Wang, Hui; (2022) Investigation on light-driven hydrogen production and storage. Doctoral thesis (Ph.D), UCL (University College London).

Text Wang_10145973_Thesis.pdf Access restricted to UCL open access staff until 1 January 2025. Download (9MB)

Abstract

Currently, commercial hydrogen (H2) has been primarily produced from natural gas (methane) reforming, which releases massive CO2 and disturbs the natural carbon cycle. In parallel methanol and ammonia (NH3) have been regarded as two major liquid H2 sources, allowing H2 to be stored and transported using the existing infrastructure. To realize a green H2 economy, methanol dehydrogenation is one of the best ways as it converts biomethanol into H2 and valuable products rather than COx under ambient conditions, thus promising for H2 release on-demand. In this context, this project targets to produce H2 at nearly zero-carbon emission by photocatalytic methanol dehydrogenation using an atomic-level catalyst design strategy to fabricate single atoms and nanodots of cocatalysts on titanium dioxide (TiO2). The synthesised platinum-copper-TiO2 (PtCu-TiO2) photocatalyst showed an extraordinary H2 generation activity (472.6 mmol g-1 h-1), together with highly selective production of formaldehyde (HCHO). The synergistic effect of Cu and Pt cocatalysts was identified, where Cu promoted H2 production and tuned the selectivity towards HCHO and Pt facilitated charge separation and photocatalyst stability. Furthermore, characterisation results indicated that photo-excited electrons were accelerated by Cu2+ ions, followed by the transfer into Pt nanodots for proton reduction. At the same time, Cu+ acted as a hole acceptor and mediated selective methanol oxidation. Though high H2 yield was achieved by methanol dehydrogenation, protons in water molecules were less used for hydrogen production. Hence, to efficiently utilize the protons from both methanol and water for H2 production, another efficient photocatalyst was developed. The self-assembled sulphur-doped carbon nitride (SCN 550) photocatalyst was used to reduce protons derived from both methanol and water, which exhibited 7 times higher H2 generation activity than bare g-C3N4. More importantly, the ratio of CO2 to H2 was close to 1:3, suggesting that protons derived from both methanol and water have been utilized successfully for H2 production. Under optimized conditions, the SCN 550 showed a H2 evolution rate of 14.7 mmol g-1 h-1, with an AQE of 29%. Moreover, the photocatalyst exhibited excellent stability for a prolonged time (up to 95 h), thus representing a strong potential for industrial application. X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and electrochemical impedance spectroscopy (EIS) results suggested that the S was doped as N-S-H and C-S-H terminals, narrowing the band gap and offering better photo-excited charge separation that was mainly due to C-S-H sites and carbon vacancies. In contrast, NH3 has a higher H2 content than methanol, which is more attractive for heavy duty vehicles as a zero-carbon fuel. However, the traditional NH3 production approaches require high temperature and high-pressure conditions, which consume about 2% of the world's annual energy, in addition to the environmental impact. Hence, photocatalytic NH3 production using a ternary heterostructure that consists of ruthenium species and g-C3N4 was explored. Under light irradiation, the Ru/RuO2/g-C3N4 system exhibited 6 times higher NH3 production activity than bare g-C3N4 and Ru/g-C3N4 systems. The characterization of these photocatalysts revealed that the enhanced activity of Ru/RuO2/g-C3N4 was due to the efficient transfer of electrons and holes to Ru and RuO2, respectively, facilitating both the reduction and oxidation reaction, in addition to the advantage of Ru active sites for better N2 adsorption and activation.

Download activity - last month

Export as JSONXMLCSV

Download activity - last 12 months

Export as JSONXMLCSV

Downloads by country - last 12 months

Export as XMLCSVJSON

Archive Staff Only

View Item