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SFB 1109:  Understanding of Oxide/Water Systems at the Molecular Scale: Structural Evolution, Interfaces and Dissolution

Subject Area Chemistry
Term from 2014 to 2018
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 234149247
 
Final Report Year 2019

Final Report Abstract

Metal oxides constitute the majority of the surface of the earth. The surface chemistry of these oxides is dominated by interactions with water and solvated ions. Metal oxide/water interactions are also critical in our engineered environment, namely for a multitude of present and emerging industrial and consumer applications: the lifetime of metal oxides employed, e.g., as construction materials, surface coatings or medical implants heavily relies on their corrosion resistance in an aqueous environment, and typically they are also produced from aqueous solution. Thus, a detailed understanding of the interactions between metal oxides and water, which determine oxide formation and dissolution, is indispensable for the design of oxides with desirable properties and ensuring the stability of these materials over time (i.e. resistance to degradation and corrosion). Despite the fact that different metal oxide/water interfaces have been studied for a long time, a fundamental understanding has not been reached yet in many cases. One reason is the profoundly multi-scale nature of metal oxide/water chemistry: a comprehensive description of the behaviour of oxide/water interfaces has to account for processes occurring on length scales ranging from individual molecules, clusters and nanoparticles to extended crystalline and amorphous bulk materials. However, analytical and computational methods have developed rapidly within the last decades. Thus, the elementary processes that control metal oxide/water interactions can now be studied for all relevant system sizes. This has been the central aim of the CRC: by studying metal oxide/water interaction over a wide range of length scale, an understanding of the complex atomic level processes underlying oxide formation, structural evolution and dissolution and how these change as a function of oxide complexity and water amount has been pursued. Exemplarily, silica, alumina and iron oxides were studied as metal oxides with the highest natural abundance and application relevance. Moreover, ternary systems composed of these oxides were investigated to account for the complexity encountered in many real applications. To investigate the different stages of oxide assembly and dissolution from or in the aqueous phase, respectively, the CRC has employed combinations of state-of-the-art methods in computational chemistry, chemical synthesis and spectroscopy, including time-resolved in-situ spectroscopy. Each applied method was suitable to address a different level of structural complexity of a studied metal oxide system in contact with water. Within the allotted run-time of five years the CRC has focussed on the investigation of the initial stages of oxide formation, interactions of gas phase models and model surfaces with small amounts of water, the generation and behaviour of defect sites, nucleation and crystallisation, and on the evaluation of the potential of molecular compounds as well as two dimensional films as models for more aggregated structures. In the course of these investigations it could be shown that during the hydrolysis of an aluminium precursor aluminium hydroxide clusters of varying size (dependent on the added water amount) can be trapped on their way to the respective solids by addition of suitable siloxide ligands, and the resulting well-defined complexes behave like small versions of zeolites, catalysing similar reactions. Microhydrated aluminium oxide clusters in the gas phase proved valuable spectroscopic models for the assignment of vibrational spectra recorded for aluminium oxide/hydroxide surfaces. The latter (as well as iron oxide and aluminosilicate surfaces) in turn could be modelled suitably by 2-dimensional oxide films, so that hydrolytic processes could be studied. Incorporation of fluoride ions as defect sites into bulk aluminium oxide significantly changed the hydration and dehydroxylation behaviour as well as the surface reactivity. With regard to iron oxide solids it could be shown that during their formation the development of pre-nucleation clusters and nanoparticles depends sensitively on the outer conditions. Mesoporous amorphous films turned out to be valuable models to study the crystallisation behaviour of iron oxides and to adjust conditions to achieve phase transformations. In attempts to combine iron ions with silicate units segregation occurred, arguing against incorporation of iron into the zeolite frameworks via impregnation with iron ions. Setting out from carbon nanotubes silica nanotubes could be accessed via step-wise deposition procedures. Monomeric silica units could be stabilised by strongly donating ligands so that their intrinsic reactivity could be investigated. Finally, computational work on oxide hydration has shown that at coverages beyond a monolayer surface reconstruction has to be taken into account. These results pave the way for future research to further deepen the understanding of the mechanisms by which metal oxides are formed and dissolved, including non-local effects, the development of electronic structure with size as well as larger numbers of water molecules, which would represent the next step to reach the long term goal of such research: the rational synthesis of metal oxides with predetermined properties.

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