JP2007057268A - Surface fractal dimension measuring method of carbon material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for measuring easily a surface fractal dimension showing the abundance of pores, especially submicron pores, on the surface of a carbon material, especially activated carbon. <P>SOLUTION: Metal atoms are adsorbed onto the carbon material, and activation energy for atomization is determined through temperature change measurement of a graphite furnace atomic absorption signal of the atoms. Then, the fractal dimension is determined from a saturated adsorption amount of the atoms. It can be determined from the values whether the atoms are adsorbed into submicron pores or not, namely, how many submicron pores exist in the carbon material. Serviceability is shown by using gold atoms as an example. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for easily measuring the surface fractal dimension of a carbon material.

  A large number of pores exist on the surface of the activated carbon used as a deodorizer and the like, and these pores have a hierarchical structure similar to a fractal structure (FIG. 1 (a)). Among these, relatively large macropores adsorb very large molecules such as proteins, intermediate size mesopores adsorb large molecules such as dyes, and smaller micropores adsorb general molecules. Furthermore, it is known that atoms, gas molecules, and low molecular weight molecules are adsorbed in even smaller sub-micropores (FIG. 1 (b)).

  Therefore, when using activated carbon as a gas adsorbent, especially when using it as a material for adsorbing methane, ethane or other lower hydrocarbon gas or hydrogen with a small gas molecule size, the specific surface area is large and the micropore volume is large. A large one is advantageous, and various activated carbons with improved adsorption activity have been studied and developed with these requirements as a guide (Patent Document 1). However, it has not been easy to determine the presence or absence of sub-micropores in activated carbon.

JP 2001-122608 A ([0002])

As described above, since the pore structure of the carbon material such as activated carbon is similar to the fractal structure, the ratio of sub-micropores on the surface of the carbon material can be expressed by the fractal dimension which is a characteristic value of the fractal structure. This fractal dimension is generally called the D value, and pores are generated on the surface, the structure becomes complicated, and the value of D increases. It is known that the saturated adsorption amount Vm of the monomolecular phase has the following relationship with this D value.
Vm = k ・ rD
Here, k is a proportional constant, and r is an effective radius of the adsorbed molecule.

  The surface fractal dimension D on the surface of activated carbon is generally close to D = 3 for heat treatment of about 1000K, but becomes about D = 2 at 1700K, and it is known that sub-micropores disappear and the surface becomes more planar. It has been.

  Conventionally, there is no simple method for measuring the fractal dimension of a carbon material, and there is only a troublesome method of determining by adsorbing and desorbing molecules and atoms of different sizes in order and measuring the amount of each adsorption. It was.

  The problem to be solved by the present invention is to provide a method for easily measuring a fractal dimension, which is an index indicating the abundance of sub-micropores on the surface of a carbon material.

The method for measuring the surface fractal dimension of the carbon material according to the present invention, which has been made to solve the above problems,
It is characterized in that a noble metal atom is adsorbed on a carbon material to be measured, the activation energy of the noble metal atom is obtained by measuring the temperature change of the graphite furnace atomic absorption signal, and the surface fractal dimension is obtained from the activation energy.

  By the method according to the present invention, the surface fractal dimension of a carbon material such as activated carbon can be easily measured, and the existence ratio of sub-micropores on the surface of the carbon material can be determined. The existence ratio of sub-micropores is indispensable information for understanding the adsorption action and the catalytic action, and the carbon material can be easily evaluated by this method, which can contribute to the improvement of the production method.

目的とする原子の黒鉛炉内当初存在量N₀は、黒鉛炉内で完全に原子化する時間τより次式で表される。

最終的には、吸光度信号の最大値A_maxは原子総数N₀に比例し、吸光度信号の出現から消失までの吸光度の積分値A_sはτに無関係で、原子総数N₀に比例する。
原子吸光度信号の変化（吸光度プロファイル）の模式図を図２に示す。 The principle of measurement according to the present invention is shown below.
Time t number of atoms the atomic state in a graphite furnace to n1, when the number of atoms went out of graphite furnace was n _2, the number of atoms N in a graphite furnace at time t is expressed as follows The

The initial abundance N ₀ of the target atom in the graphite furnace is expressed by the following equation from the time τ for complete atomization in the graphite furnace.

Finally, the maximum value A _max of the absorption signal is proportional to the atomic total N _0, the integral value A _s absorbance from the appearance to the disappearance of the absorbance signal is independent of tau, proportional to the atomic total number N _0.
A schematic diagram of the change in the atomic absorbance signal (absorbance profile) is shown in FIG.

  If the temperature of the graphite furnace is rapidly raised after the metal atoms are adsorbed on the carbon material and the time change of the absorbance signal of the metal atoms is measured, the desorption rate and activation energy of the metal atoms are obtained. be able to. When metal atoms are detached monoatically and atomized randomly, the elimination reaction is a primary reaction.

  As will be described later, a linear relationship as shown in FIG. 11 was found between the activation energy of the desorption of the monoatomic metal and the surface fractal dimension.

  Thus, if a metal atom capable of adsorbing a single atom in a sub-micropore is selected as an adsorbing species to the carbon material, knowledge about the sub-micropore can be obtained. As such a metal material, a noble metal material such as gold or platinum can be used. In particular, since gold has an atomic radius of 0.14 nm, it can be adsorbed to sub-micropores and is relatively easy to obtain. Therefore, it is suitable for use in the method according to the present invention.

  A schematic configuration of an example of the measuring apparatus is shown in FIG. As the atomic absorption device, a Z-8000 type Zeeman atomic absorption spectrophotometer manufactured by Hitachi, Ltd. was used. The gold solution was injected using an auto sampler manufactured by the same company, and the organic aqueous solution sample was injected using P-200 manufactured by Gilson. A PG furnace manufactured by Hitachi, Ltd. was used as the graphite furnace. The light source was a hollow cathode lamp made of Hitachi, Ltd. Au element.

The absorbance obtained by the data processor is recorded on an external personal computer via RS-232C, and the temperature data of the graphite furnace is the same after the output of the optical temperature controller attached to the atomic absorption spectrophotometer is A / D converted. Recorded on a PC. The temperature calibration was performed using Chino's radiation thermometer IR-AHIS and Pt-Rh thermocouple. Background correction was performed by the Zeeman background correction method. Measurements were taken at both the peak height and area of the absorbance curve.
The water used was ultra-pure water whose purity was increased by Milli-Q academic from reverse osmosis water deionized by Millipore Elix manufactured by Millipore Japan.
As the gold solution, a standard solution for atomic absorption analysis Au = 1000 ppm (1 mol / HCl) manufactured by Kanto Chemical Co., Ltd. was used. The hydrochloric acid used was a special grade hydrochloric acid manufactured by Kanto Chemical Co., diluted to 1.0 N and stored in a brown bottle.

The following three types of carbon materials were measured.
・ Particulate activated carbon Takeda Yakuhin Kogyo Morshibon X2M4 / 6 was shaved with a diamond file and then ground in an agate mortar. ・ Activated carbon fiber A
ACF FE-200A made by Toho Kako Construction, dispersed in water, activated carbon fiber B
Toyobo BW5506 cut to 3 mm length and dispersed in water Among these, granular activated carbon has a wide distribution of pore diameter distributions including sub-micropores to macropores. On the other hand, activated carbon fiber is a specific carbon material having only micropores centered on a pore diameter of 10 nm.

  The following measurements were performed on these carbon materials. A carbon material to be measured is placed in a graphite furnace, and the gold solution is dropped onto the carbon material. The temperature of the graphite furnace is increased to 120 ° C., the sample is desolvated and incinerated, and then rapidly heated to about 2500 ° C. (for example, about 2000 ° C./sec). During this time, the position and intensity of the absorption curve are measured with respect to the graphite furnace temperature (FIG. 2).

The atomic absorption profile when the gold solution is dropped onto the granular activated carbon sample is shown by the line 4 in FIG. Compared with the atomic absorption profile (line 1) measured only with the standard solution without placing the carbon material in the graphite furnace, the peak of gold atomic absorption shifted to the high temperature side. Here, the atomic absorption profile (line 2) when the granular activated carbon sample was previously immersed in a gold solution at normal pressure to adsorb gold was also shifted to the high temperature side.
Also, as shown in FIGS. 5 (a) and 5 (b), the atomic absorption profile when the gold solution is dropped onto the activated carbon fibers A and B is different from that in FIG. It did not shift to the high temperature side. The atomic absorption profile (line 2) when activated carbon fibers A and B were pre-immersed in a gold solution at normal pressure was measured, and the peak shifted to a higher temperature than the profile of only the standard solution (line 1). . In FIGS. 5A and 5B, the measurement result when the gold solution is immersed after removing the gas adsorbed on the activated carbon by reducing the pressure is also shown as line 3.

  Thus, in both granular activated carbon and activated carbon fiber, the peak of the absorption profile of gold atoms adsorbed on them shifts to a higher temperature than the gold standard solution profile adsorbed on the surface of pyrolytic graphite. This indicates that energy is required for desorption from these carbon materials, and that gold atoms are adsorbed on the micropores of these granular activated carbons and activated carbon fibers.

ここで、αは変化率であり、α=N_t/N₀=A_t/A_maxで表される。また、X=RT/Eaとするとき、P=1-2X+6X²-24X³+120X⁴+・・・ で表され、またAは頻度因子である。
アレニウスプロットにおいて良好な直線性を示す反応機構関数g(α)が最適な反応機構であると判断することができ、その傾きから金の活性化エネルギーを決定することができる。 The analysis was performed based on the solid phase reaction kinetics shown below. From the Arrhenius plot according to this Arrhenius equation, the reaction mechanism and activation parameters can be determined.

Here, α is the rate of change, and is expressed as α = N _t / N ₀ = A _t / A _max . When X = RT / Ea, it is expressed by P = 1−2X + 6X ² -24X ³ + 120X ⁴ +... And A is a frequency factor.
It can be determined that the reaction mechanism function g (α) showing good linearity in the Arrhenius plot is the optimum reaction mechanism, and the activation energy of gold can be determined from the inclination.

  The g (a) function assuming various reaction models is shown in FIG. 6 (Table 1). In this table, F1 represents a case where metal atoms are dispersed in a monoatomic state and the reaction order is n = 1. The case where metal atoms form aggregates such as islands and clusters and become diffusion-limited is D1 to D3, and the case where they are interface-limited is R1 to R3. A3 / 2 and A2 to A4 indicate nucleation and reaction growth processes.

  As described above, granular activated carbon has a wide distribution of pore diameter distribution including sub-micropores to macropores, but activated carbon fiber has only micropores centered on a pore diameter of 10 nm. On the other hand, it has already been reported that gold atoms do not cause monoatomic dispersion at pore sizes larger than mesopores.

  From the kinetic analysis of the temperature-absorbance profile, monoatomic dispersion was not confirmed when gold atoms were adsorbed on activated carbon fibers having only micropores, and only when adsorbed on sub-micropores. From this, it can be concluded that when gold is adsorbed on the activated carbon material, the gold atoms are adsorbed monomolecularly only in the sub-micropores.

Activated carbon was generated by thermal decomposition of an organic aqueous solution in a graphite furnace, and the same measurement was performed. Saccharose and ascorbic acid were used as organic substances.
Activated charcoal was allowed to cool and then allowed to cool, 20 μl of a standard solution containing 50 ppb of gold was added, and the process of drying, ashing and atomization was analyzed by measuring the temperature-absorbance profile with the case where no activated charcoal was used as a standard. An example of the measurement results when using a 5% aqueous saccharose solution is shown in FIG.

  When adsorbed on activated carbon, a shift of the absorbance peak on the high temperature side is observed as compared with the case of only the standard solution. Focusing on the second peak of gold atoms, kinetic analysis was performed, and it was concluded that gold atoms were adsorbed by monoatomic dispersion. The activation energy of gold atomization was determined, and the relationship with the activated carbon generation temperature is shown in FIG. 8 (Table 2) for an aqueous saccharose solution and in FIG. 9 (Table 3) for an ascorbic acid solution. The surface fractal dimension is determined, and the relationship between the value and the activation energy is shown in FIG. 10 (Table 3) together with the values for carbon black and pyrolytic graphite. The relationship between the two is shown in FIG.

  Gold atoms are adsorbed by simple van der Waals forces without specific interaction with carbon atoms. Here, the measured activation energy corresponds to this van der Waals force. Since the van der Waals force depends on the effective area of the carbon surface at a position where it can interact with gold atoms, the unevenness of the sub-micropores affects the activation energy. Therefore, based on the activation energy and surface fractal dimension plot, the surface fractal dimension of the sub-micropore is determined from the activation energy of atomization of gold atoms on the unknown carbon material.

Since gold atoms having an atomic radius of 0.14 nm are used as the surface probe, the sub-micropores on the surface of the carbon material evaluated by this example are considered to be unevenness of the 0.14 nm level.
It is considered that knowledge about sub-micropores having different sizes can be obtained by selecting metal atoms that adsorb single atoms other than gold.

  In the above examples, the Zeeman atomic absorption spectrophotometry was used for the analysis of gold atoms. However, if the method can measure the adsorption amount of noble metal atoms such as gold atoms and the temperature change thereof with high sensitivity, absorption spectroscopy is used. The measuring device is not limited to this.

  According to this method, it is possible to know the presence of sub-micropores that could not be measured conventionally. In the production of higher-grade activated carbon, if the production process is evaluated while measuring the abundance of sub-micropores, the production of activated carbon having high added value becomes easy. As a result, a useful carbon-based adsorbent is developed, and it is expected that useful results will be obtained for sick house countermeasures, exhaust gas countermeasures, and malodor countermeasures. In addition, it can be applied to inspection of gas masks in that it measures the ability to adsorb toxic gases.

Schematic diagram showing various pores on the surface of the carbon material (a), and schematic diagram showing how molecules of different sizes adsorb to the pores (b) Schematic diagram showing the relationship between graphite furnace temperature and atomic absorption profile in an atomic absorption spectrophotometer Configuration diagram of graphite furnace atomic absorption spectrophotometer system used for measurement Atomic absorption profile of gold atoms adsorbed on granular activated carbon Atomic absorption profile of gold atoms adsorbed on activated carbon fiber A (a) and Atomic absorption profile of gold atoms adsorbed on activated carbon fiber B (b) (Table 1) Table showing various aspects of g (α) function in kinetic analysis method Atomic absorption profile of gold atoms adsorbed on activated carbon produced by thermal decomposition of aqueous saccharose solution (Table 2) Table showing the effect of 5% sucrose baking temperature on the activation energy of gold (Table 3) Table showing the effect of 5% ascorbic acid baking temperature on gold activation energy (Table 4) Table of activation energy of gold monoatomic dispersed on various fractal surfaces Graph showing the relationship between gold activation energy and surface fractal dimension D value

Claims (4)

  Measurement of surface fractal dimension of carbon material by adsorbing noble metal atoms on the carbon material to be measured, obtaining the activation energy of the noble metal atom by measuring the temperature change of the graphite furnace atomic absorption signal, and obtaining the surface fractal dimension from the activation energy Method.   The method for measuring a surface fractal dimension of a carbon material according to claim 1, wherein the noble metal atom is a gold atom.   The precious metal atoms are adsorbed on the carbon material to be measured, the activation energy of the precious metal atoms is obtained by measuring the temperature change of the graphite furnace atomic absorption signal, and the surface of the carbon material is determined based on the surface fractal dimension value obtained from the activation energy. A method for measuring the surface sub-micropores of a carbon material that determines the existence ratio of sub-micropores. The method for measuring a surface submicropore of a carbon material according to claim 3, wherein the noble metal atom is a gold atom.

Images