Importance of trivalency and the e _g ¹ configuration in the photocatalytic oxidation of water by Mn and Co oxides

Abstract

Prompted by the early results on the catalytic activity of LiMn 2 O 4 and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn 2 O 3 , LaMnO 3 , and MgMn 2 O 4 are found to be very good catalysts with turnover frequencies of 5 × 10 −4 s −1 , 4.8 × 10 −4 s −1 , and 0.8 ×10 −4 s −1 , respectively. Among the cobalt oxides, Li 2 Co 2 O 4 and LaCoO 3 -especially the latter-exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10 −4 s −1 and 1.4 × 10 −3 s −1 , respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co 3+ in the Co oxides is in the intermediate t 2g 5 e g 1 state whereas Mn 3+ is in the t 2g 3 e g 1 state. The presence of the e g 1 electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.artificial photosynthesis | water oxidation A ny strategy for solving the energy crisis would involve the generation of fuels through artificial photosynthesis, involving the sun as the only source of energy. To complete the solar cycle, water has to act as the source of electrons, either to generate liquid fuels by the reduction of CO 2 or to yield H 2 through a complete cycle of transfer of electrons. Oxidation of water, involving the transfer of four electrons, is energy-intensive. One of the challenges with artificial photosynthesis is the development of cost-effective catalysts made of abundant elements for the efficient oxidation of water to O 2 (1). RuO 2 and IrO 2 are widely used as oxygen evolution catalysts although their availability is limited and are expensive (2-5). Oxidation of water in plants occurs in Photosystem II making use of the Mn 4 O 5 Ca cluster (6, 7) present in the protein. The structure of the water oxidation complex involves a cubic cluster with four Mn atoms bridged via oxygen atoms (8,9 Results and DiscussionWe first studied the catalytic properties of the nanoparticles of the LiMn 2 O 4 spinel, with an average crystallite size of 57 nm [see Fig. S1 for an X-ray diffraction pattern and transmission electron microscope (TEM) image], prepared by the citrate sol-gel method from their corresponding metal precursors. Oxygen evolution was studied under visible light in a standard photoexcitation system consisting of Ru(bpy) 3 2+ as the photosensitizer and Na 2 S 2 O 8 as the sacrificial electron acceptor in a solution buffered at pH = 5.8. Oxygen evolved was quantified both by a Clark-type electrode and gas chromatography. We found reasonable catalytic activity with these nanoparticles as shown in Fig. 1A. Delithiation of LiMn 2 O 4 in dilute nitric acid resulted in a decrease of unit cell volume and an increase in surface area from 20 to 55 m 2 /g (Table S1). The average crystallite size of the delithiated sample (DlLiMn 2 O 4 ) was 46 nm. Delithiation seemed to result in a slight impr...

Redox properties of birnessite from a defect perspective

“…In some trivalent Mn and Co oxides with an octahedral building block like that in birnessite (9), the localized single eg electron of the transition metal can form σ bonds with the adsorbates and hence determine the energetics of the intermediates during the OER (10). In operation, usually only a very small portion of Mn sites is actually involved in the reaction, which justifies the defect perspective, and the formation of Mn(III) within the pure MnO2 matrix can be understood with the concept of the Jahn-Teller electron small polaron: One extra electron, initially located at the conduction band minimum (CBM) and delocalized over the whole crystal, becomes finally well localized (small) around one Mn site with a strong Jahn-Teller local distortion and with the oxidation state of this specific Mn decreasing by one.…”

“…The coexistence of Mn(IV) and Mn(III), and further the "balanced equilibrium" or low energy barrier between them, has been suspected to be a critical factor for its catalytic activity (1). The importance of the high-spin Mn(III), through its e 1 g electronic configuration, has been realized in other manganese oxides for OER catalysis (2,9) and also follows the design principle of Suntivich et al (10) that a near-unity eg occupancy may imply good OER catalytic activity. This easy switching of the oxidation state is also a typical behavior for the transition metal cation undergoing back and forth changes between several oxidation states in OER catalysts during the water oxidation cycle (11)(12)(13).…”

Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction

“…A high voltage to cause the OER (2.0 V vs RHE) was applied to the Ni-Co spinel nanosheets (Ni/Co ≈1:1), and XAS analysis was performed to determine the redox centers for the OER process. Compared with the pristine states, Co exhibited almost no change during the OER, remaining a mixture of Co 2+ [23,127] When the octahedral Co 3+ was oxidized to Co 4+ by applying a voltage, the O adsorbate attached to Co 4+ exhibited large electronegativity and could thus easily form an OO bond, a step that is generally considered a bottleneck in the OER mechanism. The authors also argued that the vacancy formation resulting from the loss of tetrahedral Zn sites facilitated the increase of surface-exposed octahedral Co 3+ sites.…”

Mechanistic Parameters of Electrocatalytic Water Oxidation on LiMn2O4 in Comparison to Natural Photosynthesis

“…The λ ‐Li x Mn 2 O 4 materials system has been previously investigated using photochemical35, 36, 37 and electrochemical38, 39 methods, displaying inconsistent activities. We establish the expected room‐temperature crystal structure, valence, and covalence of LiMn 2 O 4 using X‐ray diffraction (XRD) and X‐ray absorption spectroscopy (XAS).…”