Journal of Colloid and Interface Science 441 (2015) 25–29
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Metal dependent catalytic hydrogenation of nitroarenes over water-soluble glutathione capped metal nanoparticles Sachil Sharma ⇑ Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
a r t i c l e
i n f o
Article history: Received 11 September 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Water soluble nanoparticles Glutathione Metal–substrate interaction Hydrogen adsorption Metal-dependence Catalytic hydrogenation Less densely packed branched thiol
a b s t r a c t The water soluble glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) with size 2.4 ± 0.2 nm were synthesized by borohydride reduction of metal ions in the presence of glutathione as capping ligand and used as catalyst for the hydrogenation of nitroaniline in aqueous phase. The rate of catalytic hydrogenation was dependent on metal type and the trend of catalytic activity over these M-GS nanoparticles was found to be Pd-GS (kapp = 0.0227 (±3 10 4)) s 1 Pt-GS (±1 10 4)) s 1 > Au-GS (kapp = 0.0015 (±0.2 10 4)) s 1 > Ag-GS (kapp = 0.0008 (kapp = 0.0043 (±0.2 10 4)) s 1. The similar trend of catalytic activity was found for the hydrogenation of nitrobenzene. Our experimental results, along taking into account the theoretical calculations done by other research groups, suggest that the observed catalytic activity trend is attributed to the ‘‘different rates of H2 molecule adsorption and dissociation’’ on the M-GS nanoparticles. The ‘‘high rate of H2 molecule adsorption’’ and ‘‘highly oxidized surface’’ make Pd-GS nanoparticles an ideal candidate for the rapid hydrogenation. On the basis of our experimental results, we proposed that small gaps between less densely packed branched thiol ‘‘glutathione molecules’’ provide the access to metal nanoparticle surface for the hydrogenation reaction. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Noble metal nanoparticles have drawn immense research interest owing to their importance in catalysis [1] , sensing [2], surfaceenhanced Raman scattering (SERS) [3] and biomedical applications [4]. The large surface area of ultrafine noble metal nanoparticles makes them an ideal candidate for catalyzing various reactions. The hydrogenation of nitroarenes to aniline derivatives catalyzed by noble metal nanoparticles is an important reaction from industrial, synthetic and environmental point of view. The functionalized aniline derivatives are important intermediates for pharmaceutical polymers, dyes, pigments and other fine agrochemicals [5–9]. The nitroarenes are also found as contaminants in industrial waste water and hence, hazardous to the environment. Therefore, it becomes necessary to reduce them to industrial chemicals and intermediates in order to clean the environment. The hydrogenation of nitroarenes to corresponding amino benzenes catalyzed by nanoparticles immobilized on oxide supports such as AuATiO2/Fe2O3/MgO [10,11], AgATiO2 [12] and AgArGO [13] has been studied vastly in the literature. However, the hydrogenation of nitroarenes over ligand capped fine metal ⇑ Present address: Department of Applied Chemistry, Tokyo University of Science (TUS), Shinjuku, Tokyo 162-8601, Japan. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.jcis.2014.11.030 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
nanoclusters and nanoparticles are scantily reported. Recently, ligand capped novel metal clusters [14,15] have been explored as catalyst for a number of important reactions. Very recently, Fenger and co-workers [16] studied size dependent catalysis of Au nanoparticles, respectively for the hydrogenation of 4-nitrophenol. However, most of the literature on the catalytic hydrogenation of nitroarenes is still limited to Au nanoparticles and to the best of our knowledge, there is no report yet on the metal dependent catalysis by ligand capped metal nanoparticles. Thus, it becomes important to gain better understanding of metal dependence catalysis by ligand stabilized metal nanoparticles. In present work, firstly, glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) of same size were synthesized and then, their catalytic activity toward the hydrogenation of nitroarenes was compared. Herein, we report the metal dependent catalytic activity of water soluble glutathione (GSH) capped Pd, Pt, Au and Ag nanoparticles for the hydrogenation of nitroarenes. The trend of catalytic activity of the glutathione capped metal (M-GS) nanoparticles was found to be following: Pd-GS PtGS > Au-GS > Ag-GS. Our experimental results suggest that the observed catalytic activity trend is attributed to different rates of H2 molecule adsorption and dissociation. However, high rate of H2 adsorption along with high amount of surface oxidation on Pd-GS nanoparticles make them ideal catalyst for rapid hydrogenation of nitroarenes.
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2. Materials and methods 2.1. Materials L-glutathione reduced (GSH, P98%, Aldrich), Sodium borohydride (99%, Aldrich), Palladium(II) chloride (PdCl2, 99.999%, Aldrich), Chloroplatinic acid hexahydrate (H2PtCl66H2O, ACS reagent), Hydrogen tetrachloroaurate trihydrate (HAuCl43H2O, ACS reagent), Silver Nitrate (AgNO3, ACS reagent P99.0%, Aldrich), 2-nitro aniline (>99%, Tokyo Chemical Industry), Nitrobenzene (P99%, Aldrich), were used as received. Water was purified using a Millipore MilliQ system (18.2 MX cm). Firstly, various M-GS nanoparticles with same size were synthesized and further used as catalyst for hydrogenation of nitroarenes. All synthesis and catalysis experiments were performed at room temperature and under air atmosphere. Transmission electron microscopy (TEM) images were obtained using JEOL JEM-2010 high contrast electron microscope operating at 200 kV. XPS measurements were carried out on an XPS system (VG ESCALAB 220i-XL) using monochromatic Al Ka X-ray source (1486.6 eV). Binding energies (BE) were referenced to the C 1s BE at 284.8 eV.
2.2. Synthesis of glutathione capped metal nanoparticles The water soluble Pd, Pt, Au and Ag nanoparticles with same size 2.4 ± 0.2 nm were prepared by borohydride reduction of their respective metal salts in presence of glutathione as capping ligand. The glutathione capped Pd nanoparticles were synthesized using the procedure reported in our previous work [17]. However, glutathione capped Pt, Au and Ag nanoparticles were synthesized according to literature procedures [4,18,19] with the slight modification. The experimental details are described in the supplementary material. 2.3. Catalytic experiment procedure In a typical catalytic reaction, 0.13 mL (130 ll) of aqueous solution of 5 mM nitroarene (2-nitro aniline and nitrobenzene) was first mixed with 2.5 mL aqueous solution of 0.3575 mg/L catalyst (2.4 nm M-GS nanoparticles) in a UV–Vis quartz cuvette while kept on stirring. Then, 0.87 mL (870 ll) of aqueous solution of 21 mM NaBH4 was added into the above mixture while kept on stirring. Immediately following this, in situ absorption spectral changes were recorded till the completion of catalytic hydrogenation with a USB-4000 fiber optic spectrometer (Ocean Optics) equipped with magnetic stirrer. The base line correction was performed using water as solvent before taking absorbance measurements. The UV–Vis spectra of reaction products were found to be in agreement with UV–Vis spectra of the authentic compounds. 3. Results and discussion Fig. 1 shows the characteristic UV–Vis spectrum of 2.4 nm glutathione capped Pd, Pt, Au and Ag nanoparticles. Their bright field TEM images show the narrow size distribution 2.4 ± 0.2 nm (Fig. S1 in supplementary material). Here it is important to note that the glutathione capped metal (M-GS, M = Pd, Pt, Au and Ag) nanoparticles with same size (2.4 ± 0.2 nm) were used as catalyst for the hydrogenation reaction to exclude the size effect. To test the metal dependent catalytic activity of 2.4 nm M-GS (M = Pd, Pt, Au, and Ag) nanoparticles toward the aqueous phase hydrogenation of nitroarenes, we chose the 2-nitroaniline as a model substrate. Fig. 2a shows the time dependent UV–Vis spectral changes during the catalytic reduction of 2-nitroaniline over M-GS
Fig. 1. Characteristic UV–Vis spectrum of 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles.
nanoparticles. After certain time, the peak at 412 nm was found to disappear completely, indicating the complete reduction of 2-nitroaniline. The completion of hydrogenation of 2-nitroaniline over M-GS nanoparticles was also noted manually by observing the complete fading of yellow color of 2-nitroaniline after certain time. The completion of catalytic hydrogenation reaction was confirmed by recording positive ion ESI-MS spectra of starting nitroarenes and crude final product. As shown in Fig. S2 (supplementary material), the intense peak at m/z = 139.04 for 2-nitroaniline was found to completely absent in the positive ion ESI-MS spectrum of crude final product. Whereas, the presence of most intense peak at m/z = 109.06 in the positive ion ESI-MS spectrum of crude final product indicates the formation of 2-phenyldiamine. The similar ESI-MS spectra were obtained after the complete hydrogenation of 2-nitroaniline over all M-GS (M = Pd, Pt, Au and Ag) nanoparticles. The plot of Ct/C0 versus t (s) shows the exponential decrease of Ct/C0 of 2-nitroaniline with time over various M-GS nanoparticles, which was plotted by taking into account the spectral changes at 412 nm (Fig. S3 in supplementary material). The time for 95% completion of the hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles was observed to be 1.7, 10, 23 and 47 min, respectively. Fig. 2(b) shows the kinetic pseudo-first order rate plots of the reduction of 2-nitroaniline over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. The values of slope (defined as apparent rate constants, kapp) were calculated to be 0.0227 (±3 10 4), 0.0043 (±1 10 4), 0.0015 (±0.2 10 4), and 0.0008 (±0.2 10 4) (s 1), respectively for the hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles. Thus, the order of catalytic activity of 2.4 nm M-GS nanoparticles was as follows: Pd-GS Pt-GS > AuGS > Ag-GS for the hydrogenation of 2-nitroaniline. Similar trend in catalytic activity was also found for the hydrogenation of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles (vide infra; Fig. 3). Although, induction time was not observed for the catalytic hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS nanoparticles (Fig. S3 in supplementary material). However, the induction time of 1.5, 3, and 7 min was observed for the catalytic hydrogenation over 2.4 nm Pt-GS, Au-GS and Ag-GS nanoparticles, respectively. Similar trend in the induction period was also observed for the catalytic hydrogenation of nitrobenzene (Fig. S7 in supplementary information). The induction time may be attributed: to (1) diffusion controlled adsorption of reactants on the metal nanoparticle surface, (2) the presence of dissolved oxygen in the reaction medium, and (3) metal surface restructuring induced by adsorbed substrate [15]. To check the effect of dissolved oxygen, we performed the catalytic hydrogenation reaction of 4-nitroaniline over 2.4 nm Au-GS nanoparticles after purging the reaction solutions by
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Fig. 2. (a) Time dependent UV–Vis absorption spectral changes and (b) Plots of ln (Ct/C0) versus t (s) with pseudo first order rate fitting for the reduction of 2-nitroaniline over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. Error limits are given in parentheses.
Fig. 3. (a) Time dependent UV–Vis absorption spectral changes and (b) Plots of ln (Ct/C0) versus t (s) (with pseudo first order rate fits) taking into account the spectral changes at 228 nm for the reduction of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K.
argon gas for 20 min. It was worth to note that no induction time was noticed for catalytic hydrogenation of 2-nitroaniline over 2.4 nm Au-GS nanoparticles under the inert gas atmosphere as shown in Fig. S4 (supplementary material). It indicates that induction time in our study is due to the dissolved oxygen in the reaction medium. In other words, the adsorption of nitroarenes on the surface of 2.4 nm M-GS nanoparticles is not a diffusion controlled process. Recently, Tamura and co-workers [20] reported the following order of catalytic activity: Pd/SiO2 > Pt/SiO2 > Rh/SiO2. . .. . .. . . > Co/ SiO2 > Ag/SiO2 for SiO2 supported uncapped metal nanoparticles (M-SiO2, M = Ag, Pd, Pt, Rh. . .. . .Co) toward the liquid phase hydrogenation of nitrobenzene in THF under 3.0 MPa H2 gas at 100 °C. In our work, in spite of capping with thick layer of glutathione, 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles, still show the same trend of catalytic activity (Pd-GS Pt-GS > Au-GS > AgGS) as reported for SiO2 supported uncapped metal nanoparticles [20]. We proposed a model (Vide Infra: Fig. 5) to explain the catalytic active behavior of M-GS NPs, keeping in view of highly branched nature of glutathione molecule. Further, it is also interesting to note that compared to Pt-GS, Au-GS and Ag-GS nanoparticles, Pd-GS nanoparticles were found to be highly efficient catalyst for the rapid hydrogenation of 2-nitroaniline. To gain insight into the observed trend of catalytic activity: Pd-GS Pt-GS > Au-GS > Ag-GS of MGS nanoparticles and high catalytic efficiency of Pd-GS nanoparticles for the rapid hydrogenation of nitroarenes, we need to look into the involved mechanism. Generally, the catalytic hydrogenation of nitroarenes occurs by hydrogen transfer from metal-hydride complex on metal surface to the nitrobenzene [11]. The BH4 ion (Na+ BH4 in aqueous medium)
reacts on the surface of metal nanoparticles to generate H2. The H2 molecule is known to dissociate rapidly on early and late transition metals to form MAH bonds on their surface [21]. The bulk Pd metal is well known for adsorption of molecular hydrogen on its surface to form metal hydrides [22]. Corma and co-workers [23] reported that the H2 molecule adsorption on Au and Pt is dissociative in nature; however, no activation barriers are involved in case of dissociation of H2 on Pt as compared to Au. Although, the bulk Ag metal is not known to adsorb the molecular H2 on their surfaces; however there is possibility for ultrafine nanoparticles of Ag to form MAH bonds on their surfaces, because of low coordination of surface atoms. Extensive theoretical and experimental research work done by Corma et al. [23–25] suggests that the dissociation of H2 molecule on the metal surface atoms is rate controlling step for determining the activity of catalyst toward the hydrogenation of nitroarenes. Furthermore, according to the ‘‘Sabatier principle’’, there must be optimum interactions between reactants and catalyst to obtain high catalytic activity [20]. In this regard, Toulhoat [21] calculated the metal–hydrogen (MAH) bond energy for three transition metal series based on density functional theory and their calculations suggest that the PdAH and PtAH bonds (PdAH > PtAH) have strong bond energy compared to that of AuAH and AgAH. In fact, Cardenas-Lizana and co-workers [26] observed three fold increase in catalytic activity on inclusion of Pd to Au/Al2O3 toward the hydrogenation of p-chloronitrobenzene. Similarly, Corma and co-workers [24] observed the significant increase in catalytic activity on inclusion of Pt to Au/TiO2 toward the hydrogenation of nitrobenzene. Therefore, taking into account the hydrogen adsorption on Pd, Pt, Au and Ag metal surfaces, we can conclude that the observed catalytic trend in our study is attributed to different rate of H2 molecule
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Fig. 4. (a) Pd 3d, (b) Pt 4f, (c) Au 4f and (d) Ag 3d XPS spectra of 2.4 nm glutathione capped Pd, Pt, Au and Ag nanoparticles, respectively.
adsorption and dissociation on the surface of glutathione capped M-GS (M = Pd, Pt, Au and Ag) nanoparticles. However, ‘‘interaction of model substrates such as nitroarenes on the surfaces of metal nanoparticles’’ can also significantly influence the rate of hydrogenation. Hence to gain further insight, we looked into such possible interactions, by (1) changing the model substrate without any ANH2 functionality and (2) checking the surface oxidation of M-GS nanoparticles by XPS (X-ray photoelectron spectroscopy), as following. Firstly, the nitroaniline molecules can interact with surface of noble metal nanoparticles through ANH2 group. On basis of the hard and soft acids bases (HSAB) concept [27], Pd (hardness parameter, g = 3.8) metal is hardest acid, while Ag (g = 3.1) metal is softest acid. Hence, the hard base such as ANH2 group is expected to interact efficiently with Pd than Pt, Au and Ag. This factor can also affect the rate of the catalytic hydrogenation. Hence, to check the effect of interaction strength between nitroarene molecule and nanoparticle surface on the rate of catalytic hydrogenation, we performed the same catalysis experiment using a model substrate without any ANH2 functionality such as nitrobenzene. Fig. 3(a) shows the time dependent UV–Vis spectral changes during the catalytic hydrogenation of nitrobenzene over M-GS nanoparticles. Upon the addition of NaBH4 to the mixture of catalyst and nitrobenzene, the characteristic peak of nitrobenzene at 268 nm was disappeared. Instead, the two characteristic peaks of aniline were appeared at 228 and 278 nm. The absorption intensity of the peak centered at 228 nm was found to increase with time. Whereas, the absorption intensity of the peak centered at 278 nm was found to decrease with time. However, as shown in Fig. 3. a, the broad peak centered at 278 nm was found to associate with many shoulder peaks. These shoulder peaks correspond to various unstable reaction intermediates (Figs. S5 and S6 in supplementary material) formed during the course of catalytic hydrogenation of nitrobenzene over M-GS nanoparticles. The presence of the reaction intermediates also indicates their week interaction on the surface of M-GS nanoparticles [28]. These shoulder peaks were found to disappear completely on the completion of catalytic hydrogenation of nitrobenzene. Due to the absence of any associated shoulder peak,
the spectral changes of peak centered at 228 nm (Fig. 3a) were monitored to plot the Ct/C0 versus t (s) (Fig. S7 in supplementary material). The increase in the intensity of peak at 228 nm with time (s) indicates the increase of aniline concentration with time. The time for 95% completion of the catalytic hydrogenation of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles was observed to be 7, 13, 32 and 38 min, respectively. Fig. 3(b) shows the kinetic pseudo-first order rate plots for the reduction of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. The values of apparent rate constant (kapp) were calculated to be 0.013 (±2 10 3), 0.0036 (±0.9 10 3), 0.0013 (±0.1 10 3) and 0.0015 (±0.1 10 3) (s 1), respectively for the hydrogenation of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles. The lesser completion time for the catalytic hydrogenation of 2-nitroaniline molecule compared to that of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, and Au-GS nanoparticles indicates that along with the H2 adsorption and dissociation, the interaction of nitroaniline molecules on the surface of M-GS nanoparticles through ANH2 functionality (HSAB principle) further fastens the catalytic hydrogenation. However, the trend of catalytic activity of M-GS nanoparticles for the hydrogenation of nitrobenzene: Pd-GS > Pt-GS > AuGS > Ag-GS was still found to be same as that for the hydrogenation of 2-nitroaniline. It indicates that the observed catalytic activity trend among various M-GS nanoparticles toward the hydrogenation of nitroarenes is due to different rates of H2 adsorption and dissociation on the M-GS (M = Pd, Pt, Au and Ag) nanoparticles surface. Secondly, the oxidized metal surface has large number of hydroxyl groups. Thus, the surface oxidation can also increase the interaction of metal surface with model substrate (nitroarenes) through hydrogen bonding in aqueous medium. As revealed by XPS spectra (Fig. 4) of glutathione capped M-GS (M = Pd, Pt, Au and Ag) nanoparticles, 2.4 nm Pd-GS NPs were found to have highly oxidized surface (64%). Whereas, 2.4 nm Pt-GS and Au-GS nanoparticles were found to have relatively less surface oxidation (22% and 20% respectively). The 2.4 nm Ag-GS nanoparticles were found to have unoxidized surface. The respective metal/metal oxide binding energy values have been summarized in Table S-1 (supplementary material). Here it is interesting to be noted that
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and can be modeled similar to Langmuir Hinshelwood mechanistic model [11,23,32,33]. 4. Conclusions In summary, we first prepared 2.4 nm water soluble glutathione capped Pd, Pt, Au and Ag NPs and observed following order of their catalytic activity: Pd-GS Pt-GS > Au-GS > Ag-GS toward the hydrogenation of nitroarenes. The observed catalytic activity trend was found to be completely metal dependent and is attributed to different rates of H2 adsorption/dissociation on 2.4 nm M-GS nanoparticles. The ‘‘high rate of H2 adsorption/dissociation’’ and ‘‘high oxidation’’ on the surface of Pd-GS nanoparticles make them an ideal candidate for the rapid hydrogenation of nitroarenes. We propose that small gaps between less densely packed branched thiols such as glutathione on the metal nanoparticle surface is responsible for the display of catalytic activity. Acknowledgment Fig. 5. Proposed scheme for hydrogenation of nitroarene over surface of 2.4 nm MGS NPs.
in spite of having same surface oxidation (20%) on 2.4 nm Pt-GS and Au-GS nanoparticles, they were not found to have same catalytic activity for hydrogenation. The 2.4 nm Pt-GS nanoparticles displayed superior catalytic activity than 2.4 nm Au-GS nanoparticles. It is because of the fact that rate of H2 adsorption/dissociation is high for 2.4 nm Pt-GS compared to Au-GS NPs. The high surface oxidation on Pd-GS nanoparticles probably increases the interaction of model substrate on the surface. Hence, in addition to high rate of H2 molecular adsorption and dissociation, the high amount of surface oxidation on the Pd-GS nanoparticles make them an ideal candidate for rapid hydrogenation. Recently, Suzuki and coworkers [29] also reported superior catalytic activity of porous PdO surface compared to porous Pd surface for the rapid hydrogenation of p-nitrophenol. However, the reason behind catalytic activity of M-GS nanoparticles despite having thick layer of glutathione on their surface still needs to be addressed. Moreover, these M-GS nanoparticles were found to have same tendency to catalyze the hydrogenation as reported by Tamura and coworkers [20] for SiO2 supported ligand free metal nanoparticles. Atomically precise Au25(GS)18 nanoclusters have already been proven to have high catalytic efficiency for the rapid hydrogenation of nitroarenes. Kawasaki and co-workers [15] suggested that the high catalytic activity of Au25(GS)18 nanoclusters toward the hydrogenation is basically attributed to unhindered active sites because of its unique core–shell structure. Hutchings [30] and Katz and co-workers [31] also reported that the large capping molecules pack on the surface of small metal clusters in such a way that the large gaps are left between them. The smaller reactant molecules use these gaps to access the metal surface during the catalytic reaction. Eklund and Cliffel [18] studied the catalytic activity for various thiolate protected Pt nanoparticles (including glutathione) toward the hydrogenation of C@C double bond. They found that the packing density of branched thiols such as glutathione and tiopronin on the surface of metal nanoparticles is less than that of straight chain thiols. As a result of less dense packing, the branched ligands leave more exposed metal atoms on metal surface, which render the catalytic activity to these metal nanoparticles. Based on experimental results, we propose that due to less dense packing of branched glutathione ligand on the surface of 2.4 nm metal nanoparticles; space between glutathione molecules are available for small molecules such as H2 and nitroaniline to access the metal surface. The proposed scheme is shown in Fig. 5
Author thanks Dr. D. Lee, Yonsei University, S. Korea for UV–Vis instrumental facility. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.11.030. References [1] M. Zahmakıran, S. Özkar, Nanoscale 3 (2011) 3462–3481. [2] S.S. Kumar, K. Kwak, D. Lee, Anal. Chem. 83 (2011) 3244–3247. [3] E. del Puerto, C. Domingo, J.V. Garcia Ramos, S. Sanchez-Cortes, Langmuir 30 (2014) 753–761. [4] H.C.Q. Wu, Q. Luan, J. Zhang, Z. Wang, J.H. Warner, A.A.R. Watt, Inorg. Chem. 47 (2008) 5882–5888. [5] J. Renault, Eur. J. Med. Chem. 16 (1981) 545–550. [6] Y. Zhang, X. Cui, F. Shi, Y. Deng, Chem. Rev. 112 (2012) 2467–2505. [7] H.-U. Blaser, H. Steiner, M. Studer, ChemCatChem 1 (2009) 210–221. [8] R.A. Sheldon, Fine Chemicals Through Heterogenous Catalysis, Wiley-VCH, New York, 2001. [9] T. Clausen, Hair Preparations in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2007. [10] A. Corma, Science 313 (2006) 332–334. [11] K. Layek, M.L. Kantam, M. Shirai, D. Nishio-Hamane, T. Sasaki, H. Maheswaran, Green Chem. 14 (2012) 3164. [12] H. Zhang, X. Li, G. Chen, J. Mater. Chem. 19 (2009) 8223. [13] Y. Choi, H.S. Bae, E. Seo, S. Jang, K.H. Park, B.-S. Kim, J. Mater. Chem. 21 (2011) 15431. [14] A. Taketoshi, M. Haruta, Chem. Lett. 43 (2014) 380–387. [15] H. Yamamoto, H. Yano, H. Kouchi, Y. Obora, R. Arakawa, H. Kawasaki, Nanoscale 4 (2012) 4148. [16] R. Fenger, E. Fertitta, H. Kirmse, A.F. Thünemann, K. Rademann, Phys. Chem. Chem. Phys. 14 (2012) 9343. [17] S. Sharma, B. Kim, D. Lee, Langmuir 28 (2012) 15958–15965. [18] S.E. Eklund, D.E. Cliffel, Langmuir 20 (2004) 6012–6018. [19] J. Lee, H.S. Shim, M. Lee, J.K. Song, D. Lee, J. Phys. Chem. Lett. 2 (2011) 2840– 2845. [20] M. Tamura, K. Kon, A. Satsuma, K.-i. Shimizu, ACS Catal. 2 (2012) 1904–1909. [21] H. Toulhoat, J. Catal. 216 (2003) 63–72. [22] N.N. Greenwood, Chemistry of Elements, Butterworth Heinemann, Oxford, 1998. [23] P. Serna, P. Concepción, A. Corma, J. Catal. 265 (2009) 19–25. [24] M. Boronat, A. Corma, Langmuir 26 (2010) 16607–16614. [25] A. Corma, M. Boronat, S. González, F. Illas, Chem. Commun. (2007) 3371. [26] F. Cárdenas-Lizana, S. Gómez-Quero, A. Hugon, L. Delannoy, C. Louis, M.A. Keane, J. Catal. 262 (2009) 235–243. [27] R.G. Parr, R.G. Pearson, J. Am. Chem. Soc. 105 (1983) 7512–7516. [28] A. Corma, P. Concepción, P. Serna, Angew. Chem. Int. Ed. 46 (2007) 7266–7269. [29] R. Javaid, S. Kawasaki, A. Suzuki, T.M. Suzuki, Beilstein J. Org. Chem. 9 (2013) 1156–1163. [30] G.J. Hutchings, Nat. Chem. 2 (2010) 1005–1006. [31] J.-M. Ha, A. Solovyov, A. Katz, Langmuir 25 (2009) 10548–10553. [32] S. Wunder, F. Polzer, Y. Lu, Y. Mei, M. Ballauff, J. Phys. Chem. C 114 (2010) 8814–8820. [33] P. Hervés, M. Pérez-Lorenzo, L.M. Liz-Marzán, J. Dzubiella, Y. Lu, M. Ballauff, Chem. Soc. Rev. 41 (2012) 5577–5587.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Metal dependent catalytic hydrogenation of nitroarenes over water-soluble glutathione capped metal nanoparticles Sachil Sharma ⇑ Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
a r t i c l e
i n f o
Article history: Received 11 September 2014 Accepted 10 November 2014 Available online 18 November 2014 Keywords: Water soluble nanoparticles Glutathione Metal–substrate interaction Hydrogen adsorption Metal-dependence Catalytic hydrogenation Less densely packed branched thiol
a b s t r a c t The water soluble glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) with size 2.4 ± 0.2 nm were synthesized by borohydride reduction of metal ions in the presence of glutathione as capping ligand and used as catalyst for the hydrogenation of nitroaniline in aqueous phase. The rate of catalytic hydrogenation was dependent on metal type and the trend of catalytic activity over these M-GS nanoparticles was found to be Pd-GS (kapp = 0.0227 (±3 10 4)) s 1 Pt-GS (±1 10 4)) s 1 > Au-GS (kapp = 0.0015 (±0.2 10 4)) s 1 > Ag-GS (kapp = 0.0008 (kapp = 0.0043 (±0.2 10 4)) s 1. The similar trend of catalytic activity was found for the hydrogenation of nitrobenzene. Our experimental results, along taking into account the theoretical calculations done by other research groups, suggest that the observed catalytic activity trend is attributed to the ‘‘different rates of H2 molecule adsorption and dissociation’’ on the M-GS nanoparticles. The ‘‘high rate of H2 molecule adsorption’’ and ‘‘highly oxidized surface’’ make Pd-GS nanoparticles an ideal candidate for the rapid hydrogenation. On the basis of our experimental results, we proposed that small gaps between less densely packed branched thiol ‘‘glutathione molecules’’ provide the access to metal nanoparticle surface for the hydrogenation reaction. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Noble metal nanoparticles have drawn immense research interest owing to their importance in catalysis [1] , sensing [2], surfaceenhanced Raman scattering (SERS) [3] and biomedical applications [4]. The large surface area of ultrafine noble metal nanoparticles makes them an ideal candidate for catalyzing various reactions. The hydrogenation of nitroarenes to aniline derivatives catalyzed by noble metal nanoparticles is an important reaction from industrial, synthetic and environmental point of view. The functionalized aniline derivatives are important intermediates for pharmaceutical polymers, dyes, pigments and other fine agrochemicals [5–9]. The nitroarenes are also found as contaminants in industrial waste water and hence, hazardous to the environment. Therefore, it becomes necessary to reduce them to industrial chemicals and intermediates in order to clean the environment. The hydrogenation of nitroarenes to corresponding amino benzenes catalyzed by nanoparticles immobilized on oxide supports such as AuATiO2/Fe2O3/MgO [10,11], AgATiO2 [12] and AgArGO [13] has been studied vastly in the literature. However, the hydrogenation of nitroarenes over ligand capped fine metal ⇑ Present address: Department of Applied Chemistry, Tokyo University of Science (TUS), Shinjuku, Tokyo 162-8601, Japan. E-mail addresses: [email protected], [email protected] http://dx.doi.org/10.1016/j.jcis.2014.11.030 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
nanoclusters and nanoparticles are scantily reported. Recently, ligand capped novel metal clusters [14,15] have been explored as catalyst for a number of important reactions. Very recently, Fenger and co-workers [16] studied size dependent catalysis of Au nanoparticles, respectively for the hydrogenation of 4-nitrophenol. However, most of the literature on the catalytic hydrogenation of nitroarenes is still limited to Au nanoparticles and to the best of our knowledge, there is no report yet on the metal dependent catalysis by ligand capped metal nanoparticles. Thus, it becomes important to gain better understanding of metal dependence catalysis by ligand stabilized metal nanoparticles. In present work, firstly, glutathione capped metal nanoparticles (M-GS, where M = Pd, Pt, Au and Ag; GS = glutathione) of same size were synthesized and then, their catalytic activity toward the hydrogenation of nitroarenes was compared. Herein, we report the metal dependent catalytic activity of water soluble glutathione (GSH) capped Pd, Pt, Au and Ag nanoparticles for the hydrogenation of nitroarenes. The trend of catalytic activity of the glutathione capped metal (M-GS) nanoparticles was found to be following: Pd-GS PtGS > Au-GS > Ag-GS. Our experimental results suggest that the observed catalytic activity trend is attributed to different rates of H2 molecule adsorption and dissociation. However, high rate of H2 adsorption along with high amount of surface oxidation on Pd-GS nanoparticles make them ideal catalyst for rapid hydrogenation of nitroarenes.
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2. Materials and methods 2.1. Materials L-glutathione reduced (GSH, P98%, Aldrich), Sodium borohydride (99%, Aldrich), Palladium(II) chloride (PdCl2, 99.999%, Aldrich), Chloroplatinic acid hexahydrate (H2PtCl66H2O, ACS reagent), Hydrogen tetrachloroaurate trihydrate (HAuCl43H2O, ACS reagent), Silver Nitrate (AgNO3, ACS reagent P99.0%, Aldrich), 2-nitro aniline (>99%, Tokyo Chemical Industry), Nitrobenzene (P99%, Aldrich), were used as received. Water was purified using a Millipore MilliQ system (18.2 MX cm). Firstly, various M-GS nanoparticles with same size were synthesized and further used as catalyst for hydrogenation of nitroarenes. All synthesis and catalysis experiments were performed at room temperature and under air atmosphere. Transmission electron microscopy (TEM) images were obtained using JEOL JEM-2010 high contrast electron microscope operating at 200 kV. XPS measurements were carried out on an XPS system (VG ESCALAB 220i-XL) using monochromatic Al Ka X-ray source (1486.6 eV). Binding energies (BE) were referenced to the C 1s BE at 284.8 eV.
2.2. Synthesis of glutathione capped metal nanoparticles The water soluble Pd, Pt, Au and Ag nanoparticles with same size 2.4 ± 0.2 nm were prepared by borohydride reduction of their respective metal salts in presence of glutathione as capping ligand. The glutathione capped Pd nanoparticles were synthesized using the procedure reported in our previous work [17]. However, glutathione capped Pt, Au and Ag nanoparticles were synthesized according to literature procedures [4,18,19] with the slight modification. The experimental details are described in the supplementary material. 2.3. Catalytic experiment procedure In a typical catalytic reaction, 0.13 mL (130 ll) of aqueous solution of 5 mM nitroarene (2-nitro aniline and nitrobenzene) was first mixed with 2.5 mL aqueous solution of 0.3575 mg/L catalyst (2.4 nm M-GS nanoparticles) in a UV–Vis quartz cuvette while kept on stirring. Then, 0.87 mL (870 ll) of aqueous solution of 21 mM NaBH4 was added into the above mixture while kept on stirring. Immediately following this, in situ absorption spectral changes were recorded till the completion of catalytic hydrogenation with a USB-4000 fiber optic spectrometer (Ocean Optics) equipped with magnetic stirrer. The base line correction was performed using water as solvent before taking absorbance measurements. The UV–Vis spectra of reaction products were found to be in agreement with UV–Vis spectra of the authentic compounds. 3. Results and discussion Fig. 1 shows the characteristic UV–Vis spectrum of 2.4 nm glutathione capped Pd, Pt, Au and Ag nanoparticles. Their bright field TEM images show the narrow size distribution 2.4 ± 0.2 nm (Fig. S1 in supplementary material). Here it is important to note that the glutathione capped metal (M-GS, M = Pd, Pt, Au and Ag) nanoparticles with same size (2.4 ± 0.2 nm) were used as catalyst for the hydrogenation reaction to exclude the size effect. To test the metal dependent catalytic activity of 2.4 nm M-GS (M = Pd, Pt, Au, and Ag) nanoparticles toward the aqueous phase hydrogenation of nitroarenes, we chose the 2-nitroaniline as a model substrate. Fig. 2a shows the time dependent UV–Vis spectral changes during the catalytic reduction of 2-nitroaniline over M-GS
Fig. 1. Characteristic UV–Vis spectrum of 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles.
nanoparticles. After certain time, the peak at 412 nm was found to disappear completely, indicating the complete reduction of 2-nitroaniline. The completion of hydrogenation of 2-nitroaniline over M-GS nanoparticles was also noted manually by observing the complete fading of yellow color of 2-nitroaniline after certain time. The completion of catalytic hydrogenation reaction was confirmed by recording positive ion ESI-MS spectra of starting nitroarenes and crude final product. As shown in Fig. S2 (supplementary material), the intense peak at m/z = 139.04 for 2-nitroaniline was found to completely absent in the positive ion ESI-MS spectrum of crude final product. Whereas, the presence of most intense peak at m/z = 109.06 in the positive ion ESI-MS spectrum of crude final product indicates the formation of 2-phenyldiamine. The similar ESI-MS spectra were obtained after the complete hydrogenation of 2-nitroaniline over all M-GS (M = Pd, Pt, Au and Ag) nanoparticles. The plot of Ct/C0 versus t (s) shows the exponential decrease of Ct/C0 of 2-nitroaniline with time over various M-GS nanoparticles, which was plotted by taking into account the spectral changes at 412 nm (Fig. S3 in supplementary material). The time for 95% completion of the hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles was observed to be 1.7, 10, 23 and 47 min, respectively. Fig. 2(b) shows the kinetic pseudo-first order rate plots of the reduction of 2-nitroaniline over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. The values of slope (defined as apparent rate constants, kapp) were calculated to be 0.0227 (±3 10 4), 0.0043 (±1 10 4), 0.0015 (±0.2 10 4), and 0.0008 (±0.2 10 4) (s 1), respectively for the hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles. Thus, the order of catalytic activity of 2.4 nm M-GS nanoparticles was as follows: Pd-GS Pt-GS > AuGS > Ag-GS for the hydrogenation of 2-nitroaniline. Similar trend in catalytic activity was also found for the hydrogenation of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles (vide infra; Fig. 3). Although, induction time was not observed for the catalytic hydrogenation of 2-nitroaniline over 2.4 nm Pd-GS nanoparticles (Fig. S3 in supplementary material). However, the induction time of 1.5, 3, and 7 min was observed for the catalytic hydrogenation over 2.4 nm Pt-GS, Au-GS and Ag-GS nanoparticles, respectively. Similar trend in the induction period was also observed for the catalytic hydrogenation of nitrobenzene (Fig. S7 in supplementary information). The induction time may be attributed: to (1) diffusion controlled adsorption of reactants on the metal nanoparticle surface, (2) the presence of dissolved oxygen in the reaction medium, and (3) metal surface restructuring induced by adsorbed substrate [15]. To check the effect of dissolved oxygen, we performed the catalytic hydrogenation reaction of 4-nitroaniline over 2.4 nm Au-GS nanoparticles after purging the reaction solutions by
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Fig. 2. (a) Time dependent UV–Vis absorption spectral changes and (b) Plots of ln (Ct/C0) versus t (s) with pseudo first order rate fitting for the reduction of 2-nitroaniline over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. Error limits are given in parentheses.
Fig. 3. (a) Time dependent UV–Vis absorption spectral changes and (b) Plots of ln (Ct/C0) versus t (s) (with pseudo first order rate fits) taking into account the spectral changes at 228 nm for the reduction of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K.
argon gas for 20 min. It was worth to note that no induction time was noticed for catalytic hydrogenation of 2-nitroaniline over 2.4 nm Au-GS nanoparticles under the inert gas atmosphere as shown in Fig. S4 (supplementary material). It indicates that induction time in our study is due to the dissolved oxygen in the reaction medium. In other words, the adsorption of nitroarenes on the surface of 2.4 nm M-GS nanoparticles is not a diffusion controlled process. Recently, Tamura and co-workers [20] reported the following order of catalytic activity: Pd/SiO2 > Pt/SiO2 > Rh/SiO2. . .. . .. . . > Co/ SiO2 > Ag/SiO2 for SiO2 supported uncapped metal nanoparticles (M-SiO2, M = Ag, Pd, Pt, Rh. . .. . .Co) toward the liquid phase hydrogenation of nitrobenzene in THF under 3.0 MPa H2 gas at 100 °C. In our work, in spite of capping with thick layer of glutathione, 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles, still show the same trend of catalytic activity (Pd-GS Pt-GS > Au-GS > AgGS) as reported for SiO2 supported uncapped metal nanoparticles [20]. We proposed a model (Vide Infra: Fig. 5) to explain the catalytic active behavior of M-GS NPs, keeping in view of highly branched nature of glutathione molecule. Further, it is also interesting to note that compared to Pt-GS, Au-GS and Ag-GS nanoparticles, Pd-GS nanoparticles were found to be highly efficient catalyst for the rapid hydrogenation of 2-nitroaniline. To gain insight into the observed trend of catalytic activity: Pd-GS Pt-GS > Au-GS > Ag-GS of MGS nanoparticles and high catalytic efficiency of Pd-GS nanoparticles for the rapid hydrogenation of nitroarenes, we need to look into the involved mechanism. Generally, the catalytic hydrogenation of nitroarenes occurs by hydrogen transfer from metal-hydride complex on metal surface to the nitrobenzene [11]. The BH4 ion (Na+ BH4 in aqueous medium)
reacts on the surface of metal nanoparticles to generate H2. The H2 molecule is known to dissociate rapidly on early and late transition metals to form MAH bonds on their surface [21]. The bulk Pd metal is well known for adsorption of molecular hydrogen on its surface to form metal hydrides [22]. Corma and co-workers [23] reported that the H2 molecule adsorption on Au and Pt is dissociative in nature; however, no activation barriers are involved in case of dissociation of H2 on Pt as compared to Au. Although, the bulk Ag metal is not known to adsorb the molecular H2 on their surfaces; however there is possibility for ultrafine nanoparticles of Ag to form MAH bonds on their surfaces, because of low coordination of surface atoms. Extensive theoretical and experimental research work done by Corma et al. [23–25] suggests that the dissociation of H2 molecule on the metal surface atoms is rate controlling step for determining the activity of catalyst toward the hydrogenation of nitroarenes. Furthermore, according to the ‘‘Sabatier principle’’, there must be optimum interactions between reactants and catalyst to obtain high catalytic activity [20]. In this regard, Toulhoat [21] calculated the metal–hydrogen (MAH) bond energy for three transition metal series based on density functional theory and their calculations suggest that the PdAH and PtAH bonds (PdAH > PtAH) have strong bond energy compared to that of AuAH and AgAH. In fact, Cardenas-Lizana and co-workers [26] observed three fold increase in catalytic activity on inclusion of Pd to Au/Al2O3 toward the hydrogenation of p-chloronitrobenzene. Similarly, Corma and co-workers [24] observed the significant increase in catalytic activity on inclusion of Pt to Au/TiO2 toward the hydrogenation of nitrobenzene. Therefore, taking into account the hydrogen adsorption on Pd, Pt, Au and Ag metal surfaces, we can conclude that the observed catalytic trend in our study is attributed to different rate of H2 molecule
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Fig. 4. (a) Pd 3d, (b) Pt 4f, (c) Au 4f and (d) Ag 3d XPS spectra of 2.4 nm glutathione capped Pd, Pt, Au and Ag nanoparticles, respectively.
adsorption and dissociation on the surface of glutathione capped M-GS (M = Pd, Pt, Au and Ag) nanoparticles. However, ‘‘interaction of model substrates such as nitroarenes on the surfaces of metal nanoparticles’’ can also significantly influence the rate of hydrogenation. Hence to gain further insight, we looked into such possible interactions, by (1) changing the model substrate without any ANH2 functionality and (2) checking the surface oxidation of M-GS nanoparticles by XPS (X-ray photoelectron spectroscopy), as following. Firstly, the nitroaniline molecules can interact with surface of noble metal nanoparticles through ANH2 group. On basis of the hard and soft acids bases (HSAB) concept [27], Pd (hardness parameter, g = 3.8) metal is hardest acid, while Ag (g = 3.1) metal is softest acid. Hence, the hard base such as ANH2 group is expected to interact efficiently with Pd than Pt, Au and Ag. This factor can also affect the rate of the catalytic hydrogenation. Hence, to check the effect of interaction strength between nitroarene molecule and nanoparticle surface on the rate of catalytic hydrogenation, we performed the same catalysis experiment using a model substrate without any ANH2 functionality such as nitrobenzene. Fig. 3(a) shows the time dependent UV–Vis spectral changes during the catalytic hydrogenation of nitrobenzene over M-GS nanoparticles. Upon the addition of NaBH4 to the mixture of catalyst and nitrobenzene, the characteristic peak of nitrobenzene at 268 nm was disappeared. Instead, the two characteristic peaks of aniline were appeared at 228 and 278 nm. The absorption intensity of the peak centered at 228 nm was found to increase with time. Whereas, the absorption intensity of the peak centered at 278 nm was found to decrease with time. However, as shown in Fig. 3. a, the broad peak centered at 278 nm was found to associate with many shoulder peaks. These shoulder peaks correspond to various unstable reaction intermediates (Figs. S5 and S6 in supplementary material) formed during the course of catalytic hydrogenation of nitrobenzene over M-GS nanoparticles. The presence of the reaction intermediates also indicates their week interaction on the surface of M-GS nanoparticles [28]. These shoulder peaks were found to disappear completely on the completion of catalytic hydrogenation of nitrobenzene. Due to the absence of any associated shoulder peak,
the spectral changes of peak centered at 228 nm (Fig. 3a) were monitored to plot the Ct/C0 versus t (s) (Fig. S7 in supplementary material). The increase in the intensity of peak at 228 nm with time (s) indicates the increase of aniline concentration with time. The time for 95% completion of the catalytic hydrogenation of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles was observed to be 7, 13, 32 and 38 min, respectively. Fig. 3(b) shows the kinetic pseudo-first order rate plots for the reduction of nitrobenzene over 2.4 nm M-GS (M = Pd, Pt, Au and Ag) nanoparticles at 298 K. The values of apparent rate constant (kapp) were calculated to be 0.013 (±2 10 3), 0.0036 (±0.9 10 3), 0.0013 (±0.1 10 3) and 0.0015 (±0.1 10 3) (s 1), respectively for the hydrogenation of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, Au-GS and Ag-GS nanoparticles. The lesser completion time for the catalytic hydrogenation of 2-nitroaniline molecule compared to that of nitrobenzene over 2.4 nm Pd-GS, Pt-GS, and Au-GS nanoparticles indicates that along with the H2 adsorption and dissociation, the interaction of nitroaniline molecules on the surface of M-GS nanoparticles through ANH2 functionality (HSAB principle) further fastens the catalytic hydrogenation. However, the trend of catalytic activity of M-GS nanoparticles for the hydrogenation of nitrobenzene: Pd-GS > Pt-GS > AuGS > Ag-GS was still found to be same as that for the hydrogenation of 2-nitroaniline. It indicates that the observed catalytic activity trend among various M-GS nanoparticles toward the hydrogenation of nitroarenes is due to different rates of H2 adsorption and dissociation on the M-GS (M = Pd, Pt, Au and Ag) nanoparticles surface. Secondly, the oxidized metal surface has large number of hydroxyl groups. Thus, the surface oxidation can also increase the interaction of metal surface with model substrate (nitroarenes) through hydrogen bonding in aqueous medium. As revealed by XPS spectra (Fig. 4) of glutathione capped M-GS (M = Pd, Pt, Au and Ag) nanoparticles, 2.4 nm Pd-GS NPs were found to have highly oxidized surface (64%). Whereas, 2.4 nm Pt-GS and Au-GS nanoparticles were found to have relatively less surface oxidation (22% and 20% respectively). The 2.4 nm Ag-GS nanoparticles were found to have unoxidized surface. The respective metal/metal oxide binding energy values have been summarized in Table S-1 (supplementary material). Here it is interesting to be noted that
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and can be modeled similar to Langmuir Hinshelwood mechanistic model [11,23,32,33]. 4. Conclusions In summary, we first prepared 2.4 nm water soluble glutathione capped Pd, Pt, Au and Ag NPs and observed following order of their catalytic activity: Pd-GS Pt-GS > Au-GS > Ag-GS toward the hydrogenation of nitroarenes. The observed catalytic activity trend was found to be completely metal dependent and is attributed to different rates of H2 adsorption/dissociation on 2.4 nm M-GS nanoparticles. The ‘‘high rate of H2 adsorption/dissociation’’ and ‘‘high oxidation’’ on the surface of Pd-GS nanoparticles make them an ideal candidate for the rapid hydrogenation of nitroarenes. We propose that small gaps between less densely packed branched thiols such as glutathione on the metal nanoparticle surface is responsible for the display of catalytic activity. Acknowledgment Fig. 5. Proposed scheme for hydrogenation of nitroarene over surface of 2.4 nm MGS NPs.
in spite of having same surface oxidation (20%) on 2.4 nm Pt-GS and Au-GS nanoparticles, they were not found to have same catalytic activity for hydrogenation. The 2.4 nm Pt-GS nanoparticles displayed superior catalytic activity than 2.4 nm Au-GS nanoparticles. It is because of the fact that rate of H2 adsorption/dissociation is high for 2.4 nm Pt-GS compared to Au-GS NPs. The high surface oxidation on Pd-GS nanoparticles probably increases the interaction of model substrate on the surface. Hence, in addition to high rate of H2 molecular adsorption and dissociation, the high amount of surface oxidation on the Pd-GS nanoparticles make them an ideal candidate for rapid hydrogenation. Recently, Suzuki and coworkers [29] also reported superior catalytic activity of porous PdO surface compared to porous Pd surface for the rapid hydrogenation of p-nitrophenol. However, the reason behind catalytic activity of M-GS nanoparticles despite having thick layer of glutathione on their surface still needs to be addressed. Moreover, these M-GS nanoparticles were found to have same tendency to catalyze the hydrogenation as reported by Tamura and coworkers [20] for SiO2 supported ligand free metal nanoparticles. Atomically precise Au25(GS)18 nanoclusters have already been proven to have high catalytic efficiency for the rapid hydrogenation of nitroarenes. Kawasaki and co-workers [15] suggested that the high catalytic activity of Au25(GS)18 nanoclusters toward the hydrogenation is basically attributed to unhindered active sites because of its unique core–shell structure. Hutchings [30] and Katz and co-workers [31] also reported that the large capping molecules pack on the surface of small metal clusters in such a way that the large gaps are left between them. The smaller reactant molecules use these gaps to access the metal surface during the catalytic reaction. Eklund and Cliffel [18] studied the catalytic activity for various thiolate protected Pt nanoparticles (including glutathione) toward the hydrogenation of C@C double bond. They found that the packing density of branched thiols such as glutathione and tiopronin on the surface of metal nanoparticles is less than that of straight chain thiols. As a result of less dense packing, the branched ligands leave more exposed metal atoms on metal surface, which render the catalytic activity to these metal nanoparticles. Based on experimental results, we propose that due to less dense packing of branched glutathione ligand on the surface of 2.4 nm metal nanoparticles; space between glutathione molecules are available for small molecules such as H2 and nitroaniline to access the metal surface. The proposed scheme is shown in Fig. 5
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