Monitoring Transport Phenomena of Paramagnetic Metal‐Ion Complexes Inside Catalyst Bodies with Magnetic Resonance Imaging

Abstract

An indirect magnetic resonance imaging (MRI) method has been developed to determine in a noninvasive manner the distribution of paramagnetic Co2+ complexes inside Co/Al2O3 catalyst extrudates after impregnation with Co2+/citrate solutions of different pH and citrate concentrations. UV/Vis/NIR microspectroscopic measurements were carried out simultaneously to obtain complementary information on the nature of the Co2+ complexes. In this way, it could be confirmed that the actual distribution of Co2+ inside the extrudates could be derived from the MRI images. By combining these space‐ and time‐resolved techniques, information was obtained on both the strength and the mode of interaction between [Co(H2O)6]2+ and different Co2+ citrate complexes with the Al2O3 support. Complexation of Co2+ by citrate was found to lead to a stronger interaction of Co with the support surface and formation of an eggshell distribution of Co2+ complexes after impregnation. By addition of free citrate and by changing the pH of the impregnation solution, it was possible to obtain the rather uncommon egg‐yolk and egg‐white distributions of Co2+ inside the extrudates after impregnation. In other words, by carefully altering the chemical composition and pH of the impregnation solution, the macrodistribution of Co2+ complexes inside catalyst extrudates could be fine‐tuned from eggshell over egg white and egg yolk to uniform.

Chemical Imaging of Spatial Heterogeneities in Catalytic Solids at Different Length and Time Scales

“…In other words, the proportion of paramagnetic nuclei can be monitored in space and time within a catalyst body by monitoring the quenching of the 1 H MRI signal. [35][36][37] The proof of principle is illustrated in Figure 14 Lighter shades of gray correspond to higher signal intensity. Reproduced from reference [18], copyright Elsevier, 2007. catalyst body at different times after impregnation, while Figure 14 b shows a quantitative Co 2+ -ion distribution plot derived from the 2D images.…”

Imaging Catalysts at Work: A Hierarchical Approach from the Macro‐ to the Meso‐ and Nano‐scale

“…This technique is well suited for studying catalyst preparation, as it is rapid and non‐invasive. MRI has also been applied by Bergwerff et al 15b. 22 to determine Co distributions in Co/γ‐Al 2 O 3 Fischer–Tropsch catalysts that had previously been studied using UV/Vis microspectroscopy 23.…”

Space-Resolved Profiling Relevant in Heterogeneous Catalysis

“…Furthermore, physicochemical processes during catalyst preparation and resulting distribution of catalytically active metals have been studied, giving rational reasoning for the distribution processes and patterns as shown in Fig. 4 [123][124][125][126].…”