Post-discharge: An interesting step to improve heterogeneous catalysts synthesized by glidarc plasma?

Hanon, Fanny [UCL] Gaigneaux, Eric M. [UCL]

Gliding arc plasma (GP) is an interesting method for the synthesis of heterogeneous catalysts. In this process, precursors (metal salts) undergo a reaction with plasma species, resulting in the formation of a precipitate. Previous literature documents the synthesis of catalysts such as FeOx and MnOx derived from (NH4)2Fe(SO4)2 and KMnO4, revealing that incorporating a post-discharge (PD) step can introduce valuable properties like phase transformation, increased specific surface area and enhanced catalytic activity. Hypotheses have been proposed to explain the changes that the solids undergo during this step, suggesting that the long-lived species created in PD influence the solids transformation. However, no conclusive evidence supporting these hypotheses has been provided. Therefore, the aim of the research presented here is to explore the influence of PD on various types of solids synthesized by GP and to understand the underlying mechanisms of this step. To investigate the role of PD, we exposed iron and tin precursors to the plasma, and submitted them to two types of PD. The respective synthesis conditions do not consistently impact the characteristics of the plasma-synthesized solids for the different metals. However, a common pattern emerges when examining the influence of the PD on different salts of a given metal. Specifically, solids obtained from different precursors of the same metal, identically exhibit superior catalytic activity when synthesized under similar conditions. To understand the mechanism underlying the PD step and investigate the hypothesis that the long-lived species created in PD influence the solids transformation, we exposed iron and tin precursors to a PD where no plasma species were present. To do so, the plasma-synthesized solids were recovered and placed into pure distilled water before performing the PD step. This enables a comparison of the solids obtained under these conditions with those obtained through classical PD, where plasma species are present. We understood that, even though long-lived plasma species present during the PD can still trigger the reaction of metal ions remaining in solution, they do not impact the way the solid is transformed during this stage.