Abstract:
A method for realizing carbon dioxide reduction, which is a task to be urgently accomplished from the viewpoint of global environmental problems, is provided. The method includes the steps of irradiating carbon dioxide in a supercritical or subcritical state (i.e., near the critical point) with a UV-wavelength laser beam to decompose carbon dioxide and form a carbon-particle structure.

Description:
TECHNICAL FIELD 
     This invention relates to a novel method of decomposing carbon dioxide, which is useful for solving carbon dioxide environmental problems, and a novel method of forming a carbon particle structure by the decomposition. 
     BACKGROUND ART 
     Carbon dioxide reduction is a worldwide important task to be accomplished from the viewpoint of global environmental problems. However, effective methods have not been developed for the decomposition treatment of carbon dioxide discharged, for example, from living environment and industrial environment. 
     Carbon dioxide: CO 2  is a compound of carbon and oxygen. Accordingly, if CO 2  can be decomposed and utilized as a novel carbon resource, then a great contribution to the society could be realized. 
     The current situation, however, is that studies on carbon dioxide decomposition and effective utilization of carbon dioxide as a carbon resource have not hitherto been made very eagerly. 
     SUMMARY OF THE INVENTION 
     In view of the above, an object of the present invention is to provide a novel technical method that can realize carbon dioxide reduction, which is a task to be urgently accomplished from the viewpoint of global environmental problems, that can realize carbon dioxide decomposition for the carbon dioxide reduction, and that can realize the utilization of carbon resources obtained from the carbon dioxide with a higher added value. 
     The object of the present invention can be attained by a method for decomposing carbon dioxide, comprising the step of irradiating carbon dioxide in a supercritical or subcritical state (i.e., near its critical point) with a UV-wavelength laser beam to decompose carbon dioxide. 
     Secondly, the object of the present invention can be attained by a process for producing a carbon-particle structure, comprising the step of irradiating carbon dioxide in a supercritical or subcritical state with a UV-wavelength laser beam to produce a carbon-particle structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing the construction of an apparatus used in the working example. 
         FIG. 2  is a diagram showing an image that exemplifies an SEM image of a carbon particle structure produced in Example 1. 
         FIG. 3  is a diagram showing an image that exemplifies another SEM image of the carbon particle structure produced in Example 1. 
         FIG. 4  is a diagram showing an image that exemplifies still another SEM image of the carbon particle structure produced in Example 1. 
         FIG. 5  is a diagram showing an image that exemplifies a further SEM image of the carbon particle structure produced in Example 1. 
         FIG. 6  is an image that exemplifies the results of an EDS analysis of the carbon-particle structure produced in Example 1. 
         FIG. 7  is a diagram showing an image that exemplifies an SEM image of a carbon particle structure produced in Example 2. 
         FIG. 8  is a diagram showing an EDS image of the carbon particle structure shown in  FIG. 7 . 
         FIG. 9  is a diagram showing a TEM image of the carbon particle structure shown in  FIG. 7 . 
         FIG. 10  is a diagram showing an electron beam diffraction image of the carbon particle structure shown in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The best mode for carrying out the invention having the above features will be described. 
     In the decomposition of carbon dioxide and the production of a carbon particle structure according to the present invention, carbon dioxide that is in a supercritical or subcritical state in relation to its critical point (critical density 466 kg/m 3 , critical pressure 7.38 MPa, critical temperature 304.2 K) is irradiated with a UV-wavelength laser beam. Various laser beam sources and optical system can be properly adopted for this irradiation. Typical examples of UV (ultraviolet)-wavelength laser beams include YAG-THG (tertiary high frequency): wavelength 355 nm, YAG-FHG (quaternary high frequency): wavelength 266 nm, and KrF excimer: wavelength: 248 nm. 
     In the UV-wavelength laser beam irradiation, the laser beam may not be focused, or alternatively may if necessary be focused. 
     Further, any proper apparatus may be used for bringing the carbon dioxide to a supercritical or subcritical state. 
     In the production of the carbon-particle structure, a substrate may be used, and examples of substrates usable herein include substrates formed of various metals such as aluminum, nickel, tungsten, molybdenum, magnesium, silver, gold, tin, titanium, tantalum, and silicon, various metal alloys such as stainless steel, nickel-base alloys, and magnesium alloys, or inorganic materials such as alumina, graphite, BN and SiC. Alternatively, these substrates may not be used. 
     As exemplified also in the working example which will be described later, the carbon particle structure is produced as a single particulate body or is produced in such a state that a plurality of particulate bodies have been aggregated, fused or bonded together. The size of these carbon particle structures is very small and is usually not more than several tens of micrometers (μm), for example, in the range of several tens of nanometers (nm) to several tens of micrometers (μm). This size can be regulated by irradiation energy, for example, the wavelength of the UV-wavelength laser or irradiation time. 
     Further, a carbon particle structure having a concave on its surface, a hollow carbon particle structure, or a semispherical or partially spherical carbon particle structure can also be produced. 
     At the present stage, the method and process according to the present invention are considered based on the following mechanism. 
     Near the critical point of carbon dioxide (critical density 466 kg/m 3 , critical pressure 7.38 MPa, critical temperature 304.2 K), large molecule clusters are produced and the structure becomes opaque (critical opaline luster) as light is scattered. Due to this nature, various unique behaviors regarding physical properties are exhibited. For example, the specific heat or compressibility diverges as approaching the critical point. 
     Upon exposure of carbon dioxide near the critical point to UV laser, it is considered that, when photons collide against C—O bond, the C—O bond is cleaved. That is, a carbon dioxide decomposition process (CO 2 →C+O 2 ) is considered to take place stochastically. In particular, as described above, since large molecule clusters are formed near the critical point, it is considered that the probability of collision of photons against CO 2  increases and, consequently, the decomposition process (CO 2 →C+O 2 ) is promoted. Carbon atoms are bonded to each other to form a nanostructure or microstructure. It is considered that, as the temperature difference from the critical point increases, the probability of collision of photons against CO 2  molecules is reduced, resulting in a reduced CO 2  decomposition rate. 
     It is needless to say that the above consideration will be further deepened and developed. 
     The following Examples further illustrate the present invention. 
     However, it should be noted that the present invention is not limited to the following Examples. 
     Example 1 
       FIG. 1  is a schematic diagram showing an apparatus used in this Example. The apparatus is constructed so that a substrate is disposed inside of the apparatus and a laser beam is applied through a glass window. Carbon dioxide was introduced into this apparatus, and the carbon dioxide was irradiated near the critical point (atmosphere temperature 31.4° C.) with a 266 nm-wavelength UV laser beam. As a result, the decomposition of carbon dioxide and the production of a carbon-particle structure were observed. 
       FIGS. 2 ,  3 ,  4  and  5  exemplify SEM images of the produced carbon particle structure.  FIG. 6  exemplifies the results of an EDS analysis of the carbon-particle structure produced on an aluminum substrate, indicating that the resultant structure is a carbon-particle structure. 
     It was found that the production of the carbon-particle structure is observed in the case where an aluminum, graphite or other substrate is used, as well as in the case where no substrate is used. 
     For comparison, carbon dioxide with one atom of pressure was irradiated with the above laser beam. As a result, the production of a carbon-particle structure could not be observed at all. In this case, when an aluminum substrate was used, nano-size aluminum spheres were produced. 
     Example 2 
     In the same apparatus as in Example 1, a silicon substrate is disposed within the apparatus, carbon dioxide near the critical point was irradiated with a 266 nm-wavelength UV laser beam without focusing. As a result, it was confirmed that a carbon particle nanostructure was produced on the silicon substrate. 
       FIG. 7  exemplifies an SEM image of the resultant carbon particle structure deposited on a horizontal surface and a vertical surface of the silicon substrate.  FIG. 8  is an EDS image corresponding to  FIG. 7  in which the green part shows carbon atoms. 
       FIG. 9  shows a TEM image of a carbon particle structure, and  FIG. 10  shows an electron beam diffraction image of the carbon particle structure, indicating that the carbon particle structure is an amorphous structure. 
     Example 3 
     The procedure of Example 2 was repeated, except that each of nickel, magnesium, and carbon substrates were used instead of the silicon substrate. In this case, the UV wavelength laser beam was used for irradiation as in Example 2. As a result, it was confirmed that, in all of these substrates, a carbon particle structure was produced. 
     Example 4 
     The procedure of Example 2 was repeated, except that a 248 nm-wavelength UV laser beam was used for irradiation instead of the 266 nm-wavelength UV laser beam. Also in this case, it was confirmed that a carbon particle structure was produced. 
     INDUSTRIAL APPLICABILITY 
     The first embodiment of the present invention can realize the decomposition of carbon dioxide at or around room temperature. UV wavelength laser irradiation does not cause a temperature rise upon decomposition. According to the second embodiment of the present invention, upon the decomposition of carbon dioxide as a carbon resource, a carbon particle structure useful as a functional material in various fields such as electric, electronic, medical, catalyst, lubricant, plastic, and mechanically molded product fields can be produced. The production of the carbon particle structure is also possible without the use of any substrate.