Effect of Support on CO Oxidation Performance over the Pd/CeO2 and Pd/CeO2–ZrO2 Catalyst

Recent advances in noble metal-based catalysts for CO oxidation

Carbon monoxide, one of the major pollutants in the air, is mainly produced due to the incomplete combustion of fossil fuels such as coal and oil. Among all the techniques developed for removing CO, catalytic oxidation has been considered one of the most effective approaches, and the commonly used catalysts include metal oxides and noble metals. Noble metal attracted extensive attention due to its good catalytic performance at low temperatures and high resistance to poisoning. The review summarizes the recent advances of noble metals including Pt, Pd, Au, Ru, Rh, and Ir in CO oxidation. The effect of support, metal doping, the particle size of noble metals, and the hydroxyl groups on CO oxidation is discussed. Besides, the metal-support interaction on the stability and activity is also involved in this review. Finally, the challenges and opportunities of supported noble metal catalysts in practical CO oxidation are proposed.

Macropore-Size Engineering toward Enhancing the Catalytic Performance of CO Oxidation over Three-Way Catalyst Particles

In recent years, transportation-related air pollution has escalated into a global concern, necessitating the development of a three-way catalyst (TWC) technology to address harmful emissions. However, the efficiency of TWC's performance in mitigating these emissions has been hindered because of limited mass transfer efficiency within their structures. Thus, this study attempted to overcome the existing issue by synthesizing a series of macroporous TWC particles exhibiting various macropore sizes via a template-assisted spray process, aiming to achieve optimal mass transfer efficiency and catalytic performance. The synthesis incorporated various template particles (size of 67-381 nm) to obtain various macroporous structures. Thereafter, these macroporous particles were assessed for their carbon monoxide (CO) oxidation performance, revealing a substantial influence of the macropore size on the catalytic performance of TWC structures. Interestingly, among the investigated samples, those containing the smallest and largest macropores demonstrated the highest CO oxidation performances. Based on these results, a plausible reactant diffusion mechanism was proposed to explain the effect of the macropore size on the diffusion efficiency within the macroporous structures. This work may have significant implications in optimizing the macroporous structure to enhance catalytic performance in the gas purification process.

Progress on metal-support interactions in Pd-based catalysts for automobile emission control

The interactions between metals and oxide supports, so-called metal-support interactions (MSI), are of great importance in heterogeneous catalysis. Pd-based automotive exhaust control catalysts, especially Pd-based three-way catalysts (TWCs), have received considerable research attention owing to its prominent oxidation activity of HCs/CO, as well as excellent thermal stability. For Pd-based TWCs, the dispersion, chemical state and thermal stability of Pd species, which are crucial to the catalytic performance, are closely associated with interactions between metal nanoparticles and their supporting matrix. Progress on the research about MSI and utilization of MSI in advanced Pd-based three-way catalysts are reviewed here. Along with the development of advanced synthesis approaches and engine control technology, the study on MSI would play a notable role in further development of catalysts for automobile exhaust control.

Fabrication and catalytic properties of nanorod-shaped (Pt-Pd)/CeO2 composites

Nanorod-supported (Pt-Pd)/CeO₂ catalysts were synthesized by a simple method of dealloying Al_91.7Ce₈ Pt _X Pd_0.3-X (X = 0, 0.075, 0.1, 0.15, 0.2, 0.3) alloy ribbons. SEM and TEM characterization implied that after calcination treatment, the achieved resultants exhibited interspersed nanorod structures with a rich distribution of nanopores. Catalytic tests showed that the (Pt_0.1-Pd_0.2)/CeO₂ catalyst calcined at 300 °C exhibited the highest catalyst activity for CO oxidation when compared with other catalysts prepared at different noble metal ratios or calcined at other temperatures, whose complete reaction temperature was as low as 100 °C. The outstanding catalytic performance is ascribed to the stable framework structure, rich gas pathways and collaborative effect between the noble Pt and Pd bimetals.

Enhanced catalytic performance of atomically dispersed Pd on Pr-doped CeO2 nanorod in CO oxidation

Single-atom noble metal catalysts have been widely studied for catalytic oxidation of CO. Regulating the coordination environment of single metal atom site is an effective strategy to improve the intrinsic catalytic activity of single atom catalyst. In this work, single atom Pd catalyst supported on Pr-doped CeO₂ nanorods was prepared, and the performance and nature of Pr-coordinated atomic Pd site in CO catalytic oxidation are systematically investigated. The structure characterization using AC-HAADF-STEM, EXAFS, XRD and Raman spectroscopy demonstrate the formation of single atom Pd site and abundant surface oxygen vacancies on the surface of Pr-doped CeO₂ nanorod. With the combination of the XPS characterization and DFT calculations, the oxidation state of Pd on Pr-doped CeO₂ nanorod is determined lower than that on CeO₂ nanorod. The turnover frequency of CO oxidation is markedly increased from 8.4 × 10^-3 to 31.9 × 10^-3 s with Pr-doping at 130 ºC and GHSV of 70,000 h^-1. Combined with kinetic studies, DRIFT and DFT calculations, the doped-Pr atoms reduced the formation energy of oxygen vacancies and generate more oxygen vacancies around the atomically dispersed Pd sites on the surface of cerium oxide, which reduces the dissociation energy of oxygen, thereby accelerating the reaction rate of CO oxidation.

Transformation of Highly Stable Pt Single Sites on Defect Engineered Ceria into Robust Pt Clusters for Vehicle Emission Control

Engineering surface defects on metal oxide supports could help promote the dispersion of active sites and catalytic performance of supported catalysts. Herein, a strategy of ZrO₂ doping was proposed to create rich surface defects on CeO₂ (CZO) and, with these defects, to improve Pt dispersion and enhance its affinity as single sites to the CZO support (Pt/CZO). The strongly anchored Pt single sites on CZO support were initially not efficient for catalytic oxidation of CO/C₃H₆. However, after a simple activation by H₂ reduction, the catalytic oxidation performance over Pt/CZO catalyst was significantly boosted and better than Pt/CeO₂. Pt/CZO catalyst also exhibited much higher thermal stability. The structural evolution of Pt active sites by H₂ treatment was systematically investigated on aged Pt/CZO and Pt/CeO₂ catalysts. With H₂ reduction, ionic Pt single sites were transformed into active Pt clusters. Much smaller Pt clusters were created on CZO (ca. 1.2 nm) than on CeO₂ (ca. 1.8 nm) due to stronger Pt-CeO₂ interaction on aged Pt/CZO. Consequently, more exposed active Pt sites were obtained on the smaller clusters surrounded by more oxygen defects and Ce³⁺ species, which directly translated to the higher catalytic oxidation performance of activated Pt/CZO catalyst in vehicle emission control applications.

Catalytic oxidation of CO on noble metal-based catalysts

Carbon monoxide (CO) catalytic oxidation has gained increasing interest in recent years due to its application prospects. The noble metal catalysts commonly exhibit outstanding CO catalytic oxidation activity. Therefore, this article reviewed the recent research on the application of noble metal catalysts in the catalytic oxidation of CO. The effects of catalyst support, dopant, and physicochemical properties on the catalytic activity for CO oxidation are summarized. The influence of the presence of water vapor and sulfur dioxide in the reaction atmosphere on the catalytic activity in CO oxidation is emphatically discussed. Moreover, this paper discussed several reaction mechanisms of CO catalytic oxidation on noble metal catalysts. Finally, the challenges of removing CO by catalytic oxidation in practical industrial flue gas are proposed.

Comprehensive study of the light-off performance and surface properties of engine-aged Pd-based three-way catalysts

Pd particles with a high fraction of corner sites on low-index facets are highly active for three-way catalyst reactions.