10.1002/ejic.201201429
Controlled Synthesis and High Oxidation Stability of Cobalt Nanoparticles Encapsulated in Mesoporous Silica using a Modified Stober Approach and a Pseudomorphic Transformation
Materials: Cobalt(II) nitrate hexahydrate (VWR Prolabo, 98 %), sodium borohydride (Sigma-Aldrich, 98 %), citric acid monohydrate (Sigma-Aldrich, 99.5 %), 3-aminopropyltriethoxysilane (APTES, Sigma-Aldrich, 99 %), tetraethylorthosilicate (TEOS, VWR Prolabo, 99.9 %), hexadecyltrimethylammonium bromide (CTAB, C19H42BrN, Sigma, >=98 %), poly(vinylpyrrolidone) PVP (Sigma-Aldrich, average mol. wt 40000), NaOH (Sigma-Aldrich, >=98 %), and EtOH (VWR, absolute 99.9 %) were used as received. Deionized water (18 MΩ cm) was used for all the preparations after deoxygenation. 
Synthesis of the Co NPs: Citrate-stabilized Co NPs were prepared from classical NaBH4 reduction.56 In a typical synthesis, cobalt(II) nitrate hexahydrate (115.5 mg, 0.397 mmol) and citric acid monohydrate (83-167 mg, 0.397-0.793 mmol, 1-2 equiv.) were added to deoxygenated water (175 mL) under mechanical stirring. previously solubilized in cold water (5 mL) was quickly added to the vigorously stirred solution. The mixture turned black, which is indicative of reduced cobalt NP formation. In certain cases, PVP (600 mg) was added immediately after the introduction of NaBH4 to form PVP-stabilized Co NPs. Complete dissolution of the polymer occurred rapidly (<1 min). 
Encapsulation of the Co NPs in Silica, Co@SiO2: Co@SiO2 were prepared by controlled growth of the silica shell on the Co NPs in an ethanol/water mixture according to a modified Stober method.16 After the in situ synthesis of the Co NPs, an ethanol solution (390-780 mL) containing APTES (77 μL, 0.331 mmol, 0.8 equiv.) and TEOS (881 μL, 3.97 mmol, 10 equiv.) was added to the NP solution. This one-pot reaction was allowed to proceed for 20 h before centrifuging and drying to isolate the silica-coated particles. 
Synthesis of Cobalt-Mesoporous Silica NPs (Co@m-SiO2): Co@SiO2 were used as a silica source for the synthesis of Co@m-SiO2 by a PMT.40 The appropriate quantity of Co@SiO2 was dispersed into an alkaline solution containing CTAB. The molar composition of the system was 1:0.11:0.24:395:36 SiO2/CTAB/NaOH/H2O/EtOH. After 1 h of stirring at room temperature, the gel was put in an autoclave and heated at 100 degC for 24 h under static conditions. The final solid was recovered by centrifugation, washed with water, and dried at 80 degC for 12 h. The CTAB rod-like template was removed by thermal treatment under hydrogen in a programmed heated tubular oven before use. The temperature was initially increased from room temperature to 400 degC at a rate of 5 degC min-1, and the temperature kept for 4 h to remove the organic phase. 
Synthesis of Co/SiO2: The cobalt-based reference catalyst Co/SiO2 was prepared by incipient wetness impregnation over a silica-based carrier (SBET = 241 m2 g-1, Vp = Subsequently, the impregnated catalyst was dried by direct heating of the sample from room temperature to 85 degC for one night. Finally, the catalyst was calcined at 420 degC in dry air for 4 h and activated by a reduction treatment under pure hydrogen. To achieve a Co loading of 12.7 wt.-%, two successive incipient wetness impregnations were realized (see the Supporting Information, Table S1 and Figures S1 and S2, for characterization).
sodium borohydride
98 %
%
citric acid monohydrate
99.5 %
%
3-aminopropyltriethoxysilane (APTES
99 %
%
tetraethylorthosilicate (TEOS
99.9 %
%
hexadecyltrimethylammonium bromide (CTAB, C19H42BrN
>=98 %
%
poly(vinylpyrrolidone) PVP
average mol. wt 40000
average mol. wt
Cobalt(II) nitrate hexahydrate
98 %
%
NaOH
>=98 %
%
EtOH
99.9 %
%
Deionized water
18 MΩ cm
MΩ cm
citric acid monohydrate
added
1-2 equiv
equiv
cobalt(II) nitrate hexahydrate
added
115.5 mg
mg
cobalt(II) nitrate hexahydrate
added
0.397 mmol
mmol
citric acid monohydrate
83-167 mg
mg
citric acid monohydrate
0.397-0.793 mmol
mmol
water
175 mL
mL
water
5 mL
mL
PVP
added
600 mg
mg
polymer
dissolution
<1 min
min
APTES
0.8 equiv
equiv
ethanol
added
390-780 mL
mL
APTES
added
77 μL
μL
APTES
added
0.331 mmol
mmol
TEOS
added
881 μL
μL
TEOS
added
3.97 mmol
mmol
TEOS
added
10 equiv
equiv
one-pot reaction
proceed
20 h
h
SiO2/CTAB/NaOH/H2O/EtOH
molar composition
1:0.11:0.24:395:36
gel
stirring
1 h
h
gel
heated
100 degC
degC
gel
heated
24 h
h
final solid
dried
80 degC
degC
final solid
dried
12 h
h
temperature
increased
400 degC
degC
temperature was initially increased
rate
5 degC min-1
degC min-1
temperature
kept
4 h
h
silica-based carrier
SBET
241 m2 g-1
m2 g-1
sample
direct heating
85 degC
degC
catalyst
calcined
420 degC
degC
catalyst
calcined
4 h
h
Co loading
12.7 wt.-%,
wt.-%,
incipient wetness impregnations
realized
two