WO2009052510A2

WO2009052510A2 - Method of surface modifying titania using metal and compositions therefrom

Info

Publication number: WO2009052510A2
Application number: PCT/US2008/080524
Authority: WO
Inventors: Ihor Mykhaylovych Kobasa; Mykhaylo Andriyovych Kovbasa; Wojciech Jan Strus
Original assignee: Worthington Technologies, Llc
Priority date: 2007-10-19
Filing date: 2008-10-20
Publication date: 2009-04-23
Also published as: WO2009052510A3

Abstract

A method for in-situ formation of metal surface modified titania comprises the steps of burning a titanium chloride comprising compound in the presence of oxygen and hydrogen in a reactor to form plurality of fine titania particles comprising titanium dioxide and titanium suboxide. A temperature during the burning step is generally from 700 to 1100 0C. Metal is deposited on a surface of the titania particles while in the reactor at a temperature below the temperature for the burning step, wherein the metal only partially covers the surface of said titania particles. The metal on the titania surface is generally in the form of randomly located nanoparticles.

Description

METHOD OF SURFACE MODIFYING TITANIA USING METAL AND COMPOSITIONS THEREFROM

This application claims priority to U.S. Provisional Patent Application No. 60/981,300, filed October 19, 2007, the entire content of which is incorporated by reference herein in its entirety.

BACKGROUND

[0001] Titanium dioxide (titania or ΗO₂) is found in three known crystal forms, rutile, anatase and brookite. The anatase and rutile forms are commonly used industrially. There are various known methods of synthesis, compositional variants including admixtures, and thermal processing which can modify the crystalline form(s) obtained.

[0002] Photocatalytic activity is generally the most important feature of titania. Photo- catalytic reactions do not lead to photocorrosion of the reagents and their composition remains unchanged unlike photochemical reactions, where semiconducting reagents undergo photocorrosion. Photochemical reagents absorb light and promote reactions between various substances in gaseous or liquid phases, or induce electrical current. Many semiconductors (including titania) exhibit such photochemical activity.

[0003] Semiconducting photocatalysis is a complex phenomenon with numerous promising spectro-optical, thermodynamic, kinetic, electrophysical and some other fundamental prospects. For instance, titania can be used as a basic material for highly effective photocatalytic systems for the transformation, conservation and utilization of the solar energy and for the hazardous waste neutralization and for other environment preservation solutions. Titania products also bring good prospects for the low-tonnage chemistry, for design and production of multi-functional materials (for example, materials containing thin-precipitated layer of nano-particles on various substrates), and for the production of optical sensors and materials with non-linear optical properties.

[0004] Most available titania comprises coarsely dispersed particles with low photocata- lytic activity. What is needed is a relatively simple process flow for forming new titania compositions that provide high catalytic activity and good catalytic stability, preferably from a process that generates less associated waste, such as toxic wastewater.

SUMMARY

[0005] This Summary is provided briefly indicate the nature and substance of the invention. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Embodiments of the present invention describe methods for the in-situ formation of metal surface modified titania and comprises the steps of burning a titanium chloride comprising compound in the presence of oxygen and hydrogen in a reactor to form plurality of fine titania particles comprising titanium dioxide and titanium suboxide. A temperature during the burning step is generally from 700 to 1100 ⁰C. Metal is deposited on a surface of the titania particles while in the reactor at a temperature below the temperature for the burning step, wherein the metal only partially covers the surface of said titania particles. The metal on the titania surface is generally in the form of randomly located nanoparticles.

DESCRIPTION OF THE DRAWING

[0006] A fuller understanding of the present invention and the features and benefits thereof will be obtained upon review of the following detailed description together with the accompanying drawing, in which:

[0007] Figure 1 shows a simplified reactor apparatus that can be used to produce metal surface modified titania particles according to embodiments of the invention. DETAILED DESCRIPTION

[0008] The present invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of embodiments of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The present invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present invention.

[0009] A method for in-situ formation of metal surface modified titania comprises the steps of burning a titanium chloride comprising compound in the presence of oxygen and hydrogen in a reactor to form plurality of fine titania particles comprising titanium dioxide and titanium suboxide. A maximum temperature during the burning step is generally from 700 to 1100 ⁰C. Metal is deposited on a surface of the titania particles while in the same reactor at a temperature below the temperature for the burning step. The metal on the titania surface is generally in the form of randomly located nanoparticles which only partially cover the titania surface, such as on average from 15 to 70% of the surface area of the titania particles. [0010] The method can further include the step of removing adsorbed chlorine from the surface of the particles with steam at a temperature below the temperature for the depositing step, such as in a range between 150-220 ⁰C. In one embodiment the depositing step com- prises cooling the titania particles while in the same reactor to a temperature between 300 and 700 ⁰C and in an atmosphere comprising at least one metal source and a reducing agent. The depositing can comprise injecting an aerosol comprising the metal to be deposited into a pipe-in-pipe heat exchanger within the reactor.

[0011] In certain embodiments of the invention, a molar ratio of hydrogen (H₂) with respect to oxygen (O₂) (f^iC^), is generally between 1.96: 1 to 2.61: 1, such as being in a ratio range from 2.02:1 to 2.61:1, during the burning step. The titanium chloride comprising compound can comprise titanium tetrachloride. The metal can comprise various metals, such as Pd, Pt, Zn, W, Bi and Mo, which can be provided in a salt or covalent molecular form. Other metals can also be used, include, but are not limited to, V, Al, Zr, Hf, Si, Cu, Co, Ni, Fe, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er and Tm.

[0012] In the case of Pd deposition using a PdCl₂ salt derived aerosol, it has been found that Pd ⁺ ions tend to embed into titania lattice if the deposition is at a temperature that exceeds about 600 to 700 ⁰C. This can leads to lowering of the surface concentration of Pd²⁺ and a decrease of its catalytic activity. Moreover, the specific surface area of titania may also decrease, which is generally an undesirable effect.

[0013] Methods according to embodiments of the invention are significantly simplified as compared to conventional solution-based methods for modification of titania and provides the opportunity to avoid many additional stages such as the treatment of the source titania with solutions of tin and palladium chlorides; intermediate rinsing; treatment with toxic reducing agents (hydrazinehydrate); treatment with aggregation stoppers (ethyl alcohol or acetone); and two-stage drying of the product. Unlike conventional surface modification methods, methods according to embodiments of the invention can proceed continuously in the reactor. [0014] One process for forming titania particles is a gas-plasma hydrolysis process, such as disclosed in related co-pending and commonly assigned U.S. Application No. 11/686,796 entitled "Highly photosensitive titanium dioxide and process for forming the same" which was published as U.S. Pub. No. 20080146441 on June 19, 2008. Briefly, Application No. 11/686,796 discloses methods of forming a high photosensitivity titanium oxide composition and comprises the steps of providing a titanium chloride compound, such as titanium trichloride or titanium tetrachloride, and an oxygen-containing gas and hydrogen, wherein a concentration of hydrogen is in a molar excess with respect to oxygen being from 2.02: 1 to 2.61: 1, such as 2.12: 1, 2.22: 1, 2.32:1, 2.42:1 or 2.52:1. The titanium chloride compound is burned in the presence of oxygen from the oxygen-containing gas and hydrogen to form plurality of ultrafine particles comprising titanium dioxide and titanium suboxide. The methods can include the steps prior to the burning step of mixing the titanium chloride compound, the oxygen and hydrogen, and pre-heating the titanium chloride compound, oxygen and hydrogen to 50 to 100 ⁰C, such as from 70-100 ⁰C. The steady state temperature during the burning step is generally from 700 to 1100 ⁰C, such as 800 ⁰C, 850 ⁰C, 900⁰C, 950 ⁰C, 1000 ⁰C, or 1050 ⁰C. The method can further comprise the step of steaming the particles at 150-220 ⁰C to promote desorption of HCl and CI₂ from the surface of the particles, such as in a temperature range from 170-200 ⁰C. A molar ratio of the titanium tetrachloride compound to H₂ is generally in a range from 1:4 to 1:2. A median size of the particles is generally in the range from 10-40 nm (agglomerated), while individual primary particles are generally 2-8 nm in size. The photocatalytic activity of the particles can be from 1.4 - 3.0 mg/ml minm² as measured in the reaction of methylene blue reduction at room temperature (300 K). Application No. 11/686,796 is incorporated by reference into the present application in its entirety. [0015] In one embodiment of the present invention, the process described in Application No. 11/686,796 is modified so that after the burning of the titanium chloride compound at 700-1100 ⁰C an amount of a metal sufficient to achieve a desired concentration of metal on titania surface is injected into the cooling area of the reactor at 300-700 ⁰C, such as through an additional sprayer coupled to a heat exchanger as described below relative to FIG. 1. [0016] Figure 1 shows a simplified reactor apparatus 200 that can be used to produce metal surface modified titania particles according to embodiments the invention. Source materials include a fully dried oxygen comprising gas such as air, hydrogen, and titanium tetrachloride as an exemplary titanium chloride comprising compound. The source materials can be heated up to 70-100 ⁰C and piped to the burner/combustion chamber 210, where they mix with each other, wherein a molar excess of hydrogen to oxygen can be provided. They are then piped at the laminar mode from the orifice to flame tube 215 where the air-hydrogen mixture burns at 700-1100 ⁰C causing hydrolysis of titanium tetrachloride as follows:

TiCl₄ + 2H₂ + O₂ → TiO₂ + 4HCl .

Primary particles of titanium dioxide are formed in this reaction. Primary particles are generally 2 to 8 nm in size.

[0017] Another process can also co-run:

AHCl + O₂ → 2H₂O + 2C/₂

Secondary particles of titania according to embodiments of the invention are finally formed as agglomerates in the coagulator. The agglomerated size is generally 10 to 40 nm. The titania particles then enter a heat exchanger, such as the tube-in-tube heat exchanger 250 shown in Fig. 1, where metal comprising vapors from an aerosol generated by bubbler 240 enters the front evaporator section. A tube-in-tube heat exchanger is also known as a tubular heat exchanger, or sometimes called a double-pipe heat exchanger. The tube-in-tube is the simplest form of heat exchanger and comprises two concentric (coaxial) tubes carrying the hot and cold fluids. As known in the art, heat is transferred to/from one fluid in the inner tube from/to the other fluid in the outer annulus via the metal tube wall that separates the two flu- ids. Although embodiments of the invention are generally described using a tube-in-tube heat exchanger, a variety of other heat exchangers or other means for cooling can be used. [0018] Regarding tube-in-tube heat exchangers, as known in the art, the vapors move toward the cooler distal end of the pipe. The temperature of heat exchanger 250 is generally held in a range of 300 to 700 C, such as 400 to 600 C. Metal forms on the surface of the tita- nia in the heat exchanger to form metal surface modified titania. The final product can also be steamed, such as in situ in a temperature range of 170-200 ⁰C, to remove surface adsorbed

HCl and Cl₂.

[0019] Superfine pyrogenic metal surface modified titania according to embodiments of the invention is generally the final product produced. In embodiments of the invention, the modified product is a non-stoichiometric composition. For example, the particles generally comprise Tiθ₂-_X, wherein 0.15 < x< 0.3 at a surface of said particles, and less than x in a bulk of said particles. The titania obtained as a mixture of anatase and rutile. This causes high de- fectiveness of the material, which ensures high paramagnetic susceptibility and photosensitivity. X-ray analysis performed by the present Inventors has evidenced that the resulting product is a mixture of separate particles of anatase and separate particles of rutile. However, it is possible that a very small percentage of the particles may include mixed anatase and rutile. [0020] Pre-heating up to 70-100 ⁰C helps avoid condensation of TiCl₄ vapor during piping. The heating also promotes keeping of the reacting mixture more stable and uniform. The temperature of the reacting can be higher than boiling point of the titanium chloride compound, such as for example 140 ⁰C for TiCl₄ (boiling point of -138 ⁰C). Thus, TiCl₄ should be added to the transportation air in evaporator at the temperature, which prevents condensation of TiCl₄ vapor (for instance, at 70-100 ⁰C). The pipe between evaporator and burner should also be heated to avoid the vapor condensation inside. If the condensation occurs, new liquid-drop phase appears in the gas mixture, which can significantly changes the burning process regime. This change generally leads to obtaining coarse- and poly-disperse titania powder.

[0021] The flame hydrolysis should be kept within 700 - 1100 ⁰C because the process runs too slow at the temperature lower than 700 ⁰C and reaction of hydrolysis -oxidation does not finish, which significantly lower photosensitivity of the product. On other hand, the present inventors have found that the ratio between anatase and rutile phases shifts away from an optimal range for maximizing catalytic activity at temperatures higher than 1100 ⁰C, which causes lowering of the specific surface area and photosensitivity of the product. The flame hydrolysis temperature can be measured using a thermocouple detector. [0022] The ratio of TiCl₄ (or TiCl₃) and H₂ can be within a range from 1 :4 to 1 :2. Hydrogen excess is unfavorable because it causes extra consumption of hydrogen and does not generally provide any significant improvement of dispersibility and photocatalytic activity of titania. Hydrogen deficit causes worse dispersibility and reduction of photocatalytic activity of titania.

[0023] Steam processing at the temperature lower than 150 ⁰C also lowers the product's photosensitivity while processing at the temperature higher than 200 ⁰C does not give any significant rise of photosensitivity but requires extra energy consumption. Steaming helps eliminate "acid" gases (~ 0.1-0.15 mass % of HCl, Cl₂), which can get adsorbed on the surface of the titania particles. Titania and steam are very affine, which makes possible effective elimination (high temperature desorption) of HCl and Cl₂ from the surface of titania. Titania products can be steamed with the air, which was preliminary moistened by the distilled water vapor at 400 ⁰C. Steam processing of the metal surface modified titania powder can be an in situ process using the reactor apparatus 200 shown in Fig. 1, or a separate step. [0024] Although not necessary to practice the present invention, Applicant, not seeking to be bound by any mechanism, provides the following mechanism believed to be operable. Ions of a metal derived from a metal salt, such as Pd²⁺ in the case of PdC^, deposit on surface of titania as a result of the metal salt exposure. The ions then undergo reduction in the air- hydrogen medium and transform into active metal catalytic centers. Steam processing, such as in the range 170-200 ⁰C, can be used to remove adsorbed HCl and CI₂ from the surface of metal-modified titania.

[0025] The Table below lists catalytic activity of five samples of Pd surface modified titanium dioxide prepared according to an embodiment of the invention.

Table 1 Results of determination of catalytic activity of titania samples

Note: 50 % or lower catalytic activity may be considered as unsuitable for a polymer composite product.

[0026] Table 1 evidences that embodiments of the present invention provide an opportunity to reach a sufficient increase in the catalytic activity of the surface modified material and avoid use of toxic compounds and harmful wastewater formation. Embodiments of the invention also significantly shorten production time for titania modification (due to simultaneous production and surface modification processes) and lowers energy consumption (due to utilization of the gas-flame titania synthesis heat, which is unused and dissipates in the conventional methods). [0027] In one embodiment of the invention the metal surface modified titania is mixed with another material, such as to form Ti(VMe_nO_m, where Me is a metal or a group IVA material (e.g. Si). For example, SiCl₄ can be added to the titanium chloride compound and burned to add a desired amount SiC^, which can be used to enhance specific surface area. In one embodiment, a titania-SiC^ system (SiC^ content is about 30%) modified with Pd is produced. In another embodiment, AICI₃ can be added and burned to add aluminum oxide. [0028] Metal surface modified titania according to embodiments of the invention can be used as filler for a wide variety of materials. For example, in the production of the polymer- based composite materials, paper, and films. In a typical embodiment, 2-15 weight % of metal surface modified titania according to the invention is added to another material to make it photoactive. Applications for metal surface modified titania according to the invention fillers for the rubber, and cigarette paper. However, in another embodiment, metal surface modified titania is used as a pure surface coating material to promote catalytic oxidation, such as for the oxidation of CO to CO₂

EXAMPLES [0029] The following non-limiting Examples serve to illustrate selected embodiments of the invention. It will be appreciated that variations in proportions and alternatives in elements of the components shown will be apparent to those skilled in the art and are within the scope of embodiments of the present invention.

[0030] A reactor system similar to system 200 shown in Fig. 1 was used for all Examples described herein to form titania having a surface modified with Pd. All the samples were synthesized using the process disclosed in Application No. 11/686,796 including a steam process for chlorine removal. However, unlike the titania formed using the process disclosed in Application No. 11/686,796, the titania formed was surface modified by Pd deposition. The surface Pd content was found to be 0.01, 0.05 and 0.075 mass %. The photocatalytic activity of the Pd surface modified samples was found to be 3.0-4.0 mg/ (ml min m ); magnetic susceptibility = 2.8 10^"6 to 3.2 10^"6 cm³/g; and average particle size of 10-40 nm. The particle shape was found to be generally spherical, density ofl 10-160 g/1; anatase-to-rutile ratio of 20:80 to 80:20; and the surface chlorine content was found to be 0.1-0.15 mass %. The bulk of the particles was found to generally be > 99 wt %, and generally > 99.9 wt. % titania. [0031] Example 1. The burner was fed with a gas mixture containing 100 Nm³/hr of fully dried air heated to 100 ⁰C, 100 1/hr of TiCl₄ and 40 Nm³/hr of hydrogen gas, which was burnt at 900 ⁰C. Nm³ as used above is an abbreviation for "normal". It is measured at "normal" pressure of 1 atmosphere and a normal temperature of 0° C. Then PdC^ solution was injected through a special sprayer into the cooling area of the burner at 400 ⁰C using air as a carrier gas. The calculated quantity of the solution was sufficient to form and keep a needful concentration of the modifying agent inside the reactor. Then the titania formed was treated in the next part of the cooling area of the reactor with steam at 200 ⁰C to remove adsorbed HCl and CI₂ from the surface. Metal surface modified titania having a specific surface area of 70 m²/g was found to result.

[0032] Example 2. Titania was synthesized under the conditions described above relative to Example 1. Then a PdCl₂ solution was injected as an aerosol through the special sprayer into the cooling area of the reactor at 600 ⁰C. Calculated quantity of the solution was sufficient to form and keep needful concentration of the modifying agent inside the reactor. Then the titania was treated in the next part of the cooling area with steam at 200 ⁰C to remove adsorbed HCl and CI₂ from the surface. A specific surface area of 65 m²/g was found to result. [0033] Example 3. Titania was synthesized under the conditions described above relative to Example 1. Then a PdCl₂ aerosol was injected through the special sprayer into the cooling area at 500 ⁰C. Further proceedings were similar to those described in Example 1. A specific surface area of 70 m /g was found to result. [0034] Example 4. Titania was synthesized under the conditions described above relative to Example 1. Then a PdCl₂ aerosol was injected through the special sprayer into the cooling area at 300 ⁰C. Further proceedings were similar to those described in Example 1. A specific surface area of 70 m /g was found to result.

[0035] Example 5. Titania was synthesized under the conditions described above relative to Example 1. Then a PdCl₂ aerosol was injected through the special sprayer into the cooling area at 700 ⁰C. Further proceedings were similar to those described in Example 1. A specific surface area of 62 m²/g was found to result.

[0036] While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents.

[0037] Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.

[0038] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms "including", "includes", "having", "has", "with", or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term "comprising."

[0039] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0040] The Abstract of the Disclosure is provided to comply with 37 C.F.R. §1.72(b), requiring an abstract that will allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the following claims.

Claims

1. A method for in-situ formation of metal surface modified titania, comprising: burning a titanium chloride comprising compound in the presence of oxygen and hydrogen in a reactor to form plurality of fine titania particles comprising titanium dioxide and titanium suboxide, wherein a temperature during said burning step is from 700 to 1100 ⁰C, and depositing a metal on a surface of said titania particles while in said reactor at a temperature below said temperature of said burning step, wherein said metal only partially covers said surface of said titania particles.

2. The method of claim 1, further comprising the step of removing adsorbed chlorine from said surface with steam at a temperature in a range between 150-220 ⁰C.

3. The method of claim 1, wherein said depositing comprises cooling said titania particles while in said reactor to a temperature between 300 and 700 ⁰C and in an atmosphere comprising at least one metal source and a reducing agent

4. The method of claim 1, wherein said depositing said metal comprises injecting an aerosol comprising said metal into a pipe-in-pipe heat exchanger within said reactor.

5. The method of claim 1, wherein a concentration of said hydrogen in a stoichiometric excess with respect to a molar ratio of said oxygen (H₂O₂) from 2.02:1 to 2.61:1 during said burning step.

6. The method of claim 1, wherein said titanium chloride comprising compound comprises titanium tetrachloride.

7. The method of claim 1, wherein said metal comprises at least one of Pd, Pt, Zn, W, Bi and Mo.

8. A high photosensitivity titanium oxide composition, comprising: a plurality of nanosize titania particles comprising titanium dioxide and titanium suboxide, said particles being substantially non- stoichiometric having a magnetic susceptibility value (χ) of at least 0.8 10^~6 cm³/g at 300 K and being at least 30% by weight rutile,

[0041] a surface of said titania particles including a least one metal, wherein said metal only partially covers said surface of said titania particles.

9. The composition of claim 8, wherein said metal comprises nanoparticles of said metal.

10. The method of claim 8, wherein said metal comprises at least one of Pd, Pt, Zn, W, Bi and Mo.

11. The composition of claim 8, wherein said χ is between 0.8 x 10^"6 cm³/g and 2.4 x 10^"6 cnrVg at 300 K.

12. The composition of claim 8, wherein an average size of said particles is 10-40 nm.

13. The composition of claim 8, wherein said rutile is at least 40%, balance of said composition being essentially all anatase.

14. The composition of claim 8, wherein said composition comprises 45 to 55 % of said rutile, said anatase comprising the balance.

15. The composition of claim 8, wherein a chlorine concentration at a surface of said particles is less than a chlorine concentration in a bulk of said particles.

16. The composition of claim 15, wherein said chlorine concentration at said surface of said particles is at least an order of magnitude less than said chlorine concentration in said bulk of said particles.

17. The composition of claim 8, wherein said composition comprises Tiθ₂-_X, wherein 0.15 < x< 0.3 at a surface of said particles, and less than x in a bulk of said particles.