JP3906354B2 - Method for producing high surface area silicon carbide ceramic porous body - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a novel high surface area silicon carbide (hereinafter abbreviated as SiC) ceramic porous body.
[0002]
[Prior art]
Hitherto, high surface area silicon carbide materials in which fine particles are formed by thermal decomposition of carbosilanes or their gels, or by introducing cross-links and then increasing the surface area per unit weight are known. Yes.
On the other hand, it has been known that activated carbon and a silicon source such as silicon oxide are reacted at a high temperature to obtain granules having a particle size of about 2 mm.
[0003]
However, the high surface area SiC material by thermal decomposition of carbosilanes is a powder having a particle size of about 0.01 μm, and if used as a catalyst carrier as it is, the pressure loss increases, so that it is sintered at a temperature around 2000 ° C., Although it has been necessary to process and use the molded article having a large size, at such a sintering temperature, SiC particle growth occurs and the surface area is inevitably reduced.
Further, if sintering is attempted at a lower temperature in order to suppress such particle growth, the crystallinity of SiC deteriorates, stacking faults increase, oxidation resistance at high temperatures decreases, and SiC air In the middle, the SiO ₂ generated by oxidation at a high temperature becomes glassy and the surface area is reduced.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for producing a porous ceramic body having a high surface area that does not require molding such as granulation and has excellent oxidation resistance and high strength when used as a catalyst carrier. It is a thing.
[0005]
[Means for Solving the Problems]
As a result of various studies to develop a high surface area SiC ceramic porous body suitable as a catalyst support, the present inventors carbonized rice husks that have been used as one of silica sources until now. Using this as a source of carbon and silica, this is first hot-pressed at a predetermined temperature in the presence of a metal catalyst to form a SiC sintered porous body, and then the metal silicide and SiC contained therein are acid-added. It was found that a high-strength, high surface area SiC ceramic can be obtained by elution to form pores, and the present invention has been made based on this finding.
[0006]
That is, the present invention oxidizes rice husk charcoal to obtain an oxidation product having a C / SiO ₂ weight ratio of 0.65 or more, and then a metal catalyst is added thereto and hot pressed at a temperature of 2050 ° C. After forming a SiC sintered body containing a composite of a trace amount of metal silicide and carbon, and then removing residual carbon from the sintered body , a mixture of hydrofluoric acid and nitric acid is used to remove the metal in the sintered body. The present invention provides a method for producing a high surface area silicon carbide ceramic porous body characterized by eluting silicide and eluting silicon carbide around pores formed after melting the silicide.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The high surface area SiC ceramic porous body of the present invention is produced using rice husk charcoal obtained by carbonizing rice husk. Since the rice husk charcoal contains carbon and silicon, when this is oxidized at a temperature of 300 ° C. or higher, a mixture of carbon and silica is obtained. By changing the oxidation conditions in this case, the weight ratio of C / SiO ₂ in the oxidized chaff charcoal can be changed. The C / SiO ₂ weight ratio should be 0.65 or more, preferably in the range of 0.65 to 0.79.
[0008]
A high surface area SiC ceramic is obtained from such oxidized rice bran charcoal through the following two-stage process.
[0009]
That is, in the first stage, a transition metal compound such as Fe ₂ O ₃ or CoO is added as a reaction catalyst to oxidized rice bran charcoal and hot pressed at a predetermined temperature to form a sintered porous body. This sintered porous body has pores having a pore size generated during normal sintering. At this time, the metal catalyst such as iron changes to a metal silicide such as iron silicide during the SiC conversion reaction. And carbon melt | dissolves in this metal silicide, SiC which became supersaturated precipitates. SiC produced in this manner has good oxidation resistance because of small stacking faults.
[0010]
Thus, once precipitated SiC is united with molten SiC-containing metal silicide to form a thick SiC inter-particle neck bond and become ceramic.
Next, in the second stage, the SiC ceramic containing the metal silicide formed in this way is eluted with a mixed acid of hydrofluoric acid and nitric acid (hereinafter referred to as HF-HNO ₃ ). In this case, the original porous ceramic structure is kept intact even if about 50 wt% is eluted. And when C / SiO ₂ is large, if the hot press temperature in the first stage is high, the metal silicide is eluted in the state of being contained in the SiC particles, and pores are formed. This produces a porous material.
[0011]
【Example】
EXAMPLES Next , although an Example demonstrates this invention further in detail, this invention is not limited at all by these examples.
[0012]
Reference example
After carbonizing the rice husk, it was oxidized in air at 300 ° C. or higher. After adding 2.2 mM Fe ₂ O ₃ to 18 g of this oxidized rice bran charcoal , mixing well in an ethanol slurry, drying, and hot pressing at the temperature shown in FIG. At this time, C / SiO ₂ obtained by heat-treating the oxidized rice husk charcoal at 1000 ° C. for 1 hour in a nitrogen stream had a weight ratio of 0.42. Next, the porous SiC ceramic obtained by hot pressing at the temperature shown in FIG. 1 was immersed in a mixed acid of 50H ₂ O: 25HNO ₃ : 25HF (volume ratio), and the surface area was continuously measured. The horizontal axis indicates the amount of elution at that time, and the vertical axis indicates the surface area. Thus, it was found that the surface area depends on the hot press temperature and the elution amount. When observing these electron microscopes after elution, pores are developed in the SiC particles where the temperature is relatively low, and when the hot press temperature reaches 2050 ° C., the outside of the SiC particles is eluted. I understood. This is because Fe ₂ O ₃ added as a catalyst becomes iron silicide, carbon dissolves in the iron silicide at a high temperature, and when the solubility is exceeded, SiC is precipitated, and the iron silicide is coalesced with the generated SiC to grow grains. The iron silicide is taken into the SiC particles by the so-called “ dissolution-precipitation ” mechanism. Since this iron silicide is relatively easy to dissolve in HF-HNO ₃ , further pores develop starting from this pore, but since the iron silicide comes out of the SiC particle at a high temperature, the inside of the SiC particle is eluted. However, since the outside of the particles is eluted, it is considered that the increase in the surface area with respect to the elution amount is reduced. Similarly, it has been found that the surface area of porous SiC ceramics using cobalt as a catalyst also increases the hot press temperature and decreases the surface area relative to the amount of SiC eluted. In this case, unless a catalyst such as iron or cobalt is present, the strength of the ceramic is weak, and when it is eluted, it becomes powder or lump. Furthermore, when the hot press temperature is increased, the connection between SiCs is increased, and the folding force is increased.
Thus, when the weight ratio of C / SiO ₂ is small, that is, when the carbon content is small, a metal catalyst is added, hot pressed at a temperature of 2050 ° C., and treated with HF-HNO ₃ to reduce the surface area. There is no increase.
[0013]
Example 1
Next, oxidize rice bran charcoal at 300 ° C. or higher, and mix 18 g of oxidized rice bran charcoal and Fe ₂ O ₃ in a slurry of 2.2 mM with a C / SiO ₂ ratio, dry, and hot press at 2050 ° C. did. At this time, if the oxidation loss of the carbon in the husk charcoal was small, a molded body in which SiC and carbon were combined was obtained. Thereafter, excess carbon in the composite molded body was removed at 700 ° C. in air. Thereafter, 50H ₂ O: 25HNO _3: immersed in mixed acid of 25HF (volume ratio) was measured continuously surface area. The result is shown in FIG .
As C / SiO ₂ becomes smaller, the rate of increase in surface area relative to the elution amount becomes smaller . When C / SiO ₂ was 0.79, 0.65, and 0.53, the residual carbon amounts were 26.2, 10.7, and 0 wt%, respectively. Therefore, the particle growth of SiC is inhibited by the residual carbon and, after removing carbon enters the mixed acid in efficiently porous SiC ceramics HF-HNO ₃ becomes hole portion was carbon, the SiC elution progresses Mutame, it is presumed that the surface area of the SiC ceramics as residual carbon is abundant increases.
[0014]
Example 2
Boiled charcoal having C / SiO ₂ with Co as a catalyst was hot pressed at 2050 ° C. and decarbonized at 700 ° C. Next, 50H ₂ O: 25HNO _3: wet during 25HF (volume ratio), the amount of elution was measured pore distribution of about 50 wt% of a high surface area SiC ceramics. The result is shown in FIG. As can be seen from this figure, pores having a pore diameter of 200 mm or less are formed by the elution process.
[0015]
【The invention's effect】
SiC has low sinterability, the manufacture of SiC ceramic having a high surface area has been difficult. However, according to the present invention, when a metal catalyst is added to a mixture of carbon and silica and hot pressing is performed, SiC generation and grain growth occur due to dissolution-precipitation action due to metal silicide, and porous SiC ceramics with small stacking faults are manufactured. . Thereafter, the HF-HNO ₃ elution, it is possible to produce high strength, high surface area SiC ceramics. This is particularly used in the exothermic reaction catalyst carrier and hydrophobic catalyst support.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the HF-HNO ₃ elution amount and the surface area at each hot press temperature during Fe catalyst.
FIG. 2 is a graph showing the relationship between the HF-HNO ₃ elution amount and the surface area of each C / SiO ₂ in rice bran charcoal when Fe catalyst is used.
[Figure 3] Co catalyst presence, pore distribution curve obtained when eluted 50 wt% of each C / SiO _2.

Claims (1)

After oxidation of the rice husk charcoal to an oxidation product having a C / SiO ₂ weight ratio of 0.65 or more, a metal catalyst was added thereto and hot-pressed at a temperature of 2050 ° C. to form a trace amount of metal silicide and Forming a SiC sintered body containing a composite with carbon, and then removing residual carbon from the sintered body , then eluting the metal silicide in the sintered body with a mixture of hydrofluoric acid and nitric acid; and A method for producing a high surface area silicon carbide ceramic porous body, wherein silicon carbide is eluted around pores formed after melting.

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