HYDROGEN PRODUCTION FROM OPUNTIA FICUS-INDICA CLADODES

This thesis investigates an integrated pathway combining autonomous Hydrothermal Liquefaction (HTL) and solar photocatalysis for renewable hydrogen production from Opuntia ficus-indica cladodes. The first stage involves the HTL of fresh cladodes over a temperature range of 250–320 °C, utilizing their high intrinsic moisture (83 wt%) as the sole reaction medium, eliminating the need for energy-intensive drying. Five batch runs were carried out at 250, 300 and 320 °C; biocrude yields ranged from 3.5 to 5.6 wt% on a dry basis across the temperature range tested, with the aqueous phase consistently accounting for approximately 88–93% of the reactor charge. Gas-phase analysis confirmed hydrogen formation in the HTL step across all conditions. The second stage valorizes this aqueous phase as a sacrificial electron donor for photocatalytic hydrogen evolution using graphitic carbon nitride (g-C₃N₄) under natural sunlight. Photocatalytic tests demonstrated that the uncatalysed HTL aqueous phase effectively promotes hydrogen production, with enhanced performance observed when co-sensitized with an indigo dye. However, while adding an alkaline catalyst (NaOH) during HTL increased gas-phase hydrogen generation, it completely inhibited the subsequent photocatalytic activity of the aqueous phase. This study confirms the potential of coupling wet thermochemical conversion with photocatalysis for sustainable energy generation, while highlighting the necessary chemical trade-offs for effective process integration.