Hydrogenation Of Nitrate Research Articles

Development of monolith with a carbon-nanofiber-washcoat as a structured catalyst support in liquid phase

Washcoats with improved mass transfer properties are necessary to circumvent concentration gradients in case of fast reactions in liquid phase, e.g. nitrate hydrogenation. A highly porous, high surface area (180 m 2/g) and thin washcoat of carbon fibers, was produced on a monolith support by methane decomposition over small nickel particles. Carbon fibers form a homogeneous layer less then 1 μm thin, covering the surface of the channels in the monolith. The fibers penetrated into the cordierite, which is suggested to cause a remarkable stability of the fibers against ultrasound maltreatment. The texture of the fibers is independent of both the thickness of the γ-alumina washcoat as well as the time to grow carbon fibers.

Removal of Nitrate in Water by Hydrogenation with a Noble Metal‐Free Ni‐Base Catalyst.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Denitrification of natural water on supported Pd/Cu catalysts

The influence of the support on the catalytic hydrogenation of nitrates in liquid phase was investigated. Pd/Cu supported on hydrotalcite and Pd/Cu supported on alumina catalysts were tested for this reaction and its activity and selectivity were compared. It has been observed a higher activity and a lower production of ammonium when the metals are supported on hydrotalcite than when they are supported on alumina. When rehydrated after calcination hydrotalcite recovers its layered structure. Nitrates can be located in the interlayer space diminishing the problems associated to mass transfer limitations and increasing the activity of the material. In addition, copper can be incorporated in the hydrotalcite structure by an isomorphical replacement of magnesium, obtaining thus a high dispersion of the copper centres, that is reflected in a higher activity if compared with copper impregnated samples. A XPS study was made in order to characterise the metals supported on the hydrotalcite.

Removal of Nitrate in Water by Hydrogenation with a Noble Metal-Free Ni-Base Catalyst

Abstract Porous Ni catalyst modified with a small amount of Zr was found to be highly active for hydrogenation of nitrate (200 ppm) in water at 333 K. This is the first example of noble metal-free catalyst effective for this reaction.

Hydrogenation of Nitrate in Water to Nitrogen over Pd–Cu Supported on Active Carbon

Catalytic removal of nitrate in water by hydrogenation was investigated by using supported Pd–Cu catalysts at 333 K in a flow system. Active carbon (AC) was superior in conversion, selectivity, and insolubility to other supports. Pd and Cu were not dissolved when supported on AC, even at low pH reaction conditions, while they were considerably released from the surface of γ-Al2O3, SiO2, and ZrO2. The contact time dependence of the reactant and products suggested that the hydrogenation of NO3− is a consecutive reaction with NO2− as an intermediate product and that the hydrogenation of NO2− is a key step in determining selectivity to N2 and NH3. The conversion of NO3− was enhanced greatly by the addition of a small amount of Cu to 5 wt% Pd/AC or of Pd to 3 wt% Cu/AC, indicating that the Pd–Cu site was indispensable to activate the nitrate ion. The yield of N2 went through a maximum at 0.6 wt% Cu in the 5 wt% Pd–Cu/AC. A similar dependence of the N2 yield on Cu content for NO2− hydrogenation strongly supports the proposed reaction pathways. The enhancement of N2 selectivity by the addition of small amounts of Cu can be explained by the selective poisoning of the edge or corner sites of the Pd particles by Cu atoms.

Nitrate removal in drinking waters: the effect of tin oxides in the catalytic hydrogenation of nitrate by Pd/SnO2 catalysts

The preparation of some new Pd-Cu and Pd only catalysts dispersed on ZrO2 and SnO2 is reported. All samples are characterized by BET, TPR and powder XRD analysis. The hydrogenation of 100ppm nitrate solutions shows significant differences depending on the pH control method. The use of CO2 in the gas feed allows to reduce the formation of ammonia. Pd/SnO2 catalysts represent the best combination between activity and selectivity and both can be improved by decreasing the reduction temperature of the catalyst. These results suggest a possible role for SnOx in the control of the activity and selectivity of the reaction.

New insight in the solid state characteristics, in the possible intermediates and on the reactivity of Pd–Cu and Pd–Sn catalysts, used in denitratation of drinking water

Bimetallic palladium-based supported catalysts were tested in the liquid phase hydrogenation of nitrates. They were characterised by XPS, CO chemisorption, TPD–TPR and DRIFT. The effect of the preparation method, the support, the precursors, the relative amount of active metals and their role in the formation of intermediates and products are tentatively discussed. The catalytic activity and the formation of intermediate nitrite depend on the Pd–Cu ratio. Catalysts presenting a Pd/Cu atomic ratio >1 display the highest activity and the lowest intermediate nitrite than those presenting a Pd/Cu atomic ratio <1. Sol–gel method gives catalysts with a high activity and a low nitrite formation. The Pd–Cu-based catalyst supported on zirconia is more active and selective in N2 compared to the corresponding Pd–Sn catalyst. An enrichment of the surface by Pd is responsible for a low intermediate nitrite formation and high selectivity in N2. The reduction of NO is activated on Pd–Cu catalysts, contrary to Pd–Sn catalysts. Sn promotes the formation of ammonia.

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.

Tin promoted palladium catalysts for nitrate removal from drinking water

Hydrogenation of nitrate to nitrogen using Pd/Al2O3 catalysts promoted by a second metal offers a promising process for nitrate removal in drinking water treatment. This study was aimed to elucidate the nature and function of promoting tin species in PdSn/Al2O3 catalysts obtained in different preparation routes. On one hand, a parent Pd/Al2O3 catalyst was doped via impregnation with aqueous solutions of SnCl2 of different concentrations. On the other hand, the palladium surface of the same parent Pd/Al2O3 catalyst was modified via controlled surface reaction (CSR) with hexane solutions of Sn(C4H9)4. The structure of the different PdSn/Al2O3 catalysts was investigated by 119Sn-Mössbauer spectroscopy and by means of various chemisorption techniques (static and pulse chemisorption of H2 or CO, measurement of the differential heat of CO chemisorption, FTIR spectroscopy of CO chemisorption). Catalytic properties were studied in batch experiments under atmospheric pressure. Promoting of the Pd/Al2O3 catalyst by CSR resulted in catalysts with a significantly higher activity compared to PdSn/Al2O3 catalysts obtained via incipient wetness method. Obviously, Sn(II)-species being present in the latter in high portion inhibit the nitrate reduction on bimetallic PdSn ensembles.CO chemisorption reflected a ‘palladium site blocking’ by tin species in both kinds of catalysts and indirectly indicated the generation of palladium–tin ensembles. In case of the CSR preparation the palladium is alloyed by metallic tin.The palladium surface is diluted by tin atoms, i.e. bimetallic PdSnx ensembles are generated which are able to adsorb and activate nitrate ions. There is an optimum of tin loading, i.e. the activity decreases and the undesired ammonium production in a site reaction increases when the surface becomes too tin rich. Sn(II) species, being preferentially present in catalysts obtained by SnCl2 impregnation also strongly modify the chemisorption properties of the palladium surface, but obviously these species inhibit the nitrate reduction on the bimetallic PdSn ensembles, which are also present in these catalysts.

Cloth catalysts in water denitrification: I. Pd on glass fibers

Fiber catalysts are easy to handle and free of mass-transfer resistance. This report is the first application of cloth catalysts to water denitrification. In this work, cloths woven from glass fibers (GF) of 7–10μm in diameter, impregnated with Pd, were tested in a semi-batch reactor to evaluate their effectiveness in the catalytic liquid phase hydrogenation of nitrites and nitrates.The catalytic properties of Pd-GF cloths were evaluated as a function of Pd loading as well of chemical composition of the glass, specific surface area and weaving mode of the fibrous support. Investigated catalysts showed the same level of specific activity (per g Pd) as conventional powdered catalysts for liquid-phase hydrogenation of nitrites but their activity for nitrates was about one order of magnitude lower. The nitrite and nitrate removal activities were independent of the catalyst structure; the formation of ammonium ions was highly sensitive to reactant concentration. The stability of Pd-GF cloths is discussed.

Use of palladium based catalysts in the hydrogenation of nitrates in drinking water: from powders to membranes

The behavior of Cu or Sn promoted palladium catalysts supported on zirconia and titania for the hydrogenation of nitrate in drinking water is discussed. Catalysts are prepared either as powders (microspheres) or in the form of membranes deposited on commercial alumina tubes. The performance of the former in terms of activity and selectivity towards the formation of nitrogen in a batch reactor is presented as a function of the preparation procedure, the type of precursor, the Pd/Cu ratio and the type of promoter. The use of catalysts under diffusion controlled conditions allows to reduce significantly the amount of ammonia formed while retaining a high catalytic activity. Membrane catalysts are prepared by sol–gel and characterized by SEM, EDS and hydrogen permeability tests. Their reactivity is studied either in a recirculation reactor or in a continuous flow reactor and the effect of the initial pH, the residence time and the internal hydrogen pressure are reported.

Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd–Cu catalysts

Two series of pumice-supported palladium and palladium–copper catalysts, prepared by impregnation with different palladium and copper precursors, were tested for the hydrogenation of aqueous nitrate and nitrite solutions. Measurements were performed in a stirred tank reactor, operating in batch conditions, in buffered water solution at atmospheric pressure and at 293 K. The activities of the catalysts were calculated in terms of nitrate and/or nitrite removal. With the monometallic Pd/pumice, the reduction of nitrite is highly selective; only 0.2% of the initial nitrite content is converted to ammonium ions. The activity in terms of turn over frequency (TOF) is higher as compared to a catalyst of Pd on silica. Addition of copper to the palladium catalyst is essential for the reduction of nitrates, although it decreases the nitrite reduction activity and increases the production of ammonium ions. Nitrate reduction appears to be structure-insensitive and a volcano-type dependence of the activity versus the overall Cu atomic weight percentage is observed for the two series of catalysts.

Studies on the use of catalytic membranes for reduction of nitrate in drinking water

Hydrogenation of nitrate using supported bimetallic Pd-Cu or Pd-Sn catalysts provides a promising new alternative for denitrification of drinking water with several advantages over established technologies such as reverse osmosis or biological denitrification (Sell et al., 1992). One of the major problems of this technology is the formation of undesired side products, i.e. ammonium and nitrite, both subject to a limiting value far below the admissible nitrate content. Previous studies have shown that the particle size, among other effects, is a key parameter determining the selectivity of the catalyst, pointing to a detrimental effect of pore diffusion limitation (Hähnlein et al., 1998). Moreover, activity and selectivity depend on the concentration of dissolved hydrogen, i.e. high concentration means high activity, but at the same time favoring of ammonium formation due to further hydrogenation. In this paper a porous catalytic membrane acting as a hydrogen diffuser is proposed as a means to solve these problems by creating an efficient three-phase contact between the catalytic surface, dissolved nitrate, and hydrogen gas. Such a design offers the potential to control the activity and selectivity of the process through controlled dosage of hydrogen. Experimental data on the hydrogenation of aqueous nitrate using commercial ceramic membranes modified by insertion of palladium and copper or tin into the pore-structure through impregnation and MOCVD (metalorganic chemical vapor deposition) are presented and discussed together with characterization results obtained by SEM/EDAX and XPS.

Pd-Cu Supported on Anionic Polymers - Promising Catalysts for Removal of Nitrates from Drinking Water

Removal of nitrates from drinking water by catalytic hydrogenation over polymer supported Pd-Cu catalysts was studied. Precursors of the catalysts were prepared by ion exchange using microporous polymer cation-exchange resin Dowex 50 W X 4 in acid form and acetone-water solution of Pd and Cu acetates. The metals (approximately 2 and 0.5 wt.% of Pd and Cu, respectively) were reduced with NaBH4 in ethanol at room temperature or with H2 in MeOH at 50 °C and 1 MPa. Catalysts made it possible to reduce nitrate content in water from 100 mg/l to less than 50 mg/l after 4 h of batch treatment with hydrogen-nitrogen mixture (43 vol.% H2) at ambient temperature and atmospheric pressure under vigorous stirring of the reaction mixture. The importance of acid sites of the catalyst for the selective hydrogenation of nitrates to nitrogen was demonstrated by a higher overall selectivity (about 60 mole %) obtained on catalysts containing acid sites than on catalysts without acid sites (about 20 mole %). Catalysts bearing acid sites are promising candidates for removal of nitrates, formed ammonia, as well as other cations because of parallel acting as catalysts and cation exchangers. The used catalysts were regenerated with a low concentrated mineral acid.

Selective reduction of the aromatic nitro group with retention of the nitrate group

Selective reduction of a nitro group whilst retaining the ONO2 group has been carried out for the first time in the hydrogenation of nitrates of N-ethanolamidesp-nitrobenzoic and nitrophenylacetic acids.

Sol-gel palladium catalysts for nitrate and nitrite removal from drinking water

Alumina-copper oxide supports can be obtained by cogelation yielding uniformly sized spheres with high surface areas and unimodal mesoporosity, that can be employed as supports for Pd catalysts obtained by impregnation. These are characterized by SEM, XRD, BET, H 2 chemisorption and TPR, and can be successfully used in the hydrogenation of nitrate (nitrite) in water with minor ammonia formation.

Kinetics of nitrate reduction in monolith reactor

The kinetics of nitrate reduction with hydrogen was studied in a metallic monolith reactor, where the catalytically active material was Cu-doped Pd supported on Al 2O 3. The experiments were carried out at 60°C and at hydrogen pressures of 2 atm and 4 atm. The reactor was operated in semibatch with respect to hydrogen, whereas the liquid phase was in batch recirculating through a storage tank. The main reaction products were nitrogen and ammonia; nitrite was formed as an intermediate product. The reduction kinetics was described with a rate model based on a mechanism where absorbed NO is the key intermediate on the catalyst surface. The mass transfer parameters were estimated from correlations and the mass transfer model was combined to the kinetic model. The reaction kinetics and the mass transfer were included in a plug-flow type of model for the monolith and the kinetic parameters were estimated with non-linear regression analysis. Simulation of the experimental conditions showed that the rate model can be used in nitrate hydrogenation.

Hydrogen transfer in slurries of carbon supported catalyst (HPO process)

AbstractThe effect of suspended activated carbon on gas/liquid mass transfer is studied for the Pd/C‐catalyzed hydrogenation of nitrate to hydroxylamine (HPO process). In a bubble column and in a stirred autoclave, physical mass transfer (kLa) in the concentrated salt solution is improved by more than a factor of three by the catalyst. Chemical absorption enhancement by the zero‐order catalytic reaction is insignificant at the employed catalyst diameter (40 μm), but hydroxylamine selectivity depends on the absorption regime.