Carbon deposition, with

The possible causes for the cooling from the middle Miocene to the present are (1) the change in carbonate deposition with time (2) the change in albedo due to topographic change (building of mountains such as the Himalaya, etc.) (3) the change... 

Fig. 3 shows a series of TPSR curves for reaction of the carbon deposits with H2 as a function of deposidon temperature. Results obtained from Fig.3 confirm that two forms of carbon, a relatively active and a relatively inactive form based on their reactivity toward H2, arc deposited on the surface of 10 wt% Co/y-Al203 during the dispropoTtionation of carbon monoxide. The two peaks at 190 10°C and 430 10 C are close to those for atomic carbon (Ca) and polymeric carbon (Cp) respectively, designated on Ni/Al203 by McCarty and Wise... 

Figure 2 shows the dependence of the rate of carbon deposition with initial hydrogen partial pressure. The rate depends on the hydrogen partial pressure with a negative order, i.e., the rate of deposition decreases with increased hydrogen pressure. The dependence of the rate on the initial benzene partial pressure is first order as revealed by the data on Figure 1. For fixed temperatures, benzene and hydrogen partial pressures, there is a critical gas phase residence time (t ) below which no appreciable deposition occurs. Above the critical residence time t, deposition occurs and its rate increases linearly with gas residence time. This is depicted in Figure 3.

Deactivation may be structure-sensitive. Thus Lankhorst et al. (132) have shown that variations of TOF with d for hexane reforming on Pt were caused more by increases in carbon deposition with d than by decreases in the intrinsic rate of reaction with d. For model stepped-surface single crystals of platinum, Somoijai and Blakely (133) showed that carbon deposition was favored on terraces over edges and comers, so that the reforming reactions favored by the latter sites are poisoned less rapidly than those occurring mostly on the flat surfaces. Thus small particles are expected to be poisoned less rapidly than large ones, for this example. 

Denton et al. describe a procedure for aminating alkyl aromatic hydrocarbons, particularly toluene, which is converted to benzonitrile. Toluene and ammonia, at various reactant ratios and liquid space velocities, are passed at atmospheric pressure over a supported molybdenum trioxide catalyst at about 525-550 C. The conversion per pass is -10 per cent, and the yields are 60-85 per cent based on the toluene consumed. The process, as operated commercially, involves the continuous feed of toluene and NHj to one of two reactors followed by continuous removal of ammonia and toluene, for recycle, from the benzonitrile. Catalyst in one reactor is regenerated over a 3-6 hr period by oxidizing carbon deposits with air diluted with an inert gas, while the other reactor is on stream. 

Figure 1-14. Catalyst made of carbon deposited with platinum particles...

In this case, the palladium-based MR benefits result in higher methane conversion than FBR, as shown in Table 2.6, and a reduction of the carbon deposition with respect to a conventional reformer, as above reported. 

Regarding the key alkali chemical promoters, they increase the intrinsic rate of carbon deposition, with K2O having a greater effect than the weaker base Na20 ((7), chapter 7, (14)). The presence of silica lowers the basicity of the alkali and in keeping with this, the rate of carbon deposition is lower. 

Earlier, Olson and co-workers [26] obtained Raman spectra of diamond on a scratched silicon crystal after different times of exposure to sputtered carbon and to atomic hydrogen in a CVD apparatus. Only the first monolayer of carbon deposited with each exposure was utilized, indicating that diamond growth was a surface reaction. Increasing carbon exposure increased the layer thickness but decreased the quality of the deposited film as increasingly strong broad peaks appeared at 1140, 1350, and 1450 cm . 

Afterward, the notion of unspecified carbon deposition with an olefin-like composition (CH ) has been gradually transformed to Polymethylbenzenes (PMBs) by many research groups [87,97]. Those PMBs serve as scaffolds/cocatalysts, where methanol is added and olefins are eliminated in a closed catalytic cycle [87,98]. It is therefore indicated that the interplay between the inorganic framework and the organic reaction centers dictates the activity and selectivity. However, according to Ref. [97], the role of PMBs as the major hydrocarbon pool species appears to be independent of the zeotype catalyst chosen. Haw et al. [87] provided both experimental and theoretical evidence in favor of PMBs as the driving force for the hydrocarbon pool mechanism. In 1998, by means of pulse-quench reactions on an H-ZSM-5 catalyst and GC-MS and MAS NMR analysis, it was reported... 

In some countries, including the UK, coal consumption is decreasing. The high cost of sulfur removal is one significant economic motivation towards replacing the exploitation of natural carbon deposits with natural gas (CH4) as a primary energy source. Methane fuel contains less sulfur and yields proportionately less CO2 for... 

---