Carbonates formation during catalyst deactivation

Related work includes investigations of carbon formation during hydrogenation of C5 hydrocarbons catalyzed by nickel and palladium (5P) interactions of N2O with a hydrotalcite-derived multimetallic mixed oxide catalysts (60,61) changes in mass of solid oxides (62) methanol sorption in Nafion-117 (proton-exchange) membranes (63) vanadyl pyrophosphate catalysts for butane oxidation (64-66) and deactivation/regeneration of a Rb0 c/Si02 catalyst for methylene valerolactone synthesis (67). 

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. 

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. 

Also, manganese added to cobalt on activated carbon catalysts resulted in a decrease in bulk carbide formation during reduction and a decrease in the subsequent deactivation rate.84 Magnesium and yttrium added to the support in alumina-supported cobalt catalysts showed a lower extent of carburization. This was explained by a decrease in Lewis acidity of the alumina surface in the presence of these ions.87... 

Rh, Ru, Pd) and oxides (<4wt% Fe jO4/Cr2O3, La2O3, SnO2, K2O) was recently performed by Lodeng et al. [134]. A comparison with Ni- and Fe-based catalysts was also addressed. It was found that addition of metal promoters, particularly Rh and Pt, enhanced the catalyst activity at low temperatures (which resulted in delayed extinction of the reaction during ramping at —1 Tmin ). However, addition of Ni promoted carbon formation. Addition of surface oxides typically promoted instability, deactivation and combustion (hence the formation of a stable Co metallic phase was hindered). It was found that Ni performed better than Co-based catalysts at all temperatures. However, Fe-based catalysts showed high combustion activity. 

Catalysts deactivate during use. The most common reason for deactivation is coke or carbon formation on and in the catalyst particles. The carbon can be burned off under carefully controlled conditions to regenerate the catalyst with activity and selectivity very nearly like that of a new catalyst. In the removal of carbon by oxidation, the quantity of oxygen and the temperature to which the catalyst particles are exposed must be limited. 

This work, on the other hand, is directed at understanding the kinetics of carbon formation and the species responsible for deactivation. In order to understand what is happening during deactivation, it is first necessary to have an understanding of the reaction kinetics. While reaction kinetics have been reported for iron Fischer-Tropsch catalysts, operating conditions were not sufficiently general and the catalysts were different from those used in this work. Hence, it was necessary to obtain reaction parameters over a wide range of conditions to meet the needs of this study. These parameters were obtained under conditions of little or no deactivation as well as under conditions for which deactivation plays a major role so that effects due to reaction and deactivation could be separated. 

One of the principal modes of catalyst deactivation for Fischer-Tropsch (FT) catalysts is loss of catalytic surface area due to the accumulation of carbonaceous species on metal/metal carbide surfaces and in the pores of the catalyst and/or formation of inactive carbide phases [1-3]. These carbon-containing species are probably products of the condensation/polymerization of atomic carbon or CH, reaction intermediates, formed during reaction by CO dissociation and subsequent partial hydrogenation of the atomic carbon [2]. 

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