Structural Changes of a U3O8/ZrO2 Catalyst During HCl Oxidation – a HAADF‐STEM Study

Uranium oxide finely dispersed on ZrO2 crystals represents an effective and robust catalyst for the industrially relevant oxidation of HCl to chlorine. Remarkably, the catalytic performance increases significantly during the first 80 h on stream and remains stable thereafter. According to XRD results, this self‐activation of the catalyst is accompanied by the disappearance of the starting α‐U3O8 phase. The uranium distribution on the ZrO2 crystals at different stages of the reaction was visualized by aberration‐corrected HAADF‐STEM. The mostly mono‐atomic uranium oxide layers present in the fresh catalyst broaden during reaction and often form a homogeneous coating over the support crystals comprising small uranium oxide species mostly comprising a single uranium atom. These species are found to be often located on top of columns of ZrO7 polyhedra, pointing to a strong fixation on the carrier. This observation likely explains the unique stability of the zirconia‐supported uranium oxide catalyst in the Deacon reaction.