Controlled Fabrication Of Carbon Nanomaterials By Layered Structure Catalyst

Since carbon nanofibers (CNFs) and carbon nanotubes (CNTs) have specific structures and unique properties, they can widely be used as matrix composites, catalyst supports, hydrogen storage materials, sensors, fuel cells, capacitors and other fields. The synthesis of carbon nanomaterials conventionally can be influenced by several factors, such as catalyst species, catalyst carriers, carbon sources, reaction temperature, and catalyst promoters. Carbon nanomaterials with different morphologies and structures can be formed due to different conditions.Layered double hydroxides (LDHs) are a class of highly ordered two-dimension layered materials. When LDHs are used for synthesis of carbon nanomaterials, their superiority exhibits in two aspects:the prepared nanomaterials can be highly controlled because of the high adjustability of the species and content of catalyst active constituent; additionally the active constituents are uniformly distributed in layers and the active species can highly dispersed after high temperature calcinations, which make for carbon nanomaterials with high yield and regular structure.On the basis of two advantages of adjustability and uniformity mentioned, in the present work, carbon nanomaterials were synthesized by chemical vapor deposition (CVD) over LDHs catalyst precursors. The materials were characterized by XRD, BET, TPR, XPS, SEM, TEM, HRTEM, RAMAN, and a series of conclusions are obtained.Impregnation and co-precipitation methods were used to prepare catalyst precursors while synthesizing CNTs over supported Pt catalysts. With impregnation method the calcined MgAl-LDH products were used as carrier to support Pt catalysts, and the results revealed that increasing Pt content was attributable to the high dispersion of smaller Pt nanoparticles as well as the high yield of CNTs and degree of graphitization. While with co-precipitation method, a range of Pt/MgAl-LDHs precursors were prepared, and CNFs were synthesized by CVD method over the calcined product catalysts. The results indicated:(1) the reaction temperature of 600℃was appropriate for the growth of uniform CNFs with regular shape while using acetylene as carbon source; (2) the catalyst content had a great influence on the synthesis of CNFs. Lower Pt content, lower yield of CNFs. Nanopaticles may agglomerate easily with too high Pt content. Thus it is very important for the selection of Pt content in catalyst precursors.Combination with the anion interchangeability in the interlayer of LDHs, we successfully introduced catalyst promoter W to the Co-based LDH precursors. CNTs were prepared over Co-based catalyst nanoparticles derived from calcined products. The results revealed that the addition of a small quantity of catalyst promoter W contributes to the increase of CNTs yield and improvement of the morphology and structure of CNTs. However, catalyst particles aggregated a lot with too much W catalyst promoter, which led to the great decrease of product yield.