Influence of Amorphous Silica Matrices on the Formation, Structure, and Chemistry of Iron and Iron Oxide Nanoparticles

Abstract

Fe(CO)(5) physisorbs onto Corning's code 7930 porous Vycor glass (PVG) and dried (<or=200 degrees C), base-catalyzed (NH(3)) tetramethoxysilane/methanol/water xerogels. Although chemically and structurally similar matrices, 488-nm photolysis of the physisorbed complex yields ca. equal amounts of Fe(0) and Fe(2)O(3) in PVG, but only Fe(2)O(3) in the xerogel. Mossbauer, EXAFS, and XANES results give no indication the photoproducts bind to either silica matrix, and consolidation of the PVG matrix leads to Fe(0)-Fe(2)O(3) nanoparticle formation with little change in the Fe(0)/Fe(III) ratio. PVG serves as a template defining the particle diameter and interparticle spacing, whereas consolidation of the xerogel does not result in nanoparticle formation. Instead, ca. 20% of the octahedrally coordinated Fe(III) converts to tetrahedral coordination during consolidation. The photoproducts within these porous silica matrices reflect a competition between aggregation and oxidation, where the extent and most likely the rate of aggregation are functions of the correlation lengths of these amorphous matrices. With a correlation length of 22 +/- 1 nm, aggregation exceeds oxidation in PVG and limits oxidation to the outer periphery, thereby creating particles whose Fe(0)/Fe(III) ratio is unaffected by air or water released during consolidation of the silica matrix. The correlation length of the xerogel,<or=1 nm, limits aggregation of the primary photoproduct and favors smaller particles. As a result, the primary photoproducts in the xerogel do not achieve sufficient size to limit oxidation to the outer periphery of the particle, and the primary photoproduct oxidizes, forming only Fe(2)O(3). Desorption of decomposition products derived from the xerogel precursors creates a dynamic surface that limits nanoparticle growth during annealing. Desorption also disrupts the growing silicate matrix, creating sites that facilitate the change from octahedrally to tetrahedrally coordinated Fe(III) in the xerogel.

Nature and distribution of tungsten oxides in porous Vycor glass

“…The extent of oxidation, however, does not appear to be limited by the amount of oxidant present within the matrix. Although O 2 in air can serve as an oxidant, collectively, the data point to coadsorbed water as the principal oxidant under all conditions [9,24,25]. The higher temperatures necessary for thermally induced oxidation, for example, are expected to desorb most air and O 2 from PVG, while the photoinduced CO pressure gradients are expected to sweep air and O 2 from PVG.…”

“…Nonetheless, ablation increases the roughness of the surface, and the error in the measurements is taken to be twice the initial roughness of the PVG surface. Since the silica nodules making up PVG are covered by 40 ± 20 nm high stalagmite-like features [9,24], the error in the distribution measurements is taken to be 40 ± 20 nm.…”

“…EPR spectra recorded during photolysis of Mo(CO) These experiments were undertaken to identify the tungsten oxides created photochemically in PVG, and the factors that influence their formation. With metal oxides derived from monometallic, metal carbonyl precursors, current data indicate that oxide formation within PVG and structurally similar TMOS/MeOH/H 2 O xerogels is governed by a competition between oxidation and aggregation [24]. If the rate of oxidation exceeds the rate of aggregation, the dominant product is the fully oxidized metal, or with tungsten, WO 3 .…”

“…In the absence of a persistent, measurable absorbance attributable to W(CO) 4 , the oxidation step does not appear to be photoassisted. Instead, whether the initial decarbonylation of the hexacarbonyl is initiated thermally or photochemically, the subsequent oxidation of the zero valent metal is a thermal process usually with coadsorbed water, ubiquitous in these nanoporous silicas, serving as the oxidant [24]. In spite of the different methods of preparation, i.e., oxidatively as opposed to reductively, the oxides formed in PVG are spectroscopically similar to the commercially available oxides.…”

Fabrication of microporous amorphous silica glass by pyrolysis of phenyl groups intercalated in sol–gel derived phenyl-modified silica glass

“…Due to its high rigidity and durability rooted from the firm siloxane bonding structure, amorphous glass is applicable to a variety of applications [1][2][3][4][5][6]. One of the potential usefulness of amorphous glass is the porous glass material (mainly mesopore region), which can be applied to the molecular sieve materials, selective adsorbents, and selective catalysts [7][8][9]12].…”