Patent Application
20070085053
The present invention is directed to a method for preparing oxidatively passivated superparamagnetic transition metal nanoparticles utilizing a gas stream comprising at least one inert gas and hydrogen.
It is known in the art that iron particles in the size range of about 5 nm exhibit superparamagnetic properties. That is, there is no residual magnetism so that no hysteresis is observed in a magnetization/demagnetization cycle. Superparamagnetic particles are useful in that they form stable fine particle dispersions by virtue of having no permanent magnetism but become highly magnetized when they are subject to an external field allowing them to be recognized or separated by their magnetic signature.
One long-recognized problem with very fine metallic particles, particularly iron, is that they are pyrophoric. Thus, it is necessary to chemically passivate the iron nanoparticles if they are to be useful. It has been found in the art that carbon-coated iron nanoparticles can be prepared by various methods.
Scott et al., Mat. Res. Soc. Symp. Proc. Vol 457 (1997), 219-224, discloses carbon coated iron cobalt nanoparticles synthesized in a radio-frequency plasma torch. In the process, a combination of iron and cobalt powders are combined in an argon plasma gas with acetylene. A mixture of argon and hydrogen are employed in the so-called sheath gas. The product thereof comprises highly agglomerated about 20 nm iron-cobalt particles embedded in a carbon matrix.
Gedanken et al., U.S. Patent Publication 20030017336 discloses preparation of hydrocarbon polymer coated superparamagnetic iron nanoparticles prepared by sonicating Fe(CO)5 in a polymerizable hydrocarbon solvent.
Dravid et al., U.S. Pat. No. 5,472,749 discloses preparation of graphite encapsulated nanophase particles using a tungsten arc to heat graphite in the presence of an iron or other anode.
Ruoff et al., U.S. Pat. No. 5,547,748 discloses preparation of metallic particles about 25-30 nm in size encapsulated in polyhedral graphitic shells in a carbon arc discharge process wherein one carbon electrode contains a core of a metal.
Dumitrache et al., Diamond and Related Materials, 13 (2004), 362-370, discloses preparation of graphite-coated iron nanoparticles with average size less than 10 nm and a narrow size distribution. The nanoparticles are prepared by feeding a combination of Fe(CO)5, ethylene, and acetylene through a continuous CO2 laser beam.
In view of the foregoing it is believed advantageous to provide a novel process for making carbon-coated iron nanoparticles.
The present invention provides a process comprising introducing into a gas stream a hydrocarbon and a transition metal carbonyl wherein the transition metal carbonyl is introduced downstream from the hydrocarbon; wherein at the point of introduction of the hydrocarbon the gas stream is in the form of a plasma, and wherein at the point of introduction of the transition metal carbonyl the gas stream is at a temperature of at least 1000° C.; followed by quenching to form carbon-coated transition metal nanoparticles; and wherein the gas stream consists essentially of at least one inert gas and hydrogen.
The present invention is directed to a process comprising combining a transition metal carbonyl with a hydrocarbon in a gas stream being flowing plasma or plasma-heated plasma gas, the gas comprises at least one inert gas and hydrogen followed by a quenching step to form carbon-coated transition metal nano-particles. Accordingly, a transition metal carbonyl and a hydrocarbon are fed into a flowing plasma or plasma-heated plasma gas, the plasma gas comprising an inert gas and hydrogen. The product produced comprises transition metal nanoparticles coated with turbostratic carbon. Turbostratic is a term of art referring to two-dimensionally ordered carbon. The transition metal nanoparticles so formed are oxidatively passivated by virtue of the carbon coating.
By the term “inert gas” is meant one or more gases selected from the group helium, neon, argon and xenon.
It shall be understood that all materials employed in the practice of the present invention are of high purity in order to avoid contamination in the highly reactive environment produced according to the process hereof.
In a first step of the process, plasma is formed using conventional methods in which an ionizable gas is subject to a sufficiently high voltage that ionization occurs. Any method known in the art for forming plasma is suitable for use. For example, methods may include direct current (DC) plasma torches, radio-frequency (RF) plasma torches, carbon arcs, lasers, electron beams, and the like. Carbon arcs are less preferred because of potential interference from unwanted carbon contributed by the carbon electrode.
In the process, a gas stream is used. The stream is described as follows. A plasma gas is caused to flow into a high voltage region where it undergoes ionization to form plasma. The prepared plasma is then caused to flow into a reactor chamber where a hydrocarbon and a transition metal carbonyl are introduced. The transition metal carbonyl is introduced downstream from the entry point of the hydrocarbon. In one embodiment prior to introduction the transition metal carbonyl may be cooled below room temperature. At the point of introduction of the transition metal carbonyl into the gas stream, the gas stream is at a temperature of at least 1000° C., preferably at least 2000° C. In some embodiments at least a portion of the hydrocarbon is introduced into the plasma. In some embodiments the downstream introduction of the transition metal carbonyl may be into a plasma-heated plasma gas which is reconstituted into the non-ionic form upon sufficient cooling of the plasma, so that the combination of the transition metal carbonyl and the hydrocarbon is in fact effected in the plasma-heated plasma gas rather than in the plasma itself. The plasma-heated plasma gas though not plasma is still quite hot, estimated to be about at least 1000° C. The term “plasma gas” as employed herein refers to the gas which is subject to high voltage which induces ionization thereof, thereby forming plasma—an ionized gas. That is, the plasma gas is not ionic in nature, while the plasma is ionic in nature. The flow of the plasma gas may be intermittent, but is preferably continuous.
In one embodiment a portion of the hydrocarbon is introduced into the plasma upstream from the introduction point of the transition metal carbonyl, while another portion of the hydrocarbon is employed as the carrier and make-up gases for the transition metal carbonyl input stream.
The plasma gas comprises a mixture of at least one inert gas and hydrogen, preferably argon and hydrogen. It was found that excessive amounts of hydrogen may interfere with the formation of the plasma. It is also found that the omission of hydrogen in the plasma gas results in the formation of layered sheet-like carbon structures intermixed with the desired carbon-coated transition metal nanoparticles. Hydrogen greatly reduces or eliminates the occurrence of the layered carbon by product. While a suitable range in flow rates will depend upon the scale of the apparatus, it has been found using the apparatus described herein that argon/hydrogen flow rates of about 14/1 l/min was satisfactory.
After the plasma is formed, the plasma is caused to flow into a reaction chamber wherein a hydrocarbon gas or liquid and a transition metal carbonyl vapor are introduced. Any readily vaporized hydrocarbon may be employed but alkanes having 1 to 5 carbons are preferred. Most preferred is methane. Any transition metal carbonyl is suitable. This includes but is not limited to Fe(CO)5, Ni(CO)4, and Co(CO)8. Preferred is Fe(CO)5. Flow rates of the gas stream will depend upon the scale of the apparatus. In an embodiment employed herein, methane flow rates in the range of 0.1 to 0.5 l/min have been found to be satisfactory with flow rates of Fe(CO)5 in the range of 0.01 to 0.15 g/min. The equipment used to prepare superparamagnetic carbon-coated iron nanoparticles comprises an iron pentacarbonyl vapor phase source, and a plasma torch reactor.
The means for introducing the hydrocarbon and transition metal carbonyl into the plasma is not critical. Any method known in the art for adding a known amount of vapor of a volatilizable liquid at a constant rate may be employed, as may any known method for flow monitoring and controlling of a hydrocarbon vapor. One embodiment found to be useful is to combine the transition metal carbonyl vapor with the hydrocarbon by employing a vapor entrainment device, shown in
Accordingly, the hydrocarbon is introduced into the plasma, preferably at the hottest point where the plasma exits the plasma torch. While it is not necessary to combine the transition metal carbonyl with additional hydrocarbon, employed as carrier or make-up gas, prior to introduction into the plasma, it is preferred. It is found that there is a minimum volume flow rate of the carrier plus make-up gas in order for the process to operate effectively to produce useful quantities of the nanoparticles. In the process, a volume flow rate of carrier plus make-up gas of at least 0.1 l/min is required. It is further found that excessive carrier gas flow rates can result in excessively high concentrations of the transition metal carbonyl which results in rapid plugging of the injector tip.
While the particular method employed for introducing the reactants into the plasma is not important, the order of introduction is. It was found that obtaining the smallest particle size and narrowest polydispersity dictates the order of introduction of the reactants and the interval between their respective introductions. The hydrocarbon is introduced first, that is, upstream from the point of introduction of the transition metal carbonyl. Preferably, the hydrocarbon is introduced at the hottest point, where the plasma emerges from the plasma torch. The transition metal carbonyl is introduced downstream therefrom. It is not known whether at the entry point of the transition metal carbonyl the plasma still exists or not. However, it is believed that by that point, the plasma has reverted to the non-ionized plasma gas, although it is still very hot, estimated to be at least 1000° C.
The purpose of the make-up gas stream is to provide sufficient volume flow of the Fe(CO)5-containing feed to permit good mixing to be obtained with the heated plasma or plasma gas. It is found that if insufficient volume flow is provided, sufficient mixing with the heated plasma or plasma gas is not obtained. It is possible in the practice of the invention to eliminate the make up gas stream and simply increase the feed rate of the carrier gas, but this may cause excess Fe(CO)5 to be entrained which results in clogging of the injector port.
A plasma torch reactor suitable for use in an embodiment, illustrated in
The nozzle assembly of
While the nozzle assembly, 9, provides a convenient arrangement for effecting the reaction followed by rapid quenching, it may be replaced by a simple reaction chamber, possibly with multiple ports arranged longitudinally along the flow path in order to permit variability in the position of introduction of the transition metal carbonyl.
In the following examples, the apparatus employed comprised a vapor entrainment device as illustrated in
The DC plasma torch reactor was equipped with a modified Metco type MBN plasma gun (available from Sulzer Metco Inc., Westbury N.Y.), having a maximum power of 40 kW (500 A at 80 V). The plasma torch current was set at 110 A unless otherwise noted. The plasma torch was provided with a water-cooled copper anode, Metco MB63, and a thoriated tungsten tip water-cooled copper cathode, Metco MBN430. The plasma torch was modified by placing an in-house fabricated electromagnet around the torch to produce an axial magnetic field in the direction of plasma gas flow. This ensured rotation of the electric arc between the cathode and anode, especially at low gas flow rates. The arc needed to be constantly rotated to prevent anchoring and to provide even wear to the anode. The electromagnetic was water-cooled, machine wound and housed in a plexiglass enclosure. The magnet was operated at 90% full scale voltage, which was ˜35 volts. Argon (ultra high purity, MG Industries Malvern, Pa.) and hydrogen (ultra high purity, also from MG Industries) gases were fed through the torch at 14 and 1.05 l/min., respectively.
With reference to
The main methane feed stream was introduced into the reactor below the exit of the torch and above the nozzle. The Fe(CO)5 containing feed stream was introduced into the reaction zone of the nozzle assembly. Since the Fe(CO)5 was fed into the reactor through feed ports that extended radially into the nozzle, the nano-size iron particles were formed and coated inside the nozzle. Below the nozzle holder was a water-cooled quench chamber that had three radial input ports to provide additional quench using He (scientific grade, MG Industries, see above) fed at 5 l/min through each of the ports for a total He quench of 15 l/min. Below the quench chamber was an adapter connecting the quench chamber to the water-cooled, single-filter element product collector. The collector housed a 3 micrometer sintered INCONEL® 600 filter element. The carbon-coated nanometer iron particles were collected on the filter and removed for analysis.
In the examples wherein the nozzle is not employed, it is replaced by a multiport reaction chamber (as illustrated in
It was found in performing the examples described that both the nozzle and the methane injection ports were subject to plugging. For runs of greater duration than about 30 minutes, it was usually necessary to stop to clear a plug. The situation was aggravated when the combination of carrier gas and make-up gas flow rates was below about 0.7 l/min.
A brine ice bath was employed to keep the Fe(CO)5 and methane carrier gas at −10° C. Carrier gas flow rate was 0.5 l/min., methane make-up gas flow rate was 0.2 l/min., and the main methane feed rate was 0.3 l/min. The Fe(CO)5/CH4 mixture was injected into the nozzle at the port shown in
The configuration of Example 1 was employed except that argon was employed as the carrier gas at 0.3 l/min with no make-up gas.
Besides encapsulated particles of Fe, the product was also observed to contain pure C structures such as multi-walled carbon nanotubes. A TEM is shown in
This run was an exact replicate of Example 1. BET surface area was 185 m2/g; particle size distribution by light scattering showed that 90% of the particles were less 169 nm in size.
The configuration of Example 1 was employed except that methane carrier flow rate was 0.5 l/min but there was no additional make-up gas, and no ice bath was used so that the Fe(CO)5 was at room temperature of about 22° C.
TEM image shown in
BET surface analysis showed 122 m2/g. Particle size distribution determined by light scattering showed that 90% of the particles were smaller than 169 nm.
The configuration of Example 4 was employed except that the make-up gas flow rate was 0.2 l/min. The inlet ports plugged almost immediately. Not enough product to collect. It is not known why this reaction failed.
In the following examples, the nozzle assembly shown in
Other associated equipment shown in
The process piping and feed rates were identical to those in Example 1 except where indicated. In all three examples, the main methane stream was fed in at a distance of 0.5″ from the exit of the plasma torch, at a rate of 0.3 l/min. The methane carrier flow rate was 0.1 l/min and the make up gas at 0.2 l/min. The location of the injection point for the Fe(CO)5 is shown in Table 1 as the downstream distance from the introduction point of the main methane stream.
It was observed that a plasma gun current of 130 amps produces approximately an increase of about 50° C. of the temperature at the entry port of the iron pentacarbonyl stream compared to 110 amps.
Using the apparatus and process from Examples 6 and 7, a reaction was run for 60 minutes at 130 amps plasma current. No hydrogen was added to the argon plasma gas. The iron pentacarbonyl was fed in at a port spaced 2 inches (5.1 cm) downstream from the main methane feed port. The pressure in the reaction vessel was observed to increase by 57 torr during the reaction. The total product collected was 1.2 g, 0.6 g as an even deposit along the filter element; 0.6 g deposited on the wall of the reaction chamber near the feed ports.
The reaction was run for 18 minutes with a pressure increase of 19 torr was observed. The plasma current was 110 amps. The iron pentacarbonyl injection point was located 1″ downstream from the main methane stream input point. 1 l/min of hydrogen was added to the argon flow of 14 l/min. Total product collected was 1.1 g, 0.5 g from the filter, and 0.6 g from the reactor wall near the feed ports.
The reaction was run in the same manner as in Example 6, except that the injection point for the iron pentacarbonyl was located 2″ downstream from the main methane stream input. The reaction was run for 32 minutes, with an observed pressure increase of 16 torr. The total product collected was—0.3 g, 0.2 g from the filter and 0.1 g from the reactor wall near the feed ports.
Legend for
30—Plasma Gun
31—Multi-port Reactor
32—Adapter
33—Product Collector
34—Cooling Ports
35—Fifty-two Feed Ports
36—Instrumentation Connections
37—Cooling Ports
38—Instrumentation Connection
39—Exit to Scrubber
