(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 [1][2]. 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 (E_app) 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 E_app for PtZn/SiO₂.

References:

[1] Galvita, Vladimir, et al. "Ethane dehydrogenation on Pt/Mg (Al) O and PtSn/Mg (Al) O catalysts." Journal of Catalysis 271.2 (2010): 209-219.

[2] Siddiqi, Georges, et al. "Catalyst performance of novel Pt/Mg (Ga)(Al) O catalysts for alkane dehydrogenation." Journal of Catalysis 274.2 (2010): 200-206.