We have used a single SBA-15 silica batch (mesopore diameter of 7.1 nm) and the two-solvent impregnation technique with either pentane or cyclohexane to prepare model samples in which an initial aqueous solution of salt precursor (iron nitrate or iron chloride) is deposited mainly inside silica pores. Detailed characterization by SAXS, WAXS, Ar sorption, TEM (SAED), and UV-visible-NIR spectroscopy is described. Molecular iron species and iron oxide particles are identified. Special emphasis is given to the spatial location of the oxide particles (trapped inside silica pores or dispersed outside silica grains). The formation of hematite external particles is favored when iron chloride is used as a precursor and pentane as a solvent. Spectral signatures that can be attributed either with iron oxide particles or with iron dispersed molecular species (isolated Fe(II)/(III) ions, pairs and/or clusters, films of iron silicate) are detected by UV-visible-NIR spectroscopy. Additional neutron diffraction measurements performed on a sample particularly enriched in external hematite particles indicate a very low magnetic moment of 3.9 (1) μB per iron atom and show that external hematite particles are not significantly polluted by Si atoms. Model catalysts, all containing molecular species but particularly enriched in either external or internal iron oxide particles, were selected and tested for the catalytic Fenton decomposition (with H2O2 and diluted O2 and under artificial white light) of sodium formate and of an organic copper-containing dye (Reactive Violet 2) in water. Samples enriched in external hematite particles are significantly less active and react less rapidly than samples containing mainly iron species remaining trapped inside silica grains. Preliminary UV-visible-NIR spectra suggest that strong interactions between molecular species and internal replicated oxide particles are at the origin of an enhanced reactivity.