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Palladium–tin catalysts on conducting polymers for nitrate removal
Applied Catalysis B: Environmental 93 (2009) 50–55
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Palladium–tin catalysts on conducting polymers for nitrate removal Ibrahim Dodouche a, Danns Pereira Barbosa a,b, Maria do Carmo Rangel b, Florence Epron a,* a b
Laboratoire de Catalyse en Chimie organique, UMR6503 CNRS, Universite´ de Poitiers, 40 avenue du recteur Pineau, 86022 Poitiers Cedex, France Grupo de Estudos em Cine´tica e Cata´lise, Instituto de Quı´mica, Universidade Federal da Bahia, Campus Universita´rio de Ondina, Salvador, Bahia, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 July 2009 Received in revised form 3 September 2009 Accepted 7 September 2009 Available online 12 September 2009
Palladium–tin catalysts were prepared by successive impregnation or co-impregnation onto polyaniline and polypyrrole. The catalytic tests showed that this type of catalyst is active for nitrate reduction. The use of polymer support improves the selectivity of the catalyst toward nitrogen formation compared to a classical support, and avoids the apparition of intermediate nitrite. These better performances of the catalysts supported on electroactive polymers were explained by the ion-exchange properties of the conducting polymers. The most selective catalyst was Pd–Sn/PPy catalyst prepared by successive impregnation. The lower selectivity obtained with the Pani-supported catalyst was explained by the capability of this polymer to directly reduce nitrate into ammonium ions. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Electroactive polymers Palladium–tin Catalytic properties Nitrate reduction
1. Introduction Intensive agricultural activities and specially over-fertilization induce an increase of nitrate concentration in groundwater, which is often higher than the maximum permitted level (50 mg L1 in Europe). Nitrates consumed with drinking water can be converted into nitrites in human body and may cause health problems. To comply with the legislation, a specific treatment of drinking water is necessary. Nowadays, two types of processes are developed based on physico-chemical processes (ion exchange, electrodialysis or reversed osmosis) or biological processes. In the late 1980s, catalytic denitration has been developed to obtain the selective reduction of nitrate into nitrogen, with the undesired formation of ammonia [1]. The global reaction is the following: 2NO3 þ 5H2 catalyst ! N2 þ 2OH þ 4H2 O or NO3 þ 4H2 catalyst ! NH4 þ þ 2OH þ H2 O It was demonstrated that nitrites are reduced by hydrogen on various hydrogenation catalysts, such as platinum, palladiumsupported monometallic catalysts, whereas these catalysts are inactive for nitrate reduction. To reduce nitrates, it is necessary to activate the precious metal by addition of a promoter such as tin or copper. Alumina is the most employed support used for bimetallic catalysts active in nitrate reduction. However, the use of this
* Corresponding author. Tel.: +33 5 49 45 48 32; fax: +33 5 49 45 37 41. E-mail address: fl[email protected] (F. Epron). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.011
oxide results in unacceptable amounts of ammonia as reaction by-product (maximum acceptable level 0.5 mg L1) and also to intermediate nitrite (maximum acceptable level 0.1 mg L1). Other supports have been studied with the aim of exerting a better selectivity control, namely titania and zirconia [2], SnO2 [3], hydrotalcite [4,5]. In all cases the major problem to overcome is the control of pH in the proximity of active sites. Nitrate reduction always results in the formation of OH, which causes an increase of the pH value up to 10. At pH > 8 ammonia formation becomes preferred instead of molecular nitrogen formation, which could be avoided by using a buffering system as the controlled addition of hydrochloric acid [6], carbon dioxide [2,7,8], formic acid [9,10] or polymers [11]. In a previous study, palladium monometallic catalysts supported on electroactive polymers were studied for nitrite removal [11]. It was shown that Pd/polyaniline and Pd/polypyrrole catalysts are much more active than a classical Pd/Al2O3 catalyst for nitrite reduction with less ammonium ions produced. These better performances were explained by the redox and ionexchange properties of the conducting polymers allowing the exchange between the hydroxides produced and the dopant anion of the conducting polymer. The best selectivity to nitrogen was obtained with the palladium supported on polyaniline, whereas the palladium supported on polypyrrole presented the highest activity. The aim of the present paper is to prepare bimetallic Pd–Sn catalysts supported on polypyrrole and polyaniline and to evaluate their performances in terms of activity and selectivity for nitrate reduction in comparison with those of a classical Pd–Sn/Al2O3 catalyst.
I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
2. Experimental 2.1. Preparation of the catalysts 2.1.1. Preparation of the supports Pyrrole (alpha) and aniline (Riedel-de HAE¨N) were purified by distillation and stored in a refrigerator at 4 8C before use. Both polymers were prepared by chemical polymerization. Polyaniline (Pani) was obtained from aniline using potassium peroxydisulfate K2S2O8 as an oxidant. Polypyrrole (PPy) was obtained using FeCl3 as an oxidant. The experimental procedure, given in details in [11], was chosen in order to obtain the conductive form of the polymers as a function of the data given in the literature. It was shown in [11] that the two polymers are almost completely oxidized and then conductive. Their BET surface area is within the 30–40 m2 g1 range. The classical support was a g-alumina with a surface area (BET method) of 216 m2 g1. It was ground and then sieved to retain particles with sizes between 0.04 and 0.08 mm. 2.1.2. Preparation of the bimetallic catalysts Bimetallic Pd–Sn/polymer catalysts were prepared either by successive impregnation or by co-impregnation in order to obtain a palladium loading of 5 wt.% and various tin contents, between 0.5 and 3 wt.%. 2.1.2.1. Preparation by successive impregnation (SI). At first the monometallic catalysts were prepared by an impregnation method using aqueous solution of the PdCl2 precursor salt in order to obtain a catalyst containing 5 wt.% of palladium. In a typical experiment, 5 mL of PdCl2 (2 mgPd L1 in HCl 0.1 mol L1) were added to 0.2 g of the support. The suspension was stirred for 3 h at room temperature and thereafter the water was evaporated on a sand bath. Catalyst was then dried in an oven at 90 8C. Then, a defined amount of tin precursor (SnCl4) was added to the monometallic Pd(5 wt.%)/polymer catalyst. The reaction mixture was stirred during 3 h before being evaporated and dried in a sand bath (80 8C) during a night. The activation consists of a reduction. This catalyst series was mentioned as Pd5Snx/polymer SI or Pd5Snx/ Al2O3, where x is the weight percentage of tin in the catalyst. 2.1.2.1.1. Preparation of catalyst by co-impregnation (CI). The coimpregnation consists in immersing a quantity of the polymer in ultra pure water and in adding the necessary amounts of both PdCl2 and SnCl4 precursor salts. The reaction mixture was stirred during 3 h before being evaporated on a sand bath and dried in an oven during a night (90 8C). This catalyst series was mentioned as Pd5Snx/polymer CI, where x is the weight percentage of tin in the catalyst. Whatever the method of impregnation (successive or coimpregnation), it was verified by elemental analysis that all the metals introduced in the precursor salt solution were actually deposited on the support.
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mode of the microscope using a Si–Li Super UTW detector. The particle size distribution was obtained from TEM pictures and P the mean particle diameter was calculated from dp = nidi3/ P 2 nidi . 2.3. Catalytic test Nitrate reduction reactions were performed in a semi-batch reactor, at atmospheric pressure and 298 K. The catalyst (64 mg) is at first reduced under hydrogen (373 K, 1 h, pH2 = 1 bar, flowrate = 250 mL min1). Meanwhile 90 mL of ultra pure water are placed in a vial purged with nitrogen. When the catalyst is stabilised in temperature the ultra pure water is added over the reduced catalyst. Afterwards, solution containing 10 mL of a nitrate solution (1 g/L) is purged with nitrogen and introduced in the reactor to start the reaction. The catalyst dispersion in the aqueous medium was achieved by the hydrogen flow (pH2 = 1 bar, flow-rate = 250 mL min1) through a porous glass located at the bottom of the reactor. It was checked that the hydrogen flow and the resulting stirring were sufficient to ensure that the reaction was not rate-limited by reactant diffusion. To monitor the progress of the reaction, representative aqueous samples were periodically withdrawn, immediately separated by filtration and then analyzed by high performance liquid chromatography, in order to determine the NO3, NO2 and NH4+ concentration. In order to evidence the amount of nitrate or nitrite adsorbed onto the support, 20 mL of a solution of KCl (0.08 mol L1) are added at the end of the reaction and the concentration of nitrate and nitrite is once again determined. It was verified that cations, such as ammonium ions, are not adsorbed onto the support. Nitrate concentration was determined after separation on a C18 column using an UV detector at l = 210 nm by the method described in reference [12]. Ammonium ions were quantified using an Alltech Universal Cation column coupled with a conductivity detector. The acidic mobile phase (oxalic acid) used provided the complete conversion of the ammonia basic form into ammonium ions. It was verified by gas chromatography (TCD detector) that the only gaseous product was molecular nitrogen. Nevertheless, as the experiment was performed under a hydrogen flow at high flowrate (250 mL min1), it was impossible to quantify precisely the amount of N2 produced as a function of time. However, it could be deduced from the nitrogen mass balance, taking into account the sum of all nitrogen-containing products. Considering the initial concentration of nitrate C0 and the concentration determined after addition of KCl in the solution Cf, taking into account the dilution value, concentration of adsorbed nitrate is given by the difference C0 Cf. The pH of the initial and final solution was determined by means of a digital pH meter (Consort). 3. Results and discussion 3.1. Characterization of the bimetallic catalysts
2.2. Catalyst characterization The characterization of the support alone was described in details in [11]. The composition of the bimetallic catalysts was determined by the central service of analysis of the CNRS (French National Centre of Scientific Research). The Pd–Sn supported catalysts were characterized by TEM and EDX before and after reaction. TEM measurements were carried out with a Philips CM120 electron microscope operating at 120 kV with a resolution of 0.35 nm. The catalyst was ultrasonically dispersed in ethanol, and the suspension was deposited on an aluminium grid coated with a porous carbon film. X-ray energy dispersive spectrometry (EDX) was performed with the STEM
The chemical analysis showed that all the precursor salts were deposited on the polymer supports. TEM coupled with EDX was carried out on fresh and tested Pd5Sn0.5/PPy catalysts in order to collect some information on metallic particles in terms of particle size and composition. The TEM picture of the fresh catalyst prepared by successive impregnation (Fig. 1a) shows that the particles were mainly agglomerated. Consequently, it was impossible to determine by EDX if Pd and Sn are isolated on the support in different particles or together in the same bimetallic particles. The mean particle size determined from the particle size distribution (Fig. 1c) is 2.5 nm. On catalysts prepared by co-impregnation (Fig. 2a), the particles
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I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
Fig. 1. TEM pictures (a and b) and particle size distributions (c and d) of Pd5Sn0.5/PPY prepared by successive impregnation before reaction (a and c) after reaction (b and d).
were bigger with a mean diameter of 3.6 nm, with some isolated agglomerates reaching sizes of 10–15 nm (not shown). The EDX analysis showed that agglomerates, mainly located at the edge, corresponded to tin only, whereas smaller particles containing tin and/or palladium are also present in the core. For both preparation methods, after catalytic test, particles were visible, mainly located at the support edge, with mean particle sizes of 3.5 nm (SI) and 6.07 nm (CI), without agglomerates. The EDX analysis evidenced the presence of isolated palladium or bimetallic Pd–Sn particles corresponding to the PdSn3 (SI catalysts) or Pd3Sn2 (CI catalysts) alloys. The characteristics of the Pd5Sn/Al2O3 catalyst were described in details in [13]. 3.2. Nitrate removal 3.2.1. Role of the polymer supports In was shown in [11] that PPy and Pani, which are completely oxidized, are able to remove a large part of nitrite (80%) from water, probably by ion exchange between the dopant ion and nitrite. The dopant ions are chlorine for the polypyrrole support prepared using FeCl3 and both chlorine and sulphate for the polyaniline support [11]. In order to evidence if nitrate removal by the Pani or PPy support is also possible, a blank reaction was performed using these supports instead of the catalyst in the same reaction conditions. The evolution of the nitrate concentration versus time is reported in Fig. 3 for the two polymers. It is important to note that roughly similar results were obtained under nitrogen (results not shown) or hydrogen gas. Fig. 3 shows that when nitrate solution is introduced in contact with the conducting polymers, a part of nitrates is immediately removed; then the nitrate
concentration remains unchanged. Thus, 57 and 25% of nitrates disappear from the solution, in the presence of PPy and Pani, respectively, either by redox reaction or by ion exchange, as it was concluded for nitrite [11]. In order to determine the amount of nitrate adsorbed onto the two supports after 300 min, a solution of KCl was added and the nitrate concentration was then once again determined. The pH values, the concentration of ammonium ions measured after 300 min of reaction, and the concentration of adsorbed nitrate, determined from the nitrate concentration determined after addition of KCl, are reported in Table 1 for the two polymer supports. Ammonium ions being produced, this indicates that a part of nitrates is removed by a redox reaction, in the reaction conditions. In the presence of polypyrrole and polyaniline, 3.3 and 25%, respectively, of the nitrates removed from water yield ammonium ions, thus indicating that at minimum, these percentages of nitrate are reduced and not exchanged. This possibility of redox reaction is confirmed by the values of redox potentials of the various species (nitrates, Pani and PPy), reported in Table 2, showing that polypyrrole and polyaniline, when they are in a reduced state, are able to reduce nitrate. Considering now the amount of adsorbed nitrate, it can be deduced that 33 and 25% of the disappeared nitrate were removed by adsorption onto the PPy and Pani support, respectively. Table 1 pH, concentration of ammonium ions and adsorbed nitrate after 300 min in the presence of polypyrrole and polyaniline supports. Polymers
[NH4+]f (mmol/L)
[NO3]ads (mmol/L)
Final pH
PPy Pani
0.03 0.1
0.3 0.1
3 3
I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
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Fig. 2. TEM pictures (a and b) and particle size distributions (c and d) of Pd5Sn0.5/PPY prepared by co-impregnation before reaction (a and c) after reaction (b and d).
Consequently, taking into account the nitrogen mass balance, it can be deduced that onto the two polymers, nitrate is mainly transformed into nitrogen.
Fig. 3. Nitrate abatement in the presence of PPy (^), and Pani (~) under hydrogen.
Table 2 Redox potentials of different species. Redox systems
E8 (V/SHE) pH 0
NO3/N2 Pd2+/Pd NO3/NH4+ Polyemeraldine/polyleucoemeraldinea Polypernigraniline/polyemeraldinea PPy+/PPy
1.246 0.915 0.875 0.342 0.942 0.15
a
Various types of polyaniline.
3.2.2. Performances of the bimetallic catalysts supported on PPy and Pani For the bimetallic catalysts supported on polypyrrole, a complete nitrate conversion is rapidly reached. No intermediate nitrite or hydroxylamine was evidenced in solution. After addition of KCl in solution, no nitrate was released in solution, proving that in the presence of Pd–Sn/PPy catalyst, the nitrate reduction is the only way to remove nitrate from the solution, the adsorption being negligible. For this catalyst series, the activity was calculated by dividing the initial amount of nitrate introduced in solution (in mol), by the weight of catalyst (in g) and by the time needed to completely convert nitrate (in min). The selectivity in ammonium ions is a final selectivity corresponding to the amount of ammonium ions produced (in mol) per mol of nitrate converted. The results obtained with the Pd–Sn/PPy catalysts are summarized in Fig. 4. They show that whatever the preparation method, successive impregnation or co-impregnation, the highest activity is obtained with the catalyst containing 0.5 wt.% of tin, and the activity decreases with the increase in tin content. The Pd5Sn0.5/ PPy catalyst prepared by co-impregnation is slightly more active than the one prepared by successive impregnation, with an activity 1 1 of 82 mmol min1 g1 gcata . However, cata instead of 62 mmol min the latter presents the lowest selectivity in ammonium ions among the catalyst series, 8.5%, which corresponds to 91.5% of selectivity in molecular nitrogen. Whatever the catalyst, the initial pH is around 3 and the final pH value is between 4 and 6. Roughly, the lower the pH, the lower the selectivity in ammonium ions. Then,
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I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
Fig. 4. Effect of the Sn content on the activity (histograms) and selectivity at 100% of nitrate conversion (lines) for nitrate reduction in the presence the Pd5Snx/PPy catalysts prepared by successive impregnation (in grey) or co-impregnation (in black).
the selectivity in ammonium ions may be directly linked to the capability of the polymer to exchange OH produced during nitrate reduction with the dopant ions, thus limiting the increase of pH and then the ammonium ions production. As far as the performances of the bimetallic catalysts supported on Pani are concerned, they are much less active than their PPysupported counterpart. For some catalysts, a complete conversion was not reached, and, as for the Pd–Sn/PPy series, no adsorption was observed. For this reason, the activity and selectivity were determined at 75% of nitrate conversion. As for the PPy-supported catalyst series, no intermediate nitrite was detected in solution. Results (Fig. 5) show that an optimum of activity is obtained with Pd5Sn1/Pani catalysts. The most active catalyst is the one prepared by successive impregnation, which is also the most selective for molecular nitrogen formation with only 1.8% of ammonium ions produced at 75% of nitrate conversion. The catalysts prepared by co-impregnation produce more ammonium ions than their counterpart prepared by successive impregnation. However, whatever the catalyst tested, the initial and final pH values are around 3. Contrary to what was observed for the catalysts supported on PPy, and to what is commonly admitted for catalysts supported on classical supports such as alumina, it is difficult to link the selectivity to the pH of the solution for catalysts supported onto polyaniline. This may be explained by the capability of the
Fig. 6. Evolution of the nitrate (*, &, ~), nitrite () and ammonium ions (*, &, ~) concentrations vs. time in the presence of Pd–Sn/PPy (~, ~), Pd–Sn/Pani (&, &), Pd–Sn/Al2O3 (*, *, ) (64 mg of catalyst, pH2 = 1).
Table 3 Effect of various supports of Pd–Sn bimetallic catalysts prepared by successive impregnation on their performances in nitrate reduction. Catalysts
Activity Final selectivity Ammonium Initial Final (mmol min1 g1 (ppm)a pH pH cata ) in N2
Pd5Sn0.5/PPy 150 Pd5Sn1/Pani 47 Pd5Sn2/Al2O3 32 a
95.2 98.2 55
1.4 0.5 13
3.5 2.7 5
4.8 3 10
At the end of the reaction.
Pani support to directly reduce a part of nitrate into ammonium ions, as demonstrated in Section 3.2.1. Whatever the metal content, the preparation method and the support, polypyrrole or polyaniline, no metal leaching was observed at the end of the reaction. 3.2.3. Comparison of the performances as a function of the support The lowest selectivity to NH4+, and consequently the highest selectivity to N2, was obtained with Pd–Sn catalysts prepared by successive impregnation, whatever the support used (PPy or Pani). The best catalysts, in terms of selectivity, were Pd5Sn0.5/PPy SI and Pd5Sn1/Pani SI. However, it is impossible to compare together the performances of these two catalysts from the values reported in Figs. 4 and 5 since they are determined at different levels of nitrate conversion. To favour the comparison, the evolution of nitrate and ammonium ions is reported in Fig. 6 as a function of time for the two catalysts and also for a classical Pd–Sn/Al2O3 one prepared according to the same experimental procedure. For this latter, the metal content was also optimized in order to obtain the best activity and selectivity. It contents 5 wt.% of palladium and 2 wt.% of tin. Results are summarized in Table 3. Fig. 6 shows that the activity of the Pd–Sn/Pani and Pd–Sn/Al2O3 catalysts are similar. However, the amount of ammonium ions is twice more important with the alumina supported catalyst, which also yields intermediate nitrite in solution. By far, the Pd–Sn/PPy is the most active catalyst, all the nitrates being removed in 40 min, with also the lowest amount of produced ammonium ions after complete nitrate removal and then the highest final selectivity to N2. 4. Conclusion
Fig. 5. Effect of the Sn content on the activity (histograms) and selectivity at 75% of nitrate conversion (lines) for nitrate reduction in the presence the Pd5Snx/Pani catalysts prepared by successive impregnation (in grey) or co-impregnation (in black).
Polymers with redox and ion-exchange properties, such as polyaniline and polypyrrole, were used as supports for bimetallic Pd–Sn catalysts. The Pd–Sn/PPy and Pd–Sn/Pani catalysts showed
I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
interesting performances for nitrate removal from water. In particular, the use of these polymers as supports allowed us to decrease the selectivity towards ammonium ions and to avoid the apparition in solution of intermediate nitrite during nitrate reduction, compared to a classical Pd–Sn/Al2O3 catalyst. The Pd5Sn0.5/PPy SI catalyst is by far more active and more selective to molecular nitrogen compared to the best Pd–Sn catalyst supported onto polyaniline. Acknowledgment The authors thank the CAPES/COFECUB (Ph 603/08) for the financial support.
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References [1] T. Tacke, K.-D. Vorlop, Dechema Biotechnology Conferences, vol. 3, VCH, Weinheim, 1989, p. 1007. [2] G. Strukul, R. Gavagnin, F. Pinna, E. Modafferi, S. Perathoner, G. Centi, M. Marella, M. Tomaselli, Catal. Today 55 (2000) 139. [3] R. Gavagnin, L. Biasetto, F. Pinna, G. Strukul, Appl. Catal. B 38 (2002) 91. [4] A.E. Palomares, J.G. Prato, F. Rey, A. Corma, J. Catal. 221 (2004) 62. [5] Y. Wang, J. Qu, H. Liu, J. Mol. Catal. A 272 (2007) 31. [6] A. Pintar, J. Batista, J. Levec, Catal. Today 66 (2001) 503. [7] U. Prusse, S. Horold, K.D. Vorlop, Chem. Eng. Technol. 69 (1997) 93. [8] A. Pintar, J. Batista, J. Levec, T. Kajiuchi, Appl. Catal. B 11 (1996) 81. [9] U. Prusse, M. Hahnlein, J. Daum, K.D. Vorlop, Catal. Today 55 (2000) 79. [10] A. Garron, F. Epron, Water Res. 39 (2005) 3073. [11] I. Dodouche, F. Epron, Appl. Catal. B 76 (2007) 291. [12] F. Epron, F. Gauthard, C. Pineda, J. Barbier, J. Catal. 198 (2001) 309. [13] A. Garron, K. Lazar, F. Epron, Appl. Catal. B 59 (2005) 57.
Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Palladium–tin catalysts on conducting polymers for nitrate removal Ibrahim Dodouche a, Danns Pereira Barbosa a,b, Maria do Carmo Rangel b, Florence Epron a,* a b
Laboratoire de Catalyse en Chimie organique, UMR6503 CNRS, Universite´ de Poitiers, 40 avenue du recteur Pineau, 86022 Poitiers Cedex, France Grupo de Estudos em Cine´tica e Cata´lise, Instituto de Quı´mica, Universidade Federal da Bahia, Campus Universita´rio de Ondina, Salvador, Bahia, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 July 2009 Received in revised form 3 September 2009 Accepted 7 September 2009 Available online 12 September 2009
Palladium–tin catalysts were prepared by successive impregnation or co-impregnation onto polyaniline and polypyrrole. The catalytic tests showed that this type of catalyst is active for nitrate reduction. The use of polymer support improves the selectivity of the catalyst toward nitrogen formation compared to a classical support, and avoids the apparition of intermediate nitrite. These better performances of the catalysts supported on electroactive polymers were explained by the ion-exchange properties of the conducting polymers. The most selective catalyst was Pd–Sn/PPy catalyst prepared by successive impregnation. The lower selectivity obtained with the Pani-supported catalyst was explained by the capability of this polymer to directly reduce nitrate into ammonium ions. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Electroactive polymers Palladium–tin Catalytic properties Nitrate reduction
1. Introduction Intensive agricultural activities and specially over-fertilization induce an increase of nitrate concentration in groundwater, which is often higher than the maximum permitted level (50 mg L1 in Europe). Nitrates consumed with drinking water can be converted into nitrites in human body and may cause health problems. To comply with the legislation, a specific treatment of drinking water is necessary. Nowadays, two types of processes are developed based on physico-chemical processes (ion exchange, electrodialysis or reversed osmosis) or biological processes. In the late 1980s, catalytic denitration has been developed to obtain the selective reduction of nitrate into nitrogen, with the undesired formation of ammonia [1]. The global reaction is the following: 2NO3 þ 5H2 catalyst ! N2 þ 2OH þ 4H2 O or NO3 þ 4H2 catalyst ! NH4 þ þ 2OH þ H2 O It was demonstrated that nitrites are reduced by hydrogen on various hydrogenation catalysts, such as platinum, palladiumsupported monometallic catalysts, whereas these catalysts are inactive for nitrate reduction. To reduce nitrates, it is necessary to activate the precious metal by addition of a promoter such as tin or copper. Alumina is the most employed support used for bimetallic catalysts active in nitrate reduction. However, the use of this
* Corresponding author. Tel.: +33 5 49 45 48 32; fax: +33 5 49 45 37 41. E-mail address: fl[email protected] (F. Epron). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.011
oxide results in unacceptable amounts of ammonia as reaction by-product (maximum acceptable level 0.5 mg L1) and also to intermediate nitrite (maximum acceptable level 0.1 mg L1). Other supports have been studied with the aim of exerting a better selectivity control, namely titania and zirconia [2], SnO2 [3], hydrotalcite [4,5]. In all cases the major problem to overcome is the control of pH in the proximity of active sites. Nitrate reduction always results in the formation of OH, which causes an increase of the pH value up to 10. At pH > 8 ammonia formation becomes preferred instead of molecular nitrogen formation, which could be avoided by using a buffering system as the controlled addition of hydrochloric acid [6], carbon dioxide [2,7,8], formic acid [9,10] or polymers [11]. In a previous study, palladium monometallic catalysts supported on electroactive polymers were studied for nitrite removal [11]. It was shown that Pd/polyaniline and Pd/polypyrrole catalysts are much more active than a classical Pd/Al2O3 catalyst for nitrite reduction with less ammonium ions produced. These better performances were explained by the redox and ionexchange properties of the conducting polymers allowing the exchange between the hydroxides produced and the dopant anion of the conducting polymer. The best selectivity to nitrogen was obtained with the palladium supported on polyaniline, whereas the palladium supported on polypyrrole presented the highest activity. The aim of the present paper is to prepare bimetallic Pd–Sn catalysts supported on polypyrrole and polyaniline and to evaluate their performances in terms of activity and selectivity for nitrate reduction in comparison with those of a classical Pd–Sn/Al2O3 catalyst.
I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
2. Experimental 2.1. Preparation of the catalysts 2.1.1. Preparation of the supports Pyrrole (alpha) and aniline (Riedel-de HAE¨N) were purified by distillation and stored in a refrigerator at 4 8C before use. Both polymers were prepared by chemical polymerization. Polyaniline (Pani) was obtained from aniline using potassium peroxydisulfate K2S2O8 as an oxidant. Polypyrrole (PPy) was obtained using FeCl3 as an oxidant. The experimental procedure, given in details in [11], was chosen in order to obtain the conductive form of the polymers as a function of the data given in the literature. It was shown in [11] that the two polymers are almost completely oxidized and then conductive. Their BET surface area is within the 30–40 m2 g1 range. The classical support was a g-alumina with a surface area (BET method) of 216 m2 g1. It was ground and then sieved to retain particles with sizes between 0.04 and 0.08 mm. 2.1.2. Preparation of the bimetallic catalysts Bimetallic Pd–Sn/polymer catalysts were prepared either by successive impregnation or by co-impregnation in order to obtain a palladium loading of 5 wt.% and various tin contents, between 0.5 and 3 wt.%. 2.1.2.1. Preparation by successive impregnation (SI). At first the monometallic catalysts were prepared by an impregnation method using aqueous solution of the PdCl2 precursor salt in order to obtain a catalyst containing 5 wt.% of palladium. In a typical experiment, 5 mL of PdCl2 (2 mgPd L1 in HCl 0.1 mol L1) were added to 0.2 g of the support. The suspension was stirred for 3 h at room temperature and thereafter the water was evaporated on a sand bath. Catalyst was then dried in an oven at 90 8C. Then, a defined amount of tin precursor (SnCl4) was added to the monometallic Pd(5 wt.%)/polymer catalyst. The reaction mixture was stirred during 3 h before being evaporated and dried in a sand bath (80 8C) during a night. The activation consists of a reduction. This catalyst series was mentioned as Pd5Snx/polymer SI or Pd5Snx/ Al2O3, where x is the weight percentage of tin in the catalyst. 2.1.2.1.1. Preparation of catalyst by co-impregnation (CI). The coimpregnation consists in immersing a quantity of the polymer in ultra pure water and in adding the necessary amounts of both PdCl2 and SnCl4 precursor salts. The reaction mixture was stirred during 3 h before being evaporated on a sand bath and dried in an oven during a night (90 8C). This catalyst series was mentioned as Pd5Snx/polymer CI, where x is the weight percentage of tin in the catalyst. Whatever the method of impregnation (successive or coimpregnation), it was verified by elemental analysis that all the metals introduced in the precursor salt solution were actually deposited on the support.
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mode of the microscope using a Si–Li Super UTW detector. The particle size distribution was obtained from TEM pictures and P the mean particle diameter was calculated from dp = nidi3/ P 2 nidi . 2.3. Catalytic test Nitrate reduction reactions were performed in a semi-batch reactor, at atmospheric pressure and 298 K. The catalyst (64 mg) is at first reduced under hydrogen (373 K, 1 h, pH2 = 1 bar, flowrate = 250 mL min1). Meanwhile 90 mL of ultra pure water are placed in a vial purged with nitrogen. When the catalyst is stabilised in temperature the ultra pure water is added over the reduced catalyst. Afterwards, solution containing 10 mL of a nitrate solution (1 g/L) is purged with nitrogen and introduced in the reactor to start the reaction. The catalyst dispersion in the aqueous medium was achieved by the hydrogen flow (pH2 = 1 bar, flow-rate = 250 mL min1) through a porous glass located at the bottom of the reactor. It was checked that the hydrogen flow and the resulting stirring were sufficient to ensure that the reaction was not rate-limited by reactant diffusion. To monitor the progress of the reaction, representative aqueous samples were periodically withdrawn, immediately separated by filtration and then analyzed by high performance liquid chromatography, in order to determine the NO3, NO2 and NH4+ concentration. In order to evidence the amount of nitrate or nitrite adsorbed onto the support, 20 mL of a solution of KCl (0.08 mol L1) are added at the end of the reaction and the concentration of nitrate and nitrite is once again determined. It was verified that cations, such as ammonium ions, are not adsorbed onto the support. Nitrate concentration was determined after separation on a C18 column using an UV detector at l = 210 nm by the method described in reference [12]. Ammonium ions were quantified using an Alltech Universal Cation column coupled with a conductivity detector. The acidic mobile phase (oxalic acid) used provided the complete conversion of the ammonia basic form into ammonium ions. It was verified by gas chromatography (TCD detector) that the only gaseous product was molecular nitrogen. Nevertheless, as the experiment was performed under a hydrogen flow at high flowrate (250 mL min1), it was impossible to quantify precisely the amount of N2 produced as a function of time. However, it could be deduced from the nitrogen mass balance, taking into account the sum of all nitrogen-containing products. Considering the initial concentration of nitrate C0 and the concentration determined after addition of KCl in the solution Cf, taking into account the dilution value, concentration of adsorbed nitrate is given by the difference C0 Cf. The pH of the initial and final solution was determined by means of a digital pH meter (Consort). 3. Results and discussion 3.1. Characterization of the bimetallic catalysts
2.2. Catalyst characterization The characterization of the support alone was described in details in [11]. The composition of the bimetallic catalysts was determined by the central service of analysis of the CNRS (French National Centre of Scientific Research). The Pd–Sn supported catalysts were characterized by TEM and EDX before and after reaction. TEM measurements were carried out with a Philips CM120 electron microscope operating at 120 kV with a resolution of 0.35 nm. The catalyst was ultrasonically dispersed in ethanol, and the suspension was deposited on an aluminium grid coated with a porous carbon film. X-ray energy dispersive spectrometry (EDX) was performed with the STEM
The chemical analysis showed that all the precursor salts were deposited on the polymer supports. TEM coupled with EDX was carried out on fresh and tested Pd5Sn0.5/PPy catalysts in order to collect some information on metallic particles in terms of particle size and composition. The TEM picture of the fresh catalyst prepared by successive impregnation (Fig. 1a) shows that the particles were mainly agglomerated. Consequently, it was impossible to determine by EDX if Pd and Sn are isolated on the support in different particles or together in the same bimetallic particles. The mean particle size determined from the particle size distribution (Fig. 1c) is 2.5 nm. On catalysts prepared by co-impregnation (Fig. 2a), the particles
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Fig. 1. TEM pictures (a and b) and particle size distributions (c and d) of Pd5Sn0.5/PPY prepared by successive impregnation before reaction (a and c) after reaction (b and d).
were bigger with a mean diameter of 3.6 nm, with some isolated agglomerates reaching sizes of 10–15 nm (not shown). The EDX analysis showed that agglomerates, mainly located at the edge, corresponded to tin only, whereas smaller particles containing tin and/or palladium are also present in the core. For both preparation methods, after catalytic test, particles were visible, mainly located at the support edge, with mean particle sizes of 3.5 nm (SI) and 6.07 nm (CI), without agglomerates. The EDX analysis evidenced the presence of isolated palladium or bimetallic Pd–Sn particles corresponding to the PdSn3 (SI catalysts) or Pd3Sn2 (CI catalysts) alloys. The characteristics of the Pd5Sn/Al2O3 catalyst were described in details in [13]. 3.2. Nitrate removal 3.2.1. Role of the polymer supports In was shown in [11] that PPy and Pani, which are completely oxidized, are able to remove a large part of nitrite (80%) from water, probably by ion exchange between the dopant ion and nitrite. The dopant ions are chlorine for the polypyrrole support prepared using FeCl3 and both chlorine and sulphate for the polyaniline support [11]. In order to evidence if nitrate removal by the Pani or PPy support is also possible, a blank reaction was performed using these supports instead of the catalyst in the same reaction conditions. The evolution of the nitrate concentration versus time is reported in Fig. 3 for the two polymers. It is important to note that roughly similar results were obtained under nitrogen (results not shown) or hydrogen gas. Fig. 3 shows that when nitrate solution is introduced in contact with the conducting polymers, a part of nitrates is immediately removed; then the nitrate
concentration remains unchanged. Thus, 57 and 25% of nitrates disappear from the solution, in the presence of PPy and Pani, respectively, either by redox reaction or by ion exchange, as it was concluded for nitrite [11]. In order to determine the amount of nitrate adsorbed onto the two supports after 300 min, a solution of KCl was added and the nitrate concentration was then once again determined. The pH values, the concentration of ammonium ions measured after 300 min of reaction, and the concentration of adsorbed nitrate, determined from the nitrate concentration determined after addition of KCl, are reported in Table 1 for the two polymer supports. Ammonium ions being produced, this indicates that a part of nitrates is removed by a redox reaction, in the reaction conditions. In the presence of polypyrrole and polyaniline, 3.3 and 25%, respectively, of the nitrates removed from water yield ammonium ions, thus indicating that at minimum, these percentages of nitrate are reduced and not exchanged. This possibility of redox reaction is confirmed by the values of redox potentials of the various species (nitrates, Pani and PPy), reported in Table 2, showing that polypyrrole and polyaniline, when they are in a reduced state, are able to reduce nitrate. Considering now the amount of adsorbed nitrate, it can be deduced that 33 and 25% of the disappeared nitrate were removed by adsorption onto the PPy and Pani support, respectively. Table 1 pH, concentration of ammonium ions and adsorbed nitrate after 300 min in the presence of polypyrrole and polyaniline supports. Polymers
[NH4+]f (mmol/L)
[NO3]ads (mmol/L)
Final pH
PPy Pani
0.03 0.1
0.3 0.1
3 3
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Fig. 2. TEM pictures (a and b) and particle size distributions (c and d) of Pd5Sn0.5/PPY prepared by co-impregnation before reaction (a and c) after reaction (b and d).
Consequently, taking into account the nitrogen mass balance, it can be deduced that onto the two polymers, nitrate is mainly transformed into nitrogen.
Fig. 3. Nitrate abatement in the presence of PPy (^), and Pani (~) under hydrogen.
Table 2 Redox potentials of different species. Redox systems
E8 (V/SHE) pH 0
NO3/N2 Pd2+/Pd NO3/NH4+ Polyemeraldine/polyleucoemeraldinea Polypernigraniline/polyemeraldinea PPy+/PPy
1.246 0.915 0.875 0.342 0.942 0.15
a
Various types of polyaniline.
3.2.2. Performances of the bimetallic catalysts supported on PPy and Pani For the bimetallic catalysts supported on polypyrrole, a complete nitrate conversion is rapidly reached. No intermediate nitrite or hydroxylamine was evidenced in solution. After addition of KCl in solution, no nitrate was released in solution, proving that in the presence of Pd–Sn/PPy catalyst, the nitrate reduction is the only way to remove nitrate from the solution, the adsorption being negligible. For this catalyst series, the activity was calculated by dividing the initial amount of nitrate introduced in solution (in mol), by the weight of catalyst (in g) and by the time needed to completely convert nitrate (in min). The selectivity in ammonium ions is a final selectivity corresponding to the amount of ammonium ions produced (in mol) per mol of nitrate converted. The results obtained with the Pd–Sn/PPy catalysts are summarized in Fig. 4. They show that whatever the preparation method, successive impregnation or co-impregnation, the highest activity is obtained with the catalyst containing 0.5 wt.% of tin, and the activity decreases with the increase in tin content. The Pd5Sn0.5/ PPy catalyst prepared by co-impregnation is slightly more active than the one prepared by successive impregnation, with an activity 1 1 of 82 mmol min1 g1 gcata . However, cata instead of 62 mmol min the latter presents the lowest selectivity in ammonium ions among the catalyst series, 8.5%, which corresponds to 91.5% of selectivity in molecular nitrogen. Whatever the catalyst, the initial pH is around 3 and the final pH value is between 4 and 6. Roughly, the lower the pH, the lower the selectivity in ammonium ions. Then,
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I. Dodouche et al. / Applied Catalysis B: Environmental 93 (2009) 50–55
Fig. 4. Effect of the Sn content on the activity (histograms) and selectivity at 100% of nitrate conversion (lines) for nitrate reduction in the presence the Pd5Snx/PPy catalysts prepared by successive impregnation (in grey) or co-impregnation (in black).
the selectivity in ammonium ions may be directly linked to the capability of the polymer to exchange OH produced during nitrate reduction with the dopant ions, thus limiting the increase of pH and then the ammonium ions production. As far as the performances of the bimetallic catalysts supported on Pani are concerned, they are much less active than their PPysupported counterpart. For some catalysts, a complete conversion was not reached, and, as for the Pd–Sn/PPy series, no adsorption was observed. For this reason, the activity and selectivity were determined at 75% of nitrate conversion. As for the PPy-supported catalyst series, no intermediate nitrite was detected in solution. Results (Fig. 5) show that an optimum of activity is obtained with Pd5Sn1/Pani catalysts. The most active catalyst is the one prepared by successive impregnation, which is also the most selective for molecular nitrogen formation with only 1.8% of ammonium ions produced at 75% of nitrate conversion. The catalysts prepared by co-impregnation produce more ammonium ions than their counterpart prepared by successive impregnation. However, whatever the catalyst tested, the initial and final pH values are around 3. Contrary to what was observed for the catalysts supported on PPy, and to what is commonly admitted for catalysts supported on classical supports such as alumina, it is difficult to link the selectivity to the pH of the solution for catalysts supported onto polyaniline. This may be explained by the capability of the
Fig. 6. Evolution of the nitrate (*, &, ~), nitrite () and ammonium ions (*, &, ~) concentrations vs. time in the presence of Pd–Sn/PPy (~, ~), Pd–Sn/Pani (&, &), Pd–Sn/Al2O3 (*, *, ) (64 mg of catalyst, pH2 = 1).
Table 3 Effect of various supports of Pd–Sn bimetallic catalysts prepared by successive impregnation on their performances in nitrate reduction. Catalysts
Activity Final selectivity Ammonium Initial Final (mmol min1 g1 (ppm)a pH pH cata ) in N2
Pd5Sn0.5/PPy 150 Pd5Sn1/Pani 47 Pd5Sn2/Al2O3 32 a
95.2 98.2 55
1.4 0.5 13
3.5 2.7 5
4.8 3 10
At the end of the reaction.
Pani support to directly reduce a part of nitrate into ammonium ions, as demonstrated in Section 3.2.1. Whatever the metal content, the preparation method and the support, polypyrrole or polyaniline, no metal leaching was observed at the end of the reaction. 3.2.3. Comparison of the performances as a function of the support The lowest selectivity to NH4+, and consequently the highest selectivity to N2, was obtained with Pd–Sn catalysts prepared by successive impregnation, whatever the support used (PPy or Pani). The best catalysts, in terms of selectivity, were Pd5Sn0.5/PPy SI and Pd5Sn1/Pani SI. However, it is impossible to compare together the performances of these two catalysts from the values reported in Figs. 4 and 5 since they are determined at different levels of nitrate conversion. To favour the comparison, the evolution of nitrate and ammonium ions is reported in Fig. 6 as a function of time for the two catalysts and also for a classical Pd–Sn/Al2O3 one prepared according to the same experimental procedure. For this latter, the metal content was also optimized in order to obtain the best activity and selectivity. It contents 5 wt.% of palladium and 2 wt.% of tin. Results are summarized in Table 3. Fig. 6 shows that the activity of the Pd–Sn/Pani and Pd–Sn/Al2O3 catalysts are similar. However, the amount of ammonium ions is twice more important with the alumina supported catalyst, which also yields intermediate nitrite in solution. By far, the Pd–Sn/PPy is the most active catalyst, all the nitrates being removed in 40 min, with also the lowest amount of produced ammonium ions after complete nitrate removal and then the highest final selectivity to N2. 4. Conclusion
Fig. 5. Effect of the Sn content on the activity (histograms) and selectivity at 75% of nitrate conversion (lines) for nitrate reduction in the presence the Pd5Snx/Pani catalysts prepared by successive impregnation (in grey) or co-impregnation (in black).
Polymers with redox and ion-exchange properties, such as polyaniline and polypyrrole, were used as supports for bimetallic Pd–Sn catalysts. The Pd–Sn/PPy and Pd–Sn/Pani catalysts showed
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interesting performances for nitrate removal from water. In particular, the use of these polymers as supports allowed us to decrease the selectivity towards ammonium ions and to avoid the apparition in solution of intermediate nitrite during nitrate reduction, compared to a classical Pd–Sn/Al2O3 catalyst. The Pd5Sn0.5/PPy SI catalyst is by far more active and more selective to molecular nitrogen compared to the best Pd–Sn catalyst supported onto polyaniline. Acknowledgment The authors thank the CAPES/COFECUB (Ph 603/08) for the financial support.
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