Determining adsorbate configuration on alumina surfaces with 13C nuclear magnetic resonance relaxation time analysis

Relative strengths of surface interaction for individual carbon atoms in acyclic and cyclic hydrocarbons adsorbed on alumina surfaces are determined using chemically resolved 13C nuclear magnetic resonance (NMR) T1 relaxation times. The ratio of relaxation times for the adsorbed atoms T1,ads to the bulk liquid relaxation time T1,bulk provides an indication of the mobility of the atom. Hence a low T1,ads/T1,bulk ratio indicates a stronger surface interaction. The carbon atoms associated with unsaturated bonds in the molecules are seen to exhibit a larger reduction in T1 on adsorption relative to the aliphatic carbons, consistent with adsorption occurring through the carbon-carbon multiple bonds. The relaxation data are interpreted in terms of proximity of individual carbon atoms to the alumina surface and adsorption conformations are inferred. Furthermore, variations of interaction strength and molecular configuration have been explored as a function of adsorbate coverage, temperature, surface pre-treatment, and in the presence of co-adsorbates. This relaxation time analysis is appropriate for studying the behaviour of hydrocarbons adsorbed on a wide range of catalyst support and supported-metal catalyst surfaces, and offers the potential to explore such systems under realistic operating conditions when multiple chemical components are present at the surface.

Importance of surface carbide formation on the activity and selectivity of Pd surfaces in the selective hydrogenation of acetylene

A recent experimental investigation (Kim et al. J. Catal. 306 (2013) 146-154) on the selective hydrogenation of acetylene over Pd nanoparticles with different shapes concluded that Pd(100) showed higher activity and selectivity than Pd(111) for acetylene hydrogenation. However, our recent density functional calculations (Yang et al. J. Catal. 305 (2013) 264-276) observed that the clean Pd(111) surface should result in higher activity and ethylene selectivity compared with the clean Pd(100) surface for acetylene hydrogenation. In the current work, using density functional theory calculations, we find that Pd(100) in the carbide form gives rise to higher activity and selectivity than Pd(111) carbide. These results indicate that the catalyst surface is most likely in the carbide form under the experimental reaction conditions. Furthermore, the adsorption energies of hydrogen atoms as a function of the hydrogen coverage at the surface and subsurface sites over Pd(100) are compared with those over Pd(111), and it is found that the adsorption of hydrogen atoms is always less favoured on Pd(100) over the whole coverage range. This suggests that the Pd(100) hydride surface will be less stable than the Pd(111) hydride surface, which is also in accordance with the experimental results reported.

Investigations on the Analysis and Design of Aperiodic Frequency Selective Surfaces for Space Applications

An investigation on the design of aperiodic FSS is presented. First, an accurate yet efficient method which allows the analysis of finite sized aperiodic FSS has been developed. Subsequently, an optimisation method is implemented which optimises all the FSS elements to obtain an FSS design with an aperiodic element layout. Preliminary designs of aperiodic FSS are presented and the numerical results are discussed.

Measurement of Local Sodium Ion Levels near Micellar Surfaces with Fluorescent PET (Photoinduced Electron Transfer) Sensors

Na+ near membranes controls our nerve signals, besides several other crucial bioprocesses. We demonstrate that fluorescent PET (photoinduced electron transfer) sensor molecules target Na+ in nanospaces near micellar membranes with excellent discrimination against H+. They find that Na+ near anionic micelles is concentrated by factors of upto 160. Sensor molecules which are not held tight to the micelle surface find a Na+ amplification factor of 8 only. These findings are strengthened by the employment of control compounds whose PET processes are permanently ‘on’ or permanently ‘off’.

Possibility of designing catalysts beyond the traditional volcano curve: A theoretical framework for multi-phase surfaces

The current theory of catalyst activity in heterogeneous catalysis is mainly obtained from the study of catalysts with mono-phases, while most catalysts in real systems consist of multi-phases, the understanding of which is far short of chemists' expectation. Density functional theory (DFT) and micro-kinetics simulations are used to investigate the activities of six mono-phase and nine bi-phase catalysts, using CO hydrogenation that is arguably the most typical reaction in heterogeneous catalysis. Excellent activities that are beyond the activity peak of traditional mono-phase volcano curves are found on some bi-phase surfaces. By analyzing these results, a new framework to understand the unexpected activities of bi-phase surfaces is proposed. Based on the framework, several principles for the design of multi-phase catalysts are suggested. The theoretical framework extends the traditional catalysis theory to understand more complex systems.

High-temperature stability study of 1 nm Al2O3 deposited on InAs surfaces investigated by synchrotron radiation based photoemission spectroscopy

High-resolution soft x-ray photoemission spectroscopy (SXPS) has been used to study the high-temperature thermal stability of ultra-thin atomic layer deposited (ALD) Al2O3 layers (~1 nm) on sulfur passivated and native oxide covered InAs surfaces. While the arsenic oxides were removed from both interfaces following a 600 °C anneal, a residual indium oxide signal remained. No significant differences were observed between the sulfur passivated and native oxide surfaces other than the thickness of the interfacial oxide layer while the Al2O3 stoichiometry remained unaffected by the anneals. The energy band offsets were determined for the Al2O3 on the sulfur passivated InAs surface using both valence band edge and shallow core-level photoemission measurements.

Equivalent Circuit Model of Twisted Split Ring Frequency Selective Surfaces

This paper proposes a wideband equivalent circuit model for a twisted split ring frequency selective surface (FSS). Such surfaces can be used for modelling and design of polarisation sensitive surfaces such as circularly polarized selective surfaces as well as structures with asymmetric transmission. The proposed model is based extraction of equivalent circuit parameters from a single split ring (SRR) FSS and magnetic coupling from periodic eigenmode analysis of the coupled SRR. The resulting equivalent circuit model demonstrates excellent agreement with full-wave simulations.

Design Method for Circularly Polarized Frequency Selective Surfaces

This paper proposes a design method for the realisation of circularly polarised frequency selective surfaces (CP FSS). An equivalent circuit model for a capacitive asymmetric loop FSS is proposed. For this model a set of nonlinear design equation for CP operation is obtained. Based on space mapping of the model and full-wave simulation, a fast converging design method for CP FSS synthesis is demonstrated for the first time. 

La mise en registre automatique des surfaces acquises à partir d’objets déformables

La mise en registre 3D (opération parfois appelée alignement) est un processus de transformation d’ensembles de données 3D dans un même système de coordonnées afin d’en aligner les éléments communs. Deux ensembles de données alignés ensemble peuvent être les scans partiels des deux vues différentes d’un même objet. Ils peuvent aussi être deux modèles complets, générés à des moments différents, d’un même objet ou de deux objets distincts. En fonction des ensembles de données à traiter, les méthodes d’alignement sont classées en mise en registre rigide ou non-rigide. Dans le cas de la mise en registre rigide, les données sont généralement acquises à partir d’objets rigides. Le processus de mise en registre peut être accompli en trouvant une seule transformation rigide globale (rotation, translation) pour aligner l’ensemble de données source avec l’ensemble de données cible. Toutefois, dans le cas non-rigide, où les données sont acquises à partir d’objets déformables, le processus de mise en registre est plus difficile parce qu’il est important de trouver à la fois une transformation globale et des déformations locales. Dans cette thèse, trois méthodes sont proposées pour résoudre le problème de mise en registre non-rigide entre deux ensembles de données (représentées par des maillages triangulaires) acquises à partir d’objets déformables. La première méthode permet de mettre en registre deux surfaces se chevauchant partiellement. La méthode surmonte les limitations des méthodes antérieures pour trouver une grande déformation globale entre deux surfaces. Cependant, cette méthode est limitée aux petites déformations locales sur la surface afin de valider le descripteur utilisé. La seconde méthode est s’appuie sur le cadre de la première et est appliquée à des données pour lesquelles la déformation entre les deux surfaces est composée à la fois d’une grande déformation globale et de petites déformations locales. La troisième méthode, qui se base sur les deux autres méthodes, est proposée pour la mise en registre d’ensembles de données qui sont plus complexes. Bien que la qualité que elle fournit n’est pas aussi bonne que la seconde méthode, son temps de calcul est accéléré d’environ quatre fois parce que le nombre de paramètres optimisés est réduit de moitié. L’efficacité des trois méthodes repose sur des stratégies via lesquelles les correspondances sont déterminées correctement et le modèle de déformation est exploité judicieusement. Ces méthodes sont mises en oeuvre et comparées avec d’autres méthodes sur diverses données afin d’évaluer leur robustesse pour résoudre le problème de mise en registre non-rigide. Les méthodes proposées sont des solutions prometteuses qui peuvent être appliquées dans des applications telles que la mise en registre non-rigide de vues multiples, la reconstruction 3D dynamique, l’animation 3D ou la recherche de modèles 3D dans des banques de données.