Synthesis and Analysis of Nanostructured SiO2 / TiO2 Photocatalysts for Hydrogen Release from Liquid Organic Hydrogen Carriers

Hydrogen is a promising fossil fuel alternative, providing versatility, zero direct carbon emissions, and renewable production potential. However, transporting pure H₂ is energy-intensive, technically challenging, and potential dangerous. Liquid Organic Hydrogen Carriers (LOHCs) provide a solution, allowing safe H₂ storage and transportation within common industrial chemicals. Developing catalysts that efficiently and selectively release H₂ from LOHCs using only sunlight (photocatalysts) would allow H₂ to be used in underserved and off-grid communities. TiO₂ is a promising dehydrogenation photocatalyst, however its high activity makes controlling specific reactions (selectivity) difficult. Depositing layered nanostructures on TiO₂ surfaces and using organic templates to protect certain active sites has been shown to effect selectivity, reaction rates, and turnover frequency. This study investigates two methods for synthesizing layered and templated SiO₂/TiO₂ photocatalysts, exploring their impact on material properties and photocatalytic dehydrogenation behavior.

The first method uses Atomic Layer Deposition (ALD) to deposit thin SiO₂ layers from Si and O-containing vapors. The second method places two organic templates, p-tertbutyl calix[4]arene (CAL) and 3,5 di-tert-butyl catechol (TBC), and uses Chemical Liquid Deposition (CLD) to deposit SiO₂ layers on the exposed surface, creating artificial nanopores. Nine ALD catalysts (3 temperatures x 3 cycle quantities) and ten CLD catalysts (2 template sizes x 5 cycle quantities) were studied. Catalysts were analyzed for light absorption ability, Si to Ti surface ratio, and total effective surface area. Benzyl-alcohol (a model LOHC) dehydrogenation was used to measure reaction rates, catalyst activation energies, and selectivity. Initial results suggest thicker SiO₂ layers improve dehydrogenation rates for ALD catalysts and enhance active site stability for CLD catalysts. For CLD catalysts, there are indications that an intermediate SiO₂ layer thickness leads to an optimized turnover frequency. By examining synthesis-structure-performance relationships for layered SiO₂/TiO₂ materials, this study expands the field of TiO₂-based LOHC dehydrogenation photocatalysts, contributing to a future where clean H₂ can reduce society’s reliance on carbon-emitting fossil fuels.