W.A. Diño

First principles studies for the dissociative adsorption of H2 on graphene

We investigate and discuss the interaction of H2 with graphene based on density functional (DFT) theory. We calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites on graphene. Our calculation results show that reconstructions of the carbon atoms play an important role in the H2 -graphene interactions. Activation barrier for H2 dissociation on an unrelaxed graphene is considerably high, ∼4.3 eV for a T–H–T geometry and ∼4.7 eV for a T–B–T geometry. The T–H–T(T–B–T) geometry means that the center of mass position of H2 is at the hollow(bridge) site, and the two H atoms are directed towards the top sites on the graphene. On the other hand, when the carbon atoms are allowed to relax, the activation barrier decreases, and becoming 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry. In this case, the two carbon atoms near the hydrogen atoms move 0.33 A towards the gas phase for the T–H–T geometry and 0.26 A for the T–B–T geometry.

Orientational effects in dissociative adsorption/associative desorption dynamics of H2(D2) on Cu and Pd

Abstract With the advent of sophisticated experimental techniques to study various dynamical processes on solid surfaces, e.g., initial molecular state preparation, energy- and state-resolved detection techniques, the study of dynamical processes occurring on solid surfaces is now at the stage where there is a more direct link between what experimental studies observe and what theory predicts. It would not be an exaggeration to say that, in surfaces we have a playground for physics, and the study of dynamical processes occurring on solid surfaces, such as the ones mentioned above, is a rich field for new discoveries and observations of novel physical phenomena, filled with many possibilities. The most natural test particle of choice for these reactions is hydrogen , which has always played a central role in the development of modern physics. Of the several dynamical factors that influence the dynamics of hydrogen-solid surface reactions (e.g., relative coordinates of reactions partners—hydrogen molecule and solid surface, molecular internal degrees of freedom, surface degrees of freedom), one important factor is molecular orientation. In this review, we will consider the dissociative adsorption and associative desorption dynamics of H 2 (D 2 ) molecules on/from Cu and Pd surfaces, which are typical examples of an activated and a non-activated system, respectively, and discuss how the orientation affects the dynamics of hydrogen on these surfaces and brings about such dynamical processes as steering and dynamical quantum filtering.

Dissociation and Sticking of H2 on Mg(0001), Ti(0001) and La(0001) Surfaces

We performed quantum dynamics calculations using previously obtained potential energy surfaces (PESs) for the dissociative adsorption of hydrogen molecule incident on a Mg(0001), Ti(0001), and La(0001) surface. Based on the sticking probability plots we obtained as functions of the incidence H 2 beam energy, La is the best material for hydrogen storage, followed by Ti, and then by Mg. This is due to the absence of an activation barrier in the H 2 /La(0001) system. Both H 2 /Ti(0001) and H 2 /Mg(0001) systems have activation barriers, but the H 2 /Ti(0001) system has a very small activation barrier far from the curved region of the reaction path, while the H 2 /Mg(0001) system has a high activation barrier close to the curved region along the reaction path. Our results also indicate that the sticking probability has some dependence on the vibrational state of the impending H 2 molecule for the Mg, Ti and La surfaces. The degree of dependence still varies in each metal. Vibrational effect is most observed w...

High-uptake graphene hydrogenation: a computational perspective

We review the physical mechanisms that lead toward the conversion of graphene into its fully hydrogenated counterpart, which is a material that possesses properties closer to those of diamond than graphene. These are discussed from a theoretical perspective, i.e., from calculations based on density functional theory. We first discuss stability trends in small clusters of adsorbed hydrogen, as well as surface structure and concurrent reactivity changes for graphene one-face and two-face hydrogenation. Effects of adsorbed hydrogen on graphene electronic states, which are essential to adsorbed hydrogen structure discrimination, are also discussed.

Vibrational and rotational coupling effects in the direct scattering of H2 from Cu(111)

Abstract We investigate how the coupling between molecular vibration and rotation affects the direct scattering of H 2 from Cu(111) by performing coupled channel calculations. Our calculation results show that the rotational excitation probability of H 2 scattered (vibrationally elastic) in the first vibrationally excited state ( ν =1) increases more rapidly than that of H 2 scattered (vibrationally elastic) in the vibrational ground state ( ν =0) with increasing translational energy in the low translational energy region. Furthermore, the calculation results for the ratio of the probability of H 2 scattered in ν =1 and rotational state J =2 to the probability of H 2 scattered in ν =1 and J =0 as a function of translational energy qualitatively reproduce the experimental results in the low translational energy region.

A microscopic theory of STM-induced CO desorption from Cu(111)

Abstract We propose a microscopic theory to describe the dynamics of STM-induced desorption of CO from Cu(111). In the theory, a single electron initially occupies the CO 2π ∗ orbital, and the CO–Cu stretching vibration, described by a Morse potential, is in the ground state. An excitation of the CO–Cu stretching vibration occurs when this electron subsequently transfers/tunnels from the CO 2π ∗ orbital to the metal. The excitation of the CO–Cu stretching vibration to an unbound state leads to CO desorption from Cu(111). Our calculation results for the desorption probability agree with experimental results, and show the same isotope effects as those observed experimentally.

On the Molecular Orientation Dependence of Dynamical Processes on Solid Surfaces: Dissociative Adsorption and Scattering

Recently molecular orientation dependence has been observed in several dynamical processes on solid surfaces. In the first half of this paper, the dissociative adsorption of a hydrogen molecule interacting with a metal surface is cited as an example of a dynamical process on solid surface which shows strong molecular orientation dependence in the range of translational energy comparable to the height of the activation barrier around 1 eV. On the basis of the results of numerical calculation, it is shown how the orientation dependence results in what is called steering effects. In the latter half, the dissociative scattering of a hydrogen molecule interacting with a metal surface is used as a representative dynamical process which shows strong molecular orientation dependence in the range of translational energy around a few hundred eV. On the basis of the results of numerical calculation, it is shown how the orientation dependence manifests itself.

Molecular orientation dependence of ortho-para conversion of a H2 interacting with a metal surface

In order to obtain a general conclusion regarding the steric effect on the ortho-para H2 (o-p H2) conversion on solid surfaces, we investigate the molecular orientation dependence of the o-p conversion of a H2 interacting with a metal surface. Taking the H2-surface electron interaction (Coulomb interaction) and Fermi’s contact interaction as perturbations, our calculation results indicate that the o-p H2 conversion yield for a H2 oriented perpendicular to the surface is larger than that for a H2 oriented parallel to the surface. These results and the conclusion thus obtained generalize the steric effect on the o-p H2 conversion on metal oxide surfaces obtained previously.

First-principles study on oxygen ion conduction of La2GeO5 based on the density functional theory

We performed first-principles simulations based on the density functional theory for investigations on the atomic geometry of La2GeO5, which is a fast oxygen ion conductor applicable to solid oxide fuel cells. While two experimental studies have reported contradicting results about the configuration of GeO4 tetrahedral substructures, i.e., sp2- or sp3-like, we found that only the sp3-like form is stable. We confirmed that the favorability of oxygen sites for vacancy formation is fundamentally affected by this configuration. The bonding mechanisms between atoms are discussed based on analyses of atomic distances and electronic density of states.

Ferromagnetic nanostructures of oxygen on Ag(111)

First principles calculations were performed to investigate the robustness of oxygen adsorbed structures that show ferromagnetic properties. These ferromagnetic properties are induced by ferromagnetic superexchange interactions and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Such ferromagnetic oxygen adsorbed structures appear only in the case of coverages higher than 0.5 ML, because the ferromagnetic superexchange interactions and RKKY interactions are appreciable between O-O only for short distances. In this work, we focused on single oxygen atom diffusion parallel and perpendicular to Ag(111) surfaces. The in-plane diffusion could induce associative desorption, which reduces the oxygen coverage. On the other hand, diffusion to the subsurface could quench the magnetic moment. These diffusion paths have effective diffusion barriers of 0.78 eV. Thus, such diffusion rarely occurs in low temperature regions.