Atomic Layer Deposition on Porous Materials: Problems with Conventional Approaches to Catalyst and Fuel Cell Electrode Preparation

Abstract

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given.

“Intelligent” Pt Catalysts Based on Thin LaCoO3 Films Prepared by Atomic Layer Deposition

“…La 2 O 3 , Co 3 O 4 , and LaCoO 3 films were deposited onto the MgAl 2 O 4 support by ALD using a home-built, static system that has been described in more detail elsewhere [24]. The ALD precursors used in this study were La(TMHD) 3 (Strem Chemicals, Inc., Newburyport, MA, USA) and Co(TMHD) 3 (Strem Chemicals, Inc).…”

“…CeO 2 was again added by ALD, using Ce(TMHD) 4 (Strem Chemicals, Inc.) as the precursor. The CeO 2 /Al 2 O 3 sample was identical to that used in a previous work [24]. The CeO 2 loading was 20 wt %, corresponding to a film thickness of roughly 0.5 nm, and the BET surface area of the sample was 89 m 2 /g after calcination to 773 K.…”

Nickel Supported on Mesoporous Zirconium Oxide by Atomic Layer Deposition: Initial Fixed-Bed Reactor Study

“…In 1990s, there was a strong industry-driven effort for ALD for catalysis in Finland; the technique was then called "atomic layer epitaxy" [11,[25][26][27][28][29][30][31][32]. Interest in ALD for the preparation of supported heterogeneous catalysts has again been increasing during the past decade [33][34][35][36][37][38][39]. The current interest in ALD is based for example in the ability of ALD to prepare (close to) monodisperse metal particles; to make overcoatings to temper the activity of highly active but non-selective sites; and to prepare single-atom catalysts.…”

“…Whatever the reactor type, the strength of ALD is best employed when the whole particle bed is coated with a uniform, conformal material layer. Attainment of saturation is not self-evident [39,45]; conformality in extreme aspect ratios needs process tuning and should be verified.…”

Thermal atomic layer deposition of gold nanoparticles: controlled growth and size selection for photocatalysis

“…In fact, the removal of low-vaporpressure substances, such as phosphoric acid, can be particularly slow in powders compared with flat substrates because of two phenomena: (1) capillary condensation, and (2) the fact that molecules that desorb from one particle can adsorb onto other particles before they can escape from the reactor. 47 In particular, the latter is exacerbated by operating the reactor at relatively high pressure compared with the vacuum reactors used by Griffiths et al 30 The effective removal of phosphorus in the form of phosphoric acid would probably require much longer water exposures, which would be cumbersome as they tend to disrupt the fluidization behavior of the powders. Hence, the fabrication of phosphorus-free Au/TiO 2 nano-composites via fluidized bed reactors operating at atmospheric pressure might require more aggressive treatments.…”