Competition between electron and phonon excitations in the scattering of nitrogen atoms and molecules off tungsten and silver surfaces

We investigate the role played by electron-hole pair and phonon excitations in the interaction of reactive gas molecules and atoms with metal surfaces. We present a theoretical framework that allows us to evaluate within a full-dimensional dynamics the combined contribution of both excitation mechanisms while the gas particle-surface interaction is described by an ab-initio potential energy surface. The model is applied to study energy dissipation in the scattering of N$_2$ on W(110) and N on Ag(111). Our results show that phonon excitation is the dominant energy loss channel whereas electron-hole pair excitations represent a minor contribution. We substantiate that, even when the energy dissipated is quantitatively significant, important aspects of the scattering dynamics are well captured by the adiabatic approximation.Comment: 4pages and 3 figure

Non-equilibrium Anisotropic Phases, Nucleation and Critical Behavior in a Driven Lennard-Jones Fluid

We describe short-time kinetic and steady-state properties of the non--equilibrium phases, namely, solid, liquid and gas anisotropic phases in a driven Lennard-Jones fluid. This is a computationally-convenient two-dimensional model which exhibits a net current and striped structures at low temperature, thus resembling many situations in nature. We here focus on both critical behavior and details of the nucleation process. In spite of the anisotropy of the late--time spinodal decomposition process, earlier nucleation seems to proceed by Smoluchowski coagulation and Ostwald ripening, which are known to account for nucleation in equilibrium, isotropic lattice systems and actual fluids. On the other hand, a detailed analysis of the system critical behavior rises some intriguing questions on the role of symmetries; this concerns the computer and field-theoretical modeling of non-equilibrium fluids.Comment: 7 pages, 9 ps figures, to appear in PR

Development of Suitable CuO-Based Materials Supported on Al2O3, MgAl2O4, and ZrO2 for Ca/Cu H2 Production Process

Functional materials for the sorption enhanced reforming process for H2 production coupled to a Cu/CuO chemical loop have been synthesized. The performance of CuO-based materials supported on Al2O3, MgAl2O4, and ZrO2 and synthesized by different routes has been analyzed. Highly stable materials supported on Al2O3 or MgAl2O4 synthesized by coprecipitation and mechanical mixing with sufficient Cu loads (around 65% wt) have been successfully developed. However, it has been found that coprecipitation under these conditions is not a suitable route for ZrO2. Spray-drying and deposition precipitation did not provide the best chemical features to the materials. As the Ca/Cu process is operated in fixed bed reactors, the best candidates were pelletized and their stability was again assessed. Pellets with high chemical and mechanical stability, high oxygen transport capacity, and good mechanical properties have been finally obtained by coprecipitation. The good homogeneity that provides this route would allow an easy scaling up

Density functional theory calculations of nitrogen adsorption features on Fe(111) surfaces

Trabajo presentado al Workshop on Controlled Atomic Dynamics on Solid Surfaces: Atom an Molecular Manipulation, celebrado en Donostia-San sebastián (España) del 13 al 16 de Mayo de 2013.The interaction of nitrogen with metal surfaces has been one of the most popular topics of research in surface science for the last decades. This is due in part to the industrial importance of ammonia synthesis, typically obtained from nitrogen and hydrogen catalyzed over iron-based compounds. The rate limiting step in ammonia synthesis is the adsorption and dissociation of nitrogen on the catalyst surface. In Fe surfaces, the reactivity of the process depends on the face, the Fe(111) and Fe(211) surfaces being the most reactive ones. Although Fe(111) is the most reactive iron face for N2 dissociation, the dynamics of such process has not been analyzed in detail. In this work we present exhaustive calculations of the interaction of nitrogen atoms and molecules with the Fe(111) surface. These calculations set the basis for subsequent analysis of the N2 dissociation dynamics. We perform Density functional Theory spin-polarized calculations using VASP code. We first study the relaxation of the Fe(111) surface, which was a matter of controversy in the past. From here, we calculate the interaction energy of nitrogen atoms and molecules when approaching the Fe(111) surface. Our results show the preferred adsorption paths and sites for nitrogen adsorption, as well as the adsorption energies. We finally discuss the dynamics of the dissociation process and make the link with the high reactivity properties of the surface.Peer reviewe

Composites Based on Poly(Lactic Acid) (PLA) and SBA-15: Effect of Mesoporous Silica on Thermal Stability and on Isothermal Crystallization from Either Glass or Molten State

© 2020 by the authors.Several composites based on an L-rich poly(lactic acid) (PLA) with different contents of mesoporous Santa Barbara Amorphous (SBA-15) silica were prepared in order to evaluate the effect of the mesoporous silica on the resultant PLA materials by examining morphological aspects, changes in PLA phases and their transitions, and, primarily, the influence on some final properties. Melt extrusion was chosen for the obtainment of the composites, followed by quenching from the melt to prepare films. Completely amorphous samples were then attained, as deduced from X-ray diffraction and differential scanning calorimetry (DSC) analyses. Thermogravimetric analysis (TGA) results demonstrated that the presence of SBA-15 particles in the PLA matrix did not exert any significant influence on the thermal decomposition of these composites. An important nucleation effect of the silica was found in PLA, especially under isothermal crystallization either from the melt or from its glassy state. As expected, isothermal crystallization from the glass was considerably faster than from the molten state, and these high differences were also responsible for a more considerable nucleating role of SBA-15 when crystallizing from the melt. It is remarkable that the PLA under analysis showed very close temperatures for cold crystallization and its subsequent melting. Moreover, the type of developed polymorphs did not accomplish the common rules previously described in the literature. Thus, all the isothermal experiments led to exclusive formation of the α modification, and the observation of the α’ crystals required the annealing for long times at temperatures below 80 °C, as ascertained by both DSC and X-ray diffraction experiments. Finally, microhardness (MH) measurements indicated a competition between the PLA physical aging and the silica reinforcement effect in the as-processed amorphous films. Physical aging in the neat PLA was much more important than in the PLA matrix that constituted the composites. Accordingly, the MH trend with SBA-15 content was strongly dependent on aging times.This research was funded by AEI/FEDER, UE (grant number MAT2016-79869-C2-1-P) and ALBA Synchrotron Light Facility. TMDR is also grateful for her pre-doctoral funding (contract number BES-2017-082284).Peer reviewe