First-principles elucidation of the surface chemistry of the C2Hx (x = 0–6) adsorbate series on Fe(100)

Ab initio total-energy calculations of the elementary reaction steps leading to acetylene, ethylene and ethane formation and their decomposition on Fe(100) are described. Alongside the endothermicity of all the formation reactions, the crucial role played by adsorbed ethyl as main precursor towards both ethylene and ethane formation, characterises Fe(100) surface reactivity towards C2Hx (x = 0–6) hydrocarbon formation in the low coverage limit. A comprehensive scheme based on three viable mechanisms towards ethyl formation on Fe(100), including methyl/methylene coupling, methyl/methylidyne coupling followed by one hydrogenation and methyl/carbon coupling followed by two hydrogenations, is the main result of this article

Metal–Support Interactions in Heterogeneous Catalysis: DFT Calculations on the Interaction of Copper Nanoparticles with Magnesium Oxide

Oxide supports play an important role in enhancing the catalytic properties of transition metal nanoparticles in heterogeneous catalysis. How extensively interactions between the oxide support and the nanoparticles impact the electronic structure as well as the surface properties of the nanoparticles is hence of high interest. In this study, the influence of a magnesium oxide support on the properties of copper nanoparticles with different size, shape, and adsorption sites is investigated using density functional theory (DFT) calculations. By proposing simple models to reduce the cost of the calculations while maintaining the accuracy of the results, we show using the nonreducible oxide support MgO as an example that there is no significant influence of the MgO support on the electronic structure of the copper nanoparticles, with the exception of adsorption directly at the Cu–MgO interface. We also propose a simplified methodology that allows us to reduce the cost of the calculations, while the accuracy of the results is maintained. We demonstrate in addition that the Cu nanowire model corresponds well to the nanoparticle model, which reduces the computational cost even further

Theoretical Investigation of the Size Effect on the Oxygen Adsorption Energy of Coinage Metal Nanoparticles

This study evaluates the finite size effect on the oxygen adsorption energy of coinage metal (Cu, Ag and Au) cuboctahedral nanoparticles in the size range of 13 to 1415 atoms (0.7–3.5\ua0nm in diameter). Trends in particle size effects are well described with single point calculations, in which the metal atoms are frozen in their bulk position and the oxygen atom is added in a location determined from periodic surface calculations. This is shown explicitly for Cu nanoparticles, for which full geometry optimization only leads to a constant offset between relaxed and unrelaxed adsorption energies that is independent of particle size. With increasing cluster size, the adsorption energy converges systematically to the limit of the (211) extended surface. The 55-atomic cluster is an outlier for all of the coinage metals and all three materials show similar behavior with respect to particle size

Vibrational Stark tuning rates from periodic DFT calculations : CO/Pt(111)

DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm-1/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm-1/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm-1/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm-1/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations

Reactions of acetylene and nitrogen to CN on the surface of Rh(100)

No abstrac

Reactions of acetylene and nitrogen to CN on the surface of Rh(100)

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

Adsorption energies, structures, and vibrational frequencies of CO on Fe(100) for several adsorption states and at three surface coverages are reported. A full anal. of the vibrational frequencies of CO was performed, thus detg. what structures are stable adsorption states and characterizing the transition-state structure for CO dissocn. The activation energy of dissocn. of CO at 0.25 ML (ML = monolayers) was calcd. as well as at 0.5 ML; the dissocn. at 0.5 ML was studied to quantify the destabilization effect on the CO(a3) mols. when a neighboring CO mol. dissocs. The no. and nature of likely adsorption states is coverage dependent. Evidence is presented that shows that the CO mol. adsorbs on Fe(100) at fourfold hollow sites with the mol. axis tilted away from the surface normal by 51.0 Deg. The adsorption energy of the CO mol. is -2.54 eV and the C-O stretching frequency is 1156 cm-1. This adsorption state corresponds to the a3 mol. desorption state reported in temp. programmed desorption (TPD) expts. However, the activation energy of dissocn. of CO(a3) mols. at 0.25 ML is only 1.11 eV (~25.60 kcal mol-1) and the gain in energy is 1.17 eV; thus, the dissocn. of CO is largely favored at low coverages. The activation energy of dissocn. of CO at 0.5 ML is 1.18 eV (~27.21 kcal mol-1), very similar to that calcd. at 0.25 ML. However, the dissocn. reaction at 0.5 ML is slightly endothermic, with a total change in energy of 0.10 eV. Consequently, mol. adsorption is stabilized with respect to CO dissocn. when the CO coverage is increased from 0.25 to 0.5 ML. [on SciFinder (R)

Atom-molecule interactions on transition metal surfaces : A DFT study of CO and several atoms on Rh(100), Pd(100) and Ir(100)

Density functional theory (DFT) calculations have been performed to determine the interaction energy between a CO probe molecule and all atoms from the first three rows of the periodic table coadsorbed on Rh(100), Pd(100) and Ir(100) metal surfaces. Varying the coverage of CO or the coadsorbed atom proved to have a profound effect on the strength of the interaction energy. The general trend, however, is the same in all cases: the interaction energy becomes more repulsive when moving towards the right along a row of elements, and reaches a maximum somewhere in the middle of a row of elements. The absolute value of the interaction energy between an atom-CO pair ranges from about -0.40 eV (39 kJ mol-1) attraction to +0.70 eV (68 kJ mol-1) repulsion, depending on the coadsorbate, the metal and the coverage. The general trend in interaction energies seems to be a common characteristic for several transition metals

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

Adsorption energies, structures, and vibrational frequencies of CO on Fe(100) for several adsorption states and at three surface coverages are reported. A full anal. of the vibrational frequencies of CO was performed, thus detg. what structures are stable adsorption states and characterizing the transition-state structure for CO dissocn. The activation energy of dissocn. of CO at 0.25 ML (ML = monolayers) was calcd. as well as at 0.5 ML; the dissocn. at 0.5 ML was studied to quantify the destabilization effect on the CO(a3) mols. when a neighboring CO mol. dissocs. The no. and nature of likely adsorption states is coverage dependent. Evidence is presented that shows that the CO mol. adsorbs on Fe(100) at fourfold hollow sites with the mol. axis tilted away from the surface normal by 51.0 Deg. The adsorption energy of the CO mol. is -2.54 eV and the C-O stretching frequency is 1156 cm-1. This adsorption state corresponds to the a3 mol. desorption state reported in temp. programmed desorption (TPD) expts. However, the activation energy of dissocn. of CO(a3) mols. at 0.25 ML is only 1.11 eV (~25.60 kcal mol-1) and the gain in energy is 1.17 eV; thus, the dissocn. of CO is largely favored at low coverages. The activation energy of dissocn. of CO at 0.5 ML is 1.18 eV (~27.21 kcal mol-1), very similar to that calcd. at 0.25 ML. However, the dissocn. reaction at 0.5 ML is slightly endothermic, with a total change in energy of 0.10 eV. Consequently, mol. adsorption is stabilized with respect to CO dissocn. when the CO coverage is increased from 0.25 to 0.5 ML. [on SciFinder (R)

The influence of promoters and poisons on Carbon monoxide adsorption on Rh(100) : a DFT study

Density functional theory calculations were performed to determine the pairwise lateral interaction energies between carbon monoxide and coadsorbed elements from the first three rows of the periodic table on a Rh(100) surface. The atoms were placed in a c(2×2) arrangement of fourfold hollow sites and the carbon monoxide probe molecule in a p(2×2) arrangement, so that each CO molecule had four atoms as nearest neighbours. The alkali atoms show an attractive interaction with CO while the other atoms show a repulsive interaction. For second-row elements the maximum repulsion is at nitrogen and for third-row elements at sulphur. Attempts to correlate the interaction energies with properties of the system, such as electronegativity, distances, or change in work function, failed, which implies that each combination of adsorbates needs to be calculated separately