Unraveling the Direct Decomposition of NOₓ over Keggin Heteropolyacids and Their Deactivation Using a Combination of Gas-IR/MS and In Situ DRIFT Spectroscopy
2020
Schnee, Josefine | Delannoy, Laurent | Costentin, Guylène | Thomas, Cyril
Keggin heteropolyacids (HPAs) have been known to be efficient NOₓ absorbers for many years, and to be able to decompose a significant fraction of NOₓ species preabsorbed at 100–150 °C into harmless N₂ and O₂ upon rapid heating to 450 °C. However, this capability of Keggin HPAs to directly decompose NOₓ in the absence of a reducing agent in the feed has never been studied under more realistic reaction conditions, for example, under continuous NOₓ feeding and in the presence of O₂ in the feed. This was done for the first time in the present work over the widely used H₃PW₁₂O₄₀ Keggin HPA at 380 °C. H₃PW₁₂O₄₀ was shown to be active in the direct decomposition of continuously fed NOₓ, with the decomposition process occurring through the involvement of NOH⁺ adspecies. O₂ was found to play a dual role. On the one hand, it led to the oxidation of NO into NO₂ in the feed, with NO₂ deactivating the HPA units. On the other hand, it was found to initiate the diffusion of NOₓ into the bulk of the HPA crystals, which significantly enhanced the durability of the activity of the HPA in the direct decomposition of these NOₓ, provided that the diffusion pathway in between the bulk Keggin units was already accessible, thanks to the presence of NOₓ species preabsorbed at room temperature. By providing the key factors involved in the direct decomposition of continuously fed NOₓ over H₃PW₁₂O₄₀, the present work paves the way for future investigations that might focus, for instance, on tuning the chemical composition of the heteropolyanions in order to make the HPA more resistant to deactivation.
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