TUNGSTEN CARBIDE MATERIAL

Abstract: A high-surface-area tungsten carbide material is provided. The material has a unique structure as defined by its x-ray diffraction pattern and consists of extremely small crystallites on the order of about 15 to about 30 angstroms in size. The hihg-surface-area of at least about 6 m<2>/g is desirable for catalyst applications.

Description: TUNGSTEN CARBIDE MATERIAL TECHNICAL FIELD This invention relates to tungsten carbides and methods of making same. More particularly, this invention relates to tungsten carbide catalysts . BACKGROUND ART High-surface-area tungsten and molybdenum carbide materials are known to possess catalytic properties similar to ruthenium, iridium, palladium and platinum.- For example, high-surface-area tungsten and molybdenum carbides have been described as highly efficient catalysts for the conversion of methane to synthesis gas via steam reforming and dry reforming, and for water-gas shift reactions. Like platinum, palladium and ruthenium, tungsten carbide is also known to catalyze the oxidation of hydrogen gas at room temperature which makes it a potential catalyst for low- temperature fuel cell applications such as the PEM (polymer electrolyte membrane) , sulfuric acid, and direct methanol types of fuel cells. The 2 C form has been reported as being more catalytically active than the WC form in some applications . The abundance and relatively low cost of the starting materials used to produce these carbides makes them attractive replacements for the rarer and more costly platinum metals. The main difficulty with metal carbides has been obtaining materials with sufficiently high surface areas. A high surface area is desirable for increasing the number of available catalytic sites. Original studies of preparing high-surface-area carbides used methane and hydrogen flowing over tungsten metal powder or oxides . Further improvements for tungsten and molybdenum carbides were seen in a two-step nitride-carbide formation using ammonia followed by methane. A later advancement in the art found that using ethane as a carburizing gas produced a similar effect in a one step process for molybdenum and tungsten carbides. Other attempts at producing a high specific surface included using organic intermediates. Metal carbides with surface areas as high as 200 m 2 /g have now been reported. Other applications for high-surface-area tungsten carbides include biomedical electrodes, e.g., electrodes for pacemakers. SUMMARY OF THE INVENTION It is an object of the invention to obviate the disadvantages of the prior art . It is another object of the invention to provide a tungsten carbide material having a high-surface-area. It is a further object of the invention to provide a method for forming a high-surface-area tungsten carbide material . In accordance with one object of the invention, there is provided a tungsten carbide material comprising tungsten and carbon. The material has an x-ray diffraction pattern containing a primary x-ray diffraction peak and first and second secondary x-ray diffraction peaks; the primary x-ray diffraction peak having a reflection angle corresponding to a d-spacing of 2.39 ± 0.02 A; the first secondary x-ray diffraction peak has a reflection angle corresponding to a d-spacing of 1.496 + 0.007 A 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 A and a relative peak height of 35% to 55% of the peak height of the primary x-ray diffraction ' peak. In accordance with another object of the invention, there is provided a method for forming a high-surface-area tungsten carbide material comprising heating a tungsten precursor to a temperature from about 500 °C to about 800°C in an atmosphere containing a hydrocarbon gas and, optionally, hydrogen gas for a time sufficient to convert the tungsten precursor to the tungsten carbide material . BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an x-ray diffraction pattern of the tungsten carbide material of this invention. Fig. 2 is the x-ray diffraction pattern of Fig. 1 overlaid with x-ray diffraction lines associated with W 2 (C,0) . Fig. 3 is the x-ray diffraction pattern of Fig. 1 overlaid with x-ray diffraction lines associated with WC^. DESCRIPTION OF THE PREFERRED EMBODIMENTS For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings. The tungsten carbide material of this invention achieves a high surface area desirable for catalysis and features substoichiometric levels of carbon in the active crystal matrix which are known to make the active surfaces less prone to coking. The composition of the material may be represented by the general formula WC-^.. where x is from 0 to 0.5 The x-ray diffraction (XRD) pattern of the tungsten carbide material is exemplified in Fig. 1 (Cu Ka radiation, 1.5405 A). The XRD pattern indicates that the tungsten carbide material has a face centered cubic lattice. The broad diffraction peaks are consistent with the presence of extremely small crystallites. According to the Scherrer relationship, the peak widths correspond to crystallite sizes in the range of about 15 A to about 30 A. This is a major improvement over previously reported crystallite sizes of 275 to 385 A. The peak positions in the XRD pattern indicate a similarity with W 2 (C,0) and WC X _ X . Figs. 2 and 3 respectively show the XRD line positions and relative intensities for W 2 (C,0) and WC X _ X superimposed on the diffraction pattern shown in Fig. 1. The data for the W 2 (C,0) and WC^ XRD patterns were obtained from the powder diffraction files maintained by the International Centre for Diffraction Data (PDF#22-0959 and PDF#20-1316) . Referring to Fig. 2, it can be seen that the W 2 (C,0) line positions while arising near the major peak positions for the tungsten carbide material do not exactly correspond. . Furthermore, the relative intensities of the (220) and (311) reflections of W 2 (C,0) are not in the same proportion as the two secondary diffraction peaks of the tungsten carbide material. A similar situation exists in Fig. 3. In this case, there appears to be a closer agreement with the WC^., line positions but there is a major discrepancy in the relative heights. In particular, the most intense line for ^C _ x corresponds to the (200) reflection whereas the primary peak in the XRD pattern of the tungsten carbide material is closer to the (111) reflection. With regard to Fig. 1, the XRD pattern of the tungsten carbide material of this invention is characterized by three peaks: a primary peak P and two secondary peaks SI and S2. The primary peak P occurs at a 2-theta (20) angle of 37.6 + 0.3 degrees. Applying the Bragg equation, this reflection angle corresponds to a d-spacing of about 2.39 ± 0.02 A. The two secondary peaks SI and S2 occur at 20 angles of 62.0 ± 0.3° and 74.8 ± 0.3°. These angles correspond to d-spacings of 1.496 ± 0.007 A and 1.268 ± 0.005 A, respectively. The relative peak height of the first secondary peak SI 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 peak height ratio of the first secondary peak SI to the second secondary peak S2 ranges from 0.65 to 0.80, and preferably from 0.69 to 0.75. As used herein, peak height refers to the maximum intensity of a peak after applying a simple background subtraction. The tungsten carbide material is formed by the reaction of a tungsten precursor in flowing hydrocarbon and, optionally, hydrogen gases at a temperature from about 500°C to about 800 °C. The tungsten precursor material may be ammonium metatungstate, ammonium paratungstate, tungsten metal powder, tungsten oxides, tungsten halides, absorbed tungsten species, or dissolved tungstates . 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. Other hydrocarbon gases having molecular formulas containing twelve or less carbon atoms are also believed to useable in the method of this invention. The tungsten precursor is loaded into a ceramic boat which is placed into 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 combination of hydrocarbon and, optionally, hydrogen gases. Once sufficiently reacted, the gas flow is changed back to solely argon gas and the furnace is allowed to cool to room temperature. The tungsten carbide material is then passivated by flowing nitrogen gas through the tube furnace. Passivation is achieved by impurity oxygen adsorption on the surface of the material . Preferred flow rates in standard liters per minute (slm) for these gases include: 0.05 slm to 9.5 slm for the hydrocarbon gases, 0 to 2.4 slm for the hydrogen gas, and 0 to 14.2 slm for the argon gas. In addition to the unique structure, the tungsten carbide material formed from these methods has a high surface area. In particular, the surface area is at least about 6 m 2 /g and preferably ranges from about 10 m 2 /g to about 60 m 2 /g. The following non-limiting examples are presented. XRD analyses were performed with a Rigaku D/Max X-ray Diffractometer using Cu Kc. radiation (40keV, 30ma) . The Cu Koi 2 contribution in the Cu Kc. radiation was removed mathematically from the diffraction patterns. The diffractometer was measured to be accurate to +0.04° (20) . Example 1 A 5 g amount of reagent/catalyst grade ammonium metatungstate (AMT) , (NH 4 ) s H 2 W 12 O 40 *5H 2 O, (OSRAM SYLVANIA Products Inc., Towanda, PA) was placed evenly in a ceramic boat . The ceramic boat was then loaded into a Lindberg/Blue M Model HTF55000 hinged tube furnace utilizing a 2.5 inch diameter quartz tube. An inert atmosphere was established in the tube by flowing argon gas through the tube at 0.5 slm. The furnace temperature was then raised to 650°C and the gas flow was switched to a propane flow of 0.2 slm and a hydrogen flow of 1 slm. After 2.5 hours, the gas flow was switched back to only argon at 0.5 slm and the furnace turned off. After allowing the material to cool in the furnace under the flowing argon, the material was passivated by passing flowing nitrogen gas (cryogenic grade, 99 . 998% from liquid nitrogen or generated to 0.9 ppm oxygen max., Air Liquide) through the tube at 1.0 slm for 6 hours. XRD analysis confirmed the presence of the tungsten carbide material . Example 2 Ammonium paratungstate decahydrate (APT) , (NH 4 ) 10 H 2 W 12 O 42 «10H 2 O, was generated from ammonium metatungsate by dissolving 0.5 kg of AMT in 1.5 1 of deionized water. The pH of the solution was adjusted to between 9 and 11 using concentrated ammonium hydroxide. The solution was allowed to set for 48 to 72 hours. Needle-like crystals of APT appeared along the walls of the beaker and at the solution surface. The APT crystals were harvested and allowed to dry. The highly pure APT needles were measured to have a surface area of 0.11 m 2 /g. A 5 g amount of the APT needles was placed evenly in a ceramic boat which was then loaded into a Lindberg/Blue M model HTF55-000 hinged tube furnace utilizing a 2.5 inch diameter quartz tube. An inert atmosphere was established by flowing argon gas through the tube at 0.5 slm. The furnace temperature was raised to 650°C and the gas flow was changed to a propane flow of 0.2 slm and a hydrogen flow of 1 slm. After 2.5 hours, the gas flow was changed back to only argon at 0.5 slm and the furnace turned off. After allowing the material to cool in the furnace under the flowing argon, the material was passivated by passing flowing nitrogen gas through the tube at 1.0 slm for 6 hours. XRD analysis confirmed the presence of the tungsten carbide material . Example 3 A 25 g amount of a used tungsten carbide tool was leached in 6N hydrochloric acid at 97°C for 6 hours. The resulting solid was dried and fired in air at 750 °C to an expanded yellow tungsten oxide. The yellow oxide was placed evenly in a ceramic boat which was then loaded into a Lindberg/Blue M model HTF55000 hinged tube furnace utilizing a 2.5 inch diameter quartz tube. An inert atmosphere was established by flowing argon gas through the tube at 0.5 slm. The furnace temperature was raised to 700°C and the gas flow was changed to a propane flow of 0.2 slm and a hydrogen flow of 1 slm. After 2 hours, the gas flow was changed back to only argon at 0.5 slm and the furnace turned off. After allowing the material to cool in the furnace under the flowing argon, the material was passivated by passing flowing nitrogen gas through the tube at 1.0 slm for 24 hours. XRD analysis confirmed the presence of the tungsten carbide material . Example 4 Same as Example 1 except that ethane was used instead of propane. XRD analysis confirmed the presence of the tungsten carbide material . While there has been shown and described what are at the present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims: CLAIMS I claim: 1. A tungsten carbide material comprising tungsten and carbon; the material having an x-ray diffraction pattern containing a primary x-ray diffraction peak and first and second secondary x-ray diffraction peaks, the primary x-ray diffraction peak having a reflection angle corresponding to a d-spacing of 2.39 ± 0.02 A, the first secondary x-ray diffraction peak having a reflection angle corresponding to a d-spacing of 1.496 + 0.007 A 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 having a reflection angle corresponding to a d-spacing of 1.268 + 0.005 A and a relative peak height of 35% to 55% of the peak height of the primary x-ray diffraction peak. 2. The tungsten carbide material of claim 1 wherein ratio of peak height of the first secondary x-ray diffraction peak to the peak height of the second x-ray diffraction peak is from 0.65 to 0.80. 3. The tungsten carbide material of claim 1 wherein ratio of peak height of the first secondary x-ray diffraction peak to the peak height of the second x-ray diffraction peak is from 0.69 to 0.75. 4. The tungsten carbide material of claim 1 wherein the composition of the material is represented by the general formula WC λ _ x where x is from 0 to 0.5. 5. The tungsten carbide material of claim 1 wherein the material has a surface area of at least about 6 m 2 /g. 6. The 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. 7. The tungsten carbide material of claim 1 wherein the material has crystallite sizes of about 15 A to about 30 A. 8. The tungsten carbide material of claim 7 wherein ratio of peak height of the first secondary x-ray diffraction peak to the peak height of the second x-ray diffraction peak is from 0.65 to 0.80. 9. The tungsten carbide material of claim 8 wherein ratio of peak height of the first secondary x-ray diffraction peak to the peak height of the second x-ray diffraction peak is from 0.69 to 0.75. 10. A tungsten carbide material comprising tungsten and carbon; the material having the CuKo. ! x-ray diffraction pattern shown in Fig. 1. 11. A tungsten carbide material comprising tungsten and carbon, the material having a CuKct^ x-ray diffraction pattern consisting of a primary x-ray diffraction peak and first and second secondary x-ray diffraction peaks, the primary peak having a 2-theta reflection angle of 37.3 to 37.9 degrees, the first secondary peak having a 2-theta reflection angle of 61.7 to 62.3 degrees, the second secondary peak having a 2-theta reflection angle of 74.5 to 75.1 degrees, and wherein the ratio of the peak height of the first secondary peak to the peak height of the second secondary peak is from 0.65 to 0.80. 12. The tungsten carbide material of claim 11 wherein 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 35% to 55% of the peak height of the primary x-ray diffraction peak. 13. The tungsten carbide material of claim 12 wherein the ratio of the peak height of the first secondary peak to the peak height of the second secondary peak is from 0.69 to 0.75. 14. The tungsten carbide material of claim 11 wherein the composition of the material is represented by the general formula WC^.. where x is from 0 to 0.5. 15. The tungsten carbide material of claim 11 wherein the material has a surface area of at least about 6 m 2 /g. 16. The tungsten carbide material of claim 11 wherein the material has a surface area of about 10 m 2 /g to about 60 m/g. 17. The tungsten carbide material of claim 11 wherein the material has crystallite sizes of about 15 A to about 30 A. 18. The tungsten carbide material of claim 17 wherein 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 35% to 55% of the peak height of the primary x-ray diffraction peak. 19. The tungsten carbide material of claim 18 wherein the ratio of the peak height of the first secondary peak to the peak height of the second secondary peak is from 0 . 69 to 0.75. 20. A tungsten carbide material comprising tungsten and carbon and having crystallite sizes of about 15 A to about 30 A. 21. A method for forming a high-surface-area tungsten carbide material comprising heating a tungsten precursor to a temperature from about 500 °C to ' about 800°C in an atmosphere containing a hydrocarbon gas and, optionally, hydrogen gas for a time sufficient to convert the tungsten precursor to the tungsten carbide material . 22. The method of claim 21 wherein the tungsten precursor is selected from ammonium metatungstate, ammonium paratungstate, tungsten metal powder, tungsten oxides, or tungsten halides. 23. The method of claim 21 wherein the tungsten precursor is ammonium metatungstate, ammonium paratungstate or a tungsten oxide . 24. The method of claim 21 wherein the hydrocarbon gas is selected from propane, ethane, natural gas, ethylene, acetylene, or a combination thereof. 25. The method of claim 21 wherein the hydrocarbon gas has a molecular formula containing twelve or less carbon atoms . 26. The method of claim 23 wherein the hydrocarbon gas is ethane or propane . 27. The method of claim 21 wherein the tungsten carbide material is cooled in an inert atmosphere and passivated in nitrogen after conversion. 28. The method of claim 26 wherein the tungsten precursor is ammonium paratungstate decahydrate. 29. A method for forming a high-surface-area tungsten carbide material comprising heating a tungsten precursor selected from ammonium metatungstate, ammonium paratungstate, or a tungsten oxide to a temperature from about 500 °C to about 800 °C in an atmosphere containing a hydrocarbon gas and, optionally, hydrogen gas for a time sufficient to convert the tungsten precursor to the tungsten carbide material; the tungsten carbide material comprising tungsten .and carbon; the material having an x- ray diffraction pattern containing a primary x-ray diffraction peak and first and second secondary x-ray diffraction peaks, the primary x-ray diffraction peak having a reflection angle corresponding to a d-spacing of 2.39 ± 0.02 A, the first secondary x-ray diffraction peak having a reflection angle corresponding to a d-spacing of 1.496 + 0.007 A 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 having a reflection ■ angle corresponding to a d-spacing of 1.268 ± 0.005 A and a relative peak height of 35% to 55% of the peak height of the primary x-ray diffraction peak. 30. The method of claim 29 wherein the hydrocarbon gas is ethane or propane . 31. The method of claim 30 wherein the tungsten precursor is ammonium paratungstate decahydrate. 32. The method of claim 29 wherein the material has crystallite sizes of about 15 A to about 30 A.

Inventors: CHRISTIAN JOEL B, CHRISTIAN, JOEL, B.
Applicants: OSRAM SYLVANIA INC, OSRAM SYLVANIA INC.
Application year: 2001
Main classification: C01B31
IPC codes: B01J23, B01J27, B01J35, B01J37, C01B31