Investigation of Co2AlO4/CeO2 Catalyst for N2O Abatement using Electron Microscopy Techniques
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- Institutt for fysikk 
A N2O abatement catalyst of a Co2AlO4 active phase and CeO2 support phase has been investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM). The goal of the investigation was to link properties of the support material to the activity of the final catalyst. Six samples of the catalyst were investigated. Catalysts of two grades of support, a high surface and a low surface, were provided in three different variants each: support material only, complete fresh catalyst and used catalyst. The high surface support material was found to be comprised of significantly smaller individual particles than the low surface support, as expected. This difference in support particles had evened out considerably to their respective complete unused catalysts, although the high surface catalyst still had somewhat smaller particle sizes. In the used catalysts the high surface samples had grown slightly while the low surface sample looked mostly unchanged. The Co2AlO4 phase was found to be comprised of particles that were typically in the size range 30-200 nm. Quantitative particle-size statistics were not obtained to confirm this because of low particle counts and low STEM Z-contrast. TEM images showed that Co2AlO4 particles had characteristic rough surfaces compared to the CeO2 particles. Some Co2AlO4 particles also had fine nanoparticles on their surfaces. STEM/EDS analysis was employed to demonstrate the distribution of Co2AlO4 particles on the support in the high surface catalysts, and also indicated that many Co2AlO4 particles had become embedded partially or completely into the bulk of support particles, instead of residing on the surface. It is suggested here that sintering of the support particles during calcination plays an important role for the activity and longevity of the catalyst. The active phase is also thought to undergo sintering during calcination. The surface roughness and nanoparticles observed on the Co2AlO4 particles could influence the effective surface area and thereby activity. The nanoparticles are believed to be of either the Co2AlO4 or the Co3O4 structure, but could not be conclusively determined. Their phase is believed to play an important role for the activity of the catalyst. The high degree of sintering of the high surface area support oxide during calcination could result in the observed embedding in the corresponding catalyst material. Embedding is believed to result in decreased initial activity but increased longevity of the catalyst.