What are the species involved in the gliding arc plasma synthesis of heterogeneous catalysts?

Gliding arc plasma is an innovative method for synthesizing heterogeneous catalysts. All the hypothetical reactions from the literature currently attempting to explain the catalyst formation (usually metal (hydr)oxide), when precursors (metal salts) are exposed to plasma, only involve OH degrees and NO degrees radicals. Yet, these reactions remain unverified, and other reactive plasma species generated have never been considered to exert an influence on the precipitation of (hydr)oxide particles. In this context, this contribution investigates which species are responsible for the oxidoreduction of precursors exposed to the glidarc, with a specific focus on the role of NO degrees and OH degrees in the oxidoreduction of iron salts. To investigate the role of OH degrees radicals, radical scavenger (coumarin) was used. Similar solids precipitated whenever coumarin was used or not, suggesting that other plasmagenerated species might contribute to the oxidation process, such as long-lived species. To challenge this hypothesis, the behaviour of metal precursors in demineralized water pre-exposed to plasma was studied. Similar precipitates were formed for both FeSO4 and KMnO4 when exposed to this "activated" water and when directly exposed to the discharge, showing that the predominant initiators of the precipitation are the long-lived species, that we identified in our system to be NO3 - and HNO2/NO2- . We demonstrate that HNO2/NO2- species play a prominent role in the plasma precipitation process which has never been reported before. Yet slight differences in the composition of the precipitates formed when no short-lived species were present suggests that HNO2/NO2may not be the sole agent responsible for initiating the precipitation.