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Article

Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction

by
Ines Sanchis
*,
Juan Jose Rodriguez
,
Angel F. Mohedano
and
Elena Diaz
Chemical Engineering Department, Faculty of Sciences, Universidad Autonoma de Madrid, 28049 Madrid, Spain
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(7), 729; https://doi.org/10.3390/catal12070729
Submission received: 28 May 2022 / Revised: 24 June 2022 / Accepted: 28 June 2022 / Published: 30 June 2022
(This article belongs to the Special Issue Porous Materials: Active Phases or Supports in Heterogeneous Catalysis)
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Abstract

:
In this work, we study the effect of modifying the metal loading (0.5–1.5 wt.% Pd and 0.1–1 wt.% Sn or In), the impregnation order of noble or promoter metal (Pd–Sn or Sn–Pd), and the type of promoter metal (Sn or In) during the preparation process for a Pd bimetallic catalyst, supported on γ-alumina, used in the catalytic reduction of nitrate. The deposition of the noble metal over the promoter metal, especially with Pd:Sn ratios (wt.) of 1:10 and 1:2, favored the hydrogen spillover rate and increased the H concentration on the catalyst surface, enhancing NH4+ production. On the other hand, Pd–In catalysts showed higher activity than the Sn catalysts, as well as higher NH4+ selectivity. The stability of the Pd–Sn/Al2O3 (1.5–1 wt.%) catalyst was evaluated in long-term experiments for the treatment of synthetic water (100 mg L−1 NO3−) and three different commercial drinking waters. This Pd–Sn/Al2O3 catalyst achieved a stable nitrate conversion for a duration of 50 h in the synthetic water treatment. However, the catalyst showed a significant activity loss in the presence of other ions (different to NO3−) in the reaction medium, increasing slightly the selectivity to NH4+.

Graphical Abstract

1. Introduction

Nitrate (NO3−) is one of the most widespread inorganic contaminants in groundwater sources worldwide. The presence of nitrate in drinking water raises major concerns because it is associated with health problems, such as methemoglobinemia, reproductive/developmental effects, and different types of cancer [1,2,3]. The European Union established limitations on the concentration of NO3− and its reduced forms (nitrite (NO2−) and ammonium (NH4+)) in drinking water. Directive 91/676/EEC [4] sets maximum concentrations of 11.3, 0.03, and 0.39 mg N L−1 for NO3−, NO2−, and NH4+, respectively. The World Health Organization (WHO) recommends not exceeding 10 mg L−1 N-NO3− [5]. Currently, one of the major concerns about NO3− pollution is the lack of an efficient treatment for its disposal [6]. Most of the current denitrification technologies, such as ion exchange and reverse osmosis, generate a highly concentrated residual NO3− brine, which requires further treatment [7,8,9,10], increasing economic and environmental costs. On the other hand, biological denitrification is an effective technique for NO3− removal that converts NO3− to N2, but the possibility of contamination by microorganisms prevents its use for drinking water [7].
Catalytic NO3− reduction was first reported by Vorlop and Tackle [11] as a favorable alternative to overcome the disadvantages of the above-mentioned technologies. The treatment is based on the selective reduction of NO3− to harmless N2, using a bimetallic catalyst and a reducing agent, such as H2. The main drawbacks of this treatment are the formation of NO2− as a reaction intermediate and NH4+ as a by-product. In addition, the ions present in natural waters can cause catalyst deactivation, making it difficult to use for drinking water [12,13,14,15].
The catalytic reduction of NO3− requires the use of bimetallic catalysts, composed of a noble metal (Pd or Pt) and a promoter metal (Cu, Sn, or In), which integrate the active phase, mainly supported on metal oxides (Al2O3, SiO2, or TiO2) and activated carbon [8,9,10]. Al2O3 is considered a passive support because it does not have reducible capacity, and therefore does not participate directly in the reduction process [9]. Aqueous NO3− adsorbs on the active sites of the promoter, where it is reduced to NO2−; this causes the oxidation of the promoter metal, which returns to its initial state due to hydrogen activation carried out by the noble metal [16]. Subsequently, the intermediate NO2− is reduced over the noble metal sites to the final products (N2 and NH4+), with the promoter metal not being involved in the reaction. The most used catalysts show a noble metal content between 0.1–2 wt.% and could reach values of 5 wt.%. The promoter metal usually range from 0.1 to 2.5 wt.%. The usual noble to promoter metal mass ratios are 4:1, 2:1, and 1:1 [10,16,17,18,19,20,21]. Several authors have reported that a higher amount of promoter metal could give rise to a higher coverage of the noble metal surface [21,22,23]. Thus, the H spillover from the noble site to the promoter metal decreases and therefore the rate of NO3− reduction. On the other hand, an excess of noble metal with respect to the promoter metal increases the hydrogen spillover rate and consequently decreases the N/H ratio on the catalyst surface, favoring the formation of NH4+ [15,17,21,22,23,24].
One of the most important drawbacks of catalytic NO3− reduction is catalyst deactivation during the treatment of polluted waters, which limits the large-scale application of this technology [10]. The most frequently reported causes of catalyst deactivation in the literature can be summarized as (i) irreversible oxidation of the promoter metal, (ii) fouling of the catalyst surface, (iii) leaching of the metal phase, and (iv) aggregation of metal particles [10]. Since the catalytic reduction of nitrate is based on the continuous redox reaction between NO3− and the promoter metal, the irreversible oxidation of the latter could limit the progression of the reaction, weakening the reactivity of the bimetallic catalyst and decreasing the NO3− removal efficiency [25]. On the other hand, the fouling of the catalyst surface by salt precipitation is the most common deactivation cause in NO3− removal treatments in water wherein other compounds are present [13,24,26,27,28,29,30,31,32]. The loss of metal components is highly dependent on the reaction pH [33] and can be avoided by modifying the catalyst characteristics [34] or operating conditions [33,35]. Finally, the sintering of metal particles results in an increased particle size and decreased metal dispersion, which is associated with a loss of catalytic activity [30,36,37].
The order of metal impregnation during the catalyst preparation can influence the formation of alloy particles on the catalyst surface, which have been related to decreased catalyst activity and N2 production [38,39]. Batista et al. [40] reported that the catalyst preparation procedure in which the alumina is firstly impregnated with Cu salt, followed by the deposition of Pd salt, enhanced the formation of Pd–Cu alloys. Aristizabal et al. [41] observed that the impregnation order mainly controlled the type and extent of active sites formed in the catalysts. They observed that, in catalysts prepared with the Ag-Pt order, the active sites were similar to the observed in Ag monometallic catalysts. However, the Pt-Ag impregnation order achieved a higher Pt contact with the Ag-containing phases. On the other hand, Pintar et al. [42] reported that the order of Cu–Pd on the alumina support improved N2 production (90% N2 selectivity) as compared to Pd–Cu (83% N2 selectivity), both at complete nitrate conversion.
It has been reported that an increase in the metal loading of the active phase leads to an improvement in catalyst activity [19,21]. The widely used noble metal content is in the range of 0.1–2 wt.% and, in some cases, reaches the value of 5 wt.%, while the promoter metal content usually varies between 0.1 and 2.5 wt.% [10]. An excessive increase in promoter metal bulk (>2.8 wt.%) has been related to adverse effects on catalytic denitrification [22]. The active surface of the noble metal may be overlapped by the promoter metal, which leads to a decrease in the area available for H adsorption on the catalytic surface. This reduces the chances of H spillover from the noble metal to the promoter metal and interference with the redox cycle in which the active sites are repeatedly reoxidized and reduced [21,22,23]. On the other hand, an excess of noble metal with respect to the promoter metal causes an increase in the H concentration on the catalyst surface, which decreases the N/H ratio, increasing NH4+ production.
This study aims to look for the optimal characteristics of Pd-bimetallic catalysts, evaluating the effect of modifying the metal loading, the impregnation order, and the promoter metal, both in the conversion of NO3− and in the selectivity to NH4+. In addition, the activity and stability of the catalyst are evaluated in a continuous flow using commercial drinking waters spiked with NO3−.

2. Results and Discussion

2.1. Catalyst Configuration Effect

Figure 1 shows the conversion and selectivity for ammonium over the reaction time with the tested Pd bimetallic catalysts. Table 1 reports the main characteristics of the catalysts, the reaction selectivity, and the kinetic constants (pseudo first order) obtained from the fitting to the experimental NO3− removal data. As can be observed, the real metal content of the bimetallic catalysts analyzed by TXRF was close to the nominal value in all cases, confirming effective impregnation. Bimetallic impregnation slightly decreased the surface area and the mesoporous volume of the initial alumina, with the microporous volume being negligible.
The nitrate removal obtained with all the bimetallic catalysts was close to complete conversion at the end of the reaction time. Regarding the Pd–Sn impregnation order, for a constant Pd content, increasing the Sn concentration improved the catalyst activity, although not proportionally. As the Sn concentration increased from 0.1 to 0.5 wt.%, the kinetic constants of Pd1–Snx catalysts grew slightly, from 9.1 × 10−3 to 10.1 × 10−3 L min−1 gcat−1, respectively. However, in the case of the opposite order of impregnation, this increase in Sn content did not produce a modification in the activity of Snx–Pd1 catalysts, obtaining similar kinetic constant values of 12.6 × 10−3 and 12.5 × 10−3 L min−1 gcat−1 for Sn0.1–Pd1 and Sn0.5–Pd1, respectively. However, an increase in the Sn content to 1 wt.%, with identical Pd content, improved the activity of the catalysts, reaching values of 19.5 × 10−3 and 18.8 × 10−3 L min−1 gcat−1 for the Pd1–Sn1 and Sn1–Pd1 catalysts, respectively. On the other hand, the activity of the catalysts improved with increasing Pd content when keeping the Sn content at 1 wt.%, reaching high kinetic constant values (21.7 × 10−3 and 23.6 × 10−3 L min−1 gcat−1 for the Pd1.5–Sn1 and Sn1–Pd1.5 catalysts, respectively). Pizarro et al. [19] and Mendow et al. [21] also observed an improvement in catalyst activity by increasing the metal loading of the active phase. Pizarro et al. [19] studied the effect of increasing the metal content for the same metal ratio by 2:1 with Pd–Sn catalysts (5−2.5 and 1−0.5 wt.%) supported on pillared clays. They observed that the catalyst with the highest metal loading (5−2.5 wt.%) achieved complete nitrate conversion, and its activity increased tenfold compared to that of the catalyst with 1−0.5 wt.%, which only removed 64 wt.% of the initial nitrate concentration.
Regarding selectivity, all catalysts presented a NO2− production lower than 0.7%, with the amount being negligible in several cases. The results indicated that the metal loading for maximizing N2 production is 1–1.5 wt.% for Pd and 1 wt.% for Sn. Moreover, the Sn0.1–Pd1 and Sn0.5–Pd1 catalysts showed the highest NH4+ selectivity, 20.9 and 22.0%, respectively. Therefore, particularly for this impregnation order, the Pd:Sn ratios of 10:1 and 2:1 favored NH4+ production. Mendow et al. [21] observed that Pd–Sn catalyst supported on an anionic resin with a ratio of 10:1 (1−0.1 wt.%) produced less N2 than did a catalyst with a 1:1 ratio (1−1 wt.%), producing 80.9 and 91.8%, respectively. Thus, in a catalyst with an excess of noble metal with respect to promoter metal, the active surface of Pd available for H activation is increased, decreasing the N/H ratio on the catalyst surface, which is related to the increase in NH4+ production [15,17,21,22,23,24].
The order of metal impregnation appeared to have a negligible effect on the catalyst activity, showing similar kinetic constants between Pd–Sn and Sn–Pd metal loading pairs. However, Batista et al. [40] observed that the impregnation of alumina with a promoter metal (Cu) salt followed by the deposition of a Pd salt enhanced the formation of Pd–Cu alloys, which is related to a decrease in the catalyst activity [38]. However, in this study, differences in catalyst selectivity associated with the order of metal impregnation were observed. The results indicated higher NH4+ production when the Sn salt was impregnated first. Furthermore, the largest differences in terms of NH4+ production were observed with the highest Pd:Sn ratios (10:1, 2:1) when comparing Sn0.1–Pd1 and Sn0.5–Pd1 with their Pd–Sn counterparts. Thus, the deposition of the noble metal over the promoter metal, especially when the ratios were 10:1 and 2:1, favored the hydrogen spillover rate [21], increased the H concentration on the catalyst surface, and enhanced NH4+ production. These results differ from those reported by Pintar et al. [42], who observed that impregnation with the promoter metal followed by the deposition of noble metal enhanced N2 production using a Pd-Cu catalyst supported on alumina. However, this increase in the N2 selectivity not only was attributed to the decrease in the ammonium production, but also the decrease in the selectivity to nitrite. Pintar et al. observed that the Pd-Cu clusters were located covered by a layer composed of Pd atoms when they used the impregnation order Cu-Pd. Due to the higher adsorption affinity of nitrate ions to Pd-Cu clusters, the nitrite once formed was forced to migrate into the aqueous solution. The reduction of NO2− is carried out on the Pd active sites. Therefore, if the Pd sites were more available, the NO2− reduction was more efficient and the selectivity to N2 increased. Moreover, the NO2− had a positive effect in the N/H ratio on the catalyst surface, which could explain the increase in the production of N2 instead of NH4+ production.
Regarding the promoter metal, the influence of an increase in metal loading in Pd–In catalysts seems to be less important than that in Pd–Sn ones, achieving kinetic constant values of 15.4 × 10−3 and 20.1 × 10−3 min−1 with Pd1–In0.1/Al2O3 and Pd1–In0.5/Al2O3, respectively twice those observed with Pd–Sn catalysts with the same metal content. In all cases, except for Pd0.5–In1, In-bimetallic catalysts showed higher activity than did those with Sn. However, as can be seen in Table 1, the Pd–In catalysts showed a higher selectivity to NH4+ than did Pd–Sn catalysts, and they did not exceed 89% N2 production.
One of the major limitations of catalytic nitrate reduction is the generation of NH4+ as a by-product. In our study, at 50% nitrate conversion, which represents the maximum NO3− concentration allowed in drinking water by European legislation (50 mg L−1), the Pd0.5–Sn1/Al2O3 and Pd1.5–Sn1/Al2O3 catalysts showed <0.1 mg L−1 N-NH4+ in both cases; this NH4+ production is below the limit established by Directive 91/676/EEC [4] (0.38 mg L−1 N-NH4+). Moreover, the Pd1.5–Sn1/Al2O3 catalyst exhibited a higher activity than did Pd0.5–Sn1/Al2O3 according to their kinetic constant values (21.7 × 10−3 and 11.9 × 10−3 L min−1 gcat−1, respectively). Thus, the Pd1.5–Sn1/Al2O3 catalyst was selected as the optimum catalyst to carry out further studies on the catalytic stability. In addition, Table 2 shows a comparison of the best results obtained in the batch reactions with the catalyst prepared in the present study and the results reported in the literature with similar catalysts. As can be seen, the prepared catalysts stand out for the high selectivity to N2.

2.2. Stability Experiments in Drinking Waters

The stability of the Pd1.5–Sn1/Al2O3 catalyst was tested in continuous experiments evaluating nitrate conversion and ammonium selectivity for 60 h time on stream (Figure 2). As can be seen, the Pd1.5–Sn1/Al2O3 activity remained practically constant in the experiments performed with synthetic water (W0), achieving nitrate conversion rates between 70 and 75% once the steady state was achieved after 10 h time on stream. However, when commercial drinking waters were used, a loss of activity was observed in all experiments, which can be attributed to the water composition [15]. A decrease in nitrate conversion from 60% at the beginning of the reaction to 42% at 45 h on stream, when the steady state was reached, was observed using DW2 drinking water. On the other hand, the initial nitrate conversion rates obtained with DW1 and DW3 were similar, 55 and 60%, respectively, and a continuous conversion decrease was observed to 20 and 30%, respectively, after 60 h on stream. DW1 and DW3 presented similar concentrations of HCO3− (255 and 244 mg L−1, respectively), an anion that shows a negative effect on the catalyst performance due to its competitive adsorption for the active metal sites versus nitrate ions [13,15]. The difference observed in the catalyst when treating DW1 and DW3 is related to the higher Cl− concentration of DW3, which can reduce the negative influence of HCO3− on the catalytic reduction of nitrate [15].
In the case of ammonium production, the selectivity to NH4+ stabilized after 30 h at 20, 28, 30, and 25% with W0, DW1, DW2, and DW3, respectively. As can be seen, the NH4+ production was higher than that obtained in batch experiments with the same catalyst at 60% nitrate conversion. Therefore, the presence of other ions (different to NO3−) in the reaction medium slightly increased the selectivity to NH4+. These ions could disturb the adsorption of NO3− on the active sites, decreasing the N/H ratio on the catalyst surface and favoring NH4+ formation, while decreasing the probability of the recombination of N atoms into molecular N2 [17,24,48,49].
Table 3 shows the results obtained by elemental analysis, the textural properties, and the metal contents determined by the TXRF of the fresh and used Pd1.5–Sn1/Al2O3 catalyst. The C, N, and S contents of the catalyst did not change after 60 h on stream. However, the H content was higher in the catalyst used with DW1 and DW3, which contained a higher HCO3− concentration. Thus, HCO3− can hinder the catalytic reduction of nitrate not only by competitive adsorption on the metal active sites, but also by fouling the catalyst surface. Concerning textural properties, the surface area was not affected by the catalyst’s use in the reaction. The results obtained by the TXRF analysis of the used catalysts show a negligible loss of the metal phase.
To obtain information about the evolution of the oxidation state of the active phases on the catalyst surface, X-ray photoelectron spectroscopy (XPS) studies were performed. The surface atomic concentrations and XPS spectra of catalysts are depicted in Table 4 and Figure 3, respectively. Both the fresh and used bimetallic catalysts presented a broad and asymmetric peak in the Pd 3d spectra, suggesting the presence of more than one Pd chemical environment. The Pd 3d5/2 peak appeared at 334–336 eV, while Pd 3d3/2 was exhibited at 340–342 eV. Pd 3d showed two peaks at around 335 and 340 corresponding to Pd0 species, and another two peaks at 337 and 342 eV corresponding to Pd2+. As can be seen, Pd0 increased after the reaction, irrespective of the water used, indicating that most of the Pd is available to act as a reloading agent of the Snn+ species, which is necessary during the redox catalytic cycle [10,50].
On the other hand, the Sn 3d5/2 spectra present three peaks at 485.1, 486.2, and 487.3 eV, corresponding to Sn0, Sn2+, and Sn4+, respectively. The presence of two oxidized Sn species (Sn2+ and Sn4+) suggests that NO3− reduction on active Sn surfaces could proceed in two subsequent reduction steps [23]. First, Sn0 surfaces could reduce nitrate and be oxidized to Sn2+ (Equation (1)), and second, Sn2+ could be partially oxidized to Sn4+ for the reduction (Equation (2)).
NO3− + Sn0 → SnO + NO2−
NO3− + SnO → SnO2 + NO2−
The Sn content on the catalyst surface seems to be higher than the total content determined via TXRF. However, this could be produced by an agglomeration of Sn particles. In addition, the slight oxidation of the promoter metal was observed after the treatments with the synthetic water (W0) and DW1 and DW3 waters, where the Sn0 content decreased by 8.5, 7.7, and 11.5%, respectively, with respect to that of the fresh catalyst; this did not seem to affect the catalyst activity, since there was no activity loss during the W0 treatment.

3. Materials and Methods

3.1. Preparation and Characterization of Catalysts

Bimetallic catalysts with Pd–Sn and Pd–In as metallic phases were supported on γ-Al2O3 spheres (SASOL Germany, d = 1.06 mm, ABET 158 m2 g−1, Vmeso = 0.47 cm3 g−1) by sequential impregnation using different metal contents and ratios, as follows: 1–0.1, 1–0.5, 0.5–1, 1–1, and 1.5–1 wt.% of noble metal and promoter, respectively. Bimetallic Sn catalysts were synthesized with varied impregnation order of noble–promoter metal or promoter–noble metal, while bimetallic In catalysts were prepared with the impregnation order established as optimal in the above-mentioned study. The catalysts were named Pdm–Men when the impregnation started with the noble metal and Men–Pdm for the opposite case, the subscripts “m” and “n” being their theoretical metal contents. Pd was incorporated by dissolving Na2PdCl4 (Sigma Aldrich, 99.99 %, St. Louis, MO, USA) in deionized water (1 mL gsupport−1), whereas the promoter metals, Sn or In, were added by dissolving SnCl2 (Sigma Aldrich, 99.99 %, St. Louis, MO, USA) or In(NO3)3 (Sigma Aldrich, 99.99 %, St. Louis, MO, USA) in methanol or deionized water (1 mL gsupport−1 in both cases), respectively. The solutions were evaporated in a rotary evaporator at 70 °C and atmospheric pressure or 200 mbar for salts dissolved in methanol or water, respectively. After each impregnation, the material was dried overnight at 60 °C and calcined at 500 °C for 2 h.
The metal content of the catalysts was determined via the Total Reflection X-ray Fluorescence (TXRF S2 PicoFox, Bruker spectrometer, Bremen, Germany). The porosity of the texture was assessed from the N2 adsorption–desorption isotherms at 77 K using MicromeriticsTristar 3020 automated volumetric gas adsorption equipment (Unterschleißheim, Germany). The samples were previously outgassed at 150 °C for 20 h by using a Micromeritics VacPrep 061 vacuum degassing system (Unterschleißheim, Germany). The contents of C, H, N, and S were determined by elemental analysis using a LECO CHNS-932 analyzer (St Joseph, MI, USA). X-ray photoelectron spectra (XPS) (Chanhassen, MN, USA) were obtained using a Physical Electronics 5700C Multitechnique instrument with Mg Ka radiation (1253.6 eV).

3.2. Experimental Set-Up and Procedure

The catalytic tests in semi-batch mode were performed in 250 mL glass reactors, using a thermostatic bath at 25 °C and atmospheric pressure. Magnetic stirring at 700 rpm and catalysts with a particle size of less than 80 μm were used to avoid mass diffusional effects. The initial concentration of NO3− (NaNO3, Panreac, 99 %) was 100 mg L−1, and the catalyst concentration was 1 g L−1. Prior to each reaction, the catalyst (0.2 g) was reduced under H2 flow (25 N mL min−1) at 100 °C for 1 h, outgassing with N2, and deposited in the reactor with 200 mL of the NO3− solution. A flow of CO2 (25 N mL min−1) was passed for 20 min prior to H2 feeding. The initial pH of the solution was 4. A flow of H2 + CO2 (50 N mL min−1; 1:1) was continuously fed to the reactor. Reaction samples were regularly withdrawn and filtered through regenerated cellulose filters (pore diameter of 0.45 μm). The values of NO3− conversion and selectivity to reaction products are reported as the mean of three experiments, the standard deviation being less than 10% in all cases. The experimental data were fitted to a pseudo-first-order kinetic model. Kinetic constants were calculated to nitrate conversions below 95 %.
A fixed-bed glass tubular reactor with an inner diameter of 6 mm and length of 20 cm was used for the continuous flow experiments. An NO3− solution was fed upward at a constant flow rate of 0.3 mL min−1 with a peristaltic pump (GILSON MINIPULS 3). Throughout the experiments, CO2 (100 N mL min−1) was bubbled into an NO3− solution (2 L). To enhance the mixing of NO3− and H2 (0.13 N mL min−1), a significant volume of the reactor was filled with glass spheres (30 g, 3 mm diameter). A catalytic bed composed of 0.4 g of catalyst spheres was placed inside the glass tube between glass wool. First, the catalyst stability was tested using a solution of 100 mg L−1 NO3− (NaNO3, Panreac, 99%) in milli Q water (W0). In addition, catalytic stability was evaluated using three commercial drinking waters spiked with 100 mg L−1 of NO3−, denoted DW1, DW2, and DW3. Table 5 shows the main characteristics of the commercial drinking waters.

3.3. Analytical Methods

The initial solutions and reaction samples obtained throughout the experiments were analyzed via Ionic Chromatography on a Metrohm 882 Compact IC plus chromatographer. Na+, NH4+, K+, Ca2+, and Mg2+ were determined using a Metrosep C4 column, with a mobile phase flow of 0.9 mL min−1 of C7H5NO4 (0.7 mM) and HNO3 (1.7 mM), and NO3−, NO2−, Cl−, and SO42− were quantified using a Metrosep A Supp 5 column, with a mobile phase flow of 0.7 mL min−1 of Na2CO3 (3.2 mM) and NaHCO3 (1.0 mM). The HCO3− concentration was determined using a titrator (TitroMatic 1S/2S titrator Karl Fischer) as the alkalinity (mg L−1 CaCO3). The pH was measured along the reaction runs using a pH-meter (GLP 21, CRISON), and conductivity was determined using a conductimeter (GLP 31, CRISON). In the case of drinking waters, the dry residue was determined from the weight difference after heating a 100 mL sample for 4 h at 180 °C. The hardness value was calculated from the Ca2+ and Mg2+ concentrations, obtained via Ionic Chromatography, following APHA procedure 2340B.

4. Conclusions

The effects of metal loading, the impregnation order of the metal phase during catalyst preparation, and the promoter metal of an Al2O3-supported bimetallic Pd catalyst on the catalytic reduction of nitrate were evaluated in this paper. The deposition of the noble metal over the promoter metal increased the available surface area covered by Pd, enhancing the H coating on the catalyst surface, decreasing the N/H ratio and the selectivity to N2. Consequently, the optimum impregnation order is the noble metal followed by the promoter metal. Moreover, Sn as a promoter metal was more selective to N2 than In in the bimetallic Pd catalysts. The Pd1.5–Sn1/Al2O3 catalyst showed high stability for a duration of 60 h in the catalytic reduction of nitrate in synthetic water, maintaining a nitrate conversion rate of around 70 %. However, the fouling of the catalyst surface by the water components, such as HCO3−, was responsible for the deactivation of the catalyst when it was used in the catalytic reduction of nitrate in drinking water.

Author Contributions

Conceptualization, A.F.M. and E.D.; Methodology, I.S. and E.D.; Investigation, I.S. and E.D.; Writing—Original Draft Preparation, I.S., A.F.M. and E.D.; Writing—Review and Editing, J.J.R., A.F.M. and E.D.; Supervision, J.J.R., A.F.M. and E.D.; Funding Acquisition, A.F.M. and E.D. All authors have read and agreed to the published version of the manuscript.

Funding

The authors greatly appreciate the financial support from the Spanish MINECO (PID 2019-108445RB-100 and PDC2021-120755-I00) and Comunidad de Madrid (S2018/EMT-4344). I. Sanchis wishes to thank the Comunidad de Madrid for the PEJD-2017-PRE/AMB-4616 contract.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank Dydia Tanisha González for his valuable help.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00729 g001 550
Figure 1. Time course of the NO3− concentration (solid lines) and NH4+ selectivity (dash lines) for the Pd–Sn, Pd–In, and Sn–Pd catalysts.
Figure 1. Time course of the NO3− concentration (solid lines) and NH4+ selectivity (dash lines) for the Pd–Sn, Pd–In, and Sn–Pd catalysts.
Catalysts 12 00729 g001
Catalysts 12 00729 g002 550
Figure 2. Time course of NO3− conversion and NH4+ selectivity of Pd1.5–Sn1/Al2O3 in commercial drinking waters tested in fixed-bed reactor experiments.
Figure 2. Time course of NO3− conversion and NH4+ selectivity of Pd1.5–Sn1/Al2O3 in commercial drinking waters tested in fixed-bed reactor experiments.
Catalysts 12 00729 g002
Catalysts 12 00729 g003 550
Figure 3. XPS Pd (3d) and Sn (3d5/2) spectra of the fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Figure 3. XPS Pd (3d) and Sn (3d5/2) spectra of the fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Catalysts 12 00729 g003
Table
Table 1. Catalyst characteristics and main results of nitrate reduction experiments in semi-batch runs with synthetic water with 100 mg L−1 of NO3−. Selectivity has been calculated at 95% of nitrate conversion.
Table 1. Catalyst characteristics and main results of nitrate reduction experiments in semi-batch runs with synthetic water with 100 mg L−1 of NO3−. Selectivity has been calculated at 95% of nitrate conversion.
CatalystMetal
Content (wt.%)		ABET
(m2 g−1)		Vmesopore
(cm3 g−1)		Selectivity at
X-NO3− = 95%		First-Order Kinetic Constant
(X-NO3− < 95%)
NH4+NO2−N2k × 103 (L min−1 gcat−1)R2
Sn0.1–Pd10.1−0.91510.4520.90.478.712.6 ± 0.90.97
Sn0.5–Pd10.4−1.11490.4522.00.577.512.5 ± 0.90.97
Sn1–Pd10.8−1.01510.4512.10.087.918.8 ± 1.40.97
Sn1–Pd0.50.9−0.41480.4613.00.186.912.7 ± 0.80.98
Sn1–Pd1.50.8−1.31450.439.10.090.923.6 ± 2.70.94
Pd1–Sn0.11.1−0.11520.4513.10.086.99.1 ± 0.50.98
Pd1–Sn0.51.0−0.41550.4510.10.489.510.1 ± 0.60.98
Pd1–Sn10.8−0.81540.469.50.090.519.5 ± 1.00.98
Pd0.5–Sn10.5−0.71550.466.50.393.211.9 ± 0.80.98
Pd1.5–Sn11.6−0.91540.466.60.692.821.7 ± 1.60.97
Pd1–In0.10.9−0.11520.4614.20.485.415.4 ± 0.70.99
Pd1–In0.50.8−0.41480.4513.00.786.320.1 ± 1.30.98
Pd1–In11.0−0.81520.4611.30.088.723.1 ± 1.80.96
Pd0.5–In10.3−0.71510.4514.90.684.58.1 ± 0.30.99
Pd1.5–In11.5−0.71460.4414.40.485.222.9 ± 2.70.94
Table
Table 2. Comparison of the results of this study with those reported in the literature. Selectivity calculated at reported nitrate conversion.
Table 2. Comparison of the results of this study with those reported in the literature. Selectivity calculated at reported nitrate conversion.
CatalystMetal Content (wt%)First-Order Kinetic ConstantX-NO3− (%)S-N2 (%)Reference
Pd-Sn/Al2O30.5−58.4 × 10−3 L min−1 gcat−19591.7[15]
Pd-Sn/Al2O31.5−1.536 × 10−3 L min−1 gcat−110074.7[43]
Pd-In/Al2O31−0.25 10086.0[32]
Pd-Sn/Al2O35−1.25 10044[44]
Pd-In/Al2O35−1.25 10066[44]
Pd-In/Al2O35−2 10075.4[45]
Pd-In/Al2O31−0.25 10072.3[27]
Pd-Sn/Al2O34.4−1.2 10088[46]
Pd-In/Al2O35−1.250.24 L min−1 gcat−110072.1[47]
Pd-Sn/Al2O31.5−121.7 × 10−3 L min−1 gcat−19592.8This study
Pd-In/Al2O31−123.1 × 10−3 L min−1 gcat−19588.7This study
Table
Table 3. Elemental analysis, textural properties, and TXRF results of fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Table 3. Elemental analysis, textural properties, and TXRF results of fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Fresh CatalystUsed Catalyst
W0DW1DW2DW3
C (%)0.20.20.20.20.2
H (%)0.80.92.51.02.8
N (%)<0.1<0.1<0.1<0.1<0.1
S (%)<0.1<0.1<0.1<0.1<0.1
ABET (m2 g−1)154155150152152
Vmeso (cm3 g−1)0.460.450.430.450.44
Pd (%)1.531.521.471.501.39
Sn (%)0.860.870.820.870.81
Table
Table 4. Surface concentrations (atomic/wt. %) of the fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Table 4. Surface concentrations (atomic/wt. %) of the fresh and used Pd1.5–Sn1/Al2O3 catalysts.
Fresh CatalystUsed Catalyst
W0DW1DW2DW3
C (%)4.1/2.24.5/2.44.7/2.48.2/4.35.0/2.7
O (%)50.1/35.051.2/36.150.6/35.148.0/33.850.5/35.8
Al (%)38.0/44.839.0/46.438.1/44.640.4/48.039.1/46.7
S (%)4.4/6.23.2/4.54.0/5.60.9/1.33.4/4.8
Pd (%)1.0/4.60.9/4.21.0/4.61.0/4.70.8/3.8
Pd0 (%)67.373.974.181.477.0
Pd2+ (%)32.726.125.918.623.0
Sn (%)1.4/7.31.2/6.31.5/7.71.5/7.81.2/6.3
Sn0 (%)36.433.333.636.132.2
Sn2+ (%)39.639.139.139.937.3
Sn4+ (%)24.127.627.424.030.5
Table
Table 5. Main characteristics of the commercial drinking waters.
Table 5. Main characteristics of the commercial drinking waters.
WaterDry Residue 180 °C Hardness pHConductivityNa+K+Ca2+Mg2+NH4+NO3−Cl−SO42−HCO3−
mg L−1mg CaCO3; L−1 µS cm−1mg L−1
DW12602647.54935.80.862.925.90.42.57.322.6255.1
DW212146.1323.00.23.91.10.46.7-2.68.9
DW35212578.287489.41.376.316.10.45.3169.839.1244.0
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Sanchis, I.; Rodriguez, J.J.; Mohedano, A.F.; Diaz, E. Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction. Catalysts 2022, 12, 729. https://doi.org/10.3390/catal12070729

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Sanchis I, Rodriguez JJ, Mohedano AF, Diaz E. Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction. Catalysts. 2022; 12(7):729. https://doi.org/10.3390/catal12070729

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Sanchis, Ines, Juan Jose Rodriguez, Angel F. Mohedano, and Elena Diaz. 2022. "Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction" Catalysts 12, no. 7: 729. https://doi.org/10.3390/catal12070729

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