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.

A DFT study of the adsorption and dissociation of CO on Fe(100): influence of surface coverage on the nature of accessible adsorption states.

CO adsorption structures and energetics on the iron (100), (110), (111), (210), (211), and (310) surfaces from the lowest coverage up to saturation have been computed using spin-polarized density functional theory and ab initio thermodynamics. It is found that different adsorption configurations on each of these surfaces at high coverage can coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. Only molecular CO adsorption is possible at very high coverage and only dissociative CO adsorption at very low coverage, whereas mixed molecular and dissociative CO adsorption becomes possible at medium coverage. The computed stable adsorption configurations and the respective C–O and Fe–C stretching frequencies as well as desorption temperatures on the (100), (110), and (111) surfaces agree very well with the available experimental data. Such agreements between theory and experiment validate our co...

Coverage-Dependent CO Adsorption and Dissociation Mechanisms on Iron Surfaces from DFT Computations

CO activation on Fe(100) at different coverage was systematically computed on the basis of spin-polarized density functional theory. At the saturated coverage (11CO) on a p(3 × 4) surface size (24 exposed Fe atoms), top (1CO), bridge (3CO) and 4-fold hollow (7CO) adsorption configurations coexist. The stepwise adsorption energies and dissociation barriers at different coverage reveal equilibriums between desorption and dissociation of adsorbed CO molecules. It is found that only molecular adsorption is likely for nCO = 8–11, and mixed molecular and dissociative adsorption becomes possible for nCO = 5–7, while only dissociative adsorption is favorable for nCO = 1–4. The computed CO adsorption configurations and stretching frequencies as well as desorption temperatures from ab initio thermodynamic analysis agree well with the available experimental data.

High Coverage CO Activation Mechanisms on Fe(100) from Computations

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 × 2) (θS = 0.25 ML) and on Fe(100)−S−c(2 × 2) (θS = 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 × 2), which, interestingly, is very similar to the activation energy of CO dissociation on the sulfur-free Fe(100) surface. Ho...

A DFT Study of the Adsorption and Dissociation of CO on Sulfur-Precovered Fe(100)

A theoretical study of the site of CO dissociation on Fe(100)

A theoretical comparison of CO bonding on the Fe(100) surface

Ab initio cluster calculation which model the adsorption of Al atoms on the basal plane of graphite are presented One layer clusters find the Al prefers to adsorb at the center of the carbon rings A two layer cluster determines the most stable Al adsorption site to be above the carbon atom in agreement with recent Scanning Tunneling Microscopy experiments The charge distributions on the clusters are oscillatory and largely independent of the Al position The form of the lowest unoccupied pi orbital for each cluster essentially determines the cluster charge distribution From a constrained space orbital variation we find the on top site to be favored when Al and cluster wavefunctions are superimposed However when the Al and pi orbitals interact the relaxation favors the open site

http://iopscience.iop.org/article/10.1088/0031-8949/41/6/035/pdf

Timothy E Meehan

Papers

A theoretical comparison of CO bonding on the Fe(100) surface

An ab initio MO study of aluminium atom adsorption on the basal plane of graphite

Surface science letterA theoretical comparison of CO bonding on the Fe(100) surface