JP2004510668A - Tungsten carbide material - Google Patents

Abstract

A high surface area tungsten carbide material is provided. This material has a unique structure defined by its X-ray diffraction pattern and consists of very small crystallites with dimensions of about 15-30 °. A high surface area of at least about 6 m ² / g is desirable for catalytic applications.

Description

[0001]
TECHNICAL FIELD The present invention relates to tungsten carbide and a method for producing the same. More specifically, the present invention relates to a tungsten carbide catalyst.
[0002]
High surface area tungsten carbide and molybdenum materials are known to have catalytic properties similar to ruthenium, iridium, palladium and platinum. For example, high surface area tungsten carbide and molybdenum have been described as very effective catalysts for converting methane to synthesis gas by steam and dry reforming and for water gas shift reactions. Like platinum, palladium and ruthenium, tungsten carbide also makes this a potential catalyst for low temperature fuel cell applications such as PEM (polymer electrolyte membrane), sulfuric acid and direct methanol fuel cells at room temperature. It is known to catalyze the oxidation of hydrogen gas. Form W ₂ C has been reported to be more catalytically active than form WC in some applications.
[0003]
The abundance of the starting materials used to produce these carbides and their relatively low cost make them an attractive substitute for the rarer and more expensive platinum metals. A major problem with metal carbides has been obtaining sufficiently high surface area materials. A high surface area is desirable to increase the number of effective catalyst sites. Initial work in producing high surface area carbides used flowing methane and hydrogen over tungsten metal powders or oxides. A further improvement for tungsten carbide and molybdenum was seen in a two-step nitride-carbide formation using ammonia and then methane. The latter advance in the prior art has been found to use ethane as a carbonizing gas to produce similar effects for molybdenum carbide and tungsten in a one-step process. Other attempts to produce high specific surface areas have involved the use of organic intermediates. Metal carbides with surface areas as high as 200 m ² / g have been reported. Other applications for high surface area tungsten carbide include biological electrodes, such as electrodes for pacemakers.
[0004]
SUMMARY OF THE INVENTION It is an object of the present invention to obviate the disadvantages of the prior art.
[0005]
It is another object of the present invention to provide a tungsten carbide material having a high surface area.
[0006]
It is a further object of the present invention to provide a method for forming a high surface area tungsten carbide material.
[0007]
In accordance with one object of the present invention, there is provided a tungsten carbide material comprising tungsten and carbon. This material has an X-ray diffraction pattern that includes a primary X-ray diffraction peak and first and second secondary X-ray diffraction peaks. The primary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 2.39 ± 0.02 °. The first secondary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.496 ± 0.007 ° and a relative peak height of 25% to 40% of the peak height of the primary X-ray diffraction peak. Have. The second secondary X-ray diffraction peak has a reflection angle corresponding to a d-interval of 1.268 ± 0.005 ° and a relative peak height of 35% to 55% of the peak height of the primary X-ray diffraction peak. Having.
[0008]
In accordance with another object of the present invention, sufficient to convert the tungsten precursor to a tungsten carbide material at a temperature of about 500C to about 800C in an atmosphere containing hydrocarbon gas and optional hydrogen gas. There is provided a method of forming a high surface area tungsten carbide material comprising heating for an extended period of time.
[0009]
Wherein <br/> Other and further objects of the preferred embodiments, in order to better understand the present invention, together with advantages and properties, see the scope of the following disclosure and appended claims in conjunction with the accompanying drawings.
[0010]
The tungsten carbide material of the present invention features a lower level of carbon than the stoichiometry in an active crystal matrix that is known to achieve the desired high surface area for catalysis and not to coke the active surface as much. And The composition of this material can be represented by the general formula WC1 _-X , where x is 0-0.5. X-ray diffraction pattern of the tungsten carbide material (XRD) is illustrated in FIG. 1 (CuKa ₁ radiation, 1.5405 Å). The XRD pattern shows that the tungsten carbide material has a face centered cubic lattice. This broad diffraction peak is consistent with the presence of very small crystallites. According to Schlehr's relationship, the peak width corresponds to a crystallite size in the range of about 15 ° to about 30 °. This is a significant improvement over the previously reported crystallite size of 275-385 °.
[0011]
Peak position of the XRD _figure, W 2 _(C, O) and shows a similarity of _{WC 1-X.} 2 and 3 show _W 2 (C, O) was superimposed on the diffraction pattern shown in Figure 1 and _{WC 1-X} positions and relative intensities of XRD lines for each. Data for W ₂ (C, O) and _{WC 1-X} of the XRD figure, International Center for Diffraction Data obtained from the powder diffraction files stored by (International Center for Diffraciton Data) ( PDF # 22-0959 and PDF # 20-1316). Referring to FIG. 2, although the location of the W ₂ (C, O) line occurs near the location of the main peak for the tungsten carbide material, it does not exactly match. _Furthermore, W 2 (C, O) relative intensity of (220) and (311) reflections are not at the same rate of the two secondary diffraction peaks of the tungsten carbide material. A similar situation exists in FIG. In this case, it seems that the position almost coincides with the position of the WC _1-X line, but there is a large mismatch in the relative height. In particular, the strongest line for WC _1-X corresponds to the (200) reflection, whereas the primary peak of the XRD pattern of the tungsten carbide material is closer to the (111) reflection.
[0012]
Referring to FIG. 1, the XRD pattern of the tungsten carbide material of the present invention is characterized by three peaks. A primary peak P and two secondary peaks S1 and S2. The primary peak P occurs at a 2theta (θ) angle of 37.6 ± 0.3 degrees. Applying the Bragg equation, this angle of reflection corresponds to a d-spacing of about 2.39 ± 0.02 °. Two secondary peaks S1 and S2 occur at 2θ angles of 62.0 ± 0.3 ° and 74.8 ± 0.3 °. These angles correspond to d-spacings of 1.496 ± 0.007 ° and 1.268 ± 0.005 °, respectively. The relative peak height of the first secondary peak S1 varies from 25% to 40% of the peak height of the primary peak. The relative peak height of the second secondary peak S2 varies from 35% to 55% of the peak height of the primary peak. The ratio of the peak height of the first secondary peak S1 to the second secondary peak S2 is in the range of 0.65 to 0.80, preferably in the range of 0.69 to 0.75. As used herein, peak height refers to the maximum intensity of a peak after applying a simple background subtraction method.
[0013]
Tungsten carbide materials are formed by reacting a tungsten precursor in flowing hydrocarbon and optional hydrogen gas at a temperature from about 500C to about 800C. The tungsten precursor material can be ammonium metatungstate, ammonium paratungstate, tungsten metal powder, tungsten oxide, tungsten halide, absorbing tungsten species or dissolved tungstate. Preferably, the tungsten precursor is ammonium paratungstate decahydrate. Suitable hydrocarbon gases include propane, ethane, natural gas, ethylene, acetylene, or combinations thereof. Preferably, the hydrocarbon gas is propane or ethane. It is also contemplated that other hydrocarbon gases having a molecular formula containing up to 12 carbon atoms can be used in the method of the present invention. The tungsten precursor is charged to a ceramic boat that is placed in a tube furnace. An inert atmosphere is established in the tube furnace using flowing argon gas. The furnace is then heated to the reaction temperature and the gas flow is changed to a mixture of hydrocarbons and optional hydrogen gas. Once fully reacted, the gas flow is again changed to argon gas alone, allowing the furnace to cool to room temperature. The tungsten carbide material is then passivated by flowing nitrogen gas through the tube furnace. Passivation is achieved by absorption of impurity oxygen on the surface of the material. Preferred flow rates of standard liters per minute (slm) for these gases include 0.05 slm to 9.5 slm for hydrocarbon gas, 0 to 2.4 slm for hydrogen gas, and 0 to 14.2 slm for argon gas. . In addition to its unique structure, the tungsten carbide materials formed by these methods have a high surface area. In particular, this surface area is at least about ^{6 m} 2 / g, preferably in the range of about ^{10 m} 2 / g to about ^{60 m} 2 / g.
[0014]
The following non-limiting examples are given. XRD analysis was performed on a physical D / Max X-ray diffractometer using CuKα radiation (40 keV, 30 ma). Contribution of CuKa ₂ of CuKa radiation was mathematically removed from the diffractogram. When the diffractometer was measured, the accuracy was ± 0.04 ° C. (2θ).
[0015]
(Example)
Example 1
Ammonium metatungstate 5g of reagent / catalyst grade _{(AMT), (NH 4)} 6 H 2 W 12 O 40 · 5H 2 O ( Osram Sylvania Products Inc., Towonda, Pennsylvania) was uniformly placed in a ceramic boat . The ceramic boat was then loaded into a Lindberg / Blue M type HTF 55000 hinged tubular furnace using 2.5 inch diameter quartz tubing. An inert atmosphere was established in the tube by flowing argon gas through the tube at 0.5 slm. The furnace temperature was then increased to 650 ° C. and the gas flow switched to a 0.2 slm propane flow and a 1 slm hydrogen flow. After 2.5 hours, the gas flow was switched back to argon only at 0.5 slm and the furnace was shut down. After cooling the material in a furnace under flowing argon, nitrogen gas (up to 99.998% cryogenic grade from liquid nitrogen or up to 0.9 ppm oxygen) was added to the tube at 1.0 slm. The material was passivated by passing it through for 6 hours. XRD analysis demonstrated the presence of the tungsten carbide material.
[0016]
Example 2
AMT by dissolving ammonium paratungstate decahydrate (APT), (NH ₄ ) ₁₀ H ₂ W ₁₂ O ₄₂ · 10H ₂ O in 1.5 L of deionized water and 0.5 kg of ammonium metatungstate Generated from. The pH of this solution was adjusted to 9-11 using concentrated ammonium hydroxide. The solution solidified for 48-75 hours. APT needles appeared along the beaker wall and at the surface of the solution. The APT crystals were collected and dried. This very pure APT needle was measured and had a surface area of 0.11 m ² / g. A 5 g quantity of APT needles was evenly placed in a ceramic boat and then loaded into a Lindberg / Blue M type HTF55000 hinged tubular furnace using 2.5 inch diameter quartz tubing. An inert atmosphere was established by flowing argon gas through the tube at 0.5 slm. The furnace temperature was increased to 650 ° C. and the gas flow was changed to 0.2 slm propane flow and 1 slm hydrogen flow. After 2.5 hours, the gas flow was again changed to only argon at 0.5 slm and the furnace was shut down. After allowing the material to cool in an oven under flowing argon, the material was passivated by passing flowing nitrogen gas through the tube at 1.0 slm for 6 hours. XRD analysis demonstrated the presence of the tungsten carbide material.
[0017]
Example 3
A 25 g quantity of the used tungsten carbide tool was leached in 6N hydrochloric acid at 97 ° C. for 6 hours. The resulting solid was dried and calcined at 750 ° C. to yellow tungsten oxide expanded in air. The yellow oxide was evenly placed in a ceramic boat and then loaded into a Lindberg / Blue M type HTF 55000 hinged tube furnace using 2.5 inch diameter quartz tubing. An inert atmosphere was established by flowing argon gas through the tube at 0.5 slm. The furnace temperature was increased to 700 ° C. and the gas flow was changed to 0.2 slm propane flow and 1 slm hydrogen flow. After 2 hours, the gas flow was again changed to only argon at 0.5 slm and the furnace was shut down. After allowing the material to cool in a furnace under flowing argon, the material was passivated by passing nitrogen gas through the tube at 1.0 slm for 24 hours. XRD analysis demonstrated the presence of the tungsten carbide material.
[0018]
Example 4
As Example 1, but using ethane instead of propane. XRD analysis demonstrated the presence of the tungsten carbide material.
[0019]
Having shown and described what is presently considered to be the preferred embodiments of the present invention, those skilled in the art will appreciate that various modifications can be made without departing from the scope of the invention as defined by the appended claims. It will be apparent that changes and modifications may be made.
[Brief description of the drawings]
FIG.
3 is an X-ray diffraction pattern of the tungsten carbide material of the present invention.
FIG. 2
FIG. 2 is an X-ray diffraction pattern of FIG. 1 on which X-ray diffraction lines related to W ₂ (C, O) are superimposed.
FIG. 3
It is the X-ray diffraction pattern of FIG. 1 on which the X-ray diffraction line regarding WC1 _-X was superimposed.

Claims (32)

In a tungsten carbide material consisting of tungsten and carbon,
The material has an X-ray diffraction pattern comprising a primary X-ray diffraction peak and first and second secondary X-ray diffraction peaks,
The primary X-ray diffraction peak has a reflection angle corresponding to a d-interval of 2.39 ± 0.02 °,
The first secondary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.496 ± 0.007 ° and a relative peak height of 25% to 40% of the peak height of the primary X-ray diffraction peak. And the second secondary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.268 ± 0.005 ° and a relative height of 35% to 55% of the peak height of the primary X-ray diffraction peak. A tungsten carbide material having a peak height. The tungsten carbide material according to claim 1, wherein a ratio of a peak height of the first secondary X-ray diffraction peak to a peak height of the second secondary X-ray diffraction peak is 0.65 to 0.80. The tungsten carbide material according to claim 1, wherein a ratio of a peak height of the first secondary X-ray diffraction peak to a peak height of the second secondary X-ray diffraction peak is 0.69 to 0.75. The tungsten carbide material according to claim 1, wherein a composition of the material is represented by a general formula WC1 _-x , wherein x is 0 to 0.5. Tungsten carbide material of claim 1 wherein the material having a surface area of at least about 6 m 2 ^{/ g.} Tungsten carbide material of claim 1, wherein the material has a surface area of about ^{10 m} 2 / g to about ^{60 m} 2 / g. The tungsten carbide material of claim 1 wherein said material has a crystallite size of about 15 ° to about 30 °. The tungsten carbide material according to claim 7, wherein a ratio of a peak height of the first secondary X-ray diffraction peak to a peak height of the second secondary X-ray diffraction peak is 0.65 to 0.80. The tungsten carbide material according to claim 8, wherein a ratio of a peak height of the first secondary X-ray diffraction peak to a peak height of the second secondary X-ray diffraction peak is 0.69 to 0.75. In the tungsten carbide material comprising tungsten and carbon, tungsten carbide material material is characterized by having an X-ray diffraction pattern of CuKa ₁ shown in FIG. In a tungsten carbide material consisting of tungsten and carbon,
Said material has an X-ray diffraction pattern of CuKa ₁ consisting of primary X-ray diffraction peak and first and second secondary X-ray diffraction peaks,
The primary peak has a 2θ reflection angle of 37.3-37.9 degrees,
The first secondary peak has a 2θ reflection angle of 61.7 to 62.3 degrees,
The second secondary peak has a 2θ reflection angle of 74.5 to 75.1 degrees, and the ratio of the peak height of the first secondary peak to the peak height of the second secondary peak is 0.5. A tungsten carbide material of 65-0.80. The first secondary peak has a relative peak height of 25% to 40% of the peak height of the primary X-ray diffraction peak, and the second secondary X-ray diffraction peak has a relative peak height of the primary X-ray diffraction peak. The tungsten carbide material of claim 11, having a relative peak height between 35% and 55%. 13. The tungsten carbide material according to claim 12, wherein a ratio of a peak height of the first secondary peak to a peak height of the second secondary peak is 0.69 to 0.75. The tungsten carbide material according to claim 11, wherein a composition of the material is represented by a general formula WC1 _-x , wherein x is 0 to 0.5. Tungsten carbide material of claim 11 wherein the material having a surface area of at least about 6 m 2 ^/ g. Tungsten carbide material of claim 11, wherein the material has a surface area of about ^{10 m} 2 / g to about ^{60 m} 2 / g. The tungsten carbide material of claim 11, wherein said material has a crystallite size of about 15 ° to about 30 °. The first secondary peak has a relative peak height of 25% to 40% of the peak height of the primary X-ray diffraction peak, and the second secondary X-ray diffraction peak has a relative peak height of the primary X-ray diffraction peak. 18. The tungsten carbide material of claim 17, having a relative peak height between 35% and 55%. 19. The tungsten carbide material according to claim 18, wherein the ratio of the peak height of the first secondary peak to the peak height of the second secondary peak is between 0.69 and 0.75. A tungsten carbide material consisting of tungsten and carbon and having a crystallite size of about 15 ° to about 30 °. Heating the tungsten precursor in an atmosphere containing a hydrocarbon gas and optional hydrogen gas at a temperature of about 500C to about 800C for a time sufficient to convert the tungsten precursor to a tungsten carbide material. A method for forming a high surface area tungsten carbide material. 22. The method according to claim 21, wherein the tungsten precursor is selected from ammonium metatungstate, ammonium paratungstate, tungsten metal powder, tungsten oxide or tungsten halide. 22. The method according to claim 21, wherein the tungsten precursor is ammonium metatungstate, ammonium paratungstate or tungsten oxide. 22. The method of claim 21, wherein the hydrocarbon gas is selected from propane, ethane, natural gas, ethylene, acetylene, or a combination thereof. 22. The method of claim 21, wherein the hydrocarbon gas has a molecular formula containing no more than 12 carbon atoms. The method according to claim 23, wherein the hydrocarbon gas is ethane or propane. 22. The method of claim 21 wherein the tungsten carbide material is cooled in an inert atmosphere and passivated in nitrogen after conversion. 27. The method of claim 26, wherein the tungsten precursor is ammonium paratungstate decahydrate. Carbonizing a tungsten precursor selected from ammonium metatungstate, ammonium paratungstate or tungsten oxide in an atmosphere containing hydrocarbon gas and optional hydrogen gas at a temperature of about 500 ° C to about 800 ° C. A method of forming a high surface area tungsten carbide material comprising heating for a time sufficient to convert to a tungsten material,
The tungsten carbide material comprises tungsten and carbon,
The material has an X-ray diffraction pattern comprising a primary X-ray diffraction peak and first and second secondary X-ray diffraction peaks,
The primary X-ray diffraction peak has a reflection angle corresponding to a d-interval of 2.39 ± 0.02 °,
The first secondary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.496 ± 0.007 ° and a relative peak height of 25% to 40% of the peak height of the primary X-ray diffraction peak. And the second secondary X-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.268 ± 0.005 ° and a relative height of 35% to 55% of the peak height of the primary X-ray diffraction peak. A method for forming a high surface area tungsten carbide material, characterized by having a peak height. 30. The method according to claim 29, wherein the hydrocarbon gas is ethane or propane. 31. The method of claim 30, wherein the tungsten precursor is ammonium paratungstate decahydrate. 30. The method of claim 29, wherein the material has a crystallite size of about 15 to about 30.

Images