Formation of carbon deposits

Experiments are required to distinguish between these two possibilities. Corrosion of Ni to NiO was observed [20]. A comparison [20] between the thermodynamic predictions and experiments showed good agreement for Au, Pt, Ag, Fe, and Ni within the limitation of the thermodynamic method. Only gold and platinum were found to be immune to attack. Protection of other metals will depend mainly on passivation. Cathodic protection is undesirable because of the loss in performance due to electrode polarization. 

The formation [23—29] of carbon deposits on the anode has to be avoided since it decreases the performance of carbonate fuel cells. Carbon may be deposited electrochemically by reactions 17 to 20  

The standard potentials [30] referred to a reversible oxygen electrode (Pcoz = t P02 = i given for 600 °C. The standard state corres- 

Mixtures of hydrogen and carbon dioxide are more suitable fuels than hydrogen because they suppress the formation of carbon by reaction 21, the equilibrium constant of which is equal [29] to 2.5 at 650 °C. 

The values of the standard potentials indicate that some of the reactions 17 to 21 are feasible on thermodynamic grounds as side reactions on the anode under certain conditions determined by the partial pressures of the participating gaseous species. The extent of the participation is not known. Cairns, Tevebaugh, and Holm [26] gave a detailed discus- 

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In order to maintain high energy efficiency and ensure a long service life of the materials of construction in the combustion chamber, turbine and jet nozzle, a clean burning flame must be obtained that minimizes the heat exchange by radiation and limits the formation of carbon deposits. These qualities are determined by two procedures that determine respectively the smoke point and the luminometer index. 

Over the next four years, Houdry, working closely with Sun s engineering team headed by Clarence Thayer, worked to build a commercial plant. The limitations imposed by a static catalyst bed design imposed a major obstacle, particularly in the formation of carbon deposits that fouled the catalyst mass and impeded a continuous system of production. 

High velocity steam or particles striking a metal surface and causing metal wastage by erosion. Also refers to unbumed fuel oil striking a surface and resulting in the formation of carbon deposits and smoke. 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

FORMATION OF CARBON DEPOSITS ON COBALT CATALYSTS DURING FTS AND IMPLICATIONS FOR ACTIVITY... 

One of the major problems in C02 reforming is the formation of carbon deposits [2, 3], In the C02 reforming reaction, the possible routes of carbon formation include methane decomposition (Equation 9.6) and/or CO disproportionation (Equation 9.7). The clarification of this issue is necessary because it will help to understand the mechanism of carbon formation in this reforming reaction. Such insight will also be beneficial not only for the design of reactors but also for the... 

S rk ny et al. (80) considered the formation of carbon deposition from hydrocarbons using normal pressure. They conclude that carbon deposition is decreased by the addition of tin but the rate of formation of carbon deposits through a "polyene" route is increased with the addition of tin. Wilde et al. (81), also using normal pressure conditions, also emphasize the high activity and selectivity for dehydrogenation and dehydrocyclization by 1,6 ring closure due to lower carbon deposition. 

Highly undesired is the formation of carbon deposit. This phenomenon is favored by higher wall temperature, by the presence of heavy chlorinated hydrocarbons, as well as by some heavy impurities, namely trichloroethylene (TRI). Preventing the coke formation is a major problem in operating the furnace for EDC cracking. Keeping the reaction temperature below 500 °C prevents the coke formation but decreases the reaction rate. Therefore, as already mentioned, it is rational to use initiators , such as nitromethane, chloroform or carbon tetrachloride. 

The deactivation constant, obtained by fitting the data to a first order law equation (8), decreases with the reduction temperature for all cases, and is especially low for the impregnated alumina-titania catalyst. These results suggest the formation of carbon deposits and deactivation of catalysts occurs due to the metal activity, The contribution of the acid sites to deactivation seems to be negligible, despite the fact that alumina-titania supports present higher acidity than alumina or titania single oxides (9). 

As research into gaseous photocatalysis progressed a potential major disadvantage was the possibility of catalyst deactivation. Einaga et al. [208] concluded that the key factors which influenced catalyst deactivation were the formation of carbon deposits on the photocatalyst and their decomposition to CO. The photo oxidation rate of benzene decreased with decreasing humidity due to the increasing amount of carbon deposits on the catalyst, however, photo irradiation in humidified air decomposed the deposits and regenerated the catalyst [208]. 

Although hydrogen is believed to suppress the formation of carbon deposits, coking may still occur in hydrogenation reactions as well. During ethylene hydrogenation, extensive coke deposits are noticed on the feed side of Pd-Y membranes and are believed to eventually lead to the embrittlement and rupture of the dense membranes due to carbon diffusion inside the membrane [Al-Shammary et al., 1991]. A modest deposit of carbon could actually increase the selective formation of ethane which may be indicative of some reaction taking place on the carbon deposit. 

One of the main problems associated with hydrocarbon steam reforming over Ni is the deactivation of the Ni catalyst as a result of the formation of carbon deposits on Ni. The C-induced deactivation of Ni has been studied extensively [10,18,28-35], For example, Rostrup-Nielsen reported that steam reforming of various hquid fuels on Ni leads to the formation of encapsulating, whisker-like, or pyrolytic carbon on the catalyst [18, 30], To illustrate the problem of carbon poisoning, in Fig. 13.1 we show a transmission electron micrograph (TEM) of a Ni particle taken after steam reforming of propane at steam to carbon ratio of 1.5. The micrograph shows that carbon deposits are formed on Ni [16],... 

Rather than increasing the operating S/C ratio, it is more desirable to develop reforming catalysts that are inherently more carbon-tolerant than Ni [19, 35, 37-43], For example, it has been suggested that Ru and Rh do not facilitate the formation of carbon deposits because of poor carbon solubility in these metals [30, 44]. However, Ru and Rh are prohibitively expensive. It has also been shown that the promotion of Ni with alkaline earth metals such as Mg suppresses carbon-induced catalyst deactivation [18, 36]. There have also been reports that by selectively poisoning the low-coordinated Ni sites with small amounts of sulfur, the carbon-induced deactivation of Ni can be suppressed [9, 45]. In addition, the patent literature is rich with multiple examples where numerous additives, including those mentioned below in this text (e.g., Sn and Au), have been suggested to promote the stability of Ni catalysts [46]. 

We illustrated that recent advances in the fields of quantum chemistry, experimental microscopy and spectroscopy, and chemical synthesis represent landscape-changing developments that have allowed us to pursue approaches toward the formulation of heterogeneous catalysts which are based on the understanding of the underlying molecular transformations that govern catalyst performance. These approaches rely on the ability to identify and control critical elementary reactions on active sites. It is important to stress that we view experimental and theoretical techniques as complementary to each other. For instance, in the example presented above, the results obtained from in-situ TEM measurements [34] were utilized along with DFT calculations to obtain a molecular mechanism for the formation of carbon deposits on Ni. This molecular information allowed for the development of a systematic approach, based on first-principles calculations, that was utilized to identify possible carbon-tolerant alternatives to Ni. 

Without catalyst, phenol conversion is less than 3% at 140°C at a pressure of 2.0 MPa of oxygen. In addition, a test with 50 mg/1 of Mn" (quantity eluted when an experiment is run with 4 g/1 of Mn-Ce oxide) gives a phenol conversion of lower than 3%. This confirms that the formation of carbon deposit is heterogeneous. 

The formation of carbon-mineral adsorbents containing carbon deposits in the form of dendrites, whiskers or carbon black is not advantageous because of the poor mechanical properties of these deposits. The morphology of the coke depends on the mechanism and conditions of its formation on the mineral surface. Two main mechanisms of formation of carbon deposits can be distinguished consecutive reactions and carbide-forming. The latter mode consists in the thermal decomposition of hydrocarbons. 

The binding of carbon into carbonates is related to the activity of living organisms. However, the surface runoff of Ca + ions from the land determines the formation of carbonate deposits to a significant degree. The Ca + ion stream is roughly 0.53 x 10 tons/year, which can provide for a CaCO. precipitation rate of 1.33 X 10 tons/year. This would correspond to the loss of 0.57 x 10 tons CO2, or 0.16 X 10 tons C from the carbonate-hydrocarbonate system. 

The mechanism under which this surface carbon is formed on non-catalytical surfaces is not very well understood yet. Several theories have been proposed from the observation of carbon formation on surfaces and in the gas phase under different conditions (1) From this study it can be concluded that a unique mechanism that explains the formation of carbon deposits on catalytically inert surfaces does not seem to exist. The answer has to be looked for each particular case as different operating conditions (pressure, temperature, reactants) produce different, if any, carbon deposits. 

The theory assumes the following steps in the formation of carbon deposits ... 

The formation of carbon deposits is an undesirable feature of a number of industrial processes, for example on the fuel pins in nuclear reactors, on the catalysts and structural materials of reactors in petrochemical plant, and on furnace wall linings. A greater understanding of the mechanism of deposition is required in order to develop improved methods of control. Metal-carbon interactions are also important in the catalysis of carbon gasification for the manufacture of synthetic fuels. 

Synthetic Studies. The formation of carbon deposits by hydrocarbon pyrolysis has been extensively studied. Baker et have described the results of a... 

The influence of CO2 and H2O on the reaction rates is negligible or weak. The inhibition by the hydrocarbons depends both on the nature of the hydrocarbon and on the O2 content. For low O2 contents and especially with propene it is explained by the formation of carbon deposits. For high O2 contents and especially with propane it is explained by the excess of unreacted O2. The best catalysts would be those which are able to limit the formation of carbon deposits, to oxidize CO and the hydrocarbons in the same temperature range or to oxidize CO by NO more easily than by O2. With the complex mixtures (CO, NO, O2, CO2, H2O, hydrocarbons), the best catalysts, Pd on zirconia and Pd on Ba-modified alumina, which obey on these requirements, have an activity nearly comparable to that of the reference Pt-Rh/Al203 solid. 

Mechanistic studies of formation of carbon deposits on supported Pt catalysts during wet oxidation of phenol... 

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