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

Abstract

The hierarchical pore systems of Pt/Al 2 O 3 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 cm to nm scale. Thermal aging at 750 °C in air and hydrothermal aging at 1050 °C in 10% H 2 O/air caused increasing structural degradation, which manifested as widespread sintering of Pt particles, increased volume and quantity of macropores (>20 nm), and reduction in effective surface area coupled to decreasing volume and frequency of mesopores (2-20 nm) and micropores (<2 nm). Electron tomography unraveled the 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 modelling of fluid dynamics, mass transport or diffusion properties and may readily complement bulk 1D porosimetry measurements or simulated porosity.

First ptychographic X-ray computed tomography experiment on the NanoMAX beamline

“…Comparison of published PXCT experiments (Dierolf et al, 2010;Guizar-Sicairos et al, 2011Diaz et al, 2012Diaz et al, , 2014Esmaeili et al, 2013;Trtik et al, 2013;Holler et al, 2014Holler et al, , 2017Donnelly et al, 2015;Fløystad et al, 2015;Dam et al, 2015;Fam et al, 2018;Polo et al, 2018;Kahnt et al, 2018;Sala et al, 2019;Becher et al, 2019;Weissenberger et al, 2019; in magenta, cyan and yellow with the present data in red. (Left) The average achieved 2D translational scanning speed (scan dimensions for a single projection divided by the time it took to record one projection) plotted against the imaged volume.…”

Porosity and Structure of Hierarchically Porous Ni/Al2O3 Catalysts for CO2 Methanation

“…A similar combination of FIB tomography and ET was used by Stoeckel et al to study macro-and mesoporous silica [25], while it was shown that, based on FIB tomography and ET, mass-transfer properties could be calculated [26,27]. Recently, the potential of combining multiscale imaging techniques was shown for a study of the porous structure of Pt/Al 2 O 3 -based catalysts for exhaust aftertreatment [28]. In these studies, ptychographic X-ray computed tomography (PXCT) was used to image length scales from tens of nanometer to several micrometer, therefore covering large meso-and macropores.…”

Challenges and Opportunities to Enhance Flexibility in Design and Operation of Chemical Processes

“…Examples for all scales are available. For heterogeneous catalysis, investigation of the catalyst and pore structure from the nano‐ to the centimeter scale (e.g., 76, 77) or the measurement of temperature profiles with high spatial and temporal resolution in technical fixed‐bed reactors for model validation 78 are prominent examples. Regarding multiphase flows, investigations from the scale of single bubbles (e.g., 79, 80) to the integral investigation of entire unit operation (e.g., 81–83) are presented in literature.…”