Patent Application
20110034328
This application claims priority to Korean Patent Application No. 2009-0072165, filed on Aug. 5, 2009, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a double metal-carbon nanotube hybrid catalyst capable of generating hydrogen from an ammonia-borane (NH3BH3) solution at a high speed, and a method for preparation thereof.
2. Description of the Related Art
A carbon nanotube is well known with much attention as a material having excellent thermal, mechanical and electric properties useful for a variety of applications. Especially, a carbon nanotube having a transition metal attached thereto shows improved material characteristics and/or may be used as a hybrid substance enabling expression of additional characteristics.
An example of currently employed catalysts for hydrogen generation is a noble metal-carbon nanotube hybrid catalyst containing only one noble metal such as Pt, Ru, etc., as disclosed in S. C. Amendola et al., Power Sources 25, 269, 2000; and C. Wu, H. M. Zhang et al., Catal. Today 93-95, 477, 2004. However, such a hybrid catalyst requires a complicated manufacturing process and has difficulty in mass-production, therefore, entails restrictions in time and economic aspects in view of practical application.
Meanwhile, a transition metal-carbon nanotube hybrid catalyst containing a single transition metal such as Co or Ni being relatively cheaper than noble metals such as Pt or Ru was proposed in G. G. Wildgoose et al., Small, 2, 182, 2006. However, this catalyst also has problems of low catalytic activity due to limited contact area between the transition metal-carbon nanotube hybrid catalyst and NH3BH3, although the foregoing hybrid catalyst comprises nano-scale particles.
Accordingly, it is an object of the present invention to provide a double metal-carbon nanotube hybrid catalyst with high speed hydrogen generation from an aqueous ammonia-borane (NH3BH3) solution and economic merit, and a method for preparation of the same.
In order to accomplish the above object, the present invention provides a method for preparation of a double metal-carbon nanotube hybrid catalyst comprising: adding nitrogen doped carbon nanotube (NDCNT) to a polyol solution to prepare a carbon nanotube solution; adding at least two of transition metal salts as well as sodium borohydride (NaBH4) to the prepared carbon nanotube solution to return the carbon nanotube by reduction thereof; and thermally treating the reduced carbon nanotube under hydrogen atmosphere after vacuum drying the same, so as to form the double metal-carbon nanotube hybrid catalyst.
The double metal-carbon nanotube hybrid catalyst of the present invention comprises carbon nantoubes with excellent electrical conductivity and at least two different transition metals functioning as a reactant, thus exhibiting improved catalytic activity. Accordingly, the inventive double metal-carbon nanotube hybrid catalyst containing at least two different transition metals may have higher hydrogen generation efficiency relative to the same mass, compared to a double metal-carbon nanotube hybrid catalyst comprising only one transition metal.
Also, the inventive double metal-carbon nanotube hybrid catalyst which enables generation of high capacity hydrogen from an aqueous NH3BH3 solution may store hydrogen in a simple mode, compared to conventional storage methods such as high pressure gas storage, liquefaction and storage, hydrogen storage using hydrogen storage alloys, etc. and, in addition, have advantages such as scale-down of hydrogen storage tank owing to high hydrogen storage capacity, reduction in investment costs, and the like.
Therefore, the double metal-carbon nanotube hybrid catalyst according to the present invention may be widely applied in different industrial fields using hydrogen energy including, for example, hydrogen storage systems for fuel cell, fuel storage systems for a hydrogen fuel cell vehicle, electric car, power sources for small electronic devices, and so forth.
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
According to an aspect of the present invention, there is provided a double metal-carbon nanotube hybrid catalyst comprising at least two transition metals selected from a group consisting of Mn, Fe, Co, Ni, Cu, Mo, Tc, Ru, Rh, Pd, Ag, Re, Os, Ir and Pt, which are homogeneously distributed in the catalyst. The double metal-carbon nanotube hybrid catalyst includes nitrogen with high chemical activity as a heterogeneous element added to carbon nanotubes and comprises at least two different transition metals with high catalytic activity and a nano-scale size uniformly distributed in the carbon nanotubes, so as to generate hydrogen from an aqueous ammonia-borane (NH3BH3) solution at a high speed.
According to another aspect of the present invention, there is provided a method for preparation of a double metal-carbon nanotube hybrid catalyst comprising: adding nitrogen doped carbon nanotube (NDCNT) to a polyol solution to prepare a carbon nanotube solution; adding at least two of transition metal salts as well as NaBH4 to the prepared carbon nanotube solution to return the carbon nanotube by reduction thereof; and thermally treating the reduced carbon nanotube under a hydrogen atmosphere after vacuum drying the same, so as to form the double metal-carbon nanotube hybrid catalyst. The foregoing NaBH4 as a reductant.
The NDCNT is preferably prepared by plasma chemical vapor deposition (CVD) using a gas mixture containing a hydrocarbon gas and a nitrogen gas in a ratio (%) by volume of 1:99 to 99:1 in the presence of a metal catalyst. When the hydrocarbon gas and the nitrogen gas are separately introduced into the metal catalyst, a relative ratio thereof may range from 1:99 to 99:1. On the other hand, if a mixture of the hydrocarbon gas and nitrogen gas is used, the relative ratio thereof may range from 10:90 to 90:10.
The metal catalyst includes cobalt (Co), iron (Fe), nickel (Ni), or a compound containing the same, however, is not particularly limited thereto so long as it may facilitate a catalytic reaction during preparation of NDCNT.
The plasma CVD may be performed using a microwave, RF or DC power source as a plasma generating source.
Preferably, the NDCNT is a carbon nanotube containing nitrogen in the range of 0.1 to 20 at % (atomic percentage).
The polyol may include at least one selected from a group consisting of ethyleneglycol, diethyleneglycol, polyethyleneglycol, 1,2-propanediol and dodecanediol alone or in combination with two or more thereof.
A transition metal in the transition metal salt may be at least one selected from a group consisting of Mn, Fe, Co, Ni, Cu, Mo, Tc, Ru, Rh, Pd, Ag, Re, Os, Ir and Pt.
Preferably, anions contained in the transition metal salt are acetate or chloride.
Hereinafter, the present invention will be described in greater detail with reference to the following preparative examples, experimental examples and comparative examples. However, these examples are intended for illustrative purposes and it would be appreciated by a person skilled in the art that various modifications and variations may be made without departing from the scope of the present invention. Therefore, it is not construed that the present invention is restricted to such examples.
Before preparation of NDCNT, a catalyst for growing the NDCNT was prepared by magnetron RF sputtering method. A process of preparing the catalyst for growth of NDCNT will be described in detail below.
Under an argon atmosphere of 15 torr, Fe was deposited on a SiO2/Si substrate. RF power used for deposition was set to 100 W and a thickness of Fe deposition was 10 nm. In order to form Fe particles from an Fe layer deposited on the substrate, plasma processing was carried out using a microwave enhanced CVD apparatus at a microwave power of 700 W for 1 minute.
Fe particles deposited on the substrate fabricated during the foregoing processes may be used as a catalyst for growth of NDCNT.
The catalyst for growing NDCNT was placed in a chamber, a hydrocarbon gas and a nitrogen gas were mixed in a ratio by volume of 15:85 and introduced into the chamber, followed by executing plasma CVD reaction. The chamber was maintained at 700° C. with a pressure of 21 torr. The plasma CVD reaction was performed with a microwave power of 800 W for 20 minutes, resulting in formation of the NDCNT.
Adding 10 mg of the NDCNT formed in Preparative Example 1 to 300 mL of ethyleneglycol solution, ultrasonic homogenization was performed to prepare a carbon nanotube solution.
After adding two different transition metal salts, that is, 7 mL of 10 mM NiCl2.4H2O and 1 mL of 10 mM H2PtCl6.4H2O to the prepared carbon nanotube solution, 100 mL of 0.1M NaBH4 as a reductant was further added to the mixture. From results of Inductive Coupled Plasma analysis for the above transition metal salts, it was found that a ratio by volume of the transition metal salts is defined by Ni:Pt=0.72:0.28. Therefore, a transition metal composite with such ratio will be referred to as Ni0.72Pt0.28.
After filtering the carbon nanotube solution containing the transition metal salts and NaBH4, the filtered material was sufficiently washed with acetone to obtain a pure product. The pure product was vacuum dried and thermally treated at 300° C. under a hydrogen atmosphere after vacuum drying at 60° C., thereby completing formation of a Ni0.72Pt0.28-carbon nanotube hybrid catalyst according to the present invention.
An amount of hydrogen per minute generated from an aqueous NH3BH3 solution by the inventive double metal-carbon nanotube hybrid catalyst (often abbrev. to “NiPt-NDCNT”), that is, a hydrogen generation rate was determined and compared to a target amount of hydrogen according to the US Department of Energy (DOE) (often abbrev. to “DOE Target”). Results thereof are shown in
For this purpose, a temperature of 0.5 wt. % NH3BH3 in 50 mL of aqueous solution was set to 25° C. and 10 mg of the Ni0.72Pt0.28-carbon nanotube hybrid catalyst prepared in Preparative Example 2 was added to the above solution. An amount of hydrogen generated from the prepared mixture was measured using a gas flow meter.
Referring to
A speed of generating hydrogen from an aqueous NH3BH3 solution by the inventive double metal-carbon nanotube hybrid was measured with different temperatures.
For this purpose, a temperature of 0.5 wt. % NH3BH3 in 50 mL of aqueous solution was set to 20° C., 25° C. and 40° C., respectively, and 10 mg of the Ni0.72Pt0.28-carbon nanotube hybrid catalyst prepared in Preparative Example 2 was added to each of the above solutions. A hydrogen generation amount along evolution time was measured using a gas flow meter and results thereof are shown in
Referring to
Applying the Arrhenius plot in
Arrhenius Equation: ln k=ln A−Ea/RT
wherein k is a rate constant, A is a frequency factor, Ea is an activation energy, R is a gas constant, and T is a Kelvin temperature (K).
From the X-ray diffraction analysis photograph of
A speed of generating hydrogen from an aqueous NH3BH3 solution by the inventive double metal-carbon nanotube hybrid was measured.
For this purpose, a temperature of 0.5 wt. % NH3BH3 in 50 mL of aqueous solution was set to 25° C., and 10 mg of each of the samples 1 to 4 prepared in Comparative Example 1 was added to the above solution. A hydrogen generation amount in relation to evolution time was measured using a gas flow meter and results thereof are shown in
Referring to
The major reason for such excellent hydrogen generation characteristics such as high hydrogen generation speed is that the double metal-carbon nanotube hybrid catalyst of the present invention comprises at least two different transition metals, instead of a single transition metal.
Although the present invention has been described in detail with reference to its presently preferred embodiment, it will be understood by those skilled in the art that various modifications and equivalents can be made without departing from the spirit and scope of the present invention, as set forth in the appended claims. Also, the substances of each constituent explained in the specification can be easily selected and processed by those skilled in the art from the well-known various substances. Also, those skilled in the art can remove a part of the constituents as described in the specification without deterioration of performance or can add constituents for improving the performance. Furthermore, those skilled in the art can change the order to methodic steps explained in the specification according to environments of processes or equipments. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2009-0072165 | Aug 2009 | KR | national |
