Understanding Pore Formation in ALD Alumina Overcoats

Abstract

AlO X thin films deposited by atomic layer deposition (ALD) have previously been used to increase both stability and selectivity of supported palladium catalysts and are known to develop nanoscale porosity upon heating. Understanding the factors that affect ALD thin-film porosity enables future design of layered catalytic structures with tunable nanoscale features on industrially-relevant high-surface-area materials. In this study, porous and nonporous aluminum oxide supports with and without palladium nanoparticles were overcoated with thin films of 2–7 nm AlO X by ALD deposited at temperatures of 100, 200, and 300 °C. Hydroxyl loss and changes in surface chemistry were observed upon heating the films, and changes in surface area and pore volume of the annealed films were correlated to AlO X deposition temperature and the presence of Pd. Crystallization of the overcoat to γ-Al2O3 is shown to occur separately from hydroxyl loss and pore formation. A mechanistic understanding of pore formation in AlO X ALD films is obtained by reference to studies of the structural transformations accompanying the formation of transition aluminas from hydroxide precursors. Additionally, a direct and tunable correlation is established between pore development and the overall hydroxyl content of AlO X ALD coatings.

Synthesis and Characterization of Mesoporous Silica Nanoparticles Loaded with Pt Catalysts

“…The loss of chemisorbed water and organic solvent may be the reason for weight loss at a higher temperature above 300 °C. Such results are similar to an ALD system [39].…”

Catalyst Deactivation by Carbon Deposition: The Remarkable Case of Nickel Confined by Atomic Layer Deposition

“…The rise in the 20‐ALD catalyst's activity with increasing temperature is slower than for the commercial one, as shown in Figure 1(c). The gradual increase may be attributed to two mechanisms, one of which is the well‐documented process of pore‐formation in an ALD overcoat at high temperatures, giving access to the active metal as the reaction proceeds [12] . Another possibility is the formation and reduction of NiAl 2 O 4 mediated by the crystallization of the alumina overcoat, [10,13] leading to increased Ni dispersion on the catalyst surface.…”

“…Previous studies [30,31] show that similar nanofibers arise from the hydrothermal treatment of sol‐gel‐derived boehmite (AlO(OH)). Such a transformation would be likely if a significant concentration of unreacted hydroxyl groups remained in the ALD overcoat after synthesis [12] . As the temperature ramps up during DRM, de‐hydroxylation would occur, initiating a topotactic phase transformation sequence leading to the formation of transition aluminas at higher temperatures [32,33] .…”

“…Such a transformation would be likely if a significant concentration of unreacted hydroxyl groups remained in the ALD overcoat after synthesis. [12] As the temperature ramps up during DRM, de-hydroxylation would occur, initiating a topotactic phase transformation sequence leading to the formation of transition aluminas at higher temperatures. [32,33] The alumina nanofibers are approximately 4-5 nm in diameter and vary in length (see Figures 5(d)), dimensions quite different from the nanorods observed on the surface in SEM (section 3.2).…”

Elucidating the Roles of Amorphous Alumina Overcoat in Palladium-Catalyzed Selective Hydrogenation

“… 37 George et al proposed that the structure of the as-synthesized AlO x ALD overcoat was close to amorphous boehmite, i.e., hydroxylated alumina. 38 Despite these structural characterizations, the diffusion/permeation behavior through an amorphous AlO x ALD overcoat has not been well understood. Infrared (IR) spectroscopy with CO as a probe molecule has been commonly used to evaluate the accessibility of metal sites underneath the ALD overcoat.…”