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

Compositional Evolution of Individual CoNPs on Co/TiO2 during CO and Syngas Treatment Resolved through Soft XAS/X-PEEM

The nanoparticle (NP) redox state is an important parameter in the performance of cobalt-based Fischer-Tropsch synthesis (FTS) catalysts. Here, the compositional evolution of individual CoNPs (6-24 nm) in terms of the oxide vs metallic state was investigated in situ during CO/syngas treatment using spatially resolved X-ray absorption spectroscopy (XAS)/X-ray photoemission electron microscopy (X-PEEM). It was observed that in the presence of CO, smaller CoNPs (i.e., ≤12 nm in size) remained in the metallic state, whereas NPs ≥ 15 nm became partially oxidized, suggesting that the latter were more readily able to dissociate CO. In contrast, in the presence of syngas, the oxide content of NPs ≥ 15 nm reduced, while it increased in quantity in the smaller NPs; this reoxidation that occurs primarily at the surface proved to be temporary, reforming the reduced state during subsequent UHV annealing. O K-edge measurements revealed that a key parameter mitigating the redox behavior of the CoNPs were proximate oxygen vacancies (Ovac). These results demonstrate the differences in the reducibility and the reactivity of Co NP size on a Co/TiO2 catalyst and the effect Ovac have on these properties, therefore yielding a better understanding of the physicochemical properties of this popular choice of FTS catalysts.

Binary Metal-Oxide Active Sites Derived from Cu-Doped MIL-88 with Enhanced Electroactivity for Nitrate Reduction

Nitrate-to-ammonia electrochemical conversion is important for decreasing water pollution and increasing the production of valuable ammonia. However, achieving high ammonium production without undesirable byproducts is difficult. Cu-doped MIL-88-derived bimetallic oxide catalysts with electrocatalytically active Fe-O-Cu bridges, which have high NO3- adsorption energy and facilitate N-intermediate hydrogenation, are developed for NH4+ production. Cu doping promotes hybridization between the O 2p of NO3- and Fe-Cu 3d, facilitating the adsorption and reduction of NO3- with a low Tafel slope (62.1 mV dec-1) and high ammonia yield (1698.8 μg·h-1·cm-2). The cathode efficiency is stable for seven cycles. Cu adjacent to Fe sites inhibits hydrogen evolution, promotes NO3- adsorption, and decreases the intermediate adsorption energy barrier. This study provides new opportunities for fabricating diverse binary metal oxides with new interfaces as efficient cathode materials for selective electroreduction.