Microwave-assisted synthesis of single-walled carbon nanotube-supported ruthenium nanoparticles for the catalytic degradation of Congo red dye

Accepted Manuscript Microwave-assisted synthesis of single-walled carbon nanotube-supported ruthenium nanoparticles for the catalytic degradation of Congo red dye Tirandai Hemraj-Benny, Nelson Tobar, Nicholas Carrero, Rawlric Sumner, Leandro Pimentel, Gariele Emeran PII:

S0254-0584(18)30487-5

DOI:

10.1016/j.matchemphys.2018.05.081

Reference:

MAC 20697

To appear in:

Materials Chemistry and Physics

Received Date: 21 February 2018 Revised Date:

15 May 2018

Accepted Date: 29 May 2018

Please cite this article as: T. Hemraj-Benny, N. Tobar, N. Carrero, R. Sumner, L. Pimentel, G. Emeran, Microwave-assisted synthesis of single-walled carbon nanotube-supported ruthenium nanoparticles for the catalytic degradation of Congo red dye, Materials Chemistry and Physics (2018), doi: 10.1016/ j.matchemphys.2018.05.081. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Microwave-Assisted Synthesis of Single-Walled Carbon Nanotube-Supported Ruthenium Nanoparticles for the Catalytic Degradation of Congo red Dye Tirandai Hemraj-Bennya*, Nelson Tobara, Nicholas Carreroa, Rawlric Sumnera, and Leandro Pimentela, Gariele Emerana Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364.

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*Corresponding Author: Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364. Tel: 718-281-5494. Email: [email protected]

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In this article, we describe a rapid, convenient, one-pot microwave method to synthesize Ru nanoparticles supported onto un-functionalized single-walled carbon nanotubes (SWNTs). Depending on the reaction temperature, small ruthenium nanoparticles, of 2.0 ± 0.5 nm or 3.5 ± 0.5 nm, were evenly distributed and stabilized onto SWNTs support without agglomeration. The

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structural integrity of the carbon nanotube was maintained upon microwave irradiation. The structural morphology of the SWNT-Ru nanoparticle composites was analyzed by highresolution transmission electron microscopy (HR-TEM), UV-Visible spectroscopy and Raman

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spectroscopy. The SWNT-Ru nanoparticle composites demonstrated excellent catalytic properties in the decolorization and the degradation of Congo red dye within minutes at ambient

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temperature and without additional perturbation. The support of the SWNTs played a critical role not only in the synthesis of the non-agglomerated Ru nanoparticles but also in the efficient degradation of the Congo red dye. In addition, the effective recoverability and reusability of the catalyst establish its potential for practical catalytic applications.

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Microwave-Assisted Synthesis of Single-Walled Carbon Nanotube-Supported Ruthenium Nanoparticles for the Catalytic Degradation of Congo red Dye Tirandai Hemraj-Bennya*, Nelson Tobara, Nicholas Carreroa, Rawlric Sumnera, Leandro Pimentela, and Gariele Emerana Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364.

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*Corresponding Author: Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364. Tel: 718-281-5494. Email: [email protected]

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Keywords: single-walled carbon nanotubes, ruthenium nanoparticles, microwave irradiation, Congo red dye

INTRODUCTION

Metal nanoparticles are known to exhibit high catalytic activity due to their large specific surface area and unique crystalline structures.1-2 Lately, there have been several reports on using

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Ag,3-4 Au,5 TiO26 and Ru7 nanoparticles for the degradation of azo dyes. Ru nanoparticles are one of the cheapest catalysts and exhibit unique catalytic properties in serving as a significant catalyst in several synthetic processes, such as the synthesis of diesel fuels8 and the removal of

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organic pollutants from water.7, 9 In general, two significant drawbacks of metal nanoparticles in catalytic reactions are the inherit aggregation of the nanoparticles and the poor recoverability and

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separation of the nanoparticle catalysts in reaction mixtures.1, 10 To overcome these difficulties, metal nanoparticles have been immobilized onto solid supports such as silica,11-12 polymers10, 13, and carbon nanomaterials.14-19 Specifically, carbon nanotubes (CNTs) have become attractive solid supports for heterogeneous catalysts due to their small size, high chemical stability, and large surface area.2022

There have been some reports of utilizing multi-walled carbon nanotubes (MWNTs) as

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catalyst supports for hydrogenation and dye degradation reactions.22-27 However, the effect of using single-walled carbon nanotubes (SWNTs) as catalyst support has not been equally explored and are inconclusive. Interestingly, one report suggested that SWNT-supported TiO2

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offered improved catalytic properties in the degradation of Congo red dye when compared with MWNT-supported TiO2.21 It is well known that SWNTs possess smaller diameter (or higher surface area) and higher purity than MWNTs and can offer improved mechanical properties over

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MWNTs due to higher crystallinity.16 In addition, the purity of carbon nanotube surfaces affects the formation of nanoparticles.26, 28 Lower dispersion and larger sizes of Ru nanoparticles were

performance of the hybrid catalysts.

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formed when oxygenated MWNTs were utilized as catalyst supports, which affected the

Conventional preparation methods for Ru nanoparticle-based catalysts, such as vapor deposition, are time-consuming, require multi-steps and do not yield a highly uniformed size and

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distribution of the nanoparticles.26 Microwave-assisted preparation of nanoparticles has been shown to dramatically reduce processing time and enhance product purity. This method can facilitate rapid homogenous heating of the entire sample, allowing for the formation of uniform

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nucleation centers.7, 24, 29 In addition, since microwave heating generates heat directly within the CNT supporting materials, CNTs can act as preferred nucleation sites resulting in a uniform

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dispersion of nanoparticles.24, 30 Moreover, this method of synthesizing nanoparticles is energy efficient and environmentally friendly.29 The efficient degradation of Congo red dye continues to be a challenging task for environmental chemists.31-33 Congo red dye is a well-known carcinogenic pollutant found in the effluent water from textile, paper and plastic industries.31,

34-36

Several physical and chemical

methods have been employed to eliminate Congo red dye from wastewater, which includes

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adsorption, photocatalytic degradation, electrochemical oxidation, and biodegradation.31-32, 37-40 Most of these methods have drawbacks such as slow degradation kinetics, high energy

efficient, rapid and economical ways to degrade Congo red dye.

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consumption, and use of complex synthetic steps.37 Thus, there is still a need to develop more

To the best of our knowledge, there has been no report on the catalytic degradation of Congo red dye in the presence of ruthenium nanoparticles which are stabilized onto un-

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functionalized SWNTs support. In this article, we describe a rapid, convenient, one-pot microwave assisted method to synthesize a novel SWNT-supported Ru nanoparticle hybrid

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catalyst with a high concentration of uniformed size and distribution of Ru nanoparticles. The structural morphology of the SWNT-Ru nanoparticle catalysts was analyzed by high-resolution transmission

electron

microscopy

(HR-TEM),

UV-Visible

spectroscopy

and

Raman

spectroscopy. The effects of varying microwave reaction conditions such as temperature and

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time on the formation of the ruthenium nanoparticles onto the un-functionalized SWNTs support were also investigated. All synthesized SWNT-Ru nanoparticle catalysts demonstrated useful catalytic properties, where the decolorization and the degradation of Congo red dye occurred

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within three minutes, rather than after several minutes or hours as reported in previous works.7, 21 Control experiments were conducted to understand better the role of the SWNTs support in the

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synthesis of the nanoparticles and also in the degradation of Congo red dye. This current work is an extension of the efforts of our group41-42 in understanding metal nanoparticle complexation onto nanotubes to facilitate an enhanced understanding of nanotube solubilization, catalysis, charge transfer, and processability.

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2.

MATERIALS AND METHODS

2.1. Materials: The SWNTs (CoMoCAT) with purity of >90% and diameter of 0.7 nm- 0.9 nm

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were obtained from South West Nanotechnologies, Inc. Anhydrous ethanol, ruthenium (III) chloride, sodium borohydride (NaBH4) and Congo red dye (C32H22N6Na2O6S2) were purchased

solutions were prepared with distilled water.

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from Sigma-Aldrich. All reagents were used as received without further purification. All

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2.2. Synthesis of SWNT-Ru Nanoparticle Catalysts: All synthesis reactions were carried out using a Biotage Initiator 2.5 microwave reactor. Scheme 1 depicts a typical reaction for the synthesis of the SWNT-Ru nanoparticle catalyst. To disperse the nanotube bundles, 15.0 mg of SWNTs (~5 eq.) were initially sonicated in 10 ml of anhydrous ethanol for 5 minutes. A RuCl3

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solution (51.85 mg, 1 eq.) was prepared in 5 ml of anhydrous ethanol. The two solutions (SWNTs and RuCl3) were placed together in a 20 ml Biotage microwave vial, equipped with a stirring bar. A 5 ml NaBH4 solution (28.29 mg, 3 eq.) was then added to the SWNT-RuCl3

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mixture, and the vial was capped. The mixture was initially pre-stirred for 3 minutes, after which it was microwaved at 150 oC for 15 minutes. The SWNT-Ru nanoparticle composite was

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obtained by filtration through a polycarbonate 0.2 µm membrane and was washed several times with anhydrous ethanol. The sample was subsequently dried under vacuum at room temperature. To understand the effect of Ruo nanoparticle loading on the performance of the SWNT-

Ru nanoparticle catalyst, a composite consisting of the stoichiometric ratio of SWNTs (~ 2 eq.) to RuCl3 (1 eq.) was also synthesized under similar reaction conditions described above (microwaved at 150 oC for 15 minutes in the presence of NaBH4, 3 eq. and 20 ml anhydrous

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ethanol).

To understand the effect of time and temperature on the formation of the Ru

nanoparticles onto the un-functionalized SWNTs, and ultimately their catalytic performances, two additional SWNT-Ru nanoparticle composites were synthesized under various conditions.

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With a similar stoichiometric ratio of SWNTs (~ 5 eq.) to RuCl3 (1 eq.) in the presence of NaBH4 (3 eq.) and 20 ml anhydrous ethanol, one mixture was microwaved at 150 oC for 8

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minutes, and another mixture was microwaved at 100 oC for 15 minutes.

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Scheme 1: A schematic representation for the preparation of the SWNT-Ru nanoparticle catalyst. The effects of varying the ratio of SWNT and Ru3+, and microwave temperature and

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time were investigated with constant NaBH4 and anhydrous ethanol.

2.3. Characterization of the SWNT-Ru Nanoparticle Catalysts Microscopy. HR-TEM analyses were carried out on a JEOL JEM 2100F, high-resolution transmission electron microscope (HR-TEM), operated under 200 kV and equipped with energy –dispersive X-ray spectroscopy (EDS). The sample under analysis was sonicated in anhydrous

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ethanol for one minute and was deposited onto a copper grid coated with a lacey carbon film (300 mesh). The EDS analyses were performed using a 0.2 nm spot size. Spectroscopy. UVVisible spectra were collected on a Varian Carry 50Bio UV-Vis spectrometer using quartz cells

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possessing a 10 mm path length. Raman spectra were obtained on a Thermo Scientific DXR Raman Microscope with a 10X (0.25 N.A.) objective. Solid samples, which were placed on to microscope slides, were excited at 455 nm, and 532 nm. To obtain a more accurate

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representation of the nanotube samples, averaged values of three scans were obtained for each

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sample.

2.4. Catalytic Degradation of Congo red Dye

The catalytic activity of the SWNT-Ru nanoparticle composites was monitored by UVVisible spectroscopy. Typically, the desired mass of SWNT-Ru nanoparticle catalyst, in mg, was

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added to 2 ml of aqueous Congo red dye solution. The mixture was sonicated for no more than 5 seconds to disperse the SWNT-Ru nanoparticle catalyst. The sonicated mixture was then transferred to a quartz cuvette, and 1 ml of freshly prepared 5 mM NaBH4 solution was rapidly

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injected. The progress of decolorization and degradation, at room temperature, was continuously monitored at intervals of 5 seconds, scanning from 200 nm to 800 nm, without further stirring or

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perturbation. Absorbance values were obtained until constant values were observed, which also corresponded to the time of complete disappearance of the bright red color of the Congo red dye. Calibration plots based on Beer-Lambert’s law were established by relating absorbance to concentration.37

The degradation reaction kinetics were studied by varying different parameters such as catalyst amount, and the initial concentration of the dye (Co). The feature at λ500 nm was observed

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with time, and the values were used in equation (1) to determine the % degradation of the Congo red dye in the presence of the SWNT-Ru nanoparticle catalyst.4 Degradation efficiency (%) = =

× 100

(1)

respectively.

Reaction rate = ln

=k

t

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The reaction rate was determined by equation 2 when applicable.43

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Where Co and Ct are the initial concentration and the concentration at the time, t, of the dye

(2)

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Where kapp is the apparent pseudo-first order reaction rate constant (min-1), and t is the reaction time. A plot of ln (Co/Ct) versus t yields a slope of kapp.

3. RESULTS and DISCUSSION 3.1. SWNT-Ru Nanoparticle Catalysts

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Figure 1(a) - (c) show the HR-TEM images of the SWNTs with incorporated ruthenium nanoparticles, and Figure 1 (d) shows the untreated starting SWNTs. The starting SWNT bundles were pure and relatively free of amorphous carbon and metal impurities. The individual

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SWNT ranged in sizes from 0.7 nm to 0.9 nm and were found in bundles ranged in sizes from 5

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to 60 nm in diameter. The EDS data (Figure 1 (e)) confirmed that the starting SWNTs were free of Ru and other metal nanoparticles. The Cu signal was due to the TEM sample grid. The HR-TEM images of the SWNT-Ru nanoparticle catalysts indicated that all

microwaved samples of similar stoichiometric ratio of SWNT (~5 eq.) to Ru3+ (1 eq.) yielded a highly loaded, uniformed distribution of Ruo nanoparticles onto the SWNTs. The SWNT-Ru nanoparticle catalysts microwaved at 150 oC (Figure 1 (a) and (b)) yielded Ruo nanoparticles of an averaged size of 3.5 ± 0.5 nm onto the SWNT bundles. However, the duration of microwave

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time affected the distribution of the Ruo nanoparticles onto the SWNT bundles. A denser distribution of nanoparticles was observed when the SWNT-Ru3+ mixture was microwaved for 15 minutes, Figure 1 (a), as opposed to 8 minutes, Figure 1 (b). It was noted that the microwave

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temperature affected the size range of the Ru nanoparticles. The SWNT-Ru nanoparticle catalyst microwaved at 150 oC for 15 minutes, Figure 1 (a), yielded Ruo nanoparticles of an averaged size of 3.5 ± 0.5 nm onto the SWNT bundles.

However, SWNT-Ru nanoparticle catalysts

averaged size of 2.0 ± 0.5 nm onto the SWNT bundles.

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microwaved at 100 oC for 15 minutes, Figure 1(c), yielded smaller Ruo nanoparticles of an

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Figure 1(f) shows a representative EDS spectrum of the treated SWNTs which indicated that no by-product impurities were present. This may be attributed to an effective post washing treatment of all synthesized catalysts. It should be noted that Ru nanoparticles synthesized with similar ratios of reagents (Ru3+,1 eq. in 20 ml ethanol and NaBH4, 3 eq.) and under identical

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microwave conditions (150 oC for 15 minutes) in the absence of SWNTs, led to clumping of the Ru nanoparticles (HR-TEM image in Figure S1).

Thus, the SWNT bundles prevented

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agglomeration of the Ru nanoparticles and served as an essential size and growth stabilizer.

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Figure 1: HR-TEM images of the SWNT-Ru nanoparticle catalysts, of similar stoichiometric equivalences, microwaved at (a) 150 oC for 15 mins, (b) 150 oC for 8 mins, (c) 100 oC for 15

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mins; and (d) the untreated SWNTs. Uniformed size and distribution of Ru nanoparticles onto the SWNTs were observed for all catalysts. Averaged particle sizes of 3.5 ± 0.5 nm were observed for mixtures microwaved at 150 oC for 15 mins and 150 oC for 8 mins, (a) and (b) respectively, and averaged particle size of 2.0 ± 0.5 nm was observed for mixtures microwaved at 100oC for 15 mins, (c). The insets show lattice fringes of ruthenium nanoparticles. (e) the EDS spectrum of untreated SWNTs indicated the absence of ruthenium, and (d) a representative EDS

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spectrum of treated SWNTs showed the presence of ruthenium. The Cu signal originated from the TEM sample grid. Allowable transitions between van Hove singularities in the electronic density of state of

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SWNTs are observable as spike-like features in the optical spectra of the nanotube solutions, which correspond to the specific diameter of carbon nanotubes.44 Figure 2 represents a comparison of the un-treated CoMoCAT SWNTs, pure ruthenium (III) chloride starting material,

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and SWNT-Ru nanoparticle catalysts in the UV-Visible region.

In Figure 2 (a) - (e), features at 471, 589, 665, 1050 and 1064 nm correspond to

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semiconducting nanotubes measuring 0.678, 0.84, 0.782, 1.48 and 1.55 nm in diameter respectively.45 In general, since there was a preservation of these observable features seen in the treated SWNTs spectra (Figure 2 (b) – (e)), it can be concluded that the structural integrity of the SWNTs was not negatively affected during the microwave synthesis of the Ru nanoparticles onto

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the SWNTs. The spectrum for the ruthenium (III) chloride salt (Figure 2 (f)) is characterized by a feature at 395 nm which corresponds to Ru3+. This feature completely disappeared in the plots corresponding to the synthesized SWNT-Ru nanoparticle catalysts (Figure 2 (b) – (e)). Thus, it

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can be inferred that the Ru3+ ions were not present in any of the SWNT-Ru nanoparticle catalyst samples. The reduction of the Ru3+ ions was completed in less than 15 minutes, a considerably

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shorter period compared to hours required by the traditional heating process. Based on theoretical studies of the partial density of electron state (DOS) of Ru-carbon

nanotube hybrid systems, it can be implied that the Ru-SWNT interaction mainly originates from the hybridization of the d electrons from the ruthenium atom and sp electrons from the carbon atom.46 Moreover, the interaction between the ruthenium nanoparticles and the SWNTs must have been sufficiently strong to withstand extensive rinsing and repetitive dispersion. It should

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also be noted that the dispersive properties of all SWNT-Ru nanoparticle catalysts in water were

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significantly improved when compared with the untreated SWNTs.

Figure 2: UV-Visible spectra of (a) starting material, SWNT; (b) SWNT-Ru nanoparticle catalyst (SWNT, ~5 eq. : Ru3+,1 eq.) microwaved at 150 oC for 15 mins; (c) SWNT-Ru nanoparticle catalyst (SWNT, ~5 eq. : Ru3+,1 eq.) microwaved at 100 oC for 15 mins, (d) SWNTRu nanoparticle catalyst (SWNT, ~5 eq. : Ru3+,1 eq.) microwaved at 150 oC for 8 mins; (e) SWNT-Ru nanoparticle catalyst ( SWNT, ~2 eq. : Ru3+,1 eq.) microwaved at 150 oC for 15 mins; and (f) starting material, RuCl3.

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Raman spectroscopy is widely used for the characterization of SWNT samples and for

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gaining information about their structural integrity. In this experiment two excitation wavelengths were used, 455 nm and 532 nm, which bring nanotubes of different diameter into resonance. Raman analyses of the catalysts consisting of varying stoichiometric ratios (SWNT,

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~5 eq.: Ru3+,1 eq. and SWNT, ~2 eq.: Ru3+,1 eq.) were conducted to understand the effect of metal loading on the structural integrity of the treated SWNTs and the de-bundling effect of the

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SWNTs, Figure 3.

The tangential G band mode, appearing in the 1400-1700 cm-1 range, is sensitive to charges exchanged between nanotubes and guest atoms that have intercalated into the nanotube bundles. The disorder D band, appearing in the 1320-1360 cm-1 range, is related to the presence

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of defects, as well as, nanoparticles and amorphous carbon. An obtained value closest to one, using the formula 1-D/G where D represents the intensity of the disorder D-band, and G represents the intensity of the tangential G-band, indicates the highest purity and structural

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integrity of the SWNT. 44

At 532 nm excitation, Figure 3 (a), a value of 0.879 was calculated for the starting

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untreated SWNTs. A slightly decreased value of 0.873 was noted for the SWNT-Ru nanoparticle catalyst consisted of a stoichiometric ratio of SWNT (~5 eq.) to Ru3+ (1 eq.). An increased value of 0.925 was noted for the SWNT-Ru nanoparticle catalyst consisted of a stoichiometric ratio of SWNT (~2 eq.) to Ru3+ (1 eq.). Upon 455 nm excitation, Figure 3 (b), a value of 0.796 was calculated for the starting untreated SWNTs, with a decreased valued of 0.643 for the SWNT-Ru nanoparticle catalyst consisted of a stoichiometric ratio of SWNT (~5 eq.) to Ru3+ (1 eq.) and a

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decreased value of 0.658 for the SWNT-Ru nanoparticle catalyst consisted of a stoichiometric ratio of SWNT (~2 eq.) to Ru3+ (1 eq.). In general, the observed reduced values, upon the incorporation of the Ru nanoparticles

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onto the SWNT bundles were not significant, and thus, it can be implied that there were no significant induced damages to the crystalline structure of the SWNTs, as was also confirmed from the UV-Visible data. The observed decreased values upon 455 nm excitation may be as a

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result of some degree of de-bundling of specific diameters of carbon nanotubes caused by the synthesized Ruo nanoparticles.

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Further evidence of de-bundling of the nanotubes was observed by the decreased intensities of the feature at 1540 cm-1, which can be well fitted by a Breit-Wigner-Fano (BWF) line shape. It has been reported that a reduction in intensity of this feature can be related to debundling of metallic nanotubes.44 Thus, the observed decrease in intensity of this feature for both

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SWNT-Ru nanoparticle catalysts, upon 455 nm excitation, supports the possibility that the presence of the Ru nanoparticles produced a de-bundling effect and perturbed the interactions between individual nanotubes. It should be noted that based on the calculated data obtained the

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increased metal loading did not facilitate an increased alteration to the structural integrity of the

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SWNTs or a higher degree of de-bundling of the SWNTs.

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Figure 3: Raman spectra (D band and G band regions) of untreated SWNTs (

black line); and

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SWNT-Ru nanoparticle catalysts microwaved at 150 oC for 15 mins, with a stoichiometric ratio of SWNT, ~5 eq. : Ru3+,1 eq. ( Ru3+,1 eq.(

dotted line), and with a stoichiometric ratio of SWNT, ~2 eq. :

dashed line). (a) Excitation wavelength at 532 nm, and (b) Excitation

wavelength at 455 nm. Normalization was performed concerning the feature at around 2035 cm1

.

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3.2. Catalytic Degradation of Congo red Dye using SWNT-Ru Nanoparticle Catalysts

To evaluate the catalytic properties of the well-defined Ruo nanoparticles, uniformly dispersed onto the SWNT supports, the decolorization and degradation reaction of Congo red

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dye at room temperature in the presence of NaBH4 was studied. As observed from Figure 4, absorption spectra of the original Congo red dye solution were characterized by one main band

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in the visible region with its maximum absorption at 500 nm and by another band located at 345 nm. The absorbance peak at 345 nm has been attributed to the naphthalene ring structure, while the absorbance peak at 500 nm has been assigned to the azo bonds (N=N) of the Congo red dye

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molecule, Figure 5.43

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Figure 4: UV-Visible spectra of the degradation of 0.05 mM Congo red dye in the presence of NaBH4 and the SWNT-Ru nanoparticle catalyst (SWNT, ~5 eq.: Ru3+,1 eq.; microwaved at 150 o

C for 15 minutes) showing the degradation progress with reaction time. A freshly prepared

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sample of 1 ml of 5 mM NaBH4 and 0.5 mg of the SWNT-Ru nanoparticle catalyst was used.

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Figure 5: Schematic (not drawn to scale) representing the interaction between the SWNT and the Congo red dye molecule, and the electron transfer from the borohydride ions to the

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Congo red dye molecule via the Ru nanoparticle located on the SWNT.

According to the literature, the decrease in absorbance value at 500 nm can be attributed

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to the priority destruction of the azo linkage (N=N) by the attack, leading to the decolorization of the Congo red dye due to the rapid disappearance of chromophores in the dye structure.7, 18, 43 This initial step of the destruction of the azo linkage is also considered to be the fastest step leading towards decolorization and degradation of the dye.18, 43 Thus, the degradation efficiency of the SWNT-Ru nanoparticle catalysts and the degradation rate of reactions were monitored by observing the change in the absorbance value of the peak at 500 nm. In general, in the presence

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of the SWNT-Ru nanoparticle catalysts, the disappearance of the peak at 500 nm started to occur within seconds. Since the concentration of NaBH4 was significantly larger than that of the Congo red dye, it can be assumed that even after complete reaction, the concentration of NaBH4 did not

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change substantially.

3.2.1. The Effect of Initial Dye Concentration on the Degradation of Congo red Dye

To optimize the suitable conditions and properties of the SWNT-Ru nanoparticles

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catalyst for maximum catalytic activity, degradation studies were performed with different initial

catalysts, Figure 6.

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Congo red dye concentrations in the presence of the synthesized SWNT-Ru nanoparticle The concentration of NaBH4 (5 mM) and the SWNT-Ru nanoparticle

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catalyst (0.5 mg) were kept constant in all cases.

Figure 6: Plots of maximum dye degradation efficiencies obtained for each initial Congo red dye concentration in the presence of NaBH4 (5 mM) and varying SWNT-Ru nanoparticle

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catalyst (0.5 mg): (a) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 100 oC for 15 minutes; (b) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 8 minutes; (c) SWNT (2 eq.) to Ru3+(1 eq.) microwaved at 150 oC for 15 minutes; and (d) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at

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150 oC for 15 minutes.

In general, it was observed that the optimum % degradation of the Congo red dye increased as the concentration of the dye increased, Figure 6 (a)-(d)). This increase in %

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degradation with increasing initial concentration of Congo red dye observed in the presence of all catalysts can be explained by the fact that more dye molecules were available as the

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concentration of the dye solution increased, and thus improved the catalyst-dye substrate contact and interactions.7 However, catalysts which were synthesized upon microwaving for 15 minutes, regardless of stoichiometric ratio and microwave temperature, exhibited a subsequent decreased in % degradation of Congo red dye of concentrations higher than 0.06 mM, Figure 6 (a), (c) and

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(d), indicative of the saturation or the unavailability of the catalyst adsorption sites. This trend was not observed when the SWNT-Ru nanoparticles catalyst which was synthesized upon microwaving for 8 minutes was present, Figure 6 (b). As concluded from the HR-TEM analysis,

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although this particular catalyst contained Ruo nanoparticles of similar sizes, i.e., 3.5 ± 0.5 nm, as compared with the other catalysts synthesized upon microwaving at 150 oC for 15 minutes, a less

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dense distribution of Ruo nanoparticles on the SWNT bundles was observed, Figure 1 (b). And thus, the availability of the SWNT surfaces may have played a role in the dye degradation, especially at higher concentrations where a greater amount of dye substrates was present. Although, a less dense distribution of Ruo nanoparticles of 3.5 ± 0.5 nm was observed for

the catalyst (SWNT, ~5 eq.: Ru3+, 1 eq.) synthesized by microwaving at 150 oC for 8 minutes, Figure 1 (b) and 6 (b), the amount of Ruo nanoparticles present was sufficient to facilitate efficient

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degradation of the Congo red dye. While at low concentration (0.01 mM) this catalyst exhibited a dye degradation efficiency of 29%, at higher concentration (0.07 mM) a dye degradation efficiency of 81 % was observed, which occurred after 1.6 minutes, Figure 6 (b) and 7 (c). SWNT-Ru

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nanoparticle catalyst (SWNT, ~5 eq.: Ru3+, 1 eq.; microwaved at 150 oC for 15 minutes), yielded similar nanoparticle sizes of 3.5 nm ± 0.5 nm which were more densely distributed onto the SWNT bundles. At low concentration (0.01 mM) this catalyst exhibited a lower dye degradation efficiency

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of 4%, and at high concentration (0.06 mM) a dye degradation efficiency of 81.3 % was observed after 1.6 minutes. At higher concentration (0.07 mM), a decrease in % degradation (66.8 %) was

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observed. Thus, it can be concluded that a more efficient SWNT-Ru nanoparticle catalyst of SWNT, ~5 eq.: Ru3+, 1 eq. ratio, microwaved at 150 oC can be obtained after 8 minutes. The most efficient SWNT-Ru nanoparticle catalyst, which degraded the Congo red dye substrate of both low concentration (0.01 mM, 63.8 %, 2.1 min.) and high concentration (0.06 mM,

o

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91.0 %, 3.2 min.), was the catalyst (SWNT, ~5 eq.: Ru3+, 1 eq.) synthesized by microwaving at 100 C for 15 minutes. HR-TEM analysis, Figure 1 (c), confirmed the presence of smaller uniformed

Ruo nanoparticles sizes of 2.0 nm ± 0.5 nm, which were highly distributed onto the SWNTs.

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It was noted that an increase in metal loading of the Ruo nanoparticle did not necessarily correlate to an improved % dye degradation. However, increase metal loading resulted in higher %

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dye degradation at lower dye concentrations. SWNT-Ru nanoparticle catalyst of higher metal loading (SWNT, ~2 eq.: Ru3+, 1 eq.; microwaved at 150 oC for 15 minutes), yielded similar nanoparticle sizes of 3.5 nm ± 0.5 nm which were densely distributed onto the SWNT bundles. At low concentration (0.01 mM) this catalyst exhibited a dye degradation efficiency of 25.0 %, and at high concentration (0.06 mM) a dye degradation efficiency of 75.0 % was observed after 2.2 minutes. The higher % dye degradation (25.0 % vs. 4.0 %, Figure 6 (c) and (d) respectively)

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observed at low initial dye concentration (0.01 mM) in the presence of the catalyst with higher metal loading may be due to an increase in Ruo nanoparticle on the SWNT bundles which

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facilitated improved catalyst-dye substrate interactions. In general, the observed efficient % degradation combined with the short degradation time for all SWNT-Ru nanoparticle catalysts is a significant improvement when compared with previously reported catalysts for Congo red dye degradation which occurred after several

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minutes to an hour.7, 21

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The effect of various initial concentration of Congo red dye on the reaction rate over time is shown in Figure 7 (a) – (d). The steeper the slope, the higher the rate constant, and thus, faster the reaction. In general, the initial reaction rate increased with increasing initial dye concentrations for all SWNT-Ru nanoparticle catalysts. This observed trend may also be related to the presence of increased dye molecules as the Congo red dye concentration increased. It is

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known that SWNTs, with π-conjugative structure, can easily interact with aromatic compounds via π− π stacking interactions.47-48 The chance of the Congo red dye molecules being initially attracted towards the SWNT support, via π− π interactions, and are subsequently degraded by the

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Ru nanoparticles7 increases with increasing Congo red dye molecule present in the system,

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Figure 5. And thus, a faster rate of degradation was noted for higher Congo red dye concentrations.

It was observed that data obtained for only the SWNT-Ru nanoparticle catalyst which

was synthesized with a SWNT (5 eq.) to Ru3+ (1 eq.) ratio and microwaved at 150 oC for 15 minutes, fitted the first order kinetic equation, ln At = ln Ao – kt. The highest rate constants, kapp, were observed for higher initial Congo red dye concentrations (0.03mM to 0.07 mM), with an optimum rate constant (4.562 min-1) observed with an initial dye concentration of 0.04 mM,

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Figure S 2. Since the first-order, second-order and third-order kinetic equations failed to provide agreement with the absorbance-time data obtained experimentally for the other catalysts, Figure

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7 (b)-(d), the reaction rates were determined from the slope of the steep linear part of the plots.

Figure 7: The degradation rate of different initial Congo red dye concentrations in the

presence of NaBH4 (5 mM) and different SWNT-Ru nanoparticle catalysts (0.5 mg): (a) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 15 minutes.; (b) SWNT (2 eq.) to Ru3+(1 eq.)

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microwaved at 150 oC for 15 minutes; (c) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC

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for 8 minutes; and (d) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 100 oC for 15 minutes.

The two SWNT-Ru nanoparticle catalysts microwaved at 150 oC for 15 minutes of varying stoichiometric ratios, SWNT, ~5 eq. to Ru3+, 1 eq., Figure 7 (a) and SWNT, ~2 eq. to

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Ru3+, 1 eq., Figure 7 (b) showed no sigmoidal curves, thus indicating that the catalytic reactions had no induction period for all initial dye concentrations. A significantly faster initial rate was

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observed for the catalyst of higher metal loading, Figure 7 (b), which occurred within 15 seconds for all concentrations. This trend may be attributed to an increase in Ruo nanoparticles on the SWNT bundles leading to increasing catalyst-dye substrate interactions. The two SWNT-Ru nanoparticle catalysts consisted of SWNT, ~5 eq. to Ru3+, 1 eq. and

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microwaved at 150 oC for 8 minutes, Figure 7 (c), and microwave at 100 oC for 15 minutes, Figure 7 (d), depicted varying rates of reaction consisting mostly of sigmoidal curves. This observed induction period, which occurred because of a delay in catalyst-dye substrate

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interaction, may be related to the morphology of the SWNT-Ru nanoparticle catalysts. From HRTEM data, Figure 1, the catalyst used in Figure 7 (c) consisted of well- defined Ruo nanoparticles

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of 3.5 ± 0.5 nm and were not as densely dispersed onto the SWNT bundles as observed with the other catalysts, Figure 1 (b). On the other hand, smaller diameter of Ruo nanoparticles of 2.0 ± 0.5 nm, were observed, Figure 1 (c), for the catalyst used in Figure 7 (d). Interestingly, regardless of the SWNT-Ru nanoparticle catalyst used, an initial dye concentration of 0.04 mM never produced a sigmoidal plot, and thus, never possessed an initial induction period.

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3.2.2. The Effect of SWNT-Ru nanoparticle Catalyst Amount on the Degradation of Congo red Dye The effect of the amount of SWNT-Ru nanoparticle catalysts on the degradation of

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Congo red dye has also been studied by varying the amount of the catalysts from 0.5 mg to 1.5 mg, Figure 8 and 9. The concentration of Congo red dye (0.02 mM) and NaBH4 (5 mM) were kept constant. A low Congo red dye concentration of 0.02 mM was selected for this particular

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study to ensure the efficient process of monitoring the degradation of Congo red dye by UV-

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Visible spectroscopy without scattering caused by high quantities of carbon nanotubes.

Figure 8: Effect of SWNT-Ru nanoparticle catalyst doses on the degradation of Congo

red dye. A constant dye concentration (0.02 mM) and NaBH4 (5 mM) were used for each run. Plots of maximum dye degradation efficiencies obtained for each catalyst dose: (a) SWNT (5

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eq.) to Ru3+ (1 eq.) microwaved at 100 oC for 15 minutes; (b) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 8 minutes; (c) SWNT (2 eq.) to Ru3+(1 eq.) microwaved at 150 oC for

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15 minutes; and (d) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 15 minutes.

Figure 9: The degradation rate of 0.02 mM Congo red dye in the presence of NaBH4 (5

mM) and various amount of SWNT-Ru nanoparticle catalysts: (a) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 15 minutes.; (b) SWNT (2 eq.) to Ru3+(1 eq.) microwaved at 150 oC for 15 minutes; (c) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 150 oC for 8 minutes; and (d) SWNT (5 eq.) to Ru3+ (1 eq.) microwaved at 100 oC for 15 minutes.

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It was observed that data obtained for only the SWNT-Ru nanoparticle catalyst which was synthesized with a SWNT (5 eq.) to Ru3+ (1 eq.) ratio and microwaved at 150 oC for 15

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minutes, fitted the first order kinetic equation, ln At = ln Ao – kt, Figure S 3. Since the first-order, second-order and third-order kinetic equations failed to provide agreement with the absorbance-

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time data obtained experimentally for the other catalysts, Figure 9 (b)-(d), the reaction rate was determined from the slope of the steep linear part of the plots.

In the presence of the smaller Ruo nanoparticles, 2.0 ± 0.5 nm, the % degradation of the Congo red dye decreased as the amount of catalyst increased, Figure 8 (a). However, it was

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observed that as the amount of catalyst increased, the initial rate of the reaction significantly increased, Figure 9 (d). In fact, the original induction period observed with 0.5 mg of the catalyst, Figure 7 (d) and 9 (d) completely disappeared as the amount of catalyst increased to 1.0

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mg and 1.5 mg. This trend is related to the increased catalyst-dye substrate contact as a result of the increasing amount of catalyst present. The observed decrease in % dye degradation may be

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related to the catalytic sites being blocked causing the catalyst to receive a limited amount of Congo red dye to degrade. This could potentially be due to the lack of mobility of degraded products off of the catalytic sites caused by attraction or entanglement with the increased amount of SWNT surfaces present in relation to the smaller sized Ruo nanoparticles. In the presence of the larger Ruo nanoparticles, 3.5 ± 0.5 nm, the % degradation of the Congo red dye and rate of the reaction significantly increased as the amount of catalyst

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increased. Figure 8 (d) and 9 (a). Since this particular catalyst (SWNT, 5 eq. to Ru3+, 1 eq.; microwaved at 150 oC for 15 minutes) fitted the first order kinetic equation (Figure S 3), it was possible to determine the Kapp values. The rate of degradation dramatically increased (~10

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times) when the amount of catalyst was tripled, Figure 9 (a) and Figure S (3). An observed Kapp value of 0.0829 min-1 was observed for the Congo red dye solution consisting of 0.5 mg catalyst, while an observed Kapp value of 0.799 min-1 was observed for the Congo red dye solution

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consisting of 1.5 mg catalyst. The increased amount of SWNT-Ru nanoparticle catalyst facilitated improved catalyst-dye substrate contact, which resulted in the observed increased %

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degradation efficiency and rate of degradation of Congo red dye.

Overall, an increase in % degradation and degradation rate as the catalyst amount increased was also observed with the catalyst consisting of higher metal loading, Figure 8 (c), and Figure 9 (b), which may also be related to an increase in catalyst-dye substrate contact. On

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the other hand, a decrease in % degradation and degradation rate as the catalyst amount increased was observed with the catalyst consisting of the fewer distribution of Ruo nanoparticles of the SWNTs, Figure 8 (b) and 9 (c).

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In general, the % degradation of the Congo red dye and the rate of reaction varied as the amount of the different SWNT-Ru nanoparticle catalyst increased, which may be related the

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particular environment of each case. Increased degradation rate can be related to improved catalyst-dye substrate contact by either having a greater amount of catalyst or having an environment where there is no hindrance or lack of accessibility to the Ruo nanoparticles on the SWNTs. It was noted that whenever there was an increased existence of the SWNT surfaces with relation the Ruo nanoparticles, the % degradation of the dye decreased, Figure 8 (a) and (b). It is known that electrons can be easily transferred from Ru nanoparticles to SWNTs.46 And thus, the

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observed decrease in % dye degradation may be as a result of physical blockages due to reaction products or intermediates remaining on the SWNTs or by the SWNTs interfering with the electron transfer process.

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In a control experiment, the degradation of 0.02 mM Congo red dye in the presence of 5 mM NaBH4, without the presence of any SWNT-Ru nanoparticle catalyst, was monitored. A degradation occurred with 8.9% degradation efficiency after 84 minutes, (Figure S 4). However,

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upon the addition of the synthesized SWNT-Ru nanoparticle catalysts (0.5 mg) to the system, a more rapid degradation was noticed, which occurred in less than 2.6 minutes and yielded higher

Catalyst

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% degradation efficiencies, Figure 8 and 9 and Table 1.

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NaBH4 only SWNTs only o 5:1 (100 C, 15 min.); ~2.0 nm Ru NP 5:1 (150 oC, 8 min.); ~3.5 nm Ru NP 2:1 (150 oC, 15 min.); ~3.5 nm Ru NP 5:1 (150 oC, 15 min.); ~3.5 nm Ru NP

% Degradation of 0.02 mM Congo red dye 8.9 % 48.3 % 76.6 % 52.8 % 24.0 % 4.0 %

Degradation time 84 min. 6 days 1.74 min. 1.33 min. 2.66 min. 1.20 min.

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Table 1: A comparison of % degradation and degradation time upon completion for 0.02 mM Congo red dye in the presence of various reagents. SWNT-Ru nanoparticle composites of 0.5 mg

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are represented.

It is evident from the data obtained that the synthesized SWNT-Ru catalyst plays an

essential role in the degradation of Congo red dye. The Ru nanoparticles supported on the SWNTs acted as the mediator in the ion transfer from the borohydride ions to the azo bonds of Congo red7, Figure 5. The primary role of the Ruo nanoparticle is to trap and transfer the

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electrons to the dye to facilitate the reduction process. This process may have occurred at a faster rate in the presence of the SWNTs due to the structure of the SWNTs promoting an initial attraction of the Congo red dye molecules, which were subsequently reduced by the attached Ru

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nanoparticles. An appropriate combination of the un-functionalized SWNT bundles and the Ru nanoparticles provided an excellent environment to promote rapid degradation of the Congo red dye within seconds.

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To further understand the role of the SWNTs in the decolorization and degradation of Congo red dye a controlled study was performed with 0.02 mM Congo red dye and two different

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amounts of starting pristine SWNTs (0.5 mg and 1.0 mg), Figure S 5. Interestingly, observed decolorization of the Congo red dye occurred after six days (48.3% with 0.5 mg SWNTs and 62.5% with 1.0 mg SWNTs). It is believed that the Congo red dye molecules were eventually adsorbed onto the un-functionalized SWNT bundles via π− π stacking interactions, which

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interfered with the chromophores in the dye structure.21 It was evident that at low concentration of the SWNTs (0.5 mg), the interactions were not strong due to the fluctuation of the calculated % dye degradation efficiencies, Figure S 5. These observations further strengthen the proposed

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mechanism of degradation of the Congo red dye by the synthesized SWNT-Ru nanoparticle catalyst where an initial attraction between the SWNTs and the Congo red dye molecules occurs,

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follow by an immediate degradation facilitated by the Ru nanoparticles and the NaBH4.

3.2.3. Reusability of the SWNT-Ru Nanoparticle Catalyst Herein, the recyclability of the SWNT-Ru nanoparticle catalyst, which is crucial for practical applications, was also investigated. Reusability test of the SWNT-Ru nanoparticle catalysts was performed using the catalyst isolated from the reaction solution after a previous

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degradation run of Congo red dye. The catalyst was isolated by filtration, washed with distilled water, and was dried in an oven at 120 oC overnight. The entire powder catalyst sample was then re-dispersed into 2 ml of 0.02 mM Congo red dye, which was placed into a quartz cuvette and 1

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ml of 5 mM NaBH4 was injected into the sample. The absorbance value at 500 nm was monitored over time until the absorbance value remained constant. At that point, the solution also became colorless. This process was repeated for a total of 3 cycles. The reusability tests

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revealed that the SWNT-Ru nanoparticle catalysts were still active after three re-uses for the degradation of Congo red dye with no significant decrease in % degradation efficiency. Also, it

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can be concluded that the interaction between the SWNTs and the Ru nanoparticles were strong since the SWNTs stabilized the Ru nanoparticles for at least three cycles. Recoverability of the Ru nanoparticles was fast and efficient due to the SWNT support.

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4. CONCLUSIONS

In conclusion, a fast, convenient, one-pot microwave assisted method was implemented for the synthesis of a novel single-walled carbon nanotube-supported Ru nanoparticle catalyst.

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Depending on the reaction temperature, the Ru nanoparticles averaged sizes of 2.0 nm ± 0.5 nm or 3.5 nm ± 0.5 nm were uniformly distributed on the surfaces of the SWNTs. UV-Visible data

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indicated that the structural integrity of the SWNTs was maintained upon microwave irradiation. Raman data indicated some degree of de-bundling of the nanotubes occurred upon incorporation of the Ru nanoparticles, which may have enabled the observed increased dispersion of the SWNTs.

The degradation of Congo red dye in the presence of the novel synthesized SWNT-Ru nanoparticle hybrids was studied as a model reaction. The % dye degradation and degradation

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rate increased initially with increasing dye concentration. An efficient SWNT-Ru nanoparticle catalyst of SWNT, ~5 eq.: Ru3+, 1 eq. ratio, microwaved at 150 oC was obtained after 8 minutes. The most efficient SWNT-Ru nanoparticle catalyst was obtained when the SWNT-Ru3+ mixture

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was microwaved at 100 oC for 15 minutes. Ruo nanoparticles of sizes 2.0 nm ± 0.5 nm were densely distributed onto the SWNT bundles and degraded the Congo red dye substrate of both low concentration (0.01 mM, 63.8 %, 2.1 min.) and high concentration (0.06 mM, 91.0 %, 3.2

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min.). It was noted that an increase in metal loading of the Ruo nanoparticle did not necessarily correlate to an improved % dye degradation. However, a higher % dye degradation was observed

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at low initial dye concentration which may be due to an increase in Ruo nanoparticle on the SWNT bundles.

Control experiments demonstrated that the SWNTs support plays a critical role not only in the synthesis of non-agglomerated Ru nanoparticles but also in the efficient degradation of the

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Congo red dye. In addition, the SWNT-Ru nanoparticle catalyst displayed great stability and recyclability properties, and thus shows a promising future for practical applications.

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5. ACKNOWLEDGMENTS

This work was funded by the Professional Staff Congress-City University of New York Grant

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(Award #TRADA-46-683) and by the NIH Bridges to the Baccalaureate Program (NIH 5R25GM065096). HR-TEM analyses were carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. We acknowledge Dr. Alex Rzhevskii and Ad Boyer from Thermo Fisher Scientific for the assistance in obtaining the Raman data, and Dr. Lihua Zhang and Na Li for assistance with the HR-TEM analyses.

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6. SUPPLEMENTARY MATERIAL HR-TEM image of Ru nanoparticles synthesized in the absence of SWNT, effect of initial dye

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concentration and the amount of catalyst on the degradation of Congo red dye in the presence of the SWNT-Ru nanoparticle catalyst, % dye degradation of 0.02 mM Congo red dye in the presence of only NaBH4, and % dye degradation of 0.02 mM Congo red dye in the presence of

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only SWNTs.

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ABSTRACT GRAPHICS 34

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Microwave-Assisted Synthesis of Single-Walled Carbon Nanotube-Supported Ruthenium Nanoparticles for the Catalytic Degradation of Congo red Dye

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Tirandai Hemraj-Bennya*, Nelson Tobara, Nicholas Carreroa, Rawlric Sumnera, and Leandro Pimentela, Gariele Emerana Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364.

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*Corresponding Author: Queensborough Community College, Department of Chemistry, S-443, 222-05 56th Avenue, Bayside, NY, 11364. Tel: 718-281-5494. Email: [email protected]

Highlights

Rapid microwave synthesis of a carbon nanotube-ruthenium nanoparticle composite.

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The size of the ruthenium nanoparticles varied with reaction temperature.

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Composites allowed for the degradation of Congo red dye within minutes.

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