Carbon deposition supports

Optional experiment. When all the air has been displaced, collect a test-tube of the gas over water (by appropriate inclination of the end of the delivery tube beneath the mouth of a test-tube filled with water and supported in a beaker of water). Observe the colour and odour of the gas. Ignite the test-tube of gas, and note the luminosity of the flame and the amount of carbon deposited. Pure acetylene is almost odourless the characteristic odour observed is due to traces of hydrides of phosphorus, arsenic and sulphur. 

As it was established by Geus et a/.[18, 19] the decrease of the rate of carbon deposition is a positive factor for the growth of fibres on metal catalysts. Si02 is an inhibitor of carbon condensation as was shown in Ref [20]. This support also provides possibilities for the stabilization of metal dispersion. Co and Fe, i.e. the metals that give the best results for the tubular condensation of carbon on graphite support, were introduced on the surface of siUca gel... 

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. 

Fig. 1(b) represents the selectivity to styrene as a ftmcfion of time fijr the above catal ts. It is observed that the selectivity to styrene is more than 95% over carbon nauofiber supported iron oxide catalyst compared with about 90% for the oxidized carbon nanofiber. It can be observed that there is an increase in selectivity to styrene and a decrease in selectivity to benzene with time on stream until 40 min. In particrdar, when the carbon nanofiber which has been treated in 4M HCl solution for three days is directly us as support to deposit the iron-precursor, the resulting catalyst shows a significantly lows selectivity to styrene, about 70%, in contrast to more than 95% on the similar catalyst using oxidized carbon nanofiber. The doping of the alkali or alkali metal on Fe/CNF did not improve the steady-state selectivity to styrene, but shortened the time to reach the steady-state selectivity. 

As already described (1) the inhibition of the NO reduction by CO due to carbon deposits in mixtures containing hydrocarbons depends on the support, with the most acidic supports leading to higher amounts of carbon deposits. 

On the basis of the analysis presented in Tables II, III, and IV and measurements of the mass of C02 evolved during oxidation, Figure 1 was constructed to display the fraction of original carbon mobilized by heating, the fraction of the remaining (available) carbon mobilized as incompletely oxidized hydrocarbon by oxidation, and the fraction of available carbon deposited as coke by oxidation. The distribution of available carbon between the mobile and non-mobile products of oxidation lends additional support to our proposed "two-reactions" mechanism. 

Another important factor affecting carbon deposition is the catalyst surface basicity. In particular, it was demonstrated that carbon formation can be diminished or even suppressed when the metal is supported on a metal oxide carrier with a strong Lewis basicity [47]. This effect can be attributed to the fact that high Lewis basicity of the support enhances the C02 chemisorption on the catalyst surface resulting in the removal of carbon (by surface gasification reactions). According to Rostrup-Nielsen and Hansen [12], the amount of carbon deposited on the metal catalysts decreases in the following order ... 

The catalysts supports and promoters have a significant effect on the rate of carbon deposition. In particular, Zr02 has been widely used as a support for Pt because of the lower rate of carbon deposition compared to other supports [48]. The authors demonstrated the following order of the carbon formation rate ... 

Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

Figure 4.1 summarizes the different routes that can potentially lead to carbon deposition during FTS (a) CO dissociation occurs on cobalt to form an adsorbed atomic carbon, which is also referred to as surface carbide, which can further react to produce the FT intermediates and products. The adsorbed atomic carbon may also form bulk carbide or a polymeric type of carbon. Carbon deposition may also result (b) from the Boudouard reaction and (c) due to further reaction and dehydrogenation of the FTS product (what is commonly called coke), a reaction that should be limited at typical FT reaction conditions. Carbon formed on the surface of cobalt can also spill over or migrate to the support. This is reported to readily occur on Co/A1203 catalysts.43 The chemical nature of the carbonaceous deposits during FTS will depend on the conditions of temperature and pressure, the age of the catalyst, the chemical nature of the feed, and the products formed. 

Lee, D.-K., Lee, J.-H., and Ihm, S.-K. 1988. Effect of carbon deposits on carbon monoxide hydrogenation over alumina-supported cobalt catalysts. Appl. Catal. 36 199-207. 

Choi, J. G., Rhee, H. K., and Moon, S. H. 1985. IR and TPD study of fresh and carbon-deposited aluminum oxide-supported cobalt catalysts. Appl. Catal. 13 269-80. 

Pankina, G. V., Chemavskii, P. A., Lermontov, A. S., and Lunin, V. V. 2002. Study of carbon deposits on the surface of supported cobalt catalysts in the Fischer-Tropsch process. Petrol. Chem. 42 217-20. 

Alumina is one of the most commonly used supports for nickel catalysts 111, 178,194-204). Ni/Al203 exhibits carbon deposition (180) that depends on the catalyst structure, composition, and preparation conditions. 

MgO is a basic metal oxide and has the same crystal structure as NiO. As a result, the combination of MgO and NiO results in a solid-solution catalyst with a basic surface (171,172), and both characteristics are helpful in inhibiting carbon deposition (171,172,239). The basic surface increases C02 adsorption, which reduces or inhibits carbon-deposition (Section ALB). The NiO-MgO solid solution can control the nickel particle sizes in the catalyst. This control occurs because in the solid solution NiO has strong interactions with MgO and, as indicated by TPR data (26), the former oxide can no longer be easily reduced. Consequently, only a small amount of NiO is expected to be reduced, and thus small nickel particles are formed on the surface of the solid solution, smaller than the size necessary for coke formation. Indeed, the nickel particles on a reduced 16.7 wt% NiO/MgO solid-solution catalyst were too small to be observed by TEM (171). Furthermore, two additional important qualities stimulated the selection of MgO as a support its high thermal stability and low cost (250,251). 

In contrast to MgO, the other alkaline-earth oxides, such as CaO, SrO, and BaO, were found to be poor supports for NiO, as they provided catalysts with low activities, selectivities, or stabilities (Fig. 14) (239). Although the reduced Ni0/Al203 catalyst provided high initial conversions (CH4, 91% C02, 98%) and selectivities (>95% for both CO and H2), it was characterized by the fastest carbon deposition, which led to the complete plugging of the reactor after only 6 h of reaction (197). The reduced Ni/Ti02 catalyst gave relatively low initial... 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

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. 

Carbon deposition was also found on particles smaller than 1.0 nm supported on polycrystalline gamma alumina. In this case, the decay of TPD curves and the amount of carbon deposited was even more important than in the case discussed above of 1.7 nm particles on 000l a-A O. ... 

The catalyst typically employed in this process is a high Ni content catalyst ( 12-20% Ni as NiO) supported on a refractory material such as a-alumina containing a variety of promoters. Key additives are potassium and/or calcium ions, which mainly serve to suppress excessive carbon deposition on the catalyst [9]. 

---