Zeolite Types Used in Catalytic Methane Combustion

Version	Summary	Created by	Modification	Content Size	Created at	Operation
1		Prof. Hongxing Dai	--	4797	2023-09-07 13:01:53	\|
2	format change	Peter Tang	-1 word(s)	4796	2023-09-08 04:28:29	\|

This entry is adapted from the peer-reviewed paper 10.3390/catal13091251

The emission of methane leads to the increase in the methane concentration in the atmosphere, which not only wastes resources but also intensifies the greenhouse effect and brings about serious environmental problems. Catalytic combustion can completely convert methane into carbon dioxide and water at low temperatures. However, the catalytic activities of the conventional supported palladium catalysts (e.g., Pd/Al₂O₃ and Pd/ZrO₂) are easy to decrease or the two catalysts can even be deactivated under actual harsh reaction conditions (high temperatures, steam- and sulfur dioxide-containing atmospheres, etc.). Noble metal catalysts supported on zeolites with ordered pores and good thermal stability have attracted much attention.

methane combustion zeolite supported noble metal catalyst palladium-based catalyst

1. Introduction

As an attractive energy source, methane is more economical and friendly to the atmosphere than the other fossil fuels, and furthermore, methane possesses a high energy density with a high H/C ratio. Natural gas (its main component is methane) can be used as a fuel in compressed natural gas (CNG) vehicles, and its combustion in CNG vehicles leads to lower emissions of CO₂, CO, non-CH₄ hydrocarbons, and NOx than that in traditional diesel or gasoline vehicles. It has been reported that the above emissions could be significantly reduced if the energy source was replaced by natural gas ^[1]^[2]. With a higher fuel combustion efficiency, lean-burn natural gas vehicles (NGVs) emit up to 21% less greenhouse gas than their gasoline or diesel counterparts. Complete oxidation of methane to CO₂ can effectively reduce its environmental impact by more than 96% as compared with the direct emission of methane to the atmosphere. Up to now, it has been well-recognized that CH₄ plays an indispensable role in the paradigm shift to a more sustainable planet. However, natural gas engines still emit significant amounts of unburnt CH₄ (so-called methane slip) in stationary applications (power plants). The lifetime of CH₄ in the atmosphere is much shorter than CO₂, but CH₄ is more efficient than CO₂ in trapping radiation. In the past 100 years, the global warming potential of CH₄ has been reported to be 28–36 times higher than that of CO₂ ^[3]^[4]^[5].

Catalytic combustion of CH₄ provides an alternative pathway to its thermal combustion. Under the fuel-lean conditions, due to the overheating of methane combustion, more nitrogen oxides are produced. Due to the high temperatures generated during the methane combustion process under the fuel-lean conditions, more nitrogen oxides are produced. For the combustion of low-concentration methane, the emergence of adsorption enrichment technology provides another pathway for the comprehensive utilization of lean methane. Membrane separation and pressure swing adsorption technologies are mainly used in practice, which can enrich the methane concentration. However, it is not widely used in industry because the costs of operation and preparation exceed the economic benefit of increasing the concentration. An ideal catalyst design should take into consideration the low-temperature operation and long-term stability ^[6]. The low operation temperature is important in minimizing the generation of the secondary pollutants emissions (e.g., NO_x) in CH₄ combustion or making the system achieve low CH₄ emission standards for the removal of CH₄.

Up to now, there have been a large number of catalysts (transition metal oxides, mixed metal oxides (e.g., ABO₃, A₂BO₄, and A₂B₂O₇), (conventional supports or zeolites)-supported noble metals, and hexaaluminates (MAl₁₂O₁₉)) developed in the literature. The noble metal-based catalysts used in methane combustion are mainly Pd, Pt, Rh, and Au. Generally speaking, their activity sequence is Pd > Pt > Rh > Au. Among them, Rh is expensive and easy to volatilize and lose at high temperatures, while Au has the worst activity for methane combustion and is generally used as a cocatalyst, such as Pd-Au, Pt-Au, etc. Pd shows the highest activity and light-off activity at low temperatures, and it is not volatile at high temperatures. Moreover, Pd-based catalysts exhibit better catalytic performance for the combustion of alkanes (C₃₊). In addition, some non-Pd-based catalysts are discussed later. Compared with the other noble metal catalysts, supported Pd catalysts are the most widely and deeply studied. Although the microscopic mechanism is still unclear, we can get a glimpse of catalytic methane combustion with the development of more advanced characterization methods and theoretical computational chemistry. Up to now, various catalysts have been developed to promote CH₄ combustion, including platinum group metals (PGM), transition metal (no-PGM) oxides, perovskite-type or -like oxides, and hexaaluminates ^[7]^[8]^[9]^[10]^[11]^[12]^[13]. To date, there have been some review articles on catalytic CH₄ combustion in the literature ^[3]^[7]^[14], which summarized the catalyst choice and their properties, support effects, active center nature, reaction kinetics, water resistance, and sulfur dioxide-poisoning mechanism, as well as the impact of the operating conditions. Among all the reported catalysts, platinum group catalysts show the highest intrinsic activities at low temperatures and can achieve a nearly 100% CH₄ conversion below 600 °C. However, the reaction rates of transition metal oxide catalysts are generally lower by 3–5 orders of magnitude, and methane conversion can reach 100% only at an operating temperature above 600 °C.

In the past decades, there has been a tremendous interest in developing high-performance low-temperature CH₄ oxidation catalysts ^[15]^[16], and the Al₂O₃-supported Pd-based materials remain one of the highly efficient catalysts. Of course, Pt- and Rh-based catalysts are also considered to be excellent candidates for catalytic methane combustion. Although Pt is typically doped in commercial catalysts to improve the thermal stability and sulfur dioxide resistance, Pd is the major catalytic active site. The big challenge is that H₂O (typical concentration = 5–15 vol%) and SO₂ (typical concentration = 1–30 ppm) in the exhaust gas results in severe deactivation of the catalysts that decrease the CH₄ removal efficiency. Many available supports have been reported, such as Al₂O₃ ^[17]^[18]^[19]^[20]^[21]^[22]^[23], CeO₂ ^[24]^[25]^[26]^[27], Ce_1−xZr_xO₂ solid solutions ^[28]^[29]^[30], and SnO₂ ^[31]^[32]^[33]^[34]. Zeolites with various additives have also been used in methane combustion. Each type of support possesses some beneficial features. For instance, Al₂O₃ has a relatively low cost and a large specific surface area. CeO₂- and Ce_1−xZr_xO₂-supported catalysts exhibit a large oxygen storage—release capacity due to the presence of the Ce⁴⁺/Ce³⁺ redox couple, while doping Zr into the lattice of CeO₂ to form a Ce-Zr solid solution can strongly improve the thermal stability of the catalyst. Zeolite-supported catalysts demonstrate better water tolerance than other compounds-supported catalysts. Of course, each of these catalysts possesses some obvious advantages and disadvantages. The noble metal components are generally supported on various metal oxides (e.g., Al₂O₃, CeO₂, ZrO₂, Co₃O₄, and SiO₂) for lean methane combustion. For example, the Pd/CeO₂ and Pd/Ce_xZr_1−xO₂ catalysts exhibited a better performance at low temperatures than other supported noble metal catalysts due to the support of CeO₂ or CexZr_1−xO₂ possessing the Ce⁴⁺/Ce³⁺ redox couple that could indirectly affect the performance of the catalyst, while doping Zr to the lattice of CeO₂ to generate a Ce-Zr-O solid solution could strongly improve the thermal stability of the catalyst ^[28]^[29]. Willis et al. ^[35] used SiO₂ (inert), Al₂O₃ (acidic), Ce_0.8Zr_0.2O₂ (redox), and MgO (alkaline) as support materials and found that, although the above oxide-supported Pd catalysts possessed similar activation energies for methane combustion, the activity of the MgO-supported Pd catalyst was obviously low. The authors believed that basic MgO enhanced the stability of the electron-rich PdO phase, which led to the increase in energy required to form oxygen vacancies and the decrease in the methane reaction rate. In addition, MgO could easily adsorb CO₂ to generate magnesium carbonate, which covered the active sites, influencing the surface of the Pd particles or the interface between the Pd and the carrier, and hence giving rise to a decreased catalytic activity.

It is well-known that low-temperature CH₄ oxidation over Pd-based catalysts follows a Mars—van Krevelen (MvK) redox mechanism. Garbowski et al. investigated the physicochemical properties of the Pd/γ-Al₂O₃ catalyst by combining nano-diffraction, electron microscopy, and Fourier transform IR (FT-IR) analysis ^[36]. The authors proved that the MvK reaction mechanism was suitable for explaining the kinetic behavior of CH₄ oxidation. Over the supported noble metal (e.g., Pd and Pt) catalysts, CH₄ is first adsorbed on the surface of the active nanoparticles (NPs) and dissociated into the adsorbed methyl species. Because methane has a highly symmetrical tetrahedral structure with a C–H binding energy of 435 kJ/mol, the first step of bond breaking is the rate-limiting step ^[37]. Under the action of oxygen, intermediate species (such as HCHO and carbonate) are generated and finally converted to CO₂ and H₂O as products. The catalytic activity of methane oxidation is controlled by different oxygen species, among which lattice oxygen (O_latt) and surface oxygen (O_sur) are the main oxygen species. In the low-temperature stage of CH₄ oxidation, the increase in the amount of the O_sur species can greatly enhance the catalytic activity of the catalyst, while in the high-temperature stage of CH₄ oxidation, the increase in the amount of the O_latt species can greatly improve the catalytic activity of the catalyst. In this process, the adsorption and desorption between CH₄ and CH₄^* are very fast processes, and at the same time, the gas-phase oxygen in this process combines with the catalyst to generate a series of O_sur and O_latt species, so the rapid transformation of CH₄^* and [O] is a critical factor in the catalytic combustion of low-concentration CH₄.

As pointed out above, Pd-based catalysts are thought to be the most active for combustion of CH₄ under lean conditions. It is known that the activity of the Pd/Al₂O₃ catalyst is seriously inhibited by water vapor and its gradual deactivation takes place. Promotion brought about by suitable supports has been reported. A support is important for enhancing the catalytic activity and stability of a supported noble metal catalyst. In comparison to γ-Al₂O₃, α- and θ-Al₂O₃ could offer weaker Pd-Al₂O₃ interactions that helped to expose more Pd step sites ^[38]. Upon increasing the calcination temperature, the crystalline structure of ZrO₂ was transformed from the mixed phase (monoclinic and tetragonal) to the pure tetragonal phase, and the tetragonal ZrO₂-supported Pd catalysts exhibited good hydrothermal stability ^[39]. Moreover, doping part of the Zr to the lattice of CeO₂ to form a Ce_1−xZr_xO₂ solid solution can greatly improve the thermal stability of the Ce_1−xZr_xO₂-supported noble metal catalyst. Although such a deactivation is reversible to some extent, it affects the normal working status of the catalyst in a specific environment. Schwartz et al. thought that H₂O poisoning of the supported Pd catalyst was caused by the adsorbed OH species, which covered the catalyst surface to form the stable and inactive Pd(OH)₂ phase ^[40]. However, the intrinsic CH₄ combustion activity of Pd/SiO₂ is much lower than that of Pd/Al₂O₃. Different from Pd/SiO₂, Pd/Al₂O₃ experienced severe low-temperature steam poisoning when it was exposed to an excessive steam atmosphere. As a unique class of microporous and crystalline solids, aluminosilicate zeolites have hence been considered to be one possible selection for decreasing the negative effect of water on CH₄ combustion. Aluminum oxide and zirconium oxide are usually used as supports for loading the noble metals in methane combustion. In addition to aluminum oxide and zirconium oxide, zeolites are also a kind of important support used for loading the noble metals.

Zeolites are an attractive class of shape-selective materials with regular microporous aluminosilicates, controlled acidity, and good (hydro)thermal stability. Such microporous materials have been used in adsorption and separation, petroleum refining, energy and chemical industries, environmental protection, and other fields because of their structural characteristics (pore structures and pore sizes), specific adsorption performance of small molecules, and high specific surface areas ^[41]. The framework illustrations of the support types of the main zeolites and some mesoporous silicas were used for the complete combustion of methane here, as shown in Table 1 and Figure 1. Zeolites are also effective catalytic materials, although they cannot catalyze methane combustion independently. For this reason, zeolites are often used as supports for dispersing metal NPs to increase functionality. Complex strategies are also developed to prepare composite zeolites with core-shell structures. Specifically, the design and development of zeolite-supported metal catalysts (e.g., precious metals) and NPs doped in zeolites have strengthened the application of zeolites in CH₄ activation.

Figure 1. Framework images of (a) CHA, (b) FAU, (c) LTA, (d) MFI, and (e) BEA (obtained from the IZA-SC).

Table 1. Structural properties of zeolites with different topologies and mesoporous silicas.

Framework Type	Material Name	Dimensionality	Ring Type (Pore Type)	Limiting Pore Size (nm)
CHA	SSZ-13	3D	8/6/4 (micropore)	0.38 × 0.38
LTA	LTA	3D	8/6/4 (micropore)	0.41 × 0.41
MFI	ZSM-5	3D	10/6/5/4 (micropore)	0.53 × 0.56
BEA	Beta	3D	12/6/5/4 (micropore)	0.66 × 0.67
FAU	X, Y	3D	12/6/4 (micropore)	0.33 × 0.33
SBA-15	-	1D	-/-/- (mesopore)	4.6–10.0 ^[42]
MCM-41	-	1D	-/-/- (mesopore)	10.0 ^[43]
KIT-6	-	3D	-/-/- (mesopore)	4.0–12.0 ^[44]

Note: The information for the microporous zeolites is obtained from the “Database of zeolite structures” by the Structure Commission of the International Zeolite Association (IZA-SC), whereas the information for the mesoporous zeolites is cited from the references as noted.

2. Small-Pore Zeolites with 8 Member Rings

SSZ-13 is a typical CHA-structured molecular sieve with small pores and eight member rings, which has a large specific surface area and an abundant acid amount. Zeolites with eight member rings have a high potential to inhibit the aggregation of metal cations under reaction conditions due to their small pore sizes (0.38 nm × 0.38 nm) and good hydrothermal stability compared with the medium- and large-pore zeolites. Recently, Cui et al. have reported that when the Pd loading in Pd/SZZ-13 was increased to 2 wt% or more, the sizes of the PdO particles were 2.5 ± 1.5 nm; the decrease in the CH4 oxidation rate with the drop in Pd content was attributed to the generation of a greater amount of the isolated Pd(II) ions that were less active than the PdO NPs formed at higher Pd contents ^[45]. These findings are in good agreement with those previously reported by Friberg et al. ^[46]. However, the hydrothermal aging of SSZ-13 at high temperatures (>850 °C) can cause structural destruction, giving rise to a decreased surface area, pore volume, and change in the XRD pattern ^[47]. Friberg et al. studied the effect of Pd loading on CH₄ oxidation of the thermally stable LTA zeolite with a Si/Al ratio of 44 and after hydrothermal aging at temperatures as high as 900 °C ^[48]. After hydrothermal aging at temperatures of ≤700 °C, the Pd/LTA exhibited a high and stable CH₄ combustion performance in the presence of water vapor. These authors found that the deactivation of the catalyst was not caused by the structural collapse of the LTA zeolite, and the T_50% over the Pd/LTA was 41 °C lower than that over the Pd/Al₂O₃. After thermal aging at 700 °C, the difference in the T_50% was 20 °C. Thermal aging at higher temperatures (800 and 900 °C) resulted in a higher T_50% (68 and 118 °C) over the Pd/LTA, respectively.

3. Medium-Pore Zeolites with 10 Member Rings

ZSM-5 was first synthesized in 1972 using the hydrothermal method with quaternary ammonium salts as the template agent ^[49]. The pore size of ZSM-5 is 0.55 nm × 0.51 nm, and this material shows a 10-element ring pore with a linear shape parallel to the b-axis direction and an elliptical pore with a pore size of 0.53 nm × 0.56 nm. As early as 1994, Li and Armor loaded Pd on ZSM-5 through the ion exchange approach and found that Pd/ZSM-5 exhibited excellent low-temperature methane combustion activity ^[50]. The higher catalytic activity of the Pd-based zeolite catalyst has been thought to be related to the additional lattice oxygen that can readily activate methane at low temperatures. All the zeolite catalysts reported in these early studies were prepared by the ion exchange method. Liu et al. pointed out that the glow discharge plasma treatment of the Pd-loaded zeolite catalysts before calcination could reduce the light-off temperature and improve the stability of the catalysts ^[51]. Hosseiniamoli et al. evaluated the stability of Pd/TS-1 and Pd/Silicalite-1 at 400 °C and a relative humidity (RH) of about 80% ^[52]. It was found that Pd/Silicalite-1 gradually became inactive after 30 h of operation, but the activity of Pd/TS-1 remained stable after 1900 h of reaction. The extended X-ray absorption fine structure (EXAFS) analysis of the Ti K-edge and Pd K-edge proved the anchoring effect of Ti on Pd, and the high stability of the porous catalyst was mainly assigned to the hydrophobicity and low acidity of the aluminum-free molecular sieve. Rational use of newly developed techniques could improve the dispersion of PdO and enhance its interaction with the zeolite support.

High-Si zeolites with an MFI framework have recently gained much attention, probably due to the water-related problems and the increasing interest in its hydrophobic property. For example, silicalite-1 (S-1) is an Al-free MFI zeolite and thus lacks acidic sites, but it is highly hydrophobic and hydrothermally stable. Wang et al. claimed that the Pd particles confined in S-1 and the synthesized core-shell catalysts showed high activities at low temperatures, with a shielding impact on H₂O resistance ^[53]. Furthermore, due to the hydrophobicity of S-1, Pd@S-1 was able to selectively prevent water vapor from diffusing into the Pd sites, exhibiting outstanding water resistance. In addition, Pd@S-1 and Pd_0.8Ni_0.2@S-1 showed the T_90% of 385 and 412 °C in the presence of 5 vol% water vapor ^[54], respectively. It is widely accepted in a number of the literature works that the small PdO NPs encapsulated in the zeolitic crystals are the active sites, which can prevent Pd from migrating and sintering and improve the stability of the zeolite-supported PdO catalysts under harsh conditions.

4. Large-Pore Zeolites with 12 Member Rings

In addition to the small and medium pore size zeolites mentioned above, the noble metal catalysts supported on large pore size zeolites (BEA, pore size = 0.7–0.8 nm) for CH₄ combustion applications have also been reported before. For example, Zhang et al. deposited Pd on the dealuminated zeolite support via a reduction—deposition route to generate the high-efficiency Pd/beta catalyst for lean CH₄ combustion ^[55]. The Pd/beta catalyst at a Pd loading of 0.5 wt% showed excellent catalytic activity (methane conversion > 99% at 375 °C) and good stability. The good performance of Pd/beta was ascribed to the highly dispersed PdO NPs, abundant adsorbed oxygen species, stable PdO, rich Pdⁿ⁺ (0 < n < 2) species anchored at the T-atom site, and enhanced hydrophobicity. It is generally recognized that dealumination can reduce the crystallinity of a zeolite and lead to the formation of the aluminum species outside the acidic framework, which further promotes the sintering of palladium particles. Subsequently, they also introduced an organic ligand of 3-mercaptopropyl trimethoxysilane (MPTS) to stabilize the Pd precursor during the preparation process, which was used to confine the PdO NPs in the beta zeolite ^[56]. The as-obtained sample demonstrated prominent CH₄ combustion activity (99% CH₄ conversion at 360 °C) and exhibited a remarkably better anti-sintering property and long-term (50 h) stability than the Pd/beta-IW and Pd/beta-HT samples. Kröcher et al. designed a porous catalyst based on Pd and a hierarchical zeolite with the fully Na-exchanged acid sites, which improved the support stability and prevents steam-induced Pd sintering under the reaction conditions by confining the noble metal within the zeolite ^[57]. Under the dry and wet conditions, the T_50% values over Pd/Na-MOR were 340 and 375 °C, respectively. There was no deactivation of this catalyst after 90 h of reaction in the presence of water. Ghorbel et al. prepared the Pd and Pd–Mo catalysts supported on HY and NaY and studied the effect of molybdenum doping on the activity of the catalyst ^[58]. It was found that Pd/HY and PdMo/NaY were less active than Pd/NaY at 500 °C. The catalytic activity of the monometallic sample increased with the reaction time, whereas that of the bimetallic sample decreased. The comparison of the catalytic activity of Pd/HY or PdMo/NaY revealed that the basicity of the support enhanced the oxidation ability of Pd by a rise in the electronic density of the metal particles on the sample surface.

5. Mesoporous Zeolite and Mesoporous Silicas Catalysts

The pore sizes of the conventional zeolites are generally less than 2 nm, and the reactants can be readily blocked from entering the active centers. The introduction of another set of mesopore nanochannels (so-called mesoporous zeolites) can considerably enhance the mass transfer efficiency, which structurally favors most of the reactions. Mesoporous silica zeolites are one of the most attractive catalytic materials for methane combustion because of their good chemical stability at high temperatures. More importantly, the mesoporous channels of such materials can provide limited space for the formation of Pd NPs with suitable sizes, and control the Pd species in their most active state, which shows the interesting activities of the as-designed catalysts ^[59].

Hussain et al. prepared the mesoporous silica-supported Pd catalysts and measured their catalytic activities for methane combustion ^[60]. Benefiting from the suitable structure and pore size of the silica, all the as-obtained Pd catalysts exhibited an almost complete conversion of CH₄, although the catalysts with the lowest Pd loading showed a methane conversion of 90% at 650 °C, whereas the maximum Pd loading in the KIT-6-supported catalyst allowed for a decrease in the temperature (T_90% = 405 °C for complete conversion of methane). Yuranov and coworkers successfully encapsulated Pd NPs with particle sizes of 1.0–3.6 nm within the channels of SBA-15 using the ion exchange strategy ^[61]. It was found that through the ion exchange of the surface silanol groups with [Pd(NH₃)₄]²⁺, the Pd loadings (0.1–4.4 wt%) could be further increased. The transmission electron microscopic (TEM) investigations of 2.1 wt% Pd/SBA-15-extr. indicated that most of the Pd NPs were located inside the mesoporous channels. The 2.1 wt% Pd/SBA-15 catalyst showed a stable catalytic activity in 6 h on-stream methane combustion at 520 °C.

The above research advances exemplify the feasibility of mesoporous silica (especially SBA-15) as a support for loading Pd and the effectiveness of the ion exchange method in maximizing the reactivity of the Pd species in methane combustion. Murthy et al. used the sol immobilization (SI) method instead of the conventional initial wet impregnation (IWI) method to prepare uniform Pd NPs confined in ordered mesoporous SBA-15 and determined their activities for methane combustion ^[62]. Compared with the catalyst derived from the conventional IWI method, the one derived from the SI route possessed uniform Pd NPs with a size of 5 nm in the channels of SBA-15 and still maintained such a size even after thermal aging in air at 800 °C for 3 days. The thermally stable Pd/SBA-15-SI catalyst showed excellent activity for CH₄ combustion (T_50% = 300 °C and T_90% = 350 °C), which was consistent with the result reported in the literature ^[63]. Afterward, Zribi et al. also synthesized a highly dispersed PdO (particle size = 2 nm) phase in MCM-41, and the Pd(II) complex with a monoamine ligand was used to prevent precipitation of the Pd precursor ^[64].

The doping of a substance with a redox nature is conducive to the construction of the metal—support interaction, improves stability of the target catalyst, and reduces the use of precious metals ^[10]^[19]^[21]. Liotta et al. prepared the SBA-15-supported LaMn_0.4Fe_0.6O₃ catalysts with perovskite loadings of 10, 30, and 40 wt% and found that the CH₄ combustion activity of the Pd/30 wt% LaMn_0.4Fe_0.6O₃/SBA-15 catalyst was improved compared with that of the Pd/LaMn_0.4Fe_0.6O₃ catalyst ^[65]. The Pd/30 wt% LaMn_0.4Fe_0.6O₃/SBA-15 catalyst also showed excellent SO₂ tolerance. Based on the structure and spectral characterization results, the optimized catalytic activity was due to the high dispersion of the active sites and good resistance to sintering during the reaction process. Yin et al. used the impregnation method to prepare a series of catalysts with different Pd loadings and investigated their catalytic activities and stability under the lean CH₄ combustion conditions ^[66]. The introduction of ZrO₂ as a promoter led to the enhancements in the oxygen storage ability and oxygen mobility of the catalyst. Among all the catalysts, 0.5 wt% Pd/5 wt% ZrO₂/SBA-15 exhibited the best catalytic performance and stability. After 400 h of an on-stream reaction at 450 ℃, the methane conversions remained unchanged. Ruiz et al. investigated the catalytic activities of the MCM-41-supported Pd catalysts for methane combustion ^[67]. Although the change in structure was not observed, the addition of La or Ce gave rise to a decrease in the catalytic activity of the methane combustion, and the all-silicate-based catalyst was more active than the Pd sample supported on aluminosilicate. In addition, the hydrophobic nature of the support and the reducibility of PdO were the main factors influencing the performance of the catalyst.

Peng et al. developed a facile in situ mesoporous-free strategy to synthesize Pd NPs enveloped in a single-crystal S-1 zeolite with intra-mesopores (Pd@IM-S-1) ^[68]. The Pd@IM-S-1 catalyst exhibited good performance for the deep oxidation of light alkanes (the T_90% was as low as 318 °C for CH₄ combustion), which was attributable to the confinement effect of the zeolite shell and improvement in the mass transfer efficiency and accessibility of the active metal sites. The Pd-PdO interface as a new active site could provide the active oxygen species for cleavage of the first C–H bond in light alkanes. It is proved that the restriction of zeolite channels further protects PdO NPs from sintering or hydrothermal aging under methane oxidation conditions, giving rise to high catalytic activity, stable operation durability, and good high-temperature stability. However, what can confine the active site is still controversial. In short, all kinds of zeolite supports with different skeleton architectures and pore diameters (i.e., micro- and mesopores) are suitable as supports of the Pd-based catalysts for CH₄ combustion.

6. Other Noble Metal-Based Zeolite Catalysts

Rh has been far less studied for CH₄ combustion, but it is an important component in the three-way catalysts, which can improve the catalytic activity and stability of the catalysts. Christensen et al. examined the impacts of temperature and SO₂ concentration (1–20 ppm) on the activity of the catalysts ^[69]. The deactivation induced by sulfur dioxide introduction was intensified with the drop in temperature or the rise in SO₂ concentration, and such a SO₂ poisoning could be thought to be due to partial or complete occupation of the active sites (which exhibits a Temkin adsorption isotherm). In the presence of 1–2 ppm SO₂, full coverage of the S-containing species was reached at 400 °C, but the coverage was significantly decreased at 500 °C, and significant catalytic activity was retained in the presence of SO₂. Recently, the same group reported that the 2 wt% Rh/ZSM-5 catalyst showed high methane conversions at high space velocities in the presence of water and sulfur dioxide ^[70]. The Rh-based catalysts with the presence of Rh₂O₃ were superior to the Pd-based catalysts in the presence of H₂O and SO₂. Although both water and sulfur dioxide decreased the activities of the supported Pd and Rh catalysts, water was found to exert a destabilizing effect on rhodium sulfate, i.e., the presence of water caused the sulfate to be partially decomposed below 400 °C, which could partly alleviate the sulfur dioxide poisoning of the supported Rh catalysts. The combined inhibition derived from (H₂O + SO₂) was weaker for the supported Rh catalysts than that for the supported Pd catalysts. In fact, the research works on CH₄ combustion over the supported Rh catalysts are relatively insufficient in the literature. In addition to the aluminosilicate zeolites, the silico-alumino-phosphate (SAPO) molecular sieves were also thought to be promising support materials. Nomura et al. studied CH₄ combustion over the Pd-SAPO-5 catalyst and found that steam strongly inhibited methane combustion ^[71]. Under the same conditions (10 vol% water vapor), the bimetallic Pd-Pt-SAPO-5 catalyst showed better methane combustion activity after the doping of Pt. Osman et al. investigated that the four-component catalyst containing bimetallic Pt and Pd NPs (for stability), TiO₂ (for oxygen mobility), and η-Al₂O₃ or H-ZSM-5 (acidic support) was superior to the simpler catalysts ^[72]. The multi-component catalyst with the optimized parameters obviously promoted the enhancement in methane combustion activity (T_90% = 200 °C at a GHSV of 10,000 mL/(gh)), and the long-term fluidization experiment showed that the activity of the catalyst did not decrease after more than 50 h of reaction at 250 °C. Meanwhile, Dai et al. prepared the PdO/CeO₂ nanosheets wrapped by the HZSM-5 film hybrid composite with sandwich and core-shell structures ^[73]. The as-synthesized materials exhibited excellent catalytic performance for methane combustion (T_90% = 495 °C, turnover rate (TOR) = 0.618 s⁻¹), and the HZSM-5 coating improved the resistance to sintering, oxygen, and water, while severe sintering of the conventionally prepared PdO/CeO₂ catalyst was suffered at high temperatures or oxygen concentrations. These results demonstrate that the multi-component catalysts possess better stability than the single Pd catalysts, and the doping of a suitable metal may stabilize the activity of the Pd-based catalyst. In practice, the design of a multi-component catalyst is beneficial for the enhancement of the low-temperature combustion of methane.

So far, the topological structure of a zeolite does not play the most important role in governing the activity of the zeolite-based catalyst for CH₄ combustion. Researchers are more inclined to control the molecular sieve in a beneficial way so as to obtain a high and stable methane oxidation catalyst with better performance.

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