Nanostructure Design on Porous Carbon Powders under Chemical Process and Their Physical Properties †

The carbon pore-structure formed by MgO-templated method consists of nano-carbon wall and mesopore made with MgO. From MgO and its mixtures with resin that have high yield of carbon, micro-mesoporous carbons were obtained through their carbonization at 900 °C, followed by the dissolution of MgO. The carbon powder prepared by this method possesses high surface area without any activation process. The mesopore size of carbon powders is controlled by the crystallite size of MgO, and micropore amount depends on resin structure. Crystallinities of their carbon powders with heat-treatment up to 2500 °C were very uniform.


Introduction
Various kinds of porous carbon materials are produced, which can be used as adsorbents, energy storage electrodes, and catalyst supports, etc. (Derbyshire F. et al., 1995).In order to get a high surface area, the oxidation and vapor activation process of carbon are usually employed.Through this process various activated carbons are prepared and applications developed (Derbyshire F. et al., 1995;Sanada Y. et al., 1992;Inagaki M, 2000).
On the other hand, there have been reported various precursors and processes to prepare porous carbons without any activation process.Pore size controlled carbons were prepared by using hard templates, such as zeolites and silicates (Kyotani T. et al., 2003;Shi Z-G. et al., 2003).For these template processes, however, it was pointed out that the template had to be dissolved out by strong acids after carbonization and that large amount of production was not easy, although the pores with a specific size were easily obtained.On the other hand, without template use, carbonization of organic aerogels was also reported to give mesoporous carbons (Hanzawa Y. et al., 1998;Tamon H. et al., 1997;Yamamoto T. et al., 2001).
We have developed easy process to coat metal-oxide ceramic particles by carbon; the powder mixtures of thermoplastic resin, such as poly(vinyl alcohol) and phenol resin dispersion, etc., and ceramics were heat-treated at a high temperature in an inert atmosphere.The carbon layer coated on the surface of a ceramic particle was found to be porous, and 3D-nano dimensions when using MgO as substrate metal oxide and dissolving it out by sulfuric acid (Inagaki M. et al., 2004).This result suggested us a new preparation process of porous carbons without any stabilization and activation processes even starting from thermoplastic precursors.
Following this idea, MgO-templated carbonization process was developed for preparing mesoporous carbons by heat-treatment of mixtures of carbon precursor and MgO powder or through the thermal decomposition of magnesium citrate (Morishita T. et al., 2006).Characterization of heat-treated MgO-templated carbons derived from magnesium citrate was reported.The obtained carbons show high BET surface area and superior electric conductivity even after the heat-treatment at around 2000 °C (Orikasa H. et al., 2012).These carbons also show high performance as negative electrode of a Li-ion capacitor (Soneda Y. et al., 2013).
In this study, we prepared the porous carbons derived from three different carbon precursors and MgO powders, and characterized them.Each carbon precursor's difference is reported.easy dissolving to dilute acidic aqueous solutions.MgO powders were analyzed by XRD and these crystallite sizes were calculated by a Scherrer equation.In this study, MgO powders with an average crystallite size of 11 nm were used.Carbon precursors were poly(vinyl alcohol) (PVA), Kapton-type polyimide (Imide), and phenol resin dispersion (Phenol).MgO powders/carbon precursors were mixed 4/6 by weight.Its mixtures of Phenol or Imide were cured in inert atmosphere and then ground to powders.Its mixtures thus prepared were heated at a temperature of 900 °C for 1 h in a flow of N 2 .Heating rate was 5 °C/min.After carbonization, the mixture was stirred in diluted sulfuric acid (3.5 mol/L) to dissolve out MgO.Its slurry was repeated suction filtration and water rinse until the pH of its filtrate showed 5 to 7. Finally carbons were recovered as powders after drying at 150 °C for 96 h in a flow of air.Illustration of the preparation method is shown in Fig. 1.

Highly crystalline mesoporous carbon
As-prepared carbon powders were heat-treated at 1800, 2200, 2500 °C for 1 h in Ar atmosphere for graphitization.
The obtained carbons were characterized by XRD, TEM, Thermogravimetry, and N 2 adsorption analysis.BET surface area was determined from the adsorption isotherms of N 2 gas measured at 77 K. Pore size distribution was evaluated by BJH method and HK method.Oxidation loss temperature was estimated by thermogravimetry in air up to 900 °C.

Non-crystalline mesoporous carbon
The carbons derived from PVA and Phenol had similar N 2 adsorption isotherms as seen in Fig. 2. BET surface areas and pore volumes are listed in Table 1.The pore size distributions by BJH method are shown in Fig. 3.The as-prepared carbons have high mesopore volume of 0.88-1.70ml/g, with micropore volume of 0.32-0.48ml/g slightly depending on the carbon precursor.The pore size at the peak distribution is about 11 nm on each carbon precursor.It's the same about crystallite size of MgO.This result suggests that carbons were coated along crystallite of MgO.TEM images of carbons derived from phenol and imide are shown in Fig. 4(a) and (b).Pores are formed with amorphous carbon layers.The porous structure seemed like inter-connected three dimensionally.Thus, pore volume parameters depend slightly on three different carbon precursors, but the pore size at the peak distribution depends on crystallite size of MgO, not carbon precursors.

Highly crystalline mesoporous carbon
BET surface area and pore volumes of mesoporous carbons are shown in Fig. 5 and Table 2. Pore size distributions measured by BJH method are shown in Fig. 6.The heat-treatment at 1800 °C gave only a small change in pore structure; no change in mesopore volume, a slight decrease in micropore volume, and consequently small decreases in total pore volume and BET surface area.Pore size profiles also nearly unchanged after heat-treatment at 1800 °C.However, it has to be pointed out that these changes show a sharp contrast to some commercially available activated carbons, where marked collapsing of micropores has been observed (Inagaki M. et al., 2014).After the heat-treatment above 1800 °C, all pore parameters decreased markedly.
But the peak positions at around 11 nm didn't shift even after pore volume decreased relatively.These results suggest that mesopore structures didn't collapse by heat-treatment despite decreasing mesopore volumes for some reasons such as forming closed pore by shrinking of micropores.Thus their heat-treatment temperature dependences are very similar.Fig. 5 BET surface areas and pore volumes on each carbon precursor.
(002) X-ray peak profiles of carbons derived from PVA are shown depending on each heat-treatment temperatures in Fig. 7. Obvious (002) peak was observed only the carbons heat-treated at 2500 °C.This peak profile can be separated into several peaks.These peak profiles suggest the formation of structural components having different crystallinity.The similar peak profile was also observed on the carbons derived from magnesium citrate (Orikasa H. et al., 2012).( 002) peak profiles of carbons heat-treated at 2200 °C and 2500 °C are shown in Fig. 8(a) and (b).
On the same heat-treatment temperature, (002) peak profiles depend on carbon precursors.
Thermogravimetry profiles in air are shown in Fig. 9.The beginning temperatures of oxidation of as-prepared carbons depended on carbon precursors.When reaching at about 600 °C, as-prepared carbons were almost entirely oxidized.But the carbons heat-treated at 1800 °C didn't start oxidation at about 600 °C.In addition, the TG profiles were almost the same on each carbon precursor.The starting temperatures of oxidation of carbons heat-treated above 2200 °C almost didn't shift to high temperatures despite the crystalline growth as shown in Fig. 5.
TEM images of carbons derived from polyimide and phenol resin are shown in Fig. 10 and Fig. 11, respectively.As seen in these images, before the heat-treatment, amorphous layers were observed.And pores were seemed like inter-connected in three-dimensionally.After the heat-treatment at 1800 °C, stacked layers were observed.On the other hand, pore structures were just a little changed.The carbons heat-treated above 2200 °C were more graphitized and pores were still inter-connected.Thus, the inter-connected pore structures still existed continuously after heat-treatment despite carbon walls was crystallized.Pore volumes of carbons derived from MgO-templated carbonization process didn't change after the heat-treatment at 1800 °C.Crystal growth of carbons wasn't observed by XRD analysis.But the results of TEM and TG analysis suggest that carbon wall was crystallized.These results suggest that both the high porosity and good characteristics by crystallization of carbon walls can be obtained by heat-treatment at 1800 °C.

Conclusions
Carbon coating on MgO was successfully processed using the mixtures of carbon precursors with MgO precursors prepared by physical mixing.BET surface area of the carbons isolated from MgO by dissolving out was very high, such as over 1000 m 2 /g, even though activation processes were not applied.The carbons obtained through ten-odd crystallite size MgO were rich in mesopores, of which sizes were almost the same as MgO particles.
Furthermore, we found that pore structure of asprepared MgO-templated carbon depends slightly on the carbon precursor, but their heat-treatment temperature dependences are very similar.And MgO-templated carbons heat-treated at 1800 °C satisfy both high porosity and oxidation resistance on each carbon precursor.The surface area was kept to 70-80 % even by the 1800 °C treatment.This would be due to the characteristic morphology and structure of the MgO-template carbon.
These results will be useful to pursue the method of preparation for highly porous carbon materials with a high thermal stability.The porous carbon having huge mesopores will also provide an unprecedented new function to various applications.

Fig. 1
Fig. 1 Illustration of preparation method of porous carbons.

Table 1
BET surface areas and pore volumes

Table 2
BET surface areas and pore volumes on each carbon precursor and heat-treatment temperatures Pore size distribution by BJH method on each carbon precursor.