Graphite, catalyst encapsulation

In general, encapsulated metal particles were observed on all graphite-supported catalysts. According to Ref. [4] it can be the result of a rather weak metal-graphite interaction. We mention the existence of two types of encapsulated metal particles those enclosed in filaments (Fig. 1) and those encapsulated by graphite. It is interesting to note that graphite layers were parallel to the surface of the encapsulated particles. 

As in the case of graphite-supported catalysts, some metal particles were also encapsulated by the deposited carbon (Fig. 4). However, the amount of encapsulated metal was much less. Differences in the nature of encapsulation were observed. Almost all encapsulated metal particles on silica-supported catalysts were found inside the tubules (Fig. 4(a)). The probable mechanism of this encapsulation was precisely described elsewhere[21 ]. We supposed that they were catalytic particles that became inactive after introduction into the tubules during the growth process. On the other hand, the formation of graphite layers around the metal in the case of graphite-supported catalysts can be explained on the basis of... 

CCEs have also been used in conjunction with enzymes. Glucose oxidase was encapsulated within the methyl silicate (or silica)-graphite composite [216, 220]. Reduced oxygen from the enzymatic reaction results in the production of H2O2, which is oxidized at the electrode. Palladium and rhodium catalysts have been added to the electrode to help lower the over voltage required for hydrogen peroxide oxidation [219, 221]. Glucose oxidase and horseradish peroxidase have been simultaneously entrapped within a CCE for a biosensor that does not require a... 

The details of the structural characteristics of individual constituents in the various carbon deposits were obtained by examination of a number of specimens from each experiment in a JEOL 100 CX transmission electron microscope that was fitted with a high resolution pole piece, capable of 0.18 nm lattice resolution. Suitable transmission specimens were prepared by applying a drop of an ultrasonic dispersion of the deposit in iso-butanol to a carbon support film. In many cases the solid carbon product was found to consist entirely of filamentous structures. Variations in the width of the filaments as a function of both catalyst composition and growth conditions were determined from the measurements of over 300 such structures in each specimen. In certain samples evidence was found for the existence of another type of ca naceous solid, a shell-like deposit in which metal particles appeared to be encapsulated by graphitic platelet structures. Selected area electron diffraction studies were performed to ascertain the overall crystalline order of the carbon filaments and the shell-like materials produced from the various catalyst systems. 

Encapsulated carbon is formed from the reaction of adsorbed hydrocarbons onto nonreactive areas (deposits), which polymerize on the catalyst. This forms graphitic carbon that encapsulates into the film and progressively deactivates the catalyst [1]. This degradation process occurs at mild temperatures and low steam to hydrocarbon ratios. 

Another recent report describes the large scale synthesis of ahgned carbon nanotubes, of uniform length and diameter, by passage of acetylene over iron nanoparticles embedded in mesoporous silica [107]. The latter two methods, based on the pyrolysis of organic precursors over templated/catalysts supports, are by far superior by comparison with plasma arcs, since other graphitic structures such as polyhedral particles, encapsulated particles and amorphous carbon are notably absent (Fig. 16). 

In 2011, Woo and collaborators prepared ORR catalysts from the pyrolysis at 700, 800, and 900 °C of a mixture of iron oxide supported on Vulcan and dicyandiamide (C2H4N4 a dimer of cyanamide) [111]. TEM of the catalysts revealed that at 700 °C metal particles were encapsulated with a carbon layer, while they were mostly in carbon tubes at 900 °C. The total N content was 2.2,3.5, and 6.6 at.% for the catalysts heat treated at 700,800, and 900 °C, respectively. This N content was broken down as 54 % pyridinic and 0 % graphitic nitrogen atoms at 700 °C, while it was 61.4 % pyridinic and 10.7 % graphitic at 900 C. The Fe content was also measured in these catalysts and also for the catalyst iron... 

The DC arc technique is carried out in a water-cooled chamber with a graphite cathode and a graphite anode that has been doped with the desired metal and a catalyst in the presence of a gas, usually helium. One reason for the lack of transition-metal endohedral fullerenes can be attributed to the difficulty of current DC arc-discharge methods to cause encapsulation of transition metals in fullerenes, for which the reason remains unclear. For this reason, transition metals are... 

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