Chemistry

As Thin As Clay Gets?

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Science  10 Jan 2014:
Vol. 343, Issue 6167, pp. 119
DOI: 10.1126/science.343.6167.119-d

Thin films of aluminosilicates can mimic the reactivity of zeolites but avoid the kinetic limitations of diffusion through pore networks. Włodarczyk et al. build on their recent work on creating monolayers of aluminosilicates on the surfaces of single crystals of Ru to create thin films of Fe-containing silicates similar to the layers in smectite clays. Analysis of Si-O-Si stretching bands from infrared reflection-absorption spectroscopy revealed that the addition of Fe led to the formation of two-component films containing pure silica and an iron silicate. X-ray photoelectron spectroscopy (XPS) of an oxide with a 1:1 ratio of Fe to Si revealed a high coordination of O to Fe, and low-energy electron diffraction revealed greatly increased ordering even for small amounts of Fe incorporation. Density functional theory confirmed that uniform mixing of Fe is unfavorable thermodynamically as compared to phase separation, and favored a structure in which Fe atoms substitute for Si in the layer adjacent to the substrate and the formation of bridge Fe-O-Ru bonds. Although the Fe oxidation state could not be assigned from the XPS data, assuming that the Fe is in the 3÷ oxidation state, the bilayer formed would represent a dehydroxylated form of nontronite, an Fe-rich smectite.

J. Am. Soc. Chem. 135, 19222 (2013).

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