Cluster Catalysis
Saturated hydrocarbons such as methane could be viable alternatives to traditional petroleum feedstocks if the economic and environmental cost of producing liquid fuels and value-added chemicals could be diminished. The large energy expenditure needed to overcome the inert nature of the CH bond can be reduced by improving catalyst selectivity and efficiency. Tailoring the properties of heterogeneous catalysts for improved productivity and longevity requires knowledge of the surface composition and the reaction mechanisms at play. One such reaction needed for catalyst regeneration is the formation of H2 directly following dehydrogenation. Traditional surface sensitive analytical methods are not directly sensitive to electronic structural information related to defect sites (such as oxygen vacancies or partially reduced metal centers) that are likely responsible for enhanced catalytic activity. In order to directly investigate the role of defect sites, we will generate small clusters of metal oxides with precise geometries to serve as simple model systems for such studies. Using cationic and anionic aluminum and gallium oxide clusters within one oxygen of the bulk stoichiometry to represent active sites, we will study how properties such as acidity/basicity and hydroxylation impact CH bond activation.
Metal oxide clusters will be generated in a laser vaporization source (see figure to the left), and then ions from this source will be [Link to Sarah] accumulated in an octopole trap before being mass selected and guided to a cryogenic ion trap. Inside the cryogenic ion trap, acidity/basicity probe species such as CO, CH3CN, H2, and CO2 will bind to the clusters. The characteristic stretching frequency of the probe molecule will experience a red or blue shift depending on the electron density of the local environment, as seen, for example in metal carbonyl complexes. In addition, the uptake of probe molecules can reveal the number and structure of open binding sites from undercoordinated metal atoms. Once all labile sites are filled the binding energy of the second-shell adsorbates will be significantly lower, leading either to a magic number or a sharp drop in the adduct distribution in the mass spectrum. The mass spectrometry studies will occur in conjunction with vibrational spectroscopy studies, in which first-shell adsorbates will cause larger spectral shifts than second-shell species. The addition of probe molecules will also shift the metal-oxygen stretches of the core cluster.
Laser vaporization source design. Laser ablation of a rotating metal target produces nascent clusters which mix with reactant gases in the clustering channel.
Once we have established the structure and acidity/basicity of active binding sites in metal oxide clusters, we will then turn our focus to the dissociation of H2 and CH4 over the clusters. Which of the possible dissociation products has formed – metal hydride or hydroxide / methyl-metal or methoxy species – will be obvious from the distinct signature appearing in the vibrational spectrum (see figures below). Once the binding patterns of methane on a series of metal oxide clusters are established, the dissociation of larger alkanes over the clusters will be investigated, which will allow us to extract the key properties that govern CH activation and gas phase adsorption.
A. Vibrational spectra and zero-point corrected energies of several hydroxylated Al2O3– clusters. Stepwise addition of H-atoms will provide unambiguous assignment of these bands. B. Lowest-energy structures and vibrational spectra of methyl and methoxy species bound to Al2O4– clusters. The CH3 stretching, CH3 umbrella, and OH stretching frequencies clearly respond to a change in the methyl binding site.
