(661b) Geometric and Electronic Effects of Zn Promotion on Pt for Ethane Dehydrogenation
Viktor J. Cybulskis1, Brandon C. Bukowski1, Han-Ting Tseng1, James R. Gallagher2, Zhenwei Wu1, Evan C. Wegener1, A. Jeremy Kropf2, Bruce Ravel3, Fabio H. Ribeiro1, Jeffrey Greeley1, Jeffrey T. Miller1
1Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907
2Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439
3Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899
The study of hydrocarbon C-H bond activation has become more important because of recent shale gas boom in the United States. Supported Pt catalysts have been shown to selectively promote C-H bond activation for light alkanes when alloyed with other inactive metals . We tried to correlate the structure of a PtZn/SiO2catalyst with its catalytic performance for ethane dehydrogenation.
In the present work, 100% C2H4 selectivity and six-fold turnover rate (TOR) enhancement compared to Pt were achieved during ethane dehydrogenation (EDH) at 600 °C after the addition of Zn to Pt/SiO2. The apparent activation energy (Eapp) was determined to be 72±4 kJ mol-1 for Pt/SiO2 and 99±5 kJ mol-1 for PtZn/SiO2. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) showed that Pt and PtZn metal particle sizes were 3.3±1.9 nm and 2.0±0.6 nm, respectively. The Pt1Zn1 nanoparticle alloy with geometrically isolated Pt sites was identified by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) over PtZn/SiO2. XAS also revealed that the Pt edge energy increased by 0.9 eV for PtZn/SiO2. Resonant inelastic X-ray scattering (RIXS) showed that the energy difference between the unoccupied and occupied Pt 5d valence orbitals increased from 4.0 eV for Pt/SiO2 to 6.0 eV for PtZn/SiO2, which agrees with the results from density functional theory (DFT) calculations. It is suggested that the geometrically isolated Pt sites by Zn are responsible for the enhanced C2H4 selectivity as the structure-sensitive hydrogenolysis is suppressed by breaking Pt ensembles. The change in the energy of Pt 5d electrons caused the change in the relative energy difference between Pt 5d orbitals and the ones from the adsorbate. We propose that this difference leads to the increased TOR and Eapp for PtZn/SiO2.
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