Photocatalytic transformation of water pollutants into fuels
Creators: name: Al-Madanat, Osama
Publisher: name: Hannover : Institutionelles Repositorium der Leibniz Universität Hannover
Year: 2021
Subjects: subject: TiO2-Photokatalyse
subject: Naphthalin
subject: H2-Bildung
subject: Wasserspaltung
subject: Reaktionsmechanismus
subject: solarer Brennstoff
subject: Photoreformierung
subject: Dewey Decimal Classification::500 | Naturwissenschaften::540 | Chemie

description: The exploitation of solar radiation in the remediation of organic water pollutants has the potential to be a sustainable energy source to replace fossil fuel. Conceptually, coupling H2 evolution and the degradation of organic pollutants through dual-functional TiO2 photocatalysis has been frequently nominated as one of the most sustainable and environmentally friendly technologies. However, the low quantum efficiencies achieved so far by using TiO2 photocatalysts, and the lack of a fundamental understanding of the reaction mechanism represent big challenges that need to be overcome before their potential can be fully realized. Despite the large number of studies every year that utilize the dual functional photocatalysis processes for the H2 production from simple water pollutants such as methanol, formaldehyde, and formic acid, aromatic pollutants, however, have been rarely considered. Thus, several open questions regarding the photocatalytic efficiencies and the reaction mechanism during the photoreforming of aromatic compounds still exist. In the presented work, the photocatalytic reforming of naphthalene in water has been investigated employing TiO2-based photocatalysts under solar irradiation. The effect of different crystalline phases of TiO2, namely Sachtleben Hombikat UV100 (pure anatase) and Evonik Aeroxide P25 (mixed phase of anatase and rutile), were assessed under the same experimental conditions. In order to understand the effect of Pt co-catalysts on H2 production, different fraction ratios were deposited on P25 and UV100. At the optimum loading ratio, 0.5 wt.% Pt-UV100 exhibited the higher photocatalytic activity toward the H2 formation and photooxidation of naphthalene comparing to all Pt-P25 samples. The observed decrease of the photocatalytic activity of the P25 after the platinization indicates that Pt nanoparticles act as a recombination center for the photogenerated charge carriers as revealed by electron paramagnetic spectroscopy techniques. Besides that, it has been found that the platinization method strongly affects the H2 formation during the photoreforming of naphthalene over Pt-UV100. The evolution of H2 is 40 % higher when using Pt-UV100 that was prepared by the photodeposition method than that prepared by the physical mixing of TiO2 with Pt nanoparticles. The analysis of the charge carrier dynamics employing different spectroscopic techniques revealed that the strong metal-semiconductor interaction resulting from the photodeposition process leads to a significant decrease in the charge carrier recombination rates, which increases the rate of the photocatalytic reactions and enhances the H2 formation. The analysis of the reaction intermediates employing a combination of chromatographic, and mass spectroscopic techniques reveals that 1- and 2-naphthalenol, among other hydroxylated compounds, are the main reaction products during the photoreforming of naphthalene. Moreover, the accumulation of these intermediates in the photocatalytic system was found to be the main reason for the inhibition of the H2 formation and the photooxidation of naphthalene, besides, the unequal mass balance between the half reactions. Several possible pathways for the photoreforming mechanism of naphthalene involving different reactive species has been tested. Combining the spin trapping studies with different spectroscopic techniques suggested that the holes play the significant role in naphthalene oxidation. Isotopic labeling studies showed that lattice oxygen atoms were not incorporated into the photooxidation products, and the photogenerated electrons reduced the proton originating mainly from water to molecular hydrogen, whereas, water (but not naphthalene) appears to be involved in the rate-determining step of the reaction.