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Cu catalysts
Applied Catalysis B: Environmental 41 (2003) 3–13
Denitrification of natural water on supported Pd/Cu catalysts A.E. Palomares a , J.G. Prato b , F. Márquez b , A. Corma b,∗ a
Department of Chemical and Nuclear Engineering, Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain b Instituto de Tecnolog´ıa Qu´ımica, Universidad Politécnica de Valencia, UPV-CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain Received 10 December 2001; received in revised form 31 March 2002; accepted 1 April 2002
Abstract The influence of the support on the catalytic hydrogenation of nitrates in liquid phase was investigated. Pd/Cu supported on hydrotalcite and Pd/Cu supported on alumina catalysts were tested for this reaction and its activity and selectivity were compared. It has been observed a higher activity and a lower production of ammonium when the metals are supported on hydrotalcite than when they are supported on alumina. When rehydrated after calcination hydrotalcite recovers its layered structure. Nitrates can be located in the interlayer space diminishing the problems associated to mass transfer limitations and increasing the activity of the material. In addition, copper can be incorporated in the hydrotalcite structure by an isomorphical replacement of magnesium, obtaining thus a high dispersion of the copper centres, that is reflected in a higher activity if compared with copper impregnated samples. A XPS study was made in order to characterise the metals supported on the hydrotalcite. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Denitrification; Pd/Cu catalyst; Water; Pollution; Hydrotalcite
1. Introduction The increasing rigorousity of the drinking water quality standards and the toxicity of the nitrates, has generated the urgent need to develop a new technology for its removal from aqueous solutions [1]. Nowadays, the main technologies used for nitrate removal from water are: (i) physicochemical processes such as electrodyalisis, ion exchange or reversed osmosis; and (ii) biological processes based on the nitrate reduction by micro-organisms into gaseous nitrogen [2]. The main disadvantage of the physicochemical processes is the fact that nitrates are not converted into harmless compounds but are concentrated in a secondary waste which has to be treated or disposed ∗ Corresponding author. Tel.: +34-6-3877800; fax: +34-6-3877809. E-mail address: [email protected] (A. Corma).
off. Biological denitrification is a well known treatment for wastewater but its use for denitrification of ground water presents multiple difficulties. Therefore, the development of an environmental friendly and highly efficient technology is becoming a necessity. The most favourable way to remove nitrates from the environmental point of view is to convert them into N2 . In order to do this, one of the most promising processes is the liquid phase nitrate hydrogenation over noble metal catalysts [3–7]. This reaction was first described by Vorlop and Tacke [8]. In this process nitrate reacts with hydrogen and it is converted into nitrogen and water using a solid catalyst. It is necessary a high selectivity of the catalyst to avoid the production of nitrite and ammonium ions. Typically, metal oxide supported bimetallic catalysts, consisting of a combination between a noble and a non-noble metal, were applied for this reaction. In these catalysts the bimetallic combination has a substantial
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activity for the nitrate reduction and the noble metal component is highly active for the reduction of nitrite (considered as a possible intermediate in the reaction) to nitrogen [3]. The most selective noble metal for the catalytic reduction of nitrite to nitrogen with hydrogen was found to be palladium [3–9], although other noble metals as Rh, Ir, Ru and Pt also showed some activity [10,11]. As second metallic component in the bimetallic catalysts various combinations of Pd with Ag, Fe, Hg, Ni, Cu, Zn, Sn and In had been tested [4,12,13], obtaining the best results when Cu, Sn or In were the second metallic component. The disadvantage of this process is the undesired formation of ammonia and further improvements of these catalytic systems are necessary to optimise their performance. These studies were mainly focused in the composition and preparation of the catalysts [11–15] and their application on various reactor types [16–18]. Another possibility was described by Prüse et al. [19] using formic acid instead of hydrogen as a reductant agent. Formic acid has an in situ buffering effect, that lowers the pH gradient leading to a higher selectivity to nitrogen [15]. It has to be also considered the effect of the presence of other ions in the solution, being the most important the inhibiting effect of hydrogencarbonates, probably due to a competitive adsorption of these species on Pd-Cu active sites [6]. An additional factor that may strongly influence the properties of the catalyst is the type of support used, but only a few works studying this possibility have been published [13,16,20]. In this work, we have studied the possibilities of using hydrotalcites as active materials for supporting Cu and Pd metals. Hydrotalcites are layered double hydroxides (LDH) that can be represented by: M(II)1−y M(III)y (OH)2 ]y+ [(Xy/n n− ) · mH2 O]y− , where y is typically between 0.25 and 0.33, and Xn − is a n-valent anion. These compounds present positively charged brucite-like layers, Mg(OH)2 , with trivalent cations substituting divalent cations in octahedral sites of the hydroxide sheet. A wide range of derivatives containing various combinations of M2+ , M3+ and An − ions can be synthesised [21–23]. They are thermally stable at temperatures up to 673 K, but they transform into mixed metal oxides at higher temperatures, recovering its initial structure when they are again hydrated. Therefore, calcined hydrotalcites can be a very interesting support material for the liquid phase removal
of nitrates, especially considering that when recovering the hydrotalcite structure, NO3 − anions could be in the interlayer space compensating the positively charged layers. It has to be also considered that it is possible to synthesise a hydrotalcite with copper partially substituting magnesium that will result in a very high dispersion of the catalyst copper sites [24]. In this work, we have studied the possibilities of using Mg:Al hydrotalcites with different Mg:Al ratio as active materials for supporting Cu and Pd metals. The activity of these catalysts has been compared with the activity of Cu/Pd catalysts supported on alumina and with the activity of a palladium wet impregnated hydrotalcite with copper in its structure. The oxidation state of Cu and Pd was analysed by XPS. 2. Experimental 2.1. Catalyst preparation The gamma alumina and the hydrotalcites used as support in the nitrate reduction experiments were supplied by Guilder and by Intercat, respectively. The catalysts were prepared by a standard impregnation method starting from Pd(NO3 )2 ·2H2 O and Cu(CH3 COO)2 ·4H2 O aqueous solutions. This wet impregnation sequence was followed: (i) impregnation by copper acetate; (ii) drying in an oven at 423 K and calcination in air for 1 h at 773 K; (iii) deposition of palladium nitrate followed by drying at 423 K. Finally, the resulting solid was calcined for 1 h at 773 K and then reduced in hydrogen for 4 h at the same temperature. Hydrotalcites with copper in its structure were prepared by a standard co-precipitating procedure using two solutions. The first solution contained Mg(NO3 )2 · 6H2 O, Al(NO3 )3 ·9H2 O and Cu(NO3 )2 ·5H2 O, having a (Al + Mg + Cu) molar concentration of 1.5. The second solution contained NaOH and Na2 CO3 in adequate concentration to obtain total precipitation of aluminium, magnesium and copper in the former solution and to fix the pH at a value of 13. Both solutions were added, while vigorously stirring, at a total flow-rate of 30 cm3 /h, for 4 h. The gel was aged under autogenous pressure conditions at 333 K for 14 h, then filtered and washed with distilled water until the pH was 7 and carbonate was not detected
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in the filtrate. The hydrotalcite was calcined at 823 K in air for 9 h before reaction, obtaining a Cu/Mg/Al mixed oxide. Over this material palladium was impregnated as it is described above (from step (iii)). In all the catalysts Pd, Cu, Mg and Al contents were determined by atomic absorption spectroscopy. 2.2. Catalyst characterisation The surface areas of the catalysts were obtained in an ASAP 2000 apparatus, using the BET method from the nitrogen adsorption isotherms at 77 K. X-ray diffraction (XRD) patterns were collected using a Philips X’Pert PW3719 diffractometer (Cu K␣ radiation and graphite monochromator), provided with a variable divergence slit and working in the fixed irradiated area mode. The X-ray photoelectron spectra were acquired with a surface analysis system (Escalab-210, VG-Scientific) by using the Al K␣ (1486.6 eV) radiation of a twin anode in the constant analyser energy mode, with a pass energy of 50 eV. During the spectral acquisition the pressure of the analysis chamber was maintained at 5 × 10−8 N m−2 . All samples are insulators with a very small amount of carbonaceous impurity on their surfaces. The charging effect was corrected by setting the C1s transition at 284.6 eV. The reproducibility of the peak position was ±0.1 eV. To avoid the X-ray induced reduction of Cu2+ to Cu1+ , samples were maintained at 173 K during acquisition and the X-ray power was limited to 200 W (20 mA to 10 kV). The spectral acquisition time was also reduced to a minimum to prevent damage of the sample and the possible reduction of Cu2+ . 2.3. Catalytic test The catalysts were used for batch experiments in a 1 l glass reactor equipped with a mechanical Teflon stirrer. The N2 and H2 flows were introduced into the vessel below the impeller and their flow controlled by an electronic mass flow controller. In a typical run, 0.85 g of the powder (average diameter 0.104 mm) catalyst previously reduced with hydrogen at 773 K, was charged into the reactor containing 0.6 l of distilled water. The content of the reactor was purged with hydrogen during 1 h. The reaction starts with the addition of 5 cm3 of a concentrated KNO3 solution,
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in order to achieve a total concentration of 90 mg/l inside the reactor. During the reaction a hydrogen flow of 500 cm3 /min was fed by a glass frit into the solution. The experiments were carried out at 293 K and a total pressure of 1 bar, which was equal to the hydrogen pressure. The reactions were carried out using a stirring rate of 500 rpm. The reaction progress was followed by taking small samples from the vessel for the determination of nitrate, nitrite and ammonium concentration at defined periods. The concentrations of NO3 − , NO2 − and NH4 + in the aqueous-phase samples were determined by UV-Vis spectroscopy (Shimadzu double-beam spectrophotometer, model UV-2101 PC) combined with reagent kits for photometric analysis (Merck—Spectroquant® , with concentration ranges for NO3 − , NO2 − and NH4 + of 1–90, 0.03–3.0 and 0.03–3.0 mg/l, respectively). 3. Results and discussion 3.1. Characterisation The chemical composition and surface areas of the catalysts after calcination in air are comparatively shown in Table 1. It can be seen that all the catalysts tested have surface areas up to 240 m2 /g. This surface area is comparable to that of the metals supported on alumina, then the different activity of the samples cannot be attributed to this support characteristic. Also from this table it can be conclude that the different Mg:Al ratio, copper content or catalyst preparation method does not result in a substantial difference of the catalyst surface area. The XRD patterns of the hydrotalcite supported Pd/Cu sample after calcination in air and reduction with hydrogen at 773 K is shown in Fig. 1(a). It is observed the formation of magnesium oxide poorly crystalline with peaks at 2θ = 36.9, 43.0 and 62.5◦ . These peaks are shifted to higher angles if compared with a pure magnesium oxide. This is consequence of the incorporation of aluminium in the framework of the MgO [25], resulting in the formation of a Mg/Al mixed oxide. The presence of Pd metal is also observed, indicated by the peaks at 2θ = 40.1, 46.7 and 68.3◦ . Due to the low copper content no peak related with it can be observed. After reaction (Fig. 1(b)) a substantial change is observed in the XRD spectra. As
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Table 1 Chemical composition and textural characteristics of the samples studied Sample
Cu (wt.%)
Pd (wt.%)
Support (Mg:Al ratio)
Surface area (m2 /g)
Cu/Pd-Al2 O3 Cu/Pd-HT1 Cu/Pd-HT2 Cu/Pd-HT3 Pd-HTCu1 Pd-HTCu2 Pd-HTCu3 Pd-HTCu4
1.5 1.6 1.6 1.5 1.5 1.5 1.4 7.1
5.1 5.0 5.2 5.0 4.9 4.8 4.9 5.5
␥-Alumina Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite
195 200 235 242 – 218 – 195
(Cu (Cu (Cu (Cu
in in in in
structure) structure) structure) structure)
it can be expected, the hydrotalcite structure has been recovered due to the rehydration of the mixed oxides and its characteristic peaks appear at 2θ = 11.2, 22.3, 34.5 and 60.2◦ . The peaks of Pd metal are still observed at 2θ = 40.1, 46.7 and 68.3◦ , indicating that the metal has been only partially oxidised. No peaks related with copper species have been observed. XPS was used to characterise the nature of surface copper and palladium present in the catalysts before and after reaction. Since the Cu2p(3/2) transition does
(4:1) (3:1) (2:1) (4:1) (3:1) (2:1) (3:1)
not allow to distinguish the oxidation states of copper by itself, these have been assigned using Cu2p(3/2) binding energies, the associated shake-up satellites and the kinetic energies of the CuL3 VV Auger transitions. The shake-up satellite peaks are final state effects which arise when the photoelectron imparts energy to another electron of the atom, ending up in a higher unoccupied state (shake-up). Due to this effect the photoelectron losses energy and appears at a higher binding energy in the XPS spectrum. This effect is most
Fig. 1. X-Ray diffraction patterns of hydrotalcite supported Pd/Cu catalyst (Cu/Pd-HT1): (a) after calcination in air and reduction with H2 ; (b) after reaction.
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Fig. 2. (A) Cu2p(3/2) photoelectron spectra of Pd-Cu-mixed oxides derived from hydrotalcite (with 7 wt.% Cu and 5 wt.% Pd): (a) reduced in hydrogen at 773 K; (b) after reaction. (B) Pd3d photoelectron spectra of Pd-Cu-mixed oxides derived from hydrotalcite (with 7 wt.% Cu and 5 wt.% Pd): (a) reduced in hydrogen at 773 K; (b) after reaction.
pronounced in metals with d-orbitals, being clearly observed in copper. In the case of copper, the final state effects are only observed in Cu(II) species and for this reason this characteristic can be used to identify Cu(II) species. The determination of Cu(I) or Cu metal is more difficult and requires a careful analysis. Fig. 2A shows the Cu2p(3/2) spectra obtained for the same catalyst after different treatment conditions. After reduction at 773 K (Fig. 2A (a)), a maximum at ca. 933.1 eV was observed, which, along with the satellite peak at 942.6 eV, is characteristic of Cu2+ species [24,26–28]. This is supported by the CuL3 VV spectrum (not shown), where the maximum at ca. 1151 eV (KE) is associated with Cu2+ [24,26–28]. Nevertheless, the low intensity of the satellite peak together with the presence of a shoulder at ca. 1149 eV (KE) in the CuL3 VV transition indicates the presence of Cu+ . Fig. 2B (a) illustrates the Pd3d spectrum obtained for the same catalyst after reduction in
hydrogen at 773 K. As it can be seen, the presence of a peak at ca. 335.2 eV (Pd3d5/2 ) indicates that Pd is in the metallic form [24,26–28]. After reaction the binding and kinetic energies corresponding to copper did not experience substantial changes and only the intensity corresponding to the satellite peak associated to Cu2p(3/2) transition increased slightly (see Fig. 2A (b)). These results should be associated with the increase of the Cu2+ level on the catalysts surface. After reaction, two components (at ca. 337.3 and 335.3 eV) appear generating the peak shape of the Pd3d(5/2) transition, Fig. 2B (b), indicating the presence of oxidised and metallic Pd, respectively. As the dissolution of the catalyst components is the most critical problem in this system, the amount of Pd, Cu, Mg and Al present in the solution after reaction was measured. It was proved that no metal was present in the solution, indicating that in these
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conditions the metals do no dissolve and it does no suppose any problem for this system. 3.2. Reaction studies The activity and selectivity for the hydrogenation of nitrates with a hydrotalcite supported Pd/Cu catalyst
was studied and compared with that of an alumina supported Pd/Cu catalyst. In Fig. 3, it is shown a typical course of the changes of nitrate, nitrite and ammonium ion concentrations during a batch experiment. As it can be seen the activity of the metals is quite different depending on the support. At these reaction conditions, the reaction velocity and the final
Fig. 3. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for: (䊉) Cu/Pd-Al2 O3 and (䊏) Cu/Pd-Hydrotalcite (Cu/Pd-HT1) catalysts. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 −[NO3 − ]t )/[NO3 − ]t=0 , for: (䊉) Cu/Pd-Al2 O3 and (䊏) Cu/Pd-Hydrotalcite (Cu/Pd-HT1) catalysts.
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conversion obtained when Pd and Cu are supported on hydrotalcites is much higher than those obtained when the metals are supported on alumina. The presence of nitrites and ammonium was also observed. Nitrite is considered in some studies as a primary intermediate,
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while ammonium is formed as a final product via a consecutive-parallel reaction pathway [15,17]. The nitrite conversion with the catalyst supported on alumina does not reach the maximum, usually obtained when the nitrate conversion is around 80%, after 2 h
Fig. 4. (A) NO3 − concentration profiles as a function of time for (䊊) Cu/Pd supported on alumina, Cu/Pd supported on various hydrotalcites: (䊐) HT1-Mg:Al, 4:1; (䉭) HT2-Mg:Al, 3:1; (䉫) HT3-Mg:Al, 2:1 and (∗) Cu/Pd supported on magnesium oxide. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for (䊊) Cu/Pd supported on alumina, Cu/Pd supported on various hydrotalcites: (䊐) HT1-Mg:Al, 4:1; (䉭) HT2-Mg:Al, 3:1; (䉫) HT3-Mg:Al, 2:1 and (∗) Cu/Pd supported on magnesium oxide.
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reaction. On the other hand this maximum is quickly achieved when Pd/Cu is supported on hydrotalcite. The two samples show the formation of ammonium, but the production of ammonium with Pd/Cu supported on hydrotalcite is much lower than that obtained when they are supported on alumina (Fig. 3B).
Since these two catalysts have the same Cu and Pd content and a non-significant influence of the surface area is observed, the different behaviour of the materials should be related with a possible support catalytic activity or with a support promoting effect on the metals. In order to check this, a hydrotalcite with the same
Fig. 5. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for Cu/Pd hydrotalcite catalysts: (䊉) Cu/Pd-HT1 with copper and palladium impregnated on the hydrotalcite and (䊏) Pd-HTCu1 with copper in the hydrotalcite structure. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for Cu/Pd hydrotalcite catalysts: (䊉) Cu/Pd-HT1 with copper and palladium impregnated on the hydrotalcite and (䊏) Pd-HTCu1 with copper in the hydrotalcite structure.
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chemical composition but with no metal supported on it was tested at the same reaction conditions obtaining no conversion in the removal of nitrates. This shows the absence of a hydrotalcite catalytic behaviour. Then, this promoting effect should be related with the
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structure or with the electronegativity of the hydrotalcite that is influencing the Pd and Cu active sites. To clarify these aspects, Pd/Cu catalysts supported on different hydrotalcites with different Mg:Al contents were compared and were compared with a Pd/Cu
Fig. 6. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for Cu/Pd hydrotalcite catalysts with different copper content: (䊉) 1.5% Cu weight Cu/Pd-HT1 and (䊏) 7.1% Cu weight Pd-HTCu4. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for Cu/Pd hydrotalcite catalysts with different copper content: (䊉) 1.5% Cu weight Cu/Pd-HT and (䊏) 7.1% Cu weight Pd-HTCu.
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catalyst supported on a MgO. The different Mg:Al content represents different Sanderson electronegativity, being the enriched magnesium samples the most basic materials. In Fig. 4 it can be observed the results obtained with these samples. There is some difference between the activity of the different hydrotalcites, suggesting that the support with the highest Mg:Al ratio is the most active and the one which produces less ammonium in this reaction. But this highest activity cannot be explained only in terms of electronegativity, because there are substantial differences between the catalysts supported on the hydrotalcites and the catalysts supported on MgO or Al2 O3 . Pd/Cu-MgO shows a very low activity for this reaction, probably due to severe transport limitations of NO3 − and NO2 − caused by the partial negative charge of this basic oxide [9,18]. This is not occurring with the hydrotalcites because despite being also basic materials, calcined hydrotalcites recover its structure after rehydration forming a LDH with a partial positive charge. In this structure, nitrate anions are located in the interlayer space compensating the partial positive charge and solving the problems associated to mass transfer limitations. It is well known [24] that Cu-substituted hydrotalcite can be synthesised by the isomorphical replacement of the magnesium ions by copper ions. This could be very interesting in order to obtain an adequate dispersion of copper in the catalyst. In order to check this some samples with copper incorporated in the structure of the hydrotalcite were synthesised and impregnated with palladium. When these samples were tested for the catalytic reduction of nitrates, (Fig. 5) it can be observed a higher velocity of reaction and that it is more important a less ammonium production than with the hydrotalcites with copper and palladium impregnated on them or impregnated on an alumina support. This has to be clearly related with the best dispersion of copper on this material. In order to compare the catalytic results with those obtained in the XPS analysis, where a higher copper concentration (around 7 wt.%) is necessary for an adequate analysis of the samples, the activity of a catalyst with a higher copper content was studied. The results are shown in Fig. 6. It shows that an increase on the copper concentration, enhance the initial activity of the catalyst, although it also enhances the production of ammonium that is not desired. This can
be explained by the low accessibility of palladium sites when the copper amount is too high [11].
4. Conclusions The results from this work show that hydrotalcite supported Cu/Pd catalysts are a very interesting material for the liquid phase hydrogenation of nitrates. These samples present more activity on the removal of nitrates and less production of ammonium than when Pd/Cu are supported on alumina. These results can be related with the characteristics of the hydrotalcite which, after calcination, regenerates its structure when it is in contact with water, forming a LDH with a partial positive charge. In this structure nitrate can be located in the interlayer space compensating the partial positive charge, reducing the mass transfer problems. For this reason the activity and selectivity of the catalyst increases, especially if the hydrotalcite is synthesised with copper in its structure that results in a higher dispersion of the copper active sites. These results show that not only the oxidation state of the metal but the support has a very important influence on the activity of the metals supported on it. The XPS results show that these metals are Pd(0) and Cu(II) or Cu(I) that are partially oxidised in the course of the reaction, but further experiments have to be made in order to know the exact nature of the metals supported on the hydrotalcite.
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[20] D. Gasparovicova, M. Kralik, M. Hronec, Collect. Czech. Chem. Commun. 64 (1999) 502. [21] F. Trifiró, A. Vaccari, in: J.L. Atwood, J.E.D. Davies, D.D. MacNicol, F. Vogtle, (Eds.), Comprehensive Supramolecular Chemistry, Pergamon, Oxford, vol. 7, 1995. [22] F. Cavani, F. Trifiró, A. Vaccari, Catal. Today 11 (1991) 173. [23] V. Rives, M.A. Ulibarri, Coord. Chem. Rev. 181 (1999) 61. [24] F. Marquez, A.E. Palomares, F. Rey, A. Corma, J. Mat. Chem. 11 (2001) 1675. [25] T. Sato, H. Fujito, T. Endo, M. Shimada, React. Solids 5 (1988) 219. [26] A. Corma, A. Palomares, F. Rey, F. Márquez, J. Catal. 170 (1997) 140. [27] A. Corma, A. Palomares, F. Márquez, J. Catal. 170 (1997) 132. [28] J. Batista, A. Pintar, D. Mandrino, M. Jenko, V. Martin, Appl. Catal. A 206 (2001) 113.
Denitrification of natural water on supported Pd/Cu catalysts A.E. Palomares a , J.G. Prato b , F. Márquez b , A. Corma b,∗ a
Department of Chemical and Nuclear Engineering, Universidad Politécnica de Valencia, Avenida de los Naranjos s/n, 46022 Valencia, Spain b Instituto de Tecnolog´ıa Qu´ımica, Universidad Politécnica de Valencia, UPV-CSIC, Avenida de los Naranjos s/n, 46022 Valencia, Spain Received 10 December 2001; received in revised form 31 March 2002; accepted 1 April 2002
Abstract The influence of the support on the catalytic hydrogenation of nitrates in liquid phase was investigated. Pd/Cu supported on hydrotalcite and Pd/Cu supported on alumina catalysts were tested for this reaction and its activity and selectivity were compared. It has been observed a higher activity and a lower production of ammonium when the metals are supported on hydrotalcite than when they are supported on alumina. When rehydrated after calcination hydrotalcite recovers its layered structure. Nitrates can be located in the interlayer space diminishing the problems associated to mass transfer limitations and increasing the activity of the material. In addition, copper can be incorporated in the hydrotalcite structure by an isomorphical replacement of magnesium, obtaining thus a high dispersion of the copper centres, that is reflected in a higher activity if compared with copper impregnated samples. A XPS study was made in order to characterise the metals supported on the hydrotalcite. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Denitrification; Pd/Cu catalyst; Water; Pollution; Hydrotalcite
1. Introduction The increasing rigorousity of the drinking water quality standards and the toxicity of the nitrates, has generated the urgent need to develop a new technology for its removal from aqueous solutions [1]. Nowadays, the main technologies used for nitrate removal from water are: (i) physicochemical processes such as electrodyalisis, ion exchange or reversed osmosis; and (ii) biological processes based on the nitrate reduction by micro-organisms into gaseous nitrogen [2]. The main disadvantage of the physicochemical processes is the fact that nitrates are not converted into harmless compounds but are concentrated in a secondary waste which has to be treated or disposed ∗ Corresponding author. Tel.: +34-6-3877800; fax: +34-6-3877809. E-mail address: [email protected] (A. Corma).
off. Biological denitrification is a well known treatment for wastewater but its use for denitrification of ground water presents multiple difficulties. Therefore, the development of an environmental friendly and highly efficient technology is becoming a necessity. The most favourable way to remove nitrates from the environmental point of view is to convert them into N2 . In order to do this, one of the most promising processes is the liquid phase nitrate hydrogenation over noble metal catalysts [3–7]. This reaction was first described by Vorlop and Tacke [8]. In this process nitrate reacts with hydrogen and it is converted into nitrogen and water using a solid catalyst. It is necessary a high selectivity of the catalyst to avoid the production of nitrite and ammonium ions. Typically, metal oxide supported bimetallic catalysts, consisting of a combination between a noble and a non-noble metal, were applied for this reaction. In these catalysts the bimetallic combination has a substantial
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activity for the nitrate reduction and the noble metal component is highly active for the reduction of nitrite (considered as a possible intermediate in the reaction) to nitrogen [3]. The most selective noble metal for the catalytic reduction of nitrite to nitrogen with hydrogen was found to be palladium [3–9], although other noble metals as Rh, Ir, Ru and Pt also showed some activity [10,11]. As second metallic component in the bimetallic catalysts various combinations of Pd with Ag, Fe, Hg, Ni, Cu, Zn, Sn and In had been tested [4,12,13], obtaining the best results when Cu, Sn or In were the second metallic component. The disadvantage of this process is the undesired formation of ammonia and further improvements of these catalytic systems are necessary to optimise their performance. These studies were mainly focused in the composition and preparation of the catalysts [11–15] and their application on various reactor types [16–18]. Another possibility was described by Prüse et al. [19] using formic acid instead of hydrogen as a reductant agent. Formic acid has an in situ buffering effect, that lowers the pH gradient leading to a higher selectivity to nitrogen [15]. It has to be also considered the effect of the presence of other ions in the solution, being the most important the inhibiting effect of hydrogencarbonates, probably due to a competitive adsorption of these species on Pd-Cu active sites [6]. An additional factor that may strongly influence the properties of the catalyst is the type of support used, but only a few works studying this possibility have been published [13,16,20]. In this work, we have studied the possibilities of using hydrotalcites as active materials for supporting Cu and Pd metals. Hydrotalcites are layered double hydroxides (LDH) that can be represented by: M(II)1−y M(III)y (OH)2 ]y+ [(Xy/n n− ) · mH2 O]y− , where y is typically between 0.25 and 0.33, and Xn − is a n-valent anion. These compounds present positively charged brucite-like layers, Mg(OH)2 , with trivalent cations substituting divalent cations in octahedral sites of the hydroxide sheet. A wide range of derivatives containing various combinations of M2+ , M3+ and An − ions can be synthesised [21–23]. They are thermally stable at temperatures up to 673 K, but they transform into mixed metal oxides at higher temperatures, recovering its initial structure when they are again hydrated. Therefore, calcined hydrotalcites can be a very interesting support material for the liquid phase removal
of nitrates, especially considering that when recovering the hydrotalcite structure, NO3 − anions could be in the interlayer space compensating the positively charged layers. It has to be also considered that it is possible to synthesise a hydrotalcite with copper partially substituting magnesium that will result in a very high dispersion of the catalyst copper sites [24]. In this work, we have studied the possibilities of using Mg:Al hydrotalcites with different Mg:Al ratio as active materials for supporting Cu and Pd metals. The activity of these catalysts has been compared with the activity of Cu/Pd catalysts supported on alumina and with the activity of a palladium wet impregnated hydrotalcite with copper in its structure. The oxidation state of Cu and Pd was analysed by XPS. 2. Experimental 2.1. Catalyst preparation The gamma alumina and the hydrotalcites used as support in the nitrate reduction experiments were supplied by Guilder and by Intercat, respectively. The catalysts were prepared by a standard impregnation method starting from Pd(NO3 )2 ·2H2 O and Cu(CH3 COO)2 ·4H2 O aqueous solutions. This wet impregnation sequence was followed: (i) impregnation by copper acetate; (ii) drying in an oven at 423 K and calcination in air for 1 h at 773 K; (iii) deposition of palladium nitrate followed by drying at 423 K. Finally, the resulting solid was calcined for 1 h at 773 K and then reduced in hydrogen for 4 h at the same temperature. Hydrotalcites with copper in its structure were prepared by a standard co-precipitating procedure using two solutions. The first solution contained Mg(NO3 )2 · 6H2 O, Al(NO3 )3 ·9H2 O and Cu(NO3 )2 ·5H2 O, having a (Al + Mg + Cu) molar concentration of 1.5. The second solution contained NaOH and Na2 CO3 in adequate concentration to obtain total precipitation of aluminium, magnesium and copper in the former solution and to fix the pH at a value of 13. Both solutions were added, while vigorously stirring, at a total flow-rate of 30 cm3 /h, for 4 h. The gel was aged under autogenous pressure conditions at 333 K for 14 h, then filtered and washed with distilled water until the pH was 7 and carbonate was not detected
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in the filtrate. The hydrotalcite was calcined at 823 K in air for 9 h before reaction, obtaining a Cu/Mg/Al mixed oxide. Over this material palladium was impregnated as it is described above (from step (iii)). In all the catalysts Pd, Cu, Mg and Al contents were determined by atomic absorption spectroscopy. 2.2. Catalyst characterisation The surface areas of the catalysts were obtained in an ASAP 2000 apparatus, using the BET method from the nitrogen adsorption isotherms at 77 K. X-ray diffraction (XRD) patterns were collected using a Philips X’Pert PW3719 diffractometer (Cu K␣ radiation and graphite monochromator), provided with a variable divergence slit and working in the fixed irradiated area mode. The X-ray photoelectron spectra were acquired with a surface analysis system (Escalab-210, VG-Scientific) by using the Al K␣ (1486.6 eV) radiation of a twin anode in the constant analyser energy mode, with a pass energy of 50 eV. During the spectral acquisition the pressure of the analysis chamber was maintained at 5 × 10−8 N m−2 . All samples are insulators with a very small amount of carbonaceous impurity on their surfaces. The charging effect was corrected by setting the C1s transition at 284.6 eV. The reproducibility of the peak position was ±0.1 eV. To avoid the X-ray induced reduction of Cu2+ to Cu1+ , samples were maintained at 173 K during acquisition and the X-ray power was limited to 200 W (20 mA to 10 kV). The spectral acquisition time was also reduced to a minimum to prevent damage of the sample and the possible reduction of Cu2+ . 2.3. Catalytic test The catalysts were used for batch experiments in a 1 l glass reactor equipped with a mechanical Teflon stirrer. The N2 and H2 flows were introduced into the vessel below the impeller and their flow controlled by an electronic mass flow controller. In a typical run, 0.85 g of the powder (average diameter 0.104 mm) catalyst previously reduced with hydrogen at 773 K, was charged into the reactor containing 0.6 l of distilled water. The content of the reactor was purged with hydrogen during 1 h. The reaction starts with the addition of 5 cm3 of a concentrated KNO3 solution,
5
in order to achieve a total concentration of 90 mg/l inside the reactor. During the reaction a hydrogen flow of 500 cm3 /min was fed by a glass frit into the solution. The experiments were carried out at 293 K and a total pressure of 1 bar, which was equal to the hydrogen pressure. The reactions were carried out using a stirring rate of 500 rpm. The reaction progress was followed by taking small samples from the vessel for the determination of nitrate, nitrite and ammonium concentration at defined periods. The concentrations of NO3 − , NO2 − and NH4 + in the aqueous-phase samples were determined by UV-Vis spectroscopy (Shimadzu double-beam spectrophotometer, model UV-2101 PC) combined with reagent kits for photometric analysis (Merck—Spectroquant® , with concentration ranges for NO3 − , NO2 − and NH4 + of 1–90, 0.03–3.0 and 0.03–3.0 mg/l, respectively). 3. Results and discussion 3.1. Characterisation The chemical composition and surface areas of the catalysts after calcination in air are comparatively shown in Table 1. It can be seen that all the catalysts tested have surface areas up to 240 m2 /g. This surface area is comparable to that of the metals supported on alumina, then the different activity of the samples cannot be attributed to this support characteristic. Also from this table it can be conclude that the different Mg:Al ratio, copper content or catalyst preparation method does not result in a substantial difference of the catalyst surface area. The XRD patterns of the hydrotalcite supported Pd/Cu sample after calcination in air and reduction with hydrogen at 773 K is shown in Fig. 1(a). It is observed the formation of magnesium oxide poorly crystalline with peaks at 2θ = 36.9, 43.0 and 62.5◦ . These peaks are shifted to higher angles if compared with a pure magnesium oxide. This is consequence of the incorporation of aluminium in the framework of the MgO [25], resulting in the formation of a Mg/Al mixed oxide. The presence of Pd metal is also observed, indicated by the peaks at 2θ = 40.1, 46.7 and 68.3◦ . Due to the low copper content no peak related with it can be observed. After reaction (Fig. 1(b)) a substantial change is observed in the XRD spectra. As
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Table 1 Chemical composition and textural characteristics of the samples studied Sample
Cu (wt.%)
Pd (wt.%)
Support (Mg:Al ratio)
Surface area (m2 /g)
Cu/Pd-Al2 O3 Cu/Pd-HT1 Cu/Pd-HT2 Cu/Pd-HT3 Pd-HTCu1 Pd-HTCu2 Pd-HTCu3 Pd-HTCu4
1.5 1.6 1.6 1.5 1.5 1.5 1.4 7.1
5.1 5.0 5.2 5.0 4.9 4.8 4.9 5.5
␥-Alumina Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite Hydrotalcite
195 200 235 242 – 218 – 195
(Cu (Cu (Cu (Cu
in in in in
structure) structure) structure) structure)
it can be expected, the hydrotalcite structure has been recovered due to the rehydration of the mixed oxides and its characteristic peaks appear at 2θ = 11.2, 22.3, 34.5 and 60.2◦ . The peaks of Pd metal are still observed at 2θ = 40.1, 46.7 and 68.3◦ , indicating that the metal has been only partially oxidised. No peaks related with copper species have been observed. XPS was used to characterise the nature of surface copper and palladium present in the catalysts before and after reaction. Since the Cu2p(3/2) transition does
(4:1) (3:1) (2:1) (4:1) (3:1) (2:1) (3:1)
not allow to distinguish the oxidation states of copper by itself, these have been assigned using Cu2p(3/2) binding energies, the associated shake-up satellites and the kinetic energies of the CuL3 VV Auger transitions. The shake-up satellite peaks are final state effects which arise when the photoelectron imparts energy to another electron of the atom, ending up in a higher unoccupied state (shake-up). Due to this effect the photoelectron losses energy and appears at a higher binding energy in the XPS spectrum. This effect is most
Fig. 1. X-Ray diffraction patterns of hydrotalcite supported Pd/Cu catalyst (Cu/Pd-HT1): (a) after calcination in air and reduction with H2 ; (b) after reaction.
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Fig. 2. (A) Cu2p(3/2) photoelectron spectra of Pd-Cu-mixed oxides derived from hydrotalcite (with 7 wt.% Cu and 5 wt.% Pd): (a) reduced in hydrogen at 773 K; (b) after reaction. (B) Pd3d photoelectron spectra of Pd-Cu-mixed oxides derived from hydrotalcite (with 7 wt.% Cu and 5 wt.% Pd): (a) reduced in hydrogen at 773 K; (b) after reaction.
pronounced in metals with d-orbitals, being clearly observed in copper. In the case of copper, the final state effects are only observed in Cu(II) species and for this reason this characteristic can be used to identify Cu(II) species. The determination of Cu(I) or Cu metal is more difficult and requires a careful analysis. Fig. 2A shows the Cu2p(3/2) spectra obtained for the same catalyst after different treatment conditions. After reduction at 773 K (Fig. 2A (a)), a maximum at ca. 933.1 eV was observed, which, along with the satellite peak at 942.6 eV, is characteristic of Cu2+ species [24,26–28]. This is supported by the CuL3 VV spectrum (not shown), where the maximum at ca. 1151 eV (KE) is associated with Cu2+ [24,26–28]. Nevertheless, the low intensity of the satellite peak together with the presence of a shoulder at ca. 1149 eV (KE) in the CuL3 VV transition indicates the presence of Cu+ . Fig. 2B (a) illustrates the Pd3d spectrum obtained for the same catalyst after reduction in
hydrogen at 773 K. As it can be seen, the presence of a peak at ca. 335.2 eV (Pd3d5/2 ) indicates that Pd is in the metallic form [24,26–28]. After reaction the binding and kinetic energies corresponding to copper did not experience substantial changes and only the intensity corresponding to the satellite peak associated to Cu2p(3/2) transition increased slightly (see Fig. 2A (b)). These results should be associated with the increase of the Cu2+ level on the catalysts surface. After reaction, two components (at ca. 337.3 and 335.3 eV) appear generating the peak shape of the Pd3d(5/2) transition, Fig. 2B (b), indicating the presence of oxidised and metallic Pd, respectively. As the dissolution of the catalyst components is the most critical problem in this system, the amount of Pd, Cu, Mg and Al present in the solution after reaction was measured. It was proved that no metal was present in the solution, indicating that in these
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conditions the metals do no dissolve and it does no suppose any problem for this system. 3.2. Reaction studies The activity and selectivity for the hydrogenation of nitrates with a hydrotalcite supported Pd/Cu catalyst
was studied and compared with that of an alumina supported Pd/Cu catalyst. In Fig. 3, it is shown a typical course of the changes of nitrate, nitrite and ammonium ion concentrations during a batch experiment. As it can be seen the activity of the metals is quite different depending on the support. At these reaction conditions, the reaction velocity and the final
Fig. 3. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for: (䊉) Cu/Pd-Al2 O3 and (䊏) Cu/Pd-Hydrotalcite (Cu/Pd-HT1) catalysts. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 −[NO3 − ]t )/[NO3 − ]t=0 , for: (䊉) Cu/Pd-Al2 O3 and (䊏) Cu/Pd-Hydrotalcite (Cu/Pd-HT1) catalysts.
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conversion obtained when Pd and Cu are supported on hydrotalcites is much higher than those obtained when the metals are supported on alumina. The presence of nitrites and ammonium was also observed. Nitrite is considered in some studies as a primary intermediate,
9
while ammonium is formed as a final product via a consecutive-parallel reaction pathway [15,17]. The nitrite conversion with the catalyst supported on alumina does not reach the maximum, usually obtained when the nitrate conversion is around 80%, after 2 h
Fig. 4. (A) NO3 − concentration profiles as a function of time for (䊊) Cu/Pd supported on alumina, Cu/Pd supported on various hydrotalcites: (䊐) HT1-Mg:Al, 4:1; (䉭) HT2-Mg:Al, 3:1; (䉫) HT3-Mg:Al, 2:1 and (∗) Cu/Pd supported on magnesium oxide. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for (䊊) Cu/Pd supported on alumina, Cu/Pd supported on various hydrotalcites: (䊐) HT1-Mg:Al, 4:1; (䉭) HT2-Mg:Al, 3:1; (䉫) HT3-Mg:Al, 2:1 and (∗) Cu/Pd supported on magnesium oxide.
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reaction. On the other hand this maximum is quickly achieved when Pd/Cu is supported on hydrotalcite. The two samples show the formation of ammonium, but the production of ammonium with Pd/Cu supported on hydrotalcite is much lower than that obtained when they are supported on alumina (Fig. 3B).
Since these two catalysts have the same Cu and Pd content and a non-significant influence of the surface area is observed, the different behaviour of the materials should be related with a possible support catalytic activity or with a support promoting effect on the metals. In order to check this, a hydrotalcite with the same
Fig. 5. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for Cu/Pd hydrotalcite catalysts: (䊉) Cu/Pd-HT1 with copper and palladium impregnated on the hydrotalcite and (䊏) Pd-HTCu1 with copper in the hydrotalcite structure. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for Cu/Pd hydrotalcite catalysts: (䊉) Cu/Pd-HT1 with copper and palladium impregnated on the hydrotalcite and (䊏) Pd-HTCu1 with copper in the hydrotalcite structure.
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chemical composition but with no metal supported on it was tested at the same reaction conditions obtaining no conversion in the removal of nitrates. This shows the absence of a hydrotalcite catalytic behaviour. Then, this promoting effect should be related with the
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structure or with the electronegativity of the hydrotalcite that is influencing the Pd and Cu active sites. To clarify these aspects, Pd/Cu catalysts supported on different hydrotalcites with different Mg:Al contents were compared and were compared with a Pd/Cu
Fig. 6. (A) NO3 − (—) and NO2 − (- - -) concentration profiles as a function of time for Cu/Pd hydrotalcite catalysts with different copper content: (䊉) 1.5% Cu weight Cu/Pd-HT1 and (䊏) 7.1% Cu weight Pd-HTCu4. (B) Ammonium ion concentration as a function of nitrate conversion, defined as ([NO3 − ]t=0 − [NO3 − ]t )/[NO3 − ]t=0 , for Cu/Pd hydrotalcite catalysts with different copper content: (䊉) 1.5% Cu weight Cu/Pd-HT and (䊏) 7.1% Cu weight Pd-HTCu.
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catalyst supported on a MgO. The different Mg:Al content represents different Sanderson electronegativity, being the enriched magnesium samples the most basic materials. In Fig. 4 it can be observed the results obtained with these samples. There is some difference between the activity of the different hydrotalcites, suggesting that the support with the highest Mg:Al ratio is the most active and the one which produces less ammonium in this reaction. But this highest activity cannot be explained only in terms of electronegativity, because there are substantial differences between the catalysts supported on the hydrotalcites and the catalysts supported on MgO or Al2 O3 . Pd/Cu-MgO shows a very low activity for this reaction, probably due to severe transport limitations of NO3 − and NO2 − caused by the partial negative charge of this basic oxide [9,18]. This is not occurring with the hydrotalcites because despite being also basic materials, calcined hydrotalcites recover its structure after rehydration forming a LDH with a partial positive charge. In this structure, nitrate anions are located in the interlayer space compensating the partial positive charge and solving the problems associated to mass transfer limitations. It is well known [24] that Cu-substituted hydrotalcite can be synthesised by the isomorphical replacement of the magnesium ions by copper ions. This could be very interesting in order to obtain an adequate dispersion of copper in the catalyst. In order to check this some samples with copper incorporated in the structure of the hydrotalcite were synthesised and impregnated with palladium. When these samples were tested for the catalytic reduction of nitrates, (Fig. 5) it can be observed a higher velocity of reaction and that it is more important a less ammonium production than with the hydrotalcites with copper and palladium impregnated on them or impregnated on an alumina support. This has to be clearly related with the best dispersion of copper on this material. In order to compare the catalytic results with those obtained in the XPS analysis, where a higher copper concentration (around 7 wt.%) is necessary for an adequate analysis of the samples, the activity of a catalyst with a higher copper content was studied. The results are shown in Fig. 6. It shows that an increase on the copper concentration, enhance the initial activity of the catalyst, although it also enhances the production of ammonium that is not desired. This can
be explained by the low accessibility of palladium sites when the copper amount is too high [11].
4. Conclusions The results from this work show that hydrotalcite supported Cu/Pd catalysts are a very interesting material for the liquid phase hydrogenation of nitrates. These samples present more activity on the removal of nitrates and less production of ammonium than when Pd/Cu are supported on alumina. These results can be related with the characteristics of the hydrotalcite which, after calcination, regenerates its structure when it is in contact with water, forming a LDH with a partial positive charge. In this structure nitrate can be located in the interlayer space compensating the partial positive charge, reducing the mass transfer problems. For this reason the activity and selectivity of the catalyst increases, especially if the hydrotalcite is synthesised with copper in its structure that results in a higher dispersion of the copper active sites. These results show that not only the oxidation state of the metal but the support has a very important influence on the activity of the metals supported on it. The XPS results show that these metals are Pd(0) and Cu(II) or Cu(I) that are partially oxidised in the course of the reaction, but further experiments have to be made in order to know the exact nature of the metals supported on the hydrotalcite.
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