CuPd Bimetallic Nanoparticles Supported on ...

International Journal of Green Technology, 2017, 3, 51-62

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CuPd Bimetallic Nanoparticles Supported on Magnesium Oxide as an Active and Stable Catalyst for the Reduction of 4-Nitrophenol to 4-Aminophenol María V. Morales1,*, José M. Conesa1,2, Inmaculada Rodríguez-Ramos2, Mariana Rocha3, Cristina Freire3 and Antonio Guerrero-Ruiz1,* 1Departamento

de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Senda del Rey 9, 28040, Madrid, Spain; 2Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Cantoblanco, 28049, Madrid, Spain and 3REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, 4169-007 Porto, Portugal Abstract: In this work, we report the synthesis and characterization of a catalyst based on bimetallic CuPd nanostructures generated over a magnesium oxide support. Its catalytic activity and reusability have been tested in the reduction of 4nitrophenol as a model reaction using NaBH4 as the source of hydrogen for the reduction of the nitro-group. The structure, composition and morphology of the catalysts were studied by N2 physisorption, X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). The reaction kinetics of reduction of 4nitrophenol to 4-aminophenol has been followed by UV-visible spectrophotometry, and its apparent rate constant has been determined and compared with its monometallic counterparts. All the tested catalysts exhibited remarkable high activity and excellent stability upon reuse for multiple consecutive cycles. We found out that a small loading addition of Pd to Cu catalyst greatly improved the catalytic activity in comparison with the monometallic samples. Our characterization results pointed out a higher metallic dispersion degree as the main explanation for this enhanced performance. CuPd/MgO is highly competitive or outperforms the catalytic activity of other bimetallic systems reported in literature.

Keywords: Wastewaters treatment, 4-Nitrophenol degradation, Model reaction, Heterogeneous bimetallic catalysts, Metallic particle size, Rate constant, Recyclability, Magnesium oxide support 1. INTRODUCTION Nitroarene compounds are common pollutants present in industrial wastewaters nowadays. These compounds frequently outcome from the manufacture of agrochemicals, plasticizers, synthetic dyes and pharmaceutical products. Among these nitroarene compounds, 4-nitrophenol (4-NP) is one of the most often occurring by-products and has been widely considered a subject of environmental concern, being one of the priority pollutants by the US Environmental Protection Agency (EPA) [1]. Besides, 4-NP is carcinogenic and genotoxic to human and wildlife [2]. Among the various processes developed to date for its degradation, the hydrogenation reaction to 4-aminophenol (4-AP) is currently one of the most explored and efficient methods for the treatment of the resulting wastewaters. Moreover, its reduction product, 4-AP, is a valuable raw material which serves as an intermediate for the manufacture of analgesic drugs, photographic developers and corrosion inhibitors, among others. Likewise, the reduction of 4-NP to 4-AP is highly regarded as a “model catalytic reaction” [3], frequently *

Address correspondence to this author at the Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias, UNED, Senda del Rey 9, 28040, Madrid, Spain; E-mails: [email protected] and [email protected]

used to test the catalytic activity of metal nanoparticles in aqueous solution at room temperature. The mechanism of such a reaction is well-understood and its kinetics easy to follow by UV-Vis spectroscopy. Since this reaction was first reported in 2002 by T. Pal et al. [4] and K. Esumi et al. [5], a huge number of research groups have used it in order to evaluate the catalytic properties (kinetics, stability) in the aqueous phase of lab-synthetized metal nanoparticles [6]. Although more cost-effective metal such as copper has drawn high attention in recent years [7], as we exposed in a former work [8], the noble and costly metal catalysts such as gold [9,10], Pt [11] or Ag [12], have been the focus of most of the reported literature. Apart from these precious metals, catalysts based on Pd have been reported to exhibit excellent catalytic performance in this reaction in terms of activity, stability and reusability [13]. However, palladium has a high price and is relatively scarce. In the last years bimetallic nanostructures have aroused great attention since not only can lower the cost but also effectively can improve the catalytic performance on the basis of electronic (ligand) or geometric effects (ensemble). The electronic and geometric modifications can be attained by tuning the bimetallic structure, surface composition, particle size, and its distribution. Hence, low quantities of Pd have been combined with other lower-priced metals to generate bimetallic nano-

E-ISSN-2414-2077 © 2017 International Journal of Green Technology

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International Journal of Green Technology, 2017, Vol. 3

catalysts such as CuPd [14] or NiPd [15], with promising results in the reduction reaction of 4-NP. It is well known in heterogeneous catalysis that the catalyst support plays a critical role not only as a metal nanoparticles carrier but also as a medium to stabilize them, aspects that directly interfere in their catalytic activity as well as their reusability. A wide variety of solid materials have been explored to stabilize metal nanoparticles such as, among others; zeolites, carbonaceous materials, alumina, silica, clays or metallic oxides. Among these last materials, magnesium oxide presents certain properties that make it a suitable support for metallic nanoparticles. For instance, it possesses various edges/corner defect sites with several cationic and anionic vacancies, which can act as reactive surface ions for metal nanoparticles stabilization [16]. Also MgO is an irreducible oxide and, consequently, it is relatively inactive and therefore appropriate for support applications. Thus, magnesium oxide has been extensively employed not only as a catalysts support but also directly as catalyst itself, taking advantage of its basic nature. For example, it has been widely used in basecatalyzed reactions such as aldol-condensation [17], Cannizzaro [18] or Tischenko [19] reactions, and the Knoevenagel condensation [20]. So far, to our knowledge, MgO has poorly previously tested as support of metal nanoparticles applied in the reduction of 4-NP to 4-AP. In this work we report the synthesis and characterization of bimetallic CuPd nanostructures generated on a magnesium oxide support, by the simple method of wetness impregnation. Their catalytic performance has been tested in the reduction of 4-NP as a model reaction using NaBH4 as reductant. Additionally, a comparative study of its main characteristics and catalytic performance with its monometallic counterparts is presented. 2. MATERIALS AND METHODOLOGY 2.1. Catalysts Synthesis The support, MgO, was obtained by calcination at 723 K of a commercial Mg(OH)2 (Fluka). The monometallic catalysts, Cu/MgO and Pd/MgO, were prepared by the incipient wetness impregnation using CuCl2 and PdCl2 precursors (Sigma Aldrich), respectively, with the adequate concentration to incorporate a 5 wt% metal loading on the support MgO. The bimetallic catalyst CuPd/MgO was prepared by successive impregnation with the same Cu and Pd precursors as those employed in the synthesis of the monometallic counterparts,

Morales et al.

respectively, but the weight metal loading in this case was 5% for Cu and 1% for Pd. The resulting materials were dried in air at 383 K overnight. In the next step the catalysts were activated as follows: 100 mg of the impregnated catalysts were reduced employing 100 mL of a 50 mM NaBH4 solution under stirring for 15 min at room temperature. Thereafter, they were filtered, washed with distilled water and, finally, dried in a vacuum oven at 323 K for 12 h. 2.2. Catalyst Characterization Values of specific surface area (SBET) of the supports were determined by N2 adsorption-desorption isotherms at 77 K in an automatic volumetric adsorption apparatus (Micromeritics ASAP 2020). Prior to nitrogen adsorption, the samples were outgassed for 5 h a 423 K. X-ray diffraction (XRD) patterns were obtained on a Polycristal X’Pert Pro PANalytical diffractometer with Nifiltered Cu/Kα radiation (λ= 0.1544 nm) operating at 45 kV and 40 mA. For each sample, Bragg’s angles between 4° and 90° were scanned at a rate of 0.04 degree/sec. In order to obtain information on the morphological characteristics such as shape and size of metal crystallites on the support, the reduced catalysts were subjected to a detailed Transmission Electron Microscopy (TEM) study. TEM micrographs were obtained on a JEOL 2100F electron microscope operated at 200 kV. The samples were milled and suspended in ethanol by ultrasonic treatment and a drop of the fine suspension was placed on a carbon-coated nickel grid to be loaded into the microscope. The mean diameter (d) of metal particles was calculated based on a minimum of 300 particles. X-ray photoelectron spectroscopy (XPS) was performed at Centro de Materiais da Universidade do Porto (CEMUP, Porto, Portugal) in a Kratos AXIS Ultra HSA spectrometer with VISION software for data acquisition using monochromatized Al Kα radiation (1486.6 eV) operating at 15 kV (90 W) in FAT (Fixed Analyser Transmission) mode. The powdered samples were pressed into pellets prior to the XPS studies. All binding energies (BE) were referenced to the C 1s line at 284.6 eV. XPS technique was also used to study the chemical states of Cu and Pd in the catalysts. Spectra were analyzed with Casa XPS software by fitting after Shirley background correction. The relative concentrations and atomic ratios were determined from the integrated intensities of photoelectron lines corrected with the corresponding atomic sensitivity factor.

CuPd Bimetallic Nanoparticles Supported on Magnesium Oxide

International Journal of Green Technology, 2017, Vol. 3,

2.3. Catalytic Tests

with deionized water and dried for further characterization. It should be indicated that reproducibility of these catalytic tests was verified by duplication, with different aliquots of the same catalytic material, of some of these experiments. Less than 10% results variation was obtained.

The catalytic reduction of 4-NP to 4-aminophenol 4AP was carried out at room temperature, directly on a 3 mL quartz optical cell and monitorized by UV-Vis spectroscopy. UV-Vis spectra were performed on an Agilent 8453 UV-Vis spectrometer with diode array detector in the range of 250-550 nm. The degradation of 4-NP was monitored by the decrease of the absorbance at λ=400 nm for nitrophenolate ion in basic media and development of a new band at λ=300 nm corresponding to the formation of 4-AP. A stock solution of the 4nitrophenol was prepared (0.05 mM, 50 mL); 3 mL of this solution was transferred to the UV-Vis cell and the NaBH4 was added (5.4 mg). Upon NaBH4 addition, the band at λ=400 nm remained unaltered during 15 min and the reaction only started after the addition of the catalyst. 3 mg of the catalyst was added to the cuvette and the degradation reaction was controlled at 10 seconds intervals.

3. RESULTS AND DISCUSSION 3.1. Catalysts Characterization The X-ray diffraction patterns of the commercial Mg(OH)2 and the support MgO are represented in Figure 1a. The Mg(OH)2 diffractogram can be indexed as the hexagonal brucite phase (JCPDS card 084-2164) while the thermal dehydration process at 723 K gave rise to the characteristic periclase structure of MgO (JCPDS card 45-946). The crystallite size of MgO was estimated from line broadening of (200) diffraction peak (2ϴ=42.9o) using Scherrer formula, while for Mg(OH)2 the peak at 2ϴ=58.8o corresponding to reflection plane (110) was used to determine the mean size of crystallites. The values are included in Table 1, along with some other parameters related to textural and structural properties. As was expected, the thermal decomposition of Mg(OH)2 resulted in the formation of MgO particles of a much smaller size which is in good agreement with other reported observations [21,22].

To test the reusability of all the catalysts, a scale up experiment with 100 mg of catalyst was carried out in a round bottom flask under stirring and maintaining the same ratio catalyst/4-NP as those in the cuvette experiments. In this case, the electronic spectra of the reaction mixtures were performed at different time intervals by withdrawing 3 mL aliquots from the reaction. After each catalytic cycle the reaction mixture was filtered and the catalyst was reused, without any further treatment, in a new cycle. This procedure was repeated up to six times and at the end, the material was washed

a)

Equally important to changes in the structural properties are the textural parameters (Figure 2, Table 1). It can be seen that the reduction of crystal size of

b)

c)

CuPd/MgO

CuPd/MgO

Intensity (a.u.)

53

MgO 0

Pd

0

Pd Pd/MgO

Pd/MgO

Cu/MgO

Cu/MgO

Mg(OH)2

0

20

40

2

60

80

0

20

40

2

60

80

0

20

40

60

80

2

Figure 1. XRD patterns of a) commercial Mg(OH)2 and MgO obtained after calcination, b) activated catalysts with NaBH4 and c) spent catalysts after 6 reaction cycles.

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Morales et al.

Table 1: Textural and Structural Parameters of Supports and Catalysts SBET

Sample

2

(m /g)

Average pore diametera

Total pore volumea (cm3/g)

Mg(OH)2 or MgO average crystallite size (nm)

Metal average particle size TEM (nm)

(nm)

Mg(OH)2

17

16.5

0.11

44b

-

MgO

105

9.6

0.39

9.9c

-

Pd/MgO

54

10.4

0.29

28b

7.1

0.23

b

-

b

2.7

Cu/MgO

33

CuPd/MgO

18.3

38

22.0

0.31

33 24

a

BJH desorption method, bcrystallite sizes calculated applying Scherrer formula to peak at 2ϴ=58.80 corresponding to Mg(OH)2, ccrystallite sizes calculated applying Scherrer formula to peak at 2ϴ=42.90 corresponding to MgO.

Mg(OH)2 after calcination is ccompanied by an increase of surface area (from 17 m 2/g to 105 m2/g, Table 1). Figure 2 presents the N2 adsorption/desorption isotherms of the as-received commercial Mg(OH)2, the support MgO derived by calcination of Mg(OH)2 and the resulting CuPd/MgO catalyst. All of them exhibit adsorption type-IV isotherm showing relatively narrow hysteresis loops of type H3, typical of textural porosity originated from intercrystal spaces/voids at the mesoporous scale. As confirmed in Table 1, all the samples were mesoporous with the mean pore diameters ranging from 9 to 22 nm. 300

Quantity Adsorbed (cm³/g STP)

TEM images of the bimetallic CuPd/MgO and its monometallic counterpart Pd/MgO, are shown in Figure 3 and Figure 4, respectively. Unfortunately, TEM images of Cu/MgO catalyst could not be obtained due to the low contrast achieved with the support, likely caused by the oxidation of copper and the poor degree of crystallization of metallic particles induced by the reduction process at room temperature.

Mg(OH)2

250

MgO CuPd/MgO

200

150

100

50

0 0,0

0,2

MgO-supported catalysts show the characteristic peaks of brucite structure of Mg(OH)2, which confirmed that aqueous impregnation transformed MgO back to Mg(OH)2 as a consequence of its interaction with water [23]. No diffraction peaks corresponding to metallic Cu were detected due to fact that, in the cases where they were present, they would be overhead by the Mg(OH) 2 or MgO diffraction patterns. Only in the case of Pd/MgO catalyst (Figure 1b), the (200) reflection from the Pd (fcc phase) of low intensity can be intuited at approximately 2ϴ=430, overlapped with the main peak of the periclase structure of MgO.

0,4

0,6

Relative Pressure (p/p°)

0,8

1,0

Figure 2. N2 adsorption-desorption isotherms.

The impregnation of MgO with the aqueous solution of the metallic precursors, followed by reduction with NaBH4 and drying, resulted in a remarkable decrease in the specific surface areas as well as in the pore volume with respect to the starting MgO, which is in agreement with other authors findings [22,23] (Table 1). Furthermore, it should be noted that the incorporation of metallic nanoparticles to the MgO support caused some evident changes in the crystalline structure of the material (Figure 1b, Table 1). The XRD patterns of the metallic

TEM images confirmed the formation of metallic nanoparticles on the magnesium oxide surface after the activation with NaBH4 at room temperature. In the lowmagnification images of the CuPd/MgO catalyst, nanoparticles embedded in larger agglomerated crystallites are appreciated. The darker and smaller elements correspond to bimetallic nanoparticles whereas the larger ones are the Mg(OH)2 crystals, which exhibit irregular shapes and sizes ranging from 10 to 100 nm. As observed in the high-magnification images, bimetallic PdCu nanoparticles are uniformly dispersed on the Mg(OH)2 crystallites surfaces. A statistical count of the metallic nanoparticles yielded an average diameter of 2.7 nm, and a narrow range particle size distribution was obtained as represented in Figure 3. Conversely, its monometallic Pd counterpart (Figure 4) exhibited a wide particle size distribution, with an average particle size of



55

35

CuPd/MgO

30

Average size: 2.7 nm

Frequency (%)

25 20 15 10 5 0 0

1

2

3

4

5

6

7

Particle size (nm)

Figure 3. TEM micrographs of CuPd/MgO and particle size distribution.

35

Pd/MgO

30

Average size: 7.1 nm

Frequency (%)

25 20 15 10 5 0 0

2

4

6

8

10

12

14

Particle size (nm)

Figure 4. TEM micrographs of Pd/MgO and particle size distribution.

7.1 nm, noticeably higher. This indicates that the development of bimetallic CuPd nanostructures leads to a higher dispersion of metallic nanoparticles in comparison with the monometallic counterpart, in agreement with literature findings for bimetallic CuPd systems [24].

To get further insights concerning the metal surface structures formed, the surface composition and oxidation state of the catalysts supported on MgO were investigated by XPS. First of all, it is worth mentioning that the only elements detected in the survey spectra were: Mg, O, C, Cu and/or Pd, and Cl. The detection of

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Morales et al.

this last element indicates the existence of chlorine residues likely originated from the metallic precursor; nonetheless, its atomic content is negligible, lower than 0.3 at.%. The corresponding core level XPS spectra of the main elements are represented in Figure 5. Figure 5a focuses on the Mg 1s region of the monometallic catalyst Cu/MgO, which shows a peak at 1303.9 eV, characteristic of Mg2+ species in MgO or Mg(OH)2 [25,26]. The Cu 2p core level spectrum (Figure 5b) exhibits two major peaks centered at binding energies of 932.9 and 952.8 eV, attributed to Cu 2p3/2 and Cu 2p1/2 doublets characteristic of copper, respectively [27-29]. The 2p3/2 peak was deconvoluted into two different contributions which can be attributed to Cu(II) species at 934.7 eV and Cu(0)/Cu(I) species at 932.7 eV. The shake-up peaks at 943.5 and 962.8 eV further confirmed the presence of Cu(II) species [30]. The existence of Cu(II) could be due to partial re-oxidation on the surface of Cu when exposed to an oxidizing environment such as the air, given that copper is easily oxidized. Although it is expected to be reduced to metallic state by the borohydride in reaction conditions, the accurate copper oxidation state is quite difficult to determine under in situ reaction conditions.

peaks [31] (Figure 5c and 5d). Moreover, the Pd 3s region overlaps the O1s region [32], which makes unable to determine the Pd content with significant confidence. As can be appreciated, the characteristic doublet of palladium species can be intuited, overlapped with the Mg KLL Auger, in the monometallic catalyst at binding energies of around 335.5 and 340.5 eV, attributed to 3d5/2 and 3d3/2 doublets due to spin-orbital, respectively [32] (Figure 5c). However, for the bimetallic catalyst (Figure 5d) this appreciation is not possible since the Pd loading in this catalyst is quite low in comparison (1% vs 5%). Figure 5e offers the Cu 2p core level spectrum of the bimetallic CuPd/MgO catalysts, which exhibits similar binding energies values than the monometallic one (negligible shift of binding energy is detected). Also the XPS spectrum of this catalyst shows the existence of Cu(II) species, confirmed by the presence of the satellite peaks. It is interesting to note that the ratio of Cu2+/(Cu++Cuo) (1.38) is higher in the bimetallic sample than in Cu/MgO (1.09). This is consistent with the higher dispersion of metallic nanoparticles achieved in the bimetallic catalyst as deduced by TEM analysis (Figure 3), since Cu-containing small particles are more easily oxidized compared to larger monometallic Cu particles. Similar findings have been found by other authors for CuPd bimetallic systems as reported in literature [27].

The XPS analysis of the catalysts containing palladium is challenging due to the overlap of the Pd 3d region with the Mg KLL Auger structure photoemission

Mg 1s

a)

b)

CPS (a.u.)

2p3/2 2p1/2

1310

1305

1300

1295

1290

960

950

BE (eV)

d)

Mg KLL

CPS (a.u.)

940

930

920

e) Pd 3d Mg KLL

2p1/2

Pd 3d3/2

340

+

BE (eV)

Pd 3d5/2

345

0

Cu , Cu

Satellite 2+ Cu

1315

c)

2+

Cu

2p3/2 2+

Cu

Satellite 2+ Cu

335

BE (eV)

330

345

340

335

BE (eV)

330

960

950

940

0

+

Cu , Cu

930

920

BE (eV)

Figure 5. XPS spectra of the catalysts. a) Mg1s spectrum of Cu/MgO, b) Cu2p spectrum of Cu/MgO, c) MgKLL and Pd3d overlapped regions of Pd/MgO, d) MgKLL and Pd3d overlapped regions of CuPd/MgO and e) Cu2p spectrum of CuPd/MgO.



3.2. Catalytic Tests

same time, a new electronic band around λ=300 nm appeared due to the formation of 4-aminophenolate ion. The 4-NP reduction could also be confirmed by visual inspection since the intensity of the characteristic yellow color of 4-NP in the presence of NaBH4 gradually decreases after the addition of the catalyst to the solution, indicating the reduction of 4-NP to 4-AP.

The catalytic behavior of these samples was studied in the reduction of 4-NP to 4-AP in aqueous medium using NaBH4 as a hydrogen generator. The kinetics of 4NP reduction in presence of bimetallic NPs was followed by UV–vis spectroscopy. Figure 6a displays the control experiment performed with the support MgO in order to ascertain its adsorption capacity. As shown, the 4-NP in water exhibits an electronic band at = 313 nm, but after adding MgO the electronic band is shifted to = 400 nm due to the formation of the corresponding nitrophenolate anion in a basic medium. This maximum absorption at = 400 nm did not change over time for at least 12 minutes, which indicates no adsorption took place. In the same manner, in the presence of NaBH4 (Figure 6b), the intensity decrease of the electronic band at λ= 400 nm owing to 4-nitrophenolate ion is insignificant and any other electronic band appeared, which confirms the catalytic reduction of 4-NP did not occur in the absence of metal nanoparticles. As shown, upon the increase of the reaction time in the presence of the catalyst (Figure 6c, 6d and 6e), the intensity of electronic band at λ= 400 nm owing to 4-nitrophenolate ion decreases and, at the

The CuPd/MgO and its monometallic counterparts exhibited excellent catalytic behavior in the degradation of 4-NP. The reaction was catalyzed by Cu/MgO and Pd/MgO reaching a conversion close to 100% within 140 and 160 seconds, respectively. As for the bimetallic catalyst, it is worth mentioning that the total degradation of 4-NP was achieved after only 50 seconds of reaction time, around 3 times faster than its monometallic counterparts. Since the concentration of the reducing agent, sodium borohydride, largely exceeds the concentration of 4-NP, the reduction rate can be assumed to be independent of borohydride concentration and the catalytic reduction process can be considered a pseudo-firstorder reaction [8]. Bearing in mind that the absorbance

1,2

1,0

4-nitrophenol 4-nitrophenolate ion in presence of MgO

1,0

Absorbance

Absorbance

MgO 0,8

0,6 0,4

0,6

12 min 0,4

0,2

0,2

0,0

0,0 250

300

350

400

450

500

Wavelength (nm)

250

300

350

400

1,0

0s

Cu/MgO

0,8

0,6

140 s

0,4

Absorbance

c)

CuPd/MgO

0s 0,8

d)

0,6

0,4

160 s

0,2

0,0

0,0

0,0

350

400

450

500

250

300

350

400

450

Wavelength (nm)

500

e)

50 s

0,4

0,2

Wavelength (nm)

0s

0,6

0,2

300

500

1,0

Pd/MgO

0,8

450

Wavelength (nm)

Absorbance

1,0

Absorbance

0 min

b)

a)

0,8

250

57

250

300

350

400

450

500

Wavelength (nm)

Figure 6. Time-dependent UV-Vis spectra of the reduction of 4-NP to 4-AP. Experimental conditions: aqueous solution of [4-NP] = 0.05 mM, [NaBH4] = 50 mM, [catalyst] = 1 mg/mL.


Morales et al.

at time t (At) and time t =0 (A0) are equivalent to the concentration at time t (C t) and time t = 0 (C0), the correlation between ln(Ct/C0) and reaction time is linear (Figure 7). The apparent rate constant, kapp, was obtained from the slope of these linear correlations and the values are summarized in Table 2. Given that the apparent rate constant depends directly of the mass of metal catalyst used, the normalized rate constant has been estimated for each catalyst as: K= Kapp/(mmol of metal) and reported in Table 2. Interestingly, for monometallic Cu catalyst the reaction started after an induction period, designed as ti. This period of time for the reaction to start has been previously observed in several catalytic systems applied in this reaction, for example, for Co [33], Cu [8] or Au [34] catalysts. The causes of this induction period have been deeply studied for Au faceted metal particles [3] and the authors stated that any limitation by diffusion can be discarded. This required time to the reaction to start is thought to be associated with a dynamic restructuring of the surface necessary to render the metallic particles as an active catalyst. It is worth mentioning that no Table 2:

Cu/MgO Pd/MgO CuPd/MgO

Equa

-1

Weig Resid Sum Squa Adj. R

-1

kapp = 0,042 s

-1

kapp = 0,017 s -2

Book1

-1

kapp = 0,060 s

Book1

-3

0

20

40

60

80

100

120

140

160

180

200

Time (s)

Figure 7. Pseudo-first order plots of 4-NP reduction of Cu, Pd and Cu-Pd catalysts supported on MgO.

Coming back to the analysis of the catalytic activity, as observed in Table 2, the normalized rate constant of our bimetallic catalyst is superior to those obtained with

Catalyst

Support

[4-NP] (mM)

tb (s)

tic

kdapp∙102 (s-1)

Ke (s-1 mmol-1)

Ref.

Cu

MgO

0.05

140

60

4.2

17.8

This work

Pd

MgO

0.05

160

0

1.7

12.1

This work

CuPd

MgO

0.05

50

0

6.0

32.1

This work

CuPd

-

0.16

180

0

0.48

7.6

14

CuPd

-

0.08

720

0

0.59

0.10

35

0.06

80

0

4.5

-

36

0.05

20

0

29.9

230

8

CuPd

c

Induction period 0

Catalytic Parameters for the 4-NP Reduction Catalyzed by our Catalystsa and other Catalysts Reported in Literature

CuPd

a

induction period was observed neither for Pd/MgO nor the bimetallic CuPd/MgO.

ln(At/A0)

58

rGO

CuNi

rGO

0.5

90

0

2.3

-

37

NiPd

SiO2/RGO

0.05

120

0

1.7

4.53

44

NiPd

N-rGO

0.05

160

0

1.7

288

15

AgCu

monoliths

0.14

480

0

5.5

108

45

AgPt

Sepiolite

0.18

900

0

0.25 (20 oC)

0.17

38

AgAu

rGO

5

360

0

0.35

3.02

46

NiCo

rGO

-

250

0

1.8

-

39

CuO

MgO

0.06

120

0

0.41

9.29

25

Co

LDH

0.20

300

60

-

27.8

33

Pd

HSAG

0.05

0

0

11.1

150

8

Pd

MWCNT

0.06

720

0

0.023

0.28

43

Au

MgO

43

420

-

0.76

7.45

16

Au

Mn@SiO2_NH2 Mn@SiO2_SH

0.05

720 960

0 540

0.61 0.66

102 13.8

34

Reactions performed with aqueous solution of [4-NP]=0.05 mM, [NaBH4]=50 mM, [catalyst]=1 mg/mL. b Reaction time to achieve maximum 4-NP conversion to 4-AP. Induction period. d kapp determined from the slope of ln(At/A0)= -kt plots. e K was calculated as K= kapp(s-1)/nmetal (mmol) from the data given in the article.



the monometallic counterparts. This fact indicates that the presence of the bimetallic catalyst results in a superior catalytic system, compared with both monometallic samples separately. This result is in accordance with the literature findings, which reveal superior catalytic performance in this reaction for bimetallic nanostructures in comparison with the monometallic counterparts [35-39]. Most of the referred literature explains this improvement of the catalyst performance in bimetallic systems due to two types of synergic effects. One is a consequence of the formation of isolated Pd ensembles on the metallic nanoparticle surface creating highly active sites [36,40]. The other one, known as the charge redistribution effect, is due to electronic effects, which can facilitate the hydrogenation of the nitro-group [36], an explanation for AuPd bimetallic systems can be found in references [40, 41]. Krisnha and coworkers attributed the enhancement of the catalytic performance of the bimetallic system CuNi [37] or AgCo [42] on the basis of synergetic effect via spill-over mechanism. In our work, we have shown that a small loading addition of Pd (1 %wt) to Cu (5%) greatly reduces the nanoparticle size from 7.1 to 2.7 nm, generating more active sites (higher dispersion). Although we cannot discard the contribution of other synergetic effects between both metals, since we did not notice any evidence of intimate contact between them (for instance, electron shift shown by XPS is negligible), we state that the Cu dilution by Pd is the most feasible reason for this improvement. Table 2 also summarizes the catalytic performance of several bimetallic systems along with some other monometallic catalysts reported in the literature for the catalytic reduction of 4-nitrophenol to 4-aminophenol. It

is clear that our CuPd/MgO outperforms several catalysts and is highly competitive with some monometallic noble metal catalysts such as monometallic Pd [43] or Au [16]. The comparison with other CuPd bimetallic systems reported in literature [14,35,36] indicates that a support such as our MgO is very suitable to enhance the catalytic activity. Nonetheless, it should be noted the general superiority of the catalysts containing graphene materials or high surface area graphite (HSAG) as supports as we shown in our previous work [8]. These graphitic materials provide additionally synergism effect in this reaction due to π-π interactions, which are not present in a metallic oxide such as magnesium oxide. However, given that the catalytic activity obtained by the Cu-Pd supported-MgO catalysts is highly comparable with other reported catalysts, we believe that magnesium oxide can represent a cost-effective alternative to these expensive nanocarbon supports. Also our CuPd/ MgO outperforms the catalytic activity of other bimetallic systems containing Cu or Pd such as CuNi [37] or NiPd [44-46]. 3.3. Recyclability Finally, the catalysts were reused for another 5 reaction cycles. After each run the reaction mixture was filtered and the catalyst was reused, without any further treatment, in a new cycle. The UV-Vis spectra were recorded at different time intervals by withdrawing 3 mL aliquots from the reaction. As represented in Figure 8, all tested catalysts preserved their catalytic performance with very slight decrease in conversion after six consecutive catalytic cycles.

120

Pd/MgO Cu/MgO CuPd/MgO

Conversion (%)

100

80

60

40

20

0

1

2

3

4

Run number

Figure 8. Recycling tests.

59

5

6

60


As mentioned in the introduction section, magnesium oxide has barely been studied as support for the reduction of 4-nitrophenol in the aqueous phase, which means no many references about stability of these type of materials are found. Thus after these consecutive catalytic cycles, the catalysts were characterized by powder XRD (Figure 1c). For the three used catalysts, the X-ray diffraction patterns do not show any appreciable change, proving the preservation of their crystalline structure after 6 reaction cycles. As for the surface atomic composition of the spent catalysts, as mentioned in the XPS section, Pd content in the bimetallic and monometallic counterpart could not be accurately calculated due to the overlap of spectral features. Nonetheless, for the Cu/MgO catalyst, the atomic concentrations of Cu observed in the fresh (2.0 %at.) and used catalysts (2.4 %at.) were comparable which suggests no Cu was leached during the reactions. Nonetheless, it should be noted that the O/(Mg+Cu) atomic ratio increases from 4.4 until 5.4 in the spent catalyst, which confirmed a higher degree of surface hydroxylation after several catalytic runs in the aqueous phase. Even though, this fact seems not to have any impact on the catalytic activity. To further verify the absence of metal leaching, we carried out the following experiment for each catalytic system tested in this work. After performing a reaction test, the catalyst was separated from the solution by filtration and then the same amount of 4-NP typically used in the catalytic tests was added again into that solution containing the NaBH4, but without adding the catalyst. The band at λ =400 nm corresponding to 4-NP remained unaltered during 30 min, indicating no conversion of 4-NP took place, proving that during the course of the reaction there was no leaching of metal to the solution from the solid support. These results further confirm the stability of these catalysts upon reuse.

Morales et al.

bimetallic systems described in literature, CuPd/MgO has improved the reactivity by several times, which highlights the suitability of magnesium oxide as support for metallic nanoparticles. Furthermore, we proved its reusability during six consecutive cycles with hardly any loss of efficiency. This stability was corroborated by the characterization results of the reused catalysts, which indicated that the crystalline structure and the surface composition were almost preserved upon recycling. In conclusion, bimetallic CuPd/MgO catalysts can act efficiently as recyclable heterogeneous catalyst for the degradation of 4-nitrophenol in aqueous phase and could represent an alternative to other expensive catalytic systems frequently tested in this reaction. Notice that both the catalyst synthesis and the reaction can be carried out under very mild conditions. 5. ACKNOWLEDGEMENTS Authors acknowledge financial support from the Spanish Government (projects CTQ2017-89443-C3-1R CTQ2017-89443-C3-3-R) and from Fundação para a Ciência e a Tecnologia (FCT)/MEC and FEDER under Program PT2020 (project UID/QUI/50006/2013). M.V. Morales gratefully acknowledges UNED for a predoctoral grant and M. Rocha thanks FCT for the grant SFRH/BD/52529/201. REFERENCES [1]

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Received on 25-12-2017

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Accepted on 30-12-2017

Published on 31-12-2017

© 2017 Morales et al; International Journal of Green Technology. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.