A density functional theory study of CO oxidation on Ru(0001) at low coverage

Computational investigation of N2O adsorption and dissociation on the silicon-embedded graphene catalyst: A density functional theory perspective

One of the encouraging processes to protect the environment is the catalytic conversion of N₂O and other harmful greenhouse gases. Employing heteroatom dopants into the Graphene structure for this conversion is an attractive technique owing to its relatively low price and the very low destructive impacts. DFT was applied to explore fundamental and principal reactions of N₂O adsorption and dissociation over the Silicon-embedded Graphene catalyst to contribute to the search for green catalysts in the conversion of toxic gases into less harmful ones. Forming a surface peroxy group O₂^2-, N₂O bond cleavage and oxygen atom transfer were theoretically investigated. It is found that the N₂O molecule requires +0.52, +0.88 and + 0.4 eV of activation energies through mentioned three reactions, respectively, to adsorb and decompose to N₂ and O₂. The parallel, lying-atop-011 and flat were stable forms with adsorption energies of -0.20 (-4.65), -0.19 (-4.53) and -0.18 (-4.46) and -0.19 eV (-4.53 kcal/mol), respectively. The achieved outcomes reveal that Silicon-embedded Graphene has a high potential to be used as a more efficient and green catalyst for the catalytic conversion of the air polluting gases in comparison to the Selenium-doped Graphene, Fe⁺, Manganese-embedded Graphene and Magnesium oxide (MgO) catalysts.

Density fluctuations as door-opener for diffusion on crowded surfaces

How particles can move on a catalyst surface that, under the conditions of an industrial process, is highly covered by adsorbates and where most adsorption sites are occupied has remained an open question. We have studied the diffusion of O atoms on a fully CO-covered Ru(0001) surface by means of high-speed/variable-temperature scanning tunneling microscopy combined with density functional theory calculations. Atomically resolved trajectories show a surprisingly fast diffusion of the O atoms, almost as fast as on the clean surface. This finding can be explained by a "door-opening" mechanism in which local density fluctuations in the CO layer intermittently create diffusion pathways on which the O atoms can move with low activation energy.

Ru-Catalyzed Steam Methane Reforming: Mechanistic Study from First Principles Calculations

Elucidating the reaction mechanism of steam methane reforming (SMR) is imperative for the rational design of catalysts for efficient hydrogen production. In this paper, we provide mechanistic insights into SMR on Ru surface using first principles calculations based on dispersion-corrected density functional theory. Methane activation (i.e., C-H bond cleavage) was found to proceed via a thermodynamically exothermic dissociative adsorption process, resulting in (CH _y + zH)* species ("*" denotes a surface-bound state, and y + z = 4), with C* and CH* being the most stable adsorbates. The calculation of activation barriers suggests that the conversion of C* into O-containing species via C-O bond formation is kinetically slow, indicating that the surface reaction of carbon intermediates with oxygen is a possible rate-determining step. The results suggest the importance of subsequent elementary reactions following methane activation in determining the formation of stable carbon structures on the surface that deactivates the catalyst or the conversion of carbon into O-containing species.

Vibrationally promoted electron emission from low work-function metal surfaces

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9 < or = v < or =18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10(-2) electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.

Evidence from first principles calculations for a bent CO2 intermediate in the oxidation of carbon monoxide on the Cu (110) surface

We have carried out first principles plane wave density-functional theory calculations to study the adsorption of CO molecule on a clean and unreconstructed Cu (110) surface at 1/12 monolayer coverage and have investigated the subsequent oxidation by preadsorbed oxygen atoms. As found experimentally, the CO adsorbs perpendicular to the surface plane through the carbon atom; the top site was found to be the most favorable position for CO adsorption although the short-bridge site is only slightly less stable. Surprisingly, for a sparely oxidized surface with O atoms adsorbed in hollow sites the coadsorption energy is slightly negative for only the above two CO sites which have therefore been used as starting points to explore the energy surface of the oxidation reaction. We have confirmed the existence of bent CO(2) surface intermediate as previously suggested from experimental studies. Using the nudged elastic band method, we have characterized a two step reaction which involves the formation of this intermediate. The results suggest that the rate determining step of the oxidation reaction is the formation of the intermediate and the energy barrier (200 meV) is close to although smaller than experimentally estimated values.

CO oxidation on Pd(100) and Pd(111): a comparative study of reaction pathways and reactivity at low and medium coverages

We have performed density functional theory calculations with the generalized gradient approximation to investigate CO oxidation on a close-packed transition metal surface, Pd(111), and a more open surface, Pd(100), aiming to shed light on surface structure effects on reaction pathways and reactivity, an important issue in catalysis. Reaction pathways on both surfaces at two different coverages have been studied. It is found that the reaction pathways on both surfaces possess crucial common features despite the fact that they have different surface symmetries. Having determined reaction barriers in these systems, we find that the reaction on Pd(111) is strongly coverage dependent. Surface coverages, however, have little effect on the reaction on Pd(100). Calculations also reveal that the low coverage reactions are structure sensitive while the medium coverage reactions are not. Detailed discussions on these results are given.