(544cc) Porous Titania Microspheres: Highly-Efficient Catalyst Scaffold for Green Syngas Production

Continued increase in global energy demand coupled with continued reliance on hydrocarbon fuels as well as emissions caused by other human activities necessitate development of new, environmentally-conscious sources of energy. An interesting possibility for both mitigation of emissions and as a source of fuel is the capture of CO₂ and CH₄ emissions from livestock farming operations and landfills. These gases can undergo dry reforming of methane (DRM)/ CO₂ reforming of methane (CRM) to produce syngas, which may be used for production of carbon-neutral fuels via Fischer-Tropsch upgrading.

However, DRM processes possess a prohibitive energy barrier, often requiring temperatures ranging from 1000-1500K for the reaction to proceed (reaction enthalpy = 247 kJ/mol), even with catalysts. A solution that has been tested for the reduction of the DRM energy barrier is the implementation of dual-functionality thermo/photocatalysts by impregnation of transition metal catalysts like platinum on “black” titanium oxide (TiO₂). This combination permits the reforming reaction to proceed via both the thermocatalytic and visible-light photocatalytic pathways, thus dramatically reducing the heating required for DRM. Unfortunately, implementation of these processes is affected by the undesirable polydispersity and nano-scale of the most easily available TiO₂ catalyst particles, which will cause high pressure drop across flow reactors.

This work explores the synthesis of porous, hollow TiO₂ microspheres for use in thermo/photocatalytic DRM reactions via a microfluidic synthesis process. Using this method, TiO₂ microparticles are formed via flow-focusing droplet formation in custom built glass microfluidic reactors. The inner tapered-tip capillary injects the precursor (comprised of titanium butoxide, toluene, a photocurable polymer, and a radical photoinitiator) into a constricted outlet, where precursor droplets are formed via dripping particle breakup. The continuous phase, comprised of formamide with 1 wt% Pluronic F108, is fed via independently controlled co- and counter-annular flows. The droplets then exit the reactor into a DI water bath, where the titanium butoxide undergoes rapid hydrolysis to form an amorphous titania shell. Following the hydrolysis, the collected particles are UV cured, washed, dried, and calcined in a muffle furnace to form crystalline TiO₂ hollow microspheres. After calcination, the particles are impregnated with rhodium and reduced to “black” TiO₂ to achieve the dual-functionality catalyst. Using these particles without illumination at 550°C, we have achieved yields approximately 3 orders of magnitude higher than those previously reported at similar temperatures.¹ As such, these results suggest an exciting path by which DRM processes may be made feasible both economically and environmentally.

1. “Efficient Visible Light Photocatalytic CO₂ Reforming of CH₄.” Han, B.; Wei, W.; Chang, L.; Cheng, P.; Hu, Y. H. ACS Catal. 6, pp 494-497, 2016.