Theoretical Study on the Reaction Mechanism of the Water-Splitting Process on Cobalt Oxide Catalysts

Abstract

In recent years, artificial photosynthesis has attracted attention as a mechanism for generating fuel by sunlight irradiation of a photocatalyst to split water into O and H. Elucidation of the four-electron oxidation reaction mechanism of this split is essential for the development of highly active photocatalysts. A reaction mechanism has been proposed for Co₃O₄ catalysts using time-resolved Fourier transform infrared spectroscopy to identify the intermediates in the oxidation of water under reaction. In this study, density functional theory was used to examine the mechanism of the four-electron reaction of the Co₃O₄ photocatalysts.

1 Introduction

Research into the field of new energy is accelerating because of the mass use of fossil fuels, and artificial photosynthesis has gained attention as a mechanism for fuel generation by sunlight irradiation of a photocatalyst to split water into O and H [1]. The electrons obtained from this O-generating reaction ($2H_{2}O\to O_2+4H^{+}+4e^{-}$) reduce CO₂ and produce hydrocarbons, indicating that the supply of electrons is extremely important. To develop photocatalysts with a high activity, the efficiency of multi-electron transfer reactions needs to be increased. Co₃O₄ is a first-row transition metal oxide that is abundant in the earth and is highly active as an O-generating catalyst due to its high electron conductivity; it also generates O through the four-electron oxidation reaction of water. Co sites with the cubane model of Co₄O₄ [2, 3] and complexes containing Co₄O₄ as core [4], Co-containing photocatalysts, have been intensively studied. Using time-resolved Fourier transform infrared (FTIR) spectroscopy, Frei et al. [5] identified the IR absorption at 1013 cm^–1, observed for the oxidation of water catalyzed by Co₃O₄ under reactive conditions, as being due to superoxide in the reaction intermediate and proposed a two-site reaction mechanism involving OH groups on the neighboring Co (Figure 1). In this study, we used density functional theory (DFT) calculations to confirm the four-electron oxidation pathway in the two-site reaction mechanism of Co₃O₄ photocatalysts and attributed FTIR spectra to intermediates.

Figure 1.

 Proposed two-site photocatalytic mechanism.

2 Method

The Co₃O₄ surface with adsorbed OH groups was modeled as a cluster, and Gibbs free energy calculations along the reaction pathway were calculated by DFT at the B3LYP+D3/LanL2DZ, 6-31G* level using the Gaussian16 program. In the structural optimization and frequency calculations for each step of the reaction process, the surface portion of the Co₃O₄ catalyst was fixed.

3 Results and Discussion

The change in the stable structure and Gibbs free energy at each step along the experimentally proposed reaction path of Figure 1 is shown in Figure 2. The proton-coupled electron transfer (PCET) process associated with hole injection is represented by the removal of H atoms. Frei et al. [1] proposed a model in which Co(III)–OH becomes Co(IV)=O by two-electron oxidation (3rd step), but the calculation results show an open-shell singlet electronic structure of Co(III)–O ∙ with singlet coupling of two unpaired electrons. The 4th step generated by dissociative adsorption of water is an open-shell singlet, and the 5th step generated by PCET is a quartet. Although the order of hole injection and water addition is not specified in Figure 1, when comparing the 6th and 7th steps (Figure 3), Path A (hole injection after water addition) and Path B (hole injection followed by water addition), Path A was adopted because of the energetic advantage (the 6th step is a doublet and the 7th step is a triplet). From the above, the reaction pathway with endothermic 12.9 eV can be clarified theoretically. It can be verified that the four-electron reaction mechanism can proceed exothermically because a sum of the Co₃O₄ band gap (about 2.5 eV [6]) and the standard voltage required for water splitting (1.23 eV [7]) enters the system as an external potential at each PCET process. Frei et al. [1] used time-resolved FTIR spectroscopy to identify the intermediates under reaction and identified one of the intermediates as the absorption of surface superoxide (1013 cm^–1) in the three-electron oxidation intermediate (5th step). Our calculations have yielded an IR absorption of surface superoxide (1165 cm^–1) at the intermediate (4th step) formed by dissociative adsorption of water molecules after two-electron oxidation (Figure 4).

Figure 2.

 Gibbs free energy diagram of the two-site mechanism. In the optimized structures, the blue dotted lines indicate the areas where a water molecule is dissociatively adsorbed and the light-blue dotted lines indicate areas where an H atom is released.

Figure 3.

 Comparison of Path A and Path B with different reaction orders for the 6th and 7th steps.

Figure 4.

 Calculated IR spectrum at the 4th step and results of frequency analysis at 1165 cm⁻¹. The light-blue arrows in the inset indicate the direction of vibrational frequency, 1165 cm⁻¹.

4 Conclusion

Using DFT calculations, we have explored the two-site reaction pathway of Co₃O₄ photocatalysts and identified a reaction pathway that generates O through a four-electron oxidation reaction. Experimentally, a reaction mechanism that occurs on a single Co atom (one-site) has been proposed, although it is not favorable in terms of reaction rate, and the one-site reaction pathway will be discussed in the future.

Acknowledgment

This work was partly supported by MEXT as “Program for Promoting Researches on the Supercomputer Fugaku” (Realization of innovative light energy conversion materials utilizing the supercomputer Fugaku, JPMXP1020210317). K. Y. acknowledges the support from JSPS KAKENHI in Scientific Research on Innovative Areas "Innovations for Light-Energy Conversion (^I4LEC)".

The computation was performed using Research Center for Computational Science, Okazaki, Japan (Project: 22-IMS-C064) and Tohoku University for the use of MASAMUNE-IMR (MAterials science Supercomputing system for Advanced MUlti-scale simulations towards NExt-generation-Institute for Materials Research).

Acknowledgments

References

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