Alloys platinum-based

The materials currently used in the production of medical devices include stainless steels, cobalt-base alloys, titanium-base alloys, platinum-base alloys, and nickel-titanium alloys. Steels were the first modern metallic alloys to be used in orthopedics and initial problems with corrosion were overcome by modifying the composition of the steel with the addition of carbon, chromium, and molybdenum. Carbon was added at low concentrations (ca. 0.03-0.08%) to initiate carbide formation, while the addition of chromium (17-19%) facilitated the formation of a stable surface oxide layer and the presence of molybdenum (2.0-3.0%) was found to control corrosion. The compositions of stainless steels used can vary widely. Table V shows the limits for the chemical compositions of three different alloys containing eleven different elements together with the mechanical properties for the samples after annealing and cold working. 

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

Neergat N, Sbukla AK, Gandhi KS. 2001. Platinum-based alloys as oxygen-reduction catalysts for solid-polymer-electrolyte direct methanol fuel cells. J Appl Electrochem 31 373-378. 

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. 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

Platinum-based bi-metallics (Pt M, M = Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the OER. Several rationales have been proposed including (1) enhanced chemisorption of intermediates (2) a lattice change of Pt that results in the shortening of Pt-Pt interatomic distances by alloying (3) the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer caused by the underlying alloy and the anchor effect of alloy metals on a carbon carrier.93,94... 

The MTL (Mass Transfer Limited) gauzes use Pd/Ni (95/5) or Pd/Cu (95/5) alloys. The base metals in the alloys are not stable at the nitric acid plant operating conditions. They are rapidly oxidized and vaporized in the gas stream. This rapid oxidation makes the wire swell by a factor of 2 after ten days of operation. The larger surface area results in a large improvement in the capture of platinum oxide vapor237. 

Junction beads can be made for platinum-based thermocouples, such as types R, S, or B, by welding with a high-temperature flame (e.g. oxy-acetylene). Using a flame for junction formation in alloy-based (e.g. K- or 2 -type) thermocouples does not work well since the wires tend to oxidize rather than fuse. Beads are more effectively made by electric arc for these thermocouple types. 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth-... 

The use of supported platinum-based catalysts in hydrogen cyanide synthesis has been described, and some are in use in certain plants. For instance, Merrill and Perry recommended natural beryl coated with platinum or a platinum alloy [12], and a range of alternative supports were considered by Schmidt and his co-workers [13], including coated foamed ceramic and monolithic substrates. They are claimed to have some advantages in ammonia oxidation [14,15]. 

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

XPS or AES is extensively used not only to indicate the cleanliness of the sample before transfer, but also to indicate the presence of adsorbates and their oxidation states following electrochemical experiments and transfer back into the UHV environment. In the case of model platinum-based electrocatalysts, the electron spectroscopies have been used to estimate the coverage of the adsorbate metal atoms or the alloy composition. In the case of alloys, or the nucleation and growth of metal adsorbate structures, the techniques give only the mean concentrations averaged over a depth determined by the inelastic mean free path of the emitted electrons. Adsorption and reaction at surfaces often depend on the... 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

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. 

Colon-Mercado HR, Popov BN (2006) Stabihty of platinum based alloy cathode catalysts in PEM fuel cells. J Power Sources 155 253-263... 

As far as the action of supported bimetallic catalysts is concerned, the main theories suggest either geometric and/or electronic effects to account for the improved catalytic properties. For instance, in platinum based naphtha reforming catalysts, the electronic modification of platinum particles may be induced by an interaction with an oxide layer of the promoter or by alloy formation. The electronic modification results in a change in the Pt-C bond strength of adsorp-... 

In order to improve the electrocatalytic properties of methanol electrodes, and to reduce the poisoning phenomenon usually observed with bulk platinum, different platinum based alloys were considered such as Pt-Ru, and Pt-Sn, etc. [153]. Therefore such alloys were also dispersed into electron conducting polymers. Hable et al. [53] were apparently the first authors to disperse Pt-Sn catalyst particles in a polyaniline matrix, in order to activate the oxidation of methanol. They evaluated the Pt/Sn ratio by X-ray Photoelectron Spectroscopy and found that small amounts of Sn (e.g. Pt/Sn ratios of 10/1) were sufficient to enhance the electrocatalytic oxidation of methanol. Pt was found to be in the Pt(0) state whereas Sn was in an oxidized form. Similar observations concerning the enhanced electrocatalytic activity of Pt-Sn particles incorporated in PAni films were made by Laborde et al. [154]. Such Pt-Sn alloys are also very active for the electrocatalytic oxidation of ethanol [68,154]. 

The enhancement effect due to the presence of Ru and Sn in platinum-based dispersed alloys on the oxidation rate of organic Cl molecules was related to a strong decrease in CO poisoning, so that the electro-... 

Figure 10.8 Tafel plots of the electrocatalytic oxidation of 0.1 M formic acid in 0,1 M HCIO4 at different platinum-based alloy electrodes dispersed in a polyaniline film and containing 0,1 mg cm of platinum (—) Pt (---) Pt-Ru (...) Pt-Sn (—) Pt-Ru-Sn. (Reprinted with permission from ref 69)...

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. ... 

Other approaches have focused upon using non-precious metals and their oxides as alternatives to the platinum catalysts. For example, the mixed oxide catalysts of the binary and ternary alloys of noble metals and transition metals have been investigated for the oxygen evolution reaction in solid polymer electrolyte water electrolyzers. Binary, ternary, and quaternary platinum alloys with base metals of Cu, Ni, and Co have been used as electrocatalysts in liquid acid electrolyte cells. It was also reported that a R-Cu-Cr alloy displayed better activity to oxygen reduction than R and Pt-Cr in liquid electrolyte.The enhanced electrocatalytic activity of these types of alloys has been attributed to various factors, including the decrease of the nearest neighbor distance of platinum,the formation of Raney type... 

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). 

Colon-Mercado Hector R., Popov Branko N. Stability of platinum based alloy cathode catalysts mPEM fuel cells, Journal of Power Sources , 155,253-263 (2006). 

I. Becerik and F. Kadirgan, Glucose sensitivity of platinum-based alloys incorporated in polypyrrole films at neutral media, Synth. Met., 124, 379-384 (2001). 

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. 

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