H(2) scattering and dissociative adsorption on the W(100)-c(2 × 2)Cu surface alloy is studied based on DFT calculations. A strongly site dependent reactivity is observed in line with results obtained for the density of states projected onto the W and Cu atoms of the topmost layer. H(2) dissociation on a defect free terrace of W(100)-c(2 × 2)Cu is found to be a non-activated process like on W(100), despite the reduction of the number of energetically accessible dissociation pathways at low impact energies due to the presence of Cu atoms. A prominence of dynamic trapping and a reduction of the efficacy of trapping to promote dissociation is also verified, leading to a decrease of the initial sticking probability as a function of the molecular impact energy, in qualitative agreement with experimental findings. The heterogeneous reactivity is also evidenced by two different kinds of reflection events at low energies. Its combination gives rise to a broad specular peak superimposed on a cosine-like angular distribution of scattered molecules which is in good agreement with available experimental data.
We present a Molecular Dynamics (MD) study based on Density Functional Theory (DFT) calculations for H(2) interacting with a Pd-Cu(111) surface alloy for low Pd coverages, Θ(Pd). Our results show, in line with recent experimental data, that single isolated Pd atoms evaporated on Cu(111) significantly increase the reactivity of the otherwise inert pure Cu surface. On top of substitutional Pd atoms in the Pd-Cu(111) surface alloy, the activation energy barrier for H(2) dissociation is smaller than the lowest one found on Cu(111) by a factor of two: 0.25 eV vs. 0.46 eV. Also in agreement with experiments, our DFT-MD calculations show that a large fraction of the dissociating H atoms efficiently spillover from Pd (i.e. the active sites), thanks to their extra kinetic energy due to the ~0.50 eV chemisorption exothermicity. Still, our DFT-MD calculations predict a dissociative sticking probability for low energy H(2) molecules that is much smaller than the estimated value from scanning tunneling microscopy experiments. Thus, further theoretical and experimental investigations are required for a complete understanding of H(2) dissociation on low-Θ(Pd) Pd-Cu(111) surface alloys.
Chemisorption of H and H 2 on clean W(100) and W(110) surfaces is investigated from extensive density functional theory (DFT) calculations within the generalized gradient approximation (GGA). We obtain properties of the clean surfaces [e.g., the well-known reconstructed structure of W(100) below 200 K] as well as adsorption energies and geometries of H atoms chemisorbed on both faces of W in very good agreement with available experimental data. From our DFT-GGA results, we build accurate six-dimensional potential energy surfaces (PESs) which are used to compute dissociative sticking probabilities of H 2 on both faces of W through classical trajectory calculations. For both systems, our theoretical results agree with the available experimental data at low impact energies and are larger than experiments at higher energies. The main signatures of the sticking probabilities found in molecular beam experiments are reproduced, at least qualitatively, by our calculations: for instance, (i) the larger values for W(100) than for W(110), (ii) the nonmonotonic and monotonic energy variation for W(100) and W(110), respectively, and (iii) the dependence on incidence angle as a function of W face and impact energy. Our calculations show that dissociative adsorption of H 2 on both W(100) and W( 110) is a nonactivated process. In contrast with the widely accepted assumption that H 2 chemisorption on W( 110) is a purely direct process, we obtain that, at very low energies, adsorption takes place through an indirect (dynamic trapping) and a direct mechanism on both surfaces. Thus, the qualitatively different behavior of the sticking probability at low energies arises from a smaller contribution of dynamic trapping in the case of H 2 /W(110), due to slight differences between the corresponding PESs far from the surface in the entrance channel.
Normal incidence scattering of hydrogen atoms off a H-covered tungsten W(110) surface is simulated via quasiclassical trajectories. A density functional theory (DFT) based multiadsorbate potential is developed to model a wide range of surface coverages, θ = 0.25−1 monolayer (ML), reproducing the surface arrangements observed at low temperature. The competition between hot-atom (HA) and Eley−Rideal (ER) abstraction mechanisms is studied for collision energies of the projectile atom in the range E p = 0.1− 5.0 eV (E p = 0.1−2.0 eV) for θ = 0.25 ML (θ = 0.5, 0.75, and 1 ML) coverage. Cross sections, final energies of the recombination products, and reaction times are analyzed. At low coverage and low collision energy, HA dominates the abstraction, whereas HA and ER cross-sections become similar when collision energy increases. The vibrational distribution of recombined H 2 molecules at finite coverage is found to be in better agreement with experiments than the one computed within the single adsorbate limit. At high surface coverage, ER dominates abstraction but the dynamical observables highlight the similarity between both reaction mechanisms, thus suggesting that abstraction may be considered as a unique process.
Dynamics of the Eley-Rideal (ER) abstraction of H 2 from W(110) is analyzed by means of quasiclassical trajectory calculations. Simulations are based on two different molecule-surface potential energy surfaces (PES) constructed from Density Functional Theory results. One PES is obtained by fitting, using a Flexible Periodic London-Eyring-Polanyi-Sato (FPLEPS) functional form, and the other by interpolation through the corrugation reducing procedure (CRP). Then, the present study allows us to elucidate the ER dynamics sensitivity on the PES representation. Despite some sizable discrepancies between both H+H/W(110) PESs, the obtained projectile-energy dependence of the total ER cross sections are qualitatively very similar ensuring that the main physical ingredients are captured in both PES models. The obtained distributions of the final energy among the different molecular degrees of freedom barely depend on the PES model, being most likely determined by the reaction exothermicity. Therefore, a reasonably good agreement with the measured final vibrational state distribution is observed in spite of the pressure and material gaps between theoretical and experimental conditions.
The interaction between Trichoderma pseudokoningii (Rifai) 511, 2212, 741A, 741B and 453 and the arbuscular mycorrhizal fungi Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe BEG12 and Gigaspora rosea Nicolson & Schenck BEG9 were studied in vitro and in greenhouse experiments. All T. pseudokoningii strains inhibited the germination of G. mosseae and Gi. rosea except the strain 453, which did not affect the germination of Gi. rosea. Soluble exudates and volatile substances produced by all T. pseudokoningii strains inhibited the spore germination of G. mosseae. The germination of Gi. rosea spores was inhibited by the soluble exudates produced by T. pseudokoningii 2212 and 511, whereas T. pseudokoningii 714A and 714B inhibited the germination of Gi. rosea spores by the production of volatile substances. The strains of T. pseudokoningii did not affect dry matter and percentage of root length colonization of soybean inoculated with G. mosseae, except T. pseudokoningii 2212, which inhibited both parameters. However, all T. pseudokoningii strains decreased the shoot dry matter and the percentage of AM root length colonization of soybean inoculated with Gi. rosea. The saprotrophic fungi tested seem to affect AM colonization of root by effects on the presymbiotic phase of the AM fungi. No influence of AM fungi on the number of CFUs of T. pseudokoningii was found. The effect of saprotrophic fungi on AM fungal development and function varied with the strain of the saprotrophic species tested.
The dissociative adsorption of molecular hydrogen on Pd(x)Ru(1-x)/Ru(0001) (0 ≤ x ≤ 1) has been investigated by means of He atom scattering, Density Functional Theory and quasi-classical trajectory calculations. Regardless of their surroundings, Pd atoms in the alloy are always less reactive than Ru ones. However, the reactivity of Ru atoms is enhanced by the presence of nearest neighbor Pd atoms. This environment-dependent reactivity of the Ru atoms in the alloy provides a sound explanation for the striking step-like dependence of the initial reactive sticking probability as a function of the Pd concentration observed in experiments. Moreover, we show that these environment-dependent effects on the reactivity of H2 on single atoms allow one to get around the usual constraint imposed by the Brønsted-Evans-Polanyi relationship between the reaction barrier and chemisorption energy.
The reaction of diatomic molecules on bimetallic surfaces, formed by one to few monolayers of a metal adsorbed on a different metal substrate, is relevant to understand the role of surface strain and substrate chemical binding in catalysis, which is interesting for industrial applications, challenges existing state-of-the-art theoretical methods, due to the additional complexity associated with having a molecule with triplet spin multiplicity. Here, we have studied the interaction of O2 with Cu xML/Ru(0001) (x being the number of Cu monolayers), for which experimental data are available, by means of multidimensional classical dynamics simulations based on first-principles potential energy surfaces. Our results show, on the one hand, that the inclusion of the surface temperature on the simulations is essential to reproduce the experimental observations, and therefore, to analyze the physical mechanisms behind these observations, and, on the other hand, that electronic effects due to the binding between the two metallic species are only relevant for one Cu monolayer, whereas strain is responsible for the observed reactivity in O2 interacting with Cu(x≥2)ML/Ru(0001).
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