Effective catalytic oxidation of CO, hydrocarbons (HCs), and nitrogen oxides (NOx) at low temperature is crucial for ultra-clean and energy-efficient emission control across various energy systems, including mobile and stationary sources. Existing catalytic devices, such as diesel oxidation catalysts (DOCs), three-way catalysts (TWCs), selective catalytic reduction (SCR) catalysts, and lean NOx traps (LNTs), often struggle with low-temperature efficiency. Improved low-temperature solutions could significantly enhance the energy efficiency and environmental friendliness of technologies like low-temperature combustion regime engine technology. While gold, metal oxides (like Co3O4, Co-Cu-Ce mixed oxides, and La-based perovskites), have shown promise, they often suffer from poor hydrothermal stability and susceptibility to sulfur poisoning. Recently, supported platinum-group metal (PGM) single-atom catalysts (SACs) have emerged as a potential solution due to their high metal dispersion, fewer active site types, low-coordination environments, quantum size effects, and enhanced metal-support interactions. However, a major challenge remains in improving the stability and maintaining the high activity of these SACs, as isolated metal atoms tend to aggregate. Previous research, while demonstrating some success with CO oxidation, has been limited to model reactions and lab-scale reactors, lacking real-world evaluation in field-size catalytic converters under realistic exhaust conditions. This study aims to address these limitations by developing and testing a durable single-atom Pt catalyst integrated into a full-size honeycomb monolith, evaluating its performance under simulated diesel exhaust conditions and assessing its long-term stability.

The search for effective low-temperature oxidation catalysts has led to investigations of various materials. Gold catalysts have shown promising results, but their performance is often limited. Metal oxides, such as Co3O4 and Co-Cu-Ce mixed oxides, and La-based perovskites have also been explored, but they generally lack hydrothermal stability and are susceptible to sulfur poisoning. The recent focus on supported platinum-group metal (PGM) single-atom catalysts (SACs) offers a new avenue for improving low-temperature catalysis due to their unique properties. However, challenges related to the stability and activity of these SACs remain. Studies on SACs have primarily concentrated on model reactions like CO oxidation and water-gas shift, with limited success in hydrocarbon oxidation at low temperatures. Furthermore, most research has been conducted on a laboratory scale, lacking the crucial step of evaluating these catalysts in real-world, field-size reactors under realistic exhaust conditions, which is essential for translating scientific advancements into practical applications. Previous work by Nie et al. demonstrated highly active and stable Pt SACs on CeO2, exhibiting good CO oxidation performance; however, deactivation was observed at higher temperatures. This highlights the need for catalysts that demonstrate superior stability and activity in more demanding conditions.

This study utilized a novel approach to create a durable single-atom Pt catalyst. The process begins with the growth of a densely packed forest of mesoporous rutile TiO2 nanowire arrays (NAs) directly onto the channel surfaces of full-size cordierite honeycombs. The TiO2 loading was approximately 23 wt.%. The nanowires, approximately 10-20 nm wide, were arranged in bundles of 50-100 nm width. The mesoporous nature of the TiO2 NAs was confirmed by HAADF STEM imaging and BET surface area measurements, which showed a high surface area of ~89.6 m²/g. A thin layer of mesoporous SiO2 formed during the TiO2 growth process due to leaching from the cordierite substrate. Platinum was then loaded onto the TiO2 NAs using either microwave-assisted dip-coating or Na-promoted wet incipient impregnation (WII). Aberration-corrected (ac) HAADF STEM imaging confirmed the successful atomic dispersion of Pt on the TiO2 nanowire surface. High Pt dispersions (up to 80%) were achieved by H2 chemisorption. The mechanical stability of the TiO2 NAs on the honeycomb substrate was evaluated using sonication, demonstrating improved adherence compared to conventional washcoated samples. To evaluate the catalyst's durability, hydrothermal aging at 700 °C for 100 h was performed. In situ X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) computations were used to investigate the structure and reactivity of the Pt single atoms on the TiO2 nanowire surfaces. The catalytic oxidation activity was assessed under simulated diesel exhaust conditions (CDC and LTC-D protocols from US DRIVE) using a continuous flow reactor and a customized plug-flow reactor system. Tests were conducted at a high gas hourly space velocity (GHSV) of 60,000 h⁻¹. Sulfur-poisoning effects were evaluated by exposing the catalyst to SO2, followed by a de-sulfation step. Finally, the scalability of the catalyst was demonstrated by testing a full-size monolith under highly transient conditions mimicking a heavy-duty diesel (HDD) federal test procedure.

The single-atom Pt catalyst supported on the TiO2 nanowire array exhibited exceptional low-temperature activity for CO and hydrocarbon oxidation. A 90% conversion was achieved at temperatures as low as ~160 °C under simulated diesel exhaust conditions, significantly outperforming a commercial DOC with five times the PGM loading. The catalyst demonstrated remarkable stability after hydrothermal aging at 700 °C for 100 h, retaining its single-atom Pt dispersion and catalytic activity. XAS, XPS, and DFT calculations revealed that the strong electrostatic interaction between the Pt single atoms and the TiO2 nanowire surface, particularly at Ti vacancy sites, was responsible for the exceptional hydrothermal stability. In situ DRIFTS studies showed that the CO oxidation on this catalyst favored the Eley-Rideal (ER) mechanism with a low activation energy. Comparison with Pt nanoparticle catalysts demonstrated the superior activity of the single-atom Pt catalyst. The catalyst also exhibited good NO-to-NO2 conversion, benefiting downstream NOx reduction. The sulfur tolerance testing showed only a slight deactivation after SO2 exposure, with low-temperature propane oxidation even improving. Finally, the full-size catalyst monolith showed excellent performance under highly transient conditions mimicking heavy-duty diesel engine operation, maintaining its activity even after aging.

The results of this study demonstrate a significant advancement in low-temperature diesel oxidation catalysis. The exceptional performance of the single-atom Pt catalyst on the TiO2 nanowire array is attributed to the synergistic effect of the highly dispersed and isolated Pt atoms and the unique hierarchical structure of the support. The combination of the high reactivity of single-atom Pt, favored ER mechanism and the excellent mass transport properties of the TiO2 nanowire array led to superior low-temperature activity and stability compared to traditional catalysts. The strong metal-support interaction, as evidenced by XAS and DFT calculations, ensures the stability of the single-atom Pt sites even under harsh hydrothermal conditions. The findings address the long-standing challenge of developing durable and highly active low-temperature oxidation catalysts for diesel emission control. The improved sulfur tolerance further enhances the practical applicability of this catalyst. This work opens up new opportunities for designing and developing highly efficient and environmentally friendly catalysts for various applications beyond diesel oxidation.

This research successfully demonstrated a highly active and durable single-atom Pt catalyst for low-temperature diesel oxidation. The catalyst, supported on a mesoporous rutile TiO2 nanowire array grown on a full-size honeycomb monolith, achieved 90% CO and hydrocarbon conversion at temperatures as low as 160 °C with significantly reduced PGM usage. The excellent hydrothermal and sulfur tolerance properties highlight the potential for practical applications. Future research could focus on exploring other single-atom catalysts and support materials to further enhance performance and expand the range of applications.

While the study demonstrates excellent performance under simulated conditions, further testing under real-world engine exhaust conditions is necessary to fully validate the catalyst's long-term durability and effectiveness. The precise quantification of turnover frequencies (TOFs) is challenging due to the complex flow conditions and steep light-off curves. Additional research is also needed to explore the effects of other exhaust components and potential deactivation mechanisms under prolonged operation.