Electrocatalysis and electrocatalysts for low temperature fuel cells: fundamentals, state of the art, research and development

This article deals with electrocatalysis and electrocatalysts for low temperature fuel cells and also with established means and methods in electrocatalyst research, development and characterization. The intention is to inform about the fundamentals, state of the art, research and development of noble metal electrocatalysts for fuel cells operating at low temperatures.

Electrocatalysis in wastewater treatment: recent mechanism advances

Over 50 years, several scientists and industries have developed new alternatives for wastewater treatment and remediation. Recently, electrochemical technology has been largely developed mainly because of its versatility and environmental compatibility. Scientific contributions about role of the electrode material have allowed determining that the influence of material in the selectivity is an important parameter. However, to interpret this behavior, comprehensive physical chemistry models for organics destruction, related to electrochemical phenomena and material surfaces, were proposed in the last decades. So, this paper presents a critical and comprehensive review about the principles and recent mechanism advances in electrocatalysis for wastewater treatment.

Eletrodos quimicamente modificados aplicados à eletroanálise: uma breve abordagem

Chemically modified electrodes (CMEs) have been subject of considerable attention since its inception about 23 years ago. CMEs result of a deliberate immobilization of a modifier agent onto the electrode surface obtained through chemical reactions, chemisorption, composite formation or polymer coating. This immobilization seeks transfer the physicochemical properties of the modifier to the electrode surface and thus to dictate and control the behavior of the electrode/solution interface. In recent years the interest in CMEs has increased particularly to enhance the sensitivity and/or the selectivity of electroanalytical techniques. In general higher sensitivity and/or selectivity may be achieved by exploiting one or more of the following phenomena: electrocatalysis, preconcentration and interferents exclusion. This paper deals with the application of CMEs in electroanalysis, including a brief presentation of the more general procedures that have been employed for the modification of electrode surfaces.

Electrocatalytic oxidation of beta-dicarbonyl compounds using ceric methanesulphonate as mediator

beta-dicarbonyl compounds were oxidized electrocatalytically, with fragmentation and loss of "ch2", using ceric methanesulphonate as a mediator. 2,4-pentanedione yields acetic acid (90%), methyl acetoacetate yields acetic acid (84%) plus methanol and dimethyl malonate yields methanol (64%). For 1,3-diphenyl-1,3-propanedione and 1,3-cyclohexanedione, benzoic acid (61% yield) and glutaric acid (75% yield) were obtained, respectively. Methyl cyanoacetate and malononitrile were inert.

Metodologia para o crescimento de esferas monocristalinas de metais nobres

This paper describes in detail a technique employed to grow quasi-spherical single crystals of noble metals for electrochemical applications, using platinum as an example. The metal beads were formed by melting the extremity of a wire in an oxygen / butane flame. X-ray techniques were used to check the crystallization and to determine the orientation of the crystals. Treatment with a pure hydrogen flame followed by a cooling procedure in a hydrogen / argon atmosphere were used for conditioning the well-defined platinum single crystal surfaces. Finally, electrochemical characterization of the Pt(111), Pt(110) and Pt(100) surfaces was done in diluted sulfuric acid solution in the hydrogen adsorption / desorption potential region.

Eletrocatálise e poluição ambiental

This work considers mainly the problem of environmental pollution due to the production of energy by burning fossil fuels, particularly in urban vehicles. Electrochemical energy conversion is proposed as a partial solution to this problem, through the use of hydrogen in fuel cells. In both the production of hydrogen by electrolysis and in the electrochemical processes in fuel cells, the key factor is electrocatalysis. The concept of electrocatalysis, the production of catalysts and supported catalysts and their use in practical systems for the conversion of energy is discussed.

Tecnologia de células a combustível

The fuel cell principle was discovered by Sir Grove 150 years ago. However material problems prohibited its commercialization for a long time. A change has been occurring during the last 30 years, so two types of fuel cell technologies can be distinguished: low and high temperature operation cells. Nowadays, only phosphoric acid cells are commercially offered as 200 kWel power plants. Membrane cells are more suitable for automobile electrotraction with a very low (or no) environmental impact. The fuel continues, however, to play a very particular role, since hydrogen is not easy to store and to transport. The more promising target is the utilization of liquid methanol. The Brazilian scenario concerning this kind of technology is discussed.

Análise por difração de raios x de filmes de óxidos cerâmicos compostos por IrO2/TiO2/CeO2

Independent of the sample form (powder or film), XRD analysis of Ir0,3Ti(0,7-x)Ce xO2, (nominal) mixtures, for x=0, shows the formation of a solid solution phase between IrO2 and TiO2, as well as the rutile phases of IrO2 and TiO2. The presence of the anatase phase of TiO2 is also confirmed. The introduction of 30 mol% CeO2 in the mixture reveals the presence of the CeO2 and Ce2O3 phases, besides the already mentioned ones, in the powder. In the film form, however, an amorphous phase is identified. When all of the TiO2 is substituded by CeO2, for both sample forms, the only phases found are IrO2, CeO2 and Ce2O3. This result suggests cerium oxides are not capable of forming solid solutions with either IrO2 or (Ir,Ti)O2 acting solely as a dispersant matrix for these phases. These results are consistent with the much higher electrochemically active surface area when CeO2 is introduced in the binary Ti/Ir0,3Ti0,7O2 mixture. It was possible to establish a relationship between the electrochemical stability of the supported films and their crystalline structure. The unexpected presence of TiO2 and Ti2O3 in the Ti/Ir0,3Ce0,7O2 (film sample) is attributed to oxidation of the Ti support during the calcination step.

Reação de geração de oxigênio em eletrodos de Mn2O3 suportados em aço inoxidável

A study of the kinetics of oxygen evolution in alkaline conditions from ceramic films of Mn2O3 supported on stainless steel was carried out. This study has been done through the determination of transfer coefficients, Tafel slopes and exchange currents using potentiodynamic and quasi-potentiostatic measurements. The activation energy was determined as a function of the overpotential and, additionally, the electrode active surface was estimated. The results are consistent with data already published for other electrodes, implying that the methods used in this work were reliable and precise.

Química de sistemas supramoleculares constituídos por porfirinas e complexos metálicos

Supramolecular chemistry is expected to keep a high developing pace in the next years, giving support to the advancement of molecular devices and nanotechnology. In this sense, porphyrins and their analogues should play a significant role as a consequence of their catalytic, electrocatalytic, photochemical and photoelectrochemical properties. In this review we focused on our own strategy based on coordination chemistry for the design and build-up of supermolecules and supramolecular structures constituted by polynuclear porphyrins and metalloporphyrins. Included are also their properties and potential applications.

Álcoois quirais: métodos químicos e catalíticos de obtenção por redução assimétrica

The importance of chiral alcohols as starting materials for the production of fine chemicals and as useful chirons for the building of several interesting molecules or natural products is reported. The useful and common methods of asymmetric reduction such as the chemical (with organoboron or organoaluminum reagents) and the catalytic ones (with ruthenium or rhodium complexes) for preparation of chiral alcohols are described; even the newer and much more rare electrocatalytic methods are reported.

Hidrogenação de compostos orgânicos utilizando método eletroquímico para geração de hidrogênio in situ: hidrogenação eletrocatalítica

Electrocatalytic hydrogenation (HEC) may be compared to catalytic hydrogenation (HC). The difference between these methods is the hydrogen source: HC needs a hydrogen gas supply; HEC needs a source of protons (solvent) to be reduced at a cathode surface. HEC has presented interesting advances in the last decades due to investigation of the influence of the supporting electrolyte, co-solvent, surfactant, presence of inert gas and the composition of the electrode on the reaction. Several classes of organic compounds have been hydrogenated through HEC: olefins, ketones, aldehydes, aromatics, polyaromatics and nitro-compounds. This paper shows some details about the HEC which may be regarded as a promising technique for the hydrogenation of organic compounds both in industrial processes and in laboratories.

Eletrocatálise das reações de oxidação de hidrogênio e de redução de oxigênio

This work discusses the electrocatalytic processes taking place in the polymer electrolyte fuel cell electrodes, specifically the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), because these are clear examples of electrochemical reactions favored by the use of electrocatalysts. Since the gaseous reactants are very little soluble in the electrolyte, the use of special electrodes, named gas diffusion electrodes, is required to promote easy and continuous access of reactant gases to the electrocatalytic sites. Besides this, other important aspects such as the use of spectroscopic techniques and of theoretical models to improve the knowledge of the electrocatalytic systems are shortly discussed.

Estudo do efeito de tratamentos térmicos em catalisadores de PtRu/C frente à reação de oxidação de hidrogênio na presença de CO

In this work the effects of time and temperature of thermal treatments under reducing atmosphere (H2) on PtRu/C catalysts for the hydrogen oxidation reaction (HOR) in the presence of CO on a proton exchange membrane fuel cell (PEMFC) single cells have been studied. It can be seen that the increase of the treatment temperature leads to an increasing sintering of the catalyst particles with reduction of the active area, although the catalyst treated at 550 ºC presents more CO tolerance for the HOR.

Electrocatalytic oxidation of methanol by the [Ru3O(OAc)6(py)2(CH3OH)]3+cluster: improving the metal-ligand electron transfer by accessing the higher oxidation states of a multicentered system

The [Ru3O(Ac)6(py)2(CH3OH)]+ cluster provides an effective electrocatalytic species for the oxidation of methanol under mild conditions. This complex exhibits characteristic electrochemical waves at -1.02, 0.15 and 1.18 V, associated with the Ru3III,II,II/Ru3III,III,II/Ru 3III,III,III /Ru3IV,III,III successive redox couples, respectively. Above 1.7 V, formation of two RuIV centers enhances the 2-electron oxidation of the methanol ligand yielding formaldehyde, in agreement with the theoretical evolution of the HOMO levels as a function of the oxidation states. This work illustrates an important strategy to improve the efficiency of the oxidation catalysis, by using a multicentered redox catalyst and accessing its multiple higher oxidation states.