Evolution of Hierarchically Porous Nickel Alumina Catalysts Studied by X‐Ray Ptychography

Abstract

The synthesis of hierarchically porous materials usually requires complex experimental procedures, often based around extensive trial and error approaches. One common synthesis strategy is the sol–gel method, although the relation between synthesis parameters, material structure and function has not been widely explored. Here, in situ 2D hard X‐ray ptychography (XRP) and 3D ptychographic X‐ray computed tomography (PXCT) are applied to monitor the development of hierarchical porosity in Ni/Al2O3 and Al2O3 catalysts with connected meso‐ and macropore networks. In situ XRP allows to follow textural changes of a dried gel Ni/Al2O3 sample as a function of temperature during calcination, activation and CO2 methanation reaction. Complementary PXCT studies on dried gel particles of Ni/Al2O3 and Al2O3 provide quantitative information on pore structure, size distribution, and shape with 3D spatial resolution approaching 50 nm, while identical particles are imaged ex situ before and after calcination. The X‐ray imaging results are correlated with N2‐sorption, Hg porosimetry and He pycnometry pore characterization. Hard X‐ray nanotomography is highlighted to derive fine structural details including tortuosity, branching nodes, and closed pores, which are relevant in understanding transport phenomena during chemical reactions. XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials.

Monolithic Al2O3 Xerogels with Hierarchical Meso‐/Macropore System as Catalyst Supports for Methanation of CO2

“…[14,33] Recently, we proved that the meso-/macropore system of Ni-containing Al 2 O 3 xerogels from the synthesis approach presented here is in fact of a hierarchical nature. [13,34] SEM micrographs (Figure 6), revealing mesopores within the pore walls of the macroporous gel network, confirm this finding also for the xerogels of this study. As determined by Hg porosimetry, (Figure 5b), the meso-and macropore width of the calcined xerogel are 7.1 nm and 1.5 μm, respectively.…”

“…The material exhibits a bimodal pore system, resulting from polymerization‐induced phase separation [14,33] . Recently, we proved that the meso‐/macropore system of Ni‐containing Al 2 O 3 xerogels from the synthesis approach presented here is in fact of a hierarchical nature [13,34] …”

New Dimensions in Catalysis Research with Hard X‐Ray Tomography

“…Combined with additional structural information, i.e., elemental composition, X-ray density from a determined crystal structure, or skeletal density from He pycnometry, electron density can be used to interpret and quantify features such as catalyst mesoporosity. This is applicable even if such features are below the spatial resolution of the measurement, as shown recently for the hierarchically porous Ni/Al 2 O 3 SPP2080-IMRC catalyst [54]. Furthermore, electron density information might be applied to detect and localize coking as recently shown for an artificially coked Ni/Al 2 O 3 catalyst by PXCT [55], and similarly for an FCC catalyst particle by Vesely ´et al using HXCT [56].…”

Environmental control for X-ray nanotomography

“…Environmental setups that facilitate in situ and operando studies of materials in response to changes in atmosphere, temperature and/or pressure are a hallmark of X-ray microtomography (Zhu et al, 2021;Maire et al, 2001;Marone et al, 2020;Meirer & Weckhuysen, 2018;Wood, 2018). However, equivalents for similar studies at nanometric 3D resolution are rare (Weber et al, 2022;Watanabe et al, 2007;Murata et al, 2015) because of the more stringent requirements concerning sample stability, positioning accuracy and angular sampling, especially in scanning-based imaging modalities (Pfeiffer, 2018;Mokso et al, 2007). Artifact-free tomographic reconstructions with isotropic resolution require the acquistion of angularly equally spaced projections covering the full angular span from 0 to 180 .…”

“…The few existing environmental control nanotomography setups follow the design of environmental transmission electron microscopy cells (Fam et al, 2019;Weber et al, 2022;Kahnt et al, 2021;Wu et al, 2017;Parker et al, 2022). These delimited cells are equipped with imaging windows and constrain the angular sampling range.…”