Graphite deposits

Figure 2. Graphite deposition as a function of composition and temperature...

Pressure. The curves for two temperatures (Figure 2) indicate the areas of graphite deposition. Before we consider the effects of higher... 

Figure 3. Effect of pressure and composition on graphite deposition at 700° K...

Let us first consider mixtures of pure CO and C02, all points of which lie on the X axis. The equilibrium in this system is fully described by Equation 3. As the temperature increases, the equilibrium constant decreases, and CO becomes stable. It is apparent (Figure 4) that, as the temperature increases, the intersections of the curves for different temperatures move close to pure CO, thus increasing the region of no graphite deposition. 

In the region of pure CH4, the equilibrium is governed by Equation 4. For this reaction, the equilibrium constant increases with temperature so that at high enough temperatures there will be appreciable dissociation of CH4 to H2 and graphite. In the lower temperature range considered here, the thermodynamic equilibrium indicates only a very small amount of dissociation so the intersection of the graphite deposition curve with the H2-CH4 line occurs at almost pure CH4. As the temperature increases, the point of intersection will move toward pure H2 on the H2-CH4 line. 

So far, we have discussed graphite deposition only in terms of the two reactions, Reactions 3 and 4. As the temperature increases, graphite deposition by Reaction 4 is favored, and it is retarded by Reaction 3. The net result is that the graphite deposition curves for two temperatures will intersect at some point (e.g., the two curves for 900° and 1000°K). The quantitative description of curves depends on the interactions of all the species. 

Because of the opposite effects of temperature on the stability of CO and CH4, the odd result is that, as the temperature increases, graphite deposition is less likely for starting mixtures which are near stoichiometric, but it is more difficult to produce pure methane by removing water and allowing the mixture to react further. Because of equilibrium considerations, the final approach to pure methane must be done at a relatively low temperature. 

Most recently, they have developed" a cell configuration for the study of modified electrodes that employs, as a working electrode, colloidal graphite deposited onto kapton tape (typically employed as a window material). Such an arrangement minimizes attenuation due to the electrolyte solution. 

Grades of natural graphite from Zavalie (NGZ) graphite deposit, Kirovograd Region, Ukraine ... 

Agrawal et al.33 performed studies of Co/A1203 catalysts using sulfur-free feed synthesis gas and reported a slow continual deactivation of Co/A1203 methanation catalysts at 300°C due to carbon deposition. They postulate that the deactivation could occur by carburization of bulk cobalt and formation of graphite deposits on the Co surface, which they observed by Auger spectroscopy. 

Figure 7.24 Photoelectron emission microscopy images of two Fe304 surfaces that were used as model catalyst in the dehydrogenation of ethylbenzene to styrene at 870 K, showing carbonaceous deposits (bright). These graphitic deposits grow in dots and streaks on a surface of low defect density, but form dendritic structures on surfaces rich in point and step detects (from Weiss et al. f731).

The hydrogen gas produced in the decomposition is thought to eliminate the surface-adsorbed carbon-containing species, CH c(ads)( = 0-3) via Eq.2. This would explain the relative lack of amorphous carbon and graphitic deposits found in our samples (EigureC.l), a problem that has plagued alternate CVD experiments. 

Graphite deposits are found in or at the edges of stratified rock... 

The term graphite refers to the true, well-ordered single crystal as first described by Bernal in 1924 with the structure shown in Figure 1, but is also used for polycrystalline materials and of some materials with quite a high proportion of very disordered regions. The use of the term amorphous graphite for some natural graphite deposits in Mexico, Korea, and other parts of the world may seem scientifically absurd, yet is quite common. 

Figure 6. Electron micrograph showing the shell-like appearance of graphite deposits that encapsulate iron-nickel particles when heated at temperatures of about 825°C in a C2H4/H2 (4 1) mixture.

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