Reduction Of Nitrate In Water Research Articles

Pd−Cu and Pt−Cu Catalysts Supported on Carbon Nanotubes for Nitrate Reduction in Water

The influence of the preparation conditions on the performance of Pd−Cu and Pt−Cu supported on multiwalled carbon nanotubes for the reduction of nitrates was studied and compared with that obtained using a commercial activated carbon as support. Different preparation conditions lead to different catalytic activities and selectivities. Generally, the activity decreases by increasing the calcination and reduction temperatures for the catalysts supported on the original carbon nanotubes, where the nitrate conversion varies from 67% (noncalcined and nonreduced) to 15% (calcined and reduced at 200 °C) for the pair Pd−Cu after 5 h of reaction; the inverse performance was observed for the catalysts supported on carbon nanotubes previously oxidized with HNO3 and on this same sample heat treated at 400 °C. The functional groups created during the oxidation treatment have a negative effect on the catalyst performance. For all the preparation conditions tested, the Pd−Cu pair is more selective to nitrogen than the pair Pt−Cu; 82% and 37% are the highest values obtained for each pair, respectively. Carbon nanotubes are demonstrated to be a good support for this reaction.

Pt/CuZnAl mixed oxides for the catalytic reduction of nitrates in water: Study of the incidence of the Cu/Zn atomic ratio

Pt/CuZnAl mixed oxides for the catalytic reduction of nitrates in water: Study of the incidence of the Cu/Zn atomic ratio

Pt/CuZnAl mixed oxides for the catalytic reduction of nitrates in water: Study of the incidence of the Cu/Zn atomic ratio

1%wt Pt was impregnated on CuZnAl calcined hydrotalcite-type precursors with three Cu/Zn atomic ratios (2, 1 and 0.5). The calcined hydrotalcites and the Pt impregnated materials were characterized by: XRD, BET, TGA, and TPR. These materials were tested as catalysts for the nitrate reduction in water, to study the role of the Cu/Zn atomic ratio in the conversion and selectivity of the reaction. The different Cu/Zn atomic ratios led to different catalytic behaviours. In addition, support activity and adsorptive tests were performed.

Catalytic reduction of nitrate in water: Promoted palladium catalysts supported in resin

The reactive properties of Pd–Cu and Pd–In catalysts over weak anionic exchange resin as support was investigated in the catalytic nitrate reduction. Pd–Cu/resin catalysts were prepared upon different copper fixation procedures (ion exchange method and controlled surface reaction). The samples were tested in a batch reactor with H 2 bubbling and constant pH. The fresh and reacted catalysts were characterized by TEM, EPMA, XRD and XPS. The results demonstrated a higher activity of the Pd–Cu/resin catalyst prepared upon controlled surface reaction due to a high concentration of binary sites in the external surface. On the other hand, the XRD and TEM analyses indicated that sinterization occurs during reaction, leading to an increase in the average particle size. In the same way, a Pd–In/resin was investigated showing more activity and less selectivity than the Pd–Cu/resin sample.

Comparative study of photocatalytic and non-photocatalytic reduction of nitrates in water

A Pd–Cu/TiO 2 catalyst was comparatively studied in the catalytic and phototocatalytic reduction of nitrates in water. The reaction was systematically investigated in the presence of H 2, HCOOH (which can act as reductant in catalytic experiments and/or hole scavenger in photocatalytic experiments) or mixtures of both, and in the presence or absence of UV irradiation. The catalytic reduction of nitrates was efficiently performed with H 2 or H 2 + HCOOH as the reductant, but was poorly efficient in the presence of HCOOH alone. In the photocatalytic experiments, UV irradiation significantly improved the reaction provided that both H 2 + HCOOH were present. Ammonium formation occurred at high conversion of nitrates, whatever the reaction conditions applied. The combination of UV with formic acid showed the slowest activity but prevented ammonium formation.

Bimetallic catalysts supported on activated carbon for the nitrate reduction in water: Optimization of catalysts composition

The activities and selectivities of four pairs of bimetallic catalysts (Pd–Cu, Pt–Cu, Rh–Cu and Ir–Cu) supported on activated carbon were studied in order to optimize the metals composition for the reduction of nitrate in water with hydrogen. The catalytic tests were carried out in a semi-batch reactor, working at room temperature and pressure. The activity of the catalysts is quite different depending on the copper content. The maximum activity for the catalysts with 1% of the noble metal was obtained for 1%Rh–0.6%Cu, 1%Pd–0.6%Cu, 1%Pt–0.3%Cu and 1%Ir–0.3%Cu, with nitrate conversions after 5h of reaction of 98%, 63%, 56%, and 55%, respectively. All these weight compositions correspond to an atomic ratio noble metal/copper close to 1. With the exception of the pair Ir–Cu, where the selectivity to ammonium is almost independent of the metals composition, for all the other cases it increases with the atomic copper content up to around 75%. The pair Rh–Cu was the most active among the bimetallic catalysts tested; however, significant amounts of ammonium are obtained. The nitrate conversions for the pairs Ir–Cu and Pt–Cu are similar but the former presents higher selectivities to ammonium. The pair Pd–Cu is the most selective in the transformation of nitrate to nitrogen.

Synthesis of nanoiron by microemulsion with Span/Tween as mixed surfactants for reduction of nitrate in water

Denitrification of nitrate in groundwater using iron nanoparticles has received increasing interest in recent years. In order to fabricate iron nanoparticles with homogeneously spherical shape and narrow size distribution, a simple and “green” method was developed to synthesize iron nanoparticles. The conventional microemulsion methods were modified by applying Span 80 and Tween 60 as mixed surfactants. The maximum content of water in the Water-in-oil (W/O) microemulsion and its appropriate forming conditions were found, and then the microemulsion system consisting of saturated Fe2+ solution was used to synthesize α-Fe ultrafine particles by redox reaction. The nanoparticles were characterized by using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the average diameter of the particle is about 80–90 nm. The chemical activity of the obtained iron nanoparticles was studied by the denitrification experiment of nitrate. The results show that under the experimental conditions, iron removed most of the 80 mg/L nitrate within 30 min. The mass balance of nitrate reduction with nanoscale Fe indicates that endproducts are mainly ammonia. Two possible reaction pathways for nitrate reduction by nanoscale iron particles have been proposed in this work.

Adsorption and reduction of nitrate in water on hydrotalcite-supported Pd-Cu catalyst

The hydrotalcite-supported Pd-Cu catalysts were successfully prepared by the impregnation or coprecipitation method, and their adsorption and catalytic reduction activity for nitrate in water were evaluated. The catalysts were characterized by X-ray diffraction (XRD) and surface area (BET) analysis. The results demonstrated that hydrotalcite-supported Pd-Cu catalysts could significantly adsorb nitrate ions, and then, effectively catalytically reduce them. The excellent adsorption ability for nitrate resulted from the regenerated layer structure of calcined hydrotalcite catalyst in nitrate aqueous solution. Nitrate was forced into the interlayer space and adsorbed on the external surface. The adsorption kinetics and the adsorption isotherm could be well described by pseudo-second-order model and the Langmuir model, respectively. The comparison of catalytic reduction with the adsorption for nitrate indicated that catalytic hydrogenation activity for nitrate increased with increasing adsorption capacity; nitrate reduction on hydrotalcite-supported Pd-Cu catalysts was a consecutive and dynamic adsorption and catalytic hydrogenation process. In addition, the catalyst obtained by coprecipitation method, with intact regeneration of hydrotalcite structure and a high dispersion of active metals, hold higher adsorption and catalytic activity than that prepared by co-impregnation method.

Electrocatalytic Reduction of Nitrate in Water with a Palladium‐Modified Copper Electrode

A highly active electrocatalytic electrode for nitrate reduction was prepared by the electro-deposition of palladium onto a copper electrode. The capacity of nitrate reduction by a palladium-modified copper electrode has been studied using cyclic voltammetry (CV). The existence of a reduction peak at -0.605 V versus saturated calomel electrode in 0.1-M sodium nitrate + 0.1-M perchloric acid solution (pH = 0.86) can be found in the CV measurement. The influence of solution properties, such as pH, nitrate concentration, and other anions in solution, on nitrate reduction was determined in detail. Results showed that nitrate reduction was suppressed in alkaline solution, while it was beneficial to nitrate reduction in acid or neutral solution. At low nitrate concentrations (0.01 to 0.5 M), nitrate reduction current increased with increasing nitrate concentration, but was hindered by sulfate. At high nitrate concentrations (1 to 5 M), no significant difference on nitrate reduction was observed. Compared with other different electrodes prepared in our work (copper, titanium, and palladium-modified titanium electrodes), the palladium-modified copper electrode showed the highest electrocatalytic capacity and stability in the nitrate-reduction process.

The electrocatalytic reduction of nitrate in water on Pd/Sn-modified activated carbon fiber electrode

The Pd/Sn-modified activated carbon fiber (ACF) electrodes were successfully prepared by the impregnation of Pd 2+ and Sn 2+ ions onto ACF, and their electrocatalytic reduction capacity for nitrate ions in water was evaluated in a batch experiment. The electrode was characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS) and temperature programmed reduction (TPR). The capacity for nitrate reduction depending on Sn content on the electrode and the pH of electrolyte was discussed at length. The results showed that at an applied current density of 1.11 mA cm −2, nitrate ions in water (solution volume: 400 mL) were reduced from 110 to 3.4 mg L −1 after 240 min with consecutive change of intermediate nitrite. Ammonium ions and nitrogen were formed as the main final products. The amount of other possible gaseous products (including NO and N 2O) was trace. With the increase of Sn content on the Pd/Sn-modified ACF electrode, the activity for nitrate reduction went up to reach a maximum (at Pd/Sn=4) and then decreased, while the selectivity to N 2 was depressed. Higher pH value of electrolyte exhibited more suppression effect on the reduction of nitrite than that of nitrate. However, no significant influence on the final ammonia formation was observed. Additionally, Cu ion in water was found to cover the active sites of the electrode to make the electrode deactivated.

Can TiO 2 promote the reduction of nitrates in water?

Monometallic palladium catalysts were synthesized using different titanium supports and tested for the reduction of nitrates from aqueous solutions using hydrogen as a reductant. The Pd/TiO 2 catalysts were characterized by electron paramagnetic resonance (EPR), low-temperature Fourier transform infrared (FTIR) spectroscopy of adsorbed CO, and X-ray diffraction (XRD). The catalysts studied exhibited a high activity for nitrate removal with a lower tendency for nitrite formation than the conventional bimetallic Pd catalysts. Although ammonium formation was greater than desired, the use of a monometallic catalyst for this two-step reduction process is significant and suggests that a single site may be responsible for both reduction stages. The titanium support (particularly the Ti 3+ centers generated during prereduction in the presence of Pd) appear to play an important role in the nitrate degradation process. The potential role of Pd β-hydride in generating these Ti 3+ centers is discussed.

Use of formic acid as reducing agent for application in catalytic reduction of nitrate in water

The reduction of nitrate in nitrogen using bimetallic palladium tin catalysts and hydrogen is an interesting process for water treatment. The aim of the present study is to use formic acid (FA) as a reducing agent and a pH buffer in order to substitute the mixture of hydrogen and carbon dioxide. The catalytic performances of a palladium tin catalyst supported on silica were evaluated in the presence of FA, as a function of the initial acid concentration and of the gas phase (N 2, CO 2, or H 2). Results were compared to those obtained with hydrogen in the presence of carbon dioxide. Similar mechanisms seem to explain the identical catalytic performances observed with these two reducing agents.

Using the “memory effect” of hydrotalcites for improving the catalytic reduction of nitrates in water

The role of the support in the catalytic hydrogenation of nitrates in the liquid phase using a PdCu/Mg/Al hydrotalcite is studied. The material has been characterized by X-ray diffraction, infrared spectroscopy, elemental analysis, and surface area. It is shown that the use of calcined hydrotalcites as support reduces the problems associated with mass transfer limitations observed on PdCu/Al 2O 3, by introducing a new concept of active supports. In this case by taking advantage of the “memory effect” of calcined hydrotalcites, the nitrates are forced to be located between the positively charged layers of the hydrotalcite and therefore close to the reductive active sites. The nitrates are reduced to nitrites that remain in the same position, and these are further reduced to nitrogen or in a much lower extent to ammonia. These final compounds due to their inadequate charge are released to the solution, reducing the problems related with diffusion limitations that strongly affect the selectivity of the reaction. This has been proved by studying separately the adsorption and reaction steps with different characterization techniques.

Palladium and platinum-based catalysts in the catalytic reduction of nitrate in water: effect of copper, silver, or gold addition

Supported bimetallic palladium and platinum catalysts promoted by metals of group 11 (Cu, Ag, and Au) were prepared by control surface deposition and tested in the liquid-phase reduction of nitrates. Whereas bimetallic catalysts promoted by gold are totally inactive, copper or silver deposition leads to bimetallic catalysts active for nitrate reduction. The promoting effect of the second metal can be related to its redox properties, confirming that nitrate reduction occurs through a bifunctionnal mechanism following (i) a direct redox mechanism between promoter and nitrate and (ii) a catalytic reaction between hydrogen, chemisorbed on the noble metal, and intermediate nitrite. TEM experiments, TPR, and FTIR of chemisorbed CO studies of the different systems have been used to evidence the metal–metal interaction and the localization of the promoter. The characterization results have been correlated with the catalytic behavior of the materials.

Electrocatalytic reduction of nitrate in water

Nitrate (NO 3 −) contamination of groundwater is a common problem throughout intensive agricultural areas (nonpoint source pollution). Current processes (e.g., ion exchange, membrane separation) for NO 3 − removal have various disadvantages. The objective of this study was to evaluate an electrocatalytic reduction process to selectively remove NO 3 − from groundwater associated with small agricultural communities. A commercially available ELAT (E-Tek Inc., Natick, MA) carbon cloth with a 30% surface coated Rh (rhodium) (1 μg cm −1) was tested at an applied potential of –1.5 V versus standard calomel electrode (SCE) with a Pt auxiliary electrode. Electrocatalytic reduction process (electrolysis) of NO 3 − was tested with cyclic voltammetry (CV) in samples containing NO 3 − and 0.1 M NaClO 4 −. Nitrate and NO 2 − concentrations in test solutions and groundwater samples were analyzed by ion chromatography (IC). The presence of Rh on the carbon cloth surface resulted in current increase of 36% over uncoated carbon cloths. The electrocatalysis experiments using Rh coated carbon cloth resulted in reduction of NO 3 − and NO 2 − on a timescale of minutes. Nitrite is produced as a product, but is rapidly consumed upon further electrolysis. Field groundwater samples subjected to electrocatalysis experiments, without the addition of NaClO 4 − electrolyte, also exhibited removal of NO 3 − on a timescale of minutes. Overall, results suggest that at an applied potential of –1.5 V with respect to SCE, Rh coated carbon cloth can reduce NO 3 − concentrations in field groundwater samples from 73 to 39 mg/L (16.58 to 8.82 mg/L as N) on a timescale range of 40–60 min. The electrocatalytic reduction process described in this study may prove useful for removing NO 3 − and NO 2 − from groundwater associated with nonpoint source pollution.

Influence of oxidizing and reducing treatments on the metal–metal interactions and on the activity for nitrate reduction of a Pt-Cu bimetallic catalyst

Bimetallic Pt-Cu/γAl 2O 3 catalyst has been prepared by controlled deposition of copper onto platinum particles according to a redox reaction between hydrogen, pre-adsorbed on metallic platinum, and Cu 2+ ions in water. This catalyst has been submitted to either oxidizing or reducing treatments, at ambient temperature or at 400 °C. The interaction between copper and platinum in the bimetallic catalysts was estimated from temperature-programmed reduction (TPR) profiles, determined after various pre-treatments, and compared to that of a fresh catalyst. It was demonstrated that reducing and oxidizing pre-treatments noticeably affect the interactions between platinum and copper and consequently the catalytic activity for nitrate reduction in water.

Catalytic Reduction of Nitrate in Water on a Monometallic Pd/CeO2 Catalyst

Whereas monometallic palladium catalysts deposited on classical supports such as silica, alumina, or carbon are totally inactive for nitrate reduction, we have demonstrated that a reduced Pd/CeO2 catalyst is active for this reaction (33 mmol/(min gmet)). The promoting effect of the support, based on its redox properties, was clearly identified, nitrate reduction probably occurring through the interaction of oxygen atoms of nitrate with the oxygen vacancies created at the ceria surface upon the reduction treatment. Conversely, nitrite intermediates are converted through a classical catalytic reaction by hydrogen adsorbed on palladium sites.

CVD preparation of catalytic membranes for reduction of nitrates in water

A catalytic membrane contactor for selective hydrogenation of nitrate in water to nitrogen is discussed as a promising new approach to develop a technically feasible catalytic process for nitrate reduction from ground and surface water. Metal organic chemical vapour deposition (MOCVD) was used to place catalytically active metals, i.e., palladium and tin, inside the porous top-layer of asymmetric ceramic membranes (e.g. alumina) with different pore sizes and thicknesses of the top-layer. The influence of the membrane properties, i.e., the material, pore size and thickness of the catalytic layer, and the influence of the MOCVD process parameters on the deposition of the metals was studied and catalytic membranes with different palladium loading and palladium/tin ratio were prepared. These membranes were characterised with respect to the pore structure and the distribution of the active metals. Their catalytic performance in the hydrogenation of nitrate in water was investigated in a laboratory stirred tank membrane reactor under continuous flow conditions to explore the influence of important process parameters. The results show that the catalytic membranes offer a high activity and a fair selectivity to nitrogen which may be further improved by an optimisation of the preparation procedure.

Photocatalytic Reduction of Nitrate in Water on Meso-Porous Hollandite Catalyst: A New Pathway on Removal of Nitrate in Water

Photocatalysis of a hollandite compound K x Ga x Sn8−x O16 (x = ca. 1.8) was examined for the reduction of nitrate to N2 with a reducing agent of methanol in water under UV irradiation. Hollandites have a characteristic one-dimensional tunnel structure. The meso-porous hollandite was prepared by sol-gel method. This hollandite was used as the photocatalyst and its reaction process was quantitatively analyzed by using ion chromatograph and on-line mass spectrometry. The hollandite photocatalyst showed a significant activity for the formation of N2 from NO 3 − . Two factors, an increase in UV intensity and a lowering in pH of the solution, contributed to improvement in the selectivity for N2. The selectivity for N2 was improved to reach the perfect level by adjusting the factors. Although, in the previous report, the nitrate was mainly reduced to NH 4 + or NO 2 − , the present photocatalytic conditions converted it to N2. The observed photocatalytic reduction of NO 3 − to N2 with reducing agent CH3OH was conjugated to the partial oxidation of CH3OH to HCOOH. This high selective photocatalytic decomposition of NO 3 − to N2 may be a new pathway among the others reported so far, and it would be useful for environmental protection of water.