Synthesis and characterization of CMK porous carbons modified with metals applied to hydrogen uptake and storage

MARCOS B. GÓMEZ COSTA; JULIANA M. JUÁREZ; OSCAR A. ANUNZIATA

Microporous and Mesoporous Materials

Intech Europe

Lugar: Rijeka; Año: 2016; p. 51 - 85

Introduction Hydrogen is considered as a clean energy carrier and an alternative fuel source for many applications. Some keys to use hydrogen, as fuel is its safe storage, low cost, lightweight, and reversible, with simple adsorption-desorption kinetics. However, a common limitation is found in weak van der Waals interaction between molecular hydrogen and host material. Hence, to enhance hydrogen binding, low amounts of transition metals or cations are added improving hydrogen sorption.Carbons mesostructured from Korea (CMK) comprise one such family of ordered mesoporous carbons (OMC). Produced inside the channels of mesostructured silicates or aluminosilicates, CMK have high specific surface areas and pore volumes. Thus, CMK-1 and CMK-3 were chosen as an ideal candidate as a support material for hydrogen storage due to its large surface area, high chemical stability, uniform pore diameter, accessible porosity and three-dimensional conducting network.Ordered porous carbon designated as CMK-1, synthesized by replication from MCM-48 silica, and ordered porous carbon referred to as CMK-3, synthesized by replication from SBA-15 silica, were modified with metals and metals oxides in order to gain higher hydrogen storage capacity. Pt-CMK-3, Zn-CMK-1, Ni-CMK-1, TiO2-CMK-3 and V-CMK-3 have been reported as promising hydrogen storage nanomaterials. The adsorption in these materials was fully reversible and higher than that in carbon hosts. ExperimentalPreparation of MCM-48 and SBA-15 templatesThe mesoporous silicate MCM-48 was synthesized by hydrolysis of tetraethylorthosilicate (TEOS, 98%, Sigma?Aldrich) at room temperature, in an aqueous solution, using cetyltrimethylammonium bromide (CTAB, Sigma?Aldrich) as a surfactant.Ordered mesoporous silica SBA-15 was prepared by using the triblock copolymer, (EO20PO70EO20, P123-Sigma?Aldrich), as the surfactant and tetraethyl orthosilicate (TEOS, Sigma?Aldrich, reagent grade 98%) as the silica source.Synthesis of CMK-1 and CMK-3 mesoporous carbonsMesoporous carbons CMK-1 and CMK-3 were prepared by carbonization of sucrose as carbon precursor. First, sucrose is adsorbed into the channels of silica templates (MCM-48 and SBA-15) following of carbonization process, finally pure carbonaceous product was obtained by dissolving the silica-template with HF.Synthesis of Zn-CMK-1, Ni-CMK-1, Pt-CMK-3, V-CMK-3 and TiO2-CMK-3 composites materialsMetal/cations nanoparticles were incorporated into ordered mesoporous carbons (CMK-1 or CMK-3) by wetness impregnation using different metals source (Zn (NO3)2.6H2O, NiCl2.6H2O, H2PtCl6.H2O, Tetrabutyl ortotitanate, and VCl3). Zn-CMK-1, Pt-CMK-3, Ni-CMK-1 and V-CMK-3 was activated in a hydrogen flow. TiO2-CMK-3 sample will not be made any treatment with H2.Results and DiscussionsThe silica templates indicates excellent structural order for the cubic crystallographic space group Ia3d and hexagonal P6mm crystallographic space group respectively. The mesoporous carbon CMK-3 is an exact replica of the template. However, the carbon CMK-1 was found to be not the exact negative replica of the template, due to a transformation of the mesostructure after the dissolution of the template wall. When metals/cations were incorporated to the carbon structures, the overall pore framework was maintained as indicated by the presence of low-angle diffraction peaks. Wide-angle X-ray diffraction patterns for carbons and modified ?carbons show two broad diffraction peaks which can be indexed for typical graphite carbons. The absence of prominent reflections in metal/cations/carbons clusters indicates that no crystalline bulk material has been formed, with nanometric size and high dispersion (not showed).TEM images of CMK-3 reveals particles of the nanometric carbon consisting of many rope-like domains (rod-like particles) with relatively uniform sizes of about 1 μm long and 0.2 wide. TEM images for Pt-CMK-3 and TiO2-CMK-3 show an ordered structure slightly damaged by thermal treatments; they also exhibit well-organized pores parallel to each other. The white lines correspond to the mesopores generated in the space previously occupied by the walls of SBA-15 template. Dark spot indicates platinum and anatase nanoclusters. Hydrogen adsorption/desorption isotherms of mesoporous carbons and modified-carbonsThe capacity of hydrogen storage was evaluated at cryogenic temperatures (77 K). The experimental data were fitted by Freundlich isotherm equation. At low and high pressures, the amount of hydrogen uptake is higher in modified-carbon samples than in respective carbons. We employed the term ?uptake? as storage (carbon like a sponge), thus hydrogen adsorption was completely reversible, indicating absence of no chemical reaction or strong bond between hydrogen and metals/cations nanoclusters and carbon frameworks.ConclusionsWe have shown that hopeful hydrogen storage materials can be obtained by ordered porous carbons (CMK-1 and CMK-3) modified with metal/cations species, synthesized by replication using MCM-48 and SBA-15 as a template. Incorporation of metal/cations species was carried out by wetness impregnation, supported by XRD, TEM and adsorption/desorption N2 isotherms. Carbon modified with metal/cations shows a better capacity for hydrogen uptake than that of the mesoporous carbons. The high pressure hydrogen adsorption evolution measured at 77K shows that composites can significantly enhance hydrogen adsorption capacity and hydrogen storage performance of carbon materials, proving to prospective candidates for application in hydrogen storage. The improved activity and the larger performance of composites materials is attributed to improved dispersion of uniform metal/cations nanoparticles as well as to efficient use of the support, which may probably originate a high surface area and pore volume, allowing a large dispersion of clusters.