Mapping the Pore Architecture of Structured Catalyst Monoliths from Nanometer to Centimeter Scale with Electron and X-ray Tomographies

The hierarchicalpore systems of Pt/Al₂O₃ exhaust gas aftertreatment catalysts were analyzed with a collection of correlative imaging techniques to monitor changes induced by hydrothermal aging. Synergistic imaging with laboratory X-ray microtomography, synchrotron radiation ptychographic X-ray computed nanotomography, and electron tomography allowed quantitative observation of the catalyst pore architecture from centimeter to nanometer scale. Thermal aging at 750 °C in air and hydrothermal aging at 1050 °C in 10% H₂O/air caused increasing structural degradation, which manifested as widespread sintering of Pt particles, increased volumeand quantity of macropores (>20 nm), and reduction in effectivesurfacearea coupled with decreasing volume and frequency of mesopores (2–20nm) and micropores (<2 nm). Electron tomography unraveled the three-dimensional (3D) structure with high resolution allowing visualization of meso- and macropores but with samples of maximum 300 nm thickness. To complement this, hard X-ray ptychographic tomography produced quantitative 3D electron density maps of 5 μm diameter samples with spatial resolution <50 nm, effectively filling the resolution gap between electron tomography and hard X-ray microtomography. The obtained 3D volumes are an essential input for future computational modeling of fluid dynamics, mass transport, or diffusion properties and may readily complement bulk onedimensional porosimetry measurements or simulated porosity.