Resolving the Effect of Oxygen Vacancies on Co Nanostructures Using Soft XAS/X-PEEM.

Improving both the extent of metallic Co nanoparticle (Co NP) formation and their stability is necessary to ensure good catalytic performance, particularly for Fischer-Tropsch synthesis (FTS). Here, we observe how the presence of surface oxygen vacancies (O) on TiO can readily reduce individual CoO NPs directly into CoO/Co in the freshly prepared sample by using a combination of X-ray photoemission electron microscopy (X-PEEM) coupled with soft X-ray absorption spectroscopy. The O are particularly good at reducing the edge of the NPs as opposed to their center, leading to smaller particles being more reduced than larger ones. We then show how further reduction (and O consumption) is achieved during heating in H/syngas (H + CO) and reveal that O also prevents total reoxidation of Co NPs in syngas, particularly the smallest (∼8 nm) particles, thus maintaining the presence of metallic Co, potentially improving catalyst performance.