Hydrogen adsorption on modified activated carbon

international journal of hydrogen energy 35 (2010) 2777–2780

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Hydrogen adsorption on modified activated carbon Chen-Chia Huang*, Hsiu-Mei Chen, Chien-Hung Chen Department of Chemical and Materials Engineering, National Yunlin University of Science and Technology, 123 University Road, Section 3, Douliu, Yunlin, Taiwan, ROC

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abstract

Article history:

The aim of this work is to investigate hydrogen adsorption on prepared super activated

Received 23 March 2009

carbon (AC). Litchi trunk was activated by potassium hydroxide under N2 or CO2 atmo-

Accepted 5 May 2009

sphere. Nanoparticles of palladium were impregnated in the prepared-AC. Hydrogen

Available online 3 June 2009

adsorption was accurately measured by a volumetric adsorption apparatus at 77, 87, 90 and 303 K, up to 5 MPa. Experimental results revealed that specific surface area of the prepared-

Keywords:

AC increased according to KOH/char ratio. The maximum specific surface area reached up

Hydrogen storage

to 3400 m2/g and total pore volume of 1.79 cm3/g. The maximum hydrogen adsorption

Activated carbon

capacity of 2.89 wt.% at 77 K and under 0.1 MPa, was obtained on these materials. The

Adsorption

hydrogen adsorption capacity of the 10 wt.% Pd-AC was determined as 0.53 wt.% at 303 K

Microporosity

and under 6 MPa. This amount is higher than that on the pristine AC (0.41 wt.%) under the same conditions. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Carbonaceous material has attracted attention as a potential adsorbent in hydrogen storage. The advantages of carbonaceous materials for hydrogen storage are fast adsorption/ desorption kinetics and low weight. Activated carbons with high surface area revealed a good capability in hydrogen storage, compared to other carbonaceous materials such as carbon nanotubes and carbon nanofibers [1]. For physisorption, porous texture of carbonaceous materials is a critical factor. Pores with two [2] or three [3] times diameter of hydrogen molecule were demonstrated as the optimum size for hydrogen storage, because high gas density was obtained in the narrow pores from theoretical calculations. Similar studies have been reported elsewhere [4–6]. Furthermore, hydrogen capacity exhibits a linear correlation with specific surface area and micropore volume of carbonaceous materials [7–10].

Surface complexes onto carbon samples were also considered in hydrogen adsorption. Agarwal et al. [11] reported that hydrogen capacity increased with increasing amounts of oxygen group on ACs. On the contrary, some papers [12,13] indicated that oxygen functional groups repressed hydrogen uptake on ACs. Furthermore, hydrogen storage in carbonaceous material by hydrogen spillover has been demonstrated by previous papers [14–16]. Hydrogen spillover not only improves hydrogen storage but also increases initial hydrogen adsorption kinetics. In this study, the ‘‘litchi trunk’’-based AC samples with high porosity were prepared using KOH activation method. The oxidized-AC and Pd-AC samples were prepared by acidic oxidization and wetness impregnation, respectively. Hydrogen storage measurements were performed by a volumetric apparatus. Effects of porous texture, functional groups and catalyst of ACs on hydrogen adsorption were investigated.

* Corresponding author. Tel./fax: þ886 5 534 2601x2500. E-mail address: [email protected] (C.-C. Huang). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.05.016

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2.

international journal of hydrogen energy 35 (2010) 2777–2780

Experimental

‘‘Litchi trunk’’ as a precursor was used to produce the ACs with high porosity in this study. Carbonization of ‘‘litchi trunk’’ was performed at 823 K for 2 h in nitrogen flowing of 1000 mL/min. Prior to the activation process, the mass ratios of char to KOH agents were set as 1:0.5, 1:1, 1:2 and 1:4. The mixture samples were activated at 1073 K for 2 h under N2 or CO2 atmosphere. After KOH activation, the activated products were washed by diluted HCl solution to remove potassium species. The asprepared AC was obtained after filtration and dried at 378 K. The as-prepared ACs were oxidized by acidic treatment to create the oxygen functional groups on the surface of carbon. Two grams of the as-prepared ACs were immersed in 100 mL of different concentrations (1 N or 3 N) of HNO3 solution under the temperatures of 303 K, 323 K and 373 K for 24 h. More deionic water washed the acidic AC samples to remove residual acids, referred to as the oxidized AC (O-AC). The AC samples were mixed with 10 wt.% Pd by a wetness impregnation method, referred to as the Pd-AC. The porous texture of activated carbon was characterized by physical adsorption of nitrogen at 77 K in a gas adsorption apparatus (Quantachrome Autosorb-1). The acidic and basic functional groups onto the AC samples were quantified by a Boehm titration method [17]. The hydrogen adsorption in all ACs was accurately measured by a volumetric adsorption apparatus. Prior to measurement, the samples were outgassed at 473 K for 3 h under vacuum. Moreover, the Pd-AC reduction was carried out at 473 K for 3 h in a flowing of H2, and then outgassed at 473 K for 3 h under vacuum.

3.

Results and discussion

The ‘‘litchi trunk’’ based ACs was prepared by KOH activation. The result indicated that specific surface area and micropore volume of ACs enhanced with increased the mass ratio of KOH to char. The enhancement of AC porosities under activation condition with CO2 atmosphere revealed more apparently than that with N2. Similar results have been reported elsewhere [18]. The highest porosity of ‘‘litchi trunk’’ based ACs in this study was obtained as 3400 m2/g of specific surface area and 1.46 cm3/g of micropore volume. The specific surface area (micropore volume) of AX-21 and Maxsorb as commercially activated carbons were 2513 m2/g (0.93 cm3/g) and 3178 m2/g (1.20 cm3/g), respectively [19]. From hydrogen adsorption experiments, the type I isotherm of the IUPAC classification [20] was obtained at 77– 90 K and below 0.1 MPa. It is noted that hydrogen storage on ACs was assigned to microporosity adsorption [20]. According to the Clausius–Clapeyron equation, the isosteric heat of adsorption in a range of 5.6–7.9 kJ/mol was found from these AC isotherms at 77–90 K. At 298 K and under pressure up to 6 MPa, the isotherm exhibited a linear relation between hydrogen capacity and pressure. Fig. 1 shows the relation between hydrogen capacity and micropore volume for the asprepared AC samples. The linear relation between hydrogen capacity and micropore volume was found, confirmed by previous paper [8].

Fig. 1 – Effect of micropore volume on hydrogen capacity of CNTs at 77 K and under 0.1 MPa.

Effect of surface functional groups on hydrogen storage was presented in Fig. 2. It is noted that hydrogen capacity of the O-AC samples significantly suppressed with increasing the acidic group amounts (oxygen-containing functional groups). The basic group amounts were independent of the hydrogen capacity of ACs. In this work, hydrogen isotherm data on ACs (as-prepared and oxidized) were examined by the Dubinin–Astakhov (D–A) equation [21]. q¼

  n  W A ; A ¼ RTlnðPs =PÞ ¼ exp  Wo E

Fig. 2 – Effect of surface functional groups on hydrogen storage at 77 K and 0.1 MPa.

(1)

international journal of hydrogen energy 35 (2010) 2777–2780

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where q is the adsorption ratio or the filling degree, W is the adsorbed volume(cm3/g), W0 is the limiting micropore volume (cm3/g), E is the characteristic energy of the adsorption system, A is the adsorption potential, and n is the heterogeneity factor. Dubinin’s approach [22] was used to estimate the hydrogen pseudo-vapor pressure (Ps) under supercritical conditions as: Ps ¼ ðT=Tc Þ2 Pc

(2)

The adsorbed phase specific volume was estimated by Eq. (3) proposed by Ozawa et al. [23]: Va ¼ Vs ðTb Þexp½0:0025ðT  Tb Þ

(3)

where Vs(Tb) is the molar volume of the liquid adsorbate at the boiling point, Vs is 25.87 cm3/mol for hydrogen [24]. From these O-AC hydrogen isotherms, hydrogen capacity of the O-AC samples obviously reduced with increasing acidified AC degrees. Fig. 3 reveals parameters of the D–A equation for ACs with different acidic group amounts. The characteristic adsorption energy insignificantly changed in the ACs with few acidic group amounts. When the acidic groups increased to 0.9 mmol/g, the characteristic adsorption energy enhanced from 3.5 kJ/mol to 4.0 kJ/mol, even up to 4.7 kJ/mol. The results implied that stronger interaction between oxygen– hydrogen molecules compared to that between carbon– hydrogen ones. Furthermore, the micropore volume, W0 reduced along with increasing of acidic group amounts. The cause was attributed to the steric hindrance effect of acidic groups onto the AC surface leading to diminish the chance of hydrogen molecules entering into the micropores. Other possibility was the microporosity of the ACs destruction after excess-oxygenation treatment. However, the acidic functional groups on the AC surface were negative for hydrogen adsorption. Obviously, the microporous structure in carbon was a critical factor for hydrogen storage. Fig. 4 shows isotherms of the Pd-AC and the Pd-free AC samples at 303 K and under pressure up to 6 MPa. The results

Fig. 4 – Hydrogen isotherms of Pd-AC and Pd-free AC at 303 K and under pressure up to 5 MPa.

indicated that the H2 capacity of AC and OAC samples after Pd modification enhanced from 0.41 and 0.32 wt.% to 0.53 and 0.45 wt.%, respectively. The catalyst effect of the carbonaceous material was positive on hydrogen storage, due to the hydrogen spillover occurrence [13,16]. It is also found that hydrogen storage by spillover in AC samples was insignificantly affected by the presence of oxygen functional groups.

4.

Conclusions

In this study, hydrogen storage on ‘‘litchi trunk’’ based ACs was investigated. The hydrogen capacity of the ACs was strongly depending on the micropore volume. The hydrogen storage on the ACs was suppressed by the presence of acidic groups, due to the steric hindrance effect. The catalyst effect was a way to improve hydrogen storage in ACs.

Acknowledgements This work was financially supported by the National Science Council, Taiwan, Republic of China, under contract (NSC 962221-E-224-036-MY2).

references

Fig. 3 – Parameters of D–A equation for the ACs with different amounts of acidic groups.

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