Method for producing mesoporpus nanoscale iron-containing metal particles

Abstract

A method for producing mesoporous nanoscale iron-containing metal particles includes the steps of: adding dropwise a reducing agent into an iron salt containing aqueous solution under a condition that the molar ratio of the reducing agent to the iron salt ranges from 6 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles; separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and drying the mesoporous nanoscale iron-containing metal particles.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority of Taiwanese application no. 094112548 filed on Apr. 20, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for producing mesoporous nanoscale iron-containing metal particles, especially, mesoporous nanoscale iron-containing metal particles with superior BET specific surface area and reactivity.

2. Description of the Related Art

In recent years, nanotechnology has a great impact on the fields of biotechnology, energy source, information, microelectromechanics, and environmental engineering due to the specific properties of nanoscale particles. Specifically, it is found that when the dimensions of a particulate particle are reduced to nano-scale, the physicochemical properties thereof are markedly changed, and reactivity thereof is greatly improved. Therefore, researches have been focused on particles having a size ranging from 1 to 100 nm,

In general, the methods for producing nanoscale particles include: (1) gas condensation method; (2) mechanical alloying method; and (3) solution chemistry methods. Among the solution chemistry methods, the chemical reduction method is commonly used because the nanoparticles thus formed have a relatively small size and an even distribution in the particle size.

In the known chemical reduction method for producing nanoscale iron particles, iron salt is reduced by a reducing agent, is nucleated in an over-saturated system, and grows and is precipitated so as to form nanoscale zero-valent iron. Ferric chloride (FeCl₃) reduced by sodium borohydride (NaBH₄) so as to form nanoscale zero-valent iron particles having BET specific surface area of 31.4 m²/g is disclosed in Choe, S., Y. Y. Chang, K. Y. Hwangnd, and J. Khim, 2000, “Kinetics of Reductive Denitrification by Nanoscale Zero-Valent Iron,” Chemosphere, 41(8), pp. 1307-1311, or having BET specific surface area of 33.5 m²/g is disclosed in Wang, C. B. and W. X. Zhang, 1997, “Synthesizing Nanoscale Iron Particles for Rapid and Complete Dechlorination of TCE and PCBs, ” Environmental Science & Technology, 31(7), pp. 2154-2156.

In the chemical reduction process, nanoscale particles with desired properties can be obtained by adjusting the reactant ratio, the way of mixing the reactants, the pH value, the reaction rate, the reaction temperature, the reaction pressure, and the solvent used in the reaction system. The characteristics of nanoscale particles reside in their high reactivity and high surface area-to-volume ratio. However, the nanoscale iron particles reported in the literatures or that are commercially available have BET specific surface area less than 38 m²/g such that the reactivity improvement is limited.

Therefore, there is a need in the art to provide a method for producing nanoscale particles that can enhance BET specific surface area of nanoscale particles to thereby improve reactivity thereof.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide a method for producing mesoporous nanoscale iron-containing metal particles that can overcome the aforesaid shortcoming associated with the prior art.

According to one aspect of this invention, a method for producing mesoporous nanoscale iron-containing metal particles comprises the steps of: adding dropwise a reducing agent into an iron salt containing aqueous solution under a condition that the molar ratio of the reducing agent to the iron salt ranges from 6 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles; separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and drying the mesoporous nanoscale iron-containing metal particles.

According to another aspect of this invention, a method for producing mesoporous nanoscale iron-containing metal particles comprises the steps of: adding dropwise an iron salt containing aqueous solution into a reducing agent under a condition that the molar ratio of the reducing agent to the iron salt ranges from 4 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles; separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and drying the mesoporous nanoscale iron-containing metal particles.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which:

FIG. 1 is a flow chart illustrating consecutive steps of the first preferred embodiment of a method for producing mesoporous nanoscale iron-containing metal particles according to this invention;

FIG. 2 is a flow chart illustrating detailed consecutive steps of the first preferred embodiment;

FIG. 3 shows a SEM photograph to illustrate the mesoporous nanoscale zero-valent iron particles produced according to the first preferred embodiment;

FIG. 4 is a plot showing nitrogen adsorption/desorption isotherms for the mesoporous nanoscale zero-valent iron particles produced according to the first preferred embodiment, which were determined using a specific surface area analyzer;

FIG. 5 is a plot showing a pore size distribution of the mesoporous nanoscale zero-valent iron particles produced according to the first preferred embodiment;

FIG. 6 is a flow chart illustrating detailed consecutive steps of the second preferred embodiment of the method for producing mesoporous nanoscale iron-containing metal particles according to this invention;

FIG. 7 shows a SEM photograph to illustrate the mesoporous nanoscale zero-valent iron particles produced according to the second preferred embodiment;

FIG. 8 is a plot showing nitrogen adsorption/desorption isotherms for the mesoporous nanoscale zero-valent iron particles produced according to the second preferred embodiment, which were determined using a specific surface area analyzer;

FIG. 9 is a plot showing a pore size distribution of the mesoporous nanoscale zero-valent iron particles produced according to the second preferred embodiment;

FIG. 10 is a flowchart illustrating consecutive steps of the third preferred embodiment of the method for producing mesoporous nanoscale iron-containing metal particles according to this invention; and

FIG. 11 is a flow chart illustrating detailed consecutive steps of the third preferred embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the first preferred embodiment of a method for producing mesoporous nanoscale iron-containing metal particles according to the present invention includes the steps of: 1) adding dropwise a reducing agent into an iron salt containing aqueous solution under a condition that the molar ratio of the reducing agent to the iron salt ranges from 6 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles; 2) separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and 3) drying the mesoporous nanoscale iron-containing metal particles.

Preferably, the adding rate of the reducing agent ranges from 0.185 to 0.55 ml/sec based on one liter of the iron salt containing aqueous solution.

Preferably, the iron salt is a compound selected from the group consisting of: ferric chloride (FeCl₃), ferrous chloride (FeCl₂), ferric sulfide (Fe₂(SO₄)₃), ferrous sulfide (FeSO₄), ferric nitride (Fe(NO₃)₃), ferrous nitride (Fe(NO₃)₂), ferric bromide (FeBr₃), ferrous bromide (FeBr₂), and combinations thereof.

The reducing agent is in the form of an aqueous solution, and is selected from the group consisting of: sodium borohydride (NaBH₄), potassium borohydride (KBH₄), lithium borohydride (LiBH₄), sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃), sodium hydroxide (NaOH), potassium hydroxide (KOH), lithium hydroxide (LiOH), methanol, ethanol, lithium aluminum tetrahydride (LiAlH₄), ammonium ion (NH₄⁺), hydrazine (N₂H₄), citric acid (C₆H₈O₇), sodium citrate (Na₃C₆H₅O₇), potassium citrate (K₃C₆H₅O₇), and combinations thereof.

Specifically, as shown in FIG. 2, the iron salt-containing aqueous solution used in this embodiment is a 0.09 M FeCl₃ solution, and the reducing agent is a 0.50M NaBH₄ solution. The reaction equilibrium formula for reduction of the iron salt is as follows:
2FeCl₃+6NaBH₄+18H₂O→2Fe_(s)↓+21H₂O+6B(OH)₃+6NaCl
Under vigorous agitation at ambient temperature and atmosphere, 100 ml NaBH₄ solution is added slowly and dropwise into 100 ml FeCl₃ solution under conditions that the titration rate is 0.055 ml/sec, the molar ratio of the reducing agent to the iron salt is 22.5, and the pH value of the reaction system ranges from 6 to 11. In this embodiment, the molar ratio (45/2=22.5) of the reducing agent to the iron salt is 7.5 times the stoichiometric ratio (6/2=3) of the reducing agent to the iron salt. During the process of titration, nucleuses are produced at the over-saturated condition and grow to desired nanoscale iron-containing metal particles through poly-nuclear growth of surface process. Subsequently, the nanoscale iron-containing metal particles thus formed are separated from the solution using suitable means, followed by a drying step using a freeze dryer.

The structure of the nanoscale iron-containing metal particles thus formed was observed using Field Emission Scanning Electron Microscopy (FESEM). As shown in FIG. 3, the grain size of the nanoscale iron-containing metal particles ranges from 50 to 80 nm, and has a spherical shape. The nanoscale iron-containing metal particles produced by this preferred embodiment agglomerate with each other because of stronger magnetism.

FIG. 4 shows nitrogen adsorption/desorption isotherms at 77.35 K. The adsorption curve rises rapidly and a hysteresis loop exists between the adsorption and desorption curves, which indicates that the iron-containing metal particles thus formed have a mesoporous structure. The specific surface area was calculated to be 128 m²/g using the Brunauer-Emmett-Teller (BET) method. FIG. 5 shows a pore size distribution of the iron-containing metal particles produced by this embodiment, which indicates that the surface pore size ranges from 30 to 40 Å calculated from adsorption data using Barrett-Joyner-Halenda (BJH) equation. It is clear that the nanoscale iron-containing metal particles produced by the method of this invention have a high BET specific surface area and a mesoporous structure.

FIG. 6 illustrates the second preferred embodiment of a method for producing nanoscale iron-containing metal particles according to the present invention. The second preferred embodiment differs from the first preferred embodiment in that the titration process is a reverse titration. That is, the iron salt-containing aqueous solution is added slowly and dropwise into the reducing agent solution. In this reverse titration system, the molar ratio of the reducing agent to the iron salt is 4 to 10 times the stoichiometric ratio of the reducing agent to the iron salt. In this embodiment, the molar ratio of the reducing agent to the iron salt is 7.5 times the stoichiometric ratio of the reducing agent to the iron salt. In addition, the growth of the nucleuses is controlled by diffusion. Subsequently, the nanoscale iron-containing particles thus formed are separated from the solution using suitable means, followed by a drying step using a freeze dryer.

The structure of the nanoscale iron-containing metal particles thus formed was observed using Field Emission Scanning Electron Microscopy (FESEM). As shown in FIG. 7, the grain size of the nanoscale iron-containing metal particles ranges from 30 to 50 nm and has a spherical shape. The nanoscale iron-containing particles produced by this embodiment agglomerate in a chain form, which shows weak magnetism of the particles. Hence, the mesoporous nanoscale iron-containing particles produced by this preferred embodiment have better dispersion property than those produced by the first preferred embodiment.

As shown in FIGS. 8 and 9, the iron-containing metal particles thus obtained have a mesoporous structure, the calculated BET specific surface area thereof is 77 m²/g, and the surface pore size ranges from 30 to 40 Å calculated from adsorption data using Barrett-Joyner-Halenda (BJH) equation.

FIGS. 10 and 11 illustrate the third preferred embodiment of a method for producing nanoscale iron-containing metal particles according to the present invention. As shown in FIG. 10, the third preferred embodiment differs from the first and second preferred embodiments in that the method further includes a step of 4) adding a catalytic non-iron metal salt into the iron salt containing aqueous solution after the reducing step so as to form mesoporous nanoscale bimetallic particles. The suitable catalytic non-iron metal salt is selected from the group consisting of: palladium salt, rhodium salt, platinum salt, iridium salt, ruthenium salt, osmium salt, gold salt, nickel salt, copper salt, manganese salt, zinc salt, cobalt salt, vanadium salt, and combinations thereof. The weight ratio of catalytic non-iron metal salt to Fe⁰ ranges from 1/10 to 1/3000.

As shown in FIG. 11, the catalytic non-iron metal salt used in this embodiment is Pd(NO₃)₂. The weight ratio of Pd²⁺ to Fe⁰ is less than 1/100, and the reaction is as follows:
Pd²⁺+Fe⁰→Pd⁰+Fe²⁺

Fe⁰ in the reaction serves as a carrier. Pd⁰ grows on Fe⁰ during the reaction so as to form bimetallic particles of palladized iron, which have a structure of particle-on-particle (other than core-shell structure). The BET specific surface area of the nanoscale bimetallic particles thus formed is 101 m²/g, and the pore diameter of the mesoporous structure thereof ranges from 30 to 40 Å.

The method of the fourth preferred embodiment of this invention is conducted at a large scale. That is, 2 L of 0.5M NaBH₄ solution is added dropwise into 2 L of 0.09M FeCl₃ solution under a condition that the titration rate is 0.37 ml/sec for 1.5 hrs. The obtained nanoscale iron-containing particles have a BET specific surface area ranging from 150-160 m²/g.

It is noted that, by controlling the molar ratio of the reducing agent to the iron salt within a range of from 4 to 10 times or 6 to 10 times of the stoichiometric ratio of the reducing agent to the iron salt, the nanoscale iron-containing metal particles produced by the method of this invention can have a higher BET specific surface area (45-175 m²/g) than that in the prior art (<38 m²/g) to result in improved reactivity.

While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.

Claims

1. A method for producing mesoporous nanoscale iron-containing metal particles, comprising the steps of:

adding dropwise a reducing agent into an iron salt containing aqueous solution under a condition that the molar ratio of the reducing agent to the iron salt ranges from 6 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles;

separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and

drying the mesoporous nanoscale iron-containing metal particles.

2. The method of claim 1, further comprising a step of adding a catalytic non-iron metal-salt into the iron salt containing aqueous solution after the reducing step.

3. The method of claim 1, wherein the adding rate of the reducing agent ranges from 0.185 to 0.55 ml/sec based on one liter of the iron salt containing aqueous solution.

4. The method of claim 1, wherein the iron salt is a compound selected from the group consisting of: ferric chloride, ferrous chloride, ferric sulfide, ferrous sulfide, ferric nitride, ferrous nitride, ferric bromide, ferrous bromide, and combinations thereof.

5. The method of claim 1, wherein the reducing agent is selected from the group consisting of: sodium borohydride, potassium borohydride, lithium borohydride, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, methanol, ethanol, lithium aluminum tetrahydride, ammonium ion, hydrazine, citric acid, sodium citrate, potassium citrate, and combinations thereof.

6. The method of claim 2, wherein the catalytic non-iron metal salt is selected from the group consisting of: palladium salt, rhodium salt, platinum salt, iridium salt, ruthenium salt, osmium salt, gold salt, nickel salt, copper salt, manganese salt, zinc salt, cobalt salt, vanadium salt, and combinations thereof.

7. The method of claim 1, wherein the pH value in the mixture of the reducing agent and the iron salt containing aqueous solution ranges from 6 to 11.

8. A method for producing mesoporous nanoscale iron-containing metal particles, comprising the steps of:

adding dropwise an iron salt containing aqueous solution into a reducing agent under a condition that the molar ratio of the reducing agent to the iron salt ranges from 4 to 10 times the stoichiometric ratio of the reducing agent to the iron salt to produce mesoporous nanoscale iron-containing metal particles;

separating the mesoporous nanoscale iron-containing metal particles from the aqueous solution; and

drying the mesoporous nanoscale iron-containing metal particles.

9. The method of claim 8, further comprising a step of adding a catalytic non-iron metal salt into the iron salt containing aqueous solution after the reducing step.

10. The method of claim 8, wherein the iron salt is a compound selected from the group consisting of: ferric chloride, ferrous chloride, ferric sulfide, ferrous sulfide, ferric nitride, ferrous nitride, ferric bromide, ferrous bromide, and combinations thereof.

11. The method of claim 8, wherein the reducing agent is selected from the group consisting of: sodium borohydride, potassium borohydride, lithiumborohydride, sodium carbonate, potassium carbonate, sodium hydroxide, potassium hydroxide, lithium hydroxide, methanol, ethanol, lithium aluminum tetrahydride, ammonium ion, hydrazine, citric acid, sodium citrate, potassium citrate, and combinations thereof.

12. The method of claim 9, wherein the catalytic non-iron metal salt is selected from the group consisting of: palladium salt, rhodium salt, platinum salt, iridium salt, ruthenium salt, osmium salt, gold salt, nickel salt, copper salt, manganese salt, zinc salt, cobalt salt, vanadium salt, and combinations thereof.

13. The method of claim 8, wherein the pH value in the mixture of the reducing agent and the iron salt containing aqueous solution ranges from 6 to 11.