Project Overview

This computational chemistry project investigates the fundamental interactions between metalloporphine molecules (containing Iron or Manganese) and carbon dioxide (CO2​) when these molecules are anchored to a copper metal surface (Cu(111)). The study of such systems is crucial as they serve as models for developing advanced materials for catalysis, chemical sensing, and environmental applications like CO2​ capture. By using high-level quantum mechanical simulations, the project aims to provide a detailed atomic-scale understanding of how the copper surface influences the chemical and electronic properties of the porphine molecule and its ability to bind CO2​. 

Research Objectives 

• To investigate the chemical and electronic properties of Fe-porphine and Mn-porphine when adsorbed on a Cu(111) surface.

• To determine the nature, geometry, and energetic favorability of CO2​ adsorption onto the metalloporphine-Cu(111) system.

• To elucidate the specific orbital interactions responsible for the binding between the central metal atom of the porphine and the CO2​ molecule.

• To analyze how the spatial arrangement and proximity of multiple porphine molecules on the copper surface affect their interaction with CO2​.

• To understand the role of the underlying copper substrate in mediating the overall host-guest interaction.

Methodology & Computational Details

• Theoretical Framework: The study was conducted using Density Functional Theory (DFT) with periodic boundary conditions (PBC), an accurate method for simulating crystalline surfaces and adsorbates.

• Software: All calculations were performed using the Vienna Ab initio Simulation Package (VASP).

• System Model: The Cu(111) surface was modeled as a three-layer slab of 180 copper atoms. The top layer, along with the porphine and CO2​ molecules, was allowed to fully relax during simulations.

• DFT Functional: The rev-vdW-DF2 exchange-correlation functional was used to properly account for the non-covalent van der Waals forces, which are critical for describing the physisorption process accurately.

• Analysis Techniques: Geometry optimizations were performed to find the most stable structures. Adsorption energies were calculated to quantify binding strength. Density of States (DOS) and partial charge (PARCHG) analyses were used to dissect the electronic structure and orbital interactions.

Key Results & Findings

• Nature of Adsorption: The binding of CO2​ is a physisorption process, with calculated adsorption energies ranging from -4.0 to -4.7 kcal/mol, indicating a relatively weak but significant interaction.

• Binding Geometry: In the most stable configuration, the CO2​ molecule lies parallel to the plane of the porphine ring, positioned directly above the central metal atom (Fe or Mn).

• Orbital Interaction: The binding is primarily driven by the orbital overlap between the axial 3d-π orbital of the central metal atom and the 2p-π orbital of the carbon atom in the CO2​ molecule.

• Cooperative Effects: Placing two porphine molecules in close proximity (~2.3 Å) on the copper surface resulted in a slightly stronger adsorption energy per molecule compared to when they were far apart (~4.0 Å), suggesting a cooperative electronic effect mediated by the surface.

• Substrate Influence: The presence of the Cu(111) surface was found to be crucial, as it significantly modulates the potential energy landscape for CO2​ rotation and influences the overall binding strength.

B.E. -4.3 kcal/mol                                                                                                                                       B.E. -4.0 kcal/mol 

DFT optimized geometries of the calculated structures of the CO2 adsorbed on Fe-porphine and Mn-porphine separately

B.E. -4.6 kcal/mol                                                                                                                                       B.E. -4.7 kcal/mol 

DFT optimized geometries of the calculated structures of the  CO2 adsorbed on Fe-porphine and Mn-porphine together when both porphine situated in close (2.36 Å) and far (3.95 Å) from each other. 

Conclusion & Impact

This computational study successfully elucidated the key electronic and structural factors governing the interaction between CO2​ and metalloporphines on a Cu(111) surface. The results demonstrate that the underlying metal substrate plays a critical, non-innocent role in modulating the properties of the adsorbed molecular layer. The primary interaction is a d-π to p-π orbital overlap. The impact of this research lies in providing a fundamental, atomic-level understanding that can guide the rational design of new, more efficient surface-based materials for gas capture, separation, and heterogeneous catalysis.

Fe-Pn & Mn-Pn (Close)                                                                                                                                                                         Fe-Pn & Mn-Pn (Far)

Research Significance & Future Directions

• Significance: This work contributes to the fundamental understanding of surface science and molecule-substrate interactions. It provides a detailed picture of a model system that is highly relevant to the development of functional materials for energy and environmental technologies.

• Future Directions: Future work could expand on these findings by exploring other catalytically relevant metal surfaces (e.g., Ag, Au, Pt), investigating different substituent groups on the porphyrin ring to tune its electronic properties, or studying the interaction with other small molecules of industrial importance (e.g., CO, NO, H2​).

Funding & Acknowledgements

This research project was supported by the World Premier Research Initiative (WPI) promoted by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) for the Institute of Integrated Cell-Material Sciences (iCeMS) at Kyoto University