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Visible-light-driven reduction of nitrostyrene utilizing plasmonic silver nanoparticle catalysts immobilized on oxide supports
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Review
Visible-light-driven reduction of nitrostyrene utilizing plasmonic silver nanoparticle catalysts immobilized on oxide supports Priyanka Vermaa, Yasutaka Kuwaharaa,b, Kohsuke Moria,b,c, Hiromi Yamashitaa,b,
⁎
a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 606-8501, Japan c JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Silver nanoparticles Plasmonic catalysis and chemoselective reduction
The localized surface plasmon resonance (LSPR) mediated enhanced chemical activity can be entitled as a promising strategy for efficient solar to chemical energy conversion. To tune the selectivity of a desired product in a chemical reaction is of paramount importance yet a great challenge. In this paper, a new strategy to effectively enhance the selectivity of the product formation under visible light irradiation is reported. A series of Ag catalysts deposited on metal oxide support materials (TiO2, ZrO2, Al2O3 and CeO2) along with their preparative techniques, optimum metal content ratio and effect of different wavelength of light is explored for the chemoselective reduction of p-nitrostyrene to p-aminostyrene under visible light irradiation. The prepared catalysts were characterized by a range of physicochemical techniques including UV–vis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The reduction reaction was carried out in ethanolic suspension at room temperature and pressure utilizing ammonia borane (AB) as an in-situ source of H2. The reaction results displayed 100% conversion with a maximum chemoselectivity of 81% shown by Ag/TiO2 under light irradiation conditions. The high chemoselectivity could be attributed to the preferential alignment of polar nitro group on the surface of plasmonic silver under light irradiation conditions.
1. Introduction Plasmon mediated catalysis has received significant attention in recent years due to the excellent light trapping ability of metal nanostructures [1–8]. Plasmonic photocatalysis involves the localized surface plasmon resonance of metal nanostructures to efficiently harvest the solar energy into chemical energy [9–11]. There have been many reports employing the plasmonic metal nanostructures to carry out catalytic reactions like Suzuki coupling, nitrophenol reduction and hydrogen production from storage materials like ammonia borane and formic acid [12–18]. However, very few reports explore the improved chemoselective performance utilizing the light absorption ability of these nanostructures. Tuning the selectivity of metal catalysts is highly beneficial for the development of industrially important and efficient processes [19–22]. The strong reducing agents like LiAlH4 or NaBH4 which are most commonly employed in organic reactions non-selectively reduces all the functional groups such as carbonyl, carboxyl, vinyl and nitro groups. Also, conventional active metal (platinum-group metal, PGM) catalysts non-selectively reduces all the functional groups.
Hence, protecting group is introduced in the functional group to carry out chemoselective reaction. Further, deprotection is done in order to obtain the desired product which decreases the overall yield of the reaction. Functionalized anilines form an important class of intermediate compounds which has found its application in pharmaceuticals, polymers, herbicides, dyes, pigments and many other useful compounds in fine chemical industries [23–27]. Hence, there is a strong incentive to test, design and develop heterogeneous catalytic system for the reduction of nitro group, in accordance with the 12 principles of the green chemistry. The reduction of nitroaromatics to corresponding anilines is very simple and straightforward process but it is challenging to reduce only –NO2 group in the presence of other reducible functionalities. Till now, the chemoselective reduction of nitrobenzenes is carried out by using excess of Zn, Sn, Fe or NaS2O4 based catalysts which generates large amounts of harmful chemical waste materials and therefore making them environmentally unsustainable. From practical and environmental perspectives, it is highly desirable to develop and design efficient catalysts which can selectively reduce functionalized
⁎ Corresponding author at: Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 5650871, Japan. E-mail address: [email protected] (H. Yamashita).
https://doi.org/10.1016/j.cattod.2019.03.058 Received 29 December 2018; Received in revised form 6 March 2019; Accepted 25 March 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Priyanka Verma, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.058
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nitrobenzene to amino benzenes. Although this process is of commercial importance, still very few reports have been reported developing the catalytic chemoselective reduction process at ambient conditions. It would be more attractive if reducing agents other than gaseous H2 and toxic chemicals can be employed at room temperature and atmospheric pressure conditions. Employing gaseous hydrogen (H2) especially requires high pressure of hydrogen and special apparatus to carry out the process. Another method of electrochemical hydrogenation requires immense energy to supply sufficient voltage to the reaction system. Corma et al. reported that Au/TiO2 catalyzes the selective hydrogenation of nitrostyrene attributing to the cooperative effect between Au and TiO2 [21]. It was proposed that NS is adsorbed on the interface of TiO2 and Au and H2 dissociation takes place on Au. Currently very few catalytic systems have been developed involving Ag or Au NPs to study the chemoselective hydrogenation. The reason for the high selectivities is explained due to the formation of H+/H− pair at the metal-support interface. This pair is active towards the reduction of polar nitro group and is completely inactive in the reduction of non-polar C]C bond. Mitsudome et al. has reported a core-shell Ag-CeO2 nanostructure with high chemoselective performance due to the large number of interfacial sites within the catalytic system [28]. Scheme 1 illustrates the typical product formation observed during the hydrogenation of p-nitrostyrene (i) using AB as an insitu source of hydrogen. The reaction pathway proceeds via two different routes involving preferential attack of the nitro group or the C]C functionality. Preferential reduction of –NO2 group leads to the formation of the desired product i.e. p-aminostyrene (ii), whereas the attack on the C]C group leads to the formation of p-ethyl nitrobenzene (iii). The fully hydrogenated product (p-ethylaniline, iv) would be formed by the complete reduction of both the functionalities [29,30]. In this study, we have reported the photocatalytic reduction of nitrostyrene having reducible functional group at room temperature and atmospheric pressure conditions by using Ag/MxOy (TiO2, ZrO2, Al2O3 and CeO2) catalyst. Silver has not been much explored for selective hydrogenation reactions by plasmonic absorption exploiting visible light irradiation. Knowing the fact that silver exhibits lower hydrogenation ability, it might be helpful in carrying out the chemoselective hydrogenation reaction than active metal catalysts. The catalyst was prepared by simple impregnation method in the inert atmosphere. Nitro group was selectively reduced in a major proportion in comparison to C]C bond. Also, we observed the improved chemoselective performance under visible light irradiation conditions. In many such studies the chemoselective reduction is carried out at harsh chemical conditions involving high temperature and pressure [31,32]. In the present
study, the reaction is carried out in ethanolic suspension at room temperature and atm pressure conditions without any additional agent. Further, the mechanism of improved chemoselective performance under light irradiation attributing to the effect of surface plasmon resonance is discussed in the later section. 2. Experimental section 2.1. Materials P25 TiO2 (JRC-TIO-4, anatase: rutile = 7:3), anatase TiO2 (JRC-TIO3), rutile TiO2 (JRC-TIO-8), were kindly supplied by the Catalysis Society of Japan. 1-hexanol (C6H13OH), acetone, silver nitrate (AgNO3), biphenyl (C6H5C6H5), Zirconium oxide (ZrO2) and ethanol were purchased from Nacalai Tesque Inc. Ammonia borane (NH3BH3; AB) and Cerium (IV) oxide (CeO2) was obtained from Sigma-Aldrich Co. p-nitrostyrene (C8H7NO2) was purchased from Tokyo Chemical Industry Co. Ltd. Aluminium oxide (α-Al2O3) was purchased from Wako Pure Chemical Industries Ltd. All chemicals were used as received without any further purification. 2.2. Preparation of Ag/MxOy catalysts by impregnation The catalyst was prepared by a simple impregnation method. 500 mg of metal oxide support materials, MxOy (TiO2, ZrO2, CeO2 and Al2O3) and sodium laurate (5 mg) was dispersed and ultrasonicated in 100 mL ethanol solution. The mixture was bubbled with argon gas in order to maintain inert atmosphere. Subsequently desired amount of aqueous AgNO3 solution was added into the mixture and stirred continuously for about 6 h. The suspension was evaporated under vacuum and the obtained powder was dried overnight at 80 °C. The obtained sample was named as Ag/MxOy and series of catalysts with varied amount of Ag (0.5, 1.0, 2.0 and 5.0 wt percentage) were prepared. 2.3. Catalytic reaction The chemoselective reduction of p-nitrostyrene (p-NS) to p-aminostyrene (p-AS) reaction is carried out. 20 mM solution of p-NS and 60 mM solution of ammonia borane (AB) in ethanol were separately prepared. 20 mM solution of biphenyl in ethanol was also prepared to be employed as an external standard reagent for calibrating the concentration of reactants and products. 5 mg of catalyst was weighed and dispersed in the sealed reactor tube. 5 mL of p-NS and 1 mL of biphenyl was injected through the rubber septum followed by Ar bubbling for 15 min to ensure the inert atmosphere conditions. Reaction was initiated upon addition of 5 mL of AB solution through the rubber septum in the reactor tube. The mixture was stirred continuously in dark or under light irradiation conditions. For carrying out reaction in dark conditions, the reactor was covered with aluminum foil in order to ensure the complete absence of light during the reaction. For light irradiation conditions, Xenon lamp was used with and without filter cutting off the undesired portion of light irradiation. The time course of reaction products were analyzed by using Shimadzu GC-2010 chromatograph installed with Shimadzu Mass spectrometer GC–MS-QP2010 Plus. The reaction products were analyzed at 0, 5, 15, 30, 60, 90 and 180 min intervals of time. An external fan was used in order to maintain the constant temperature of reaction vessel during the course of the reaction. 2.4. Characterization Shimadzu UV-2450 spectrophotometer was used to collect the reflectance UV–vis spectra of powdered samples. BaSO4 was used as a reference solid and the spectra were collected by employing KubelkaMunk function. Brunauer–Emmett–Teller (BET) surface area measurement was performed by using a BEL-SORP max system (Microtrac BEL)
Scheme 1. Reaction pathway for p-nitrostyrene (p-NS; i) reduction to p-aminostyrene (p-AS; ii) along with byproduct formation p-nitroethylbenzene (pNEB; iii) and p-aminoethylbenzene (p-AEB; iv). 2
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at -196 °C. Degassing of the 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 H-800 TEM 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 (hν = 1253.6 eV) was used as the excitation source. The binding energy of the spectra was calibrated using the C 1 s core level for the contaminant at 284.5 eV. SAN-EI ELECTRIC Super Bright 500, Model XEF-501S Xenon lamp was used as light source to carry out reactions under light irradiation conditions. GC-2010 chromatograph installed with Shimadzu Mass spectrometer GC–MS-QP2010 Plus was used to analyze reaction products employing biphenyl as an external standard reagent.
(Table 1). The correlation between the size of NPs and the chemoselective performance is shown in Fig. S2. The catalyst was found to be stable and could be easily recycled and recovered after the reaction. No leaching of NPs was observed after the reaction. A TEM micrograph of Ag/TiO2 before and after the reaction is shown in Fig. S1 (a). 3.1.3. XPS analysis The prepared Ag based catalysts were analyzed by X-ray photoelectron spectroscopy analysis to study the effect of local chemical environment depending on the different support materials. Fig. 3 (A) display the spectra of Ag NPs deposited on different metal oxides i.e. TiO2, ZrO2, Al2O3 and CeO2 respectively. The core-level 3d spectra splits into a doublet separated by 6.0 eV, confirming the metallic nature of Ag. As per the literature, the standard values of binding energies are 368.2 and 374.2 eV ascribing to 3d5/2 and 3d3/2 core levels of Ag [37]. We observed doublet peaks at binding energies of (367.5, 373.5), (367.7, 373.7), (367.7, 373.7) and (367.4, 373.4) for Ag/TiO2, Ag/ ZrO2, Ag/Al2O3 and Ag/CeO2, respectively. These values are plotted after calibrating the obtained data with respect to C 1 s spectral peak at 284.8 eV. We expect that all Ag NPs are deposited and are present in zero oxidation state as further confirmed by FT-EXAFS analysis. The higher electronegativity values of Ag (1.93) than metal in oxides affirms the reduced state of Ag NPs. However, the difference in the observed shift values can be explained due to the different electronegativity values of Ti (1.54), Zr (1.33), Al (1.61) and Ce (1.12). The lower B.E. values of Ag/CeO2 amongst others accounts for the least electronegative character of Ce and hence keeping the Ag in most reduced state than others.
3. Results and discussion 3.1. Characterization results 3.1.1. UV–vis absorption measurement Till date, various strategies have been developed to extend the light absorption of photocatalysts from UV to visible light regime. In one such strategy, UV-active photocatalysts are being hybridized with plasmonic metal nanostructures, which are capable of showing strong photoabsorbtion arising from the localized surface plasmon (LSPR) effect [33–36]. In the present study, the light absorption ability of the prepared catalysts was evaluated by measuring their UV–vis spectra. As shown in Fig. 1 (a), the optical absorption of metal oxide support lies in the UV region with a peak maximum at ˜ 270 nm for TiO2 and CeO2. ZrO2 displays less intense peak in the region of 200–250 nm while Al2O3 displays no absorption in the spectrum. Fig. 1 (b) displays the absorption spectrum of Ag deposited on metal oxides support materials. All samples exhibited the strong intrinsic plasmon peak in the visible region at around 400 nm, attributing to the LSPR effect of Ag NPs. A single broad peak absorption suggests the spherical morphology of the prepared NPs which was further confirmed by TEM. The LSPR peak maximum was observed at 516, 450, 463, 430 nm for Ag/TiO2, Ag/ CeO2, Ag/ZrO2 and Ag/Al2O3 respectively. The Ag/TiO2 absorption peak was found to be significantly red shifted in comparison to other catalysts. However, an absorption maximum peak at 416 nm was observed after the reaction, as discussed in the later section (Fig. S1).
3.2. Catalytic reaction The catalytic activity of the prepared catalysts was tested in the chemoselective reduction of p-NS to p-AS utilizing AB as an insitu source of H2 as shown in Scheme 2. The reaction was carried out for 3 h and its catalytic activity was measured in dark and under light irradiation conditions. In our previous reports, we have explored this reaction for the size and morphology controlled Ag NPs supported on mesoporous silica and Ag NPs deposited on ceria modified silica [38–40]. In the present study, several Ag catalysts were synthesized using commercially available inorganic support materials (Al2O3, ZrO2, CeO2 and TiO2) by various preparative techniques including MW, impregnation followed by reduction and photo-assisted deposition. The chemoselective reduction of p-nitrostyrene was conducted in dark and under light irradiation conditions, as summarized in Table 1. We obtained p-AS as the major product of the reaction with selectivity of the major product varying from 47 to 81% in dark and under light irradiation conditions. Fig. 4 displays the results obtained on the prepared catalysts for 3 h in dark conditions. All the catalysts showed almost complete conversion
3.1.2. TEM measurement Fig. 2 shows the TEM micrographs along with the pertinent histograms of the prepared Ag catalysts deposited on oxide support materials. All the NPs were found to be spherical on different metal oxide support materials. The average particle size of Ag/TiO2, Ag/ZrO2, Ag/ Al2O3 and Ag/CeO2 was found out to be 11.8 ± 3.4, 6.84 ± 2.1, 20.7 ± 5.2 and 9.0 ± 3.2 nm respectively. The catalyst showing largest (Ag/Al2O3) and smallest NPs (Ag/ZrO2) size displayed intermediate catalytic performance under light irradiation conditions
Fig. 1. UV–vis spectra for (a) metal oxide support and (b) Ag deposited on metal oxide support materials. 3
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Fig. 2. TEM micrographs of (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2.
with varied selectivities obtained for different catalysts. Under light irradiation conditions, the kinetics and overall selectivity was found to be significantly enhanced as shown in Fig. 5. It was observed that Ag deposited on TiO2 found to exhibit superior catalytic activity and better selectivity under light irradiation conditions. The order of chemoselectivity was found to be Ag/TiO2 > Ag/ZrO2 > Ag/Al2O3 > Ag/CeO2 under visible light irradiation conditions. A comparison of p-AS selectivity in dark and under light irradiation conditions is shown in Fig. S3. The reaction was optimized by different methods of preparative techniques as shown in Fig. S4. The detailed procedure for the preparation of catalysts is mentioned in the supporting information section. The obtained catalysts were tested for the catalytic activity under light irradiation conditions with filter cutting off the UV region of spectrum in order to study the effect arising completely from plasmonic Ag NPs. The order of selectivities follows the trend; Ag/TiO2 (impregnation: 81) > Ag/TiO2 (NaBH4 reduction: 70) > Ag/TiO2 (microwave: 68) > Ag/TiO2 (H2 reduction:59) > Ag/TiO2 (photo-assisted deposition: 47). We expected that the Ag NPs reduced by the H2 generated insitu during the reaction might be effective in the selective reduction of p-NS. Fig. S5 summarizes the GCeMS spectra before (t = 0 min, inset) and after the reaction (t = 180 min) using Ag/TiO2 (P25, impregnation) catalyst. As can be seen in the spectra, there appears two major peaks (reactant; p-NS and external standard; biphenyl) before the reaction at t = 0 min and three major peaks (byproduct; p-AEB, main product; pAS, external reagent; biphenyl) after the reaction at t = 180 min. Upon monitoring the reaction profile using Ag/TiO2 (microwave), we found the formation of several other coupling products at the retention time of 2.1–2.2 min and hence leading to the decrease in the selectivity of the overall reaction. The obtained GC and MS spectra is shown in Fig. S6. The Ag/TiO2 prepared by impregnation method was employed in the catalytic reduction reaction with a purpose that Ag will be reduced by the H2 generated insitu during the reaction. The photoabsorbtion spectrum before and after the reaction is shown in Fig. S1. It was found out that the plasmonic absorption of Ag was blue shifted by 63 nm in the UV–vis spectrum. Also, a much higher intense peak was observed after the reaction at 453 nm. This can be accountable to the reduced Ag NPs which shows characteristic plasmonic absorption nearly at 400 nm.
Fig. 3. Ag 3d XPS spectra for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2.
Further, the TEM micrographs were measured to observe any morphological changes taking place in the NPs. The uniform spherical morphology with a similar average NPs size of 11.8 ± 3.4 nm and 13.2 ± 2.1 nm before and after the reaction respectively was observed. In all the prepared catalysts, the optimized amount of 1.0 wt % of Ag was deposited on the oxide support materials. The study of chemoselective reaction was also explored and carried out on different types of TiO2 including rutile, anatase and P25 under visible light irradiation. The anatase, rutile and P25 TiO2 showed almost complete conversion of 98, 97 and 100% and similar selectivities of 70, 79 and 81% respectively for a period of 3 h under light irradiation conditions as shown in Fig. S7. Therefore, the crystallinity of the support material had no effect in the reaction activity and selectivity. Amongst the three different TiO2, P25 was used as a support in all reaction studies. The similar activities of the three catalysts can be accounted due to the use of cut-off filter (λ > 420 nm) and hence the light absorption ability arising from plasmonic Ag NPs is monitored. In order to have a detailed insight of the mechanism and absorption of light by plasmonic Ag NPs, we carried out reactions on Ag/TiO2
Table 1 Size of NPs, LSPR peak absorption and reaction results summarized for all catalysts. Sample No.
1 2 3 4
Catalyst
Ag/TiO2 Ag/ZrO2 Ag/Al2O3 Ag/CeO2
Ag NPs size (nm)
11.8 ± 3.4 6.8 ± 2.1 20.7 ± 5.2 9.0 ± 3.2
LSPR λ (nm)
Dark
516 463 430 450
Light
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
100 100 95 100
47 50 54 42
100 100 100 100
81 76 73 60
Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure. 4
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Scheme 2. Reduction of p-nitrostyrene over Ag/MxOy catalysts.
(P25) utilizing LED light of different wavelengths and compared the obtained results in dark and under visible light irradiation conditions (λ > 420 nm). The LED and their corresponding wavelengths employed in the present study are Blue (470 nm), Green (530 nm) and Red (627 nm). All the LED lamps are placed at a particular distance from the reactor with a fixed power of 66.7 mW. The obtained results are shown and compared in Fig. 6 (a). The order of catalytic activity follows the trend Green LED > Red LED > Blue LED > Dark conditions. The selectivities changed significantly upon varying the source of monochromatic LED light. The green LED was found to be the most effective amongst all. Fig. 6 (b) summarizes the action spectrum in the performance increase over Ag/TiO2 catalyst using monochromatic light (λ = 470, 530 and 627 nm). The increasing rate of catalytic performance activities was found to be highly consistent with the LSPR absorption intensity of Ag NPs. This result concludes that LSPR plays an important role in increasing the p-AS selectivity under light irradiation conditions. The conversion amount, selectivity along with the wavelength of light source is also summarized in Table 2. To understand the mechanistic pathway of photocatalytic H2 production from AB and further reduction of p-NS to p-AS under light irradiation condition, effect of charge scavenger addition was studied to investigate the photogenerated electrons in the catalytic reaction. NaHCO3 and oxalic acid, a positive charge scavenger and K2Cr2O7, a negative charge scavenger was added to the suspension of Ag/TiO2 (impregnation) catalyst under visible light irradiation [41–43]. As shown in Fig. S8, the conversion and selectivity was dramatically decreased in the presence of NaHCO3 and oxalic acid. HCO3− can interact
with the photogenerated holes on the surface of Ag NPs under illumination conditions. The addition of oxalic acid, also a positive charge scavenger decrease more significantly the overall conversion and reaction selectivity. These results indicate that the dehydrogenation of AB is activated by the electron deficient site of Ag NPs, which is produced by the charge separation effect derived from LSPR oscillation. The dissociation of B–N bond leads to the generation of NH3δ+ and BH3δ-, where the hydrolysis of BH3 δ- leads to the generation of H2 on the h+ site [44]. No reaction was observed when K2Cr2O7 was added as a negative charge scavenger. This process depicts the importance of electrons and holes involvement in the reaction mechanism. The H2 produced is further used in-situ to carry out the reduction of p-NS to pAS. Under visible light irradiation, the polar nitro group in the reactant p-NS preferably aligns towards the dipole generated on the surface of Ag NPs, whereas the non-polar, C]C groups aligns away from the dipole. This preferable alignment induces the enhanced chemoselectivity of reaction product under light irradiation conditions (Fig. S9). The chemoselectivity in the absence of light can be explained by the conventional catalytic cycle as shown in Fig. S10. The reusability of the catalyst was tested using Ag/TiO2 catalyst. It can be easily recovered by simple filtration and was reused again for the catalytic reaction under visible light irradiation. However, an overall decrease in the catalytic performance was observed with consecutive cycles as shown in Fig. S11. Further structural modifications and investigations are needed to implement the catalyst for practical applications.
Fig. 4. Time profile of p-NS reduction in dark conditions for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2. Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) under inert argon atmosphere at room temperature and atmospheric pressure. 5
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Fig. 5. Time profile of p-NS reduction under light irradiation conditions (λ > 420 nm) for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2. Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) under inert argon atmosphere at room temperature and atmospheric pressure.
Fig. 6. (a) Effect of using different wavelengths of LED (power = 66.7 mW) in the reaction conversion and selectivity utilizing Ag/TiO2 (impregnation), (b) Wavelength dependent performance increase in the p-AS selectivity over Ag/TiO2 catalysts upon irradiation with LED light (blue light λ = 470 nm, green light λ = 530 nm and red light λ = 627 nm). Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
different wavelength of light (green, blue and red) used during the reaction, green LED was found to be most active and highly consistent with the Ag LSPR absorption for p-AS synthesis. The different method of preparation, types of TiO2 and metal content were optimized carefully for the reaction. We anticipate that such reaction results will help in the further tuning of reaction selectivity and enhance the potential of such reactions utilizing solar light irradiation. These plasmonic photocatalysts will assist in opening a new promising avenue in the visible light driven heterogeneous catalysis.
Table 2 Reaction results on Ag/TiO2 (P25) with different wavelength of light sources. Light source
Wavelength, λ (nm)
Conversion (%)
Selectivity (%)
Dark Blue LED Green LED Red LED
N/A 470 530 627
100 86 100 100
47 63 83 71
Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure. LED power (66.7 mW).
Acknowledgements The present work was partially supported by Grants-in-Aid for Scientific Research (Nos. 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 Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with the TEM measurements. YK, KM and HY thank MEXT program “Elements Strategy Initiative to Form Core Research Center.
4. Conclusion In summary, we have prepared and explored a series of Ag based catalysts on metal oxide support materials viz. TiO2, ZrO2, Al2O3 and CeO2 for chemoselective reduction of p-NS to p-AS. The reaction results were compared in dark and under light irradiation conditions. The most active catalyst was found to be Ag/TiO2 depicting an improved chemoselective performance under light irradiation conditions. Amongst 6
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Appendix A. Supplementary data
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Review
Visible-light-driven reduction of nitrostyrene utilizing plasmonic silver nanoparticle catalysts immobilized on oxide supports Priyanka Vermaa, Yasutaka Kuwaharaa,b, Kohsuke Moria,b,c, Hiromi Yamashitaa,b,
⁎
a
Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Unit of Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Kyoto 606-8501, Japan c JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan b
ARTICLE INFO
ABSTRACT
Keywords: Silver nanoparticles Plasmonic catalysis and chemoselective reduction
The localized surface plasmon resonance (LSPR) mediated enhanced chemical activity can be entitled as a promising strategy for efficient solar to chemical energy conversion. To tune the selectivity of a desired product in a chemical reaction is of paramount importance yet a great challenge. In this paper, a new strategy to effectively enhance the selectivity of the product formation under visible light irradiation is reported. A series of Ag catalysts deposited on metal oxide support materials (TiO2, ZrO2, Al2O3 and CeO2) along with their preparative techniques, optimum metal content ratio and effect of different wavelength of light is explored for the chemoselective reduction of p-nitrostyrene to p-aminostyrene under visible light irradiation. The prepared catalysts were characterized by a range of physicochemical techniques including UV–vis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The reduction reaction was carried out in ethanolic suspension at room temperature and pressure utilizing ammonia borane (AB) as an in-situ source of H2. The reaction results displayed 100% conversion with a maximum chemoselectivity of 81% shown by Ag/TiO2 under light irradiation conditions. The high chemoselectivity could be attributed to the preferential alignment of polar nitro group on the surface of plasmonic silver under light irradiation conditions.
1. Introduction Plasmon mediated catalysis has received significant attention in recent years due to the excellent light trapping ability of metal nanostructures [1–8]. Plasmonic photocatalysis involves the localized surface plasmon resonance of metal nanostructures to efficiently harvest the solar energy into chemical energy [9–11]. There have been many reports employing the plasmonic metal nanostructures to carry out catalytic reactions like Suzuki coupling, nitrophenol reduction and hydrogen production from storage materials like ammonia borane and formic acid [12–18]. However, very few reports explore the improved chemoselective performance utilizing the light absorption ability of these nanostructures. Tuning the selectivity of metal catalysts is highly beneficial for the development of industrially important and efficient processes [19–22]. The strong reducing agents like LiAlH4 or NaBH4 which are most commonly employed in organic reactions non-selectively reduces all the functional groups such as carbonyl, carboxyl, vinyl and nitro groups. Also, conventional active metal (platinum-group metal, PGM) catalysts non-selectively reduces all the functional groups.
Hence, protecting group is introduced in the functional group to carry out chemoselective reaction. Further, deprotection is done in order to obtain the desired product which decreases the overall yield of the reaction. Functionalized anilines form an important class of intermediate compounds which has found its application in pharmaceuticals, polymers, herbicides, dyes, pigments and many other useful compounds in fine chemical industries [23–27]. Hence, there is a strong incentive to test, design and develop heterogeneous catalytic system for the reduction of nitro group, in accordance with the 12 principles of the green chemistry. The reduction of nitroaromatics to corresponding anilines is very simple and straightforward process but it is challenging to reduce only –NO2 group in the presence of other reducible functionalities. Till now, the chemoselective reduction of nitrobenzenes is carried out by using excess of Zn, Sn, Fe or NaS2O4 based catalysts which generates large amounts of harmful chemical waste materials and therefore making them environmentally unsustainable. From practical and environmental perspectives, it is highly desirable to develop and design efficient catalysts which can selectively reduce functionalized
⁎ Corresponding author at: Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 5650871, Japan. E-mail address: [email protected] (H. Yamashita).
https://doi.org/10.1016/j.cattod.2019.03.058 Received 29 December 2018; Received in revised form 6 March 2019; Accepted 25 March 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Priyanka Verma, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.058
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nitrobenzene to amino benzenes. Although this process is of commercial importance, still very few reports have been reported developing the catalytic chemoselective reduction process at ambient conditions. It would be more attractive if reducing agents other than gaseous H2 and toxic chemicals can be employed at room temperature and atmospheric pressure conditions. Employing gaseous hydrogen (H2) especially requires high pressure of hydrogen and special apparatus to carry out the process. Another method of electrochemical hydrogenation requires immense energy to supply sufficient voltage to the reaction system. Corma et al. reported that Au/TiO2 catalyzes the selective hydrogenation of nitrostyrene attributing to the cooperative effect between Au and TiO2 [21]. It was proposed that NS is adsorbed on the interface of TiO2 and Au and H2 dissociation takes place on Au. Currently very few catalytic systems have been developed involving Ag or Au NPs to study the chemoselective hydrogenation. The reason for the high selectivities is explained due to the formation of H+/H− pair at the metal-support interface. This pair is active towards the reduction of polar nitro group and is completely inactive in the reduction of non-polar C]C bond. Mitsudome et al. has reported a core-shell Ag-CeO2 nanostructure with high chemoselective performance due to the large number of interfacial sites within the catalytic system [28]. Scheme 1 illustrates the typical product formation observed during the hydrogenation of p-nitrostyrene (i) using AB as an insitu source of hydrogen. The reaction pathway proceeds via two different routes involving preferential attack of the nitro group or the C]C functionality. Preferential reduction of –NO2 group leads to the formation of the desired product i.e. p-aminostyrene (ii), whereas the attack on the C]C group leads to the formation of p-ethyl nitrobenzene (iii). The fully hydrogenated product (p-ethylaniline, iv) would be formed by the complete reduction of both the functionalities [29,30]. In this study, we have reported the photocatalytic reduction of nitrostyrene having reducible functional group at room temperature and atmospheric pressure conditions by using Ag/MxOy (TiO2, ZrO2, Al2O3 and CeO2) catalyst. Silver has not been much explored for selective hydrogenation reactions by plasmonic absorption exploiting visible light irradiation. Knowing the fact that silver exhibits lower hydrogenation ability, it might be helpful in carrying out the chemoselective hydrogenation reaction than active metal catalysts. The catalyst was prepared by simple impregnation method in the inert atmosphere. Nitro group was selectively reduced in a major proportion in comparison to C]C bond. Also, we observed the improved chemoselective performance under visible light irradiation conditions. In many such studies the chemoselective reduction is carried out at harsh chemical conditions involving high temperature and pressure [31,32]. In the present
study, the reaction is carried out in ethanolic suspension at room temperature and atm pressure conditions without any additional agent. Further, the mechanism of improved chemoselective performance under light irradiation attributing to the effect of surface plasmon resonance is discussed in the later section. 2. Experimental section 2.1. Materials P25 TiO2 (JRC-TIO-4, anatase: rutile = 7:3), anatase TiO2 (JRC-TIO3), rutile TiO2 (JRC-TIO-8), were kindly supplied by the Catalysis Society of Japan. 1-hexanol (C6H13OH), acetone, silver nitrate (AgNO3), biphenyl (C6H5C6H5), Zirconium oxide (ZrO2) and ethanol were purchased from Nacalai Tesque Inc. Ammonia borane (NH3BH3; AB) and Cerium (IV) oxide (CeO2) was obtained from Sigma-Aldrich Co. p-nitrostyrene (C8H7NO2) was purchased from Tokyo Chemical Industry Co. Ltd. Aluminium oxide (α-Al2O3) was purchased from Wako Pure Chemical Industries Ltd. All chemicals were used as received without any further purification. 2.2. Preparation of Ag/MxOy catalysts by impregnation The catalyst was prepared by a simple impregnation method. 500 mg of metal oxide support materials, MxOy (TiO2, ZrO2, CeO2 and Al2O3) and sodium laurate (5 mg) was dispersed and ultrasonicated in 100 mL ethanol solution. The mixture was bubbled with argon gas in order to maintain inert atmosphere. Subsequently desired amount of aqueous AgNO3 solution was added into the mixture and stirred continuously for about 6 h. The suspension was evaporated under vacuum and the obtained powder was dried overnight at 80 °C. The obtained sample was named as Ag/MxOy and series of catalysts with varied amount of Ag (0.5, 1.0, 2.0 and 5.0 wt percentage) were prepared. 2.3. Catalytic reaction The chemoselective reduction of p-nitrostyrene (p-NS) to p-aminostyrene (p-AS) reaction is carried out. 20 mM solution of p-NS and 60 mM solution of ammonia borane (AB) in ethanol were separately prepared. 20 mM solution of biphenyl in ethanol was also prepared to be employed as an external standard reagent for calibrating the concentration of reactants and products. 5 mg of catalyst was weighed and dispersed in the sealed reactor tube. 5 mL of p-NS and 1 mL of biphenyl was injected through the rubber septum followed by Ar bubbling for 15 min to ensure the inert atmosphere conditions. Reaction was initiated upon addition of 5 mL of AB solution through the rubber septum in the reactor tube. The mixture was stirred continuously in dark or under light irradiation conditions. For carrying out reaction in dark conditions, the reactor was covered with aluminum foil in order to ensure the complete absence of light during the reaction. For light irradiation conditions, Xenon lamp was used with and without filter cutting off the undesired portion of light irradiation. The time course of reaction products were analyzed by using Shimadzu GC-2010 chromatograph installed with Shimadzu Mass spectrometer GC–MS-QP2010 Plus. The reaction products were analyzed at 0, 5, 15, 30, 60, 90 and 180 min intervals of time. An external fan was used in order to maintain the constant temperature of reaction vessel during the course of the reaction. 2.4. Characterization Shimadzu UV-2450 spectrophotometer was used to collect the reflectance UV–vis spectra of powdered samples. BaSO4 was used as a reference solid and the spectra were collected by employing KubelkaMunk function. Brunauer–Emmett–Teller (BET) surface area measurement was performed by using a BEL-SORP max system (Microtrac BEL)
Scheme 1. Reaction pathway for p-nitrostyrene (p-NS; i) reduction to p-aminostyrene (p-AS; ii) along with byproduct formation p-nitroethylbenzene (pNEB; iii) and p-aminoethylbenzene (p-AEB; iv). 2
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at -196 °C. Degassing of the 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 H-800 TEM 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 (hν = 1253.6 eV) was used as the excitation source. The binding energy of the spectra was calibrated using the C 1 s core level for the contaminant at 284.5 eV. SAN-EI ELECTRIC Super Bright 500, Model XEF-501S Xenon lamp was used as light source to carry out reactions under light irradiation conditions. GC-2010 chromatograph installed with Shimadzu Mass spectrometer GC–MS-QP2010 Plus was used to analyze reaction products employing biphenyl as an external standard reagent.
(Table 1). The correlation between the size of NPs and the chemoselective performance is shown in Fig. S2. The catalyst was found to be stable and could be easily recycled and recovered after the reaction. No leaching of NPs was observed after the reaction. A TEM micrograph of Ag/TiO2 before and after the reaction is shown in Fig. S1 (a). 3.1.3. XPS analysis The prepared Ag based catalysts were analyzed by X-ray photoelectron spectroscopy analysis to study the effect of local chemical environment depending on the different support materials. Fig. 3 (A) display the spectra of Ag NPs deposited on different metal oxides i.e. TiO2, ZrO2, Al2O3 and CeO2 respectively. The core-level 3d spectra splits into a doublet separated by 6.0 eV, confirming the metallic nature of Ag. As per the literature, the standard values of binding energies are 368.2 and 374.2 eV ascribing to 3d5/2 and 3d3/2 core levels of Ag [37]. We observed doublet peaks at binding energies of (367.5, 373.5), (367.7, 373.7), (367.7, 373.7) and (367.4, 373.4) for Ag/TiO2, Ag/ ZrO2, Ag/Al2O3 and Ag/CeO2, respectively. These values are plotted after calibrating the obtained data with respect to C 1 s spectral peak at 284.8 eV. We expect that all Ag NPs are deposited and are present in zero oxidation state as further confirmed by FT-EXAFS analysis. The higher electronegativity values of Ag (1.93) than metal in oxides affirms the reduced state of Ag NPs. However, the difference in the observed shift values can be explained due to the different electronegativity values of Ti (1.54), Zr (1.33), Al (1.61) and Ce (1.12). The lower B.E. values of Ag/CeO2 amongst others accounts for the least electronegative character of Ce and hence keeping the Ag in most reduced state than others.
3. Results and discussion 3.1. Characterization results 3.1.1. UV–vis absorption measurement Till date, various strategies have been developed to extend the light absorption of photocatalysts from UV to visible light regime. In one such strategy, UV-active photocatalysts are being hybridized with plasmonic metal nanostructures, which are capable of showing strong photoabsorbtion arising from the localized surface plasmon (LSPR) effect [33–36]. In the present study, the light absorption ability of the prepared catalysts was evaluated by measuring their UV–vis spectra. As shown in Fig. 1 (a), the optical absorption of metal oxide support lies in the UV region with a peak maximum at ˜ 270 nm for TiO2 and CeO2. ZrO2 displays less intense peak in the region of 200–250 nm while Al2O3 displays no absorption in the spectrum. Fig. 1 (b) displays the absorption spectrum of Ag deposited on metal oxides support materials. All samples exhibited the strong intrinsic plasmon peak in the visible region at around 400 nm, attributing to the LSPR effect of Ag NPs. A single broad peak absorption suggests the spherical morphology of the prepared NPs which was further confirmed by TEM. The LSPR peak maximum was observed at 516, 450, 463, 430 nm for Ag/TiO2, Ag/ CeO2, Ag/ZrO2 and Ag/Al2O3 respectively. The Ag/TiO2 absorption peak was found to be significantly red shifted in comparison to other catalysts. However, an absorption maximum peak at 416 nm was observed after the reaction, as discussed in the later section (Fig. S1).
3.2. Catalytic reaction The catalytic activity of the prepared catalysts was tested in the chemoselective reduction of p-NS to p-AS utilizing AB as an insitu source of H2 as shown in Scheme 2. The reaction was carried out for 3 h and its catalytic activity was measured in dark and under light irradiation conditions. In our previous reports, we have explored this reaction for the size and morphology controlled Ag NPs supported on mesoporous silica and Ag NPs deposited on ceria modified silica [38–40]. In the present study, several Ag catalysts were synthesized using commercially available inorganic support materials (Al2O3, ZrO2, CeO2 and TiO2) by various preparative techniques including MW, impregnation followed by reduction and photo-assisted deposition. The chemoselective reduction of p-nitrostyrene was conducted in dark and under light irradiation conditions, as summarized in Table 1. We obtained p-AS as the major product of the reaction with selectivity of the major product varying from 47 to 81% in dark and under light irradiation conditions. Fig. 4 displays the results obtained on the prepared catalysts for 3 h in dark conditions. All the catalysts showed almost complete conversion
3.1.2. TEM measurement Fig. 2 shows the TEM micrographs along with the pertinent histograms of the prepared Ag catalysts deposited on oxide support materials. All the NPs were found to be spherical on different metal oxide support materials. The average particle size of Ag/TiO2, Ag/ZrO2, Ag/ Al2O3 and Ag/CeO2 was found out to be 11.8 ± 3.4, 6.84 ± 2.1, 20.7 ± 5.2 and 9.0 ± 3.2 nm respectively. The catalyst showing largest (Ag/Al2O3) and smallest NPs (Ag/ZrO2) size displayed intermediate catalytic performance under light irradiation conditions
Fig. 1. UV–vis spectra for (a) metal oxide support and (b) Ag deposited on metal oxide support materials. 3
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Fig. 2. TEM micrographs of (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2.
with varied selectivities obtained for different catalysts. Under light irradiation conditions, the kinetics and overall selectivity was found to be significantly enhanced as shown in Fig. 5. It was observed that Ag deposited on TiO2 found to exhibit superior catalytic activity and better selectivity under light irradiation conditions. The order of chemoselectivity was found to be Ag/TiO2 > Ag/ZrO2 > Ag/Al2O3 > Ag/CeO2 under visible light irradiation conditions. A comparison of p-AS selectivity in dark and under light irradiation conditions is shown in Fig. S3. The reaction was optimized by different methods of preparative techniques as shown in Fig. S4. The detailed procedure for the preparation of catalysts is mentioned in the supporting information section. The obtained catalysts were tested for the catalytic activity under light irradiation conditions with filter cutting off the UV region of spectrum in order to study the effect arising completely from plasmonic Ag NPs. The order of selectivities follows the trend; Ag/TiO2 (impregnation: 81) > Ag/TiO2 (NaBH4 reduction: 70) > Ag/TiO2 (microwave: 68) > Ag/TiO2 (H2 reduction:59) > Ag/TiO2 (photo-assisted deposition: 47). We expected that the Ag NPs reduced by the H2 generated insitu during the reaction might be effective in the selective reduction of p-NS. Fig. S5 summarizes the GCeMS spectra before (t = 0 min, inset) and after the reaction (t = 180 min) using Ag/TiO2 (P25, impregnation) catalyst. As can be seen in the spectra, there appears two major peaks (reactant; p-NS and external standard; biphenyl) before the reaction at t = 0 min and three major peaks (byproduct; p-AEB, main product; pAS, external reagent; biphenyl) after the reaction at t = 180 min. Upon monitoring the reaction profile using Ag/TiO2 (microwave), we found the formation of several other coupling products at the retention time of 2.1–2.2 min and hence leading to the decrease in the selectivity of the overall reaction. The obtained GC and MS spectra is shown in Fig. S6. The Ag/TiO2 prepared by impregnation method was employed in the catalytic reduction reaction with a purpose that Ag will be reduced by the H2 generated insitu during the reaction. The photoabsorbtion spectrum before and after the reaction is shown in Fig. S1. It was found out that the plasmonic absorption of Ag was blue shifted by 63 nm in the UV–vis spectrum. Also, a much higher intense peak was observed after the reaction at 453 nm. This can be accountable to the reduced Ag NPs which shows characteristic plasmonic absorption nearly at 400 nm.
Fig. 3. Ag 3d XPS spectra for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2.
Further, the TEM micrographs were measured to observe any morphological changes taking place in the NPs. The uniform spherical morphology with a similar average NPs size of 11.8 ± 3.4 nm and 13.2 ± 2.1 nm before and after the reaction respectively was observed. In all the prepared catalysts, the optimized amount of 1.0 wt % of Ag was deposited on the oxide support materials. The study of chemoselective reaction was also explored and carried out on different types of TiO2 including rutile, anatase and P25 under visible light irradiation. The anatase, rutile and P25 TiO2 showed almost complete conversion of 98, 97 and 100% and similar selectivities of 70, 79 and 81% respectively for a period of 3 h under light irradiation conditions as shown in Fig. S7. Therefore, the crystallinity of the support material had no effect in the reaction activity and selectivity. Amongst the three different TiO2, P25 was used as a support in all reaction studies. The similar activities of the three catalysts can be accounted due to the use of cut-off filter (λ > 420 nm) and hence the light absorption ability arising from plasmonic Ag NPs is monitored. In order to have a detailed insight of the mechanism and absorption of light by plasmonic Ag NPs, we carried out reactions on Ag/TiO2
Table 1 Size of NPs, LSPR peak absorption and reaction results summarized for all catalysts. Sample No.
1 2 3 4
Catalyst
Ag/TiO2 Ag/ZrO2 Ag/Al2O3 Ag/CeO2
Ag NPs size (nm)
11.8 ± 3.4 6.8 ± 2.1 20.7 ± 5.2 9.0 ± 3.2
LSPR λ (nm)
Dark
516 463 430 450
Light
Conversion (%)
Selectivity (%)
Conversion (%)
Selectivity (%)
100 100 95 100
47 50 54 42
100 100 100 100
81 76 73 60
Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure. 4
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Scheme 2. Reduction of p-nitrostyrene over Ag/MxOy catalysts.
(P25) utilizing LED light of different wavelengths and compared the obtained results in dark and under visible light irradiation conditions (λ > 420 nm). The LED and their corresponding wavelengths employed in the present study are Blue (470 nm), Green (530 nm) and Red (627 nm). All the LED lamps are placed at a particular distance from the reactor with a fixed power of 66.7 mW. The obtained results are shown and compared in Fig. 6 (a). The order of catalytic activity follows the trend Green LED > Red LED > Blue LED > Dark conditions. The selectivities changed significantly upon varying the source of monochromatic LED light. The green LED was found to be the most effective amongst all. Fig. 6 (b) summarizes the action spectrum in the performance increase over Ag/TiO2 catalyst using monochromatic light (λ = 470, 530 and 627 nm). The increasing rate of catalytic performance activities was found to be highly consistent with the LSPR absorption intensity of Ag NPs. This result concludes that LSPR plays an important role in increasing the p-AS selectivity under light irradiation conditions. The conversion amount, selectivity along with the wavelength of light source is also summarized in Table 2. To understand the mechanistic pathway of photocatalytic H2 production from AB and further reduction of p-NS to p-AS under light irradiation condition, effect of charge scavenger addition was studied to investigate the photogenerated electrons in the catalytic reaction. NaHCO3 and oxalic acid, a positive charge scavenger and K2Cr2O7, a negative charge scavenger was added to the suspension of Ag/TiO2 (impregnation) catalyst under visible light irradiation [41–43]. As shown in Fig. S8, the conversion and selectivity was dramatically decreased in the presence of NaHCO3 and oxalic acid. HCO3− can interact
with the photogenerated holes on the surface of Ag NPs under illumination conditions. The addition of oxalic acid, also a positive charge scavenger decrease more significantly the overall conversion and reaction selectivity. These results indicate that the dehydrogenation of AB is activated by the electron deficient site of Ag NPs, which is produced by the charge separation effect derived from LSPR oscillation. The dissociation of B–N bond leads to the generation of NH3δ+ and BH3δ-, where the hydrolysis of BH3 δ- leads to the generation of H2 on the h+ site [44]. No reaction was observed when K2Cr2O7 was added as a negative charge scavenger. This process depicts the importance of electrons and holes involvement in the reaction mechanism. The H2 produced is further used in-situ to carry out the reduction of p-NS to pAS. Under visible light irradiation, the polar nitro group in the reactant p-NS preferably aligns towards the dipole generated on the surface of Ag NPs, whereas the non-polar, C]C groups aligns away from the dipole. This preferable alignment induces the enhanced chemoselectivity of reaction product under light irradiation conditions (Fig. S9). The chemoselectivity in the absence of light can be explained by the conventional catalytic cycle as shown in Fig. S10. The reusability of the catalyst was tested using Ag/TiO2 catalyst. It can be easily recovered by simple filtration and was reused again for the catalytic reaction under visible light irradiation. However, an overall decrease in the catalytic performance was observed with consecutive cycles as shown in Fig. S11. Further structural modifications and investigations are needed to implement the catalyst for practical applications.
Fig. 4. Time profile of p-NS reduction in dark conditions for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2. Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) under inert argon atmosphere at room temperature and atmospheric pressure. 5
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Fig. 5. Time profile of p-NS reduction under light irradiation conditions (λ > 420 nm) for (a) Ag/TiO2, (b) Ag/ZrO2, (c) Ag/Al2O3 and (d) Ag/CeO2. Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) under inert argon atmosphere at room temperature and atmospheric pressure.
Fig. 6. (a) Effect of using different wavelengths of LED (power = 66.7 mW) in the reaction conversion and selectivity utilizing Ag/TiO2 (impregnation), (b) Wavelength dependent performance increase in the p-AS selectivity over Ag/TiO2 catalysts upon irradiation with LED light (blue light λ = 470 nm, green light λ = 530 nm and red light λ = 627 nm). Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
different wavelength of light (green, blue and red) used during the reaction, green LED was found to be most active and highly consistent with the Ag LSPR absorption for p-AS synthesis. The different method of preparation, types of TiO2 and metal content were optimized carefully for the reaction. We anticipate that such reaction results will help in the further tuning of reaction selectivity and enhance the potential of such reactions utilizing solar light irradiation. These plasmonic photocatalysts will assist in opening a new promising avenue in the visible light driven heterogeneous catalysis.
Table 2 Reaction results on Ag/TiO2 (P25) with different wavelength of light sources. Light source
Wavelength, λ (nm)
Conversion (%)
Selectivity (%)
Dark Blue LED Green LED Red LED
N/A 470 530 627
100 86 100 100
47 63 83 71
Reaction conditions: catalyst (5 mg), p-NS (20 mM), biphenyl (20 mM), AB (60 mM) in ethanol (11 mL) for 3 h under inert argon atmosphere at room temperature and atmospheric pressure. LED power (66.7 mW).
Acknowledgements The present work was partially supported by Grants-in-Aid for Scientific Research (Nos. 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 Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with the TEM measurements. YK, KM and HY thank MEXT program “Elements Strategy Initiative to Form Core Research Center.
4. Conclusion In summary, we have prepared and explored a series of Ag based catalysts on metal oxide support materials viz. TiO2, ZrO2, Al2O3 and CeO2 for chemoselective reduction of p-NS to p-AS. The reaction results were compared in dark and under light irradiation conditions. The most active catalyst was found to be Ag/TiO2 depicting an improved chemoselective performance under light irradiation conditions. Amongst 6
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Appendix A. Supplementary data
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