Tubular and Rodlike Ordered Mesoporous Silicon (Oxy)carbide Ceramics and their Structural Transformations

Abstract

Ordered mesoporous silicon carbide ceramics with hexagonal p6mm (OM-SiC-3, OM-SiC-5), cubic Ia3d (OM-SiC-8) and cubic I41/a or lower (OM-SiC-8) pore arrangement symmetries were prepared via nanocasting of SBA-15 and KIT-6 ordered mesoporous silica templates with polycarbosilane (PCS) precursor (numbering of the OM-SiC materials corresponds to their CMK ordered mesoporous carbon analogues). Four different PCS were used for the OM-SiC preparation (liquid SMP-10, low-molecular-weight PCS with M W = 800 melted at 320 °C and PCS with M W = 800, 1400, 3500 dissolved in heptane/butanol). Infiltration of liquid vinyl functionalized SMP-10 precursor, and subsequent pyrolysis in vacuum resulted in OM-SiC-5 materials with tubular structure similar to that of the CMK-5 carbons (according to the TEM, low angle X-ray diffraction and nitrogen physisorption measurements). No significant differences were observed between the direct melt (PCS-800 at 320 °C) and wet impregnation procedure (PCS-800 in heptane/butanol) for OM-SiC-3, whereas significant differences were measured for materials prepared using different molecular weights of the PCS precursors. The influence of different precursor loadings and final pyrolysis temperature was also investigated. In the case of materials prepared from cubic KIT-6, transformation of the OM-SiC-8 symmetry could be observed by small-angle X-ray diffraction.

Preparation of cubic ordered mesoporous silicon carbide monoliths by pressure assisted preceramic polymer nanocasting

“…As expected, in addition to the CSi bonds, Si-O stretching vibrations are present at about 1073 cm À1 , attributed to the presence of silicon oxycarbides (Si-O-C) on the surface of SiC. The existence of Si-O-C is quite common in the polymer-derived high-surface-area SiC ceramics [31], especially in the mesoporous silica-templated SiC ceramics [21,29]. The introduction of oxygen into SiC ceramics should be due to the oxidation of preceramic precursor in air at ambient temperature, and/or the oxidation of SiC by residual oxygen from argon flow, and/or the large SiC/SiO 2 interface during the pyrolysis at high temperatures.…”

“…The monoliths also had very large pore volumes (0.769 cm 3 /g for OMSCM-1073, 0.864 cm 3 /g for OMSCM-1273, and 0.697 cm 3 /g for OMSCM-1473) determined at relative pressure p/p 0 = 0.98. The specific surface area (942 m 2 /g) and pore volume (0.864 cm 3 /g) of OMSCM-1273 were evidently greater than those of mesoporous SiC powders via nanocasting [21,22,29] and mesoporous SiC monoliths (632 m 2 /g and 0.40 cm 3 /g) replicated from CMK-3 carbon [19]. The hysteresis loops in three isotherms do not close at higher relative pressure (p/p 0 = 0.7-1.0), which might be a result of interparticle textural pores in the monoliths.…”

“…Synthesis of cubic ordered mesoporous SiC monoliths (OMSCMs): The polycarbosilane precursor (PCS-800) was introduced into the mesopores of KIT-6 silica via the wet impregnation method with organic solvents [21,24]. Typically, 2 g of as-prepared KIT-6 was added into the solution of 2.96 g of PCS-800 in 40 g of heptane and 1.8 g of 1-butanol.…”

“…We have synthesized ordered mesoporous SiC powdery materials by nanocasting ordered mesoprous silica with the preceramic precursors (PCS-800 and SMP-10) [21,22]. PCS-800 demonstrates good solubility in mixed organic solvents of hexane and 1-butanol, and can easily fill in the nanochannels of ordered mesoporous silica to prepare ordered mesoporous SiC materials.…”

Nanoporous silicon carbide as nickel support for the carbon dioxide reforming of methane

“…Those act as nanoscaled mould to shape a precursor which can be transformed to the desired product and after SiO 2 removal porous SiC is obtained. [7][8][9][10][11][12][13] Although such preparation methods are known and specific surface areas of several hundred square meter per gram were reported, SiC was used as support only in some applications so far and usually the materials were not porous and hence, had low specific surface areas (<< 100 m 2 g -1 ). [14][15][16][17][18][19][20][21][22][23][24][25][26] Nevertheless, a performance increase in the propane oxidative dehydrogenation was reported by Xu and co-workers compared to conventional supports as alumina or silica.…”