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Solid-state synthesis of metal nanoparticles supported on cellulose nanocrystals and their catalytic activity Wael Hosam Eisa, Abdelrahman Abdelgawad, and Orlando J. Rojas ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04333 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Sustainable Chemistry & Engineering

Solid-state synthesis of metal nanoparticles supported on cellulose nanocrystals and their catalytic activity Wael H. Eisaa,e,*, Abdelrahman M. Abdelgawadb,e, Orlando J. Rojasc,d,e

a

Spectroscopy Department, Physics Division, National Research Centre (NRC), Elbuhouth st., Giza, Egypt. b

c

Textile Research Division, National Research Centre (NRC), Elbuhouth st., Giza, Egypt.

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, P.O. Box 16300, FI-00076, Finland. d

Department of Applied Physics, School of Science, Aalto University, P.O. Box 13500, FI00076, Finland. e

Departments of Forest Biomaterials and Chemical and Biomolecular Engineering, North Carolina State University, Raleigh 27695, United States.

* Corresponding author: Wael H. Eisa, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Heterogeneous catalysis has played a critical role in environmental remediation, for example, in processes that generate toxic streams. Thus, there is an ever-increasing need for green, costeffective routes to synthesize highly active catalysts. In this study, a cellulose nanomaterial (cellulose nanocrystals, CNC) was employed as solid support for the nucleation of silver and gold nanoparticles via solid-state synthesis. The process involved solvent-free reduction in ambient conditions of metal precursors on the surface of CNC and in the presence of ascorbic acid. Surface plasmon resonance and X-ray diffraction indicated the successful formation of the metal nanoparticles, in the form of organic-inorganic hybrids. A strong hydrogen bonding was observed between CNC and the metal nanoparticles owing to the high density of hydroxyl groups in CNC, as determined by Fourier transform infrared spectroscopy. Electron microscopies indicated that the silver and gold precursors formed nanoparticles of hexagonal and spherical shape, respectively. The organic-inorganic hybrids were demonstrated as the potential catalyst for the reduction of 4-nitrophenol to 4-aminophenol. Overall, we introduce a green, solvent-free and facile method for the production of noble metal nanoparticles supported on CNC, which offer promise in the scalable synthesis and for application in heterogeneous catalysis.

KEYWORDS: Solid-state reaction; heterogeneous catalyst; silver nanoparticles; gold nanoparticles; cellulose nanocrystals; organic-inorganic hybrids.


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INTRODUCTION Metal nanoparticles have been studied as efficient catalysts in many relevant industrial processes, including those for the production of fine chemicals and pharmaceuticals.1 Minimizing the size of solid catalysts to the nano-scale improves the specific surface area and availability of active sites to facilitate heterogeneous conversion. For instance, bulk gold is considered inert for redox reactions but it does show high catalytic activity if it is nano-sized. Indeed, very much progress has been made in the fabrication of metal nanoparticles and nanoalloys with precise control of their shape and size.2 In general, nanoparticles may expose a large number of unsaturated and dangling bonds at their surfaces, which favors rapid and efficient adsorption. This guarantees that the catalytic material is utilized viably. However, given their very high surface energy, metal nanoparticles are subject to association and aggregation, especially in aqueous-phase catalytic reactions3 This may result in changes in shape and growth in particle size, which leads to reductions of available surface area and catalytic activity.4 Moreover, there are no practical ways to restore the nanoparticles from the reaction medium, which presents an additional challenge that limits the use of such materials in catalysis. In this context, many efforts have been conducted to overcome these bottlenecks in heterogeneous catalysis. For example, the immobilization of metal nanoparticles onto solid supports is known to enhance their stability.5 The most widely used substances for the stabilization of metal nanoparticles are ligands and polymers, specially natural or synthetic polymers

with

a

certain

affinity

toward

metals,

which

are

soluble

in

suitable

solvents.Additionally, this facilitates separation of the metal nanoparticles from the reaction medium and allows for recovery and recycling of the materials in catalytic cycles.


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Polymers, either natural or synthetic, are widely used matrices for supporting and stabilizing the metal nanoparticles.6 Owing to their excellent mechanical properties, shape, high surface-tovolume ratio and chemical stability, cellulose nanocrystals (CNC) have been investigated as biocompatible and renewable matrix in composites and as support of metal particles.7,8 For instance, CNC has been considered as solid supports for inorganic nanocatalysts, such as Fe3O4, 9,10

Ag,7 Au,11,12 Ag-Au alloy,13 Pd14,15 and Pt.16 Other forms of nanocelluloses, including their

hydrogels, have been employed as a sustainable support for the synthesis of Ag nanoparticles catalysts.17,18 The reported methods for metal nanoparticle loading on CNC in aqueous phase consider three routes. In the first one, external reducing agents are added to reduce adsorbed metal ions on the surface of CNC.11 The second route is based on surface functionalization of CNC via chemical bonding having reducing and/or coordinating capabilities for holding the metal nanoparticles. Polydopamine,

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CTAB,20 β-cyclodextrin,21 and grafted PAMAM (poly(amidoamine)-dendrimer12

have been applied to prepare and control the size of CNC/metal hybrids. Finally, the third method uses the available surface half-sulfate ester of CNC and carboxyl groups in other nanocelluloses, including CNC, to reduce the metal ions into metal nanoparticles. Thus, in this case CNC serve as both reducing and stabilizing agent.22,23 While the aforementioned synthesis routes in liquid media facilitate the synthesis of metal-organic hybrids, a large-scale production of such systems is very difficult to realize. Indeed, lack of control in the state of aggregation, low metal concentrations, toxic reagents, and the demand for large amount of solvents (water) are among some of the main obstacles. Thus, such facts reflect on complicated processing routes and large production costs.


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Herein, we propose a simple and facile synthesis method based on the solid-state production of silver and gold nanoparticles supported on CNC, thereafter referred to as Ag@CNC and Au@CNC, respectively. This expands from our previous work where soy protein was employed as reducing and stabilizing agent for Ag nanoparticles.24 Here, CNC is regarded as superior given its nano-scale characteristic as well as its suitability for functionalization, due to the abundant surface hydroxyl and sulfate groups. Also, the size-dependent properties of silver and gold nanoparticles make them extensively used compounds for wide category of applications.25 The synthesis of such hybrids, suitable for heterogeneous catalysts, was performed by simply milling freeze-dried CNC in ambient conditions and in the presence of the respective precursor salt and a reducing agent (ascorbic acid). As a proof of concept, Ag@CNC and Au@CNC hybrids were investigated with regards to their synthesis efficiency. As a proof-of-concept for their application, we explore the activity of the nanoparticles for the degradation of 4-nitrophenol (4NP) and, also for catalysts recovery (see scheme 1). As will be shown, the reusability of Ag@CNC and Au@CNC nanocatalyst was accomplished successfully with a minimum loss of catalytic activity. We note that the inherent agglomeration of CNC in its dried form and the resultant uneven distribution of metallic nanoparticles onto the surface of the cellulose nanocrystals are some of the main drawbacks of the proposed solid state reaction. In this regard, freeze-drying (the method used by for CNC drying in this work) is a well established technique for minimizing irreversible binding of CNC and allowing its redispersability in aqueous media.26,27 Nevertheless, preventing any level of CNC aggregation is highly desirable in order to better control metal nanoparticle uniformity; this is an aspect that can be addressed by utilizing other methods for water removal. At any rate, the minimization of environmental pollution caused by solvents and the interest in solid-solid reactions are the main benefits and reasons to


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explore related methodologies, i.e., solvent-free synthesis. The proposed solid state synthetic route, combined with the utilization of a sustainable resource, allows the possibility of an ecofriendly and scalable process. As such, the introduced, novel catalyst systems may open new possibilities for fine-chemical synthesis in the pharmaceutical and other industries.

Scheme 1. Schematic presentation of the solid-state synthesis of Ag@CNC and Au@CNC nanohybrids.

EXPERIMENTAL Materials. The cellulose nanocrystals, CNC (20 nm in width and 150-200 nm in length) were isolated via sulfuric acid hydrolysis from dissolving-grade pulp and obtained in freeze-dried form.28 The CNCs (CAS No. 7789-20-0) were produced at the USDA’s Forest Products Laboratory (FPL, Madison, WI) and acquired through the University of Maine. The metal


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precursors consisted of hydrogen tetrachloroaurate-(III) trihydrate, (HAuCl4.3H2O), obtained from Alfa Aesar and silver nitrate (AgNO3), purchased from Sigma-Aldrich. Ascorbic acid, which was used as reducing agent, was supplied by Sigma-Aldrich. 4-nitrophenol (4-NP) was supplied from Loba Chemie, India. Sodium borohydride (NaBH4) was provided by Merck Co. Preparation of Ag@CNC and Au@CNC hybrids. The CNC and metal salt precursors were used for the preparation of hybrids in the dry form, with no addition of solvent. One gram of CNC was milled manually for 30 min in an agate mortar in the presence of the respective metal precursor (0.05 g). The samples containing the two different metal precursors were kept for 24 h to achieve adsorption of the metal ions on the surface of the CNCs. Thereafter, 0.1 g of ascorbic acid was added to the powder mixture and grounded for 30 min. The as-prepared samples were washed repeatedly with distilled water and filtered to remove unreacted species. The obtained samples (Ag@CNC and Au@CNC hybrids) were dried in air at 40 oC for 24 h. The calculated volume fraction of Ag and Au in the corresponding hybrids was 1.66% and 0.63%, respectively. Characterization methods. UV-vis spectra of powdered samples were recorded in the solid state by using a Jasco V-570 spectrophotometer equipped with integrating sphere reflectance unit. A zeta potential unit, Zeta sizer (Malvern Instruments), was used to determine the zeta potential (ζ–potential) and the size distribution of metal@CNC in aqueous suspensions. The surface morphology and composition were accessed by using scanning electron microscopy (Quanta FEG 250 electron microscope) equipped with Energy dispersive X-ray scattering (EDX) unit. The hybrid samples were imaged using transmission electron (TEM, JEOL JEM 2010). The metal@CNC suspensions were dropped onto carbon-coated copper grids. The samples were stained with 2 % phosphotungstic acid (PTA) to improve the contrast. Fourier transform infrared (FTIR) spectra of the powdered hybrids were generated by a Jasco 6100 FT/IR spectrometer


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using KBr method. X-ray diffraction (XRD) profiles of Ag@CNC and Au@CNC hybrids were recorded in the 2θ range of 4–80o using a (Schimadzu 7000 diffractometer, Japan). Thermal degradation was tested using (TGA Q500, TA instruments). Catalytic activity of free-standing films comprising Ag@CNC and Au@CNC for the reduction reaction of 4-nitrophenol (4-NP). First, 0.25 g of the given hybrid material samples (Ag@CNC and Au@CNC hybrids) was dispersed in 20 mL distilled water and sonicated for 15 min. The respective suspension containing metal-CNC hybrid was cast in a Petri-dish and dried in an oven at 60 oC for 4 h. The dried films were peeled off from the dishes and used in the experiments introduced next (see Figure S1). The metal content was determined by using atomic absorption spectrometry and found to be 0.12 and 0.17 mg in the Ag@CNC and Au@CNC films, respectively. 100 µL of 0.1 M NaBH4 was added to a quartz cuvette filled with 2.77 mL of 0.4 mM 4nitrophenol (4-NP). To this mixture, a film of either Ag@CNC (0.05 g) or Au@CNC (0.05 g) was incorporated as a catalyst. The catalytic reaction was followed by recording the decay in the intensity of the absorption peak at 400 nm using a UV-vis spectrophotometer.

RESULTS AND DISCUSSION Ag@CNC and Au@CNC hybrids. Mixtures comprising the organic phase, CNC support and the precursor salt (Ag+1 and Au+3), in the absence of the reducing agent, indicated that the two samples preserved their original white color for several months, with no distinguishable change. This indicated that under these conditions the cellulose nanocrystals alone cannot act as reducing agent for the formation of Ag and Au nanoparticles. The incorporation of ascorbic acid induced the development of characteristic colors for the Ag and Au nanoparticles, as can be observed in


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Figure 1a. The brown color of Ag@CNC developed within few minutes after adding ascorbic acid while the violet color in Au@CNC required about 30 min to become evident. Figure 1b includes the UV-vis diffuse reflectance spectra for Ag@CNC and Au@CNC hybrids. CNC shows no significant absorption in the visible region; however, its absorption intensity increased markedly as a result of the reduction of Ag and Au nanoparticles. The Ag@CNC hybrid powder showed two absorption peaks, at 408 and 608 nm, while the Au@CNC powder displayed only one peak at 550 nm. These absorption peaks in the visible region originated from localized surface plasmon resonance (SPR), characteristics of Ag and Au nanoparticles, respectively.4,12. The spectral profile showing the SPR bands give information related to the shape of nanoparticles.29 According to Mie's theory, spherical nanoparticles usually display only a single SPR peak in the visible range, while anisotropic ones show two or more SPR peaks, according to the specific shapes of the particles.30 Consequently, it can be speculated that the prepared Au nanoparticles were spherical in shape while the Ag nanoparticles were anisotropic. This is confirmed in the next section.


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Figure 1. (a) Photo images of CNC and the as-prepared samples of Ag@CNC and Au@CNC hybrids. (b) UV–vis diffused reflectance spectra of Ag@CNC and Au@CNC hybrids. Suspension stability and size distribution. Figure S2 shows the photo-images captured at different time intervals for Ag@CNC and Au@CNC suspensions after ultrasonication. All the hybrids suspensions were stable (no signs of settling) for at least 48 h. Electrostatic repulsion is a typical mechanism involved in the stabilization of nanoparticle suspensions and therefore the ζpotential of the particles in aqueous media relates to the stability. As such, the average ζpotential values of CNC, Ag@CNC, and Au@CNC suspensions were determined to be –45, –


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28, –31 mV, respectively. These negative ζ-potential values explain the stability of the hybrid nanoparticle suspensions,31 other mechanisms for stabilization may be also involved. The size distribution of CNC and its hybrids was measured using DLS technique. Figure 2 shows the hydrodynamic diameter distributions of CNC and its hybrids by the intensity profiles. Three peaks at around 3, 15, and 220 nm were observed for the neat CNC size distribution profile while two peaks at around of 18 and 122 nm were observed for Ag@CNC . One peak at around 190 nm was determined for Au@CNC. The characteristic sizes of Ag@CNC and Au@CNC were smaller than those for neat CNC, which may be due to improved dispersity and/or the slight degradation of CNC during the reduction process.

Figure 2. Size distribution of CNC as well as Ag@CNC and Au@CNC hybrids.


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Morphology of CNC hybrids. SEM imaging was performed to inquire into the metal loading on the surfaces of CNC, Figure 3 (a, b). Bright features correspond to Ag or Au nanoparticles that are assembled on the surface of the interlinked network of CNC, which is created from its rod-like morphology. Additional SEM images of the free-standing films are displayed in Figure S3 of the Supporting Information document. The images show dense aggregates of metal nanoparticles with “necking” between individual particles. This necking may be attributed to the presence of CNC. The SEM size of the metal nanoparticles is quite larger than that determined by the TEM technique. This mismatch between SEM and TEM results has been discussed in detail elsewhere.32 The EDX spectra analysis of Ag and Au nanoparticles loaded at the surface of CNC powder are displayed in Figure 3 (c, d). Three peaks were recorded in the EDX spectrum of Ag@CNC hybrids (Figure 3c) between 2.5 and 3.6 keV due to Agα, Agβ, and Agβ2. The Ag@CNC hybrids consisted mostly of carbon (45%), oxygen (43%) and smaller amounts of Ag (10%). The elemental composition of Au@CNC hybrids (Figure 3d) included carbon (60%), oxygen (30%), and Au (5%). The corresponding EDX spectra of Ag@CNC and Au@CNC hybrids further supports the fact that Ag and Au nanoparticles were loaded on the CNC supports.


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Figure 3. SEM micrographs of (a) Ag@CNC,(b) Au@CNC and their corresponding EDX spectra (c and d). The scale bar in the images corresponds to 1 µm. Typically, observation of metal nanoparticles on the surface of CNC is challenging, due to the different electron density and stability against beam damage of the two components.33 Some TEM images for CNC and supported metal nanoparticles have been reported (see for example Ref.12,34,35). The stability of metal nanoparticles is much higher than that of CNC, which can be damaged by the electron beam. Negative staining using heavy metal such as phosphotungstic acid can be used to overcome these issues. As such, Figure 4 shows representative TEM images of Ag@CNC and Au@CNC hybrids with and without staining. The stained samples (Figure 4 a,


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b) facilitate the simultaneous observation of CNC and metals nanoparticles. The CNC had a needle shape with length of ca. 100–170 nm and a width of 15–20 nm. The images give clear evidence of the retention of nanometric scale of the dried CNC. In order to acquire TEM images of Ag and Au nanoparticles, with better resolution, the unstained samples were imaged and displayed in Fig. 4 c, d, respectively. Ag nanoparticles with diameters from 6 to 35 nm were clearly observed. The as-prepared Ag nanoparticles had irregular hexagonal shapes with broad size distribution. This is in good agreement with predictions based on the UV-vis results. Meanwhile, the TEM images of Au@CNC (Figure 4d) revealed the formation of round Au nanoparticles, with diameters between 18 and 25 nm. It was apparent that the Au particle size distribution was more uniform compared to that of Ag particles in the Ag@CNC hybrids. To a large degree, the images showed little or no aggregation. Finally, the TEM images indicated that the Ag and Au nanoparticles were coupled or bound to the CNC phase. In light of the above findings, we suggest that Ag@CNC and Au@CNC hybrids were effectively assembled via the proposed solid state pathway.


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Figure 4. TEM micrographs of (a) stained Ag@CNC, (b) stained Au@CNC, (c) unstained Ag@CNC and (d) stained Au@CNC. Crystal structure and crystallinity. X-ray diffraction was used to determine the variations in crystal structure and crystallinity as a result of nanoparticles growth on the CNC surfaces. The diffraction patterns of CNC, Ag@CNC, and Au@CNC are included in Figure 5. The diffraction pattern of neat CNC displayed five diffraction peaks located at 2θ= 14.72o (1-10), 16.32o (110), 20o (110), 22.56o (200), and 34.4o (004), which correspond to diffraction pattern of cellulose Iβ and cellulose II.36-38 The growth of Ag and Au nanoparticles at the cellulose nanocrystals induced two significant variations in the diffraction pattern of CNC. First, three additional peaks


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appeared at 2θ~38º (111), ~44º (200), and

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~65º (220), which were ascribed to the FCC

crystalline structure of both Ag (JCPDS 87-0717) and Au nanoparticles (JCPDS 04-0784).13 Second, compared to the peaks observed for neat CNC, Ag@CNC, and Au@CNC XRD signals became less intense, broader, and shifted. The crystallinity ratio (Cr%) in the cellulosic materials was calculated from the XRD pattern using the approximate and empirical method of peak height:39

Cr% = (I 200 - I am /I 200 ) × 100

(1)

where I200 is the intensity of the peak at 2θ= 22.56o corresponding to the plane (200) and Iam represents the peak height of the amorphous phase 2θ= 18o. The calculated Cr% for neat CNC, Ag@CNC, and Au@CNC were 84, 75 and 80%, respectively. The average crystal sizes of Ag and Au nanoparticles were calculated using Scherrer formula40 based on the peak width from crystalline plane (111) (2θ=38o). The average crystal size was found to be 27 and 21 nm for Ag and Au nanoparticles, respectively.


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Figure 5. XRD patterns of CNC, Ag@CNC and Au@CNC.

Structural changes. FTIR data for neat CNC, Ag@CNC, and Au@CNC hybrids were followed by FTIR (Figure 6). The neat CNC displayed an intense and broad peak at 3447 cm-1 due to stretching mode of (–OH) group. The inter- and/or intra-molecular hydrogen bonding explains the OH peak broadening.41 The vibrational signal at 2903 cm-1 was attributed to asymmetric vibration of (–CH2). The bending vibration of (–OH) due to physically absorbed water was observed at 1637 cm-1.42 Three adjacent peaks were recorded at 1429, 1375, and 1320 cm-1 which are assigned to (–CH2) and (–CH) symmetric bending and (–CH2) rocking mode, respectively.43 The vibrational peaks 1161, 1111, 1057, 1030, 895 cm-1 are attributed to


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asymmetric stretching of (–COC) bridge, asymmetric stretching of anhydroglucose ring, (–CO) stretching, in-plane (–CH) bending, and (–CH) deformation of cellulose, respectively.44,45 The FTIR data were further extended to study the structural changes that occurred on the CNC due to loading and reduction of Ag and Au nanoparticles. The stretching vibration of (–OH) group was shifted from 3447 cm-1 for CNC to 3421 and 3407 cm-1 for Ag@CNC, and Au@CNC, respectively. Also, the bending of hydroxyl group of CNC at 1637 cm-1 was observed at 1637 and 1668 cm-1 for Ag@CNC, and Au@CNC, respectively. The spectral shift may be a result of the increased hydrogen bonds formed between (–OH) of CNC and metal ions.46 It was reported that the existence of hydroxyl groups in cellulose are suitable reaction sites for in situ nucleation and growth of nanoparticles.47,48 Hydrogen bonding plays a crucial role in the nature of attachment between CNC and other materials present in the reaction medium.49 The energies of the hydrogen bonds EH for the (–OH) stretching band of CNC and its hybrids were calculated using the following relation:50

E

H

= K −1 (ν -ν/ν ) 0 0

(2)

where νo is the standard frequency of free (–OH) (≈3650 cm-1), ν is the frequency of the bonded (–OH), and K is a constant (3.8×10-3 kJ-1). The calculated values of EH for CNC, Ag@CNC, and Au@CNC were 14.8, 15.2, and 17.3 kJ, respectively. The spectral shift of hydroxyl peaks as well as the increased energies of hydrogen bonds of Ag@CNC, and Au@CNC indicated that hydrogen bonds formed between (–OH) of CNC and metal nanoparticles in the system.


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Figure 6. FTIR spectra for CNC, Ag@CNC, and Au@CNC hybrids.

Considering the UV-vis, TEM, SEM, EDX, XRD, and FTIR data, a primary mechanism for the formation of Ag@CNC and Au@CNC is proposed. CNC is notable for its high surface-tovolume ratio, as well as large number of hydroxyl groups, sites for metal ion adsorption.51 It is suggested that the metal ions (Ag+1 or Au+3) were bound to the CNC via its hydroxyl groups, implying uniform and tight attachment to the surface. Upon addition of ascorbic acid to the powder comprising the CNCs and the metal precursor, the color of the samples was changed to brown (Ag@CNC) and pink (Au@CNC), due to the successful reduction of metal ions. The strong bonding of metal ions and CNC as well as solid phase nature of the present synthetic


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pathway impeded the mobility of metal ions, inhibited the particle growth, and stabilized the metal nanoparticles. Thermal Properties. Figure 7 includes the thermograms and their derivatives of neat CNC, Ag@CNC, and Au@CNC. The initial weight loss (3%), in the temperature range of (60-105 oC), was due to the evaporation of physically adsorbed water. The main thermal degradation was recorded in the range of 220–400 oC for CNC while for Ag@CNC and Au@CNC it occurred in the 110-400 oC range, respectively. The weight loss in this range is attributed to the thermal degradation of the CNC support. The faster degradation rate of CNC/metal hybrid might be attributed to surface interactions of metal nanoparticles and CNC, that enhanced the heat transfer between the two components.52 Hence, it is hypothesized that Ag and Au nanoparticles acted as catalysts for the thermal degradation of the CNC support.

At the end of the thermal

degradation, the organic component was burned off, leaving 23, 28, and 30% solid residues for CNC, Ag@CNC, and Au@CNC, respectively. The increased value of the solid residues was pointed to the fact that Ag and Au nanoparticles were successfully loaded onto the CNC support.


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Figure 7. TGA thermograms of neat CNC, Ag@CNC, and Au@CNC.

Static catalytic performance of Ag@CNC and Au@CNC. Figure 8 (a, b) includes UV-vis spectra to show the evolution with time of the reduction of 4-NP to 4-AP using NaBH4 in the presence of Ag@CNC and Au@CNC. At the beginning of the reaction, the intensity of 4nitrophenolate peak (~400 nm) faded gradually.53 Meanwhile, a new absorption peak at 298 nm emerged and exhibited increased intensity with reaction time. The appearance of such peak was taken as evidence for the formation of 4-AP.20 These findings agree well with an apparent change in color, from yellow to colorless (see Figure S4). The catalytic degradation at ambient conditions was complete within 15 min. It is worth mentioning that in absence of the metal catalyst, i.e., using a system comprising 4-NP, CNC and NaBH4, and even after several days, no significant change in the spectra was observed, indicating that no reaction occurred.


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The molar ratio of NaBH4 to 4-NP was about 40; hence, the reaction rate (K) did not depend on the concentration of NaBH4. The catalytic degradation reaction was considered as pseudofirst-order and K depended only on the concentration of 4-NP.54 A linear relationship between ln(At/A0) and time t, for both Ag@CNC and Au@CNC, and for three successive cycles is shown in Figure 8 (d, c). The values of K were determined by calculating the slope of the straight lines (Table 1). Both, Ag@CNC and Au@CNC hybrids showed efficient catalytic performance after three cycles. The number of moles of 4-NP reduced per mole of metal catalyst per hour (turnover frequency, TOF), was 420 and 531 h-1 for Ag@CNC and Au@CNC, respectively. Table 2 compares literature data relative to 4-NP degradation using Ag and Au nanoparticles in the presence of CNC supports together with the hybrid, solid state system presented here, which uses a minimum molar ratio of NaBH4 to 4-NP i.e. less amount of the reducing agent is used which minimizes its toxicity. Furthermore, the amount of metal nanoparticles leached out from the hybrid film in solution was detected using atomic absorption spectrometer. No trace of metal was detected after 12 h contact time, which supports our assertion of an efficient fixation of the metal nanoparticle onto CNC matrix. The catalytic degradation of 4-nitrophenol using metal@CNC hybrids, in the powder form, has been studied by several authors. However, engineering facile and efficient recovery processes is still an issue that needs attention in the development of novel catalysts.55,56 The recovery processes that have been used so far include mainly filtration,20 centrifugation, magnetic separation,56 and precipitation by pH shift.12 Significantly, such processes may involve expensive, time and energy consuming steps and/or complex routes. In contrast, our main goal was to introduce an effective and practical way for separating the catalyst from the reaction medium. As a result, it is possible to minimize possible health and ecological issues associated


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with nanoparticle release. Therefore, free standing films of the nanocatalysts are proposed here to facilitate the separation from the reaction medium, without needing additional processes such as those mentioned before. It is possible that the hybrids films could be utilized, for instance, as components in fixed beds under flow of the reacting molecules. The overall implication is that the recovery of the catalyst from the reacting medium is built-into the system used for their synthesis. The efficient catalytic activity of the Ag@CNC and Au@CNC hybrids may be attributed to accessible, well-dispersed, and active metal nanoparticles present on the CNC support. In this way, sustainability demands, including those associated with environmental, economic and remediation issues can be addressed by the proposed approaches.


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Figure 8. Time-resolved UV-vis absorption spectra of the reduction of 4-NP by NaBH4 in the presence of (a) Ag@CNC, (b) Au@CNC and the corresponding logarithm of the absorbance at 400 nm as a function of time (c and d). Table 1. Rate constant k of the reaction based on Ag@CNC and Au@CNC nanocatalysts Rate constant (k, sec-1) 1st cycle

2nd cycle

3ed cycle

Ag@CNC

4×10-3

3.6×10-3

3.4×10-3

Au@CNC

3.31×10-3

2.74×10-3

2.2×10-3


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Table 2. Comparison with recent reports on the catalytic reduction of 4-NP with CNC loaded with Ag and Au nanoparticles

Support system

Catalyst NaBH4/4-NP/(Ag or Au) TOF (h−1) (mol/mol/mol)

Reference s

CNC/CTABa CNC@PDAb CNC CNC/PAMAMc CNC CNC CNC/ PDDAd CNFe CNC

Ag Ag Ag Au Au Au Au Au Au

20

15000/150/1 30723/970/1 9/0.23/1 5000/13/1 9794/3093/1 9720/30/1 36585/ 37/ 1 150 000/150/1 12/0.3/1

545 1077.3 420 740000 641 109 212 563 531

19

This work 12 57 22 34 58

This work

a

Cetyltrimethylammonium bromide, bPolydopamine, cPolyamidoamine, d Poly(diallyldimethyl ammonium chloride), e Cellulose nanofibril

CONCLUSIONS We successfully synthesized heterogenous catalysts consisting of silver and gold nanoparticles grown onto cellulose nanocrystals (CNC), Ag@CNC and Au@CNC, respectively, by using a facile, solvent-free, solid state route. The chemical affinity and physical nanostructure of CNC enhanced the loading of Ag and Au nanoparticles on their surface. Electron microscopy indicated the formation of hexagonal Ag (6 to 35 nm) and spherical Au (18 to 25 nm) nanoparticles. The hybrid Ag@CNC and Au@CNC films showed efficient catalytic activity for the degradation of 4-NP to 4-AP in the presence of NaBH4 with calculated rate constants of 4×10-3 s-1 (Ag@CNC) and 3.31×10-3 s-1 (Au@CNC). In addition, the investigated nanocatalysts were separated and recovered easily as freestanding nanocatalyst films that were reused in several catalytic cycles. The present approach enabled a robust immobilization of metal nanoparticles on the surface of


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CNC with the following advantages: (1) no need for multi-steps reactions; (2) mild reaction conditions; (3) no need for toxic and/or expensive reagents; (4) suitable for large scale production, and (5) minimum chemical waste. This facile room-temperature solid-state route may provide an excellent platform to prepare other eco-friendly metal@CNC nanomaterials with great promise in green heterogeneous catalysis.

ASSOCIATED CONTENT Supporting Information Additional materials are provided from the web as supporting information. These include photo and SEM images of Ag@CNC and Au@CNC and their suspensions at different time intervals; images related to experiments with 4-NP/NaBH4 in absence of catalyst and in the presence of Ag@CNC and Au@CNC. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS W.H. E. and A.M.A. are thankful for the support of the Spectroscopy Department and the Textile Research Division of the National Research Centre (NRC) as well as the USAID program for support during the exchange with the group of O.J.R. O.J.R is grateful to the Academy of Finland for funding support through its Centres of Excellence Programme (2014-2019) and under Project 132723612 (HYBER).


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Synopsis: the present work meets sustainability demands, including those associated with environmental, economic and remediation issues.

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FOR TABLE OF CONTENTS USE ONLY

We propose metal nanoparticle synthesis in solid state to meet sustainability demands, including those associated with environmental, economic and remediation issues.


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We propose metal nanoparticle synthesis in solid state to meet sustainability demands, including those associated with environmental, economic and remediation issues. 128x95mm (300 x 300 DPI)


Solid-State Synthesis of Metal Nanoparticles ... - ACS Publications

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