Performance of a NiFeO@Co Core-Shell Fischer-Tropsch Catalyst: Effect of Low Temperature Reduction.

In situ TEM gas-cell imaging and spectroscopy with in situ XRD have been applied to reveal morphological changes in NiFeO@CoO core-shell nanoparticles in hydrogen. The core-shell structure is retained upon reduction under mild conditions (180 °C for 1 h), resulting in a partially reduced shell. The core-shell structure was retained after exposing these reduced NiFeO@CoO core-shell nanoparticles to Fischer-Tropsch conditions at 230 °C and 20 bar. Slightly harsher reduction (230 °C, 2 h) resulted in restructuring of the NiFeO@CoO core-shell nanoparticles to form cobalt islands in addition to partially reduced NiFeO. NiFeO underwent further transformation upon exposure to Fischer-Tropsch conditions, resulting in the formation of iron carbide and nickel/iron-nickel alloy. The turnover frequency in the Fischer-Tropsch synthesis over NiFeO@CoO core-shell nanoparticles reduced in hydrogen at 180 °C for 1 h was estimated to be less than 0.02 s (cobalt-time yield of 8.40 μmolgs) with a C selectivity of 38 C-%. The low turnover frequency under these conditions in relation to the turnover frequency obtained with unsupported cobalt is attributed to the strain in the catalytically active cobalt.