Noninvasive Spatiotemporal Profiling of the Processes of Impregnation and Drying within Mo/Al₂O₃ Catalyst Bodies by a Combination of X‑ray Absorption Tomography and Diagonal Offset Raman Spectroscopy

A combination of X-ray absorption microcomputed tomography (μ-CT) and diagonal offset raman spectroscopy (DORS) have been used to follow in real time the 2-D and 3-D evolution of Mo species within 3 mm γ-Al₂O₃ extrudates during catalyst impregnation and drying processes. In a first set of experiments, we have followed the real-time incipient wetness impregnation process using an aqueous solution of ammonium heptamolybdate (AHM). We observed that during the equilibration period, singly impregnated samples formed Al­(OH)₆Mo₆O₁₈^3– (Al–Mo) hot spots distributed over the entire sample volume and that these heterogeneities grow in number and size as a function of time. A second set of measurements focused on the coimpregnation of AHM with H₃PO₄ and the subsequent equilibration and drying stages. It was found that the presence of phosphorus in the impregnating solution prevented the formation of the hot spots via the formation of weakly bound H_xP₂Mo₅O₂₃^(6–x)– species that were uniformly distributed over the sample after 70 min of equilibration. During drying, however, these species migrated to the periphery of the sample, resulting in an egg shell distribution of H_xP₂Mo₅O₂₃^(6–x)–. We show that by performing these studies noninvasively with a sufficiently high time resolution, the behavior and evolution of the Mo species were reproduced more faithfully than by using more conventional and invasive cut-and-measure approaches.