Quantitative Chemical Mapping of Pt/Rh Gauze Catalysts for Ammonia Oxidation using Resonant X-ray Tomography

The loss of platinum group metals during the industrial Ostwald process for catalytic ammonia oxidation is a century-old concern. Understanding catalyst degradation requires knowledge of physicochemical changes occurring on stream, particularly on technical Pt/Rh gauze catalysts. However, such catalysts are mainly studied with surface-sensitive methods or as model systems since their composition of noble metals (e.g., 95/5 wt % Pt/Rh) makes it challenging to characterize the bulk catalyst. Here, we exploit the high-resolution and elemental sensitivity of synchrotron-based hard X-ray resonant tomography to quantify the 3D distribution of Rh and Pt in technical gauze catalysts. Resonant tomography performed above and below the Rh K-edge (23.300 and 23.208 keV, respectively) allowed computation of elemental distribution. Absorption contrast tomography was successful despite beam transmission of <0.1%. This was achieved using a single-photon counting detector. Rh segregation, Pt loss, and bulk material degradation leading to redistribution of elements were visualized and quantified within intact similar to 76 mu m diameter wires, with 2 mu m resolution, and up to 50 days on stream. The estimated local Pt and Rh wt % changes showed significant material loss from the wire exterior and characteristic protrusions. However, the wire core was unaffected by the reaction conditions even after 50 days on stream. No volume diffusion from the wire center to the exterior was observed to mitigate material loss from the latter. This flexible and high-throughput quantitative imaging approach is uniquely possible with synchrotron hard X-ray tomography, opening further routes of study for compositionally challenging industrial catalysts.