1,721,130 research outputs found
Templating induced behavior of platinum-free carbons for oxygen reduction reaction
No full textThe aim of this work is to investigate the effect that pore shape and morphology may introduce in the behavior of some platinum free catalysts for the oxygen reduction reaction (ORR). Samples of the catalyst are obtained heat-treating a reactant mixture with constant composition and using a variety of templating procedures, i.e. shape-imprinting materials (like high surface area silica and Black Pearls 2000), and some self-templating processes (freeze-drying and in-situ generation of gas bubbles). Reaction products are characterized by XPS, BET and electrochemical methods. Analysis of results shows that the ORR catalyst activity depends in a complex manner on structural and morphological features, being not only affected, as expected, by surface area and pore width, but also by the pore shape. In a detailed examination a relation in fact emerges between ORR activity and relative abundance of micro- and mesopores of different size and pore shape
Supercritical CO2 and Green Extraction methods for Added Value Products : Carotenoids, Chlorophylls and Phycocyanin from Spirulina Microalgae
No full textNowadays, the blue-green microalgae of the genus Arthrospira, commonly known as Spirulina, are commercially grown all around the world for their nutritional properties. The popularity of Spirulina as a food supplement is mainly due to its high protein content (up to about 70% by dry weight) and its richness in minerals, vitamins and provitamins, phytochemicals, essential amino acids, fibres and pigments 1. Among them, carotenoids, chlorophylls and phycocyanins are of high relevance as food and feed dyes. In particular, phycocyanin has been widely considered as a precious protein target because of its rare intense-blue colour, due to the presence of linear tetrapyrrole chromophores, covalently bound to cysteine residues via thioether bonds. Its protein-based structure is arranged in αβ protomers associated into trimers (αβ)3 and hexamers (αβ)6. Its absorption in the visible region (max=620 nm) and its natural fluorescence account for its application as marker in the medical field. The presence of the protein in the algae, carrying specific chromophores, is able to enhance the absorption range in the visible spectrum of light, facilitating the photosynthesis.
Different strategies were developed for the isolation and purification of phycocyanin in the last decade, all of them however discarding the residual pigment fraction 2.
This study suggests an integrated and “green” extraction chain that only leads to phycocyanin at the end. The body of the strategy involves two consecutive steps of extraction of carotenoids and chlorophylls through supercritical-CO2, a well-recognised “green” extraction method, before phycocyanin extraction 3.
The biomass residue, exhausted in terms of carotenoids and chlorophylls, is finally extracted in water to yield phycocyanin. On the basis of recent and past literature on the topic, a strategy to yield the blue pigment with high purity was developed, keeping an eye on the scalability of the overall process in terms of cost and time consumption. Consecutive steps were carried out in order to enhance the phycocyanin purity, including electrocoagulation, dialysis and protein salting-out. These processes yielded 250 mg g−1 of phycocyanin (by dry Spirulina weight). A potentially scalable strategy to obtain the blue pigment with high purity (A620/A280 = 2.2) was set up. The practical application of the extracted blue phycocyanin pigment as a cotton-based tissue colorant was also experimented.
References:
[1] G. Chamorro-Cevallos, Int. J. Food Nutr. Sci., 2016, 3, 1-10.
[2] R. Chaiklahan, N. Chirasuwan, V. Loha, S. Tia, B. Bunnag, Bioresour. Technol., 2011, 102, 7159–7164.
[3] S. Marzorati, A. Schievano, A. Idà, L. Verotta, Green Chemistry, 2020, 22, 187-196
Templating-Induced Enhancement Of The Electrocatalytic Activity Of Pt-free Carbons For Oxygen Reduction Reaction
No full textFor decades scientific research has been devoted to develop efficient systems to produce electrical energy by means of fuel cells. The engineering of such devices is one of the greatest challenges of the present time, however with ensuing drawbacks due to their relatively short lifetime, prohibitive cost of many involved processes and materials. Focusing on the cathodic oxygen reduction reaction (ORR), it is kinetically hindered and catalysts are compulsory. Among them platinum and platinum alloys are nowadays the best credited ones, nevertheless there are many problems arising from platinum availability and cost. Platinum-free catalysts are highly desirable and actively searched for by worldwide investigation.
The aim of the work was not simply to develop carbon-based, platinum-free catalysts for ORR, but mostly to investigate the effects that any variation in the templating procedure may introduce on their final ORR behaviour. A constant reactant mixture (comprising a given sugar and a nitrogen-containing base) was 900°C heat-treated following a number of different templating techniques. Shape-imprinting materials (e.g. high surface area silica and Black Pearls 2000) were used with the aim of obtaining a replica of template porosity and surface features in the synthesis products. Some auto-templating processes (e.g. freeze-drying and in-situ generation of gas) were also experimented.
Reaction products were characterized by physico-chemical methods comprising BET surface area and cyclic voltammetry for ORR by rotating disk electrode. An accurate analysis of these results show that the catalyst activity depends deeply on its structural and morphological features. ORR improves with increasing sample surface area, which is a general effect related to the increased number of available reacting centres. Other interesting aspects may have more specific origins. The first one is related to the relative abundance of mesopores with sizes between 5 and 20 nm, facilitating mass transport processes from/to recessed active sites. Whereas the last distinctive feature of samples with a satisfying ORR behavior is imprinted in their nitrogen adsorption isotherm: bad catalysts isotherms have the typical shape of bottle-neck pores.
The above considerations let us infer that a smart choice of the proper templating procedure, besides the obvious compositional aspects, might lead to a final sample with an enhanced catalytic activity towards ORR deriving from an optimized merging of surface properties and pores shape
Platinum-free electrocatalysts for oxygen reduction reaction
No full textPlatinum and Pt-alloys are at present the best catalysts for oxygen reduction in PEM fuel cells. Since the precious metal is scarce and expensive, there is a strong effort to find substitute catalysts. Catalysts based on iron and cobalt ions seem to be a good and promising alternative to platinum. Moreover, the nature and behaviour of the catalyst support are also of the utmost importance. Activated carbons are used as catalyst and electrocatalyst supports in many chemical and electrochemical processes. This is due to the fact that through their structural and morphological features (high surface area, controlled porosity with selected pore size dimensions, high adsorption power and complex surface chemistry) they generally improve the intrinsic catalyst behavior. Improved performances of such composite catalyst-substrate materials are desirable and can be obtained by modifying the nature and number of the native functional groups of the carbon surface and by introducing new surface species, as metal centres, heteroatoms, etc.. In this work we present some data concerning electrochemical oxygen reduction onto nitrogen-modified activated carbon also bearing transition metal cations. Different nitrogen-bearing precursors and carbon sources will be considered. Catalysts characterisation was carried out by physical, chemical and electrochemical methods. Results are given in terms of the influence that nitrogen and carbon precursors exert on the electrocatalytic activity of the obtained materials
Templating effects onto electrocatalytic properties of Pt-free carbons for oxygen reduction reaction
No full textThe aim of this work was to investigate morphologic effects that templating may exert on the synthesis of nitrogen-doped Pt-free carbons and any resulting variation in the electrochemical behavior of these products for the oxygen reduction reaction (ORR). This latter reaction is kinetically slow and limits the energy efficiency of currently investigated Polymer Electrolyte Membrane Fuel Cells. The starting point of this work was a silica templating method, in which organic precursors reacted at high temperature in the presence of high surface area silica. Other simple templating methods were projected and developed. Among these, some catalyst synthesis procedures required solid supports as shape-imprinting materials. The aim was to obtain a replica of template porosity and surface features in the synthesized products. In other synthesis, auto-templating processes of catalyst precursors, e.g. freeze-drying and in-situ generation of gas, were studied. Synthesized catalysts have been characterized by physico-chemical methods as surface area analysis and cyclic voltammetry, which allowed the study of the kinetics of the electrochemical ORR by rotating disk electrode and rotating ring disk electrode in acidic and alkaline solutions. A relation between pore size distribution and electrochemical activity of the synthesized materials seemingly exists. Overall, a BET porosity analysis suggests that templating procedure can be actually effective to improve the electrocatalytic behaviour for ORR, only if it ensures a homogeneous distribution of macropores and small and medium mesopores (Figure 1). This surface feature is necessary to catalyst performances because it favours mass transport processes to/from the electrode surface
PT-FREE NANO- AND MICRO-STRUCTURED CARBONS FOR ELECTROCHEMICAL OXYGEN REDUCTION REACTION
Get PDFOxygen reduction reaction (ORR) catalysts are of crucial importance in developing low- and medium-temperature fuel cells, as PEMFCs (Polymer Electrolyte Membrane Fuel Cells), from which sizeable energy saving and reduction of greenhouse gas emission are expected in comparison with the use of coal and oil based fuels in thermal engines. The same electrochemical oxygen reduction reaction takes place in oxygen depolarized cathodes (ODC) in chlor-alkali electrolysis, replacing the conventional hydrogen evolving cathode, gaining about 30% energy consumption reduction in the overall process. At present carbon-supported Pt and Pt-rich alloys are best credited to the ORR purpose. However, Pt-based catalysts are not free from certain drawbacks, such as oxide formation and Pt particle coarsening through Ostwald ripening, that decrease the overall cell energy conversion efficiency. Furthermore, attendant problems concerning natural availability, geographic distribution and cost of platinum, render platinum supply strategic and fuel cells hardly scalable to mass production. At present, projections on platinum usage for PEMFCs are estimated at ~15 ton y-1 in addition to the current ones, at a cost of ~40 $ g-1.
Therefore, non-precious metal catalysts are actively searched for, such as to meet already established operational benchmarks for conventional platinum PEMFC vehicular requirements (0.5 W cm-2; 5500 h durability) with the additional target of significant cost reduction.
Several papers on non-precious ORR catalysts have been published after a first report by Jasinski in 1964 demonstrating the ORR activity of metal substitutes-phthalocyanines. Then, research on metal-nitrogen macrocycles significantly expanded, leading to the picture that ORR catalytic activity can be related to N4-Me and N2-Me moieties. However, for precursors cost and unsatisfactory lifetime performance, research was steered toward more simple nitrogen-containing reactants and preparation procedures. Significant steps in this direction were obtained by Dodelet et al. who demonstrated that ORR overpotentials almost linearly decrease with increasing nitrogen content in carbon. Positive results were obtained on a series of samples prepared by high temperature treatment of carbon precursors in NH3/H2/N2 mixtures; doping of these modified carbons with iron rather than cobalt salts was shown to be preferable for better efficiency in oxygen reduction, even though still lower than that of platinum. Further improvements both in terms of incipient ORR potentials and currents were obtained by Maruyama et al. using carbons from hemoglobin and adenine-glucose pyrolysis in the presence of added Fe(II) and Cu(II)/Fe(II) mixtures, respectively. The ORR promoting role of nitrogen in carbon was independently demonstrated both theoretically and experimentally. Indeed, it was found that substitutional nitrogen at a few, specific, peripheral positions of graphene layers in well-ordered carbon nanostructures is in itself able to promote ORR activity even in the absence of accompanying metal centers.
Besides the above examined composition-dependent factors, catalyst activity also depends on structural and morphological carbon support features. In fact, many electrocatalytic reactions show faster kinetics on carbon edge planes compared with basal ones. This is related to the ability of the edges to more readily chemisorb O2 (this is the same reason why O2 combusts faster from edges and defects). On the other hand, an optimized porosity of carbon supports is beneficial for an easy access of the oxygen to the catalyst layer in contact with the proton exchange electrolyte membrane. Despite carbon materials of different textural morphology are widely used at an industrial level as supports for precious metal catalysts, in the PEMFC field, electrocatalysts are by far supported on the same VULCAN XC72 carbon. Given that improved catalytic activity of Pt-based catalysts has been achieved by the use of carbons with pore size centered in the mesoporous region, even developed with advanced synthesis, including template methods, such strategy should be pursued also for Pt-free catalytic systems.
In this project a number of Pt-free N-doped C-based catalysts have been synthesized on the basis of different synthetic and templating strategies aiming to understand how compositional, morphological and textural aspects of the end material can affect the electrochemical behaviour of ORR. Materials have been characterized using different physico-chemical methods including a study of the kinetics and mechanism of the electrochemical oxygen reduction reaction. Electrochemical results were obtained by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE). Surface and bulk analyses have been performed by BET technique, XPS and XRPD (sometimes data were recorded at synchrotron facilities). (HR) TEM, SEM (combined with FIB milling) imaging was also performed to characterize samples morphology.
Many types of samples have been synthesized, starting with mesoporours N-, Fe- doped carbons obtained by heat treatment of a solution of precursors, using silica as a templating agent. Outstanding results in terms of ORR electroactivity in acidic and alkaline conditions have been recorded. Some samples, especially in alkaline media, catalyze ORR even better than commercial Pt-based catalysts. Then, attempting to prepare materials with a precise and defined order and trying to emphasize some of their properties such as surface area and conductivity, ordered carbonaceous nano- and microstructures were synthesized. By chemical vapor deposition N-, Fe- doped carbon nanotubes (N-CNTs) were prepared and some interesting aspects related to the aging of the Fe-doped MgO catalyst used to grow N-CNTs were evidenced. Then, a modified method, but very similar to that used in the synthesis of nanotubes, surprisingly allowed the synthesis of innovative N-doped hollow carbon nanocubes (N-CNCs). This is the great novelty of the work. Due to nanocubes endothermal transformation, happening at 37°C, as detected by DSC, and to the empty space available in the internal part of each cube, many applications can be thought for example involving cubes as nano-reactors that can be opened/close in correspondence of body’s temperature changes. This feature could be taken into considerations for medical applications, after testing and verifying the biocompatibility of nanocubes. Finally, a completely different technique, an ultraspray pyrolysis method (USP) was used to obtain N-, Fe- doped carbon microspheres. The inherent scalability of continuous flow methods such as USP represents a significant advantage compared to alternative synthetic strategies requiring batch processing or surface catalyzed deposition of nanostructured carbon materials (e.g. CVD growth), this feature might be useful in order to improve electrode packing and, consequently, mass transport electrocatalytic applications.
The last results section, apparently diverging from the main goals of the present work, was thought to better understand the electronic C-surface behavior in charge transfer reactions. This is actually strongly connected to the oxygen reduction reaction, for which all the catalysts, hereby synthesized, were designed. However, instead of starting from complicated systems involving porous and doped-carbons, the choice was addressed to the simplest but closest material: annealed and non-annealed amorphous carbon thin films prepared by DC-magnetron sputtering technique.
Part of the work described was carried out at Trinity College Dublin in the laboratory of Prof. Colavita as a part of an academic collaboration and 5 months exchange granted by the European LLP Erasmus Program
Oxygen cathode for fuel cells. ORR activity of mesoporous N-modified carbon doped with non-noble metals
No full textFuel cells for low temperature applications are the subject of intensive research because they offer promise of reducing consumption of primary fossil fuels and greenhouse gas emissions. A difficulty comes from the kinetic behaviour of cathodic oxygen reduction that requires some form of effective catalytic promotion to proceed fast enough to meet fuel cell power requirements as for instance in the case of automotive transportation. Platinum and Pt-alloys are at present the best catalysts for oxygen reduction in PEM fuel cells. Nevertheless, due to the natural scarcity and cost of platinum, a very interesting challenge is to find an alternative non-precious, though less catalytically performing, material. Among others, nitrogen-modified carbons doped with non-precious transition centres (Fe, Co, etc.) are of interest because of the flexibility by which composition and morphology can be tailored by preparation.
In this work we present some results on oxygen reduction by various Pt-free catalysts obtained by carbonization and calcination of nitrogen bases/sugar mixtures in the presence of a metal salt and a silica gel as mesopore templating agent.
Catalysts were characterized by physical, chemical and electrochemical methods. Results are given in terms of the influence that sugars, nitrogen bases and metal centres exert on oxygen reduction potential, on the presence of a limiting current and on reaction mechanism. The best materials have been obtained by using fructose, a guanidine derivative and iron acetate. They are characterized by a well-defined limiting current, an onset potential approaching Pt ORR starting potential and a number of exchanged electrons n>3.9
Low-temperature intermediates to oxygen reduction reaction catalysts based on amine-modified metal-loaded carbons : an XPS and ss-NMR investigation
No full textCarbon functionalization is a major subject of interest in a number of project applications. Herein we report results on the characterization of nitrogen- and metal-loaded (Me = Fe, Co) carbon derivatives from low-T reaction steps before they are converted to catalysts for electrochemical oxygen reduction by later high-T treatments. The aim is to shed light on the state of carbon and carbon-bonded moieties before thermal modifications take place during any chosen high-T treatment. Though necessary for end catalyst activation, such thermal treatments make difficult to establish a relation between the starting reactants and finally obtained catalysts. Of interest to the paper are 13C, 15N solid-state NMR (ss-NMR) and high-resolution X-ray Photoelectron Spectroscopy (XPS) results on a commercial carbon that was reacted first with aliphatic di- and tri-amines and then with Fe, Co ions in room-T water. Data from natural abundance ss-15N NMR in combination with XPS analysis were found especially relevant to assess that, in the adopted conditions, amines preferentially bind to carbon by creating alkylimino functional groups, which spontaneously form hydrous surface metal complexes with soluble Fe and Co ions. A chemical model is thus proposed for metal coordination in such C-N species
Cerebral CO2 vasoreactivity evaluation by transcranial Doppler ultrasound technique: a standardized methodology
No full textObjective: In normal subjects
cerebral CO2 vasoreactivity is
measured during spontaneous hyperventilation,
breathholding, or
adding CO2 to inspiratory gases.
The correlation between CO2 and
cerebral blood flow may, however,
be invalidated by the effects of a
modified respiratory pattern on venous
return, sympathovagal balance,
and cathecolamine release.
Moreover, the duration of the test,
usually not considered, may play an
important role. This may justify the
scattering of values found in literature.
We evaluated a new standardized
method for overcoming these
confounding factors.
Design: Experimental.
Participants: Twenty-one healthy
volunteers.
Methods: Subjects were connected
through a mouthpiece to a mechanical
ventilator set in the intermittent
positive pressure ventilation mode.
The ventilator was fed by two 40-l
tanks, one of which contained 5%
CO2. The inspiratory CO2 concentration
was varied at fixed time intervals
from 0% to 5% without
modifying ventilator settings. Endtidal
CO2 was measured at the mouthpiece. Mean blood velocity
(Vm) and pulsatility index (PI) in the
middle cerebral artery were measured
by means of transcranial
Doppler ultrasound.
Results: The test was easily applicable
and well tolerated. No hemodynamic
alterations were observed
during the tests. The correlation
between CO2 and Vm was always
linear and highly significant
(R2 > 0.8, p < 0.0001). A low intersubject
variability was observed. No
difference was found between the
two hemispheres, nor between the
sexes.
Conclusions: The strict standardization
of the technique, avoiding
hemodynamic interference, may explain
the low intersubject variability.
The value of this technique in ventilated
neurosurgical patients is still
speculative, but it might allow the
collecting of valuable data together
with a reduction in exposure to CO2,
and hence cerebral blood flow modifications
Sugar-based iron-doped N-carbons for oxygen reduction reaction
No full textFuel cells for low temperature applications are the subject of intensive research because they could reduce consumption of primary fossil fuels and greenhouse gas emissions. However, performances of these devices are severely limited by the kinetics of the oxygen reduction reaction (ORR) at the cathode, so that the use of high-cost catalysts, such as Pt, is so far mandatory. Although catalytically optimal, Pt catalysts suffer drawbacks such as formation of poisoning surface Pt-oxides and particle coarsening (by dissolution/re-precipitation, Ostwald ripening), that cause loss of both active surface area and activity. Other negative aspects have economic origin due to the metal natural scarcity and cost. A very interesting research challenge is to find an alternative non-precious, though less catalytically performing, catalyst. Among others, nitrogen-modified carbons doped with non-precious transition metal centres, typically iron, are of interest because of reduced costs of precursors and easiness by which composition and morphology can be modulated by preparation.
In this work we present some results on oxygen reduction reaction by a series of Pt-free catalysts obtained by pyrolysis of guanidine-based nitrogen compounds/sugar mixtures in the presence of a metal salt and a templating agent.
Catalysts were characterized by physical, chemical and electrochemical methods. Results are presented in terms of the influence that precursors nature exerts on oxygen reduction onset potential, presence of defined limiting current and reaction mechanism. They are characterized by a well-defined limiting current, an onset potential approaching Pt ORR starting potential and a number of exchanged electrons n>3.9. An attempt of explanation of different electrocatalytic behavior obtained varying the nature of precursor is also done
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