Platinum nanoparticle | EPFL Graph Search

Platinum nanoparticles are usually in the form of a suspension or colloid of nanoparticles of platinum in a fluid, usually water. A colloid is technically defined as a stable dispersion of particles in a fluid medium (liquid or gas). Spherical platinum nanoparticles can be made with sizes between about 2 and 100 nanometres (nm), depending on reaction conditions. Platinum nanoparticles are suspended in the colloidal solution of brownish-red or black color. Nanoparticles come in wide variety of shapes including spheres, rods, cubes, and tetrahedra. Platinum nanoparticles are the subject of substantial research, with potential applications in a wide variety of areas. These include catalysis, medicine, and the synthesis of novel materials with unique properties. Synthesis Platinum nanoparticles are typically synthesized either by the reduction of platinum ion precursors

Investigation of platinum-based nanoparticles on boron-doped diamond substrate for applications in electrocatalysis

Guillaume Siné

The search of new energy sources has motivated during the last few years numerous research projects due to environmental, economical and political reasons. One of the main subjects of these works is the development of fuel cells. Within this framework the Direct Alcohol Fuel Cell (DAFC), mainly fed with methanol or ethanol, has received great attention. However one of the major limitations of the DAFC is the electrocatalysis of fuel oxidation; the electrocatalyst (dispersed Pt-based particles) is indeed readily poisoned by adsorbed intermediates, hence decreasing the fuel cell efficiency. Consequently the electrocatalysis of methanol and ethanol oxidation on different types of Pt-based nanoparticles, deposited on synthetic boron-doped diamond (BDD) thin film, has been studied during this thesis. This substrate has been chosen due to its outstanding properties of chemical inertness, illustrated by a very low capacitive current. In the first part of this thesis, electrodeposited Pt particles on BDD have been studied. Electrodeposition of Pt particles has been carried out both on bare BDD and on diamond pre-modified with gold nanoparticles deposited via a thermal decomposition technique. In both cases it was established that Pt particles were electrodeposited following a progressive mechanism during which formation of new nuclei and growth of primary nuclei simultaneously occur. The electrocatalytic activity of electrodeposited particles was also studied, and it was shown that the presence of Au particles together with Pt particles does not influence markedly the electrocatalytic behaviour of the later ones. Preliminary calcination of the co-deposit results in a dramatic decrease of its electrocatalytic activity, presumably due to the formation of a core (Pt)–shell (Au) structure less capable of alcohols dehydrogenation. Finally electrodeposition of Pt on BDD leads to particles of diameter in the 150-700 nm domain with very large size distribution. Obviously such particles are not consistent with the definition of nanoparticles in catalysis and electrocatalysis. In a second part of this work, Pt-based nanoparticles synthesized via the microemulsion method have been investigated once deposited on a diamond substrate. Attention has been mainly focused on Pt, Pt/Ru and Pt/Sn nanoparticles. All the synthesized nanoparticles are in the 2-5 nm size range with narrow size distribution. Moreover the compositions of bimetallic particles were very close to those expected. The electrocatalytic behaviour of bimetallic Pt-based nanoparticles toward both methanol and ethanol electrooxidation has been investigated. Bimetallic nanoparticles are more efficient than pure Pt, and it was shown that Pt/Ru nanoparticles were more indicated for electrocatalysis of methanol oxidation whereas Pt/Sn were more efficient in the case of electrocatalysis of ethanol oxidation. Both cooperative and electronic effects are involved in the enhancement of the electrocatalytic activity of bimetallic nanoparticles, and Pt-rich nanoparticles are the most efficient ones due to their superior alcohols adsorption properties. However the electronic effect is not of same nature in Pt/Ru than in Pt/Sn (Pt/Ru nanoparticles are alloyed ones, on the contrary of Pt/Sn), and this may explain the probable activation of the ethanol C-C bond scission by Pt/Sn electrocatalysts. Due to the different action modes of Pt/Ru and Pt/Sn electrocatalysts, a ternary Pt/Ru/Sn sample has also been synthesized by the microemulsion technique. This ternary electrocatalyst has exhibited very good activity toward methanol electrooxidation whereas that toward ethanol electrooxidation was significantly lower. It is believed that the electronic effect mainly occurs between Sn and Ru components of the sample, creating a new state of adsorbed oxygenated species that should be of higher mobility and reactivity than on bimetallic surfaces. In addition, a model for methanol electrooxidation at Pt/M (M = Ru, Sn...) surfaces has been developed in order to generalise the obtained results. The concepts and theories of heterogeneous catalysis have been extended to the specific case of this electrochemical reaction involving adsorbed intermediates. The donor-acceptor theory of heterogeneous catalysis has been more particularly considered and adapted, and within this framework Pt has been considered as the acceptor whereas Sn was regarded as the donor. The development of this model has made the establishment of a relation between the measured current to the applied potential possible in a number of real limiting cases. Validity and limitations to the proposed model has in due course been discussed in view of the obtained results.

EPFL2006

Self and induced stabilized nanoparticles for electrocatalysis

Mériadec Limat

Since few decades the search of new catalysts formulations for fuel cell applications has motivated numerous projects especially due to the future lack of fossil fuel, environmental and economical reasons. Consequently, Direct Alcohol Fuel Cell (DAFC) or Direct Formic Acid Fuel Cell domains have received great attention. In spite of the promising results reported recently, these types of fuel cell still exhibit some problems. In fact, platinum, bulk or nanoparticles, undergoes surface poisoning involved by intermediates (as acetaldehyde, acetic acid, carbon monoxide) which are strongly bonded to the surface. These poisoning species block the active sites at the surface electrode and drastically decrease the catalytic activity and also the life-time of the cell. Probable solutions are suggested essentially in Pt-based alloys catalysts or nanocatalysts in order to avoid this phenomenon. Therefore, multicomponent catalysts deposited at carbon electrodes (Boron-Doped Diamond and Highly Oriented Pyrolytic Graphite) has been investigated during this thesis. These two substrates have been chosen due to their outstanding properties. In the first part of this study, self stabilized nanoparticles at BDD substrate have been studied. Ex-situ ternary Pt-Ru-Sn microemulsion-synthesized nanoparticles deposited at BDD substrate were investigated towards both methanol and ethanol electrooxidation. Such type of composite electrode seemed to possess a greater efficiency towards methanol oxidation than observed for ethanol, leading essentially for the latter to C2 oxidation products. The inability of this catalyst to activate the C-C bond scission can explain this observation. However, the major enhancement was observed for the methanol electrooxidation. Recent studies have shown that gold nanoparticles exhibited surprising catalytic activity, very different from the bulk, making them a privileged candidate to replace or be added to platinum. Gold nanoparticles prepared with a twofold technique (sputtering followed by a heat treatment in air) have been characterized with both outer and inner-sphere redox reactions. It has been observed that the resulting electrochemical behavior of such type of composite electrode was similar to a gold microelectrodes array, due to the high difference between the two material electrochemical rate constants. In a morphological point of view, the processes of nucleation and growth engaged in the stabilized and well-dispersed gold nanoparticles formation have especially attracted a great attention. At BDD surface, the gold nanoparticles were located at specific sites (grain boundaries, defects, impurities,…). This phenomenon has motivated further investigations at HOPG substrate, known to possess a quite smooth and perfect surface. In the second part of this thesis, induced stabilized nanoparticles at HOPG surface were studied. In a first time, HOPG was morphologically and electrochemically characterized. The surface state has appeared to be a key parameter, essentially with the presence of the two surface species, the edge plane and the basal plane. High modifications in the electrochemical behavior of HOPG electrode were observed after an anodic pretreatment in acidic media, due to surface concentration increase of the edge plane species, which are highly reactive. Based on these results a tailoring of the HOPG has been performed in order to create nanocontainers for a further gold deposition. HOPG surface has been bombarded by gold clusters and then oxidized (etching step) under well-defined experimental conditions, leading in a tailored pattern of nanopits with high reliability and reproducibility. Gold deposition was performed by evaporation. The gold nanoparticles were located in the created nanocontainers and were found to be stable under cyclic voltammetric measurements in acidic media. A new synthesis method for a bimetallic or alloy system formed by Au and Pt nanoparticles stabilized at tailored HOPG surface was developed. Pt electrodeposited at Au/HOPG electrode surface have been observed to preferentially deposit on Au. In fact, gold nanoparticles were used as nucleation sites for the platinum particles. The resulting Pt-Au/HOPG electrodes were then heated at 400 °C in vacuum conditions to improve interactions between both metals. The appearance of an alloy was observed and characterized by a negative shift in the peak potential of Pt oxides reduction. Thanks to these results, Pt-Au/HOPG composite electrodes were applied for electrocatalytic oxidation. At a surface composition of P55-Au45 alloy, Pt-Au/HOPG after the heat treatment has demonstrated a higher electrocatalytic activity than before the heat treatment towards formic acid electrooxidation. It seemed that electronic and geometrical effect was responsible of such enhancement. Finally, bismuth adsorption at both P55-Au45/HOPG and polycrystalline platinum was investigated towards formic acid electrooxidation. The effect of Bi adatoms has been essentially ascribed to geometrical restrictions for CO to adsorb at platinum active sites. Drastic enhancement in current density at a fixed potential and dramatic decrease of the onset potential of formic acid oxidation were observed. Thus, it seemed that avoiding the CO poisoning can be the most judicious development for new electrocatalysts in fuel cell applications.

EPFL2009

Boron-doped diamond-supported Pt nanoparticles synthesized in compartmentalized nanoreactors for electrocatalysis

Christos Comninellis, Guillaume Siné

Platinum nanoparticles have here been synthesized via the microemulsion method that isolates the reactive species from the whole medium into compartmentalized nanoreactors. Such synthesized Pt nanoparticles have diam. included in the 2-5 nm range domain with narrow size distribution. It was also shown that, when deposited on Boron-Doped Diamond electrode and stabilized by a Nafion adlayer, microemulsion-synthesized Pt nanoparticles can be activated by OH radicals electrogenerated on BDD at high overpotentials. Activated nanoparticles has exhibited higher activity towards methanol electrooxidn., due to both addnl. cleaning of the surface and participation of the radicals to the oxidn. process. [on SciFinder (R)]

2005