Mariangela Longhi

Surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic halide at mono- and polycrystalline silver in acetonitrile

Abstract The silver surface screening effects by specifically adsorbed halide anions in the electrocatalytic reduction of a model organic bromide (acetobromoglucose) have been studied by cyclic voltammetry on controlled mono- and polycrystalline silver surfaces in acetonitrile+0.1 M tetraethylammonium perchlorate medium as a function of the concentration c X of added TEAX (X=Cl, Br, or I, TEA, tetraethylammonium). The reduction peak potentials, E p , are regularly shifted in the negative direction with increasing c X , typically tending to an asymptotic value for c X ≈0.1 M. Several literature models describing adsorption/desorption equilibria have been applied to justify the above experimental E p versus c X trends (being logarithmic in the iodide cases) for the three halides and the four silver surfaces tested.

An old workhorse of oxide investigations: new features of Co3O4

Abstract The electrochemical behaviour of Co 3 O 4 from exhaustive cobalt nitrate decomposition ( T =260–850°C) is investigated on graphite-supported electrodes, mainly relying on quasi-reversible results. Two well defined 1−e − redox systems are observed related to Co 2+ /Co 3+ and Co 3+ /Co 4+ reactions of non-equivalent oxide surface sites. Oxide stoichiometry is directly estimated by voltammetry in ‘wet’ conditions and depends on preparation temperature as in ex situ solid state data. Adsorption of surface intermediates involved in charge transfer is examined. Langmuir and Temkin-type adsorption isotherms are obeyed for the Co 2+ /Co 3+ and Co 3+ /Co 4+ redox systems, respectively. Unusually negative (attractive) lateral interaction parameters are calculated for the latter system. Separation of oxide and graphite currents is achieved to permit real electrode surface area determination.

The solvent friction mechanism for outer-sphere electron exchange at bare metal electrodes. The case of Au/Ru(NH3)63+/2+ redox system

The heterogeneous rate constant for the Ru(NH3)63+/2+ electron exchange at the bare gold electrode displays a power-law dependence on the solution viscosity (varied by addition of glucose, 0–600 g l−1) with a negative power index of δ=1, indicative of the “full” solvent friction (adiabatic) mechanism for the intrinsic charge-transfer step. Comparison with related processes suggests that this mechanism mainly occurs at bare metal electrodes irrespective of the reactants’ charged state and the method of viscosity variation. The adiabatic regime operates notwithstanding the presence of at least one layer of solvating water, and/or specifically adsorbed ions, contributing to the charge-transfer distance.

Template-free ultraspray pyrolysis synthesis of N/Fe-doped carbon microspheres for oxygen reduction electrocatalysis

Ultrasonic spray pyrolysis was used in a continuous flow apparatus for the template-free synthesis of iron- and nitrogen-doped porous carbon materials. Solutions of glucose, histidine and Fe(CH3COO)2 were nebulized and pyrolyzed yielding carbon microspheres. Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and Focused Ion Beam (FIB) milling revealed that microspheres initially possess empty cores and a smooth shell. Further annealing leads to a collapse of this shell, and formation of porous microspheres with high roughness and iron-rich aggregates. X-ray Diffraction (XRD) and Photoelectron Spectroscopy (XPS) were used to investigate bulk and surface chemistry: microspheres were found to undergo graphitization; Fe and Fe3C particles form and become encapsulated within the carbon phase, while the nitrogen present in the precursor solution results in the formation of pyridinic/pyrrolic N-centers. The microspheres were tested as electrocatalysts for the oxygen reduction reaction (ORR) in acidic solution. Polarization curves using a Rotating Disk Electrode (RDE) yielded electrocatalytic behavior, and the number of exchanged electrons n = 3.7 ± 0.2 calculated from Koutecky–Levich plots suggests that direct formation of H2O is the preferred ORR mechanism. These results indicate that this synthetic approach offers a simple and scalable strategy for the preparation of electrode materials for polymer electrolyte membrane fuel cells.

Fe-based heterogeneous catalysts for the Fischer-Tropsch reaction: Sonochemical synthesis and bench-scale experimental tests.

The sonochemical synthesis of nanostructured materials owes its origins to the extreme conditions created during acoustic cavitation, i.e., the formation of localized hot spots in the core of collapsing bubbles in a liquid irradiated with high intensity ultrasound (US). In particular, in the present work a sonochemical synthesis has been investigated for the production of three different iron-based samples supported on SiO2 and loaded with different metals and promoters (10 %wt of Fe; 30 %wt of Fe; 30 %wt of Fe, 2 %wt of K and 3.75 %wt of Cu) active in the Fischer-Tropsch (FT) process. Sonochemically synthesized heterogeneous catalysts were characterized by BET, XRPD, TPR, ICP, CHN, TEM, SEM and then tested in a fixed bed FT-bench-scale rig fed with a mixture of H2 and CO at a H2/CO molar ratio equal to 2, at activation temperatures of 350-400°C and reaction temperatures of 250-260°C. The experimental results showed that the ultrasonic samples are effective catalysts for the FT process. Notably, increasing the activation temperature increased CO conversion, while product selectivity did not diminish. All the sonochemically prepared samples presented in this work provided better catalytic results compared to the corresponding traditional FT impregnated catalysts.

Charge-Transfer Patterns for [Ru(NH3)6]3+/2+ at SAM Modified GoldElectrodes: Impact of the Permeability of a Redox Probe

Electrochemical performance of a (Ru(NH3)6) 3+/2+ redox couple at gold electrodes modified by alkanethiol self assembled monolayer (SAM) films of the type (-SH -(CH2)n - CH3) with different number of methylene units (n = 2 to 10) in the presence and absence of glucose additives has been studied using fast scan cyclic and steady-state voltammetry. Specific scatter of measured rate constants caused by enhanced sensitivity of this probe to minor defects of SAMs has been observed in a general agreement with the published data for thicker SAMs (n = 9 to 18). In addition, we have disclo- sed the anomalous viscosity-imposed drop of the heterogeneous rate constant for the case of Au electrodes modified by thinner n-alkanethiol SAMs (n = 2, 4). Taking into the account the fact of (Ru(NH3)6) 3+/2+ couples capability to penetrate into the SAM interior, we ascribe the obtained results to the manifestation of the solvent-friction mechanism under the condition where the redox species presumably together with a few of solvating water molecules reside in a SAMs peri- pheral interior marked by much higher local viscosity (slower dielectric relaxation) compared to the electrolyte solution.

Oxide electrodes. A new technique to bind oxide powders onto Au substrates

A technique is described to obtain supported oxide powder electrodes. Preformed powders are first embedded into a gold substrate, as reported by Totir et al. [D.A. Totir, B.D. Cahan, D.A. Scherson, Electrochim. Acta 45 (1999) 161], and then made to adhere in place by heating at T∼260–300 °C in flowing oxygen. This latter step is of the greatest importance to obtain reliable electrodes for use in aqueous electrolytes. Electrode stability and reproducibility are examined on voltammetric and O2 evolution results using Co3O4 as a sensitive test oxide material. Many independent Au–Co3O4 electrodes compare favourably with one another and with graphite-supported electrodes from an unrelated, room temperature binding technique. Substrate and oxide-substrate junction effects are minimised.

Tailored N-Containing Carbons as Catalyst Supports in Alcohol Oxidation

The introduction of N-containing functionalities in carbon-based materials is brought to stable and highly active metal-supported catalysts. However, up to now, the role of the amount and the nature of N-groups have not been completely clear. This study aims to clarify these aspects by preparing tailored N-containing carbons where different N-groups are introduced during the synthesis of the carbon material. These materials were used as the support for Pd nanoparticles. Testing these catalysts in alcohol oxidations and comparing the results with those obtained using Pd nanoparticles supported on different N-containing supports allowed us to obtain insight into the role of the different N-containing groups. In the cinnamyl alcohol oxidation, pyridine-like groups seem to favor both activity and selectivity toward cinnamaldehyde.

Comparative toxicity of three differently shaped carbon nanomaterials on Daphnia magna: does a shape effect exist?

Abstract The acute toxicity of three differently shaped carbon nanomaterials (CNMs) was studied on Daphnia magna, comparing the induced effects and looking for the toxic mechanisms. We used carbon nano-powder (CNP), with almost spherical primary particle morphology, multi-walled carbon nanotubes (CNTs), tubes of multi-graphitic sheets, and cubic-shaped carbon nanoparticles (CNCs), for which no ecotoxicological data are available so far. Daphnids were exposed to six suspensions (1, 2, 5, 10, 20 and 50 mg L−1) of each CNM, and then microscopically analyzed. Ultrastructural analyses evidenced cellular uptake of nanoparticle in CNP and CNT exposed groups, but not in samples exposed to CNCs. Despite this difference, very similar effects were observed in tissues exposed to the three used CNMs: empty spaces between cells, cell detachment from the basal lamina, many lamellar bodies and autophagy vacuoles. These pathological figures were qualitatively similar among the three groups, but they differed in frequency and severity. CNCs caused the most severe effects, such as partial or complete dissolution of the brush border and thinning of the digestive epithelium. Being the cubic shape not allowed to be internalized into cells, but more effective than others in determining physical damages, we can conclude that shape is an important factor for driving nanoparticle uptake by cells and for determining the acute toxicological endpoints. Shape also plays a key role in determining the kind and the severity of pathologies, which are linked to the physical interactions of CNMs with the exposed tissues.

Fe local structure in Pt-free nitrogen-modified carbon based electrocatalysts: XAFS study

The paper presents a new results on the bonding environment (coordination number and geometry) and on oxidation states of Fe in nitrogen-modified Fe/C composites used as Pt-free catalysts for oxygen reduction in Direct Hydrogen Fuel Cells. Starting from glucose or fructose, two catalysts displaying different electrochemical performance were prepared and studied in the form of pristine powder and thin catalytic layer of electrode by Fe K-edge XAFS spectroscopy. The results show how the Fe local structure varies as a function of different synthesis conditions and how changes in the structural properties of the catalysts are related to fuel cell electrochemical performance increase during a cell activation period.