Influence of Alumina Support Crystallinity on ALD-Synthesized Cobalt Catalysts 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 calcination pretreatment (temperature 750°C – 1050°C) of the Al₂O₃ support influences the FTS activity of the resulting catalyst via altering cobalt deposition. 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 γ-Al₂O₃ is in fact active, suggesting an alternative ALD deposition regime not previously recognized. Extensive characterization using aberration corrected STEM/EELS imaging shows that in addition to nanoparticle deposition, cobalt has also deposited on γ-Al₂O₃ as semi-continuous planar layers likely less than 3 monolayers thick, which are likely responsible for the catalyst’s FTS activity. We further demonstrate that increasing calcination temperature increases the activity of ALD Co catalysts, suggesting that the fraction of Co deposited as semi-continuous planar layers is positively correlated with the degree of surface crystallinity and providing a means of controlling surface nanostructure formation. Supporting catalyst characterization using elemental mapping and density functional theory is discussed.