
Chemistry
Regulation of functional groups on graphene quantum dots directs selective CO2 to CH4 conversion
T. Zhang, W. Li, et al.
This groundbreaking research, conducted by Tianyu Zhang and colleagues, reveals how functionalized graphene quantum dots can efficiently steer CO2 electroreduction towards high selectivity for CH4 production. Their innovative approach enhances methane yield significantly, promising to reshape catalyst design for sustainable chemical synthesis.
Playback language: English
Introduction
Electrochemical CO2 conversion into valuable chemicals and fuels offers a promising path towards closing the anthropogenic carbon cycle and storing renewable energy. However, this technology is currently hindered by the lack of highly selective and active catalysts for converting CO2 into specific, energy-intensive hydrocarbon products. Most transition metals favor the competing hydrogen evolution reaction (HER), while others like Au, Ag, Sn, and Bi only produce CO or HCOO⁻ through two-electron reduction pathways. Copper-based catalysts are unique in their ability to catalyze CO2 electroreduction to various C1-C2 hydrocarbons and C2+ oxygenates. Nevertheless, improving the selectivity and productivity of a single high-order product remains a significant challenge, necessitating further research into tuning existing Cu-based materials or developing new alternatives.
Nanostructured carbon materials, with their low cost, large specific surface area, and tunable electronic structures, are attractive metal-free electrocatalysts. However, pristine carbon materials are chemically inert, requiring the introduction of active sites. Heteroatom doping (e.g., S, N, B, P) is a common strategy, altering charge density at heteroatoms or adjacent carbon atoms. This approach has improved CO2 reduction activity towards CO and HCOO⁻ in various carbon nanostructures (carbon nanotubes, nanofibers, graphene, 3D graphene foam), rivaling metal catalysts. However, the formation of high-order products beyond the two-electron transfer pathway remains elusive with heteroatom-doped carbon materials. First-principle calculations suggest that heteroatom dopants lead to strong *COOH binding energy but weaker CO adsorption than CO-selective metal catalysts. Subsequent hydrogenation to hydrocarbons or oxygenates requires stronger *CO adsorption, necessitating different active sites.
Tuning the allotrope or topological nanostructure of carbon has shown promise in creating active sites for high-order product formation. For example, N-doped diamond (NDD) selectively reduces CO2 to CH3COO⁻, while N and B co-doped diamond (NBDD) favors C2H5OH. This selectivity is attributed to synergistic effects of heteroatom dopants and sp³ hybridized carbon. Cylindrical N-doped mesoporous carbon (c-NC) also shows selectivity towards C2H5OH due to its ordered channel surface, which stabilizes CO intermediates and promotes C–C coupling. Graphene quantum dots (GQDs), with their high edge site density, offer another promising approach. N-doped GQDs have demonstrated high Faradaic efficiency (FE) for multi-carbon products and CH4, showcasing Cu-like performance in a metal-free catalyst. This selectivity arises from the increased number of edge sites and pyridinic N dopants. However, further improvement in selectivity and activity for a single product is necessary to achieve industrial relevance. Surface functionalization, in addition to doping, provides a powerful tool to modify electronic structure and adsorbate binding, potentially enhancing catalytic performance.
Literature Review
The literature extensively explores strategies to enhance the electrochemical reduction of CO2. Significant efforts focus on the development of efficient and selective catalysts. Transition metal-based catalysts, especially copper, have been extensively investigated due to their ability to produce a variety of products. However, achieving high selectivity towards a specific product remains a major hurdle. Studies have shown that the morphology and size of the catalyst significantly influence the product distribution. The use of nanomaterials, such as nanoparticles, nanowires, and nanosheets, is gaining traction due to their enhanced surface area and catalytic activity. The modification of catalyst surfaces through doping or functionalization has also emerged as a promising approach to enhance selectivity and activity. Heteroatom doping, for instance, introducing nitrogen or boron atoms into carbon-based materials, has shown improvements in CO2 reduction performance. The choice of electrolyte, reaction conditions, and cell design also play a crucial role in the overall efficiency of the electrochemical CO2 reduction process. The development of metal-free catalysts is an active area of research, with carbon-based materials, such as graphene and carbon nanotubes, showing potential for efficient and selective CO2 reduction.
Methodology
The researchers synthesized pristine graphene quantum dots (p-GQDs) using a hydrothermal molecular fusion method with nitrated pyrene as the precursor. They then modified the p-GQDs through oxidation (o-GQDs) and reduction (r-GQDs) treatments to investigate the effect of surface functional groups on CO2 electroreduction. The oxidized GQDs were prepared by annealing p-GQDs in a dilute air stream, while the reduced GQDs were obtained by hydrothermal reduction in a reducing gas environment. The -NH2 functionalized GQDs (GQD-NH2-L and GQD-NH2-H) were synthesized by introducing different amino group precursors (NH3·H2O and N2H4) during the p-GQDs synthesis. Similarly, -SO3 functionalized GQDs (GQD-SO3) were prepared using Na2SO3 during the p-GQDs synthesis. The electrocatalytic activity and selectivity of these modified GQDs were evaluated in a custom-designed flow cell equipped with a gas diffusion electrode (GDE) containing the GQDs catalyst layer. The electrochemical CO2 reduction was performed in 1 M KOH electrolyte, with CO2 continuously supplied to the cathode compartment. Gas products (CH4, CO, H2) were analyzed using gas chromatography (GC), and liquid products were quantified using 400 MHz H NMR. Faradaic efficiency (FE) and partial current density (j) for each product were calculated. The structural and surface characterization of the GQDs was performed using transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. To understand the effect of surface functional groups on the electronic structure and catalytic activity of the GQDs, density functional theory (DFT) calculations were conducted to study the charge distribution and reaction mechanisms.
Key Findings
The study revealed a strong correlation between the type of surface functional group on the GQDs and their catalytic performance in CO2 electroreduction.
1. **Electron-Donating Groups Enhance CH4 Production:** GQDs functionalized with electron-donating groups (-OH, -NH2) exhibited significantly higher Faradaic efficiency (FE) and partial current density (j) for CH4 compared to pristine or electron-withdrawing group-functionalized GQDs. The highest CH4 FE (70%) was achieved with -OH and -NH2 functionalized GQDs at -200 mA cm⁻² partial current density.
2. **Electron-Withdrawing Groups Suppress CO2 Reduction:** GQDs functionalized with electron-withdrawing groups (-COOH, -SO3) showed considerably lower CO2 reduction activity and selectivity, favoring the competing hydrogen evolution reaction (HER).
3. **Correlation between EDG Content and CH4 Production:** The CH4 production activity demonstrated a positive correlation with the content of electron-donating groups (-OH, -NH2). Higher content of EDGs led to higher CH4 FE and j.
4. **Isotopic Labeling Confirms CO2 Origin of CH4:** Isotopic labeling experiments using ¹³CO2 confirmed that the CH4 produced originated from CO2 reduction and not catalyst decomposition.
5. **Role of Charge Density and Intermediate Stabilization:** The enhanced CH4 yield on EDG-functionalized GQDs was attributed to two main factors: (1) higher charge density of potential active sites (neighboring C or N atoms) maintained by EDGs, and (2) stabilization of key reaction intermediates through interaction with EDGs.
6. **DFT Calculations Support the Proposed Mechanism:** DFT calculations provided further support for the proposed mechanism, indicating that the EDGs modify the electronic structure of the active sites, leading to altered adsorption energies of reaction intermediates and a lower energy barrier for CH4 formation, particularly at specific nitrogen sites. The adsorption of *CO and *CH2O intermediates were crucial and these were stronger when the substrate was negatively charged.
7. **Long-Term Stability:** The -NH2 functionalized GQDs demonstrated good long-term stability, maintaining high CH4 FE and j for an extended period (10 hours).
Discussion
The findings of this study demonstrate the significant impact of surface functionalization on the catalytic performance of graphene quantum dots in CO2 electroreduction. The superior selectivity and activity of EDG-functionalized GQDs towards CH4 production, compared to both pristine and EWG-functionalized GQDs, highlights the importance of precisely controlling the electronic properties of the catalyst surface. The observation that stronger EDGs lead to enhanced CH4 production suggests a clear structure-activity relationship. This work significantly contributes to the field of electrochemical CO2 reduction by providing a deeper understanding of the role of surface functional groups in influencing catalytic activity and selectivity. The high FE and j achieved with the functionalized GQDs are comparable to state-of-the-art CH4-selective metal catalysts, showcasing the potential of these metal-free catalysts for industrial applications. Further research is needed to fully elucidate the reaction mechanism and explore other potential functional groups and carbon-based materials for achieving even higher efficiency and selectivity in CO2 electroreduction. The DFT calculations show the complexity of the mechanism but suggest a favoured reaction pathway for the CO2 to CH4 process.
Conclusion
This research successfully demonstrated the ability of surface-functionalized graphene quantum dots to achieve highly efficient and selective electrochemical CO2 reduction to CH4. The systematic investigation revealed a strong influence of electron-donating and electron-withdrawing groups on catalytic performance. Electron-donating groups significantly enhanced CH4 production, while electron-withdrawing groups suppressed CO2 reduction. This study provides crucial insights into designing carbon-based catalysts at a molecular level and opens avenues for developing more efficient and sustainable technologies for CO2 conversion.
Limitations
While this study provides compelling evidence for the role of surface functional groups in directing CO2 electroreduction selectivity, several limitations should be noted. The DFT calculations employed simplified models of the GQDs, which may not fully capture the complexity of the real catalytic system. Further investigation into the exact nature of the active sites and the detailed reaction mechanism using advanced characterization techniques is necessary. The study primarily focused on CH4 production; further research is needed to understand the influence of surface functional groups on the selectivity towards other valuable products. The long-term stability of the catalyst under industrial conditions requires further evaluation.
Related Publications
Explore these studies to deepen your understanding of the subject.