Cobalt-promoted Iron Oxide Nanoparticles for the Selective Oxidative Dehydrogenation of Cyclohexane.
work has shown that both cobalt and iron oxide nanoparticles are active for the
oxidative dehydrogenation (ODH) of cyclohexane to benzene, the former more active than
the latter. Further study has shown that the addition of gold species as a minority
component into iron oxide nanocrystals increases the selectivity of the reaction to
benzene. Since a primary motivation for thi... read mores work is the addition of catalysts in jet
fuels to facilitate the dehydrogenation and cracking reactions preceding their
combustion, a low-cost, sacrificial catalyst is sought after. In this application,
catalyst nanoparticles suspended in the fuel stream will dehydrogenate cyclic alkanes
(cyclohexane) to their aromatic counterparts (benzene). Alkenes and aromatics have a
much higher rate of combustion, which decreases the amount of uncombusted fuel in the
exhaust, thereby increasing performance. As these catalysts are not recyclable, there is
significant impetus to substitute cheaper base metals for expensive noble metals. In
this work, iron oxide nanoparticles are doped with varying levels of cobalt to examine
the effect of cobalt content and oxidation state on the selectivity and activity of the
iron oxide for the oxidative dehydrogenation of cyclohexane, used a s a model cyclic
alkane in jet fuel. We have shown previously that small (~5nm) cobalt oxide
nanoparticles favor the production of benzene over the partial dehydrogenation products
cyclohexene and cyclohexadiene, or the complete oxidation product carbon dioxide. It is
the aim of this work to examine the surface of these cobalt-iron oxide nanoparticles to
determine the conditions most favorable for this selective oxidative dehydrogenation.
Cobalt-doped iron nanoparticles were prepared by a surfactant-free hydrothermal
co-precipitation technique that enabled a high degree of composition control and size
control. These samples were characterized via Transmission Electron Microscopy (TEM),
powder X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), BET N2
Physisorption, and CO Temperature Programmed Reduction (CO-TPR). These characterizations
helped us to correlate the selectivity and activity data for each catalyst in an attempt
to understand what roles the surface species played. It was found that iron oxide
nanoparticles doped with 2-10 mol% cobalt formed a stable surface phase, enriched in Co
(20 at%), independent of the bulk concentration. XPS measurements indicate this phase is
rich in octahedral Co2+ cations. The selectivity to benzene was much higher in the
Co-promoted iron oxide samples. This high concentration of octahedral cobalt(II) cations
appear to have a strong promotional effect on the weakly held surface oxygen sites which
have been shown previously to be the active sites for this reaction. Addition of cobalt
also promoted the activity of the iron oxide nanoparticles; and stabilized them against
particle growth under the reaction conditions, typically held at several different
temperatures up to 370 °C, in a mixture of 0.4% cyclohexane and 4% oxygen, for
Thesis (M.S.)--Tufts University, 2013.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Matthew Panzer, and Terry Haas.
Keywords: Chemical engineering, and Materials Science.read less