The Nature of the Isolated Gallium Active Center for Propane Dehydrogenation on Ga/SiO2
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Single-site Ga/SiO2 catalysts exhibit up to 99% C3H6 selectivity at 4% propane conversion with an initial rate of 5.4 × 10−4 (mole C3H6) (mole Ga)−1 s−1 during propane dehydrogenation (PDH) at 550 °C. Following pre-treatment in H2 at 550 °C, only four-coordinate, Ga3+–O Lewis acid sites are observed under reaction conditions. At 650 °C in H2, an additional isolated Ga site with lower Ga–O coordination (N Ga−O < 4) is formed and leads to a 30% decrease in the initial PDH rate per total moles of Ga. The PDH rates are equivalent when normalized by the amount of surface, four-coordinate Ga3+–O, regardless of catalyst pre-treatment conditions, which indicates that these isolated Ga3+ centers are the catalytically relevant sites.
Isolated, Lewis acidic Ga3+ cations present as four-coordinate Ga3+–O centers exhibit up to 99% C3H6 selectivity during propane dehydrogenation (PDH) at 550 °C. An additional isolated Ga site with lower Ga–O coordination is formed during H2 treatment at elevated temperatures, but is inactive for PDH and reversibly decomposes under reaction conditions.
KeywordsDehydrogenation EXAFS NEXAFS Heterogeneous catalysis XPS
Support for this research was provided by Qatar National Research Fund No. 13121024. Funding for A.S.H., B.H., G.Z., and J.T.M. was provided by the U.S. Department of Energy, Division of Chemical Sciences, Geosciences, and Biosciences, under contract DE-AC0-06CH1137. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. The authors would like to thank Andrew “Bean” Getsoian for assistance with analysis of Ga K edge XANES spectra. Also, the authors acknowledge Dmitry Zemlyanov in the Birck Nanotechnology Center at Purdue University for performing the XPS characterization.
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