DESIGN OF NOVEL MATERIALS FOR PHOTO-THERMO-CATALYTIC SOLAR FUELS PRODUCTION

The global challenge of transitioning to sustainable energy sources underscores the critical need for innovative technologies that can harness solar energy for fuel production. Among various strategies, the generation of solar fuels, such as hydrogen (H2) through water splitting or alcohol reforming, and the conversion of carbon dioxide (CO2) into valuable chemicals, has emerged as a highly promising route. Photocatalysis, which uses light to drive chemical reactions, offers a sustainable approach, but it faces significant limitations, primarily due to the low efficiency of light absorption and conversion by conventional semiconductor catalysts with wide bandgaps. A novel approach to overcome these limitations involves integrating heat with photocatalysis, a process known as photo-thermo-catalysis (PTC). PTC advantage the combined effects of light and heat, enhancing reaction rates and improving overall solar energy utilization. This PhD project aimed to develop and optimize catalytic systems for PTC, focusing on methanol steam reforming (MSR) for hydrogen production and CO2 reduction, while also deepening the understanding of reaction mechanisms through advanced in-situ techniques. The first phase of the project involved designing and building a bench-scale experimental setup capable of simultaneously studying the effects of light and heat on catalytic processes. Titanium dioxide (TiO2)-based materials, a well-known photocatalyst, were chosen as the initial focus. These materials were tested for hydrogen production via MSR, with and without the addition of noble metal nanoparticles like platinum (Pt), which are known to enhance charge carrier separation and improve catalytic activity. The study revealed that TiO2, both in its pure form and decorated with Pt nanoparticles, exhibited significant photocatalytic activity at room temperature. However, the introduction of heat substantially boosted the hydrogen production rate. Notably, Pt/TiO2 demonstrated two distinct activity regions: at lower temperatures, photocatalysis dominated, while at higher temperatures, thermocatalytic activity became predominant, and light still contributed in increasing the H2 productivity and the modulating the selectivity of the process. Despite the enhanced hydrogen production, a key challenge remained: the undesired formation of carbon monoxide (CO) as a byproduct, which limits the H2 productivity in the process. To address this, cerium oxide (CeO2) was incorporated into the catalyst formulation to promote the Water Gas Shift (WGS) reaction, which converts CO and water into CO2 and additional H2. Various synthesis methods and compositions were explored, revealing that moderate amounts of CeO2 improved catalytic efficiency and selectivity towards CO2, especially at mid-range temperatures. Interestingly, mechanical mixing of TiO2-based catalysts with higher CeO2 content provided the best overall performance, significantly enhancing hydrogen yield and selectivity through effective WGS activity. In the second phase, research expanded to explore LaFeO3 (LFO) based materials, a class of perovskite oxides noted for their visible-light activity, stability, and cost-effectiveness. Doping LFO with copper (Cu) and forming heterojunctions with other semiconductors were investigated as strategies to improve charge carrier dynamics and catalytic efficiency. The LFO-based catalysts showed promising results in both CO2 reduction and methanol reforming. The formation of heterojunctions with TiO2 enhanced CO2 reduction efficiency, while Cu-doping significantly improved hydrogen production via methanol steam reforming. These materials benefited from the combined effects of light and heat, underscoring the potential of PTC to enhance reaction rates and selectivity. Moreover, it was observed that introducing structural defects through mechanochemical treatments, while increasing photothermal conversion, i.e., the conversion of light into heat. To gain a deeper understanding of the reaction mechanisms, in-situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) was employed. This technique allowed real-time observation of surface species under reaction conditions, shedding light on how heat, light, and catalyst composition influence the MSR process. The studies showed that methanol adsorbs onto the TiO2 surface, forming intermediates such as methoxy and formate species. Under light irradiation, the formation of photo-oxidation products like formaldehyde and formic acid was enhanced, confirming the photocatalytic activity. The addition of Pt nanoparticles intensified these processes, facilitating the production and subsequent oxidation of adsorbed intermediates, especially at higher temperatures, where different reaction pathways can be observed due to the synergistic effect of heat and light. This research successfully demonstrated the potential of photo-thermo-catalysis as a promising route to enhance solar fuel production. TiO2-based materials, when combined with noble metals and co-catalysts like CeO2, showed significant improvements in hydrogen production and selectivity. LaFeO3-based systems further expanded the scope of materials suitable for PTC applications, offering new pathways for visible-light-driven reactions. The in-situ mechanistic studies provided critical insights, revealing how heat and light influence reaction pathways, offering a valid mechanistic description of the PTC process involved.