117,612 research outputs found
Cyclic Voltammetry as a powerful tool for the study of the evolution in time of bimetallic Au/Pt nanoparticles
No full textBimetallic systems have attracted more and more interest in recent years due to their novel optical, catalytic, magnetic, and sensing properties, often different from the corresponding monometallic counterparts. Novel ways to fabricate, characterize and explore these fascinating systems represent an important research topic in this field. In fact, studies directed towards the size, shape, composition, and functionalization of the particles allow leading to sophisticated nanomaterials specifically designed for their intended applications. For these reasons, not only the type and the quantity of the two metals are to be evaluated, but also the distribution of the two components, when passing from an alloy to a core-shell system or vice versa. In this context, techniques such as transmission electron microscopy (TEM), dynamic light scattering (DLS), and optical spectroscopy are generally used for this routine characterization.
Recently, electrochemistry has been employed as an alternative and complementary technique with very promising results [1-3], allowing the discrimination of an alloy from a broken or perfect core-shell system after synthesis. However, a remaining challenge in this area lies in the difficulty of following the formation of these nanomaterials during the synthetic procedure. The formation of these bimetallic systems is dependent on the rate of production of each component and can show different intermediate structures. Cyclic voltammetry (CV) can be used to cover also the gap in this field.
In the present work, bimetallic systems based on Au and Pt nanoparticles, in the form of alloy (Au+P) or core-shell (Au@Pt and Pt@Au) structures, are deeply investigated. HR-TEM images confirm the expected alloyed structure for Au+Pt; Au@Pt particles tend be alloyed, while Pt@Au systems present an alloyed core with an external shell Pt enrichment. These considerations are confirmed by CV experiments, which can also permit to follow the synthetic evolution of these systems in time.
References
[1] L. R. Holt, B.J. Plowman, N.P. Young, K. Tschulik, R.G. Compton, Angew. Chem. Int. Ed. 55 (2016) 397–400.
[2] K. Tschulik, K. Ngamchuea, C. Ziegler, M.G. Beier, C. Damm, A. Eychmueller, R.G. Compton, Adv. Funct. Mater. 25 (2015) 5149–5158.
[3] V. Pifferi, C. Chan-Thaw, S. Campisi, A. Testolin, A. Villa, L. Falciola, L. Prati, Molecules 21 (2016) 261
Voltammetric characterization of gold-based bimetallic (AuPt; AuPd; AuAg) nanoparticles
No full textBimetallic nanoparticles are nowadays some of the most promising materials for catalytic, electrocatalytic and electroanalytical applications thanks to their novel optical, catalytic, magnetic, and sensing properties. Such novel features, often different and enhanced with respect to the monometallic counterparts, make these systems good candidates to be conveniently applied in a wide range of fields. The possibility to obtain different kinds of bimetallic composites (in terms of composition, structure, metal loading, morphology, etc.) goes in parallel with the need of powerful and accurate characterization tools. Among the commonly involved techniques like Optical Spectroscopy and Dynamic Light Scattering (DLS), also the more powerful Transmission Electron Microscopy (HR-TEM) and Extended X-Ray Absorption Fine Structure (EXAFS) are widely used. However, these analytical tools present some drawbacks in terms of high costs and low accessibility. In this context, electrochemistry and particularly Cyclic Voltammetry, is here proposed as an alternative, low cost, easy to use and simple characterization technique.
The possibility to use electrochemical methods to study the final structure of bimetallic nanocomposites was already demonstrated in the Literature [1-2], but there is still lack of information on how such systems change and evolve in time and after aging periods. Therefore, Cyclic Voltammetry is here used, as a complementary technique to HR-TEM and EXAFS not only to investigate the structure of alloyed or core-shell gold-based (Au-Pt; Au-Pd; Au-Ag) systems (by studying the quantity and type of metals present in the materials), but also to elucidate the evolution and growth in time of such bimetallic samples. Time evolution characterization allows to control the morphology and to fix it at the desired point.
Finally, the characterized gold-based nanocomposites are used in electrochemical sensing and electrocatalytic applications. A strong improvement in the response, in terms of higher peak currents and electrocatalytic effects, of the bimetallic systems with respect to the monometallic counterparts is evidenced, due to the intimate contact between the two metals, which is responsible of synergistic effects. Also, the effects of an eventual carbonaceous support on the properties of the metal nanoparticles and the possible synergistic effects between composites and supports are investigated [3].
[1] K. Tschulik, K. Ngamchuea, C. Ziegler, M.G. Beier, C. Damm, A. Eychmueller, R.G. Compton, Adv. Funct. Mater., 2015, 25, 5149–5158.
[2] V. Pifferi, C. Chan-Thaw, S. Campisi, A. Testolin, A. Villa, L. Falciola, L. Prati, Molecules, 2016, 21, 261.
[3] A. Testolin, S.Cattaneo, W. Wang, D. Wang, V. Pifferi, L. Prati, L. Falciola, A. Villa, Surfaces, 2019, 2, 205-215
Electrochemical characterization of Au/Pd catalysts
No full textGold-based catalysts have attracted more and more interest in recent years due to their peculiar activity in catalytic reactions, such as low-temperature oxidation of CO, hydrochlorination of alkyne, liquid phase oxidation of alcohols and polyols [1]. However, concerning selective liquid phase oxidation, despite the peculiar selectivity shown, gold catalysts suffer from a severe limitation consisting in the use of a basic environment. This drawback has a strong impact on the use of gold and promoted the evolution of monometallic into bimetallic systems.
In the present work, an electrochemical characterization of homemade Au/Pd bimetallic catalysts is performed. Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) demonstrate the presence of an electronic interaction between the two metals providing a strong support in the determination of the nature of the synergy between Au and Pd in the liquid phase oxidation of alcohols [2].
Fig.1 shows the voltammetric patterns, recorded in 0.1M H2SO4, for Au, Pd and alloyed Au/Pd nanoparticles deposited for drop casting on a glassy carbon electrode (GC) used as inert support. The nanoalloy presents a larger oxidation peak due to the oxidation of the alloy, which comprises the two oxidation peaks of the single metals, while the reduction peak (at 0.63V) has an intermediate behaviour between Au and Pd. This behaviour indicates that the oxide formed on the alloy is more stable with respect to the gold one but it is really less stable than the palladium one. Charge transfer resistance (RCT) obtained from impedance data are in the order Pd > Alloy >> Au, confirming the intermediate behaviour shown in the voltammetric characterization.
Correlating these results to the catalytic behaviour observed in the glycerol selective oxidation of Pd, Au and AuPd catalysts we can conclude that the activities of the different systems seems to be connected not only to the redox behavior. In fact, the higher activity of the AuPd bimetallic system in the liquid phase oxidation of glycerol seems to be due to a compromise between the stability of the oxidic species (decreased with respect to Pd) and the facility of hydride formation (increased with respect to Au).
References
[1] L. Prati, A. Villa, Gold Catalysis: Preparation, Characterization and Applications, Pan Stanford Publishing Pte. Ltd.: Singapore City, Singapore, (2015).
[2] V. Pifferi, C. Chan-Thaw, S. Campisi, A. Testolin, A. Villa, L. Falciola and L. Prati, Molecules, 21, (2016), 261
Do estimates of numerosity really adhere to Weber’s law? A reexamination of two case studies
No full textBoth humans and nonhuman animals can exhibit sensitivity to the approximate number of items in a visual array or events in a sequence, and across various paradigms, uncertainty in numerosity judgments increases with the number estimated or produced. The pattern of increase is usually described as exhibiting approximate adherence to Weber’s law, such that uncertainty increases proportionally to the mean estimate, resulting in a constant coefficient of variation. Such a pattern has been proposed to be a signature characteristic of an innate “number sense.” We reexamine published behavioral data from two studies that have been cited as prototypical evidence of adherence to Weber’s law and observe that in both cases variability increases less than this account would predict, as indicated by a decreasing coefficient of variation with an increase in number. We also consider evidence from numerosity discrimination studies that show deviations from the constant coefficient of variation pattern. Though behavioral data can sometimes exhibit approximate adherence to Weber’s law, our findings suggest that such adherence is not a fixed characteristic of the mechanisms whereby humans and animals estimate numerosity. We suggest instead that the observed pattern of increase in variability with number depends on the circumstances of the task and stimuli, and reflects an adaptive ensemble of mechanisms composed to optimize performance under these circumstances
Host blood monocytes play an important role, both by themselves and interacting with allo-Langerhans cells, in presenting allo-epidermal cell antigens in vitro.
No full textHost blood monocytes play an important role, both by themselves and interacting with allo-Langerhans cells, in presenting allo-epidermal cell antigens in vitro
Modelling acidic corrosion of aluminium foil in contact with foods
No full textThe possible interaction between non-coated aluminium and foods was studied using the corrosion behaviour of an aluminium household foil under different conditions of acidity, dissolved oxygen and temperature of the contacting phase.
The aluminium foil was kept in contact with acetic acid solutions of pH values varying from 2.5 to 3.5 and dissolved oxygen concentrations between 0 and 8 ppm, under different conditions of temperature (from 0.3 to 50 °C) and contact time between 24 and 120 h. Corrosion was always detected and it was shown that all three variables influence the rate, and consequently the extent, of the interaction but pH and temperature have a greater effect since their relationships with rate are of an exponential nature. The relationship between corrosion rate and dissolved oxygen, however, is linear and has a minor accelerating effect on the corrosion rate.
By applying the Arrhehius equation it was possible to evaluate the thermal sensitivity of the phenomenon with respect to the different variables.
The activation energies vary between 31–635 and 88–471 J mol−1, indicating a higher thermal sensitivity at the lowest pH values and highest dissolved oxygen amounts
Investigating the Generative Dynamics of Energy-Based Neural Networks
No full textGenerative neural networks can produce data samples according to the statistical properties of their training distribution. This feature can be used to test modern computational neuroscience hypotheses suggesting that spontaneous brain activity is partially supported by top-down generative processing. A widely studied class of generative models is that of Restricted Boltzmann Machines (RBMs), which can be used as building blocks for unsupervised deep learning architectures. In this work, we systematically explore the generative dynamics of RBMs, characterizing the number of states visited during top-down sampling and investigating whether the heterogeneity of visited attractors could be increased by starting the generation process from biased hidden states. By considering an RBM trained on a classic dataset of handwritten digits, we show that the capacity to produce diverse data prototypes can be increased by initiating top-down sampling from chimera states, which encode high-level visual features of multiple digit classes. We also find that the model is not capable of transitioning between all possible digit states within a single generation trajectory, suggesting that the top-down dynamics is heavily constrained by the shape of the energy function. We also study the generative dynamics on a more challenging dataset containing pictures of faces, showing that the exploration of stable states also partially depends on complexity of the training data distribution
Metal-free Doped Carbons for Electroanalytical Sensors
No full textIn this chapter, a quick excursus of metal-free doped carbon materials for the modification of electrodes to be used in electroanalytical sensors has been made. After a brief introduction to the different methods available for modified electrode preparation, the most used carbonaceous materials doped with heteroatoms (B, N, O, P, S) have been considered: carbon paste and ordered carbons, glassy carbon, boron-doped diamond and amorphous carbon, carbon nanotubes, and graphene. The effect of the presence of metals, graphitic and amorphous carbon impurities, which is not completely avoidable when considering these materials, has also been discussed
The last to die is hope: prolonged mechanical circulatory support with a Novacor left ventricular assist device as a bridge to transplantation.
No full textAbstractJ Thorac Cardiovasc Surg 2003;125:417-
Polyphenol content and total antioxidant potential of selected Italian wines.
No full textThe total antioxidant activity (TAA) of 13 typical Italian wines was determined (average 12.3 and 1.6 mM Trolox equivalents for red and white wines, respectively), and the resulting values were correlated with total phenols (1365-3326 and 96-146 mg/L for red and white wines, respectively), flavanols (203-805 and 11-49 mg/L, for red and white wines, respectively), and flavonols. Only the red wines contained appreciable amounts of flavonols (average 15.3 mg/L), with quercetin and
rutin being the most abundant, followed by myricetin, kaempferol, and isorhamnetin accounting for only 0.7-3% of TAA. The TAA of investigated wines are well correlated with phenol (r=0.9902) and flavanol (r=0.9270) content. These results confirm that red wine polyphenols are in vitro
significant antioxidants and may explain the beneficial effects of a moderate daily intake of red wines, probably through a sparing action of highly bioavailable vitamins C, E, and beta-carotene
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