Bi-dimensional metal-organic frameworks (2D-MOF) are a relatively new class of materials that are gaining particular interest in surface science. Their dimensionality naturally allows the exploitation of many surface-sensitive techniques, beneficial to study with great detail their geometry, electronic structure, and chemical reactivity. On the other hand, the well-defined chemical environment of the active sites makes them ideal single-atom catalysts for heterogeneous catalysis. In this thesis, we focus on the Ultra High Vacuum (UHV) growth and characterization, and on the investigation of the reactivity from UHV to Near Ambient Pressure (NAP) of M1TPyPM2 2D Metal-organic Frameworks (MOFs, with M1,2 = Fe, Co) self-assembled on almost free-standing Graphene/Ir(111). In the first part, some selected monolayers have been investigated in UHV with a combination of vibrational- (IR-Vis SFG), real-space imaging and spectroscopy (STM/STS), and electronic- (NEXAFS, XPS) sensitive techniques, and the results compared with the literature and ab initio DFT. In the second part, we exposed a CoTPyP/Gr/Ir(111) monolayer to a mixture of water and oxygen gases (up to fractions of mbar), and an intense bending mode appeared at 1712 cm-1 in Sum Frequency Generation spectra collected in situ: this is associated with the hydroperoxyl molecule, an important reaction intermediate for the Oxygen Reduction Reaction. NAP-XPS measurements confirmed our thesis through O 1s and Co 2p core levels analysis, while pump-probe SFG allowed us to directly inspect the coupling between the hydroperoxyl and the surrounding water molecules representing the solvent. In the last part, carbon monoxide has been exploited as a probe to investigate the CoTPyPCo/Gr/Ir(111) local sites' electronic structure and coordination. The IR-Vis SFG spectra collected in situ showed evidence of both long-range MOF-driven interactions, including the anti-cooperativity of the CO adsorption process, provided that active sites are sufficiently close to each other. This experiment has been revealed to be an effective approach to investigate subtle effects in the MOF chemical affinity, possibly responsible for the remarkable non-linear increase of the electrocatalytic performances of such M1TPyPM2 compounds, which is one of the scientific interests at the basis of this thesis.

I materiali bidimensionali costituiti da atomi metallici incorporati in una struttura organica (2D-MOF) stanno guadagnando sempre più interesse nel panorama della scienza delle superfici. La loro dimensionalità consente in maniera naturale di sfruttare le tante tecniche sperimentali con sensitività superficiale, che consentono di studiare in dettaglio la loro geometria, struttura elettronica e reattività chimica. L’ambiente chimico locale dei siti attivi li rende infatti catalizzatori a singolo atomo ideali per lo studio della catalisi eterogenea. In questa tesi, ci si è concentrati sulla crescita e caratterizzazione in Ultra Alto Vuoto (UHV) e sull’investigazione della reattività da UHV a pressione quasi ambiente di M1TPyPM2 2D-MOF (con M1,2 = Fe, Co) auto-assemblati su Grafene/Ir(111). Nella prima parte, alcuni monolayer sono stati selezionati ed investigati in UHV con una combinazione di tecniche vibrazionali (IR-Vis SFG), di microscopia e spettroscopia elettronica (STM/STS, NEXAFS, XPS), confrontando i risultati con la letteratura e simulazioni DFT ab initio. Nella seconda parte, si è esposto un monolayer di CoTPyP/Gr/Ir(111) ad una atmosfera di acqua e ossigeno gassosi (fino a frazioni di millibar). Una intensa risonanza una intensa risonanza vibrazionale è apparsa a 1712 cm-1 negli spettri di Spettroscopia a Frequenza Somma acquisiti in situ, associata alla molecola di idroperossile, un importante intermedio di reazione per la Reazione di Riduzione dell’Ossigeno (ORR). Misure NAP-XPS hanno confermato la nostra tesi grazie all’analisi dei livelli di core di ossigeno e cobalto, mentre un esperimento SFG time-resolved ci ha consentito di ispezionare il legame tra l’idroperossile e le molecole di acqua che lo circondano. Nell’ultima parte, il monossido di carbonio è stato sfruttato come una sonda per investigare la struttura elettronica e la coordinazione locale dei siti attivi in un monolayer di CoTPyPCo/Gr/Ir(111). Gli spettri IR-Vis SFG misurati in situ hanno evidenziato la presenza di interazioni a lungo raggio promosse dal MOF stesso, includendo l’anti-cooperatività del processo stesso di adsorbimento del CO, a patto che i siti attivi siano sufficientemente vicini. L’esperimento si è rivelato essere un potente strumento per investigare deboli effetti legati all'affinità chimica del layer, potenzialmente responsabili del notevole incremento delle prestazioni elettrochimiche di questi materiali, uno degli aspetti scientifici alla base di questo lavori di tesi.

Monolayer mono- e bi-metallici di tetra-pyridyl porfirine dall’UHV a pressione quasi-ambiente / Armillotta, Francesco. - (2023 Feb 22).

Monolayer mono- e bi-metallici di tetra-pyridyl porfirine dall’UHV a pressione quasi-ambiente

ARMILLOTTA, FRANCESCO
2023-02-22

Abstract

Bi-dimensional metal-organic frameworks (2D-MOF) are a relatively new class of materials that are gaining particular interest in surface science. Their dimensionality naturally allows the exploitation of many surface-sensitive techniques, beneficial to study with great detail their geometry, electronic structure, and chemical reactivity. On the other hand, the well-defined chemical environment of the active sites makes them ideal single-atom catalysts for heterogeneous catalysis. In this thesis, we focus on the Ultra High Vacuum (UHV) growth and characterization, and on the investigation of the reactivity from UHV to Near Ambient Pressure (NAP) of M1TPyPM2 2D Metal-organic Frameworks (MOFs, with M1,2 = Fe, Co) self-assembled on almost free-standing Graphene/Ir(111). In the first part, some selected monolayers have been investigated in UHV with a combination of vibrational- (IR-Vis SFG), real-space imaging and spectroscopy (STM/STS), and electronic- (NEXAFS, XPS) sensitive techniques, and the results compared with the literature and ab initio DFT. In the second part, we exposed a CoTPyP/Gr/Ir(111) monolayer to a mixture of water and oxygen gases (up to fractions of mbar), and an intense bending mode appeared at 1712 cm-1 in Sum Frequency Generation spectra collected in situ: this is associated with the hydroperoxyl molecule, an important reaction intermediate for the Oxygen Reduction Reaction. NAP-XPS measurements confirmed our thesis through O 1s and Co 2p core levels analysis, while pump-probe SFG allowed us to directly inspect the coupling between the hydroperoxyl and the surrounding water molecules representing the solvent. In the last part, carbon monoxide has been exploited as a probe to investigate the CoTPyPCo/Gr/Ir(111) local sites' electronic structure and coordination. The IR-Vis SFG spectra collected in situ showed evidence of both long-range MOF-driven interactions, including the anti-cooperativity of the CO adsorption process, provided that active sites are sufficiently close to each other. This experiment has been revealed to be an effective approach to investigate subtle effects in the MOF chemical affinity, possibly responsible for the remarkable non-linear increase of the electrocatalytic performances of such M1TPyPM2 compounds, which is one of the scientific interests at the basis of this thesis.
22-feb-2023
VESSELLI, ERIK
35
2021/2022
Settore FIS/03 - Fisica della Materia
Università degli Studi di Trieste
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11368/3041021
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