Direct Synthesis and Structural Characteristics of Ordered SBA-15 Mesoporous Silica Containing Tungsten Oxides and Tungsten Carbides

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• 作者：Linhua Hu ; Shengfu Ji ; Zheng Jiang ; Huanling Song ; Pingyi Wu ; Qianqian Liu
• 刊名：Journal of Physical Chemistry C
• 出版年：2007
• 出版时间：October 25, 2007
• 年：2007
• 卷：111
• 期：42
• 页码：15173 - 15184
• 全文大小：577K
• 年卷期：v.111,no.42(October 25, 2007)
• ISSN：1932-7455

文摘

A series of WO₃-SBA-15 materials with different Si/W ratios have been hydrothermally synthesized usingtetraethyl orthosilicate (TEOS) as silica precursor, ammonium paratungstate as tungsten precursor, and EO₂₀PO₇₀EO₂₀ (P123) as structure-directing reagent. After temperature-programmed carburization (TPC) in flowingCH₄/H₂ (20/80 v/v mixture), the materials were converted to the corresponding W_xC-SBA-15 materials. Thestructure of the oxide and carbide materials has been characterized using X-ray diffraction (XRD), X-rayfluorescence (XRF), nitrogen adsorption-desorption measurements, ²⁹Si magic-angle spinning (MAS) NMRspectroscopy, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), andthermogravimetric and differential scanning calorimetric analysis (TG-DSC) measurements. The results showthat after hydrothermal synthesis using different amounts of tungsten and subsequent carburization, the materialsretain the mesopore structure of SBA-15. When Si/W = 30-15, the majority of the tungsten is dispersed inthe channels of SBA-15 with the remainder being incorporated into the framework of SBA-15 with theformation of Si-O-W bonds. The tungsten carbide exists as a single W₂C phase after carburization. Athigher tungsten content (Si/W = 7.5), the amount of tungsten in the framework of SBA-15 increases with theformation of both Si-O-W bonds and W-O-W bonds. The tungsten carbide formed after carburizationexists as a mixture of W₂C and WC phases. A model for the distribution of tungsten in SBA-15 is proposedinvolving three different tungsten species: -W inside SBA-15 channels, -W embedded in the internalsurfaces of the SBA-15 channels, and -W inside the framework of SBA-15. After temperature-programmedcarburization, -W sites are transformed into W₂C, whereas -W sites afford WC; in contrast, -W sitesshow little change after carburization.