(583ag) Theoretical Insights Into the Surface Reactivity of Au/Co3O4 Catalysts in the Oxidative Dehydrogenation of Cyclohexane

The catalytic combustion of jet fuels shows promise in the application of hypersonic propulsion systems where catalysts are used to lower the ignition temperature and reduce soot formation [1]. In previous efforts [2] we have shown that Co₃O₄ nanoparticles are active for the oxidative dehydrogenation of cyclohexane (a jet fuel surrogate) at 300 °C. Recent experimental studies [3] have explored the influence on catalytic activity by doping Co₃O₄ with ~1 nm size Au nanoparticles. In this work, we carry out first-principles theoretical studies on the oxidative dehydrogenation of cyclohexane over model Au nanoparticles (approximated by a nanometer-sized Au rod) supported on Co₃O­₄.

Density functional theory with on-site correction for Coulomb interactions of Co d-electrons (DFT+U method) calculations were carried out to examine the adsorption energies, reaction energies and activation barriers for a series of different elementary steps involved in the oxidative dehydrogenation of cyclohexane in order to compare the catalytic reactivity of Au/Co₃O­₄ with that of pure Co₃O₄. The results show that the Au nanorod transfers electrons to the Co atoms at the interface between the Au and the Co₃O₄ support. The increased electron density at the interfacial Co^octa cations weakens the Co-O bond strength and increases the hydrogen affinity of the neighboring O site. The increase in hydrogen affinity at the interfacial O sites results in a lower activation barrier for the initial C-H bond activation of cyclohexane at Au-Co₃O₄ interface. The interfacial hydroxyl intermediate reacts further to form water which ultimately desorbs from the surface and creates an O vacancy at the interface. The partially reduced Co^octa cation at the vacancy allows for the adsorption and easy activation of O₂ from the gas phase to form reactive mono-oxygen surface species. The results provide a fundamental understanding of the experimentally observed reactivity increase in the oxidative dehydrogenation of cyclohexane on Au/Co₃O₄ compared to that on pure Co₃O₄ surfaces.

References

1. Wickham, D.T., et al.  Journal of Propulsion and Power, 2001. 17(6): p. 1253-1257.
2. Tyo, E.C., et al. ACS Catalysis, 2012. 2(11): p. 2409-2423.
3. Tyo, E.C., et al. Prepr. - Am. Chem. Soc., Div. Energy & Fuels, 2013. 58: p. 1059