Aqueous-phase aerobic oxidation of ethylene glycol on Co3O4: A facet specific investigation in alkaline free conditions

We have studied partial oxidation and proton exchange reactions for ethylene glycol (EGLY) molecules solvated in a liquid film of adsorbed water on cobalt oxide surfaces via ab initio molecular dynamics (AIMD) simulations. Specifically, the catalytic behavior at an elevated temperature of 393 K was investigated on the (110) and (111) low index surfaces under oxidative conditions, which were obtained by creating a hydrogen deficient system by replacing some water molecules by hydroxyl groups, and adsorbing some additional atom oxygen species. On the (110) surface, EGLY adsorbs with both oxygen atoms coordinating the same C o 3 + site in an octahedral bidentate complex (together with oxygen atoms from the solid). Six molecules (75% of the total) are modified by dissociating at least one proton in an acid-base type reaction. Out of these, three molecules in addition are partially oxidized by C–H cleavage to form hydroxyaldehyde species. Surface proton transfer occurs exclusively by transfering the proton to two-fold coordinated surface oxygen atoms. This reaction becomes coupled to an oxygen evolution reaction (OER) to form a molecular oxygen species. The (111) surface, on the other hand, undergoes reconstruction during the early phases of the simulations and exposes additional C o 2 + sites, that lead to molecules adsorbing on two different cobalt ions, thereby bridging two ions (usually one being a C o 3 + and one a C o 2 + ). On this surface all four adsorbate molecules loose at least one of the hydroxyl group protons to form charged ethylene dioxy intermediates, but no C–H cleavage has been observed. Hydrogen atoms from water or EGLY dissociation are, on the (111) surface, predominantly transferred to the adsorbed atomic oxygen, while on the (110) surface the bridging surface oxygen atoms are the preferred acceptor sites. On the (111) surface, side reactions such as hydrogen peroxide formation and hydrogen evolution have been observed, whereas the (110) surface shows much less reactions of this kind. Our simulations highlight possible reasons for the different reactivity of cobalt oxide surfaces, with the (110) surface showing higher efficiency for partial EGLY oxidation and the (111) surface having a higher proclivity for unwanted side reactions.