(400r) 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 pretreatment (either via steam treatment or calcination temperature [650Â°C â 1050Â°C]) of the Al2O3 support influences the FTS activity of the resulting catalyst via altering cobalt deposition. Over this temperature range, Al2O3 will undergo phase transitions from the Î³- (~750Â°C), Î´- (~900Â°C), and Î¸- (~1000Â°C) phases. 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 Î³-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 Î³-Al2O3 as semi-continuous planar layers likely less than 3 monolayers thick, which must be responsible for the catalystâs FTS activity. We further demonstrate that increasing calcination temperature increases the activity of ALD Co catalysts while also altering catalyst selectivity, 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.