(722f) Microkinetic Study of CO Adsorption and Dissociation On Fe Catalysts

CO
adsorption and dissociation are important reaction steps in Fischer-Tropsch
synthesis on Fe catalysts.  CO is known to adsorb in the molecular state at
200-450 K, while at temperatures above 350 it dissociates, at least in part, to
C and O atoms, the latter of which participate in CO hydrogenation and
water-gas-shift reactions. C and O atoms also recombine at T > 600 K. In
previous kinetic studies of CO adsorption and dissociation on Fe surfaces by
TPD, kinetic parameters were estimated using shape factors or other semi-empirical
analytical methods based on the Redhead equation. Kinetic parameters for CO
adsorption and dissociation on polycrystalline or supported Fe derived from TPD
were not reported previous to this study.   

A quantitative microkinetic model has been developed for CO adsorption
and dissociation occurring during temperature-programmed desorption of CO on
unsupported and supported Fe catalysts.  Kinetic parameters (activation
energies and preexponential factors) and surface coverages were obtained by
solving simultaneously a set of differential equations coupled with a robust
nonlinear regression routine for various models of the reaction including the
following sequence of elementary steps (where _* represents a surface
site).

                                    CO_*            =      CO(g)    +   _*                                             (1)

                                CO_*  +   _*      =      C_*          +   O_*                                         (2)

                                CO_*  +  O_*    =      CO_2**                                                         (3)

                                    CO_2**        =     CO₂(g)   +   2*                                          (4)

Effects of coverage, immobile species, and multiple adsorption,
desorption, or dissociation sites were also considered in different forms of
the rate equations for the elementary steps.  Starting values of preexponential
factors were calculated from Transition State theory, while initial guesses for
activation energies were selected from previous literature estimates.

TPD data were obtained as a function of adsorption temperature, K or Pt
promoter level, and metal dispersion.  Results of fitting these data to various
models provides new insights into the effects of these variables on (1) ΔH_ad of CO on Fe; (2) activation energies
for CO desorption and dissociation and for C + O recombination; and (3) CO, C,
and O coverages.  For example, the E_act
for CO adsorption increases, E_act
for CO dissociation decreases, and C & O coverages increase with increasing
adsorption temperature from 298 to 423 K.