Carbonaceous layer

In general, pyrolants composed of a polymeric material and AN particles are smokeless in character, their burning rates are very low, and their pressure exponents of burning rate are high. However, black smoke is formed as i decreased and carbonaceous layers are formed on the burning surface. These carbonaceous layers are formed from the undecomposed polymeric materials used as the matrix of the pyrolant. When crystalline AN particles are mixed with GAP, GAP-AN pyrolants are formed. Since GAP burns by itself, the GAP used as a matrix for AN particles decomposes completely and bums with the oxidizer gases generated by the AN particles. 

In many cases ordering is no longer observable in the presence of steps. Ordered carbonaceous layers form on the Ir(l 11) crystal face, for example, while ordering is absent on the stepped iridium surface. Ordering is absent on stepped Pt surfaces for most molecules that would order on the low Miller-Index (111) or (100) surfaces. 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

Thereby, it is obvious that dealing with FR acting in the condensed phase, the properties and the structure of the resulting carbonaceous layers will affect the FR properties and that is the reason why it is very important to be able to characterize this layer in detail. 

Similar synergistic effects leading to the reduction of PHRR and THR up to 30% (Figure 12.9) and to the increase of time to ignition were observed between nanometric alumina and phosphinate additives,96 using the same protocol. It has been proposed that phosphinates acted principally in the condensed phase, and the presence of oxides played a reinforcement role in the carbonaceous layer promoted by the phosphinate additives. 

Although the hydrogenation of various acetylenic compounds is industrially very important and scientifically an interesting problem to study, the information gathered up to now is still insufficient to answer all questions. So, for example, the debate is still continuing on the problem whether the hydrogen diluted in metals plays a role or not, or whether the carbonaceous layer (instead of unoccupied metal sites) is the locus of hydrogenation of acetylene or not, etc. (see e.g. the discussion in ref. 80). 

Hydrogenolytic reactions can also be suppressed by carbonaceous layer deposition or by modification of the active surfaces by — for example — sulphur. Suppression of hydrogenolysis gives more chance to a reaction the rate of which decreases only proportionally to the active metal surface concentration, like hydro/dehydrogenation, or some forms of isomerisation (5C cyclic mechanism see the chapter on elementary steps). 

ID CNTs inserted in the 2D clay platelet network is believed to be responsible for this, as this unique nanostructure provides larger tortuosity and obstacle for the heat transport. The large improvement in thermal stability of chitosan may arise from following two reasons (1) good heat barrier properties of CNTs and clay for polymer matrix during formation of chars and (2) formation of carbonaceous layer. 

Cu addition leads to an enhanced rate of benzene production with little or no induction time. That is, the initial rate of cyclohexane hydrogenolysis, relative to the Cu-free surface, is suppressed. Further, Cu reduces the relative carbon buildup on the surface during reaction. Thus, Cu may play a similar role as the carbonaceous layer in suppressing cyclohexane hydrogenolysis while concurrently stabilizing those intermediates leading to the product... 

However, as an electron spectroscopic technique, core-level XPS is typically operated under UHV (or at pressures up to 10 mbar) and generally not suited to elevated pressure measurements. Of course, XPS can be used for pre- and postreaction analysis if the surface species are stable (such as carbonaceous layers). However, if the adsorbed species are in equilibrium with the gas phase, they desorb upon evacuation, and the limitations discussed in the introduction are significant. It is preferable to carry out XPS under reaction conditions, at mbar pressures. 

The cause of catalyst deactivation has been addressed by Iglesia et al. These authors observe by electron microscopy that after catalytic operation the Pt particles that are located outside the L zeolite are covered with a carbonaceous layer, whereas intrachannel Pt particles are apparently free of coke. For these particles the active catalytic life is limited by their agglomeration with concomitant loss of Pt surface area. Sulfur not only poisons active sites on the Pt particles, but also accelerates the rate of agglomeration (334). 

The carbonaceous layer acts as hydrogen donor to the reactants. 

The carbonaceous layer leconfigures and stabilizes the structure of the metallic suiface. 

Selective surface poisoning may involve impurities or reaction intermediates, such as carbonaceous layers formed during hydrocarbon reactions. The order or disorder of this carbonaceous layer appears to affect selectivity, with demanding reactions favored by an ordered layer (575). Although alkene hydrogenation is assumed to occur on this layer (575), further characterization of its significance for catalyst selectivity is necessary. 

Figure 8.5 shows a working metal catalyst surface. On the surface there are reactants, products, coke in the form of polymers, and different hydrocarbon fragments that may react to coke if they are not removed from the surface [15]. The carbonaceous layer is not the only form of coke. On iron surfaces, coke can grow as whiskers with a little metal crystallite. 

Organic phosphorus compounds, irrespective of their structure, decompose to polyphosphoric and methaphosphoric acids, which retain the acidity at higher temperatures and catalyse the rapid decomposition of the organic substrate to carbon. A carbonaceous layer containing phosphorus is formed, which is very difficult to burn, which is a true protective layer for the rest of material and the process of burning is stopped [1, 2, 6-8, 11, 15]. 

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