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

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 Co₄O_x and Co₂₇O_x 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 Co_nO_x 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 Co₂₇O_xclusters 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 Co_xMg_yO_z 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 Co_nO_x clusters in low-temperature activation of O₂.

References

1. “Oxidative Dehydrogenation of Cyclohexane on Cobalt Oxide (Co₃O₄) NanoparticlesThe Effect of Particle Size on Activity and Selectivity”, by E. C. Tyo, C. Yin, M. Di Vece, Q. Qian, S. Lee, B. Lee, S. Seifert, R. E. Winans, R. Si, B. Ricks, S. Goergen, M. Rutter, B. Zugic, M. Flytzani-Stephanopoulos, Z. Wang, R. E. Palmer, M. Neurock, and S. Vajda, ACS Catal.  2, p. 2409−2423 (2012)

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)

4. L. Curtiss, J. Greeley, M. Flytzani-Stephanopoulos, G. Haller, S. Lee, M. Neurock, R. Palmer,  L. Pfefferle, Q. Qian, R. Schlögl, D. Teschner, E. Tyo, S. Vajda, R. Winans et al, papers in preparation

5. “Subnanometre Platinum Clusters as Highly Active and Selective Catalysts for the Oxidative Dehydrogenation of Propane”, S. Vajda, M. J. Pellin, J. P. Greeley, C. L. Marshall, L. A. Curtiss, G. A. Ballentine, J. W. Elam, S. Catillon-Mucherie, P. C. Redfern, F. Mehmood and P. Zapol, Nat. Mater.  8, 213-216 (2009)