Determination of Kinematic Viscosity of Mg(ClO₄)₂ and KOH Brines Saturated with CO₂ at Sub-Zero Temperatures

The current race for space exploration has hastened the development of electrochemical technologies for the in-situ utilisation of planetary resources for the synthesis of vital chemicals such as O2 and fuels. Understanding the physicochemical properties, such as the density and kinematic viscosity, of aqueous solutions is essential for the design of electrochemical devices for the electrolysis of water and CO2, particularly at low temperatures. The density and kinematic viscosity of highly concentrated Mg(ClO4)2 and KOH solutions have been determined, both at low temperatures and in the presence of CO2 gas. It was found that, for all of the solutions, independent of the concentration or nature of the electrolyte, as the temperature was decreased to 255 K, the density and the viscosity of the solutions increased. Upon saturation with CO2, no significant change to the density and viscosity of Mg(ClO4)2, at all of the temperatures measured, was observed. Conversely, the CO2 saturated solutions of KOH showed significant changes in density and viscosity at all temperatures, likely due to the formation of carbonates. The effects of these changes on the diffusion coefficient for dissolved CO2 is also discussed

Electrochemical Oxidation of Small Organic Molecules on Au Nanoparticles with Preferential Surface Orientation

The surface orientation effect on the oxidation of small organic molecules such as methanol, formaldehyde, ethanol, and glycerol has been studied on Au nanoparticles in alkaline medium. Two sets of Au nanoparticles enriched in (100) and (111) facets were synthetized by using colloidal methods in presence of cetyltrimethylammonium bromide. The nanoparticles were physically characterized by using TEM and XRD and electrochemically characterized by using Pb underpotential deposition as a surface-structure probe. It is reported that, although methanol oxidation was similar in both types of nanoparticles, the oxidation of formaldehyde presented a clear surface orientation effect. For this reaction, nanoparticles with (111) preferential orientation presented higher current densities at low potentials, whereas Au(100) nanoparticles exhibited higher activity at potentials more positive than 1.0 V versus RHE. On the other hand, for glycerol and ethanol oxidations, the onset of the reaction was similar in both types of particles, although Au(111) nanoparticles showed higher current densities than the Au(100) ones.P.R. acknowledges financial support from the Netherlands Organization for Scientific Research (NWO) through a VENI grant and the University of Birmingham through a Birmingham Fellowship

Enhanced electrocatalytic activity of Au@Cu core@shell nanoparticles towards CO2 reduction

The development of technologies for the recycling of carbon dioxide into carbon-containing fuels is one of the major challenges in sustainable energy research. Two of the main current limitations are the poor efficiency and fast deactivation of catalysts. Core–shell nanoparticles are promising candidates for enhancing challenging reactions. In this work, Au@Cu core–shell nanoparticles with well-defined surface structures were synthesized and evaluated as catalysts for the electrochemical reduction of carbon dioxide in neutral medium. The activation potential, the product distribution and the long term durability of this catalyst were assessed by electrochemical methods, on-line electrochemical mass spectrometry (OLEMS) and on-line high performance liquid chromatography. Our results show that the catalytic activity and the selectivity can be tweaked as a function of the thickness of Cu shells. We have observed that the Au cubic nanoparticles with 7–8 layers of copper present higher selectivity towards the formation of hydrogen and ethylene; on the other hand, we observed that Au cubic nanoparticles with more than 14 layers of Cu are more selective towards the formation of hydrogen and methane. A trend in the formation of the gaseous products can be also drawn. The H2 and CH4 formation increases with the number of Cu layers, while the formation of ethylene decreases. Formic acid was the only liquid species detected during CO2 reduction. Similar to the gaseous species, the formation of formic acid is strongly dependent on the number of Cu layers on the core@shell nanoparticles. The Au cubic nanoparticles with 7–8 layers of Cu showed the largest conversion of CO2 to formic acid at potentials higher than 0.8 V vs. RHE. The observed trends in reactivity and selectivity are linked to the catalyst composition, surface structure and strain/electronic effects

Electrochemical characterization and regeneration of sulfur poisoned Pt catalysts in aqueous media

Understanding the poisoning and recovery of precious metal catalysts is greatly relevant for the chemical industry dealing with the synthesis of organic compounds. For example, hydrogenation reactions typically use platinum catalysts and sulfuric acid media, leading to poisoning by sulfur-containing species. In this work, we have applied electrochemical methods to understand the status and recovery of Pt catalysts by studying the electro-oxidation of a family of sulfur-containing species adsorbed at several types of Pt electrodes: (i) polycrystalline Pt foil; (ii) Pt single-crystal electrodes; and (iii) Pt nanoparticles supported on Vulcan carbon. The results obtained from polycrystalline Pt electrodes and Pt nanoparticles supported on Vulcan carbon demonstrate that all sulfur-containing species with different oxidation states (2-, 3+ and 4+) lead to the poisoning of Pt active sites. X-ray photoelectron spectroscopy (XPS) analysis was employed to elucidate the chemical state of sulfur species during the recovery process. The degree of poisoning decreased with increased sulfur oxidation state, while the rate of regeneration of the Pt surfaces generally increases with the oxidation state of the sulfur species. Finally, the use of Pt single-crystal electrodes reveals the surface-structure sensitivity of the oxidation of the sulfur species. This information could be useful in designing catalysts that are less susceptible to poisoning and/or more easily regenerated. These studies demonstrate voltammetry to be a powerful method for assessing the status of platinum surfaces and for recovering catalyst activity, such that electrochemical methods could find applications as sensors in catalysis and for catalyst recovery in-situ

Anomalous phase transition of layered lepidocrocite titania nanosheets to anatase and rutile

In this study, phase transformations from lepidocrocite titania (L-TiO2) nanosheets to rutile (R-TiO2) and anatase (A-TiO2) have been systematically investigated as a function of the preparation conditions, such as pH and freeze-drying, and as a function of the temperature treatment. We have found that the transformation of (L-TiO2) into rutile takes place upon freeze-drying treatment. We report that temperature determined the final phase structure in the transition phase of the L-TiO2 nanosheets into TiO2 nanoparticles, while the pH determined the final morphology and particle size. On the basis of the experimental results, two different transition pathways of dissolution–recrystallization and topologically rolling transition have been proposed. Our results give a full map of phase transition and morphology evolution of L-TiO2 to R-TiO2/A-TiO2 that can provide guideline to new materials design, especially for photocatalysts