Coke formation from methane

Elementary steps of coke formation from methane cracking and of the gasification of coke by H2O and Hz. Net rate equation for coke formation accounting also for diffusion of carbon through the Ni particles of catalyst. 

Nickel catalysts are also used for steam methane reforming. Moreover, nickel catalysts containing potassium to inhibit coke formation from feedstocks such as LPG and naphtha have received wide appHcation. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

In any case, one of the most important issues to be prevented in SOFC systems is carbon deposition (coke formation) from the fuels. Figure 6.21 shows the equilibrium products for (a) methane- and (b) methanol-based fuels with the steam-to-carbon (S/C) ratio of 1.5 at elevated temperatures [251]. Assuming thermochemical equilibrium, carbon deposition is not expected to occur within a wide temperature range. The calculated results for various other fuels mentioned above have been shown elsewhere [251]. The minimum amounts of H2O (water vapor) necessary to prevent carbon deposition are shown in Fig. 6.22 for hydrocarbon fuels. While S/C of 1.5 is enough for CH4, higher S/C is needed with increasing carbon number of hydrocarbons, especially at lower temperatures. Such dependencies have also been revealed for O2 (partial oxidation) and CO2 (CO2 reforming) [251] to prevent carbon deposition. 

It has been indicated by several investigators that strong acids rather than weak acids, Lewis acids rather than Bronsted acids favor coke formation, and that the presence of transition metal ions as impurities, e.g., Fe and Ni ions, accelerate the formation of coke. It was reported that coke formation on a nonacidic silica was less than one-twentieth that on acidic silica —alumina. Coke deposition became less serious as the strong acid sites of silica —alumina were weakened by NaOH treatments. Coke formation from hydrocarbons is usually less serious in the case of solid bases. It is reported that the deactivation of MgO and Li/MgO for methane coupling was due to sintering and loss of alkali. ... 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

In addition to coke formation and formation of carbon monoxide, the most familiar by-product is methane, which may be formed from carbon monoxide via the mefhanation reaction ... 

A feed of 100 Ncm3 min-1 methane and 50 Ncm3 min-1 oxygen was introduced into the reactor at a pressure loss of < 2.5 mbar. The residence time of the reaction was 50 ms. 60% conversion was achieved along with a high carbon monoxide selectivity of 70% at 700 °C reaction temperature. Owing to the short residence times applied, no coke formation was observed and carbon monoxide selectivity was higher than expected from the thermodynamic equilibrium [46],... 

It is also of interest to observe that the coke laydown observed experimentally is more than an order of magnitude, less than would be predicted from equilibrium calculations. That is, the amount of coke on the catalyst per liter of methane on Figure 4 at a given temperature and steam methane ratio is about 5% of that shown on Figure 2 formed under equilibrium conditions suggesting that coke formation is rate limited. 

The results show that the specificities of catalyst deactivation and it s kinetic description are in closed connection with reaction kinetics of main process and they form a common kinetic model. The kinetic nature of promotor action in platinum catalysts side by side with other physicochemical research follows from this studies as well. It is concern the increase of slow step rate, the decrease of side processes (including coke formation) rate and the acceleration of coke transformation into methane owing to the increase of hydrogen contents in coke. The obtained data can be united by common kinetic model.lt is desirable to solve some problems in describing the catalyst deactivation such as the consideration of coke distribution between surfaces of metal, promoter and the carrier in the course of reactions, diffusion effects etc,. 

Clearly the rate equation for the cracking of methane, i.e. for the coke formation is not fundamentally different from that of one of the main reactions (12). What remains to be done is to link the coke content of the catalyst to the rate of the main reactions. Thereby a specific aspect of coke formation on Ni/alumina catalysts has to be accounted for, namely whisker formation. The rate equation (16) is not directly applicable because it contains the concentration of coke adsorbed on the Ni-surface, which is not accessible, just like Cc 4., C. i,... The latter are eliminated through adsorption-isotherms in favor of the measurable gas phase partial pressures PcH4. Ph2> but this is not possible for coke. What is done in the derivation of (5) and (6), where the same problem is already encountered, is to manipulate the expressions so as to factor out the Cq, thus yielding the... 

---