Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 180

d n 9 r 4 n g | 7

Figure 7.13 For the preparation of catalysts by ALD, temperature variations are
sometimes introduced in the ALD cycle.

mesoporous silica** ,6,7,59 and alumina yy ,60 powder for the hydroformylation
of ethene 7 and the hydrogenation of toluene. 6,59,60 After preheating of the
support, the Co-precursor, cobalt( III )acetylacetonate or Co(acac) 3 , was che-
misorbed on the surface at a temperature of 180 1C. Removal of the re-
maining ligands occurred by calcination in synthetic air at 450 1C leading to
CoO x species (Figure 7.13). This ALD cycle was repeated up to five times,
after which the CoO x species were partially reduced to Co in flowing H 2 at
500-600 1C. It was shown that omitting the calcination step after the last
Co(acac) 3 reaction step led to an enhanced reducibility of the CoO x species.
The ALD-synthesized Co catalysts showed improved activities compared to
catalysts prepared by impregnation from Co(NO 3 ) 2 , due to the higher dis-
persion of the active species. 6 A similar ALD approach was applied to fab-
ricate catalysts for alkane dehydrogenation by Cr(acac) 3 /air reactions on the
mesoporous alumina support. 5,61 Although the active species were more
uniformly distributed in catalysts prepared by ALD than by impregnation
from Cr(NO 3 ) 3 , the catalysts were equally active in the dehydrogenation of
isobutane. 61
Significant efforts were also made to modify the support prior to catalyst
loading. 62-64 Porous silica and alumina powders covered with AlN by ALD
from TMA and NH 3 were evaluated as supports for the preparation of Co
hydroformylation catalysts 65 and Cr dehydrogenation catalysts, 66 respect-
ively. The active species were generated by the chemisorption and sub-
sequent reduction in H 2 of Co(acac) 3 or Cr(acac) 3 precursor molecules. In the
considered reactions, however, the AlN-coated supports did not offer add-
itional benefits for the catalytic activity.

.

**Grace 432 Silica: particle size 0.5-1 mm, surface area 320 m 2 g 1 , pore volume 1.2 cm 3 g 1 , and
average pore diameter 10-15 nm.

yy
AKZO g-alumina: particle size 0.2-0.4 mm, surface area 120-195 m 2 g 1 , pore volume

0.45-0.5 cm 3 g 1 , and average pore diameter 10-15 nm.