Research progress on low-concentration methane oxidation using palladium-based catalysts

. There is low concentration (0.05% to 0.5%) of CH4 leakage in marine natural gas engine exhaust. It is difficult to capture and utilize low concentration CH4 because of its stable chemical structure and high ignition temperature. Catalytic oxidation is the main method to remove low concentration CH4 from tail gas. The noble metal palladium (Pd) is the best catalytic material for the complete oxidation of low concentration CH4. In this paper, the main research progress of Pd-based catalysts is reviewed for their low-temperature activity, reliability and development cost. Initially, the impact of Pd particle dimensions and valence distribution, Pd dispersion, carrier identity and strong metal-carrier interaction (SMSI) on the catalyst's methane oxidation activity was elucidated. Then, the mechanism of methane oxidation on the surface of Pd-based catalysts is summarized; In addition, the deactivation mechanisms of Pd-based catalysts, such as high temperature sintering, water poisoning and sulphur poisoning, are described in detail. In conclusion, the major hurdles encountered in achieving full oxidation of methane at low concentrations are outlined, along with the future development direction for methane oxidation catalysts. Additionally, strategies to enhance the performance of Pd-based catalysts are briefly suggested. The modified Text maintains a clear, professional, and concise style with minimal changes.


Introduction
International shipping greenhouse gas emission control is an important part of global greenhouse gas emission reduction, and is also a common responsibility and challenge faced by the global shipping industry. Compared with conventional fuel power, the use of liquefied natural gas (LNG) can achieve an immediate carbon reduction of more than 20% over the entire industry chain cycle, while reducing more than 85% of NOx emissions and 99% of SO x emissions. The escape of unburned CH 4 (about 0.05%-0.5% concentration) caused by insufficient combustion in the cylinder of natural gas engines has become one of the most important sources.

Low concentration methane catalytic oxidation technology
The core of catalytic oxidation technology is to obtain a highly active and reliable catalyst for the target reaction. The following technical challenges are usually faced.
• Problem of catalyst activity at low temperature.
• Problem of catalyst reliability under harsh working conditions. • Problem of expensive catalyst development.
Ford developed Pd/Rh catalyst for the first time in 1989. Compared with other precious metal catalysts, PDbased catalysts not only have high economy, excellent heat resistance and sintering property, but also have better oxidation efficiency for hydrocarbons [1]. Research on PD-based catalysts focuses on solving such problems as low temperature ignition of CH4, inhibition inactivation of H 2 O, CO or SO x poisoning, high temperature stability and durability, and cost [2].

Effect of Pd particle size distribution
The change of Pd particle size will affect exposed crystal surfaces and unsaturated sites. Moreover, the bond energy between Pd and O is changed, which reflects the structural sensitivity. Hicks [3] first explored the influence of Pd particle size on the catalytic oxidation process of catalyst. Studies have shown that compared with the catalyst, the unit activity check point turnover frequency (TOF) of Pd particles with 4.0mm particle size is higher, and the apparent activation energy is almost the same. Stakheev [4] found that when Pd particle size increased from 1nm to 20nm under lean combustion conditions, TOF of the catalyst increased linearly. This further confirmed the structural sensitivity of methane oxidation in terms of Pd particle size under oxygen-enriched conditions. PdO particles with large particle size has better catalytic performance.

Effects of Pd dispersion and valence distribution
The dispersion state and valence state distribution on the surface are related to reaction activity. Huang [5] prepared La-Al 2 O 3 catalyst modified by Ce-Zr by the initial wet impregnation method. It was found that Pd and PDO can be used as active check point of CH 4 oxidation, while PDO only plays a leading role under rich combustion conditions. Murata [6] pointed out that the core-shell structure formed Pd@PDo metal oxide support has the best activation effect on CH 4 .

Effect of carrier on catalytic activity
The properties of carrier also affect the oxidation process of CH 4 . Studies by Yoshida [7] show that Pd species are more reactive than Pd species on strong acids and basic oxides, as shown in Figure 1. The exposed crystal surface of carrier also affects the oxidation performance of CH 4 . Chen [8] synthesized CeO 2 carriers with nano-rod-like, cube and octahedral morphology and further loaded Pd. The result is shown in Figure 2.

Effect of strong metal-support interaction
SMSI is the focus of catalyst design. SMSI usually involves such processes as charge transfer between surface interface, species overflow, carrier coating of metals, and component morphological changes, as shown in FIG. 3 [9].

Study on Methane Oxidation Mechanism of Pd-based Catalyst
Eley-Rideal and Langmuir-Hinshelwoo models are the earliest known mechanisms for CH 4 oxidation of PD-based catalyst. Pd species on the catalyst surface first adsorbed CH 4 and O 2 at the active check point following Langmuir-Hinshelwoo mechanism, and then catalyzed the oxidation of CH 4 [10], which is also a generally accepted reaction process at present. The mechanism is shown in Figure 4.

Mechanism of high temperature deactivation
Although using SMSI effect between Pd and CeO 2 can effectively inhibit the sintering agglomeration of Pd species, SMSI effect may also initiate the deactivation of the catalyst. The mode of deactivation associated with SMSI effect may be electronic effect [10]. The mechanism is shown in Figure 5.

Mechanism of water intoxication
Cullis [7] proposed that surface hydroxyl groups may inhibit active check point. Another study by Ciuparu [7] found that accumulation of surface Pd(OH) 2 would interfere with the migration process of surface reactive oxygen species, as shown in Figure 6.

Mechanism of sulfur poisoning
Catalysts can be poisoned and deactivated by long-term exposure to sulfur-containing exhaust gases. It has been confirmed that SO x in the exhaust gas can lead to the sulfurization of catalyst support or active components under the condition of dilute combustion above 200℃ [6], as shown in Figure 7.

Conclusion
Pd-based metal oxides is the most widely used catalyst for CH 4 oxidation. Low-cost design and development of more catalysts with low temperature and high efficiency, excellent water resistance and sulfur resistance, and high durability are the key research and development directions in the future. Therefore, it is necessary to continue to design the catalyst structure at the molecular level.