The formation of filamentous carbon on iron and nickel catalysts III. Morphology

Abstract

The microstructure of primary carbon filaments formed on supported iron and nickel catalysts has been investigated using transmission electron microscopy, dark-held imaging, and (selectedarea) electron diffraction. It has been established that the filaments consist of cone-shaped graphite layers, stacked with their c-axes in a direction normal to the metal-carbon interface. A growth mechanism is proposed involving the excretion of cone-shaped graphite layers. To explain the constancy of the filament diameter, slippage of these layers over one another is invoked. Edge dislocations are brought about by differences in the rate of carbon excretion.

Development of an impedance-based sensor for detection of catalyst coking in fuel reforming systems

“…showed that metastable carbides were present during filament growth and that the heat of reaction (∆h R ) agreed with Alstrup's theoretical calculations within experimental uncertainty [17,40]. Gues et.…”

“…Pores are also present in the layer which allow gas to enter the pore structure and coke to form below the surface of the material. Since the nickel nodules do not form an electrically conductive path across the layer, the electrical resistance of the material is high, While carbon nanofiber growth has been observed in nickel catalysts, [3,16,17,18] it was not expected to be the dominate mechanism in lowering the resistance of the Ni-YSZ. In the course of evaluating the sensor design, the carbon filaments were found to be more prevalent than expected through electron microscopy, which will be discussed in a later section.…”

“…A field-effect scanning electron microscope (FESEM) was used to examine the surface of the coked sensor and obtained the images in Filamentous carbon has been observed to form under similar conditions on nickel catalysts [3,17,18]. Since the carbon filaments are hundreds of times longer than their diameter, only a small mass of carbon is needed for a measurable decrease in sensor resistance.…”

Transitional behaviour in the transformation from active end planes to stable loops caused by annealing

“…We postulate that during the growth process, an envelope of truncated conical graphene layers is formed at once from the catalyst surface, consisting of four or more cups. This envelope-type formation occurs due to an interruption in the supply of the critical level of carbon supersaturating the metal catalyst and the difference in rate of carbon supply to the catalyst, which relies on the position of the metal introduced at edge dislocations [28]. In other words, four or more conical graphene layers (about 4-6) are released from the surface of the catalyst when the carbon concentration within it is at a critical level.…”

Using Mechanical Alloying to Create Bimetallic Catalysts for Vapor-Phase Carbon Nanofiber Synthesis

“…For example, a 10 mg catalyst load will result in a final mass of 300-500 mg of carbon during a 1 h reaction. The unmilled, blended catalyst powders also achieved growth, but the activity is attributed to Ni alone, which is known to be a catalytic metal for carbon deposition [7,28,[64][65][66][67][68][69]. The rate is comparable to pure Ni, which was tested as a control sample with growth rates shown in Figure 8.…”