Catalyst carbon deposition on nickel

Higher Hydrocarbons. - A number of papers describing the steam reforming of higher hydrocarbons are particularly concerned with the subject of carbon deposition on the catalysts. The subject of carbon deposition on nickel catalysts is considered to be somewhat outside the subject of this review, especially as the subject is covered by two excellent recent discussions of papers on carbon deposition and coking during steam reforming, methanation, and other reactions.202 203... 

TPSR CHARACTERIZATION OF CARBON DEPOSITED ON NICKEL CATALYSTS FOLLOWING ETHYLENE EXPOSURE... 

Carbon Deposited on Nickel Catalysts by Exposure to Carbon Monoxide. In a previous TPSR study (9) carbon was deposited on at 25-wt% Ni/A Oj catalyst (G-65) by exposure to CO at temperatures between 550 and 610 K. TPSR (t ) of carbon produced by the dissociation/Qisproportionation of chemisorbed CO showed the presence of large a and p states (Figure 7). Although some chemisorbed CO may have contributed to the a states, it was, conclusively shown that most of the a carbon state was more reactive than a monolayer of chemisorbed CO. In this study, we extended the deposition of carbon by CO exposure to higher temperatures to deter-mine if the 6 and 6 filament carbon states in addition to the a and p states can be populated by a hydrogen-free source. The TPSR (H2) results (Figure 8) clearly show the presence of the filament carbon (6T carbon) state at 773 K. Comparison of the peak temperatures for carbon deposited by C2H and CO exposure (Table IV) shows little difference in the reactivity of the carbon states. 

Inui (180) and Rostrup-Nielsen et al. (175) reported that the amount of carbon deposited on metal catalysts decreases in the order Ni Rh > Ir = Ru > Pt = Pd at 773 K and Ni > Pd = Rh > Ir > Pt Ru at 923 K. Thus, the noble metals exhibit higher selectivities for a carbon-free operation than nickel. Nevertheless, carbon deposition does also occur on noble metals. 

Fig. 10. Carbon deposition on nickel-containing catalysts at 973 K as determined by TGA. Before reaction, the catalysts were reduced at 1073 K for 3 h. Reaction conditions temperature, 973 K feed gas molar ratio, C02/CH4 = 1/1 GHSV, 144,000 mL (g catalyst)-1 h-1 (214).

Avoiding carbon deposition on the catalyst is a major challenge [2, 3]. Carbon can be present as graphite-like coke and in the form of whiskers, or carbon nanofibers. The latter lead to detachment of the nickel crystallites from the support and breaking of the catalyst pellets. This may cause blockage of the reformer reactor tubes and the formation of hot spots. Higher hydrocarbons exhibit a larger tendency to form... 

Figure 2.33. TGA data showing the effect of alloying of nickel with gold on the amount of carbon deposited on the catalyst surface during steam reforming of n-butane.10...

A dependence of the amount of carbon deposition on the nickel loading was observed forNi/Al203 catalysts (197). For example, a l-wt% Ni/Al203 exhibited much less carbon deposition than a 13.6 wt% Ni/Al203 catalyst (197). 

De Deken and his colleagues197 have studied the nature of the carbon deposited on a commercial CCE catalyst (12 wt% Ni on a-Al203) and have concluded that it has diffused into the bulk of the nickel and that some of it is present as carbide. A more applied article from the same group198 presents intrinsic kinetic data from a tubular reactor in the temperature range 823-953K. 

When steam reforming of saturated hydrocarbons was carried out on nickel catalysts at a high steam - carbon ratio (eg >1.5 mol/atom) [1,8], no carbon deposition was observed after several hours of reaction. The same results (on N1/AI2O3) were observed during CO methanation at H2 CO ratios of 1 to 3 [4]... 

In the discussion of the subject Balandin mentions (15) that Fischer previously postulated that methylene radicals may be produced as an intermediate in the formation of hydrocarbons by his method (116). This mechanism of carbon deposition on platinum supported on oxides of nickel and chromium (oxidized nichrome) through the intermediate formation of methylenes was thought by Balandin to be similar to the mechanism of dehydrogenation over this type of catalyst in that both occur on the boundaries of platinum-nickel and of platinum-chromia and were brought in agreement by him with his multiplet theory (26). 

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