FIB
‐
SEM
tomography imaging and
3D
structure quantification of supported metal catalysts with bi‐ and tri‐modal meso‐macroporosity
Authors: Duyckaerts Nicolas , Bartsch Mathias , Lorke Axel , Schüth Ferdi , Prieto Gonzalo 
Published: December 2016

Materials with hierarchical porosities extending over several length scales, from the nanometer to the macrometer range, attract great attention as solid catalysis. They offer possibilities to enhance the rate of diffusional access of reactants to, and products from, the catalyst active sites. Even though several methodologies based on e.g. soft‐ and hard‐templating, or spontaneous assembly, have been demonstrated to synthesize meso‐macroporous catalysts [1], their structure is typically assessed on a qualitative basis and not much is known about how to deliberately control key structural features of their different porosity levels such as modality, pore size or spatial interconnectivity. Here we have applied FIB‐SEM tomography, coupled to quantitative 3D image analysis, to visualize and quantify the internal architecture of high‐surface area, multi‐modal meso‐macroporous RuCo/γ‐Al
2
O
3
catalysts, which are highly interesting in the production of synthetic liquid fuels from natural gas.[2] The spatial resolution offered by FIB‐SEM tomography is particularly suitable to image macroporous networks, with pores ranging from 50 nm to several micrometer in size.

Different series of gamma‐Al
2
O
3
support materials, displaying either highly interconnected, channel‐like or mostly isolated, globule‐like macroporous networks, and essentially identical specific surface areas and mesoporities, were synthesized via soft‐ and hard‐templating from nanosized boehmite precursors. Next, highly dispersed RuCo nanoparticles (10‐15 nm) were incorporated by incipient wetness impregnation with nitrate precursors in solution to obtain the supported catalysts. The overall texture (specific surface area, pore volume, mesopore size distribution) of the materials was assessed by means of N
2
adsorption and Hg intrusion porosimetries. Volumes of ca. 400 µm
2
of resin‐embedded catalyst particles (400‐600 µm) were imaged using a ETD(SE) detector (2kV, 0.17 nA) and Ar‐FIB milling (30 kV, 21 nA) in a Helios Nanolab dual‐beam microscope (FEI).

3D reconstruction of the tomograms allowed a precise visualization of the internal macropore networks in the catalysts, overcoming limitations of alternative intrusion porosimetry methods which probe exclusively unconstrained macropores connected to the outer surface of the particles. Access to the internal structure of the materials made it possible to identify composition ranges for the synthesis gels suitable to prepare catalyst materials with either bi‐modal meso‐macroporous or tri‐modal meso‐macro‐macroporous structures. In addition, several structural parameters of relevance for molecular diffusion could be quantified by image analysis after tomogram segmentation (Figure 1). The skeleton of the macropore system was computed through a morphology‐preserving thinning algorithm. Macropore connectivities were quantified using an algorithm based on the Euler characteristic of the corresponding set of voxels, while a local trabecular thickness algorithm was applied on the set of voxels corresponding to the mesoporous Al
2
O
3
phase to quantify the diffusion distances for primary reaction products through the mesoporous network towards the connected macropores.[3]

Our imaging and structure quantification approach guided the synthesis of catalysts with similar macropore volumes and mesopore diffusion distances albeit notably different macropore connectivities (Figure 2), enabling a systematic investigation of the relative significance of these parameters for the catalytic performance under industrially relevant reaction conditions.