Identifying Influential Parameters of Octahedrally Coordinated Cations in Spinel ZnMn_xCo_2–xO₄ Oxides for the Oxidation Reaction

Abstract

Transition metal oxides are of potential alternatives to precious metal catalysts for oxidation reactions. Among these earth abundant oxide catalysts, cobalt-or manganese-based spinel oxides have gained consistent interests due to their superior catalytic performances. It has been found that the octahedral sites in spinels are responsible for their catalytic activities. However, little is known to date about the parameters of the octahedrally coordinated cations that influence their activity. Herein, a series of ZnMnxCo2-xO4 (x=0~2.0) spinel oxides are investigated, employing CO oxidation as the model reaction, with the particular attention on the activity variation caused by tuning the ratio of octahedrally occupied Mn to Co. Both Mn and Co contribute to the activity with Mn cations as the primary active species when they co-exist, the intrinsic specific activity is found composition-dependence and the highest activity is given by the Mn/Co molar ratio of 0.11. The presence of Mn 4+ and Mn 3+ in a proper ratio is another key for achieving high oxidation activity and can be rationalized by the moderate oxygen adsorption during the CO oxidation, which facilitates the O vacancy refilling. This is also supported by the density function theory (DFT) calculation, showing that the high activity of ZnMn0.2Co1.8O4 originates from having the O p-band center neither too far nor too close to the Fermi level. The eg occupancy of Mn cations and the O p-band center relative to the Fermi level, which are the indexes of how the electronic structure influences the oxygen addition-and removal-related processes, are proposed to serve as the activity descriptors. This work may provide a different insight into the understanding of the activity of transition metal spinel oxides for oxidation reactions.

Highly efficient mixed-metal spinel cobaltite electrocatalysts for the oxygen evolution reaction

“…This might be the reason why the FeCo2O4 can easily form composite hybrid spinel structures. Our findings are supported by previous reports that the cations A' valence values depend on the occupied positions [12,14,17]. As stated above, the dominant factor for the OER performance of electrode materials is the spinel crystal field.…”

“…[10]. Recently, there has been an increased interest in using mixed-metal spinel oxides for the OER applications [11][12][13][14].…”

Self-Supporting Clusters Constituted of Nitrogen-Doped CoMoO4 Nanosheets for Efficiently Catalyzing the Hydrogen Evolution Reaction in Alkaline Media

“…Among various non-noble metal electrocatalysts, plenty of research has been reported on transition metal oxides (TMOs) catalysts due to their attractive properties including low price, abundant reserves, low toxicity, and high activity. Some works found that pure TMOs exhibited low overpotential toward the oxygen evolution reaction (OER). − On the other hand, to date, their applications as HER electrocatalysts have been rarely reported due to their poor HER activity . To address the above issues, diverse modification methods have been explored to boost the HER performance of TMOs: (1) Elemental doping can efficiently accelerate the HER kinetics of TMOs by modulating the electronic structure and conductivity. , (2) Also, oxygen vacancy engineering can modulate electronic structure and free energy for H adsorption. , (3) Selectively exposing favorable crystal planes can obtain higher HER catalytic activity of TMOs. , (4) The construction of hierarchical heterostructure can enhance electrolyte diffusion and ionic transportation .…”

Hollow Mesoporous Co3O4–CeO2 Composite Nanotubes with Open Ends for Efficient Catalytic CO Oxidation

“…Meanwhile, the amount of adsorbed O 2 is increased as a result of the abundant oxygen vacancies in CeO 2 , and the dissociation into O 2 − ion radical is promoted, which is a key factor for the enhancement of the CO oxidation. Finally, the O 2 − ion radical reacts with a CO molecule to form a CO 2 molecule …”