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* = CO2** (3)
CO2** = 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) ΔHad 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 Eact for CO adsorption increases, Eact for CO dissociation decreases, and C & O coverages increase with increasing adsorption temperature from 298 to 423 K.