(643c) Deactivation Model for Co Fischer-Tropsch Catalysts

            One
of the foremost technical barriers to the successful, economical practice of
Fischer-Tropsch Synthesis (FTS) is uncertainty regarding catalyst life.
Understanding the chemistry of deactivation pathways and modeling catalyst
deactivation rates are imperatives to the design of more stable catalysts and
the development of robust, realistic reactor models which facilitate FT
reactor/process design and optimization. Carbon deposition on FT catalysts,
however, is not sufficiently quantitatively understood to effectively enable
its management. Our goal is to develop a deactivation rate model which predicts
the deactivation of Co FT catalysts by carbon deposition as a function of
reactant partial pressures and temperature.

The FTS reaction kinetics and
deactivation kinetics can be linked by the activity of the catalyst. Activity (a)
is a function of time, temperature, and concentrations and is defined as the CO
depletion rate normalized by the initial rate. The rate of the main reaction (r_m)
and deactivation rate (r_d) are defined as follows:

-r_m
= k(T) f₁(T,C) a

-r_d
= -da/dt = k_d(T) f₂(T,C) a^d

f₁(T, C) and f₂(T,
C) are potentially functions of reactant and product concentrations and
temperature which can lead to models with shifting reaction orders (e.g.
Langmuir-Hinshelwood) which are more complex than models traditionally used
such as power-law and generalized power-law models [1]. To verify and determine
the constants for such a model, a wide range of rate data as a function of CO
and H₂ partial pressures is needed.

The
proposed mechanism for carbon deposition is shown Figure 1. It consists of 1)
dissociation of CO on the surface to atomic carbon (C_α), 2)
transformation of C_α to polymeric carbon (C_β),
3) formation of graphitic carbons (C_C), 4) hydrogenation of C_α,
5) hydrogenation of C_β. Reactions 1, 2, 4, and 5 are
considered in this study.

Figure 1. Formation, transformation, and hydrogenation
of carbon on cobalt ((a), (g), (s) refer to adsorbed, gaseous, solid states,
respectively).

Kinetic
parameters of the first elementary step can be estimated by CO-TPD [2] and the
other elementary steps can be found by temperature-programmed hydrogenation (TPH) [2]. CO-TPD and TPH spectra on a cobalt catalyst are shown
in Figure 2.

Figure 2.
(a) CO-TPD spectra after CO adsorption at 150 °C, (b) TPH spectra after FTS for 1
h at 200 °C,
H₂/CO=1.

References:

1- Fuentes, G.A.
Applied Catalysis, 15, 33 (1985).

2- Paul, U.,
Microkinetic model of Fischer-Tropsch synthesis on iron catalysts, in Chemical
Engineering. 2008, Brigham Young University: Provo.