Catalysts For Methane Combustion Research Articles

A highly active and stable palladium zeolite catalyst for wet methane combustion

Natural gas engines are the most viable alternative to diesel and gasoline ones. However, the current state-of-art catalyst, palladium (Pd) on alumina, for eliminating unburnt methane from engine exhaust suffers from high light-off temperature (> 400 °C) and poor water tolerance. Here we systematically investigated the effects of zeolite structure and Si/Al ratio, catalyst calcination temperature and extra-framework cation type on catalytic performance of zeolite-supported Pd catalysts for wet methane combustion to overcome these limitations. We found that a 3.0 wt% Pd catalyst supported on Na+-post-exchanged 500 °C-calcined IWV zeolite with a Si/Al ratio of 45 has a light-off temperature as low as 290 °C for methane combustion in the presence of 10 % water vapor while maintaining ca. 85 % methane conversion at 330 °C for 100 h. This study provides a new direction for bringing supported Pd catalysts close to real-world applications.

Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion

Effect of Ba Addition on the Catalytic Performance of NiO/CeO2 Catalysts for Methane Combustion

Silicate-induced high-temperature-resistant small-crystallite ceria support enhancing palladium-catalyzed low-concentration methane combustion

High-performance Pd/CeO2 catalysts for the purification of low-concentration methane in waste gases by catalytic combustion have long been pursued, given that methane is a potent greenhouse gas. The reduction in ceria oxygen supply and palladium dispersion, caused by ceria crystallite sintering, deactivates the catalyst. Here, we report an enhanced palladium catalyst supported on a silicate-induced small-crystallite ceria (7.2 nm) calcined at 800 °C, compared to the pristine ceria (50.7 nm). The catalyst was coated on a cordierite monolith for 0.1 vol% methane combustion, achieving 90 % methane conversion at 336 °C, which is 84 °C lower than the temperature required for Pd/CeO2 reference. The small-crystallite ceria exhibited a high oxygen supply capacity due to abundant oxygen vacancies, facilitating the conversion of carbon-containing intermediates from methane dissociation on palladium species, thereby enhancing the catalyst’s intrinsic activity. The catalyst, with increased palladium dispersion, demonstrated a methane reaction rate 6.7 times higher than that of the Pd/CeO2 at 280 °C. This study provides new insights into designing highly efficient Pd/CeO2-based catalysts for low-concentration methane combustion.

Electron transferring with oxygen defects on Ni-promoted Pd/Al2O3 catalysts for low-temperature lean methane combustion

Methane (CH4) is the second most consequential greenhouse gas after CO2, with a substantial global warming potential. The CH4 catalytic combustion offers an efficient method for the elimination of CH4. However, improving the catalytic performance of Pd-based materials for low-temperature CH4 combustion remains a big challenge. In this study, we synthesized an enhanced Pd/5NiAlOx catalyst that demonstrated superior catalytic activity and improved water resistance compared to the Pd/Al2O3 catalyst. Specifically, the T90 was decreased by over 100 °C under both dry and wet conditions. Introducing Ni resulted in an enormously enhanced number of oxygen defects on the obtained 5NiAlOx support. This defect-rich support facilitates the anchoring of PdO through increased electron transfer, thereby inhibiting the production of high-valence Pd(2+δ)+ and stimulating the generation of unsaturated Pd sites. Pd0 can effectively activate surface oxygen and PdO plays a significant role in activating CH4, resulting in high activity for Pd/5NiAlOx. On the other hand, the increased water resistance of Pd/5NiAlOx was mainly due to the generation of *OOH species and the lower accumulation of surface -OH species during the reaction process.

Crystalline phase-modulated PtOx nanoparticles exhibit superior methane combustion performance and sulfur poisoning resistance: Multi-interaction regulation

Up to now, methane combustion catalysts generally suffer from issues such as poor reaction stability and susceptibility to sulfur poisoning, which have become the key factors restricting their application, so it is necessary to construct catalysts with excellent chemical stability of the active center and strong sulfur resistance. Here, we showed the crystal phase-modulated state of PtOx nanoparticles (PtOx NPs) on TiO2 and investigated their effects on the reactivity of methane catalytic combustion and anti-SO2 poisoning properties. It was found that Pt/r-TiO2 showed superior activity, stability, and resistance to SO2 poisoning. Transmission electron microscope (TEM) images showed that the PtOx NPs on the surface of r-TiO2 were much more stable than a-TiO2 for the reason that the Ostwald ripening of PtOx NPs on a-TiO2 was facilitated due to the strong metal-support interaction (MSI) between PtOx NPs and TiO2 and the low migration energy of the PtOx NPs on TiO2, while they were moderate of Pt/r-TiO2, and then the active sites were stabilized during methane combustion. The activity was controlled by the size and electronic structure of PtOx NPs, which were stable on the surface of r-TiO2 with a higher Pt4+ content, and the density functional theory (DFT) calculations further revealed the role of r-TiO2 in increasing the quantities of surface oxygen vacancies. Moreover, the SO2 tolerance of Pt/r-TiO2 was also the best among all kinds of Pt-based catalysts prepared, which was attributed to the minimal adsorption energy (Ead) of SO2 on r-TiO2, and the absence of sulfate and sulfite deposition on the Pt/r-TiO2 catalyst confirmed by TGA and in-situ DRIFTS studies on SO2 adsorption. In this research, the anti-SO2 poisoning and reaction stability were realized by adjusting the interaction between SO2, active components, and supports, which provided a new insight for the design of stable Pt-based SO2-resistant catalysts.

Synthesis and testing of active and water resilient low temperature methane combustion catalysts

Synthesis and testing of active and water resilient low temperature methane combustion catalysts

Inhibition effect of sulfation on Pt/TiO2 catalysts in methane combustion

Inhibition effect of sulfation on Pt/TiO2 catalysts in methane combustion

Mechanistic insights into the deactivation of Pd/BEA methane combustion catalysts by hydrothermal aging

Three Pd/Beta zeolite catalysts with low, intermediate and high support Si/Al2 (SAR) ratios were synthesized for methane combustion in both fresh and hydrothermally aged forms. SAR was found to dramatically influence catalyst performance and hydrothermal stability. Via an array of structural and textural characterizations, it is discovered that (1) at low SAR, catalyst deactivation is due primarily to the transformation of catalytically active PdO to inactive Pd2+-ions via protonolysis; (2) at intermediate SAR, catalyst deactivation during aging is due partly to the protonolysis chemistry, and partly to Al-decoration of the remaining PdO particles; (3) at high SAR, both of these deactivation mechanisms are greatly mitigated, leading to much improved catalyst hydrothermal stability. Our work here demonstrates that PdO particles stabilized by silanol nests, sites that are highly active for methane combustion, can only survive harsh hydrothermal aging at very high support SAR.

Acid treatment enhances the methane combustion activity of LaFeO3 perovskite catalyst

The catalytic combustion of methane has gained much attention as a sustainable technology, due to concerns about the environmental impact of unburnt methane emissions from the natural gas vehicles. Researchers have explored alternatives to precious metal catalysts for methane combustion, with a focus on perovskite oxides known for their excellent activity and sintering-resistance capacity. In this study, LaFeO3 perovskite material was synthesized and subsequently subjected to a nitric acid etching treatment to modify its surface. It was found that the acid preferentially dissolved La cations from LaFeO3 surface while the perovskite phase was unchanged, as a consequence, the etched LaFeO3 shows improvement in methane combustion activity. The best performance was achieved in the sample with one-hour surface etched treatment, which suggests that the appropriate surface reconstruction induced by acid treatment promotes the formation of more active Fe sites on the surface and reduces the energic barriers of methane activation. This work presents a straightforward and effective approach to tailor the exposed surface properties of perovskites, thereby enhancing their activity in hydrocarbon combustion reactions.

Cobalt–Magnesium Oxide Catalysts for Deep Oxidation of Hydrocarbons

Co–Mg catalysts for methane combustion were synthesized and studied, revealing the transformation of MgCo2O4 spinel into a CoO–MgO solid solution with oxygen release from the spinel lattice as the calcination temperature increased. Repeated heat treatment of the calcined solid solution at lower temperatures led to spinel regeneration with segregation of the solid solution phase. A TPR of the samples showed the presence of two characteristic peaks, the first of which relates to the transition of Co3+Oh spinel to the Co2+Oh structure of CoO, and the second to the reduction of CoO to Co°. The second peak was observed at 540–620 °C for samples calcined at temperatures below spinel decomposition, and for high-temperature samples at 900–1100 °C. Taking into account the identity of the structure of phases obtained in both cases, the formation of not a true CoO–MgO solid solution, but rather a mixture of ordered oxides (“pseudo-solid solution”) in the low-temperature region, was postulated. A study of the activity of the samples showed the high activity of the spinel systems and a linear relationship between the activation energy of methane oxidation and the heat treatment temperature.

Uniformly Dispersed Nano Pd-Ni Oxide Supported on Polyporous CeO2 and Its Application in Methane Conversion of Tail Gas from Dual-Fuel Engine

The development of catalysts for low-temperature methane combustion is crucial in addressing the greenhouse effect. An effective industrial catalyst strategy involves optimizing noble metal utilization and boosting metal–metal interaction. Here, the PdNi-H catalyst was synthesized using the self-assembly method, achieving the high dispersion and close proximity of Pd and Ni atoms compared to the counterparts prepared by the impregnation method, as confirmed by EDS mapping. The XRD and TEM results revealed Pd2+ and Ni2+ doping within the CeO2 lattice, causing distortions and forming Pd-O-Ce or Ni-O-Ce structures. These structures promoted oxygen vacancy formation in CeO2, and this was further confirmed by the Raman and XPS results. Consequently, the PdNi-H catalyst demonstrated an excellent redox ability and catalytic activity, achieving lower ignition and complete methane burning temperatures at 282 and 387 °C, respectively. The highly dispersed PdNi species played a pivotal role in activating methane for enhanced redox ability. Additionally, the narrow size distribution range contributed to more vacancies on the surface of CeO2, as confirmed by the XPS results, thereby facilitating the activation of gas phase oxygen to form oxygen species (O2−). This collaborative catalytic approach presents a promising strategy for developing efficient and stable methane combustion catalysts at low temperatures.

Effects of dealumination on the methane-combustion activity of Pd/SSZ-13 catalysts

This study investigated the effects of dealumination on Pd/SSZ-13 catalysts for methane combustion. Pd(1)/SSZ-13 (HTA), prepared by treating the catalyst hydrothermally at 750 °C, displayed extremely low catalytic activity. In contrast, Pd(1)/SSZ-13 (DeAl), prepared by treating the SSZ-13 support hydrothermally followed by Pd impregnation, exhibited superior catalytic activity for methane oxidation. Catalytic properties, including zeolite hydrophobicity and Pd location and states, were investigated by using various characterization techniques to clarify the differences between the catalysts. Most Pd species in the Pd/SSZ-13 (HTA) catalyst existed as Pd ions, which were not very active for the reaction. On the contrary, most Pd species in the Pd(1)/SSZ-13 (DeAl) catalyst existed as PdO nanoparticles at the external surface, which were highly active for methane oxidation. The study results suggested that the amount of external PdO nanoparticles should be maximized in order to make Pd/SSZ-13 catalyst highly active for methane combustion, which was achieved by dealuminating the SSZ-13 support before Pd impregnation.

Influence of the drying mode of support on the properties of Pd/Al2O3–ZrO2 materials used for methane combustion

This work constitutes a new trial to enhance the properties of palladium supported on alumina modified with zirconium used as catalysts for methane combustion. The effect of the support drying mode is studied. For this aim, Al2O3-ZrO2 binary oxides with zirconium loading of 2 and 5% in weight were prepared using sol–gel process then dried under ordinary or supercritical conditions. Palladium with a loading of 0.5% was deposited on the support by wet impregnation. Several techniques have been used to investigate differences between the two types of the derived catalysts.

Bayesian Optimization-guided Discovery of High-performance Methane Combustion Catalysts based on Multi-component PtPd@CeZrOx Core-Shell Nanospheres.

Formula regulation of multi-component catalysts by manual search is undoubtedly a time-consuming task, which has severely impeded the development efficiency of high-performance catalysts. In this work, PtPd@CeZrOx core-shell nanospheres, as a successful case study, is explicitly demonstrated how Bayesian optimization (BO) accelerates the discovery of methane combustion catalysts with the optimal formula ratio (the Pt/Pd mole ratio ranges from 1/2.33-1/9.09, and Ce/Zr from 1/0.22-1/0.35), which directly results in a lower conversion temperature (T50 approaching to 330 °C) than ones reported hitherto. Consequently, the best sample obtained could be efficiently developed after two rounds of iterations, containing only 18 experiments in all that is far less than the common human workload via the traditional trial-and-error search for optimal compositions. Further, this BO-based machine learning strategy can be straightforward extended to serve the autonomous discovery in multi-component material systems, for other desired properties, showing promising opportunities to practical applications in future.

Bayesian Optimization‐guided Discovery of High‐performance Methane Combustion Catalysts based on Multi‐component PtPd@CeZrOx Core–Shell Nanospheres

AbstractFormula regulation of multi‐component catalysts by manual search is undoubtedly a time‐consuming task, which has severely impeded the development efficiency of high‐performance catalysts. In this work, PtPd@CeZrOx core–shell nanospheres, as a successful case study, is explicitly demonstrated how Bayesian optimization (BO) accelerates the discovery of methane combustion catalysts with the optimal formula ratio (the Pt/Pd mole ratio ranges from 1/2.33–1/9.09, and Ce/Zr from 1/0.22–1/0.35), which directly results in a lower conversion temperature (T50 approaching to 330 °C) than ones reported hitherto. Consequently, the best sample obtained could be efficiently developed after two rounds of iterations, containing only 18 experiments in all that is far less than the common human workload via the traditional trial‐and‐error search for optimal compositions. Further, this BO‐based machine learning strategy can be straightforward extended to serve the autonomous discovery in multi‐component material systems, for other desired properties, showing promising opportunities to practical applications in future.

Boosting Methane Combustion over Pd/Y2O3-ZrO2 Catalyst by Inert Silicate Patches Tuning Both Palladium Chemistry and Support Hydrophobicity.

Supported palladium (Pd) catalysts are widely utilized to reduce the emission of exhaust CH4 from lean-burn engines by catalytic combustion. A large amount of water vapor in the exhaust makes hydroxyls accumulate on the catalyst surface at temperatures below 450 °C, leading to severe catalyst deactivation. Tuning palladium chemistry and inhibiting water adsorption are critical to developing active catalysts. Modifying the support surface with inert silicates would both change the palladium-support interaction and decrease water adsorption sites. This study reports an improved Pd/Y2O3-ZrO2 catalyst by constructing silicate patches on yttria-stabilized zirconia (Y2O3-ZrO2) support. The silicates hindered electron transfer from Y2O3-ZrO2 oxygen vacancies to palladium, which optimized palladium chemistry, especially the reducibility of active PdO species, and thereby boosted CH4 conversion under dry conditions. The temperature of 90% methane conversion (T90) over the catalyst decreased from 386 to 309 °C. Moreover, the inert silicates decreased surface oxygen vacancies of Y2O3-ZrO2 to improve support hydrophobicity, thereby inhibiting hydroxyl accumulation. The poisoning effect of water on the active sites located on the palladium-silicate interface was alleviated. When reaction gases contained 10 vol % water, the silicate-modified catalyst still showed higher activity with T90 of 404 °C, which is lower than T90 of 452 °C for unmodified catalyst. This work represents a step forward in preparing high-performance palladium catalysts for low-temperature wet methane combustion.

Synthesis of high-performance Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalysts for low-temperature methane combustion with the ultrasound-assisted field

In this work, ultrasound-assisted sol–gel (UASG) and wet incipient impregnation (WIP) methods were employed to synthesize a series of Ce0.4Zr0.5La0.05Y0.05O1.95 oxygen storage materials (OSM) and Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalysts under different frequency ultrasound external field. The effect of the ultrasonic external field on the low-temperature catalytic activity of the catalyst was investigated. The BET and TEM results demonstrated that ultrasound field can increase the catalyst's specific surface area by 25% and decrease the particle size from 10.1 nm to 7.3 nm. The XPS results showed that the concentrations of surface Pd2+, Oads, and Ce3+ species increased from 21%, 44%, and 43% to 42%, 65%, and 46%, respectively, enhancing the formation of Pd2+, Oads and Ce3+ species on the catalyst surface. The performance tests revealed that T90(CH4) of the prepared Pd/Ce0.4Zr0.5La0.05Y0.05O1.95 catalyst by UASG method decreased from 536 ℃ to 331 ℃, improving its low-temperature catalytic activity. Additionally, the catalysts synthesized using by UASG method exhibited the enhanced resistance to water and sulfur poisoning and maintained their catalytic performance for 20 h.

Kinetic investigation of the promoting effect of cobalt on Pd-Pt/SnO2 catalyzed wet methane combustion

This study is focused on developing a stable and active catalyst for lean-burn methane combustion in the presence of water. By varying the cobalt loading, it was found that 10 wt% Co deposition enhanced the hydrothermal stability of the 1 wt% Pd-Pt (1:1 molar ratio)/SnO2 catalyst. Kinetic investigation of methane combustion on the hydrothermally aged Pd-Pt/10Co/SnO2 catalyst under 5 and 10 vol% water was performed. While SnO2 dictated the kinetics in terms of activation energy (89 ± 9 kJ mol−1), Co addition altered the reaction order to water (n = −0.37). In terms of turnover frequency, the Pd-Pt/10Co/SnO2 catalyst surpassed the Al2O3-supported catalyst formulations. Reaction rates normalized per total catalyst mass at 773 K and 10 vol% added water were found in the order of Pd/Al2O3(“x”) < Pd-Pt/Al2O3 (1.5x) < Pd-Pt/SnO2 (1.6x) < Pd-Pt/10Co/SnO2 (14x), consequently suggesting that the utilization of the Pd-Pt/10Co/SnO2 catalyst could result in small catalytic converter volumes.

Lean methane combustion over zeolite-supported Pd catalysts: Structure-performance relationship and deactivation mechanism

Zeolites are a promising support for Pd catalysts in lean methane (CH4) combustion. Herein, three types of zeolites (H-MOR, H-ZSM-5 and H-Y) were selected to estimate their structural effects and deactivation mechanisms in CH4 combustion. We show that variations in zeolite structure and surface acidity led to distinct changes in Pd states. Pd/H-MOR with external high-dispersing Pd nanoparticles exhibited the best apparent activity, with activation energy (Ea) at 73 kJ/mol, while Pd/H-ZSM-5 displayed the highest turnover frequency (TOF) at 19.6 × 10−3 sec−1, presumably owing to its large particles with more step sites providing active sites in one particle for CH4 activation. Pd/H-Y with dispersed PdO within pore channels and/or Pd2+ ions on ion-exchange sites yielded the lowest apparent activity and TOF. Furthermore, Pd/H-MOR and Pd/H-ZSM-5 were both stable under a dry condition, but introducing 3 vol.% H2O caused the CH4 conversion rate on Pd/H-MOR drop from 100% to 63% and that on Pd/H-ZSM-5 decreased remarkably from 82% to 36%. The former was shown to originate from zeolite structural dealumination, and the latter principally owed to Pd aggregation and the loss of active PdO.

Promoting effect of Pt on the activity and stability of Pd/MgFe2O4 for catalytic combustion of methane

Catalytic combustion is a promising way to remove low-concentration methane. Developing catalysts with good low-temperature activity and excellent catalytic stability toward catalytic combustion of methane is critical but challenging. In this work, we report that the addition of Pt into the 1Pd/MgFe2O4 in a suitable amount (4 wt%) can result in a significant enhancement in the catalytic activity and stability. High and stable methane combustion performances were achieved at 395 °C on 4Pt–1Pd/MgFe2O4 catalyst under WHSV of 30,000 mL/(g–h) during 20 h on stream. With comprehensive characterizations by using ICP, XRD, TPO, XPS, STEM and EDS, we reveal that the high catalytic activity and stability is due to that the addition of Pt could decrease the size of metallic nanoparticles, increase the proportion of Pd0 and adsorbed oxygen species. These results provide guidance for designing highly active and stable Pd-based catalysts for catalytic combustion of methane.