
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Abstract
Plasmonic photocatalysis caused by surface plasmon resonance (SPR) has emerged out as an
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impressive way for efficient light absorption and solar light conversion. A series of catalysts
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consisting of plasmonic Ag nanoparticles (NPs) deposited on CeO2 coated mesoporous silica,
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SBA-15, prepared by MW-assisted alcohol reduction method were tested for the chemoselective hydrogenation of p-nitrostyrene. A remarkable increase in the reaction rate occurred when the
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catalyst support contained specific composition (2 wt %) of CeO2 on silica. The structure and properties of the prepared catalysts were studied by using a range of physicochemical
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characterization tools; UV–vis, transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS) and N2-physisorption measurement
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studies. The challenging reduction of p-nitrostyrene containing an easily reducible group was examined in an ethanol suspension under mild reaction conditions using ammonia borane (AB) as
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an in-situ source of hydrogen. The highest conversion (100 %) and 90 % product yield in the hydrogenation of p-nitrostyrene to p-aminostyrene was achieved on the catalyst with 1.0 weight %
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loading of Ag NPs and 2 weight % of CeO2 supported on silica.
Keywords: Ag-plasmon, mesoporous silica, plasmonic catalysis, CeO2 and nitrostyrene reduction
1. Introduction The efficient utilization of solar energy to carry out photocatalytic chemical transformation reactions is one of the most promising approaches to reduce the burden of consuming
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nonrenewable resources [1]. Recently, plasmonic nanomaterials are under the subject of immense research focus for their synthesis and characterization because of their unique ability to concentrate light at nanoscale regimes [2-3]. The collective oscillation of the conduction electrons in the plasmonic nanoparticles (Au, Ag) generated by the incident electromagnetic irradiation is known as Localized Surface Plasmon Resonance (LSPR) [4-7]. This effect in nanomaterials can lead to strong electric fields around the NPs that causes near-field enhancement effects, finding its
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applications in surface-enhanced Raman scattering (SERS), optical metamaterials, nanophotonics
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devices, biosensing and photocatalysis [8-13]. Therefore, detailed study and tuning of LSPR is substantial to expand the applications and to develop new technologies using plasmonic
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nanomaterials.
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In the recent times, plasmon-enhanced photocatalysis has shown great promise and attracted significant attention of the researchers worldwide [14]. The purpose of utilizing such
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nanostructures in heterogeneous catalytic reactions is to employ the interaction of reactants with
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plasmon-excited hot electrons to increase the reaction conversion efficiency and decrease the activation energy [15-17]. Different size, shape and composition of plasmonic nanostructures can
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be synthesized by various new different methods and processes in order to study their chemical and optical properties. In the recent report of Ding Ma et al., hybrid Ag-Au nanoparticles and
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nanochains structure have been synthesized for selective hydrogenation reaction under visible light irradiation. A superior catalytic performance of nanochain structures with their stronger SERS activity was observed [18]. Moores et al. explained the plasmonic excitation phenomenon in Ag
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nanocubes for the activation of molecular hydrogen in the reduction of ketones and aldehydes to their corresponding alcohols [19]. In another report, the synergistic effect of gold/bismuth hybrid, Au/(BiO)2CO3 was utilized to carry out solar-driven photoconversion of nitrogen to ammonia [20]. The simple hydrogenation of nitro compounds to their corresponding amines poses only a few problems and is therefore successfully accomplished on a commercial scale. But the selective
reduction of nitroaromatic compounds in the presence of other reducible functionalities like C=O and C=C is still a challenge for its implication on a large scale synthesis [21]. This reaction is important for the formation of intermediates utilized in industries like agrochemicals, pharmaceuticals, polymers, herbicides and dyes [22-27]. The selective reduction necessitates the heterolytic cleavage of H2 (H+/H- pair) to reduce polar –NO2 group preference to nonpolar C=C
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bond. Many such initial studies reported by Corma et al. have shown increased selectivity but often led to decrease in the overall catalytic activity [28-30]. For example, they reported the studies of Au NPs supported on titanium oxide or iron oxide using H2 gas under mild reaction condition for the reduction of a variety of functionalized aromatic nitro compounds. The result led to the significant high chemoselectivity and no generation of unwanted byproducts. An alternative approach was employed to enhance the adsorption and control the selectivity of the products. The
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surface of the support material was modified with organic thiols or the NPs modified with
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phosphine ligands or self-assembled monolayers. Few recent research works have reported the importance of fine-tuning of the morphology of metal catalysts in controlling the chemoselectivity
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of nitro group reduction, avoiding the usage of toxic species. As per theoretical and experimental
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research results, Ag NPs interacts very weakly with H2 because of its d10 configuration but recent research reports reveal that it can act as a heterogeneous catalyst in liquid phase reactions [31].
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Our research group has reported the synthesis and photoconversion activity of size and
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color controlled plasmonic Ag NPs and their bimetallic combination of Ag with catalytically active Pd NPs [32-36]. These were tested in the tandem hydrogenation reaction of p-nitrophenol to p-
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aminophenol and p-nitrostyrene to p-aminostyrene with ammonia borane (AB) as a source of insitu H2 [37-39]. In this work, we have synthesized a series of Ag-based plasmonic catalysts on
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CeO2 modified mesoporous silica to investigate the effect of CeO2 on the catalytic activity. The doping amount of CeO2 was varied from 0.5, 1.0, 2.0 and 5.0 weight percentage on the surface of silica. Thereafter, 1.0 weight percentage of Ag NPs were deposited on CeO2(x)/SBA-15 support
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material. These catalysts were tested for the chemoselective reduction of p-nitrostyrene to paminostyrene using AB as a mild source of reducing agent under light irradiation conditions.
2. Experimental section
2.1. Materials Tetraethyl orthosilicate ((C2H5O)4Si)), hydrochloric acid (HCl), 1-hexanol (C6H13OH), acetone, silver nitrate (AgNO3) and ethanol were purchased from Nacalai Tesque Inc. Triblock Pluronic P123® (Mw = 5800, PEO20PPO70PEO20) and ammonia borane (NH3BH3; AB) were obtained from
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Sigma-Aldrich Co. Ce(NO3)3.6H2O (> 98%) was purchased from Wako Pure Chemical Industries Ltd. p-nitrostyrene (C8H7NO2) was purchased from Tokyo Chemical Industry Co. Ltd. All chemicals were used as received without any further purification. 2.2. Synthesis of SBA-15
Mesoporous silica SBA-15 was synthesized according to the method reported in literature utilizing Pluronic P123® as a structure directing agent and tetraethyl orthosilicate (TEOS) as a silica source
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under acidic conditions (pH < 1) [40].
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2.3. Synthesis of CeO2(x)/SBA-15
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CeO2/SBA-15 samples with different CeO2 contents were synthesized through an impregnation
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method. At first, SBA-15 was degassed overnight at 150 °C before impregnating with cerium precursor solution. Aqueous solutions of Ce(NO3)3·6H2O (>98 %, Wako) containing different
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amounts of CeO2 was prepared and 1.0 g of SBA-15 was dispersed onto it. After stirring for
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overnight at room temperature, the above mixture was evaporated by rotary evaporation under vacuum at 70 °C. The obtained powder with different concentration of CeO2 was then calcined at 500 °C with a heating rate of 4 °C min-1 for 4 h and named as CeO2(x)/SBA-15, where x varies
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from 0.0, 0.5, 1.0, 2.0 and 5.0 wt % of CeO2 loaded on SBA-15.
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2.4. Synthesis of Ag/CeO2(x)/SBA-15 The loading of 1 wt % Ag nanoparticles (NPs) within SBA-15 (0.396 g) was carried out by
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microwave assisted alcohol reduction method. At first, the ceria coated silica support was dispersed and ultrasonicated well with 1-hexanol (40 mL) for 30 min. Further, the surface directing agent, sodium laurate (10 mg) and precursor AgNO3 aqueous solution (0.037 mmol) was injected into the solution followed by Ar bubbling for 15 min. The resultant mixture was then exposed to microwave irradiation (500 W) for a period of 3 min. The solution was then filtered and dried in air at 80 ˚C. The obtained yellow colored Ag nanocatalysts were named as Ag/CeO2(x)/SBA-15.
2.5. Synthesis of Ag/CeO2 Ag NPs deposited on bulk CeO2 was also synthesized as a reference sample. The method of preparation was similar to that of depositing Ag NPs on silica. 1 wt % of Ag was deposited via microwave assisted alcohol reduction method as mentioned above.
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2.6. Catalytic reaction
Chemoselective reduction of p-nitrostyrene (p-NS) to p-aminostyrene (p-AS): 5 mg of catalyst was weighed and dispersed in the sealed reactor tube followed by Ar bubbling for about 30 min to maintain inert atmosphere. 20 mM solution of p-NS and 0.06 M solution of ammonia borane (AB) in ethanol were separately prepared. Reaction was initiated upon addition of 5 mL of each solution (p-NS and AB) through the rubber septum in the reactor tube. The mixture was stirred continuously
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in dark or under light irradiation conditions. The time course of reaction and products were
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analyzed by using Shimadzu GC-2010 chromatograph installed with Shimadzu Mass spectrometer
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GC-MS-QP2010 Plus. An external fan was used in order to maintain the constant temperature of
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reaction vessel during the course of the reaction. 2.7. Characterization
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Shimadzu UV-2450 spectrophotometer was used to collect the reflectance UV-vis spectra of
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powdered samples. BaSO4 was used as a reference solid and the spectra were collected by employing Kubelka-Munk function. Brunauer–Emmett–Teller (BET) surface area measurement was performed by using a BEL-SORP max system (Microtrac BEL) at -196 ˚C. Degassing of the
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samples was done in a vacuum at 150 ˚C for 3 h in order to remove the adsorbed impurities. TEM micrographs were obtained with a Hitachi Hf-2000 FE-TEM equipped with Kevex energy-
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dispersive X-ray detector operated at 200 kV. Shimadzu ESCA-3400 electron spectrometer was used to characterize samples for X-ray photoelectron spectroscopy (XPS). Mg Kα X-ray radiation
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(hν = 1253.6 eV) was used as the excitation source. The binding energy of the spectra was calibrated using the C 1s core level for the contaminant at 284.5 eV. Ag K-edge XAFS spectra were recorded by using a fluorescence-yield collection technique at the beam line 01B1 station with an attached Si (111) monochromator at SPring-8, JASRI, Harima, Japan (prop. No. 2017A1063 and 2017A1057). SAN-EI ELECTRIC Super Bright 500, Model XEF-501S Xenon lamp was used as light source to carry out reactions under light irradiation conditions.
3. Results and discussions 3.1. Characterization results 3.1.1. N2 physisorption analysis
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The textural properties including surface area, pore size and pore volume, calculated by Brunauer– Emmett–Teller (BET) method, are summarized in Table 1. All samples exhibited characteristic type IV isotherm of mesoporous materials with a hysteresis loop as shown in Fig. 1, concluding the preservation of the ordered mesoporous structure of SBA-15 even after surface modification and deposition of Ag NPs. Fig. 1 (A, B) displays adsorption-desorption isotherms for CeO2(x)/SBA-15 and Ag/CeO2(x)/SBA-15 catalysts respectively. The pore size distribution of catalysts is shown in Fig. S1. It was observed that CeO2 incorporation onto SBA-15 and Ag
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incorporation onto CeO2(x)/SBA-15 lead to decrease in the pore volume of mesoporous channels,
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suggesting the filling of internal cavities of mesoporous silica. The pore size values remain
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constant for almost all the catalysts within the range of 7-8 nm. The surface area values decreased from 638 m2g-1 for SBA-15 to 601 m2g-1 for CeO2(0.5)/SBA-15 and 596 m2g-1 for
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Ag/CeO2(0.5)/SBA-15. The pore volume decreased from 1.04 cm3g-1 to 0.96 to 0.73 cm3g-1 for SBA-15, CeO2(0.5)/SBA-15 and Ag/CeO2(0.5)/SBA-15 samples, respectively. However, the trend
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is not applicable for all samples and had no clear effect on size of NPs and their catalytic activity.
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An unusual increment can be explained due to the excessive particle growth by two possible mechanisms, viz., particle migration and Ostwald ripening. The growth of NPs can lead to wall
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deformation and hence overall increment in surface area and the pore size values. Table 1 also summarizes the LSPR peak maximum shown by the plasmonic Ag NPs with different
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concentrations of CeO2 deposited on SBA-15 silica. The shift of LSPR peak towards high or lower wavenumbers depends on their NP size. The size of NPs as observed by TEM analysis is also shown in Table 1. The BET C-constant was calculated from a straight line plot of p/p0 vs. p/Va(p0-
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p) and is shown in Table S1. Also a t-plot was calculated for N2 adsorption at 77 K to investigate the microporous volume in the prepared catalysts. Fig. S2 summarizes the Vmicro = 0.0 cm3g-1 for SBA-15, CeO2(2.0)/SBA-15 and Ag/CeO2(2.0)/SBA-15. The surface area values calculated from t-plot (661.2, 607.1 and 578.8 m2g-1) was also found to be in accordance with the BET method.
3.1.2. UV-vis absorption measurement In our previously reported study, the size and color controlled Pd/Ag bimetallic NPs supported on mesoporous silica were studied for the chemoselective reduction reaction of pnitrostyrene [38]. In the present study, Ag NPs deposited on different weight percentage loading
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of CeO2 coated on mesoporous silica, SBA-15 is explored for its catalytic activity under visible light irradiation. The optical response of the prepared catalysts was recorded by UV-vis spectrometer and is shown in Fig. 2. SBA-15 showed complete transparency to light absorption while CeO2 exhibited strong peak at around 280 nm. On increasing the CeO2 content from 0 to 5.0 wt %, the absorption intensity also showed significant enhancement as can be seen evidently from the absorption spectra in Fig. 2 (A). Fig. 2 (B) summarizes the optical response of Ag deposited samples with a characteristic plasmonic peak at 400 nm. The intensity of plasmonic peak tends to
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continuously decrease on increasing the CeO2 content in the catalysts, which is opposite to what
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observed in the case of CeO2(x)/SBA-15 samples. The possible interaction between Ag and CeO2
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might also be responsible for hindering the light absorption and hence decreased light intensity in the optical spectra. Ag/CeO2 displayed a relatively broader peak which is red-shifted with respect
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to the silica supported samples. This might be due to the larger sized Ag NPs grown on the surface
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of CeO2 which was further confirmed by TEM micrographs in the later section.
3.1.3. TEM measurement
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To study the effect of size of plasmonic Ag NPs, TEM micrographs of all samples were measured. The size of NPs plays a decisive role in the positioning of LSPR wavelength maximum
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peak. Fig. 3 (A-F) shows the TEM micrographs of all samples along with their pertinent histograms of the size of Ag NPs in the inset. The spherical morphology of the Ag NPs grown within the channels of mesoporous silica can be clearly observed in all the samples. The average
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particle size as calculated from the inset representing particle size histograms is summarized in Table 1. The size of the NPs was found to be 3.9±1.3, 4.8±1.1, 5.2±1.0, 4.8±0.9 and 3.9±0.9 nm for Ag/SBA-15,
Ag/CeO2(0.5)/SBA-15,
Ag/CeO2(1.0)/SBA-15,
Ag/CeO2(2.0)/SBA-15
and
Ag/CeO2(5.0)/SBA-15, respectively. Table S2 summarizes the NPs size along with the distribution width mentioning maximum and minimum values (nm). A plot of the size of Ag NPs vs CeO2 wt % loading is shown in Fig. S3. The effect of CeO2 coating on silica seems to have no
special effect on the size of NPs. On increasing the CeO2 wt % loading from 0.0 to 1.0, the size of Ag NPs also increased and decreased on further increasing from 1.0 to 5.0 wt %. On the other hand, Ag NPs deposited on bare CeO2 exhibited larger NPs size with an average diameter of around 6.1±1.2 nm. The red-shifted plasmonic peak of Ag/CeO2 (~ 465 nm) is well justified from its larger NPs size as observed from TEM micrographs. All samples from (A-E) displayed
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beautifully grown spherical Ag NPs on silica, however, Ag/CeO2(2.0)/SBA-15 displayed the most uniform and narrow size distribution. This can be one of the reasons for its high selectivity in the
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product formation as explained in the catalytic reaction section.
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3.1.4. X-ray absorption measurement
The electronic structure and chemical environment of Ag NPs were investigated by
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carrying out X-ray absorption measurement. Fig. 4 (A) and (B) summarizes the normalized Ag Kedge X-ray absorption near-edge structure (XANES) and Fourier Transform extended X-ray
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absorption fine structure (FT-EXAFS) spectra, respectively. AgO and Ag foil were used as the
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reference samples to study the electronic state of Ag in prepared samples. The observable two peaks and edge position in the XANES spectra of Ag foil resemble to those of Ag/SBA-15 and
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Ag/CeO2(x)/SBA-15, confirming their similar electronic structure. The different peak spectra of AgO from all samples conclude the absence of Ag2+ species. Hence, all the prepared catalysts
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possess similar electronic properties and Ag in zero oxidation state. In the FT-EXAFS spectra, all samples exhibited a single peak at approximately 2.7 Å similar to the reference Ag foil. This main peak can be assigned to the contiguous Ag-Ag metallic bonding. The spectral peak responsible for
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Ag-O bonding at 1.7 Å in reference AgO sample found to be absent in all the samples, confirming the absence of silver oxide species.
3.1.5. XPS analysis To further investigate the chemical property, surface composition and electronic state of Ag NPs in all prepared samples, X-ray photoelectron spectroscopy (XPS) measurement study was performed. Fig. 5 (a-f) displays the Ag 3d XPS spectra for Ag/SBA-15, Ag/CeO2(0.5)/SBA-15,
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Ag/CeO2(1.0)/SBA-15, Ag/CeO2(2.0)/SBA-15, and Ag/CeO2(5.0)/SBA-15 samples respectively. The XPS spectra of metallic Ag 3d3/2 and Ag 3d5/2 is reported to show two peaks at 374.2 eV and 368.2 eV (theoretical values) respectively, representing the splitting of a 3d doublet separated by 6.0 eV [41]. Ag/SBA-15 was studied as the reference sample to study the effect of CeO2 deposited samples in the peak shifts towards lower or higher binding energies. The core level peaks of Ag/SBA-15 due to 3d3/2 and 3d5/2 are observed at binding energy values of 375.7 and 369.7 eV
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respectively. The integration of Ag with CeO2 leads to the peak shift towards higher binding energy values with respect to Ag/SBA-15. This shift might be due to the charge transfer from CeO2 to Ag
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atoms owing to their net difference in their electronegativities. The peak shift in the Ag 3d XPS
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spectra can be visualized easily in Fig. S4. Hence, XPS can prove to be a helpful technique to
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display the existence of heterojunction interaction between neighboring atoms situated adjacent to each other. The Ce 3d XPS spectra is shown in Fig. S5 for all catalysts. Fig. S5 (A) shows two
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multiplets of a total of six peaks separated by 18.6 eV corresponding to CeO2 and Ag/CeO2. In Fig. S5 (B) the two peaks corresponding to the spin-orbit coupling Ce 3d3/2 and 3d5/2 appear at
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887.3 eV and 905.9 eV with the spin-orbit splitting of about 18.6 eV was observed for Ag/CeO2(x)/SBA-15 samples. The low percentage loading of CeO2 followed by deposition of Ag
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NPs lead to the low intense two peaks assigned to Ce 3d3/2 and 3d5/2 in the XPS spectra.
3.2. Catalytic reaction
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The catalytic behaviors of the Ag catalysts were evaluated in the liquid-phase chemoselective
hydrogenation of p-NS to p-AS using AB as a reducing agent. AB (NH3BH3) is a white colored
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solid material containing 19.6 wt % of hydrogen and has recently gained attention to be used as a promising hydrogen storage carrier [42]. Under the reaction conditions used, the hydrogenation products of p-NS obtained are p-aminostyrene (p-AS), p-nitroethylbenzene (p-NEB) and paminoethylbenzene (p-AEB) as shown in Scheme 1. In this reaction, p-AS is the desired product, while p-AEB and p-NEB are formed as byproducts due to the complete and incomplete reduction of p-NS, respectively.
The reduction of p-NS is comparatively a complex but commercially important process. There are still very few reports on the chemoselective reduction of a nitro group having a reducible C=C double bond exploiting visible light irradiation. During the course of reaction, two consecutive
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parallel pathways occur simultaneously viz. hydrogenation of a C=C double bond and a nitro N=O group. The conversion process of nitro to amino group takes place via formation of intermediates, which are hard to detect due to their extremely short lifespans. As represented in the equation (1), the reactive intermediates are nitroso and hydroxylamines derivatives. R-NO2 + 2e-
R-NO + 2e-
R-NH(OH) + 2e-
R-NH2
(1)
Fig. 6 summarizes the amount of p-AS formed (%) in the reduction reaction carried out in an
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ethanol suspension under dark and light irradiation conditions. The reaction was carried out for 3
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h under an inert argon atmosphere at room temperature and atmospheric pressure conditions
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utilizing 5 mg catalyst, 5 mL of 20 mM p-NS and 5 mL of 0.06 M AB. The amount of reactant decreased continuously and entirely used up within the first hour after injection of reaction solution.
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For instance, a time profile graph for Ag/CeO2 is shown in Fig. S6 for the first 60 min of reaction to understand the kinetics of the reaction. In the series of blank test experiments, no products were
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observed in the absence of catalyst, showing the inherent stability of p-NS in ethanol. All the Ag
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nanocatalysts showed activity in the reduction reaction and underwent complete conversion to produce p-AS and by-products. The Ag/SBA-15 catalyst with no coating of CeO2 showed 17 %
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yield of p-AS in dark conditions in 3 h reaction time. On increasing the CeO2 wt % loading on silica, the amount of p-AS formed also showed increased values. Among all, Ag/CeO2(2.0)/SBA15 exhibited the highest catalytic activity in the product formation with 90 % yield of p-AS. This
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might be attributed to the specific interaction of Ag and CeO2 due to its uniform and narrow size distribution of Ag NPs on the surface of silica (as can be seen from TEM micrographs). Increasing
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beyond 2.0 wt % loading of CeO2, a significant decrease in the activity was observed. Ag deposited on bare CeO2 exhibited superior activity than Ag/SBA-15 but still less active than Ag/CeO2(2.0)/SBA-15. This showed the importance of optimum doping of CeO2 on the surface of silica. Under light irradiation conditions, it was found that all catalysts exhibited an enhancement in the amount (%) of p-AS formed during the catalytic reduction reaction as displayed in Fig. 6.
This can be attributed to the Ag-plasmonic effect absorbing visible portion of light irradiation. Ag/SBA-15 improved the amount of product yield from 17 to 32 %. Ag/CeO2(2.0)/SBA-15 displayed 90 % of the desired product under light irradiation conditions as compared to 72 % in the absence of light irradiation conditions. An enhancement ratio was calculated by dividing the amount of desired product produced in light (YL) and dark (YD) conditions. The enhancement
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follows the following order: Ag/SBA-15 > Ag/CeO2(2.0)/SBA-15 > Ag/CeO2(1.0)/SBA-15 > Ag/CeO2(0.5)/SBA-15 > Ag/CeO2(5.0)/SBA-15 > Ag/CeO2. The observed least enhancement in case of Ag/CeO2, depicting the importance of using mesoporous silica materials in our research study. It might be due to large sized, not so-defined nanoparticles grown on the surface of CeO2. The higher % yields under visible light irradiation can be ascribed to the efficient transfer of H+/Hpair to the polar bonds in the –NO2 group instead of non-polar C=C group of p-NS. It can be
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envisaged that under light irradiation conditions, charge polarization of plasmonic metal NPs takes
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place and enhances the efficient adsorption of polar molecules. The metal-support interaction and
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synergistic effect of Ag-CeO2 can’t be ignored in the efficient catalysis reaction [35].
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The effect of addition of positive charge scavenger was studied in order to understand the mechanistic pathway under light irradiation conditions. In the present study, 100 µmol of NaHCO3, a positive charge scavenger was added to the suspension of Ag/CeO2(2.0)/SBA-15 under light
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irradiation conditions. The time course profile of reaction show complete conversion within first
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30 min of reaction as can be seen in Fig. S7 (A). The amount of p-AS formed is approximately 50 % in comparison to 90 % in the absence of NaHCO3 as summarized in Fig. S7 (B). The
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inhibition of catalytic activity was observed to be negligible under dark conditions. The observed retarded catalytic reaction indicates that the dehydrogenation of AB and further hydrogenation of
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p-NS is activated due to the transitory deficiency of electronic population on the surface of plasmonic NPs. This deficiency is arising due to the charge separation process derived from the
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Ag-LSPR oscillation [35, 43].
4. Conclusions
In summary, we have demonstrated the facile and easy synthesis of small-sized plasmonic Ag NPs supported on different weight percentages of CeO2 coated on mesoporous silica. The growth of Ag NPs took place within the confined mesoporous channels of SBA-15 silica. Ag NPs deposited on 2.0 wt % of CeO2 loaded on silica found to display superior catalytic performances in the reduction of p-nitrostyrene to p-aminostyrene. All catalysts showed significant enhancement
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effects under light irradiation conditions owing to the charge separation derived from localized surface plasmon resonance of Ag NPs. The effect of scavenger addition was also studied in order to understand the mechanistic pathway and role of electron/holes in the enhanced catalytic activity under light irradiation conditions. We hope that such progress and promotion of LSPR-assisted catalysis, especially in the reactions with commercial importance, will surely help in understanding this not-so explored area of research. We hope this work can stimulate further advances and can
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solve the large-scale energy and environmental aspects in the near future.
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Acknowledgements
The present work was partially supported by Grants-in-Aid for Scientific Research (Nos.
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26220911, 26630409, 26620194, and T16K14478) from the Japan Society for the Promotion of Science (JSPS) and MEXT. We acknowledge Dr. Eiji Taguchi and Prof. H. Yasuda at the Research
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Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with
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the TEM measurements. YK, KM and HY thank MEXT program “Elements Strategy Initiative to
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Form Core Research Center.
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Figures:
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Fig. 1. N2 adsorption-desorption isotherms for (A) CeO2(x)/SBA-15 and (B) Ag/CeO2(x)/SBA-15,
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where x = 0.5, 1.0, 2.0 and 5.0 wt %.
Fig. 2. UV-vis spectra of (A) SBA-15, CeO2, CeO2(x)/SBA-15 and (B) Ag/SBA-15, Ag/CeO2 and
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Ag/CeO2(x)/SBA-15 where x varies from 0.5, 1.0, 2.0 and 5.0 wt %.
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Fig. 3. TEM micrographs along with their pertinent histograms of the size of Ag NPs for (A) Ag/SBA-15, (B) Ag/CeO2(0.5)/SBA-15, (C) Ag/CeO2(1.0)/SBA-15, (D) Ag/CeO2(2.0)/SBA-15,
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(E) Ag/CeO2(5.0)/SBA-15 and (F) Ag/CeO2.
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Fig. 4. (A) Ag K-edge XANES and (B) FT-EXAFS spectra for (a) AgO, (b) Ag foil, (c) Ag/SBA-
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15, (d) Ag/CeO2(0.5)/SBA-15, (e) Ag/CeO2(1.0)/SBA-15, (f) Ag/CeO2(2.0)/SBA-15, (g)
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Ag/CeO2(5.0)/SBA-15 and (h) Ag/CeO2.
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Fig. 5. XPS Ag 3d spectra for (a) Ag/SBA-15, (b) Ag/CeO2(0.5)/SBA-15, (c) Ag/CeO2(1.0)/SBA-
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15, (d) Ag/CeO2(2.0)/SBA-15, (e) Ag/CeO2(5.0)/SBA-15 and (f) Ag/CeO2.
Fig. 6. Comparison of yields of p-aminostyrene (p-AS) for all catalysts in dark and under light irradiation conditions for 3 h. Reaction conditions: catalyst (5 mg), p-NS (5 mL, 20 mM) and AB
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(5 mL, 0.06 M) under an inert argon atmosphere at room temperature and atmospheric pressure.
Scheme:
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Scheme 1. Reduction of p-Nitrostyrene over Ag/CeO2(x)/SBA-15 catalysts
Table:
Table 1. Textural properties, NPs size and LSPR absorption of the prepared catalysts determined
Pore size (nm) 8.3
Average NPs size (nm) ---
LSPR ?? (nm)
8.3
---
---
8.8
---
---
8.8
---
---
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from N2-physisorption measurements, TEM analysis and UV-vis spectra, respectively.
8.3
---
---
660
8.3
3.9±1.3
391
596
7.2
4.8±1.1
399
769
7.2
5.2±1.0
398
0.94
570
8.3
4.8±0.9
405
BET surface area (m2g-1)
SBA-15
1.04
638
CeO2(0.5)/SBA-15
0.96
601
CeO2(1.0)/SBA-15
1.02
599
CeO2(2.0)/SBA-15
0.99
587
CeO2(5.0)/SBA-15
1.1
699
Ag/SBA-15
0.98
Ag/CeO2(0.5)/SBA-15
0.73
Ag/CeO2(1.0)/SBA-15
1.34
Ag/CeO2(2.0)/SBA-15
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---
1.11
702
7.2
3.9±0.9
402
Ag/CeO2
0.25
37
2.08
6.1±1.2
465
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Ag/CeO2(5.0)/SBA-15
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Pore volume (cm3g-1)
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Catalyst
S0920-5861(18)30116-0 https://doi.org/10.1016/j.cattod.2018.06.051 CATTOD 11540
To appear in:
Catalysis Today
Received date: Revised date: Accepted date:
23-2-2018 24-4-2018 30-6-2018
Please cite this article as: Verma P, Kuwahara Y, Mori K, Yamashita H, Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light irradiation conditions, Catalysis Today (2018), https://doi.org/10.1016/j.cattod.2018.06.051 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Plasmonic catalysis of Ag nanoparticles deposited on CeO2 modified mesoporous silica for the nitrostyrene reduction under light
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irradiation conditions
Priyanka Vermaa, Yasutaka Kuwaharaa,b, Kohsuke Moria,b,c and Hiromi Yamashitaa,b*
a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka
Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 606-
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b
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University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
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8501, Japan
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JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan
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Email: [email protected]
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Graphical abstract
Highlights
Ag-based plasmonic nanocatalyst supported on ceria modified mesoporous silica is designed. The prepared catalysts showed excellent catalytic performances in the p-nitrostyrene
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reduction.
Significant enhancements in the product formation were observed under light irradiation conditions attributing to the Ag-plasmonic effect.
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Abstract
Plasmonic photocatalysis caused by surface plasmon resonance (SPR) has emerged out as an
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impressive way for efficient light absorption and solar light conversion. A series of catalysts
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consisting of plasmonic Ag nanoparticles (NPs) deposited on CeO2 coated mesoporous silica,
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SBA-15, prepared by MW-assisted alcohol reduction method were tested for the chemoselective hydrogenation of p-nitrostyrene. A remarkable increase in the reaction rate occurred when the
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catalyst support contained specific composition (2 wt %) of CeO2 on silica. The structure and properties of the prepared catalysts were studied by using a range of physicochemical
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characterization tools; UV–vis, transmission electron microscopy (TEM), X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS) and N2-physisorption measurement
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studies. The challenging reduction of p-nitrostyrene containing an easily reducible group was examined in an ethanol suspension under mild reaction conditions using ammonia borane (AB) as
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an in-situ source of hydrogen. The highest conversion (100 %) and 90 % product yield in the hydrogenation of p-nitrostyrene to p-aminostyrene was achieved on the catalyst with 1.0 weight %
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loading of Ag NPs and 2 weight % of CeO2 supported on silica.
Keywords: Ag-plasmon, mesoporous silica, plasmonic catalysis, CeO2 and nitrostyrene reduction
1. Introduction The efficient utilization of solar energy to carry out photocatalytic chemical transformation reactions is one of the most promising approaches to reduce the burden of consuming
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nonrenewable resources [1]. Recently, plasmonic nanomaterials are under the subject of immense research focus for their synthesis and characterization because of their unique ability to concentrate light at nanoscale regimes [2-3]. The collective oscillation of the conduction electrons in the plasmonic nanoparticles (Au, Ag) generated by the incident electromagnetic irradiation is known as Localized Surface Plasmon Resonance (LSPR) [4-7]. This effect in nanomaterials can lead to strong electric fields around the NPs that causes near-field enhancement effects, finding its
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applications in surface-enhanced Raman scattering (SERS), optical metamaterials, nanophotonics
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devices, biosensing and photocatalysis [8-13]. Therefore, detailed study and tuning of LSPR is substantial to expand the applications and to develop new technologies using plasmonic
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nanomaterials.
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In the recent times, plasmon-enhanced photocatalysis has shown great promise and attracted significant attention of the researchers worldwide [14]. The purpose of utilizing such
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nanostructures in heterogeneous catalytic reactions is to employ the interaction of reactants with
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plasmon-excited hot electrons to increase the reaction conversion efficiency and decrease the activation energy [15-17]. Different size, shape and composition of plasmonic nanostructures can
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be synthesized by various new different methods and processes in order to study their chemical and optical properties. In the recent report of Ding Ma et al., hybrid Ag-Au nanoparticles and
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nanochains structure have been synthesized for selective hydrogenation reaction under visible light irradiation. A superior catalytic performance of nanochain structures with their stronger SERS activity was observed [18]. Moores et al. explained the plasmonic excitation phenomenon in Ag
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nanocubes for the activation of molecular hydrogen in the reduction of ketones and aldehydes to their corresponding alcohols [19]. In another report, the synergistic effect of gold/bismuth hybrid, Au/(BiO)2CO3 was utilized to carry out solar-driven photoconversion of nitrogen to ammonia [20]. The simple hydrogenation of nitro compounds to their corresponding amines poses only a few problems and is therefore successfully accomplished on a commercial scale. But the selective
reduction of nitroaromatic compounds in the presence of other reducible functionalities like C=O and C=C is still a challenge for its implication on a large scale synthesis [21]. This reaction is important for the formation of intermediates utilized in industries like agrochemicals, pharmaceuticals, polymers, herbicides and dyes [22-27]. The selective reduction necessitates the heterolytic cleavage of H2 (H+/H- pair) to reduce polar –NO2 group preference to nonpolar C=C
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bond. Many such initial studies reported by Corma et al. have shown increased selectivity but often led to decrease in the overall catalytic activity [28-30]. For example, they reported the studies of Au NPs supported on titanium oxide or iron oxide using H2 gas under mild reaction condition for the reduction of a variety of functionalized aromatic nitro compounds. The result led to the significant high chemoselectivity and no generation of unwanted byproducts. An alternative approach was employed to enhance the adsorption and control the selectivity of the products. The
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surface of the support material was modified with organic thiols or the NPs modified with
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phosphine ligands or self-assembled monolayers. Few recent research works have reported the importance of fine-tuning of the morphology of metal catalysts in controlling the chemoselectivity
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of nitro group reduction, avoiding the usage of toxic species. As per theoretical and experimental
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research results, Ag NPs interacts very weakly with H2 because of its d10 configuration but recent research reports reveal that it can act as a heterogeneous catalyst in liquid phase reactions [31].
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Our research group has reported the synthesis and photoconversion activity of size and
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color controlled plasmonic Ag NPs and their bimetallic combination of Ag with catalytically active Pd NPs [32-36]. These were tested in the tandem hydrogenation reaction of p-nitrophenol to p-
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aminophenol and p-nitrostyrene to p-aminostyrene with ammonia borane (AB) as a source of insitu H2 [37-39]. In this work, we have synthesized a series of Ag-based plasmonic catalysts on
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CeO2 modified mesoporous silica to investigate the effect of CeO2 on the catalytic activity. The doping amount of CeO2 was varied from 0.5, 1.0, 2.0 and 5.0 weight percentage on the surface of silica. Thereafter, 1.0 weight percentage of Ag NPs were deposited on CeO2(x)/SBA-15 support
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material. These catalysts were tested for the chemoselective reduction of p-nitrostyrene to paminostyrene using AB as a mild source of reducing agent under light irradiation conditions.
2. Experimental section
2.1. Materials Tetraethyl orthosilicate ((C2H5O)4Si)), hydrochloric acid (HCl), 1-hexanol (C6H13OH), acetone, silver nitrate (AgNO3) and ethanol were purchased from Nacalai Tesque Inc. Triblock Pluronic P123® (Mw = 5800, PEO20PPO70PEO20) and ammonia borane (NH3BH3; AB) were obtained from
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Sigma-Aldrich Co. Ce(NO3)3.6H2O (> 98%) was purchased from Wako Pure Chemical Industries Ltd. p-nitrostyrene (C8H7NO2) was purchased from Tokyo Chemical Industry Co. Ltd. All chemicals were used as received without any further purification. 2.2. Synthesis of SBA-15
Mesoporous silica SBA-15 was synthesized according to the method reported in literature utilizing Pluronic P123® as a structure directing agent and tetraethyl orthosilicate (TEOS) as a silica source
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under acidic conditions (pH < 1) [40].
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2.3. Synthesis of CeO2(x)/SBA-15
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CeO2/SBA-15 samples with different CeO2 contents were synthesized through an impregnation
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method. At first, SBA-15 was degassed overnight at 150 °C before impregnating with cerium precursor solution. Aqueous solutions of Ce(NO3)3·6H2O (>98 %, Wako) containing different
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amounts of CeO2 was prepared and 1.0 g of SBA-15 was dispersed onto it. After stirring for
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overnight at room temperature, the above mixture was evaporated by rotary evaporation under vacuum at 70 °C. The obtained powder with different concentration of CeO2 was then calcined at 500 °C with a heating rate of 4 °C min-1 for 4 h and named as CeO2(x)/SBA-15, where x varies
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from 0.0, 0.5, 1.0, 2.0 and 5.0 wt % of CeO2 loaded on SBA-15.
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2.4. Synthesis of Ag/CeO2(x)/SBA-15 The loading of 1 wt % Ag nanoparticles (NPs) within SBA-15 (0.396 g) was carried out by
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microwave assisted alcohol reduction method. At first, the ceria coated silica support was dispersed and ultrasonicated well with 1-hexanol (40 mL) for 30 min. Further, the surface directing agent, sodium laurate (10 mg) and precursor AgNO3 aqueous solution (0.037 mmol) was injected into the solution followed by Ar bubbling for 15 min. The resultant mixture was then exposed to microwave irradiation (500 W) for a period of 3 min. The solution was then filtered and dried in air at 80 ˚C. The obtained yellow colored Ag nanocatalysts were named as Ag/CeO2(x)/SBA-15.
2.5. Synthesis of Ag/CeO2 Ag NPs deposited on bulk CeO2 was also synthesized as a reference sample. The method of preparation was similar to that of depositing Ag NPs on silica. 1 wt % of Ag was deposited via microwave assisted alcohol reduction method as mentioned above.
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2.6. Catalytic reaction
Chemoselective reduction of p-nitrostyrene (p-NS) to p-aminostyrene (p-AS): 5 mg of catalyst was weighed and dispersed in the sealed reactor tube followed by Ar bubbling for about 30 min to maintain inert atmosphere. 20 mM solution of p-NS and 0.06 M solution of ammonia borane (AB) in ethanol were separately prepared. Reaction was initiated upon addition of 5 mL of each solution (p-NS and AB) through the rubber septum in the reactor tube. The mixture was stirred continuously
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in dark or under light irradiation conditions. The time course of reaction and products were
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analyzed by using Shimadzu GC-2010 chromatograph installed with Shimadzu Mass spectrometer
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GC-MS-QP2010 Plus. An external fan was used in order to maintain the constant temperature of
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reaction vessel during the course of the reaction. 2.7. Characterization
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Shimadzu UV-2450 spectrophotometer was used to collect the reflectance UV-vis spectra of
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powdered samples. BaSO4 was used as a reference solid and the spectra were collected by employing Kubelka-Munk function. Brunauer–Emmett–Teller (BET) surface area measurement was performed by using a BEL-SORP max system (Microtrac BEL) at -196 ˚C. Degassing of the
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samples was done in a vacuum at 150 ˚C for 3 h in order to remove the adsorbed impurities. TEM micrographs were obtained with a Hitachi Hf-2000 FE-TEM equipped with Kevex energy-
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dispersive X-ray detector operated at 200 kV. Shimadzu ESCA-3400 electron spectrometer was used to characterize samples for X-ray photoelectron spectroscopy (XPS). Mg Kα X-ray radiation
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(hν = 1253.6 eV) was used as the excitation source. The binding energy of the spectra was calibrated using the C 1s core level for the contaminant at 284.5 eV. Ag K-edge XAFS spectra were recorded by using a fluorescence-yield collection technique at the beam line 01B1 station with an attached Si (111) monochromator at SPring-8, JASRI, Harima, Japan (prop. No. 2017A1063 and 2017A1057). SAN-EI ELECTRIC Super Bright 500, Model XEF-501S Xenon lamp was used as light source to carry out reactions under light irradiation conditions.
3. Results and discussions 3.1. Characterization results 3.1.1. N2 physisorption analysis
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The textural properties including surface area, pore size and pore volume, calculated by Brunauer– Emmett–Teller (BET) method, are summarized in Table 1. All samples exhibited characteristic type IV isotherm of mesoporous materials with a hysteresis loop as shown in Fig. 1, concluding the preservation of the ordered mesoporous structure of SBA-15 even after surface modification and deposition of Ag NPs. Fig. 1 (A, B) displays adsorption-desorption isotherms for CeO2(x)/SBA-15 and Ag/CeO2(x)/SBA-15 catalysts respectively. The pore size distribution of catalysts is shown in Fig. S1. It was observed that CeO2 incorporation onto SBA-15 and Ag
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incorporation onto CeO2(x)/SBA-15 lead to decrease in the pore volume of mesoporous channels,
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suggesting the filling of internal cavities of mesoporous silica. The pore size values remain
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constant for almost all the catalysts within the range of 7-8 nm. The surface area values decreased from 638 m2g-1 for SBA-15 to 601 m2g-1 for CeO2(0.5)/SBA-15 and 596 m2g-1 for
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Ag/CeO2(0.5)/SBA-15. The pore volume decreased from 1.04 cm3g-1 to 0.96 to 0.73 cm3g-1 for SBA-15, CeO2(0.5)/SBA-15 and Ag/CeO2(0.5)/SBA-15 samples, respectively. However, the trend
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is not applicable for all samples and had no clear effect on size of NPs and their catalytic activity.
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An unusual increment can be explained due to the excessive particle growth by two possible mechanisms, viz., particle migration and Ostwald ripening. The growth of NPs can lead to wall
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deformation and hence overall increment in surface area and the pore size values. Table 1 also summarizes the LSPR peak maximum shown by the plasmonic Ag NPs with different
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concentrations of CeO2 deposited on SBA-15 silica. The shift of LSPR peak towards high or lower wavenumbers depends on their NP size. The size of NPs as observed by TEM analysis is also shown in Table 1. The BET C-constant was calculated from a straight line plot of p/p0 vs. p/Va(p0-
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p) and is shown in Table S1. Also a t-plot was calculated for N2 adsorption at 77 K to investigate the microporous volume in the prepared catalysts. Fig. S2 summarizes the Vmicro = 0.0 cm3g-1 for SBA-15, CeO2(2.0)/SBA-15 and Ag/CeO2(2.0)/SBA-15. The surface area values calculated from t-plot (661.2, 607.1 and 578.8 m2g-1) was also found to be in accordance with the BET method.
3.1.2. UV-vis absorption measurement In our previously reported study, the size and color controlled Pd/Ag bimetallic NPs supported on mesoporous silica were studied for the chemoselective reduction reaction of pnitrostyrene [38]. In the present study, Ag NPs deposited on different weight percentage loading
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of CeO2 coated on mesoporous silica, SBA-15 is explored for its catalytic activity under visible light irradiation. The optical response of the prepared catalysts was recorded by UV-vis spectrometer and is shown in Fig. 2. SBA-15 showed complete transparency to light absorption while CeO2 exhibited strong peak at around 280 nm. On increasing the CeO2 content from 0 to 5.0 wt %, the absorption intensity also showed significant enhancement as can be seen evidently from the absorption spectra in Fig. 2 (A). Fig. 2 (B) summarizes the optical response of Ag deposited samples with a characteristic plasmonic peak at 400 nm. The intensity of plasmonic peak tends to
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continuously decrease on increasing the CeO2 content in the catalysts, which is opposite to what
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observed in the case of CeO2(x)/SBA-15 samples. The possible interaction between Ag and CeO2
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might also be responsible for hindering the light absorption and hence decreased light intensity in the optical spectra. Ag/CeO2 displayed a relatively broader peak which is red-shifted with respect
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to the silica supported samples. This might be due to the larger sized Ag NPs grown on the surface
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of CeO2 which was further confirmed by TEM micrographs in the later section.
3.1.3. TEM measurement
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To study the effect of size of plasmonic Ag NPs, TEM micrographs of all samples were measured. The size of NPs plays a decisive role in the positioning of LSPR wavelength maximum
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peak. Fig. 3 (A-F) shows the TEM micrographs of all samples along with their pertinent histograms of the size of Ag NPs in the inset. The spherical morphology of the Ag NPs grown within the channels of mesoporous silica can be clearly observed in all the samples. The average
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particle size as calculated from the inset representing particle size histograms is summarized in Table 1. The size of the NPs was found to be 3.9±1.3, 4.8±1.1, 5.2±1.0, 4.8±0.9 and 3.9±0.9 nm for Ag/SBA-15,
Ag/CeO2(0.5)/SBA-15,
Ag/CeO2(1.0)/SBA-15,
Ag/CeO2(2.0)/SBA-15
and
Ag/CeO2(5.0)/SBA-15, respectively. Table S2 summarizes the NPs size along with the distribution width mentioning maximum and minimum values (nm). A plot of the size of Ag NPs vs CeO2 wt % loading is shown in Fig. S3. The effect of CeO2 coating on silica seems to have no
special effect on the size of NPs. On increasing the CeO2 wt % loading from 0.0 to 1.0, the size of Ag NPs also increased and decreased on further increasing from 1.0 to 5.0 wt %. On the other hand, Ag NPs deposited on bare CeO2 exhibited larger NPs size with an average diameter of around 6.1±1.2 nm. The red-shifted plasmonic peak of Ag/CeO2 (~ 465 nm) is well justified from its larger NPs size as observed from TEM micrographs. All samples from (A-E) displayed
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beautifully grown spherical Ag NPs on silica, however, Ag/CeO2(2.0)/SBA-15 displayed the most uniform and narrow size distribution. This can be one of the reasons for its high selectivity in the
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product formation as explained in the catalytic reaction section.
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3.1.4. X-ray absorption measurement
The electronic structure and chemical environment of Ag NPs were investigated by
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carrying out X-ray absorption measurement. Fig. 4 (A) and (B) summarizes the normalized Ag Kedge X-ray absorption near-edge structure (XANES) and Fourier Transform extended X-ray
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absorption fine structure (FT-EXAFS) spectra, respectively. AgO and Ag foil were used as the
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reference samples to study the electronic state of Ag in prepared samples. The observable two peaks and edge position in the XANES spectra of Ag foil resemble to those of Ag/SBA-15 and
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Ag/CeO2(x)/SBA-15, confirming their similar electronic structure. The different peak spectra of AgO from all samples conclude the absence of Ag2+ species. Hence, all the prepared catalysts
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possess similar electronic properties and Ag in zero oxidation state. In the FT-EXAFS spectra, all samples exhibited a single peak at approximately 2.7 Å similar to the reference Ag foil. This main peak can be assigned to the contiguous Ag-Ag metallic bonding. The spectral peak responsible for
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Ag-O bonding at 1.7 Å in reference AgO sample found to be absent in all the samples, confirming the absence of silver oxide species.
3.1.5. XPS analysis To further investigate the chemical property, surface composition and electronic state of Ag NPs in all prepared samples, X-ray photoelectron spectroscopy (XPS) measurement study was performed. Fig. 5 (a-f) displays the Ag 3d XPS spectra for Ag/SBA-15, Ag/CeO2(0.5)/SBA-15,
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Ag/CeO2(1.0)/SBA-15, Ag/CeO2(2.0)/SBA-15, and Ag/CeO2(5.0)/SBA-15 samples respectively. The XPS spectra of metallic Ag 3d3/2 and Ag 3d5/2 is reported to show two peaks at 374.2 eV and 368.2 eV (theoretical values) respectively, representing the splitting of a 3d doublet separated by 6.0 eV [41]. Ag/SBA-15 was studied as the reference sample to study the effect of CeO2 deposited samples in the peak shifts towards lower or higher binding energies. The core level peaks of Ag/SBA-15 due to 3d3/2 and 3d5/2 are observed at binding energy values of 375.7 and 369.7 eV
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respectively. The integration of Ag with CeO2 leads to the peak shift towards higher binding energy values with respect to Ag/SBA-15. This shift might be due to the charge transfer from CeO2 to Ag
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atoms owing to their net difference in their electronegativities. The peak shift in the Ag 3d XPS
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spectra can be visualized easily in Fig. S4. Hence, XPS can prove to be a helpful technique to
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display the existence of heterojunction interaction between neighboring atoms situated adjacent to each other. The Ce 3d XPS spectra is shown in Fig. S5 for all catalysts. Fig. S5 (A) shows two
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multiplets of a total of six peaks separated by 18.6 eV corresponding to CeO2 and Ag/CeO2. In Fig. S5 (B) the two peaks corresponding to the spin-orbit coupling Ce 3d3/2 and 3d5/2 appear at
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887.3 eV and 905.9 eV with the spin-orbit splitting of about 18.6 eV was observed for Ag/CeO2(x)/SBA-15 samples. The low percentage loading of CeO2 followed by deposition of Ag
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NPs lead to the low intense two peaks assigned to Ce 3d3/2 and 3d5/2 in the XPS spectra.
3.2. Catalytic reaction
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The catalytic behaviors of the Ag catalysts were evaluated in the liquid-phase chemoselective
hydrogenation of p-NS to p-AS using AB as a reducing agent. AB (NH3BH3) is a white colored
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solid material containing 19.6 wt % of hydrogen and has recently gained attention to be used as a promising hydrogen storage carrier [42]. Under the reaction conditions used, the hydrogenation products of p-NS obtained are p-aminostyrene (p-AS), p-nitroethylbenzene (p-NEB) and paminoethylbenzene (p-AEB) as shown in Scheme 1. In this reaction, p-AS is the desired product, while p-AEB and p-NEB are formed as byproducts due to the complete and incomplete reduction of p-NS, respectively.
The reduction of p-NS is comparatively a complex but commercially important process. There are still very few reports on the chemoselective reduction of a nitro group having a reducible C=C double bond exploiting visible light irradiation. During the course of reaction, two consecutive
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parallel pathways occur simultaneously viz. hydrogenation of a C=C double bond and a nitro N=O group. The conversion process of nitro to amino group takes place via formation of intermediates, which are hard to detect due to their extremely short lifespans. As represented in the equation (1), the reactive intermediates are nitroso and hydroxylamines derivatives. R-NO2 + 2e-
R-NO + 2e-
R-NH(OH) + 2e-
R-NH2
(1)
Fig. 6 summarizes the amount of p-AS formed (%) in the reduction reaction carried out in an
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ethanol suspension under dark and light irradiation conditions. The reaction was carried out for 3
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h under an inert argon atmosphere at room temperature and atmospheric pressure conditions
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utilizing 5 mg catalyst, 5 mL of 20 mM p-NS and 5 mL of 0.06 M AB. The amount of reactant decreased continuously and entirely used up within the first hour after injection of reaction solution.
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For instance, a time profile graph for Ag/CeO2 is shown in Fig. S6 for the first 60 min of reaction to understand the kinetics of the reaction. In the series of blank test experiments, no products were
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observed in the absence of catalyst, showing the inherent stability of p-NS in ethanol. All the Ag
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nanocatalysts showed activity in the reduction reaction and underwent complete conversion to produce p-AS and by-products. The Ag/SBA-15 catalyst with no coating of CeO2 showed 17 %
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yield of p-AS in dark conditions in 3 h reaction time. On increasing the CeO2 wt % loading on silica, the amount of p-AS formed also showed increased values. Among all, Ag/CeO2(2.0)/SBA15 exhibited the highest catalytic activity in the product formation with 90 % yield of p-AS. This
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might be attributed to the specific interaction of Ag and CeO2 due to its uniform and narrow size distribution of Ag NPs on the surface of silica (as can be seen from TEM micrographs). Increasing
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beyond 2.0 wt % loading of CeO2, a significant decrease in the activity was observed. Ag deposited on bare CeO2 exhibited superior activity than Ag/SBA-15 but still less active than Ag/CeO2(2.0)/SBA-15. This showed the importance of optimum doping of CeO2 on the surface of silica. Under light irradiation conditions, it was found that all catalysts exhibited an enhancement in the amount (%) of p-AS formed during the catalytic reduction reaction as displayed in Fig. 6.
This can be attributed to the Ag-plasmonic effect absorbing visible portion of light irradiation. Ag/SBA-15 improved the amount of product yield from 17 to 32 %. Ag/CeO2(2.0)/SBA-15 displayed 90 % of the desired product under light irradiation conditions as compared to 72 % in the absence of light irradiation conditions. An enhancement ratio was calculated by dividing the amount of desired product produced in light (YL) and dark (YD) conditions. The enhancement
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follows the following order: Ag/SBA-15 > Ag/CeO2(2.0)/SBA-15 > Ag/CeO2(1.0)/SBA-15 > Ag/CeO2(0.5)/SBA-15 > Ag/CeO2(5.0)/SBA-15 > Ag/CeO2. The observed least enhancement in case of Ag/CeO2, depicting the importance of using mesoporous silica materials in our research study. It might be due to large sized, not so-defined nanoparticles grown on the surface of CeO2. The higher % yields under visible light irradiation can be ascribed to the efficient transfer of H+/Hpair to the polar bonds in the –NO2 group instead of non-polar C=C group of p-NS. It can be
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envisaged that under light irradiation conditions, charge polarization of plasmonic metal NPs takes
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place and enhances the efficient adsorption of polar molecules. The metal-support interaction and
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synergistic effect of Ag-CeO2 can’t be ignored in the efficient catalysis reaction [35].
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The effect of addition of positive charge scavenger was studied in order to understand the mechanistic pathway under light irradiation conditions. In the present study, 100 µmol of NaHCO3, a positive charge scavenger was added to the suspension of Ag/CeO2(2.0)/SBA-15 under light
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irradiation conditions. The time course profile of reaction show complete conversion within first
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30 min of reaction as can be seen in Fig. S7 (A). The amount of p-AS formed is approximately 50 % in comparison to 90 % in the absence of NaHCO3 as summarized in Fig. S7 (B). The
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inhibition of catalytic activity was observed to be negligible under dark conditions. The observed retarded catalytic reaction indicates that the dehydrogenation of AB and further hydrogenation of
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p-NS is activated due to the transitory deficiency of electronic population on the surface of plasmonic NPs. This deficiency is arising due to the charge separation process derived from the
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Ag-LSPR oscillation [35, 43].
4. Conclusions
In summary, we have demonstrated the facile and easy synthesis of small-sized plasmonic Ag NPs supported on different weight percentages of CeO2 coated on mesoporous silica. The growth of Ag NPs took place within the confined mesoporous channels of SBA-15 silica. Ag NPs deposited on 2.0 wt % of CeO2 loaded on silica found to display superior catalytic performances in the reduction of p-nitrostyrene to p-aminostyrene. All catalysts showed significant enhancement
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effects under light irradiation conditions owing to the charge separation derived from localized surface plasmon resonance of Ag NPs. The effect of scavenger addition was also studied in order to understand the mechanistic pathway and role of electron/holes in the enhanced catalytic activity under light irradiation conditions. We hope that such progress and promotion of LSPR-assisted catalysis, especially in the reactions with commercial importance, will surely help in understanding this not-so explored area of research. We hope this work can stimulate further advances and can
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solve the large-scale energy and environmental aspects in the near future.
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Acknowledgements
The present work was partially supported by Grants-in-Aid for Scientific Research (Nos.
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26220911, 26630409, 26620194, and T16K14478) from the Japan Society for the Promotion of Science (JSPS) and MEXT. We acknowledge Dr. Eiji Taguchi and Prof. H. Yasuda at the Research
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Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with
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the TEM measurements. YK, KM and HY thank MEXT program “Elements Strategy Initiative to
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Form Core Research Center.
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Figures:
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Fig. 1. N2 adsorption-desorption isotherms for (A) CeO2(x)/SBA-15 and (B) Ag/CeO2(x)/SBA-15,
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where x = 0.5, 1.0, 2.0 and 5.0 wt %.
Fig. 2. UV-vis spectra of (A) SBA-15, CeO2, CeO2(x)/SBA-15 and (B) Ag/SBA-15, Ag/CeO2 and
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Ag/CeO2(x)/SBA-15 where x varies from 0.5, 1.0, 2.0 and 5.0 wt %.
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Fig. 3. TEM micrographs along with their pertinent histograms of the size of Ag NPs for (A) Ag/SBA-15, (B) Ag/CeO2(0.5)/SBA-15, (C) Ag/CeO2(1.0)/SBA-15, (D) Ag/CeO2(2.0)/SBA-15,
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(E) Ag/CeO2(5.0)/SBA-15 and (F) Ag/CeO2.
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Fig. 4. (A) Ag K-edge XANES and (B) FT-EXAFS spectra for (a) AgO, (b) Ag foil, (c) Ag/SBA-
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15, (d) Ag/CeO2(0.5)/SBA-15, (e) Ag/CeO2(1.0)/SBA-15, (f) Ag/CeO2(2.0)/SBA-15, (g)
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Ag/CeO2(5.0)/SBA-15 and (h) Ag/CeO2.
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Fig. 5. XPS Ag 3d spectra for (a) Ag/SBA-15, (b) Ag/CeO2(0.5)/SBA-15, (c) Ag/CeO2(1.0)/SBA-
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15, (d) Ag/CeO2(2.0)/SBA-15, (e) Ag/CeO2(5.0)/SBA-15 and (f) Ag/CeO2.
Fig. 6. Comparison of yields of p-aminostyrene (p-AS) for all catalysts in dark and under light irradiation conditions for 3 h. Reaction conditions: catalyst (5 mg), p-NS (5 mL, 20 mM) and AB
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(5 mL, 0.06 M) under an inert argon atmosphere at room temperature and atmospheric pressure.
Scheme:
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Scheme 1. Reduction of p-Nitrostyrene over Ag/CeO2(x)/SBA-15 catalysts
Table:
Table 1. Textural properties, NPs size and LSPR absorption of the prepared catalysts determined
Pore size (nm) 8.3
Average NPs size (nm) ---
LSPR ?? (nm)
8.3
---
---
8.8
---
---
8.8
---
---
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from N2-physisorption measurements, TEM analysis and UV-vis spectra, respectively.
8.3
---
---
660
8.3
3.9±1.3
391
596
7.2
4.8±1.1
399
769
7.2
5.2±1.0
398
0.94
570
8.3
4.8±0.9
405
BET surface area (m2g-1)
SBA-15
1.04
638
CeO2(0.5)/SBA-15
0.96
601
CeO2(1.0)/SBA-15
1.02
599
CeO2(2.0)/SBA-15
0.99
587
CeO2(5.0)/SBA-15
1.1
699
Ag/SBA-15
0.98
Ag/CeO2(0.5)/SBA-15
0.73
Ag/CeO2(1.0)/SBA-15
1.34
Ag/CeO2(2.0)/SBA-15
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---
1.11
702
7.2
3.9±0.9
402
Ag/CeO2
0.25
37
2.08
6.1±1.2
465
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Ag/CeO2(5.0)/SBA-15
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Pore volume (cm3g-1)
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Catalyst