Crossed Molecular Beams and Theoretical Studies of the O(P-3)+1,2-Butadiene Reaction: Dominant Formation of Propene plus CO and Ethylidene plus Ketene Molecular Channels

Detailed understanding of the mechanism of the combustion relevant multichannel reactions of O(P-3) with unsaturated hydrocarbons (UHs) requires the identification of all primary reaction products, the determination of their branching ratios and assessment of intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). This can be best achieved combining crossed-molecular-beam (CMB) experiments with universal, soft ionization, mass-spectrometric detection and time-of-flight analysis to high-level ab initio electronic structure calculations of triplet/singlet PESs and RRKM/Master Equation computations of branching ratios (BRs) including ISC. This approach has been recently demonstrated to be successful for O(P-3) reactions with the simplest UHs (alkynes, alkenes, dienes) containing two or three carbon atoms. Here, we extend the combined CMB/theoretical approach to the next member in the diene series containing four C atoms, namely 1,2-butadiene (methylallene) to explore how product distributions, branching ratios and ISC vary with increasing molecular complexity going from O(P-3)+propadiene to O(P-3)+1,2-butadiene. In particular, we focus on the most important, dominant molecular channels, those forming propene+CO (with branching ratio similar to 0.5) and ethylidene+ketene (with branching ratio similar to 0.15), that lead to chain termination, to be contrasted to radical forming channels (branching ratio similar to 0.35) which lead to chain propagation in combustion systems.

Influence of Residual Water and Carbon Containing Molecules in Focused Electron Beam Induced Processes

Bamdad Afra

Focused Electron Beam Induced Processing (FEBIP) is a technique used for etching and deposition of micro- and nano-structures using a focused electron beam. FEBIP is commonly carried out in a Scanning Electron Microscope (SEM) with a Gas Injection System (GIS) allowing precursor molecule vapors to be introduced on the substrate where they interact with the focused electron beam. Results of this work shine light on the influence of residual Water and Carbon containing species in the SEM chamber on etching and deposition processes. For etching HOPG (Highly Oriented Pyrolytic Graphite) and PMMA (Polymethyl methacrylate) using water vapor as precursor, it is shown that water participates in the principal chemical reaction for removal of these materials. However, when using Oxygen as precursor for etching these materials, Oxygen has a significantly reduced role in chemical reactions compared to residual water desorbing from surfaces in the FEBIP set-up. Degrees of involvement of Oxygen and Water in chemical reactions are demonstrated by combining experimental results and existing FEBIP Models. The Focused Electron Beam Induced Etching (FEBIE) contribution of Oxygen molecule Gas Phase Ionization prior to adsorption on the substrate surface is determined to be approximately 10% of the etching process for a micron size focused electron beam. A Precursor Gas Preparation System (PGPS) was designed and developed for in-situ fabrication of Metal Fluoride precursors from Xenon di-fluoride, XeF2. PGPS provided means of safe execution of FEBIP experiments without having to store large quantities of various dangerous and toxic materials. Using PGPS, Tungsten Hexafluoride precursor, WF6, was created in-situ and used in FEBIP experiments for deposition of Tungsten containing materials. This precursor allowed the elimination of both Oxygen and Carbon atoms in the precursor molecule in order to study the influence of residual Water (containing Oxygen) and residual Carbon containing species by measuring the amount of these atoms in deposit composition. Installation of a Liquid Nitrogen Cooled Plate over Substrate (without any direct contact with the substrate or GIS in the FEBIP set-up) proved to significantly reduce Oxygen and Fluorine content in deposits by eliminating the contribution of residual molecules from the chamber. Carbon content in deposits was also reduced when using Liquid Nitrogen Cooled Plate over Substrate (LNC-PoS). Non-FEB induced reactions (resulting in build-up of a contamination film on the substrate) are also significantly reduced or eliminated when using LNC-PoS in FEBIP experiments with WF6 precursor. In experiments with LNC-PoS, it is shown that the Carbon content in deposits mostly originates from substrate surface and arrives to FEB location via surface diffusion.

EPFL2011

Bond-selective and surface-site-specific dissociation of methane on platinum

Ana Gutiérrez González

I present a molecular beam study of methane dissociation on differ-ent surface sites of several platinum single crystal surfaces (Pt(111), Pt(211), Pt(210), Pt(110)-(2x1)). The experiments were performed in a molecular beam/surface-science apparatus that combines rovibrational state-selective excitation of reactants with detection of the reaction products by reflection absorption infrared spectroscopy (RAIRS), Auger electron spectroscopy (AES) and King and Wells (K&W) technique. First, I explore how the barrier for methane dissociation depends on the Pt surface site (terrace, step, kink, and ridge atoms). For that, I compare the average reaction probabilities of methane on the different crystals using K&W. Moreover, I used the site-specific detection of chemisorbed methyl species by RAIRS to obtain the site-specific reactivity on steps, ridges and terraces. The highest methane reactivity and therefore the lowest barrier was observed for the surface atoms with the lowest coordination. The trans-lational energy dependence of the reactivity shows clear evidence for a direct activated mechanism on all the surface sites studied. The decreasing catalyt-ic activity for Pt atoms with increasing coordination number agrees well with theoretical predictions using first principles quantum theory calculations. Second, I present the role of rovibrational excitation of the incident methane on the chemisorption on different surface sites. For CH4, excitation of one quantum of the antisymmetric C-H stretch vibration (Îœ3) was found to be less efficient than an equivalent amount of translational energy normal to the sur-face for promoting the dissociation on all the surface sites. The vibrational efficacy was seen to be lower for dissociation on the low coordinated sites, in accordance with calculations of transition state geometries. For CH3D, the excitation of the antisymmetric C-H stretch vibration (Îœ4) led to bond selec-tivity on the steps and terraces of the Pt(211) surface. These results show how careful control of the translational energy and rovibrational state of the incident CH3D can be used for site-specific and bond-selective dissociation of methane. As predicted previously by theory but not observed before experi-mentally, the C-H bond selectivity is shown to decrease with increasing translational energy. Third, I explore the effect of surface temperature on the methane dissocia-tion. The sticking coefficient for a direct chemisorption reaction such as CH4 on Pt, might be expected to be independent of surface temperature, because the reaction happens very fast leaving very little time for the molecule to equilibrate with the surface. However, I observed an increase in methane reactivity with increasing surface temperature at low incident CH4 energies. This observation is interpreted by a âloss of coordinationâ from the atoms displaced out of the plane due to their vibrational motion when the surface is heated. The surface-site and quantum-state-specific data presented in this thesis are ideally suitable for testing theoretical models that aim to describe the me-thane-surface reaction at the microscopic level. Moreover, the characteriza-tion of different catalytically active sites for methane dissociation contrib-utes to advancing the understanding of methane reactivity towards real cata-lytic conditions, where surfaces with several different atomic terminations are used.

EPFL2019

Influence of Residual Water and Carbon Containing Molecules in Focused Electron Beam Induced Processes

Bamdad Afra

EPFL2011

Bond-selective and surface-site-specific dissociation of methane on platinum

Ana Gutiérrez González

EPFL2019