Electrochemical Oxidation Of Methane Research Articles

Indirect Electrooxidation of Methane to Methyl Bisulfate on a Boron‐Doped Diamond Electrode

AbstractAlthough highly desired and studied for decades, direct methane functionalization to liquid products remains a challenge. We report an electrochemical system using a boron‐doped diamond (BDD) anode in concentrated sulfuric acid that is able to convert methane to methyl bisulfate and methanesulfonic acid without the use of a catalyst by indirect electrochemical oxidation. Due to its high material stability, BDD can be operated at high current densities. High temperature (140 °C) and pressure (70 bar) support the formation of methyl bisulfate to concentrations as high as 160 mM in 3 h and methanesulfonic acid to concentrations of up to 750 mM in 8 h. We present a novel way of catalyst‐free electrochemical methane oxidation and show general trends and limitations of this reaction.

Fundamental insight into electrochemical oxidation of methane towards methanol on transition metal oxides

Electrochemical oxidation of CH4 is known to be inefficient in aqueous electrolytes. The lower activity of methane oxidation reaction (MOR) is primarily attributed to the dominant oxygen evolution reaction (OER) and the higher barrier for CH4 activation on transition metal oxides (TMOs). However, a satisfactory explanation for the origins of such lower activity of MOR on TMOs, along with the enabling strategies to partially oxidize CH4 to CH3OH, have not been developed yet. We report here the activation of CH4 is governed by a previously unrecognized consequence of electrostatic (or Madelung) potential of metal atom in TMOs. The measured binding energies of CH4 on 12 different TMOs scale linearly with the Madelung potentials of the metal in the TMOs. The MOR active TMOs are the ones with higher CH4 binding energy and lower Madelung potential. Out of 12 TMOs studied here, only TiO2, IrO2, PbO2, and PtO2 are active for MOR, where the stable active site is the O on top of the metal in TMOs. The reaction pathway for MOR proceeds primarily through *CH x intermediates at lower potentials and through *CH3OH intermediates at higher potentials. The key MOR intermediate *CH3OH is identified on TiO2 under operando conditions at higher potential using transient open-circuit potential measurement. To minimize the overoxidation of *CH3OH, a bimetallic Cu2O3 on TiO2 catalysts is developed, in which Cu reduces the barrier for the reaction of *CH3 and *OH and facilitates the desorption of *CH3OH. The highest faradaic efficiency of 6% is obtained using Cu-Ti bimetallic TMO.

Active oxygen promoted electrochemical conversion of methane on two-dimensional carbide (MXenes): From stability, reactivity and selectivity

The electrochemical oxidation of methane is promising under mild conditions, while the Faraday efficiency is restricted at low temperatures due to very considerable required overpotentials activating the parasitic formation of oxygen. Herein, by means of large-scale density functional theory (DFT) computations, we systematically explored the electrochemical methane conversion promoted by active oxygen (O*) on 66 two-dimensional carbides (MXenes). Most MXenes are stabilized with O-terminated surface under external potential. Through the established OER activity volcano map, we showed that the materials on the right side of the volcano (corresponding to ΔEOH*–ΔEO* > −1.80 eV) satisfy the stability condition of O*. Taking into account the CH activation, we presented that there is a trade-off between the stability of O* and the reactivity toward CH activation, where TaHf2C2O2 and CrHf2C2O2 can be the promising materials for electrochemical methane conversion with a low energy barrier (<1 eV) for CH activation while inhibiting oxygen evolution. Furthermore, we found that the p-band center of O* is an effective descriptor for catalyst design in terms of stability and reactivity. And we showed that the selectivity on the screened MXenes varies under different potentials, mainly oxygenated derivatives, which stems from more metal-O* bonding orbital filling at the active site. These results demonstrate the application potential of MXenes for electrochemical conversion of methane and provide a feasible strategy for rational design and rapid screening of efficient catalysts.

Shuttle-like core-shell gold nanorod@Ag-Au nanostructures: Shape control and electrocatalytic activity for formaldehyde oxidation

Bimetallic nanostructures have attracted much attention due to their special properties in optics, surface plasmon and catalysis. In this work, two synthesis routes of core/shell gold nanorod/silver (AuNR@Ag) nanostructures were revisited to pursue the control factor of shuttle-like shapes, which demonstrated the key role of residual Au precursor in the formation of shuttle-like shapes. A novel route was adopted to verify the shape control effect of Au and fabricate shuttle-like AuNR@Ag-Au nanostructures. The presence of gold in the shell growth solution leads to shuttle-like shapes of bimetallic nanostructures, and the Au/Ag molar ratio controls shape details of the shuttle-like nanostructures. Then, the electrocatalytic activity of these core-shell nanostructures in the electrochemical oxidation of formaldehyde in alkaline solutions was investigated. In the forward scan the peaks at +0.22–0.3 V for all samples correspond to the electrochemical oxidation catalyzed by monometallic Ag or Au atoms, but the low potential peak at about −0.1 V for AuNR@Ag-Au sample with less Au alloy shell displays the oxidation reaction catalyzed by the adjacent Ag and Au atoms. The sample with less Au alloy shell also shows higher catalytic activity. Additionally, the surface reaction mechanism is proposed for the electrocatalytic oxidation of formaldehyde in alkaline conditions.

Electrochemical CH4 Oxidation on Spinel NiCo2O4-Based Composite Catalysts

CH4 is the main component of natural gas, which represents 21.4 % of the total primary sources of energy in the world; it is an essential fuel for both daily human life and chemical industry. The increasing emissions of methane is considered as a crucial issue because its effect as a greenhouse gas is more than 30 times potent than that of CO2. One attractive solution to cut-off the impact of methane emission is performed by oxidizing methane into liquid high-value fuels, such as methanol. For decades, the conversion of methane into liquid fuels has been a significant challenge for heterogeneous catalysis. Nevertheless, to yield higher efficiencies, complicated multistage processes should be utilized, which require higher pressures and temperatures and consequently extended infrastructure. On the contrary, the electrochemical oxidation of methane could be a far more promising approach. The challenge of producing liquid intermediate products at elevated rates and selectivity lies in activating CH4 while suppressing the competing oxygen evolution reaction (OER) and its complete oxidation into carbon dioxide, which is the most thermodynamically favorable product. It is worth mentioning that the direct electrochemical oxidation of CH4 at mild conditions is extremely sluggish. Alternatively, the CH4 oxidation could be achieved via exploiting the partial OER. The reaction would take place purely through a thermal catalytic step, whereby coupling the adsorbed oxygen atom (*O) on the active sites governed by the first two elementary reactions in OER with CH4 in the vicinity to it. Here, nickel-cobalt oxide (NiCo2O4)-based nanowire electrodes for partial oxidation of CH4 are reported. The NiCo2O4-based materials could achieve elevated electrochemical performance by its resilience in terms of controlling the pore properties and morphology. Nanowire catalysts with porous structures are attained by the facile decomposition of the adsorbed cetrimonium bromide during the calcination process. The spinel NiCo2O4 is further improved by incorporating metal oxides, i.e., RuO2 and V2O5, oxygen delivery sources. Moreover, the synergistic effects of the NiCo2O4 and metal oxides would enhance the conductivity and its bi-functional catalytic activity towards favoring CH4 and O* coupling. The electrocatalysts are investigated at room temperature in a liquid-phase electrolyzer, where the CH4 gas and anolyte flows are separated by a carbon-based gas diffusion layer that sandwiches the reaction interface between separate conductive and hydrophobic supports. Keywords : methane oxidation; NiCo2O4; synergistic effects; oxygen evolution reaction; gas diffusion electrode; spinel.

An optimized nickel phosphate/carbon composite electrocatalyst for the oxidation of formaldehyde

The electrocatalytic performance of highly conducting Nickel phosphate (NiPh)/carbon composite catalyst is investigated for the oxidation of formaldehyde in alkaline solution. The NiPh nanoparticles are synthesized by a cost-effective one-pot method, which is based on refluxing nickel and phosphate precursors at 90 °C. Inks of the composite catalyst are produced by mixing NiPh nanoparticles with carbon conductive additives (CCA) and Nafion oil. Then, the ink is cast then dried on the glassy carbon electrode. Systematic study is performed to investigate the effect of varying the CCA loading on the electrochemical oxidation of formaldehyde. The best catalytic performance is obtained for NiPh/CCA composite catalyst containing 20 wt% of CCA (NiPh/CCA-20 wt%). The physicochemical properties of the composite catalysts are investigated and analyzed by field-emission scanning electron microscopy (FE-SEM), Energy Dispersive x-ray Spectroscopy (EDX) and X-ray diffraction (XRD). Also, the N2 adsorption/desorption isotherms are recorded and the Brunauer–Emmett–Teller (BET) and Barrett-Joyner-Halenda (BJH) methods are used to calculate the specific surface area and pore size distribution. The electrocatalytic performance of the NiPh/CCA composite was compared to the pristine NiPh for the oxidation of formaldehyde in alkaline solution. Electrochemical impedance spectroscopy technique is used to study the electrical conductivity of the composite catalysts. Additionally, cyclic voltammetry and chronoamperometry techniques are used to calculate key parameters such as surface coverage (Γ) of Ni2+/Ni3+ species, the diffusion coefficient of the formaldehyde (D) and the catalytic rate constant (kcat). Ã, D and kcat values for the NiPh/CCA-20 wt% catalyst are 5.95 × 10−5 mmol cm−2, 1.08 × 10−4 cm2 s−1 and 2.59 × 107 cm3 mol−1 s−1 respectively. Both Γ and kcat parameters are used to identify the optimum composition of the catalyst.

Communication—Electrocatalytic Coupling of Methane at Platinum Oxide Electrodes in Superacids

Electrochemical oxidation of methane in the absence of chemical oxidants offers the potential to directly synthesize high value hydrocarbons at low temperatures. Experimental results demonstrate methane oxidization at oxidized Pt electrodes in liquid superacid electrolytes. Ethane and ethylene are observed as products under chronoamperometric experiments at room temperature. Faradaic efficiencies for C2 products range from 5% to 25% at anodic potentials between 0.8 to 1.4 V vs RHE. A generalized Lewis-acid Pt2+ mechanism including methylidene coupling is proposed.

Electrochemical Reoxidation Enables Continuous Methane-to-Methanol Catalysis with Aqueous Pt Salts.

The direct conversion of methane to methanol would enable better utilization of abundant natural gas resources. In the presence of stoichiometric PtIV oxidants, PtII ions are capable of catalyzing this reaction in aqueous solutions at modest temperatures. Practical implementation of this chemistry requires a viable strategy for replacing or regenerating the expensive PtIV oxidant. Herein, we establish an electrochemical strategy for continuous regeneration of the PtIV oxidant to furnish overall electrochemical methane oxidation. We show that Cl-adsorbed Pt electrodes catalyze facile oxidation of PtII to PtIV at low overpotential without concomitant methanol oxidation. Exploiting this facile electrochemistry, we maintain the PtII/IV ratio during PtII-catalyzed methane oxidation via in situ monitoring of the solution potential coupled with dynamic modulation of the electric current. This approach leads to sustained methane oxidation catalysis with 70% selectivity for methanol.

Electrocatalyst Performance for Selective Methanol Formation in Methane Electrolyzer

Methane’s abundance and low cost makes it an optimal raw material for chemical precursors and other energy dense fuels. Traditional methods of converting methane to methanol require large amounts of energy through thermal catalysis and high capital costs. However, electrochemical oxidation of methane is a cleaner and cheaper way to produce methanol, and a systematic study of an electrochemical cell could help assure selectivity over undesired products. Electrochemical oxidation of methane is an underdeveloped field, but electrochemical cells have been employed with high selectivity towards methanol.[1,2] Promising transition state metal-oxide catalysts have shown activity towards the selectivity of methanol over other by products, such as carbon dioxide, carbon monoxide, formic acid, formaldehyde.[2, 3] Further investigation of single site metal oxide catalysts is required to improve conversion and activity, but system optimization is also required. Reports of electrolyzer and fuel cell systems show varying assemblies, ranging from various solid and liquid electrolytes, temperatures, pressures, and current densities. [1-3] Few reports have been consistent on the operating current density and potential windows of such systems. Little work has been done on understanding how various components for these systems and understanding the phenomena of creating methanol in this partial oxidation pathway. In this study, we explore the activity, selectivity, efficiency of transition metal oxide catalyst in operation in an electrolyzer. The electrolyzer used was tested under a variety of relative humidity, operating temperatures, current densities, and membrane electrode assemblies. Through electrochemical and conductivity measurements, and gas phase analysis of the effluent, a better understanding of the partial electrochemical oxidation of methane to methanol is elucidated. The results of this study provide a systematic approach to this challenging problem and provide insights on new catalyst and electrochemistry pathways.

Adsorption of Methane at Platinum Electrodes Under Potentiodynamic Control at Ambient Conditions, and Characterization of Adsorbed Intermediates Via ATR-FTIR Spectroscopy

Electrochemical oxidation of methane is a promising route towards the direct production of high value oxygenates such as methanol, formaldehyde, and formic acid. Compared to traditional catalytic processes such as methane steam reforming and Fischer-Tropsch reactions that operate at high temperatures and pressures, electrochemical oxidation of methane has the benefit of operating at ambient temperatures and pressures. However, there are still many challenges associated with electrochemical methane oxidation including catalyst activity and selectivity. Previous work has established platinum as one of the few catalysts able to oxidize methane to CO2 at room temperature and pressure. By applying a constant potential within the capacitance region of platinum in methane saturated aqueous electrolytes an ad layer can be built up on the platinum surface. The adsorbed intermediates can then be stripped from the platinum surface by sweeping to more anodic potentials. In this work, the aforementioned electrochemical methods are coupled with in-situ infrared spectroscopy to identify reaction intermediates and determine their dependence on applied potential, pH, and time. Improving our understanding of the mechanism of methane oxidation on platinum is necessary for the design of more active and selective catalysts. With better catalysts, fuel cell technologies could allow for the utilization of stranded natural gas for the production of electricity and oxygenates.

Electrocatalytic oxidation of formaldehyde on nanoporous nickel phosphate modified electrode

Nanoporous nickel phosphate (nano-NiPh) is synthesized using a hydrothermal-based method from nickel phosphate precursors. It is applied for electrocatalytic oxidation of formaldehyde (FAL) in alkaline solution. The produced material is characterized using different techniques such as field emission scanning electron microscopy (FE-SEM), transmission electron micrograph (TEM) and X-ray diffraction (XRD) pattern. The nano-NiPh particles nearly reveal one-dimensional nanosize (200×800nm) and demonstrate nanopores of ∼30nm size. The chemical identity is found to be Ni3(PO4)2.8H2O. A nano-NiPh modified glassy carbon electrode (GC/nano-NiPh) is characterized by various techniques including cyclic voltammetry (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS). The fabricated electrode demonstrates significant electrocatalytic activity towards the electrochemical oxidation of formaldehyde. The activity and stability of the electrode are discussed and some transport and kinetic parameters for FAL electrochemical oxidation have been determined.

The synthesis of analcime zeolite nanoparticles using silica extracted from stem of sorghum Halepenesic ash and their application as support for electrooxidation of formaldehyde

In this work, a free template method is used to synthesize nanostructured analcime (ANA) zeolite from natural source and the prepared zeolite is applied as substrate to fabricate a modified electrode. To this aim, ANA zeolite nanoparticles are synthesized by hydrothermal method using amorphous silica extracted from stem of sorghum ash (SSA) with approximately 92.34% purity in the absence of organic template. The effects of different parameters such as Si/Al ratio, heating time, temperature and stirring time on synthesis and crystallinity of zeolite are optimized. The synthesized zeolite is characterized using X-ray diffraction, Fourier-transform infrared and scanning electron microscopy (SEM) techniques. SEM shows that ANA zeolite nanoparticles appear as spherical particles contains some smaller nanoparticles which arranged in regular pattern to each other. Also, some fibers consist of nanoparticles in range of 30–48 nm are observed.The modified carbon paste electrode (CPE) is prepared based on ANA modified with Ni species (Ni/ANACPE). The electrochemical oxidation of formaldehyde is studied at Ni/ANACPE in alkaline solution. From cyclic voltammetry studies, it is shown that Ni/ANACPE can facilitate the oxidation of formaldehyde compared to CPE. The effects of some parameters such as scan rate of potential, concentration of formaldehyde, amount of Ni-zeolite are investigated on the oxidation of formaldehyde. Also using chronoamperometry technique, the catalytic rate constant (k) for oxidation of formaldehyde is found to be 7.56 × 104 cm3 mol−1 s−1.

Research on Electrochemical Oxidation of Formaldehyde on the Nanoporous Silver Electrode in Alkaline Solution

were masked by epoxy resin and the exposed end was ground with sand paper and polished up to 1 μm with diamond paste. Electrochemical measurement was carried out in a traditional three-electrode cell in PARSTAT 2273 electrochemical workstation. The working electrode, assistant electrode and reference electrode were the alloy, Pt electrode and saturated KCl calomel electrode, respectively. The applied potential is relative to the saturated KCl calomel electrode potential (SCE). 0.9 M KOH aqueous solution was utilized to estimate the active surface area of electrochemical catalysts. Electrochemical oxidation of formaldehyde was carried out by cyclic voltammetry in the electrolyte containing 0.9 M KOH and 0.3 M HCHO with scanning rate of 20 mV/s. The surface morphology of nanoporous silver was characterized by Zeiss Supra 55 fieldemission gun scanning electron microscope (FESEM).

Electrocatalytic oxidation of formaldehyde on Ni/Poly(N,N-Dimethylaniline) (sodium dodecylsulfate) modified carbon paste electrode in alkaline medium

In this work, an aqueous solution of sodium dodecylsulfate (SDS) surfactant is used as an additive for electropolymerization of N,N-dimethylaniline (DMA) onto carbon paste electrode (CPE), which is investigated as a novel matrix for deposition of nickel. The electrochemical oxidation of formaldehyde is studied at the surface of this modified electrode. The electrooxidation of formaldehyde was found to be more efficient on CPE modified with Ni/Poly(N,N-Dimethylaniline) (SDS), Ni/PDMA (SDS), than deposition Ni on CPE in alkaline solution. The electrochemical behavior and electrocatalytic activity of the electrode were studied using cyclic voltammetry and chronomethods studies. Also, the transfer second-order rate constant (k = 5.5 × 103 cm3 mol−1 s−1) between formaldehyde and nickel hydroxide was calculated. Moreover, in order to optimize of electrode and variables for efficient performance of Ni/PDMA (SDS)/CPE towards formaldehyde oxidations, the effect of various parameters such as number of potential cycles for preparation of polymer, nickel and formaldehyde concentration and accumulation time have been investigated.

Electrodeposited nanostructured Pt–Ru co-catalyst on graphene for the electrocatalytic oxidation of formaldehyde

The electrochemical oxidation of formaldehyde over graphene surfaces modified with Pt–Ru co-catalyst is presented. Graphene was chemically converted from graphite and Pt–Ru co-catalyst was electrochemically deposited using cyclic voltammetry. The hybrid surface is prepared using “green approaches” and displayed electrocatalytic activity towards formaldehyde in the form of current oscillations. The current oscillations that were mainly due to adsorption/desorption of carbonaceous oxidative products are a factor of several parameters such as the concentrations of both formaldehyde and supporting electrolyte in solution, the amount of catalyst loading, scan rate of potential, upper potential limit, and the temperature change. CCG/Pt–Ru exhibited higher electrocatalytic activity toward formaldehyde electro-oxidation, and intense electrochemical current oscillations were obtained at relatively low HCHO concentrations compared to other work mentioned in literature for CCG/Pt–Pd.

Bis(trifluoromethylsulfonyl)imide (NTf2)-Based Ionic Liquids for Facile Methane Electro-Oxidation on Pt

Methane electro-oxidation was investigated and systematically characterized on three different electrodes (i.e. platinum, gold and carbon electrodes) with six different ionic liquids (ILs) by Cyclic Voltammetry (CV). At platinum electrode in bis(trifluoromethylsulfonyl)imide (NTf2) based ionic liquids (ILs), facile methane electro-oxidation is observed suggesting a unique catalytic Pt-NTf2 interface for electron transfer reaction of methane. Little methane electro-oxidation signal is observed in ILs with other anions. The final methane oxidation products in [C4mpy][NTf2] is confirmed to be CO2 and water by in-situ infrared spectroelectrochemistry. In contrast to many reported methane catalytic oxidation systems, there is no incomplete methane oxidation products observed. The electrochemical oxidation of methane at Pt and [C4mpy][NTf2] interface depends on the methane concentration linearly (from 1%∼10 vol.% methane concentration). The characteristic of peak potential shifts of the oxygen redox processes in the multiple cycle CV study is consistent with the electrode reaction of oxygen coupled with chemical reactions of CO2 and water which further confirms the products of methane electrode oxidation are CO2 and water. The cumulative results lead us to propose a mechanism suggesting a potential catalytic system of methane oxidation in NTf2 based ILs that strongly depends on the properties of the ILs and the electrode materials.

Effect of Sm0.2Ce0.8O1.9 on the carbon coking in Ni-based anodes for solid oxide fuel cells running on methane fuel

To directly use hydrocarbon fuel without a reforming process, a new microstructure for Ni/Sm0.2Ce0.8O2−δ (Ni/SDC) anodes, in which the Ni surface of the anode is covered with a porous Sm0.2Ce0.8O2−δ thin film, was investigated as an alternative to conventional Ni/YSZ anodes. The porous SDC thin layer was coated on the pores of the anode using the sol–gel coating method. The cell performance was improved by 20%–25% with the Ni/SDC anode relative to the cell performance with the Ni/YSZ anode due to the high ionic conductivity of the Ni/SDC anode and the increase of electrochemical reaction sites. For the SDC-coated Ni/SDC anode operating with methane fuel, no significant degradation of the cell performance was observed after 180 h due to the surface modification with the SDC film on the Ni surface, which opposes the severe degradation of the cell performance that was observed for the Ni/YSZ anode, which results from carbon deposition by methane cracking. Carbon was hardly detected in the SDC-coated Ni/SDC anode due to the catalytic oxidation of the deposited carbon on the SDC film as well as the electrochemical oxidation of methane in the triple-phase-boundary.

Enhanced Electrode Activity of Ni/(CeO2)1−x(TiO2)x Anode in Electrochemical Oxidation of Methane

The electrode activity of a newly developed Ni/(CeO2)1−x(TiO2)x cermet anode has been investigated at intermediate temperatures. (CeO2)1−x(TiO2)x mixed oxide powders were synthesized by the sol-gel method. A single cell, Ni/(CeO2)1−x(TiO2)x|Sm0.2Ce0.8O3|Sm0.5Sr0.5CoO3, was fabricated using a samarium-doped ceria disk. The effect of incorporating TiO2 into the CeO2 matrix was studied with the goal of improving the anode micro structure. The modified micro structure of the new Ni/(CeO2)0.8(TiO2)0.2 cermet anode, which consists of fine Ni particles distributed on (CeO2)0.8(TiO2)0.2 mixed oxide particles, enhances the electrode activity in methane oxidation.

Increases in the Electrochemical Oxidation of Formaldehyde and Ethylene Glycol in Perchloric Acid by Adding Inhibiting Anions

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Performance of Ce/Fe oxide anodes for SOFC operating on methane fuel

The performance of Ce 1− x Fe x O 2− δ ( x = 0.1, 0.2) was studied as anode for solid oxide fuel cell running on methane in this paper. The oxides were prepared by the citrate method. The polarization resistance of electrode was measured in the two-electrode symmetric cell configuration and an electrolyte-supported cell was prepared to assess Ce 0.8Fe 0.2O 2− δ anode performance of running on methane fuel. The results suggested that Ce 0.8Fe 0.2O 2− δ has a significant effect on the electrochemical oxidation of methane. A maximum power density of single cell was 52 mW/cm 2 at 800 °C and little carbon deposition was shown on the anode operating in humidified methane for 20 h. Therefore, Ce 0.8Fe 0.2O 2− δ is a promising candidate for anode of solid oxide fuel cell running on methane fuel, which breaks a new path to solve the problem of carbon deposition in methane.