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

Abstract

Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation.

Beneficial Effects of Low Iron Contents on Cobalt‐Containing Spinel Catalysts in the Gas Phase 2‐Propanol Oxidation

“…Specific BET surface areas of the catalysts are reproduced from the previous work [6] with the exception of Co : Fe 64 : 1 and are shown in Table 1. For Co‐rich samples, the exposed surface areas ranged between 103 and 128 m 2 g −1 .…”

“…The 3 rd run was performed with the 10-fold co-dosing of water compared to 2-propanol and O 2 into the feed to study its effect compared to the dry runs. Before each 118 [6] 96.9 95.2 96.0 Co : Fe 16 : 1 103 [6] 94.1 93.1 93.7 Co : Fe 7 : 1 113 [6] 87.5 84.8 84.0 Co : Fe 3 : 1 125 [6] 75.0 75.9 76.8 CoFe 2 O 4 178 [6] 33.3 27.8 46.6…”

“…[5] From the mechanistic point of view, the O 2 activation and the formation of reactive oxygen species (ROS) at the surface of these cobalt oxide catalysts play an important role in CO and methane oxidation. [6] Zasada et al reported that two fivefold coordinated Co 3 + centers in octahedral sites of the spinel activate the oxygen as a cooperative tandem. [7] In the temperature range below 450 °C, they reported μ-superoxo and metaloxo species.…”

“…From the mechanistic point of view, the O 2 activation and the formation of reactive oxygen species (ROS) at the surface of these cobalt oxide catalysts play an important role in CO and methane oxidation [6] . Zasada et al.…”

Cobalt Oxide Nanowires with Controllable Diameters and Crystal Structures for the Oxygen Evolution Reaction

“…Cobalt oxide has been extensively examined and it is a prime candidate to be utilized as an electrocatalyst for OER due to its tunable defect sites, good stability, and robust crystalline structure. [4,[14][15][16][17][18] The ordered mesoporous cobalt oxide nanowires templated by SBA-15 are also recently investigated as the model catalyst for other gas and liquid phase catalytic reactions and it shows very good activity in CO, [19] 2-propanol, [6] aromatics alcohols, and styrene oxidation. [20] Among the crystalline cobalt oxide phases, Co 3 O 4 adopts a cubic Fd-3 m space group and normal spinel-type structure with high spin Co 2 + occupying tetrahedral sites and low spin Co 3 + occupying octahedral sites.…”