Kinetics of palladium nano-particles catalyzed reduction of Methylene Green by hydrazine: Role of induction period in determining mechanistic pathway

Inorganica Chimica Acta 428 (2015) 185–192

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Kinetics of palladium nano-particles catalyzed reduction of Methylene Green by hydrazine: Role of induction period in determining mechanistic pathway Ranendu Sekhar Das a,⇑, Bula Singh b, Arabinda Mandal c, Rupendranath Banerjee a, Subrata Mukhopadhyay a a

Department of Chemistry, Jadavpur University, Kolkata 700 032, India Department of Chemistry, Visva-Bharati, Santiniketan 731 235, India c Department of Chemistry, Haldia Government College, Purba Medinipur 721 657, India b

a r t i c l e

i n f o

Article history: Received 10 December 2014 Received in revised form 29 January 2015 Accepted 1 February 2015 Available online 9 February 2015 Keywords: Redox Kinetics Catalysis Surface Adsorption

a b s t r a c t Methylene Green (MG), a thiazine dye, is catalytically reduced by N2H4 in presence of palladium nanoparticles (Pdn) in buffer media. The observed rate, ko increases with increase in [Pdn], [N2H4] and pH but decrease with increase in ionic strength (maintained with NaNO3). Unlike other thiazine dyes, the catalyzed reduction of MG shows an induction period which is caused by the restructuring of Pdn surface. The rate of adsorbate-induced surface restructuring which increase with increase in [N2H4] but independent of [MG], suggest that only N2H4 is adsorbed on Pdn surface during the catalyzed reaction. Interestingly, NO 3 ions are also adsorbed on the Pdn surface and turn the surface negatively charged. The reductant, N2H4 is adsorbed on this negatively charged Pdn surface before it reacts with MG. The surface restructuring energy of Pdn, though is constant (78.8 ± 1.6 kJ M1) over the temperature range of 288–313 K but the activation energy for the catalyzed reduction varies with temperatures. The convex Arrhenius plot illustrates that activation energy is different in lower (288–304 K, 123.8 ± 12.6 kJ M1) and higher temperature range (304–313 K, 69.2 ± 2.1 kJ M1) and around the inversion temperature, Tinv (304 K) a change in the nature of transition state takes place. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Dye molecules usually contain diverse types of functional groups [1] and thus, far from being a mere coloring agent in textile, leather and paper industries [2–4], dye molecules have also found significant applications in the fields of biological and chemical sciences [5]. Among these dyes with different functional groups, derivatives of phenothiazine or thiazine group are especially important because they find exceptional uses in biological fields [6–9] due to its non-toxic nature. Moreover, thiazine dyes have also found use in solar cell technology [10], corrosion inhibition [11], anti-malarial activity [12], photobactericidal activity [13] and in analytical detection of sulfide [14]. The most studied thiazine dye in redox chemistry is undoubtedly Methylene blue (3,7-bis(dimethylamino)phenothiazinium chloride) [15–17] but one another dye, Methylene Green ⇑ Corresponding author. Tel.: +91 94331 63075; fax: +91 33 2414 6223. E-mail addresses: [email protected] (R.S. Das), smukhopadhyay@ chemistry.jdvu.ac.in, [email protected] (S. Mukhopadhyay). http://dx.doi.org/10.1016/j.ica.2015.02.001 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

(MG, Basic Green 5,[7-(dimethylamino)-4-nitrophenothiazin-3ylidene]-dimethylazanium chloride) belonging to same class is comparatively much less studied. Though Methylene Green is largely used in electrochemical bio-sensors for NADH and H2O2 [18–21] and in recent past few redox reactions between Methylene Green and reductants like, urea and glucose [22–26] have been studied but detailed kinetics of any metal nano-particles catalyzed reduction of Methylene Green is yet not performed to the best of our knowledge. In this present work we have prepared hexagonal nanoparticles of noble metal like palladium by a one pot wet reduction method and in the presence of catalytic amount of these palladium nano-particles (Pdn), Methylene Green is quickly reduced by hydrazine which is otherwise extremely slow under similar condition. The catalyzed reduction reaction shows the presence of an initial induction period (vide infra) which indicates the phenomenon of restructuring of the nano-particles surface. Analysis of induction period and the effects of other reactants on it reveal important threads of information about the probable path of reaction mechanism.

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

2. Experimental 2.1. Materials Palladium nano-particles were prepared by reducing PdCl2 (Aldrich) with sodium citrate dihydrate (Rankem) and gelatin (Merck) was used to stabilize it. For kinetic studies Methylene Green (Aldrich), hydrazine (Merck), NaNO3, NaCl, KH2PO4 and NaOH (Merck) were used as received. Throughout the nanoparticles preparation and kinetic experiments, Milli-Q water was used. All other chemicals for standardization of reagents were of analytical grade. 2.2. Preparation of palladium nano-particles (Pdn) coated with gelatin Noble-metal nano-particles, like palladium have attracted lot of interest recently in the fields of H2 and O2 adsorption [27–29], hydrogenation [30–32], oxidation of alcohol and methane [33,34], etc. Here an easy, one-pot wet-synthesis process has been adopted to prepare palladium nano-particles (Pdn). Palladium nano-particles (Pdn) were prepared following a simple procedure [35]: an aqueous solution of PdCl2 (50 mL, 2.5  104 M) was heated to boiling with vigorous stirring. Then 2 ml of aqueous tri sodium citrate solution (1% w/v) was added to this solution and the solution mixture was allowed to boil for one hour with vigorous stirring. The formation of the Pd nano-particles can be followed by the appearance of yellow color in the solution. Here, citrate ions satisfy both the role of reductant by reducing the precursor Pd(II) ions to Pd(0) and the role of primary stabilizing agent for the palladium nano-particles. Pdn are formed from Pd(0) by nucleation process and citrate ions adsorbed at the particles interfaces form an electrical double layer which provides an electrostatic stabilization from coagulation [36]. For further stabilization of Pdn, 5 mL of 1% (w/v) aqueous gelatin solution was added to the preformed Pd nano-particles solution and the boiling was continued for another 30 min with stirring. Gelatin is a non-toxic naturally occurring denatured product of collagen which provides better and prolonged stabilization. Moreover, gelatin does not react with the reactants or passivate the nano particle surfaces [37–39]. Finally, this yellow gelatin coated Pdn solution was cooled to room temperature and was stored in a cold, dry and dark place. This stock Pdn solution was always used after sonication.

The phosphate buffer solutions were prepared with KH2PO4 and NaOH following literature method [40] and were measured with a Toshniwal pH-meter (CL-54, India), calibrated as usual. 2.4. Kinetics In aqueous phosphate buffer media (pH 6.0–8.0) at ambient temperature and fixed ionic strength (I = 0.2–1.0 M), Pdn catalyze the reduction of MG to colorless leuco-Methylene Green (l-MG, MGH) by hydrazine (N2H4). The reaction was studied spectrophotometrically and the time-resolved spectra of such catalyzed reduction reaction of MG are shown in Fig. S1 (see Electronic Supplementary Information). The uncatalyzed reaction between MG by N2H4 is extremely slow (Fig. 1, inset) and the reaction contributes almost nothing to the observed rate of the catalyzed reaction under similar condition (details are provided in the legend of Fig. 1). The reduction reaction of MG was performed in situ in spectrophotometer coupled with an electrically controlled thermostat where a reaction mixture of measured amount of Pdn solution (from stock solution), sodium nitrate solution (calculated amount of added sodium nitrate along with KH2PO4 and NaOH maintain the ionic strength, I of the reaction media) and buffered solution of MG is quickly mixed with N2H4. In all the instances a large excess of [N2H4] is maintained over [MG]. Moreover, from independent experiments it is ensured that under the specified kinetic condition alone Pdn solution and/or aqueous gelatin solution do not reduce MG. The addition of mere aqueous gelatin solution also does not catalyze the reduction of MG by N2H4. In both cases the absorbance of MG at 658 nm does not change at least for three hours. Pdn and the gelatin molecules thus function as the catalyst and the stabilizing agent of Pdn, respectively. The progress of the catalyzed reduction is followed by the change in absorbance of the reaction mixture over 400–800 nm which mainly portrays the absorbance of MG since no other reactants absorb in this wavelength range. Kinetics was particularly measured at 658 nm which is the characteristic absorption peak of MG. A non-linear, standard least-square fit to the lnA versus time data (A is the absorbance value at time t) yielded the observed first-order rate constants, ko. Here, it is important to mention that an initial time delay, i.e., an induction period (t0) has been observed to precede the catalyzed

0.5

0.5

t0

uncatalysed

0.0

0.0

lnA

The present findings are significant because it can open up the possibility of using the phenothiazine or thiazine drugs in conjugation with noble nano-particles for better activity in resistance against bacterial attack. Moreover, implementation of this catalytic reaction can result in modified new biosensors with faster detection capacity. The present work also attempts to illustrate that how different kinetic information can be put together to achieve the idea of the most plausible reaction mechanism.

-0.5

lnA

186

-0.5

catalysed

-1.0 0

100

200

300

400

500

600

time (s)

2.3. Instrumentation Absorbance and UV–VIS spectra were recorded using a Shimadzu spectrophotometer (UV-1700) with 1.00 cm quartz cuvettes. Transmission electron microscopic (TEM) analyses were performed in a Hitachi (H-9000 NAR) instrument on samples prepared by placing a drop of Pdn solution on Cu grid, pre-coated with carbon films, followed by solvent evaporation under vacuum. Pdn solution was centrifuged in Heraeus, Biofuge primo R machine and Fourier Transform Infrared Spectroscopy (FTIR) data were collected using a Shimadzu FT-IR 8400S. Pdn solution was regularly sonicated in Rivotek ultrasonic bath (50 Hz) before any experimentation.

catalysed

-1.0 0

100 200 300 400 500 600 700 800 900 1000 time (s)

Fig. 1. Plot of lnA vs. time for uncatalyzed and Pdn catalyzed reduction of MG (inset) where A is the corresponding absorbance values of MG at time t measured at 658 nm. The catalyzed reduction follows a first-order kinetics (represented by black line) with an induction period (t0). Condition: (uncatalyzed) [MG] = 0.025 mM; [N2H4] = 10.0 mM; pH 7.0; I = 0.5 M (NaNO3); T = 298 K. Condition: (catalyzed) [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3); T = 298 K.

187

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

Fig. 2. HRTEM image of palladium nano-particles (Pdn) have average diameter of nearly 4.5 nm.

time, t0. The presence of this induction period does not arise due to the presence of dissolved oxygen [41] or any impurities present in the reaction medium since on passing argon or on addition of chelating agent, like dipicolinic acid [42], the induction period remains as usual. This induction period spontaneously originates from restructuring of the nano-particles surface [35] which will be addressed later in details. We assume that during this short period of induction time the adsorption of the reactants and the rate of the reduction reaction was yet to achieve full pace.

3.0 2.5

-1

10 ko (s )

2.0

3

1.5 1.0

3. Results and discussions

0.5 3.1. Characterization of Pdn

0.0 0

2

4

6

8 10 12 14 16 18 20 22 10

10 [Pdn] (M) Fig. 3. The observed rate, ko values of the Pdn catalyzed reduction of MG linearly increase with increasing [Pdn]. Condition: [MG] = 0.025 mM; [N2H4] = 5.0 mM; I = 0.5 M (NaNO3); pH 7.0; T = 298 K.

reaction (Fig. 1) and this time period has been excluded during the determination of ko values. The first point of time where the linear plots of lnA over the initial and subsequent experimental points intersects each other was taken as the measure of the induction

The formation of Pdn was characterized by UV–VIS spectroscopy and their size is determined from TEM imaging. Fig. S1 presents the UV–VIS spectra of aqueous solution of PdCl2 and aqueous solution of Pdn nano-particles. Spectrum of Pdn nano-particles show the absence of characteristic absorbance peaks of PdCl2 at 235 nm and 413 nm where the former one arises due to the corresponding LMCT transition [43–45]. Thus it can be logically assumed that Pd(II) ions have been mostly reduced to Pd(0) during the formation of nano-particles. Moreover, Fig. S2 also describes the absorption of MG in aqueous solution and it can be seen that Pdn does not absorb in the wavelength range 500–750 nm where the characteristic absorption of the former lies.

-2.2

25

a

20

b

logko

3

-1

10 ko (s )

-2.4 15 10

-2.6 5 0 0

10

20

30

[N2H4] (mM) Fig. 4. Variation of ko values with [N2H4]. Condition: (a) [MG] = 0.015 mM; [Pdn] = 16.7  1010 M; I = 0.5 M (NaNO3); pH 7.0; T = 298 K. (b) [MG] = 0.025 mM; [Pdn] = 16.7  1010 M; I = 0.5 M (NaNO3); pH 7.0; T = 298 K.

-2.8 0.4

0.5

0.6

0.7 1/2

I

0.8

0.9

1.0

1/2

(M )

Fig. 5. Linear plot of log ko vs. I1/2 which has a slope of 0.67 ± 0.07. Conditions: [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; T = 298 K.

188

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

½Pdn ¼ ½Pd

10

8

6 -1

3

4

6 4

ions=ðV Pdn =V Pd atom Þ

ð1Þ

Here, VPdn and VPd are the corresponding volumes of palladium nano-particles and atoms where the atomic radius of Pd atom was taken as 0.137 nm [50]. Following the above equation, the calculated approximate concentration of stock Pdn solution was 5.0  108 M from which aliquots were used in the reaction media to maintain the concentration of Pdn catalyst in the order of 1010 M. Though, the concentration values of Pdn are approximate but still it can guide us to understand and follow the trend of variation of the observed reaction rates with Pdn concentration and other factors.

10

10 ko (s )

3

-1

10 ko (s )

8

2þ

2

2 0 0

20

0 6

40 60 80 -6 + -1 10 /[H ] (M )

8

10

100

3.3. Effect of [Pdn] and [N2H4] on ko

12

pH Fig. 6. Variation of ko vs. 1/[H+] (inset) and vs. pH of the reaction media. Conditions: [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3); T = 298 K.

In Fig. 2 the HR-TEM images of the Pdn are shown at two different magnifications. High resolution images show that most of the Pdn nano-particles are polygonal indicating a shell structure [43] and the average diameter of a single nano-particle is about 4.5 nm. Palladium nano-particles are also subjected to FTIR analysis to ascertain the presence of gelatin molecules which was used to protect nano-particles surface and to inhibit any agglomeration (Fig. S3). FTIR spectrum of Pdn was recorded after the centrifugation of the stock Pdn solution (at 12 000 rpm for 30 min), followed by washing with Milli-Q water to remove excess gelatin. The presence of absorption bands at 1631 cm1 (C@O stretching of amide) and 1595 cm1 (N–H bending) confirm the presence of amide groups of gelatin molecules [46–48]. These gelatin molecules do not catalyze the reaction but only provides the necessary stability to the nano-particles. 3.2. Concentration of Pdn The HRTEM image shows that the hexagonal Pdn have an average diameter of 4.5 nm (vide supra). If we assume that all the Pd(II) were reduced to Pd(0) to form nano-particles and they are near spherical with the same diameter as mentioned, Eq. (1) can provide us an ideas of approximate concentration for Pdn [49].

The variation of the observed rate constant, ko values with increasing [Pdn] and [N2H4] are shown in Figs. 3 and 4, respectively (Tables S1 and S2 respectively). In Fig. 3 the plot passes through the origin which signifies that Pdn is essential for the reduction of MG by N2H4 and the rate of the uncatalyzed reduction reaction is virtually negligible. The plot also reveals that within the experimental range of variation, the increasing gelatin concentration (0.0015–0.0075%) along with the increasing [Pdn] does not result either in the slow diffusion rate of the reagents or the passivity the catalyst surface [35]. The plots of variation of ko with [N2H4] at two different [MG] in Fig. 4 are both linear as the general criteria of pseudo first-order reaction requires. Moreover, the plots also pass through the origin confirming the inactivity of Pdn and/or gelatine as reducing agent under the experimental conditions. 3.4. Effect of ionic strength, I and media pH on ko Ionic strength, I of the reaction media was varied by the addition of calculated amount of NaNO3 and the observed ko values have been found to decrease with the increase in I (Fig. 5, Table S3). The plot of log ko versus I1/2 is linear and the negative slope value of the line (0.68 ± 0.07) indicates that the reacting species carry ionic charges of opposite sign. Since MG is a cationic dye, the other reactant must bear negative charge. This is striking as the reducing agent N2H4 cannot bear negative charge under the experimental condition. The reductant, N2H4 would rather have positive charge due to the protonation of the molecule to form N2H+5. In fact as the protonated form of hydrazine, N2H+5 has the

6

10

a a. intercept = 2.9 + 0.2

4

6 4

b. intercept = 3.4 + 0.4

b

2

3

3

-1

10 /t0 (s )

-1

10 (1/t0 - 1/ts,p) (s )

8

2

0

0 0

5

10

15

20

25

30

[N2H4] (mM) Fig. 7. Plot of 1/t0 vs. [N2H4]. Condition: (a) [MG] = 0.015 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3); T = 298 K; (b) [MG] = 0.025 mM; 10 [Pdn] = 16.7  10 M; pH 7.0; I = 0.5 M (NaNO3); T = 298 K.

0

5

10

15

20

25

30

[N2H4] (mM) Fig. 8. Plot of (1/t0-1/t0,sp) vs. [N2H4]. Condition: (a) [MG] = 0.015 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3); T = 298 K; (b) [MG] = 0.025 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M; T = 298 K.

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

5

4

3

-1

10 /t0 (s )

3 mean = 3.4 intercept = 3.5 + 0.1

2

1

0 0.00

0.01

0.02

0.03

0.04

[MG] (mM) Fig. 9. Plot of 1/t0 vs. [MG]. Condition: [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; I = 0.5 M (NaNO3); pH 7.0; T = 298 K.

pKa value of 7.95 (Eq. (2)) [51], it is expected that most of the hydrazine will remain protonated over the experimental pH range (6.0–8.0). This observation thus indicates that the rate step of the title catalyzed reaction does not constitute a simple reaction between the cationic MG and N2H+5. K

N2 Hþ5 ¢ N2 H4 þ Hþ

ð2Þ

The situation was further made complex as the plot of observed rate, ko versus media pH (Fig. 6, Table S4) shows that ko increases with increase in pH of the media. The higher pH of the medium must be facilitating the release of proton from the reactant(s) and thus favoring the forward reaction. Since MG does not have any ionisable proton, N2H+5 is the only other reactant that must take part in the protic equilibria. Thus, though N2H+5 is the prime form of the reductant that takes part in the reaction but it must have released the proton before the rate step and the reactive reductant is N2H4. 3.5. Analysis of induction time, t0 As mentioned earlier the catalyzed reduction of MG is preceded by a time period of delay, called the induction period (t0) when the

7

6

-1

(10 /t0) (s )

4

3

3

-1

(10 /t0) (s )

6

2

5

4 0.2

0.4 0.6 [N aN O 3 ] (M )

0 0

1

2

3

4

5

0.8

6

7

189

absorbance of reaction mixture does not change with time. Even on removing the Pdn-surface adsorbed or dissolved oxygen by purging argon gas this delay time cannot be removed. The origin of this induction period, t0 is now believed to be the phenomenon called surface restructuring of the nano-particles [52,53]. The surface of a small particle, as small as in nano dimension is unstable since the atoms resting on the nano-particle surface posses more potential energy than the atoms comprising the bulk of the nano-particle. This results in the spontaneous reshuffling of the atomic arrangements which lowers the excess surface energy [54]. Along with this inherent and spontaneous surface restructuring process the surface structure is also influenced by the presence of adsorbates and the later is called the adsorbate-induced process [54– 56]. Nano-particles, especially which is made of the platinum group metals, for instance, Pdn, are more prone to suffer this restructuring process due to the relativistic effects [55]. The extent of surface restructuring can be measured from the initial time period, t0 and the rate of such restructuring process can be taken as 1/t0 [54,56]. Fig. 7 shows the plot of 1/t0 versus [N2H4] which displays that the overall restructuring process linearly increases with increase in [N2H4] (Table S5). The intercept in Fig. 7 that is the rate of surface restructuring at zero [N2H4] is noteworthy and this actually represent the rate of spontaneous surface restructuring (t0,sp) process [54,56]. Thus, the plot of (1/t0  1/t0,sp) versus [N2H4] (Fig. 8) describes only the effect of adsorbateinduced surface restructuring process. It can be seen that the rate increases linearly with the increasing adsorbate concentration and at zero [N2H4] the rate is also zero. This suggests that the protonated reductant, N2H4 is adsorbed on the Pdn surface. Similarly, we can plot 1/t0 versus. [MG] (Fig. 9) and remarkably it is found that the rate of total surface restructuring process is practically invariant in [MG]. Moreover, the corresponding 1/t0 values are clustered around the average value of 3.4  103 s1 (intercept = (3.5 ± 0.1)  103 s1) which is close to the spontaneous surface restructuring rate yielded from the intercept of Fig. 7 ((2.9 ± 0.2)  103 and (3.4 ± 0.4)  103 s1). Since the adsorbate-induced surface restructuring process is not affected by the presence of MG, it can be assumed that MG is not adsorbed on the nano particle surface [52]. Interestingly, the presence of NaNO3 (0.17–0.84 M) affects the duration of the induction period though it does play any role in the catalyzed reaction. The plot of 1/t0 versus 1/[NaNO3] (Fig. 10, Table S6) shows that the total surface restructuring rate decreases with increasing [NaNO3] and reaches a saturation value at higher [NaNO3]. This probably suggests that the adsorption of NO 3 ion on Pdn surface lowers the potential energy and hence stabilizes the surface structure. Interestingly, NaCl, instead of NaNO3 if is used in reaction media to maintain I under otherwise alike conditions, similar plot of 1/t0 versus 1/[NaCl (Fig. S4, Table S7) can be achieved but in this case the extent of adsorbate-induced surface restructuring is much lower than in the case of NaNO3. The spontaneous surface restructuring rate, (3.1 ± 0.1)  103 s1 as obtained from the intercept of Fig. 10 is however close to the values obtained from Figs. 7 and 9. Here it is also important to mention that though both N2H4 and NO 3 are adsorbed on the nano-particle surface but it can be assumed that they do not compete for the same surface sites. If it has been a case of competitive adsorption then the variation of [N2H4] at constant I (Fig. 4) and vice versa (Fig. 5) would have resulted in an optimum rate [35] instead of the observed monotonous increase and monotonous decrease respectively.

-1

1/[NaNO3] (M )

3.6. Proposed reaction mechanism 3

1

Fig. 10. Linear plot of (1/t0) vs. 1/[NaNO3] with an intercept of 3.1 ± 0.1  10 s . Conditions: [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; T = 298 K.

The reduction of MG to l-MG (MGH) involves the overall transfer of two electrons and one proton [57] as shown in Scheme 1.

190

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

N (H3C)2N

N(CH3)2

S

H N

NO2

I

+ 2e-

S+

N(CH3)2

S

+H

N (H3C)2N

(H3C)2N

+

NO2

l-MG (MGH)

N(CH3)2 NO2

II

MG Scheme 1. Reduction of MG to l-MG.

N2H5+ + PdnmN2H4

Pdn

m+ MG+

N2H4 Pdn ... MG N2H3

+ MG fast

N2H2

fast

2N2H2

K1

N2H4 K2

(m-1)-

fast

k slow

Pdn

m-

+ H+

(3) (m-1)-

N2H4 Pdn ... MG

(4)

MG + N2H3 + H+ + Pdnm-

(5)

MGH + N2H2

(6)

N2 + H2

(7)

N2 + N2H4

(8)

Scheme 2. Plausible mechanistic scheme for the Pdn catalyzed reduction of MG by N2H4.

-4

-2 -3 -4 lnko

ln(1/t0)

-5

High temp. -1 69.2 + 2.1 kJ M

-6

-5 -6

-7

Low temp.

3

10 /Tinv

-7 3.2

3.3 3

3.4

-1

123.8 + 12.6 kJ M

3.5

-1

10 /T (K )

3.2

3.3 3

Fig. 11. Plot of ln(1/t0) vs. 103/T. Condition: [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3).

The kinetic observations mentioned herein suggest that the reaction occurs between two oppositely charged species. The oxidant MG is cationic and is not adsorbed on the Pdn nano-particles. The reductant, N2H4 though is adsorbed on Pdn but bears no charge. To find the species with negative charge, thus we turn to the phenomenon of adsorption of NO 3 ions on the Pdn surface. In our earlier works [35] it has been found that the increasing

3.4

3.5

-1

10 /T (K ) Fig. 12. Plot of lnko vs. 103/T. Condition: [MG] = 0.025 mM; [N2H4] = 10.0 mM; [Pdn] = 16.7  1010 M; pH 7.0; I = 0.5 M (NaNO3).

alkalinity of the reaction media imparts negative charge on the nano-particle surface. Similarly, in this present instance we propose that adsorption of NO 3 ions on the nano-particles surface make the Pdn surface negatively charged. We denote this negatively charged nano-particles as Pdnm where m may be an integer as well a fraction. It seems that NO 3 ions which was assigned a role

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192

of simple ionic strength modulator ultimately dictates the course of the reaction, though indirectly. In this context it is noteworthy that the catalyzed reaction was performed in phosphate buffer media and thus PO3 4 ions (0.05 M) and OH ions (for pH 6.0–8.0, [NaOH] = 0.005–0.046 M) [40], though present in much smaller concentration can also be  adsorbed on the Pdn surface. Such adsorption of PO3 4 and OH ions along with NO ions can enhance the negative charge of the 3 surface and that further strengthen the above made proposition. The protonated reductant, N2H+5 is first adsorbed on the Pdnm surface with a release of the proton (nano-particles assisted deprotonation) and ultimately reacts with MG to produce l-MG. The overall mechanistic scheme for the title reaction may simply be described by the following Scheme 2. Here, MG+ and MGH have been used for cationic MG and l-MG respectively to represent the actual charge of the corresponding molecules. In Scheme 2, N2H+5 is truly adsorbed on Pdnm surface (represented by Lewis type bond ‘–’ in Eq. (3), Scheme 2) which is evident from the effect of [N2H4] on surface restructuring rate. During the adsorption, N2H+5 releases a proton and forms the catalyst bound N2H4 entity ([N2H4–Pdn]m) that form an associate with MG+ (Eq. (4). Since MG+ is not truly adsorbed on Pdn, association of MG+ is represented by a broken (  ) bond in [N2H4–Pdn  MG](m1). The association, [N2H4–Pdn  MG](m1) is a transient reaction complex entrapped in the solvent cage [58] and after the electron transfer reaction, this reaction complex breaks down to the oneelectron reduced MG and the N2H3 [59] (Eq. (5)). N2H3 further reduces MG to MGH (l-MG) (Eq. (6)) and itself is oxidized to unstable N2H2 which disintegrates either to N2 and H2 (Eq. (7)) [60] or to N2 and N2H4 (Eq. (8)) [59,60]. Both of the decomposition reaction shown in Eqs. (7) and (8) are competitive [60] and we have not further attempted to the detect the presence of small amount of H2, if any, generated in the product mixture. The rate of the reaction can simply be expressed as follows: þ

þ

Rate ¼ d½MG =dt ¼ ko ½MG  ¼ kK 1 K 2 ½MGþ ½N2 Hþ5 ½Pdn

m

=½Hþ 

ð9Þ

With the assumption that under the experimental condition, most of the N2H4 remain as N2H+5 and essentially with [Pdn] = [Pdnm], Eq. (9) becomes Eq. (10). þ

ko ¼ kK 1 K 2 ½N2 H4 ½Pdn=½H 

ð10Þ

According to Eq. (10), the observed rate, ko is proportional to [Pdn], [N2H4] and [H+]1 and the validation of this relation is evident from Figs. 3, 4 and 6 respectively. 3.7. Effect of temperature on catalysis and surface restructuring With the increase in temperature of reaction media, both the rate of Pdn-catalyzed reduction of MG by N2H4 and the rate of overall surface restructuring process increase. The surface restructuring energy and the activation energy (Ea) for the reduction reaction can be calculated from the slope of corresponding plot of ln(1/t0) versus 103/T and lnko versus 103/T (Figs. 11 and 12 respectively, Tables S8 and S9, respectively) using Arrhenius equation, lnko = lnA  Ea/RT (ko and A are, respectively, the rate constant and pre-exponential constant). Plot of ln(1/t0) versus 103/T (Fig. 11) is linear and the corresponding value of the surface restructuring energy was 78.8 ± 1.6 kJ M1. Similar, plot of lnko versus 103/T (Fig. 12) is not linear as expected and the plot represents for an example of the convex Arrhenius plot. Ea for the catalyzed reduction decreases from low temperature range (288–304 K, 123.8 ± 12.6 kJ M1) to higher temperature range (304–313 K, 69.2 ± 2.1 kJ M1). The reason for this change in activation energy can be accounted by the

191

change in nature of the transition state [61] and the temperature at which this change takes place, called the inversion temperature (Tinv) is approximately 304 K. The change in transition state probably arises due to the change in extent of adsorption of reactants on Pdn surface [42]. 4. Conclusion In buffer media (pH 6.0–8.0), cationic dye Methylene Green (MG) is catalytically reduced by N2H4 in presence of palladium nanoparticles (Pdn). The observed rate, ko is proportional to [Pdn], [N2H4] and pH but it is inversely proportional to media ionic strength, I. The negative slope of the plot of log ko versus I1/2 suggests a reaction between cationic MG and a negatively charged species. Further analysis of the initial induction period (t0) reveals that the adsorbate-induced surface restructuring of Pdn is independent of [MG] but increases with [N2H4]. Moreover, it was found that [NaNO3] which had the role of ionic strength modulator also proportionally affects the surface restructuring process. All the observations suggest that NO 3 , on adsorption imparts negative charge on nano-particles (Pdnm). N2H+5 is adsorbed on Pdnm to form (N2H4–Pdn)m. MG forms transient reaction complex with (N2H4–Pdn)m as (N2H4–PdnMG)(m1) which finally breaks down to products. The degree of association of the complex is temperature sensitive and the activation energy of the reaction decreases over the inversion temperature, Tinv (304 K). The title study exemplifies the dual role of NO 3 in determining the course of the catalyzed reaction. Furthermore the study illustrates the importance of the kinetic information in understanding the underlying mechanism of the process of surface-induced catalysis by nano-particles. Acknowledgements Authors thankfully acknowledge the financial help received from the UPE II fund of Jadavpur University sanctioned by University Grants Commission, New Delhi. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2015.02.001. References [1] [2] [3] [4]

[5] [6] [7] [8]

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

J.A. Kiernan, Biotech. Histochem. 76 (2002) 261. H. Zollinger, Color Chemistry, VCH, Weinheim, 1991. R.M. Christie, Colour Chemistry, Royal Society of Chemistry, 2001. W. Herbst, K. Hunger, G. Wilker, H. Ohleier, R. Winter, Industrial Organic Pigments: Production, Properties, Applications, Wiley-VCH, Verlag GmbH & Co, KGaA, 2004. P. Bamfield, Chromic Phenomena, The Technological Applications of Colour Chemistry, The Royal Society of Chemistry, 2001. S.P. Massie, Chem. Rev. 54 (1954) 797. K. Pluta, B. Morak-Młodawska, M. Jelen´, Eur. J. Med. Chem. 46 (2011) 3179. J.P. Tardivo, A.D. Giglio, C.S.D. Oliveira, D.S. Gabrielli, H.C. Junqueira, D.B. Tada, D. Severino, R.D.F. Turchiello, M.S. Baptista, Photodiagn. Photodyn. Ther. 2 (2005) 175. J. McClaskey, M. Xu, E.L. Snyder, C.A. Tormey, Transfus. Apher. Sci. 41 (2009) 217. A.K. Jana, J. Photochem. Photobiol. A 132 (2000) 1. E.E. Ebenso, M.M. Kabanda, L.C. Murulana, A.K. Singh, S.K. Shukla, Ind. Eng. Chem. Res. 51 (2012) 12940. M. Wainwright, L. Amaral, Tropical Med. Int. Health 10 (2005) 501. M. Wainwright, D.A. Phoenix, J. Marland, D.R.A. Wareing, F.J. Bolton, FEMS Immunol. Med. Microbiol. 19 (1997) 75. N.S. Lawrence, J. Davis, R.G. Compton, Talanta 52 (2000) 771. N.R. Jana, Z.L. Wang, T. Pal, Langmuir 16 (2000) 2457. S. Kundu, S.K. Ghosh, M. Mandal, T. Pal, New J. Chem. 27 (2003) 656. S.K. Ghosh, S. Kundu, M. Mandal, T. Pal, Langmuir 18 (2002) 8756. Q. Chi, S. Dong, Anal. Chim. Acta 285 (1994) 125. D. Zhou, H. Fang, H. Chen, H. Ju, Y. Wang, Anal. Chim. Acta 329 (1996) 41. C. Lei, J. Deng, Anal. Chem. 68 (1996) 3344.

192 [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

R.S. Das et al. / Inorganica Chimica Acta 428 (2015) 185–192 B. Wang, S. Dong, Talanta 51 (2000) 565. R. Azmat, S. Ahmed, S. Qureshi, J. Appl. Sci. 6 (2006) 2784. R. Azmat, N. Qamar, A. Saeed, F. Uddin, Chin. J. Chem. 26 (2008) 631. T. Ahmed, F. Uddin, R. Azmat, Chin. J. Chem. 28 (2010) 748. R. Azmat, N. Qamar, R. Naz, Nat. Sci. 3 (2011) 566. T. Ahmed, R. Azmat, F. Uddin, J. Environ. Chem. Ecotoxicol. 4 (2012) 143. N.A. Yashtulova, A.A. Revinab, M.V. Lebedevaa, V.R. Flida, Kinet. Catal. 54 (2013) 322. I. Cabria, M.J. López, S. Fraile, J.A. Alonso, J. Phys. Chem. C 116 (2012) 21179. M.J. López, I. Cabria, J.A. Alonso, J. Phys. Chem. C 118 (2014) 5081. R.B.N. Baig, R.S. Varma, ACS Sustain. Chem. Eng. 2 (2014) 2155. D.B. Bagal, B.M. Bhanage, RSC Adv. 4 (2014) 32834. Q. Chen, Z. Ye, Y. Duan, Y. Zhou, Chem. Soc. Rev. 42 (2013) 497. A. Mondal, A. Das, B. Adhikary, D.K. Mukherjee, J. Nanopart. Res. 16 (2014) 1. J. Yin, S. Shan, M.S. Ng, L. Yang, D. Mott, W. Fang, N. Kang, J. Luo, C. Zhong, Langmuir 29 (2013) 9249. R.S. Das, B. Singh, R. Banerjee, S. Mukhopadhyay, Dalton Trans. 42 (2013) 4068. J.S. Bradley, The chemistry of transition metal colloids, in: G. Schmid (Ed.), Clusters and Colloids, VCH, Weinheim, 1994. T.G. Shutava, S.S. Balkundi, P. Vangala, J.J. Steffan, R.L. Bigelow, J.A. Cardelli, D.P. O’Neal, Y.M. Lvov, ACS Nano 3 (2009) 1877. M.P. Neupane, S.J. Lee, I.S. Park, M.H. Lee, T.S. Bae, Y. Kuboki, M. Uo, F. Watari, J. Nanopart. Res. 13 (2011) 491. Y. Liu, X. Liu, X. Wang, Nanoscale Res. Lett. 6 (2011) 22. D.D. Perrin, B. Demsey, Buffers for pH and Metal Ion Control, Chapman and Hall, London, 1974. G. Sharma, M. Ballauff, Macromol. Rapid Commun. 25 (2004) 547. R.S. Das, B. Singh, S. Mukhopadhyay, R. Banerjee, Dalton Trans. 41 (2012) 4641.

[43] T. Teranishi, M. Miyake, Chem. Mater. 10 (1998) 594. [44] Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385. [45] R. Redon, S.K. Rendon-Lara, A.L. Fernandez-Osorio, V.M. Ugalde-Saldivar, Rev. Adv. Mater Sci. 27 (2011) 31. [46] S. Liu, Z. Zhang, M.Y. Han, Adv. Mater. 17 (2005) 1862. [47] B. Gaihrea, M.S. Khil, D.R. Lee, H.Y. Kim, Int. J. Pharm. 365 (2009) 180. [48] M.P. Neupane, S.J. Lee, I.S. Park, M.H. Lee, T.S. Bae, Y. Kuboki, M. Uo, F. Watari, J. Nanopart. Res. 13 (2011) 491. [49] R. Narayanan, M.A. El-Sayed, J. Phys. Chem., B 107 (2003) 12416. [50] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, second ed., Butterworth-Heinemann, Oxford, 1997. [51] R.L. Hinman, J. Org. Chem. 23 (1958) 1587. [52] S. Wunder, F. Polzer, Y. Lu, Y. Mei, M. Ballauff, J. Phys. Chem., C 114 (2010) 8814. [53] J. Kaiser, L. Leppert, H. Welz, F. Polzer, S. Wunder, N. Wanderka, M. Albrecht, T. Lunkenbein, J. Breu, S. Kummel, Y. Lu, M. Ballauff, Phys. Chem. Chem. Phys. 14 (2012) 6487. [54] S. Titmuss, A. Wander, D.A. King, Chem. Rev. 96 (1996) 1291. [55] M. Kiskinova, Chem. Rev. 96 (1996) 1431. [56] G.A. Somorjai, G. Rupprechter, J. Chem. Educ. 75 (1998) 161. [57] R.P. Akkermans, S.L. Roberts, F. Marken, B.A. Coles, S.J. Wilkins, J.A. Cooper, K.E. Woodhouse, R.G. Compton, J. Phys. Chem., B 103 (1999) 9987. [58] M.I. Page, A. Williams, Organic and Bio-organic Mechanisms, Pearson Education Ltd., 2010. [59] D.M. Stanbury, Prog. Inorg. Chem. 47 (1997) 511. [60] F. Albert Cotton, Geoffrey Wilkinson, Carlos A. Murillo, Manfred Bochmann, in: Advanced Inorganic Chemistry, sixth ed., Wiley-Interscience, 1999. [61] D.G. Truhlar, A. Kohen, PNAS 98 (2001) 848.