Catalytic Reduction Of Nitrate Research Articles

Electrochemical reduction of aqueous nitrate using metallic silver particles as spatially suspended catalyst

Increasing nitrate concentrations in water sources is a serious concern. The current study demonstrates the electrochemical reduction of nitrate in the presence of silver metallic particles (AgMPs). Using Ti/RuO2 anode and a Ti cathode, nitrate removal in undivided and divided cells was limited to 18% and 77%, respectively, in the absence of AgMPs. Total Nitrogen (TN) removal in undivided and divided cells was nil and 29.5%, respectively, in the absence of AgMPs. Under identical conditions, the presence of 8 mM AgMPs increased nitrate removal to 63% and 99%, respectively, with significant improvement in nitrate conversion to N2. The effect of various parameters on nitrate removal and selectivity of products was studied using a divided cell: concentration of AgMPs, inter-electrode distance, cathode material, current density, initial nitrate concentration, and reusability of AgMPs. In the presence of 8 mM AgMPs, nitrate removal using different cathodes was Ti ≈ Fe > Cu > SS > Fe/Sn. The cathode type also affected the selectivity, with the maximum conversion to N2 achieved with the Ti cathode. Nitrate removal and conversion to N2 improved with the increase in current density up to 10 mA/cm2. Overall, the presence of AgMPs substantially improved nitrate removal and conversion to N2. The production of reductive species (AgHx) by the intercalation of nascent hydrogen on metallic silver appears to promote catalytic nitrate reduction. AgMPs could be reused up to 10 times with consistent performance. This work offers a new approach to nitrate reduction using an efficient and reusable catalyst.

(Invited) Understanding Trends for Electrocatalytic and Catalytic Nitrate Reduction

Fixation of nitrogen such as for ammonia production through the Haber-Bosch process is imperative for feeding the world’s population, but is causing an imbalance in the nitrogen cycle as oxidized nitrogen species such as nitrate builds up. This build up of nitrate pollution from agricultural and industrial waste poses an immediate threat to environmental and human health. While separation of this nitrate is a method to produce clean water, it does not chemically address the issue of oxidized nitrate. In addition, it does not form nitrogen species in a valuable form. One method is to use nitrate waste as a feedstock for production of fertilizers such as ammonia or ammonium nitrate. In this work we discuss the use of electrocatalysis to convert nitrate to ammonia, where renewable electricity can be used to drive the reaction, minimizing carbon dioxide emissions from current methods to produce ammonia. In this talk we will discuss (1) trends in electrocatalytic nitrate reduction, (2) similarities and differences in electrocatalytic nitrate reduction and catalytic nitrate reduction (i.e., using molecular H2 rather than an applied potential), and (3) practical challenges in systems to commercially convert nitrate. Through the use of kinetic studies, in situ spectroscopy, microkinetic modeling, and density functional theory calculations, we show how the adsorption energetics of nitrate and hydrogen are descriptors for nitrate reduction on metals and alloy surfaces. We will discuss the importance of operando spectroscopy to understand the catalyst under reaction conditions. We will show how alloying Pt with Ru to increase the nitrate adsorption energy shows an increase in the rates of nitrate reduction at low Ru content. We will also show that the nitrate reduction rate is decreased when the Ru content is too high, because nitrate binds to the catalyst too strongly. We will compare the electrocatalytic results to thermal nitrate catalytic and show that while there are numerous similarities between electrocatalysis and thermal catalysis, there are also factors unique to electrocatalysis that impact the rate of reaction. These differences motivate a greater understanding of electrochemical steps and the effect of the applied potential, solvent, and ions. We will discuss how the best performing catalysts in batch systems translate to flow reactors more amenable to commercialized processes and other challenges in realistic nitrate streams containing potential catalyst poisons such as chlorides.

Practical application of Pd-based bimetallic catalysts with enhanced selectivity supported by chelating resin for catalytic nitrate reduction in real water

An innovative Pd-Me (Pd-Cu, Pd-In and Pd-Sn) bimetallic catalyst supported on porous chelating DOWEX M4195 resin (D) was established to reduce the nitrate almost entirely and achieved high selectivity to the expected harmless form of nitrogen. In this study, sodium borohydride (NaBH4) was applied in preparing bimetallic catalysts by liquid-phase reduction as the prestoring reductant. Pd-In/D and Pd-Sn/D groups performed well in N2 selectivity (all above 97%). In addition, Pd-In and Pd-Sn bimetallic catalysis yields higher selectivity towards N2 than the Pd-Cu pair in the presence of HCO3 −, Cl−, SO4 2− and humic acid. Likewise, in terms of N2 selectivity, Pd-In/D and Pd-Sn/D bimetallic catalysts were superior to that of Pd-Cu/D (72.16%) in the municipal wastewater treatment plant sewage. The current results provide insight into the superb reactivity, excellent stability, and most important—extremely high harmless N2 selectivity of Pd-In and Pd-Sn-based bimetallic catalysts in practical application and provide new ideas for enhancing the feasibility of the catalytic reduction of nitrate by minimizing environmentally harmful by-products.

Thermodynamic Constraints on the Catalytic Reduction of Nitrates in Drinking Water

Thermodynamic Constraints on the Catalytic Reduction of Nitrates in Drinking Water

N-doped activated carbon as support of Pd-Sn bimetallic catalysts for nitrate catalytic reduction

This work studies the catalytic reduction of nitrate using N-doped activated carbon catalysts. Activated carbons were prepared by hydrothermal carbonization (220 °C, 80 % water, 16 h) of garden and park waste and olive stones using (NH4)2SO4 as N source, followed by chemical activation with H3PO4 at 500 °C (N2 atmosphere, 1 h). These were impregnated with Pd and Sn to prepare bimetallic catalysts for nitrate catalytic reduction. N-doping improved the BET surface area up to 1369 m2 g−1 and increased the N content on the catalyst surface to 3 wt%. The N-doped catalysts showed better catalytic performance than the non-doped ones, showing high stability for 100 h on stream, reaching even higher activity than a catalyst supported on a commercial activated carbon. N-doping also showed a positive effect by decreasing NH4+ selectivity. Finally, natural and drinking waters spiked with NO3- were treated in continuous flow, exhibiting the N-doped activated carbon catalysts, prepared from garden and park waste, a high tolerance to ions other than NO3- present in the water solution.

Interference effects of oxyanions commonly found in natural waters on the catalytic reduction of nitrate in water.

In this work, the influence of oxyanions on the catalytic reduction of nitrates using formic acid as the reducing agent was studied as well as the influence of bicarbonate, sulfate, and phosphate co-anions on the catalytic nitrate reduction with Pd:In/Al2O3 (1:0.25 wt.%). A negative effect on nitrate conversion was observed in the following order: phosphate > sulfate > bicarbonate, showing a strong influence of electrostatic adsorption on the catalytic reduction of nitrate. However, no direct trend was observed relating the levels of interferents to the impact on the selectivity of the bimetallic catalyst using formic acid as a reducing agent. For both bicarbonate and phosphate, at lower levels, higher selectivity to nitrogen was obtained than for the reaction in the absence of interferents. On the other hand, increasing sulfate concentration led to a decrease in nitrate conversion. The mixtures of co-anions also showed a decrease in the catalytic activity. At 120min, a N2 selectivity higher than 95% was obtained, except for the C50-S20 (bicarbonate 50ppm-sulfate 20ppm) mixture which showed the lowest selectivity to N2 value (87.3%). The loss of catalyst activity was found to be reversible and not permanent.

Phosphomolybdic acid-modified polyaniline nanowires with abundant acidic sites for efficient photocatalytic reduction of nitrate to nitrogen in water

Although polymeric conductive polyaniline (PANI) is an effective material for catalytic nitrate reduction in water, its catalytic efficiency is limited. Here, we report that phosphomolybdic (PMoA) acid-modified PANI nanowires exhibit superior activity in the catalytic reduction of nitrate to harmless N2. Excellent catalytic efficiency is attributed to the construction of Z-scheme heterojunctions and the combined contribution of chemical-photocatalytic reduction. Pyridine infrared analysis confirms that the introduction of PMoA adds an abundance of acidic sites to PANI, which explains the extraordinary N2 selectivity. Additionally, DFT calculations are used to determine the band structures and surface electron transfer pathways of the two materials. Impressively, the effectiveness of catalytic nitrate reduction under various conditions is evaluated together with the practicality of PMoA-PANI by simulating saltwater. This work sheds new light on the mechanism of polymer for efficient reduction of nitrate and develops innovative technology for the remediation of practical wastewater.

Activity and Stability of Pd Bimetallic Catalysts for Catalytic Nitrate Reduction

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+.

A Bioinspired Iron-Centered Electrocatalyst for Selective Catalytic Reduction of Nitrate to Ammonia

Nitrate has been commonly used in fertilizers, synthetic materials, and explosives; however, its irregular release leads to extensive contaminants in many sources of drinking water because of its high solubility and mobility. Unfortunately, efficient nitrate removal at low concentrations has been challenging for a long time. Inspired by nitrate reductase with iron as the active center in plants, we constructed a novel nanozyme catalyst, FeNPs@MXene, and proved that the confinement effect of Cl size facilitates the formation of FeOOH nanoshuttles. The FeNPs@MXene electrocatalyst can achieve selective catalytic reduction of nitrate to ammonia, including up to 97.8% conversion of nitrate and high selectivity toward ammonia (76.8%). By precisely analyzing in situ electrochemical online differential electrochemical mass spectrometry (DEMS) and 15N isotope labeling experiments, the electroreduction of nitrate to ammonia was clearly proven. Using density functional theory (DFT) calculations and conductive atomic force microscopy (C-AFM), the built-in electric field formed between FeOOH and MXene is proven to be beneficial to nitrate adsorption and can lower the reaction barrier of key intermediates. This design of iron-based nanozymes offers new illuminating insight into the NO3RR.

In-situ polymerization of polyaniline modified by phosphotungstic acid on the surface of hollow carbon for two-way efficient reduction of nitrate in water

• Chemical and photocatalytic reduction are combined to remove nitrate in water. • H 3 PW 12 O 40 -PANI-C shows superb photocatalytic reduction activity with less hole scavenger. • Interface electron transport station is built to realize the fast electron conduction. • The Lewis acidic site is the key active site for nitrate reduction. Catalytic reduction of nitrate to harmless nitrogen is a significant and promising green environmental protection technology. In this paper, the reduction of nitrate by two pathways is realized without light (Reaction I) and under light (Reaction II) conditions by wrapping polyaniline modified by phosphotungstic acid on the hollow carbon surface (H 3 PW 12 O 40 -PANI-C, HPC). Modified PANI chain and Z-scheme heterojunction make the hollow carbon surface form an interfacial electron transport network, which not only shortens electron migration distance but also promotes the carrier separation and transfer. HPC catalysts concentrate the advantages of adsorption, high conductivity and abundant catalytic sites to promote the reduction efficiency. Satisfied nitrogen generation and fewer hole scavengers using make HPC catalysts more practical. This work emphasizes the rapid conduction of interfacial electrons and the effect of catalytic sites on nitrate removal, which provides new guidance for the treatment of pollutants in water.

Competitive inhibition of catalytic nitrate reduction over Cu–Pd-hematite by groundwater oxyanions

The presence of various oxyanions in the groundwater could be the main challenge for the successive application of Cu–Pd-hematite bimetallic catalyst to aqueous NO3− reduction due to the inhibition of its catalytic reactivity and alteration of product selectivity. The batch experiments showed that the reduction kinetics of NO3− was strongly suppressed by ClO4−, PO43−, BrO3− and SO32− at low concentrations (>5 mg/L) and HCO3−, CO32−, SO42− and Cl− at high concentrations (20–500 mg/L). The presence of anions significantly changing the end-product selectivities influenced high N2 selectivity. The selectivity toward N2 increased from 55% to 60%, 60%, and 70% as the concentrations of PO43−, SO32−, and SO42− increased, respectively. It decreased from 55% to 35% in the presence of HCO3− and CO32− in their concentration range of 0–500 mg/L. The production of NO2− was generally not detected, while the formation of NH4+ was observed as the second by-product. It was found that the presence of oxyanions in the NO3− reduction influenced the reactivity and selectivity of bimetallic catalysts by i) competing for active sites (PO43−, SO32−, and BrO3− cases) due to their similar structure, ii) blockage of the promoter and/or noble metal (HCO3−, CO32−, SO42−, Cl− and ClO4− cases), and iii) interaction with the support surface (PO43− case). The results can provide a new insight for the successful application of catalytic NO3− reduction technology with high N2 selectivity to the contaminated groundwater system.

Boosted selective catalytic nitrate reduction to ammonia on carbon/bismuth/bismuth oxide photocatalysts

Using solar energy to catalytically convert nitrate into ammonia is attractive for waste recycling and sustainable development. However, the rapid recombination of electron-hole pairs and the poor selectivity are obstructing photocatalytic nitrate reduction to ammonia to be mass applied currently. In this work, we reported a facile synthesis of carbon/bismuth/bismuth oxide photocatalyst via a one-pot hydrothermal reaction without using reducing reagent. Compared with α-bismuth oxide (α-Bi2O3), the photocatalytic ammonia yield of the optimum sample increased 3.65 times. In addition, the ammonia selectivity increased from 65.21% to 95.00%. The highly enhanced photocatalytic performance was attributed to the surface plasmon resonance of metallic bismuth. Meanwhile, the formation of carbon enables to boost the transfer of electrons significantly. Under light irradiation, electrons can be accumulated on metallic bismuth, effectively boosting the reduction of nitrate. The findings of this work will contribute to the recycling of nitrate for ammonia synthesis and sustainable environmental development.

Enhancement of activity and selectivity to nitrogen in catalytic nitrate reduction by use of conductive carbon catalytic supports and control of hydrogen mass transfer regime

Carbon materials with different structural properties (activated carbon, carbon nanofibers, reduced graphene oxide, graphite, and carbon black) were used as supports of Pd-Cu (5 wt%) catalysts for the catalytic reduction of NO3- in batch reactors. In general, those catalyst with smaller metal nanoparticles and carbon supports with higher specific surface area were more active, although remarkable activity was also observed for catalysts with relatively large nanoparticles and conductive supports. Conductivity of carbon supports also contributed to a lower selectivity to NH4+, which can be ascribed to higher charge transfer between metal and support reducing activation of intermediate species. The decrease of H2 feed into the reaction system resulted in external (gas-liquid) mass transfer constrains, as evidenced by the parameters describing control regime, and led to diminished NH4+ selectivity, particularly in the case of catalysts with conductive supports. Approaches combining control of mass transfer regime and conductive carbon catalytic supports can be useful to enhance selectivity to N2, which is a major challenge in catalytic NO3- reduction.

Influence of H2 availability in the catalytic reduction of nitrates in fixed bed reactors

The effect of H2 availability in the catalytic reduction of NO3− was studied through experimental tests and theoretical estimations of mass transfer regime for H2, CO2 and NO3−. The experiments were carried out in a fixed bed reactor packed with bimetallic 0.75% (w) Pd-Cu catalysts supported on granular activated carbon at 25 °C. External mass transfer limitations were only significant in the case of H2 and resulted in lower selectivity to NH4+, particularly in the low gas surface velocity range (0.09–0.019 cm/s). Low availability of H2, becoming the limiting reagent for the reaction, contributed to lower selectivity to NH4+ due to lower H:N ratio at the catalyst surface. Decreasing space–time also led to lower selectivity to NH4+, probably due to the neutralization of OH− by the available H+ at catalytic sites, thus controlling local pH and favoring N2 production. Reduction of selectivity to NH4+ was achieved with lower loss of activity and NO3− conversion when gas flow was decreased than when liquid flow was decreased. The results indicate that the design and operation of reactors for the catalytic reduction of NO3− should consider conditions leading to H2 transfer control in order to reduce selectivity to NH4+, even though this could be at the expense of reaction rate and NO3− conversion.

Influence of organic matter formed during oxidative processes in the catalytic reduction of nitrate

The application of the catalytic reduction process to remove nitrate formed during the ozonation process of organic compounds in water was assessed to study the possibility of integrating both technologies to perform a simultaneous removal of organic and inorganic species present in water. Single ozonation of two model organic compounds (4-nitrobenzaldehyde and ampicillin) was performed in order to decompose the parent organic molecule. After the treatment with ozone, the obtained solution was forwarded to a catalytic reduction process, to convert nitrate (NO3-) formed during the previous process. Organic matter present in a solution after the treatment with ozone proved to negatively affect the efficiency of nitrate reduction due to possible adsorption on the catalyst active sites, inhibiting the conversion process. However, for solutions with a low concentration of organic matter (around 2 mg L−1) it was possible to achieve NO3- conversion, although not complete during the reaction time used, and with high selectivity for ammonia formation. Several experiments were carried out to understand the effect of the organic matter on the catalyst performance, allowing to conclude that its adsorption on the catalyst surface would be the main cause of its deactivation. This issue was overcome by the application of bimetallic catalysts with high metal content (5% Pd-2.5% Cu) in different supports in the presence of organic matter, and an improvement in NO3- conversion was achieved, meaning that the presence of organic matter in solution does not completely deactivate the catalyst.

Catalytic reduction of nitrate by an oxidorhenium (V) complex

The previously published oxidorhenium(V) complex [ReOCl(L1)2] (2), equipped with the bidentate phenol-dimethyloxazoline ligand HL1 (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol), shows catalytic activity for the reduction of nitrate to nitrite under mild, ambient and aqueous conditions. The reaction operates under an oxygen atom transfer (OAT) mechanism, using dimethyl sulfide SMe2 (DMS) as oxygen acceptor. Experiments with catalytic amounts of 2 and labelled 15NO3– proved the full reduction of 15NO3– to 15NO2– by 15N NMR spectroscopy.For the second reduction step of nitrite NO2–, we have evidence for a single electron reduction to yield paramagnetic NO, as from one nitrate reduction experiment the paramagnetic cis-dioxidorhenium(VI) complex [Re(O)2(L1)2] (3) could be isolated and characterized by single-crystal X-ray diffraction analysis. Such a single electron reduction of nitrite NO2– would yield NO and complex 3 as the oxidation product.In a stoichiometric experiment of 2, 15NO3– and DMS, nitrous oxide 15N2O could be detected as the only 15N containing product by 15N NMR spectroscopy, proving that further reduction beyond NO is possible with pre-catalyst 2. The rhenium species responsible for the reduction to N2O is currently unknown. Most likely, N2O is formed via an intermediate rhenium nitrosyl complex. Experimental data was gathered by 1H and 15N NMR, IR- and UV–Vis spectroscopy, HR-ESI mass spectrometry, X-ray crystallography, and supported by theoretical computations (DFT).

Synthesis of polyoxometalates (POM)/TiO2/Cu and removal of nitrate nitrogen in water by photocatalysis

Nitrate nitrogen is one of the vital issues needed to be addressed in the water purification process utilizing groundwater as a drinking water source. In this study, a sandwich-type polyoxometalate Na [α-SiW9O34] has been synthesized and SiW9/TiO2/Cu composite has been prepared by sol-gel method. Samples have been characterized by XRD, BET, FTIR, SEM, ED-Mapping, UV–Vis, XPS. The catalytic reduction of nitrate was performed in the presence of SiW9/TiO2/Cu composite as photocatalyst. The effects of catalyst loading, initial nitrate concentration, sandwich-type polyoxometalate loading, dissolved O2, and concentration of formic acid on nitrate removal have been investigated. The results showed that the prepared composite catalyst had better photocatalytic activity than the TiO2. 76.53% of nitrate removal with 82.09% of N2 selectivity was obtained under the following conditions: catalyst dosage 0.8 g/L, initial nitrate concentration of 30 mg/L, SiW9/Cu loading percentage of 10%, 30 mmol/L of formic acid, in presence of N2, and 6 h reaction time.

Study on reaction mechanism and Langmuir-Hinshelwood kinetic model of catalytic denitrification by Fe0 and bimetallic catalyst

The focus of this research was on the catalytic reduction of nitrate to nitrogen gas for the water conservation. Zero-valent iron (Fe0) with bimetallic catalyst that carrier supported palladium (Pd) and copper (Cu) was innovatively applied in this study. First, XPS (X-ray photoelectron spectroscopy) analyses and experiments were conducted to study the mechanism of the catalytic reduction of nitrate. In the catalytic reaction, which is regarded as a stepwise process, Fe0 was the electron provider; Pd and Cu supported on carrier played indispensable but distinct roles. The kinetics suggested that the process was better reflected by first-order kinetics of the Langmuir-Hinshelwood model. Additionally, first-order kinetics of the catalytic reaction under the effect of catalysts with different carriers (SiO2, silica gel, kaolin, diatomite, γ-Al2O3, graphene) were further studied. Pd-Cu/graphene catalyst showed higher catalytic performance compared with other catalysts.

Effect of Nanoparticle Size in Pt/SiO2 Catalyzed Nitrate Reduction in Liquid Phase

Effect of platinum nanoparticle size on catalytic reduction of nitrate in liquid phase was examined under ambient conditions by using hydrogen as a reducing agent. For the size effect study, Pt nanoparticles with sizes of 2, 4 and 8 nm were loaded silica support. TEM images of Pt nanoparticles showed that homogeneous morphologies as well as narrow size distributions were achieved during the preparation. All three catalysts showed high activity and were able to reduce nitrate below the recommended limit of 50 mg/L in drinking water. The highest catalytic activity was seen with 8 nm platinum; however, the product selectivity for N2 was highest with 4 nm platinum. In addition, the possibility of PVP capping agent acting as a promoter in the reaction is highlighted.

Effect of water composition on catalytic reduction of nitrate

This work studies catalytic reduction of nitrate with bimetallic catalysts supported on γ-alumina (Pd-Sn/Al2O3 and Pd-In/Al2O3). Pd-Sn/Al2O3 yielded higher NO3− conversion and selectivity towards N2 than Pd-In/Al2O3 in synthetic water (deionized water with 100 mg/L NO3−). The Pd-Sn/Al2O3 catalyst showed highly stable behaviour, without signs of deactivation upon ten consecutive runs of 6 h each, where almost equal low selectivity to NH4+ (absence of NO2−) was maintained at high nearly constant nitrate conversion (≈ 90%). The presence of anions (Cl−, SO42− and HCO3−) in the reaction medium decreased NO3− conversion and the selectivity towards N2. Chloride showed a moderately negative effect at relatively low concentration. The effect of SO42− and HCO3− was more pronounced, being the second the most detrimental to the catalytic activity. The selectivity towards N2 was also negatively affected by the presence of those anions following the sequence HCO3− > SO42− > Cl−. However, the joint presence of Cl− or SO42− with HCO3− reduced the negative effect of the latter. The Pd-Sn/Al2O3 catalyst also showed high activity in NO3− reduction from drinking waters of weak mineralization, with a slightly increase of the selectivity towards NH4+ with respect to the obtained in deionized water with NO3− as the only anion.