Three Miller index surfaces of copper, Cu(111), Cu(211), and Cu(643) were evaluated for spontaneous carbon dioxide dissociation. DFT (Density Functional Theory) was used to characterize the initial and final adsorption states and Climbing Image Nudged Elastic Band (cNEB) calculations were used to identify the dissociation transition sites. A simple kinetic model was formulated and used to quantitatively compare the three surfaces and determine which facilitated CO₂ dissociation most readily.

Master of Science / Department of Physics / Itzhak Ben-Itzhak / Imaging the interaction of molecular ion beams with ultrafast intense laser fields is a very powerful method to understand the fragmentation dynamics of molecules. Femtosecond laser pulses with different wavelengths and intensities are applied to dissociate and ionize molecular ions, and each resulting fragmentation channel can be studied separately by implementing a coincidence three-dimensional (3D) momentum imaging method. The work presented in this master’s report can be separated into two parts. First, the interaction between molecular ion beams and femtosecond laser pulses, in particular, the dissociation of CO[superscript]+ into C[superscript]++O, is studied. For that purpose, measurements are conducted at different laser intensities and wavelengths to investigate the possible pathways of dissociation into C[superscript]++O. The study reveals that CO[superscript]+ starts to dissociate from the quartet electronic state at low laser intensities. Higher laser intensity measurements, in which a larger number of photons can be absorbed by the molecule, show that the doublet electronic states with deeper potential wells, e.g. A [superscript]2Π, contribute to the dissociation of the molecule. In addition, the three-body fragmentation of CO[subscript]2[superscript]+ into C[superscript]++O[superscript]++O[superscript]+ is studied, and two breakup scenarios are separated using the angle between the sum and difference of the momentum vectors of two O[superscript]+ fragments. In the second part, improvements in experimental techniques are discussed. Development of a reflective telescope setup intended to increase the conversion efficiency of ultraviolet (UV) laser pulse generation is described, and the setup is used in the studies of CO[superscript]+ dissociation described in this report. The other technical study presented here is the measurement of the position dependence of timing signals picked off of a microchannel plate (MCP) surface. The experimental method is presented and significant time spread over the surface of the MCP detector is reported [1].

No description available.

La réaction d’oxydation sélective du CO par O2 en présence d’un excès d’hydrogène (PROX) est considérée comme une étape de purification essentielle de l’H2 à utiliser dans des piles à combustible. L’objectif de cette thèse est de mieux comprendre le mécanisme de cette réaction sur des catalyseurs bimétallique s à base de Pt et Sn. Des catalyseurs modèles Pt et Pt-Sn ont été synthétisés en deux étapes : (i) formation de nanoparticules (NP) métalliques colloïdales en suspension suivi par ( ii) l’imprégnation de ces particules sur des supports. L’adsorption du CO suivit par spectroscopie FT-IR en réflexion diffuse (DRIFTS) a été utilisée pour caractériser ces solides après une réduction permettant de reformer des phases d’alliage PtSn. L’analyse DRIFTS permet de caractériser la nature des sites de Pt présents, soit dans l’alliage, soit dans des phases pures de Pt. La chaleur d’adsorption du CO sur la phase d’alliage a été mesurée par DRIFTS, pour la première fois, et apparait bien plus faible que celle sur le Pt seul. De manière surprenante, la ségrégation de l’alliage en présence de CO/H 2 à des températures inférieur es à 175°C a été mise en évidence. Des mesures in situ DRIFTS de la réaction d’oxydation préférentielle du CO (PROX) indiquent que l’alliage se transforme rapidement en Pt et SnOx de par la présence de l’O2. Aucune indication de la présence d’alliage n’a jamais pu être obtenue sous PROX, indiquant que les meilleures propriétés catalytiques associés aux phases Pt-Sn sont dues à leur habilité à générer une nouvelle phase active Pt+SnOx lors de leur ségrégation. Un modèle microcinétique du PROX sur Pt+SnOx a été développé sur la base de ceux pertinents à l’oxydation du CO et PROX sur Pt seul, permettant une modélisation satisfaisante des données. Ce travail montre l’intérêt du couplage des méthodes spectroscopiques et cinétiques pour la compréhension de la structure des catalyseurs « au travail » et des mécanismes de réactions complexes / The selective oxidation of CO in the presence of a large excess of H2 (PROX) is considered as a crucial step in the purification of H 2 to be used in low-temperature fuel cells, which are clean sources of energy. The objective of this thesis was to better understand the reaction mechanisms taking place over promising catalysts based on Pt and Sn. Model Pt-Sn catalysts were prepared by a two-step method: (i) synthesis of metallic nanoparticules (NP) in a colloidal suspension followed by (ii) the deposition of these NPs onto a support. The first step of the method enabled to produce well-controlled Pt-Sn NPs in terms of size and composition. However, the NPs were partly destroyed during the deposition step followed by calcination, due to the reoxidation of Sn. The adsorption of CO followed by diffuse reflectance spectroscopy (DRIFTS) was used to characterize the nature of these solids following a reduction, which was able to regenerate an alloyed phase. The DRIFTS analysis enabled to discriminate between Pt in an alloyed phase and Pt on monometallic surfaces. The heat of CO adsorption measured by DRIFTS appeared to be much lower than that associated with the pure Pt phase. Surprinsingly, a segregation of Pt and Sn was observed under a CO/H2 mixture below 175°C. In situ analysis by DRIFTS of the PROX reaction indicated that the Pt-Sn alloy rapidly decomposed in the presence of O2, forming an intimate mixture of Pt and SnOx. No evidence of the presence of Pt -Sn alloyed phases could be obtained under PROX conditions, suggesting that the superior catalytic activity of the Pt –Sn materials were related to the Pt+SnOx mixture. A detailed PROX microkinetic model was developed over Pt+SnOx, based on those relevant to CO oxidation and PROX over pure Pt. This work epitomises the benefits in combining in situ spectroscopic study with kinetic modelling to better understand the structure of catalysts “at work” and reaction mechanisms