Surface-catalysed dehydrogenation and aromatisation reactions of hydrocarbons
- Autores: Daniel Arribas Mercado
- Directores de la Tesis: José Ángel Martín Gago (dir. tes.), Pablo Merino Mateo (dir. tes.)
- Lectura: En la Universidad Autónoma de Madrid ( España ) en 2024
- Idioma: inglés
- Número de páginas: 132
- Títulos paralelos:
- Reacciones de deshidrogenación y aromatización de hidrocarburos catalizadas en superficies
- Tribunal Calificador de la Tesis: Arantzazu Mascaraque Susunaga (presid.), Amadeo L. Vázquez de Parga (secret.), Martina Corso (voc.), Martin Kalbác (voc.), Rubén Pérez Pérez (voc.)
- Programa de doctorado: Programa de Doctorado en Materiales Avanzados y Nanotecnología por la Universidad Autónoma de Madrid
- Calificación: Sobresaliente Cum Laude
- Materias:
- Enlaces
- Tesis en acceso abierto en: Biblos-e Archivo
- Resumen
The hydrocarbon family comprises all those organic molecules that are constituted exclusively of carbon and hydrogen atoms. Thanks to the remarkable versatility of the carbon chemistry, hydrocarbons are present in a wide range of molecular shapes, size and matter states (gas, liquid and solid). The combustion of these molecules is nowadays a strategical source of energy, despite the environmental impact derived from it. Additionally, hydrocarbons constitute a crucial source of carbon atoms for organic chemical industries and the high hydrogen content of some of them makes of these molecules a valuable source of hydrogen.
On one hand, aliphatic hydrocarbons are particularly interesting due to their abundance. In general, these molecules represent more than half the total hydrocarbon content of petroleum and almost the totality of natural gas. When they are saturated, i.e. composed exclusively by sp3-hybridised carbon atoms, their stability rends them highly unreactive and difficult to process as precursors in the chemical industry.
On the other hand, graphene is a highly stable and inert carbon allotrope. This polyaromatic flat monolayer of sp2-hybridised carbon atoms is arranged in a characteristic honeycomb lattice with its pz electrons highly delocalised. It displays extremely appealing properties in terms of mechanical resistance or thermal and electric conductivity. Unfortunately, the inertness of this material hinders its application in electronic devices and the modification of its properties via chemical functionalisation.
In this thesis, we focus on the fundamental aspects of the chemical reactions that enable for chemical modification of unreactive hydrocarbon precursors without addition of heteroatoms into their structure, i.e. only hydrogen and carbon atoms are involved. This leaves us with a reduced group of possibilities, which includes dehydrogenation cyclisation, and aromatisation among others. Therefore, our main goal is to advance in our understanding of mechanisms underlying C-C and C-H chemical interactions.
The research presented over the following pages has been conducted following a surface science approach. A well-controlled ultra-high vacuum environment permits us to characterise our samples at the nanoscale without interferences from atmospheric contaminants. Our conclusions mainly rely on a combination of high-resolution scanning tunnelling microscopy images with x-ray photoelectron spectroscopy and temperature programmed desorption experiments. These techniques provide us with a full morphological and chemical characterisation of the molecules and materials we deposit on the model surface of a metallic single-crystal. The complementary insights gained from ab initio calculations and kinetic modelling of the reactions reinforce our conclusions with a theoretical basis.
For our investigation, we use a linear alkane (n-octane or n-C8H18) as a saturated aliphatic precursor to study the thermal dehydrogenation, aromatisation, and intermolecular bonding of these molecules on a catalytic Pt(111) model surface. The catalytic dehydrogenation of alkanes is key for the conversion of these inert hydrocarbon chains into other value-added unsaturated and aromatic chemicals, and platinum-based catalysts are among the most widely used for this purpose. After probing a wide range of reaction temperatures (from 100 K to 1200 K), we identify several sequential dehydrogenation stages, starting from the physisorption of the molecule without chemical modification at low temperatures. Above 330 K, n-octane releases a hydrogen atom from one of its methyl ends, yielding an n-octyl radical that strongly bonds to the surface. Above 600 K, further dehydrogenation induces the formation of unsaturated double bonds in the carbon backbone of the adsorbates. At this temperature, thermal diffusion of the adsorbates favours the aggregation of the molecules, which arrange in parallel clusters. A dehydrogenative intermolecular coupling reaction produces intermolecular C-C bonds, leading to the apparition of hexagonal rings and the formation of graphene patches after completion of the dehydrogenation above 1000 K.
Additionally, we study the deposition of hydrogen atoms on polycrystalline epitaxial graphene monolayers grown on a Pt(111) surface by thermal decomposition of fullerenes at elevated temperatures (above 1100 K). This permits us to investigate the inverseprocess, i.e. the hydrogenation of a fully aromatic and inert material to produce sp3 carbon atoms. Due to the mismatch between the substrate and the graphene lattice, domains of up to 22 differently corrugated moiré superstructures coexist on the samples. This represents an ideal template for assessing the effect of graphene morphology on the efficiency of hydrogen adsorption and its stability over time. We find experimentally that hydrogen chemisorbs to the graphene forming clusters, preferentially on those areas of the moiré with local convex curvature. The most corrugated moiré superstructures tend to accumulate more hydrogen than flat graphene. Thanks to kinetic Monte-Carlo simulations of hydrogen adsorption, we identify a significative enhancement of the adsorption speed and the thermal stability of the chemisorbed hydrogen on the corrugated moiré superstructures.
Overall, the results obtained in this thesis provide key fundamental insights into the hydrogenation and dehydrogenation reactions of sp2 and sp3carbon derivatives. This has allowed us to chemically modify the otherwise inert saturated hydrocarbons and describe a dehydrogenative coupling reaction ultimately leading to the on-surface aromatisation of inert aliphatic precursors. In this way, using the system n-octane / Pt(111) as a model, we advance in the fundamental understanding of the C-C and C-H interactions in aliphatic precursors when a catalytic substrate intervenes.
