Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 177

d n 9 r 4 n g | 7

Figure 7.10 Percentage ALD coating of a spherical porous particle with radius R 0
against penetration depth d of the precursor.
Reproduced from ref. 11.

during ALD on powders. 42-44 It should, however, also be noted that al-
ready a significant fraction of the interior surface of a mesoporous particle
is reached when only an outer rim is coated. 45 For a simple model of a
spherical porous particle, deposition to a depth of 100 nm in a particle
with a radius of 0.5 mm su ces to cover already 50% of the internal por-
ous volume (Figure 7.10). Moreover, also in catalytic processes, the react-
ants have to diffuse into the pores of the catalysts, and, as for the ALD
precursors, only a minority of them will reach the core of the nanoporous
catalyst particles. With this in mind, it is valid to conclude that the con-
formality of ALD can be exploited to apply conformal coatings on nano-
porous materials for catalytic applications.

.

7.1.3 Opportunities of ALD in Supported Catalyst
Preparation
The versatility of ALD as a method to design supported catalysts is illustrated
in Figure 7.11. In this figure, the nanoporous support is schematically
represented by means of a cross-section covering three nanopores.
Figure 7.11a represents the situation where ALD is used as a vapor phase
grafting method to engineer the presence of active sites on the pore walls. 4,46
Very few (or even only one) ALD cycles are typically applied in this case.
In Figure 7.11b, ALD is used to synthesize catalytic active nanoparticles
nicely dispersed on the large surface area support. For nucleation-controlled
ALD processes, islands are formed at the start of the deposition instead of a
continuous layer. 17 This is, for example, the case for many noble metal ALD
processes on metal oxide surfaces. By carefully controlling the nucleation