3 Catalysts via Atomic Layer Deposition
97
provides an effective approach for catalytic particle ALD with both high efficiency
and precursor utilization.
Besides conventional ALD reactors for particles which all need to be in vacuum
work conditions, a spatial ALD process on particles has been reported by Van Ommen
et al. [158], Spencer et al. [159], and Elam et al. [160]. In a spatial ALD process,
the particles are continuously moving through the reaction areas with alternate ALD
precursors, which are separated by inert purge gas. The spatial ALD process is
suitable for the scale-up of particle surface modification. For instance, platinum
(Pt) nanoclusters with the size of about 1 nm are deposited onto titania (TiO 2 ) P25
nanoparticles resulting in a continuous production of an active photocatalyst (0.12–
0.31 wt% of Pt) [158].
3.5 Conclusion and Outlook
This chapter summarizes recent advances in catalyst preparation via ALD processes.
The ALD method allows precise catalyst synthesis of size and composition at atomic
level and creates well-defined structures, which is the superiority compared with
conventional wet chemistry methods. Strategies to control the structures of metal
active center and metal/metal oxide composites by ALD have been reviewed, with
a focus on enhancing the performance of catalysts. These strategies of selective
ALD have demonstrated unique advantages to design and fabricate the catalysts in
atomic scale and provided insights to understand the structure–activity relationship
in a more direct way. There are still plenty of challenges lying ahead, however. It
is essential to focus on the fundamental understanding of surface reaction mechanisms such as interactions and reactions of precursors with different substrates,
the reaction energetic routes of the selective ALD processes. The current choice
of precursors for ALD approaches is quite limited at this moment. For instance,
gold and silver catalysts prepared by ALD are barely reported, which are important in the catalytic family. On the other hand, it is necessary to develop precursors
for catalytic nanoparticles. Various oxides and materials rely on plasma-enhanced
ALD, which are not quite compatible with large quantity of nanoparticles as well as
three-dimensional growth. For selective ALD to construct different structures, it is
important to develop a fundamental theoretical understanding to guide the process
and obtain various structures. The combination of studies with both in situ and ex situ
experiments and characterizations are essential to understand the structural evolution
and reaction processes.
Furthermore, the readers need to be aware that most of the reported results are
laboratory-scale demonstrations of proofs of concept showing superior catalytic
properties of ALD catalysts. The practical industrial applications are needed to
continue motivating research on scale-up of this technology. The main problems
originate from the huge specific surface area of practical catalysts. The deposition and decoration of catalysts on powder substrates consume long period of time
for precursor diffusion and reaction, and agglomeration of nano-/micron powder
97
provides an effective approach for catalytic particle ALD with both high efficiency
and precursor utilization.
Besides conventional ALD reactors for particles which all need to be in vacuum
work conditions, a spatial ALD process on particles has been reported by Van Ommen
et al. [158], Spencer et al. [159], and Elam et al. [160]. In a spatial ALD process,
the particles are continuously moving through the reaction areas with alternate ALD
precursors, which are separated by inert purge gas. The spatial ALD process is
suitable for the scale-up of particle surface modification. For instance, platinum
(Pt) nanoclusters with the size of about 1 nm are deposited onto titania (TiO 2 ) P25
nanoparticles resulting in a continuous production of an active photocatalyst (0.12–
0.31 wt% of Pt) [158].
3.5 Conclusion and Outlook
This chapter summarizes recent advances in catalyst preparation via ALD processes.
The ALD method allows precise catalyst synthesis of size and composition at atomic
level and creates well-defined structures, which is the superiority compared with
conventional wet chemistry methods. Strategies to control the structures of metal
active center and metal/metal oxide composites by ALD have been reviewed, with
a focus on enhancing the performance of catalysts. These strategies of selective
ALD have demonstrated unique advantages to design and fabricate the catalysts in
atomic scale and provided insights to understand the structure–activity relationship
in a more direct way. There are still plenty of challenges lying ahead, however. It
is essential to focus on the fundamental understanding of surface reaction mechanisms such as interactions and reactions of precursors with different substrates,
the reaction energetic routes of the selective ALD processes. The current choice
of precursors for ALD approaches is quite limited at this moment. For instance,
gold and silver catalysts prepared by ALD are barely reported, which are important in the catalytic family. On the other hand, it is necessary to develop precursors
for catalytic nanoparticles. Various oxides and materials rely on plasma-enhanced
ALD, which are not quite compatible with large quantity of nanoparticles as well as
three-dimensional growth. For selective ALD to construct different structures, it is
important to develop a fundamental theoretical understanding to guide the process
and obtain various structures. The combination of studies with both in situ and ex situ
experiments and characterizations are essential to understand the structural evolution
and reaction processes.
Furthermore, the readers need to be aware that most of the reported results are
laboratory-scale demonstrations of proofs of concept showing superior catalytic
properties of ALD catalysts. The practical industrial applications are needed to
continue motivating research on scale-up of this technology. The main problems
originate from the huge specific surface area of practical catalysts. The deposition and decoration of catalysts on powder substrates consume long period of time
for precursor diffusion and reaction, and agglomeration of nano-/micron powder