SI9500357A - Process and catalyst for reduction of nitrate ion in aqueous solution - Google Patents

Abstract

The invention relates to the purification of drinking and technological water and discusses a new process and catalyst for hydrogenation of nitrate ion in aqueous solution and its selective conversion to nitrogen. It's a catalyst consists of two metal phases which are: a) applied to the same carrier, with the particle being on the outside catalyst is that phase at which it dissociates adsorbs hydrogen; b) separated from each other and for reaction forms the required active site in situ by collisions between particles of both phases.

Description

PROCEDURE AND CATALYST FOR NITRATE ION V Reduction

WATER SOLUTIONS

CHEMICAL INSTITUTE

The present invention relates to a novel process and catalyst for the reduction of a nitrate ion in an aqueous solution. It specifically addresses a novel process and catalyst for the hydrogenation of a nitrate ion in a liquid phase and its selective conversion to nitrogen. The invention is particularly useful in the field of drinking and process water treatment.

Excessive use of natural or artificial fertilizers on agricultural land is known to significantly increase the concentration of nitrate ion in surface water and groundwater, up to 200 mg / L. The use of such waterways for the public supply of drinking water causes carcinogenic diseases and damage to the central nervous system.

There are several known methods in the art that are capable of removing nitrate ion from aqueous media, such as:

ion exchange, ultrafiltration, reverse osmosis, biological denitrification and heterogeneously catalyzed reduction in the liquid phase.

The first three methods listed above have several drawbacks, but the main ones, of course, are the generated wastewater with a very high nitrate ion concentration. In a time of high ecological awareness, only biological denitrification and heterogeneously catalyzed reduction of nitrate ion have such a further perspective, since both techniques allow selective conversion of nitrate ion via intermediates into harmless nitrogen without any wastewater production. The process of biological denitrification also has drawbacks, which are reflected in the slowness and difficulty of managing the reaction. Therefore, the heterogeneously catalyzed reduction method, which is still in various developmental stages, is an alternative to biological denitrification, since it is up to 100 times faster.

The process of heterogeneously catalyzed hydrogenation of the nitrate ion is carried out in a three-phase reactor system in which the contaminated aqueous phase and the reducing agent (i.e., hydrogen) at the active site of the solid catalyst are contacted with the nitrate ion. Reduction of the nitrate ion via nitrogen intermediates takes place under very mild reaction conditions: temperatures up to 293 K and atmospheric pressure, with the partial pressure of hydrogen also being significantly lower. Horold et al. (Catal. Today, 17, 21-30 (1993)) used a bimetallic catalyst, ie a bimetallic catalyst, for experiments in a slurry reactor. Pd (5 wt%) and Cu (1.25 wt%) metal phase applied to γ-Αΐ2θ3. As can be seen from the results of their work, the catalyst used shows both high activity for hydrogenation of nitrate ion in aqueous solution as well as chemical stability. Its disadvantage, however, is in the formation of ammonium ion as a by-product of the reaction, which reduces the selectivity of the reaction (this is 82 mol% under the following experimental conditions: reaction temperature 283 K; partial hydrogen pressure: 1.0 bar; total operating pressure: 1.0 bar; initial nitrate ion concentration: 100 mg / L; catalyst concentration: 1.6 g / L).

However, it has now been found to avoid the disadvantages of the present methods using the present invention, i. a new process and a new solid catalyst consisting of two metal phases and effectively reduce the nitrate ion concentration to the statutory value.

The first object of the invention is a novel process for the reduction of a nitrate ion in an aqueous solution, for the hydrogenation of an ion in a liquid phase and its selective conversion to nitrogen. According to the invention, the reaction of heterogeneously catalyzed hydrogenation of the nitrate ion is preferably carried out in a three-phase reactor at room temperature and atmospheric pressure, with the partial pressure of hydrogen as a reducing agent being less than or equal to 1 bar and the total operating pressure equal to atmospheric. The process can be used to remove excess amounts of nitrate ion from both drinking water and process water.

Another object of the invention is a new catalyst for the reduction of a nitrate ion in aqueous solution, for the hydrogenation of an ion in a liquid phase and its selective conversion to nitrogen. The catalyst of the invention comprises two metal phases. They can be applied in the same order on the same support (in this case, so-called bimetallic catalysts), with the metal phase on which the gaseous reactant is dissociatively adsorbed on the outside of the catalyst particle.

Alternatively, metal phases may be constituents of the physical mixture. In this case, for the heterogeneously catalyzed chemical reaction, the required active site is formed by contact between the particles of the two phases. Catalytic systems containing e.g. Pd and Cu metal phases are extremely effective for removing excess amounts of nitrate ion from drinking and process waters, since the rate of conversion of nitrate ion to harmless nitrogen in a three-phase reactor system is very high even at reaction temperatures below 293 K and hydrogen partial pressures below 1 bar, with the total operating pressure equal to atmospheric. The metal phases can be used in powder or granular form.

The invention is illustrated by the accompanying Figures 1 and 2.

Figure 1 shows the time dependence of the nitrate ion concentration measured in the slurry reactor in the presence of different types of catalyst.

Figure 2 shows the ammonium ion concentration as a function of time determined in the slurry reactor for the Pd / Cu bimetallic catalyst.

EXECUTIVE EXAMPLES

Example 1: Preparation of a catalyst

The solid Pd and Pd / Cu catalyst of the present invention was prepared as follows:

Γ-Α ^ Οβ (type NST-3H; surface area (BET method): 180 m ² / g; mean particle size: 25 μΐη; pore size: 10-25 nm), supplied from Nikki- was used as the carrier of the active components. Universal Co., Ltd., Japan. The precursors selected for the formation of the metal phases were copper and palladium nitrate. The aqueous solutions of both salts were applied to the support by the impregnation method. Two Pd / Cu bimetallic catalysts were manufactured using the following procedures:

KAT-1: The carrier was first impregnated with copper nitrate. This was followed by drying T = 523 K and then palladium nitrate was applied to the resulting material.

KAT-2: γ-Αΐ2θ3 was first impregnated with copper nitrate. This was followed by drying at 423 K and firing for one hour in the dry air flow at T = 773 K, leading to the formation of copper oxide. Palladium nitrate was then applied to the resulting material.

The solids produced by the KAT-1 and KAT-2 procedures were then dried at T = 423 K and first calcined for three hours in a stream of dry air at T = 773 K and finally calcined for one hour under hydrogen at the same temperature. . Both Pd / Cu bimetallic catalysts contain 5 wt.% Pd phase and 1.5 wt. % Cu metal phase.

In addition to differently prepared Pd / Cu bimetallic catalysts, Pd (5 wt%) / γ-Αΐ2θ3, designated KAT-3, was also synthesized. It was made according to the same procedure as KAT-1, of course without copper nitrate pre-application.

Pd / Cu bimetallic catalysts (KAT-1 and KAT-2) were used as separate substances in the catalytic hydrogenation process of nitrate ion, while Pd / 7-Al2O3 (KAT-3) was an integral part of the physical mixture and thus used in combination with a separate Cu metal phase (i.e., copper dust in this case).

Example 2: Testing the catalyst of the invention

This embodiment demonstrates the effectiveness of the present invention in the case of heterogeneously catalyzed reduction of nitrate ion in aqueous solution. The catalysts presented in Example 1 were tested: KAT-1, KAT-2 and a physical mixture consisting of KAT-3 and Cu metal phase. The size of the copper particles used in the latter case was less than 100 μτη. The laboratory experiments were carried out in an isothermal, suction-based slurry reactor. Reaction conditions in which the external and internal material resistances are negligible compared to the rate of chemical reaction are presented in Table 1. In the characteristic experiment, the weighed amount of catalyst was shaken into a reactor in which a known volume of bi-distilled water was already stored. The system was then purged with nitrogen and hydrogen. When the temperature of the reaction mixture was constant and equal to the prescribed value, a concentrated nitrate ion solution prepared from potassium nitrate and bidestilled water was poured into the reactor. The reaction was followed by taking representative samples at certain intervals from the reactor system, the solid and liquid phases of which were separated by centrifugation. The concentrations of nitrate, nitrite and ammonium ion in the aqueous phase were measured using a UV / VIS spectrophotometer (Hitachi, model U-3210) equipped with FIA technique, using standardized analytical methods.

Table 1.

Experimental process conditions of the invention - catalytic hydrogenation of aqueous nitrate ion solutions

Reaction temperature, K 293 Total pressure, bar 1.0 Hydrogen partial pressure, bar 1.0 Mixing speed, min ' ¹ 450 Gas phase flow, mlN / min 400 Initial nitrate ion concentration, mg / L 200 Concentration of catalyst, g / L 1.0 Average particle size, μτη 25 Concentration of Cu metal phase, g / L 1.0 * Volume of reactor, L 2.0

* only in case of reduction of aqueous nitrate ion solution with physical mixture of Pd and Cu metal phases

Figure 1 shows the nitrate ion concentration as a function of the reaction time determined in the presence of different catalysts in a slurry reactor. As can be seen, these results demonstrate the high efficiency of the catalysts presented in Example 1 for removing excess amounts of nitrate ion from drinking and process waters. Of the only Pd / Cu bimetallic catalyst mentioned in the professional literature (Horold et al. (Catal. Today, 17, 21-30 (1993)) used so far in the process of catalytic hydrogenation of aqueous solutions of nitrate ion, the catalysts are the subject of the present invention, In addition to the better activity, the Pd / Cu bimetallic catalysts synthesized by KAT-1 and KAT-2 procedures (Example 1) allow more selective conversion of the reactant to the reference material (Horold et al. (1993)). nitrogen, so that the final amount of water-dissolved ammonium ion as a by-product of the reaction was reduced by 45 percent under the experimental conditions listed in Table 1. (Figure 2) The selectivity of the catalytic reduction process of the nitrate ion was thus made using new Pd / Cu bimetallic The KAT-1 and KAT-2 catalysts were raised from 92 mole percent to 92 and 93 mole percent respectively.

Using a physical mixture consisting of KAT-3 and Cu metal phase, the Pd / Cu active sites where the nitrate ion is converted to nitrogen are formed in situ by collisions between the Pd and Cu metal phase particles in the slurry reactor. Although the activity of the physical mixture of the two metal phases for the hydrogenation of the nitrate ion is equivalent to that of the KAT-1 and KAT-2 Pd / Cu bimetallic catalysts, however, the selectivity of the conversion of the nitrate ion to nitrogen in this case is lower and is numerically identical to that indicated ( Horold et al (1993).

Claims (8)

A process for the reduction of a nitrate ion in an aqueous solution, characterized in that it consists of contacting a nitrate ion containing aqueous medium and hydrogen or hydrogen containing gas on solid catalysts. Process according to claim 1, characterized in that it is preferably carried out in a three-phase reactor at room temperature and atmospheric pressure, the partial pressure of hydrogen being an angle. reducing agent less than or equal to 1 bar and total operating pressure equal to atmospheric pressure. 3. A catalyst for the reduction of nitrate ion in aqueous solution, characterized in that the catalyst is solid and consists of two metal phases, which are: a) deposited on the same support, with the exterior of the catalyst particle being the phase at which hydrogen is dissociatively adsorbed; b) separated from each other and the active site required for the reaction is formed in situ by collisions between the particles of the two phases. Catalyst according to claim 3, characterized in that it consists of a physical mixture of Cu, Ag or Fe metal phase and one of the following metals: Pd, Pt or Ni. Catalyst according to claim 3, characterized in that the metallic Pd, Pt or Ni phase is supported on the carrier, with a concentration of 0.3 to 10% by weight. Catalyst according to claim 3, characterized in that it consists of a combination of Pd, Pt or Ni with one of the metals, such as Cu, Ag, Au, Rh, Ru or Fe, the metals being supported on the support such that the layer Pd, Pt, or Ni covers the Cu, Ag, Au, Rh, Ru, or Fe metal phase. Catalyst according to claim 3, characterized in that the concentration of Pd, Pt, or Ni of the metal phase on the support is from 0.3 to 10% by weight. A catalyst according to claim 3, characterized in that the concentration of Cu, Ag, Au, Rh, Ru or Fe of the metal phase on the support is from 0.05 to 5% by weight.