Article pubs.acs.org/JPCC
Experimental and Theoretical Studies of Hydrogen/Deuterium Spillover on Pt-Loaded Zeolite-Templated Carbon Hirotomo Nishihara,*,† Somlak Ittisanronnachai,† Hiroyuki Itoi,†,‡ Li-Xiang Li,† Kimichi Suzuki,§ Umpei Nagashima,§ Hiroshi Ogawa,§ Takashi Kyotani,† and Masashi Ito†,∥ †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Department of Applied Chemistry, Aichi Institute of Technology, Yachigusa 1247, Yakusa-cho, Toyota 470-0392, Japan § Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8568, Japan ∥ Advanced Materials Laboratory, Nissan Research Center, Nissan Motor Company, Ltd., Yokosuka 237-8523, Japan ‡
S Supporting Information *
ABSTRACT: Hydrogen storage in metal-doped carbons through a spillover mechanism has attracted a great attention. However, the data reported so far are lacking in consistency among different research groups, and the mechanism has not been fully revealed yet. In this work, we prepare a model Pt-loaded carbon by a simple and reproducible method in which alreadysynthesized Pt-nanocolloid is directly doped on zeolite-templated carbon. The Pt-loaded carbon thus obtained provides reliable data as for the temperature effects (273−353 K) on hydrogen/deuterium (H/D) adsorption isotherms, which contain the contributions of H 2 /D 2 physisorption, H/D chemisorption on Pt surface, and H/D storage by the spillover mechanism. We extracted the last contribution (spillover H/D) from the isotherms, and found that the amount of the spillover H/D increases with increasing pressure (up to 100 kPa) and temperature (273− 353 K). Detailed analysis of the number of H/D atoms stored by the spillover mechanism reveals that H/D radicals spilling from the Pt surface migrate on the carbon surface. The path integral molecular dynamics simulation also demonstrates the migration of atomic H/D on a model fragment of the zeolite-templated carbon, and suggests the enhancement of migration at higher temperature. stored.18,19 Yang’s group has intensively investigated the hydrogen storage of metal-doped ZTCs and observed the enhanced effect through this mechanism.20−22 We have also prepared Pt-loaded ZTC and examined the storage capacity.11 However, the enhancement was not as remarkable as that reported by Yang’s group. A lot of other groups have also examined hydrogen storage of metal-doped porous materials so far. Some of them have reported significant enhancement,23−29 but some others have observed a slight or no enhancement30−33 at room temperature. One of the reasons for the disagreement may be due to the differences in metal structures, metal surface conditions, and carbon structures.34 Additionally, the difficulty of the volumetric hydrogen sorption measurements sometimes causes serious error in experimental results.35 Thus, the mechanism of spillover storage is still far from full understanding, and further investigation from a fundamental point of view is significantly important.36−43 Recently, we have investigated the mobility of H• attached on a model fragment of ZTC (buckybowl C36H12, in Figure
1. INTRODUCTION Hydrogen-powered fuel cell vehicles are one of the nextgeneration automobiles that use an energy source other than crude oil derivatives. Most current fuel cell vehicles adopt a high-pressure (typically 35 or 70 MPa) hydrogen tank, which is a bulky and heavy cylinder made from costly carbon fibers, and therefore significant improvement for on-board hydrogen storage system is highly required.1−4 One promising method is to embed hydrogen storage materials into a hydrogen tank to store an adequate amount of hydrogen even at lower pressure. Among hydrogen storage materials, nanoporous materials, especially carbons, could be an interesting choice from ease of hydrogen release and long cycle life.5−14 We have previously demonstrated that zeolite-templated carbon (ZTC),15−17 an ordered microporous carbon having a unique framework structure (Figure 1a), exhibited hydrogen storage capacity of 2.2 wt % at 303 K and 34 MPa, due to its very high surface area (>3000 m2 g−1) and suitable pore size (1.2 nm).11 In addition to physisorption, hydrogen spillover18,19 could further enhance hydrogen storage amount. In the spillover mechanism, hydrogen molecules dissociatively adsorb on the surface of metal particles, such as Pt, and then hydrogen radicals (H•) move to the carbon surface and are reversibly © 2014 American Chemical Society
Received: February 17, 2014 Revised: April 14, 2014 Published: April 16, 2014 9551
dx.doi.org/10.1021/jp5016802 | J. Phys. Chem. C 2014, 118, 9551−9559
The Journal of Physical Chemistry C
Article
hydrogen/deuterium spillover. In addition, the experimental data thus obtained are compared to the results of the PIMD simulation.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. ZTC was synthesized with the method reported elsewhere.11 A composite of polyfurfuryl alcohol and NaY zeolite was gradually heated to 973 K under N2 flow. Subsequently, a chemical vapor deposition of propylene (7 vol % in N2) was performed for 2 h at 973 K, followed by a heat-treatment at 1173 K for 3 h under N2 flow. Next, the zeolite template was dissolved away by HF etching to obtain ZTC. Pt-doping into ZTC was carried out by a very simple method, in which ZTC was just mixed with an aqueous solution of Pt nanocolloid (PtNC; the concentration of Pt is 200 ppm). The PtNC solution was purchased from Nippon Sheet Glass Co. Ltd. About 0.3 g of well-dried ZTC was mixed with 50 mL of the PtNC solution, and the resulting mixture was stirred at 298 K for 15 h. The mixture then was dried up stepwise under a reduced pressure at 318 K for 1 h, at 353 K for 1 h, and finally at 423 K for 6 h. The stoichiometric Pt content is 3.23 wt %, and the inductively coupled plasma analysis revealed that the actual Pt content in PtNC/ZTC was 3.2 wt %, almost the same as the stoichiometric one. The present Ptloading method enables highly reproducible sample preparation. The Pt-loading amount was not optimized for achieving the highest hydrogen uptake in this work, because the purpose of this work is to understand the basis of the spillover mechanism. 2.2. Characterization. Nitrogen physisorption measurements were carried out at 77 K using a volumetric sorption analyzer (BEL Japan, BELSORP-mini). The specific surface areas (Sα) were calculated by the subtracting pore effect method47 because the amount of H2 physisorption in porous carbon is better correlated with Sα rather than BET surface area.11 Powder X-ray diffraction (XRD) patterns of ZTC and PtNC/ZTC were recorded with an X-ray diffractometer (Shimadzu, XRD-6100) with Cu Kα radiation generated at 30 kV and 20 mA. PtNC/ZTC was observed with a transmission electron microscope (TEM; JEOL, JEM-2010). H2 and D2 isotherms at 273, 298, 323, and 353 K were measured with a static volumetric technique (using BEL Japan, BELSORP-max). Isotherm measurements were performed at low-pressure region (
