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.
  1. 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]