90
R. Chen et al.
created. In the calcination process, the stress relaxation of the coating layers can also
be helpful for the formation of pores.
For example, Stair et al. fabricated 8 nm porous oxide-coated Pd metal NPs with
enhanced catalytic activity and selectivity toward dehydrogenation of ethane reaction [83]. Firstly, the 8 nm conformal Al 2 O 3 layer was deposited on the catalyst.
After ALD coating, the as-prepared catalyst was calcined under high temperature at
oxidation environment to create ~2 nm pores. After calcination, CO chemisorption
FTIR measurements indicated the porous structure was formed, the metallic Pd sites
were re-exposed, and the porous layer could improve the sintering and coking resistance at high temperature. The porous Al 2 O 3 coated Pd catalyst showed improved
catalytic selectivity of ethylene (23%), and the by-product yields of CH 4 , CO 2 , and
CO were reduced to 0.9%, 3.9%, and 5.1%, respectively. At the same time, the carbon
formation was also reduced significantly by 94%. For uncoated Pd catalyst, after the
catalytic reaction tests, heavy coking was observed and the activity was degraded.
The porous coating structures formed via ALD have also utilized for other metalbased catalysts like Cu as shown in Fig. 3.8a (especially The Cu/γ-Al 2 O 3 system)
[86], Co for solid oxide fuel cell cathode, aqueous-phase hydrogenation reactions
[89], Au for CO oxidation [130], Ni in the dry reforming of methane [94, 135], Pd
for methane oxidation [121, 137], and Ag for plasmonic photocatalysis [14].
An alternative way of creating porous structures was to utilize the initial nucleation
growth of metal oxide via ALD. It was reported that in the initial growth stage,
porous Al 2 O 3 was formed and part of the active metal surface was exposed to the
reactants (Fig. 3.8b). FTIR measurements confirmed that Al 2 O 3 started its growth
on low coordinated sites of Pd NPs in the first few cycles, instead of forming a
conformal coating layer (Fig. 3.8c) [82, 140, 141]. The Al 2 O 3 protective layers with
an optimized thickness showed enhanced catalytic activity, while as the coating layer
thickness increased and became continuous that blocked the reactants reaching to
the Pd surface, the catalytic activity also decreased (Fig. 3.8d). The ALD Al 2 O 3
also effectively improved the stability of the Pd NPs at 500 °C. On the other hand,
Detavernier et al. elaborated the formation of porous coating structures with tailored
pore sizes by applying TiO 2 ALD [142]. Weimer et al. investigated the molecular
layer deposition (MLD) using TMA and ethylene glycol to fabricate alumina alkoxide
hybrid films on Pt NPs (~2 nm). The alumina alkoxide hybrid overcoatings could be
transformed to porous Al 2 O 3 film on Pt NPs since the contained hydrocarbon groups
were combusted at high temperature. Although some metal surface sites were reexposed, it was found that the porous layer still decreased the catalytic performance
which might be caused by the small pore size [143].
3.3.3 Site-Selective ALD Coating of Metal Oxide
A more delicate way to perform ALD is exploiting inherently selective ALD process
to directly modify the active sites of nanoparticles. Lu et al. showed that during the
TiO 2 ALD on Au particles, the TiO 2 layer firstly covered Au’s low coordinated sites
R. Chen et al.
created. In the calcination process, the stress relaxation of the coating layers can also
be helpful for the formation of pores.
For example, Stair et al. fabricated 8 nm porous oxide-coated Pd metal NPs with
enhanced catalytic activity and selectivity toward dehydrogenation of ethane reaction [83]. Firstly, the 8 nm conformal Al 2 O 3 layer was deposited on the catalyst.
After ALD coating, the as-prepared catalyst was calcined under high temperature at
oxidation environment to create ~2 nm pores. After calcination, CO chemisorption
FTIR measurements indicated the porous structure was formed, the metallic Pd sites
were re-exposed, and the porous layer could improve the sintering and coking resistance at high temperature. The porous Al 2 O 3 coated Pd catalyst showed improved
catalytic selectivity of ethylene (23%), and the by-product yields of CH 4 , CO 2 , and
CO were reduced to 0.9%, 3.9%, and 5.1%, respectively. At the same time, the carbon
formation was also reduced significantly by 94%. For uncoated Pd catalyst, after the
catalytic reaction tests, heavy coking was observed and the activity was degraded.
The porous coating structures formed via ALD have also utilized for other metalbased catalysts like Cu as shown in Fig. 3.8a (especially The Cu/γ-Al 2 O 3 system)
[86], Co for solid oxide fuel cell cathode, aqueous-phase hydrogenation reactions
[89], Au for CO oxidation [130], Ni in the dry reforming of methane [94, 135], Pd
for methane oxidation [121, 137], and Ag for plasmonic photocatalysis [14].
An alternative way of creating porous structures was to utilize the initial nucleation
growth of metal oxide via ALD. It was reported that in the initial growth stage,
porous Al 2 O 3 was formed and part of the active metal surface was exposed to the
reactants (Fig. 3.8b). FTIR measurements confirmed that Al 2 O 3 started its growth
on low coordinated sites of Pd NPs in the first few cycles, instead of forming a
conformal coating layer (Fig. 3.8c) [82, 140, 141]. The Al 2 O 3 protective layers with
an optimized thickness showed enhanced catalytic activity, while as the coating layer
thickness increased and became continuous that blocked the reactants reaching to
the Pd surface, the catalytic activity also decreased (Fig. 3.8d). The ALD Al 2 O 3
also effectively improved the stability of the Pd NPs at 500 °C. On the other hand,
Detavernier et al. elaborated the formation of porous coating structures with tailored
pore sizes by applying TiO 2 ALD [142]. Weimer et al. investigated the molecular
layer deposition (MLD) using TMA and ethylene glycol to fabricate alumina alkoxide
hybrid films on Pt NPs (~2 nm). The alumina alkoxide hybrid overcoatings could be
transformed to porous Al 2 O 3 film on Pt NPs since the contained hydrocarbon groups
were combusted at high temperature. Although some metal surface sites were reexposed, it was found that the porous layer still decreased the catalytic performance
which might be caused by the small pore size [143].
3.3.3 Site-Selective ALD Coating of Metal Oxide
A more delicate way to perform ALD is exploiting inherently selective ALD process
to directly modify the active sites of nanoparticles. Lu et al. showed that during the
TiO 2 ALD on Au particles, the TiO 2 layer firstly covered Au’s low coordinated sites