Enhanced Photocatalysis via Self-Assembled Architectures—Record Low-Temperature Methane Reforming over Ordered Mesoporous and Asymmetrically Porous Block Copolymer-Templated Semiconductors

The recent discovery of low-temperature photocatalytic conversion of methane and carbon dioxide to syngas (photocatalytic dry reforming of methane; photo-DRM) transformed an expensive high-temperature process into an appealing low-temperature sustainable process for carbon conversion. This simple reaction, which can proceed under illumination without external heating, could provide an alternative to crude oil for supplying the organic precursors our modern world is built upon, without the coking and catalyst failure of high-temperature solutions. While much attention has been paid to alternative support chemistries or exotic metallic promoters, the 3-D architecture of the semiconductor support itself has been largely overlooked. Our work has developed chemically-simple highly accessible photocatalysts with novel mesoscale architectures, delivering record catalyst activities though self-assembly derived mesostructure.<br/><br/>By utilizing block-copolymer self-assembly templating of common semiconductor supports, we have studied TiO₂ and Ta₂O₅ photocatalyst supports in a range of architectures: from hexagonally packed cylinders to 3-D co-continuous gyroids to asymmetrically porous thin films. This mesoporosity provides enhanced activity across a wide range of volumetric flow rates, delivering record low-temperature performance beyond the expectations of enhanced surface area alone. Beyond surface area, the 3-D accessibility of co-continuous architectures greatly reduces tortuosity, leading to fast facile transport of reactive species through the structure.<br/><br/>Further, we developed a novel TiO₂ thin film catalyst architecture derived from liquid filtration membranes that has a thin mesoporous top layer with macroporous support layer. This highly-active low-density membrane architecture delivers the highest reported activity per gram for low-temperature photo-DRM to-date solely through changes in support architecture, delivering 2500x the mass-normalized photo-DRM activity of non-structured TiO₂ powder. This combination self-assembly and non-solvent-induced phase separation (SNIPS) technique creates catalyst architectures highly suited to photocatalysis, concentrating highly active material in only the thin illuminated surface region, minimizing non-illuminated mass in the supporting substructure. These self-assembly-based polymer processes for creating 3-D semiconductor architectures are simple, low-cost, and scalable routes to create world-class photocatalyst supports, applicable to a wide-range of gas-phase catalytic reactions.