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 junctions in composite guest-host aerogels in which ∼5 nm Au nanoparticles are incorporated either directly into the anatase TiO network (Au "in" TiO, Au-TiO aerogel) or deposited onto preformed TiO aerogel (Au "on" TiO, Au/TiO aerogel). The metal-meets-oxide nanoscale interphase as visualized by electron tomography feature extended three-dimensional (3D) interfaces, but Au-TiO aerogels impose a greater degree of Au contact with TiO particles than does the Au/TiO form. Both aerogel variants enable transport of electrons over micrometer-scale distances across the TiO network to Au||TiO 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 network more effectively disperses trapped electrons. Density functional theory (DFT) calculations find that increased physical contact between Au and TiO, 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 junctions in Au-TiO 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 to O at the interface. Solid-state H nuclear magnetic resonance experiments show that proton spin-lattice relaxation times and possibly proton diffusion are strongly coupled to Au||TiO interfacial design, likely through spin coupling of protons to unpaired electrons trapped at the TiO network. Taken together, our results show that Au||TiO interfacial design strongly impacts charge carrier (electron and proton) transport over mesoscale distances in catalytic aerogel architectures.