76
R. Chen et al.
Iridium
Unlike Pt and Pd, many kinds of Ir precursors are used in the thermal Ir ALD
process including
Ir(acac) 3 (acac=acetylacetone), (ethylcyclopentadienyl)(1,5-cyclooctadiene)iridium (EtCpIr(COD)), and (ethylcyclopentadienyl)(1,3cyclohexadiene)iridium (Ir(MeCp)(CHD), where the ligands are removed by O 2 at
225–400 °C with a rate of 0.02–0.08 nm/cycle [31, 32]. The highly active iridium
catalysts prepared by ALD using Ir(acac) 3 and O 2 show smaller metal particle
size and higher activity for decalin conversion and toluene hydrogenation than that
prepared by traditional wet chemistry methods [33, 34].
Rhodium
For Rh ALD, Rh(acac) 3 is the exclusive precursor, followed by O 2 combustion,
and the temperature window shows a wide range of 225–325 °C with a growth rate
of 0.05–0.19 nm per cycle [35]. To our knowledge, Rh catalyst prepared by ALD
had not been reported till Li et al. deposited Rh NPs on Al 2 O 3 used for methane
dry reforming reaction (DRM) by using Rh(acac) 3 and O 3 [36]. Compared with the
sample prepared by impregnation method, the Rh nanoparticles prepared by ALD
method exhibit a narrower size distribution. Other noble metals, such as Ru, Ag, and
Au, which have been successfully synthesized by ALD, might also be prepared as
catalysts in the future.
Non-noble metals
Besides the direct reaction between metal precursor and the reducing agent like H 2 or
NH 3 , some non-noble metal nanoparticles could be formed from the corresponding
oxides by a reduction process in H 2 flow considering their strong affinity with
oxygen. Taking Ni ALD as an example, two deposition routes are viable, including
Ni(Cp) 2 –H 2 and Ni(Cp) 2 –O 2 –H 2 [37, 38]. The Ni nanoparticles have been deposited
on alumina by using Ni(acac) 2 and air as precursors. The temperature window for Ni
ALD is in the range of 200–300 °C with a growth rate of about 0.5–1.2 nm/cycle [39,
40]. By using Ni(Cp) 2 and water as precursor in the Ni ALD process, the formation
temperature of Ni nanoparticles is lowered (under 250 °C), which is usually above
400 °C in impregnation methods [41]. Ni catalysts prepared by ALD are usually used
as hydrogenation and reforming catalysis [37, 38, 41, 42].
Cu ALD has also attracted great attention due to the variety of industrial applications, and various Cu ALD processes have been developed:
CuCl–H 2 , Cu(thd) 2 –H 2 (thd=2,2,6,6-tetramethyl-3,5-heptanedionate), Cu(acac) 2 –
H 2 , Cu(dialkylacetamidinate) 2 –H 2, etc. [43]. In an ALD process using Cu(
s Buamd)) 2 (
s Bu-amd=N,N’-disec-butylacetamidinate) and hydrogen as precursors, the
growth rate varies from 1.5 to 2 Å/cycle on SiO 2 or Si 3 N 4 at a relative low temperature range of 150–190 °C [44]. Highly dispersed Cu catalysts prepared by ALD can
be applied in (reverse) water–gas shift reaction and photodegradation of methylene
blue and phenol [45, 46].
Co catalysts are prepared by ALD where the pulse of Co(acac) 2 and O 3 precursors
was performed at 200 and 400 °C [47]. The ALD catalysts show up to 2.3 times
higher dispersion than cubic cobalt nanoparticles. The Co catalysts prepared by
R. Chen et al.
Iridium
Unlike Pt and Pd, many kinds of Ir precursors are used in the thermal Ir ALD
process including
Ir(acac) 3 (acac=acetylacetone), (ethylcyclopentadienyl)(1,5-cyclooctadiene)iridium (EtCpIr(COD)), and (ethylcyclopentadienyl)(1,3cyclohexadiene)iridium (Ir(MeCp)(CHD), where the ligands are removed by O 2 at
225–400 °C with a rate of 0.02–0.08 nm/cycle [31, 32]. The highly active iridium
catalysts prepared by ALD using Ir(acac) 3 and O 2 show smaller metal particle
size and higher activity for decalin conversion and toluene hydrogenation than that
prepared by traditional wet chemistry methods [33, 34].
Rhodium
For Rh ALD, Rh(acac) 3 is the exclusive precursor, followed by O 2 combustion,
and the temperature window shows a wide range of 225–325 °C with a growth rate
of 0.05–0.19 nm per cycle [35]. To our knowledge, Rh catalyst prepared by ALD
had not been reported till Li et al. deposited Rh NPs on Al 2 O 3 used for methane
dry reforming reaction (DRM) by using Rh(acac) 3 and O 3 [36]. Compared with the
sample prepared by impregnation method, the Rh nanoparticles prepared by ALD
method exhibit a narrower size distribution. Other noble metals, such as Ru, Ag, and
Au, which have been successfully synthesized by ALD, might also be prepared as
catalysts in the future.
Non-noble metals
Besides the direct reaction between metal precursor and the reducing agent like H 2 or
NH 3 , some non-noble metal nanoparticles could be formed from the corresponding
oxides by a reduction process in H 2 flow considering their strong affinity with
oxygen. Taking Ni ALD as an example, two deposition routes are viable, including
Ni(Cp) 2 –H 2 and Ni(Cp) 2 –O 2 –H 2 [37, 38]. The Ni nanoparticles have been deposited
on alumina by using Ni(acac) 2 and air as precursors. The temperature window for Ni
ALD is in the range of 200–300 °C with a growth rate of about 0.5–1.2 nm/cycle [39,
40]. By using Ni(Cp) 2 and water as precursor in the Ni ALD process, the formation
temperature of Ni nanoparticles is lowered (under 250 °C), which is usually above
400 °C in impregnation methods [41]. Ni catalysts prepared by ALD are usually used
as hydrogenation and reforming catalysis [37, 38, 41, 42].
Cu ALD has also attracted great attention due to the variety of industrial applications, and various Cu ALD processes have been developed:
CuCl–H 2 , Cu(thd) 2 –H 2 (thd=2,2,6,6-tetramethyl-3,5-heptanedionate), Cu(acac) 2 –
H 2 , Cu(dialkylacetamidinate) 2 –H 2, etc. [43]. In an ALD process using Cu(
s Buamd)) 2 (
s Bu-amd=N,N’-disec-butylacetamidinate) and hydrogen as precursors, the
growth rate varies from 1.5 to 2 Å/cycle on SiO 2 or Si 3 N 4 at a relative low temperature range of 150–190 °C [44]. Highly dispersed Cu catalysts prepared by ALD can
be applied in (reverse) water–gas shift reaction and photodegradation of methylene
blue and phenol [45, 46].
Co catalysts are prepared by ALD where the pulse of Co(acac) 2 and O 3 precursors
was performed at 200 and 400 °C [47]. The ALD catalysts show up to 2.3 times
higher dispersion than cubic cobalt nanoparticles. The Co catalysts prepared by