Catalytic Activity For Methane Oxidation Research Articles

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.

In Situ Synthesis of Encapsulated Pd@silicalite-2 for Highly Stable Methane Catalytic Combustion.

Methane emissions from vehicles have made a significant contribution to the greenhouse effect, primarily due to its high global warming potential. Supported noble metal catalysts are widely employed in catalytic combustion of methane in vehicles, but they still face challenges such as inadequate low-temperature activity and deactivation due to sintering under harsh operating conditions. In the present work, a series of encapsulated structured catalysts with palladium nanoparticles confined in hydrophobic silicalite-2 were prepared by an in situ synthesis method. Based on various characterization methods, including XRD, HR-TEM, XPS, H2-TPR, O2-TPD, H2O-TPD, CH4-TPR, Raman, and in situ DRIFTS-MS, it was confirmed that PdOx nanoparticles were mainly encapsulated inside the silicalite-2 zeolite, which further maintained the stability of the nanoparticles under harsh conditions. Specifically, the 3Pd@S-2 sample exhibited high catalytic activity for methane oxidation even after harsh hydrothermal aging at 750 °C for 16 h and maintained long-term stability at 400 °C for 130 h during wet methane combustion. In situ Raman spectroscopy has confirmed that PdOx species act as active species for methane oxidation. During this reaction, methane reacts with PdOx to produce CO2 and H2O, while simultaneously reducing PdOx to metallic Pd species, which is further reoxidized by oxygen to replenish the PdOx catalyst.

Catalytic Activity of Methane Oxidation on Nickel-doped Lanthanum Strontium Titanate Perovskite from Three Synthesis Methods

BackgroundThree perovskites of nickel-doped lanthanum strontium titanate (LSTN) were made and compared for their activity toward methane oxidation. MethodThe perovskite was synthesized separately via solid-state reaction, citrate complexation, and microwave-assisted methods. Significant FindingsThe perovskite from solid state reaction gave the highest activity of syngas production in repetitive redox cycles without any deterioration, demonstrating a great potential for use in chemical looping technology. Nevertheless, the material required activation in methane, as evidenced in studies of temperature-programmed surface reaction. On the contrary, the perovskite from the complexation method behaved completely different while the microwave-synthesized one gives the poorest activity. The variation of the activity was correlated with the distinct structure from different synthesis routes, which influences where a metal phase resides and how the metal anchors onto host oxides. The information obtained here would be of great benefit in making good material with desirable properties for catalysis and potential applications.

Photocatalytic oxidation of methane to methanol by tungsten trioxide-supported atomic gold at room temperature

Atomic-scale metals as active center have been widely investigated for efficient photocatalysis. Understanding the specific electronic structure of atomic-scale center is of profound fundamental importance for superior catalytic performance. Here, we report an atomically dispersed gold on tungsten trioxide (Au1/WO3) catalyst for photocatalytic oxidation of methane toward value-added methanol. The Au1 species reveal a specific tip-enhanced local electrons field which favors the C-H dehydrogenation of methane and thus form methanol (up to 589 µmol g−1 h−1). Both experimental and theoretical results demonstrate such tip-enhanced effect enhance the catalytic activity of methane oxidation. The theoretical calculations further reveal a lower adsorption energy of product methanol on Au1, in contrast to Au particles, which suppresses the overoxidation of methanol, and thus promotes its selectivity. Establishing the relationship between electronic density and catalytic activity may create a platform for designing efficient atomic-scale catalysts for C1 catalysis and green chemistry.

Total Oxidation of Methane on Oxide and Mixed Oxide Ceria-Containing Catalysts

Methane, discovered in 1766 by Alessandro Volta, is an attractive energy source because of its high heat of combustion per mole of carbon dioxide. However, methane is the most abundant hydrocarbon in the atmosphere and is an important greenhouse gas, with a 21-fold greater relative radiative effectiveness than CO2 on a per-molecule basis. To avoid or limit the formation of pollutants that are dangerous for both human health and the atmospheric environment, the catalytic combustion of methane appears to be one of the most promising alternatives to thermal combustion. Total oxidation of methane, which is environmentally friendly at much lower temperatures, is believed to be an efficient and economically feasible way to eliminate pollutants. This work presents a literature review, a statu quo, on catalytic methane oxidation on transition metal oxide-modified ceria catalysts (MOx/CeO2). Methane was used for this study since it is of great interest as a model compound for understanding the mechanisms of oxidation and catalytic combustion on metal oxides. The objective was to evaluate the conceptual ideas of oxygen vacancy formation through doping to increase the catalytic activity for methane oxidation over CeO2. Oxygen vacancies were created through the formation of solid solutions, and their catalytic activities were compared to the catalytic activity of an undoped CeO2 sample. The reaction conditions, the type of catalysts, the morphology and crystallographic facets exposing the role of oxygen vacancies, the deactivation mechanism, the stability of the catalysts, the reaction mechanism and kinetic characteristics are summarized.

The robust catalysts (Ni1−x–Mox/doped ceria and Zn1−x–Mox/doped ceria, x = 0.1 and 0.3) for efficient natural gas reforming in solid oxide fuel cells

Nickel is a promising catalyst in Solid Oxide fuel cell (SOFC) due to its electrocatalytic performance, however, the practical utilization of Ni-based materials is hindered by the undesirable carbon deposition during methane decomposition. Herein, molybdenum is incorporated into the Ni- and Zn-based cermets (Ni1−x–Mox/GDC and Zn1−x–Mox/GDC, x = 0.1 and 0.3) to enhance electrocatalytic properties and avoid the carbon deposition during cell operation. The desired composites are synthesized by the impregnation method and adopted as anode in SOFCs. The catalytic activity for methane oxidation has been significantly improved due to the introduction on Mo, which hindered the carbon deposition due to higher graphitization and abundant active sites accessible to fuel. The detailed Raman spectroscopy and conductivity analysis revealed that addition of Mo reduced the amount of deposited carbon and enhanced the electrical conductivity. By using natural gas, as a fuel, the as-prepared Mo-doped Ni–GDC rendered a maximum power density of 690 mW cm−2 at 600 °C. It is worth mentioning that the achieved stable power density is one of the best in existing literature. The current study presents a novel strategy to improve the catalytic behavior of electrode materials and demonstrate the optimal performance at low operating temperature.

Outstanding stability and highly efficient methane oxidation performance of palladium-embedded ultrathin mesoporous Co2MnO4 spinel catalyst

Exploring the state-of-the-art combustion catalysts that efficiently stabilize precious metal has been a wide concern to limit rising levels in methane emissions. Herein we exhibit that utilizing ultrathin mesoporous spinel catalysts (Co2MO4, M = Co, Mn, and Al) as a support can stabilize PdO cluster by strengthening the metal − support interactions at being close to atomic scale. These are found that the as-synthesized Pd-embedded catalysts possesses highly active surface lattice oxygen, enhanced Co3+ in the surface layers, and shows outstanding low-temperature reducibility. More importantly, in-situ XRD results reveals that Pd-Co2MnO4 is more sensitive to thermal effects than that of Co2MnO4, which leads to the surface phase migration of PdO clusters and lattice oxygen release on Pd-Co2MnO4 catalyst. Among these catalysts, the Pd-Co2MnO4 catalyst performs the best catalytic activity for methane oxidation (T90 = 324 °C, Ea=37.4 kJ mol−1), especially with excellent cycle stability, and water-resistances. This study can be instructive for fabricating desirable heterogeneous catalysts and bring new insight into the design of highly efficient Pd-based catalysts.

3D printed catalytic converters with enhanced activity for low-temperature methane oxidation in dual-fuel engines

Catalytic converters with non-linear channel structures were prepared using 3D printing and tested in the oxidation of methane in a simulated dual-fuel engine exhaust stream. The design used a simple repeating angular offset between adjacent layers, which was sufficient to introduce complexity with minimal software programming. All 3D printed substrates were mechanically stable and, following washcoating with a composite catalyst, demonstrated higher catalytic activity in methane oxidation than a commercial honeycomb substrate. The methane conversion at e.g. 510 °C was 12.6% on the commercial sample, 72.6% for 90°, 80.1% for both 30° and 45°, and 89.6% for the 60° oriented structures. This enhancement is attributed to the increased turbulence/mass transfer and surface area than are possible using conventional straight-channelled substrates. Computational fluid dynamics (CFD) analysis confirmed that the higher methane conversion over 3D printed substrates is due (at least partially) to its higher turbulence kinetic energy. Backpressures over the 3D printed structures were also experimentally measured and compared with the conventional honeycomb monolith.

Steady state isotopic transient kinetic analysis of the combustion of CH4 over Co-Mn-O catalysts

The main objective of the research was the analysis of the number and intrinsic activity of active sites present on the surface of working cobalt-manganese-based catalytic systems for complete methane oxidation. The obtained values of these parameters strongly depend on the reaction temperature and chemical composition of studied catalysts. The highest number of active sites is present on the surface of the cobalt-manganese catalyst, which demonstrates the best catalytic activity in methane oxidation. Nevertheless, in some temperature ranges intrinsic activity of its active sites is lower than that of the active sites present on the surface of other catalysts.

Combustion Synthesis of Non-Precious CuO-CeO₂ Nanocrystalline Catalysts with Enhanced Catalytic Activity for Methane Oxidation.

In this study, xCuO-CeO2 mixed oxide catalysts (Cu weight ratio x = 1.5, 3, 4.5, 6 and 15 wt.%) were prepared using solution combustion synthesis (SCS) and their catalytic activities towards the methane (CH4) oxidation reaction were studied. The combustion synthesis of the pure CeO2 and the CuO-CeO2 solid solution catalysts was performed using copper and/or cerium nitrate salt as an oxidizer and citric acid as a fuel. A variety of standard techniques, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were employed to reveal the microstructural, crystal, thermal and electronic properties that may affect the performance of CH4 oxidation. The CuO subphase was detected in the prepared solid solution and confirmed with XRD and Raman spectroscopy, as indicated by the XRD peaks at diffraction angles of 35.3° and 38.5° and the Ag Raman mode at 289 cm−1, which are characteristics of tenorite CuO. A profound influence of Cu content was evident, not only affecting the structural and electronic properties of the catalysts, but also the performance of catalysts in the CH4 oxidation. The presence of Cu in the CeO2 lattice obviously promoted its catalytic activity for CH4 catalytic oxidation. Among the prepared catalysts, the 6% CuO-CeO2 catalyst demonstrated the highest performance, with T50 = 502 °C and T80 = 556 °C, an activity that is associated with the availability of a fine porous structure and the enhanced surface area of this catalyst. The results demonstrate that nanocrystalline copper-ceria mixed oxide catalysts could serve as an inexpensive and active material for CH4 combustion.

Synthesis of a Highly Stable Pd@CeO2 Catalyst for Methane Combustion with the Synergistic Effect of Urea and Citric Acid.

Making use of synergy between urea and citric acid, a core–shell Pd@CeO2 catalyst with spherical morphology was facilely synthesized by a hydrothermal method. The formation mechanism of the core–shell structure in the presence of citric acid and hydrogen peroxide was studied. Results showed that the Pd@CeO2 catalyst exhibited high catalytic activity in methane oxidation. Pd nanoparticles were well stabilized by CeO2 shell encapsulation, resulting in high stability of the catalyst. A high CH4 conversion of 99% was retained after 50 h on-stream reaction at 500 °C. Additionally, many tiny pores on the CeO2 shell surface were beneficial for the full contact between reactants and active components. Pd nanoparticles were highly dispersed inside the shell, improving the utilization efficiency of active components. The results also demonstrated that the Pd species in the catalyst existed in the form of oxidation state, mainly in PdO (ca. 66.6%), which played an essential part in methane combustion.

Catalytic activity in methane oxidation of La1-xCaxCoO3-δ (х = 0-1) perovskites prepared by mechanochemical route

Catalytic activity in methane oxidation of La1-xCaxCoO3-δ (х = 0-1) perovskites prepared by mechanochemical route

Influence of the metal/metal oxide redox cycle on the catalytic activity of methane oxidation over Pd and Ni doped hydroxyapatite

The methane oxidation reaction was investigated over Pd and Ni doped hydroxyapatite (HAP) catalysts. All catalysts were evaluated in both their reduced and oxidised forms, and the behaviour of the metals in the reaction was investigated. The obtained results demonstrated that the reaction proceeds through a redox cycle between metal oxide and reduced metal, however more stable catalysts with higher conversions were observed when the catalysts were pre-treated in oxygen. After CH4 oxidation, all the samples were tested towards coke formation using TGA analysis; results showed that all the catalysts were resistant to the coking process under the conditions studied.

Effect of transition metal Fe adsorption on CeO2 (110) surface in the methane activation and oxygen vacancy formation: A density functional theory study

Methane activation and oxygen vacancy formation over transition metal Fe adsorption on CeO2 (110) are studied by using the method of density functional theory (DFT)+U method. A set of model configurations are generated by placing Fe at five surface sites, viz., O-top site, O-bridge site, Ce-bridge site, Ce-top and double oxygen-bridge sites. The study shows that the energetically most favorable configuration is Fe adsorption at the double oxygen-bridge site. Based on the calculated surface, subsurface and the second oxygen vacancies formation energy with (or without) Fe adsorption, it shows that the Fe adsorption is in favor of the surface, subsurface and second oxygen vacancies formation. For the surface and subsurface oxygen vacancy on the Fe/CeO2 (110) surface, the main factor responsible for lowering of Evac is that the adsorption induces structural distortions, whereas, for the second oxygen vacancy, half can be attributed to the large structural relaxation, half can be attributed to the electronic effects. After calculating and discussing about the CH4 activation on CeO2 (110) and Fe/CeO2 (110) surface with (or without) the surface or subsurface oxygen vacancies at the possible adsorption sites, the results show that when the CH4 adsorbed on the Fe/CeO2 (110) with the surface oxygen vacancy at the Ce1 and Ce2 sites, the CH4 decomposed into the CH(ads) and H(ads), its belongs to the chemical absorption, whereas, when the CH4 adsorbed on the other possible sites, the mentioned phenomenon is not occurred, its belongs to the physical absorption. This study reveals the correlation between surface reducibility and catalytic activity for methane oxidation on cerium-based materials, which might be beneficial in developing improved catalysts for methane combustion.

Surface tuned La0.9Ca0.1Fe0.9Nb0.1O3-δ based anode for direct methane solid oxide fuel cells by infiltration method

It has been found in our previous study that La0.9Ca0.1Fe0.9Nb0.1O3-δ-Sm0.2Ce0.8O1.9 (LCFNb-SDC) anode shows high catalytic activity and stability for solid oxide fuel cells (SOFCs) under H2 and CO atmospheres. In this study, LCFNb-SDC is further tuned into a promising anode for direct methane SOFCs through surface modification with Ni. The LCFNb, SDC and Ni materials are found to be thermal-mechanically and chemically compatible with each other up to 850°C. Although LCFNb-SDC anode shows poor catalytic activity for methane oxidation, further modification of such anode with a small amount of Ni particles significantly improves the cell performance in CH4. The maximal power densities of 1031 and 729mWcm−2 in H2 and CH4 have been achieved for the electrolyte supported SOFC with LCFNb-SDC-Ni|SSZ (Sc0.2Zr0.8O2−δ)|LSM (La0.8Sr0.2MnO3) configuration at 850°C, respectively. Besides, the same cells also exhibit good stabilities in both H2 (500h) and CH4 (100h) with no obvious performance degradation and carbon deposition. Surface tuned LCFNb anode by impregnating SDC and a small amount of Ni is thus highly promising as an anode for direct methane SOFCs to deliver favorable performance with reasonable stability.

The Effect of the Preparation Method of Pd-Doped Cobalt Spinel on the Catalytic Activity in Methane Oxidation Under Lean Fuel Conditions

In this work a series of cobalt spinel catalysts was synthesized by different methods and doped with 1 wt% of palladium via incipient wetness impregnation or solution combustion synthesis (SCS). Both undoped and Pd-doped spinels were characterized from the structural and surface point of view (XRD, XRF, μRS, BET, FESEM) and tested toward the oxidation of methane in lean conditions, in gas mixtures containing from 0.5 to 2 vol% of CH4. These methane concentrations are typical of processes involving combustion of CH4 in lean conditions as the after-treatment of unconverted CH4 in compressed natural gas-fed vehicle exhausts, or the treatment of fugitive CH4 from leaks in coal mines. The temperature window for the catalytic activity of the undoped Co3O4 materials spread from 350 to 600 °C. The preparation method affected the catalytic activity of undoped Co3O4, with the best catalytic activity related to the spinel prepared by SCS employing urea. The addition of 1 wt% of Pd lowered the temperature window for the catalytic activity to the range of 250–500 °C, with insignificant differences for the various preparation methods. The beneficial effect of Pd-doping was assigned to the enhancement of the reduction of cobalt in the oxide structure.

Preparation and characterization of the non-stoichiometric La–Mn perovskites

Six La–Mn oxide samples with La/Mn atomic ratio x = 1.03–0.56 (denoted as sample LaxMn) were prepared by the citrate method with calcination at 700 °C for 5 h, and characterized by X-ray diffraction (XRD), N2 adsorption–desorption, temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). It is confirmed that the four samples with La/Mn atomic ratio at 1.03–0.72 are all single phase perovskites by XRD patterns. Lattice parameters of the perovskites are varying with the La/Mn atomic ratio. As the La/Mn atomic ratio further lowers to 0.63 and 0.56, Mn3O4 phase is formed besides the main phase of perovskite. Lattice vacancy at the A-sites of the perovskites is present for all the six samples, and there are an appreciable number of Mn4+ ions in the perovskite crystal according to the refinement results of the Rietveld method. XPS analyses indicate that Mn ions are enriched on the surfaces of all the samples. In addition, catalytic activity for methane oxidation is in an order of sample La0.89Mn > La1.03Mn > La0.81Mn > La0.72Mn > La0.63Mn > La0.56Mn.

Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation

Four La-Fe oxide samples with La/Fe atomic ratio y=1.02∼0.68 (denoted as LayFe) were prepared by the citrate method. The samples had a decreased specific surface area with the La/Fe atomic ratio decreasing. XRD pattern proved that the sample La0.94Fe is single phase perovskite La0.94FeO3−d. Phase composition of the samples was estimated by the Rietveld refinement method. XPS analyses indicate that La3+ ions are enriched on surface of crystallites for all the samples, and surface carbonate ions are relatively abundant on the samples La1.02Fe and La0.94Fe. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La0.68Fe>La0.76Fe>La0.94Fe>La1.02Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe3+ ion on the surface of the samples La0.68Fe and La0.76Fe.

Microwave synthesis of LaBO3 (B = Co, Fe) perovskites using graphite and citric acid additions

We studied the effects of graphite and citric acid additions on the formation of LaFeO3 and LaCoO3 perovskites upon microwave treatment of crystalline hydrates or nitrate solutions, respectively. The addition of graphite was shown to increase the yield of LaCoO3 perovskite and to result in the formation of crystalline LaFeO3 perovskite even at the microwave treatment stage. Subsequent thermal treatment at 800°C yields single-phase lanthanum ferrite with a high specific surface area (11 m2/g). Due to the addition of citric acid to nitrate solutions, a highly viscous gel forms, which allows preparation of single-phase perovskites with a high specific surface area (up to 34 m2/g) after microwave treatment and calcination. The samples obtained using the admixtures are characterized by a high catalytic activity in methane oxidation. No single-phase oxides form without introduction of these admixtures.

ChemInform Abstract: Perovskite‐Supported Palladium for Methane Oxidation — Structure‐Activity Relationships

AbstractReview: 30 refs.