Synchrotron Operando Spectroscopy for Electrochemical reduction of Single-Atom Alloy Ca talysts

2022 Fiscal Year Annual Research Report

• Project/Area Number: 21F41066
• Allocation Type: Single-year Grants
• Research Institution: The University of Tokyo
• Principal Investigator: 原田 慈久 東京大学, 物性研究所, 教授 (70333317)
• Co-Investigator(Kenkyū-buntansha): LIU DAOBIN 東京大学, 物性研究所, 外国人特別研究員
• Project Period (FY): 2021-09-28 – 2024-03-31
• Keywords: Cobalt phthalocyanine / CO2 reduction reaction / Operando measurement / HERFD-XAS / Soft-XAS

Outline of Annual Research Achievements

The well-known M-N4 coordinated catalysts show a superior catalytic activity for electrochemical CO2 reduction (ECR). However, the catalytic mechanism and the role of centered metal sites are still remaining a bitter controversy due to the lack of experimental evidence under the operating ECR conditions. Here, the cobalt phthalocyanine (CoPC) is used as a model catalyst to investigate the electronic structure evolution of reactive Co sites by HERFD-XAS and soft-XAS.
(1) The graphene supported CoPc (CoPc/G) shows a potential-dependent behavior for H2 and CO production. The selectivity for CO products is increasing as the applied potentials and reaches the maximum value of ~96% Faradic efficiency at-0.8 V (vs. RHE).
(2) The Co K HERFD-XAS shows that, as increasing the bias in CO2-saturated electrolyte, the peak at 7708 eV (1s→3d-4p hybridization) is gradually raising the intensity from -0.6 V and is back to the initial without the bias . Inversely, the intensity of the peak at 7715 eV (1s→4pz) is decreased. It implies that the Co site of CoPc/G could be interacted with intermediates (*COOH/*CO) and shows a reversible behavior during the reaction.
(3) The Co L soft-XAS shows the intensity of the peak at 777.8 eV regarding to the transitions with components out of CoPc plane decreases at -0.6 V. It is ascribed to the intermediates absorbed on the Co site and results in a large distortion of the D4h symmetry. The spectra show same characteristic features but difference in peak intensities, indicating that the Co site of CoPc/G remains the CoII oxidation state.

Current Status of Research Progress

2: Research has progressed on the whole more than it was originally planned.

Reason

We identify the electronic structure of Co sites in CoPc under the ECR reaction by operando XAS measurement. From the spectra, the oxidation state of Co sites is almost no change, suggesting that the ligand is in the charge state instead of Co sites under ECR conditions. Accordingly, we proposed an ECR catalytic mechanism for CoPc/G. At the first two steps, the phthalocyamine (Pc) ligand is charged and absorbed with protons. After that, CO2 is attached on the Co site to form *COOH intermediates by a proton-coupled electron transfer (PCET) step and then further reduced to *CO before desorption.

Strategy for Future Research Activity

As mentioned, we performed multiple X-ray techniques to track the electronic structure evolution of Co sites in CoPc/G under the ECR reaction. To end of this research, we make a plan from the following three aspects.
(1) The operando ATR-FTIR measurement will be performed to identify the key intermediates on CoPc/G during the ECR reaction. The obtained spectra will be analyzed for acquiring the fingerprint of key intermediate species and the corresponding applied potential. by the combination of the Co K/L edge XAS spectra, the structural distortion caused by the intermediate adsorbates (*COOH or *CO) will be carefully clarified.
(2) To further clarify whether the CoPc ligand stay in the charge state at the low overpotential, The CV measurement will be conducted in aqueous or non-aqueous electrolytes. It is expected to give more insights into the redox behavior of CoPc ligands by the comparison of the molecular without Co sties.
(3) The multiplet XAS calculation will be done to clarify the electronic structure of Co sites with *CO or *COOH adsorbates. Moreover, the CoPc-CO/COOH structure evidenced by XAS and FTIR spectra will be optimized by the DFT calculation. The Gibbs free energies and redox potentials of each reaction step will be calculated and thus determines the most possible catalytic mechanism and the RDS step for selective CO2 electroreduction to CO.

Research Products

• [Presentation] Operando X-ray Spectroscopy Study on the Electronic Structure Evolution of Cobalt Phthalocyanine under Electrochemical CO2 Reduction Reaction (2023)
  • Author(s): Liu, D., Samarai, M.A., Zhang, W., Zhao, H., Kiuchi, H., Harada, Y.
  • Organizer: JSR 2023
• [Presentation] Identification of Electronic Structure of Atomic Cobalt Sites in CoPc under Electrochemical CO2 Reduction Reaction (2022)
  • Author(s): Liu, D., Samarai, M.A., Zhang, W., Zhao, H., Kiuchi, H., Harada, Y.
  • Organizer: AO-SRI 2022
  • Int'l Joint Research