Enhancement of Hydrogen Storage Capacity of Zeolite- Templated Carbons by Chemical Activation Muthukrishnan. I Sevilla, M.; Alam, N.; and Mokaya, R. J. Phys. Chem. C 2010, 114, 11314–11319

Introduction Porous carbons that have high surface area and well-ordered pore systems are potentially useful as hydrogen storage materials Zeolite-templated carbons (ZTCs) are well-ordered porous materials with tailored structural characteristics, including controlled pore size, surface area, pore volume, particle size,and morphology However, the rigid framework of zeolites and narrow range of pore wall thickness rather limit the extent to which the porosity of ZTCs can be varied. Activation of carbons via physical or chemical methods is a well-established method for generating activated carbon materials with a high surface area, pore volume Chemical activation with potassium hydroxide may, however, in some cases, generate carbon materials with enhanced porosity and uniform pores via the control of activation parameters (carbon/KOH ratio, activation temperature, heating rate, activation time, and gas flow rate) Here the effects of chemical activation (with KOH) on the textural properties of ZTCs, paying particular attention to changes in pore size distribution and how these changes relate to the hydrogen storage capacity of the activated ZTCs

Preparation of Activated Zeolite templated carbon materials 0.6 g of zeolite Y in an alumina boat zeolite/carbon composite 900°C, 3 h in flow of argon only / cooling to room temperature under argon 800 °C under a flow of argon / Maintained 3 h under a flow of argon saturated with acetonitrile to allow CVD Zeolite templated carbon (ZTC) washed with 10% (HF) acid several times / refluxed at 60 °C in concentrated HCl / oven-dried at 120 °C. mixed with KOH (ZTC/KOH weight ratio of 1:4) 800 °C in N 2 flow for 1 h /washed several times with 2 M HCl and distilled water /oven-dried at 120 °C. Activated ZTC

Figure 1. Powder XRD patterns of zeolite-templated carbon (111) (002) ZTC1,ZTC2, and ZTC3 for CVD temperatures of 800, 850, and C respectively. 850 (ZTC4) and C (ZTC5) zeolite template was in contact with the acetonitrile during both the temperature ramping and the CVD steps

Figure 2. Nitrogen sorption isotherms of zeolite-templated carbons before (a) and after (b) chemical activation with KOH

Figure 3. Pore size distribution curves of zeolite-templated carbons before (solid symbols) and after (open symbols) chemical activation with KOH

TABLE 1: Textural Properties and Hydrogen Uptake Capacity of Zeolite-Templated Carbons (ZTCs) and Their Activated Derivatives (Ac-ZTC)

Figure 4. Hydrogen uptake isotherms of zeolite-templated carbons before (solid symbols) and after (open symbols) chemical activation with KOH: (a) ZTC3, (b) ZTC2, (c) ZTC1, (d) Ac-ZTC3, (e) Ac-ZTC2, and (f) Ac-ZTC1

Figure 5. Plot of hydrogen storage capacity as a function of (A) surface area or (B) pore volume of zeolite-templated carbons before (o) and after (●) chemical activation with KOH.

Conclusions Chemical activation (with KOH) of ZTCs can be used to prepare activated (Ac- ZTC) porous carbon materials with an enhanced and tailored porosity. The KOH activation (at C for 1 h) and a carbon/KOH weight ratio of 1:4 generate carbons with a surface area in the range of m 2 /g and a pore volume of between 1.5 and 1.75 cm 3 /g. These values represent increases of up to 84% in surface area and a doubling of pore volume. An increase in the proportion of pores with sizes between 1.5 and 3.5 nm, via creation of new pores with a size of 2.2 nm and slight expansion of already existing 2.2 nm pores. The size of “new” pores generated by activation depends on the level of graphitic ordering in the ZTC with high graphitization favoring formation of larger pores. The increase in hydrogen uptake is strongly linked to rises in surface area and pore volume.