Electrospinning Machine| Manipulating d-Band Center and d–p Orbital Coupling to Mitigate Adsorption–Desorption Trade-Off for Efficient CO2-to-CO Photoreduction over MOF Nanofibers

In the reaction pathway of photocatalytic CO2 reduction to CO, strong CO2 adsorption and easy CO desorption are prerequisites for efficient conversion but represent a trade-off. Simultaneously optimizing these steps remains a key challenge, with limited related research.

Fuzhou University's Yu Yan, Chen Feifei, and Zhang Dantong from Qiqihar University reported an O/N hybrid-coordinated cobalt-nitrilotriacetic acid (NTA) MOF ultralong nanowire. DFT results show that O/N mixed coordination modulates the cobalt metal d-band center position and adjusts the d-p orbital coupling with coordinated atoms, optimizing both CO2→*CO2 and *CO→CO steps, making the CO2→CO reduction pathway thermodynamically favorable. In contrast, classic nitrogen-coordinated cobalt-2-methylimidazole (MeIm) and oxygen-coordinated cobalt-terephthalic acid (BDC) face challenges in spontaneous CO2 adsorption and difficult CO desorption, respectively.

Additionally, NTA molecules guide the growth of Co-NTA into 1D nanowires, while Co-BDC and Co-MeIm form 2D nanosheets and 3D polyhedrons, respectively. In situ and ex situ characterization techniques reveal that axial electron transport in 1D Co-NTA enables the highest charge transfer kinetics efficiency, resulting in a 1.8-fold increase in CO yield.

The study, titled *"Manipulating d-Band Center and d–p Orbital Coupling to Mitigate Adsorption–Desorption Trade-Off for Efficient CO2-to-CO Photoreduction over MOF Nanofibers,"* was published in Applied Catalysis B: Environment and Energy.

Highlights of the Study

1. Using nitrilotriacetic acid (NTA) as the organic ligand, a 1D ultralong nanowire-structured Co-NTA with a unique CoO6N decahedral coordination mode was constructed. In contrast, classic terephthalic acid (BDC) and 2-methylimidazole (MeIm) ligands yielded 2D Co-BDC nanosheets (CoO6 octahedral coordination) and 3D Co-MeIm polyhedrons (CoN4 tetrahedral coordination).

2. DFT calculations show that compared to O-coordinated Co-BDC and N-coordinated Co-MeIm, O/N hybrid-coordinated Co-NTA exhibits moderate d-band center positioning and d-p orbital coupling, synchronously optimizing CO2 adsorption and CO desorption, aligning the CO2→CO pathway with the Sabatier principle.

3. In situ and ex situ techniques (EIS, transient photocurrent, TRPL, in situ XPS, EPR) demonstrate that 1D Co-NTA has more efficient charge carrier kinetics than 2D Co-BDC and 3D Co-MeIm, attributed to rapid axial electron transfer along the nanowire.

Graphical Analysis

The molecular configuration of organic ligands and their coordination with metal sites significantly influence MOF morphology. Using NTA, BDC, and MeIm as ligands, three Co-MOFs—1D nanowires, 2D nanosheets, and 3D polyhedrons—were synthesized. XPS, XANES, and EXAFS characterized their coordination environments.

Fig. 1: Structures, SEM, and TEM of 1D Co-NTA, 2D Co-BDC, and 3D Co-MeIm.

Gas chromatography and NMR confirmed CO as the primary CO2 reduction product, with 1D Co-NTA showing 1.9x and 1.8x higher CO yields than 2D Co-BDC and 3D Co-MeIm, respectively, alongside excellent cyclability and stability.

Fig. 2: Photocatalytic CO2 reduction performance.

1D Co-NTA exhibits superior charge carrier kinetics: lower charge transfer resistance (EIS), higher photocurrent density (transient photocurrent), shorter fluorescence lifetime (TRPL), larger peak shifts (in situ XPS), and stronger superoxide radical signals (EPR).

Fig. 3: Charge carrier kinetics characterization.

DFT analyzed the electronic localization function, Bader charge, density of states, and crystal orbital Hamiltonian population for 1D Co-NTA, 2D Co-BDC, and 3D Co-MeIm. Results show that O-coordinated Co-BDC has the largest d-up/d-down spin splitting and d-p distance, while N-coordinated Co-MeIm has the smallest. O/N hybrid-coordinated Co-NTA exhibits moderate d-d splitting and d-p coupling. Pearson correlation confirms d-up, d-down, and d-p distance as key descriptors for *CO2 adsorption free energy.

Fig. 4: d-d splitting and d-p coupling analysis.

In situ DRIFTS detected the key *COOH and *CO intermediates in the CO2→CO reduction pathway, followed by DFT calculations of the reaction free energies for each step. CO2 activation and adsorption on the O-coordinated Co-BDC were highly favorable (−2.32 eV), indicating an extremely strong surface affinity for CO2. However, the strong binding of intermediates on Co-BDC inevitably led to difficulties in CO desorption, requiring a high energy barrier of 3.97 eV to overcome. Thus, CO desorption became the rate-limiting step for O-coordinated Co-BDC.

In contrast, the formation of *CO2 on N-coordinated Co-MeIm was non-spontaneous (+1.36 eV), suggesting an unfavorable CO2 adsorption and activation process, which further increased the difficulty of CO2 protonation. The energy barrier for *COOH formation was as high as 1.82 eV. However, due to the minimal d-d orbital splitting and d-p orbital spacing in N-coordinated Co-MeIm, its CO desorption process was more spontaneous compared to Co-NTA and Co-BDC.

Both Co-BDC and Co-MeIm exhibited thermodynamically unfavorable coordination environments for the CO2-to-CO reduction reaction. In comparison, the O/N mixed-coordinated Co-NTA possessed the optimal d-band center position and moderate d-p orbital coupling. Unlike the 3D Co-MeIm, the formation of *CO2 and *COOH on Co-NTA proceeded spontaneously, while compared to the 2D Co-BDC, the energy barrier for its rate-limiting step (*CO→CO) was significantly reduced to 1.56 eV.

Overall, the 1D Co-NTA better adhered to the Sabatier principle in terms of CO2 adsorption, protonation, and CO desorption processes, thereby achieving a higher CO production rate.

Fig. 5: In situ DRIFTS spectra and Gibbs free energy of the CO2→CO pathway.

Conclusions and Perspectives

This study aims to overcome the trade-off between CO₂ adsorption and CO desorption in the CO₂ reduction pathway by modulating the d-band center of metal sites and their orbital coupling with coordinating atoms. The NTA molecule not only induces preferential crystal growth orientation but also effectively regulates the electronic structure of Co-MOF. The research reveals that the d-band center position and d-p orbital spacing serve as primary descriptors for photocatalytic activity. In the one-dimensional Co-NTA, moderate d-d orbital splitting and appropriate d-p orbital coupling synergistically optimize the CO₂ adsorption, protonation, and CO desorption processes, making the CO₂→CO reduction pathway thermodynamically more favorable. Furthermore, compared to two-dimensional Co-BDC nanosheets and three-dimensional Co-MeIm polyhedrons, the one-dimensional Co-NTA nanowires exhibit significantly enhanced electron transfer capability. This study provides novel insights and approaches for regulating molecular adsorption and desorption in CO₂ reduction processes.

Link: https://doi.org/10.1016/j.apcatb.2025.125643