Single-atom catalysts (SACs) have emerged as a frontier in heterogeneous catalysis due to their maximized atomic efficiency and unique electronic properties. When applied to methane dry reforming (MDR), where CH4 and CO2 are converted into syngas (H2 + CO), oxide-stabilized Ni or Pt single atoms offer exceptional activity and coke resistance. CeO2 and Al2O3 serve as ideal supports due to their redox properties and Lewis acidity, respectively, which stabilize metal atoms and modulate reaction pathways. This article examines the synthesis, dynamic behavior, and long-term performance of these catalysts, with emphasis on coke suppression and syngas ratio stability.
**Synthesis via Strong Electrostatic Adsorption**
Strong electrostatic adsorption (SEA) is a precise method for anchoring single atoms onto oxide supports. For CeO2, the surface hydroxyl groups provide binding sites for cationic Ni or Pt precursors (e.g., [Ni(H2O)6]²⁺ or [PtCl6]²⁻). By adjusting the pH to the point of zero charge (PZC) of CeO2 (~6.5), electrostatic attraction ensures uniform deposition. Al2O3, with a higher PZC (~8–9), requires anionic precursors at lower pH. After adsorption, calcination at 500–600°C in air converts precursors into isolated metal-oxo species, while H2 reduction at 400°C generates metallic single atoms. Aberration-corrected HAADF-STEM confirms atomic dispersion, with Ni/Pt loadings typically below 1 wt% to prevent clustering. XANES spectra reveal oxidation states: Ni²⁺ or Pt²⁺ before reduction, transitioning to Ni⁰/Pt⁰ under reaction conditions.
**Dynamic Restructuring Under Reaction Conditions**
During MDR (700–800°C), operando studies show dynamic restructuring of single atoms. On CeO2, Ni atoms migrate to step edges or oxygen vacancies, forming transient Ni–Ce³⁺ interfaces that activate CO2 via redox cycling: Ce⁴⁺ + Ni⁰ ↔ Ce³⁺ + Ni²⁺. This mitigates carbon deposition by oxidizing CHx fragments. Pt on Al2O3 remains anchored at Lewis acid sites but exhibits electronic modulation, with charge transfer from Pt to Al2O3 enhancing CO2 dissociation. Quasi in situ XPS detects Ptδ+ (0 < δ < 2) under CO2 flow, indicating partial oxidation. The stability of these configurations hinges on the oxide’s ability to buffer lattice oxygen without phase change. CeO2’s oxygen storage capacity (OSC) is critical, with 10–15% Ce³⁺ content optimal for Ni stabilization.
**Coke Suppression Strategies**
Coke resistance is paramount for MDR catalysts. Three mechanisms dominate in oxide-stabilized SACs:
1. **Geometric confinement**: Single atoms limit C–C coupling by isolating CH4 decomposition sites. TEM after 100 h on stream shows negligible filamentous carbon on Ni/CeO2, contrasting with Ni nanoparticles which form 20–30 nm fibers.
2. **Redox buffering**: CeO2 donates lattice oxygen to gasify surface carbon. TPO profiles reveal coke combustion peaks below 400°C for Ni/CeO2, versus 550°C for Ni/Al2O3, indicating weaker carbon bonding.
3. **Electronic tuning**: Pt/Al2O3 weakens CO adsorption (DRIFTS shows ν(CO) at 2060 cm⁻¹ vs. 2085 cm⁻¹ on Pt nanoparticles), reducing Boudouard reaction (2CO → C + CO2) rates.
Adding 1–2% La to CeO2 further enhances OSC, lowering coke accumulation from 5 mgC/gcat·h (undoped) to <1 mgC/gcat·h. Similarly, sulfating Al2O3 increases Pt dispersion and suppresses CH4 cracking.
**H2/CO Ratio Stability Over Extended Operation**
Syngas ratio (H2/CO) reflects catalyst balance between CH4 decomposition (H2 producer) and CO2 activation (CO producer). For Ni/CeO2, the ratio stabilizes at 0.92–0.98 over 120 h at 750°C, close to the stoichiometric value (1.0). The consistency arises from CeO2’s ability to equilibrate surface oxygen, ensuring CO2 conversion tracks CH4 consumption. Pt/Al2O3 maintains H2/CO at 0.88–0.93 due to slightly favored CO2 dissociation. Deviations below 0.85 indicate onset of RWGS (CO2 + H2 → CO + H2O), observed when CeO2 is reduced excessively (Ce³⁺ > 25%).
Long-term tests reveal gradual deactivation modes:
- **Ni/CeO2**: Activity drops 8% after 100 h, linked to CeO2 sintering (BET surface area decreases from 120 to 95 m²/g).
- **Pt/Al2O3**: Loss of 12% conversion correlates with Pt aggregation (STEM detects 1–2 nm clusters), mitigated by pre-treating Al2O3 with phosphoric acid to anchor Pt.
Regeneration in 5% O2 at 600°C restores 95% of initial activity for both systems, confirming reversible deactivation.
**Conclusion**
Oxide-stabilized single-atom Ni/Pt catalysts combine high MDR activity with exceptional durability. CeO2’s redox activity and Al2O3’s acidic sites tailor metal-support interactions to suppress coke and maintain syngas ratios. SEA enables precise synthesis, while dynamic restructuring under reaction conditions ensures sustained performance. Future work may explore dopants (e.g., Zr in CeO2) or bimetallic single atoms to further enhance stability beyond 200 h. These systems exemplify the potential of SACs for industrial syngas production.