Carbon dioxide (CO2) is a greenhouse gas that plays a significant role in global climate change. This is why new technologies for capturing, storing and processing CO2 are the subject of the most recent studies. Among these, the photocatalytic reduction of carbon dioxide (CO2) is a promising approach to mitigate its effects on the climate. The importance of studies in the field of photocatalytic reduction of carbon dioxide (CO2) into fuel with water as reducing agent, is linked to the development of green technologies that aims to mimic the photosynthesis process in nature and to the development of an efficient, stable and cost-effective photocatalyst. Various semiconductor photocatalysts have been proposed in recent years, including metal oxides such as TiO2, Cu2O, and CeO2; oxysalts such as KTaO3 or Bi2WO6; and metal chalcogenides such as ZnS, Bi2S3, etc. Non-metallic materials such as graphene oxide and carbon nitride have lately been investigated for photocatalytic CO2 conversion. Graphitic carbon nitride (g-C3N4) has recently received increasing attention as an easy-to-obtain, low-cost, visible-light-responsive non-metal semiconductor photocatalyst with high stability and non-toxicity. Despite the fact that many efforts have been made to improve the catalytic performances of g-C3N4 in the CO2 reduction reaction with water, an in-depth comprehension of the concerned processes is still lacking. The aim of this thesis is, therefore, the study the mechanisms of CO2 reduction on g-C3N4 through the combined use of XAS and FTIR operando spectroscopies. In order to have a detailed comprehension of the mechanisms of heterogeneous reactions involved, it is necessary to investigate both the pertinent electronic states at the surface of the catalyst and the chemical nature of the species adsorbed on the surface. This was both done by monitoring the N K-edge during the course of the reaction under flowing CO2 and mixtures of CO2 and H2O, with or without illumination, using XAS spectroscopy, and, in the same conditions, by detecting carboxylated and/or hydroxylated species on the surface using IR spectroscopy. The experiment was performed on g-C3N4 samples differently doped with alkali and transition metal ions, at the NFFA-Trieste facility. Operando soft X-ray analysis have been carried out at the beamline APE-HE and operando FTIR measurements were performed at the SISSI-Bio beamline. The results, although still at the preliminary stage of analysis, show significant effects in the IR spectra of some samples with the increase in intensity of peaks related to species adsorbed on the surface of the photocatalyst. On the other hand, the XAS analyses will have to be repeated as they did not provide significant results.
L'anidride carbonica (CO2) è un gas serra che svolge un ruolo significativo nel cambiamento climatico globale. Ecco perché le nuove tecnologie per la cattura, lo stoccaggio e la lavorazione della CO2 sono oggetto degli studi più recenti. Tra queste la riduzione fotocatalitica dell'anidride carbonica (CO2) è un approccio promettente per mitigare i suoi effetti sul clima. L'importanza degli studi nel campo della riduzione fotocatalitica dell'anidride carbonica (CO2) in combustibili con l'acqua come agente riducente è legato allo sviluppo di tecnologie green che mirano ad imitare il processo di fotosintesi in natura e allo sviluppo di un fotocatalizzatore efficiente, stabile ed economicamente sostenibile. Vari fotocatalizzatori a semiconduttore sono stati proposti negli ultimi anni, tra questi ossidi metallici come TiO2, Cu2O e CeO2; Sali ternari come KTaO3 o Bi2WO6; e calcogenuri metallici come ZnS, Bi2S3, ecc. Materiali non metallici come l'ossido di grafene e il nitruro di carbonio sono stati recentemente studiati per la conversione fotocatalitica di CO2. Il nitruro di carbonio grafitico (g-C3N4) ha recentemente ricevuto una crescente attenzione come fotocatalizzatore a semiconduttore non metallico facile da ottenere, a basso costo e sensibile alla luce visibile con elevata stabilità e non tossicità. Nonostante siano stati compiuti molti sforzi per migliorare le prestazioni catalitiche del g-C3N4 nella reazione di riduzione della CO2 con l'acqua, manca ancora una comprensione approfondita dei processi interessati. Lo scopo di questa tesi è quindi lo studio dei meccanismi di riduzione della CO2 sul g-C3N4 attraverso l'uso combinato delle spettroscopie operando XAS e FTIR. Per avere una comprensione dettagliata dei meccanismi delle reazioni eterogenee coinvolte è necessario indagare sia gli stati elettronici pertinenti alla superficie del catalizzatore, sia la natura chimica delle specie adsorbite sulla superficie. Questo è stato fatto monitorando la N K-edge durante il corso della reazione, flussando CO2 e miscele di CO2 e H2O con e senza illuminazione, utilizzando la spettroscopia XAS e, nelle stesse condizioni, rilevando specie carbossilate e/o idrossilate sulla superficie utilizzando la spettroscopia IR.
Riduzione fotocatalitica della CO2 sul g-C3N4: uno studio condotto tramite l’utilizzo combinato di spettroscopie XAS e FTIR operando.
BALLERINI, FILIPPO
2022/2023
Abstract
Carbon dioxide (CO2) is a greenhouse gas that plays a significant role in global climate change. This is why new technologies for capturing, storing and processing CO2 are the subject of the most recent studies. Among these, the photocatalytic reduction of carbon dioxide (CO2) is a promising approach to mitigate its effects on the climate. The importance of studies in the field of photocatalytic reduction of carbon dioxide (CO2) into fuel with water as reducing agent, is linked to the development of green technologies that aims to mimic the photosynthesis process in nature and to the development of an efficient, stable and cost-effective photocatalyst. Various semiconductor photocatalysts have been proposed in recent years, including metal oxides such as TiO2, Cu2O, and CeO2; oxysalts such as KTaO3 or Bi2WO6; and metal chalcogenides such as ZnS, Bi2S3, etc. Non-metallic materials such as graphene oxide and carbon nitride have lately been investigated for photocatalytic CO2 conversion. Graphitic carbon nitride (g-C3N4) has recently received increasing attention as an easy-to-obtain, low-cost, visible-light-responsive non-metal semiconductor photocatalyst with high stability and non-toxicity. Despite the fact that many efforts have been made to improve the catalytic performances of g-C3N4 in the CO2 reduction reaction with water, an in-depth comprehension of the concerned processes is still lacking. The aim of this thesis is, therefore, the study the mechanisms of CO2 reduction on g-C3N4 through the combined use of XAS and FTIR operando spectroscopies. In order to have a detailed comprehension of the mechanisms of heterogeneous reactions involved, it is necessary to investigate both the pertinent electronic states at the surface of the catalyst and the chemical nature of the species adsorbed on the surface. This was both done by monitoring the N K-edge during the course of the reaction under flowing CO2 and mixtures of CO2 and H2O, with or without illumination, using XAS spectroscopy, and, in the same conditions, by detecting carboxylated and/or hydroxylated species on the surface using IR spectroscopy. The experiment was performed on g-C3N4 samples differently doped with alkali and transition metal ions, at the NFFA-Trieste facility. Operando soft X-ray analysis have been carried out at the beamline APE-HE and operando FTIR measurements were performed at the SISSI-Bio beamline. The results, although still at the preliminary stage of analysis, show significant effects in the IR spectra of some samples with the increase in intensity of peaks related to species adsorbed on the surface of the photocatalyst. On the other hand, the XAS analyses will have to be repeated as they did not provide significant results.È consentito all'utente scaricare e condividere i documenti disponibili a testo pieno in UNITESI UNIPV nel rispetto della licenza Creative Commons del tipo CC BY NC ND.
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https://hdl.handle.net/20.500.14239/16439