Synthesis, characterization and catalytic properties of polypyrrole-supported catalysts

Synthesis, characterization and catalytic properties of polypyrrole-supported catalysts

Catalysis Communications 4 (2003) 435–439 www.elsevier.com/locate/catcom Synthesis, characterization and catalytic properties of polypyrrole-supporte...

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Catalysis Communications 4 (2003) 435–439 www.elsevier.com/locate/catcom

Synthesis, characterization and catalytic properties of polypyrrole-supported catalysts Eric Gautron, Anthony Garron, Emmanuelle Bost, Florence Epron

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Laboratoire de Catalyse en Chimie Organique, UMR 6503 CNRS et Universit e de Poitiers, 40 avenue du Recteur Pineau, Poitiers cedex 86022, France Received 10 April 2003; accepted 18 June 2003 Published online: 12 July 2003

Abstract Polypyrrole, a conductive polymer, was used as support of noble metals, namely, copper and palladium, for application in heterogeneous catalysis. Catalysts were characterized by TEM coupled with EDS and XRD. Their activity and selectivity were determined in the reduction of nitrate and intermediate nitrite in water under 1 bar of hydrogen and at room temperature. In nitrite reduction, these novel catalysts demonstrated a better activity and a higher selectivity towards nitrogen formation than their Pd/Al2 O3 counterpart. On the other hand, the bimetallic catalyst tested for nitrate reduction showed an activity similar to the one of a classical Pd–Cu/Al2 O3 catalyst, but no intermediate nitrite was observed. Ó 2003 Elsevier B.V. All rights reserved.

1. Introduction Polymers represent a class of materials that plays a role of growing importance in catalysis, due to the possibility of controlling in a simple way their morphology and their physico-chemical properties. Until now, most of the studies have been devoted to the use of two types of polymers for the preparation of metallic catalyst, namely: (i) synthetic cross-linked macromolecular materials,

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Corresponding author. Fax: +33-5-4945-3499. E-mail address: fl[email protected] (F. Epron).

such as ion-exchange resins and (ii) hydrosoluble polymers. Resins have been used as support for catalytically active polydispersed noble metal phases since 1969 [1]. They are employed for example in the industrial synthesis of methyl-isobutyl ketone [2]. Noble metals supported on resins, comprising acid centers as well as hydrogenation active centers have been recently developed. They are generally prepared by ionic-exchange between the resin and the metal precursor followed by a chemical reduction [3–8]. Hydrosoluble polymers are used to prepare suspension of metal nanoparticles in order to prevent their coagulation and precipitation. These catalysts, named metal colloids, are generally prepared by reduction of metal

1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1566-7367(03)00109-2

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salts in the presence of protective polymers such as poly(vinylalcohol) [9] or poly(N-vinyl-2-pyrrolidone) [10–15]. Noble-metal colloids are active and selective in the hydrogenation of alkenes [10–13]. During the last decade, conductive polymers have received increased interest for commercial applications such as sensors [16,17], conducting wires [18], antielectrostatic coatings [19], etc. Although various metals have already been introduced into the conductive polymer matrix by electrochemical deposition [20,21] for applications in electrocatalytic reactions, very few papers report the use of conducting polymers as support of noble metals for applications in catalysis [22,23]. In these papers, the deposition of palladium into the conducting polymer matrix was operated by treatment of the polymer powder with an aqueous solution of Pd(II) ions. The oxidation state of palladium into the polymer was dependent on the experimental conditions (pH, nature of the anions in the palladium salt) and the polymer-supported palladium catalyst was used as prepared or after chemical reduction in the hydrogenation of 2ethylanthraquinone [22] or of nitrobenzene [23]. The purpose of the present paper is to describe the preparation, the characterization and the catalytic properties for nitrate and nitrite reduction in water of novel bimetallic catalysts supported on polypyrrole. Among the intrinsic conducting polymers with conjugated double bonds, polypyrrole is one of the most promising because of its good environmental stability, facile synthesis and high conductivity. Polypyrrole can be easily prepared either by chemical or electrochemical polymerization. The chemical polymerization, yielding to powders, is carried out by oxidation using, for example, transition metal ions (Fe3þ , Cu2þ , etc.) as oxidants. This paper deals with the preparation of new polymer-supported bimetallic Pd–Cu catalysts, obtained by direct reduction of copper and/ or palladium ions during the oxidative polymerization of pyrrole. These new catalysts were characterized by transmission electron microscopy (TEM) coupled with energy dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), and thermogravimetric analysis. The resulting catalysts were tested in the catalytic reduction of nitrate and nitrite in water and their catalytic performances

were compared with those of classical Pd or Pd–Cu catalysts supported on alumina.

2. Experimental The oxidative polymerization of pyrrole was carried out in water using copper chloride or an equimolar mixture of copper chloride and palladium chloride as an oxidant. The resulting catalysts were named Cu–PPy or Pd–Cu–PPy, respectively. In a typical experiment, 2.1 ml of pure pyrrole (Fluka) was quickly added to an aqueous solution of metal salt under vigorous magnetic stirring to facilitate the dispersion of pyrrole. The mole ratio of CuCl2 to pyrrole was 2.3 (1 for the mixture CuCl2 + PdCl2 ) (CuCl2  2H2 O from Fluka and PdCl2 from Johnson Matthey). The reaction mixture was stirred for 6 h at room temperature. The resulting black powder, corresponding to polypyrrole as identified by infra-red spectroscopy, was separated by filtration, repeatedly washed with water and dried at 90 °C for 5 h. After this preparation step, Cu–PPy catalyst contained 10 wt% of copper, and Pd–Cu–PPy 7 wt% of palladium and 5 wt% of copper. The Pd– Cu–PPy catalyst was tested directly after preparation. To obtain a bimetallic catalyst, Cu–PPy was impregnated with Pd(NH3 )4 (NO2 )2 (Johnson Matthey), in order to obtain a palladium loading of 7 wt%. Then it was either (i) directly reduced by NaBH4 or (ii) calcined in flowing air (80 ml min1 ) at 190 °C for 1 h and reduced in flowing hydrogen (150 ml min1 ) at 150 °C. The resulting catalyst was named Pd/Cu–PPy. It was demonstrated by thermogravimetric analysis that the polymer is not degraded after such treatments. Catalysts were characterized by TEM coupled with EDS and XRD. Their activity and selectivity were determined in the reduction of nitrate and intermediate nitrite in water under hydrogen and at room temperature, according to the procedure described in [24]. Products of the reaction are gaseous nitrogen and ammonium ions, which are undesired products. Selectivity towards ammonium ions was determined by HPLC analysis according to [25].

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3. Results and discussion Fig. 1 shows a representative sample of palladium catalyst supported on Cu–PPy support, after , reduction. Particles, with an average size of 20 A are distributed within the polymer framework and correspond either to palladium alone or to palladium–copper entities, as characterized by EDX analysis. A large amount of chloride is also present principally associated with polypyrrole or with copper. Chloride probably acts as counterions to the Nþ in the polymer. Copper (I) chloride was also evidenced by XRD in the Cu–PPy support. This is characteristic of an incomplete reduction of copper (II) chloride into metallic copper. Conversely no crystallite corresponding to either copper or palladium species was evidenced by XRD on the Pd–Cu–PPy catalyst. This could be explained by the high dispersion of both metals in the polymer framework. The activity and selectivity of catalyst for nitrite and nitrate were determined and compared with those of classical monometallic Pd/Al2 O3 or bimetallic Pd–Cu/Al2 O3 catalysts. Pd/Cu–PPy is only active for nitrite reduction. As palladium is

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active for nitrite reduction whereas Pd–Cu bimetallic entities are needed to reduce nitrate, the inactivity for nitrate reduction of the Pd/Cu–PPy catalyst can be explained by the absence of metallic copper in interaction with palladium, copper being in CuCl form to a large extent. The activities of this catalyst for nitrite reduction and selectivities towards ammonium ions are reported in Table 1 as a function of the pretreatment and compared with those of a classical Pd/Al2 O3 catalyst. These novel catalysts are more active than the classical one for nitrite reduction with a lower selectivity towards ammonium ions, which are undesired products. The differences of activity for Pd/Cu– PPy as a function of the pretreatment confirm that the conditions of the reduction are of major importance, as in any catalytic hydrogenation process involving a noble metal. The best result is observed when the reduction is performed in situ under hydrogen that avoids any contact with air of the reduced catalyst before reaction. The most promising result concerns the lower selectivity towards ammonium ions of catalyst supported on conductive polymer compared to the one obtained with a classical catalyst. The difference of the size

Fig. 1. TEM image of Pd/Cu–PPy.

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Table 1 Activities and selectivities of 7 wt% Pd/Cu–PPy and 1.6 wt% Pd/Al2 O3 catalysts Catalyst

Pretreatment

Pd/Cu–PPy Pd/Cu–PPy

Reduced by NaBH4 Calcined (air, 150 °C, 6 h) and reduced ex situ (H2 , 150 °C, 6 h) Calcined (air, 150 °C, 6 h) and reduced in situ b (H2, 150 °C, 1 h) Calcined (air, 150 °C, 6 h) and reduced ex situ (H2 , 150 °C, 6 h) then in situ

Pd/Cu–PPy Pd/Al2 O3

Initial activity of Pd (mmol min1 g1 )

Selectivity of a NHþ 4 (%)

Time (min) for 100% of nitrite converted

1 8

8 0.7

600 20

11

0.15

15

2.25

44

240

3 Reaction conditions: T ¼ 25 °C, pH2 ¼ 1 bar, hydrogen flowing ¼ 250 ml min1 , ½NO mol l1 , reaction 2 0 ¼ 1:6  10 volume ¼ 100 ml, and catalyst weight ¼ 64 mg for Pd/Cu–PPy and 200 mg for Pd/Al2 O3 . a At the end of the reaction. b In the reactor.

of the metallic particle could explain this result, the  in the Pd/Cu–PPy catalyst particle size being 20 A  in the Pd/Al2 O3 catalyst. Indeed, it was and 72 A demonstrated [26] that, by varying the palladium

dispersion from 10% to 45% on the Pd/Al2 O3 catalyst, the selectivity towards ammonium ions is decreased. However, the changes are not very important, the lower selectivity being around 25% for

Table 2 Activities and selectivities of Pd–Cu–PPy and Pd–Cu/Al2 O3 catalysts Catalysts

Pretreatment

Initial activity of catalyst (mmol min1 g1 )

Selectivity of a NHþ 4 (%)

Maximum selectivity of NO 2 (%)

Pd–Cu–PPyb Pd–Cu/Al2 O3 c

– Calcined (air, 150 °C, 6 h) and reduced ex situ (H2 , 150 °C, 6 h) then in situ

0.034 0.04

39 60

0 64

3 Reaction conditions: T ¼ 25 °C, pH2 ¼ 1 bar, hydrogen flowing ¼ 250 ml min1 , ½NO mol l1 , reaction 3 0 ¼ 1:6  10 volume ¼ 100 ml, and catalyst weight ¼ 64 mg. a At the end of the reaction. b 7 wt% Pd and 5wt% of Cu. c 5 wt% Pd and 2wt% of Cu, prepared by successive impregnation.

Fig. 2. Scheme of the oxidative polymerization of pyrrole (I) and of the oxidation of the polypyrrole framework leading to a conductive polymer (II).

E. Gautron et al. / Catalysis Communications 4 (2003) 435–439

). Then, the the smallest metallic particles (21 A support is likely to play a role of major importance in the ammonium ions production and the conducting polymer support seems to be more suitable for this type of reaction to favor the nitrogen formation. As far as nitrate reduction is concerned (Table 2), the activity of the polymer-supported catalyst Pd–Cu–PPy is similar to that of the classical one (PdCu/Al2 O3 ), but no intermediate nitrite is observed and the selectivity towards ammonium ions is lower. As the polymer supported catalyst was directly used after preparation, without reductive pretreatment and as nitrate reduction occurs on bimetallic Pd–Cu entities [24], the activity of Pd– Cu–PPy catalyst for nitrate reduction indicates that the oxidative polymerization of pyrrole in the presence of CuCl2 and PdCl2 directly yields to polypyrrole and reduced metals. This is in accordance with the results of Henry et al. [27] obtained by oxidizing pyrrole monomer with chlorauric acid and other noble metal precursors. The proposed mechanism of oxidative polymerization of pyrrole by PdCl2 is presented in Fig. 2. 4. Conclusion In conclusion, we have shown that electroactive polymers can be used as supports for noble metals and that the resulting materials have promising catalytic properties for reactions carried out at ambient temperature, such as nitrate and nitrite reduction in water.

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