Chemistry at surfaces: from ab initio structures to quantum dynamics

Recent years have witnessed an ever growing interest in theoretically studying chemical processes at surfaces. Apart from the interest in catalysis, electrochemistry, hydrogen economy, green chemistry, atmospheric and interstellar chemistry, theoretical understanding of the molecule–surface chemical bonding and of the microscopic dynamics of adsorption and reaction of adsorbates are of fundamental importance for modeling known processes, understanding new experimental data, predicting new phenomena, controlling reaction pathways. In this work, we review the efforts we have made in the last few years in this exciting field. We first consider the energetics and the structural properties of some adsorbates on metal surfaces, as deduced by converged, first-principles, plane-wave calculations within the slab-supercell approach. These studies comprise water adsorption on Ru(0001), a subject of very intense debate in the past few years, and oxygen adsorption on aluminum, the prototypical example of metal passivation. Next, we address dynamical processes at surfaces with classical and quantum methods. Here the main interest is in hydrogen dynamics on metallic and semi-metallic surfaces, because of its importance for hydrogen storage and interstellar chemistry. Hydrogen sticking is studied with classical and quasi-classical means, with particular emphasis on the relaxation of hot–atoms following dissociative chemisorption. Hot atoms dynamics on metal surfaces is investigated in the reverse, hydrogen recombination process and compared to Eley–Rideal dynamics. Finally, Eley–Rideal, collision-induced desorption, and adsorbate-induced trapping are studied quantum mechanically on a graphite surface, and unexpected quantum effects are observed.     

Ag(I) solid solutions in zirconium dihydrogen phosphate. Thermal behaviour and phase changes

The thermal and redox behaviours of some Ag-low-converted zirconium phosphates have been investigated by thermoanalytical and X-ray methods: the conditions for obtaining Ag solid solutions in phases are reported.

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Zusammenfassung Das thermische und Redoxverhalten einiger Zirkoniumphosphate niedriger Ag-Konversion wurde durch thermoanalytische und Röntgenmethoden studiert; über die Bedingungen zum Erhalten von Ag-Festphasenlösungen in HH-Phasen wird berichtet

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Résumé On a étudié le comportement thermique et oxydo-réducteur de quelques phosphates de zirconium faiblement converti par Ag, en se servant des méthodes d'analyse thermique et des rayons X: on communique les conditions permettant d'obtenir des solutions solides d'Ag dans les phases .

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- Ag- . HH.

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Paper in part presented at the 6th ICTA, in Bayreuth (Germany), July 1980.

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We thank Dr. M. A. Massucci, of our Institute, for helpful discussions and suggestions.     

Quantum effects in an exoergic, barrierless reaction at high collision energies

The exoergic Eley-Rideal hydrogen recombination on graphite surfaces is known to produce vibrationally hot product molecules. Realistic quantum scattering calculations at normal incidence over a wide range of collision energies show that the degree of vibrational excitation of the reaction product is a steep decreasing function of the collision energy. This results in selective population of the lower-lying vibrational levels and gives rise to an oscillating structure in the total reaction cross-sections at high energies, which cannot be reproduced by classical and quasi-classical trajectory calculations. An analogous quantum structure appears in the total collision-induced desorption cross-sections and is assigned to selective population of the metastable states of the transient molecular hydrogen.     

3-D numerical simulation of fully-coupled particle heating in ICPTs

Numerical simulations of the trajectory and heating history of powders injected with a carrier-gas into inductively coupled plasma torches working at atmospheric pressure have been performed in the framework of a 3-D FLUENT -based model, taking into account the effects of coupling between particles and plasma. In this paper, we present a selection of the results obtained for different operating conditions of the torch and for different characteristics of the injected particles.Received: 5 January 2004, Published online: 10 February 2004PACS: 52.75.Hn Plasma torches - 52.65.-y Plasma simulation - 52.80.Pi High-frequency and RF discharges - 81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation     

Front propagation in chaotic and noisy reaction-diffusion systems: a discrete-time map approach

We study the front propagation in reaction-diffusion systems whose reaction dynamics exhibits an unstable fixed point and chaotic or noisy behaviour. We have examined the influence of chaos and noise on the front propagation speed and on the wandering of the front around its average position. Assuming that the reaction term acts periodically in an impulsive way, the dynamical evolution of the system can be written as the convolution between a spatial propagator and a discrete-time map acting locally. This approach allows us to perform accurate numerical analysis. They reveal that in the pulled regime the front speed is basically determined by the shape of the map around the unstable fixed point, while its chaotic or noisy features play a marginal role. In contrast, in the pushed regime the presence of chaos or noise is more relevant. In particular the front speed decreases when the degree of chaoticity is increased, but it is not straightforward to derive a direct connection between the chaotic properties (e.g. the Lyapunov exponent) and the behaviour of the front. As for the fluctuations of the front position, we observe for the noisy maps that the associated mean square displacement grows in time as t ^1/2 in the pushed case and as t ^1/4 in the pulled one, in agreement with recent findings obtained for continuous models with multiplicative noise. Moreover we show that the same quantity saturates when a chaotic deterministic dynamics is considered for both pushed and pulled regimes. Received 17 July 2001     

Analytical prediction and experimental validation for longitudinal control of cable oscillations

The paper summarizes the knowledge acquired from the analytical studies and the experimental implementation of a longitudinal non-collocated control strategy for the reduction of cable oscillations. The control is introduced by imposing a longitudinal action at one support based on the knowledge of transverse displacements and velocities of a few selected points. A spatially one-dimensional continuous model of a suspended cable has been used to describe the main features of the non-collocated longitudinal active control strategy. A discrete modal representation has permitted the introduction of suitable non-linear state-feedback controllers. The results have been used to derive an implementable strategy, based on direct output feedback, which preserves the main previous control features. A physical model of an actively controlled cable has been used to demonstrate the control effectiveness of the proposed strategy through a large campaign of experiments, conducted in various frequency ranges and amplitude levels including meaningful external resonance conditions. The responses predicted by the analytical model and the experimental results show good qualitative agreement with one another, in both the uncontrolled and controlled experienced cable dynamics.