Fabrication of Highly Dispersed Pt Nanoparticles in Tubular Carbon
Mesoporous Materials for Hydrogen Energy Applications
Shou-Heng Liu,aRong-Feng Lu,b Shing-Jong Huang,a An-Ya Lo,aWen-Hua Chen,a Wen-Yueh Yu,b Shu-Hua Chien,band Shang-Bin Liua,*
a
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan b
Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan
Tel: +886-2-2366-8230, Fax: +886-2-2362-0200, E-Mail: [email protected]
A novel route for synthesizing carbon mesoporous materials (CMMs) with well-dispersed, highly stable platinum nanoparticles (2-3 nm) favorable for applications as carriers for hydrogen storage and as supported electrodecatalysts for fuel cell (FC) has been developed.
1. INTRODUCTION
The R&D of safe and effective hydrogen storage systems is one of the key issues in future realization of hydrogen energy applications, particularly in commercialization of fuel cell vehicles [1]. Among various adsorbents, nano-structured carbon materials are promising candidates for hydrogen storage due to their high surface area and light-weight characteristics, however, no available storage materials are capable of meeting the US-DOE target (6.5 wt%) for commercial exploitation at present [2]; a further 40-50% increase in storage capacity is needed for currently available carbon-based materials, such as carbon nanotubes, activated carbon and carbon nanofibers [3]. Recent development in carbon mesoporous materials (CMMs) have drawn some attention in R&D [4] owing to their potential applications as catalytic supports [5], hydrogen fuel storage [3b,6], and fuel cells [7]. In particular, fabrications of metal-incorporated CMMs, which normally invoke methods such as adsorption [5], impregnation [7], and ion-exchange [8], also received much attention. However, these conventional synthesis routes normally lead to uncontrollable growth of metal-particles particularly in terms of their sizes and shapes. We report herein a novel synthesis route to fabricate tubular CMMs with well-dispersed Pt nano-particles studded on the pore walls. Their performances on hydrogen adsorption, carbon monoxide tolerance, and cyclic voltammetry test were also investigated.
2. EXPERIMENTAL
The SBA-15 template used in this work was synthesized following the procedures reported earlier [9]. Platinum-incorporated CMMs (Pt-CMMs) were prepared by mixing ca. 0.5 g of calcined SBA-15 template with desirable amounts of platinum acetylacetonate (98%, Acros), furfuryl alcohol (FA) (98%, Acros), trimethylbenzene (TMB) (98%, Acros), and oxalic acid (98%, Acros). The latter was used as the acid catalyst for polymerization of carbon sources, which was carried out first at 333 K then 353 K for 12 h under air. The resultant composite was treated at 423 K for 3 h under vacuum, followed by raising the temperature to 573 K at a rate of 1 K/min, then with a rate of 5 K/min to 1073 K and kept for 3 h. The resultant black powders were leached with HF (1%) aqueous solution for at
Figure 3 displays the cyclic voltammograms for methanol oxidation activities of a commercial Johnson Matthey PtC catalyst (20 wt% Pt on Vulcan XC-72) and the Pt-CMM-11.5 sample. It is indicative that the our sample, which possesses well-dispersed, highly stable Pt nano-sized particles on porous carbon supports, exhibits catalytic activities surpassing that of the commercial Johnson Matthey catalyst. In particular, a lower oxidation peak potential was observed. In terms of the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib), the If/Ib
ratio obtained for the Pt-CMM-11.5 and the commercial Johnson Matthey PtC were found to be 4.23 and 1.03, respectively, indicating that the latter is more vulnerable to coking by carbonaceous deposits [12] and less tolerance towards CO poisoning.
4. CONCLUSIONS
By incorporating carbon-rich metal presursor, such as Pt(CH(COCH3)2), as co-feeding carbon
source during replicated synthesis using mesoporous silicas with well-defined pore sizes as templates, tubular CMMs with well-dispersed metal nanoparticles and tailored sizes can be obtained. These metal-CMMs, which can be easily fabricated with controllable loading even with multifunctional metal characteristics, were found to possess high surface areas, highly accessible and stable active sites, improved hydrogen adsorption capacities, and superior electrocatalytic properties. Moreover, in view of the high metal dispersion that favor reduction of metal required to reach the same catalytic activity per unit mass, these novel metal-CMMs should render future practical and cost-effective commercial applications in hydrogen-energy related areas, for examples, as adsorbents for hydrogen fuel storage and as supported electrocatalysts for proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs).
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