Abstract

Mono- and bimetallic catalysts have been evaluated in the catalytic reduction of NO2 and NO3. The activity and selectivity of the catalysts based on Pd, Pd-In and Pd-Sn supported on different materials such as basic Al2O3, SiO2 and SnO2 have been evaluated under mild operation conditions (25 °C, 1 atm). NO2 hydrogenation was efficiently achieved with monometallic Pd catalyst supported on SnO2 and the bimetallic Pd-In catalysts supported on Al2O3 and SiO2 which led to the highest NO2 conversion (80%). Pd/SnO2 and bimetallic Pd-In supported catalysts completely transformed NO3 while Pd-Sn catalyst showed a lower activity. Initial rates between 0.5 and 2.6 mmol·min−1·gPd−1 were obtained for NO3 reduction. The lowest selectivity to NH4+ was observed with the Pd-In/Al2O3 catalyst, which also showed a high stability in long-term experiments. The operation at low relative H2 fractions in the gas feed greatly diminishes the selectivity to NH4+, reaching concentrations below the maximum concentration allowed in drinking water (0.5 mg/L).

INTRODUCTION

Nitrate pollution is one of the most important issues in water quality (Gupta & Ali 2012). The European Commission highlighted the situation of some hypertrophic rivers, lakes and ground waters where NO3 exceeds legal limits in more than 10% of the sampling points in the European Union (EU) (Eurostat 2012). Supranational organizations such as the European Commission and the World Health Organization (WHO) have developed some regulations and guidelines regarding NO3 accumulation in ground waters. The guideline values for NO3 and NO2 in drinking water dictated by the WHO are 50 and 3 mg/L, respectively. Several nations have also limited the maximum NH4+ levels for drinking water to 0.5 mg/L (Della Rocca et al. 2007).

The removal of NO3 from water has been traditionally carried out by biological nitrification-denitrification which is a well-known, effective and cost-effective method. However, this method is not recommended for the treatment of drinking water because of the risk of biological contamination. The application of other economically feasible, effective and environmentally friendly solutions is required. Other systems such as electroreduction can have drawbacks, such as the production of chlorates and perchlorates (Sánchez-Carretero et al. 2011). Some other technologies based on the reduction of NO3 and perchlorate such as advanced reduction processes have also been tested recently (Vellanki et al. 2013).

The catalytic reduction of NO3 is one of the more suitable technologies for the potabilization of ground water polluted with NO3. The catalytic reduction of NO3 uses H2 as reactant in the presence of a bimetallic catalyst containing a noble metal from group VIII, usually palladium, and a transition metal, usually copper, as catalytic promoter for the conversion of NO3 to N2 (Hörold et al. 1993; Barrabés & Sá 2011; Pizarro et al. 2015). The reported costs of NO3 catalytic reduction are in the same range as other technologies such as biological denitrification, reverse osmosis and ionic exchange, and there are other advantages such as no brine production, innocuous end products, no biohazard risks, stability, and ease of fine-tuning and start-up, which have to be considered in the life cycle analysis comparison (Centi & Perathoner 2003). The main cost is the use of hydrogen that is used in very low concentration when the H2 is limited in the reaction medium. The cost of this process decreases with the life of the catalyst; therefore the main objective is to obtain catalysts with high stability. Hence, NO3 can be converted selectively to N2 using Pd and other metals as promoters under adequate operation conditions. Several works have studied the denitration process using monometallic Pd supported on SnO2 (Gavagnin et al. 2002) and ZrO2 (Marella et al. 2000), showing some advantages over traditional supports such as Al2O3 or SiO2. These conventional supports present narrow pores that allow the formation of an internal OH gradient that diminishes their activity and selectivity to N2, increasing the undesirable NH4+ production. The development of catalysts with a higher pore width (i.e. Pd/SnO2, Pd/ZrO2) decreases the effect of alkalinity inside the pore system, causing a noticeable increase of the selectivity to N2 compared with catalysts with narrow pores.

Previous works have reported low NH4+ selectivity using Al2O3, activated carbon, TiO2, SiO2 and SnO2 as catalytic supports. The selectivity and activity greatly depends on the catalyst properties (active phase and surface charge) and the operation conditions, mainly pH, N:H ratio (liquid phase) and the competition between different species present in the feed stream. The catalytic supports also play an important role since they present significant differences in their surface charge, area, particle size, pore width and hydrophobicity. NH4+ selectivity is also reported as 5% to 10% using these supports (Hörold et al. 1993; Barrabés & Sá 2011; Pizarro et al. 2015). The interaction between Pd and the promoting metal such as Cu, In or Sn in bimetallic catalysts supported on inert materials is also more improbable than that present in reducible metal oxides where Pd nanoparticles are in full contact with the active supports such as SnO2 (Gavagnin et al. 2002; et al. 2007; Bošković et al. 2008; Guo et al. 2012), CeO2 (Epron et al. 2002; Devadas et al. 2011; Kim et al. 2014; Lee et al. 2015) or TiO2 (Gao et al. 2003; Sá & Anderson 2008; Zhang et al. 2008; Krawczyk et al. 2011; Chen et al. 2013; Kim et al. 2013, 2014).

The majority of studies (close to 75%) focused on the catalytic reduction of NO3 concluded that Pd is the most active phase to reduce NO2. In addition, Pd is more selective to N2 production than Rh, Ru or Pt catalysts (Hörold et al. 1993; Barrabés & Sá 2011). Studies on the use of other less toxic metals such as zero valent iron for NO3 reduction reported an undesirable high selectivity to NH4+ and iron leaching caused by its corrosion. Pd has demonstrated the highest stability so far and scarce leaching in the pH range for water potabilization. The aim of this work is to study and compare the catalytic reduction of NO2 and NO3 with H2 as reducing agent using Pd catalysts supported on Al2O3, SiO2 and SnO2. The effect of the H2 flow and the supplementation of CO2 have also been evaluated. A novel Pd-In/γ-Al2O3 catalyst has been also prepared and evaluated in terms of activity, selectivity to N2 and stability treating NO3 bearing water.

MATERIALS AND METHODS

Catalyst preparation

Al2O3 Brockmann (Sigma Aldrich), SiO2 (Scharlab), and SnO2 (Sigma) were used as supports to prepare the monometallic catalysts tested in the batch experiments. Pd was incorporated in the oxides by wet impregnation of PdCl2 (Sigma, 99.99%) dissolved in an acidic solution (0.1 M HCl) or Na2PdCl4 (Sigma, 99.99%) dissolved in water (1 mL/g of support). The bimetallic catalysts supported on Al2O3 or SiO2 were co-impregnated using PdCl2 (Sigma, 99.99%) (0.1 M HCl) with InCl3 or SnCl4 (liquid). The impregnated materials were dried overnight at 100 °C and calcined at 500 °C for 2 h. The resulting catalysts were reduced in H2 atmosphere (35 NmL/min) at 100 °C for 1 h before the catalytic experiments. The Pd content in the monometallic catalyst was 5 wt%. Bimetallic catalysts were prepared with 5 wt% of Pd with a ratio Pd:Me 4:1 (w:w), where Me is Sn or In.

Catalyst characterization methods

Room temperature powder X-ray diffraction (XRD) patterns were obtained with a Bruker D8 Advance A25 diffractometer with a Bragg Brentano configuration and a lineal detector Lynxeye (Bruker). The X-ray source was a copper long fine focus X-ray diffraction tube operating at 40 kV and 30 mA, with an angular step of 0.015° and a time per step of 0.1 s. Crystalline phases were identified by referencing the PDF 2014 database.

The porous structure of the catalyst was assessed from the −196 °C N2 adsorption-desorption isotherms, obtained in a Micromeritics ASAP 2020. The samples were previously outgassed at 160 °C and 0.66 Pa for 16 h. The specific surface area was calculated from the BET method (SBET). The particle size distribution of the metallic phase was determined by transmission electron microscopy (TEM) using a Phillips CM-200 microscope with a point of resolution of 2.8 Å coupled with an energy dispersive EDAX X-ray spectrometer.

Catalytic reduction of nitrite and nitrate

The reduction experiments in batch mode were performed in batch glass reactors (0.6 L capacity) at 25 °C and atmospheric pressure using 1.61 mM initial concentration of NO2 (NaNO2, Scharlau, 98%) or NO3 (NaNO3, Scharlau, 99%) with 1 g/L of catalyst. Internal and external mass transfer limitations were discarded after preliminary experiments where the effects of agitation (700 rpm), bubbling and particle size were checked. The reactor was placed in a thermostatic bath and the experiments were conducted under continuous H2 or H2 + CO2 flow.

The catalyst was previously reduced at 100 °C under H2 flow (35 N mL/min) for 1 h and outgassed with N2 for 30 min at ambient temperature. The temperature of the reactor was maintained at 25 °C before and during the catalytic tests. The catalyst was placed inside the reactor with 450 mL of distilled water. The catalyst was maintained in this aqueous suspension under continuous stirring and bubbling H2 or H2 + CO2 for 20 min prior to the catalytic test in order to favour the catalyst wetting. A H2 gas flow of 15 N mL/min was used, which was reduced to 8 N mL/min when CO2 was supplemented (8 N mL/min). Then, a concentrated solution containing (16.1 mM of NO2 (NaNO2) or NO3 (NaNO3) was fed to the catalytic reactor, reaching an initial concentration of 1.61 mM. Catalytic tests of 1 and 3 h were performed for NO2 and NO3 reduction, respectively. Samples were withdrawn from the reactor and filtered through regenerated cellulose filters with a pore diameter of 0.45 μm.

The stability of the catalysts was checked upon long-term continuous experiments (up to 500 h) carried out in a fixed-bed reactor (Pyrex glass, 30 cm length, 9 mm inner diameter). The catalysts were deposited on a porous glass support placed in the middle part of the reactor. The catalyst used consisted of 0.3 g of Pd-In/Al2O3 synthetized according to the aforementioned method. An aqueous solution of 100 mg/L of NO3 was fed (0.5 mL/min) with H2 and CO2 at different flow rates (0.3–1.5 N mL/min) and fractions (3.75–100% H2/CO2, v/v). Liquid samples were taken from the reactor at different times on stream.

Analytical methods

NO3, NO2, NH4+ and total nitrogen (NT) were analyzed by colorimetric titration using Hach Lange reagents and a benchtop VIS spectrophotometer DR 3900™ with RFID (Hach Lange). The pH, electrical conductivity (EC) and temperature were analyzed using a Crison MM 40 equipment.

RESULTS AND DISCUSSION

Characterization of mono- and bimetallic catalysts

The XRD diffractograms of the Pd catalysts prepared by impregnation of PdCl2 on SiO2, Al2O3 and SnO2 are depicted in Figure 1(a)–1(c), respectively. The mean Pd particle size was measured using the Scherrer equation giving values of 14, 9 and 11 nm for Pd/Al2O3, Pd/SiO2 and Pd/SnO2, respectively. The crystallinity of these catalysts samples resulted in 40%, 73% and 77%, respectively. Table 1 shows the BET area and the average pore diameter of these monometallic catalysts. A high increase of the pore width upon the impregnation stage was observed for Pd supported on Al2O3 probably due to the acidic nature of the PdCl2 solution. Contrarily, pore size diminished when Pd was incorporated to SiO2, which seems to be related to pore system blockage. The effect of Pd incorporation on SnO2 was negligible. A similar response was observed with the bimetallic catalysts (Table 2), but the effect of the impregnation was more severe in this case. The main difference with respect to the monometallic catalysts is the significant decrease in the BET surface area values obtained; this is because a high percentage of the metals are co-impregnated. The metal loads obtained are close to the expected nominal values, which indicates that the impregnation method proposed is highly effective.

Figure 1

XRD diffractograms of Pd/SiO2 (a), Pd/Al2O3 (b), and Pd/SnO2 (c).

Figure 1

XRD diffractograms of Pd/SiO2 (a), Pd/Al2O3 (b), and Pd/SnO2 (c).

Table 1

Characterization of the monometallic catalysts

Support/catalyst BET surface area (m2 g−1External area (m2 g−1VPa (cm3 g−1DPb (nm) 
Al2O3 145 149 0.27 2.4 
SiO2 500 508 0.67 5.3 
SnO2 30 28 0.19 18.0 
Pd/Al2O3 145 143 0.26 4.8 
Pd/SiO2 497 513 0.62 3.9 
Pd/SnO2 30 27 0.18 18.1 
Support/catalyst BET surface area (m2 g−1External area (m2 g−1VPa (cm3 g−1DPb (nm) 
Al2O3 145 149 0.27 2.4 
SiO2 500 508 0.67 5.3 
SnO2 30 28 0.19 18.0 
Pd/Al2O3 145 143 0.26 4.8 
Pd/SiO2 497 513 0.62 3.9 
Pd/SnO2 30 27 0.18 18.1 

aVP (pore volume) Adsorption total volume of pores.

bDP (pore diameter) Adsorption average pore width (4 V/A by BET).

Table 2

Characterization of the bimetallic catalysts tested and the catalytic activity in NO3 reduction

Catalyst BET surface area (m2 g−1External area (m2 g−1VPa (cm3 g−1DPb (nm) 
Pd-Sn/Al2O3 131 133 0.23 7.2 
Pd-In/Al2O3 122 120 0.22 6.1 
Pd-Sn/SiO2 435 448 0.55 4.0 
Pd-In/SiO2 420 434 0.54 5.1 
Catalyst BET surface area (m2 g−1External area (m2 g−1VPa (cm3 g−1DPb (nm) 
Pd-Sn/Al2O3 131 133 0.23 7.2 
Pd-In/Al2O3 122 120 0.22 6.1 
Pd-Sn/SiO2 435 448 0.55 4.0 
Pd-In/SiO2 420 434 0.54 5.1 

aVP (pore volume) Adsorption total volume of pores.

bDP (pore diameter) Adsorption average pore width (4 V/A by BET).

TEM images of the monometallic catalysts are depicted in Figure 2. As can be observed in Figure 2(a) and 2(b), the high load of Pd achieved in the Al2O3 catalysts favoured the agglomeration of the Pd nanoparticles. The micrographs of Pd/SiO2 (Figure 2(c) and 2(d)) and of Pd/SnO2 (Figure 2(e) and 2(f)) show a high dispersion of the metallic active phase. The average nanoparticle diameters of the Pd nanoparticles calculated were 62, 20 and 5 nm for Pd-Al2O3, Pd/SiO2 and Pd/SnO2, respectively, following the procedure described elsewhere (Pizarro et al. 2014).

Figure 2

TEM images of monometallic Pd catalysts on different supports: Al2O3 (a) and (b), SiO2 (c) and (d) and SnO2 (e) and (f).

Figure 2

TEM images of monometallic Pd catalysts on different supports: Al2O3 (a) and (b), SiO2 (c) and (d) and SnO2 (e) and (f).

Catalytic reduction of nitrite using monometallic catalysts

Figure 3 shows the NH4+ concentration observed at different NO2 conversion with monometallic catalysts impregnated with PdCl2. The analysis of the selectivity to NH4+ formation revealed a broad response of the catalysts tested. The Pd/SiO2 catalyst was especially attractive since a concentration of NH4+ lower than 3.5 mg/L was obtained for a NO2 conversion close to 50%. This has been related to its small nanoparticle size, which favours the selectivity to N2 (Matatov-Meytal et al. 2003). The use of Al2O3 and SnO2 as supports led to NH4+ concentrations of 7.0 and 8.5 mg/L, respectively, for a similar NO2 conversion value (50%).

Figure 3

NO2 reduction with monometallic Pd catalysts (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO2]0 = 1.61 mmol/L , QH2 = 15 N mL/min).

Figure 3

NO2 reduction with monometallic Pd catalysts (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO2]0 = 1.61 mmol/L , QH2 = 15 N mL/min).

The Pd/SnO2 catalyst showed the highest reaction rates and conversion (70%), which reveals the positive impact of highly dispersed Pd nanoparticles (Figure 2). However, this catalyst led to an NH4+ concentration higher than that obtained using Pd/Al2O3 and Pd/SiO2 catalysts. This fact could be related to the synergistic interaction between Pd and the SnO2 reducible support.

Catalytic reduction of nitrite using bimetallic catalysts

The catalytic reduction of NO2 was also evaluated using bimetallic Pd-Sn and Pd-In catalysts supported on Al2O3 and SiO2. As can be observed in Figure 4(a), the addition of co-metals, especially In, increased the conversion extension up to 0.82. Figure 4(a) shows the NH4+ concentration produced with Pd-Sn/Al2O3 and Pd-Sn/SiO2 catalysts. The conversion obtained with these catalysts exceeds the values obtained with the monometallic catalysts supported on Al2O3 or SiO2. Previous works have reported that bimetallic catalysts favour the dispersion of the Pd metallic phase due to the strong interaction between Pd and the SnO2 phase, expanding the active surface of Pd, especially when a co-impregnation method is used (Bošković et al. 2008; Pizarro et al. 2015).

Figure 4

NO2 reduction with bimetallic Pd catalysts supported on inert oxides; (a) catalysts based on Pd-Sn; (b) catalysts based on Pd-In (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO2]0 = 1.61 mmol/L, Q H2 = 15 N mL/min).

Figure 4

NO2 reduction with bimetallic Pd catalysts supported on inert oxides; (a) catalysts based on Pd-Sn; (b) catalysts based on Pd-In (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO2]0 = 1.61 mmol/L, Q H2 = 15 N mL/min).

The addition of Sn (Pd-Sn/Al2O3) as co-metal also enhanced the selectivity to N2, reducing the NH4+ concentration from 6.7 to 3.6 mg/L at a NO2 conversion of 50%. Unlike the results observed with the monometallic catalysts, the use of SiO2 in bimetallic catalysts produced more NH4+ than those supported in Al2O3.

The results obtained in the catalytic reduction of NO2 with bimetallic Pd-In catalysts supported on Al2O3 and SiO2 are shown in Figure 4(b). The presence of In led to a higher activity than that obtained with Sn doped catalysts. This enhancement of the catalytic activity seems to be related to a strong interaction between Pd and In2O3 phases that results in a close contact between the two active phases. The production of NH4+ with Pd-In catalysts yielded concentrations of 5.0 and 12.5 mg/L at a NO2 conversion of 50% with Pd-In/Al2O3 and Pd-In/SiO2, respectively.

Catalytic reduction of nitrate using monometallic catalysts

The catalytic reduction of NO3 using a monometallic Pd-SnO2 catalyst was tested using two different salts as precursors: PdCl2 and Na2PdCl4 (Figure 5). First, the NO3 reduction was performed with H2 as the only gas fed (Figure 5(a)). The conversion reached with the catalyst synthesized with PdCl2 (90%) was higher than that synthesized with Na2PdCl4 (60%) while a slight difference was observed on the selectivity to NH4+ at a similar NO3 conversion. Nitrite production was favoured with the catalyst impregnated with Na2PdCl4.Figure 5(b) shows the conversion of NO3 and the concentration of NH4+ reached when feeding H2:CO2 (50%, v/v), where a significant enhancement in the catalytic performance was observed. The addition of CO2 in the inlet gas led to a total NO3 conversion and decreased the selectivity to NH4+ as a consequence of the OH neutralization in the pore system. The use of PdCl2 as salt precursor resulted in a higher catalytic activity and a high NO3 reduction rate due to the close interaction between Pd and the reducible support. In addition, NO2 accumulation in the reaction medium and the increase of the pH medium, were mitigated in the presence of CO2.

Figure 5

NO3 reduction with monometallic Pd/SnO2 catalyst synthesized with PdCl2 and Na2PdCl4; (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 NmL/min or QH2 + CO2 = 16 N mL/min). Dashed lines represent NO2 concentration.

Figure 5

NO3 reduction with monometallic Pd/SnO2 catalyst synthesized with PdCl2 and Na2PdCl4; (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 NmL/min or QH2 + CO2 = 16 N mL/min). Dashed lines represent NO2 concentration.

Catalytic reduction of nitrate with bimetallic catalysts

The catalytic reduction of NO3 was also tested with Pd-Sn and Pd-In catalysts impregnated with PdCl2 supported on Al2O3 and SiO2 in absence and presence of CO2. Figure 6(a) shows the NO3 conversion and the concentration of NO2 and NH4+, corresponding to the catalytic reduction of NO3 with Pd-Sn/SiO2 and Pd-In/SiO2 catalysts without pH buffering. The Pd-In catalyst yielded total conversion of NO3 in less than 3 h while the Pd-Sn catalyst reached less than 25% conversion. The latter was deactivated by the alkaline medium generated in the catalytic reaction studied.

Figure 6

NO3 reduction with Pd-Sn and Pd-In supported on SiO2 (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 NmL/min or QH2 + CO2 = 8 and 8 NmL/min). Dashed lines represent NO2 concentration.

Figure 6

NO3 reduction with Pd-Sn and Pd-In supported on SiO2 (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 NmL/min or QH2 + CO2 = 8 and 8 NmL/min). Dashed lines represent NO2 concentration.

The performance of these bimetallic Pd-Sn and Pd-In catalysts supported on SiO2 was improved significantly when adding CO2 to control the pH value between 5.0 and 5.5 in the reaction medium (Figure 6(b)). The NO3 conversion was significantly increased by supplementing CO2, reaching a conversion of 65% with the Pd-Sn/SiO2, and total conversion was achieved with the Pd-In/SiO2 catalyst. The selectivity to NH4+ obtained with the latter clearly diminished, with concentrations from 21 to 14 mg/L and from 5 to less than 3 mg/L at a NO3 conversion of 100% and 50%, respectively. The initial rate of NO3 reduction reached with the Pd-In/SiO2 catalyst was fivefold higher than that of Pd-Sn/SiO2 (Table 3).

Table 3

Selectivity to NH4+ (at different NO3 conversion values) and initial rates calculated in the catalytic reduction of NO3 with and without CO2 addition

Catalyst SNH4+ [H2] % (NO3 conversion) SNH4+ [H2 + CO2] % (NO3 conversion) Specific initial conversion rate (mmolNO3 min−1 gPd−1
Pd/SnO2 46 (91) 49 (100) – 
Pd/SnO2 (Na) 73 (91) 51 (100) – 
Pd-Sn/Al2O3 74 (50) 56 (100) 0.74 
Pd-In/Al2O3 92 (100) 34 (100) 1.44 
Pd-Sn/SiO2 48 (24) 64 (64) 0.50 
Pd-In/SiO2 94 (100) 59 (100) 2.57 
Catalyst SNH4+ [H2] % (NO3 conversion) SNH4+ [H2 + CO2] % (NO3 conversion) Specific initial conversion rate (mmolNO3 min−1 gPd−1
Pd/SnO2 46 (91) 49 (100) – 
Pd/SnO2 (Na) 73 (91) 51 (100) – 
Pd-Sn/Al2O3 74 (50) 56 (100) 0.74 
Pd-In/Al2O3 92 (100) 34 (100) 1.44 
Pd-Sn/SiO2 48 (24) 64 (64) 0.50 
Pd-In/SiO2 94 (100) 59 (100) 2.57 

NO3 conversion and selectivity to NH4+ was significantly improved when CO2 was added using the Pd-Me bimetallic catalysts supported on Al2O3. Figure 7(a) shows the results obtained during the catalytic reduction of NO3 experiments with Pd-Sn/Al2O3 and Pd-In/Al2O3 in absence of CO2. Bimetallic Pd-Sn/Al2O3 catalyst led to a higher conversion level (50%) than the Pd-Sn/SiO2 catalyst (25%) showed in Figure 6(a). Pd-In/Al2O3 catalyst reached total conversion and a similar selectivity to NH4+ to that obtained with the Pd-In/SiO2 catalyst (Figure 6(a)). NO2 is accumulated in the absence of a pH buffering source (Figure 6(a)), which is effectively transformed by the addition of CO2 (Figure 6(b)).

Figure 7

NO3 reduction with Pd- Sn and Pd-In supported on Al2O3 (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 N mL/min or QH2 + CO2 = 8 and 8 N mL/min). Dashed lines represent NO2 concentration.

Figure 7

NO3 reduction with Pd- Sn and Pd-In supported on Al2O3 (a) with H2 and (b) with H2 + CO2 (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 15 N mL/min or QH2 + CO2 = 8 and 8 N mL/min). Dashed lines represent NO2 concentration.

The results obtained with the Pd-Sn/Al2O3 and Pd-In/Al2O3 catalysts without and with CO2 in the gas feed are depicted in Figure 7(a) and 7(b), respectively. A total NO3 conversion was achieved with both catalysts when CO2 was added, the Pd-In catalyst being less selective to NH4+, reaching a concentration of 7.5 mg/L in the final effluent (Figure 7(b)). The initial rate of NO3 reduction reached by the Pd-In/Al2O3 catalyst was twice the observed with the Pd-Sn/Al2O3 catalyst (Table 3). Again, the pH control by adding CO2 was an effective strategy to diminish the production of NO2 and NH4+. These results indicate that the bimetallic catalysts supported on Al2O3 are the most appropriate for the reduction of NO3; the Pd-In/Al2O3 catalyst was selected for the next experiments. The wider pores of Al2O3 than that observed for SiO2 could present a positive effect on the neutralization of OH when CO2 is supplemented.

Influence of H2 flow on the catalytic activity

It has been reported that the H:N ratio in the gas feed can have a decisive influence on the selectivity to N2 obtained during the catalytic reduction of NO3 (Hörold et al. 1993). With the aim of analyzing the influence of the amount of H2 present in the reaction medium, the H2 flow was varied from 0.3 to 8.0 N mL/min maintaining a CO2 gas flow of 8.0 N mL/min with the Pd-In/Al2O3 catalyst. Figure 8(a) shows the concentration of NH4+ obtained at different NO3 conversions. The amount of NH4+ produced with the lowest H2 flow rate (0.3 N mL/min) was 0.33 mg/L of NH4+ (selectivity to NH4+ of 2.7%) for a NO3 conversion of 55%, reaching the legal limits established by the environmental legislation. This result improves those obtained under H2 and CO2 saturation as reflected in Table 3. This shows that selectivity towards NH4+ can be effectively diminished by controlling the H:N ratio in the gas feed. High values of NO3 conversion (around 80%) were reached at increasing H2 flows (4.0 and 8.0 N mL/min).

Figure 8

(a) NO3 reduction with Pd-In/Al2O3 with different gas flows (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 0.3, 4 and 8 N mL/min or QCO2 = 8 NmL/min). (b) Long-term NO3 reduction test with Pd-In/Al2O3 (25 °C, 1 atm, pH0 6, Wcat = 0.3 g, [NO3]0 = 1.61 mmol/L (100 mg/L), different QH2:QCO2 in each stage). (c) Long-term NO3 reduction test with Pd-In/Al2O3 (25 °C, 1 atm, pH0 6, Wcat = 0.3 g, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 0.75 N mL/min or QCO2 = 0.75 N mL/min).

Figure 8

(a) NO3 reduction with Pd-In/Al2O3 with different gas flows (25 °C, 1 atm, pH0 6, [catalyst]0 = 1 g/L, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 0.3, 4 and 8 N mL/min or QCO2 = 8 NmL/min). (b) Long-term NO3 reduction test with Pd-In/Al2O3 (25 °C, 1 atm, pH0 6, Wcat = 0.3 g, [NO3]0 = 1.61 mmol/L (100 mg/L), different QH2:QCO2 in each stage). (c) Long-term NO3 reduction test with Pd-In/Al2O3 (25 °C, 1 atm, pH0 6, Wcat = 0.3 g, [NO3]0 = 1.61 mmol/L (100 mg/L), QH2 = 0.75 N mL/min or QCO2 = 0.75 N mL/min).

The availability of chemisorbed hydrogen promotes the formation of NH4+ from NO2 (Ebbesen et al. 2008). This fact determines the selectivity when different relative concentrations of NO3/NO2 are treated, different sizes of Pd nanoparticles are used or the solubility of H2 is modified by changing the operation conditions.

Stability of the Pd-In/Al2O3 catalyst

The stability of the Pd-based catalysts has been scarcely studied, showing high and constant activities treating synthetic NO3-polluted waters (Wang et al. 2009). The influence of the amount or relative fraction of the gases fed has been also analyzed during long-term stability tests. Other long-term experiments with Pd-In catalysts using different experimental conditions such as powdered Pd/Al2O3 catalysts, H2 or formic acid as reducing agents and lower temperatures have been conducted in previous studies (Prüsse et al. 2000; Chaplin et al. 2007, 2009).

The Pd-In/Al2O3 was used in a fixed-bed reactor varying the gas flow and the proportion of H2 to CO2. Four different gas mixtures were evaluated, stages I–IV (Figure 8(b)). A gas composition of 0.3 N mL/min of H2 diluted in 8 N mL/min of CO2 (3.6% H2) was fed during stage I. This experiment lasted for more than 50 h, detecting a very low amount of NH4+ in the resulting effluent (0.2 mg/L) but reaching a low NO3 conversion of around 11%. Higher NO3 conversions (28%) were obtained when the CO2 flow was reduced to 1.2 N mL (25% H2) where no more than 0.5 mg/L of NH4+ was detected. The addition of a higher amount of H2 (0.75 N mL/min), keeping the relative H2:CO2 fraction constant (50%), also led to an increase of NO3 conversion up to 32% and an NH4+ concentration of around 1.0 mg/L. During the last stage H2 was fed solely to the reactor at a flow rate of 1.5 N mL/min. The catalytic activity showed a significant increase of the NO3 conversion by 31–37% with respect to the previous experimental stages. Nevertheless, the selectivity to NH4+ increased, detecting 2.0 mg/L in the discharge due to the increase of the pH values from 4–5 to around 9 (Figure 8(b)). Decreasing H2 feed flow values reduces the production of NH4+ dramatically, and the pH of the resulting effluents are leveled to neutral values.

Finally, a catalytic test was performed under the same conditions evaluated in stage III to analyze the stability of the catalyst (Figure 8(c)). The space-time used was 6.21 kgcat·h/mol. The results showed herein demonstrate that the Pd-In/Al2O3 catalyst was stable during 500 h on stream. The conversion values were maintained between 30% and 42% with a production of NH4+ between 1.6 and 2.2 mg/L during the whole experiment. The pH values directly depend on the amount of CO2 added, concluding that values lower than 5.5 are optimal for the catalytic reaction. The concentration of H2 is one of the main limiting steps that determines the N:H ratio in this process which can be fine-tuned in future applications.

CONCLUSIONS

The activity and selectivity to N2 of monometallic Pd catalysts used for NO2 removal in water have been evaluated with SnO2, Al2O3 and SiO2 as supports, respectively. The use of bimetallic Pd-Sn and Pd-In catalysts improved the NO2 conversion, especially with the Pd-In catalysts where the metallic phases showed a strong interaction. Pd-In catalysts showed high activity in the NO3 reduction, leading to low NH4+ concentration effluents. The Pd-In/Al2O3 prepared in this work has shown promising stability (more than 20 days in continuous operation) and a high selectivity to N2. The addition of CO2 in the gas feed as buffering agent and the limitation of H2 dose are effective to control NH4+ formation, and offer an efficient strategy to ensure a long-term operation satisfying drinking water quality standards.

ACKNOWLEDGEMENTS

The authors would like to express their gratitude to A. Buenaventura, Director of Abengoa Research, for his help. The authors also thank Santiago Medina Carrasco and the XRD laboratory team from the University of Seville for their help in the development of the work. The authors would also like to express their gratitude to Maria Elena Guillén, Manuel Antonio Díaz and Javier Echave for their help in this work.

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