Identification of Active Sites in the Catalytic Oxidation of 2‐Propanol over Co_1+xFe_2–xO₄ Spinel Oxides at Solid/Liquid and Solid/Gas Interfaces

Abstract

A series of Co1+xFe2–xO4 (0≤x≤2) spinel nanowires was synthesized by nanocasting using SBA‐15 silica as hard template, which was characterized by X‐ray powder diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy. The Co1+xFe2–xO4 spinels were applied in the aerobic oxidation of aqueous 2‐propanol solutions to systematically study the influence of exposed Co and Fe cations on the catalytic properties. The activity of the catalysts was found to depend strongly on the Co content, showing an exponential increase of the reaction rate with increasing Co content. Ensembles of Co3+cus (coordinatively unsaturated) sites were identified as the active sites for selective 2‐propanol oxidation, which are assumed to consist of more than six Co ions. In addition, gas‐phase oxidation with and without water vapor co‐feeding was performed to achieve a comparison with liquid‐phase oxidation kinetics. An apparent activation energy of 94 kJ mol−1 was determined for 2‐propanol oxidation over Co3O4 in the liquid phase, which is in good agreement with the gas‐phase oxidation in the presence of water vapor. In contrast to gas‐phase conditions, the catalysts showed high stability and reusability in the aqueous phase with constant conversion in three consecutive runs.

Dynamics of Reactive Oxygen Species on Cobalt-Containing Spinel Oxides in Cyclic CO Oxidation

“…[34] Furthermore, X-ray photoelectron spectroscopy (XPS) analyses also confirmed the ratio between Co:Fe, which suggested a uniform cation distribution in the bulk and at the surface. The Fe species on the surface were found to be Fe 3+ for all catalysts, whereas the Co species were Co 3+ and Co 2+ despite only Co 2+ containing CoFe 2 O 4 , as expected from the stoichiometry [35].…”

“…Furthermore, upon incorporating Fe into the spinel structure, higher occupancy of Co 2+ in tetrahedral sites was reported and correlated with a distortion of the Co 3+ -O bonds [34,37]. In another study on the herein presented SBA-15 templated nanowires, indications for the presence of a Co 3 O 4 secondary phase were reported on CoFe 2 O 4 based on Rietveld refinements [35].…”

“…As already stated in the introduction, the incorporation of Fe has different effects on the activity for different oxidation reactions. In the herein investigated CO oxidation, Fe did not have a beneficial effect on the SBA-15 templated catalyst materials, similar to liquid-phase oxidation of 2-propanol with oxygen as the oxidant [35]. The effect of different synthesis routes resulting in different micro-and mesostructures for S500, C800, and P600 with the same nominal Co 3 O 4 composition is shown in Figure 2b.…”

“…In the electrochemical oxygen evolution reaction (OER), a beneficial effect of Fe was observed [34]. In liquid-phase 2-propanol oxidation, however, no effect on the activity of the nominal Co:Fe ratio was detected [35]. Interestingly, if tert-butyl hydroperoxide was used as the oxidant, a beneficial effect of Fe was observed, which was related to different activation pathways [36].…”

“…The samples differed in their mesostructure, namely interconnected nanowires, sintered previously hexagonal particles, and a non-homogeneous sample. Low-surface area Co 3 O 4 samples were characterized by powder X-ray diffraction (XRD), physisorption methods, and scanning electron microscopy (SEM) and compared to the SBA-15 templated materials, whose properties were already reported elsewhere [34][35][36]. All samples were tested in cyclic CO oxidation in 2% CO and 20% O 2 in three consecutive runs, while between the first and the second run no further pretreatment was performed, while between the second and the third run an oxidative pretreatment was performed.…”