Mechanism
The primary mechanism for metal incorporation by co-condensation of
silica precursors and metal salts was disclosed by Vinu et
al.9 For example, Al was successfully incorporated
into SBA-15 by increasing the pH of the gel above the aqueous
isoelectric point for silica. At pH > 2 silica precursor
species in solution possess a negative charge due to deprotonation,
which favors interaction with cationic metal hydroxide species found at
the elevated pH. Initially, we believed that DHP could increase the pH
of the solution by way of NH3 release, promoting Fe
incorporation in a similar manner.
To investigate this hypothesis, we prepared four control vessels filled
with identical quantities of H2O, HCl, DHP, and
(NH4)2Fe(SO4)2
as those used in preparation of our Fe-SBA-15 samples and monitored
their pH. Upon introduction of HCl the pH of each vessel was acidified
to 1.16. Upon equilibration, an amount of DHP ranging from 0.25 g to
2.00 g was added to each of the vessels. For introduction of 0.25, 0.50,
or 1.00 g of DHP, the solution pH remained below 1.67. However, upon
adding 2.22 g of DHP, the pH rose from 1.16 to 3.72. Addition of 0.55,
1.11, or 2.22 g
(NH4)2Fe(SO4)2
further increased the pH to a maximum of 1.87. When mixing 4.44 g of
iron salt, pH decreased from 3.73 to 3.59. While DHP is a suitable
ammonia source in the agricultural industry, we did not see evidence of
a significant increase in solution pH or bubbling in the form of
NH3 (g). Since the pH of the gel predominately remained
below 2 for all samples, yet significant metal incorporation was still
observed we believe a different mechanism for Fe incorporation must
occur compared to that of the “pH-adjusting”
method.10, 11
ICP-AES and EDAX show P as the only elemental component in the material
outside of Fe, Si, and O in the final MSN. Since direct interaction
between SBA-15 and Fe seems unlikely under the higher acidity
conditions, we were prompted to investigate the role of P. Similar to
SBA-15 prepared here, porous phosphosilicates are often synthesized from
TEOS and orthophosphate by HCl catalyzed sol-gel
practices .12, 13 It has been shown by
31P NMR, 29Si NMR, Raman, and FTIR
that phosphoric acid is capable of co-hydrolyzing and co-condensing with
TEOS during the formation of mesoporous materials.13
Here, Fe incorporation could have been due to the in situ formation of
oligomeric iron polyphosphates. Biologically important polyphosphates
such as ATP have been shown to have high affinity for metal
chelation.14 Additionally, Huo, Stucky, and Schuth
describe the higher affinity of polydentate silicate oligomers towards
cations as mechanism for self-assembly in base catalyzed ionic templated
systems.3 If negatively charged polyphosphates were to
form in solution they could chelate iron species and co-condense with
the forming silica network.
However, the formation of polyphosphates at low pH is unfavorable and we
found by XPS, ICP-OES, and XRD that the
nP:nFe was consistently 1:1, whereas
polyphosphate species would produce
nP:nFe >
1.15 Additionally, if a phosphosilicate material was
forming, we would also expect the nP:nFe
to exceed one since the excess of TEOS would afford abundant
opportunities for phosphorous-silica interaction compared to
phosphorous-iron. Another alternative could have been phosphates playing
the role of counterions during the counterion mediated assembly process
of SBA-15.4, 16 A variety of counterions have been
investigated including NO3, Br, Cl, SO4,
but not phosphates.6 Typically the counterion
influences the degree of ordering of the MSN or the overall particle
morphology. By delaying addition of our DHP until after sufficient
induction period for the formation of
S+X- by protonated P-123
(S+) and chloride anions (X-),
surplus of H2PO4-
could not participate as X- reducing the possibility
of Fe enrichment at the silica-template interface through phosphate-iron
complexation.