Complete Oxidation Of Methane Research Articles

Identification of Active Phase for Complete Oxidation of Methane on Palladium Surface

Identification of Active Phase for Complete Oxidation of Methane on Palladium Surface

Electrochemical Promotion of Bimetallic Palladium‐Cobalt Nano‐Catalysts for Complete Methane Oxidation

AbstractThe catalytic activity and electrochemical promotion of catalysis (EPOC) of the bimetallic Pd−Co nanoparticles (10 nm) deposited on yttria‐stabilized zirconia were investigated for complete methane oxidation. The reaction was conducted under open‐circuit and under polarization in a temperature range of 320–400 °C in reducing, stoichiometric, and oxidizing conditions. The catalytic activity of the catalyst nanoparticles increased to 40 % upon positive polarization in all gaseous compositions. A comparison of three reaction conditions revealed that the highest reaction rate increase (enhancement ratio, ρ=1.4) occurs under reducing conditions. The reaction rate increased upon anodic polarization, resulting in non‐Faradaic electrochemical modification of catalytic activity (Λ≫1) in reducing and oxidizing conditions and Faradaic or electrochemical enhancement (Λ<1) in stoichiometric reaction condition. This work demonstrates that the formation of different Pd and Co oxide phases can be accurately controlled by electrochemical stimuli and, in reducing conditions, result in pseudo‐capacitor behaviour.

The interaction between Pd/CeO2 crystal surface and electric field: Application to complete oxidation of methane

Pd/CeO2 with different crystal faces can significantly affect the methane oxidation process, but the catalytic behavior in electric field is not clear yet. The supported Pd/CeO2 with rod, octahedral and cubic morphologies were synthesized by hydrothermal method, and their methane oxidation efficiency in electric field was measured. The structure of Pd/CeO2 and reaction mechanism in electric field were investigated by XRD, BET, XPS, H2-TPR, TEM and in situ DRIFTs. Studies show that the synergistic effect between electric field and (111) crystal plane in octahedral CeO2 is the strongest and (110) in rod CeO2 is the main active restraint crystal plane. The Pd atoms on the surface of Pd/CeO2-oct are exposed to the most amount of Pd atoms and the main Pd nanoparticles are present. Pd species on CeO2-cube and CeO2-rod surface with relatively low activity exist mainly in the form of Pd2+ and Pd4+, whereas Pd0 and Pd2+ on the Pd/CeO2-oct surface are more favorable for CH4 oxidation in the catalytic cycle. Pd4+ is inactive for CH4 oxidation. In addition, H migration from Pd active site to the support induced by hydrogen spillover can enhance the oxidation activity of CH4 in electric field. The conversion of carbonate/hydrogencarbonate to formate is an advantageous step in the methane oxidation process, with the active sequence being octahedral (111) > cubic (100) > rod (110) faces in/without electric field.

Understanding the reversible and irreversible deactivation of methane oxidation catalysts

Catalytic oxidation is a promising technology for controlling methane emissions from natural gas engines, but fast and severe deactivation prevents implementation. We investigated a commercial Pd on alumina oxidation catalyst under realistic conditions and identified two deactivation phenomena: fast, reversible inhibition and slow, irreversible loss of active sites. The loss of active sites occurs only during methane conversion, fortunately a brief oxygen cut-off is enough to regenerate the catalyst. Both types of deactivation increase the reduction temperature of PdO. From 36 kinetic experiments we propose a simple kinetic model encompassing both types of deactivation. The inhibition is confirmed to be due to water coverage of the active sites whereas dispersion of Pd on the surface is the cause of the irreversible loss of active sites. The new insight shows a pathway toward designing more durable catalysts for complete methane oxidation.

Catalytic Activity of Highly Dispersed Systems PdО/Mn0.20Ce0.80O2–δ and PdО/M0.05Mn0.15Ce0.80O2–δ, where M = Cu, Bi, in Complete Oxidation of Methane

Catalytic Activity of Highly Dispersed Systems PdО/Mn0.20Ce0.80O2–δ and PdО/M0.05Mn0.15Ce0.80O2–δ, where M = Cu, Bi, in Complete Oxidation of Methane

Doped Mn modulates the local charge distribution of cobalt-based spinel catalysts to promote the availability of ligand lattice oxygen for complete oxidation of methane

Doping other metals such as Mn, Ni, In, et al. is known as an effective strategy to improve the environmental catalytic performance of Co3O4. In our works, Mn ions were successfully incorporated into the lattice of Co3O4 via the oxalic acid synchronous complexation method. The Mn ions preferentially occupy octahedrally coordinated Co3+ sites and constructing the Mn-O-Co structure. In addition, theoretical calculation further verified that the enhanced catalytic performance of Co5Mn1 originated from highly reactive lattice oxygen coordinated with both Mn and Co ions. Significantly, Mn ions transfer more electrons to lattice oxygen. This oxygen sites have the lower oxygen vacancy formation energy (EOv = 1.94 eV), allowing the lattice oxygen to participate in the reaction following the MvK mechanism in the high temperature range. Collectively, this work is helpful to understand the mechanism of effect of Mn ion doping on the physical and chemical properties of Co-based catalysts.

Magnesium-enhanced redox property and surface acidity-basicity of LaMnO3 perovskites for efficient methane purification

Catalytic combustion has been applied to purify methane emissions from natural gas vehicle exhaust, yet it still requires the development of catalysts with both high activity and robust stability. Herein, non-equivalent promoter-magnesium ions (Mg2+) were introduced into LaMnO3 perovskites via a facile sol–gel method to partially substitute Mn sites and boost the catalytic performance. The incorporation of Mg induced the transition of Mn3+ to Mn4+ and generated oxygen vacancies, which improved the reactivity of lattice oxygen and facilitated the formation of adsorbed oxygen species. Turning the coordination state of Mn and O led to an augmentation of surface acid-basic sites on LaMn1-xMgxO3, which was beneficial for the activation of methane and adsorption of dissociated intermediates. The presence of abundant highly-reducible Mn4+ and surface-adsorbed oxygen species accelerated the complete oxidation of methane. Consequently, LaMn0.7Mg0.3O3 with the optimized surface acidity-basicity and redox property exhibited greatly enhanced catalytic activity, selectivity, water-/sulfur-resistance and thermal stability compared to LaMnO3. This study emphasizes the importance of synergistically adjusting surface properties to enhance catalytic purification performance.

Electrochemical Characterization of Pd-Based Catalysts Deposited on Yittria Stabilized Zirconia Solid Electrolyte for Methane Oxidation Reaction

Methane, the main constituent of natural gas, has received increased attention since it has a higher energy density than other fossil fuels while producing less particulates containing sulphur and nitrogen. However, although methane emits less CO2 than other fossil fuels, it has 28 times more global warming potential [1]. Therefore, complete methane oxidation via heterogeneous catalysis that prevents escaping unburned methane from the engine exhaust is a key solution to this issue.Palladium is the most effective conventional noble metal for methane catalytic oxidation, which is susceptible to deactivation, especially in the presence of water. The stability and activity of a monometallic Pd catalyst could be improved upon adding a second less expensive and more abundant non-noble metal or metal oxide[2]. The interaction of Pd nanoparticles and the second metal can be characterized using electrochemical means to understand the catalytic and electrocatalytic activity of catalysts[3,4].In this work, the electrochemical behaviour of PdM (M= Co, Sn, Fe and ZnO) nanoparticles deposited on YSZ solid electrolyte was evaluated for complete methane oxidation and compared to free-standing Pd catalyst. To this end, the nanoparticles were synthesized via the polyol method and tested for their open circuit catalytic oxidation and electrochemical performance in a temperature range from 320 to 400 °C under reducing, stoichiometric and oxidizing reaction conditions.The light-off experiments in open circuit condition revealed that the presence of Sn increased the catalytic rate of the reaction more than two other metals, Co and Fe, which resulted in lowering the reaction rate per unit mass of active catalyst. Furthermore, ZnO-supported Pd demonstrated the highest catalytic reactivity due to metal-support interactions. Lastly, the results demonstrate a higher level of catalytic reactivity with higher temperatures and oxygen partial pressures (pO2).Using Tafel plot calculations from the linear sweep voltammetry measurements, the exchange current density (io) was obtained for each catalyst, as well as the apparent activation energy of the reaction in the individual condition. It was found that higher exchange current density corresponds to lower open circuit catalytic rates.The catalyst was characterized thoroughly by several physicochemical techniques, such as TEM, SEM, XRD and ICP-MS.

Methane Catalytic Combustion under Lean Conditions over Pristine and Ir-Loaded La1-xSrxMnO3 Perovskites: Efficiency, Hysteresis, and Time-on-Stream and Thermal Aging Stabilities.

The increasing use of natural gas as an efficient, reliable, affordable, and cleaner energy source, compared with other fossil fuels, has brought the catalytic CH4 complete oxidation reaction into the spotlight as a simple and economic way to control the amount of unconverted methane escaping into the atmosphere. CH4 emissions are a major contributor to the 'greenhouse effect', and therefore, they need to be effectively reduced. Catalytic CH4 oxidation is a promising method that can be used for this purpose. Detailed studies of the activity, oxidative thermal aging, and the time-on-stream (TOS) stability of pristine La1-xSrxMnO3 perovskites (LSXM; X = % substitution of La with Sr = 0, 30, 50 and 70%) and iridium-loaded Ir/La1-xSrxMnO3 (Ir/LSXM) perovskite catalysts were conducted in a temperature range of 400-970 °C to achieve complete methane oxidation under excess oxygen (lean) conditions. The effect of X on the properties of the perovskites, and thus, their catalytic performance during heating/cooling cycles, was studied using samples that were subjected to various pretreatment conditions in order to gain an in-depth understanding of the structure-activity/stability correlations. Large (up to ca. 300 °C in terms of T50) inverted volcano-type differences in catalytic activity were found as a function of X, with the most active catalysts being those where X = 0%, and the least active were those where X = 50%. Inverse hysteresis phenomena (steady-state rate multiplicities) were revealed in heating/cooling cycles under reaction conditions, the occurrence of which was found to depend strongly on the employed catalyst pre-treatment (pre-reduction or pre-oxidation), while their shape and the loop amplitude were found to depend on X and the presence of Ir. All findings were consistently interpreted, which involved a two-term mechanistic model that utilized the synergy of Eley-Rideal and Mars-van Krevelen kinetics.

Highly effective CO2 splitting in a plasma-assisted membrane reactor

Membrane reactors offer a promising approach to converting CO2 into chemicals, fuels, and value-added products. However, challenges remain in its high energy input and deactivation of the catalyst. Here, we report a breakthrough strategy that uses dielectric barrier discharge (DBD) plasma to enhance the CO2 splitting in a perovskite-based La0.6Sr0.4Co0.5Fe0.5O3-δ (LSCF) membrane reactor where a traditional catalyst is not required however instead relies on the highly reactive species generated by the CO2 plasma. The CO2 conversion, physical and chemical evolution of membranes at different temperatures, CO2 plasma power, and purging gas were systematically investigated. The maximum CO2 conversion was achieved at 800 °C and 25 W while reaching 10.6%, 20.0%, and 28.0% via helium, complete oxidation of methane, and partial oxidation of methane purging on the permeate side, respectively. The experimental results obtained under low plasma power consumption suggest that the plasma-assisted membrane reactor strategy has the potential to be energy-efficient and sustainable, particularly when combined with renewable energy sources.

Unlocking High-Efficiency Methane Oxidation with Bimetallic Pd-Ce Catalysts under Zeolite Confinement.

Catalytic complete oxidation is an efficient approach to reducing methane emissions, a significant contributor to global warming. This approach requires active catalysts that are highly resistant to sintering and water vapor. In this work, we demonstrate that Pd nanoparticles confined within silicalite-1 zeolites (Pd@S-1), fabricated using a facile in situ encapsulation strategy, are highly active and stable in catalyzing methane oxidation and are superior to those supported on the S-1 surface due to a confinement effect. The activity of the confined Pd catalysts was further improved by co-confining a suitable amount of Ce within the S-1 zeolite (PdCe0.4@S-1), which is attributed to confinement-reinforced Pd-Ce interactions that promote the formation of oxygen vacancies and highly reactive oxygen species. Furthermore, the introduction of Ce improves the hydrophobicity of the S-1 zeolite and, by forming Pd-Ce mixed oxides, inhibits the transformation of the active PdO phase to inactive Pd(OH)2 species. Overall, the bimetallic PdCe0.4@S-1 catalyst delivers exceptional outstanding activity and durability in complete methane oxidation, even in the presence of water vapor. This study may provide new prospects for the rational design of high-performance and durable Pd catalysts for complete methane oxidation.

Variation in Metal-Support Interaction with TiO2 Loading and Synthesis Conditions for Pt-Ti/SBA-15 Active Catalysts in Methane Combustion.

The control of catalytic performance using synthesis conditions is one of the main goals of catalytic research. Two series of Pt-Ti/SBA-15 catalysts with different TiO2 percentages (n = 1, 5, 10, 30 wt.%) were obtained from tetrabutylorthotitanate (TBOT) and peroxotitanate (PT), as titania precursors and Pt impregnation. The obtained catalysts were characterized using X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), N2 sorption, Raman, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), hydrogen temperature-programmed reduction (H2-TPR) and H2-chemisorption measurements. Raman spectroscopy showed framework titanium species in low TiO2 loading samples. The anatase phase was evidenced for samples with higher titania loading, obtained from TBOT, and a mixture of rutile and anatase for those synthesized by PT. The rutile phase prevails in rich TiO2 catalysts obtained from PT. Variable concentrations of Pt0 as a result of the stronger interaction of PtO with anatase and the weaker interaction with rutile were depicted using XPS. TiO2 loading and precursors influenced the concentration of Pt species, while the effect on Pt nanoparticles' size and uniform distribution on support was insignificant. The Pt/PtO ratio and their concentration on the surface were the result of strong metal-support interaction, and this influenced catalytic performance in the complete oxidation of methane at a low temperature. The highest conversion was obtained for sample prepared from PT with 30% TiO2.

Methane Oxidation over the Zeolites-Based Catalysts

Zeolites have ordered pore structures, good spatial constraints, and superior hydrothermal stability. In addition, the active metal elements inside and outside the zeolite framework provide the porous material with adjustable acid–base property and good redox performance. Thus, zeolites-based catalysts are more and more widely used in chemical industries. Combining the advantages of zeolites and active metal components, the zeolites-based materials are used to catalyze the oxidation of methane to produce various products, such as carbon dioxide, methanol, formaldehyde, formic acid, acetic acid, and etc. This multifunction, high selectivity, and good activity are the key factors that enable the zeolites-based catalysts to be used for methane activation and conversion. In this review article, we briefly introduce and discuss the effect of zeolite materials on the activation of C–H bonds in methane and the reaction mechanisms of complete methane oxidation and selective methane oxidation. Pd/zeolite is used for the complete oxidation of methane to carbon dioxide and water, and Fe- and Cu-zeolite catalysts are used for the partial oxidation of methane to methanol, formaldehyde, formic acid, and etc. The prospects and challenges of zeolite-based catalysts in the future research work and practical applications are also envisioned. We hope that the outcome of this review can stimulate more researchers to develop more effective zeolite-based catalysts for the complete or selective oxidation of methane.

Catalytic Activity of Highly Dispersed Systems PdО/Mn0.20Ce0.80O2–δ and PdО/M0.05Mn0.15Ce0.80O2–δ, where M = Cu, Bi, in Complete Oxidation of Methane

Catalytic Activity of Highly Dispersed Systems PdО/Mn0.20Ce0.80O2–δ and PdО/M0.05Mn0.15Ce0.80O2–δ, where M = Cu, Bi, in Complete Oxidation of Methane

Reaction pathways of oxidative coupling of methane on lithiated lanthanum oxide

The reaction pathways of oxidative coupling of methane (OCM) on lithium-doped lanthana were investigated using CH4CH4/O2 step reactions. As compared with those of the undoped catalysts, the results demonstrate that the lithiated lanthana first initiates activation of methane on surface oxygen species and then catalyzes the OCM, followed by production of oxidation products via chain reaction pathways. The role of doped lithium and effect of CH4/O2 ratios for the OCM were also studied. The presence of lithium was believed to promote the reaction via formation of carbonates, which gives rise to the oxidized surface that catalyzes conversion of ethane to ethylene together with complete oxidation of methane. This was also confirmed by experimental results and theoretical DFT calculations, which indicates that C2H6 and C2H4 are favored in O2-lean and O2-rich atmospheres, respectively. The activation energy of deoxyhydrogenation for C2H6 to C2H4 follows an order of La2O3 > Li/La2O3 > O2-Li/La2O3, explaining the variation of product selectivity in different CH4/O2 compositions.

Colloidally Engineered Pd and Pt Catalysts Distinguish Surface- and Vapor-Mediated Deactivation Mechanisms

Noble metal-based catalysts are ubiquitous because of their high activity and stability. However, they irreversibly deteriorate over time especially in high-temperature applications. In these conditions, sintering is the main reason for deactivation, and understanding how sintering occurs gives the opportunity to mitigate these detrimental processes. Previous studies successfully distinguished between two fundamental sintering modes, namely, particle migration and coalescence (PMC) and Ostwald ripening (OR). However, differentiation between surface- and vapor-mediated Ostwald ripening processes has not been demonstrated yet, even though it is crucial information to tune metal/support interactions and stabilize catalysts. Here, we demonstrate that surface- and vapor-mediated ripening occur in two distinct regimes of temperature with some overlap using Pt and Pd catalysts prepared from colloidal nanocrystals as precursors. By either co-impregnating the two metal nanocrystals on the same grain of alumina support or by physically mixing powders of the two distinct metal catalysts, we tune the intermetal particle distance between nanometers and micrometers. We then use methane complete oxidation as a reporter reaction that occurs at higher rates on pure Pd and lower rates on alloyed Pd/Pt catalysts to trace the movement of Pt in the system. Aging the catalysts at different temperatures allows us to reveal that Pt initially sinters by surface-mediated ripening until ∼750 °C, but at temperatures above 800 °C, vapor-mediated ripening by PtO2 becomes the main sintering mechanism. This work demonstrates how colloidal catalysts allow unique insights into the working and deactivation mechanisms of supported systems.

Autonomous kinetic model identification using optimal experimental design and retrospective data analysis: methane complete oxidation as a case study

An uncertainty-aware autonomous flow reactor platform was developed by combining automation and feedback optimization. The platform was applied to identify appropriate kinetic models online for a gas–solid catalytic reaction.

Effect of electric field and water on Pd-based catalyst for complete oxidation of methane: Study on promotion and deactivation

Methane (CH4) is an important greenhouse gas, and its ability to cause greenhouse effect is 25 times of the same amount of CO2. Electric field is one of the effective ways to reduce the ignition temperature of CH4 catalytic oxidation. In this paper, a Pd-based catalyst with Ce0.75Zr0.25-γ-Al2O3 composite support was synthesized by impregnation and its activity and water resistance were evaluated. The effect of water and electric field on the properties of catalyst was investigated by XRD, XPS, BET, TEM, H2-TPR and in-situ DRIFT. The results showed high CH4 conversion was maintained in the presence of water because the aggregation of Pd specie was prevented by electric field. After 24 h operation in electric field, catalyst’s lattice structure remained stable, however, a loss of surface Pd active sites due to the strong metal-support interaction and the formation of Pd-OH were observed, and finally reduced its thermal activity. Pd-OH, the inhibited site in thermal reaction, can be used as active site to participate in CH4 oxidation process, which was the key factor in maintaining high activity of the catalyst in electric field.

Robust Direct Hydrocarbon Solid Oxide Fuel Cells with Exsolved Anode Nanocatalysts

Perovskite anodes with in situ exsolved nanocatalysts have been proven to overcome carbon deposition and increase anode catalytic activity as an alternative to conventional Ni/YSZ anodes for direct hydrocarbon solid oxide fuel cells (SOFCs). This study, for the first time, demonstrates the state-of-the-art exsolution over cathode-supported SOFCs, which achieve the highest cell performance compared to conventional electrolyte-supported SOFCs with perovskite anodes using CH4 as a fuel. The dendritic channel structure of cathode supports retains a high active surface during high-temperature electrolyte sintering. Sr2Ti0.8Co0.2FeO6-δ perovskite ceramic is employed as anodes, and Co-Fe alloy nanoparticles are exsolved after reduction, which increases the cell power output by about 40%. The peak power densities of the cells are 0.82, 0.59, 0.43, and 0.33 W cm-2 at 800 °C using hydrogen, methane, methanol, and ethanol, respectively. The SOFCs with the exsolved nanocatalysts demonstrate stable power generation up to 110 h using methane, methanol, and ethanol fuels. Interestingly, the perovskite anodes show high methane fuel utilization by the complete oxidation of methane, which is in contrast to the partial oxidation over Ni catalysts. Robust hydrocarbon SOFCs have been developed by coupling anode catalyst exsolution with dendritically channeled cathode supports.

On the Effect of the Synthesis Route of the Support in Co3O4/CeO2 Catalysts for the Complete Oxidation of Methane.

Six ceria supports synthesized by various synthesis methodologies were used to deposit cobalt oxide. The catalysts were thoroughly characterized, and their catalytic activity for complete methane oxidation was studied. The supports synthesized by direct calcination and precipitation with ammonia exhibited the best textural and structural properties as well as the highest degree of oxidation. The remaining supports presented poorer textural properties to be employed as catalytic supports. The cobalt deposited over the first two supports presented a good dispersion at the external surface, which induced a significant redox effect that increased the number of Co3+ ions on their surface. Consequently, the presence of highly active lattice oxygen species on the surface of these catalysts was favored. Additionally, the optimal active catalyst (Co-DC) revealed a significant resistance to water vapor inhibition, owing to the high hydrophobicity of the ceria support.