A microbial fuel cell using manganese oxide oxygen reduction catalysts

Microbial fuel cells (MFCs) are a potential method for enhanced water and waste treatment, which offer the additional benefit of energy generation. Manganese oxide was prepared by a simple chemical oxidation using potassium permanganate. Carbon-supported manganese oxide nanoparticles were successfully characterised as cathode materials for MFCs. The manganese oxide particles when used in a two-chamber MFC, using inoculum from an anaerobically digested sewage sludge, were found to exhibit similar oxygen reduction performance to that in separate electrochemical tests. MFC tests were conducted in a simple two chamber cell using aqueous air-saturated catholytes separated from the anode chamber by a Nafion membrane. MFC peak power densities were ca. 161 mW m^?2 for MnO _x /C compared to 193 mW m^?2 for a benchmark Pt/C, in neutral solution at room temperature. The catalyst materials demonstrated good stability in the 7.0–10.0 pH range. Theoretical (IR free) peak power densities were 937 mW m^?2 for MnO _x /C compared with 1037 mW m^?2 for Pt/C in the same experimental conditions: showing the MFCs performances can easily be improved by using more favourable conditions (more conductive electrolyte, improved cathode catalyst etc.). Our studies indicated that the use of our low cost MnO _x /C catalysts is of potential interest for the future application of MFC systems.     

Non-platinum electrocatalysts: Manganese oxide nanoparticle-cobaltporphyrin binary catalysts for oxygen reduction

This study is concerned with the development of non-platinum electrocatalysts for the efficient 4-electron reduction of molecular oxygen to water in acidic media. A binary catalyst composed of electrodeposited manganese oxide nanoparticles (nano-MnO _x ) and cobalt porphyrin macro complex (CoP) has been proposed in. The modification of glassy carbon (GC) electrode with CoP alone resulted in a significant positive shift of the oxygen reduction reaction (ORR) compared to the unmodified GC electrode while maintining a 2-electron reduction. That is a positive shift of the onset potential of the ORR of ca. 450 mV was achieved at the former electrode. The modification of the GC electrode with nano-MnO _x alone did not affect the ORR peak potential, but caused a remarkable increase in the reduction peak current due to the catalytic disproportionation of the electrogenerated hydrogen peroxide into water and oxygen. The modification of a GC electrode with CoP and nano-MnO _x (utilizing the advantages of the individual catalysts) resulted in the occurrence of the ORR at a significantly positive potential with almost double peak current compared to the unmodified GC electrode, suggesting a promising procedure for developing electrocatalysts for oxygen reduction in replacement of costly Pt. XPS and SEM techniques were employed to probe the structural and morphological characterization of the proposed binary catalysts.     

Electrochemical reduction of cerium oxide into metal

The Fray Farthing and Chen (FFC) and Ono and Suzuki (OS) processes were developed for the reduction of titanium oxide to titanium metal by electrolysis in high temperature molten alkali chloride salts. The possible transposition to CeO₂ reduction is considered in this study. Present work clarifies, by electro-analytical techniques, the reduction pathway leading to the metal. The reduction of CeO₂ into metal was feasible via an indirect mechanism. Electrolyses on 10 g of CeO₂ were carried out to evaluate the electrochemical process efficiency. Ca metal is electrodeposited at the cathode from CaCl₂–KCl solvent and reacts chemically with ceria to form not only metallic cerium, but also cerium oxychloride.     

氮掺杂非贵金属氧还原催化剂研究进展

综述了近几年来用于直接甲醇燃料电池阴极氧还原反应的含氮碳催化材料发展状况,包括催化剂制备过程中热处理条件、金属的种类及负载量、碳载体、新型合成方法等影响催化剂氧还原活性的几个因素。针对当前研究工作的局限展开了评论,指出催化机理和优化制备过程是氮掺杂碳材料负载非贵金属氧还原催化剂深入研究的重点和主要方向。     

Electrochemical reduction of graphene oxide in organic solvents

Graphene oxide (GO) cast on conductive substrates was electrochemically reduced in some organic solvents. The amount of electricity required for the almost complete reduction of GO was 2.0 C for 1 mg GO, corresponding to attaching of a one-electron reducible species to each benzene ring in graphene. The electrochemically reduced GO film gave an electrical conductivity of about 3 S cm⁻¹ and exhibited a relatively high specific capacitance of 147.2 F g⁻¹ in propylene carbonate. The electrochemical reduction of GO was feasible on Al foils as well.     

Effects of methane and oxygen on decomposition of nitrous oxide over metal oxide catalysts

Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −ΔH_f⁰ around 450 kJ (O mol)⁻¹ was observed in N₂O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)⁻¹, CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃, Cr₂O₃, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO₂, Al₂O₃, La₂O₃, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N₂O+CH₄→2N₂+CO₂+2H₂O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Methanol-resistant cathodic oxygen reduction catalysts for methanol fuel cells

Efforts were made to simplify the structure of Ru-based catalysts, and to tailor industrially practicable methanol insensitive oxygen reduction catalysts both by thermolysis of Ru-carbonyls in organic solvents and by modified preparation techniques of Ru colloids. Selective catalysis was found to be essentially independent of the chalcogene (Se) used which, however, is a crucial factor for facilitating efficient electron transfer. All preparations contained Ru-metal particles of nm size, the surfaces of which were modified by carbonyl and carbido-carbonyl complexes or carbon compounds. The role of carbon as ligand to Ru clusters stabilizing the Ru interface against oxidation and in promoting catalytic electron exchange via nonbonding Ru d-states is theoretically analysed in a model calculation. An analogy is drawn to a biological Fe – only hydrogenase centre in order to discuss projected key experiments for optimizing reduction catalysis: the stabilization of small, inherently unstable catalytic metal clusters by CO or CN and their linking via electron bridges such as S and Se to electron reservoirs (metal colloids).     

Selective reduction of nitric oxide by methanol and dimethyl ether over promoted alumina catalysts in excess oxygen

Selective catalytic reduction (SCR) activity for NO conversion to N₂ over γ-alumina, vanadia/alumina and molybdena/alumina catalysts has been investigated with methanol (MeOH) and dimethyl ether (DME) as reductants under lean conditions. Molybdena/alumina catalysts showed high efficiency for NO reduction with either reductant, especially at low temperature, which may involve surface formyl produced by oxidative dehydrogenation. Sulphated γ-alumina remains active for NO reduction with MeOH, while sulphated 5 wt.% MoO₃/Al₂O₃ remains active with both MeOH and DME over a broad temperature range.     

Electrocatalysis of oxygen reduction on carbon-supported PtCo catalysts prepared by water-in-oil micro-emulsion

Synthesis of carbon-supported PtCo/C using micro-emulsion method including simultaneous procedure and sequential procedures in both acid and alkaline media was reported. UV-vis and electron microscopy were used to characterize the formation, surface morphology and distribution of PtCo nanoparticles. Crystallite structure of catalysts was analyzed from XRD patterns. Catalytic properties of PtCo/C catalysts synthesized were compared with commercial Pt/C using RDE based on both mass activity (MA) and specific activity (SA). PtCo/C catalysts prepared in both acidic and basic conditions showed better performance than commercial Pt/C catalyst. High-temperature heat treatment was found useful only to PtCo/C by sequential procedure. The peroxide yield was also explored using RRDE technique. The H₂O₂ yield results were correlated with SA and R values (ratio of charge transferred about Co and Pt on the surface of catalyst) obtained from CVs in 1 M KOH solution. A sacrificial Co oxidized effect on impediment of adsorption of OH may cause higher catalytic properties and higher H₂O₂ yield to Pt base alloy catalysts.     

Electrochemical reduction of oxygen on palladium nanocubes in acid and alkaline solutions

The electrocatalytic properties of cubic palladium nanoparticles towards the electrochemical reduction of oxygen were studied in acid and alkaline solutions and compared with those of spherical nanoparticles and bulk Pd. The synthesised Pd nanoparticles were characterised by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrooxidation of pre-adsorbed CO was employed for cleaning the palladium catalyst surface. Oxygen reduction was studied using the rotating disk electrode (RDE) method and enhanced electrocatalytic activity of Pd nanocubes was revealed both in acid and alkaline solutions, which was attributed to the prevalence of Pd(1 0 0) facets. The mechanism of oxygen reduction on Pd nanoparticles was similar to that on bulk Pd, the first electron transfer being the rate-limiting step, and the reaction predominantly followed a four-electron pathway in both solutions.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

Platinum-alloy nanostructured thin film catalysts for the oxygen reduction reaction

In an effort to study advanced catalytic materials for the oxygen reduction reaction (ORR), a number of metallic alloy nanostructured thin film (NSTF) catalysts have been characterized by rotating disk electrode (RDE). Optimal loadings for the ORR and activity enhancement compared to conventional carbon supported nanoparticles (Pt/C) were established. The most efficient catalyst was found to be PtNi alloy with 55 wt% of Pt. The enhancement in specific activity is more than one order of magnitude, while the improvement factor in mass activity is 2.5 compared to Pt/C. Further lowering of the platinum to nickel ratio in NSTF catalysts did not lead to increased mass activity values.     

Electrochemical reduction of oxygen with iron phthalocyanine in neutral media

Recent interest in electricity production using microbial fuel cells makes it important to better understand O₂ reduction in neutral solutions with non-precious metal catalysts. Higher O₂ reduction activity was obtained using iron phthalocyanine supported on Ketjen black carbon (FePc-KJB) than with a platinum catalyst in neutral pH. At low overpotentials, a Tafel slope close to −0.06 V/dec in both acid and neutral pH suggested that the mechanism of O₂ reduction on FePc is not changed with the change of pH, and the reaction is mainly controlled by Fe^II/Fe^III redox couple. This behaviour gives us new insight into catalysis using FePc, and further supports the use of FePc as a promising catalyst for the oxygen reduction applications in neutral media.     

Electrochemical reduction of thick oxide film on platinum electrode in alkaline solutions

The electrochemical reduction of the thick oxide film formed on Pt electrode by severe preanodization has been studied in LiOH, NaOH and KOH solutions of different concentrations (0.001 ～ 1.0 M) using a galvanostatic technique.An outermost monolayer oxide and an inner multilayer oxide of the thick oxide film exhibit different potential behaviors in the cathodic reduction. In dilute solution, both the oxides are completely reduced in a potential range of 0.6-0.4 V (vs rhe in the test solution) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide only shifts rapidly into a H-electrosorption potential region and the amount of the oxide reduced at this potential decreases. The remaining oxide is slowly reduced at H₂-evolution potential. The retardation of the reduction of the inner oxide is related to cations adsorbed on Pt electrode. This retardation effect increases in the order, K⁺ < Na⁺ < Li⁺.     

Electrochemical reduction of oxygen catalyzed by Pseudomonas aeruginosa

Pseudomonas aeruginosa has already been shown to catalyze oxidation processes in the anode compartment of a microbial fuel cell. The present study focuses on the reverse capacity of the bacterium, i.e. reduction catalysis. Here we show that P. aeruginosa is able to catalyze the electrochemical reduction of oxygen. The use of cyclic voltammetry showed that, for a given range of potential values, the current generated in the presence of bacteria could reach up to four times the current obtained without bacteria. The adhesion of bacteria to the working electrode was necessary for the catalysis to be observed but was not sufficient. The electron transfer between the working electrode and the bacteria did not involve mediator metabolites like phenazines. The transfer was by direct contact. The catalysis required a certain contact duration between electrodes and live bacteria but after this delay, the metabolic activity of cells was no longer necessary. Membrane-bound proteins, like catalase, may be involved. Various strains of P. aeruginosa, including clinical isolates, were tested and all of them, even catalase-defective mutants, presented the same catalytic property. P. aeruginosa offers a new model for the analysis of reduction catalysis and the protocol designed here may provide a basis for developing an interesting tool in the field of bacterial adhesion.     

Catalytic activity of dual catalysts system based on nano-manganese oxide and cobalt octacyanophthalocyanine toward four-electron reduction of oxygen in alkaline media

The electrocatalysis of the dual functional catalysts system composed of electrolytic nano-manganese oxide (nano-MnOx) and cobalt octacyanophthalocyanine (CoPcCN) toward 4-electron reduction of oxygen (O₂) in alkaline media was studied. Nano-MnOx electrodeposited on the CoPcCN monolayer-modified glassy carbon (GC) electrode was clarified as the nano-rods with ca. 10-20 nm diameter by scanning electron microscopy. The peak current for O₂ reduction at the dual catalysts-modified GC electrode increases largely and the peak potential shifts by ca. 160 mV to the positive direction in cyclic voltammograms compared with those obtained at the bare GC electrode. The Koutecký-Levich plots indicate that the O₂ reduction at the dual catalysts-modified GC electrode is an apparent 4-electron process. Collection efficiencies obtained at the dual catalysts-modified GC electrode are much lower than those at the GC electrode and are almost similar to those at the Pt nano-particles modified GC electrode. The obtained results demonstrate that the dual catalysts system possesses a bifuctional catalytic activity for redox-mediating 2-electron reduction of O₂ to HO₂⁻ by CoPcCN as well as catalyzing the disproportionation of HO₂⁻ to OH⁻ and O₂ by nano-MnOx, and enables an apparent 4-electron reduction of O₂ at a relatively low overpotential in alkaline media. In addition, it has been found that the cleaning of the dual catalysts-modified electrode by soaking in 0.1 M sulfuric acid solution enhances its catalytic activity toward the reduction of O₂.     

The catalytic reduction of nitric oxide over supported ruthenium catalysts

The activity of supported ruthenium catalysts for reducing NO to N₂ in an exhaust-like feedstream has been examined in laboratory experiments. The rate and temperature of NO removal is largely dependent on the NO inlet concentration and independent of the concentration of reducing agents in the system. The selectivity for nitrogen formation, however, is dependent on the concentration of the reducing agents CO and H₂ as well as the concentration of NO. No evidence was found for an ammonia intermediate in the conversion of nitric oxide to elemental nitrogen over ruthenium. The high selectivity of ruthenium for the NO to N₂ conversion is explained and compared with the behavior of platinum and palladium catalysts.     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.