Surface carbonaceous compounds

Characterization of Deactivated Catalysts by SOj BET Surface Area and Sulfur Content. Surface areas of mordenite-type zeolite catalysts after SCR of NO with hydrocarbons without SOj were quite similar to those of the fresh catalysts, as listed in Table 2. This supports the hypothesis that little or no surface carbonaceous compounds formed on the catalyst surface. About 10% of the surface area of the HM catalyst was lost after exposure to SO2, while more than 50% of the surface area of the CuHM catalyst was lost after... 

By comparing the initial activities of I and E samples and the TEM results, it can be concluded that the Pt amount on the outer surface of E sample is at least 10 times lower than that on I sample. With Pt/Al203 alone or mixed with HMCM-22 zeolite, a very fast initial deactivation can be observed, followed by a plateau in activity. This behaviour suggests a very fast initial formation of carbonaceous compounds, most likely with the co-participation of adjacent platinum and acid sites. 

Carbonaceous compounds have the peculiarity of being nondesorbed products.178,791 Therefore, their formation, besides reaction steps, requires the molecules to be retained either within the pores or on the outer surface of the molecular sieve. 

The decreasing trend for oxidation of C2H4 and CjHg in the presence of SOj was similar to that for NO conversion, regardless of the types of catdyst employed in this study. No activity loss by carbonaceous compounds deposited on the surface of the catalysts is anticipated during the course of reaction without SO,. 

The rate of the oxidation process is determined by the reactivity of the starting carbon and oxidizer. The greater the reactivity of the substrates the lower the temperature of the process in which uniform formation of the pores in the granules is observed. In the case of carbonaceous materials the cokes of brown coals show the greatest reactivity, and the cokes of hard coals the smallest activity. The cokes of pit coals show an intermediate reactivity. This is connected with the earlier mentioned ordering of the crystallographic structure of carbon, which is of significant importance in the case of modification of carbon deposits contained in the carbon-mineral adsorbents in which the carbonaceous compound may be characterized by a differentiated chemical and physical structure. Thus the surface properties of hydrothermally modified complex adsorbents are defined by the course of three processes ... 

During mesityiene coking, the carbonaceous compounds are found to be exclusively deposited on the outer surface. For n-hexane, two stages of the coke deposition become visible At shorter coking times n-hexane is mainiy deposited in the intracrystalline space, tnus simultaneously affecting a retardation of intracrystaliine diffusion ana tracer desorption. In a second stage, similar to the behaviour observed with mesityiene, coke is predominantly deposited on the crystallite surface. 

Some carbonaceous compounds are adsorbed over the surface of the catalyst. An increment on the carbon content with the time-on-stream is observed. Nevertheless, the amount of residue remaining on the surface is less than that expected from the Sr values. Likewise, processes for residue decomposition such as oxidation to CO2 or desorption with further condensation in the cooler line should also be considered, since some heavy compounds have been detected in the reactor outlet. 

Under most conditions, oxygen release into rhizosphere should be adequate to oxidize excessive levels of reduced compounds in order for wetland plants to survive soil anaerobiosis. Release of oxygen into the rhizosphere is demonstrated by the observation of oxidation of Fe + to Fe + and precipitation on the root surface, oxidation of carbonaceous compounds, and nitrification of ammonium nitrogen. A detailed discussion on the fate of oxygen in the rhizosphere is presented in Chapter 6. 

The deposit of carbonaceous compounds inside the pores or on the outer surface of zeolites (coke) is the main cause of their deactivation during the transformation of organic reactants [1-4]. The cost of this deactivation is very high and great efforts have been and are being made to find methods i) for limiting the formation of coke and its effect on the zeolite activity and ii) for regenerating the activity. The aim of this paper is to illustrate these methods by several examples chosen in the transformation of hydrocarbons. 

Magnoux, P., Roger, P., Canaff, C., Fouche, V., Gnep, N.S. and Guisnet, M. (1987), New technique for the characterization of carbonaceous compounds responsible for zeolite deactivation, in B. Delmon and G.F. Froment (eds.). Studies in Surface Science and Catalysis, Catalyst Deactivation, Elsevier, Amsterdam, pp. 317-330. 

Surface properties of the support material influence the adsorption states of Pt(acac)2, Cr(acac)3 and V(acac)3 as well as the decomposition pathways of the adsorbates and finally the dispersion of the catalytic compound. The deposited particles are more mobile on silica and hence, more capable to agglomerate. Alumina-supported Pt may be stabilized by coordinativdy unsaturated Al " " surface ions similar arguments may apply for the stabilization of amorphous Cr203 on alumina. Because the metal acetyl acetonate decomposition is accompanied by deposition of carbonaceous compounds an additional air treatment of the samples is required. Finally, the fluidized-bed technique has been proven to be applicable for preparation of catalyst particles of uniform dispersion of the catalytic compound throughout the whole bed of particles. 

A possible process for the SiOx film deposition at low temperatures below 70°C may be a special plasma CVD method which involves some elimination of carbonaceous compounds. In the plasma CVD, we believe that SiOx films are formed in two reactions (1) the bond scission of Si-OC bond in TEOS to form Si or SiO radicals, and (2) the recombination between two radicals to form Si-O-Si linkage. The repetitious combination of the two reactions leads to the Si-O-Si network, and as a result, a SiOx film is deposited. On the other hand, fragments eliminated from TEOS, ethyl or ethoxy radicals, also are recombined to form carbonaceous compounds and incorporated into the SiOx film. In the conventional plasma CVD process, the carbonaceous compounds in the deposited SiOx film are eliminated by the pyrolysis of the SiOx film at high temperature (500 ) (2). Therefore, in the SiOx-deposition on the PET film surface, some special process for the elimination of the carbonaceous compounds instead of the pyrolysis treatment should be investigated. 

In this study, we have investigated three possible processes for the elimination of the carbonaceous compounds from the deposited SiOx films (1) Choice of silane compounds used as a starting material for the SiOx film deposition by the plasma CVD, (2) Oxidation processes by oxygen plasma, and (3) Etching processes occurring on an electrode surface. 

Numerous soot oxidation catalysts have been reported since the 1980s, because soot oxidation is fundamentally a simple complete oxidation reaction (carbonaceous compounds CO2 + H2O), so that sophisticated catalysts with high selectivity are not required. However, there is a critical problem in establishing contact and interaction, directly or indirectly, between the reactant (soot) and the catalyst, both of which are solid materials. Therefore, soot oxidation catalysts reported to date can be classified according to the assumed working mechanism that solves this problem. In this review the authors classify the catalysts into the four types shown in Fig. 2.5, based on the mediator for the oxidation reaction that connects the active sites of catalyst and soot surfaces mobile catalysts, mobile oxygen catalysts, NO2 mediating... 

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