An atomic layer deposition diffusion-reaction model for porous media with different particle geometries.

This work presents a diffusion-reaction model for atomic layer deposition (ALD), which has been adapted to describe radial direction reactant transport and adsorption kinetics in a porous particle. Specifically, we present the effect of three particle geometries: spherical, cylindrical and a slab in the diffusion-reaction model. The reactant diffusion propagates as a unidimensional front inside the slab particle, whereas with cylinder and spherical particles, the reactant diffusion approaches the particle centre from two and three dimensions, respectively. Due to additional reactant propagation dimensions, cylindrical and spherical particles require less exposure for full particle penetration. In addition to the particle geometry effect, a sensitivity analysis was used to compare the impact of the particles' physical properties on the achieved penetration depth. The analysis evaluates properties, such as the combined porosity and tortuosity factor, mean pore diameter, specific surface area, pore volume, and particle radius. Furthermore, we address the impact of the reactant molar mass, growth-per-cycle (GPC), sticking probability, reactant exposure and deposition temperature on the simulated diffusion and surface coverage profiles. The diffusion-reaction model presented in this work is relevant for the design and optimization of ALD processes in porous media with different particle geometries.

Modelling atomic layer deposition overcoating formation on a porous heterogeneous catalyst.

Atomic layer deposition (ALD) was used to deposit a protective overcoating (Al2O3) on an industrially relevant Co-based Fischer-Tropsch catalyst. A trimethylaluminium/water (TMA/H2O) ALD process was used to prepare ∼0.7-2.2 nm overcoatings on an incipient wetness impregnated Co-Pt/TiO2 catalyst. A diffusion-reaction differential equation model was used to predict precursor transport and the resulting deposited overcoating surface coverage inside a catalyst particle. The model was validated against transmission electron (TEM) and scanning electron (SEM) microscopy studies. The prepared model utilised catalyst physical properties and ALD process parameters to estimate achieved overcoating thickness for 20 and 30 deposition cycles (1.36 and 2.04 nm respectively). The TEM analysis supported these estimates, with 1.29 ± 0.16 and 2.15 ± 0.29 nm average layer thicknesses. In addition to layer thickness estimation, the model was used to predict overcoating penetration into the porous catalyst. The model estimated a penetration depth of ∼19 µm, and cross-sectional scanning electron microscopy supported the prediction with a deepest penetration of 15-18 µm. The model successfully estimated the deepest penetration, however, the microscopy study showed penetration depth fluctuation between 0-18 µm, having an average of 9.6 µm.

Effect of Co-fed Water on a Co-Pt-Si/γ-Al2O3 Fischer-Tropsch Catalyst Modified with an Atomic Layer Deposited or Molecular Layer Deposition Overcoating.

Atomic layer deposition (ALD) and molecular layer deposition (MLD) methods were used to prepare overcoatings on a cobalt-based Fischer-Tropsch catalyst. A Co-Pt-Si/γ-Al2O3 catalyst (21.4 wt % Co, 0.2 wt % Pt, and 1.6 wt % Si) prepared by incipient wetness impregnation was ALD overcoated with 30-40 cycles of trimethylaluminum (TMA) and water, followed by temperature treatment (420 °C) in an inert nitrogen atmosphere. MLD-overcoated samples with corresponding film thicknesses were prepared by using TMA and ethylene glycol, followed by temperature treatment (400 °C) in an oxidative synthetic air atmosphere. The ALD catalyst (40 deposition cycles) had a positive activity effect upon moderate water addition (P H2O/P H2 = 0.42), and compared with a non-overcoated catalyst, it showed resistance to irreversible deactivation after co-fed water conditions. In addition, MLD overcoatings had a positive effect on the catalyst activity upon moderate water addition. However, compared with a non-overcoated catalyst, only the 10-cycle MLD-overcoated catalyst retained increased activity throughout high added water conditions (P H2O/P H2 = 0.71). All catalyst variations exhibited irreversible deactivation under high added water conditions.