Materials platinum-based

Chen, A. and Holt-Hindle, P. (2010) Platinum-based nanostructured materials synthesis, properties, and applications. Chemical Reviews, 110, 3767-3804. 

At present, the only material that shows reasonable electrocatalytic activity is platinum and platinum based alloys. A msgor problem of these catalyst, aside fiom the high price of platinum, is rapid decrease of the catalytic activity after starting oxidation. 

For these low-temperature fuel cells, the development of catalytic materials is essential to activate the electrochemical reactions involved. This concerns the electro-oxidation of the fuel (reformate hydrogen containing some traces of CO, which acts as a poisoning species for the anode catalyst methanol and ethanol, which have a relatively low reactivity at low temperatures) and the electroreduction of the oxidant (oxygen), which is still a source of high energy losses (up to 30-40%) due to the low reactivity of oxygen at the best platinum-based electrocatalysts. 

Aqueous phase reforming of glycerol in several studies by Dumesic and co-workers has been reported [270, 275, 277, 282, 289, 292, 294, 319]. The first catalysts that they reported were platinum-based materials which operate at relatively moderate temperatures (220-280 °C) and pressures that prevent steam formation. Catalyst performances are stable for a long period. The gas stream contains low levels of CO, while the major reaction intermediates detected in the liquid phase include ethanol, 1,2-pro-panediol, methanol, 1-propanol, propionic acid, acetone, propionaldehyde and lactic acid. Novel tin-promoted Raney nickel catalysts were subsequently developed. The catalytic performance of these non-precious metal catalysts is comparable to that of more costly platinum-based systems for the production of hydrogen from glycerol. 

The amount of catalyst in such cases is rather high 1000-5000 ppm and selectivity towards anti-Markovnikov addition is lower (80-90%), compared to hydrosilylation in the presence of platinum based catalyst. The synthesis of phenylethenyl substituted siloxanes is of commercial importance, driven by potential application in personal care products. Such materials should be in the form of fluids and thus in order to preserve this requirement two approaches have been exploited. One of them involved substitution of less than 100% phenylethenyl moieties, the other made use of 1-hexene as a co-reactant, leading to decreased crystallinity of the final materials. Depending on the structure of (methylhydrido)siloxanes and reaction conditions the resulting silicon fluids exhibited refraction indices ranging from 1.527 to 1.574 (Table 1). 

The materials for PEMFC electrodes should have good electrical conductivities and be stable in contact with electrolyte. Platinum-based electrodes have shown excellent electrochemical activities for PEMFC. 

In recent years, the interest toward PM catalysts has grown from essentially academic to much more practical, as a result of efforts to develop on-board fuel processors for cars. Operational conditions envisaged for such fuel processors preclude the existing Cu-Zn-Al catalysts from being applicable, while PM catalysts are believed to be a viable candidate. Platinum-based catalysts with a great variety of promoters, modifiers, and supports, as well as their preparation conditions seem to be most widely explored in research papers and patent applications. Pt supported on ceria containing support materials have been widely explored for the WGS reaction. - ... 

Pt has the highest adsorption of methanol on its surface, but its catalytic properties are low due to the formation of poison species (most notably CO) that can be oxidized only after the Pt is covered with OH. Platinum-based bimetallic electrocatalysts, such as Pt-Ru alloys and Ru-decorated Pt materials, are the most active ones. The bi-functional mechanism is to a large extent operative in these catalysts. Most commercial Pt-Ru catalysts are based on 1 1 Pt-Ru alloy. While the alloys typically show enhanced activity in comparison with pure Pt, there is significant Pt loading in the bulk of the alloy in which catalysis does not proceed because the sites are inaccessible for methanol adsorption hence, the need for reducing the Pt content. 

Catalytic hydrogenation of benzene to cyclohexane (see Section 12.1), normally in the liquid phase, around 200°C, at 4.10 Pa absolute, in the presence of a uckel or platinum base catalyst, which is highly sensitive to the existence of shlfur compounds in the raw material... 

Platinum and chlorine (samples made with chloride precursors) contents of the catalyst samples were determined with X-ray fluorescence spectroscopy (XRF) (Phillips PW 1480 spectrometer). BET surface areas of catalysts were within 5% that of the silica support material. Platinum dispersion was measured with hydrogen chemisorption in a volumetric set-up, using a procedure described elsewhere [3]. Stoichiometry of H/Pt = 1 was assumed for calculating the platinum dispersion [4]. Transmission electron microscopy (TEM) (Phillips CM 30, 300kV) was used to check the platinum particle size in some of the catalysts. Average platinum particle size was determined based on analysis of about 100 platinum crystallites. 

Improved kinetics at the negative electrode. Research to date has found that platinum-based electrocatalysts are the only materials that are able to activate methanol. Even then, the overall reaction at the negative... 

For polymer electrolyte membrane fuel cell (PEMFC) applications, platinum and platinum-based alloy materials have been the most extensively investigated as catalysts for the electrocatalytic reduction of oxygen. A number of factors can influence the performance of Pt-based cathodic electrocatalysts in fuel cell applications, including (i) the method of Pt/C electrocatalyst preparation, (ii) R particle size, (iii) activation process, (iv) wetting of electrode structure, (v) PTFE content in the electrode, and the (vi) surface properties of the carbon support, among others. ... 

E. Antolini, T. Lopes, E. R. Gonzalez An overview of platinum-based catalysts as methanol-resistant oxygen reduction materials for direct methanol fuel cells, Journal of Alloys and Compounds , 461, 253-262 (2008). 

Platinum based catalysts supported on carbon black allowed to significantly increase the power density per electrode area as compared to platinum black type catalysts. The pore system of the support material allows to increase the platinum dispersion and partially prevents migration and the agglomeration of nanoparticles thus leading to a higher specific surface area. 

Presently, the oxidation of methanol on pure platinum has more academic interest than practical application once DMFC universally employs platinum based materials having two or more metals as an anodic catalyst In absence of methanoUc inteimediate readsorption, the maximum reactiOTi rate for CO oxidation is 100-fold smaller than maximum reaction rate for CO adsorption from methanol dehydrogenation steps [11]. Indeed, the mechanism of methanol oxidation on platinum is expected to be equal to that on its alloys despite different kinetics which would result in a selection of pathway. In terms of complex activation theory, alloyed Pt is intend to lower the Ea barrier for CO adsorption, thus driving methanol oxidation to completion. As previously established [3], there are several factors that affect the calculated activation energy for the MOR at a given potential, such as coverage of methanoUc intermediates and anion adsorption from the electrolyte as well as pH and oxide formation processes. 

On a basis of trial and error it was noticed that a practical fuel cell attains higher performance employing ternary platinum based materials than employing the binary catalysts. During the last decade, the global observation reveals an increasing of performance for the H2/CO oxidation as well as for the MOR when a third element was added to the best bimetallic catalyst, the Pt-Ru [57] or Pt-Sn [58] based material. An overview of the preparation and structural characteristics of Pt-based ternary catalysts [59] and their electrochemical performance [60] was presented by AntoUni. Therein, all the relevant works before 2007 are found. In summary, many ternary Pt-Ru-M catalysts (M = Wi Wox or W2C form. Mo, Ir, Ni, Co, Rh, Os, V) perform better than commercial standard Pt-Ru catalysts and/or Pt-Ru catalysts prepared by the same method than the ternary. 

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