Carbon formation, catalyst deactivation

Rather than increasing the operating S/C ratio, it is more desirable to develop reforming catalysts that are inherently more carbon-tolerant than Ni [19, 35, 37-43], For example, it has been suggested that Ru and Rh do not facilitate the formation of carbon deposits because of poor carbon solubility in these metals [30, 44]. However, Ru and Rh are prohibitively expensive. It has also been shown that the promotion of Ni with alkaline earth metals such as Mg suppresses carbon-induced catalyst deactivation [18, 36]. There have also been reports that by selectively poisoning the low-coordinated Ni sites with small amounts of sulfur, the carbon-induced deactivation of Ni can be suppressed [9, 45]. In addition, the patent literature is rich with multiple examples where numerous additives, including those mentioned below in this text (e.g., Sn and Au), have been suggested to promote the stability of Ni catalysts [46]. 

A Model for Reforming on Ni Catalyst with Carbon Formation and Deactivation... 

However, a direct link between catalyst properties and kinetics is missing in Aparicio s model (6) and our previous work (7). The present work deals with the development of a microkinetic model for syngas production including carbon formation and deactivation, where the activation energies are related to C-Ni, 0-Ni and H-Ni bond strengths determined by the BOC-MP method. 

The experiments were performed in the Tapered Element Oscillating Microbalance (TEOM) reactor (7,8), in which carbon formation and deactivation could be measured simultaneously by coupling with on-line GC analysis. The dry reforming of methane was studied on an industrial Ni (11 wt%)/(Ca0)a-Al203 catalyst at temperatures of 500 °C and 650 °C, total pressures of 0.1 MPa and 0.5 MPa and a CO2/CH4 ratio of 1. The BET surface area of the catalyst was 5.5 m /g, and the Ni surface area 0.33 m /g. The detailed experimental procedures were similar to that reported previously (7). 

Elnashaie and Elshishini (4) have shown that the rate of steam reforming is nonmonotonic with respect to steam. Steam reforming reactions have a strong tendency to carbon formation, causing deactivation of the catalyst as follows ... 

Chen, D., Lodeng, R., Omdahl, K., Anundskas, A., Olsvik, O., and Holmen, A. A model for reforming on Ni catalyst with carbon formation and deactivation. Studies Surface Sci. Cataly. 139, 93-100, 2001. 

Four pilot plant experiments were conducted at 300 psig and up to 475°C maximum temperature in a 3.07-in. i.d. adiabatic hot gas recycle methanation reactor. Two catalysts were used parallel plates coated with Raney nickel and precipitated nickel pellets. Pressure drop across the parallel plates was about 1/15 that across the bed of pellets. Fresh feed gas containing 75% H2 and 24% CO was fed at up to 3000/hr space velocity. CO concentrations in the product gas ranged from less than 0.1% to 4%. Best performance was achieved with the Raney-nickel-coated plates which yielded 32 mscf CHh/lb Raney nickel during 2307 hrs of operation. Carbon and iron deposition and nickel carbide formation were suspected causes of catalyst deactivation. 

The activity and stability of catalysts for methane-carbon dioxide reforming depend subtly upon the support and the active metal. Methane decomposes to carbon and hydrogen, forming carbon on the oxide support and the metal. Carbon on the metal is reactive and can be oxidized to CO by oxygen from dissociatively adsorbed COj. For noble metals this reaction is fast, leading to low coke accumulation on the metal particles The rate of carbon formation on the support is proportional to the concentration of Lewis acid sites. This carbon is non reactive and may cover the Pt particles causing catalyst deactivation. Hence, the combination of Pt with a support low in acid sites, such as ZrO, is well suited for long term stable operation. For non-noble metals such as Ni, the rate of CH4 dissociation exceeds the rate of oxidation drastically and carbon forms rapidly on the metal in the form of filaments. The rate of carbon filament formation is proportional to the particle size of Ni Below a critical Ni particle size (d<2 nm), formation of carbon slowed down dramatically Well dispersed Ni supported on ZrO is thus a viable alternative to the noble metal based materials. 

Temperature plays an important role in determining the amount and type of the carbon deposit. Generally during FTS at higher temperatures the amount of carbon deposited will tend to increase,30-31 but the case is often not so straightforward. An example of temperature dependence on the rate of carbon deposition and deactivation is the case of nickel CO hydrogenation catalysts, as studied by Bartholomew.56 At temperatures below 325°C the rate of surface carbidic carbon removal by hydrogenation exceeds that of its formation, so no carbon is deposited. However, above 325°C, surface carbidic carbon accumulates on the surface... 

Carbon formation/deposition is a difficult deactivation mechanism to characterize on cobalt-based FTS catalysts. This is due to the low quantities of carbon that are responsible for the deactivation (<0.5 m%) coupled with the presence of wax that is produced during FTS. Furthermore, carbon is only detrimental to the FT performance if it is bound irreversibly to an active site or interacts electronically with it. Hence, not all carbon detected will be responsible for deactivation, especially if the carbon is located on the support. 

Carbon deposition is much greater on Co/A1203 catalysts than on Ni/Al203 (240). The presence of MgO markedly decreased the carbon deposition on the surface of the cobalt catalyst (241). The role of MgO may be attributed to the formation of strongly adsorbed C02 species, which can easily react with the deposited carbon, thus preventing catalyst deactivation (241). 

A systematic study of differently supported Ru catalysts showed that carbon catalysts provide very high selectivities to higher hydrocarbons (C10-C20) and the CNT-supported catalyst is among the most active systems of all [138]. In parts this is related to the inertness of carbon preventing the formation of hardly reducible mixed metal oxides with the support, such as CoAl204 [139,140], which is, besides coking, the main reason for catalyst deactivation. The carbon surface functionalized with oxygen... 

---