Modified Carbon Catalysts Research Articles

Dipole-Dipole Tuned Electronic Reconfiguration of Defective Carbon Sites for Efficient Oxygen Reduction into H2O2.

Metal-free carbon-based materials are one of the most promising electrocatalysts toward 2-electron oxygen reduction reaction (2e-ORR) for on-site production of hydrogen peroxide (H2O2), which however suffer from uncontrollable carbonizations and inferior 2e-ORR selectivity. To this end, a polydopamine (PDA)-modified carbon catalyst with a dipole-dipole enhancement is developed via a calcination-free method. The H2O2 yield rate outstandingly reaches 1.8mol gcat -1 h-1 with high faradaic efficiency of above 95% under a wide potential range of 0.4-0.7 VRHE, overwhelming most of carbon electrocatalysts. Meanwhile, within a lab-made flow cell, the synthesized ORR electrode features an exceptional stability for over 250h, achieved a pure H2O2 production efficacy of 306g kWh-1. By virtue of its industrial-level capabilities, the established flow cell manages to perform a rapid pulp bleaching within 30min. The superior performance and enhanced selectivity of 2e-ORR is experimentally revealed and attributed to the electronic reconfiguration on defective carbon sites induced by non-covalent dipole-dipole influence between PDA and carbon, thereby prohibiting the cleavage of O-O in OOH intermediates. This proposed strategy of dipole-dipole effects is universally applicable over 1D carbon nanotubes and 2D graphene, providing a practical route to design 2e-ORR catalysts.

Electro-Fenton process with scalable modified carbonaceous electrode material processed by direct calcination for refractory organic pollutant degradation

Electro-Fenton (EF) regarded as a green substitution to traditional homogeneous Fenton, produces highly oxidizing OH through in-situ H2O2 electrosynthesis and electrochemical Fe2+ regeneration. One of obstacles that hinders translation of bench-scale EF performance to water treatment practices is the difficulty of scaling up catalyst/electrode preparation in low cost while producing high performance. Here we report scalable preparation of modified carbon catalysts for high-efficiency EF by directly calcinating commercial carbonaceous materials in air. The EF with modified carbon cathode shows high kinetics of 0.296–0.569 min−1 for levofloxacin removal in much wider pH range of 3.0–6.6 using less Fe dosage compared with homogeneous Fenton. This EF system can reduce chemical oxygen demand (COD) of coking wastewater from 312.0 to 99.5 mg L−1 to satisfy wastewater discharge standard in China while consuming low electricity of 1.8–4.9 kWh kg−1 COD−1. We demonstrated the scalability of this EF process by scaling up preparative throughput of catalyst (30 g that can be processed into electrodes with total working areas of 54000 cm2), enlarging electrode size (230 cm−2) and reactor volume with wastewater treatment capacity of 7.2 L/h. This scalability demonstration of EF technology shows the promising potential of achieving low-cost advanced wastewater treatment.

Biodiesel Production from Waste Cooking Oil, Using an Acid Modified Carbon Catalyst Derived from Cassava Peels

Biodiesel Production from Waste Cooking Oil, Using an Acid Modified Carbon Catalyst Derived from Cassava Peels

Sulfation of Wheat Straw Soda Lignin with Sulfamic Acid over Solid Catalysts.

Soda lignin is a by-product of the soda process for producing cellulose from grassy raw materials. Since a method for the industrial processing of lignin of this type is still lacking, several research teams have been working on solving this problem. We first propose a modification of soda lignin with sulfamic acid over solid catalysts. As solid catalysts for lignin sulfation, modified carbon catalysts (with acid sites) and titanium and aluminum oxides have been used. In the elemental analysis, it is shown that the maximum sulfur content (16.5 wt%) was obtained with the Sibunit-4® catalyst oxidized at 400 °C. The incorporation of a sulfate group has been proven by the elemental analysis and Fourier-transform infrared spectroscopy. The molecular weight distribution has been examined by gel permeation chromatography. It has been demonstrated that the solid catalysts used in the sulfation process causes hydrolysis reactions and reduces the molecular weight and polydispersity index. It has been established by the thermal analysis that sulfated lignin is thermally stabile at temperatures of up to 200 °C. According to the atomic force microscopy data, the surface of the investigated film consists of particles with an average size of 50 nm. The characteristics of the initial and sulfated β-O-4 lignin model compounds have been calculated and recorded using the density functional theory.

Template-assisted polymerization-pyrolysis derived mesoporous carbon anchored with Fe/Fe3C and Fe−NX species as efficient oxygen reduction catalysts for Zn-air battery

Constructing highly efficient and durable non-noble metal modified carbon catalysts for oxygen reduction reaction (ORR) in the whole pH range is essential for energy conversion devices but still remains a challenge. Herein, the Fe/Fe3C nanoparticles and Fe-NX species anchored on the interconnected mesoporous carbon materials are fabricated through an economical and facile template-assisted polymerization-pyrolysis strategy. The catalyst exhibits unique features with the electronic interaction between Fe/Fe3C and Fe−NX, large specific surface area and high mesoporous structure as well as nitrogen doping in porous carbon skeletons, which can effectively catalyze ORR over the full pH range. In an alkaline electrolyte, the optimized catalyst displays favorable ORR performance with positive onset potential (Eonset = 0.91 V), half-wave potential (E1/2 = 0.83 V), long-term cycles stability and methanol tolerance, exceeding those for the commercial Pt/C. Furthermore, the prepared catalyst could be directly assembled into the alkaline Zn−air battery that exhibits the open-circuit voltage of 1.44 V, high power density of 96.0 mW cm−2 and long-term durability. Therefore, the template-assisted polymerization-pyrolysis strategy provides a promising route for designing high-performance non-noble metal decorated ORR electrocatalysts.

Overall Oxygen Electrocatalysis on Nitrogen-Modified Carbon Catalysts: Identification of Active Sites and In Situ Observation of Reactive Intermediates.

The recent mechanistic understanding of active sites, adsorbed intermediate products, and rate‐determining steps (RDS) of nitrogen (N)‐modified carbon catalysts in electrocatalytic oxygen reduction (ORR) and oxygen evolution reaction (OER) are still rife with controversy because of the inevitable coexistence of diverse N configurations and the technical limitations for the observation of formed intermediates. Herein, seven kinds of aromatic molecules with designated single N species are used as model structures to investigate the explicit role of each common N group in both ORR and OER. Specifically, dynamic evolution of active sites and key adsorbed intermediate products including O2 (ads), superoxide anion O2 −*, and OOH* are monitored with in situ spectroscopy. We propose that the formation of *OOH species from O2 −* (O2 −*+H2O→OOH*+OH−) is a possible RDS during the ORR process, whereas the generation of O2 from OOH* species is the most likely RDS during the OER process.

Carbon Defect Characterization of Nitrogen-Doped Reduced Graphene Oxide Electrocatalysts for the Two-Electron Oxygen Reduction Reaction

Numerous modified-carbon catalysts have been developed for the direct synthesis of hydrogen peroxide through electrochemical oxygen reduction. However, given the complex structure of most porous ca...

Acidic activated carbons as catalysts of biodiesel formation

Biodiesel is recognized as one of the most attractive motor vehicle fuels of the future. Therefore, technologies of its production are continuously improved and optimized. The research work reported concerns the process of biodiesel production via transesterification of vegetable oil over modified carbon catalysts obtained from waste material. In order to introduce strongly acidic groups into the carbon structure, the initial sample was treated with concentrated or fuming sulfuric acid, sodium sulfide, 4-aminobenzenesulfonic acid, phosphoric acid, or H2SO4/HNO3 mixture. The carbon materials prepared in this way were thoroughly characterized (elemental and textural analysis, potentiometric titration, TG, NMR, SEM, and XPS measurements) and then used in transesterification of rapeseed oil in the presence of methanol at 130 °C under elevated pressure. The results obtained revealed that not only sulfonic groups are capable of catalyzing transesterification reaction (FAME yield up to 60% for the best SO3H bearing sample), but also materials containing phosphate groups can successfully catalyze this process (FAME yield 43%). It was established that the carboxyl functionalities, which can be generated on the catalyst surface upon its reaction with some modification agents (e.g., H2SO4 or H2SO4/HNO3 mixture), block the active sites of the process and reduce the reaction effectiveness. The stability of the catalysts was rather poor, as they underwent deactivation in subsequent reaction cycles (mainly due to neutralization of —SO3H groups but also by their leaching); however, the catalyst regeneration was possible.

Modified solid acids derived from biomass based cellulose for one-step conversion of carbohydrates into ethyl levulinate

A series of metal salt-modified carbon catalysts had been prepared to study the selective transformation of various carbohydrates into ethyl levulinate in an ethanol medium. The specific textural and chemical characteristics of prepared carbon samples were identified by Raman, XRD, XPS, NH3-TPD, FT-IR and nitrogen physisorption. Various parameters such as ethanol/water volume ratio, NaCl addition, reaction temperature, and catalyst dosage played a great role in ethyl levulinate production. A desirable ethyl levulinate yield of 58.0 mol% with a highest ROF (rate of ethyl levulinate formation per gram of catalyst per hour) value of 2148.3µmol/(gcat·h) was achieved at 468K over FeCl3 modified carbon catalyst with respect to fructose conversion. The recycling experiments revealed that the sulfonated carbon catalysts exhibited relatively satisfied activity and stability.

Preliminary Testing of Hybrid Catalytic-Plasma Reactor for Biodiesel Production Using Modified-Carbon Catalyst

Preliminary testing of hybrid catalytic-plasma reactor for biodiesel production through transesterification of soybean oil with methanol over modified-carbon catalyst was investigated. This research focused on synergetic roles of non-thermal plasma and catalysis in the transesterification process. The amount of modified-carbon catalyst with grain size of 1.75 mm was placed into fixed tubular reactor within discharge zone. The discharge zone of the hybrid catalytic-plasma reactor was defined in the volume area between high voltage and ground electrodes. Weight Hourly Space Velocity (WHSV) of 1.85 h-1 of reactant feed was studied at reaction temperature of 65 oC and at ambient pressure. The modified-carbon catalyst was prepared by impregnation of active carbon within H2SO4 solution followed by drying at 100 oC for overnight and calcining at 300 oC for 3 h. It was found that biodiesel yield obtained using the hybrid catalytic-plasma reactor was 92.39% and 73.91% when using active carbon and modified-carbon catalysts, respectively better than without plasma. Therefore, there were synergetic effects of non-thermal plasma and catalysis roles for driving the transesterification process.

Influence of Temperature on the Oxygen Electroreduction Activity at Micro-Mesoporous Carbon Support

Influence of temperature on the kinetics of oxygen electroreduction on molybdenum carbide derived carbon based catalysts has been analyzed in O2 saturated 0.1M KOH solution using rotating disc electrode and cyclic voltammetry methods. The calculated kinetic current density value (|jkin| = 62.6 A m−2) at the half-wave potential (−0.05 V vs. Hg|HgO|0.1M KOH) of platinum nanoparticles modified catalyst was maximal at 60°C. Two different linear areas in the Tafel-like plot have been observed for unmodified and Pt nanoparticles modified catalysts within the studied temperature range from 10 to 80°C. The calculated experimental apparent activation energy values for the oxygen reduction were within a range from 19 to 27 kJ mol−1 for Pt nanoparticles modified carbon catalyst within potential range from 0.05 to −0.07 V vs. Hg|HgO|0.1M KOH. The oxygen reduction peak current density values decrease with increasing the temperature from 10 to 80°C.

Oxygen Electrocatalysis on High-Surface Area Non-Pt Metal Modified Carbon Catalysts

Series of carbon supported non-noble d-metal Fe or d-metal-nitrogen (Fe-N- or Co-N-) nanocluster activated electrocatalysts have been synthesized by sodium borohydride reduction method. Synthesized d-metal-nitrogen carbon catalysts were studied using X-ray diffraction, scanning electron microscopy, time of flight secondary ion mass-spectrometry and Raman spectroscopy methods. The electrochemical parameters of synthesized catalysts were investigated in alkaline media using cyclic voltammetry and rotating disk electrode methods. The high catalytic activity toward oxygen electroreduction reaction was demonstrated and found to be increasing in the following order: Fe-Vulcan; Co-N-Vulcan; Co-N-C(Mo2C); Fe-N-C(Mo2C). The possibility to replace the Pt metal catalyst with non-noble metal–nitrogen carbon electrocatalyst has been discussed.

ChemInform Abstract: Metal‐Free Carbon Catalysts for Oxidative Dehydrogenation Reactions

AbstractReview: 50 refs.

Metal-Free Carbon Catalysts for Oxidative Dehydrogenation Reactions

Catalysis over carbon, especially nanocarbon, is an attractive topic in material science and chemical engineering fields due to its significant advantages compared with conventional metal or metal oxide catalysts. This paper summarizes the recent developments, basic concepts, and commonly accepted understandings on the nature of carbon catalysis in oxidative dehydrogenation reactions, including: introduction and comparison of various reaction systems; identity and quantity of active sites on carbon catalysts; mechanism for the reactions; and structure− selectivity relations for modified carbon catalysts. These fruitful conclusive achievements are the basis for in-depth comprehension of carbon-catalyzed oxidative dehydrogenation process at the molecular level, and many other efforts, such as detailed kinetic study, precisely controllable synthetic technique for nanocarbon catalysts, are still needed to further push carbon catalysis fields to practical applications.

Oxygen Electrocatalysis on High-Surface Area at Non-Pt Modified Carbon Catalysts

Although a Pt/C electrocatalyst is the most widely used cathode material, there is a great interest in the development of more active catalysts for the oxygen electroreduction reaction (ORR). Promising results have been achieved by alloying Pt with non-noble metals [1]. In order to lower the cost of cathode catalysts, employing bi-metallic catalysts that combine platinum with other less expensive metals are under investigation [2,3].The main aim of this work is to extend the previous study in order to characterize the ORR kinetics on non-Pt and Fe-Pt nanoparticles in alkaline media (0.1M KOH solution) using crystallographically and structurally different carbon supports.The novel non-conventional micro-mesoporous carbide derived carbon powder, synthesized from molybdenum carbide (Mo2C) at 750°C using the high-temperature chlorination method, and carbon aerogel (CAG) have been used as electrocatalyst supports for oxygen electroreduction. For comparison, the commercially available VulcanXC72® has been used. XRD, XRF and SEM methods were applied for structural and electronic characterization of the materials. Gas adsorption data at liquid nitrogen temperature were used for the porosity analysis of the materials under discussion (Table).XRD patterns of the electrode materials were recorded on a Bruker D8 diffractometer (Bruker Corp.) employing CuKα radiation with a step size of 0.01° and counting time of 2 s using a position sensitive LynxEye detector. The diffraction spectrums were recorded at 25 °C.XRD patterns of the obtained catalysts show the characteristic peaks (2q = 24.5o) of carbon supports (presented in Figure). Analysis shows that the Fe-Pt nanoclusters have been deposited mainly as the face-centered cubic and face-centered tetragonal internal crystals at all carbon supports studied. The metal particles were distributed uniformly on the carbon supports. Both C(Mo2C) and CAG based catalysts are practically amorphous. However, there is a small amount of graphitic structure in all materials as well [4]. The RDE measurements were carried out in a three-electrode electrochemical cell. Several important reaction parameters, such as the number of electrons involved in the ORR, Tafel slopes, and the kinetic rate constant for the ORR, were obtained [5-9].Potentials were measured against Hg|HgO|0.1M KOH reference electrode. RDE data were measured at rotation rates from 0 to 3000 rpm (v=10 mV∙s-1) and in the region of potentials from +0.21 to -0.75 V vs. Hg|HgO|0.1M KOH. CVs were measured at potential scan rates (v mV/s) 5, 10, 20, 30, 50, 70, 100, 150 and 200, in both Ar and O2 saturated solutions. The solutions were saturated with Ar or O2, respectively, between measuring of each voltammogram. Acknowledgements: This work was supported by the Estonian Centre of Excellence in Science Project TK117T "High-technology Materials for Sustainable Development", the Estonian Energy Technology Program project SLOKT 10209T, the Materials Technology project SLOKT12180T.

Metal-free catalysts for oxygen reduction in alkaline electrolytes: Influence of the presence of Co, Fe, Mn and Ni inclusions

Metal-free nitrogen modified carbon catalysts (NC) are very closely related to MNC catalysts which contain a transition metal(s) (M), usually Fe or Co as an essential constituent. We investigated the influence of metal inclusions on the activity of nitrogen-doped carbon black in the electrocatalysis of the oxygen reduction reaction (ORR). A reference metal-free NC catalyst was prepared by pyrolysis of a polypyrrole/Vulcan XC72 composite at 800°C for 2h under helium. Controlled amounts of Co, Fe, Mn and Ni in low concentrations were then introduced into NC by impregnating it with the corresponding meso-tetra(4-pyridyl) porphyrin metal complex followed by further pyrolysis at 650°C for 2h under helium. The resulting catalysts were investigated for ORR using rotating disk electrode and rotating-ring disk electrode voltammetry in 0.1M KOH. Additionally, the rate of decomposition of hydrogen peroxide by the different catalysts was determined in order to probe the influence of the metal inclusions on the mechanism and selectivity of the ORR. The results show that Fe, Co and Mn inclusions cause a substantial decrease of the overpotential of the reaction and enhance the catalytic current, whereas the presence of Ni has a poisoning effect on ORR. In the presence of Fe, the catalysts apparently reduce oxygen selectively to OH− in a direct four electron transfer process as opposed to the two-step, two electron pathway involving hydrogen peroxide as an intermediate for the case of the NC catalyst.

Ni–P and Ni–Cu–P modified carbon catalysts for methanol electro-oxidation in KOH solution

The electrocatalytic oxidation of methanol was studied on Ni–P and Ni–Cu–P supported over commercial carbon electrodes in 0.1 M KOH solution. Cyclic voltammetry and chronoamperometry techniques were employed. Electroless deposition technique was adopted for the preparation of these catalysts. The effect of the electroless deposition parameters on the catalytic activity of the formed samples was examined. They involve the variation of the deposition time, pH and temperature. The scanning electron micrography showed a compact Ni–P surface with a smooth and low porous structure. A decreased amount of nickel and phosphorus was detected by EDX analysis in the formed catalyst after adding copper to the deposition solution. However, an improvement in the catalytic performance of Ni–Cu–P/C samples was noticed. This is attributed to the presence of copper hydroxide/nickel oxyhydroxide species. It suppresses the formation of γ-NiOOH phase and stabilizes β-NiOOH form. Linear dependence of the oxidation current density on the square root of the scan rate reveals the diffusion controlled behaviour.

Development of tellurium-modified carbon catalysts for oxygen reduction reaction in PEM fuel cells

Tellurium (Te)-modified carbon catalyst for oxygen reduction reaction was prepared through chemical reduction of telluric acid followed by the pyrolysis process at elevated temperatures. The catalyst was found to be active for oxygen reduction reaction. High-temperature pyrolysis plays a crucial role in the formation of the active sites of the catalysts. When the pyrolysis was conducted at 1000 °C, the catalyst exhibited the onset potential for oxygen reduction as high as 0.78 V vs. NHE and generated less than 1% H 2O 2 during oxygen reduction. The performance of the membrane–electrode assembly prepared with the Te-modified carbon catalyst was also evaluated.

Heterogeneous catalytic racemization of 4,4'-disubstituted 1,1'-binaphthyls by active carbons and by modified carbon catalysts

Heterogeneous catalytic racemization of 4,4'-disubstituted 1,1'-binaphthyls by active carbons and by modified carbon catalysts