Power of Aerogel Platforms to Explore Mesoscale Transport in Catalysis.

We describe the opportunity to deploy aerogels-an ultraporous nanoarchitecture with co-continuous networks of meso/macropores and covalently bonded nanoparticulates-as a platform to address the nature of the electronic, ionic, and mass transport that underlies catalytic activity. As a test case, we fabricated Au||TiO ₂ junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO ₂ network (Au "in" TiO ₂ , Au _IN -TiO ₂ aerogel) or deposited onto preformed TiO ₂ aerogel (Au "on" TiO ₂ , Au _ON /TiO ₂ aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but Au _IN -TiO ₂ aerogels impose a greater degree of Au contact with TiO ₂ particles than does the Au _ON /TiO ₂ form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO ₂ network to Au||TiO ₂ junctions, as evidenced by electron paramagnetic resonance (EPR) and ultrafast visible pump-IR probe time-resolved absorption spectroscopy. The siting of gold nanoparticles in the TiO ₂ network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO ₂ , induced by oxygen vacancies, produces increased hybridization of midgap states and quenches unpaired trapped electrons. We assign the apparent differences in electron-transport capabilities to a combination of the relatively better-wired Au||TiO ₂ junctions in Au _IN -TiO ₂ aerogels, which have a greater capacity to dilute accumulated charge over a larger interfacial surface area, with an enhanced ability to discharge the accumulated electrons via catalytic reduction of adsorbed O ₂ to O ^2- at the interface. Solid-state ¹ H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO ₂ interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO ₂ network. Taken together, our results show that Au||TiO ₂ interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.