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The last two decades have brought significant progress in synthesis and design of meso-/macroporous materials with potential applications in heterogeneous catalysis or adsorption. Introducing macropores into a uniform mesoporous system can provide higher mass transfer velocity by promoting convective flow over diffusion. Here, we focus on meso-/macroporous systems with the predominant transport processes of Knudsen diffusion in mesopores and molecular diffusion or convective flow in macropores. One promising approach is to design catalysts with hierarchical rather than uniform pore structures, generally including some combination of solids with micro- (50 nm) pore size distribution. Therefore, strategies have to be found for a knowledge-based design of porous catalysts with optimized mass transport properties and improved catalyst performance. Diffusion limitations of reactants or products are highly relevant both in normal operating conditions and following changes induced by catalyst deactivation. Similarly to the above examples, mass transport properties strongly influence the performance of porous materials used in heterogeneous catalysis, particularly in terms of catalytic activity, product selectivity, and material stability. Hierarchical structures are omnipresent in our environment and are particularly found in connection with transport processes, for example, the evolved human respiratory system for oxygen delivery in the lungs, or modern designed road networks for managing traffic flow.
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XRP and PXCT are enabling technologies to understand complex synthesis pathways of porous materials. 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. The X-ray imaging results are correlated with N 2-sorption, Hg porosimetry and He pycnometry pore characterization. Complementary PXCT studies on dried gel particles of Ni/Al 2O 3 and Al 2O 3 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. In situ XRP allows to follow textural changes of a dried gel Ni/Al 2O 3 sample as a function of temperature during calcination, activation and CO 2 methanation reaction. 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/Al 2O 3 and Al 2O 3 catalysts with connected meso- and macropore networks. One common synthesis strategy is the sol–gel method, although the relation between synthesis parameters, material structure and function has not been widely explored.
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The synthesis of hierarchically porous materials usually requires complex experimental procedures, often based around extensive trial and error approaches.