(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 catalystâs 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.