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Title of the thesis: Porous media modification with atomic layer deposition: Catalyst performance protection with an inert coating on a Cobalt-based Fischer-Tropsch catalyst

Thesis defender: Niko Heikkinen 
Opponent: Prof. J. Will Medlin, University of Colorado Boulder, United States
Custos: Prof. Riikka Puurunen, Aalto University School of Chemical Engineering

Atomic layer deposition (ALD) is a gas phase thin film preparation method with many practical applications, from planar surface microelectronics to porous and tortuous structures such as catalysts. Precise control of reactant deposition inside porous media is achieved with self-limiting chemisorption, where instead of the amount of introduced reactant, the surface itself controls the deposition process. At full reactant saturation, the thin film formation is limited by the number of surface reaction sites and all excess reactants and by-products are removed from the surface prior to the next chemisorption cycle. Although ALD is a method of preparing conformal coatings on even complex structures, the process conditions must be appropriate and de-signed for each deposition surface and set of structural properties (e.g. pore size and surface area).

One important consideration in ALD process design related to particulate materials such as heterogeneous catalysts is that the particle shape has a distinctive effect on the ALD reactant diffusion inside the particle. For this thesis the ALD process was studied for three particle geometries: slab, cylinder and sphere. Comparison of the different particle geometries was conducted with a one dimensional, coupled partial differential equation ALD diffusion–reaction model. The most significant finding was that in comparison to slab particles, only ~1/3 and ~1/2 the reactant exposure (Pa s) was required to fully coat spherical and cylindrical particles, respectively. The lower exposure required to achieve fully coated particles is due to the spherical and cylindrical particles decreasing in volume towards the core.

Before the Fischer-Tropsch reaction experiments, a post-ALD annealing process was conducted to create porosity in the ALD coating and partially recover the coating-covered active metal (cobalt) sites. The experimental results show that with both the γ-Al2O3 and TiO2-supported catalysts the ALD coating decreased Co removal through leaching during the Fischer-Tropsch reaction. The extent of leaching was suppressed by increasing ALD cycles. However, increasing the number of ALD cycles reduced the cobalt active site recovery and active site availability. Also, a general trend in terms of catalyst selectivity was that increasing the number of ALD cycles decreased the desired C5+ hydrocarbon selectivity and increased undesired CH4 selectivity.

Keywords: atomic layer deposition, Fischer-Tropsch synthesis, cobalt, catalysis, dif-fusion-reaction modelling

Thesis available for public display 10 days prior to the defence at . 

Contact information: 
niko.heikkinen@vtt.fi
040 484 8751