Carbon fibrils

The final important form of carbon is the carbon fibre formed from polyacrylonitrile (PAN), cellulose or pitch and which is finding increasing ase in fibre-teinfbrced Composites. The corrosion of carbon fibril in composites designed for use under high temperature conditions is currently a severe limitation on their use. 

Hyperion Catalysis International, US 4663230, H.G. Tennent, 1986, Carbon Fibrils, Method for Producing Same, and Compositions Containing Same... 

Moy D., Chishti A. Methods and catalysts for the manufacture of carbon fibrils. US Patent Application 20010014307 A1, 2001. 

The most important forms of carbon are summarized in Table 9.1. We will elaborate on the two materials for catalyst support, viz. activated carbon which is commonly used and carbon fibrils, a relatively new material with potential as catalyst support. 

Carbon fibrils Decomposition of CH4 on small metal particles 100-300 meso-pores... 

Hyperion Catalysis International, Cambridge, MA, USA Hollow carbon fibrils... 

Hyperion Catalysis International developed a new technology to produce hollow carbon fibrils. The patented technology produces hollow fibrils of very small diameter in a catalytic process using ethylene gas as the raw material. The fibril structure is given in Figure 2.78. The striking feature of these fibrils is... 

Typically, practical graded dielectric RAMs are constructed of discrete layers, with properties changing from layer to layer. A technique for reducing the reflection from the front face of a flat absorber is to use semiconductive foams obtained by impregnation with carbon or dispersion of carbon fibrils. A careful combination of these foams with an internal magnetic layer of the iron-filled elastomer type leads to wide bandwidth materials which can operate readily between 4 and 18 GHz [29] as shown in Figure 8.3,... 

Figure 8.3. Evolution of reflexion coefficient vs. frequency for a three-layer material containing an external layer which is a low density PUR foam containing carbon fibrils a middle layer which is a medium density PUR foam containing carbon fibrils an inner layer which is an elastomer loaded with ex-carbonyl iron powder. The total thickness of the architecture is 16 mm.

Teiment, H.G. (1987) Carbon fibrils, method for producing same and compositions containing same, US Patent No. 4,663,230. 

Therefore, we examined the properties of the bare carbon fibrils, as well as the properties of catalysts supported on carbon fibrils. To obtain carbon fibril-supported palladium catalysts, a homogeneous deposition-precipitation procedure was utilized. Varying preparation conditions, such as, the atmosphere during drying and the pretreatment of the carbon fibrils, affect the activity in the liquid-phase hydrogenation of nitrobenzene significantly. 

Carbon fibrils, on the other hand, do not bear these constraints and a study on the application of this material as a support in liquid phase processes was estimated to be worthwhile. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

Experiments have been executed in order to compare the filterability of the carbon fibrils with that of commercially available activated carbon products. One gram of each support material contained in an equal amount of water was filtered in the same filter-device. The time required for filtering was measured. 

The preparation of palladium catalysts supported on carbon fibrils. 

In order to study the properties of the carbon fibrils as a support material for an active component in a catalytic reaction, the liquid-phase hydrogenation of nitrobenzene was chosen as a test reaction and palladium as the active component. 

The inertness of the carbon fibrils, advantageous with respect to aggressive environments, makes it difficult to apply small particles of the active component that exhibit an interaction sufficiently high with the carbon surface to resist sintering or mechanical separation. To raise the reactivity, the surface of the carbon fibrils has first to be activated. ... 

For deposition of the active component, palladium, onto the carbon fibrils a precipitation method is utUized . The method consists of four steps, viz., pretreatment of the carbon surface in boiling nitric acid, suspension of the treated fibrils in an aqueous solution and addition of the solvated palladium precursor, injection of a formaldehyde solution to reduce the palladium ions, and, finally, filtering and drying. 

Firstly the carbon fibrils were kept in refluxing 65%, nitric acid for 10 or 30 minutes. Surface groups are thus formed that, subsequently, are capable to anchor the precursor of the active components. After thoroughly washing of the carbon fibrils, they were dried at 120°C for one hour in air. 

After this pretreatment the carbon fibrils were suspended in water under a nitrogen... 

Testing of carbon fibril-supported paliadium catalysts. 

In the TEM micrograph we can observe that the carbon fibrils all exhibit approximately the same diameter. This is caused by the uniform size of the supported metal particles. It turned out that the size distribution of the supported iron particles not only determines the distribution of the diameter of the fibrils, but also the texture of the tangled skins. For example, the skein... 

Figure 1. Growth of carbon fibrils out of supported iron particles. Course of the concentrations of Hj, CO, CH, and COj measured beyond the reactor as a function of time...

Figure 2. TEM micrograph of carbon fibrils out of supported iron particles typical result of growth...

Characterization of the carbon flbriis (CF) and the carbon fibril supported palladium catalyst. 

These tangled skeins do form an open network of pores, thus providing fast transport of reactants and products. No micropores are measured to be present. The specific surface area (BET) is approximately 230 m per gram, while the pore volume turned out to be 1.6 ml/gram. The pore size distribution is shown in figure 4. Another important feature of the carbon fibrils is the fact that they are readily wetted by organic solvents. 

Rgure 3. Structure of carbon fibrils out of iron particles... 

Figure 4. Pore volume distribution plot as determined using nitrogen desorption, of carbon fibrils grown out of iron particles.

As mentioned above, an important feature is a good filterability. The results of r resoitative filtration experiments are shown in figure 5. In this figure the relative filtration times of equal weights of a commercial, activated, carbon support sample and a sample of the carbon fibrils contained in equal amounts of water have been plotted. From this figure it can be concluded... 

The filtration experiments were carried out with fresh samples. Also, the tendency of a support material to produce fines during vigorous stirring is important. To examine the attrition, the carbon fibrils were vigorously stirred at 2200 rpm for 48 hours in a vessel provided with baffles. After this ejqreriment the sample was studied with TEM. No fines could be observed, and the filterability as compared with that of the fresh sample had not changed significantly. 

Earlier we emphasized the advantageous features of carbon with respect to its resistance towards strong acidic and basic environments. Equally important is its behavior under oxidmng and reducing conditions. Therefore thermogravimetric (TG) experiments were carried out in which samples of carbon fibrils were exposed to flows of oxygen or hydrogen. 

Figure 6. TG experiment of carbon fibrils in reducing and oxidizing environment...

Hydrogenation activity of carbon fibril supported palladium catalysts. 

For testing the carbon fibril supported catalysts, the liquid phase hydrogenation of nitrobenzene was studied. The test reaction was also performed using a commercially available catalyst, i.e. palladium supported on activated carbon (Engelhard ESCATIO). 

---