Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd Cu catalysts

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1 See discussions, stats, and author profiles for this publication at: Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd Cu catalysts Article in Applied Catalysis B Environmental February 2000 DOI: /S (99) CITATIONS 117 READS authors, including: Francesca Deganello Italian National Research Council 39 PUBLICATIONS 472 CITATIONS Leonarda Liotta Italian National Research Council 140 PUBLICATIONS 3,330 CITATIONS SEE PROFILE SEE PROFILE Anna Maria Venezia Italian National Research Council 148 PUBLICATIONS 4,153 CITATIONS SEE PROFILE Available from: Francesca Deganello Retrieved on: 17 September 2016

2 Applied Catalysis B: Environmental 24 (2000) Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd Cu catalysts F. Deganello a, L.F. Liotta a, A. Macaluso b, A.M. Venezia a, G. Deganello a,b, a ICTPN-CNR, via Ugo La Malfa 153, I Palermo, Italy b Dipartimento di Chimica Inorganica, Università di Palermo, Viale delle Scienze, I Palermo, Italy Received 20 March 1999; received in revised form 14 September 1999; accepted 19 September 1999 Abstract Two series of pumice-supported palladium and palladium copper catalysts, prepared by impregnation with different palladium and copper precursors, were tested for the hydrogenation of aqueous nitrate and nitrite solutions. Measurements were performed in a stirred tank reactor, operating in batch conditions, in buffered water solution at atmospheric pressure and at 293 K. The activities of the catalysts were calculated in terms of nitrate and/or nitrite removal. With the monometallic Pd/pumice, the reduction of nitrite is highly selective; only 0.2% of the initial nitrite content is converted to ammonium ions. The activity in terms of turn over frequency (TOF) is higher as compared to a catalyst of Pd on silica. Addition of copper to the palladium catalyst is essential for the reduction of nitrates, although it decreases the nitrite reduction activity and increases the production of ammonium ions. Nitrate reduction appears to be structure-insensitive and a volcano-type dependence of the activity versus the overall Cu atomic weight percentage is observed for the two series of catalysts Elsevier Science B.V. All rights reserved. Keywords: Catalytic reduction; Nitrite; Nitrate; Turn over frequency 1. Introduction The increasing nitrate concentration in the ground water has stimulated intensive research on the denitrification of drinking water because nitrate causes a serious health risk. The toxicity of nitrates for human beings is due to the body s reduction of nitrate to nitrite, responsible for the blue baby syndrome and a precursor to carcinogenic nitrosamines as well as to other N-nitroso compounds [1]. For these reasons, the Corresponding author. Tel.: ; fax: address: deganello@ictpn.pa.cnr.it (G. Deganello). European Community limits nitrate and nitrite concentrations in drinking water to 50 mg/l and 0.1 mg/l, respectively. The metal-catalysed reduction of nitrates and nitrites is an alternative route to the use of ionic exchange and biological means by microorganisms [2,3]. Several arguments favour the catalytic process: (i) the possibility of a single operation, (ii) the specificity of the catalytic hydrogenation which reduces interference with other compounds present in natural water; and (iii) the absence of any additive which can introduce secondary contamination [4]. A drawback of the catalysed hydrogenation reaction is represented by dissolved ammonia, which is formed /00/$ see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S (99)

3 266 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) from the reaction along with gaseous nitrogen. The permitted level of ammonia in drinking water is indeed 0.5 mg/l [5]. The catalytic system most used for the nitrates and nitrites reduction is represented by Pd Cu-supported on silica or alumina [5 9]. By employing a Pd Cu/ -Al 2 O 3, Pintar et al. [8,9] were able to convert nitrate to nitrogen with a selectivity equal to 91%. Such achievement although quite good is not completely satisfactory due to a high production of ammonia. Relatively few investigations have been published on the reduction of nitrites in aqueous solutions. It is reported by Hörold et al. [5,6] that among several noble metals capable of promoting nitrite reduction, only the supported Pd catalysts give satisfactory performances. Furthermore, it was found that the nitrite removal activity, as well as the formation of ammonia, strongly depend on the ph value of the aqueous solution [6]. Recently, Pintar et al. [10] reported a detailed kinetic study of the reduction of nitrites in aqueous solutions over Pd and Pd Cu/ Al 2 O 3 catalysts. The effects of copper addition on the activity and selectivity of palladium catalysts and the influence of nitrates on the formation of ammonium ions in the course of catalytic nitrite reduction were also investigated [10]. Our recent results on the use of pumice as support for metallic catalysts had shown a positive influence of the particular carrier on the catalytic activity of the supported metals in the hydrogenation reaction of unsaturated hydrocarbons [11 17]. The alkali metal ions in the framework of the support were considered responsible for this peculiar behaviour. According to XPS and Auger analysis [13,14] such ions would increase the electron density on the metal. The increased negative charge would limit the strength of adsorption of electron-rich substrates at very high metal dispersion favouring the subsequent hydrogenation step. On the basis of these results and considering that the nitrates and nitrites are electron-rich species, it was considered interesting to support Pd Cu on pumice and use the catalysts for the reduction of these substrates in a water solution. Different metal precursors were used in the impregnation procedures for the catalyst preparation. 2. Experimental 2.1. Catalysts preparation and characterization Pumice, characterised by a surface area of 5 m 2 /g, extracted from the caves of Lipari, was treated with boiling 25% HNO 3 [18], before being used as a support. The monometallic catalyst, containing 0.75 wt% of palladium, was prepared by wet-impregnating the pumice support with a benzene solution of palladium acetylacetonate [Pd(C 5 H 7 O 2 ) 2 ] according to a modified literature procedure [19]. After impregnation, the solvent was eliminated under vacuum in a rotating evaporator and the resulting material was dried in an oven at 353 K, calcined and reduced at 623 K for 6 h. The obtained catalyst is labelled as Pd(A). Portions of this Pd catalyst were subsequently impregnated with aqueous solutions of Cu(acetate) 2 at various concentrations [20]. The resulting samples were dried at 353 K overnight, calcined and reduced at 623 K for 6 h. They are labelled Cat A, Cat B, Cat C. Another set of Pd Cu pumice catalysts was obtained by co-impregnation of the support with aqueous solution of Pd(NH 3 ) 4 (NO 3 ) 2 and Cu(NO 3 ) 2 at different concentrations. After impregnation the solvent was removed under vacuum and the catalysts were dried at 353 K, then calcined and reduced at 623 K for 6 h. These samples are labelled Cat 1, Cat 2, Cat 3. For reasons of comparison a catalyst of 1.0 wt% Pd on silica (600 m 2 /g) was also prepared using the palladium nitrate precursor. The palladium and copper contents of all samples were determined by Atomic Absorption Spectroscopy with an estimated precision of ±10%. All the catalysts previously mentioned are listed in Table 2. The X-ray diffraction (XRD) measurements, performed with a Philips X-ray diffractometer using nickel-filtered Cu K radiation, allowed to obtain the crystallite sizes of the metal phase. The metal particle sizes of the reduced catalysts were estimated as volume-average crystallite dimension, through the line-broadening (LB) of the available reflection peaks, using the Scherrer equation according to the method reported in literature [21]. Estimated errors on the particles sizes are of the order of 10%.

4 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) Catalytic tests The catalytic reductions of nitrates and/or nitrites were studied in a pyrex glass reactor, equipped with a mechanical stirrer (Heidolph), working in batch conditions [22]. The reactor was connected to a vacuum line and to a gas adsorption system operating at total pressure of 1 atm. The gas mixture H 2 /CO 2 previously stabilised, was admitted in the bulk of the solution through the holes in the bar of the stirrer. The gas flow rate of 400 ml/min was well above the hydrogen limited condition, ensuring independence of activity and selectivity from this rate. The reactions were carried out using a stirring rate of 2500 rpm, which allowed conditions of chemical regime, by eliminating any problem of gas diffusion [23]. Before the catalytic runs the catalysts were purged with nitrogen and then with hydrogen to remove any adsorbed oxygen. The extent of the reaction was monitored by withdrawing samples periodically and analysing them for nitrate and nitrite concentration after removing the catalyst by centrifugation. The reaction conditions used in this work are listed in Table 1. The solutions of nitrates and nitrites were prepared from reagent-grade KNO 3 and NaNO 2. The temperature of the reaction mixture was set at 293 K, controlled within ±0.1 K. The ph value of the aqueous solution, determined with a digital ph meter (Crison 2002), was kept constant at a value of 5.5 ± 0.1, by means of CO 2. De-ionized water was the reaction medium for the nitrate and nitrite reductions. The apparent activation energy was also calculated for the catalyst Cat A in the temperature range K using the Arrhenius equation. Table 1 Operating conditions Reaction temperature 293 K Total operating pressure 1 atm H 2 /CO 2 flow rate 400 ml/min CO 2 (% vol.) in the gas mixture 13% ph value of the aqueous solution 5.5 ± 0.1 Reaction volume 200 ml Initial nitrate concentration 100 mg/l initial nitrite concentration 60 mg/l N total /Pd molar ratio Analysis Concentration of nitrate and nitrite ions was determined by the HPLC method, employing an anionic column (Sarasep AN1), and two detectors in series (UV and conductivity). A solution of NaHCO mm and Na 2 CO mm was used as a mobile phase, the flow rate was set at 2 ml/min and the temperature of the column was maintained at 301 K. After the complete removal of nitrate and nitrite ions in aqueous solution, the final concentration of ammonium ions was determined by using HPLC, with a cationic column. A solution of acid methanesulfonic 20 mm was used as a mobile phase, the flow rate was equal to 1 ml/min. The reproducibility of this type of measurements was higher than 99%. 3. Results and discussion All the catalysts with weight percentages, the size of monometallic Pd and Cu and of the Pd Cu alloyed particles with the correspondent atomic composition as derived from X-ray diffraction experiments [24] are reported in Table 2. For the catalysts Cat 2 and Cat 3, along with monometallic particles of Cu and Pd, two alloy phases of different composition and differ- Table 2 Weight percentage (%) of Pd and Cu with the corresponding Cu atomic percentages in parentheses, particle sizes of the corresponding metallic phases and at% of Cu in the alloy phase for all catalysts Catalyst % Pd % Cu (at% Cu) d (Å) Pd d (Å) Cu at% Cu alloy d (Å) alloy Pd/SiO Pd (A) Cat A (34) Cat B (52) Cat C (76) Cat (14) Cat (43) Cat (62)

5 268 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) ent particle sizes are found. As indicated in the table, the presence of the alloy does not seem to be related to the preparation procedure. Overall the method of preparation from nitrates produces larger Pd Cu particles. The reduction of nitrates and nitrites to nitrogen or ammonia involves a quite complicated stepwise process. In a recently proposed mechanism NO is considered a key intermediate on the metal surface [25]. It may react with adsorbed hydrogen giving rise to NH-adsorbed species which may lead to NH 4 + by stepwise addition of hydrogen or to N 2 by reaction with another NH species [25]. A general scheme for the formation of the gaseous nitrogen and of ammonium ion would be the following [6]: 2NO 3 + 5H 2 4H 2 O + N 2 + 2OH (1) NO 2 + 3H 2 NH OH (2) NO 2 is a measurable intermediate species during the reduction of nitrates. The electroneutrality of the aqueous phase is maintained by the production of hydroxide ions. Preliminary experiments were carried out to see the influence of the initial ph in the reduction of nitrites in the presence of the Pd/pumice catalyst. As already observed the increase in the concentration of OH groups produced a decrease in the activity and selectivity to N 2 [10,26]. After a preliminary test at various initial ph values, the best catalytic results were obtained with solutions at ph = 5.5, kept constant with an acetate buffer in molar ratio [buffer]/[nitrite] = 5.3. Similar rates were also obtained by adding carbon dioxide to the hydrogen flow. The effect of CO 2 as a diluting agent of H 2 has already been described to enhance the activity of nitrate reduction due to the neutralization of the formed hydroxide [26,27]. All of the following experiments on nitrite and nitrate reductions were carried out using CO 2. The monometallic Pd/pumice sample prepared by the Pd(acac) 2 precursor was tested in the reduction of nitrates and of mixtures of nitrates and nitrites. In both cases no nitrate conversion is observed, nor does the presence of nitrate influence nitrite reduction over Pd sites. Based on these findings, it can be concluded that nitrate is neither adsorbed nor activated on Pd alone, which is in good agreement with other results [6,7,10]. The reduction of nitrites was studied on the two series of Pd Cu/pumice catalysts; Cat A, Cat B and Cat C, prepared from Pd(acac) 2 and Cu(II)acetate, and Cat 1, Cat 2 prepared from Pd(NH 3 ) 4 (NO 3 ) 2 and Cu(NO 3 ) 2. In Fig. 1 the variation of the nitrite concentration with time during the catalytic hydrogenation of aqueous nitrite solutions is shown for the various catalysts, at a constant ph value of 5.5. The corresponding specific activities calculated from the initial rates versus the atomic percentage of copper are reported in Fig. 2. By taking into consideration the dispersion of the catalysts obtained from the particle sizes of the Pd metal present as monometallic and as alloyed particles, listed in Table 2, the TOF were calculated and reported in Fig. 3 as function of the copper atomic percentage. For reasons of comparison the activities of the monometallic Pd(A) on pumice and Pd/SiO 2, prepared from Pd(NH 3 ) 4 (NO 3 ) 2 are also included. Both figures indicate that the reduction of nitrites decreases progressively with copper increase, according to a previous finding on the detrimental effects of copper on palladium nitrite reduction [10]. It should be noted that in terms of specific activity the monometallic Pd/SiO 2 is superior to the catalyst on pumice Pd(A); however, if the metal surface sites are considered the activity expressed as TOF is higher for the pumice catalyst. Although the XRD-derived dispersion may not account for the very small particles not detectable by the technique, previous study on monometallic catalyst of Pd supported on pumice had shown a good agreement between the dispersion arisen from the hydrogen chemisorption and XRD experiments [18]. The superior activity of the monometallic Pd(A) on pumice as compared to the Pd/SiO 2 could be due to the influence of pumice which enhances the electron density on the supported palladium [15] and decreases the strength of interaction with nitrites facilitating their release after activation. With the monometallic Pd/pumice, the reduction of nitrite is highly selective; only 0.2% of the initial nitrite content is converted to ammonium ions, in good agreement with literature results [5,8,10]. Assuming the complete conversion of nitrites to N 2 and NH 4 +, the selectivity to N 2 of Pd Cu bimetallic catalysts is of the order of 90 92%, comparable with that found previously over Pd Cu/ Al 2 O 3 catalysts in a similar condition of ph control [10]. The progressive decrease in the activity of Pd Cu/pumice catalysts

6 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) Fig. 1. Variation of nitrite ions concentration (mg/l) with time over different Pd Cu/pumice catalysts, in the presence of a mixture H 2 /CO 2, at constant solution ph value of 5.5. Fig. 2. Dependence of the initial nitrite removal catalytic activity [mg NO 2 /min g Pd)] on the atomic percentage of Cu in the metallic phase. with the increasing copper content in the metal phase, is probably due to the segregation of copper at the surface of the active palladium sites. Indeed such effect was previously determined by XPS analysis [24] on similar systems. On the basis of previous experimental data [24], the decrease of activity and selectivity in the nitrite reduction upon addition of copper could be due to geometrical effects. The presence of copper could dilute the Pd surface sites needed for the activation of the nitrites [6,7] and for the formation of N 2. The formation of a bulk alloyed phase Pd Cu, does not seem to be important. The reduction of nitrates was studied on the same Pd Cu/pumice catalysts. In Fig. 4 the variations of the nitrates and nitrites concentration with time over

7 270 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) Fig. 3. TOF for nitrite reduction vs. atomic percentage of Cu in metallic phase. The TOF values were calculated with respect to all Pd sites present as metal and as alloyed phase. Fig. 4. Variations of nitrate and nitrite ions concentration (mg/l) with time during hydrogenation of nitrate over Cat 2 in aqueous solution at constant ph value of 5.5, by using a H 2 /CO 2 gas mixture. catalyst Cat 2 are plotted. As appears from the figure, the variation of the nitrite ion concentration with time is typical of consecutive reactions. The rate equation derived recently from a Langmuir Hinshelwood mechanism described in literature [7] did not fit the experimental data of Fig. 4. As previously indicated, due to the presence of HCO 3 in the aqueous solution the proposed L H kinetic model was not valid because of the competitive adsorption between HCO 3 and NO 3 anions on the same Pd Cu active sites [25]. The dependence of the initial nitrate removal activity on the atomic percentage of copper in the metal

8 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) Fig. 5. (a) Dependence of the initial nitrate removal catalytic activity [mg NO 3 /min g Pd)] on the atomic percentage of Cu, for the two series of Pd Cu catalysts (Cat A C and Cat 1 3). (b) Dependence of TOF for nitrate reduction on overall atomic percentage of Cu. The TOF values were calculated with respect to all Pd sites present as metal and as alloyed phase.

9 272 F. Deganello et al. / Applied Catalysis B: Environmental 24 (2000) Table 3 Final ammonium concentration and rate constant ratio of NO 2 and NO 3 reduction on Pd Cu/pumice samples at different at% Cu Catalyst at% Cu NH + 4 (mg/l) Rate constant ratio (NO2 /NO 3 ) Cat Cat A Cat Cat B Cat n.d. a Cat C 76 n.a. b n.a. b a Not determined. b Not available due to the lack of activity. phase for the two series of Pd Cu catalysts, (Cat A, B, C and Cat 1, 2, 3) is illustrated in Fig. 5a. Two volcano curves are obtained. The series, Cat A C, prepared from Pd(acac) 2 and Cu(II)acetate, is found to be more active than the series Cat 1 3, prepared from nitrates. The activities expressed as TOF, using the dispersions of palladium present as monometallic and as alloyed particles, follow the same volcano curve, as shown in Fig. 5b. Even considering the uncertainty on the dispersion derived from the XRD measurements and used to calculate the TOFs, the results, in accord with a previous study [26] suggest that the reduction of nitrates on Pd Cu catalysts is structure-insensitive inasmuch as it does not depend on the size of the particles. A distinct dependence on the overall atomic percentage of copper is found. The volcano curve indicates that the amount of copper is quite critical and there is a maximum, located between 30 and 40 at%, beyond which the activity drastically decreases. A maximum at 30 at% of Cu was previously found for Pd Cu catalysts supported on silica and alumina [5]. The present results of the decreased nitrate reduction activity with high copper content can be explained if we assume that in the pumice-supported catalysts nitrite reduction occurs preferentially on palladium sites and the nitrate reduction occurs on Pd Cu sites. Therefore one can speculate that a copper loading higher than 40 at% causes a decreased nitrate removal rate due to the presence of unreacted nitrite ions adsorbed on the Pd Cu sites. In Table 3 the production of ammonium ions, calculated when all the nitrates and nitrites are reduced, are reported for selected samples. The ratios of the initial reaction rates calculated for the reduction of nitrites and nitrates from independent experiments are also listed. The high value, found for Cat 1 is related to the very low activity in the reduction of nitrates owing to insufficient copper in the metal phase. In this case the presence of nitrites in the solution is very low since they are immediately reduced by the catalyst with high content of palladium on the surface and they tend to form nitrogen due to the presence of contiguous Pd sites. As appears from Table 3, there is a direct relationship between the reaction rate ratio and the production of ammonium ions. The production of ammonium ions is lower for the catalysts with a higher nitrite reduction rate. The apparent activation energy for the catalytic liquid phase nitrate reduction on Cat A was calculated in the temperature range K. From the Arrhenius plot of the initial rate constant versus temperature a value of 46.4 kj/mol, very close to that obtained over a Pd Cu/ -Al 2 O 3 catalyst [7], was found. 4. Conclusions The results of this study indicate that pumice is a suitable support for palladium catalysts to be used in the hydrogenation of nitrates and nitrites. The electronic effects of this particular support may contribute to the higher activity of the supported palladium, in the hydrogenation of nitrite, as compared to a silica-supported palladium catalyst. The addition of copper is detrimental for the reduction of nitrites since it decreases the activity and increases the production of ammonium ions. Like all the other palladium catalysts reported so far, Pd on pumice is inactive for the removal of nitrates. To ensure the reduction of nitrates the presence of a second metal, namely copper, is necessary. A maximum of nitrate reduction activity versus copper is found for Cat A with 34 at% of Cu giving a production of ammonium ions of 1.16 mg/l after complete removal of 100 mg/l of nitrate ions. The decrease of the nitrate reduction activity with higher copper content can be explained if we assume that in the pumice-supported catalysts nitrite reduction occurs preferentially on palladium sites and nitrate reduction occurs on Pd Cu sites. Since contiguous Pd sites are needed in order to obtain nitrogen molecules, the dilution of the palladium sites by copper also determines a decrease in nitrogen selectivity.

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