Hydrogen production via sulfur-based thermochemical cycles: Part 3: Durability and post-characterization of silicon carbide honeycomb substrates coated with metal oxide-based candidate catalysts for the sulfuric acid decomposition step

This work is a follow-up of previous efforts reported on the synthesis of various single and mixed oxide materials and their evaluation as catalysts for the sulfuric acid dissociation reaction for the production of SO2 and O2. The current work concerns the comparative assessment of Fe2O3, CuO, Cu–Fe, Fe–Cr, Cu–Al and Cu–Fe–Al mixed oxides coated as catalysts on silicon carbide monolithic honeycomb structures, with respect to sulfuric acid decomposition reaction conditions for 100 h at 850 °C and ambient pressure, as well as their ex-situ characterization after such operation. The exposure conditions are representative to a potential future real application. The exposure time, although of relatively short-term, is adequate to extract safe conclusions on the stability and therefore to a large extent also on the suitability of the candidate oxide-based catalysts. All catalytic systems tested exhibited high SO3 conversions reaching or exceeding 70%, for space velocities in the range of 5–35 h−1. For some of the samples, the relatively high initial activity decreased by about 5–10 percentage points in the course of the 100 h testing, reaching stable mean values. It was concluded that Fe2O3, CuO and Fe–Cr mixed oxide retained their chemical and structural stability after exposure to reaction conditions, while the other three mixed oxides studied suffered from significant phase decomposition phenomena. Based on the fact that the initial catalytic activity of the Fe–Cr mixed oxide, as identified in a previous comparative study among several materials, was found higher than the ones of Fe2O3 and CuO and relatively close to the one of the highly active but costly Pt/Al2O3 catalyst, the particular mixed oxide is considered a promising catalyst for the SO3 dissociation reaction.

► Mixed metal oxides were tested as catalysts for SO3 dissociation (100 h of exposure).
► All materials showed high initial activities with moderate catalytic deactivation over time.
► Fe–Cr mixed oxide retained its initial structure (Fe0.7Cr1.3O3).
► All other mixed oxides suffered from phase decomposition phenomena.