In the present article, we report adsorption energies, structures, and vibrational frequencies of CO on Fe(100) for several adsorption states and at three surface coverages. We have performed a full analysis of the vibrational frequencies of CO, thus determining what structures are stable adsorption states and characterizing the transition-state structure for CO dissociation. We have calculated the activation energy of dissociation of CO at 0.25 ML (ML = monolayers) as well as at 0.5 ML; we have studied the dissociation at 0.5 ML to quantify the destabilization effect on the CO(alpha3) molecules when a neighboring CO molecule dissociates. In addition, it is shown that the number and nature of likely adsorption states is coverage dependent. Evidence is presented that shows that the CO molecule adsorbs on Fe(100) at fourfold hollow sites with the molecular axis tilted away from the surface normal by 51.0 degrees. The asorprton energy of the CO molecule is -2.54 eV and the C-O stretching frequency is 1156 cm(-1). This adsorption state corresponds to the alpha3 molecular desorption state reported in temperature programmed desorption (TPD) experiments. However, the activation energy of dissociation of CO(alpha3) molecules at 0.25 ML is only 1.11 eV (approximately 25.60 kcal mol(-1)) and the gain in energy is -1.17 eV; thus, the dissociation of CO is largely favored at low coverages. The activation energy of dissociation of CO at 0.5 ML is 1.18 eV (approximately 27.21 kcal mol(-1)), very similar to that calculated at 0.25 ML. However, the dissociation reaction at 0.5 ML is slightly endothermic, with a total change in energy of 0.10 eV Consequently, molecular adsorption is stabilized with respect to CO dissociation when the CO coverage is increased from 0.25 to 0.5 ML.

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Sulfur is known to be a poison to several catalytic reactions, e.g., the Fischer-Tropsch synthesis (FTS), in which it affects drastically the performance of both iron- and cobalt-based catalysts. However, despite the importance of this industrial process, little is known about what elementary steps are poisoned by sulfur. In the present article, we report, using density functional theory, the effect of sulfur on one of the most relevant reactions in the FTS: the dissociation of carbon monoxide over iron surfaces. We have studied the adsorption and dissociation of CO on Fe(100)-S-p(2 x 2) (theta(S) = 0.25 ML) and on Fe(100)-S-c(2 x 2) (theta(S) = 0.50 ML). We have found surface configurations that correlate well with the desorption features observed in temperature-programmed desorption mass spectroscopy. In addition, we have calculated the activation energy of CO dissociation on Fe(100)-S-p(2 x 2), which, interestingly, is very similar to the activation energy of CO dissociation on the sulfur-free Fe(100) surface. However, the sign of the reaction changes by the presence of sulfur; CO dissociation is highly exothermic on the sulfur-free Fe(100) surface, whereas on the Fe(100)-S-p(2 x 2) surface, it is slightly endothermic. Moreover, according to our results, the influence of sulfur in the CO dissociation seems to be short-ranged.

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