Deposits on the Catalyst Surface

The blocking of catalyst pores by polymeric components, especially coke, is another widely encountered cause of catalyst deactivation. In many reactions of hydrocarbons, side reactions lead to formation of polymers. If these are deposited near the pore openings, catalyst activity and selectivity can be influenced due to unpaired mass transport into and out of the pores. 

At high temperatures (above 200 °C) these polymers are dehydrogenated to carbon, a process known as coking. Especially catalysts with acidic or hydrogenating/ dehydrogenating properties cause coking. Coking on acid centers is observed with zeolite and aluminosilicate catalysts and with acidic supports. The extent of coke formation depends directly on the acidity. 

The precursors for coke formation are mainly aromatic and olefinic hydrocarbons, which are either contained in the starting materials or are formed as intermediate products in the process. 

In some processes, 5 -10 % zeohte is added to amorphous cracking catalysts. This increases the activity by several orders of magnitude and drastically reduces coke formation. This is another example of shape-selective catalysis by zeolites, in which the coke-forming intermediates are restricted by the zeohte pores (see Section 7.3.1). 

Therefore, heavy metals must be removed from crude oil fractions. Various processes are used chemical or adsorptive removal of the porphyrins, or demetallation by hydrogenation and binding the metals on AI2O3. Another effective method is the use of additives. For example, the heavy metals can be alloyed by adding antimony, after which they are deposited on the catalyst in a different form. 

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Another approach used to reduce the harmful effects of heavy metals in petroleum residues is metal passivation. In this process an oil-soluble treating agent containing antimony is used that deposits on the catalyst surface in competition with contaminant metals, thus reducing the catalytic activity of these metals in promoting coke and gas formation. Metal passivation is especially important in fluid catalytic cracking (FCC) processes. Additives that improve FCC processes were found to increase catalyst life and improve the yield and quality of products. ... 

In the previous section we have assumed that AO, thus n, is an independently controllable variable, such as pj. This is true both in electrochemical promotion experiments, since AO=eAUWR and in classical promotion experiments where AO can largely controlled, albeit not in situ, by the amount of promoter species deposited on the catalyst surface. 

It was previously reported that magnesium oxide with a moderate basicity formed reactive surface carbonate species, which reacted with carbon deposited on foe support by foe methane decjomposition [6]. Upon addition of Mg to foe Ni/HY catalyst, reactive carbonate was formed on magnesium oxide and carbon dioxide could be activated more easily on the Mg-promoted Ni/HY catal t. Reactive carbonate species played an important role in inhibiting foe carbon deposition on the catalyst surface. 

As described in Section 3.2.3, the use of acidic supports such as A1203 favors the dehydration of ethanol to ethylene, which leads to a severe carbon deposition.66,76,78,85 Reactions with lower H20/ethanol ratio can also favor several side reactions mentioned above and result in carbon deposition on the catalyst surface. Possible strategies to reduce the carbon deposition include (i) neutralization of acidic sites responsible for ethanol dehydration to ethylene and/or modification of the support nature, including less acidic oxides or redox oxides, (ii) use of a feed containing higher H20/ethanol molar ratio, and (iii) addition of a small concentration of air or 02 in the feed. 

Alloying of the active metal such as Ni with Cu has also been known to suppress the carbon deposition on the catalyst surface and this improved the stability of the Ni-Cu/Si02 catalyst during ethanol reforming.34... 

For example, the most noteworthy disadvantage of catalytic wet oxidation is the severe catalyst deactivation (Larachi el al., 1999). Hamoudi el al. (1998, 1999) systematically studied the deactivation of Mn02/Ce02 catalyst during wet catalytic oxidation of phenol and the catalyst-surface modifications. It was observed that deactivation was induced mainly by the formation of carbonaceous deposits on the catalyst surface. Ohta et al. (1980) reported that the size of the catalyst particles affected the stabilization of catalytic activity. For granular particles of supported copper oxide, the catalytic activity was decreased after each inn, even after six successive experiments. In contrast, for larger particles the catalytic activity was stabilized after the first three runs. 

The reactions take place only in active catalytic layer, the rates Rj are considered individually for each type of the converter (DOC, SCR, NSRC, TWC). The development of suitable reaction schemes and the evaluation of kinetic parameters are discussed generally in Section IV. The details for DOC, NSRC and SCR of NOx by NH3 are given in Sections V, VI and VII, respectively. The important species deposited on the catalyst surface are balanced (e.g. HC adsorption in DOC, oxygen and NOx storage in NSRC, NH3 adsorption in SCR). Heat transfer by radiation and homogeneous reactions... 

As described above, the presence of H20 not only retards the formation of the carbon deposits on the catalyst surface, but also enhances their oxidation to C02 and CO. H20 regenerates the Ti02 surface hydroxyl groups which are consumed in the photoreaction. Based on these results, the following reaction mechanism is proposed focusing on the role of the hydroxyl groups. 

Large asphalthenes crack and decompose nearly instantaneously, and metals are deposited on the catalyst surface. 

Coke is a typical example of a reversible catalyst poison. The deactivation influence of coke depends very much on the nature of the coke, its structure and morphology and the exact location of its deposition on the catalyst surface [42, 43, 44]. 

Carbon and metal sulfide deposits are the two main causes of deactivation of residue hydrodemetallization (HDM) catalysts. During a catalytic test, the metals contained in the feed (Ni, V) are slowly deposited on the catalyst surface leading to the build up of large particles of metal sulfides which ultimately plug the catalyst pores. Carbon, on the other hand, is known to accumulate quickly on the catalyst surface within the first days of a run until a steady state is reached (1-20). At the beginning of a run, a strong deactivation of the residue HDM catalyst rapidly occurs to which both types of deposits may contribute. However at the present time it is not clear whether this initial deactivation is mainly due to coke or metal sulfide deposits. 

The used catalysts of the TS series contain low metal contents. However in view of the possible deactivating effect of 0.7 or 1.3 wt % of vanadium, samples containing even less vanadium were required. These samples have been prepared in batch reactor where the catalyst/oil ratio determines the maximum amount of metal deposited. In this work, with a catalyst/oil ratio of 0.4 and an the SAR containing 106 wt ppm Ni + V, the maximum amount of metal which could be deposited on the catalyst surface is 265 wt ppm Ni + V. ... 

Allowing for some spread in the data it seems as if little deactivation is caused by the first 4-5% wt carbon deposited, after which there is an exponential activity decline. This deactivation behaviour is of course indicative of the way in which coke is deposited on the catalyst surface, The initial deposition of coke mainly takes place on the bare A1203 surface, i,e. does not interfere with the active phase as demonstrated in a previous paper [6], At higher coke levels we observe an exponential activity decline indicative of a fouling type of deactivation rather than selective poisoning. 

Fig. 4 shows the TPR profiles of the fresh and spent catalyst. Curve C shows the desorption of hydrocarbons during reduction of the spent catalyst, formed by reduction of carbonaceous deposits on the catalyst surface. The hydrogen consumption profiles of the catalyst (see Curve A and B) show the two peaks, characteristic of palladium sulfate-based catalysts, with a vanadium oxide reduction peak at approximately 400 K and a sulfate reduction peak at 600 K [11,13,16]. The peak position of the sulfate reduction peak is comparable for both catalysts. For the spent catalyst, however, an additional small hydrogen consumption is observed at 700 K, which coincides with the large peak in the FID signal,... 

This can in pan be answered because Pt/alumina, e,g. EUROPT-3 (and Pt/Re/alumina) has also been studied [7]. In n-butane hydrogenolysis on Pt/alumina the accumulation of carbonaceous deposits on the catalyst surface suppressed ethane formation (i.e. relative to that of propane formation (i.e. S3), Thus for Pt/alutnina sites responsible for central C-C bond scission in n-butane may be selectively deactivated, e.g. at 603K sample S2 S3... 

Coke preferentially deposits on the alumina support, growing in thickness, and blocks the active sites (11). As shown in Table II, a slight decrease in the amount of coke on the catalyst in the third bed with time implies that metal sulfides may reveal the coke which blocks the active sites when they deposit on the catalyst surface. The electron microprobe analysis also showed a decrease in coke with metal deposition (1). Therefore, we assume that part of the coke which blocks the active sites may be hydrogenated by metal sulfides and taken off. 

Fresh and recvcled liquid ethylbenzene combine and are heated from 25°C to 500°C and the heated ethylbenzene is mixed adiabaiically with steam at 700 C to produce the feed to the reactor at 600 C (The steam suppresses undesired side reactions and removes carbon deposited on the catalyst surface.) A once-through conversion of 35% is achieved in the reactor(2). and the products emerge at 560 C The product stream is cooled to 25°C , condensing essentially all of the water, ethylbenzene, and styrene and allowing hydrogen to pass out as a recoverable by-product of the process. 

On this process, carbon is deposited on the catalyst surface and causes the deactivation. However, the high activity of CO2 reduction was sustained over 60 h examined. The weight of Ni/SiOj catalyst increased from 0.4 to 2.04 g after CO2 reduction for 60 h. The molar amount of carbon deposited on catalysts was 200 times larger than that of supported Ni. SEM observation and XRD analysis suggest that the formed carbon on the catalyst surface was a filament shaped graphite. Figure 4 shows the TEM photograph of the... 

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