Oxidative Dehydrogenation of Cyclohexane on Cobalt Oxide (Co3O4) Nanoparticles: The Effect of Particle Size on Activity and Selectivity

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• 作者：Eric C. Tyo ; Chunrong Yin ; Marcel Di Vece ; Qiang Qian ; Gihan Kwon ; Sungsik Lee ; Byeongdu Lee ; Janae E. DeBartolo ; S枚nke Seifert ; Randall E. Winans ; Rui Si ; Brian Ricks ; Simone Goergen ; Matthew Rutter ; Branko Zugic ; Maria Flytzani-Stephanopoulos ; Zhi Wei Wang ; Richard E. Palmer ; Matthew Neurock ; Stefan Vajda
• 刊名：ACS Catalysis
• 出版年：2012
• 出版时间：November 2, 2012
• 年：2012
• 卷：2
• 期：11
• 页码：2409-2423
• 全文大小：840K
• 年卷期：v.2,no.11(November 2, 2012)
• ISSN：2155-5435

文摘

The oxidative dehydrogenation of cyclohexane by cobalt oxide nanoparticles was studied via temperature programmed reaction combined with in situ grazing incidence X-ray absorption spectroscopy and grazing incidence small-angle X-ray scattering and theoretical calculations on model Co₃O₄ substrates. Both 6 and 12 nm Co₃O₄ nanoparticles were made through a surfactant-free preparation and dispersed on an Al₂O₃ surface formed by atomic layer deposition. Under reaction conditions the nanoparticles retained their oxidation state and did not sinter. They instead underwent an assembly/disassembly process and could reorganize within their assemblies. The selectivity of the catalyst was found to be size- and temperature-dependent, with larger particles preferentially producing cyclohexene at lower temperatures and smaller particles predominantly resulting in benzene at higher temperatures. The mechanistic features thought to control the oxidative dehydrogenation of cyclohexane and other light alkanes on cobalt oxide were established by carrying out density functional theory calculations on the activation of propane, a surrogate model alkane, over model Co₃O₄ surfaces. The initial activation of the alkane (propane) proceeds via hydrogen abstraction over surface oxygen sites. The subsequent activation of the resulting alkoxide intermediate occurs at a second surface oxygen site to form the alkene (propene) which then desorbs from the surface. Hydroxyl recombination results in the formation of water which desorbs from the surface. Oxygen is necessary to regenerate the surface oxygen sites, catalyze C鈥揌 activation steps, and minimize catalyst degradation.

Keywords:

oxidative dehydrogenation; cyclohexane; cobalt oxide; Co₃O₄; size-effect; in situ X-ray scattering; GISAXS; in situ X-ray absorption; GIXANES; temperature-programmed reaction; transmission electron microscopy; scanning transmission electron microscopy; X-ray diffraction; mass spectrometry; assembly; density functional theory; alkane activation; alkene activation