Importance of trivalency and the e_g¹ configuration in the photocatalytic oxidation of water by Mn and Co oxides

Abstract

Prompted by the early results on the catalytic activity of LiMn₂O₄ 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, Mn₂O₃, LaMnO₃, and MgMn₂O₄ 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, Li₂Co₂O₄ and LaCoO₃—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 Co³⁺ in the Co oxides is in the intermediate t_2g⁵e_g¹ state whereas Mn³⁺ is in the t_2g³e_g¹ state. The presence of the e_g¹ electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.

Any 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 CO₂ or to yield H₂ 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 O₂ (1). RuO₂ and IrO₂ 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 Mn₄O₅Ca 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). To study the principles governing photosynthesis, molecular systems with structures comparable to the Mn₄O₅Ca cluster having a [Mn₄O₄] core have been studied (10, 11). Mn is not only an abundant element, but also occurs in easily switchable oxidation states. [Co₄O₄] cubanes are also shown to be active for the oxygen evolution reactions (OERs) (12, 13). Nanocrytalline Co₃O₄ (14) and Mn₂O₃ (15) as well as “Co-Pi” and Co–phosphates (16, 17) are reported to be OER catalysts. Marokite-type oxides, CaMn₂O₄ and CaMn₂O₄⋅xH₂O exhibit good activity for water oxidation (18). Based on X-ray spectroscopic studies, mixed valency of Mn (III/IV) was considered to be a criterion for good catalytic activity (19). Electrochemical water oxidation by Ca₂Mn₃O₈, which is structurally analogous to CaMn₄O₅ cluster, has also been investigated (20). In two interesting publications, Greenblatt, Dismukes, and coworkers (21, 22) have recently reported that nanoparticles of λ-MnO₂ obtained by delithiation of LiMn₂O₄ shows a much higher water oxidation catalytic activity with turnover frequency (TOF) of 3 × 10⁻⁵ s⁻¹ compared with the parent oxide. The extra flexibility of the [Mn₄O₄] cubic unit in λ-MnO₂ was considered to be an important factor. Studies on nanoparticles of the Li₂Co₂O₄ spinel containing the [Co₄O₄] cubic unit were also found by them to exhibit OER activity, but without delithiation (22). Note that Co in Li₂Co₂O₄ exists as Co (III) whereas Mn in λ-MnO₂ is primarily in the IV state. These workers did not report the properties of LiCo₂O₄ where Co is mixed-valent. To understand the crucial factors responsible for catalytic oxidation of water and find a superior catalyst in the process, we have examined OER catalytic properties of nanoparticles of LiMn₂O₄, LiMnCoO₄, LiCo₂O₄, Li₂Co₂O₄ as well as a few other Mn and Co oxides, especially LaCoO₃, Mn₂O₃, and LaMnO₃, keeping in mind that strict comparisons of catalyst performance are best made on the basis of per mol of transition metal per unit surface area. The present study demonstrates that oxides of Mn and Co in the trivalent state containing e_g¹ electrons such as Mn₂O₃, LaMnO₃, MgMn₂O₄, Li₂Co₂O₄, and LaCoO₃, especially the latter, exhibit high catalytic activity for the OER reaction.

Results and Discussion

We first studied the catalytic properties of the nanoparticles of the LiMn₂O₄ 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)₃²⁺ as the photosensitizer and Na₂S₂O₈ 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 LiMn₂O₄ in dilute nitric acid resulted in a decrease of unit cell volume and an increase in surface area from 20 to 55 m²/g (Table S1). The average crystallite size of the delithiated sample (Dl-LiMn₂O₄) was 46 nm. Delithiation seemed to result in a slight improvement in the catalytic activity. However, O₂ evolved per mol of Mn per unit surface area of the catalyst showed this not to be true (Fig. 1A, Inset). The delithiated catalyst had a very small proportion of Mn (III), if any. We have plotted the rate of oxygen evolved per mol per unit time in Fig. 1B. With both the catalysts, the rate of oxygen evolution increases initially almost equally and then slowly decreases. Thus, both the catalysts have a TOF of 2.2 × 10⁻⁵ s⁻¹ when calculated from the initial slope of O₂ evolved vs. time plot. For the delithiated catalyst, the rate of oxygen evolution increases after the reaction has proceeded for ∼3 min plausibly because, as the reaction proceeds, it takes electrons from water generating Mn (III). The results suggested that the presence of Mn in its 3+ state was essential for the activity of the catalyst. Accordingly, we have found that nanoparticles of ZnMn₂O₄ and MgMn₂O₄ exhibit good catalytic properties for water oxidation as shown in Fig. 2 (see Fig. S2 for characterization). For example, MgMn₂O₄ particles prepared at relatively low temperatures (350 °C) show high O₂ evolution with a TOF of 8.2 × 10⁻⁵ s⁻¹_. We shall be discussing the catalytic activity of other Mn (III) oxides later in the article.

Fig. 1.

(A) Amount of oxygen evolved per mol of Mn and (B) rate of oxygen evolved per mol of Mn per unit time by LiMn₂O4 and Dl-LiMn₂O₄. (A, Inset) Amount of oxygen evolved per mol of transition metal per unit surface area of catalyst.

Fig. 2.

Amount of oxygen evolved per mol of Mn by (i) MgMn₂O₄ and (ii) ZnMn₂O₄.

To understand the role of the transition metal ion and its oxidation state, we examined the catalytic activity by substituting one Mn in LiMn₂O₄ with Cr, Fe, Co, and Ni, all of which crystallize in the spinel structure (Fig. S3). In the case of Ni substitution, LiNiMnO₄ could not be prepared, but we did obtain LiNi_0.5Mn_1.5O₄. Brunauer–Emmett–Teller (BET) surface areas and crystallite sizes of these oxides are listed in Table S1. Of these oxides, LiMnCoO₄ shows maximum activity with a TOF of 8.3 × 10⁻⁵ s⁻¹. The overall order of activity is Co > Ni > Cr > Mn > Fe, as shown in Fig. 3A, the oxygen evolved per mol of transition metal per unit surface area being in the order Co > Ni > Mn > Cr > Fe (Fig. 3B). This trend can be rationalized in terms of the electronic configurations of the trivalent transition metal ions (23–26) [Cr (III) t_2g³e_g⁰, Fe (III) t_2g³e_g²(HS, high spin), Ni (III) t_2g⁶e_g¹ (LS, low spin), Co (III) t_2g⁵e_g¹(IS), and Mn (III) t_2g³e_g¹(HS)]. In the case of Co (III), we have shown the intermediate spin state, an aspect that we will discuss later.

Fig. 3.

Amount of O₂ evolved (A) per mol of transition metal and (B) per mol of transition metal per unit surface area by (i) LiCoMnO₄, (ii) LiNi_0.5Mn_1.5O₄, (iii) LiCrMnO₄, (iv) LiMn₂O₄, and (v) LiFeMnO₄.

Encouraged by the performance of LiMnCoO₄, we prepared nanoparticles of the composition LiMn_2-xCo_xO₄ (where x = 0.2, 0.5, 1, and 2), all of which crystallize in the spinel structure, with a small gradual decrease in unit cell dimensions with increasing Co content. Raman spectra of the samples show the gradual evolution of Co (III) and Co (IV) states [Co⁴⁺O₆ octahedra at 580 cm⁻¹ (red) and tetragonally distorted Co³⁺O₆ octahedra at 660 cm⁻¹ (green)] from the Mn (III) and Mn (IV) [Mn⁴⁺O₆ at 575 cm⁻¹ (red) and Mn³⁺O₆ at 620 cm⁻¹ (blue)] of LiMn₂O₄ with increasing Co content (Fig. S4). Oxygen evolution activity of these catalysts increases with the increase in Co content (Fig. 4A), Li_1.1Co₂O₄ showing the highest activity with a TOF of 1.6 × 10⁻⁴ s⁻¹. The amount of oxygen evolved per mol of transition metal per unit surface area of the catalyst also follows the same order (Fig. 4A, Inset). The increase in the catalytic activity on Co substitution in LiMn₂O₄ can arise from the higher oxidation potential of Co (III) compared with Mn (III). It is however noteworthy that all these oxides show good catalytic activity even without delithiation.

Fig. 4.

(A) Amount of O₂ evolved per mol of transition metal by LiMn_2-xCo_xO₄ (where x = 0, 0.2, 0.5, 1, and 2). (Inset) Amount of O₂ evolved per mol of transition metal per unit surface area. (B) Amount of O₂ evolved per mol of Co by Li_xCo₂O₄ (x = 0.5, 1.1, 1.5, and 2). (Inset) Amount of O₂ evolved per mol of transition metal per unit surface area by (○) Li₂Co₂O₄ and (□) Li_1.1Co₂O₄.

Li_1.1Co₂O₄ has Co (III) and Co (IV) in equal proportions and it is difficult to pin down the role of the Co oxidation state on catalytic activity. To understand the role of the electronic configuration of Co on the ability of the catalyst to oxidize water, we investigated the catalytic activities of nanoparticles of Li_xCo₂O₄ (x = 0.5, 1.1, 1.5, and 2), all with the spinel structure (Fig. S5) showing lattice contraction with increase in the Li or the Co (III) content [verified by Raman and X-ray photoelectron spectroscopy (XPS) measurements; see Figs. S6 and S7]. N₂ adsorption measurements revealed no appreciable difference in the surface area, although the crystallite size showed a slight increase with increasing Li content. We found the catalytic activity to increase with increasing Li or Co (III) content (Fig. 4B), with Li₂Co₂O₄ showing the highest O₂ evolution (Fig. 4B), slightly higher than that reported earlier (22). Even after taking the surface area of the catalyst into consideration, Li₂Co₂O₄ turns out to be a better catalyst than Li_1.1Co₂O₄, suggesting the importance of trivalency of Co. Magnetic measurements on Li₂Co₂O₄ showed Co³⁺ to be in the intermediate spin state (t_2g⁵e_g¹). Based on the above results, we felt that the presence of the e_g¹ electron could play a significant role in determining catalytic activity. To investigate the possible role of intermediate spin Co (III), we have studied the catalytic activity of LaCoO₃, which has Co (III) in the t_2g⁵e_g¹ state at room temperature (23, 24) and crystallizes in the perovskite structure, because the available information in the literature on oxides (27) could not be used for proper comparisons. Furthermore, it was necessary to measure the catalytic properties of LaCoO₃ under similar conditions as those of the other oxide systems studied by us.

LaCoO₃ shows remarkably high catalytic activity for the oxidation of water and is even superior to Li₂Co₂O₄ in performance (Fig. 5A). The TOF for LaCoO₃ is 1.4 × 10⁻³ s⁻¹ whereas that for Li₂Co₂O₄ is 9 × 10⁻⁴ s⁻¹. The commonality between these two oxides is that Co (III) in both these oxides is in the intermediate t_2g⁵e_g¹ state. Interestingly, Mn₂O₃ in the bixbyite structure, which is reported to be a good catalyst for water oxidation (15), has Mn (III) in the t_2g³e_g¹ configuration. LaMnO₃ which is a perovskite with Mn (III), shows good catalytic activity, only slightly lower than Mn₂O₃ as shown in Fig. 5A, the TOF values being 4.8 × 10⁻⁴ s⁻¹ and 5 × 10⁻⁴ s⁻¹, respectively. Even after taking the surface area of the catalysts into consideration, the OER catalytic activity follows the order LaCoO₃ > Li₂Co₂O₄ > Mn₂O₃ > LaMnO₃, as shown in Fig. 5B. Fig. 5A Inset shows the rate of O₂ evolved per mol of transition metal, which follows the same order [see Fig. S8 for X-ray diffraction (XRD) of LaCoO₃, LaMnO₃, and Mn₂O₃]. The electronic configuration of the transition metal and the associated metal–oxygen bond distances appear to be important factors in determining the OER catalytic activity as indicated by Robinson et al. (28) Intermediate-spin Co (III) and Mn (III), both with one electron in the e_g orbital, give rise to Jahn–Teller distorted metal–oxygen octahedra.

Fig. 5.

Amount of O₂ evolved (A) per mol of transition metal and (B) per mol of transition metal per unit surface area by (i) LaCoO₃, (ii) Li₂Co₂O₄, (iii) Mn₂O_3, and (iv) LaMnO₃.

Conclusions

In conclusion, the present study shows that the cobalt oxides, the spinel Li₂Co₂O₄ and the perovskite LaCoO₃, both containing Co³⁺ ions in the intermediate spin state (t_2g⁵e_g¹), are good catalysts for water oxidation. Among the manganese oxides, Mn₂O₃ with the bixbyite structure and LaMnO₃ with the perovskite structure, as well as MgMn₂O₄ with the spinel structure containing Mn³⁺ (t_2g³ e_g¹) ions, show high catalytic activity. The commonality between these Co and Mn oxides is that they have Jahn–Teller distorted metal–oxygen octahedra due to the presence of e_g¹electrons. The best catalyst found so far is LaCoO₃. Thus, our study of Mn and Co oxides of spinel and perovskite structures indicates that trivalency and the electronic configuration of the B-site transition metal cation to be important factors in determining photocatalytic OER activity. Trivalency ensures easy electron transfer from the metal ion. In particular, oxides with B-site cation having d⁴ and d⁶ configurations with one electron in the antibonding e_g orbital show high activity for oxygen evolution irrespective of the crystallographic structure of the catalyst. The e_g orbital of the transition metal ions can form σ-bonds with anion adsorbates and influence the binding of oxygen-related intermediate species to the catalyst during electrochemical oxygen evolution and oxygen reduction reactions (ORRs) (29, 30). The presence of a single electron in the antibonding e_g orbital is expected to yield just the appropriate strength of interaction between O₂ and the catalyst required for high OER and ORR activity. The localized single e_g electron in the antibonding σ*-state can readily be donated during the photocatalytic OER cycle.

Experimental Procedures

Nanoparticles of LiMn₂O₄, Li_1.1Co₂O₄, Li_xCo₂O₄, and LiMn_2-xA_xO₄ (A = Cr, Co, Fe, Ni) were all prepared by citrate sol–gel method using metal ions:citric acid:urea molar ratio of 1:2:2. A few drops of dilute HNO₃ were often added to dissolve the metal precursors. Metal acetates were used as metal precursors. This solution was heated at 80 °C until the entire solvent had evaporated to form a gel. The resultant resin was heated at 180 °C for 14 h followed by heating at 350 °C for 12 h in air. For delithiation of LiMn₂O₄, the nanoparticles were treated with dil HNO₃ (pH 1.2) for 48 h and washed by centrifugation. Zn(Mg)Mn₂O₄ samples were prepared similarly by the citrate sol–gel method starting from corresponding metal acetates, with metal:citric acid ratio of 1:2 in the gel. Two-step heating, first at 180 °C for 12 h followed by 350 °C for 8 h in air, was carried out to obtain pure nanoparticles of ZnMn₂O₄ and MgMn₂O₄. LaMnO₃ and LaCoO₃ were also obtained by citrate sol–gel method as described above starting from LaNO₃0⋅6H₂O and Mn and Co nitrates. The gel was prepared with metal:citric acid ratio of 1:10. It was first heated at 200 °C for 12 h followed by heating for 5 h in air at 500 and 800 °C for LaCoO₃ and LaMnO₃, respectively. In the case of Mn₂O₃, the gel was prepared with Mn(CH₃COO)₂:citric acid ratio of 1:2 with the final heating of 500 °C for 8 h in air.

Powder XRD measurements were carried out with a Bruker D8 Advance diffractometer using Cu Kα radiation. The average crystallite size was calculated by using the Debye–Scherrer formula t = 0.9λ/(β cos θ), where β is the full width at half maxima in radians, λ is the wavelength of X-rays, θ is the Bragg angle. Inductively coupled plasma optical emission spectroscopy was used to determine the Li to transition metal ratio using a Perkin-Elmer Optima 7000 DV instrument. Particle size was determined from TEM images obtained with a JEOL Transmission Electron Microscope 3010 operating at an accelerating voltage of 300 kV. Surface area was determined from N₂ adsorption measurements carried out in a Quanta-chrome Autosorb instrument at 77 K. Raman spectra were recorded with a Horiba-JobinYvon LabRAM HR high-resolution Raman spectrometer using Ar laser (λ = 514.5 nm) with D1 filter. Oxidation states of transition metal ions were measured with XPS using an Omicron nanotechnology spectrometer.

Oxygen evolution measurements were carried out using an oxygraph instrument from Hansatech Ltd., equipped with a Clark-type oxygen electrode with 0.022 M Na₂SiF₆ and 0.028 M NaHCO₃ buffer, 1.5 mM [Ru(bpy)₃]Cl₂·6H₂O, 20 mM Na₂S₂O₈, and 80 mM Na₂SO₄ and 100 ppm catalyst in 2 mL solution. A 100-W halogen lamp with a BG 38 filter was used as a light source, with the light intensity being kept at 25,000 Lux. Catalysts were collected after water oxidation reaction by ultracentrifugation, washed thoroughly, characterized, and checked for catalytic activity.