Pd and Pd,In nanoparticles supported on polymer fibres as catalysts for the nitrate and nitrite reduction in aqueous media

Journal Pre-proof Pd AND Pd,In NANOPARTICLES SUPPORTED ON POLYMER FIBRES AS CATALYSTS FOR THE NITRATE AND NITRITE REDUCTION IN AQUEOUS MEDIA ´ Moreno, Nuria F. Albana Marchesini, Vanina Aghemo, Ivan ´ Silvia Irusta, Laura Gutierrez Navascues,

PII:

S2213-3437(19)30774-2

DOI:

https://doi.org/10.1016/j.jece.2019.103651

Reference:

JECE 103651

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

1 October 2019

Revised Date:

26 December 2019

Accepted Date:

30 December 2019

´ N, Irusta S, Please cite this article as: Albana Marchesini F, Aghemo V, Moreno I, Navascues Gutierrez L, Pd AND Pd,In NANOPARTICLES SUPPORTED ON POLYMER FIBRES AS CATALYSTS FOR THE NITRATE AND NITRITE REDUCTION IN AQUEOUS MEDIA, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103651

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Pd AND Pd,In NANOPARTICLES SUPPORTED ON POLYMER FIBRES AS CATALYSTS FOR THE NITRATE AND NITRITE REDUCTION IN AQUEOUS MEDIA

F. Albana Marchesinia, Vanina Aghemoa, Iván Morenob, Nuria Navascuésb, Silvia Irustab,* [email protected], Laura Gutierreza,* [email protected]

a Instituto

de Investigaciones en Catálisis y Petroquímica, INCAPE, (FIQ, UNL-CONICET),

b Institute

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Santiago del Estero 2829, S3000 Santa Fe, Argentina

of Nanoscience of Aragon (INA) and Department of Chemical and Environmental Engineering,

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*Corresponding authors: , +34876555437, , +543424536861.

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Edificio I+D+i, Campus Río Ebro, 50018, Zaragoza, Spain.

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Graphical abstract

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Highlights Polymeric fibres loaded with In and Pt nanoparticles were obtained by electrospinning. The prepared catalysts were active for water pollutants nitrite and nitrate reduction. Lower Pd/in surface ratio prevent nitrate and nitrite over reduction improving nitrogen selectivity.

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  

Abstract

The presence of high concentrations of nitrate and nitrite ions in water sources is an actual

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environmental problem. Nowadays, there are several technologies for nitrate and nitrite treatment, but most of them involve the concentration of the ions which are later necessary to be decomposed in a second

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step. The catalytic elimination of nitrates and nitrites from water is a very promising method because the ions are transformed into the harmless N2(g). Two types of synthetic polymeric fibers were proposed as

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supports of palladium and indium catalytic species with low metallic loading (Pd:In=1:0.25 wt.%). They were synthesized by electrospinning method with a Pd/In weight ratio 1:0.25. PdInDA catalyst was prepared by dissolving poly (methyl methacrylate) (PMMA) with a mixture of acetone/dimethylformamide,

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whereas the polymer (PMMA) was dissolved in pure dimethylformamide for the synthesis of PdD and PdInD catalysts. Both bimetallic solids gave rise to good selectivity to N2(g). However, the PdInDA reached middle nitrate conversions (around 50 %) with low ammonia production (under 0.1Nppm). The

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characterization results suggested that the active sites remained unchanged after the reaction. This proposal involves a green chemical method with low consumption of reactants and producing a small

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quantity of waste.

Keywords: PMMA supported catalysts, Nitrate and nitrite reduction, electrospinning, nanoparticles, fibers.

1. Introduction 2

The high concentrations of nitrate ions in water sources conduce to an important environmental problem. Water pollution with these ions is a consequence of the intensive use of fertilizers in agriculture, of leachate from urban waste dump, and of the incorrect industrial wastewater treatments [1]. The elimination of this contaminant is difficult as the nitrate could be easily converted into nitrite or N-nitrous compounds, which could conduce to even more toxic species for human health. It is well known, that drinking water with nitrate ions, produces serious problems such as blue baby syndrome, cancer, hypertension, among others [2]–[5]. Because of these risks, the USEPA (U.S. Environmental Protection Agency) established that the maximum concentration of nitrates in water must be 10 mg N/L (nitrate concentration evaluated as nitrogen) [6].

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In an attempt to find possible solutions, during the last years several techniques have been applied for nitrates and nitrites treatment such as: reverse osmosis [7], [8], electrodialysis [9] and ion

exchange [10][11]–[15]. The final product of all these technologies is a pollutant concentrate which final destination is still an unsolved technical problem. Biological treatments, on the other hand, are not

recommended for treating drinking water since they do not concentrate reaction by-products as membrane

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and electrodialysis processes do [11], [15]. Consequently, it is necessary to find a reliable method in which the nitrates could been chemically transformed into environmentally friendly compounds. In this vein, the

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ion catalytic removal) has been proposed as a promising method to convert nitrates to gaseous N2 [16]. In conventional processes.

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general, catalytic technology proves easy application whit similar or lower investment costs compared to

However, finding catalytic formulations that conduce to high yields and good activity and selectivity to nitrogen is a difficult task. In fact, several systems with high activity but low selectivity, or on

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the contrary low activity but high selectivity have been reported. Low reaction selectivity means the production of harmful ammonia. Several noble metals combined with other promoter species have been

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tested as catalytically active sites. For example, palladium and platinum noble metals combined with Cu, In, Sn among other elements, were proposed [17]–[19].

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Catalytic performance of palladium-based formulations has been extensively studied. Several studies proposed that the superficial Pd could not activate by itself the nitrate reduction reaction with hydrogen [20]–[24]. In fact, it is necessary an intimate contact with a second promoter specie to conduce the redox process which ends in the reduction of nitrates. It is generally established that the metallic promoter interacts with oxygen atoms from nitrate ions and then nitrite species are formed by the dissociated H2 previously adsorbed on the superficial palladium atoms. Then the reduction of nitrite to Ncontaining compounds takes place on palladium entities. The NO species formed from the nitrite reduction could conduce to the generation of gaseous nitrogen or to ammonia ion. The main final reaction products 3

could be N2 or NH4+, the selectivity of one or another would depend on the catalyst physicochemical properties and on the reaction conditions. Obviously, the production of ammonia is not desired in the process [24]–[26]. Looking for an optimal active site combination for the water nitrate reduction is not the only task, but also finding an adequate support to immobilize the active metal couple. Several supports were probed; alumina and silica were largely studied, while activated carbon, zeolites, and titanium dioxide were also proposed [24], [26]. However, nowadays-new catalytic supports are being tested in order to improve not only physical-chemical properties but also the economic and environmental aspects [27]–[31]. In this vein, Durking et al., reported the use of lignocellulose fiber as a support of Pd-In catalytic active sites for the

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mentioned reaction [19]. This promising proposal conduced to a very good result for the nitrate reduction. However, these authors used high metal loading (Pd: 5 wt.%, In: 2 wt.%) and low nitrates initial concentration (0.1mM) in comparison with the present work.

Electrospinning is a straightforward and versatile technique to obtain polymer or inorganic fibers

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with diameters ranging from tens of nanometers to several micrometers. The characteristics of these fibers mats make them promising materials for application in many fields, including catalysis. The incorporation of

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metal nanoparticles precursors into the polymer fibers could prevent the agglomeration that can cause catalytic activity decrease [32]. A large variety of polymers can be electrospun, for instance polycaprolactone, polyvinyl alcohol, polyacrylonitrile and poly methyl methacrylate (PMMA). PMMA

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presents very interesting physical and mechanical properties that make it an appropriate structured support for catalytic applications. It possesses great mechanical resistance, high Young´s modulus and low capacity to absorb water. As a result, PMMA fibers have excellent dimensional stability and can be easily

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recovered from the fluidic catalytic media by a standard filtration process as Zoppas et al. reported with activated carbon fibers [33]. It is important to highlight that a good stability of metal loading on the fiber

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support is required in order to attain an adequate catalyst lifetime and to avoid possible water contamination [34]. On the other hand, the fiber synthesis involves few waste generation in agreement with

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the Green Chemistry principles [35]. In this study, we proposed two types of synthetic polymeric fibers prepared by electrospinning as

support for palladium and indium catalytic species, using low metallic (Pd and In) loading. Pure dimethylformamide (DMF) and a mixture of DMF and acetone were used for the PMMA pellets dissolution giving rise to the two studied supports. The produced materials were tested in nitrate and nitrite elimination reactions and then characterized by XRD, XPS and TEM in order to understand the catalytic behavior.

2. Materials and methods 4

2.1. Catalyst preparation Electrospun supported catalyst were prepared using an Y-flow 2.2.D-500 electrospinner (Coaxial Electrospinning Machines/R&D Microencapsulation, Malaga, Spain). PMMA pellets were dissolved in N,Ndimethylformamide (DMF, D), resulting in a 20 wt.% solution to prepare PdD and PdInD catalysts. At the same time, PdInDA catalyst was prepared with a solution of PMMA in a 50 v/v % mixture DMF/acetone (DA) solvent. The catalytic precursors, Pd(NO3)2 and In(NO3)3, were added to the corresponding solutions in order to obtain 1 wt.% of Pd and 0.25 wt.% of In as final fibers loading. The solutions were loaded in a syringe connected to a positive voltage of 6.5 KV for PdD and PdInD samples and of 5 KV for sample PdInDA. The resulting fibers were collected on a static plate (covered with an aluminum foil) connected to a

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negative voltage power supply of -3 KV. The solution injection rate was 1mL/h. The tip to collector distance (TCD) was fixed at 15 cm.

All prepared catalysts were reduced with a water hydrazine solution (20 M, p.a.) before the catalytic tests. The reduction pretreatment was performed at 40°C for 2 h. After that, the catalysts were washed 3

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times with water and then, dried on vacuum at 60°C during 12 h. The final catalysts were named as: PdD (Pd 1 wt.%), PdInD (Pd:In 1:0.25 wt.%) and PdInDA (Pd:In 1:0.25 wt.%), where D and DA indicate the

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solvent in which PMMA was dissolved (D: DMF and DA: mixture DMF + Acetone). Pd and In fibers loads measured by ICP technique are shown in Table 1.

Catalysts PdD

Solvent

Pd (wt.%)

In (wt.%)

DMF

1.00

-

DMF

1.00

0.25

DMF-Acetone

1.00

0.25

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PdInD

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Table 1. Used solvents and fibers composition*.

PdInDA

by ICP measurements.

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* Determined

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2.2. Nitrate and nitrite Catalytic reduction test The catalytic tests were performed in accordance with previous studies [36]. Briefly, a spherical

borosilicate three-necked round bottom flask batch reactor (250 ml) with a magnetic stirrer (800 rpm) and a temperature controller set at 25°C was used. Deionized water (80 ml) and 200 mg of catalyst were added into the reactor. A hydrogen flow (400 mL/min) was used as reducing agent. HCl 0.1M solution was added to maintain the pH of the reaction system at 5. The reaction test started when 100 ppm of N as NO3- or NO2- were added into the flask.

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0.1 ml aliquots were taken from the reactor at fixed time intervals and analyzed. The nitrates and nitrites concentrations were quantified following the Cadmium reduction method and the Gries colorimetric method respectively [37]. While the ammonia production was determined by the Modified Berthelot method [38]. Catalytic results were expressed as conversion X (%) and selectivity to nitrogen S (%). They were defined below. Equation 1

X% = [(1 − (C ⁄ C0 )] × 100

Equation 2

S% =

C0 −C−CA (C0 −C)

× 100

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Where C0 is N-ppm of nitrates or nitrite at the beginning of the reduction process, C is N-ppm from nitrates or nitrites at time t, and CA is N-ppm from products (nitrites and/or ammonia) at time t.

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2.3. Catalyst characterization

The quantitative chemical composition of the catalysts was performed by inductively coupled

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plasma atomic emission spectroscopy (ICP-AES) on an ICP-OPTIMA 2100 DV Perkin Elmer instrument. The crystallinity of the samples was studied by XRD analyses using a XRD-Diffractometer

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PANalytical X′Pert PRO (PANalytical Holland) with CuKα radiation run at 40 kV and 30 mA. In order to evaluate the surface composition of the fibers and the oxidation state of the catalyst active sites, X-ray photoelectron spectroscopy (XPS) analyses were carried out using an Axis Ultra DLD,

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Kratos Tech. The spectra were excited by a monochromatized Al Kα source (1486.6 eV.) run at 15 kV and 10 mA. For the individual peak regions, pass energy of 20 eV was used. The survey spectra were measured at 160 eV pass energy. The analysis of the peaks was performed with the CasaXPS software,

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using a weighted sum of the Lorentzian and Gaussian components curves after Shirley background subtraction. The binding energies were referenced to the internal C 1s (284.9 eV) standard.

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The catalyst morphology and the size distribution of the active site particles were analyzed by

recording TEM images using a FEI Tecnai F30 microscope operating at 300 kV and in scanning transmission mode. This instrument was equipped with a High Angle Annular Dark Field detector (STEMHAADF) for energy-dispersive X-ray (EDX) analysis. Scanning electron microscopy was carried out on a FEI Inspect F30 scanning electron microscope used to visualize the fibers.

3. Results and discussion 6

3.1. Catalyst activity It has been reported that catalysts containing only palladium or indium as active sites, present low activity for the nitrate reduction [36], [39]–[41]. However, the monometallic Pd based solids result good catalysts for the nitrite elimination reaction [36]. In this context, and in order to evaluate the fibers behavior as catalytic site support, the monometallic PdD fibers were tested for nitrite abatement. Figure 1 shows that after 20 min the nitrite conversion reached 32 % and after 70 min the final conversion was 37 %. The higher ammonia production during the reaction was 2.2 N ppm, resulting almost lower than the obtained with Pd,In/Al2O3 at the same conversion level [36]. 100

10

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90

8

Nitrites

60

6

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50 40

4

30

Ammonia

20 10 0

20

40

60

80

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0

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Nitrite (N ppm)

70

Ammonia (N ppm)

80

Time (min)

100

2

0 120

Figure 1. PdD catalytic activity. Δ: Nitrite elimination and : Ammonia production.

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This result, confirm that fibers could derive in a good support for catalytic purposes. In fact, Salvo et al. assert that the immobilization of catalysts on polymeric supports is a challenging field of research. In

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this vein, researchers are looking for new catalytic system designs and optimal protocols that conduce to better materials [42].

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Consequently, the bimetallic catalysts were tested for the nitrate elimination. The nitrate concentration on the PdInD system decreased rapidly during the first 20 min under reaction (Fig. 2A). Then, the catalyst deactivation occurred and the final conversion resulted 36 %. The nitrite and ammonia production on PdInD fiber are shown in Fig. 2B and 2C. The maximum nitrite concentration was reached after 40 min on reaction (0.33 N ppm NO2-), showing a low nitrite elimination rate. On the other hand, the final ammonia production resulted in 0.07 N ppm as NH4+ at 120 min in reaction. This value is within the levels established by USEPA (0.1 N ppm NH4+), and it is lower than the previously obtained with PdIn based catalysts prepared with inorganic supports as SiO2, Al2O3 and resins [33], [36], [43], [44]. 7

100

0,4

A

Nitrates

0,08

B

Nitrites

C

Ammonia

80

0,2

40

0,04

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60

Concentration (N ppm)

Concentration (N ppm)

Concentration (N ppm)

0,06

0,02 20 PdInD PdInDA

PdInD PdInDA 0

40

80

120

0,0

0,00 40

80

0

40

80

120

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Time (min)

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0

PdInD PdInDA

Figure 2. Catalytic activity. PdInD (--) and PdInDA (--).

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A: Nitrate elimination, B: Nitrite production, C: Ammonia production.

The final nitrate conversion after 120 min under reaction with the PdInDA catalyst resulted in 53 % and the deactivation started just after 20 min (Fig. 2A). The maximum nitrite production was obtained

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after 40 min and it resulted lower than the observed with PdInD (0.11 N ppm). Then, 0.05 N ppm NH 4+ was the final ammonia production, which evidences a good selectivity to nitrogen. This result also meets the

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standard set by the international normative for ammonia concentration in water [25]. It should be remarked that, after reaction the liquid media was filtered and the presence of metallic

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(Pd and In) was not detected by ICP technique. The control of the ammonia generation in the catalytic nitrate reduction technology is still a

current challenge. In order to show that PMMA fibers promote the catalytic performance of Pd/In base catalysts, a comparison between previous prepared catalysts was carried out (Supplementary information, Table S1). It was observed that the ammonia production with PdInD and PdInDA resulted much lower than with catalysts supported on SiO2, Al2O3 and carbon fibers (PdIn/C). Even though the nitrite production is lower, the selectivity to nitrogen is higher than the obtained with the other PdIn based catalysts [33], [36], [45]. The catalyst supported on carbon fibers was added in order to show the efficiency of this catalyst 8

when the initial concentration of NO3- was 25 N ppm NO3- (PdIn/C) [33]. The PdIn/C system reached 100% of conversion after 25 min on reaction stream, but the selectivity resulted very low. Besides, it could be remarked that the initial reaction rate resulted 23 and 6 ppmN.min-1.g catalyst-1 for PdInDA and PdInD respectively. These reaction rate values resulted similarly to the ones previously observed [36], and the selectivity to ammonia resulted lower for these PMMA catalysts. These results would indicate that the catalysts prepared on PMMA supports give rise to good selective to nitrogen. The deactivation process in these reaction systems could be generally explained [36] by two mechanisms: (i) an oxidation process over the active site that could not be reversed by the catalytic system, i.e. the In oxidized by the nitrate reduction cycle, cannot be re-reduced by the hydrogen adsorbed

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on the near metallic Pd and (ii) a superficial electric phenomenon. On the other hand, it was reported that PMMA is a partially hydrophobic material due to its highwater contact angle [46] and because of the Z-potential value that could probably produce negative

superficial charge at the similar reaction conditions used in this study [47]. Aditionally, this material is also

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considered as an inert substrate [48] because it does not involve reactive surface groups which could

adsorb OH- ions. Falahati et al. demonstrated that these kind of inert and hydrophobic surfaces can adsorb

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exclusively Cl- in acidic and neutral aqueous media, and both Cl- and OH- in alkaline conditions [47]. In our case the reaction media pH was controlled at 5 by the addition of HCl aliquots forming a negative electric layer from Cl- ions. But, while the reaction occurs, for each nitrate converted to nitrite, a OH- ion is

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generated [49]. As a result, the active site surroundings could present more alkaline behavior favoring the OH- ions adsorption. Thus, the nitrate and nitrite adsorption step on the catalytic surface could be disrupted. Consequently, after 20 min under reaction the PMMA surface could be covered by a negative

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ion layer blocking the active sites and conducing to the catalyst deactivation. In order to understand the catalytic results of the prepared materials, the characterization of the

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solids was performed.

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3.2. Catalyst characterization XRD results

Figure 3 shows the obtained XRD patterns of fresh, reduced with hydrazine and used in reaction of

both PdInD and PdInDA catalysts. The PDF cards that correspond with the designation of the species are: PdO: 27.4°, 29.9° and 44.7°, PDF 46-1211, Pd0: 39.5°, 47.7°, 67.5° and 81.9°, PDF 46-1043, Pd2O: 28.4°, PDF 34-1101, PDF 5-642 and In2O3: 29.4° and 32.3°, PDF 21-406.

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PdInDA B

PdInD A

PdO

In2O3

F

In2O3

0

Pd

In2O3

A.U.

F

R

Pd2O PdO

Pd2O

U 20

25

30

35

R

PdO

40

45

50

55

20

25

0

Pd

30

35

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A.U.

PdO

40

U

45

50

55

60

2

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Figure 3. XRD Patterns. A: PdInD and B: PdInDA. F: Fresh, R: Reduced, U: Used.

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------ PMMA signals (30° and 44°).

The diffractograms of PdInD and PdInDA solids showed similar DRX patterns for fresh, reduced

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and used catalysts, the broad peaks at 2θ c.a. 30° and 44° are related to the PMMA polymer [50]. The diffractograms of the fresh solids revealed the presence of In 2O3, PdO and Pd0. Reduced palladium in fresh fibers could be associated with the reducing properties of the DMF solvent, as it was

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reported by Pastoriza et al.[51]. Then, the reduced fibers displayed the typical signals of Pd0 and Pd2O. Whereas in the catalysts used under reaction the observed species were: PdO, Pd 0 and In2O3. The

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palladium oxide peaks are more important in the PdInD catalysts while in the PdInDA solids the most important peak is the related to Pd0. This fact would be related to the size of the palladium nanoparticles (see TEM results). The oxide would be only on the surface of particles while the core of the bigger particles (PdInDA) remain as metallic palladium. On the other hand, the small particles of solid PdInD could be easily reoxidized under atmospheric conditions. XPS results The chemical oxidation states of the elements and their atomic concentration were determined by XPS. Table 2 shows the results obtained for the fresh, reduced and used samples. Both fresh bimetallic 10

catalysts presented a broad and asymmetric peak in the Pd 3d spectra (not shown), suggesting the presence of more than one palladium chemical environment. The Pd 3d 5/2 peak that appeared at higher binding energy (337.5-337.6 eV) could be assigned to palladium nitrate species remaining from the synthesis of the fibers [47]. It should be remarked that this signal decreased after reduction treatment. However, it is unlikely that this specie remains after reaction, and according to Simon et al. [53] the presence of the peak at high binding energy could be attributed to interactions between palladium (Pd δ+) and carboxylate (coming from PMMA). On the other hand, the contribution at 336.3-336.2 eV could be related to a different chemical environment due to electronic interaction with In [54] or even the formation of an alloy [55]. Nevertheless, the monometallic Pd samples also presented this peak; consequently, the InPd interaction effect could be discarded. Consequently, the contribution in this binding energy range could

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be due to highly dispersed PdO species [56]. The higher concentration of these species (64%) in the fiber prepared with DMF/acetone (PdInDA) than in the support obtained using only DMF (42%) could probably be associated to the oxidant properties of the acetone solvent. The absence of metallic palladium signals, observed in XRD patterns, could be related to the high surface sensitivity of XPS technique.

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After the fiber reduction with hydrazine, a third peak was detected at 335.2 eV in the bimetallic catalysts and at 335.5 eV in the monometallic one. This signal, with the highest concentration (59 %) on

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PdInDA, could be assigned to metallic palladium particles [56]. The PdO presence after reduction could be associated to redox process promoted by the oxygen atoms present in the In2O3 compound [28].

Treatment

Binding energy (eV) Atomic % Pd Pd0 Pd2+ 336.3 64% 335.2 336.6 59% 19% 335.3 336.5 36% 43% 336.4 42% 335.4 336.2 42% 30% 335.2 336.3 37% 40% 335.5 336.6 33% 21% 335.4 336.8 35% 44%

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Sample

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Table 2. XPS results of the catalysts: fresh, reduced and after been used under reaction.

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Fresh PdInDA

Reduced

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Used

PdInD

Fresh Reduced Used Reduced

PdD Used

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Pdδ+ 337.5 36% 337.6 22% 337.8 21% 337.6 58% 337.6 28% 337.6 22% 337.6 45% 337.7 21%

In

Pd/In Mass ratio

445.1

0.84

444.8

0.54

444.9

0.61

444.9

4.03

444.9

3.72

444.6

2.79

-

-

-

-

The PdO species concentration increased after the catalytic reaction, however around 36% of metallic palladium remained after reaction; this result is in agreement with XRD patterns. The partial oxidation of metallic palladium could be related to the redox process that takes place in the catalyst during reaction. As regard indium species, the In 3d spectra showed the characteristic double emission peaks at the 445.0 and 452.6 eV range assigned to core-level In 3d5/2 and In 3d3/2 photoemissions, respectively. The In 3d5/2 binding energy centered between 444.6 and 445.1 eV could be associated to In3+ species according to the literature [57]. No changes in the indium chemical environment after reduction or reaction conditions were observed.

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The surface atomic ratios of both bimetallic fibers resulted significantly different. The Pd/In ratio in the PdInD catalysts is 4.03, close to the theoretical proportion (Pd/In = 4.33). After the reduction treatment and after reaction this parameter decreased to 3.72 and 2.79 respectively. In contrast, in the fresh PdInDA fiber, the Pd/In ratio resulted 0.84, while in the reduced and used solids were 0.54 and 0.61 respectively,

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which are substantially lower than the nominal value. This effect could be related to the use of acetone during the synthesis of PdInDA. The low density and low viscosity of acetone could improve the migration

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of ions to the outer part of the fibers, specially the indium (III) ions because of the electric field applied during the catalyst synthesis [58].

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Similarly, the obtained XPS results could explain the catalytic activity of the solids. The accessibility of the reactants to the bimetallic active sites is an important parameter for reaction performance in the nitrate reduction [16], [36], [59], [60]. In this vein, the XPS results suggested that there

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are two different particle arrangements. The lower surface Pd/In ratio in the PdInDA could avoid the overreduction of nitrite to ammonia because the low quantity of adsorbed hydrogen available as a result of the lower amount of Pdδ+ sites that enables the absorption of the dissociated hydrogen. However, the Pd and

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In atomic concentrations would be sufficient to activate the reduction of nitrates and the consecutive nitrite reduction. The opposite situation would take place in the PdInD, in which 4-5 fold palladium on the surface

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was observed compared with the PdInDA. The high Pd surface content on the PdInD would lead to the over-reduction of the nitrite or the NO to ammonia [20], [21], [52]. At the end of the reaction, the deactivation of both catalysts could be explained in terms of the loss of Pd0 able to acts as reloading agent of the Ind+ species, which are necessary for the nitrate reduction. This behavior, could be due to the formation of the βHPd as it was also observed by Zoppas et al. [61]. Electronic Microscopic (SEM, TEM) and EDX results

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SEM and TEM images show (Fig. 4 to 7) the morphology of the fibers. The diameter of the fibers was measured from SEM images, while the particle size distribution was measured from TEM micrographs (histograms shown in the Supplementary data S1). Figure 4 presents the results obtained for as prepared PdInD fibers. These fibers presented

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straight, cylindrical morphology with a homogeneous diameter (1.25 ± 0.01 μm).

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Figure 4. SEM a) and b) and TEM c) images of as prepared PdInD.

200 nm

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TEM micrographs of fresh catalyst showed a nanoparticle distribution with a particle size of 3.6 ± 1.2 nm (See Figs. 5 A and 5 A1 and S1-A). The EDX analysis indicated a homogenous In and Pd

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nanoparticles distribution with a Pd/In ratio (Pd/In = 4.3) closed to the theoretical ratio and in agreement with XPS results. The regions analyzed by EDX suggested that the Pd and In entities were close to each

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other. Reduction process does not seem to affect the Pd/In ratio nor particle size distribution (Pd/In = 4.31, 3.1 ± 1.2 nm) (Figs.5 B, 5 B1 and S1-B).

Similar results were obtained for the used fibers (Figs.5 C and 5 C1). In fact, the particle size

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resulted in 3.0 ± 1.1 nm (Fig. S1-C) and Pd/In atomic ratio remained equal to 4.1. These results suggest

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the stability of the active sites on the fiber surface after all catalyst treatments.

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70

70

A

Pd

Pd

A1

PdInD - Fresh

PdInD Fresh

Pd

Counts

Counts

Pd 35

In

0

2

4

35

In

0

6

2

4

Energy (KeV)

70

B

6

Energy (KeV)

70

B1

Pd

Pd

PdInD Reduced

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PdInD Reduced

35

In In

In

0

2

4

6

Energy (KeV) 70

C

2

-p

0

Pd

Counts

Counts

Pd 35

4

70

Pd

C1

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Pd

6

Energy (KeV)

PdInD Used Pd

Pd

Counts

35

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Counts

PdInD Used

In 35

In

In

0

2

4

6

2

4

Energy (KeV)

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Energy (KeV)

0

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Figure 5. TEM and EDX on the PdInD fiber. A, A1: Fresh, B, B1: Reduced, C, C1: Used. SEM and TEM images from the PdInDA as prepared solid resulted in ribbon-like fibers with a

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mean width of 3.12 ± 0.01 μm (Figure 6). The particle size distribution obtained from TEM images are shown in the Supplementary data S2.

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6

5m

30 m

1m

Figure 6. SEM a) and b) and TEM c) images of as prepared PdInDA Figures 7 A and A1 show TEM images and EDX analysis of fresh bimetallic PdInDA. In this

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catalyst, the mean particles size resulted 28.1 ± 6.5 nm (Fig. S2-A) which are higher than the one observed in fresh PdInD fibers. EDX analysis of these particles showed that they are composed of Pd, while

chemical analysis of the complete area confirms the presence of both species, Pd and In, suggesting the presence of small highly dispersed In nanoparticles. The EDX measurements give a media composition of

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79.7 wt.% Pd and 20.3 wt.% In, which was near to atomic relation Pd/In = 3.9. The atomic ratio was much higher than the obtained by XPS (0.84) what would point to a surface enrichment in In previously observed

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in this bimetallic catalysts [62]. Figs.7 B and B1 show TEM results of reduced bimetallic PdInDA. After the reduction treatment, Pd particles (27.2 ± 5.4 nm) and smaller In particles (3.0 ± 0.6 nm) were detected (Figs. S2-B and S2-B2). These results showed the advantages of the reduction method used in this case

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compared to the conventional reduction process in which flowing H2, together with high temperature, promote the Pd agglomeration [36]. In fact, the mild reduction method used in this work, would slightly

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agglomerate the smaller indium particles (up to around 3 nm) but the Pd particle size does not change. EDX analysis of large areas also gave an atomic Pd/In ratio (Pd/In = 4.2) in agreement with the theoretical one. Fig.7 C and C1 present the TEM and EDX results of PdInDA used in the nitrates elimination reaction.

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In this case, two kinds of particles were also detected, one group with 27.1 ± 5.0 nm and the other set with 3.2 ± 0.7 nm mean diameters (Figs. S2-C and S2-C2). The EDX test of a large surface of the sample

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detected both species with a Pd/In = 4.1. It should also be highlighted that in a previous work, in which PdIn catalysts supported on alumina and silica were studied, the Pd/In ratio decreased after reaction [36]. As a result, the studies performed by TEM and EDX could also contribute to justify the observed

catalytic behavior of the prepared fibers. In fact, the catalytic activity could not be related straightly to the sizes of the active particles. Indeed, it was observed that the higher particle size, the better nitrate conversion (Fig. 2A) and the quickly nitrite degradation, and beside the high reduction activity observed in PdInDA, it is resulted more selective to N2 production than PdInD. It could be inferred that the higher Pd particles size in the PdInDA catalyst could improve availability of hydrogen on the surface (by spill-over 15

mechanism), and the consequent regeneration of the indium active sites, resulting in a more efficient catalytic cycle. On the other hand, it was reported that PdIn/SiO2 and PdIn/Al2O3 showed a drastic agglomeration after 100 minutes under reaction [63]. This effect was not observed on the PMMA fiber obtained catalysts. These results are very interesting as the polymeric support might protect nanoparticles from agglomeration effects. 210

70

PdInDA Fresh

Pd

PdInDA Fresh Pd

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Pd

Counts

In

105

35

Pd

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B

0

2

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PdInDA Reduced

Pd 35

In

B1 0

6

4

6

Energy (KeV) 70

PdInDA Used

PdInDA Used Pd

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Counts

2

Energy (KeV)

Pd

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6

Pd

20

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4

70

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PdInDA Reduced

Pd

2

Energy (KeV)

Counts

2

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C1 0

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0

2

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Figure 7. TEM and EDX on the PdInDA fiber. A, A1: Fresh, B, B1: Reduced and C, C1: Used under reaction. 16

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4. Conclusions The results show that polymeric fibers resulted a very promising material to support active sites for catalytic water treatment. It should be remarked that using these materials shows: (i) an easier separation of the catalyst from the aqueous phase, (ii) no leaching of active metal during the reaction process (ICP and EDX), (iii) no active particle agglomeration and (iv) that the electrospinning method implies very few chemist wastes, in close accordance with Green Chemistry principles. Among the main observation of the catalysts studied in this work, it should be remarked that the active site distribution and the particle size are sensible to the fiber synthesis protocols, consequently, different catalytic performances were observed. Both prepared polymeric catalysts showed good selectivity

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to nitrogen compared with Pd/In catalysts prepared on inorganic supports (SiO2, Al2O3, carbon fibers). The fiber prepared with DMF and acetone (PdInDA) gave rise to a better nitrate conversion. The lowest surface Pd/In ratio on the PdInDA solid, could prevent the nitrate and nitrite over reduction to ammonia. The solids characterization indicates, that the fibers are very adequate to preserve unchanged the active site ratio

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during all the catalyst treatments.

The deactivation observed after 20 min. under reaction could be explained by an oxidation

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process on the active site that could not be reverse by catalytic system. These challenging results increased the motivation to find further insights about these promising hybrid materials in order to improve

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Author s contribution

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their catalytic performance.

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Ivan Moreno have synthesised the materials, Albana Marchesini, Nuria Navascues and Vanina Aghemo have characterized the material. Laura Gutierrez and Silvia Irusta have developed the idea and write manuscript.

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Conflict of interest

The authors do not have any conflict of interest.

4. Acknowledgments The authors from Argentina wish to acknowledge the financial support received from ANPCyT (PICT/16 – 2284), CONICET (PIP/14 - 406), UNL (CAI+D 50120110100417LI, and CAI+D 50420150100037LI) and to 17

the Agency of Science, Technology and Innovation from Santa Fe (ASaCTeI). Silvia Irusta acknowledges to the “Subsidio Cesar Milstein” from the Ministerio de Educación, Cultura, Ciencia y Tecnología, Argentina.

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Thanks are also given to Prof. Guillermina Amrein for the English language editing.

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