Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 181

Ker¨nen et al. published several papers on the synthesis of vanadium-
based catalysts. 67-70 In their early work, they used the VO(acac) 2 precursor to
modify non-porous silica zz and alumina yy powders. 67,68 One chemisorption
step at 180 1C and subsequent air treatment at 350 1C resulted in highly
dispersed V 2 O 5 species on the support materials. Compared to catalysts
prepared by impregnation, the ALD catalysts were found to have stronger
acid sites and a higher activity in the dehydrogenation of propane. 67,68 Later,
by exposing mesoporous silica zz to titanium isopropoxide and calcining it
under O 2 flow, Ker¨nen et al. synthesized a highly dispersed titania/silica
material. 69 The acidic character of this material was further enhanced by a
vanadyl isopropoxide/O 2 ALD treatment. The generated isolated vanadium
species were shown to exhibit higher acidic strength than the less dispersed
and more crystalline sites prepared with solutions of NH 4 VO 3 and
oxalic acid.
The one-step VO(acac) 2 /air reaction was also used to introduce VO x species
in ordered mesoporous MCM-48 material. 71,72 Prior to the ALD reaction, the
MCM-48 support was treated in the liquid phase with a bifunctional silane
and subsequently stirred in water. This treatment rendered the material
hydrophobic while also creating OH groups for the reaction with the
VO(acac) 2 precursor. Due to their partial hydrophobicity, these materials
were found to be highly stable towards structural collapse and leaching.
Furthermore, the VO x /MCM-48 catalysts were active in the gas phase oxi-
dation of methanol and the liquid phase selective oxidation of toluene. 72

d n 9 r 4 n g | 7

7.2.2 Supported Noble Metal Catalysts by ALD
The increase in catalysis-related ALD research from 2004 onward
(Figure 7.12) can be partly ascribed to the successful development of ALD
processes for the growth of noble metals that started in 2003. 73,74 As illus-
trated in the next paragraphs, the island growth mode that is typically pre-
sent at the start of these ALD processes can be exploited for the fabrication of
catalytic nanoparticles (Figure 7.14).
Krause and co-workers published several papers on the synthesis of noble
metal catalysts using beta-diketonate 88 ALD precursors, e.g., Pd(thd) 2 ,
Ru(thd) 3 , and Ir(acac) 3 . 47,75-79 In one of these studies, H-beta zeolites were
used as the support material for Ir catalysts prepared by one Ir(acac) 3
chemisorption step at 190 1C and subsequent ligand removal through cal-
cination or reduction in H 2 . 78 The deposition of Ir(acac) 3 was shown to be
limited to the mesoporous part of the zeolites. Two H-beta zeolites with an
effective surface area of 580 m 2

.

g 1 but with a different mesoporosity

zz
Aerosil 200 silica (Degussa): particle size 12 nm and surface area 208 m 2 g 1 .
yy Oxid C g-alumina (Degussa): particle size 20 nm and surface area 108 m 2 g 1 .
zz EP10 Silica (Crosfield): surface area 300 m 2 g 1 , pore volume 1.2 cm 3 g 1 , and average pore
size 20 nm.
88 thd: 2,2,6,6-tetramethyl-3,5-heptanedione, acac: pentane-2,4-dionate or acetylacetonate.