1,720,991 research outputs found
Preparation and characterization of electrocatalysts for high temperature polymer electrolyte fuel cells
Full text linkE’ noto che per una effettiva immissione nel settore automotive delle celle a combustibile ad elettrolita polimerico (PEFCs) alimentate a idrogeno, una delle principali barriere da superare è legata durata dei materiali adoperati. Il fenomeno di sintering del platino e la corrosione che quest’ultimo assieme al supporto, subiscono operando alle alte temperature richieste dal settore automotive (120 ÷ 130 °C), si traduce in una riduzione del tempo di vita del catalizzatore nel suo complesso. Gli effetti del sintering e della corrosione del catalizzatore si traducono in una perdita di area superficiale attiva del platino e di conseguenza una diminuzione della potenza erogata dal sistema.
Un’altra problematica da superare, per una diffusione concorrenziale nel settore automotive delle PEFCs rispetto ai classici motori a combustione interna, è il carico totale di platino. Ad oggi, le potenze erogate da questi apparati (ca. 100 kW) richiedono, soprattutto nel comparto catodico, una quantità notevole di metallo prezioso che, di riflesso, causa un aumento del costo complessivo del prodotto finito. Risulta ovvio che l’abbassamento del tenore di platino nel catalizzatore catodico gioca un ruolo determinante per una effettiva ed efficace immissione delle celle a combustibile all’interno di questo vasto mercato.
Scopo di questo dottorato di ricerca è stato lo sviluppo di nuovi elettrocatalizzatori supportati in grado resistere al sintering ed alla corrosione. Per ridurre il carico di platino questi materiali devono anche essere caratterizzati da una maggiore efficienza. Si è deciso, quindi, di sviluppare catalizzatori a base di platino e di leghe platino-cobalto con caratteristiche chimico-fisiche ben definite. La scelta del cobalto deriva dalle numerose indagini effettuate su questi catalizzatori; queste hanno mostrato che alcuni catalizzatori di leghe binarie di Pt-M (dove M = Co, Fe, Ni, Cu, ecc) presentano una maggiore attività elettrocatalitica per la reazione di riduzione dell’ossigeno rispetto ai catalizzatori di solo Pt. Tra queste la lega PtCo sembra risultare quella più promettente.
Ancora, vari studi hanno indicato che per i catalizzatori supportati Pt/C si ha un’attività massima a circa 3 nm come compromesso adatto tra il numero di siti e la fase cristallografica con basso indice di Miller caratterizzata da alta attività intrinseca. Obiettivo di questa attività è stato, quindi, quello di sintetizzare catalizzatori supportati a base di Pt e leghe PtCo con dimensione di particelle minore di 3 nm e contraddistinti da un adeguato grado di lega.
I catalizzatori sviluppati sono stati infine caratterizzati dal punto di vista chimico-fisico ed elettrochimico (in semicella ed in cella singola) ad elevate temperature (75 °C ÷ 130 °C) con lo scopo di analizzarne il comportamento e approfondire le cause della loro degradazione.Large scale application of polymer electrolyte fuel cell (PEFC) system technology requires a reduction of its high cost as well as improvement of performance and stability. The catalysts employed for operation in the present PEMFC are nanosized platinum particles supported on carbon. The reduction of the noble metal content and enhancement of catalytic stability are significant challenges for this fuel cell technology on the way to become cost competitive. High temperature PEFC operation (130°C) requires the development of catalyst with proper resistance to sintering and corrosion under working conditions.
In order to reach these goals, the aim of this research was the development of practical carbon supported Pt anode and cathode catalysts with high metal surface area capable of operation at high temperature (130°C) with suitable resistance to corrosion, Pt dissolution, thermal and electrochemical sintering. Another aspect was the development of new alloyed electrocatalysts. Several papers in the current literature address to the development of binary and ternary Pt-alloys. In fact, results show more active and less expensive Pt-alloy for oxygen reduction reaction (ORR) catalysts with better stability than pure Pt based catalysts. Accordingly, the work was focused on the development of practical carbon supported Pt-Co alloyed catalysts. The choice of cobalt as a transition metal in alloy with platinum was derived from literature, as many of investigations shown that it is the most promising catalyst for high temperature PEFC operation.
The purpose of this work was addressed on the achievement of a proper crystalline structure with face-centered-cubic phase and a high degree of alloying for the catalysts with small mean particle size (< 3 nm). The synthesis method was optimized to obtain a suitable dispersion of the metal particles on the support for high metal concentration catalysts (50 wt.%) with a mean particle size smaller than 3 nm for both Pt and Pt-Co alloyed electrocatalysts. Moreover, further studied were addressed to different pre-leaching methods on the performance and stability of synthesized Pt-Co alloy catalysts supported on carbon. These treatments were adopted to reduce the dissolution of cobalt from the particle surface in PEFCs.
To understand catalysts behaviour in terms of resistance to corrosion and electrochemical sintering, different analyses (XRD, cycling voltammetry and polarization) were carried out before and after various accelerated degradation tests (ADTs) performed in a gas-fed sulphuric acid electrolyte half cell at 75°C and in a PEM single cell in a temperature range of 80 – 130°C.
In conclusion, an appropriate synthesis method was developed to obtain Pt and PtCo electrocatalysts with a small particle size (< 3 nm) and suitable degree of alloying in the bimetallic electrocatalysts. Pre-leaching treatments were adopted to improve the catalytic activity and the resistance towards sintering and degradation. Accelerated degradation tests in sulphuric acid gas-fed half cell were carried out in order to investigate the influence of different pre-leaching treatments on the electrochemical behaviour. These analyses showed that the particle size of all catalysts increased after the tests. It was observed that after the Pt degradation test, the HClO4 pre-leached catalyst showed the best performance in terms of electrochemical activity and resistance to sintering
Effetto della dealcolazione mediante contattore su vini Italiani: valutazioni compositive e sensoriali.
No full textEffetto della dealcolazione mediante contattore su vini Italiani: valutazioni compositive e sensoriali
Development of a flexible low cost system of lab scale calorimeters
No full textA simple, low cost system of laboratory scale calorimeters is being developed in our laboratory for research, teaching and routine purposes. The system is computer controlled and has been assembled from commercially available components. Its use will be illustrated by a study of the hydrolysis of acetic anhydride
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Almond oil extraction by supercritical CO2: experiments and modelling
No full textOil from crushed almond seeds was extracted with supercritical CO2 at 350 bar and 40 degrees C. Almond particles of three different mean sizes were tested. Extraction of the smaller particles was performed at two different solvent flow rates. Oil yields were obtained with asymptotic values at large extraction times that were close to the values obtained by Soxhlet extraction. An extraction model based on the physical evidence of broken and intact oil cells has been developed. It accounts for a former equilibrium regime and a latter finite mass transfer regime. All model parameters except the internal mass transfer coefficient and the oil concentration at solvent saturation have been determined with independent experiments. The model solution was calculated with a finite difference numerical technique. A good agreement was obtained between model curves and the experimental data for an internal mass transfer coefficient of 7.5 x 10(-9) m/s. Solute concentration profiles within the extractor were evaluated with our model. (C) 1998 Elsevier Science Ltd. All rights reserved
Assessment of solubility of ketoprofen and vanillic acid in supercritical CO2 under dynamic conditions
No full textOn-line calibration and determination of the heat of reaction for laboratory scale heat transfer calorimeters
No full textAbstract A simple method for the on-line calibration, in which both the heat transfer coefficient and the heat capacity of the reactor contents are determined, is described for laboratory scale heat transfer calorimeters. The calorimeter is operated in the isoperibolic mode for the calibration and a constant power is supplied to a resistor placed inside the reactor. The reactor heat balance differential equation is used to produce a set of linear simultaneous equations with each data acquisition cycle giving one equation. The heat transfer coefficient and the heat capacity are obtained from this set of equations by linear least squares. The application of the calibration procedure is illustrated by experiments in which the heat of reaction is determined on-line fora simulated reaction with first order kinetics and for the hydrolysis of acetic anhydride.
Keywords
calibration, hydrolysis of acetic anhydride, reaction calorimetr
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