Photocatalytic versus Stoichiometric Hydrogen Generation using Mesoporous Silicon Catalysts: The Complex Role of Sacrificial Reagents

Mesoporous silicon has emerged as a promising photocatalyst for solar-driven hydrogen production via water splitting. However, its cyclability and stability are poor due to oxidation of silicon during the reaction, which also results in stoichiometric amounts of hydrogen. Despite this, the exact contribution of stoichiometric component in the reaction remains unknown. This study demonstrates that the stoichiometric hydrogen contribution when using silicon photocatalysts is dependent on the type of sacrificial reagent. In the presence of triethanolamine and sodium sulfite, which increase the solution's pH, over 90% of the hydrogen produced originates from the stoichiometric reaction. In contrast, when alcohol-based sacrificial reagents are used, the ratio of catalytic to stoichiometric hydrogen depends on the size of the alcohol molecule. Smaller alcohols, such as methanol and ethanol, result in higher overall hydrogen production; however, more than 40% of it originates from the stoichiometric reaction of silicon with water. As the alcohol size increases, the amount of water near the catalyst surface is limited, leading to decreased hydrogen production rates but improved photocatalyst stability. This study highlights the major role of the undesirable side reactions in silicon based photocatalysis and the need for more rigorous hydrogen quantification protocols to determine true photocatalytic activity.