X-ray Absorption Spectroscopy Studies of a Molecular CO₂-Reduction Catalyst Deposited on Graphitic Carbon Nitride

Abstract

Metal–ligand complexes have been extensively explored as well-defined molecular catalysts in small molecule activation reactions such as carbon dioxide (CO2) reduction. Many hybrid photocatalysts have been prepared by coupling such complexes with photoactive surfaces for use in solar CO2 reduction. In this work, we employ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, density functional theory (DFT) and computational XANES modeling to interrogate the structure of a hybrid photocatalyst consisting of a macrocyclic cobalt complex deposited on graphitic carbon nitride (C3N4). Results show that the cobalt complex binds on C3N4 through surface OH or NH2 groups. By refining the local geometry and binding sites of this well-defined molecular cobalt complex on C3N4, we established an important benchmark for modeling a large class of molecular catalysts that can be adapted to in situ/operando studies and further enhanced by applying chemometrics-based approaches and machine learning methods of XANES data analysis.

Well-defined surface catalytic sites for solar CO2reduction: heterogenized molecular catalysts and single atom catalysts

“…The Co–O scattering path was chosen to fit the first peak of CoX/C 3 N 4 , resulting in a coordination number of 6.3 ± 1.4 for Co–O nearest neighbors. 105 Since the bond lengths of Co–N and Co–O are very close to each other, our experimental results were not able to distinguish between contributions from N and O atoms. We further carried out density functional theory (DFT) simulations, in which anchoring of CoX to C 3 N 4 through either a surface –O– or –NH– group represents the most probable modes.…”

“…A TON of more than 160 for CO production was obtained using Co-cyclam deposited on a modified C 3 N 4 after CO 2 reduction for an extended period under visible-light irradiation ( λ > 420 nm). 105 Under the same reaction condition, no CO was produced using a physical mixture of Co-cyclam and C 3 N 4 .…”

“…A TON of more than 160 for CO production was obtained using Co-cyclam deposited on a modified C 3 N 4 after CO 2 reduction for an extended period under visible-light irradiation (l 4 420 nm). 105 Under the same reaction condition, no CO was produced using a physical mixture of Co-cyclam and C 3 N 4 . In combination with computational modeling, X-ray absorption spectroscopy (XAS) was employed to investigate the structure of CoX on C 3 N 4 .…”

“…In combination with computational modeling, X-ray absorption spectroscopy (XAS) was employed to investigate the structure of CoX on C 3 N 4 . 105,106 For metal-based catalytic materials, the extended X-ray absorption fine structure (EXAFS) analysis can provide valuable insights into the local geometric structure and coordination environment of the metal sites, 107 while the X-ray absorption near edge structure (XANES) spectroscopy is particularly sensitive to the electronic structure of the metal center and its local environment. 108 As can be seen in Fig.…”