Patent Application
20090317308
Not Applicable
Not Applicable
1. Field of the Invention
This invention provides catalysts for converting ammonia in an aqueous solution directly to nitrogen gas at about or above ambient temperature. It also provides a method for water treatment to lower its ammonia content by converting the ammonia to nitrogen directly in aqueous phase.
2. Prior Art
Ammonia found in aqua system at an elevated level presents health hazard. Industrial and residential waste streams containing ammonia at the elevated level need to be properly treated to lower the ammonia concentration within the legally allowable limits before being discharged.
The prior art methods in removing ammonia from the waste streams require expensive machines and use complicated procedures. For example, in one method used to remove ammonia from industrial waste streams that contain ammonium sulphate ((NH4)2SO4 (aq)), the pH of the waste stream is raised to a value greater than 10.5 by adding sodium hydroxide to convert the ammonium (NH4+(aq)) into ammonia (NH3 (aq)), and then the ammonia in solution is extracted into gas phase to gaseous ammonia. After the addition of more combustible fuel such as hydrogen, the ammonia gas is flamed in air. Because of the high temperature involved in flaming ammonia in air, pollutants such as NOx could be produced, posing potential damage to the environment.
Catalytic oxidation of ammonia in the gas phase with oxygen at an elevated temperature has been extensively studied. It is found that at a temperature higher than 350° C., ammonia oxidation promotes the formation of nitric oxides with a variety of catalysts, such as those revealed in U.S. Pat. No. 4,812,300, U.S. Pat. No. 5,242,882, and U.S. Pat. No. 3,853,790. To minimize the formation of nitric oxides in the gas phase ammonia oxidation reaction, U.S. Pat. No. 7,410,626 teaches the formation and use of layered catalyst containing a refractory metal oxide inner layer, a platinum middle layer and a vanadium top layer. With the layered catalyst, the catalytic oxidation of ammonia in the gas phase with an oxygen containing gas stream carried out at a temperature between 200 to 375° C. proceeds preferentially to nitrogen gas. US patent application 20070059228 revealed the formation of Pt on silica support as catalyst for converting gaseous ammonia with oxygen to nitrogen between 125 and 200° C. The Pt on silica support catalyst needs to be activated at a temperature above 125° C., while holding the reaction temperature below 200° C. to avoid the nitric oxide formation. In the above mentioned patent application, extracting ammonia into the gas phase from an aqueous solution is an energy intensive step, involving using a heater and a vaporizer.
In the commonly used method to treat the residential sewage water containing ammonia at a low level, multistep bacteria assisted biological ammonia decomposition processes are used. However, there are limitations in using the bacteria processes, such as the low level of ammonia in the waste stream allowable to the bacteria processes, and the rate of the bacteria processes being strongly affected by the ambient temperature. For this reason, the processes of ammonia decomposition by bacteria would be very difficult to carry out in the winter months compared to in the summer months.
Removal of ammonia by electrolysis using an electrochemical device has been documented in literatures (Frederic Vitse, Matthew Cooper, and G. G. Botte, “On the Use of Ammonia Electrolysis for Hydrogen Production,” J. Power Sources, 142, 18-26 (2005)) and in patents, (U.S. Pat. No. 6,083,377, U.S. Pat. No. 7,160,430 B2). In this method, the electrochemical device used to carry-out the electrolysis consists of two electrodes electrically separated by a hydroxide anion (OH−) conducting medium, and a DC power source connected to the two electrodes. The electrical energy input drives the ammonia electrolysis reactions. At the anodic electrode, which is connected to the positive terminal of the power supplier, ammonia is electro-oxidized to N2 gas on the surface of the electrocatalysts, according to the following electrode reaction carried out in an alkaline medium:
2NH3(aq)+6OH−(aq)→N2(g)+6H2O(I)+6e− (1)
At the cathodic electrode, which is connected to the negative terminal of the power supplier, the electrical energy input drives an electro-reduction process of evolving hydrogen gas on the surface of the electrocatalysts, according to the following electrode reaction carried out in an alkaline medium:
6H2O(I)+6e−→6OH−(aq)+3H2(g) (2)
The combined electrode reactions of eq. (1) and (2) driven by the electrical energy input is the conversion of ammonia to nitrogen gas (N2) and evolution of hydrogen gas (H2):
2NH3(aq)+Electrical energy→N2(g)+3H2(g) (3)
As the result, ammonia is removed by converting it to harmless nitrogen gas directly in aqueous solution at near ambient temperature.
To carry out the electrochemical reactions of ammonia electrolysis for removal of ammonia, several key requirements must be met for the electrochemical device, as described below:
6OH−(aq)→3/2O2(g)+3H2O(I)+6e− (4)
and at the opposite electrode the same electro-reduction of hydrogen evolution occurs as shown in Eq. (3). The overall reaction involved with oxygen evolution is:
3H2O(I)+Electrical energy→3/2O2(g)+3H2(g) (5)
In such a case, the electrolysis is ineffective in converting ammonia to nitrogen gas, and the electrical energy input is wasted in splitting water to oxygen gas and hydrogen gas.
NH4+(aq)+OH−(aq)→NH3(aq)+H2O(I).
It may now become obvious to those skilled in the art that to engineer such an electrochemical device for ammonia removal from aqueous streams by electrolyzing ammonia to nitrogen gas, one has to overcome significant barriers which may be too high to deem the method to be practically useful. For example, it is difficult to minimize the required area of electrodes and the space between the electrodes, resulting in a bulky electrochemical unit. Additionally, there are high costs associated with building the electrodes, maintaining and running the electrochemical device that needs electrical energy input, addition of alkaline electrolyte, and means for corrosion protections.
In viewing these difficulties of above mentioned methods of prior art in removing ammonia from aqueous streams, it is the intention of this invention to provide significant improvements in a novel approach.
It is an object of the invention to provide a catalyst that oxidizes ammonia directly in aqueous solution to nitrogen at about or above ambient temperature.
It is a further object of the invention to provide a process to treat an aqueous stream containing undesirable level of ammonia in a manner such that the ammonia is converted to nitrogen gas directly in aqueous phase at about or above ambient temperature.
The invention describes a novel method for removing ammonia from an aqueous solution by converting ammonia to harmless nitrogen gas directly and spontaneously at about or above ambient temperature. The ammonia containing aqueous stream is mixed with an oxygen containing stream, such as air, oxygen, or hydrogen peroxide, in a chemical reactor containing a specially formulated catalyst. When the ammonia and oxidant molecules in the mixture are exposed to the catalyst, the catalyst performs electro-oxidization of ammonia to nitrogen gas and electro-reduction of oxygen to water simultaneously. The result is a spontaneous chemical combination of the ammonia and oxidant molecules on the surface of the catalyst particles to form nitrogen and water. The ammonia conversion process on the catalyst surface resembles a miniaturized direct ammonia/oxidant fuel cell at nanoscale level operated at short circuit mode in spontaneously converting ammonia to nitrogen at the maximum rate by releasing the chemical energy stored in the ammonia molecules as waste heat. The catalysts are packed and confined in catalyst beds in a chemical reactor. The entire process of ammonia removal occurs spontaneously in the aqueous phase at a temperature significantly below that found in a combustion process or in a catalytic ammonia oxidation process in prior art technologies, and without the high cost associated with building electrodes, maintaining and running a bulky electrolysis device that needs electrical energy input.
In order that the manner in which the above-recited and other advantages and objects of the invention are obtained will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments thereof which are illustrated in the pending drawings. Understanding that these drawings depict only typical-embodiments of the invention and are not therefore to be considered to be limiting of its cope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
Based on thermodynamics, an ammonia molecule has a high energy content. In a combustion process, ammonia molecules react with O2 to form water and nitrogen, and heat is released:
2NH3(g)+3/2O2(g)→N2(g)+3H2O(g)+heat (6)
In practice, ammonia and air have a very narrow composition window of flammability, and a very high ignition energy is required to start the combustion process. As the result, the addition of other fuels, such as H2, is required to assist the combustion process of ammonia in air. Other drawbacks of flaming ammonia include the high temperature involved that tends to encourage the formation of NOx as air pollutants.
Ammonia as a fuel can be converted to N2 at a temperature significantly below that found in the combustion process and directly in aqueous phase, as demonstrated in a direct ammonia/air fuel cell operated at room temperature. As shown in
The electrode reaction at the fuel cell anode is ammonia electro-oxidation reaction:
2NH3(aq)+6OH−(aq)→N2(g)+6H2O(I)+6e− (1)
which is the same as the ammonia electrolysis anode. The electrode reaction at the fuel cell cathode is the oxygen reduction reaction:
3H2O(I)+3/2O2(g)+6e−→6OH−(aq) (7)
The combined reaction of these two electrode reactions, eqs. (1) and (7), is:
2NH3(aq)+3/2O2(g)→N2(g)+3H2O(I)+electrical energy output (8)
In
In the direct ammonia/air fuel cell, the ammonia molecules are converted to nitrogen gas at the cell anode as evidenced by the N2 gas bubbles emerged, and the conversion rate is measured by the cell current density. As show by the cell voltage current curve in
To explore the ammonia electro-oxidation rate at near short circuit condition, a set of experiments were conducted with a direct ammonia/air fuel cell operated at room temperature with a cathode by oxygen diffusion from ambient air.
A plot of the steady state cell current at 0.05 V as a function of ammonia concentration is shown in
A plot of cell current at 0.05 V as a function of KOH concentration is shown in
To verify that the ammonia conversion to nitrogen gas in the direct ammonia/air breathing fuel cell proceeds quantitatively according to the anodic electrode reaction eq. (1), the nitrogen gas bubbles emerging from the anode compartment were collected while holding the direct ammonia/air fuel cell at various constant current for a period of time. In
where:
Table 1 summarizes the conversion efficiency of ammonia to nitrogen gas at the cell anode of a direct ammonia/air fuel cell determined by linear regression of the slope shown in
To assist the ammonia electro-oxidation electrode reaction eq. (1), various catalysts, such as Pt, or Pt based alloys, such as Ptlr, PtRu, PtRd, PtNi, have demonstrated to be effective. For oxygen-reduction electrode reaction eq. (7), Pt, Ni, Ag, graphite, Co-Tetramethoxyphenylporphrine, Fe—N-carbon, MnO2 and TiO2 etc have demonstrated to be effective. Usually, these catalysts are nanoparticles in size deposited on an electronically conductive support such as carbon, graphite, indium doped TiO2, conducting polymer materials, tungsten oxides, metal powder, metal mesh and metal sheet materials etc.
In principle, one can use a direct ammonia/air fuel cell to convert ammonia in aqueous solution directly to nitrogen gas at ambient conditions. Compared to the prior art of using ammonia electrolysis, the approach of using direct ammonia/air fuel cell for the conversion process eliminates the need of an electrical energy input. However, there are other barriers shared with ammonia electrolysis method, such as the high cost associated with building electrodes, maintaining and running an electrochemical device, and the large electrode area required to achieve a reasonable ammonia conversion rate. These barriers could still be prohibitive for practical applications in lowering the ammonia level in an aqueous solution by a direct ammonia/air fuel cell.
To overcome the remaining barriers of the high cost and large size of an electrochemical device, this invention builds a direct ammonia/air fuel cell at short circuit on a single catalyst particle as illustrated in
With catalyst of this invention, the electrode reactions of a direct ammonia/air fuel cell operated in short circuit mode are carried out at a substantially miniaturized scale (a few nanometers) on the surface of a single catalyst particle 10, as illustrated in
Alternatively, as illustrated schematically in
Alternatively, as illustrated schematically in
The two electrode reactions eqs. (1) and (7) are carried out by two adjacent catalyst subparticles on a single catalyst particle as illustrated in
A chemical reactor containing a plural of the catalyst particles equating multiple miniaturized direct ammonia/air fuel cells operated in short circuit mode will thus spontaneously convert ammonia molecules to nitrogen at the maximum reaction rate directly in aqueous phase. With the catalysts packed and confined in a catalyst bed in the reactor, the direct conversion of ammonia in an aqueous stream to nitrogen gas can be carried out continuously in a compact package, and without the high cost of constructing and maintaining an electrochemical device.
In
If desired, other oxidant such as pure oxygen, or hydrogen peroxide, air diluted with inert gas can be used.
If desired, the ammonia conversion to nitrogen can be carried at elevated pressure and temperature within the reactor. A higher temperature and pressure will speed up the conversion reaction, and thus reduce the size of the reactor required.
If desired, the ammonia in the waste stream can be collected and enriched by standard cation exchanger column before being introduced to a reactor for conversion to nitrogen.
In this example, a catalyst was made by pyrolizing a precursor of Fe-phthalocyanine deposited on a high surface area carbon.
In a 2 L beaker, a batch of 40 grams of Ketjen Black EC300J carbon powder was added, flowed by adding 1.5 L ethanol and 20 grams of Fe-phthalocyanine. The mixture was stirred for 30 min, followed by intense pulses of sonication with ultrasonic probe for 30 min. The mixture was stirred overnight. After that, the solvent is removed using a rotary evaporator at 60° C. to obtain the precursor of Fe-phthalocyanine deposited on the carbon powder. The precursor was then transferred to a quartz reactor for the pyrolytic treatment. The quartz reactor was placed inside a furnace equipped with a programmable temperature controller. The pyrolysis was carried out under argon flow at 100 SCCM. The reactor was ramped to 150° C. over a period of 15 min. and held at 150° C. for 20 min., then ramped to 445° C. over a period of 30 min. and held at 445° C. for one hour, then ramped to 785° C. over a period of 45 min and held at 785° C. for two hours, and then cooled to room temperature by natural heat dissipation. The pyrolic product was collected and ball milled. Elemental analysis by ICP showed that the catalyst contained 4.2 wt. % Fe. This catalyst is designated as FeNC/C.
In this example, a Ptlr catalyst was made by incipient wetness method to distribute Pt and Ir ionic species from solution to a high surface area carbon, followed by reduction in the gas phase.
In a 250 mL beaker, 0.7 grams of IrCl3.3H2O and 1.2 grams of H2PtCl6 were added. A 20 mL of water and methanol mixture at 50/50 volume ratio was then added. Part of the resulting solution was added to a 2 L beaker containing 20 grams of Ketjen Black EC300J carbon powder to form a paste at incipient wetness. The paste was stirred and pressed with a spatchula, and dried at room temperature for about 90 minutes, followed by drying under vacuum (0.5 torr (65 Pa), 60° C.) for about 2.5 hours. The powder mixture was then transferred to quartz reactor for reduction treatment. The reduction was carried out under a reforming gas (95% N2, 5% H2) flow at 100 SCCM. The reactor is ramped to 100° C. over a period of 15 min and held at 100° C. for 20 min., then ramped to 255° C. over a period of 30 min and held at 255° C. for 30 min, and then cooled to room temperature by natural heat dissipation. The reduced product was collected and ball milled. Elemental analysis by ICP showed that the catalyst contained 1.9 wt. % Ir and 2.7 wt. % Pt. This catalyst was designated as Ptlr/C-support.
In this example, a Ptlr catalyst was made by incipient wetness method to distribute Pt and Ir ionic species from solution to the FeNC/C-support catalyst formed in Example 1, followed by reduction in the gas phase.
In a 250 mL beaker, 0.7 grams of IrCl3.3H2o and 1.2 grams of H2PtCl6 were added. A 50 mL of water and methanol mixture at 50/50 volume ratio was then added. Part of the resulting solution was added to a 2 L beaker containing 20 grams of the catalyst FeNC/C to form a paste at incipient wetness. The paste was stirred and compressed with a spatchula, and dried at room temperature for about 90 minutes, followed by drying under vacuum (0.5 torr (65 Pa), 60° C.) for about 2.5 hours. The powder mixture was then transferred to quartz reactor for reduction treatment. The reduction was carried out under a reforming gas (95% N2, 5% H2) flow at 100 SCCM. The reactor is ramped to 100° C. over a period of 15 min and held at 100° C. for 20 min., then ramped to 255° C. over a period of 30 min and held at 255° C. for 30 min, and then cooled to room temperature by natural heat dissipation. The reduced product was collected and ball milled. Elemental analysis by ICP showed that the catalyst contained 4.2 wt % Fe, 1.9 wt. % Ir and 2.7 wt. % Pt. This catalyst was designated as Ptlr/FeNC/C-support.
In this example, the three catalysts prepared in examples 1-3 and the Ketjen Black EC300J carbon support were tested for oxidation of ammonia in aqueous solution.
An ammonia solution containing 250 ppm ammonia was prepared. The solution had a pH value of 10.698, measured using an Orion 230A digital pH meter with a 9107BN pH electrode. The measured pH value was in good agreement with the value from equilibrium calculation of ammonia dissociation in water at room temperature. A batch of 15 grams of catalyst sample was loaded in a glass chromatography column with a fritted disc (10 μm pore diameter) at column bottom and a reservoir at column top. A 15 cc ammonia solution containing 250 ppm ammonia was added to the column. An oxygen flow pre-saturated with water at room temperature was introduced at the bottom of the column. After the initial steady state of flow of oxygen through the column was achieved, the oxygen flow rate was held steady at 4 SCCM. After a period of 30 min, the oxygen flow was stopped, and the ammonia solution was extracted by vacuum from the column into a filtering flask. The pH of the treated solution was measured to determining the remaining ammonia concentration. Table 2 summarizes the test results for the three catalyst samples plus the Ketjen Black EC300J carbon support sample. The test results showed that the carbon support and FeNC/C-support are not effective in oxidizing ammonia for its removal. As expected, the FeNC/C-support catalyst contains only type-C catalyst subparticles useful for the oxygen reduction reaction. The Ptlr/C-support catalyst works well since the Ptlr catalyst subparticles can function as both type-A catalyst subparticles for ammonia oxidation, and type-C catalyst subparticles for oxygen reduction. The best performance is achieved with the Ptlr/FeNC/C-support catalyst, which contains the high performance Ptlr as type-A catalyst subparticles for ammonia oxidation, and the FeNC as type-C catalyst subparticles for oxygen reduction.
These examples are no by means in limiting the scope of this patent in performance and applications. It is expected that a better performance can be achieved with a better reactor design for improved gas/liquid distribution within the catalyst column, at a higher operating temperature, and with optimizations of other contributing factors.
In view of the disclosure presented herein, yet other modifications and variations of the invention will be apparent to those of skill in the art. The foregoing discussion, description and examples are illustrative of specific embodiments of the invention, but they are not meant to be limitations upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention.
This application claims the benefit of provisional patent application Ser. No. 61/132,733 filed on Jun. 19, 2008 by the present inventor.
| Number | Date | Country | |
|---|---|---|---|
| 61132733 | Jun 2008 | US |
