A study on pore formation of high surface area activated carbon prepared by microwave-induced plasma with KOH (MiWP-KOH) activation: Effect of temperature-elevation rate

https://doi.org/10.1016/j.cep.2021.108511Get rights and content

Highlights

  • Fast pore formation to produce activated carbon (AC) by MiWP-KOH activation.

  • High temperature-elevation rate in MiWP-KOH activation for minute-time AC synthesis.

  • Micropore domination in AC synthesized by MiWP-KOH activation.

Abstract

The activation using microwave-induced plasma with KOH (MiWP-KOH activation) can realize extremely fast synthesis of activated carbon (AC), which needed only several minutes. The present work focusses the effect of temperature-elevation rate on pore formation. For the discussion, AC was prepared using a conventional heat conduction method with various temperature-elevation programs, and the structural analysis results from these experiments were compared with those observed in AC prepared by the MiWP-KOH activation. As a feature in the pore structure of AC induced by the MiWP-KOH activation, micropore volume highly dominates over mesopore volume with (micropore/mesopore) ratio = 4.3–4.9. This property can be explained by the experiments with the controlled temperature-elevation rate, suggesting that rapid temperature-elevation rate is the key factor for such micropore-dominating structure. Additionally, it was suggested that the temperature of the feed material in the MiWP-KOH activation can become about 800 °C within 75 s. MiWP-KOH activation can generate significantly higher surface area than the conventional heating with the similar temperature changing program, suggesting the effect of microwave irradiation to enhance the pore formation. Based on these investigations, a characteristic pore formation model in the MiWP-KOH activation is considered.

Introduction

High surface area activated carbon (AC) is a carbon material that has specific surface area from 1000 to 3000 m2 g  1. It consists of micropores (pore diameter < 2 nm), mesopores (2 nm < pore diameter < 50 nm), and macropores (pore diameter > 50 nm) [1], [2], [3], [4], [5]. Due to its porous structures and high surface area, AC has been used in various chemical processes. It can serve as adsorbents for water treatment [6], biogas purification [7], carbon dioxide (CO2) purification [8], and catalyst supports for biodiesel production [9], methane reformation [10], and so forth.

High surface area AC can be produced using a variety of activating agents, for example, gas activating agents such as steam [11], [12], [13] and CO2 [14], [15], or chemical activating agents such as zinc chloride (ZnCl2) [16], sodium hydroxide (NaOH) [17], [18], potassium hydroxide (KOH) [19], [20], and potassium carbonate (K2CO3) [21]. To prepare high surface area AC in conventional ways, feed carbon materials must be mixed with the activating agents and heated at the high temperature for several hours or days [12,[22], [23], [24]]. The long activation time limits the AC preparation process to be operated in batch systems, and causes the difficulty to maintain large-scale production. Therefore, the activating process must require a long period to startup and shutdown [25]. If these limitations are overcome, namely if activation period can be substantially short, the process can be changed from batch to continuous system to produce AC.

It was reported that some kinds of plasma such as dielectric barrier discharge plasma [26], [27] and microwave-induced plasma [28], [29] have been used to modify surface structures of AC. Microwave-induced plasma can be made by irradiating microwave to electrically-conductive materials with specific structures such as metals with sharp-edge or porous carbon materials. In case of the porous carbon materials, it can facilitate plasma generation because electrons can be discharged in microporous structures [30].

A previous study reported that microwave-induced CO2 activation within 90 min can be used to prepare AC that had specific surface area of 1036 m2 g  1, where plasma generation was not reported [31]. In contrast, the present work uses plasma-generating microwave system, so-called Micowave-Plasma-KOH (MiWP-KOH) activation system, aiming to produce AC of which surface area is approaching to 2000 m2 g  1 with the drastically-reduced activation time within 80–330 s [32]. For its short reaction time, the results are expected to cause a revolutionary change in the reactor design concept by changing AC preparation process from bath to continuous system. To develop a new reactor system using MiWP-KOH activation, the mechanism of this method must be investigated.

Temperature-elevation rate can be one of the key factors governing the pore formation in AC prepared by MiWP-KOH activation because microwave-induced plasma is expected to rise-up the temperature of reacting carbon material rapidly. Therefore, the effect of temperature-elevation rate on the pore formation of AC was investigated in this work. To discuss this effect, the pore structure analysis on the AC prepared by MiWP-KOH activation is compared with the results obtained from the activation by conventional heat conduction method. Here, the micropores and mesopores are focused to investigate because these pores mainly contribute to realize large surface area and important adsorption characteristics.

Section snippets

Preparation of precursor (Carbonized Carbon Gel)

In this research, Carbonized Carbon Gel (CCG) was used as a precursor material to prepare AC. The synthetic carbon gel was prepared as reported by Mukai et al. [33]. In short, phenol and formaldehyde were mixed in distillated water in the presence of sodium carbonate, which served as a catalyst. Phenol concentration was fixed at 500 mol m  3. Molar ratio of phenol to formaldehyde and sodium carbonate were set at 0.5 and 1.5, respectively. This mixture was stirred well before being kept at

N2 sorption isotherms

Fig. 2 illustrates N2 adsorption-desorption isotherms of AC prepared using different heating methods. Fig. 2(a) presents the sorption isotherms of AC prepared by MiWP-KOH activation using microwave-activation time of 40–600 s. The results show that the volume of N2 adsorbed at low relative pressure tends to increase with the increase of microwave-activation time. The adsorption of N2 at low relative pressure here indicates the formation of highly microporous structures [38], [39]. The results

Conclusions

This study investigates the pore formation of high-surface area AC prepared using MiWP-KOH activation by observing the effects of temperature-elevation rate on pore structure of AC. It is revealed from N2 adsorption-desorption isotherms that micropore and mesopore volume and BET surface area of AC can increase as the microwave-activation time increases. In addition, microporous structures become more dominant as the microwave-activation time is longer.

By the experiment using conventional

Author contribution

Purichaya Kuptajit worked as main researcher to make plan, to conduct experiment, to make calculation, and make draft of manuscript.

Noriaki Sano supervised Purichaya Kuptajit. He revised the draft to complete writing the paper. He deepen the discussion in the paper.

Kyuya Nakagawa supported the discussion about adsorption analysis.

Tetsuo Suzuki supported the calculation analysis.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Asahi Glass Foundation (AF) and the Japan ASEAN Science, Technology and Innovation Platform (JASTIP) under the Japan Science and Technology Agency (JST).

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