(750c) Design of Catalysts At the Subnanometer to Nanometer Scale: Tuning Performance Via Size, Composition, Doping, Support and Assembly

Authors: 
Vajda, S., Argonne National Laboratory



The elucidation of the size/composition/shape/structure and function relationship, the effect of support along with the determination and control of the nature of the catalytic particles under reaction conditions are instrumental for addressing fundamental aspects of catalysis on the way to the design of new classes of catalytic materials. Highly uniform particles on technologically relevant supports are prerequisites for such studies.

Our experimental studies are based on 1) fabrication of technologically relevant supports, 2) physical size- and composition selected cluster deposition with atomic precision control, 3) ex situ and in situ microscopies and 4) in situ synchrotron X-ray characterization of cluster size/shape and oxidation state under realistic working conditions, combined with mass spectroscopy analysis of the reaction products. The experimental studies are complemented with DFT calculations. Using model size-selected clusters, this contribution will outline the applicability of the outlined approach on example of selective C-H bond activation in the oxidative and non-oxidative dehydrogenation of cyclohexane and cyclohexene. Select examples will be presented on coupling studies of model and practical catalysts.

Oxidative and non-oxidative dehydrogenation of cyclohexane on subnanometer Co4Ox and Co27Ox clusters and nanometer size Co3O4 and (Au)Co3O4particles. In the absence of oxygen, the clusters became active around 300 ºC, with the larger clusters being about 2x as active as the smaller ones. The clusters were active already around 100 ºC – with somewhat different selectivities for the two cluster sizes. The performance of the subnanometer ConOx clusters will be compared with that of various size nanometer Co3O4 particles prepared by wet-chemical methods - bare as well as Au-doped. Notably, the activity of cobalt clusters under oxidative conditions makes them as potential candidates to replace Pt-based dehydrogenation catalysts, while the spectrum of investigated cobalt oxide clusters is also offering a tunable selectivity and activity with particle size, doping and oxidation state.

Dehydrogenation of cyclohexene on subnanometer Co27Oxclusters and their nanoscale assemblies: Increased activity through assembly under reaction conditions and the effect of the support and reaction conditions on catalyst performance. 27-atom cobalt clusters were deposited on TiO2, ZnO, Al2O3, MgO and ultrananocrystalline diamond (UNCD). While the activity and selectivity of TiO2, ZnO and Al2O3 supported clusters were comparable, MgO-supported clusters were 2 to 3 times more active under oxygen rich conditions. In situ data reveal that a formation of a fluxional ~2-3 nm structure of a CoxMgyOz composition is responsible for the dramatic increase in activity. Under oxygen lean conditions, UNCD-supported clusters possessed the highest activity as well as highest toward selectivity towards dehydrogetanted C6 products. Along with the dehydrogenation of cyclohexane, these results indicate extraordinary propensities of sub-nm ConOx clusters in low-temperature activation of O2.

References

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2. “Support-Dependent Performance of Size-Selected Subnanometer Cobalt Cluster-Based Catalysts in the Dehydrogenation of Cyclohexene", S. Lee, M. Di Vece, B. Lee, S. Seifert, R. E. Winans and S. Vajda, Chem. Cat. Chem. 4,  p. 1632-1637 (2012)

3. “Oxidative Dehydrogenation of Cyclohexene on Size Selected Subnanometer Cobalt Clusters: Improved Catalytic Performance via Evolution of  Cluster-Assembled Nanostructures", S. Lee, M. Di Vece, B. Lee, S. Seifert, R. E. Winans and S. Vajda, Phys. Chem. Chem. Phys., 14, p. 9336 - 9342 (2012)

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