3 Catalysts via Atomic Layer Deposition
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pulse time is usually essential to achieve surface saturation and good conformity or
overcome the slow nucleation rates on the supports with highly specific surface area
such as Al 2 O 3 or carbon materials [15, 16].
Platinum
Platinum is the most widely investigated noble metal catalysts prepared by ALD.
Most Pt nanoparticles or films prepared by ALD utilize the alternating pulse
of trimethylplatinum(methylcyclopentadienyl) (MeCpPtMe 3 , Cp=Cyclopentadiene)
and oxidizing agent of ozone or oxygen at 200–300 °C. The MeCpPtMe 3 is the dominant platinum ALD precursor, the ligands of which can be completely removed by
oxidizing agents (O 2 or O 3 ) during the other half reaction of the ALD cycle [7].
The corresponding growth rate usually varies from 0.03 to 0.05 nm/cycle at 200 and
300 °C [17]. It has been reported that the Pt precursor could hardly be removed by O 2 ,
when the deposition temperature is lower than 200 °C. At higher temperature around
350 °C, this particular Pt precursor suffers from thermal decomposition, which will
lead to a slow growth rate of Pt [18, 19]. Besides the oxidizing agents, the reducing
H 2 is also utilized to remove the ligands in Pt ALD, although such chemical process
has not been reported for the synthesis of Pt catalysts. Recently, CO molecules are
introduced into Pt ALD process as a passivation agent to modify the surface energy
of already deposited Pt. The CO treatment is beneficial for Pt’s direct deposition on
the carbon support, leading to a 40% promotion in Pt surface-to-volume ratio [20].
Sufficient adsorption of gaseous Pt precursor could generate a high distribution of
Pt cluster on the carbon substrate even when the Pt size is continuously increasing,
which can be hardly achieved by traditional wet chemistry methods [16]. Pt ALD
has been conducted on a number of different supports, such as alumina, carbon,
molybdic sulfide, and SrTiO 3 which can be used in various catalytic reactions like
CO oxidation, water–gas shift reaction, styrene hydrogenation, and oxygen reduction
reaction [15, 16, 20, 21].
Palladium
Similar to the Pt precursor, palladium hexafluoroacetylacetonate (Pd(hfac) 2 ) is the
major metal precursor in Pd ALD process to react with the reducing agent of formalin
or H 2 . Different from most noble metal ALD processes, the Pd processes rely mostly
on the reducing agents rather than the common molecular oxygen. Due to the instability of Pd(hfac)2 above 230 °C, formalin was used to react with Pd(hfac) 2 to
complete one Pd ALD cycle at 200 °C, with a growth rate of 0.02–0.03 nm per cycle
[22]. At lower temperature of 80–100 °C, limited by its activity, formalin can be
replaced by H 2 in the ALD process with a similar growth rate generated on SiO 2
or TaO x [23, 24]. The distribution of Pd nanoparticles could be easily controlled
by changing the exposure time of Pd precursor onto the support in the ALD procedure [25], which is more difficult to achieve by traditional impregnation or other
deposition methods. The Pd catalysts synthesized by ALD have been widely used
in different reactions like methanol decomposition, ethanol oxidation, and formate
oxidation [25–30].
75
pulse time is usually essential to achieve surface saturation and good conformity or
overcome the slow nucleation rates on the supports with highly specific surface area
such as Al 2 O 3 or carbon materials [15, 16].
Platinum
Platinum is the most widely investigated noble metal catalysts prepared by ALD.
Most Pt nanoparticles or films prepared by ALD utilize the alternating pulse
of trimethylplatinum(methylcyclopentadienyl) (MeCpPtMe 3 , Cp=Cyclopentadiene)
and oxidizing agent of ozone or oxygen at 200–300 °C. The MeCpPtMe 3 is the dominant platinum ALD precursor, the ligands of which can be completely removed by
oxidizing agents (O 2 or O 3 ) during the other half reaction of the ALD cycle [7].
The corresponding growth rate usually varies from 0.03 to 0.05 nm/cycle at 200 and
300 °C [17]. It has been reported that the Pt precursor could hardly be removed by O 2 ,
when the deposition temperature is lower than 200 °C. At higher temperature around
350 °C, this particular Pt precursor suffers from thermal decomposition, which will
lead to a slow growth rate of Pt [18, 19]. Besides the oxidizing agents, the reducing
H 2 is also utilized to remove the ligands in Pt ALD, although such chemical process
has not been reported for the synthesis of Pt catalysts. Recently, CO molecules are
introduced into Pt ALD process as a passivation agent to modify the surface energy
of already deposited Pt. The CO treatment is beneficial for Pt’s direct deposition on
the carbon support, leading to a 40% promotion in Pt surface-to-volume ratio [20].
Sufficient adsorption of gaseous Pt precursor could generate a high distribution of
Pt cluster on the carbon substrate even when the Pt size is continuously increasing,
which can be hardly achieved by traditional wet chemistry methods [16]. Pt ALD
has been conducted on a number of different supports, such as alumina, carbon,
molybdic sulfide, and SrTiO 3 which can be used in various catalytic reactions like
CO oxidation, water–gas shift reaction, styrene hydrogenation, and oxygen reduction
reaction [15, 16, 20, 21].
Palladium
Similar to the Pt precursor, palladium hexafluoroacetylacetonate (Pd(hfac) 2 ) is the
major metal precursor in Pd ALD process to react with the reducing agent of formalin
or H 2 . Different from most noble metal ALD processes, the Pd processes rely mostly
on the reducing agents rather than the common molecular oxygen. Due to the instability of Pd(hfac)2 above 230 °C, formalin was used to react with Pd(hfac) 2 to
complete one Pd ALD cycle at 200 °C, with a growth rate of 0.02–0.03 nm per cycle
[22]. At lower temperature of 80–100 °C, limited by its activity, formalin can be
replaced by H 2 in the ALD process with a similar growth rate generated on SiO 2
or TaO x [23, 24]. The distribution of Pd nanoparticles could be easily controlled
by changing the exposure time of Pd precursor onto the support in the ALD procedure [25], which is more difficult to achieve by traditional impregnation or other
deposition methods. The Pd catalysts synthesized by ALD have been widely used
in different reactions like methanol decomposition, ethanol oxidation, and formate
oxidation [25–30].