(29b) ALD-Formed Cobalt/Alumina Nanostructures Active for Fischer-Tropsch Synthesis

Cobalt metal was deposited onto alumina (Al₂O₃) supports using atomic layer deposition (ALD) to create dispersed catalysts for Fischer-Tropsch Synthesis (FTS). We show that the phase of the Al₂O₃ support greatly influences the FTS activity of the resulting catalyst. ALD literature predicts that Co deposited on Al₂O₃ with low cycle numbers should only produce Co nanoparticles less than 2 nm in diameter, which fall into the well-characterized non-active region of nanoparticle sizes for FTS. Our FTS experiments confirm that this inactivity is indeed the case for Co deposited on amorphous Al₂O₃. However, Co deposited on calcined Al₂O₃ is in fact active, suggesting an alternative ALD deposition regime not previously recognized. Aberration corrected STEM/EELS imaging shows that in addition to nanoparticle deposition, cobalt has also deposited on Al₂O₃ as semi-continuous planar layers likely less than 3 monolayers thick, which are likely responsible for the catalyst’s FTS activity. Further, it was found that the number of ALD cycles alters the activity of these catalysts, with 4 and 8 cycle catalysts being more active than the 1 cycle catalyst. These data suggest Co requires the presence of crystalline Al₂O₃ surfaces in order to achieve a thin semi-continuous planar growth regime. Finally, density functional theory (DFT) calculations were carried out to understand the thermodynamics of Co small particle vs. planar layer (monolayer and bilayer) growth on Al₂O₃ and explain the activity shown by these catalysts.