Dynamic X‐Ray Diffraction Computed Tomography Reveals Real‐Time Insight into Catalyst Active Phase Evolution

Abstract

Dedicated to the Fritz Haber Institute, Berlin, on the occasion of its 100th anniversary Metals and metal oxides anchored to porous support materials are widely used as heterogeneous catalysts in a number of important industrial chemical processes. These catalysts owe their activity to the formation of unique metal/metal oxide support interactions, typically resulting in highly dispersed actives stabilized in a particular electronic or coordination state. [1][2][3] They are employed in fixed-bed reactors as extruded or pelletized millimeter-sized "catalyst bodies" minimizing pressure drops along the length of the reactor. Since the efficiency of the whole catalytic system depends on the behavior and efficiency of the catalyst body per se, its design has very great importance. Crucial to this design is an understanding of the factors which influence the distribution and nature of the active phase during preparation. The type of desired distribution is very much dependant on catalytic process and required products; for example, an egg-shell distribution (as opposed to uniform, egg-white, or egg-yolk), where the active phase is located at the edges of the catalyst body, can be favored if the product forms readily. [4,5] Herein, we study catalyst bodies of nickel supported on gAl 2 O 3 . These catalysts are widely employed for hydrogenation. [1][2][3]6] They are often prepared using highly soluble nickel nitrate or chloride precursor salts, which allow for the preparation of catalysts with high metal loadings in a single impregnation step. Two problems are commonly encountered with the preparation of these catalysts: the formation of unwanted metal aluminate (spinel) phases and poor metal dispersion within the catalyst body. The dispersion of the metal ions within the alumina can be controlled by the addition of complexing agents to the impregnation solution, thereby changing the molecular structure of the precursor (through ligand exchange) as well as the interactions of the precursor with the support. Additionally, the presence of chelating ligands in the precursor increases both the metal oxide reducibility and dispersion. [7][8][9][10] In recent times, absorption, spectroscopic, and scattering techniques have been developed to obtain spatial information on the distribution of chemical species in catalyst bodies, which allows the phenomena taking place during their preparation to be monitored. [7] These techniques include Raman, IR, and UV/Vis microspectroscopy; magnetic resonance imaging (MRI); tomographic energy dispersive diffraction imaging (TEDDI); and micro-computed tomography (mCT). [11][12][13][14][15][16][17] Importantly, both MRI and TEDDI are able to probe in a non-invasive manner in two dimensions, thus providing time-resolved information on both the impregnation and calcination/activation processes. Whilst the former is limited to the study of paramagnetic species and is therefore almost exclusively limited to probing the impregnation stage, TEDDI provides detailed information regarding elemental and ...

Quantitative X-ray tomography

“…176 It has been applied recently with a reasonable speed (acquisition of a slice in 20 min) for the study of structural change during high temperature modification of catalysts. 177 Alvarez-Murga et al 178 reviewed some recent results on diffraction/scattering computed tomography. They showed that the method yields an enhancement in the detection of the weak signals coming from minor phases.…”

Diffraction scattering computed tomography: a window into the structures of complex nanomaterials

“…In the in operando study the DSCT has been combined with absorption μCT offering a strong way to distinguish voids from areas composed of weakly scattering materials and relating high resolution imaging with the diffraction signals, this way revealing inactive parts of the catalyst body. 5,49,60 …”

Developing a Molecular-Level Understanding of Organic Chemistry and Physics at the Gas–Surface Interface

“…In addition, new approaches are needed that effectively bridge the so-called materials and pressure gaps in our understanding of how to extend studies and phenomena observed in wellcharacterized systems to real-world applications. Continued advances in scanning probe techniques, 56 electron microscopy, 57 scattering and diffraction methods, 58 and high-pressure electron spectroscopies 59,60 will be critical tools in further advancing our understanding of the chemistry and physics of organic molecules on surfaces.…”