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Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water
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Water Research 39 (2005) 3073–3081 www.elsevier.com/locate/watres
Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water Anthony Garron, Florence Epron Laboratoire de Catalyse en Chimie Organique, UMR6503 CNRS—Universite´ de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 18 January 2005; received in revised form 10 May 2005; accepted 12 May 2005 Available online 27 June 2005
Abstract The reduction of nitrate in nitrogen using bimetallic palladium tin catalysts and hydrogen is an interesting process for water treatment. The aim of the present study is to use formic acid (FA) as a reducing agent and a pH buffer in order to substitute the mixture of hydrogen and carbon dioxide. The catalytic performances of a palladium tin catalyst supported on silica were evaluated in the presence of FA, as a function of the initial acid concentration and of the gas phase (N2, CO2, or H2). Results were compared to those obtained with hydrogen in the presence of carbon dioxide. Similar mechanisms seem to explain the identical catalytic performances observed with these two reducing agents. r 2005 Elsevier Ltd. All rights reserved. Keywords: Pd–Sn catalysts; Nitrate; Catalytic reduction; Drinking water; Formic acid
1. Introduction The concentration of nitrate in the soil, and subsequently in ground and surface water, for years has constantly been increasing all around the world. This compound is responsible for river and sea eutrophication. In drinking water, it is potentially harmful to human health (blue baby syndrome, cancer, etc.). Sources of nitrate in groundwater can be subdivided into four categories: (i) natural sources (ii) waste materials (iii) row crop agriculture and (iv) irrigated agriculture (Canter, 1996). Biological and physicochemical treatments allow effective removal of nitrates but have several economical and ecological disadvantages. Catalytic reactions conCorresponding author. Tel.: +33 5 49 45 48 32;
fax: +33 5 49 45 37 41. E-mail address: fl[email protected] (F. Epron).
stitute promising approaches for the destruction of pollutants in water. Since Tacke and Vorlop’s (1989) first paper on the use of palladium–copper bimetallic catalysts for nitrate reduction, numerous studies have been aiming at the development of suitable catalysts for the selective reduction of nitrate into nitrogen gas. Their application in various reactor types such as hollow fibres (Ha¨hnlein et al., 1998) and membrane reactors (Pintar et al., 2001) has been studied. However, few papers have been devoted to nitrate reduction in tap water (Pintar and Batista 1999). Palladium catalysts have proven to be the most active and selective for nitrite reduction. To reduce nitrate, it is necessary to activate the precious metal by adding a second metal (Pru¨sse et al., 1997). Copper was mostly used as a promoting second metal (Epron et al., 2001; Yoshinaga et al., 2002) but other suitable promoters, such as tin, indium or zinc have demonstrated interesting performances (Pru¨sse et al., 2000; Pru¨sse and Vorlop, 2001). The best promoter of
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this reaction in term of selectivity is tin (Vorlop and Pru¨sse, 1999). The reduction of nitrate leads to the formation of hydroxide ion that causes an unavoidable increase of the pH value up to 10; this pH gradient is unacceptable for drinking water. Furthermore, basic pH induces a polarization of the support and has a repulsive effect on nitrates and nitrites, inducing a decrease of the activity and the selectivity in nitrogen (Vorlop and Pru¨sse, 1999). Different solutions have been used to reduce the formation of hydroxides, (Pintar et al., 1998, 2001; Gautron et al., 2003; Roveda et al., 2003; Gavagnin et al., 2002; Palomares et al., 2004). The use of formic acid (FA) instead of hydrogen as reducing agent is one possibility (Vorlop and Pru¨sse, 1999; Pru¨sse and Vorlop, 2001). In this case, the FA decomposition can be used as a source of hydrogen and carbon dioxide. Indeed, on a noble metal FA can be easily decomposed according to the following reaction: HCOOH
Noble metal
!
H2 þ CO2 .
FA can be also used either for its acidic properties or as a reducing agent. Indeed, the use of this acid (pKA(HCOOH/HCOO) ¼ 3.74), allows one to work with an initial pH included between 3 and 4 in order to improve the catalytic performances of the catalyst. Its redox potential (E0(H2,CO2)/HCOOH) ¼ 0.199 V/ERH) allows nitrate to be reduced into nitrite. Recently, Coq et al. (2003) studied the use of FA in denitration of concentrated nitric acid over monometallic platinum supported on silica catalyst. The reaction mechanism proposed consisted of a redox reaction leading to the transformation of nitric acid in nitrous acid over passivated sites of platinum during the decomposition of FA in hydrogen and carbon dioxide. These authors have also observed a reduction of nitrous acid in the homogeneous phase. The intent of the present study is to gain further information on the mechanism of nitrate and nitrite reduction in the presence of FA. For this purpose, the catalytic performances obtained with FA in nitrate and nitrite reduction in distilled water will be compared with those obtained with a mixture of hydrogen and carbon dioxide. The catalyst used for this study is a PdSn catalyst supported on silica and prepared by controlled surface reaction. The characterization by Mo¨ssbauer spectroscopy of this bimetallic catalyst (Garron et al., 2005) has demonstrated that on the non-reduced catalyst, tin is only in an oxidized form with an oxidation degree of +IV as SnO2. On reduced catalyst tin is in a metallic state with around 20% corresponding to a solid solution (Pd(x43)Sn) and 80% to an alloy (Pd2Sn). All the tin is in interaction with palladium on the reduced catalyst.
2. Experimental Details of the preparation method and the characteristics of this catalyst are described elsewhere (Garron et al. (2005). 2.1. Catalyst preparation 2.1.1. Monometallic catalysts Monometallic catalysts were prepared by an impregnation method using aqueous solutions of the precursor salt: Pd(NH3)4(NO3)2. Monometallic catalysts Pd/SiO2 had a metal loading of 5 wt%. This loading was determined by Vorlop and Pru¨sse (1999) as optimal for palladium-based catalysts to an application of denitration in water. 2.1.2. Bimetallic catalysts Bimetallic catalysts were prepared by a controlled surface reaction in order to obtain great metal–metal interactions. The bimetallic catalyst has a specific weight ratio at 1.5 wt% in tin verified by elementary analysis. Generally, the experimental value gave an incertitude error not exceeding 5%. 2.2. Nitrate and nitrite reduction Nitrate and nitrite reduction reactions were performed in a semi-batch reactor, at atmospheric pressure and 25 1C. The catalyst (64 mg) can be reduced before reaction under hydrogen (130 1C; 0.5 h). During this time, 90mL of ultra pure water (18.2 MO) were purged with nitrogen. When the catalyst was stabilized in temperature, the ultra-pure water was added over the reduced catalyst. Then the reaction was performed under a mixture of 50% of hydrogen in carbon dioxide or in the presence of a known amount of FA acid under pure nitrogen, hydrogen or carbon dioxide. The CO2 or FA is used to buffer the solution at pH around 5. Afterwards, 10 mL of a solution (16 mmol L1) of nitrate (KNO3) or nitrite (KNO2) were introduced in the reactor to start the reaction. The catalyst dispersion in the aqueous medium was achieved by the gas flowing (flow rate ¼ 250 mL min1) through a porous glass located at the bottom of the reactor. It was checked that the resulting stirring is sufficient to ensure that the reaction is not rate-limited by the reactant diffusion. Representative aqueous samples were periodically withdrawn, separated by filtration and analyzed as described in Section 2.3. Catalysts were compared as a function of (i) their activity, corresponding to the disappearance rate of nitrate or nitrite, determined at 75% of nitrogenous compounds (NO 3 and NO2 ) conversion in the solution, and (ii) their selectivity towards ammonium ions, determined at 75% of conversion. This selectivity
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corresponds to the ratio between the concentration of ammonium ions at 75% of conversion and the initial concentration of nitrate or nitrite. Starting from a 1.6 mmol L1, i.e. 100 mg L1, nitrate solution, 75% of nitrate conversion leads to the residual nitrate level of 25 mg L1 corresponding to the EU recommendation. 2.3. Analysis of reaction products The reaction components were analyzed by the HPLC method. Nitrate and nitrite concentrations were determined after separation at 40 1C on a Zorbax Eclipse XDB-C18 column using an UV detector at l ¼ 210 nm. Ammonium ions were quantified using an Alltech Universal Cation column at 30 1C coupled with a conductivity detector. FA was analyzed at 30 1C on an Organic Acid Analysis Column, Aminex HPX-87 H and analyzed by a diode array UV/Visible detector (UV6000LP, Thermofinnigan) coupled with a Refractive Index Detector (RI-150, Thermofinnigan). It was checked that no hydroxylamine appears in solution during nitrate and nitrite reduction, although it is a possible intermediate product at the pH of the reaction.
3. Results and discussion 3.1. Characterization of the catalyst The catalyst composition, determined by the central service of analysis of the CNRS (French National Center of Scientific Research), and the palladium particle size, determined by TEM, are reported in Table 1 as well as the nomenclature of the different catalysts. These results show that the monometallic palladium catalyst presents a larger particle size than the bimetallic palladium/tin. This has been explained by a redispersion of palladium during tin deposition (Garron et al., 2005). 3.2. Effect of the nitrate and nitrite on formic acid decomposition First of all, it is important to specify that without nitrate or nitrite, no FA conversion has been observed in
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the presence of mono or bimetallic catalyst under nitrogen at 298 K. This result shows that nitrate and nitrite promote the catalytic conversion of FA. This phenomenon has already been observed by King et al. (1996) in the presence of nitrites and by Kopinke et al. (2004) during the hydrodechloration of chlorinated organic compounds. The kinetics of the FA decomposition on palladium black has been studied by Ruthven and Upadhye (1971) in liquid phase liquid between 30 and 60 1C. It was shown that the FA decomposition requires two adjacent sites allowing the dissociative adsorption and that this reaction is performed in three steps: HCOOH þ 2MðsÞ 2M H þ M OOCH;
(1)
Fast equilibrium, M OOCH ! M H þ CO2 ;
(2)
Slow determining step, M H þ M H ! H2 þ 2MðsÞ ;
(3)
Fast desorption. Then the promoting effect of nitrates and nitrites could be explained by the reduction of these species directly (reduction of nitrites by chemisorbed hydrogen atoms) or indirectly (reduction of the promoter oxidized during nitrates reduction) allowing the uptake of the hydrogen species generated during step (2) that is the kinetically determining step, thus increasing the global reaction rate of the FA decomposition. 3.3. Effect of the metallic phases 3.3.1. Effect on nitrate reduction The activity and the selectivity toward ammonium ions during nitrate reduction, in the presence of 0.8 mmol of FA under nitrogen flow are presented in Fig. 1, without catalyst (blank), with monometallic tin or palladium catalyst supported on silica or with the bimetallic PdSn/Silica catalyst in situ pre-reduced at 130 1C (PR) or not (nPR). First, these results showed that the reaction does not occur without catalyst, indicating that the reaction between nitrates and FA cannot be performed in homogeneous phase in the operating conditions.
Table 1 Characteristics of the fresh mono and bimetallic catalysts supported on silica Catalyst
Pd5/silica PdSn/silica
Metal content (wt%)
TEM results
Pd
Sn
Cl
Dispersion of palladium (%)
Particle size (nm)
4.7 4.7
— 1.5
0 0.9
5.1 8.4
19.8 14.4
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8
0.025
6
0.015 4
0.01
Selectivity in NH4+ (%)
Activity (mmol.min-1.gcata-1)
0.02
2
0.005
0
0 Blank
Sn/Silica
PdSn/Silica PR
PdSn/Silica nPR
Pd/Silica
Fig. 1. Activity (&) and selectivity toward ammonium ion (’) for nitrate reduction of different catalysts in the presence of formic acid as a function of the catalyst (PR: prereduced, nPR: not prereduced).
Furthermore, monometallic catalysts of tin or palladium are also inactive for this reaction. On the contrary, bimetallic catalyst allows the reaction to be performed, the palladium/tin phase being necessary to perform the reaction, as in the case of the use of hydrogen as reducing agent. Finally, it can be observed that the catalytic performances of the bimetallic palladium/tin catalyst supported on silica depend on the pre-treatment. Indeed, the in situ pre-reduction of the catalyst leads to a better activity associated to a lower amount of ammonium ions formed compared to the results obtained on a non-pre-treated catalyst. This points out that nitrate reduction in presence of FA is favored on the metallic part of the catalyst. Subsequently the results presented in the following have been obtained with the in-situ pre-reduced catalyst. 3.3.2. Effect on nitrite reduction The activities and selectivities toward ammonium ions obtained during nitrite reduction on the conditions presented above are shown in Fig. 2. Note that for the blank and the Sn/silica catalyst, a maximum of 30% of nitrite conversion has been observed. This figure shows that a small amount of nitrite can be reduced during the blank reaction, which means this reaction occurs in homogeneous phase. It should be noted that during nitrite reduction, an initial formation of nitrate is observed. This formation of nitrate can be explained by the following reactions proposed by
King et al. (1996): HCOOH þ NO 2 2HNO2 þHCOO ,
(4)
3HNO2 þHCOO ! 2NO þ HNO3 þH2 O;
(5)
HNO3 þHCOO ! NO 3 þ HCOOH:
(6)
The same activity and selectivity were observed with the monometallic catalyst as during the blank test, which could be attributed to the reaction in homogeneous phase. Then the monometallic tin catalyst is not active, as classically observed under hydrogen. On the contrary, the monometallic palladium catalyst is very active, with a reaction rate 10 times higher compared to that obtained during the blank reaction, and nitrogen is the only product. This result proves that nitrite reduction in the presence of FA is catalyzed by palladium. The bimetallic catalysts supported on silica allow nitrite reduction, but the selectivity toward ammonium ion is significant. It should be noted that the non-reduced bimetallic palladium/tin catalyst supported on silica is more active compared to the reduced catalyst. That means that the presence of metallic tin has a poisoning effect on this reaction. 3.4. Effect of the initial amount of formic acid The global redox reactions concerning the conversion of nitrate into nitrogen or ammonium ion
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2.5
8
6
1.5 4 1
Selectivity in NH4+ (%)
Activity (mmol.min-1.gcata-1)
2
2 0.5
0 Blank
Sn/Silica
PdSn/Silica PR
PdSn/Silica nPR
Pd/Silica
0
Fig. 2. Activity (&) and selectivity toward ammonium ion (’) for nitrite reduction of different catalysts in the presence of formic acid as a function of the catalyst (PR: prereduced, nPR: not prereduced).
Table 2 Initial and converted amount of formic acid (at 100% of nitrate conversion), final pH of the media and catalytic performances for nitrate reduction Initial amount of formic acid (mmol)
Conversion of formic acid (%)
Final pH (pH unit)
Activity (105 mol min1 gcata1)
Selectivity toward NH+ 4 (%)
0.4 0.8 1.6 3.2 6.4
100 100 100 62.5 25
3.9 2.7 2.7 2.5 2.3
2.25 2.58 2.99 2.68 1.92
3.8 5.3 6.8 6.2 4.1
At 75% of conversion, conversion of nitrate not completed.
are, respectively 2NO 3
þ 5HCOOH ! N2 þ 3CO2 þ
2HCO 3
þ 4H2 O, (7)
þ NO 3 þ 4HCOOH ! NH4 þ 2CO2 þ 2HCO3 þ H2 O. (8)
Then to achieve the complete conversion of nitrate into nitrogen, more than 5/2 equivalents of FA that corresponds to 0.4 mmol of FA for 0.16 mmol of nitrate are necessary. In order to determine the optimum ratio between initial amounts of FA introduced in the reaction media and nitrate, the initial and converted
amount of FA, the final pH of the media and the catalytic performances in nitrate reduction are reported in Table 2. These results show that a high activity is obtained whatever the initial amount. However, the selectivity toward ammonium ions is always higher than 0.7% and then the concentration of NH+ is higher than the 4 admissible level. Moreover, a complete mineralization of FA is only obtained when its initial amount is lower than 1.6 mmol. For the initial amount of FA of 0.4 mmol, the totality is converted before the end of the reaction and the total conversion of nitrate is not reached. This result is logical if one considers that a part of the hydrogen produced by FA decomposition is
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It also shows that when the reaction is performed in the presence of FA under hydrogen the activity is higher than that obtained in the presence of FA under inert gas or without FA under pure hydrogen. It seems that there is a synergetic effect between these two reducing agents. In Fig. 4 are presented the experimental curves obtained in nitrate reduction with a bimetallic palladium/tin catalyst supported on silica pre-reduced in situ, with FA under nitrogen (FA+N2) or without FA under the mixture of hydrogen and carbon dioxide (H2+CO2). This figure points out that the nitrate disappearance rate and the ammonium appearance rate obtained with the bimetallic catalyst supported on silica are similar with FA under nitrogen or without FA under the mixture of hydrogen and carbon dioxide. This result confirms that the mechanism should be similar in these two conditions and it seems that FA only acts as a source of hydrogen and carbon dioxide and not as a direct reducing agent. It is important to note that there is no intermediate nitrite in solution during nitrate reduction in the presence of FA. This could be ascribed to the low pH value of the reaction mixture. Indeed, Centi and Perathoner (2003) demonstrated that the amount of intermediate nitrite in solution decreases with the pH and that nitrite is not formed at pH ¼ 3.
consumed to maintain tin in the metallic form. Moreover, the final pH of the solution is close to the pKA of FA/formate and far from the one of carbon dioxide/ hydrogencarbonate ion. A low amount of FA (8.104 mol) allows to obtain a complete conversion of nitrate, a high activity and a low amount of ammonium ion. The other advantage is that this low concentration of FA allows also its complete conversion. Subsequently, in the following the initial amount of FA will be 0.8 mmol.
3.5. Influence of the gas phase Fig. 3 represents the activity and the selectivity in ammonium ions obtained during nitrate reduction in five different experimental conditions: in presence of FA and nitrogen (1) carbon dioxide (2) or hydrogen (3) or without FA under hydrogen (H2) (4) or under the mixture of hydrogen and carbon dioxide (H2+CO2) (5). This figure shows that the activities and selectivities are equivalent with FA under nitrogen or carbon dioxide and without FA under the mixture of hydrogen and carbon dioxide. This result indicates that the reaction involved in these three experimental conditions should be similar. That means that FA should be decomposed in hydrogen and carbon dioxide.
0.1
0. 0923
35
32
30
25 +
0.06
20
15
0.04
0.0225 0.02
Selectivity in NH4 (%)
-1
-1
Activity (mmol.min .gcata )
0.08
0.0248
10
0.0192 4.3
3.8
3.9 0.0105
2.3
5
0
0
0.8 mmol FA + N2
0.8 mmol FA + CO2
0.8 mmol FA + H2
H2
H2 + CO2
Fig. 3. Catalytic performances for nitrate reduction of Pd5Sn1.5/SiO2 (Degussa) catalyst with or without formic acid (FA) as a function of the gas flow (&: activity; ’: selectivity toward ammonium ion).
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1.6
Concentration (mmol.L-1)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
10
20
30
40
50
60
70
80
90
100
110
120
Time (min) Fig. 4. Typical nitrate concentration/times curves for nitrate reduction of Pd5Sn1.5/SiO2 with, respectively, formic acid under nitrogen (FA+N2: solid symbol ’) or under the mixture of hydrogen and carbon dioxide (H2+CO2: open symbol W) (’: nitrates; m: ammonium ion).
This figure also shows that contrary to the results obtained by Vorlop and Pru¨sse (1999) there is no induction period at the beginning of the nitrate reduction in the presence of FA. The difference could be explained by the way the FA was added. Indeed, in the present study the total amount of FA is introduced before nitrate while in the study of Vorlop, FA is progressively added during the reaction in order to stabilize the pH at a value of 6. Moreover, these authors deduced from their experimental curve, a zero-order kinetics with regard to nitrate. However, it seems that the linear concentration/time curve could also be attributable to the decomposition kinetics of FA, which could limit the nitrate reduction rate. Indeed, as evidenced in Fig. 4, when FA is totally added before nitrate, the concentration/time curve is non-linear indicating a kinetic order different from zero. Fig. 5 represents the activity and selectivity in ammonium ions obtained during nitrite reduction in the five different experimental conditions. This figure shows that the activities in nitrite reduction observed with FA are 3–4 times higher than those observed without FA under the mixture of hydrogen and carbon dioxide, but the selectivity toward ammonium ion is 6 times lower. These results should indicate that there are two parallel mechanisms, one corresponding to the reaction between nitrite and FA in homogeneous phase, leading to nitrates and ammonium
ion, and the other to catalytic reduction of nitrite by the hydrogen provided by the FA decomposition.
4. Conclusion This study suggests that
FA
allows the catalytic reduction of nitrate and nitrite in distilled water. The selectivity in ammonium ion is 10 times lower with FA than with hydrogen as a reducing agent. However, the concentration of this final product exceeds the maximum admissible level. This is not a real problem if one considers that the last step of drinking water treatment is a chlorination step that would easily convert the residual ammonium ions into nitrogen. The best molar ratio between FA and nitrate is 5 to obtain drinking water. It leads to a complete mineralization of FA, a good activity and a low ammonium ion formation. For nitrate reduction, FA should only act as a source of hydrogen and carbon dioxide and not as a direct reducing agent. A following paper will examine the catalytic reduction in the presence of FA in tap water.
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1.2
35 31
1.16
Activity (mmol.min-1.gcata-1)
0.8
25 0.821
0.818 20
0.6 15 0.4 10 0.2
0.248
3.1 0.191 0
0
0.8 mmol FA + CO2
0.8 mmol FA + H2
5
0.5
0
0.8 mmol FA + N2
Selectivity in NH4+ (%)
30
1
H2
H2 + CO2
0
Fig. 5. Catalytic performances for nitrite reduction of Pd5Sn1.5/SiO2 (Degussa) catalyst with or without formic acid (FA) in function of the gas flow (&: activity; ’: selectivity toward ammonium ion).
References Canter, L.W., 1996. Nitrates in Groundwater. CRC Press, Boca Raton,FL. Centi, G., Perathoner, S., 2003. Remediation of water contamination using catalytic technologies. Appl. Catal. B: Environ. 41, 15–29. Coq, B., Guenais, S., Bouyer, C., Boudic, J.C., 2003. Catalytic denitration by HCO2H of HNO3 concentrated media in the presence of Pt/SiO2 catalysts: mechanism and influence of the Pt particle size. Appl. Catal. B: Environ. 45, 205–211. Epron, F., Gauthard, F., Pine´da, C., Barbier, J., 2001. Catalytic reduction of nitrate and nitrite on Pt–Cu/Al2O3 catalysts in aqueous solution: role of the interaction between copper and platinum in the reaction. J. Catal. 198, 309–318. Garron, A., Lazar, K., Epron, F., 2005. Effect of the support on tin distribution in Pd–Sn/Al2O3 and Pd–Sn/SiO2 catalysts for application in water denitration. Appl. Catal. B: Environ. 59, 57–69. Gautron, E., Garron, A., Bost, E., Epron, F., 2003. Synthesis, characterization and catalytic properties of polypyrrole supported catalysts. Catal. Commun. 4, 435–439. Gavagnin, R., Biassato, L., Pinna, F., Strukul, G., 2002. Nitrate removal in drinking water: the effects of tin oxides on the catalytic hydrogenation of nitrate by Pd/SnO2 catalysts. Appl. Catal. B: Environ. 38, 91–99. Ha¨hnlein, M., Pru¨sse, U., Daum, J., Morawsky, V., Kro¨ger, M., Schro¨der, M., Schnabel, M., Vorlop, K.-D., 1998. Preparation of microscopic catalysts and colloids for catalytic nitrate and nitrite reduction and their use in a hollow fiber dialyser loop reactor. In: Delmon, B., et al.
(Eds.), Preparation of Catalysts, vol. VIII. Elsevier, Amsterdam, pp. 99–107. King, R.B., Bhattacharayya, N.K., Smith, H.D., Wiemers, K.D., 1996. Noble metal-catalyzed homogeneous and heterogeneous processes in treating simulated nuclear waste media with formic acid. J. Mol. Catal. A: Chem. 107, 145–152. Kopinke, F.D., Machenzie, K., Koehler, R., Georgi, A., 2004. Alternative sources of hydrogen for hydrodechlorination of chlorinated organic compounds in water on Pd catalyst. Appl. Catal. A. 271, 119–128. Palomares, A.E., Prato, J.G., Rey, F., Corma, A., 2004. Using the ‘‘memory effect’’ of hydrotacites for improving the catalytic reduction of nitrates in water. J. Catal. 221, 62–66. Pintar, A., Batista, J., 1999. catalytic hydrogenation of aqueous nitrate solutions in fixed-bed reactors. Catal. Today 53, 35–50. Pintar, A., Setinc, M., Levec, J., 1998. Hardness and salt effects on catalytic hydrogenation of aqueous nitrate solution. J. Catal. 174, 72–87. Pintar, A., Batista, J., Levec, J., 2001. Catalytic denitrification: direct and indirect removal of nitrates from potable water. Catal. Today 66, 503–510. Pru¨sse, U., Vorlop, K.-D., 2001. Supported bimetallic palladium catalysts for water-phase nitrate reduction. J. Mole. Catal. A: Chem. 173, 313–328. Pru¨sse, U., Ho¨rold, S., Vorlop, K.-D., 1997. Catalytic denitration of water by formic acid used as reducing agent (German). Chem. Ing. Tech. 69, 87–91. Pru¨sse, U., Hahnlein, M., Daum, J., Vorlop, K.D., 2000. Improving the catalytic nitrate reduction. Catal. Today 55, 79–90.
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Water Research 39 (2005) 3073–3081 www.elsevier.com/locate/watres
Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water Anthony Garron, Florence Epron Laboratoire de Catalyse en Chimie Organique, UMR6503 CNRS—Universite´ de Poitiers, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France Received 18 January 2005; received in revised form 10 May 2005; accepted 12 May 2005 Available online 27 June 2005
Abstract The reduction of nitrate in nitrogen using bimetallic palladium tin catalysts and hydrogen is an interesting process for water treatment. The aim of the present study is to use formic acid (FA) as a reducing agent and a pH buffer in order to substitute the mixture of hydrogen and carbon dioxide. The catalytic performances of a palladium tin catalyst supported on silica were evaluated in the presence of FA, as a function of the initial acid concentration and of the gas phase (N2, CO2, or H2). Results were compared to those obtained with hydrogen in the presence of carbon dioxide. Similar mechanisms seem to explain the identical catalytic performances observed with these two reducing agents. r 2005 Elsevier Ltd. All rights reserved. Keywords: Pd–Sn catalysts; Nitrate; Catalytic reduction; Drinking water; Formic acid
1. Introduction The concentration of nitrate in the soil, and subsequently in ground and surface water, for years has constantly been increasing all around the world. This compound is responsible for river and sea eutrophication. In drinking water, it is potentially harmful to human health (blue baby syndrome, cancer, etc.). Sources of nitrate in groundwater can be subdivided into four categories: (i) natural sources (ii) waste materials (iii) row crop agriculture and (iv) irrigated agriculture (Canter, 1996). Biological and physicochemical treatments allow effective removal of nitrates but have several economical and ecological disadvantages. Catalytic reactions conCorresponding author. Tel.: +33 5 49 45 48 32;
fax: +33 5 49 45 37 41. E-mail address: fl[email protected] (F. Epron).
stitute promising approaches for the destruction of pollutants in water. Since Tacke and Vorlop’s (1989) first paper on the use of palladium–copper bimetallic catalysts for nitrate reduction, numerous studies have been aiming at the development of suitable catalysts for the selective reduction of nitrate into nitrogen gas. Their application in various reactor types such as hollow fibres (Ha¨hnlein et al., 1998) and membrane reactors (Pintar et al., 2001) has been studied. However, few papers have been devoted to nitrate reduction in tap water (Pintar and Batista 1999). Palladium catalysts have proven to be the most active and selective for nitrite reduction. To reduce nitrate, it is necessary to activate the precious metal by adding a second metal (Pru¨sse et al., 1997). Copper was mostly used as a promoting second metal (Epron et al., 2001; Yoshinaga et al., 2002) but other suitable promoters, such as tin, indium or zinc have demonstrated interesting performances (Pru¨sse et al., 2000; Pru¨sse and Vorlop, 2001). The best promoter of
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this reaction in term of selectivity is tin (Vorlop and Pru¨sse, 1999). The reduction of nitrate leads to the formation of hydroxide ion that causes an unavoidable increase of the pH value up to 10; this pH gradient is unacceptable for drinking water. Furthermore, basic pH induces a polarization of the support and has a repulsive effect on nitrates and nitrites, inducing a decrease of the activity and the selectivity in nitrogen (Vorlop and Pru¨sse, 1999). Different solutions have been used to reduce the formation of hydroxides, (Pintar et al., 1998, 2001; Gautron et al., 2003; Roveda et al., 2003; Gavagnin et al., 2002; Palomares et al., 2004). The use of formic acid (FA) instead of hydrogen as reducing agent is one possibility (Vorlop and Pru¨sse, 1999; Pru¨sse and Vorlop, 2001). In this case, the FA decomposition can be used as a source of hydrogen and carbon dioxide. Indeed, on a noble metal FA can be easily decomposed according to the following reaction: HCOOH
Noble metal
!
H2 þ CO2 .
FA can be also used either for its acidic properties or as a reducing agent. Indeed, the use of this acid (pKA(HCOOH/HCOO) ¼ 3.74), allows one to work with an initial pH included between 3 and 4 in order to improve the catalytic performances of the catalyst. Its redox potential (E0(H2,CO2)/HCOOH) ¼ 0.199 V/ERH) allows nitrate to be reduced into nitrite. Recently, Coq et al. (2003) studied the use of FA in denitration of concentrated nitric acid over monometallic platinum supported on silica catalyst. The reaction mechanism proposed consisted of a redox reaction leading to the transformation of nitric acid in nitrous acid over passivated sites of platinum during the decomposition of FA in hydrogen and carbon dioxide. These authors have also observed a reduction of nitrous acid in the homogeneous phase. The intent of the present study is to gain further information on the mechanism of nitrate and nitrite reduction in the presence of FA. For this purpose, the catalytic performances obtained with FA in nitrate and nitrite reduction in distilled water will be compared with those obtained with a mixture of hydrogen and carbon dioxide. The catalyst used for this study is a PdSn catalyst supported on silica and prepared by controlled surface reaction. The characterization by Mo¨ssbauer spectroscopy of this bimetallic catalyst (Garron et al., 2005) has demonstrated that on the non-reduced catalyst, tin is only in an oxidized form with an oxidation degree of +IV as SnO2. On reduced catalyst tin is in a metallic state with around 20% corresponding to a solid solution (Pd(x43)Sn) and 80% to an alloy (Pd2Sn). All the tin is in interaction with palladium on the reduced catalyst.
2. Experimental Details of the preparation method and the characteristics of this catalyst are described elsewhere (Garron et al. (2005). 2.1. Catalyst preparation 2.1.1. Monometallic catalysts Monometallic catalysts were prepared by an impregnation method using aqueous solutions of the precursor salt: Pd(NH3)4(NO3)2. Monometallic catalysts Pd/SiO2 had a metal loading of 5 wt%. This loading was determined by Vorlop and Pru¨sse (1999) as optimal for palladium-based catalysts to an application of denitration in water. 2.1.2. Bimetallic catalysts Bimetallic catalysts were prepared by a controlled surface reaction in order to obtain great metal–metal interactions. The bimetallic catalyst has a specific weight ratio at 1.5 wt% in tin verified by elementary analysis. Generally, the experimental value gave an incertitude error not exceeding 5%. 2.2. Nitrate and nitrite reduction Nitrate and nitrite reduction reactions were performed in a semi-batch reactor, at atmospheric pressure and 25 1C. The catalyst (64 mg) can be reduced before reaction under hydrogen (130 1C; 0.5 h). During this time, 90mL of ultra pure water (18.2 MO) were purged with nitrogen. When the catalyst was stabilized in temperature, the ultra-pure water was added over the reduced catalyst. Then the reaction was performed under a mixture of 50% of hydrogen in carbon dioxide or in the presence of a known amount of FA acid under pure nitrogen, hydrogen or carbon dioxide. The CO2 or FA is used to buffer the solution at pH around 5. Afterwards, 10 mL of a solution (16 mmol L1) of nitrate (KNO3) or nitrite (KNO2) were introduced in the reactor to start the reaction. The catalyst dispersion in the aqueous medium was achieved by the gas flowing (flow rate ¼ 250 mL min1) through a porous glass located at the bottom of the reactor. It was checked that the resulting stirring is sufficient to ensure that the reaction is not rate-limited by the reactant diffusion. Representative aqueous samples were periodically withdrawn, separated by filtration and analyzed as described in Section 2.3. Catalysts were compared as a function of (i) their activity, corresponding to the disappearance rate of nitrate or nitrite, determined at 75% of nitrogenous compounds (NO 3 and NO2 ) conversion in the solution, and (ii) their selectivity towards ammonium ions, determined at 75% of conversion. This selectivity
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corresponds to the ratio between the concentration of ammonium ions at 75% of conversion and the initial concentration of nitrate or nitrite. Starting from a 1.6 mmol L1, i.e. 100 mg L1, nitrate solution, 75% of nitrate conversion leads to the residual nitrate level of 25 mg L1 corresponding to the EU recommendation. 2.3. Analysis of reaction products The reaction components were analyzed by the HPLC method. Nitrate and nitrite concentrations were determined after separation at 40 1C on a Zorbax Eclipse XDB-C18 column using an UV detector at l ¼ 210 nm. Ammonium ions were quantified using an Alltech Universal Cation column at 30 1C coupled with a conductivity detector. FA was analyzed at 30 1C on an Organic Acid Analysis Column, Aminex HPX-87 H and analyzed by a diode array UV/Visible detector (UV6000LP, Thermofinnigan) coupled with a Refractive Index Detector (RI-150, Thermofinnigan). It was checked that no hydroxylamine appears in solution during nitrate and nitrite reduction, although it is a possible intermediate product at the pH of the reaction.
3. Results and discussion 3.1. Characterization of the catalyst The catalyst composition, determined by the central service of analysis of the CNRS (French National Center of Scientific Research), and the palladium particle size, determined by TEM, are reported in Table 1 as well as the nomenclature of the different catalysts. These results show that the monometallic palladium catalyst presents a larger particle size than the bimetallic palladium/tin. This has been explained by a redispersion of palladium during tin deposition (Garron et al., 2005). 3.2. Effect of the nitrate and nitrite on formic acid decomposition First of all, it is important to specify that without nitrate or nitrite, no FA conversion has been observed in
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the presence of mono or bimetallic catalyst under nitrogen at 298 K. This result shows that nitrate and nitrite promote the catalytic conversion of FA. This phenomenon has already been observed by King et al. (1996) in the presence of nitrites and by Kopinke et al. (2004) during the hydrodechloration of chlorinated organic compounds. The kinetics of the FA decomposition on palladium black has been studied by Ruthven and Upadhye (1971) in liquid phase liquid between 30 and 60 1C. It was shown that the FA decomposition requires two adjacent sites allowing the dissociative adsorption and that this reaction is performed in three steps: HCOOH þ 2MðsÞ 2M H þ M OOCH;
(1)
Fast equilibrium, M OOCH ! M H þ CO2 ;
(2)
Slow determining step, M H þ M H ! H2 þ 2MðsÞ ;
(3)
Fast desorption. Then the promoting effect of nitrates and nitrites could be explained by the reduction of these species directly (reduction of nitrites by chemisorbed hydrogen atoms) or indirectly (reduction of the promoter oxidized during nitrates reduction) allowing the uptake of the hydrogen species generated during step (2) that is the kinetically determining step, thus increasing the global reaction rate of the FA decomposition. 3.3. Effect of the metallic phases 3.3.1. Effect on nitrate reduction The activity and the selectivity toward ammonium ions during nitrate reduction, in the presence of 0.8 mmol of FA under nitrogen flow are presented in Fig. 1, without catalyst (blank), with monometallic tin or palladium catalyst supported on silica or with the bimetallic PdSn/Silica catalyst in situ pre-reduced at 130 1C (PR) or not (nPR). First, these results showed that the reaction does not occur without catalyst, indicating that the reaction between nitrates and FA cannot be performed in homogeneous phase in the operating conditions.
Table 1 Characteristics of the fresh mono and bimetallic catalysts supported on silica Catalyst
Pd5/silica PdSn/silica
Metal content (wt%)
TEM results
Pd
Sn
Cl
Dispersion of palladium (%)
Particle size (nm)
4.7 4.7
— 1.5
0 0.9
5.1 8.4
19.8 14.4
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8
0.025
6
0.015 4
0.01
Selectivity in NH4+ (%)
Activity (mmol.min-1.gcata-1)
0.02
2
0.005
0
0 Blank
Sn/Silica
PdSn/Silica PR
PdSn/Silica nPR
Pd/Silica
Fig. 1. Activity (&) and selectivity toward ammonium ion (’) for nitrate reduction of different catalysts in the presence of formic acid as a function of the catalyst (PR: prereduced, nPR: not prereduced).
Furthermore, monometallic catalysts of tin or palladium are also inactive for this reaction. On the contrary, bimetallic catalyst allows the reaction to be performed, the palladium/tin phase being necessary to perform the reaction, as in the case of the use of hydrogen as reducing agent. Finally, it can be observed that the catalytic performances of the bimetallic palladium/tin catalyst supported on silica depend on the pre-treatment. Indeed, the in situ pre-reduction of the catalyst leads to a better activity associated to a lower amount of ammonium ions formed compared to the results obtained on a non-pre-treated catalyst. This points out that nitrate reduction in presence of FA is favored on the metallic part of the catalyst. Subsequently the results presented in the following have been obtained with the in-situ pre-reduced catalyst. 3.3.2. Effect on nitrite reduction The activities and selectivities toward ammonium ions obtained during nitrite reduction on the conditions presented above are shown in Fig. 2. Note that for the blank and the Sn/silica catalyst, a maximum of 30% of nitrite conversion has been observed. This figure shows that a small amount of nitrite can be reduced during the blank reaction, which means this reaction occurs in homogeneous phase. It should be noted that during nitrite reduction, an initial formation of nitrate is observed. This formation of nitrate can be explained by the following reactions proposed by
King et al. (1996): HCOOH þ NO 2 2HNO2 þHCOO ,
(4)
3HNO2 þHCOO ! 2NO þ HNO3 þH2 O;
(5)
HNO3 þHCOO ! NO 3 þ HCOOH:
(6)
The same activity and selectivity were observed with the monometallic catalyst as during the blank test, which could be attributed to the reaction in homogeneous phase. Then the monometallic tin catalyst is not active, as classically observed under hydrogen. On the contrary, the monometallic palladium catalyst is very active, with a reaction rate 10 times higher compared to that obtained during the blank reaction, and nitrogen is the only product. This result proves that nitrite reduction in the presence of FA is catalyzed by palladium. The bimetallic catalysts supported on silica allow nitrite reduction, but the selectivity toward ammonium ion is significant. It should be noted that the non-reduced bimetallic palladium/tin catalyst supported on silica is more active compared to the reduced catalyst. That means that the presence of metallic tin has a poisoning effect on this reaction. 3.4. Effect of the initial amount of formic acid The global redox reactions concerning the conversion of nitrate into nitrogen or ammonium ion
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2.5
8
6
1.5 4 1
Selectivity in NH4+ (%)
Activity (mmol.min-1.gcata-1)
2
2 0.5
0 Blank
Sn/Silica
PdSn/Silica PR
PdSn/Silica nPR
Pd/Silica
0
Fig. 2. Activity (&) and selectivity toward ammonium ion (’) for nitrite reduction of different catalysts in the presence of formic acid as a function of the catalyst (PR: prereduced, nPR: not prereduced).
Table 2 Initial and converted amount of formic acid (at 100% of nitrate conversion), final pH of the media and catalytic performances for nitrate reduction Initial amount of formic acid (mmol)
Conversion of formic acid (%)
Final pH (pH unit)
Activity (105 mol min1 gcata1)
Selectivity toward NH+ 4 (%)
0.4 0.8 1.6 3.2 6.4
100 100 100 62.5 25
3.9 2.7 2.7 2.5 2.3
2.25 2.58 2.99 2.68 1.92
3.8 5.3 6.8 6.2 4.1
At 75% of conversion, conversion of nitrate not completed.
are, respectively 2NO 3
þ 5HCOOH ! N2 þ 3CO2 þ
2HCO 3
þ 4H2 O, (7)
þ NO 3 þ 4HCOOH ! NH4 þ 2CO2 þ 2HCO3 þ H2 O. (8)
Then to achieve the complete conversion of nitrate into nitrogen, more than 5/2 equivalents of FA that corresponds to 0.4 mmol of FA for 0.16 mmol of nitrate are necessary. In order to determine the optimum ratio between initial amounts of FA introduced in the reaction media and nitrate, the initial and converted
amount of FA, the final pH of the media and the catalytic performances in nitrate reduction are reported in Table 2. These results show that a high activity is obtained whatever the initial amount. However, the selectivity toward ammonium ions is always higher than 0.7% and then the concentration of NH+ is higher than the 4 admissible level. Moreover, a complete mineralization of FA is only obtained when its initial amount is lower than 1.6 mmol. For the initial amount of FA of 0.4 mmol, the totality is converted before the end of the reaction and the total conversion of nitrate is not reached. This result is logical if one considers that a part of the hydrogen produced by FA decomposition is
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It also shows that when the reaction is performed in the presence of FA under hydrogen the activity is higher than that obtained in the presence of FA under inert gas or without FA under pure hydrogen. It seems that there is a synergetic effect between these two reducing agents. In Fig. 4 are presented the experimental curves obtained in nitrate reduction with a bimetallic palladium/tin catalyst supported on silica pre-reduced in situ, with FA under nitrogen (FA+N2) or without FA under the mixture of hydrogen and carbon dioxide (H2+CO2). This figure points out that the nitrate disappearance rate and the ammonium appearance rate obtained with the bimetallic catalyst supported on silica are similar with FA under nitrogen or without FA under the mixture of hydrogen and carbon dioxide. This result confirms that the mechanism should be similar in these two conditions and it seems that FA only acts as a source of hydrogen and carbon dioxide and not as a direct reducing agent. It is important to note that there is no intermediate nitrite in solution during nitrate reduction in the presence of FA. This could be ascribed to the low pH value of the reaction mixture. Indeed, Centi and Perathoner (2003) demonstrated that the amount of intermediate nitrite in solution decreases with the pH and that nitrite is not formed at pH ¼ 3.
consumed to maintain tin in the metallic form. Moreover, the final pH of the solution is close to the pKA of FA/formate and far from the one of carbon dioxide/ hydrogencarbonate ion. A low amount of FA (8.104 mol) allows to obtain a complete conversion of nitrate, a high activity and a low amount of ammonium ion. The other advantage is that this low concentration of FA allows also its complete conversion. Subsequently, in the following the initial amount of FA will be 0.8 mmol.
3.5. Influence of the gas phase Fig. 3 represents the activity and the selectivity in ammonium ions obtained during nitrate reduction in five different experimental conditions: in presence of FA and nitrogen (1) carbon dioxide (2) or hydrogen (3) or without FA under hydrogen (H2) (4) or under the mixture of hydrogen and carbon dioxide (H2+CO2) (5). This figure shows that the activities and selectivities are equivalent with FA under nitrogen or carbon dioxide and without FA under the mixture of hydrogen and carbon dioxide. This result indicates that the reaction involved in these three experimental conditions should be similar. That means that FA should be decomposed in hydrogen and carbon dioxide.
0.1
0. 0923
35
32
30
25 +
0.06
20
15
0.04
0.0225 0.02
Selectivity in NH4 (%)
-1
-1
Activity (mmol.min .gcata )
0.08
0.0248
10
0.0192 4.3
3.8
3.9 0.0105
2.3
5
0
0
0.8 mmol FA + N2
0.8 mmol FA + CO2
0.8 mmol FA + H2
H2
H2 + CO2
Fig. 3. Catalytic performances for nitrate reduction of Pd5Sn1.5/SiO2 (Degussa) catalyst with or without formic acid (FA) as a function of the gas flow (&: activity; ’: selectivity toward ammonium ion).
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1.6
Concentration (mmol.L-1)
1.4 1.2 1 0.8 0.6 0.4 0.2 0 0
10
20
30
40
50
60
70
80
90
100
110
120
Time (min) Fig. 4. Typical nitrate concentration/times curves for nitrate reduction of Pd5Sn1.5/SiO2 with, respectively, formic acid under nitrogen (FA+N2: solid symbol ’) or under the mixture of hydrogen and carbon dioxide (H2+CO2: open symbol W) (’: nitrates; m: ammonium ion).
This figure also shows that contrary to the results obtained by Vorlop and Pru¨sse (1999) there is no induction period at the beginning of the nitrate reduction in the presence of FA. The difference could be explained by the way the FA was added. Indeed, in the present study the total amount of FA is introduced before nitrate while in the study of Vorlop, FA is progressively added during the reaction in order to stabilize the pH at a value of 6. Moreover, these authors deduced from their experimental curve, a zero-order kinetics with regard to nitrate. However, it seems that the linear concentration/time curve could also be attributable to the decomposition kinetics of FA, which could limit the nitrate reduction rate. Indeed, as evidenced in Fig. 4, when FA is totally added before nitrate, the concentration/time curve is non-linear indicating a kinetic order different from zero. Fig. 5 represents the activity and selectivity in ammonium ions obtained during nitrite reduction in the five different experimental conditions. This figure shows that the activities in nitrite reduction observed with FA are 3–4 times higher than those observed without FA under the mixture of hydrogen and carbon dioxide, but the selectivity toward ammonium ion is 6 times lower. These results should indicate that there are two parallel mechanisms, one corresponding to the reaction between nitrite and FA in homogeneous phase, leading to nitrates and ammonium
ion, and the other to catalytic reduction of nitrite by the hydrogen provided by the FA decomposition.
4. Conclusion This study suggests that
FA
allows the catalytic reduction of nitrate and nitrite in distilled water. The selectivity in ammonium ion is 10 times lower with FA than with hydrogen as a reducing agent. However, the concentration of this final product exceeds the maximum admissible level. This is not a real problem if one considers that the last step of drinking water treatment is a chlorination step that would easily convert the residual ammonium ions into nitrogen. The best molar ratio between FA and nitrate is 5 to obtain drinking water. It leads to a complete mineralization of FA, a good activity and a low ammonium ion formation. For nitrate reduction, FA should only act as a source of hydrogen and carbon dioxide and not as a direct reducing agent. A following paper will examine the catalytic reduction in the presence of FA in tap water.
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1.2
35 31
1.16
Activity (mmol.min-1.gcata-1)
0.8
25 0.821
0.818 20
0.6 15 0.4 10 0.2
0.248
3.1 0.191 0
0
0.8 mmol FA + CO2
0.8 mmol FA + H2
5
0.5
0
0.8 mmol FA + N2
Selectivity in NH4+ (%)
30
1
H2
H2 + CO2
0
Fig. 5. Catalytic performances for nitrite reduction of Pd5Sn1.5/SiO2 (Degussa) catalyst with or without formic acid (FA) in function of the gas flow (&: activity; ’: selectivity toward ammonium ion).
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