Photocatalytic microreactors for ‘green’ hydrogen

RESEACHERS at the University of Southampton, UK have developed photocatalytic microreactors that convert water into hydrogen fuel using solar energy.

The photocatalytic microreactors were made using microstructured optical fibre canes (MOFCs). Microstructured optical fibres are optical fibre waveguides, made for global telecommunications. Waveguides are structures which enable the transmission of waves with minimal energy loss by restricting transmission in one direction, and optical fibres serve as waveguides for light. The researchers at Southampton repurposed this technology to produce microstructured optical photoreactors.

University of Southampton

To enable hydrogen production, the researchers employed a palladium/titanium dioxide catalyst. Titanium dioxide (TiO₂) is often used in this field due to its availability, low cost, and high activity. MOFCs were coated internally and externally using a suspension of TiO₂ nanopowder in water, and then palladium (Pd) nanoparticle deposited onto the fibre.

The MOFCs were developed as high-pressure microfluidic reactors, each housing multiple capillaries that pass a chemical reaction along the length of the cane. They simultaneously serve as both host and catalyst for continuous, indirect water-splitting, in which methanol serves as a ‘sacrificial’ reagent.

Matthew Potter, Chemistry Research Fellow at Southampton, explained that Pd helps to split methanol (CH₃OH) into carbon monoxide (CO) and hydrogen (H₂). Photons absorbed by TiO₂ briefly reduce the titanium (ie cause the gain of an electron) which then either removes the CO from the Pd site, refreshing it, or converts water into H₂.

Despite the low loading of the MOFC systems (<0.1 wt%), the researchers achieved hydrogen production efficiency of 70.4 mL/h/g of catalyst. This surpasses analogous liquid-phase powdered systems (14 mL/h/g of catalyst) by a factor of five.

H₂ and carbon dioxide (CO₂), the products of the reaction, are generated with a 3:1 stoichiometric ratio. According to researchers, this improves upon conventional hydrogen production methods. For example, a 2014 assessment of steam methane reforming – the main hydrogen production technology used by industry – suggested that for every kilogram of H₂ produced, 7 kg of CO₂ is produced.

According to the researchers, the system offers scope for optimising the photocatalytic performance of the coated MOFCs towards a variety of processes. Alongside hydrogen production, they are also investigating photochemical conversion of CO₂ into synthetic fuel.

Potter said that further development of this technology requires expanded testing, and scaleup into pilot process to confirm the lifetime of the catalyst inside the MOFC. According to Pier Sazio, Principal Research Fellow in Photonics at Southampton, the microfluidic reactors are highly scalable and can be manufactured to any length, allowing for highly desirable chemical engineering scaleup and scaleout. Sazio explained that scaleout would involve the addition of more MOFCs in parallel to spread out load.

Currently, the researchers are expanding the technology to incorporate a wider range of materials within the optical fibres for other photocatalytic applications.

Potter commented: “In this work our …photoreactor shows significant improvements in activity compared to existing systems. This is an ideal example of chemical engineering for a 21^st century green technology.”

ACS Photonics: http://doi.org/ds6z