Gas-Phase Cryogenic Vibrational Spectroscopy of Metal Oxide Cluster Ions: Structure-Reactivity Relationship

Gas phase cryogenic ion trap vibrational spectroscopy in combination with high level quantum chemical calculations provides an ideal arena to investigate structure- reactivity relationships of pure- and bi- metallic oxide clusters as a function of size, charge-state and coordination environment. In the last decades, characterization of binary metal oxide nanomaterials has received special attention, mainly because catalytically inactive materials can be activated by doping with a second metal. Precisely controlled conditions and the absence of perturbing interactions with an environment allow the gas phase clusters to serve as powerful model systems for nanomaterials. Moreover, the active site(s) of these reactive intermediates can be unambiguously identified by their characteristic vibrational signatures. Such insights ultimately allow for a molecular level understanding of the reaction mechanisms at play at reactive surfaces in heterogeneous catalysis. Iron is the most common impurity in naturally occurring zeolites. Atomic Fe- substituted small Al-oxide clusters [(Al2O3)nFeO]+ function as model system for Fe- doped zeolites. The influence of an Fe-atom in an Al-oxide network is investigated in terms of structural change and preferred coordination site in Chapter 4. The results demonstrate that the Fe-atom prefers to occupy a position in the outer ring of the cluster and induces substantial change in the cluster structure for the smallest cluster studied (n=1), but not for the larger ones. Furthermore, a structural evolution from planar (n=1) over quasi-2D (n=2) to cage type (n≥3) structures is observed with increasing cluster size. The insights correlate with reported results of Fe-doped nanoparticles and nanocrystals, where the dopant Fe-atom is mostly found to replace the under-coordinated surface Al-atoms of the Al2O3 network. In Chapter 5 the active site(s) of heteronuclear metal oxide clusters towards oxygen atom transfer (OAT) reactions is identified. [AlVOx=3,4]●+ and [VPOx=3,4]●+ radical cations are studied in the context of CO to CO2 conversion (chapter 5.1) and ethylene to formaldehyde oxidation (chapter 5.2), respectively. In both cases, the oxygen atom bound to the main group atom, either Al or P, in contrast to the transition metal atom (V) takes part in the OAT cycle. The results presented in Chapter 6 reveal that the oxygen-deficient Ti3+ centre, which represents a model system for an oxygen vacancy at a titania surface, is the active site for CO2 adsorption. The first two CO2 molecules adsorb chemically on [Ti3O6] ̅, forming asymmetric bidentate-bridged and symmetric tridentate-bridged binding motifs. The tridentate-bridged binding motif, which is reported here for the first time, plays a central role in the oxygen exchange mechanism on a defective anatase surface and activation of CO2 on wet titania surfaces