Auto-exhaust carbon particles (soot) are a significant source of air pollution, demanding efficient catalytic after-treatment. Platinum (Pt) and palladium (Pd) are currently the most effective catalysts, but their high cost drives the search for alternatives. Ruthenium (Ru) is a cheaper option, but its oxide volatility at high temperatures poses a challenge. Strong metal-support interactions (SMSIs) are explored to improve Ru-based catalyst stability. Single-atom catalysts (SACs), particularly precious metal SACs, offer exceptional atomic utilization and uniform active sites, making them attractive for deep oxidation reactions. The three-phase interface in soot oxidation (catalyst, soot, gases) necessitates careful design of SACs to ensure both high intrinsic activity and efficient soot-catalyst contact. Ceria (CeO₂) is a promising support due to its oxygen storage/release properties. This research focuses on synthesizing a single-atom Ru catalyst anchored on the surface lattice of single-crystal CeO₂ to address the limitations of existing Ru-based catalysts and provide a cost-effective alternative to Pt/Pd for soot purification.

Existing literature highlights the challenges in developing efficient and stable Ru-based catalysts for soot oxidation due to the volatility of Ru oxides at high temperatures. Studies have shown that strong interactions between Ru and CeO₂ can enhance catalytic activity and thermal stability. The use of single-atom catalysts (SACs) has gained significant attention due to their high atomic utilization and uniform active site structure. Research also emphasizes the importance of strong metal-support interactions (SMSIs) and the impact of interfacial sites on catalytic performance and stability. Ceria (CeO₂) has been identified as an excellent support material due to its oxygen storage and release capabilities. However, limited research exists on single-atom Ru anchored at the surface lattice of single-crystal CeO₂ for soot purification, which is the focus of this study.

The study employed a gas bubbling-assisted membrane deposition (GBMD) method for synthesizing the single-atom Ru catalyst (Ru₁/CeO₂). Nanoflower-like CeO₂ microspheres were used as the support material. A reference catalyst with Ru nanoparticles (Ruₙ/CeO₂) was prepared using the gas bubbling-assisted membrane reduction method. Various characterization techniques were used to analyze the catalysts' structure and properties, including inductively coupled plasma optical emission spectroscopy (ICP-OES), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), N₂ adsorption-desorption, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and aberration-corrected STEM. The catalytic activity was evaluated by soot temperature-programmed oxidation (soot-TPO) under loose contact conditions. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to investigate surface species during NO oxidation. Density functional theory (DFT) calculations were performed to understand the reaction mechanism. Isothermal soot oxidation experiments at 280 °C were used to calculate the turnover frequency (TOF). The apparent activation energy (Ea) was determined by the Coats-Redfern method. The stability of the catalysts was assessed using time-on-stream TOF measurements and cyclic soot-TPO tests.

ICP-OES analysis confirmed the Ru loading in Ru₁/CeO₂ (0.46 wt.%) and Ruₙ/CeO₂ (3.80 wt.%). XRD patterns showed only CeO₂ peaks, indicating high Ru dispersion. EXAFS data confirmed the single-atom nature of Ru in Ru₁/CeO₂, with a Ru₁O₅ coordination environment. XPS analysis showed that Ru species in Ru₁/CeO₂ are in a positively charged state, unlike Ruₙ/CeO₂ which exhibits both Ru⁰ and Ruⁿ species. CO-DRIFTS revealed distinct adsorption peaks for Ru₁/CeO₂, indicating single-atom Ru sites forming Ru-O-Ce bonds. STEM-ADF imaging and EDX mapping confirmed the atomic dispersion of Ru in Ru₁/CeO₂. The Ru₁/CeO₂ catalyst demonstrated superior catalytic performance for soot oxidation compared to Ruₙ/CeO₂ and a commercial Pt-based catalyst. The turnover frequency (TOF) for Ru₁/CeO₂ was 0.218 h⁻¹, significantly higher than Ruₙ/CeO₂ (0.023 h⁻¹). The apparent activation energy for Ru₁/CeO₂ (75.2 kJ mol⁻¹) was lower than for Ruₙ/CeO₂. In-situ DRIFTS revealed the formation of surface nitrate intermediates (NO₃⁻) crucial for NO oxidation. DFT calculations showed that Ru₁/CeO₂ exhibits stronger electronic interactions and facilitates the formation of NO₂ intermediates, which is the rate-determining step in NO oxidation. Cyclic stability tests showed that Ru₁/CeO₂ maintained its catalytic activity after six cycles, unlike Ruₙ/CeO₂, demonstrating its superior stability. The high CO₂ selectivity (>99%) and durability of the Ru₁/CeO₂ catalyst were also confirmed.

The superior performance of the single-atom Ru catalyst (Ru₁/CeO₂) can be attributed to the unique Ru₁O₅ coordination structure and the strong Ru-O-Ce interfacial charge transfer. This enhances the adsorption and activation of both NO and O₂, leading to the efficient formation of NO₂ intermediates that are essential for the NO₂-assisted soot oxidation mechanism. The DFT calculations support these findings, highlighting the lower activation energy for NO oxidation in Ru₁/CeO₂ due to the facilitated formation of NO₂. The results demonstrate the significant advantages of single-atom catalysts for soot oxidation, offering a pathway for the development of cost-effective and high-performance auto-exhaust catalysts. The high stability of the single-atom Ru sites contrasts sharply with the nanoparticle catalyst, highlighting the importance of atomic dispersion and strong metal-support interactions for catalyst longevity.

This study successfully synthesized a highly active and stable single-atom Ru catalyst for soot oxidation. The Ru₁/CeO₂ catalyst exhibited superior performance compared to Ru nanoparticle and commercial Pt-based catalysts, offering a cost-effective alternative for auto-exhaust purification. Future research could explore different support materials and single-atom catalyst designs to further optimize catalytic activity and stability. Investigating the influence of different reaction conditions and exploring potential synergistic effects with other catalytic components also present promising research avenues.

The study primarily focused on soot oxidation under loose contact conditions, which might not fully represent the tight contact conditions in a real-world catalytic converter. The DFT calculations were performed on simplified models, and further investigation might be needed to fully capture the complexity of the reaction mechanism. Long-term stability testing over extended periods might provide additional insights into the long-term durability of the catalyst.