This paragraph should focus on kinetic matching and use v minteq to
support that.
To asertain information of the mechanism of Fe incorperation by way of
Fe-P interaction, we considered all species present in solution. We fed
our predicted speciation into visual Mintwq for the calculation of the
complex equilibria distribution of products. The complete hydrolysis
products of the DHP and (NH4)2 Fe
(SO4)2 could consist of
NH4+, Fe2+,
HPO4- and
SO42-; interediate hydrolysis products
may be possible, however we consider only the complete hydrolysis
products in our model. At the pH range of this synthesis theses entities
persist as NH4+,
Fe2+, H3PO4 and
HSO4-. Ina greement with our pH
measurment, the ammonium ions sourced from DHP and the Fe precursor
carry there own proton and are unlikely to raise solution pH. The
protonation of the sulfate and phosphate anions are then responsible for
the observed increase in pH. Figure y shows the speciation of Fe as a
percentage of the total Fe concentation based on complex equlibria
calculations. There is a clear increase in
[FeH2PO4]+
directly correlated with the amount of DHP present. By comparrison,
figure r shows the behavor of the system in the absence of DHP, where
aqueous Fe2+ dominates with a slight trend towards
FeSO4 with increasing Fe content. Add in calculation
for dhp con and fe con, do these match our icp data? These calcualtions
illustate the validity of the formation of a iron phosphate phase under
our Fe-SBA-15 synthetic conditions when DHP is present.
kinetic matching
We believe the markedly highere Fe incorperation is due to encapsulation
of iron phosphate domains within the forming silica network. This is
enabled by the kinnetic match of rapid Fe precursor and DHP hydrolysis,
reaction of Fe cations and phosphates, and the condensation and
hydrolysis of TEOS.
Introduce kinetics of each process
Based on kinetic modeling by Pouxviel et al. of TEOS hydrolysis under
high water content and acid catalysis over 40 % of Si existis as
Si(OH)4 within 30 minutes.18 Brinker
et al. describe the early chemisty of the sol-gel process consiting of
both hydrolysis and condensation of alkoxides and
silanols.19 We observe that imediately upon
introduction of either DHP or ferrous ammonium sulfate to our synthesis
solution, the pH immediately increases appreciably, obtaining a maxima
within 3 minutes. This suggests that the dissociation of DHP and ferrous
ammonium sulfate into there respective contituents happens nearly
instantaneously. Additionaly, the portonation of phosphates and sulfate
anions must also rapidly occur. Regardless of the nuances of the
reaction, the production of aqueous Fe and phosphoric acid species
necessary to form iron phosphate occurs within the time frame of TEOS
hydrolysis and condnesation.
Within the scheme of mesoporous silcia synthesis, association of
silicate species with tempalte moleucles happens
instantaneously.20 During proposal of the counterion
mediated assembly mechanism, ordered mosoporous zinc phosphates were
prepared.3 This occurred by th association of
[H2ZnPO4]+ as
I+ with the negativly charged double layer formed by
S+Cl‑ in the vicinity of the
template micelle. Here we find computational support for the presence
[H2FePO4]+ and if
we infer that it forms rapidly, the kinetics of precursor formation
place the formation of
[H2FePO4]+ within
the time range necessary for its interaction, albe it in competition
with silicates, with the isolated template micelle before floculation
occurs. Both XPS and EDAX support this assembly mechanism. P 2P XPS data
of mesoporous iron phosphates often exhibit peak values below that of
bulk crystaline FePO4.21 We found that
for the lowest DHP sample, P2p was 133.9 eV which steadly decreased with
DHP content to a minimum of 133.1 eV. Initally with low concetration of
DHP, the driving force for dissociation is reduced, leading to less
nucleation of iron phosphate species at the surface of the meicell, and
the majority of the phase existing as encapsulations within the forming
silica wall. As DHP concentration is increased, progressivley more iron
phosphate precursors are avilable to interact at the micell interfact in
place of silicates, leading more hydratably iron phosphate surfaces.
Since the decrease in binding energy of the P2p electron is attributed
to hydration of phosphates, it trends with the hydratable surface area
of the iron phosphate domain.22 With our ironphosphate
domain only partialy constituting the overall particle, we have less
iron phospahte surface compared to pure mesoporous iron phosphate
materials and see a comparibly diminished decrease in P2p binding
energy. EDAX mapping, shows distinct vibrance of phosphorus map compared
ot that of iron, increasing in intensity as DHP is increased. XPS, shows
that Fe and P exist at a slightly P rich ratio at high DHP
concentration. If the iron phosphate phase origionates from the surfae
of the template instead on predomiantely as an encapsualted phase within
th e forming silica network, Fe could be leached over the course of the
synthesis.
The rate of this precipitation is on the order of the condensation of
the silica wall leading to incorperation of second phase within the MSN.
Add image summing up mechanism
Since the counteranion-micelle complex has already formed, and
P-SiO2 condensation is limited, the precipitation of
iron phosphate hydrates within the forming silica walls is likely the
mechanism of iron incorperation in these materials. At these pH levels,
Fe is incapable of forming a solidphase. However, the presennce of
phophate ions enables the formation of thermodynamically stable hydrated
iron phosphates similar to strengite or metastrengite.
-
Conclusion
In general synthesis of our material proceeds by an acid catalyzed
silica sol-gel process during which iron precusors and diammonium
phosphate react to produce incorperated FePO4 domains
withing the forming SiO2 network. During synthesis,
(NH4)2Fe(SO4)2
dissociates to a series of species, Fe exists as a distribution of
Fe2+, FeCl+, and
FeSO4 (aq). However, when the sol-gel process is
executed with (NH4)2HPO4
the distribution of species in solution, particularly Fe changes to
include a significant portion as
[H2FePO4]+. The
formation of an iron phosphate phase enables a several order of
magnitude increase in Fe incorperation within the SBA-15 framework.
While Fe incorperated SBA-15, mesoporous FePO4, and
mesoproous silicophosphates have been prepared before this is the first
report on the combination of theses synthetic techniques. We believe
that these materials could serve as battery cathodes upon carburization,
ionic conductors, while the metallophosphate regions may have catalytic
activity towards oxidative dehdrogenation.12, 23-25
\soutAs early as 1995, FePO4 demonstrated an affinity for the selective
conversion of methane to formaldehyde and methanol.[20] The
catalytic performance in methane oxidation of FePO4 catalyst is
bolstered by tetrahedral Fe sites flanked by phosphorous and oxygen
which leads to enhanced acidity of the site.[21]