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

Abstract

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 smallangle X-ray scattering and theoretical calculations on model Co 3 O 4 substrates. Both 6 and 12 nm Co 3 O 4 nanoparticles were made through a surfactant-free preparation and dispersed on an Al 2 O 3 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 Co 3 O 4 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−H activation steps, and minimize catalyst degradation.

Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions

“…12 Hence the predominant role of oxygen gas is to create suitable reaction sites by oxidizing the clusters. Since the amount of oxygen is much higher compared to cyclohexane, possible reduction effects by cyclohexane are overwhelmed by oxidation processes at the particle surface.…”

“…29À32 The main goal of this work is to follow the evolution of the oxidation state and morphology of supported size-selected and partially oxidized Co clusters during the oxidative dehydrogenation of cyclohexane, thus linking the particle properties to available chemical studies. 12 The clusters are produced by gas aggregation representing a physical method that avoids the formation of ligands or protective layers at the particles' surface. Subsequent soft landing on Al 2 O 3 and UNCD substrates takes place under high vacuum conditions.…”

Characterization of Photo-Induced Charge Transfer and Hot Carrier Relaxation Pathways in Spinel Cobalt Oxide (Co3O4)

“…(11) If the composition is changed from fluorine to oxygen, however, considering the electronegativity difference alone is not sufficient to define the type of semiconductor, because of the larger extent of orbital mixing through interatomic covalent bonding in the oxide compounds. Among various transition-metal oxides, the thermodynamic stability and surface redox reactivity make cobalt oxide (Co 3 O 4 ) an important catalyst for CO oxidation, (12) Fischer-Tropsch synthesis, (13) cyclohexane dehydrogenation, (14) and the oxygen evolution reaction in water splitting. ( interacts with visible photons, including the identity of the initially photoinduced excited state and the subsequent carrier relaxation, can provide insightful explanations for the photocatalytic activity of the material.…”

Kinetics and Mechanism of the Reaction of Coherently Synchronized Oxidation and Dehydrogenation of Cyclohexane by Hydrogen Peroxide

“…The cyclohexane oxidation is the most commonly studied process, where the valuable for organic synthesis compounds are achieved during oxidative dehydrogenation or during oxidation [4][5][6][7]. The implementation of thermodynamically hindered reactions is of great interest for petrochemical and organic synthesis.…”