Catalytic Oxidation Of Methane Research Articles

Effect of Fe doping on the surface properties of δ-MnO2 nanomaterials and its decomposition of formaldehyde at room temperature

In this paper, δ-MnO2 and Fe/δ-MnO2-X (X = 0, 1, 2 and 5) nanomaterials were prepared by a one-step water bath co-precipitation method, and then their performance in catalytic oxidation of HCHO at room temperature was investigated. The catalytic oxidation performance of Fe/δ-MnO2-5 nanomaterials for formaldehyde was significantly improved by the modification with iron doping and sulfuric acid, and the conversion of 3.35 ppm HCHO reached 99.4% within 8 h at room temperature. Next, the nanomaterials were characterized by X-ray diffraction (XRD), spectroscopy scanning electron microscope (SEM), High-resolution transmission electron microscopy (HR-TEM)，Brunauer-Emmett-Teller (BET), temperature-programmed reduction by H2 (H2-TPR), X-ray photoelectron spectroscopy (XPS) methods to investigate the structural properties of the nanomaterials. It was found that the specific surface area of the composite nanomaterials increased, the particle distribution was more uniform, and the distribution of Mn on the catalyst surface changed significantly. Among them, the content of Mn3+ increased significantly, which was beneficial to the catalytic oxidation of formaldehyde.

Selective catalytic oxidation of formaldehyde on single V- and Cr-atom decorated magnetic C4N3 substrate: A first principles study

Formaldehyde (HCHO) is the most common indoor hazardous pollutant and has attracted great concern because its long-term exposure has adverse health effects on humans. Retention and catalytic oxidation of highly hazardous HCHO is an efficient and environmentally friendly method to use for air remediation, but a major obstacle to this procedure is the lack of an appropriate catalyst. Herein, two-dimensional magnetic C4N3 material with a 3d-transition metal as activate sites was systemically investigated in HCHO oxidation using density functional theory calculations. The results show that V-C4N3 and Cr-C4N3 have high structural stability and shallow activation barriers for O2 decomposition; these characteristics provide the necessary precursors for the subsequent oxidation reaction. Moreover, the V-C4N3 and Cr-C4N3 catalysts have unique selective adsorption and catalysis toward HCHO in a mixture of some typical in-door volatile organic compounds (VOCs) and air. The corresponding dynamic barrier for each reaction step was investigated and the mechanism involved in HCHO oxidation was revealed in detail. Aggregation of metal atoms in the V-C4N3 and Cr-C4N3 catalysts is prevented by enormous diffusion resistance, and this is further confirmed by AIMD simulations. These results provide insightful guidance for developing advanced magnetic catalysts for HCHO oxidation to improve the remediation of air contaminants.

Interfacial dependent reactive oxygen species generation over Pt-ZrO2 nanoparticles for catalytic oxidation of formaldehyde at room temperature

Producing the strong metal-support interaction (SMSI) interface and highly reactive oxygen species is a great challenge for catalytic oxidation of formaldehyde (HCHO) at room temperature over supported metal catalysts. Herein, we report a conceptual oxygen vacancy (VO) associated SMSI interface of Pt-ZrO2 nanoparticles for HCHO catalysis at room temperature, exhibiting interfacial dependent reactive oxygen species (O*) formation. The VO on the ZrO2 support captures and activates the molecular O2, then proceeds to generating the reactive oxygen atom (O*) with a lower activation barrier of 0.3 eV. The generated O* tends to link the (110) surface of ZrO2 and the (111) surface of Pt as an electronic transmission channel for the selective catalytic oxidation of HCHO adsorbed on the (110) surface of ZrO2, accelerating the direct generation of formate species and mineralization of HCHO. The Pt-VO-ZrO2 achieves high HCHO removal and HCHO conversion (>95%) at 20 °C. These findings will consolidate the fundamental theories of room temperature catalytic reactions via constructing the SMSI interface engineering for wide environmental applications.

Evaluation of three-dimensionally ordered macroporous LaFe1−xCoxO3 perovskites and their performance for catalytic oxidation of methane

Nowadays, excess methane emissions have caused serious environmental problems, of which catalytic oxidation is a practical and economically feasible technique for its decomposition and harm reduction. Here, three‐dimensionally ordered macroporous (3DOM) LaFe1−xCoxO3 (x = 0, 0.1, 0.3, 0.5) were prepared by colloidal crystal template (CCT) method using polynomial methacrylate (PMMA) as templates in order to evaluate their catalytic activities for methane. Numerous analytical techniques were used to characterize the physicochemical properties of as-prepared perovskite catalysts. The results showed that the morphology of the catalysts depended strongly on the PMMA template and an appropriate calcination temperature was decisive for the formation of a high-quality 3DOM structure, which affected the textural parameters, the surface compositions of the catalysts and further determined their catalytic activities for methane. Along with the doping of Co for Fe at the B site, the BET surface areas and crystallite sizes could be optimized, and the ratios of Fe4+/Fe3+, absorbed oxygen (Oads)/lattice oxygen (Olat) both increased with the increment of Co substitution, which were beneficial to improve the catalytic activity on CH4. However, the surface carbonate or hydroxide compounds also increased with the increment of Co doping and did harm to the catalytic activity in an opposite way. The final result was that the 3DOM LaFeO3 calcined at 700 ℃ (Ea = 70.9 kJ/mol, T50% = 548 ℃, T90% = 652 ℃), with the optimal pore size, pore volume, Fe4+/Fe3+ ratio, H2-TPR reducibility, on stream testing stability and carbon deposition resistance in low temperature, showed much better performance than the Co-doped 3DOM LaFe1−xCoxO3 (x = 0.1, 0.3, 0.5) in the methane catalytic process. However, it was also predictable that the performance of methane oxidation by Co-doped perovskites can be improved if the surface carbonate or hydroxide compounds can be effectively controlled in the future by surface engineering methods like acid treated or others to eliminate La segregation on its surface.

Impact of Pd:Pt ratio of Pd/Pt bimetallic catalyst on CH4 oxidation

Bimetallic Pd/Pt catalysts have recently attracted considerable attention in controlling emission control of methane. Here, we show the impact of Pd/Pt ratio (1:0, 9:1, 3:1, 1:1, 1:3, 1:9, 0:1) on the catalytic methane oxidation under simulated exhaust environments from the CH4-rich to -lean feed. Under dry feed conditions, the CH4 oxidation activity was linear to the Pd:Pt ratio, where the Pd-only (1:0) exhibited the best performance. However, under wet feed conditions containing 5% of water vapor, bimetallic Pd/Pt catalysts having a small fraction of Pt showed superior overall activities in a wide range of reaction environments. The interaction of Pd-Pt, forming active PdO-Pt pair sites, was confirmed by a series of analysis such as STEM-EDS, XPS and H2-TPR. The evidence by DFT calculations relates the distinct properties of bimetallic Pd/Pt sites to (i) stable CH4 adsorption, (ii) facile C–H bond cleavage and (iii) weaker adsorption of water on Pt site.

Guiding questions for catalyst synthesis and use in selective catalytic oxidation reactions

This paper presents a brief history of catalysis, as well as of the processes of selective oxidation of hydrocarbons. On the other hand, the basic concepts involved in heterogeneous catalysis are mentioned, emphasizing the role of catalytic materials in chemical oxidation processes and posing a series of guiding questions to be followed when approaching a process catalyzed by solid materials. In the same way, the methods of synthesis of catalysts called in the literature as sol-gel and impregnation are shown, identifying the influence of each stage of preparation with the physical and chemical properties of the materials. Finally, a case study applied to the selective catalytic oxidation of methane and methanol using iron, molybdenum and vanadium catalytic materials synthesized by the sol-gel method is presented.

Morphology effects of CeO2 for catalytic oxidation of formaldehyde

Formaldehyde (HCHO) is a primary indoor air pollutant, and can cause serious environmental and health problems. In this study, CeO2 spherical-like aggregate of nanoplates (CeO2-S) were prepared for the HCHO oxidation, and the efficiency of CeO2-S was compared with those of CeO2 nanorods and nanocubes. CeO2-S showed excellent activity, with a maximum HCHO conversion of 87% at 120 °C using a gas hourly space velocity of 10 000 mL·gcat−1·h−1 and HCHO concentration of approximately 500 ppm. The catalysts were characterized using thermal and spectroscopic techniques, revealing that the excellent catalytic performance of CeO2-S originated in its abundant surface hydroxyl and oxygen groups. In situ FT-IR results indicated that dioxymethylene and formates were the main intermediates in HCHO oxidation. Further oxidation of formates produced CO2 and H2O, and CO2 reacted with surface hydroxy groups to form carbonate species, which accumulated on the catalyst surface, thus deactivating the CeO2 catalysts and inhibiting complete conversion of HCHO.

Optimization of Non-thermal Plasma-Assisted Catalytic Oxidation for Methane Emissions Abatement as an Exhaust Aftertreatment Technology

While methane-powered vehicles produce fewer greenhouse gas emissions in comparison to conventional fuel vehicles, there is a significant amount of methane slip in their exhaust that needs to be treated. This study investigates non-thermal plasma (NTP) assisted catalytic methane oxidation as an alternative method for the low temperature methane slip abatement applicable to the exhaust of biogas methane-powered vehicles. It is concluded that high CH4 conversion and CO2 selectivity can be obtained using NTP-catalysis at low temperature with Pd/Al2O3 found to be the most promising candidate among all catalysts tested. In addition, it was found that CH4 conversion efficiency was dependent on the feed gas components and gas hourly space velocity as well as how the activation energy is introduced. For example, a combination of plasma and external heat supply provides advantages in terms of CH4 conversion along with lower plasma energy consumption. The presence of N2 and O2 in the feed gas during NTP-catalytic methane oxidation results in unfavourable NOX formation which linearly increases with CH4 conversion. These results conclude that the most suitable aftertreatment option involves the combination of an oxidation catalyst with plasma to target the hydrocarbon and CH4 oxidation, followed by an ammonia-SCR system to convert the NOX formed in plasma assisted zone.

SnO2 doped NiO heterostructure nanofibers prepared by electrostatic spinning: A novel sensor for catalytic oxidation of formaldehyde

Up to now, there were a large number of literatures on the detection of gaseous formaldehyde (HCHO), however the liquid HCHO, which had the same harmfulness as gaseous HCHO, was rarely paid attention to by people. In this work, the electrospinning nanomaterial preparation process was used to fabricate the liquid HCHO electrochemical sensor. Firstly, five different sets of electrode materials were set up, and the material with the best results (SnO2/NiO NFs (No. 3)) was tested by scanning electron microscopy (SEM), X-ray diffraction (XRD), cyclic voltammetry (CV), and electrochemical frequency impedance (EIS). Then the detection principle of the electrochemical sensor and the electrode reaction process of catalytic oxidation of HCHO were analyzed. Finally, the limit of detection (LOD), detection range, selectivity, and stability of the SnO2/NiO NFs (No.3) for the catalytic oxidation reaction of HCHO were analyzed by CV and potential polarization methods.

Manganese Catalyzed Partial Oxidation of Light Alkanes

The catalytic partial oxidation of methane is achieved at low temperatures (<200 °C) using manganese oxides and manganese salts in mixtures of trifluoroacetic acid and trifluoroacetic anhydride. Dioxygen is used as the in situ terminal oxidant. For Mn oxides (e.g., MnO2, Mn2O3, and Mn3O4), we studied stoichiometric methane partial oxidation in HTFA (TFA = trifluoroacetate). Using a Mn trifluoroacetate salt, at 180 °C and under 25 psig of methane, product selectivity for the mono-oxidized product methyl trifluoroacetate (MeTFA) is observed to be >90% at ∼35% methane conversion at approximately 6 turnovers. Under our catalytic methane oxidation reaction conditions, MeTFA is stable against overoxidation, which explains the likely high selectivity at conversions >15%. Using combined experimental studies and DFT calculations, a mechanism involving soluble and molecular Mn species in the catalytic cycle is proposed. The proposed reaction pathway involves initial activation of MnII by dioxygen, cleavage of a methane C–H bond by a MnIII hydroxo intermediate, rebound of the methyl radical to generate MeTFA, and finally regeneration of the starting MnII complex. Also, this process is shown to be applicable to the oxidation of ethane, favoring the mono-oxidized product ethyl trifluoroacetate (EtTFA) and reaching ∼46% conversion.

Guiding questions for the synthesis of catalyst and its application in selective catalytic oxidation reactions

This paper presents a brief history of catalysis, as well as the processes of selective oxidation of hydrocarbons. On the other hand, the basic concepts involved in heterogeneous catalysis are mentioned, emphasizing the role of catalytic materials in chemical oxidation processes and posing a series of guiding questions to be followed when approaching a process catalyzed by solid materials. In the same way, the methods of synthesis of catalysts known in the literature as sol-gel and impregnation are shown, identifying the influence of each stage of preparation with the physical and chemical properties of the materials. Finally, a case study applied to the selective catalytic oxidation of methane and methanol using iron, molybdenum, and vanadium catalytic materials synthesized by the sol-gel method is presented.

A rod-like Co3O4 with high efficiency and large specific surface area for lean methane catalytic oxidation

We used SBA-15 as a hard template to successfully prepare a Co3O4 (Co3O4-r) catalyst with a large specific surface area for the catalytic oxidation of lean methane. The catalyst showed excellent performance for the catalytic oxidation of methane, with T50 at 320 °C and T90 at 385 °C. The activity was much higher than that of the Co3O4 cube and Co3O4 sphere. Co3O4-r showed good activity not only in humid conditions but also in high space velocity. Based on the XPS, H2-TPR, O2-TPD and BET analyses, it indicates that Co3O4-r has a larger surface area and more oxygen vacancies than Co3O4 cube and Co3O4 sphere catalysts. For the possible mechanism of the reaction, the carbonate intermediates in the reaction were determined by in-situ DRIFTS.

Ultra-light 3D MnO2-agar network with high and longevous performance for catalytic formaldehyde oxidation

Under the background of indoor air formaldehyde decontamination, a freestanding ultra-light assembly was fabricated via ice-templating approach starting from MnO2 nanoparticles and environmentally benign agar powder. The 3D composite of agar and MnO2 (AM-3D) was comparatively studied with powdered counterparts (including pure MnO2 and mixture of agar and MnO2) and the 3D-structured agar for formaldehyde oxidation, and their physicochemical properties were examined with XRD, ATR, SEM, XPS, isothermal N2 adsorption, ESR, Raman, CO-TPR and O2-TPD. For the single test of formaldehyde oxidation, the AM-3D catalyst exhibited 62.0%–67.0% removal percentage for ~400 mg/m3 formaldehyde, which did not demonstrate significant advantage over the control samples. However, thanks to the porous 3D agar scaffold with large spatial volume that could promote a rapid gas-phase formaldehyde concentration reduction, and the strong interaction between the dispersed MnO2 particles and agar substrate that could afford a large amount of reactive oxygen species to further oxidize the adsorbed formaldehyde, the AM-3D composite was a much better HCHO-to-CO2 converter and possessed much more advantageous stability for repeated cycles of formaldehyde oxidation: even after ten cycles, there was still 41.7% of formaldehyde removed. Furthermore, the viable sunlight irradiation could easily restore the activity of the used AM-3D catalyst back to the level approaching that of the fresh one. Finally, reaction pathways were put forward via the infrared spectroscopic and ion chromatographic investigations on the surface intermediates of the spent materials.

Insight into deactivation of the carbon-/sintering-resistant Ni@Silicalite-1 for catalytic partial oxidation of methane to syngas

Ni-based catalysts show good activity and selectivity for the catalytic partial oxidation of methane (CPOM) to syngas, but still suffer from carbon deposition and Ni-sintering. Herein, Ni nanoparticles of 3–7 nm are successfully embedded inside the crystal of silicalite-1 (S-1) zeolite via one-pot hydrothermal synthesis. The Ni@S-1 catalysts, with high activity/selectivity, show high carbon-/sintering-resistance but still deactivates fast. The surface of Ni nanoparticles involved in the reaction is oxidized to form intermediate NiO species, which may react with the circumambient SiO2 (from S-1) in the presence of water vapor (by-product and/or intermediate-product of CPOM reaction) to form a dense Ni3(Si2O5)2(OH)2 layer around Ni nanoparticles. The formed Ni3(Si2O5)2(OH)2 compounds are hard to be reduced (even in H2 at 800 °C; thus losing capacity to involve catalytic reaction cycle) while completely blocking the Ni0 sites from contacting with reactants, which is the main cause for the Ni@S-1 deactivation. Not surprisingly, such deactivation problem can be solved easily by replacing Ni with highly anti-oxidative metals such as Rh.

Selective Catalytic Oxidation of Methane to Methanol in Aqueous Medium over Copper Cations Promoted by Atomically Dispersed Rhodium on TiO2

AbstractDirect conversion of methane into value‐added chemicals, such as methanol under mild conditions, is a promising route for industrial applications. In this work, atomically dispersed Rh on TiO2 suspended in an aqueous solution was used for the oxidation of methane to methanol. Promoted by copper cations (as co‐catalyst) in solution, the catalysts exhibited high activity and selectivity for the production of methanol using molecular oxygen with the presence of carbon monoxide at 150 °C with a reaction pressure of 31 bar. Millimole level yields of methanol were reached with the selectivity higher than 99 % using the Rh/TiO2 catalysts with the promotion of the copper cation. CO was the reductive agent to generate H2 from H2O, which led to the formation of H2O2 through the reaction of H2 and O2. Atomically dispersed Rh activated the C−H bond in CH4 and catalyzed the oxidation using H2O2. Copper cations maintained the low‐valence state of Rh. Moreover, copper acted as a scavenger for suppressing the overoxidation, thus leading to the high selectivity of methanol.

Selective Catalytic Oxidation of Methane to Methanol in Aqueous Medium over Copper Cations Promoted by Atomically Dispersed Rhodium on TiO2.

Direct conversion of methane into value-added chemicals, such as methanol under mild conditions, is a promising route for industrial applications. In this work, atomically dispersed Rh on TiO2 suspended in an aqueous solution was used for the oxidation of methane to methanol. Promoted by copper cations (as co-catalyst) in solution, the catalysts exhibited high activity and selectivity for the production of methanol using molecular oxygen with the presence of carbon monoxide at 150 °C with a reaction pressure of 31 bar. Millimole level yields of methanol were reached with the selectivity higher than 99 % using the Rh/TiO2 catalysts with the promotion of the copper cation. CO was the reductive agent to generate H2 from H2 O, which led to the formation of H2 O2 through the reaction of H2 and O2 . Atomically dispersed Rh activated the C-H bond in CH4 and catalyzed the oxidation using H2 O2 . Copper cations maintained the low-valence state of Rh. Moreover, copper acted as a scavenger for suppressing the overoxidation, thus leading to the high selectivity of methanol.

Formaldehyde oxidation on Pd/USY catalysts at room temperature: The effect of acid pretreatment on supports

The complete catalytic oxidation of formaldehyde (HCHO) to CO2 and H2O at room temperature is a green route for indoor HCHO removal. Zeolite is an excellent carrier material for HCHO oxidation due to its large surface area, intricate pores and high adsorption capacity. However, the zeolite-supported noble metal catalysts have currently shown relatively low activity especially at room temperature. In this work, we present a facile acid treatment strategy for zeolite catalysts to improve the hydroxyl concentration and further enhance their catalytic activity for HCHO oxidation. Activity tests illustrated that HCHO could be completely oxidized to CO2 and H2O at a nearly 100% conversion rate with a weight hourly space velocity (WHSV) of 150,000 mL/(g∙hr) at 25°C, when the support of Pd/USY catalysts was pretreated by hydrochloric acid with a concentration of 0.20 mol/L. The characterization results revealed that the active hydroxyl groups originated from the dealumination in the acid treatment play a key role in the HCHO oxidation reaction. The deduced reaction mechanism suggests that bridging hydroxyl groups may oxidize HCHO to dioxymethylene (DOM) species and terminal hydroxyl groups are responsible for the transformation of DOM groups to formate (HCOO) species.

Amino acid-deep eutectic solvents/LaCoO3 mutualism system:Forming La-Co-C-O hybrid for low temperature methane catalytic oxidation

Low-temperature high-efficiency catalysts are key for complete methane oxidation, whereas common perovskite catalysts still have much room to improve. Herein, a unique La-Co-C-O hybrid (phase composition of La2O2CO3/LaCoO3/Co3O4) was successfully synthesized via an amino acid-deep eutectic solvent/LaCoO3 mutualism system. The C-O bond connects each phase and decreases the bond energy of active Co-O, as well as a higher Co valence state and lattice oxygen content in the hybrid. The light-off temperature of the hybrid for complete methane oxidation is 366 °C, which is 112 °C lower than the light-off temperature of LaCoO3. The high conversion rate remains in the presence of water vapour or long-term reactions. Such excellent catalytic performance of the hybrid can be attributed to the broad path of methane adsorption and C–H cleavage on acid-basic sites. This work hence provides a facile process to tailor highly active catalysts for complete methane oxidation.

Increase the number of active sites in Cu-MOR through NO/NH3 pretreatment for catalytic oxidation of methane to methanol

Copper-zeolites have been reported to catalyze the direct conversion of methane to methanol. In this work, we report that pretreatment of copper-mordenite (Cu-MOR) using a gas mixture of NO and NH3 (NO/NH3) results in remarkably increased activity for the direct oxidation of methane to methanol. The methanol yield over the pretreated catalysts reaches as high as 106 μmol/gcat, compared to 28 μmol/gcat over the un-pretreated Cu-MOR. The NO/NH3 gas pretreatment redisperses CuO nanoparticles to Cu ions with catalytically favorable Cu oxidation state.

Metal–organic frameworks derived Ag/Co3O4–MnO2 for the catalytic oxidation of formaldehyde

Metal–organic frameworks derived Ag/Co3O4–MnO2 for the catalytic oxidation of formaldehyde